KR102337382B1 - Membrane-based Processes and Systems for Recovering Lithium from Lithium-containing Brine Solution - Google Patents

Abstract

In the present disclosure, in order to overcome the inefficiency of the membrane distillation process due to multi-valent ions, particularly divalent ions, present in the lithium-containing aqueous solution (brine), a nanofiltration step is performed at the front end. A membrane-based process and system capable of effectively concentrating and recovering lithium contained in an aqueous solution or brine within a short period of time by performing the method are described.

Description

Membrane-based Processes and Systems for Recovering Lithium from Lithium-containing Brine Solution

The present disclosure relates to membrane-based processes and systems for recovering lithium from lithium-containing aqueous solutions or brine. More specifically, the present disclosure provides a method for overcoming the inefficiency of a membrane distillation process due to multi-valent ions, particularly divalent ions, present in a lithium-containing aqueous solution or brine, It relates to a membrane-based process and system capable of effectively concentrating and recovering lithium contained in brine within a short time by performing a filtration step.

Lithium is the 25th most abundant element on Earth and is one of the alkali metal elements belonging to Group 1 of the periodic table. The conductivity is 11.2, which is the highest among metals of the same type. In addition, lithium is known to have the highest negative potential of -3.043 V as well as the largest electrochemical equivalent of 2.98 A·h/g.

Lithium is currently essential in lithium-ion batteries (LIBs) for cell phones, laptop computers, electric vehicles, and energy storage systems. In particular, since lithium-ion batteries have high energy density and light weight characteristics, they are applied from small mobile device power sources to medium and large-sized electric vehicles (e.g., EV, HEV, etc.) and energy storage device (ESS) power sources. is expanding, and the demand for lithium is also rapidly increasing. With the increase in lithium demand, the price of lithium has risen sharply from $50,000 per ton to $200,000 per ton in the past four years.

In this regard, as a lithium source, a lithium raw material that exists in nature is mainly used, and ore, seawater, and salt water can be exemplified. Since the method of extracting lithium from ores (lepidolite, spodumene, petalite and ambligonite) is uneconomical and disadvantageous in terms of environmental friendliness, seawater and geothermal water are mostly used as lithium sources. In particular, the proportion of manufacturing using salt water in salt lakes within the continent is high.

However, since the lithium concentration in the lithium source is at a remarkably low level (seawater: about 0.17 ppm, geothermal water: about 1 to 100 ppm), the source from which lithium can be economically extracted is limited. To produce high-quality lithium for batteries, the lithium-containing brine is required to be concentrated to 6,000 ppm (Li). Conventionally, a suitable lithium concentration could be obtained through evaporation using sunlight after pulling up lithium-containing brine from the surface of the earth to the lake using a pump. After concentration of the brine, various kinds of undesirable ions such as sodium, potassium, calcium, magnesium and sulfate are removed. In the case of sodium and potassium, they can be removed by precipitation above their respective saturation points during evaporation. Another inclusion, borate, may be removed by solvent extraction using an aliphatic alcohol. Residual magnesium, calcium and sulfate can be removed by precipitation in the form of calcium hydroxide, sodium carbonate, oxalate and barium chloride (Science of The Total Environment, 2018, 639, 1188-1204).

Nevertheless, the conventional lithium manufacturing process is difficult to meet the demand in terms of productivity and price due to the rapid rise in lithium price. In the case of the evaporation process using sunlight, strict climatic conditions are required, and for this reason, although high concentrations of lithium salt lakes exist in regions such as the United States and China, they are only applied in some regions of South America. Moreover, the evaporation process using sunlight requires a long time (approximately 12 to 24 months) to obtain an appropriate lithium concentration.

In addition, a large amount of chemical reagents and fresh water are consumed for precipitation of calcium and magnesium and extraction of borate in the process of removing undesirable ions, and it has been reported that these reagent costs account for about 40% of the total operating cost. Therefore, there is a need for a technology capable of performing stable and efficient lithium enrichment in a narrow installation area without being limited by climatic conditions.

In one embodiment of the present disclosure, it is intended to provide a method capable of overcoming the technical limitations of the conventional evaporation process using sunlight to recover lithium from a lithium-containing aqueous solution, particularly brine.

According to the first aspect of the present disclosure,

providing a lithium-containing aqueous solution comprising monovalent ions and divalent ions;

pretreating the lithium-containing aqueous solution by membrane-based nanofiltration to form a pretreated lithium-containing aqueous solution having a reduced divalent ion concentration;

forming a concentrate having an increased lithium concentration by subjecting the pretreated lithium-containing aqueous solution to membrane distillation; and

recovering lithium from the concentrate;

includes,

There is provided a membrane-based lithium recovery method, wherein the nanofiltration membrane exhibits a divalent ion rejection of at least 30% and a monovalent ion rejection of 20% or less.

According to a second aspect of the present disclosure,

a supply part of a lithium-containing aqueous solution containing monovalent ions and divalent ions;

a membrane-based nanofiltration module communicating with the supply of the lithium-containing aqueous solution and forming a pretreated lithium-containing aqueous solution having a reduced divalent ion concentration;

a membrane distillation module communicating with the nanofiltration module at a rear end and forming a concentrate having an increased lithium concentration; and

a lithium recovery unit in the concentrate;

includes,

A membrane-based lithium recovery system is provided, wherein the nanofiltration membrane exhibits a removal rate of divalent ions of at least greater than 30% and a removal rate of monovalent ions of 20% or less.

According to an embodiment of the present disclosure, in concentrating lithium in a lithium-containing aqueous solution by membrane distillation treatment, polyvalent ions (particularly, divalent ions) contained together with lithium in the aqueous solution form crystals on the membrane surface, resulting in wetting phenomenon Although this causes a phenomenon that rapidly degrades the performance of the membrane during the membrane distillation process by causing Efficient membrane distillation can be performed by selectively separating and removing (pre-treatment) the ions.

Through the above-described process configuration, when lithium is recovered by concentration using sunlight, it not only takes a long period of time (approximately 12 to 24 months), but also overcomes the technical limitations of using a large amount of reagents, etc., It provides the advantage of recovering lithium within 2 months. Furthermore, when a downstream process of converting lithium in the concentrate obtained by membrane distillation into a lithium precursor form used for manufacturing lithium ion batteries is added, the convenience of transportation and storage can be improved.

