阅读说明：本技术 回收锂值的方法 (Method for recovering lithium values ) 是由 A·纳皮耶 C·格里菲思 于 2019-05-30 设计创作，主要内容包括：一种从含锂溶液如盐水或工艺母液中回收磷酸锂和硫酸锂的方法。该方法包括向含锂溶液中添加磷酸盐以沉淀磷酸锂,然后从所述溶液中分离出所得的磷酸锂沉淀。然后将分离出的磷酸锂沉淀在硫酸中消化以产生消化混合物,从中分离出硫酸锂沉淀。将碱金属氢氧化物添加到分离后的溶液中以产生碱金属磷酸盐溶液,并将其回收以在该方法的第一步中用作磷酸盐。(A method for recovering lithium phosphate and lithium sulfate from a lithium-containing solution, such as brine or process mother liquor. The process comprises adding a phosphate to a lithium-containing solution to precipitate lithium phosphate and then separating the resulting lithium phosphate precipitate from the solution. The separated lithium phosphate precipitate is then digested in sulfuric acid to produce a digested mixture from which the lithium sulfate precipitate is separated. An alkali metal hydroxide is added to the separated solution to produce an alkali metal phosphate solution, which is recovered for use as a phosphate in the first step of the process.)

1. A method of recovering lithium phosphate and lithium sulfate from a lithium-containing solution, comprising:

a) adding a phosphate to the lithium-containing solution to produce a lithium phosphate precipitate;

b) separating said lithium phosphate precipitate from the solution produced in step a);

c) digesting the separated lithium phosphate precipitate in sulfuric acid to produce a digested mixture, precipitating lithium sulfate therefrom, and separating the lithium sulfate precipitate from said digested mixture; and

d) adding an alkali metal hydroxide to the digested mixture separated in step c) to produce an alkali metal phosphate solution and recovering the alkali metal phosphate solution as phosphate for step a).

2. The method of claim 1, wherein a stoichiometric excess of phosphate is added to the lithium-containing solution in step a) such that less than 500mg/L of soluble lithium remains in solution.

3. The method of claim 1 or claim 2, wherein a stoichiometric excess of phosphate is added to the lithium-containing solution in step a) such that the residual P in solution is greater than 100 mg/L.

4. A process according to any one of claims 1 to 3, wherein the separated lithium phosphate precipitate is reprecipitated from phosphoric acid prior to step c).

5. The method of any one of claims 1-4, wherein the digestion mixture in step c) comprises 10-50 wt.% lithium phosphate solids such that the amount of lithium remaining in solution reaches the solubility limit of lithium sulfate in phosphoric acid.

6. A process according to any one of claims 1 to 5, wherein the precipitation of lithium phosphate separated by digestion in sulphuric acid occurs at a temperature of from ambient to 80 ℃ for a period of 1 to 4 hours.

7. The method of any one of claims 1-6, wherein the digestion mixture is concentrated to provide up to 70 wt.% H₃PO₄And (4) concentration.

8. The method of claim 7, wherein the digestion mixture is concentrated to provide 25-65 wt% H₃PO₄。

9. The method according to any of the preceding claims, wherein prior to performing step a), the method comprises softening the lithium-containing solution by reducing its calcium content to less than 25 ppm.

10. The method of claim 9, wherein the softening step comprises adding potassium carbonate or phosphate to the lithium-containing solution to produce a calcium precipitate comprising calcium carbonate or apatite.

11. The method of claim 10, wherein when the lithium-containing solution comprises fluoride, the calcium precipitate comprises fluorapatite and apatite.

12. The method according to claim 11, wherein more than 100% of the stoichiometric amount of potassium phosphate (relative to fluorapatite) is added to the lithium-containing solution.

13. The method of claim 9, wherein the softening step comprises adding sodium phosphate to the lithium-containing solution to produce apatite and/or fluoroapatite.

14. The method according to any one of claims 10 to 13, wherein the calcium precipitate is separated from the softened solution prior to step a).

15. The method according to any of the preceding claims, wherein the method further comprises the step of:

e) recovering phosphate in the form of tricalcium phosphate and/or apatite from the separated solution of step b).

16. The process according to claim 15, wherein the tricalcium phosphate and/or apatite is separated from the solution produced in step e).

17. The process according to claim 15 or claim 16, wherein recovering phosphate from the separated solution of step b) in the form of tricalcium phosphate and/or apatite comprises adding calcium hydroxide thereto.

