CA2917497A1

CA2917497A1 - Methods and apparatus for crystallization of salts

Info

Publication number: CA2917497A1
Application number: CA2917497A
Authority: CA
Inventors: Robert W. Besant; Richard W. Evitts; Xiaodong Nie; Jiban Podder
Original assignee: University of Saskatchewan
Current assignee: University of Saskatchewan
Priority date: 2016-01-13
Filing date: 2016-01-13
Publication date: 2017-07-13

Abstract

Method or apparatus for increasing the production rate, size, and/or quality of salt crystals produced from salt containing solutions by controlling temperature, evaporation, and/or circulation of the salt-containing solution. The salt-containing solution may be a salt-containing solution in a crystallization pond obtained as a product of solution mining, such as a KCl-containing solution obtained as a product of solution mining of a potash mine. Apparatuses may be provided to actively cool layers of the salt-containing solution, to increase the evaporation rate of water from the salt-containing solution, to increase or sustain circulation of the salt-containing solution and/or to form or maintain predetermined circulation cells within the salt-containing solution.

Description

METHODS AND APPARATUS FOR CRYSTALLIZATION OF SALTS
Technical Field [0001] Some embodiments of the present invention pertain to methods for crystallizing salts from a solution. Some embodiments pertain to apparatus for crystallizing salts from a solution. Some embodiments pertain to apparatus or methods for increasing the rate of salt crystal formation in a crystallization pond.
Some embodiments pertain to apparatus or methods for enhancing the quality of salt crystals formed in a crystallization pond.
Background

[0002] The crystallization of salts from solution is important in a number of processes. Crystallization from solution can be used in the production of many important inorganic crystals, for example, potassium chloride (KC1), sodium chloride (NaC1), sodium sulphate (Na2SO4), potassium sulphate (K2504), magnesium sulphate (Mg504.7H20), zinc sulphate (ZnSO4), copper sulfate (CuSO4), urea, thiourea, potassium di-hydrogen phosphate (KH2PO4), ammonium di-hydrogen phosphate (NH4PO4), or the like.

[0003] One example of a commercially important crystallization process is the crystallization of KC1 (potassium chloride). Crystallized KC1 is widely used in various fertilizers (for example, as applied to agricultural land to increase crop yields). One source of KC1 is mined potash. Examples of patents pertaining to the production of KC1 using crystallization include US 4,386,936 to Geesen; US

4,276,117 to Geesen; US 3,966,541 to Abraham; and US 3,384,459 to Carter, each of which is incorporated by reference herein.

[0004] Two techniques that can be used to produce crystalline salt products, including crystallized KC1products, include pond crystallization (typically used for solution mining operations and in naturally occurring salt lakes) and flotation bubble separation of mined potash crystals (typically used for mechanically mined small particles from potash mines).

[0005] Solution mining is achieved by pumping heated water or weak brine solutions down distributed and directionally drilled injection wells and extracting a nearly equal volume of saturated or nearly saturated salt solutions from nearby production wells. The produced saturated or nearly saturated solution is often warm, e.g. at a temperature between 35 to 40 C (which is, for example, the mine salt layer rock temperature in example operations in Saskatchewan, Canada). In some deposits, the ore body contains 50 to 55% wt/wt NaC1 and 40 to 45% wt/wt KC1 plus other salts and other impurities. Solution mining at the ore body temperatures common in Saskatchewan, Canada typically results in high concentrations of K + Cl- ions in the solution, low concentrations of Na' ions, and trace mass fractions of other materials.

[0006] During mining, as the KC1 salt deposit becomes depleted in one underground area, the KC1 concentration in the pumped-out solution will decrease and the relative concentration of NaC1 will increase slowly. At some time duration after the start of production in this production area, which has been producing saturated solution at a selected rate, it will be advantageous to move the supply and production wells to new positions. This decision is mostly based on a cost-benefit analysis for any given operation. Using horizontal drilling in the pay-zone or production¨zone of the geological formation, the total production from each particular set of well pipes can be large before new wells are required;
however the rate of production from a set of wells may have to be limited to get the maximum total mass production.

[0007] Differential crystallization is used to produce the desired KC1 crystal products from the solution and separate them from the Na ions in solution.
This can be done by cooling and evaporating the solution water such that the solution state remains within the metastable saturation state of KC1 on a phase diagram. Crystalline KC1 products can be produced from this solution in large outdoor ponds or indoors in evaporated, circulated and cooled crystallizers that have large energy input rates to control the state of the solution. Outdoor crystallization ponds have been operated in batch modes of production and have little or no control of conditions ¨ so there is a much greater variation in the crystal product sizes, quality and rate of crystal production as compared with indoor crystallizers. Since weather conditions over a typical year are so variable, often for any given operation both indoor plant and outdoor means of salt crystal production are used. The cost of producing crystallized product using an outdoor crystallization pond is typically low compared with the cost of production in an indoor processing plant.

[0008] In one example installation in Saskatchewan, Canada, the outdoor salt ponds are open ponds which are each approximately 3.5 m deep, covering about 0.5 km2. KC1 crystals are produced as the solution cools, primarily by natural pond cooling (i.e. heat convection to the air). After KC1 crystals grow to a certain size, they settle to the bottom of the pond from the open surface as the pond temperature drops. Over time a thick layer of KC1 crystals accumulates on the bottom and the solution pond becomes stratified in average pond liquid density, with density increasing toward the bottom. The temperature increases with depth because almost all the cooling takes place at the open surface of the pond.
This pond stratification restricts any natural circulation of the liquid solution (for example due to wind and destabilizing temperature gradients) in the pond to a fraction of the pond depth. Only a negligible fraction of the heat loss occurs at the bottom of the pond compared to that lost on the top surface of the pond.
Energy transfer by evaporation from the top surface of the pond is not very significant compared to convective heat transfer because, in cold or cool weather, the moisture carrying capacity of air is low, and so convective heat transfer dominates over the entire surface area of the ponds.

[0009] This situation is illustrated schematically for a hypothetical example in Figure 1, showing different regions of temperature and salt concentration vertically within a typical prior art crystallization pond. The variations of these properties in the horizontal direction are likely to be negligible except for in the pond perimeter region and near the pond surface. Upper region 18 of the illustrated crystallization pond is at a cooler temperature than the middle region 19 or bottom region 21. Upper region 18 also has a lower concentration of salt in solution, e.g. KH and a ions. The concentration of KC1 increases with increasing depth through middle region 19 and bottom region 21. Salt crystals accumulate at the bottom of bottom region 21. During variable wind, ambient air temperature and solar irradiation conditions, the solution in upper pond region 18 may move and mix. Solution temperatures in this region will vary spatially and temporally. The portion of the crystallization pond with conditions within the metastable zone width region (MSZW) where salt crystals may be produced is illustrated as region 23. Region 23, although shown as an exact known location, is in fact uncertain and variable in location and size. As well, each of the interfaces shown in Figure 1 will vary with the time duration of exposure.

[00101 When a large fraction of the dissolved KC1 has formed crystals that have been deposited on the bottom of the pond, the pond is drained and the bed of crystals is mechanically harvested. This is a batch production process. In the example installation described above, this batch process may be staged at three levels of crystal production, with varying factions of the crystal volume taken out at each stage. That is, when about 1/2 of the crystals are deposited in the first pond, the brine solution is pumped into another pond and crystals are harvested from the first pond, when about the next 1/3 of crystals are deposited in the second pond, the second pond is drained, mechanically harvested and the solution is pumped into the third pond where the last 1/6 of the crystals are produced. In this cascade of ponds the average temperature of each of these ponds decreases by about the same absolute amount over time as the solution cools before being pumped to the next pond (e.g. 10 to 15 C in winter and 5 to 10 C in summer). The duration of time the salt solution spends in each pond in the cascade is approximately the same, but pond production of KC1 crystals is several times slower in summer than in winter when the temperature differences between the solution and the atmospheric air are large.
[0011] Although the above-described example process using outdoor crystallization ponds produces crystalline KC1, it is not optimal because, at any time, only a small fraction of the pond volumes are producing crystals and of the produced crystals only a small fraction are considered to be of good quality (i.e. to be generally transparent and clear with cubic morphology). The duration the salt solution remains in each pond varies from a minimum of about 6 days for cold weather periods in winter to about 18 days for warm weather in summer. The size and quality of crystals produced increases from the first to the last pond in the sequence of three ponds; the first pond produces crystals with a size range from 0.2 to 1.2 mm, the second produces crystals with a size range from 0.9 to 2.0 mm, and the third pond produces crystals with a size range from 1.6 to 3.0 mm. The first pond crystals are slightly colored and opaque due to impurities and particles comprised of agglomerated small crystals, while the last pond produces a high fraction of crystals that are mostly transparent and clear. Pure large KC1 crystals, which have the best market price, have a cubic morphology, with three nearly equal characteristic lengths, or they have a truncated square morphology with only two sides equal. These high quality crystals are clear and transparent.
[0012] In the above-described example installation, the rate of production is limited primarily by the total surface area of the crystallization ponds and the temperature differences between the supply feed inlet of solution and the ambient air.
[0013] It is desirable to provide new methods and apparatus for increasing the production rate of crystalline salt products, particularly methods and apparatus applicable to an outdoor crystallization pond.
[0014] It is desirable to provide new methods and apparatus for increasing the crystal size and/or quality of crystalline salt products produced in a crystallization pond, including in an outdoor crystallization pond. For example, the typical crop seeds used in many agricultural applications may have a size in the range of, for example, 1.5 to 5 mm. In agricultural applications, it may be desirable to distribute fertilizer and crop seeds concurrently and of nearly the same size and/or to reduce any dust caused by handling fertilizers of very small particle size (i.e. less than 0.1 mm). As well, pure large salt crystals have a lower risk of caking during storage, and, because they are mechanically stronger, are not as easily damaged by typical mechanical bulk handling. Thus, larger salt crystals resist mechanical damage and the formation of dust, and also have a slightly longer shelf life than smaller salt crystals.
[0015] It is desirable to provide new methods and apparatus for improving the quality of salt crystals produced in a crystallization pond, including in an outdoor crystallization pond. Higher quality crystals include crystals having a particular desired morphology, having an absence of inclusions, and/or not comprising small agglomerated particles.
[0016] The foregoing examples of the related art and limitations related thereto are intended to be illustrative and not exclusive. Other limitations of the related art will become apparent to those of skill in the art upon a reading of the specification and a study of the drawings.
Brief Description of the Drawings [0017] Exemplary embodiments are illustrated in referenced figures of the drawings. It is intended that the embodiments and figures disclosed herein are to be considered illustrative rather than restrictive.
[0018] Figure 1 shows schematically a hypothetical cross-sectional elevation view of the different regions of temperature and salt concentration within a typical prior art crystallization pond. This figure shows regions in the pond where the temperature and salt concentration may vary versus the pond depth in a typical prior art crystallization pond.

[0019] Figure 2A shows the saturation line for the equilibrium phase diagram of a solution of KC1 and water.
[0020] Figure 2B shows the preferred metastable region of crystallization for a KC1 + H20 system in the super-saturation region adjacent to the equilibrium saturation line for phase change from solution-to-(KC1 crystals + solution) for quasi-equilibrium crystal production superimposed on part of the thermodynamic phase diagram for KC1 solutions with and without the addition of a chelating agent.
[0021] Figure 3 shows schematically an example embodiment of a crystallization pond having an apparatus for actively cooling the bottom of the crystallization pond.
[0022] Figure 4A shows schematically an exemplary configuration for the cooling tubes provided at the bottom of a crystallization pond. Figure 4B
shows schematically a second exemplary configuration for the cooling tubes that allows for position-dependent control of the rate of cooling using the distribution of the cooling tubes and a plurality of valves.
[0023] Figure 5 shows schematically an example embodiment of a crystallization pond having an auxiliary cooling system.
[0024] Figure 6 shows schematically an example embodiment of an apparatus for sub-cooling the groundwater used as a heat sink for the auxiliary cooling system.

[0025] Figure 7 shows schematically an example embodiment of a crystallization pond having a bubble injection apparatus for enhancing circulation within the crystallization pond and/or for enhancing the water vapor evaporation rate of the pond.
[0026] Figures 8A and 8B show schematically a plan view of bubble injection tubes on the bottom of a crystallization pond having bubble injection apparatus for generating hexagonal circulation cells (Figure 8A) or parallel circulation cells (Figure 8B).
[0027] Figures 9A and 9B show a schematic diagram of a parallel circulation cell showing one streamline or particle pathline for each of the counterclockwise and clockwise helical circulation cells in a plan isometric view (Figure 9A, viewed along line A-A indicated in Figure 9B) and elevation view (Figure 9B, viewed along line B-B indicated in Figure 9A).
[0028] Figure 10 shows a partial top view of a portion of an example embodiment of a bubble injection tube having two parallel bubble injection tubes each adapted to provide a different size of bubble to the solution in a crystallization pond.
[0029] Figures 11A and 11B show schematically a plan view of two example embodiments of a warm solution supply system.
[0030] Figures 12A, 12B and 12C show a schematic elevation view, elevation view and plan view, respectively, of an example embodiment of a warm solution supply system.