1 is a diagram schematically illustrating (a) a sequence of a membrane-based process for recovering lithium from lithium-containing brine and (b) an exemplary system;
2 is a diagram schematically illustrating the principle of nanofiltration;
3 is a diagram schematically illustrating the principle of membrane distillation;
Fig. 4 schematically shows a lab-scale evaporation apparatus for simulating an evaporation test by sunlight;
5 is a membrane-based lithium recovery process for performing membrane distillation without performing nanofiltration pretreatment, (a) concentration profiles of various ions during operation of the membrane distillation module, (b) permeate flow rates (30° C. and 50° C.) (shown together with the case of solar evaporation at digital photographs and SEM photographs of crystals;
6 is a graph showing the ionic conductivity of feed and permeate in (a) a membrane distillation experiment without nanofiltration pretreatment, and (b) a membrane distillation experiment using brine pretreated with nanofiltration, respectively;
7 shows (a) and (b) a permeate flow rate and salt removal rate profile of a nanofiltration (NF) test using three types of commercially available membranes (NE40, 70 and 90), respectively, and (c) a membrane subjected to nanofiltration pretreatment. performance (permeate flow rate and salt removal rate) of the -based lithium recovery process, (d) the ion concentration profile in the brine during the nanofiltration operation, and (e) the SEM picture of the crystals formed on the membrane surface after the nanofiltration test;
8 is a result of (a) XPS, (b) XRD, and (c) EDX analysis on the chemical composition of crystals formed on the membrane surface during the membrane-based lithium recovery process; and
Figure 9 (a) and (b) the concentration of each sodium ion and potassium ion during the experiment in the membrane distillation process without nanofiltration (NF), and (c) and (d) are nanofiltration (NF) It is a graph showing the concentration of each sodium ion and potassium ion during an experiment in the membrane distillation process performed.

The present invention can all be achieved by the following description. It is to be understood that the following description describes preferred embodiments of the present invention, and the present invention is not necessarily limited thereto. In addition, the accompanying drawings are provided to aid understanding, and the present invention is not limited thereto, and details regarding individual configurations may be properly understood by the specific purpose of the related description to be described later.

"Brine" may mean water, which is usually contained as a salt, which typically corresponds to a source of potassium, bromine, boron, lithium, iodine, magnesium, or the like.

"Nanofiltration" may refer to a membrane technique that uses a membrane to separate different components within a fluid mixture. The pore size of the nanofiltration membrane may be generally determined within the range of approximately 1 to 100 nm, and a separation process may be entailed based on the molecular size and/or physicochemical interaction with the fluid component in contact therewith. have.

"Membrane distillation (MD)" uses a vapor pressure difference between permeate components resulting from a temperature difference between both ends of the membrane through a hydrophobic porous membrane as a driving force so that a specific component (usually water vapor) in the mixture selectively dissolves the membrane. It may refer to a separation process in which the mixture is separated/purified in a pass-through manner.

"Contact angle" may mean an angle of an interface formed when a liquid comes in contact with an immiscible material. A higher surface energy indicates hydrophilicity, while a lower surface energy means hydrophobicity. In this regard, in the case of hydrophilicity, the water contact angle is 90° or less, in the hydrophobic case, the water contact angle is 90° or more, and in the case of 160° or more, it may be classified as superhydrophobic.

"Saturation index (SI)" is an index indicating whether a solution is in a supersaturated, unsaturated, or equilibrium state with respect to a specific substance, and may be expressed by Equation 1 below.

[Equation 1]

In the above formula, IAP is the ion activity product (ion activity product), Ksp means the solubility product (solubility product).

When the saturation index has a negative value, a specific substance can be dissolved in an unsaturated state, whereas when the saturation index has a positive value, it means that a specific substance is precipitated or precipitated as a supersaturated state.

In the present specification, when a component is "included", it means that other components and/or steps may be further included, unless otherwise specified.

A membrane-based process for recovering lithium from lithium-containing brine according to an embodiment of the present disclosure and an exemplary system thereof are schematically illustrated in FIGS. 1A and 1B .

Provision of a lithium-containing aqueous solution

According to the illustrated embodiment, first a lithium-containing aqueous solution comprising monovalent ions and divalent ions, specifically a brine solution is provided. The brine as the lithium-containing aqueous solution is not particularly limited as long as it contains lithium, but may be brine secured by pumping from a natural salt lake or the like.

In this regard, lithium-containing aqueous solutions are typically available from salt lakes distributed mainly in South America, China, the United States, etc., which can use brine with a relatively high lithium-content, for example, the Atacama salt lake in the Andes (Chile ), Onbrème Salt Lake (Argentina), and Uyuni Salt Lake (Bolivia) are representative examples.

The concentration of lithium ions in this lithium-containing aqueous solution may range, for example, from about 10 to 1,600 ppm, typically from about 100 ppm to 1,000 ppm, more typically from 150 to 500 ppm, which in itself is a low lithium concentration In order to concentrate to a desired level by means of evaporation by sunlight, a long period of time is required.

On the other hand, in one embodiment, the lithium-containing aqueous solution (brine) typically contains lithium ions (Li ⁺ ) as well as other monovalent ions and two or more polyvalent ions. Illustratively, monovalent ions other than lithium ions may be monovalent cations such as ^{Na +} and K ⁺ and monovalent anions such as ^{Cl − .} Further, the divalent ion may be, for example, a divalent cation such ^{as Ca 2+} and Mg ²⁺ and a divalent anion such as _{SO 4} ^{2- .}

In this regard, the concentration of monovalent ions other than lithium ions in the brine, specifically, the concentration of monovalent cations, for example, the concentration of Na ⁺ is about 10,000 to 100,000 ppm, specifically about 20,000 to 80,000 ppm, more specifically It may range from about 30,000 to 75,000 ppm. In addition, ^{the concentration of K +} may be, for example, about 1,000 to 30,000 ppm, specifically about 2,000 to 12,000 ppm, more specifically about 3,000 to 5,000 ppm.

