AU2010201962A1

AU2010201962A1 - Salt purification process

Info

Publication number: AU2010201962A1
Application number: AU2010201962A
Authority: AU
Inventors: Marcus John Fabig
Original assignee: Osmoflo Pty Ltd
Current assignee: Osmoflo Pty Ltd
Priority date: 2009-05-19
Filing date: 2010-05-17
Publication date: 2010-12-09

Description

Regulation 3.2 AUSTRALIA PATENTS ACT 1990 COMPLETE SPECIFICATION FOR A STANDARD PATENT ORIGINAL Name of Applicant: Osmoflo Pty Ltd Actual Inventor: Marcus John Fabig Address for Service: C/- MADDERNS, GPO Box 2752, Adelaide, South Australia, Australia Invention title: SALT PURIFICATION PROCESS The following statement is a full description of this invention, including the best method of performing it known to us.

2 SALT PURIFICATION PROCESS Field of Invention 5 This invention relates to a salt purification process and more particularly to a process for treating saline ground waters such as mine waste water to extract commercially useful salts therefrom. Background 10 Saline ground waters typically contain a wide variety of dissolved salts. If typical saline ground water is simply left to fully evaporate the resultant crystals will be a complex mixture of little commercial value. Operating an evaporation process, either solar evaporation ponds or thermal evaporation systems, on a selective crystallization basis is sometimes practised, i.e. allow salt products to precipitate and then removing the supernatant liquid for further 15 evaporation and crystallization. This requires careful management of the evaporation process to ensure the required precipitation events occurred accurately to minimize cross contamination of the salt products. This invention will be discussed in particular to waste water from coal-seam gas operations 20 but the invention is not so limited and may be applied to other waters, for instance saline ground waters, which have commercially useful salts therein. In coal-seam gas operations gas is extracted from seams of coal but in the process large quantities of waste water are extracted and this water is high in salts and cannot be easily disposed of. 25 It is the object of this invention to provide a process for treating such water to extract commercially useful salts therefrom. Brief Description of the Invention In one form therefore the invention is said to reside in a process for treating saline ground 30 water comprising the steps of; treating the saline ground water to at least substantially remove hardness salts; passing the treated water through a reverse osmosis apparatus to produce a pre-concentrate and a permeate; passing the pre-concentrate into an evaporation process and carrying out fractional 35 crystallization therein to separate out commercially useful salts and to produce an overflow of other salts.