[0031] Figure 13 shows schematically a cross-sectional view illustrating the different regions of temperature and salt concentration within an example embodiment of a crystallization pond.
[0032] Figure 14 shows a schematic view of a crystal vacuum harvesting device according to an exemplary embodiment.
[0033] Figure 15 shows schematically a method for controlling features of an example embodiment of a crystallization pond based on various exemplary inputs.
Description [0034] Throughout the following description specific details are set forth in order to provide a more thorough understanding to persons skilled in the art.
However, well known elements may not have been shown or described in detail to avoid unnecessarily obscuring the disclosure. Accordingly, the description and drawings are to be regarded in an illustrative, rather than a restrictive, sense.
[0035] Some embodiments of the present invention provide methods or apparatus for increasing the production rate of salt crystals produced from salt-containing solutions in crystallization ponds. In some embodiments, the increased production rate of salt crystals does not result in a significant decrease in the size range of salt crystals produced in the crystallization ponds. In some embodiments, the methods or apparatus provide an increase in the size range of salt crystals produced in the crystallization ponds. In some embodiments, the quality of salt crystals produced in the crystallization ponds is improved. In some embodiments, the crystallization ponds are outdoor crystallization ponds.
[0036] In some embodiments, apparatus is provided to actively cool sub-surface regions of the crystallization pond. In some embodiments, apparatus is provided to actively cool layers of solution within the pond that are below a surface layer of the pond.
[0037] In some embodiments, apparatus is provided to increase or sustain circulation of the salt-containing solution in the crystallization pond. In some embodiments, apparatus is provided to increase the evaporation rate of water from the salt-containing solution and/or to increase circulation of the salt-containing solution in the pond by the injection of air at selected pond locations. In some embodiments, air is injected in the form of bubbles. In some embodiments, the bubbles are injected as clusters of small bubbles. In some embodiments, the distribution pattern of the injected air is selected to produce or sustain a predetermined circulation cell pattern within the crystallization pond. In some embodiments, injection of air to provide a predetermined circulation cell pattern within the crystallization pond minimizes the energy input required to sustain a desired circulation rate within the pond.
[0038] In some embodiments, apparatus is provided to inject warm solution into the cooling pond in a manner that increases circulation of the salt-containing solution in the cooling pond. In some embodiments, the warm solution is injected generally upwardly into the cooling pond. In some embodiments, apparatus is provided to inject warm solution generally horizontally at an inlet end of a pond. In some such embodiments, the injection apparatus can be rotated to adjust the direction in which the warm solution is injected generally horizontally in the pond. In some embodiments, the direction of injection of warm solution into the pond is selected to assist in the formation and maintenance of predetermined circulation cells within the pond. In some embodiments, formation and maintenance of predetellnined circulation cells within the pond minimizes the amount of energy required to sustain a desired circulation rate within the pond.
[0039] In some embodiments, the crystallization process is actively controlled by controlling some or all of: active cooling of sub-surface regions of the crystallization pond; injecting clusters of small bubbles at selected pond locations; injecting walin salt-containing solution into the crystallization pond;
controlling a concentration gradient of the salt within the salt-containing solution so that a large portion of the pond solution remains within a metastable region for enhanced crystal growth by adjusting the pond circulation rate; or, adding a chemical chelating agent to the circulated region of the pond.
[0040] In some embodiments, the process of producing and/or harvesting salt crystals from the cooling pond is carried out continuously. In some embodiments, a vacuum harvester is provided to harvest salt crystals. In some embodiments, the vacuum harvester operates continuously.
[0041] In some embodiments, conditions within the crystallization pond are controlled so that at least 60%, at least 70%, at least 80%, at least 90%, at least 91%, at least 92%, at least 93%, at least 94%, at least 95%, at least 96%, at least 97%, at least 98%, or at least 99% of the volume of the crystallization pond is at conditions of temperature and salt concentration that fall within the metastable zone width (MSZW). In some such embodiments, the crystallization pond is exposed to ambient weather conditions (e.g. ambient temperature, humidity, precipitation and/or wind conditions) while the conditions within the crystallization pond are controlled.
[0042] In some embodiments, the salt crystals are KC1 crystals and the salt-containing solution is obtained as a product of solution mining of a potash mine.
[0043] In some embodiments, the overall size of crystallization ponds required to process a given outflow (e.g. to handle production from a single solution mine) is reduced as compared with crystallization ponds operated according to previously known protocols.
[0044] As used herein, enhancing the formation of salt crystals means any or all of: increasing the purity of the salt crystals produced, increasing the size of the salt crystals produced, producing relatively more salt crystals having a desired morphology, producing salt crystals with fewer inclusions, producing salt crystals with less agglomeration, or producing relatively more salt crystals having a desired size range.
[0045] For the creation of crystal nuclei leading to macroscopic crystal growth, an important prerequisite is the degree of super-saturation of the solution, which is the driving chemical potential and is defined to be the super-saturated concentration (SSC) divided by the saturation line concentration (SLC) minus 1.0 at the solution temperature and pressure (i.e. S-1.0, where S is the super-saturation ratio,¨ssrc). The concentration of salt in solution is measured to yield the value of S. Controlling the degree of super-saturation over time will usually result in a particular growth rate and size distribution of the crystals. At a constant degree-of-saturation, the nucleation rate will also increase with increasing solubility of the surrounding solution. Solubility is defined to be the saturation concentration and it is a function of the solution temperature. Solubility affects the probability that one ion in solution will encounter another of dissimilar charge, and therefore affects the rate of inter-molecular and inter-ion collisions. When changes in solution composition lead to increases in solubility, the interfacial or Gibbs energy decreases since the affinity between crystallizing medium and crystal increases [1].
Consequently, the degree of super-saturation required for spontaneous nucleation decreases with increasing solubility [2].
[0046] In view of the above thermodynamics, the metastable zone width (MSZW) is an important parameter for the growth of large size crystals from a solution, since it is the direct measure of stability of the solution in its supersaturated region. The metastable zone width is the difference (i.e.
region) between the saturation temperature at a given salt concentration and the temperature at which crystals are first detected under a constant cooling rate for that salt concentration. Larger metastable zone-widths imply more stability for crystal growth [3, 4].
[0047] Stability for crystal growth implies the formation of a high fraction of perfect or high quality, clear and strong crystals having a desirable morphology (e.g. cubic or truncated-cubic for KC1 crystals), generally without inclusions, impurities, or complex and fragile morphology. Stability for crystal growth generally means the solution is super-saturated and local diffusional processes occur around each nucleus or crystal in the solution. Since these diffusional physical processes are not equilibrium processes, the crystallization process is described as a quasi-equilibrium process when it is carried out slowly (i.e., slow cooling is conducted to match the crystallization rate), so a greater proportion of the crystals formed are perfect. When this process is carried out too quickly, as occurs during rapid cooling of a super-saturated solution, the crystals formed tend to be very small and the resultant particles are often weak agglomerates with many inclusions.
[0048] An assessment of crystal quality can involve many different attributes, for example, the size, size distribution, morphology, agglomeration, clarity or transparency, and/or percent inclusion of impurities. For different applications, the desirable ranges of these properties may differ. As an example, in embodiments where suitability of a product for long-term storage is desirable, clearer crystals may be desirable because substantially clear crystals can be less subject to mechanical damage when bulk materials are handled or shipped, and will be less subject to caking when exposed to high humidity during shipping or storage. As an example, substantially clear crystals, when protected from high or super-deliquescence humidity (e.g. 85% relative humidity for KC1) during storage, may have very long shelf lives (e.g. one year or more). As well, crystal particles with a narrow range of sizes (i.e. a generally uniform size distribution) can result in more uniform distributions when distributed as a soil fertilizer.
[0049] The metastable zone width depends on a number of parameters such as temperature and thermal diffusivity of the solution, diffusivity of ions in solution, rate of generating super-saturation crystal generation conditions (e.g. if the solution is cooled too quickly, poor quality crystals will be produced because the salt concentration may go well above the saturation limit), any fluid dynamic macroscopic motions, and the presence of foreign chemical particles or impurities [5-9]. There are several reports in the literature on the effect of some specific impurities on the nucleation kinetics of crystal growth processes [10-15].
[0050] The process of crystal growth consists of several stages through which growth units (e.g. ions) pass. These include (a) transport from the bulk solution to a site at the crystal surface, (b) adsorption of the growth unit onto the impingement site, or (c) diffusion from the impingement region to a growth site, and (d) incorporation of charge pairs of ions into the crystal lattice.
Dissolution can take place in steps b-d; however, the solvent may possibly be adsorbed into the crystal structure, and avoiding this requires careful control of these parameters.
Any one of the above steps may be rate-limiting depending on the growth conditions, such as the degree of super-saturation, temperature, presence of additives or solvent, and transport or diffusional controlled properties of the system. Consequently, crystal growth mechanisms fall into two main categories [16]: volume diffusion controlled and surface or interface integration controlled reactions. The goals of crystal growth theories are to determine the best conditions to achieve the most desirable crystal growth.
[0051] At equilibrium conditions the state of a salt solution is determined only by its temperature, concentration and phase of each chemical component.
Figure 2A shows the saturation line for the equilibrium phase diagram of a KC1+
H20 solution.
[0052] Crystallization occurs during non-equilibrium conditions. When the solution is close to thermodynamic equilibrium, crystals of high quality and market value can be grown; but, when the solution is supersaturated and far from equilibrium, the phase changes will result in phase transitions with a large fraction of included H20 molecules and random small crystal orientations in the resulting solid structure, and a weak chemical crystal structure with agglomerated particles.
A weak chemical crystal structure can arise from the presence of crystal inclusions such as water and/or impurities, and/or a complex morphology of interconnected bonded small crystals forming an agglomeration mass. Such particles will not have a good market value. In some embodiments, such crystals with low market value are recycled by putting them into the warm supply solution inlet. For example, harvested crystals can be screened to separate out crystals having lower than a desired size, and such crystals can be partially or wholly redissolved and recycled back into the crystallization pond.
[0053] Figure 2B shows the preferred metastable region of crystallization 15 for a KC1 + H20 system adjacent to the equilibrium saturation line 14 for phase change from solution-to-(KC1 + solution) for quasi-equilibrium crystal production superimposed on part of the theimodynamic phase diagram for KC1 solutions, with (16) and without (17, shaded region) the addition of a chelating agent. Region forms part of the metastable zone width for solutions including a chemical chelant.
Outside this region and into the region above the metastable region for the solid crystal formation (i.e. above the supersaturation limit for nucleation of crystals, indicated by dashed lines 16 with chelating agent and 17 without chelating agent) the solution will produce mostly low quality crystals. Within the metastable region, nucleation of crystals will occur and these can grow beyond a critical size as high quality KC1 crystals so that subsequent crystal growth can be achieved by cooling the solution at a controlled rate to maximize the high quality crystal production throughout the crystallization pond.
[0054] The metastable zone width (or region) is the region of transition on a plot of salt concentration versus temperature between the saturation line and the supersaturation line. The metastable zone width (or region) is the region where high quality crystals will grow. On the saturation line (i.e. line 14 in Figure 2B), crystals will just start to grow slowly, and may dissolve if they fall into non-saturated solution (i.e. below line 14 in Figure 2B). On and above the supersaturation line (i.e. dashed line 16 with chelating agent and dashed line without chelating agent in Figure 2B), nucleation will be rapid, which can lead to clusters of poor-quality agglomerated crystals. The metastable zone width (MSZW) is the zone between the saturation curve and the labile region where well-controlled and spontaneous nucleation takes place. It is the most stable zone of a critical supersaturation level where nucleation begins.
[0055] The entire metastable zone width (MSZW) area shown in Figure 2B (i.e. region 15 in the case without chelating agent, and both regions 15 and 15A
in the case with chelating agent, i.e. the region between the equilibrium saturation line 14 and saturation limit for nucleation of crystals, 17 without chelating agent or 16 with chelating agent, on a plot of salt concentration versus temperature) can be used to produce good quality KC1 crystals. Around each nucleation site or small crystal undergoing growth (e.g. crystal particle diameter, dõ about 0.01 to 2.0 mm in one example) there will be a small liquid zone of mass diffusion interaction (4)), diameter equal to about 2 to 6 mm. This particle-surrounding salt solution is a nearly isolated diffusion cell with a crystal particle growing near its center. The number of these cells in a pond could be as large as the volume of the pond divided by the average volume of 4). Within one typical cell, or mass diffusion zone, 4), with a crystal growing near its center, radial concentration gradients in the supersaturated solution will be such that, at any time, t, the concentration in the solution (c*) will decrease slightly the closer one gets to the growing crystal [i.e.

gradient (c*) will be positive with respect to the radial distance from the center of each crystal 6].
[0056] The total mass of KC1 within the diffusion cell, 4), will tend to be invariant or nearly constant over time until such time that the particle buoyancy or gravity forces cause the particle to descend rapidly with respect to the surrounding solution (e.g. at relative speeds greater than 0.1 mm/s with respect to the cell liquid) even though the bulk mass of pond contents may also be in motion with speeds greater than 1.0 mm/s. Although each diffusion cell is somewhat isolated for mass diffusion over this crystal growth duration, these cells are not isolated for heat conduction and convection. Temperature variations and gradients will exist throughout the pond. As well, some diffusion of water or salt concentration may take place between cells when there are significant gradients of water or salt concentration between different areas of the pond. In general, measurement of the concentration and temperature variations within a diffusion cell, 4), is impractical but property variations throughout a large pond are readily measured.
[0057] At any time, t, there will exist an average temperature, T, of solution and average concentration of KC1 solution ions in 4) of (C) such that, as t and dc increases, C decreases. In general, both T and C are functions of the independent spatial coordinates (x, y, z) in the pond and time (t) (i.e., vary any one of these independent variables in a pond and the value of C and T may well be different in a pond). In some embodiments, the local pond temperature T(x, y, z, t) is controlled such that C(x, y, z, t) is always or as much as possible within the MSZW as shown in Figure 2B for good quality crystallization.
[0058] For a typical prior art outdoor crystallization pond operation, temperature distribution T is controlled by its initial inlet conditions of the solution and the surrounding ground temperature distribution, ambient weather, evaporation rate of water vapor, and solar irradiation history. For a passive pond with no controls, this means that T is not controlled anywhere in the pond such that C
will be within the MSZW. All that is known is that, if a large pond is permitted to cool from say, 40 C inlet conditions to 5 C above ambient temperature, there is a good probability that each region of the pond will spend a short fraction of the total period of time of pond batch production within the MSZW. During this fraction of the total time, crystals will grow; but, they are unlikely to be good quality large crystals. Most of the time, each region of the passive pond solution will be either producing small-size agglomerated crystals with many impurities or there will be no significant crystal growth. In contrast, in some embodiments of the present invention, the local pond temperature T(x, y, z, t) is controlled such that C(x, y, z, t) is always or as much as possible within the MSZW as shown in Figure 2B for good quality crystallization. In some embodiments, conditions within the crystallization pond are controlled so that at least 60%, at least 70%, at least 80%, at least 90%, at least 91%, at least 92%, at least 93%, at least 94%, at least 95%, at least 96%, at least 97%, at least 98%, or at least 99% of the volume of the crystallization pond is at conditions of temperature and salt concentration that fall within the metastable zone width (MSZW).
[0059] The state of salt-containing solution within the pond and of crystals within the pond at any point are determined by the thermodynamic, chemical and mechanical states. By assessing conditions that affect the crystallization pond such as pond temperature, salt concentration, crystal properties (when particles exist as crystals), atmospheric pressure, solar irradiation, pond fluid velocity (e.g.
three components), turbulence (if turbulence exists), pond depth and other geometric factors such as pond shape and circulation cell size, and bubble size distribution and concentration where bubbles float to the surface of the pond; conditions that affect the top surface of the pond such as ambient air temperature, humidity ratio, wind speed and direction, together with the effect of any control measures taken as described in this specification to control the flux of heat or moisture transfer on the top surface of the pond; and conditions that subsurface regions of the pond such as heat flux induced by various methods of control as disclosed in this specification.
In some embodiments, only a few of the foregoing properties are monitored, and other properties are simulated, for example based on the results of experimental tests or simulations. Certain properties, such as transport properties (e.g.
properties for heat and mass convection) cannot be directly measured but can be determined indirectly.
[0060] In some embodiments, a chemical chelating agent is used to enhance the growth of salt crystals. There are several chelating agents that may be used and each has somewhat different effects. Chelating agents generally act to increase the size range of the metastable region for crystallization without changing the solution composition significantly (i.e. only very low concentrations, e.g. mole or mass fractions much less than 10-6, of the chelating agent are typically used). Exemplary chelating agents used to enhance salt crystal formation include, for example, EDTA (ethylene-diamine-tetra-acetic acid) (C10H16N208), DTPA
(diethylene-triamine-penta-acetic acid) (C14H23N3 01 0), DMSO (dimethyl sulfoxide) ((CH3)2S0), DMSA (dimercapto-succinic acid) (C4H604S2), NTA (nitrile-triacetic acid) (C6H9N06), citric acid (C6H807), oxalic acid (C2H204), acetic acid (CH3COOH), and the like. The chelating agent enlarges the metastable region by preventing heavy metals from being included within the formed crystals. The enlarged metastable region provided by addition of a chelating agent can allow for more or better control of the salt crystal production rate and resulting crystal sizes, and/or slower circulation rates in each circulation cell. Faster cooling rates depend on the metastable zone width (MSZW) to ensure large-sized crystals are grown.
The addition of chelating agents can not only enhance the metastable zone width, but can also suppress the contamination of produced salt crystals by trace amounts of heavy metals in the solution (i.e. the fraction of perfect crystals can be increased).
[0061] According to some embodiments of the present invention, provision is made to increase and/or control the cooling rate of a crystallization pond by actively cooling sub-surface regions of the pond. In some embodiments, the sub-surface regions of the pond that are cooled are the bottom and/or sides of the pond.
In some embodiments, the surface area for heat transfer from a crystallization pond is increased by actively cooling the bottom and/or sides of the pond. This provides an additional dimension for cooling the pond (i.e. in addition to the surface of the pond).
[0062] In some embodiments, to maintain production of high quality salt crystals, the temperature of a solution in a crystallization pond is reduced by active controlled cooling at a rate that is directly related to the rate of production of high quality crystals. In some embodiments, as the concentration of the desired salt in solution decreases, the temperature of the solution is decreased by an amount sufficient to keep the solution in the metastable zone width. In some embodiments, both salt concentration and temperature are measured at one or more locations to determine whether the solution is within the metastable zone width for crystallization of a desired salt product. In some embodiments, the rate of cooling of the solution in the crystallization pond is maintained at a rate that is directly related to the rate of production of high quality crystals at all or nearly all positions in the crystallization pond so that most or all regions of the crystallization pond remain within the metastable region.
[0063] In some embodiments, to maintain production of high quality salt crystals, the concentration of salt in a solution in a crystallization pond is controlled. In some embodiments, the salt concentration is controlled by suppressing water vapor evaporation rate when it is larger than a desired level. In some embodiments, both the pond evaporation rate (i.e. moisture flux) and cooling rate (i.e. heat flux) are controlled. In some embodiments, the solution temperature and concentration of a desired salt in solution is controlled throughout substantially the entire pond volume so that, collectively, the majority of locations within the crystallization pond lie within the metastable zone width region (i.e. within regions 15/15A in the example embodiment illustrated in Figure 2B). In some embodiments, the amount of auxiliary energy required to achieve control of temperature and salt concentration is minimized. In some embodiments, the amount of auxiliary energy required to achieve a desired circulation rate within the crystallization pond is minimized by encouraging fluid flow in a manner that produces circulation cells within the crystallization pond.
[0064] In conventional outdoor cooling ponds, cooling is provided only at the top surface of the pond at the air-liquid interface by wind and natural convection. Thus, the cooling rate of such ponds is highly dependent on factors such as environmental air temperature, relative humidity or moisture content of the air, wind speed and direction, and solar heat gains. Surface convective heat loss and evaporation rates are determined by the surface water to air convective heat transfer coefficient times the air to surface temperature difference, and, similarly, vapor mass transfer convection coefficient times the moisture content difference between the air and the surface of the liquid in the cooling pond.
[0065] For the case of the ambient air moisture content greater than the pond surface equivalent moisture content, water vapor will condense from the air onto the pond surface which will dilute the solution contained in the pond, so the surface salt content will decline near the pond surface. In some embodiments, if this situation takes the solution out of the metastable region, the active cooling rate for the pond is reduced to return the solution back to the metastable region.
In some embodiments, if the temperature and salt concentration measured in the pond at one or more regions indicate that precipitation has caused the solution to move outside the metastable zone width, active cooling, e.g. using apparatus 50 described below, is reduced or stopped. In some circumstances, if the moisture content difference cannot be corrected as aforesaid (e.g., during rain), the active cooling rate for the pond is reduced, or active cooling is stopped altogether, prior to the forecasted event (e.g. the start of precipitation), and active cooling is increased or resumed only after the precipitation has stopped and the air humidity has dropped. In some embodiments, active cooling is increased or resumed only after the dew-point temperature of the air has decreased below the surface temperature of the crystallization pond.
[0066] In some embodiments, a salt solution feed is supplied to a crystallization pond at a concentration and temperature such that it has a salt concentration close to saturation conditions for its particular supply temperature.
A supply jet momentum is provided at the solution inlet supply in such a manner that a circulating motion in the liquid solution is induced through substantially the entire volume of the solution pond in such a manner that the viscous or frictional energy dissipation rate for the pond is reduced or minimized for a selected number of liquid rotations. In some embodiments, this is achieved by inducing fluid flow in circulation cells, as described in greater detail below. In some embodiments, the resulting circulatory liquid motion is used to control the temperature variations for the entire pond such that the solution at substantially each and every point within the crystallization pond is within or very close to the metastable zone width (MSZW). In some embodiments, the metastable zone width (MSZW) region is enhanced by adding a chelating agent. In some embodiments, the crystallization pond is subjected to a wide range of environmental conditions for the ambient air, precipitation, solar irradiance and surrounding soil. In some embodiments, an apparatus for collecting or harvesting and selecting high quality salt crystals from the bottom of the crystallization pond is provided. In some embodiments as a result of one or more of the foregoing, the rate of production of the most valuable or high quality salt crystals is high per unit mass through-put within the crystallization pond.
[0067] With reference to Figure 3, an exemplary embodiment of a cooling pond 20 has a bottom 22 and sides 24. Pond 20 has a liquid-impermeable pond liner 26. In some embodiments including the illustrated embodiment, pond 20 includes a layer of clay 28 outside of liner 26, although depending on external site conditions, layer of clay 28 may be omitted in some embodiments.
[0068] In the illustrated embodiment, cooling pond 20 further has a layer of coarse gravel 30 interposing the layer of clay 28 and impermeable liner 26 on the bottom 22 of pond 20. Impermeable liner 26 contains the liquid contents of cooling pond 20 and prevents environmental contamination to and from the adjacent soil. In some embodiments, coarse gravel layer 30 is comprised of coarse-sized gravel with generally uniform particle size. In some embodiments, ambient air is circulated through coarse gravel layer 30, for example using one or more fans, shown schematically as 31. Circulation of ambient air through coarse gravel layer 30 may act to isolate heat gain in the surrounding soil from cooling tubes 60 described below, and/or may carry away any small amounts of accumulated water collecting in gravel layer 30 from ground sources or from leaks in liner 26. In some embodiments, the coarse gravel layer 30 is omitted.
[0069] In the illustrated embodiment, the bottom 22 of pond 20 is provided with a gravel layer 34 above impermeable liner 26, and with a bottom screen 36 above gravel layer 34. Crystals 38 of the desired salt form and accumulate above bottom screen 36. In the illustrated embodiment, gravel layer 34 is a layer of approximately pea-sized gravel. Bottom screen 36 has a pore size smaller than the particles comprising gravel layer 34 to contain gravel layer 34. Bottom screen may be made from any suitable inert material, for example plastic.
[0070] A salt-containing solution 32 from which a desired salt is to be crystallized is introduced into and contained within pond 20. Salt-containing solution 32 is typically a warm salt-containing solution. In some embodiments, warm salt-containing solution 32 is at least 10 C, at least 20 C, at least 30 C, or at least 40 C warmer than the ambient air temperature in the geographic location of pond 20. In some embodiments, warm salt-containing solution 32 has a temperature difference between the supply inlet and the pond surface dew-point temperature in the range of 10 C to 80 C, or any value therebetween, e.g. 15 C, 20 C, 25 C, 30 C, 35 C, 40 C, 45 C, 50 C, 55 C, 60 C, 65 C, 70 C, or 75 C. In some embodiments, warm salt-containing solution 32 is injected at conditions of temperature so that salt-containing solution 32 is near the saturation point given the salt concentration of solution 32.
[0071] In some embodiments, pond 20 has a depth in the range of 0.5 m to m, or any value therebetween, for example, 1.0 m, 1.5 m, 2.0 m, 2.5 m, 3.0 m, 3.5 m, 4.0 m, or 4.5 m. Control of operating conditions in some embodiments of the invention may allow for the use of deeper crystallization ponds than could be used in situations where operating conditions are not controlled. In some embodiments, the depth of pond 20 could be variable across its length and/or width. In some embodiments, the depth of pond 20 is maintained at approximately a constant level for a given set of inlet flow temperature and salt concentration conditions, and/or for given ambient air conditions. In some embodiments, the depth of pond 20 is varied based on changing operating conditions, for example, in winter versus summer conditions.
[0072] In some embodiments, pond 20 has a surface area in the range of 0.1 km2 to 1.0 kin2, or any value therebetween, for example, 0.2 km2, 0.3 km2, 0.4 km2, 0.5 km2, 0.6 km2, 0.7 km2, 0.8 km2, or 0.9 km2. In some embodiments, pond 20 has a length in the range of 10 m to 100 m, or any value therebetween, e.g.