On the other hand, the concentration of divalent ions, specifically divalent cations, for example, the concentration of Ca ²⁺ is, for example, about 100 to 2,000 ppm, specifically about 200 to 1,000 ppm, more specifically about 300 to 600 ppm can be a range. In addition, ^{the concentration of Mg 2+} may be, for example, about 20 to 25,000 ppm, specifically about 150 to 10,000 ppm, more specifically about 500 to 7,000 ppm. Exemplary compositions of the main sources of lithium-containing brine are shown in Table 1 below (content: % by weight).

salt lake nation Na ⁺ K ⁺ B Li ⁺ Mg ²⁺ Ca ²⁺ Cl ^- SO ₄ ^2- great salt lake United States of America 3.7 0.26 0.007 0.0018 0.5 0.026 7 0.94 Clayton Valley United States of America 4.69 0.4 0.005 0.0163 0.019 0.045 7.26 0.34 Atacama Chile 9.1 2.36 0.04 0.157 0.965 0.045 18.95 1.59 Uyuni Bolivia 7.06 1.17 0.071 0.0321 0.65 0.0306 5 - Zabuye China 7.29 1.66 - 0.0489 0.0026 0.0106 9.53 - Taijinaier China 5.63 0.44 - 0.031 2.02 0.02 13.42 3.41

Nanofiltration (NF) pretreatment

1A and 1B , the lithium-containing aqueous solution (brine) is transferred to a nanofiltration step (module). However, brine obtained from natural salt lakes, etc. contains foreign substances such as various suspended matter, and thus, when subjected to nanofiltration as it is, problems such as clogging may occur. In consideration of this, a pretreatment step (not shown) may be optionally performed prior to the nanofiltration step. Examples of such pretreatment steps may be microfiltration, ultrafiltration, chemical deposition, and the like. When the pretreatment step exemplified above involves a filtration process, the operating pressure may be, for example, about 5 bar, specifically about 4 bar or less, and more specifically about 3 bar or less. In addition, the pretreatment process temperature is not particularly limited, for example, about 10 to 30 ℃, specifically about 20 to 25 ℃, more specifically room temperature may be sufficient.

In this regard, the principle of nanofiltration is schematically shown in FIG. 2 .

As shown, water substantially passes through the nanofiltration membrane, and a significant portion of monovalent ions are also permeable. On the other hand, polyvalent ions, saccharides, amino acids, proteins, polysaccharides, particles, bacteria, etc. do not penetrate the membrane.

In an exemplary embodiment, nanofiltration uses a pressure difference as a driving force, and the driving pressure (pressure difference) during nanofiltration is, for example, about 3 to 15 bar, specifically about 4 to 12 bar, more specifically about It may range from 5 to 10 bar. To this end, as shown in Figure 1b, the process of pressing by at least one pump prior to the nanofiltration module may be performed. As such, the lithium-containing aqueous solution may generate a pressure difference as it comes into contact with the membrane in a pressurized state. In addition, the temperature of the nanofiltration step is not particularly limited, for example, about 10 to 40 ℃, specifically about 20 to 30 ℃, more specifically room temperature conditions are sufficient.

The membrane used for nanofiltration may exhibit a pore size in the range of, for example, about 10 nm or less, specifically about 5 nm or less, and more specifically about 0.1 to 2 nm, the pore size affects the ion selectivity. Considering the efficiency of excluding (removing) polyvalent ions and the like in the lithium-containing aqueous solution in this embodiment as a factor affecting it, it may be advantageous to use a membrane having a value adjusted in the above-mentioned range.

In an exemplary embodiment, such a membrane for nanofiltration may have a separation ability for an ionic substance having a divalent or higher value and/or an organic component having a molecular weight of 1,000 or higher in a lithium-containing aqueous solution (brine). Also, the thickness of the membrane may range, for example, from about 10 to 1000 μm, specifically from about 40 to 500 μm, and more specifically from about 60 to 300 μm. However, the present disclosure is not limited to the numerical range related to the above-described properties of the membrane, and various modifications are possible.

According to an exemplary embodiment, the nanofiltration membrane may be, for example, a polyamide (for example, a resin polymerized using diamine and dicarboxylic acid as main raw materials) manufactured by an interfacial polymerization method. Specifically, diamine-based monomers (eg, tetramethylenediamine, hexamethylenediamine, 2-methylpentamethylenediamine, nonamethylenediamine, undecamethylenediamine, dodecamethylenediamine, 2,2,4-/2,4, Aliphatic diamines such as 4-trimethylhexamethylenediamine and 5-methylnonamethylenediamine; Aromatic diamines such as metaxylylenediamine and paraxylylenediamine; and/or 1,3-bis(aminomethyl)cyclohexane, 1 ,4-bis(aminomethyl)cyclohexane, 1-amino-3-aminomethyl-3,5,5-trimethylcyclohexane, bis(4-aminocyclohexyl)methane, bis(3-methyl-4-aminocyclo alicyclic diamines such as hexyl)methane, 2,2-bis(4-aminocyclohexyl)propane, bis(aminopropyl)piperazine and aminoethylpiperazine) and dicarboxylic acids (e.g., adipic acid, suberazine) aliphatic dicarboxylic acids such as acid, azelaic acid, sebacic acid, and dodecanedioic acid: terephthalic acid, isophthalic acid, 2-chloroterephthalic acid, 2-methylterephthalic acid, 5-methylisophthalic acid, 5-sodiumsulfoisophthalic acid, hexahydro Aromatic dicarboxylic acids, such as terephthalic acid and hexahydroisophthalic acid, and/or 1, 4- cyclohexanedicarboxylic acid, 1, 3- cyclohexanedicarboxylic acid, 1,2-cyclohexanedicarboxylic acid, etc. may be a polyamide prepared by interfacial polymerization of an alicyclic dicarboxylic acid, particularly sebacic acid).

In a specific embodiment, the nanofiltration membrane may be made of a polyamide produced by interfacial polymerization of sebacic acid chloride, and more specifically, a poly(piperazinamide)-based semi-aromatic polyamide material. As representative products of such a membrane, Toray Chemicals' NE series (specifically, NE70), Synder Filtration's NF series (specifically, NFS, NFX, NFW and NFG), etc. can be exemplified.