3 Preferably the ground water comprises salts, being high in carbonates and bicarbonates and sodium ions, having some chloride ions and low in calcium and magnesium ions. The saline ground water may, for instance, be mine waste water such as from a coal-seam gas mine. 5 Preferably the treatment of the saline ground water to at least substantially remove hardness salts comprises passing the water through an ion exchange resin. Preferably the ion exchange resin is used to reduce the quantity of magnesium and calcium ions in the water. Preferably the overflow of other salts comprises substantially sodium chloride and this 10 sodium chloride can at least in part be used to regenerate the iron exchange resin. Preferably the evaporation process and fractional crystallization is carried out in an the evaporation system comprising a series of solar evaporation pans or a thermal evaporator. 15 Preferably the step of passing the treated water through a reverse osmosis apparatus to produce a pre-concentrate and a permeate comprises producing the permeate as substantially potable water and the pre-concentrate comprising a selected concentration of salts including carbonates and bicarbonates. 20 Preferably the reverse osmosis pre-concentration step is controlled by measurement of salt concentration in the pre-concentrate. The process can further comprise the step of initially filtering the saline ground water to remove insoluble solids. The filtration step can be carried out in a through an ultra-filtration 25 device. In an alternative form the invention comprises a process for extracting commercially useful salts from saline ground water, the saline ground water being high in carbonates and bicarbonates and sodium ions and low in calcium and magnesium ions, the process 30 comprising the steps of; filtering the waste water through an ultra-filtration device to remove insoluble solids; treating the filtered saline ground water through an ion exchange resin to reduce the magnesium and calcium ions in the water; passing the treated water through a reverse osmosis apparatus to produce a pre-concentrate 35 and a permeate, the permeate comprising substantially potable water and the pre-concentrate comprising a selected concentration of salts including the carbonates and bicarbonates; 4 passing the pre-concentrate into an evaporation pan and carrying out fractional crystallization therein to separate out commercially useful carbonates and bicarbonates and to produce an overflow of substantially sodium chloride; and at least a proportion of the sodium chloride being used to regenerate the ion exchange resin. 5 The process can further comprise the step of redissolving the commercially useful carbonates and bicarbonates and re-evaporating and fractionally crystallising in an evaporation pan to improve the grade of the carbonates and bicarbonates. 10 Preferably the selected concentration of the salts in the pre-concentrate is from 5,000 to 50,000 mg/l total dissolved solids. The reverse osmosis apparatus can be controlled by measurement of concentration of salts in the pre-concentrate leaving the reverse osmosis apparatus. 15 In an alternative form the invention comprises an apparatus for extracting commercially useful salts from saline ground water, the saline ground water being high in carbonates and bicarbonates and sodium ions and low in calcium and magnesium ions, the apparatus comprising; 20 a filtration apparatus to remove insoluble solids from the saline ground water; an ion exchange apparatus to treat the filtered saline ground water resin to reduce the magnesium and calcium ions in the water; a reverse osmosis apparatus to produce a pre-concentrate and a permeate, the permeate comprising substantially potable water and the pre-concentrate comprising a selected 25 concentration of salts including the carbonates and bicarbonates; an evaporation system to evaporate the pre-concentrate to carry out fractional crystallization of the pre-concentrate to separate out commercially useful carbonates and bicarbonates and to produce an overflow of substantially sodium chloride and wherein the sodium chloride can be used to regenerate the ion exchange resin in the ion exchange apparatus. 30 Preferably the filtration apparatus comprises an ultra-filtration device. Preferably the evaporation system to evaporate the pre-concentrate is selected from a series of solar evaporation pans or a thermal evaporator. 35 Preferably the reverse osmosis apparatus comprises a controller to measure the concentration of salts in the pre-concentrate to thereby control the reverse osmosis apparatus. Preferably the 5 selected concentration of the salts in the retentate is from 5,000 to 50,000 mg/I total dissolved solids (TDS) and the reverse osmosis assembly is controlled by measurement of concentration in the pre-concentrate flow from the reverse osmosis assembly. 5 Salts from saline ground waters are often at concentrations too low for economical commercial harvesting by evaporation, so using the process of the present invention to concentrate of the saline water by a pre-concentration step is beneficial in reducing the size of the evaporation basins and reducing the cost of salt harvesting. 10 A typical application of salt harvesting from saline groundwater is that which arises from the coal bed methane extraction process. In this process natural gas is drawn from a coal bed beneath the surface and with the gas comes water that is also held in the coal seam. The water is typically found to have a water analysis as presented in Table I below. This water varies in salinity from as low as 1,000 mg/I TDS up to 15,000 mg/I TDS. There are some valuable 15 salts that can be harvested from this water that is otherwise a by-product of the coal bed methane extraction process. This particular ground water contains high levels of sodium carbonate/sodium bicarbonate and sodium chloride which can be harvested by the process described herein. Typical of multiple ground water supplies such as those found in coal bed methane fields, the raw water quality can vary from well to well, making the blended ground 20 water from a range of well quite variable. The process and apparatus of the present invention provides process of extracting salts from the saline groundwater in such a way as to produce commercial purity grades of salts in a cost effective manner. 25 Essentially the process according top the present invention is one whereby the saline groundwater is pre-treated to substantially strip out contaminating hardness salts from the groundwater before pre-concentrating in a reverse osmosis system. The pre-concentration stage can be operated in a specific way to ensure constant supply of a selected concentrate of 30 salts to the evaporation stage. The concentrated saline stream is then evaporated in either a solar evaporation basin or a thermal evaporation system that allows for selective crystal removal. A portion of the harvested salts, typically sodium chloride, is then reused to regenerate the hardness removal pre-treatment system. In this way the efficiency of the system is very high as very little additional chemical is required for the pre-concentration 35 stage and the resultant salts produced are of acceptable quality for commercial use.