m, 30 m, 40 m, 50 m, 60 m, 70 m, 80 m, or 90 m. In some embodiments, pond 20 has a width in the range of 10 m to 100 m, or any value therebetween, e.g. 20 m, 30 m, 40 m, 50 m, 60 m, 70 m, 80 m, or 90 m. Pond 20 may have any suitable shape. In some embodiments, pond 20 is rectangular, square, elliptical or circular in shape. In some embodiments, pond 20 has an asymmetrical shape.
[0073] In some embodiments having parallel circulation cells (as described below), pond 20 has a generally rectangular planar shape with a generally constant pond depth (with the exception of locations adjacent sloping interfaces between pond liner 26 and the bottom of pond 20). In some such embodiments, the inlet flow of solution enters on one side of the pond width (indicated as inlet end 44), providing a net or circulation flow average velocity normal to the inlet end 44 (i.e.
in the direction of the length of pond 20, towards an outlet end 46).
[0074] In some embodiments, the geometric size of the crystallization pond is characterized by two dimensionless ratios: the pond width (w) divided by the pond depth (d) or the pond width ratio (w/d) and the length ratio (//d) where (1) is the pond length. In some embodiments, these ratios have the ranges 2 < w/d <
50 and 5 < l/d < 50. For example, for a pond depth of 3 m the pond width may range from 6 to 150 m and the pond length range will be from 15 to 150 m.
[0075] In some embodiments having hexagonal circulation cells, the pond 20 may be rectangular or square. In some embodiments having hexagonal circulation cells, the pond 20 may be somewhat circular or elliptical. In some embodiments having hexagonal circulation cells, the ratio for pond width (w) or length (/) to depth (d) ranges from about 2 < w/d< 80 and 2 < l/d < 50.
[0076] With reference to Figure 3, according to one exemplary embodiment, an apparatus 50 for actively cooling the bottom and/or sides of a cooling pond using a coolant 52 is provided. The apparatus 50 has a heat exchanger in pond 20. In the illustrated embodiment, the cooling apparatus is provided with a tubing network 54 for moving the coolant 52 along the bottom of the cooling pond 20 to produce a warm coolant. In some embodiments, including the illustrated embodiment, the warm coolant is directed to one or more surface heat exchangers 56. In some such embodiments, the cooled coolant is then recycled back to tubing network 54. While the exemplary embodiment described herein uses tubing network 54 to exchange heat between solution 32 and coolant 52, other suitable heat exchangers could be used, for example, cooling plates.
[0077] Coolant 52 is circulated through tubing network 54 by one or more pumps 58. In one example embodiment, coolant 52 is an aqueous solution of glycol. Any suitable substance may be used for coolant 52, for example, saline water solutions using a suitable salt (e.g. NaCl, CaCl2, LiBr, or the like) or any suitable refrigerant used in the refrigeration or HVAC industries, for example those listed in the ASHRAE Handbook of Fundamentals (2013) published by ASHRAE
ISBN 9781936504459), which is incorporated by reference herein. In some embodiments, two or more pumps 58 are provided, and pumps 58 are operated in parallel to circulate coolant 52 through tubing network 54.
[0078] To cool salt-containing solution 32, tubing network 54 extends through the bottom 22 and/or sides 24 of pond 20. In one example embodiment shown in Figure 4A, tubing network 54 includes a plurality of approximately evenly spaced parallel tubes 60 extending over the bottom 22 of pond 20, above impermeable liner 26 and below pond bottom screen 36. In the illustrated embodiment, tubes 60 are buried within gravel layer 34. While tubes 60 have been shown as extending over the bottom 22 of the pond, it will be appreciated that tubes 60 could be positioned at any vertical location within salt-containing solution 32 and still be used to cool pond 20. However, placement of tubes 60 beneath bottom screen 36 facilitates harvesting of salt crystals unimpeded by tubes 60.
[0079] In a second example embodiment illustrated in Figure 4B, a configuration of tubes 60 and valves 62 that allows for position-dependent control of the rate of cooling of the bottom of pond 20 is shown. A plurality of valves 62A, 62B, 62C and 62D are provided to regulate the rate of flow of coolant 52 into a plurality of regions 60A, 60B, 60C and 60D of tubes 60. The rate of flow of coolant 52 through each of valves 62A, 62B, 62C and 62D can be controlled, for example in response to a signal provided by a controller, shown schematically as 64, to independently regulate the rate of cooling in each of regions 60A, 60B, and 60D. In the illustrated embodiment, region 60A is closest to the inlet end of the pond, and region 60D is closest to the outlet end 46.
[0080] In some embodiments, one or more temperature sensors 66 are provided to measure the temperature at various locations within pond 20. In the illustrated embodiment, one temperature sensor 66 is provided in each of regions 60A, 60B, 60C and 60D. In some embodiments, temperature sensors 66 provide feedback to controller 64 to allow controller 64 to regulate the rate of cooling in each of regions 60A, 60B, 60C and 60D by independently controlling the rate of flow of coolant 52 through each of valves 62A, 62B, 62C and 62D, respectively to provide a desired rate of cooling in each region.
[0081] While in the embodiments illustrated in Figures 4A and 4B, tubes 60 have been shown as being parallel and evenly spaced, any desired configuration could be used for tubes 60 that allows the coolant 52 carried inside tubes 60 to absorb heat from salt-containing solution 32. For example, in some embodiments, for example those having parallel circulation cells as described below, the pond 20 has a unidirectional flow from inlet end 44 to outlet end 46, and the resulting pond mean bulk flow temperature will decrease from inlet end 44 to outlet end 46 (i.e.
pond 20 will have a negative bulk mean temperature gradient along the axial direction of the circulation vortices). This, along with surface cooling and evaporation, may make it desirable to provide at least some control over the pond bottom cooling as a function of distance from the pond solution supply inlet or inlet end 44 in order to maintain the bulk mean pond temperature at every position at its most desired value for its particular bulk mean concentration of salt ions in solution.
[0082] For example, in the embodiment illustrated in Figure 4B, controller 64 may control the flow of coolant through valve 62A to be higher than the flow of coolant through valve 62B, which in turn is controlled to be higher than the flow of coolant through valve 62C and so on, so that the rate of cooling is higher in region 60A than in region 60B, which is higher than in region 60C, and so on, to counteract the naturally occurring pond mean bulk flow temperature that tends to decrease from inlet end 44 to outlet end 46.
[0083] In some embodiments, coolant 52 may be passed through tubes 60 in a direction that allows for control over the pond bottom cooling as a function of distance from the inlet end 44. For example, in one embodiment coolant 52 flows through tubes 60 in a direction from inlet end 44 to outlet end 46 to provide a warm coolant with a higher temperature at outlet end 46 as compared with inlet end 44.
[0084] Several flow configurations could potentially be used to provide the desired control over the cooling rate within pond 20, including the example configuration shown in Figures 4A and 4B. In other embodiments, for example those having hexagonal circulation cells as described below, there may be no appreciable mean bulk flow temperature across the length of pond 20, and the operation of cooling apparatus 50 is configured and regulated to provide a relatively consistent or homogeneous rate of cooling on the bottom of pond 20.
It is within the expected ability of one of ordinary skill in the art to determine a desirable configuration for the placement of tubes 60, for example by conducting appropriate pond temperature simulation studies.
[0085] In some embodiments, the size, spacing, and positioning of tubes 60 is determined based on the need to provide a selected range of cooling rates for the salt-containing solution 32 in pond 20. In some embodiments, the size, spacing, and positioning of tubes 60 is determined to provide a small pressure drop to allow for high rates of flow of coolant 52 through tubes 60. In some embodiments, the size, spacing and positioning of tubes 60 is determined based on the provision of a selected range of circulation cell size (as described below) to pond depth. In some embodiments, the ratio of circulation cell diameter to pond depth is close to or approximately 2.0, e.g. in the range of 1.5 to 2.5 or any value therebetween. In some such embodiments, the circulation cells are hexagonal circulation cells or parallel circulation cells.
[0086] In some embodiments having parallel circulation cells as described below, different cooling rates are used for different parts of the bottom of pond 20.
In some such embodiments, the bottom surface cooling rate is varied with distance from the inlet end 44. In some embodiments, the flow of coolant 52 through tubing network 54 is controlled by using pumps or valves to regulate the rate of cooling along the bottom of pond 20, for example using the configuration shown in Figure 4B, for example by stopping the flow of coolant 52 through some tubes in tubing network 54 in regions where it is desired to decrease the rate of cooling by shutting off appropriate valves 62. In some embodiments, the degree of cooling is increased with increasing distance from the inlet end 44. In some embodiments, the degree of cooling is decreased with increasing distance from the inlet end 44.