According to a specific embodiment, the nanofiltration membrane may be in the form of a composite membrane in which a thin film (activation layer) made of a polymer (specifically, polyamide) is coated (attached) to a porous membrane structure having good mechanical strength. In this regard, the material of the porous membrane structure may be at least one selected from polysulfone (PSf), polyethersulfone (PES), polyethylene terephthalate (PET), and the like, and the thin film of polymer material is interfacial polymerization ), wherein the polymer may be the aforementioned polyamide-based polymer. Optionally, in order to remove a component having a low molecular weight in the polyamide, a process of treatment with DMF or the like may be performed after the polymerization reaction.

In an exemplary embodiment, the nanofiltration membrane may be in the form of a flat membrane or a hollow fiber membrane.

According to an exemplary embodiment, the nanofiltration membrane is, for example, about 10 to 60 L m ^-2 h ^-1 , specifically about 20 to 50 L m ^-2 h ^-1 , more specifically about 25 to 40 L It can represent a water flux of ^{m -2} h ^{-1 .}

Referring to the drawings, the membrane of the nanofiltration (NF) module exhibits high permeability to moisture. On the other hand, in the case of monovalent ions and divalent ions or more, steric exclusion (ion larger than the pore opening is difficult to permeate) and dielectric exclusion (ion hydration to introduce into the membrane) Removal (exclusion) mechanisms such as the need to shake off the cell (hydration shell) and the effect of the membrane charge due to the Donnan effect may occur together. For example, with respect to steric exclusion, ^{the diameters of Li +} , Na ⁺ , K ⁺ are 1.55 Å, 1.78 Å, and 2.01 Å, respectively, and the diameters of Ca ²⁺ and Mg ²⁺ are 2.53 Å and 3.00 Å, respectively. In addition, in relation to the theft effect, when the surface of the membrane has a negative charge, cations receive electrostatic attraction, whereas anions are difficult to permeate due to electrostatic repulsion.

Permeation of ions in the lithium-containing aqueous solution is affected by the above-mentioned various influencing factors. As much as possible, polyvalent ions of divalent or higher are removed as much as possible (not permeable through the membrane), whereas the necessity of permeating monovalent ions is reduced. have. Thus, according to one embodiment, the nanofiltration membrane is for example at least about 30%, specifically at least about 35%, more specifically at least about ^{divalent ions (especially Ca 2+} and Mg ^{2+ ).} It can represent a removal rate of 40%. As such, the concentration of divalent ions in the lithium-containing aqueous solution (brine) is significantly reduced by the nanofiltration treatment. However, when only divalent ions exist in the aqueous solution, a removal rate of 90% or more can be achieved in theory, but brine and the like are mutually influenced because other ions are mixed together, which can be explained as showing a low removal rate. ^{According to an exemplary embodiment, the removal rate of monovalent ions (especially Li +} and Na ⁺ ) in the lithium-containing aqueous solution by nanofiltration is, for example, about 20% or less, specifically about 15% or less, more specifically It may range from about 2 to 10%.

On the other hand, the divalent ions removed in the nanofiltration step may be discarded, but in some cases, they may be recovered by converting them into a specific compound form. As an example, in the case of Mg ²⁺ ions, magnesium hydroxide (Mg(OH) ₂ ) may be precipitated in the form by reacting with sodium hydroxide and/or calcium hydroxide.

Optionally, in the case of non-precipitated divalent ions, magnesium oxalate (MgC ₂ O ₄ ), calcium oxalate (CaC ₂ O ₄ ), magnesium carbonate (MgCO ₃ ), calcium carbonate (CaCO) by reacting with sodium carbonate, sodium oxalate, carbon dioxide, etc. ₃ ), etc., can be additionally converted to resources.

Membrane distillation (MD)

According to the illustrated embodiment, the nanofiltered lithium-containing aqueous solution is sent to a membrane distillation stage (module). The principle of membrane distillation is schematically illustrated in FIG. 3 .

Referring to the drawings, membrane distillation can typically use a hydrophobic polymer separation membrane. Since the surface tension of the solvent or solute (hydrophilic material) is greater than that on the surface of the membrane, the membrane pore is formed in a liquid state. It does not pass through, and it repels at the surface of the membrane.

However, the material (moisture in this embodiment) separated from the inlet of the surface pores of the membrane evaporates (or volatilizes), phase transitions to a vapor phase, and diffuses into the pores while permeating, so that it can be finally condensed and separated from the permeation side. On the other hand, a component that is not evaporated (or volatilized), that is, a salt component (mainly, a salt containing a monovalent ion such as lithium) does not permeate through the membrane and remains concentrated on the supply side. As such, membrane distillation is a thermally driven process that can increase salt removal by more than 99%, even substantially 100%, and can be performed under relatively low operating pressure and temperature conditions. In particular, unlike the conventional solar evaporation method, the dependence on climatic conditions is low.

According to an exemplary embodiment, the temperature (T _f ) of the feed side of the membrane during membrane distillation is, for example, in the range of about 40 to 90 °C (specifically about 45 to 85 °C, more specifically about 50 to 80 °C) On the other hand, the temperature (T _p ) of the transmission surface may be selected, for example, in the range of about 10 to 40 ℃ (specifically about 15 to 35 ℃, more specifically about 20 to 30 ℃). In this case, the temperature difference (ΔT) between the temperature of the supply side and the transmission side may be, for example, about 20 to 50°C, specifically, about 25 to 35°C.

Further, according to an exemplary embodiment, in the membrane distillation process, pressure rather than permeate flow rate is an important consideration. The pressure may be adjusted, for example, in the range of about 2 bar or less, specifically about 1 bar or less, and more specifically about 0.1 to 0.5 bar.

On the other hand, in the case of a membrane used for membrane distillation, typically a porous membrane can be used. In the membrane distillation process using this, separation is made by the difference in shape and size between permeate materials, and the separation efficiency is determined by that of the separation membrane. It may be determined by the pore size and porosity.