18. The method according to any one of claims 15 to 17, wherein the method further comprises the steps of:

f) recovering potassium from the separated solution of step e) in the form of potassium sulfate.

19. The method of claim 18, wherein recovering potassium from the separated solution of step e) as potassium sulfate comprises concentrating and/or cooling the separated solution of step e) followed by separation of the potassium sulfate precipitate.

20. The method of any one of the preceding claims, wherein the phosphate salt is selected from phosphoric acid, potassium phosphate, sodium phosphate, or a combination thereof.

21. The method of any preceding claim, wherein the alkali metal hydroxide and the alkali metal phosphate comprise potassium hydroxide and potassium phosphate, respectively.

Technical Field

The present disclosure relates to a method for recovering lithium values, and more particularly, to a method for recovering lithium phosphate and lithium sulfate from a lithium-containing solution, such as brine or process mother liquor.

Background

Lithium salts, such as lithium carbonate or lithium hydroxide, are used for the production of lithium ion batteries, glass, ceramics, pharmaceuticals, lubricants, air treatment or aluminum smelting. It also has potential applications in electric vehicles, lithium aluminum alloys for aircraft, and smart grid storage systems.

Lithium carbonate and hydroxide can be recovered from lithium silicates (such as spodumene and lepidolite) or from brine, salt ponds, salt lakes, salt mines, and geothermal resources. The lithium-containing solution from which lithium carbonate and lithium hydroxide are recovered also contains other alkali metal and alkaline earth metal cations in comparable or higher concentrations, resulting in separation difficulties. For example, calcium will accumulate with magnesium during evaporation, and both alkaline earth metals must be removed before lithium carbonate can be separated from the solution. Also, sodium and potassium salts are difficult to separate from such mixed metal solutions. Also, sodium and potassium salts are difficult to separate from such mixed metal solutions. Thus, the recovery process is designed to control the relative proportions of magnesium, calcium, sodium and potassium so that undesirable impurities remain in solution, precipitating a viable amount of the desired lithium salt of the desired purity.

It would be economically advantageous if the alkali and alkaline earth metal cations in the lithium-containing solution could be recovered as saleable by-products or reused in an upstream process.

Further, the solubilities of lithium carbonate and lithium hydroxide in water at 25 ℃ were 1.3g/100mL and 12.7g/100mL, respectively. Thus, recovery of the solid form of these salts from brine and process mother liquor may require a complex multi-stage process that concentrates the purified solution above the solubility limit of these lithium salts. The evaporation to handle water balance or crystallization of soluble salts (e.g., hydroxides) in the commonly used lithium (Li) processing cycles is energy intensive and costly. This problem becomes more complicated when the grade of the lithium source is low.

Accordingly, there is a need to develop a method for recovering lithium salts from lithium-containing solutions that overcomes at least some of the above-mentioned problems.

The discussion of the background to the disclosure is intended to facilitate an understanding of the disclosure. However, it should be appreciated that the discussion is not an acknowledgement or admission that any of the material referred to was published, known or part of the common general knowledge as at the priority date of the disclosure.

Disclosure of Invention

The present disclosure provides a method for recovering lithium values, in particular a method for recovering lithium phosphate and lithium sulfate from a lithium-containing solution, such as brine or process mother liquor.

The method for recovering lithium phosphate and lithium sulfate from a lithium-containing solution comprises the following steps:

a) adding a phosphate to the lithium-containing solution to produce a lithium phosphate precipitate;

b) separating said lithium phosphate precipitate from the solution produced in step a);

c) digesting the separated lithium phosphate precipitate in sulfuric acid to precipitate lithium sulfate and separating the lithium sulfate precipitate therefrom; and

d) adding an alkali metal hydroxide to the digested mixture separated in step c) to produce an alkali metal phosphate solution and recovering the alkali metal phosphate solution as phosphate for step a).

In some embodiments, a stoichiometric excess of phosphate is added to the lithium-containing solution in step a) such that the soluble lithium remaining in solution may be less than 500mg/L and/or the phosphate (P) remaining in solution may be greater than 100 mg/L. In some embodiments, the soluble lithium remaining in the solution may be 50-100mg/L and the P remaining in the solution may be 500-3000 mg/L.

In one embodiment, the separated lithium phosphate precipitate may be reprecipitated from phosphoric acid prior to step c). In this way, the major impurities such as K, Na and S can be reduced by an order of magnitude.