In some embodiments, the degree of cooling in a particular region of pond 20 is decreased by putting fewer tubes of tubing network 54 in that particular region of pond 20 where it is desired to provide a lower rate of cooling.
[0087] Tubing network 54, including tubes 60, can be made from any suitable inert material. In an example embodiment, tubing network 54 is made from plastic tubing.
[0088] In the illustrated embodiment, surface heat exchanger 56 is a plurality of heat-pipe heat exchangers. The heat-pipe heat exchangers release heat captured from the salt-containing solution 32 by a circulating coolant 52 as sensible energy or heat to the atmospheric air. It is within the expected ability of one skilled in the art to design and build a heat-pipe heat exchanger suitable for a given application. In some embodiments, a fluid-to-fluid heat exchanger could be used to cool warm coolant, for example warm coolant could be cooled by a fluid-to-fluid heat exchanger cooled by a natural coolant such as a flowing water source or groundwater.
[0089] In the illustrated embodiment, the only auxiliary energy input for cooling apparatus 50 is the electrical energy required to operate pumps 58. In such embodiments, surface heat exchangers 56 are entirely passive devices that transport sensible energy within them by gravity-induced internal flows coupled with two-phase energy transfer using a suitable available refrigerant within each heat pipe.
[0090] In operation, the flow through pumps 58 is controlled to meet the cooling requirement needs for pond 20 at any given time. In some embodiments, the rate of flow of coolant through pumps 58 is controlled to provide a desired cooling rate for salt-containing solution 32 in pond 20. In some embodiments, physical modeling, numerical modeling, simulation studies and/or laboratory scale studies are, for a given set of typical hourly or time averaged ambient air properties, solar irradiance, and soil conditions, used to select the parameters for pond depth, length, width, supply solution feed rate and solution feed recirculation rate, vortex circulation rate, and/or pond bottom cooling rate for the salt-containing solution.
[0091] In some embodiments, sub-surface regions of pond 20 could be cooled by circulating groundwater directly through coarse gravel layer 30.
[0092] With reference to the embodiment illustrated schematically in Figure 5, in another example embodiment, an auxiliary cooling system 80, using groundwater as a heat sink, is provided to assist in cooling pond 20. In the embodiment of Figure 5, groundwater 82 is used as an auxiliary heat sink. Use of groundwater 82 as an auxiliary heat sink may be particularly desirable during periods of high external air temperatures (e.g. during summer) to further cool coolant 52. In the embodiment of Figure 5, moderately deep well pumping by a pump (shown schematically as 83) is used to supply groundwater to the surface 42, where additional heat exchangers (shown schematically as 84) are used to cool coolant 52 after it exits heat exchanger 56 and before coolant 52 is returned to tubes 60 below pond 20. Slightly heated groundwater 86 produced by the exchange of heat from coolant 52 is disposed of in any suitable manner, for example by injection into wells some distance away from pond 20 and the source of groundwater 82. Injection into wells some distance from pond 20 and the source of groundwater 82 can avoid returning heat removed from salt-containing solution 32 back to either pond 20 or the source of groundwater 82.
[0093] Without being bound by theory, it is anticipated that the temperature of coolant 52 during summer periods could be reduced by up to about C to 15 C beyond what could be achieved by the use of apparatus 50 in the absence of auxiliary cooling system 80 because the source of groundwater 82 is anticipated to have a maximum temperature ranging from about 5 C to 15 C in late summer. Thus, auxiliary cooling system 80 may facilitate enhanced cooling of pond 20 cooling rates during warmer periods (e.g. during summer), and allow for improved year-round production rates of salt crystals 38.
[0094] With reference to Figure 6, in a further example embodiment illustrated schematically, groundwater 82 is sub-cooled to less than 5 C, or even cooler depending on the salt content of groundwater 82 which may allow for even cooler temperatures, during periods of low prevailing atmospheric temperature (e.g. during winter) using a sub-cooling apparatus 90. Sub-cooling apparatus has one or more heat-pipe heat exchangers (shown schematically as 92) embedded in the ground and extending down to a low level of groundwater 82. For example, where groundwater 82 is at a depth of 100 to 200 feet, a fluid conduit 93 extends approximately 100 to 200 feet to heat exchanger 92. Below-ground heat-pipe exchangers 92 are coupled in fluid communication with above-ground air-cooled heat-pipe exchangers 94 via fluid conduit 93. A suitable coolant, for example an aqueous solution of glycol, saline water solutions using a suitable salt (e.g.
NaCl, CaCl2, LiBr, or the like), or any suitable refrigerant used in the refrigeration or HVAC industries, for example those listed in the ASHRAE Handbook of Fundamentals (2013) published by ASHRAE ISBN 9781936504459), is circulated between heat-pipe exchangers 92 and 94. Coolant is circulated between the heat-pipe exchangers using a pump (shown schematically as 96) or set of pumps in parallel. Thus, heat is transferred from the source of groundwater 82 to the coolant by below-ground heat-pipe exchangers 92, and then released to atmospheric air by air-cooled heat-pipe exchangers 94 to sub-cool groundwater 82 below the native temperature of the source of groundwater 82.
[0095] Operation of heat-pipe exchangers 92, 94 during periods of prevailing low temperatures, e.g. during winter, will result in the production of a sub-cooled heat sink that can be advantageously used as a source of cold groundwater 82 for auxiliary cooling system 84 during periods of prevailing high temperatures (e.g. during summer). Stopping the flow of coolant between heat-pipe exchangers 92, 94 during periods of prevailing high temperatures (e.g.
during summer) will help to avoid additional warming of the source of groundwater 82 during periods of prevailing high temperatures. In some embodiments, summer months are June, July, August and September. In some embodiments, during the summer months, ground water temperatures are below the air temperatures most of the time (for example, in some embodiments, the groundwater temperature is expected to vary between about 5 C and 10 C, or between just over 0 C and 5 C
in embodiments in which sub-cooling apparatus 90 is used to sub-cool the source of groundwater 82 during the winter months).
[0096] In some embodiments, heat-pipe heat exchangers 92, 94 are operated during periods of prevailing low atmospheric temperatures during the winter months, for example, from November to March or any interval therebetween, including December, January and February. During this period, coolant is circulated between heat-pipe heat exchangers 92, 94 to sub-cool the source of groundwater 82. The flow of coolant between heat-pipe heat exchangers 92, 94 is then stopped. Groundwater 82 can continue to be used as an auxiliary heat-sink to cool coolant 52.
[0097] Determination of whether to operate auxiliary cooling system 80 and/or sub-cooling apparatus 90 can be made by assessing the trade-off between cooling coolant 52 below the ambient air wet-bulb temperature conditions on one hand, and the rate of evaporation from the top surface of pond 20 (which will change with changing weather conditions) and circulation rate within pond 20, on the other hand. In some embodiments, the operation of auxiliary cooling system 80 and sub-cooling apparatus 90 is controlled to approximately match the cooling rate of pond 20 with the crystallization rate, so that most or all of the solution 32 throughout pond 20 remains within the metastable zone width (MSZW). Thus, as crystallization proceeds, the concentration of salt in solution 32 will decline, and the temperature of solution 32 should be cooled to keep the solution within the MSZW. Cooling of solution 32 can then continue until the pond air-liquid surface temperature declines to the dew-point temperature of the ambient air to avoid excessive air to liquid condensation of airborne water vapor on the surface of the pond. In some embodiments, during periods of prevailing low atmospheric temperatures (e.g. winter), the ambient air dew-point temperature may be well below 0 C. In some embodiments, during periods of prevailing high atmospheric temperatures (e.g. summer), the dew-point temperature may be well above 0 C, e.g. 5 C to 10 C.
[0098] In some embodiments, the cooling of solution 32 is controlled so that the surface temperature of pond 20 is maintained slightly above the dew-point for the adjacent atmosphere at the lowest expected temperature of the day. In one example embodiment, the lowest expected temperature of the day is the temperature expected at approximately 4:00 a.m. in the absence of changes in weather.
[0099] In some embodiments, operation of auxiliary cooling system 80 and/or cooling apparatus 50 is controlled based on changes in precipitation.
Precipitation such as rain falling on pond 20 will decrease the concentration of salt in solution 32 near the pond surface 40. This may remove solution 32 from the metastable zone width region, and slow the rate of crystal formation. However, decreasing the concentration of salt in solution 32 will not generally decrease the quality of salt crystals 38 formed. In some embodiments, operation of auxiliary cooling system 80 and/or cooling apparatus 50 is stopped or reduced shortly before the forecasted arrival of precipitation, e.g. one to two days before the forecasted arrival of precipitation. In some embodiments, the expected amount of precipitation is factored in to the determination of when to stop or reduce operation of auxiliary cooling system 80 and/or cooling apparatus 50. In some embodiments, operation of auxiliary cooling system 80 and/or cooling apparatus 50 is resumed when the dewpoint temperature of the atmospheric air decreases to below the surface temperature of the pond 20.
[0100] In some embodiments, operation of cooling apparatus 50 and/or auxiliary cooling system 80 is controlled based on the time of day and/or the level of sunlight prevailing on a particular day. Sunlight will result in solar gain, i.e. the temperature of solution 32 in pond 20 will increase as a result of sunlight shining on the surface 40 of the pond 20. In some embodiments, cooling apparatus 50 is operated during the day to provide a higher degree of cooling of pond 20 than at night to counteract solar gain (e.g. by increasing the rate of flow of coolant through tubing network 60). In some embodiments, auxiliary cooling system 80 is operated during the day but not at night to counteract solar gain.
[0101] In some embodiments, a removable cover, shown schematically as 65 in Figure 3, is provided to shelter pond 20 from precipitation, wind and/or sun.
In some embodiments, the cover is removable, so that the cover can be put in place over the pond when the expected precipitation, wind or sun is being experienced, and so that the cover can be removed from pond 20 when the precipitation, wind and/or sun are no longer affecting pond 20. In some embodiments, pond 20 can be sheltered by a permanent structure or enclosure, with ventilation air provided through large apertures in the side of the structure or enclosure, or through air pumped into the structure or enclosure by auxiliary power means.
[0102] In some embodiments, air is injected into pond 20 to enhance circulation and/or to increase the evaporation rate from pond 20. In some embodiments, as illustrated in Figure 7 and Figures 8A and 8B, a bubble injection apparatus 110 for enhancing circulation in pond 20 by promoting formation or sustenance of natural liquid circulation cells is provided.
[0103] Two types of natural liquid circulation cells may be exploited for the low rate of input energy required to sustain their circulation rate in a crystallization pond: (a) hexagonal cylindrical circulation cells and (b) parallel circulation cells, each comprised of two counter-rotating circulation vortices within a square or rectangular cross-section and a length equal to the length of the pond.
In some embodiments, each of these circulation cells extends through substantially the entire depth of the pond, top to bottom, and together the circulation cells cover substantially the entire volume of the pond.

[0104] For natural convection, induced by liquid body-force-viscous-force instabilities, cell (a) is more common than cell (b) for natural convection arising from bottom heated pond heat transfer. Nonetheless, for some crystallization ponds there may be an advantage for configuration (b) because it favors a unidirectional net flow along the axis of each parallel circulation cell for a solution supply feed inlet to the pond while the outlet is distributed along the floor of the pond where crystals are deposited and where crystal harvesting occurs, for example as described below with reference to crystal vacuum harvesting device 150. In contrast, in embodiments using hexagonal circulation cells, incoming salt-containing solution 32 is supplied relatively evenly across the bottom of pond and in some embodiments is removed by a crystal vacuum harvesting device 150 as outlined below, and in such embodiments there is no net flow of solution across the pond. Hence, the temis "inlet end" and "outlet end" as used with respect to ponds having parallel circulation cells are not applicable to such embodiments having hexagonal circulation cells.
[0105] In some embodiments having parallel or hexagonal circulation cells, the amount of input energy required to sustain a given circulation rate is approximately equal to the minimum rate of viscous energy dissipation in the solution 32 in pond 20.
[0106] Release of bubbles continuously or at regular intervals at a controlled rate and allowing the bubbles to rise to the top surface of pond 20 in a manner that entrains the surrounding solution provides a flow that forms circulation cells that are stable at low Reynolds number. In some embodiments, the release of bubbles is not continuous. In some embodiments, the release of bubbles is approximately steady. In some embodiments, the rate of flow of circulation-enhancing bubbles is chosen to sustain the required bulk flow rotational rate of the circulation cells. These cells can cover the entire pond volume and can cause the temperature differences from the top to the bottom of pond 20 to be controlled to a small variance at any distance from the solution feed inlet end 44.
The circulation cells can also enhance the bottom and top surface cooling and evaporation.
[0107] With reference to Figures 8A and 8B, respectively, bubble injection apparatus 110A for injecting bubbles to produce hexagonal circulation cells and bubble injection apparatus 110B for injecting bubbles to produce parallel circulation cells are illustrated. Each parallel circulation cell has a pair of counter-rotating vortices with rectangular cross-section, as illustrated in Figures 9A
and 9B. Elements that perfoini the same function in apparatus 110A and 110B are referred to with identical reference numerals.
[0108] Bubble injection apparatus 110, including apparatus 110A/110B, includes a plurality of bubble injection tubes 112 for releasing bubbles that rise through the salt-containing solution 32 to induce the development of large natural circulation cells and to sustain their motion within pond 20. The circulation cells so formed have an effective surface plane vortex diameter nearly equal to about two times the depth of the pond (i.e. each vortex is approximately equal to the pond depth), and in some embodiments, cover the entire pond. The presence of natural circulation cells produced by apparatus 110AJ110B may reduce the power required to operate the air compressors that feed bubble injection tubes 112, for example, as compared with the power that would be required to operate the air compressors that feed bubble injection tubes 112 if the apparatus was not configured to induce formation of natural circulation cells.

[0109] In the illustrated embodiment of Figure 8B, bubble injection tubes 112 are generally equally spaced and extend generally parallel to one another.

Bubble injection tubes 112 are positioned on the bottom 22 of pond 20, below bottom screen 36 (Figure 7). In some embodiments in which cooling apparatus 50 is used together with bubble injection apparatus 110, bubble injection tubes 112 are positioned above tubes 60 so that tubes 60 do not interfere with the release of bubbles. In some embodiments, bubble injection tubes 112 are embedded within pea gravel 34, but are positioned very close to the lower surface of bottom screen 36 so that the bubbles do not have to travel a significant distance through pea gravel 34.
[0110] In the embodiment of a bubble injection apparatus 110A
illustrated in Figure 8A, hexagonal circulation cells are produced by the positioning of two sets of parallel, spaced apart bubble injection tubes 112A, 112B near the bottom of pond 20. A first set of bubble injection tubes 112A extend in a generally diagonal direction across pond 20. Bubble injection tubes 112A are spaced apart at approximately equal intervals and extend generally parallel to one another in a plane to form first and second opposite sides of a hexagon. A second set of bubble injection tubes 112B extend across pond 20 in the same plane as but at an angle to bubble injection tubes 112A to form third and fourth opposite sides of a hexagon.
Bubble injection tubes 112B are spaced apart at approximately equal intervals and extend generally parallel to one another. Apertures 118 are provided at spaced apart intervals on bubble injection tubes 112 so that the apertures on adjacent portions of bubble injection tubes 112A, 112B define four sides of a hexagon, as shown within dashed hexagonal outline 120 for illustrative purposes. In some embodiments, the fifth and sixth sides of hexagon 120 are provided by the warm solution supply system 130, described below. In some embodiments, the hexagonal circulation cell can be produced by bubble injection tubes 112A, alone (i.e. with no additional flow component providing the fifth and sixth sides of hexagon 120). In some embodiments, apertures 118 provided on bubble injection tubes 112 could be used to define all six sides of hexagon 120 by providing an additional set of parallel bubble injection tubes (not shown) intersecting tubes 112A and 112B.
[0111] In some embodiments having hexagonal circulation cells, the waini salt-containing solution 32 is supplied directly by an inlet within each hexagonal cell. In some such embodiments, the wainl solution is supplied directly through an inlet located at approximately the center of each hexagonal cell. In some such embodiments, waini solution is introduced only at or near the center of each hexagonal cell.
[0112] In the embodiment of a bubble injection apparatus 110B
illustrated in Figure 8B, parallel circulation cells, each comprised of two counter-rotating circulation vortices within a rectangular cross-section with a length equal to the length of pond 20, are produced by the positioning of one set of bubble injection tubes 112C that are spaced apart at approximately equal intervals and extend generally parallel to one another in a direction parallel to the path of the mean cell flow or travel from the inlet end 44 to the outlet end 46 of pond 20.
[0113] A parallel circulation cell produced by the apparatus shown in Figure 8B is shown in greater detail in Figures 9A and 9B, which illustrates typical streamlines or particle path lines in solution flow downstream of the inlet flow.
Each parallel circulation cell has two rotating helical flows, each resulting in the net circulation of fluid in a helical path from the inlet end 44 to the outlet end 46 of pond 20. One helical flow vortex 100 rotates in a clockwise direction, and the second helical flow vortex 102 rotates in a counterclockwise direction. Each of helical flow vortices 100 and 102 extends from substantially the bottom 22 to the top surface 40 of pond 20 (i.e. throughout the pond depth 104). When the streamlines are viewed in helical axis elevation view (Figure 9B), in some embodiments they appear elliptical or circular, depending on the depth 104 of the pond 20 relative to the width of the circulation cell. In some embodiments, at the interfaces between each vortex 100, 102, the sides, top and bottom of the outside surface of each vortex are nearly square, as shown in Figure 9B.
[0114] In some embodiments, a number of complete parallel circulation cells (N) are formed, each of which has one parallel circulation clockwise vortex 100 and one counter-clockwise vortex 102. In some embodiments, a half circulation cell (i.e. having only a clockwise vortex 100 or a counter-clockwise vortex 102) is provided at one or both sides of pond 20.
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[0115] In some embodiments, the circulation rate is measured at one or more locations within pond 20. As illustrated in Figure 9A, one or more sensors 106 can be provided at predetermined locations within pond 20 to measure the flow rate to evaluate the circulation rate within pond 20. Examples of sensors that can be used to measure the pond circulation include velocity meters, flow meters, turbine flow meters, or particle position change sensors. In some embodiments, flow rate sensors 106 provides feedback to controller 64 that is used to control the operation of some or all of the features of pond 20.
[0116] With reference to Figure 7, one or more air compressors 114 pumps compressed air through bubble injection tubes 112. In some embodiments, a dryer 116 is used to dry the supplied compressed air before it passes into bubble injection tubes 112. In some embodiments, a controller is provided to control the amount of compressed air supplied by air compressors 114. In some embodiments, the amount of air supplied by air compressors 114 is controlled based on the need to sustain a selected circulation rate in the pond. In embodiments utilizing hexagonal circulation cells, it is anticipated that the feed rate of incoming warm salt-containing solution 32 will not provide as much inlet momentum and mixing as for embodiments utilizing parallel circulation cells, for which the supply feed inlet flow will provide most or all of the vortex momentum needed to initiate circulatory cell flow. For both parallel and hexagonal circulation cells, once the circulation cells have been generated, the amount of air supplied by air compressors 114 can be reduced to the level required to sustain circulation in each circulation cell at a rate that will result in a good production rate of crystals.
[0117] Circulation within a vortex or within the pond 20 generally can be measured in any suitable manner, for example using velocity meters, flow meters, turbine flow meters, or particle position change sensors.
[0118] The spacing between bubble injection tubes 112 is selected to enhance the formation of and to sustain the natural convection cells that circulate the salt solution throughout pond 20. In some embodiments, the spacing between bubble injection tubes 112 is selected to minimize the auxiliary energy input required to create and sustain fluid flow of the convection cells. Apertures 118 of a predetermined size, which in some embodiments is optimized empirically, are positioned along the length of bubble injection tubes 112 to provide bubble flows that define the shared interfaces between adjacent circulation cells as the bubbles rise to the surface 40 of pond 20. The rising bubbles cause an upward buoyancy shear stress on adjacent circulation cells, which results in a cell circulation flow with an axis of rotation that is parallel to the bottom 22 or top 40 surface of pond 20 for any vertical cross section through solution 32. The number and length of circulation cells for any given pond 20 can be deteimined using simulation studies, model studies, laboratory scale studies, or a combination thereof.
[0119] In some embodiments, the size of bubbles released by bubble injection tubes 112 is used to regulate the solution water vapor evaporation rate.
Small bubbles tend to form rafts of small bubbles that can accumulate on the surface 40 of pond 20 and reduce the surface heat loss rate when these rafts are sufficiently large relative to the pond surface area. Large bubbles tend to rise to the surface 40 of pond 20 more quickly, and cause greater momentum transfer to the liquid and enhanced mixing of solution 42 and greater circulation rates and vertical shear forces in the circulation cells.
[0120] In some embodiments, the size of bubbles released by bubble injection tubes 112 is regulated by regulating the size of apertures 118. With reference to Figure 10, in some embodiments, a set of apertures 118 are provided to produce bubbles of a size sufficient to establish and maintain the flow of solution 32 in circulation cells, and a second set of apertures 122 are provided to produce small bubbles that can float to and remain on the surface 40 of pond 20 to provide surface rafts (or a surface foam). In the illustrated embodiment, apertures 118 are provided on a first bubble injection tube 112D and apertures 122 are provided on a second bubble injection tube 112E extending parallel and adjacent to tube 112D. In some embodiments, the flow of compressed air through bubble injection tubes 112D and 112E can be independently controlled, to allow for injection of bubbles having a desired size at appropriate times.
[0121] In some embodiments, bubble injection tubes 112 comprise a pair of bubble injection tubes bound together in a side-by-side fashion. For example, in the embodiment illustrated in Figure 10, bubble injection tubes 112D and 112E
are parallel to one another and are bound together by suitable fasteners 124. The use of two adjacent bubble injection tubes 112 can provide greater control over momentum flux in solution 32 and/or the accumulation of surface bubbles on the surface 40 of pond 20. In some embodiments, the size of apertures 118 and/or provided on each of the two adjacent bubble tubes can be different or varied, to provide a broader final mixed bubble diameter range. In some embodiments, the flow of air through each adjacent bubble tube can be independently controlled, to facilitate greater control over the circulation rate within each circulation cell. In some embodiments, the flow of air through each adjacent bubble tube is regulated by controller 64.
[0122] In some embodiments, these adjacent bubble injection tubes 112 can be used to produce either parallel circulation cells or hexagonal circulation cells by passing air only through the bubble injection tubes 112 having a configuration that will result in production of the desired circulation cell.
In embodiments having hexagonal circulation cells, apertures 118 are intermittently distributed along bubble injection tube 112. In some embodiments having parallel circulation cells, apertures 118 are substantially continuously spaced apart along bubble injection tube 112.
[0123] In some embodiments, bubble injection tubes 112 having large apertures 118 and/or small apertures 122 are operated without producing circulation cells to enhance the evaporation rate of pond 20. In some such embodiments, one or more of the location of bubble injection tubes 112, apertures 118 and/or apertures 122 and the timing of release of bubbles are randomly selected.
[0124] The flow rate of compressed air supplied by air compressors 114 is selected to meet the circulation requirements of pond 20 to enhance the rate of formation of salt crystals 38. In some embodiments, the rate of flow of compressed air supplied by air compressors 114 is controlled to provide a desired flow rate of salt-containing solution 32 in pond 20. In some embodiments, the flow rate of compressed air supplied by air compressors 114 is controlled by controller 64. In some embodiments, physical modeling, numerical modeling, simulation studies and/or laboratory scale studies are used to select the desired flow rate for the salt-containing solution.
[0125] Without being bound by theory, the circulation cells provided by bubble injection apparatus 110 can provide an increase in the evaporation rate as the warm liquid salt-containing solution 32 is brought to the surface 40 by the bubble flow, which induces the adjacent liquid to flow up to the top surface where the saturated bubbles release water vapor directly into the air. Thus, the induced-flow warm-solution evaporates more quickly at the surface. Without being bound by theory, it is believed that bubble flow rates are directly related to the solution water evaporation rates.
[0126] In some embodiments, small apertures such as apertures 122 are used to produce small bubbles. In some embodiments, small bubbles produced through small apertures 122 can be used to form rafts of small bubbles (e.g.
in the nature of a surface foam) that can reduce surface heat loss rate when the rafts are sufficiently large to cover all or most of the surface 40 of pond 20. In contrast, the larger bubbles produced by apertures 118 will rise more rapidly to the surface, causing enhanced mixing of solution 32, greater circulation speeds within the circulation cells, and vertical shear forces on the circulation cells. The rate of production of both small bubbles through small apertures 122 and large bubbles through apertures 118 can be controlled in any suitable manner, for example by regulating the volume flow rate of compressed air supplied by air compressor 114.
In some embodiments, desirable ranges of diameters for small bubbles and large bubbles are determined empirically. In some embodiments, a non-toxic foaming agent is added to pond 20 to stabilize the surface foam provided by rafts of small bubbles.
[0127] In some embodiments, an evaporation suppressing surface liquid is added to pond 20 to decrease the rate of evaporation from pond 20. Examples of evaporation suppressing surface liquids include oils, including organic oils.
In some embodiments, the oil is a plant-based oil, such as canola oil, corn oil, soybean oil, flax oil, or the like.
[0128] In some embodiments, the circulation requirements of pond 20 are determined partly by the need to decrease at a selected rate the average pond temperature, and/or by the need to decrease the temperature variations at any particular distance from the solution supply inlet in each circulation cell, and/or by the need to cause surface evaporation with time and to maintain a predetermined average salt solution concentration within the metastable region adjacent the equilibrium saturation line on the phase diagram for the salt solution throughout a large region of the pond.