According to an exemplary embodiment, the material used for the membrane distillation may be any type known in the art without particular limitation, and may typically exhibit hydrophobicity. In an exemplary embodiment, the membrane used for membrane distillation may be a polymer material, typically a microporous hydrophobic polymer material. Illustratively, the water contact angle of the membrane may be, for example, in the range of at least about 90°, specifically about 100 to 160°, and more specifically about 130 to 150°.

For membrane distillation, it may be required to use a membrane having a pore structure and a material that allows water vapor to permeate the membrane wall while preventing liquid and solid substances from passing through the pores. As such materials, polypropylene (PP), polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), heat-converted aromatic polyimide (specifically heat-converted poly(benzoxazole-imide) air coalescing), polyethylene (PE), and the like, and may be at least one of them. In a specific embodiment, PVDF can be used as a membrane material. As a hydrophobic thermoplastic polymer showing stability, which is a characteristic of a fluorinated polymer, -CH ₂ and -CF ₂ groups are alternately connected along the polymer chain to show polarity, and trade name It is marketed as GVHP, HVHP (Millipore), etc.

Further, the pore size of the membrane may be, for example, about 1 μm or less, specifically about 0.5 μm or less, and more specifically about 0.05 to 0.4 μm. In addition, the porosity of the membrane may be, for example, controlled within the range of about 30 to 90%, specifically about 50 to 85%, and more specifically about 70 to 80%.

On the other hand, the thickness of the membrane may be, for example, about 30 to 700 μm, specifically about 40 to 200 μm, and more specifically about 50 to 130 μm. It may be advantageous to adjust the thickness range, but is not necessarily limited thereto.

According to an exemplary embodiment, the low thermal conductivity of the membrane for membrane distillation treatment may be advantageous in terms of maintaining a temperature gradient to maintain driving force, and exemplary thermal conductivity is about 0.1 W/mK or less, specifically about 0.01 to 0.05 W/mK, specifically about 0.02 to 0.04 W/mK, may be in the range, but is not necessarily limited thereto.

In an exemplary embodiment, the membrane may be in the form of a flat membrane or a hollow fiber membrane. In addition, the membrane distillation module can be configured in various ways, for example, Direct Contact Membrane Distillation (DCMD), Sweeping Gas Membrane Distillation (SGMD), Air Gap Membrane Distillation (AGMD), and Vacuum Membrane Distillation (VMD). and may be DCMD in a specific embodiment. In this regard, the DCMD method can be configured with the simplest structure, and is suitable for lithium concentration in lithium-containing aqueous solutions and can be operated at a low operating cost. In particular, for salts (especially lithium) contained in a nanofiltered lithium-containing aqueous solution (brine), for example, at least about 99% (specifically at least about 99.99%) will be concentrated on the feed side without penetrating the membrane. can

When wetting occurs on the membrane surface in the membrane distillation process, the processing capability of the membrane distillation process is lost because the aqueous solution containing the salt passes through the membrane.

This phenomenon can be explained using the saturation index (SI), and as described above, the formation of crystals in the solution can be predicted according to the sign of the saturation index. Since the saturation index of divalent ions is typically higher than that of monovalent ions, as lithium is concentrated on the feed side during membrane distillation, divalent ions precipitate first to form crystals. In this case, the crystal of the divalent ion may be, for example, CaSO ₄ , CaSO ₄ ·2H ₂ O, Ca(OH) ₂ , Mg(OH) ₂ , MgSO ₄ ·7H ₂ O or the like.

In this embodiment, if such divalent ions and the like are not removed in advance to a certain level or more prior to membrane distillation, crystal wetting is induced in the lithium concentration process and membrane distillation performance is rapidly reduced, so that further lithium concentration is not possible. get in trouble However, in the case of this embodiment, the nanofiltration module disposed at the front end of the membrane distillation module selectively removes divalent or higher ions in the lithium-containing aqueous solution, and permeates the majority of monovalent ions to allow divalent ions during the subsequent membrane distillation step By suppressing the formation of crystals of ions, lithium in the aqueous solution can be concentrated to a high concentration. In particular, it is noteworthy that the mere placement of the nanofiltration stage (or module) upstream of the membrane distillation stage (module) can increase the concentration level of lithium in aqueous solution or brine to unexpected levels.

According to an exemplary embodiment, when the concentration of lithium ions in the initial lithium-containing aqueous solution is 100 ppm, the concentration of lithium ions in the lithium concentrate separated from moisture through nanofiltration and membrane distillation is, for example, about 1000 to 1500 ppm, specifically about 1100 to 1400 ppm, more specifically about 1150 to 1350 ppm range, compared to the point that can be concentrated to about 700 to 800 ppm level by membrane distillation without pretreatment by nanofiltration remarkably high.

lithium recovery

Referring back to FIG. 1A , in an exemplary embodiment, the concentrate containing high concentration lithium separated by membrane distillation is in the form of a lithium precursor compound suitable for use as a positive electrode active material such as a lithium secondary battery while having good handling and transportability can be recovered with As such a lithium precursor compound, lithium carbonate can be typically exemplified.

As an example, in order to prepare lithium carbonate from a lithium concentrate, an aqueous solution of ammonium carbonate in which ammonia and carbon dioxide are mixed with water is first prepared, and this can be contacted with the lithium concentrate to react as shown in Scheme 1 below (carbonation reaction). .

[Scheme 1]

2Li ⁺ + (NH ₄ ) ₂ CO ₃ → Li ₂ CO ₃ + 2NH ₄ ⁺

Alternatively, carbonate (for example, one or a combination of two or more from sodium carbonate, potassium carbonate, etc.) is added to the lithium concentrate together with a base (for example, sodium hydroxide, etc.) to carry out a carbonation reaction as shown in Scheme 2 below. can do.

[Scheme 2]

2Li ⁺ + Na ₂ CO ₃ → Li ₂ CO ₃ + 2Na ⁺

Then, the resulting lithium carbonate is precipitated (precipitated), and the solid lithium carbonate can be recovered through common solid-liquid separation means such as filtration, for example, reduced pressure filtration. Illustratively, since the solubility of lithium carbonate is inversely proportional to the temperature, the reaction temperature may be adjusted to, for example, about 60 to 95°C, specifically about 80 to 90°C, in order to promote precipitation.