6 This then generally describes the invention but to assist with understanding reference will now be made to the detailed description of one embodiment of the invention and the accompanying drawing which shows one possible flow sheet. 5 Description of Preferred Embodiment The process according to one embodiment of the invention consists of four main steps: 1) Solids removal; 2) Hardness removal in a pre-treatment stage; 3) Pre-concentration of the saline water by reverse osmosis; 10 4) Selective evaporation to separate the various forms of target salts. Each one of these steps will be described in more detail. I) Solids Removal If necessary, depending upon the amount of solids in the saline ground water, in the solids 15 removal stage the ground water is passed through a filter system to remove suspended solids which may otherwise foul the resin in the later ion exchange stage or the reverse osmosis membranes in the pre-concentration stage. The filter system can for instance be an ultra filtration stage. 20 2) Hardness Removal Hardness salts are usually referred to as calcium and magnesium dissolved in water. These salts tend to be sparingly soluble, which means they have a tendency to precipitate readily when their concentration increases. In saline ground waters the salts associated with calcium and magnesium are usually the first salts to precipitate. These can be calcium carbonate, 25 calcium sulphate and magnesium sulphate. These hardness salts are contaminants in a downstream evaporation processes to harvest the salt. For example if the target was to harvest sodium carbonate/sodium bicarbonate and sodium chloride from the saline ground water shown in Table 1, the presence of calcium ions 30 would be a significant barrier to allowing crystallization of the target salt products. These hardness salts can form complexes with some of the target salts, leading to a loss of volume of the target salts and contamination of the target salts in subsequent evaporation stages. Removal of the hardness salts prior to the pre-concentrator stage means that the concentrated 7 brine going to evaporation is relatively pure in terms of the target salts, so that the evaporation process can simplified and produce relatively pure salts. In addition the hardness salts are often known as the limiting design constraint in 5 concentration process like reverse osmosis, i.e. the reverse osmosis system is usually designed to ensure these sparingly soluble hardness salts do not precipitate in the reverse osmosis modules causing rapid scale build up and consequent failure. By removing the sparingly soluble harness salts prior to the reverse osmosis system, this design constraint can be lifted and the reverse osmosis system can be operated at a much higher concentration factor. For 10 example a reverse osmosis system operating without hardness removal upstream could usually operate in the range of a 2 to 5 concentration factor, depending on the other scaling salts in the water. One the same saline ground water with hardness removal upstream, the reverse osmosis system could typically operate in the range of a 5 to 20 concentration factor. 15 Removal of the hardness salts effectively allows the reverse osmosis pre-concentrator to operate at a far higher concentration factor than would otherwise be the case. This means the downstream evaporation process will be a fraction of the size of that required with no hardness removal upstream, reducing capital and operating costs significantly. 20 Whilst other forms of hardness removal are available, the most common method is by ion exchange. When salts are dissolved in water, ions dissociate into their respective positive and negatively charged ions, known as cations and anions. Cations comprise mainly calcium, magnesium and sodium, whereas the anions comprise bicarbonates, chlorides and sulphates. 25 Ion exchange resins have the ability to exchange one ion for another of the same charge. Thus cations can be exchanged by one resin, and anions by another. The resins hold the ions temporarily in chemical combination, and give them up to a strong regenerating solution. Thus a cation resin, which contains predominantly sodium ions, will exchange these for calcium and magnesium ions when water containing these is passed through the resin. If 30 strong sodium chloride brine is then passed through the resin, the calcium and magnesium ions on the resin are then replaced by sodium ions. Hence the ion exchange stage of the present invention thus contains a bed of cation resin in the sodium form, and water passing through it trades calcium and magnesium ions (if present) 35 for these sodium ions, thus reducing the level of hardness ions and replacing them instead with an equivalent amount of non-hardness sodium ions. After a period, the resin is depleted 8 of sodium ions and the softening process ceases. The bed must then be regenerated with sodium chloride in the form of brine to restore its capacity. The process of the present invention preferably utilizes the purified sodium chloride from the 5 evaporation stage (see below) to regenerate the ion exchange resin bed. The spent regenerant solution, essentially a concentrated solution of calcium chloride and magnesium chloride, is another form of separation of the salts. The calcium chloride and magnesium chloride can also be harvested by evaporation of the spent regenerant solution. 