[0129] In some embodiments, apparatus is provided to measure the temperature and/or salt concentrations at one or more locations in pond 20. In some embodiments, apparatus is provided to measure the temperature and/or salt concentrations at one or more locations within one or more typical circulation cells within pond 20. Any suitable apparatus to measure temperature can be provided, for example, a thermocouple, thermistor, resistance temperature detector, or the like. In some embodiments, any metal components of such apparatus are not in direct contact with salt solution 32. Any suitable apparatus to measure salt concentration can be provided, for example, a salt meter using an optical refractometer or a conductivity meter or a liquid density meter.
[0130] As shown in Figure 4B, one or more temperature sensors 66 can be provided at different locations around pond 20. In some embodiments, the temperature sensors are provided at a plurality of different depths within pond 20, as an alternative or in addition to being provided at a plurality of different positions across the length and width of pond 20. In some embodiments, data collected by temperature sensors 66 is returned to controller 64 and used to control the operation of cooling apparatus 50, auxiliary cooling system 80, bubble injection apparatus 110, warm solution supply system 130, and/or crystal vacuum harvesting device 150. In some embodiments, a reading is taken from one or more temperature sensors placed within pond 20 continuously or periodically.
[0131] As shown in Figure 7, one or more devices for measuring the salt concentration in solution 32, shown schematically as 68, can be provided at a plurality of different locations in pond 20, including at different depths within pond 20. In some embodiments, the salt concentration is determined by continuously or periodically removing samples of solution 32 from pond 20 at one or more different locations via a sample tube and then measuring the salt concentration using meters located outside pond 20. In some embodiments, the salt concentration measured by devices 68 or measured in a sample taken from pond is returned to controller 64 and used to control the operation of cooling apparatus 50, auxiliary cooling system 80, bubble injection apparatus 110, warm solution supply system 130, and/or crystal vacuum harvesting device 150. In some embodiments, information regarding both temperature and salt concentration at various locations within pond 20 is used by controller 64 to control the operation of cooling apparatus 50, auxiliary cooling system 80, bubble injection apparatus 110, waini solution supply system 130, and/or crystal vacuum harvesting device 150. In some embodiments, information regarding temperature and/or salt concentration at one or more locations within a single circulation cell is used by controller 64 to control the apparatus as aforesaid. In some embodiments, infoimation regarding temperature and/or salt concentration at one or more location within a single vortex of a parallel circulation cell is used by controller 64 to control the apparatus as aforesaid.
[0132] In some embodiments, the number and location of apparatus for measuring temperature 66 and/or salt concentrations 68 is determined for a given pond through model pond measurements and/or simulation studies. In some embodiments, each sensor and its sampling method is periodically calibrated.
[0133] In some embodiments, a controller 64 is provided that receives feedback from the apparatus for measuring temperature 66 and/or salt concentrations 68 at one or more locations throughout pond 20. In some embodiments, the controller 64 controls the various operating parameters of pond 20 based on such feedback, for example by adjusting the rate of cooling provided by cooling apparatus 50 by adjusting the flow rate of coolant 52 therethrough, by adjusting (e.g. turning on or off) the operation of auxiliary cooling system 80 or sub-cooling apparatus 90, by adjusting the rate of air supplied by air compressor 114 to bubble injection apparatus 110, by adjusting the rate of air supplied by air compressor 114 to bubble injection tubes 112 to adjust the rate of production by small apertures 122, by adjusting the rate of inflow of wain' salt-containing solution 32 through warm solution supply system 130 described below, and/or by regulating the rate of harvesting of salt crystals 38 by crystal vacuum harvesting device 150 described below.
[0134] In one example embodiment, the temperature and salt concentration are measured in at least one region of pond 20. The temperature and salt concentration measured are returned to controller 64. Controller 64 then determines based on the known metastable zone width whether the measured conditions of temperature and salt concentration are within the metastable zone width. In some embodiments, controller 64 determines whether the measured conditions of temperature and salt concentration are within a desired portion of the metastable zone width, for example, at least 2 C higher than the supersaturation limit and at least 2 C lower than the equilibrium saturation line. As used herein, a reference to determining whether conditions are within the metastable zone width includes determining whether conditions are within a selected portion of the metastable zone width.
[0135] In one embodiment, if controller 64 determines that, given the salt concentration measured in the region of pond 20, the temperature is too high relative to the metastable zone width or a desired portion thereof, for example, the temperature is above the metastable zone width or within about 2 C of the equilibrium saturation line, active cooling via apparatus 50 is initiated and/or increased for that region of the pond by controller 64. In some embodiments, the amount of coolant 52 passed through tubing network 54 is adjusted based upon the difference between the measured temperature and the metastable zone width for the measured salt concentration in the region of pond 20. For example, more coolant will be passed through tubing network 54 in the region of the pond if the temperature is above the equilibrium saturation line than if the temperature is near or just below the equilibrium saturation line. In some embodiments, the rate of cooling of pond 20 is adjusted for any given region of the pond 20 or for pond as a whole depending on how far the measured temperature is from the coolest temperature that still falls within the metastable zone width for the measured concentration of salt, so that the farther the temperature is above the coolest temperature within the metastable zone width, the greater the rate of active cooling. In some embodiments, if controller 64 determines that the temperature is too high relative to the mestastable zone width, and if the temperature measured at a second location within pond 20 is too low relative to the metastable zone width or is within a desired portion of the metastable zone width, the rate of flow of air through bubble injection tubes 112 is increased to increase the circulation rate within pond 20. In some embodiments, controller 64 takes some or all of the foregoing actions.
[0136] In one embodiment, if controller 64 determines that, given the salt concentration measured in the region of pond 20, the temperature is too low relative to the metastable zone width, operation of active cooling via apparatus 50 is decreased and/or stopped for that region of the pond by controller 64. In another embodiment, if controller 64 determines that, given the salt concentration measured in the region of pond 20, the temperature is too low relative to the metastable zone width, the flow of warm salt-containing solution 32 through warm solution supply system 130 is increased. In another embodiment, if controller determines that the temperature is too low relative to the mestastable zone width and the measured air atmospheric air temperature is lower than the temperature of solution 32, controller 64 activates the introduction of small bubbles via small apertures 122 into pond 20 to fat in a surface foam. In another embodiment, if controller 64 determines that the temperature is too low relative to the metastable zone width and a measurement of temperature at a second location within pond is within a desired region of the metastable zone width, the rate of air flow through bubble injection tubes 112 is increased to increase the circulation rate in the pond 20. In some embodiments, controller 64 takes some or all of the foregoing actions.
[0137] In some embodiments, circulation rates of salt-containing solution 32 within the circulation cells are chosen so that the conditions within the crystal growth region in the pond can be selected to be as optimal as practical for each operating and weather condition while maintaining production of high quality crystals and producing a selected size range of crystals. Operating conditions can be optimized through mathematical and/or laboratory modeling and simulation of the bubble induced and maintained circulation flows provided by bubble injection apparatus 110. Larger scale tests may be conducted using selected scaling factors to assist in the design and operation of full scale ponds 20.
[0138] In some embodiments, during periods of prevailing low atmospheric temperature (e.g. during winter), the rate of cooling at the surface of pond 20 tends to be increased by both wind speed and surface to air temperature difference. In some embodiments, during periods of prevailing low atmospheric temperature, the rate of sub-surface cooling of pond 20 is decreased. In some embodiments, the rate of cooling at the surface of pond 20 is decreased by flooding the surface of pond 20 with small bubbles to produce a foam. In some embodiments, the foam so produced covers most of the surface of pond 20. In some embodiments, the small bubbles used to produce the foam are produced by bubbling air through small apertures 122 in bubble injection apparatus 110. In some embodiments, the determination of whether a surface foam should be produced on pond 20 is made by a person assessing prevailing conditions. In some embodiments, weather conditions with low ambient air temperatures and strong winds are most likely to lead to higher-than-desired cooling rates for the top surface of pond 20, and under such conditions deployment of an upstream wind barrier (e.g. wind barrier 70) and/or formation of a surface foam are likely to be desirable.
[0139] In some embodiments, a non-toxic foaming agent is added to pond 20 to increase the surface tension for bubbles. In some embodiments, addition of a foaming agent increases the lifespan of the bubbles, helping to retain a surface foam on pond 20. Example foaming agents that may be used in some embodiments include octadecylamine (ODA), dodecylamine (DDA), sodium dodecyl sulphate (SDS), polyphenylsulfone (PPSF), carboxylated polysulfone (CPSF), and the like.
[0140] In some embodiments, the rate of cooling at the surface of pond is decreased by providing a wind suppression device, shown schematically in Figure 7 as 70, such as a wind barrier along at least a windward side of the surface of pond 20. In some embodiments, the wind barrier is a wind suppression fence or a physical barrier (e.g. in the nature of a snow fence) that reduces the wind speed above and across the surface 40 of pond 20. In some embodiments, the wind speed adjacent to the pond surface 40 is reduced by deploying a wind suppression device along at least a windward side of pond 20.
[0141] In some embodiments, the wind barrier reduces the wind speed over the pond surface by a factor of two or more, and reduces the convective cooling rate at the surface of pond 20 by a similar ratio. In some embodiments, a wind barrier is used if design data for a particular crystallization pond 20 shows that typical wind speeds in the region of the particular crystallization pond 20 are likely to cause excessive cooling rates.
[0142] In some embodiments, the wind suppression device, e.g. a wind barrier, is controllable, i.e. can be erected or taken down in an automated fashion either upon receiving a manual signal and/or a signal from a controller. In some embodiments, the wind barrier is controlled by controller 64. In some embodiments, an apparatus for measuring prevailing wind speed and direction provides feedback to controller 64, which raises or lowers wind barrier 70 on the basis of such feedback.
[0143] In some embodiments, the wind suppression device, e.g. a wind barrier, is used with crystallization ponds that are not too large (i.e. do not have a length and/or width that is too large) relative to the height of the wind barrier. In some embodiments, the ratio of the pond length or width to the height of the wind barrier is less than 10, including any value between 1 and 10, e.g. 2, 3, 4, 5, 6, 7, 8 or 9. For example, in one exemplary embodiment, if the wind barrier has a height of 3 m, then the pond length and width should be 30 m or less.

[0144] In some embodiments, pond 20 is operated in a batch process. In some such embodiments, an active cooling apparatus 50, auxiliary cooling system 80, sub-cooling apparatus 90, bubble injection apparatus 110, cover 65 and/or wind barrier 70 are provided, and are operated as described in this specification to control the conditions within pond 20. In some embodiments, pond 20 is operated in a continuous process. In some such embodiments, a system is provided for supplying waini solution to pond 20 and a system is provided for removing excess solution and produced crystals from pond 20.
[0145] In some embodiments, as illustrated in Figures 11A and 11B, a warm solution supply system 130 is provided to distribute warm incoming salt-containing solution 32 (e.g. as obtained from a solution mining operation) approximately equally to each pond circulation cell in pond 20. In some embodiments, solution is supplied through warm solution supply system 130 at a salt concentration such that the solution 32 is close to saturation conditions for its particular supply temperature.
[0146] In some embodiments, warm solution supply system 130 is used together with parallel circulation cells in pond 20. In such embodiments, a plurality of bottom inlet flow spreaders 132 are connected to a bottom inlet solution supply pipe network 134. In some embodiments, bottom inlet flow spreaders 132 are apertures foimed within pipe network 134. Bottom inlet solution supply pipe network 134 receives warm salt-containing solution 32 from a supply pipe 136, which receives input from the source of salt-containing solution 32 for pond 20. In some embodiments, supply pipe 136 is fed from the output of a potash solution mine. In some embodiments, supply pipe 136 also receives chelant from a chelant supply source, to provide a desired concentration of chelant within pond 20. In some embodiments, supply pipe 136 also receives recycled salt crystals (for example, salt crystals 38 that are considered too small or low quality to be sent for further processing) and salt-containing solution 32.
[0147] In the illustrated embodiment of Figures 8A and 8B, bottom inlet solution supply pipe network 134 (shown as 134A and 134B, respectively) is positioned above bubble injection tubes 112. Bottom inlet flow spreaders 132 (shown as 132A and 132B, respectively) are oriented to inject salt-containing solution 32 upwardly into pond 20 at a small angle with respect to the vertical direction, so that the resulting flow of solution will have both horizontal and vertical components of momentum along the length of each circulation cell. In some embodiments, bottom inlet flow spreaders 132 are located on opposite sides of each pond circulation cell as compared with the apertures 118 of bubble injection apparatus 110.
[0148] Bottom inlet flow spreaders 132 and bottom inlet solution supply pipe network 134 may be made from any suitable material, for example, plastic.
In some embodiments, bottom inlet solution supply pipe network 134 is clamped to bubble injection tubes 112.
[0149] In one example embodiment illustrated in Figure 11A, the bottom inlet flow spreaders 132C of warm solution supply system 130A are configured to supply incoming waini salt-containing solution 32 at approximately equally spaced-apart intervals. In some embodiments having parallel rectangular circulation cells, inlet flow spreaders 132 extend parallel to bubble injection tubes 112. In some embodiments having hexagonal circulation cells, at least one inlet flow spreader 132 is positioned within each circulation cell 120. In some such embodiments, one inlet flow spreader 132 is positioned at the center of each circulation cell 120. In some such embodiments, air is bubbled in to the circulation cell at or near the center of the circulation cell, for example through bubble injection tubes 112 (not shown in Figure 11A).
[0150] In an example embodiment having parallel rectangular circulation cells within pond 20 illustrated in Figure 11B, the bottom inlet flow spreaders 132D of warm solution supply system 130B are configured to supply incoming salt-containing solution 32 at spaced-apart intervals along bubble injection tubes 112 in the first approximately 10% to 15% of the length of pond 20 at the inlet end 44 of pond 20. The remaining 85% to 90% of the length of pond 20 is dominated by crystal deposition flux and, in some embodiments as described below, vacuum harvesting of such crystals by a crystal vacuum harvesting device. Once formed, salt crystals 38 tend to sink toward the bottom 22 of pond 20, where they are deposited in an accumulative layer on bottom screen 36.
[0151] In some embodiments, the size and number of inlet flow spreaders 132 is selected to provide exponentially decreasing flow rates at each inlet flow spreader 132 location farther from the inlet end 44 of pond 20. In some embodiments, the flow of salt-containing solution 32 through inlet flow spreaders 132 enhances the natural flow for adjacent circulation cells (i.e. carries the incoming salt-containing solution 32 toward the surface 40 where it is cooled by ambient air cooled convection and evaporation), and has a net flow along each circulation cell from the inlet end 44 of the pond 20 to the other end of the circulation cell at the outlet end 46 of pond 20.
[0152] In some embodiments, warm solution supply system 130 is used in the absence of a bubble injection apparatus 110.
[0153] In some embodiments, for example that shown in Figures 12A, 12B
and 12C, waim solution supply system 130 delivers wailil salt-containing solution 32 to pond 20 via two different and potentially alternative mechanisms. As described above, solution 32 is supplied through the bottom 22 of pond 20 by a bottom inlet solution supply pipe network 134. Solution 32 is additionally or alternatively supplied throughout the depth of pond 20 at inlet end 44 by one or more secondary distribution nozzles that extend through at least a portion of the depth of pond 20. In the illustrated embodiment, the secondary distribution nozzle is a rotary inlet 138. Additionally, a recycle supply pipe 137 feeds recycled crystals and/or solution 32 (for example, as recovered from vacuum harvesting device 150 described below) to supply pipe 136. In some embodiments, rotary inlets 138 are used together with parallel circulation cells within pond 20.
[0154] With reference to Figure 12B, rotary inlet 138 has a solution supply tube 140, which has one or more apertures 142 for allowing solution 32 to flow into pond 20 as a stream. Rotary inlet 140 is coupled through a rotatable coupling 144 to supply pipe 136.
[0155] In use, as described above, bottom inlet flow spreaders 132 are used to initiate and mix warm salt-containing solution 32 into each circulation vortex within pond 20. In some embodiments having parallel circulation cells, rotary inlet 138 is additionally or alternatively used to induce flow throughout each circulation cell. In such embodiments, solution supply tube 140 is rotated and solution is pumped therethrough by pump 146 so that solution 32 exits apertures 142 and induces flow.