In addition, in some cases, for the purification of the precipitated lithium carbonate, other components (sodium, potassium, etc.) mixed in the lithium carbonate crystals may be removed using a conventional purification means, for example, a water washing process. . As a result, lithium carbonate of high purity (eg, about 99% or more) can be obtained.

On the other hand, prior to the recovery of lithium, other ions other than lithium contained in the concentrate, particularly monovalent ions ^{such as Na +} , may be separated by a deposition method due to concentration. Separation or removal of ions other than lithium may be performed prior to the carbonation reaction.

The present invention may be more clearly understood by the following examples, which are only for illustrative purposes of the present invention and are not intended to limit the scope of the present invention.

Example

A. Chemical substances and equipment used in the examples are as follows.

- Lithium chloride (LiCl), sodium chloride (NaCl), sodium sulfate anhydrous (Na ₂ SO ₄ ), magnesium sulfate hexahydrate (MgSO ₄ .6H ₂ O) ), calcium sulfate anhydrous (CaSO ₄ ), boric acid (B(OH) ₃ ), and potassium chloride (KCl) were all obtained from DaeJung Chem. Co. Ltd. (Siheung, Republic of Korea) was used, which ^{was dissolved in deionized water (DI water, Milli-Q ㄾ} Direct, Merck Millipore, Darmstadt, Germany). Commercial TFC NF membranes (NE 40, 70, and 90), and hydrophobic porous membranes (GVHP) were prepared from Toray Chemicals (Tokyo, Japan) and Merck Millipore (Darmstadt, Germany), respectively.

- The morphological characteristics of the membrane surface were analyzed using a field-emission scanning electron microscope (FE-SEM; Hitachi, Tokyo, Japan) equipped with an energy-dispersive X-ray spectroscopy (EDX) device.

The dried sample was coated with Pt for 30 seconds using a Pt sputter (Hitachi E-1045, Tokyo, Japan). The chemical composition of the crystals was analyzed using EDX, XRD (SmartLab, Rigaku Corporation, Tokyo, Japan), and XPS (X-ray photoelectron spectroscopy; Theta probe, Thermo Scientific, MA, USA).

Concentrations of cations (Li, Na, K, B, Ca, and Mg) were measured by ICP-OES (ion coupled plasma optical emission spectrometry; iCAP 7000, Thermo Scientific, MA, USA). Each solution was measured through a series of dilutions to an appropriate concentration.

B. Membrane Performance

- Nanofiltration (NF) test

A multi-cell cross-flow apparatus was used to evaluate the nanofiltration performance of the membrane at a flow rate of 70 ml min ^{-1 and 10 bar.} The effective membrane area was 0.00062 m2. The membrane was compressed with deionized water at 10 bar overnight. After compression, artificial saline was prepared as a feed and the NF performance was measured.

The water flux was measured every 1 hour, and 3 ml of a sample was collected after 6 hours to analyze the solute removal rate.

- Membrane Distillation (MD) Test

A lab-scale direct-contact membrane distillation (DCDM) apparatus was purchased from Philos Company (Guangmyeong, Republic of Korea) to measure membrane distillation performance. The effective membrane area was 0.00181 m 2 .

The properties of the membrane used for membrane distillation are shown in Table 2 below.

In the table above, LEP _w is the liquid entry pressure, which means the pressure value at which the liquid passes through the open pores of the membrane to the hydrophobic layer on the permeate side.

The temperatures of the feed solution and the permeate solution were maintained at 50° C. and 20° C., respectively, and were measured at the inlet and outlet of the cell. The operating pressure and the flow rates of the feed solution and the permeate solution were measured. Artificial saline and deionized water were prepared and used as feed solution and permeate solution, respectively.

The permeate flow rate was measured in an automated fashion from the permeate solution that overflowed every 10 minutes. The conductivity of the bulk feed solution and the permeate solution was measured every 10 minutes to calculate the salt removal rate. Due to the limitations of the lab-scale membrane distillation apparatus, the rust generated from the apparatus needed to be washed to prevent clogging of pores, so it was further concentrated after washing with deionized water after 100 hours. .

- Solar evaporation test

A lab-scale solar evaporation test was performed to compare the conventional lithium enrichment method with the membrane distillation process. As shown in FIG. 4 , a rectangular parallelepiped acrylic box (550 mm wide, 500 mm long and 350 mm high) was manufactured. A ventilation fan was installed to maintain a low relative humidity similar to the real environment. A lab-scale evaporation pan (diameter: 23 cm, effective area: 0.0415 m 2 ) was fabricated. The temperature and concentration of the solution were controlled using a heating plate and recorded every 60 minutes using an ionic conductivity meter. The temperature and relative humidity of the internal atmosphere were measured using a thermo-hygrometer and an ionic conductivity meter. The water flux of the solar evaporation arc was calculated based on the moisture level of the lab-scale evaporation pan.

On the other hand, as shown in Table 1, based on the composition of a representative lithium-containing brine containing various types of salts, artificial brine having the composition shown in Table 3 below (% by weight) was prepared.

Na K B Li Mg Ca Cl SO ₄ artificial salt water One 0.1 0.005 0.01 0.01 0.01 1.6 0.1

- Analysis of the results of membrane distillation experiments without nanofiltration

In this embodiment, when membrane distillation is performed without nanofiltration pretreatment at the front end, (a) concentration profiles of various ions during operation of the membrane distillation module, (b) permeate flow rate, (c) simulation using PHREEQC software Saturation index (SI) of the precipitated crystals, and digital photographs and SEM photographs of the crystals attached to the membrane surface after (d) and (e) demonstration of the process are shown in FIG. 5 . In addition, in FIG. 5b, the water flux measured in the solar evaporation experiment at 30° C. and 50° C. is also shown.