10 Sometimes the quality of the raw water requires more extensive pre-treatment be included, for example for solids removal prior to the hardness softeners to ensure efficient operation of all downstream process equipment. 3) Pre-concentration of the saline water by reverse osmosis 15 Once the hardness salts are substantially stripped out of the saline ground water, a reverse osmosis system is used to pre-concentrate the ground water prior to selective evaporation. As the hardness salts are removed the reverse osmosis system can be operated at a far higher concentration factor than would otherwise be the case. Concentration factors as high as 20 are achievable depending on the quality of the raw water entering the reverse osmosis pre 20 concentrator. With very little hardness in the raw water the reverse osmosis system will operate efficiently without significant formation of hardness scale that is typical of such systems. Typical of multiple ground water supplies such as those found in coal bed methane fields, the 25 raw water quality can vary from well to well, making the blended ground water quite variable. It is standard to operate a reverse osmosis plant by controlling the recovery rate, i.e. the ratio of treated water to feed water to the system, to a design ratio set-point of the two flow rates. The reverse osmosis pre-concentrator in this present invention is preferably operated in such a way as to have a design set-point based on the outlet brine concentration, subject to the 30 blended raw water quality. In this way a constant brine concentration can be fed to the next evaporation stage, ensuring consistency of the evaporation process. 4) Selective Evaporation The pre-concentrated brine entering the evaporation stage has two key benefits: 9 a) Substantially no hardness salts, so the quality of the brine is excellent quality for further evaporation for targeted harvesting by fractional crystallization of the remaining salts. b) The concentration is consistent by the method of control of the reverse osmosis pre 5 concentrator according to the present invention. In this process description a solar evaporation system will be discussed, but this can also be accomplished by a thermal evaporation/crystallization system. The concentrated brine is fed to a first stage evaporation pan or basin, where the first target 10 species will precipitate upon solar evaporation. For example in the raw water described in Table I it is expected that a sodium carbonate/sodium bicarbonate complex will precipitate. These two salt products precipitate at similar total ionic strengths and can be processed by downstream techniques to convert them into the desired form. Both sodium carbonate and sodium bicarbonate are sought after salts with significant commercial value. 15 The further processing of the precipitated sodium carbonate/sodium bicarbonate complex can include re-solution using the permeate of the reverse osmosis step and re-crystallization in a further evaporation pan or other form of thermal evaporator. 20 Once the sodium carbonate/sodium bicarbonate salts have precipitated the supernatant liquid is removed from the solar evaporation cell to another cell for further evaporation. In this cell the dominant salt in solution is sodium chloride. The quality of this sodium chloride will be acceptable for commercial use when it is harvested. Based on the raw water in Table 1, there would be an excess of sodium chloride to that used for regenerating the hardness removal 25 softeners, and this excess would be the net harvested amount. Harvesting of the salts from the evaporation process is known technology. However what the process of the present invention does is to purify the brine at various stages of the process to produce commercial quality grades of harvested salts. In the example of the raw water in 30 Table I these harvested salts would be: a) Sodium carbonate/sodium bicarbonate b) Sodium chloride c) Calcium chloride 10 Table I Typical Raw Water Analysis of Saline Ground Water Resulting from Coal Bed Methane Extraction Process Parameter Units Value Temperature 0 C 25-35 Total Suspended Solids mg/L <20 Total Organic Carbon mg/L as C 0.67 -20 Ammonia mg/L as N < 0.5 Inorganic iron (total) mg/L Fe 0.5 Manganese (total) mg/L Mn 0.01 pH pH unit 8.4- 8.8 Conductivity jS/cm 3910 Total dissolved solids mg/L 3000 Dissolved Organic Carbon mg/L as C 0.5 - 5 Total Alkalinity mg/L as CaCO 3 980 Bicarbonate alkalinity mg/L as CaCO 3 940 Carbonate Alkalinity mg/L as CaCO 3 33 Chloride mg/L 750 Sulphate mg/L <2 Fluoride mg/L 2.6 Nitrite + Nitrate mg/L as N 0.012 Phosphorus mg/L as P 0.027 Calcium mg/L 2.7 Magnesium mg/L <0.2 Sodium mg/L 980 Potassium mg/L 4.2 Aluminium mg/L 0.54 Arsenic mg/L <0.01 Boron mg/L 0.93 Barium mg/L 0.41 Copper (total) mg/L 0.022 Silica mg/L as SiO 2 23 Zinc (total) mg/L 0.026 Strontium mg/L 0.65 Process Flow Diagram Description 5 The following process description refers to the process flow diagram in Figure 1. In this embodiment the saline ground water I comprises substantially sodium, calcium and magnesium cations and carbonate, bicarbonate and chloride anions. 10 The raw ground water 1, perhaps from a number of sources, enters a first solids removal pre treatment 2 stage via an inlet line Ia. The pre-treatment stage can comprise a filtration or ultra-filtration device. Solids are removed in line 2a.