[0156] In some embodiments having parallel circulation cells, the initial flow induced by warm solution supply system 130 persists for about one rotation of the circulation vortex. Without being bound by theory, beyond this region, the effects of friction on the bottom surface of the pond and viscous dissipation within the vortex due to the nearly square elevation shape of the vortex would dissipate the momentum imparted by warm solution supply system 130. In some embodiments, such dissipation of the inlet rotational energy is partly or fully counterbalanced by mechanical energy supplied by bubbles from bubble injection apparatus 110 rising through solution 32 on one of the vertical sides of each vortex.
In some embodiments, the amount of rotational energy to be supplied by warm solution supply system 130 and the bubble flow rate through bubble injection apparatus 110 are determined by scale model experimental and/or simulation studies.
[0157] As illustrated in Figure 12C, in embodiments having parallel circulation cells, a pair of solution supply tubes 140 can be provided in each circulation cell to induce flow along each outer side of the circulation cell (illustrated by dashed outline 148). Solution 32 entering pond 20 through rotary inlet 138 is directed horizontally so that the stream of liquid output by each rotary inlet 138 is directed to a vertical supply jet impact baffle plate 149. The momentum of the stream of liquid output by one rotary inlet 138 for one circulation vortex is thus balanced by the stream of liquid output by the adjacent rotary inlets 138. In some embodiments, the length of baffle plate 149 is equal to the axial distance (i.e. the distance in the direction of arrow 45 in Figure 9A) traveled across one rotation of the circulating vortex particles. The angular rotational speed of each vortex is approximately equal for any circulation vortex in the pond 20 at the inlet end 44. Thus, the momentum of the stream of liquid output by rotary inlet 138 will increase from zero at the center of the vortex to a maximum near the outer perimeter of each vortex. In some embodiments, this result is achieved by varying the spacing or diameter of the stream of liquid output by rotary inlet 138, or a combination thereof, to provide a rotational momentum distribution for a viscous flow rotation with approximately the lowest rate of energy dissipation for the stream of liquid output by rotary inlet 138. In one embodiment having parallel circulation cells, the circulation vortices are approximately square in cross section (i.e. the depth of the pond is approximately 1/2 of the width of each parallel circulation cell).
[0158] In some embodiments, solution supply tube 140 extends across substantially the entire depth of pond 20. In some embodiments, including the illustrated embodiment, solution supply tube 140 extends across only approximately the top half of the depth of pond 20, or across some other portion thereof, e.g. the top one third, top two thirds, top quarter, or the like.
[0159] With reference to Figure 13, through the use of cooling apparatus 50, auxiliary cooling system 80, bubble injection apparatus 110, and/or warm solution supply system 130, conditions such as temperature and/or salt concentration can be controlled throughout the volume of pond 20. Accordingly, in contrast to the exemplary prior art crystallization pond illustrated in Figure 1A, pond 20 illustrated schematically in Figure 13 has only small regions 18 and where conditions are outside a desired temperature and/or salt concentration within the metastable zone width region. In region 18, the concentration of salt may be slightly lower than in the rest of pond 20, and the temperature may be somewhat cooler than the rest of pond 20. In region 21, the salt concentration may be somewhat higher than in the rest of pond 20, and the temperature may be slightly higher or slightly lower than the rest of pond 20, depending on whether cooling apparatus 50 is being operated. Most of the solution 32 in pond 20 has a relatively uniform temperature and/or salt concentration, illustrated as a relatively large middle region 19. Moreover, because the temperature, salt concentration and/or rate of cooling can be controlled to remain within the metastable zone width, most of the solution 32 is within the metastable zone width region 23.
[0160] In some embodiments, the harvesting of salt crystals 38 and salt-containing solution 32 from pond 20 is conducted on a continuous or approximately continuous basis. In some such embodiments, the warm solution supply system 130 supplies a volume of warm salt-containing solution 32 that is approximately equal to the volume removed by harvesting, such that the volume flow rate of solution 32 into pond 20 is approximately equal to the volume flow rate of solution 32 out of pond 20.
[0161] In some embodiments, at least a portion of the warm salt-containing solution 32 harvested from pond 20 together with salt crystals 38 is re-injected back into pond 20 through warm solution supply system 32. In some embodiments, at least a portion of the warm salt-containing solution 32 harvested from pond 20 together with salt crystals 38 is re-injected back into an underground solution mining operation. The decision to return a portion of the harvested solution 32 back to pond 20 or into a solution mine, or to direct the harvested solution to waste, can be made based on the remaining fraction of the desired salt still remaining in the harvested solution 32. In some embodiments, although a portion of the harvested warm salt-containing solution 32 is re-injected back into pond 20 or back into a solution mine, the overall size of crystallization ponds required to handle the output of a given solution mining operation can still be less than would be required using previous crystallization apparatus and methods.
[0162] In some embodiments in which a waini solution supply system 130 is used together with bubble injection apparatus 110, the warm solution supply system 130 is configured to distribute waiin salt-containing solution 32 approximately equally to each circulation cell within pond 20.
[0163] In some embodiments, the warm solution supply system 130 is used to inject salt-containing solution on a substantially continuous basis so that cooling pond 20 can be operated in a continuous (rather than a batch) process.
[0164] In some embodiments, a crystal vacuum harvesting device 150 is provided to harvest salt crystals 38 from the bottom of pond 20. Salt crystals tend to be more dense than salt-containing solution 32. For example, crystals of KC1 have a density that is approximately 20% to 50% higher than the surrounding solution during crystallization. Thus, as salt crystals 38 grow in size, they will drop toward bottom screen 36 with a free fall rate that is predictable in a stationary pond (e.g. the free fall rate is very slow for small particles, and this free fall rate increases with increasing particle diameter). In embodiments in which circulation is provided in pond 20, the rate of free fall will be greater in regions of pond 20 experiencing a down-flow, and less in regions of pond 20 experiencing an up-flow.
[0165] With reference to Figure 14, crystal vacuum harvesting device 150 has a flow-driven sweeper reel 152 that helps feed deposited salt crystals 38 into a vacuum intake 154 of vacuum harvesting device 150. Vacuum intake 154 feeds material to vacuum tube 156, which is connected to a liquid and crystal vacuum system pump 158 at the surface 42 adjacent pond 20.
[0166] Vacuum harvesting device 150 has set of wheels 160 that are positioned to support part of the weight of vacuum harvesting device 150 on bottom screen 36. Wheels 160 facilitate movement of vacuum harvesting device 150 around the bottom 22 of pond 20 on screen 36.
[0167] In the illustrated embodiment, a pond float 162 is coupled to vacuum tube 156. Pond float 162 helps to at least partially support the weight of the vacuum tube 156 in the salt-containing solution 32.
[0168] In some embodiments, pond float 162 is used to guide the vacuum intake 154. In one example embodiment, a corrosion-protected pond harvester system is provided to guide vacuum harvesting device 150 around the bottom 22 of pond 20 by moving pond float 162. For example, one or more plastic-coated positioning guide cables can be provided above the surface 40 of pond 20 and operated from the edges of the pond at the surface 42 to move device 150 around pond 20.
[0169] In some embodiments, crystal vacuum harvesting device 150 moves along the bottom 22 of pond 20, on top of bottom screen 36, to harvest produced crystals. In some embodiments, crystal vacuum harvesting device 150 moves in generally parallel paths. In some embodiments, crystal vacuum harvesting device 150 moves in paths that generally cover the entire bottom 22 of pond 20. In some embodiments, crystal vacuum harvesting device 150 is moved over the bottom of the pond 20 so that over one traversing cycle, the entire bottom surface of the pond is traversed. Since the deposition depth of salt crystals 38 on bottom screen 36 is expected to vary across the pond bottom (e.g. within each circulation cell) and along the mean flow direction for supply feed flow, variations in the deposition depths may be significant. Thus, in some embodiments, the feed velocity of the vacuum harvesting device 150 is controlled by controlling the operation of vacuum system pump 158. In some embodiments, the vacuum force caused by vacuum system pump 158 is increased at locations within pond 20 where it is anticipated that salt crystals 38 will accumulate to a greater depth, and decreased at locations within pond 20 where it is anticipated that salt crystals 38 will accumulate to a lesser degree.
[0170] In some embodiments, the position, direction of motion, and rate of advance of vacuum intake 154 are externally controlled by a controller. In some embodiments, the controller is controller 64 that receives feed back from temperature sensors 66 and/or apparatus for measuring salt concentration 68 located at a plurality of different spatial locations within pond 20. In some embodiments, the operation of vacuum harvesting device 150 are determined by one or more of: the mean depth variation of crystal deposition at any given time;
the maximum depth of crystal deposition at any time; the average rate of crystal deposition at any given time; the variation in rate of crystal deposition over time;
the maximum rate of crystal deposition at any given time; expected diurnal variations in crystal deposition; or expected weather-related variations in any of the foregoing factors. In some embodiments, the operation of vacuum harvesting device 150 is regulated to minimize the energy expended per unit of crystal production based on measured conditions such as salt concentration, temperature, or the rate of cooling at selected locations throughout pond 20.
[01711 Crystal vacuum harvesting device 150 harvests salt crystals 38, together with salt-containing solution 32 that is sucked up by vacuum intake 154, and transports this mixture to the surface 42. The salt crystals 38 are then partially separated from the liquid fraction, for example by gravity separation in settling vessel 166, to achieve a high concentration of crystals in the produced crystal solution 164. Excess liquid from the liquid fraction, as well as salt crystals that are smaller than a desired size, can be returned to pond 20 as part of the supply solution (for example, through wailil solution supply system 130), or can be pumped to a different pond for further crystallization. The produced crystal solution 164 is conveyed for further processing, e.g. to a processing plant (not shown) through a suitable outlet pipe.
[0172] In some embodiments, excess liquid obtained from settling vessel 166 is used as a carrier for crystals in produced crystal solution 164, is pumped back to pond 20, is pumped to a different crystallization pond, or is pumped underground (e.g. by being returned to a solution mine). The deteimination of where excess liquid should be sent can be made based on the amount or fraction of solution needed for pumping and transporting the produced crystals in crystal solution 164 over a known distance and elevation changes to a processing plant, and the concentration of salt ions remaining in the excess liquid. In some embodiments, parameters such as the concentration of salt ions remaining in the excess liquid and the size of crystals in produced crystal solution 164 are periodically measured to deteimine the most efficient use for the excess liquid. In some embodiments, the amount of excess liquid that is combined with produced crystal solution 164 is periodically adjusted to maintain approximately a fixed mass fraction of liquid:crystal solids to facilitate transportation of the produced crystals to the processing plant.

101731 The crystal vacuum harvesting device 150 can be operated to continually harvest salt crystals 38 from the bottom of pond 20. For example, when crystal vacuum harvesting device 150 has completed one traverse of the bottom of pond 20, it can initiate a new traverse of the bottom of pond 20 to initiate another crystal harvesting cycle. The cycle rate and flow rate for vacuum harvesting device 150 can be controlled based on the need to control the depth of crystals 38 foiming on the bottom screen 36 of pond 20, and/or to reduce any inter-crystal bonding or caking among salt crystals 38 deposited on bottom screen 36. In some embodiments, caking among perfect crystals is avoided by controlling the solution conditions in the region of the deposited crystals 38 to be very close to saturation conditions for the salt-containing solution 32, and/or to be slightly cooler than the remainder of the pond. Caking is expected to be most problematic in situations where imperfect crystals are formed and deposited in contact with one another, and further are surrounded by a highly supersaturated solution.
[0174] In some embodiments, one crystal vacuum harvesting device 150 is provided in a single pond 20. In some embodiments, two or more crystal vacuum harvesting devices 150 are provided in a single pond 20.
[0175] At the processing plant, the two phases of the produced crystal solution 164, liquid waste and salt crystal product particles, are mechanically separated. The salt crystal product particles are dried to yield the desired crystalline product, while the liquid waste is disposed of in any suitable manner, for example delivery to another salt pond, pumping back underground for deposition and mixing with underground solution flows to dissolve more of the desired salt, or the like.

[0176] In some embodiments where the salt-containing solution 32 is produced from a potash mine, the resulting liquid waste is primarily a weak solution of NaC1 with a residual concentration of KC1.
[0177] In some embodiments, some or all of the rate of cooling, salt concentration gradients, or circulation within pond 20 is controlled.
Controlling the rate of cooling, salt concentration gradients, and/or circulation within crystallization ponds can increase the production rate, quality and/or size of salt crystals produced by a particular crystallization pond. For example, it is desirable that KC1 crystals be transparent and have a cubic morphology, and crystals that have these characteristics would be considered to be of higher quality. Higher quality crystals can have a higher market value than crystals of lower quality.
[0178] The formation of salt crystals from a salt-containing solution are non-equilibrium, locally time-dependent processes. Although it is accepted practice to show the metastable region of crystallization on a phase diagram, these processes occur only in quasi-equilibrium conditions because, for example, crystallization of KC1 requires the simultaneous diffusion of I(' and Cr ions toward each nucleation site for crystal growth, and the diffusion of H20 away from these sites. Also, the heat of the crystallization phase change to form KC1 is diffused away from these sites. These diffusion processes are time dependent and coupled. They will differ from site-to-site and over regions of a crystallization pond, and especially along the length of the pond from supply inlet to the outlet end of the pond. Models can be developed to help understand the crystallization processes occurring within the crystallization pond, and to assist in controlling the conditions within the pond to enhance the foimation of salt crystals therein.