According to FIG. 5A, the concentration of lithium in the artificial brine increased from 100 ppm to 900 ppm after about 80 hours by the membrane distillation process. In addition, as shown in FIG. 5b , the permeate flow rate in the membrane-based process was 22.5 L m ⁻² h ⁻¹ in the case of solar evaporation (0.37 L m ⁻² h ^{−1 at 30° C. and 0.56 L m −2 h −1 at} 50° C.). L m ^-2 h ^-1 ) compared to about 60 times higher. Although the effective evaporation area in the solar evaporation experiment was 20 times wider than that of the membrane distillation process, the time required for lithium concentration was about 4 times longer than that of the membrane-based process (90 hours). However, the permeate flow rate in the membrane distillation process gradually decreased during the experiment, because the water vapor pressure decreased as the solution concentration increased.

However, as shown in Fig. 5b, after the lithium concentration reached 900 ppm, the membrane distillation process did not work anymore, because crystals of divalent ions were formed on the surface of the membrane, which caused the wetting of the membrane. to be. However, according to FIG. 6 , the permeate conductivity did not show significant change before the critical point was reached. This indicates that the ions in the brine were successfully concentrated without loss of lithium ions without penetrating to the permeation side of the membrane.

As shown in Figure 5c, in order to predict the principle and timing of crystal formation by the precipitation (precipitation) of saturated ions, simulation using PHREEQC software was performed. not formed. However, after the lithium concentration in the brine exceeded 900 ppm, calcium-based crystals (anhydrite and gypsum) started to precipitate. Interestingly, as predicted in the simulation, the membrane surface was covered with some crystals after operation of the membrane distillation module ( FIGS. 5c and 5d ).

In the course of the membrane distillation experiment, the concentrations of all ions in the feed solution except for calcium ions were continuously increased. On the other hand, at the end of the experiment, the calcium concentration decreased as crystals were precipitated on the membrane surface (see FIG. 5A ).

5E shows rod-shaped crystals known as representative shapes of calcium sulfate crystals. As such, it can be seen that calcium ions must be removed from the feed of the membrane distillation process in order to further increase the lithium concentration to an extractable level.

- Analysis of the results of membrane distillation experiments with nanofiltration

A nanofiltration step was added prior to the membrane distillation to remove divalent ions in the artificial brine to increase the lithium concentration and inhibit crystal formation in the feed to the membrane distillation.

Three commercial nanofiltration membranes (NE40, NE70 and NE90) were screened based on the permeate flow rate and ion removal rate. As shown in Fig. 7a, the NE40 membrane showed the highest permeate flux, but a very low divalent ion removal rate (<5%). On the other hand, the NE90 membrane showed the highest removal rate for all ions, and showed low selectivity between monovalent ions and divalent ions. Moreover, the permeate flow rate (7.3 L m ⁻² h ⁻¹ ) of NE90 was not sufficient for application in lithium recovery systems.

Considering the above results, it was determined that the NE70 membrane was most suitable for the pretreatment of brine, and the membrane had an appropriate permeate flow rate (25.1 L m ⁻² h ⁻¹ ) and a removal rate of divalent ions (>30%), and A low monovalent ion removal rate (<10%) was shown.

Compared to the initial artificial saline, the nanofiltration-pretreated artificial saline ^{exhibited 40% lower Ca 2+} concentrations (100 ppm to 60 ppm) and Mg ²⁺ concentrations (100 ppm to 60 ppm), but similar Li ⁺ concentrations ( 100 ppm to 92 ppm) and Na ⁺ concentrations (10000 ppm to 9200 ppm) (see Table 4 below; wt%).

Na K B Li Mg Ca Cl SO ₄ NF-treated artificial saline 0.92 0.09 0.004 0.0096 0.006 0.006 1.2 0.06

Then, a membrane distillation process was performed using the nanofiltration-pretreated artificial brine.

According to FIG. 6b , the initial ionic conductivity (37.7 mS cm ⁻¹ ) of the nanofiltration-pretreated artificial saline was somewhat lower than that of the initial artificial saline (41.0 mS cm ⁻¹ ), but the brine was similarly concentrated. However, the membrane distillation apparatus was washed to remove rust after 100 hours of operation, which is due to the limitation of the lab-scale membrane apparatus. After rust removal, as the membrane distillation process was continued, the permeate flow rate was immediately restored and the ion concentration was slightly diluted (see FIGS. 7c and 7d ).

Finally, after 140 hours of operation, Li ⁺ ions in the brine were concentrated from 100 ppm to 1200 ppm. However, the membrane distillation process was stopped due to the formation of crystals on the membrane surface, which resulted in a sharp decrease in the permeate flow rate and an increase in the permeate conductivity.

As shown in FIG. 8 , different types of crystals were observed on the membrane surface after running the membrane distillation process using the nanofiltration-pretreated brine. As previously described in relation to FIG. 5E , rod-shaped crystals were formed without nanofiltration pretreatment. According to XPS _{analysis, Ca 2p (347.8 eV),} S 2p (169.2 eV), and O _1s (532.08 eV) was peaks are mainly observed, which is not hydroxy light (CaSO ₄₎ and gypsum (CaSO ₄ · 2H ₂ O) dictate the decision

According to FIG. 8A , small Na _1s (1,071.82 eV) and Cl _2p peaks (292.97 eV) were also detected, suggesting that halite (NaCl) crystals were formed.

As a result of XRD analysis, as shown in FIG. 8b, strong and narrow peaks corresponding to gypsum crystals were observed at 11.72°, 20.82°, 23.56°, 29.29°, 31.25°, and 33.59°, which were respectively (020) of gypsum crystals. , (-121), (031), (-141), (121), and (022) faces. These peaks were observed within an error range of about 0.1 °C.

As shown in the EDX mapping data according to Figure 8c, Ca (9.12%), S (7.60%), and O (50.03%) were observed.

Based on the analysis results of the ion concentration in the brine described above, ^{the decrease in Ca 2+} concentration (FIG. 5A) can be explained by precipitation of gypsum crystals (FIG. 5D). Regarding the precipitated crystals, as predicted in FIG. 5c , anhydrite and gypsum crystals were precipitated during operation. However, not hydroxy light (CaSO ₄₎ is as has a tendency in a metastable state upon contact with water, crystals are easy to hydrate gypsum _{(CaSO 4 · 2H 2 O)} . As a result, it can be seen that, although anhydrite crystals are predicted to be formed in FIG. 5c , an anhydrite crystal peak is not observed in XRD and XPS analysis.