I I The pre-treated raw water leaves the solids removal pre-treatment 2 via line 3 and enters a second pre-treatment stage, being a hardness removal stage 4, such as an ion exchange resin bed. The ion exchange resin bed is used to at least substantially remove hardness salts such as calcium and magnesium ions. The pre-treated water leaving the second pre-treatment stage 5 via line 5 essentially now has the hardness related salts substantially removed by the hardness removal stage 4. The pre-treated water is then fed to a reverse osmosis pre-concentrator 6. The treated water passing through the reverse osmosis membrane of the pre-concentrator as permeate then passes through line 7 to discharge. The permeate 7 can be substantially potable water. 10 The concentrate from the reverse osmosis pre-concentrator 6 passes through line 8 and into a first evaporation basin 9. In this evaporation basin 9 the primary target salt to be harvested is precipitated 11 at the bottom of the basin. In the example of raw water as shown in Table I this harvested salt would be sodium carbonate/sodium bicarbonate. 15 After precipitation of the primary target salt 11, the supernatant 10 is the transferred via line 12 to a secondary evaporation basin 15. In this secondary evaporation a secondary salt product precipitates after further evaporation. In the example of raw water in Table I this harvested salt would be sodium chloride 17. A portion of this salt stream would be fed via 20 line 13 to regenerate the hardness removal ion exchange resin 4, with the spent regenerant exiting via line 16. In the example of raw water in Table 1 this spent regenerant can contain calcium chloride. Excess sodium chloride 13a can be used commercially. If required, the primary target salt 11 can be redissolved using water 18 from permeate 7 of 25 the pre-concentrator stage and recrystallised in a further evaporation system such as a further evaporation pan 19 to improve its grade for commercial sale. The pre-concentrator step 6 can be controlled by a design set-point based upon output concentration of salt in line 8 using controller 20 controlled by concentration measuring 30 device 22. Throughout this specification various indications have been given as to the scope of this invention but the invention is not limited to any one of these but may reside in two or more of these combined together. The examples are given for illustration only and not for limitation. 35 Throughout this specification and the claims that follow unless the context requires otherwise, the words 'comprise' and 'include' and variations such as 'comprising' and 'including' will be 12 understood to imply the inclusion of a stated integer or group of integers but not the exclusion of any other integer or group of integers.