[0179] In developing analytical/numerical models for crystallization ponds, it is convenient to refer to the bulk mean properties [i.e. temperature T(K), solution salt concentration C, particle crystal size dp(mm), particle crystal concentration Cc, solution velocity V(n-i/s), particle crystal velocity Vp(m/s)] of a circulation cell as the planar averaged values at any position, x, along the parallel bulk mean flow paths of each vortex from inlet to outlet. For steady supply flows and operating conditions these bulk mean properties are expected to be only a function of x. Within the circulation cells there will be a cyclic variation of properties as the solution mixture rotates about an axis of rotation and the solution moves toward the top or bottom. This circulation rate Rc (rpm) is another variable.
Modelling these processes will require complex numerical models for systems of equations, which may be quasi-steady at any point in the pond. In order to keep the numerical analysis tractable, it will be necessary to make justifiable assumptions about the boundary conditions and how they vary with time. The relative size of each term is important when using a volume averaging method of modeling for the solution space for the set of governing physical balance equations (i.e.
continuity of each chemical species, energy balance including phase change, solar gains and heat transfer, and momentum including gravitational and internal fluid shear forces).
[0180] When several salts are present in the same solution, the metastable region for each salt is different. These are each somewhat complex crystallization processes particularly when they are coupled with more than one crystallization process and several salts in the solution. There will be some three-dimensional liquid and crystal movements that will change over time and space for a crystallization pond. Laboratory models may be used to quantify the solution processes. Expanded scale models may require theoretical/numerical models and simulations to determine the optimum dimensionless parameters for each operating condition. Thus, it is anticipated that both physical scaled modeling and model analysis and simulations may be used to develop models that can be used to better control crystallization within a crystallization pond. Particularly when chelating agents are added to the solution to better control the size of the metastable region for enhanced crystal growth and size distribution and increased rate of production of KC1 crystals, these relationships are not linear and there is coupling between temperature and circulation flow controls.
101811 High quality crystallization of salt from a salt-containing solution in a crystallization pond occurs only in a metastable crystallization region of temperature and salt concentration. The variables that can be controlled to maintain the salt-containing solution within the metastable zone and thereby control the crystallization rate and crystal quality and size distribution include the cooling rate of the salt-containing solution, the circulation rate of the salt-containing solution within the pond, and the concentration of any chelating agent added to the crystallization pond. In some embodiments, the cooling rate of the salt-containing solution is controlled by controlling the rate of coolant supply to cooling apparatus 50 by pumps 58. In some embodiments, the circulation rate of the salt-containing solution within the pond is controlled by controlling either or both of the amount of air supplied to bubble injection apparatus 110 by air compressor 114 or the rate of supply of solution 32 to watin solution supply system 130. In some embodiments, the concentration of a chelating agent added to a crystallization pond 20 is controlled by adding a desired concentration of the chelating agent to incoming salt-containing solution.
[0182] In some embodiments, the concentration of chelating agent added to crystallization pond 20 is sufficient to give a final concentration in pond 20 in the range of 300 and 3000 ppm, including any value therebetween, e.g. 500, 1000, 1500, 2000 or 2500 ppm. Lower concentrations of chelant can be used in embodiments where the concentration of impurities in salt-containing solution is lower.
[0183] Other factors that may influence the rate and quality of crystal formation in a crystallization pond include the depth of the pond, the evaporation rate from the top surface of the pond, local pond temperature and salt concentration, and the gradients of the temperature and salt concentration at that location, relative velocity of the pond liquid and the salt crystals, the feed rate of incoming salt-containing solution and the removal rate of salt crystal product and solution. Thus, in some embodiments, the average pond depth is controlled. In some embodiments, the rate of removal of salt crystal product and solution by crystal vacuum harvesting device 150 is controlled.
[0184] With reference to Figure 15, a schematic diagram of a method 200 for controlling an exemplary embodiment of pond 20 is shown. At 202, the temperature of solution 32 in pond 20 can be measured at one or more locations, for example using temperature sensors 66A, 66B, 66C disposed at a plurality of different locations within pond 20. At 204, the concentration of salt in solution 32 in pond 20 can be measured at one or more locations, for example using apparatus for measuring solution salt concentration 68A, 68B, 68C disposed at a plurality of different locations within pond 20, or by assaying solution samples withdrawn from different locations within pond 20. At 206, wind speed and/or direction can be measured. At 208, an assessment of the forecast can be made to determine if precipitation (e.g. rain, snow or hail) is anticipated and if so, how much, and/or current conditions can be assessed to determine if precipitation is presently occurring. At 210, the dew point temperature of the external atmosphere can be deteunined. At 212, atmospheric humidity can be determined. At 213, current atmospheric temperature can be determined. At 211, the temperature of a source of groundwater used by auxiliary cooling system 80 can be detelmined. At 209 atmospheric pressure can be measured. At 207, the level of solar irradiation occurring or expected to be occurring can be assessed. At 205, the circulation rate within one or more circulation cells within the pond can be assessed, for example using appropriately placed velocity meters, flow meters, turbine flow meters, or particle position change sensors.
[0185] At 214, input from some or all of the foregoing steps can be used to determine how to operate the control features of pond 20. In some embodiments, a controller is provided at 214 to receive input from the foregoing steps and control some or all of the features of pond 20, including as described below. In some embodiments, some or all of the features of pond 20 are controlled to maintain substantially all of solution 32 in pond 20 within the metastable zone width (MSZW) region. In some embodiments, some or all of the features of pond 20 are controlled to minimize the auxiliary energy demand rate associated with the operation of pond 20. In some embodiments, some or all of the features of pond 20 are controlled to both maintain substantially all of solution 32 in pond 20 within the metastable zone width and minimize the auxiliary energy demand rate associated with the operation of pond 20.
[0186] In some embodiments, some or all of the control features of pond 20 are adjusted in a slow manner and on a somewhat continuous basis (i.e.
abrupt and/or significant changes in the operation of the control features of pond 20 are avoided). In some embodiments, for example where the conditions within pond 20 deviate significantly from ideal crystallization conditions within the metastable zone width region, some or all of the control features of pond 20 are regulated more aggressively, and more rapid and/or extreme changes in the operation of some or all control features of pond 20 can be used to return conditions within pond 20 to the metastable zone width region throughout as much of the volume of pond 20 as possible.
[0187] At 216, the operation of cooling apparatus 50 is controlled to provide a desired rate of cooling of solution 32 within pond 20, including at specific regions within pond 20 if desired. For example, if the solution temperature measured at 202 is above the metastable zone width for the salt concentration measured at 204, or is above a desired region of the metastable zone width (e.g. is higher than 2 C less than the equilibrium saturation line for the salt concentration measured at 204), the rate of cooling provided by cooling apparatus 50 can be increased, or cooling apparatus 50 can be activated, at least at the region where the temperature was measured.
[0188] In some embodiments, the rate of cooling provided by cooling apparatus 50 is increased (or cooling apparatus 50 is activated) only if the ambient air temperature measured at 213 is warmer than a predetermined amount below the temperature of the solution measured at 202, e.g. warmer than about 30 C below the temperature of the solution measured at 202. In some embodiments, cooling apparatus 50 is activated only if the ambient air temperature is above about

10 C
(e.g. 15 C, 20 C, 25 C, 30 C, 35 C, 40 C or higher).
[0189] In some embodiments, the rate of cooling provided by cooling apparatus 50 is adjusted depending on how far the temperature measured at 202 is from the metastable zone width for the salt concentration measured at 204. For example, cooling apparatus 50 could be activated to 50% cooling capacity if the temperature measured at 202 is close to the equilibrium saturation line temperature, but could be activated to 100% cooling capacity of the temperature measured at 202 is more than 5 C above the equilibrium saturation line temperature.
[0190] In some embodiments, if the solution temperature measured at 202 is below the metastable zone width for the salt concentration measured at 204, or is below a certain predetermined portion of the metastable zone width, e.g. less than 2 C above the supersaturation limit concentration for nucleation of crystals for the salt concentration measured at 204, the operation of cooling apparatus 50 is reduced or stopped at 216.
[0191] At 218, the operation of auxiliary cooling system 80 can be controlled to achieve a higher degree of cooling of coolant 52 within cooling apparatus 50 if desired, for example based on the temperature difference between solution 32 and the external atmosphere, and/or the relative humidity of the atmosphere.
[0192] At 220, the operation of sub-cooling apparatus 90 can be controlled, for example based on the temperature difference between the external atmosphere measured at 213 and a source of groundwater used for auxiliary cooling system 80 measured at 211. In some embodiments, sub-cooling apparatus 90 is operated only if the temperature difference between the source of groundwater measured at 211 and the atmospheric air measured at 213 is at least 15 C. In some embodiments, sub-cooling apparatus 90 is operated only if the temperature of the source of groundwater used for auxiliary cooling system 80 rises close to or above the typical air dew-point temperature of the ambient atmosphere.
[0193] At 222, operation of bubble injection apparatus is controlled. In some embodiments, including the illustrated embodiment, the flow of air through large apertures 118 can be controlled at 224 independently of the flow through small apertures 122, controlled at 226.
[0194] In some embodiments, the flow of air through large apertures 118 controlled at 224 is controlled based on the concentration of salt and/or temperature measured at a plurality of different locations within pond 20 at steps 202, 204, for example to provide a faster rate of flow when different regions of pond 20 are at appreciably different temperatures and/or salt concentrations.
For example, if the temperature measured at a first location in pond 20 at step 202 is more than 2 C or more than 5 C different than the temperature measured at a second location in pond 20, the flow of air through large apertures 118 may be increased at 224 to increase the circulation rate in pond 20 and thereby reduce temperature differences across the pond.
[0195] In some embodiments, if the temperature measured at a first location in pond 20 at step 202 is either above the metastable zone width or above a predetermined region of the metastable zone width (e.g. higher than 2 C less than the saturation line temperature or below 2 C more than the supersaturation limit for nucleation of crystals) for the salt concentration measured at 204, but the temperature measured at a second location in pond 20 at step 202 is within the metastable zone width (or the predetermined region of the metastable zone width), the flow of air through large apertures 118 may be increased at 224 to increase the circulation rate in pond 20.
[0196] In some embodiments, if the salt concentration measured at a first location in pond 20 at step 204 is appreciably different than the salt concentration measured at a second location in pond 20, the flow of air through large apertures 118 may be increased at 224 to increase the circulation rate in pond 20.
[0197] In some embodiments, the flow of air through small apertures 122 is controlled at 226 to provide rafts of small bubbles that form a foam on the top 40 of pond 20, for example to reduce the rate of cooling at the surface 40 of pond 20 or to reduce the rate of evaporation from pond 20. In some embodiments, the flow of air through small apertures 122 is initiated or increased when the atmospheric temperature measured at 213 is more than 40 C below the temperature of solution 32 measured at 202, or when the atmospheric humidity measured at 212 is less than 70%. In some embodiments, the flow of air through small apertures 122 is stopped or decreased when the atmospheric temperature measured at 213 returns to less than 40 C below the temperature of solution 32 measured at 202, or when the atmospheric humidity measured at 212 returns to greater than 70%.
[0198] At 228, operation of warm solution supply system is controlled.
In some embodiments, the flow of warm salt-containing solution 32 through bottom inlet solution supply pipe network is controlled independently of the flow of warm salt-containing solution 32 through rotatable solution supply tube 140. In some embodiments, the flow of warm salt-containing solution 32 is increased if there are significant differences in temperature and/or salt concentration measured at different locations within pond 20 at 202, to increase the rate of circulation within pond 20. In some embodiments, if the temperature measured at a region of pond 20 at 202 is below the metastable zone width for the salt concentration measured at 204, or below a desired region of the metastable zone width, e.g. 2 C above the supersaturation limit for crystal nucleation, the flow of warm salt containing solution 32 thorough rotatable supply tube 140 and/or bottom inlet flow spreaders 132 is increased to increase the temperature of pond 20. In some embodiments, if the temperature measured at a region of pond 20 at 202 is above the metastable zone width for the salt concentration measured at 204, or above a desired region of the metastable zone width, e.g. higher than 2 C below the equilibrium saturation line for the salt concentration measured at 204, the flow of warm salt containing solution 32 through rotatable supply tube 140 and/or bottom inlet flow spreaders 132 is decreased.
[0199] At 230, the operation of crystal vacuum harvesting device 150 is controlled. In some embodiments, harvesting device 150 is controlled to remove solution at approximately the same rate that solution 32 is introduced by warm solution supply system 130. In some embodiments, the operation of crystal vacuum harvesting device 150 is controlled to spend more time harvesting crystals at locations along the bottom of pond 20 where a higher salt concentration is measured, and/or where a higher volume of deposited crystals 38 is expected to form based on anticipated flow patterns within pond 20, and/or where a higher depth of deposited crystals 38 occurs.
[0200] In some embodiments, when it is determined that the depth of pond 20 should be increased, the rate of solution supply by warm solution supply system 130 is increased at 228 and/or the rate of removal of solution by harvesting device 150 is decreased at 230. In some embodiments, when it is determined that the depth of pond 20 should be decreased, the rate of solution supply by warm solution supply system 130 is decreased at 228 and/or the rate of removal of solution by harvesting device 150 is increased at 230. In some embodiments, once the pond has reached a desired depth, the rate of solution supply by system 130 and the rate of solution removal by harvesting device 150 are adjusted to be approximately the same.
[0201] At 232, operation of cover 65 is controlled. For example, where precipitation is expected or occurring at step 208, cover 65 can be extended to cover some or all of the surface 40 of pond 20. Where precipitation has stopped and/or is forecast to stop, cover 65 can be retracted to expose the surface 40 of pond 20 to the atmosphere.
[0202] At 234, operation of wind suppression device 70 is controlled.
For example, where the wind speed measured at 206 indicates significant wind is occurring (e.g. greater than about 20 km/h or greater than about 30 km/h), wind barrier 70 can be erected on the upwind side of pond 20. In some embodiments, wind barrier 70 is erected manually. In some embodiments, wind barrier 70 is erected automatically, for example by a controller activating a mechanism to raise wind barrier 70. In some embodiments, the terrain surrounding pond 20 (e.g.
the presence of hills or other natural wind barriers) is taken into account in determining when wind barrier 70 should be used. In some embodiments, wind barrier 70 is erected only if the atmospheric temperature measured at 213 is above or below a predetermined value, e.g. below 0 C or above 30 C.
[0203] At 236, the depth 104 of pond 20 is controlled. In some embodiments, the depth 104 of pond 20 is increased by increasing the flow of solution 32 through warm solution supply system 130 and/or decreasing the rate of flow of solution 32 and crystals 38 through vacuum harvester 150. In some embodiments, the depth 104 of pond 20 is decreased by decreasing the flow of solution 32 through warm solution supply system 130 and/or increasing the rate of flow of solution 32 and crystals 38 through vacuum harvester 150. In some embodiments, the depth 104 of pond 20 is altered based on prevailing weather or climate conditions. In general, larger diameter vortex flows present in a deeper pond will respond more slowly to temperature changes on the top and bottom surfaces than will smaller diameter vortices present in a shallower pond.
[0204] At 238, the flow of recycled crystals to wann solution supply system 130 through recycle supply pipe 137 is controlled. In some embodiments, crystals harvested by crystal vacuum harvester 150 are screened, and any crystals that pass through the screen together with the solution 32 sucked up by crystal vacuum harvester 150 are wholly or partially redissolved and returned to pond via a pump or pumps used for recirculation via recycle supply pipe 137.
[0205] At 240, the concentration of chelant in the pond is controlled, for example by adding additional chelant to pond 20 and/or to inlet supply pipe 136.
In some embodiments, chelant is added to pond 20 from a chelant supply tank (Figure 12A).
[0206] At 242, a non-toxic foaming agent is added to the pond, for example in situations where it is desired to foini a surface foam as described above.

[0207] At 244, an evaporation suppressing surface oil is added to the pond, for example to reduce the evaporation rate from the surface of the pond.
[0208] It is anticipated that in some embodiments, the expected production rate of quality crystalline salt (e.g. kg of KC1 per m2 of pond and time s (kg.m-2.s-1)) can be more than doubled for a given depth of pond, and that the size and quality of the salt crystals obtained can be increased significantly over previous methods.
[0209] While a number of exemplary aspects and embodiments are discussed herein, those of skill in the art will recognize certain modifications, permutations, additions and sub-combinations thereof. It is therefore intended that the following appended claims and claims hereafter introduced are interpreted to include all such modifications, permutations, additions and sub-combinations as are within their broadest interpretation consistent with the specification as a whole.

References [0210] The following references are incorporated by reference herein in their entireties.
[1] Blagden, N.; Davey, R. J.; Lieberman, H. F.; Williams, L.; Payne, R.;
Roberts, R.; Rowe, R.; Docherty, R., J. Chem. Soc., Faraday Trans. 94,.(1998) 1035.
[2] Garti, N.; Tibika, F., Drug Dev. Ind. Pharm. 6, (1980) 379.
[3] Buckley, H. E. (Ed.), Crystal Growth, Wiley, New York, 1951.
[4] Zaitseva, N. P., Rashkovich, L. N. and Bogatyreva, S. V., J. Cryst.
Growth, 148 (3), (1995) 276.
[5] Jaroslav I\ITvlt, The Kinetics of Industrial Crystalization, Academia Prague, 1985.
[6] Ulrich, J. and Strege, C., J. Crystal Growth, 237-239, (2002) 2130.
[7] Mullin, J. W. Crystallization; Butterworth-Heinemann Ltd.: Oxford, 1993.
[8] So-hnel, O.; Garside, J. Precipitation. Basic Principles and Industrial Applications; Butterworth-Heinemann Ltd: Oxford, 1992.
[9] Myerson, A. S. Handbook of Industrial Crystallization; Butterworth-Heinemann Ltd: Oxford, 1993.

[10] Mersmann, A. Crystallization Technology Handbook; M. Dekker: New York, 1995.

[11] Davey, R.J., Mullin, J.W., J. Cryst. Growth, 23 (1974) 89.

[12] Shanmugam, M., Gnanam, F.D., Ramasamy, P., J. Mater. Sci. 19 (1984) 2837.

[13] Owczarek, I., Sangwal, K., J. Cryst. Growth, 99 (1990) 827.

[14] Podder, J., J. Crystal Growth 237-239, (2002) 70.

[15] Srinivasan, et.al., Crystal Research and Technology, 35, (2000) 291.

[16] Nyvlt, J.; So-hnel, 0.; Matuchova', M.; Broul, M. The Kinetics of Industrial Crystallization; Elsevier: New York, 1985.

[17] Chandrasekhar, S. Hydrodynamic and Hydromagnetic Stability; Oxford, Clarendon Press, 1961.

[18] Koschmieder, E. 1. Benard cells and Taylor Vortices; Cambridge University Press, 1993.

[19] Meyer-Spasche, R. Pattern Formation in Viscous Flows: The Taylor-Couette Problem and Rayleigh-Benard Convection; Birkhauser Verlag, 1999.

[20] Boudhiaf, R. et al. A Two-Dimensional Numerical Study of Hydrodynamic, Heat and Mass Transfer and Stability in a Salt Gradient Solar Pond; Energies, 5, (2012) 3986-4007.

[21] Unsworth, P.J. et al. A salt gradient pond for solar heat collection and long term storage; EUR 9838 EN Commission of the European Communities; Energy

[22] Lu, H. et al. Advancements in salinity gradient solar pond technology based on sixteen years of operational experience; J of Solar Energy Engineering;

(126) 759-767.

[23] MESSO-GEA Process Engineering Division; Crystallization (in theory and practice); 2013.

[24] Aimone-Martin, C.T. and Martell, M.-A., Salinity gradient solar pond technology applied to potash solution mining; SAND2000-0336C Sandia National Laboratories Carlsbad NM.

Claims

WHAT IS CLAIMED IS:

1. A crystallization pond for producing crystals of a salt from a warm solution containing the salt, the crystallization pond comprising a cooling apparatus, the cooling apparatus comprising:
a network of cooling tubes for circulating coolant through or adjacent to the pond so that the coolant absorbs heat from the solution to produce a warm coolant;
a first heat exchanger in fluid communication with the network of cooling tubes for cooling the warm coolant to produce a cooled coolant; and a pump for pumping the coolant through the network of cooling tubes and through the first heat exchanger to move the warm coolant through the first heat exchanger and to move the cooled coolant through the network of cooling tubes.