On the other hand, when the feed was nanofiltered, only Na _1s (1071.82 eV) and Cl _2p (292.97 eV) peaks were observed in the crystals in XPS analysis. Also. According to FIG. 8B, the peaks at 32.06° and 45.92° corresponded to the (200) and (220) planes of the halite crystal, respectively. Moreover, Na (10.28%) and Cl (11.42%) were confirmed in the result of EDX mapping analysis (FIG. 8c).

As such, since divalent ions have already been removed from the brine by nanofiltration, the formation of anhydrite and gypsum crystals on the membrane surface could be prevented by using nanofiltration-pretreated brine as a feed. However, the formation of halite crystals composed of monovalent ions (Na ⁺ and Cl ⁻ ) could not be suppressed, because nanofiltration membrane treatment can remove only divalent ions.

simulation test

The performance of membrane distillation in a large-scale process suitable for commercialization was evaluated through simulation tests using open-source software. The results are shown in Table 5 below.

lab-scale Effective Membrane Length/Width (cm) Membrane thickness (mm) Channel height of feed and permeate (mm) Feed and permeate inlet temperature (°C) Feed and permeate inlet flow rates (kg/s) 8 / 2 0.125 5 of 5 53.2 / 19.2 0.017 / 0.017 Commercial scale simulation conditions Effective Membrane Length/Width (cm) Feed and transmission channel height (mm) Feed and permeate inlet temperature (°C) Feed and permeate inlet flow rates (kg/s) 200/100 5/5 50/25 0.85/0.85 Commercial scale simulation results temperature of feed and permeate
(℃) Feed and permeate outlet flow rates (kg/s) cross-membrane permeate thermal efficiency Temperature Polarization Coefficient (TPC) 41.4 / 28.2 0.84 / 0.86 10.6 0.53 0.60

According to the above table, it can be seen that the experimental conditions and results on a commercial scale can be predicted using the conditions and results tested in the laboratory, and can be used for economic evaluation similar to the actual results using this bar.

As described above, when the membrane distillation process is performed without nanofiltration pretreatment, it is necessary to remove divalent ions that cause crystal formation. As in the example, the artificial lithium-containing brine could be concentrated 12 times by the process combining nanofiltration-membrane distillation, and the permeate flow rate was 60 times higher than that of the solar evaporation method. This suggests that compared to solar evaporation, the cost of capital can be significantly lowered while keeping the annual operating cost similar.

All simple modifications or changes of the present invention fall within the scope of the present invention, and the specific scope of protection of the present invention will be made clear by the appended claims.

Claims (17)

providing a lithium-containing aqueous solution comprising monovalent ions including lithium ions, sodium ions and potassium ions, and divalent ions including calcium ions and magnesium ions;
pretreating the lithium-containing aqueous solution by membrane-based nanofiltration to form a pretreated lithium-containing aqueous solution having a reduced divalent ion concentration;
forming a concentrate having an increased lithium concentration by subjecting the pretreated lithium-containing aqueous solution to membrane distillation; and
recovering lithium from the concentrate;
includes,
Concentrations of lithium ions, sodium ions and potassium ions, respectively, as monovalent ions in the lithium-containing aqueous solution range from 10 to 1,600 ppm, 10,000 to 100,000 ppm, and 1,000 to 30,000 ppm, and calcium ions and magnesium as divalent ions, respectively the concentration of each of the ions ranges from 100 to 2,000 ppm and from 20 to 25,000 ppm, and
Membrane-based lithium, wherein the nanofiltration membrane ^{exhibits a permeate flow rate of 20 to 50 Lm -2} h ^-1 and also exhibits a divalent ion rejection of at least 30% and a monovalent ion removal rate of 10% or less. recovery method. delete The membrane-based lithium recovery method according to claim 1, wherein the nanofiltration is performed under a driving pressure of 3 to 15 bar. delete The method of claim 1, wherein the nanofiltration membrane has a pore size of 10 nm or less and a thickness of 10 to 1000 μm. The method of claim 1, wherein the nanofiltration membrane is a composite membrane in which a thin film made of polyamide is coated with an activation layer on a porous membrane structure, wherein the porous membrane structure is polysulfone (PSf) or polyethersulfone (PES) and at least one material selected from the group consisting of polyethylene terephthalate (PET). The method according to claim 1, wherein in the membrane distillation treatment, the temperature of the feed side and the temperature of the permeate side of the membrane are selected in the range of 40 to 90°C and 10 to 40°C, and
The membrane-based lithium recovery method, characterized in that the temperature difference between the supply side and the permeation side is in the range of 20 to 50 ℃. The membrane-based lithium recovery method according to claim 6, wherein the membrane for membrane distillation treatment is made of a microporous hydrophobic polymer material, and its water contact angle is at least 90°. 9. The method of claim 8, wherein the hydrophobic polymer is selected from the group consisting of polypropylene (PP), polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), thermally converted aromatic polyimide and polyethylene (PE). Membrane-based lithium recovery method, characterized in that at least one selected. The method of claim 9, wherein the membrane for membrane distillation has a pore size of 1 μm or less and a porosity of 30 to 90%. The membrane-based lithium recovery method according to claim 10, wherein the thermal conductivity of the membrane for membrane distillation treatment is 0.1 W/mK or less. The membrane-based lithium recovery method according to claim 1, wherein each of the nanofiltration membrane and the membrane distillation membrane is in the form of a flat membrane or a hollow fiber membrane. The membrane-based lithium recovery method according to claim 1, wherein the membrane distillation treatment is performed in a DCMD method. The method according to claim 1, wherein when the concentration of lithium ions in the lithium-containing aqueous solution is 100 ppm, the concentration of lithium ions in the concentrate is in the range of 1000 to 1500 ppm. The membrane-based lithium recovery method according to claim 1, wherein the recovering of the lithium comprises converting lithium ions in the concentrate into lithium carbonate by a carbonation reaction. The membrane-based lithium recovery method according to claim 1, wherein the lithium-containing aqueous solution pretreated by nanofiltration exhibits at least 40% divalent ion removal rate compared to the lithium-containing aqueous solution prior to nanofiltration pretreatment. delete

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