Claims

1. A process for treating saline ground water comprising the steps of; treating the saline ground water to at least substantially remove hardness salts; 5 passing the treated water through a reverse osmosis apparatus to produce a pre-concentrate and a permeate; passing the pre-concentrate into an evaporation process and carrying out fractional crystallization therein to separate out commercially useful salts and to produce an overflow of other salts. 10

2. A process as in Claim I wherein the ground water comprises salts, being high in carbonates and bicarbonates and sodium ions, having some chloride ions and low in calcium and magnesium ions. 15

3. A process as in Claim I wherein the treatment of the saline ground water to at least substantially remove hardness salts comprises passing the water through an ion exchange resin.

4. A process as in Claim 3 wherein the ion exchange resin is used to reduce the quantity 20 of magnesium and calcium ions in the water.

5. A process as in Claim 3 wherein the overflow of other salts comprises substantially sodium chloride and the sodium chloride is used to regenerate the ion exchange resin. 25

6. A process as in Claim I wherein the saline ground water is mine waste water.

7. A process as in Claim I wherein the evaporation process and fractional crystallization is carried out in an the evaporation system comprising a series of solar evaporation pans or a thermal evaporator. 30

8. A process as in Claim 1 wherein the step of passing the treated water through a reverse osmosis apparatus to produce a pre-concentrate and a permeate comprises producing the permeate as substantially potable water and the pre-concentrate comprising a selected concentration of salts including carbonates and bicarbonates. 35

9. A process as in Claim I wherein the reverse osmosis pre-concentration step is controlled by measurement of salts in the pre-concentrate. 14

10. A process as in Claim I further comprising the step of initially filtering the saline ground water to remove insoluble solids. 5

11. A process as in Claim 10 wherein the filtration step is carried out in a through an ultra-filtration device.

12. A process for extracting commercially useful salts from saline ground water, the saline ground water being high in carbonates and bicarbonates and sodium ions and low in 10 calcium and magnesium ions, the process comprising the steps of; filtering the waste water through an ultra-filtration device to remove insoluble solids, treating the filtered saline ground water through an ion exchange resin to reduce the magnesium and calcium ions in the water, passing the treated water through a reverse osmosis apparatus to produce a pre-concentrate 15 and a permeate, the permeate comprising substantially potable water and the pre-concentrate comprising a selected concentration of salts including the carbonates and bicarbonates; passing the pre-concentrate into an evaporation pan and carrying out fractional crystallization therein to separate out commercially useful carbonates and bicarbonates and to produce an overflow of substantially sodium chloride; and 20 at least a proportion of the sodium chloride being used to regenerate the ion exchange resin.

13. A process as in Claim 12 wherein the saline ground water is from a coal-seam gas mine. 25

14. A process as in Claim 12 further including the step of redissolving the commercially useful carbonates and bicarbonates and re-evaporating and fractionally crystallising in an evaporation pan to improve the grade of the carbonates and bicarbonates.

15. A process as in Claim 12 wherein the selected concentration of the salts in the pre 30 concentrate is from 5,000 to 50,000 mg/l total dissolved solids.

16. A process as in Claim 12 wherein the reverse osmosis apparatus is controlled by measurement of concentration in the pre-concentrate leaving the reverse osmosis apparatus. 35

17. An apparatus for extracting commercially useful salts from saline ground water, the saline ground water being high in carbonates and bicarbonates and sodium ions and low in calcium and magnesium ions, the apparatus comprising; 15 a filtration apparatus to remove insoluble solids from the saline ground water; an ion exchange apparatus to treat the filtered saline ground water resin to reduce the magnesium and calcium ions in the water; a reverse osmosis apparatus to produce a pre-concentrate and a permeate, the permeate 5 comprising substantially potable water and the pre-concentrate comprising a selected concentration of salts including the carbonates and bicarbonates; an evaporation system to evaporate the pre-concentrate to carrying out fractional crystallization of the pre-concentrate to separate out commercially useful carbonates and bicarbonates and to produce an overflow of substantially sodium chloride and wherein the 10 sodium chloride can be used to regenerate the ion exchange resin in the ion exchange apparatus.

18. An apparatus as in Claim 17 wherein the filtration apparatus comprises an ultra filtration device. 15

19. An apparatus as in Claim 17 wherein the evaporation system to evaporate the pre concentrate is selected from a series of solar evaporation pans or a thermal evaporator.

20. An apparatus as in Claim 17 wherein the reverse osmosis apparatus comprises a 20 controller to measure the concentration of salts in the pre-concentrate to thereby control the reverse osmosis apparatus.