2. A crystallization pond as defined in claim 1, wherein:
the first heat exchanger comprises a surface heat exchanger;
the first heat exchanger comprises a heat-pipe heat exchanger;
the first heat exchanger comprises a heat-pipe heat exchanger using gravity-induced internal flows coupled with two-phase energy transfer of a refrigerant contained within the heat-pipe heat exchanger;
the coolant is an aqueous glycol solution;
the coolant is a saline solution of a suitable salt, wherein the suitable salt comprises NaCl, CaCl2, or LiBr;
the coolant circulates in a closed loop through the network of tubes and the first heat exchanger;
the coolant has a higher temperature at an outlet end of the pond as compared to an inlet end of the pond;
the network of cooling tubes comprises a series of parallel tubes extending along a bottom of the crystallization pond;
the network of cooling tubes comprises a series of approximately evenly spaced tubes extending along the bottom of the crystallization pond;
at least a portion of the network of cooling tubes is disposed within a layer of gravel supported on a bottom of the crystallization pond; and/or the crystallization pond comprises a controller for controlling the rate of flow of coolant produced by the pump.

3. A crystallization pond as defined in any one of the preceding claims comprising an auxiliary cooling system for further cooling the coolant.

4. A crystallization pond as defined in the preceding claim, wherein the auxiliary cooling system comprises:
an auxiliary heat sink; and a second heat exchanger in fluid communication with the network of cooling tubes for transferring heat from the coolant to the auxiliary heat sink.

5. A crystallization pond as defined in the preceding claim, wherein the auxiliary heat sink comprises groundwater provided from a groundwater source, and wherein the groundwater is optionally supplied to the second heat exchanger by moderately deep well pumping.

6. A crystallization pond as defined in either one of claims 4 or 5, wherein the first and second heat exchangers are positioned in series so that the warm coolant is cooled by the first heat exchanger to produce the cooled coolant, and the cooled coolant is then further cooled by the second heat exchanger.

7. A crystallization pond as defined in any one of claims 5 or 6, comprising an apparatus for pumping groundwater warmed by the second heat exchanger into a disposal well, the disposal well being positioned a sufficient distance from the groundwater source and from the crystallization pond to avoid transferring heat to the groundwater source or to the crystallization pond.

8. A crystallization pond as defined in any one of claims 4 to 7, wherein the second heat exchanger comprises a heat pipe heat exchanger, and wherein the second heat exchanger optionally uses gravity-induced internal flows coupled with two-phase energy transfer of a refrigerant contained within the heat-pipe heat exchanger to cool the coolant.

9. A crystallization pond as defined in any one of claims 4 to 8, comprising apparatus for sub-cooling the auxiliary heat sink.

10. A crystallization pond as defined in the preceding claim, wherein the apparatus for sub-cooling the auxiliary heat sink comprises:
a third heat exchanger at the surface;
a fourth heat exchanger for cooling the auxiliary heat sink;
pipes for circulating a sub-cooling coolant between the third and fourth heat exchangers so that heat absorbed by the sub-cooling coolant from the auxiliary heat sink is transferred to atmospheric air by the third heat exchanger; and a pump for circulating the sub-cooling coolant in the pipe.

11. A crystallization pond as defined in the preceding claim, wherein the third and fourth heat exchangers comprise heat-pipe heat exchangers, and wherein and the third and fourth heat exchangers optionally independently use gravity-induced internal flows coupled with two-phase energy transfer of a refrigerant contained within the heat-pipe heat exchanger to cool the sub-cooling coolant.

12. A crystallization pond for producing crystals of a desired salt from a warm solution containing the salt, the crystallizing pond comprising a bubble injection apparatus for injecting bubbles into the solution.

13. A crystallization pond as defined in the preceding claim, wherein the apparatus for injecting bubbles into the solution comprises:
a network of air supply tubes in the pond, the air supply tubes containing apertures for releasing air into the solution; and an air supply in fluid communication with the network of air supply tubes for injecting air into the network of air supply tubes.

14. A crystallization pond as defined in the preceding claim, wherein the network of air-supply tubes comprises a series of parallel spaced apart tubes, and wherein the network of air supply tubes optionally extends in a direction parallel to the direction of fluid flow from an inlet end to an outlet end of the crystallization pond.

15. A crystallization pond as defined in claim 13, wherein the network of air-supply tubes comprises a first series of parallel spaced apart tubes and a second series of parallel spaced apart tubes, the first series of parallel tubes extending along a plane in a direction to provide first and second opposite sides of a hexagon, the second series of parallel tubes extending along the plane and being angled relative to the first series of parallel tubes to define third and fourth opposite sides of a hexagon, and the apertures are provided at spaced apart intervals so that apertures on adjacent portions of the first and second series of parallel tubes define four sides of a hexagon.

16. A crystallization pond as defined in any one of claims 13 to 15, wherein:
the network of air supply tubes is disposed at a bottom of the pond;
the air supply comprises an air compressor;
the air supply comprises a dryer for drying air before it is injected into the network of air supply tubes;
the apertures are oriented to release air upwardly into the solution;
the apertures comprise a predetermined size;
the apertures comprise large apertures for establishing a flow of the solution in circulation cells and small apertures for producing small bubbles that can float on a surface of the crystallization pond;
the network of air supply tubes comprises a plurality of pairs of adjacent tubes bound together;
the apertures are positioned at spaced-apart locations on the tubes;
and/or the air supply comprises a controller for regulating the supply of air provided by the air supply based on the circulation requirements of the crystallization pond.

17. A crystallization pond for producing crystals of a salt from a warm solution containing the salt, the crystallization pond comprising a warm solution supply system, the warm solution supply system comprising:
an inlet solution supply pipe network for supplying the solution to the crystallization pond; and a plurality of inlet flow spreaders connected to the inlet solution supply pipe network for injecting the solution into the crystallization pond, and wherein the inlet flow spreaders are optionally oriented to inject the solution upwardly into the crystallization pond at a small angle with respect to a vertical direction.

18. A crystallization pond as defined in the preceding claim, wherein the inlet flow spreaders are positioned to supply the solution in approximately the first 10% to 15% of the length of the crystallization pond as measured from an inlet end to an outlet end of the crystallization pond.

19. A crystallization pond as defined in either one of claims 17 or 18, wherein the inlet flow spreaders are positioned and configured to provide exponentially decreasing flow rates of injected solution at each adjacent inlet flow spreader relative to an inlet end of the crystallization pond.

20. A crystallization pond as defined in any one of claims 17 to 19, comprising a plurality of secondary distribution nozzles connected to the inlet solution supply pipe network for injecting the solution into the crystallization pond, wherein the secondary distribution nozzles are provided at an inlet end of the crystallization pond and are oriented to inject the solution horizontally into the crystallization pond at a small angle with respect to a plane parallel to the inlet end of the pond.

21. A crystallization pond as defined in the preceding claim, wherein the secondary distribution nozzle comprises a rotary inlet comprising a rotatable solution supply tube and a plurality of apertures formed in the rotatable solution supply tube for releasing incoming solution into the crystallization pond.

22. A crystallization pond as defined in any one of the preceding claims, wherein the crystallization pond comprises an outdoor crystallization pond, and wherein the crystallization pond is optionally exposed to ambient weather conditions including ambient temperature, humidity, precipitation and/or wind.

23. A crystallization pond as defined in any one of the preceding claims, wherein the crystallization pond comprises:
a layer of clay on the sides and bottom of the crystallization pond;
a liquid impermeable pond liner, the pond liner optionally being positioned inside the layer of clay and containing the solution within the crystallization pond;
a first layer of gravel on the bottom of the crystallization pond, wherein the first layer of gravel optionally comprises approximately pea-sized gravel, and wherein the first layer of gravel is optionally disposed above the liquid impermeable pond liner;
a bottom screen positioned above the first layer of gravel, the bottom screen having a pore size smaller than the particles comprising the first layer of gravel;
a second layer of gravel under the bottom of the crystallization pond, the second layer of gravel optionally interposing the layer of clay on the bottom of the crystallization pond and the liquid impermeable pond liner, wherein a fan is optionally provided to circulate air through the second layer of gravel, and wherein the second layer of gravel optionally comprises coarse-sized gravel with generally uniform particle size;
a chelant supply tank in fluid communication with the crystallization pond for supplying chelant to the pond;
a removable cover; and/or a wind barrier that can be raised and lowered.

24. A crystallization pond as defined in any one of the preceding claims, wherein a chemical chelating agent and/or a non-toxic foaming agent is added to the solution.

25. A crystallization pond as defined in the preceding claim, wherein the chemical chelating agent comprises EDTA (ethylene-diamine-tetra-acetic acid)(C10H16N2O8), DTPA (diethylene-triamine-penta-acetic acid) (C14H23N3O10),DMSO (dimethylsulfoxide) ((CH3)2SO), DMSA
(dimercapto-succinic acid) (C4H6O4S2), NTA (nitrile-triacetic acid) (C6H9NO6), citric acid (C6H8O7), oxalic acid (C2H2O4), acetic acid (CH3COOH), or a combination thereof, and/or wherein the non-toxic foaming agent comprises octadecylamine (ODA), dodecylamine (DDA), sodium dodecyl sulphate (SDS), polyphenylsulfone (PPSF) or carboxylated polysulfone (CPSF).

26. A crystallization pond as defined in any one of the preceding claims, comprising an organic oil that floats on the surface of the crystallization pond to reduce the rate of evaporation of water vapor from the surface.

27. A crystal vacuum harvesting device for harvesting salt crystals deposited in a crystallization pond, the crystal vacuum harvesting device comprising:
a vacuum intake;
an inlet sweeper coupled to the vacuum intake to feed deposited salt crystals into the vacuum intake; and a vacuum pump connected to generate a vacuum in the vacuum intake.

28. A crystal vacuum harvesting device as defined in the preceding claim, comprising:
wheels coupled to the vacuum intake to support the vacuum intake on a bottom of the crystallization pond and to assist movement of the vacuum intake on the bottom; and/or a float coupled to the vacuum intake to at least partially support the weight of the vacuum intake.

29. A crystal vacuum harvesting device as defined in the preceding claim, comprising a pond harvester system coupled to the float to guide the crystal vacuum harvesting device around the crystallization pond, wherein the pond harvester system optionally comprises positioning guide cables for attachment to a surface of the crystallization pond, and wherein the positioning guide cables are optionally plastic-coated.

30. A crystallization pond having a cooling apparatus as defined in any one of claims 1-11, further comprising:
a bubble injection apparatus as defined in any one of claims 12 to 16;
a warm solution supply system as defined in any one of claims 17 to 21;
the features of any one of clams 22 to 26; and/or a crystal vacuum harvesting device as defined in any one of claims 27 to 29.

31. A crystallization pond as defined in any one of the preceding claims, wherein at least 60%, at least 70%, at least 80%, at least 90%, at least 91%, at least 92%, at least 93%, at least 94%, at least 95%, at least 96%, at least 97%, at least 98%, or at least 99% of the volume of the warm solution containing the salt within the crystallization pond is at conditions of temperature and salt concentration that fall within the metastable zone width (MSZW) of the salt.

32. A method of crystallizing a salt from a warm solution containing the salt comprising:
placing the solution in a crystallization pond; and actively cooling sub-surface regions of the crystallization pond.

33. A method of crystallizing a desired salt from a warm salt-containing solution comprising:
placing a warm salt-containing solution in a crystallization pond and;
actively cooling layers of warm salt-containing solution within the crystallization pond that are below a surface layer of the crystallization pond.

34. A method as defined in any one of claims 32 or 33, wherein regions of the crystallization pond that are closer to an inlet end of the crystallization pond are cooled to a greater extent than regions of the crystallization pond that are closer to an outlet end of the crystallization pond.

35. A method as defined in any one of claims 32 to 34, wherein the rate of active cooling is controlled to provide a desired rate of cooling in the crystallization pond.

36. A method as defined in the preceding claim, wherein the active cooling is controlled based on a temperature measured at one or more locations within the cooling pond.

37. A method as defined in any one of claims 32 to 36, wherein a coolant used to provide the active cooling is cooled using an auxiliary heat sink.

38. A method as defined in the preceding claim, wherein the auxiliary heat sink is sub-cooled during periods of prevailing low atmospheric temperature.

39. A method as defined in the preceding claim, comprising:
in winter, sub-cooling the auxiliary heat sink by circulating a sub-cooling coolant between a first heat exchanger at the surface and a second heat exchanger disposed within the auxiliary heat sink; and in summer:
stopping circulation of the sub-cooling coolant between the first and second heat exchangers; and cooling a coolant used for the active cooling by providing a third heat exchanger in fluid communication with the coolant used to provide the active cooling so that the third heat exchanger transfers heat from the coolant to the auxiliary heat sink.

40. A method of crystallizing a salt from a warm solution containing the salt comprising:
placing the solution in a crystallization pond and;
injecting air into the solution so that the air rises upwardly through the solution.

41. A method as defined in the preceding claim, comprising injecting the air in the form of bubbles to form circulation cells within the crystallization pond.

42. A method as defined in the preceding claim, wherein the circulation cells are parallel or hexagonal.

43. A method as defined in any one of claims 40 to 42, wherein the rate at which the air is injected is controlled to produce a desired circulation rate within the crystallization pond.

44. A method as defined in any one of claims 40 to 42, comprising injecting small bubbles of air into the solution so that the small bubbles form rafts on a surface of the crystallization pond.

45. A method of crystallizing salt from a warm solution containing the salt, comprising:
placing the solution in a crystallization pond and;
injecting additional warm solution through a warm solution supply system so that a flow of the injected additional warm solution is directed generally upwardly within the crystallization pond.

46. A method as defined in the preceding claim, wherein the warm solution is injected to enhance the formation of circulation cells within the crystallization pond.

47. A method as defined in either one of claims 45 or 46, wherein the rate of injection of the additional warm solution is controlled to produce a desired circulation rate within the crystallization pond.

48. A method as defined in any one of claims 45 to 47, wherein the additional warm solution is injected in approximately the first 10% to 15% of a length of the crystallization pond as measured from an inlet end to an outlet end of the crystallization pond.

49. A method as defined in any one of claims 45 to 48, wherein the additional warm solution is injected at exponentially decreasing flow rates along a length of the crystallization pond running from an inlet end to an outlet end of the crystallization pond.

50. A method as defined in any one of claims 45 to 49, comprising injecting at least a portion of the additional warm solution through a rotatable solution supply tube provided at an inlet end of the crystallization pond, wherein the rotatable solution supply tube is optionally positioned to induce flow in an outer surface of each of a parallel circulation cell within the crystallization pond.

51. A method of harvesting salt crystals formed on a bottom of a crystallization pond from a solution containing the salt comprising:
applying vacuum pressure to a vacuum intake connected to a discharge hose;
sucking the salt crystals and a portion of the solution from the bottom of the crystallization pond into the vacuum intake;
moving the salt crystals and the portion of the solution through the discharge hose; and discharging the salt crystals and the portion of the solution from the discharge hose.

52. A method as defined in the preceding claim, wherein the salt crystals are harvested continuously.

53. A method as defined in either one of claims 51 or 52, wherein the vacuum pressure is controlled to be higher at locations within the crystallization pond where salt crystals are expected to accumulate to a greater depth.

54. A method of crystallizing a salt from a warm solution containing the salt comprising:
actively cooling sub-surface regions of the crystallization pond in accordance with any one of claims 32 to 36;

cooling a coolant used to perform the active cooling using an auxiliary heat sink;
sub-cooling the auxiliary heat sink in accordance with any one of claims 38 to 39;
injecting air into the solution in the crystallization pond in accordance with any one of claims 40 to 44;
injecting additional warm solution through a warm solution supply system in accordance with any one of claims 45 to 50; and/or harvesting salt crystals formed on the bottom of the crystallization pond using a method as defined in any one of claims 51 to 53.

55. A method as defined in any one of the preceding claims, comprising erecting a wind barrier on at least a windward side of the surface of the crystallization pond, wherein a ratio of the length of the crystallization pond to the height of the wind barrier is less than 10.

56. A method as defined in any one of the preceding claims, comprising covering the crystallization pond to prevent precipitation from falling into the crystallization pond, prevent sunlight from reaching the pond, and/or prevent wind from blowing across the surface of the pond.

57. A method as defined in any one of the preceding claims, comprising halting cooling of sub-surface regions of the crystallization pond shortly before the forecasted arrival of precipitation, and resuming cooling of sub-surface regions of the crystallization pond after a dewpoint temperature of the atmospheric air decreases to below a surface temperature of the crystallization pond.

58. A method as defined in any one of the preceding claims, wherein the crystallization pond comprises an outdoor crystallization pond, and wherein the crystallization pond is exposed to one or more ambient environmental conditions including ambient temperature, ambient humidity, ambient precipitation, or ambient wind.

59. A method as defined in any one of the preceding claims, comprising adding a chemical chelating agent to the solution.

60. A method as defined in any one of the preceding claims, comprising controlling conditions within the crystallization pond so that at least 60%, at least 70%, at least 80%, at least 90%, at least 91%, at least 92%, at least 93%, at least 94%, at least 95%, at least 96%, at least 97%, at least 98%, or at least 99% of the volume of the crystallization pond is at conditions of temperature and salt concentration that fall within the metastable zone width (MSZW).

61. A pond as defined in claim 31 or a method as defined in claim 60, wherein the metastable zone width comprises the region between the equilibrium saturation line and the supersaturation limit for nucleation of crystals on a plot of concentration of the desired salt versus temperature.