In one embodiment, the digestion mixture described in step c) may comprise 10 to 50 weight percent lithium phosphate solids, such that the amount of lithium remaining in solution reaches the solubility limit of lithium sulfate in phosphoric acid, in particular between 30 and 35 g/L.

In some embodiments, the digestion of the lithium phosphate precipitate separated in step c) in sulfuric acid occurs at a temperature from ambient temperature to 80 ℃ within 1 to 4 hours, in particular within 1 to 2 hours. .

In some embodiments, the digestion mixture may be concentrated to provide up to 70 wt.% H₃PO₄Concentrations, in particular of 25 to 65% by weight of H₃PO₄And (4) concentration.

In some embodiments, prior to performing step a), the method may comprise softening the lithium-containing solution by reducing its calcium content to less than 25 ppm. In some embodiments, the softening step may include adding potassium carbonate or potassium phosphate to the lithium-containing solution to produce a calcium precipitate comprising calcium carbonate or apatite. In other embodiments, the softening step may comprise adding an alkali metal phosphate (e.g., sodium phosphate) to the lithium-containing solution to produce a calcium precipitate comprising apatite. In a particular embodiment, when the lithium-containing solution comprises fluoride, the calcium precipitate may comprise fluorapatite and apatite. The calcium precipitate may be separated from the softened solution prior to step a).

In some embodiments, the process may further comprise: e) recovering phosphate in the form of tricalcium phosphate and/or apatite from the separated solution of step b). The tricalcium phosphate and/or apatite may be separated from the solution produced in step e). In these embodiments, recovering phosphate from the separated solution of step b) in the form of tricalcium phosphate and/or apatite may comprise adding calcium hydroxide to said separated solution.

In further embodiments, the process may further comprise:

f) recovering potassium from the separated solution of step e) in the form of potassium sulfate. In these particular embodiments, recovering potassium from the separated solution of step e) as potassium sulfate comprises concentrating and/or cooling the separated solution of step e) followed by separation of the potassium sulfate.

Drawings

Although any other form may fall within the scope of the method described in the summary of the invention, specific embodiments will be described exemplarily with reference to the following drawings:

FIG. 1 is a process flow diagram depicting a method for producing lithium phosphate from a lithium-containing solution.

Detailed Description

The present disclosure relates to a method for producing lithium phosphate, and in particular, to a method for producing lithium phosphate and lithium sulfate from a lithium-containing solution, such as brine or process mother liquor.

General terms

Throughout this specification, unless specifically stated otherwise or the context requires otherwise, reference to a single step, composition of matter, group of steps or group of compositions of matter shall be taken to include one or more (i.e., one or more) of such steps, compositions of matter, groups of steps or groups of compositions of matter. Thus, as used herein, the singular forms "a," "an," and "the" include plural aspects unless the context clearly dictates otherwise. For example, reference to "a" includes a single as well as two or more; reference to "a" includes a single species as well as two or more species; reference to "the" includes singular as well as two or more; and so on.

Unless specifically stated otherwise, each embodiment of the present disclosure described herein applies, mutatis mutandis, to each other embodiment. The present disclosure is not to be limited in scope by the specific embodiments described herein, which are intended as examples only. Functionally equivalent products, compositions, and methods are clearly within the scope of the disclosure described herein.

The term "and/or", e.g., "X and/or Y", is understood to mean "X and Y" or "X or Y", and is understood to provide express support for both meanings or for either meaning.

Throughout this specification the word "comprise", or variations such as "comprises" or "comprising", will be understood to imply the inclusion of a stated element, integer or step, or group of elements, integers or steps, but not the exclusion of any other element, integer or step, or group of elements, integers or steps.

Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. Although methods and materials similar or equivalent to those described herein can be used in the practice or testing of the present invention, suitable methods and materials are described below. In case of conflict, the present specification, including definitions, will control. In addition, the materials, methods, and examples are illustrative only and not intended to be in any way limiting.

Specific terminology

The term "lithium-containing solution" as used herein is understood to mean an aqueous liquid containing lithium. Such liquids may be from natural sources, such as saline. Alternatively, such liquids may be byproducts of mining, drilling, dewatering and excavation activities, particularly in the form of produced water or wastewater streams. For example, the lithium-containing solution may be produced water from oil and gas drilling, coal bed methane production, and the like. Alternatively, the aqueous liquor may be a hydrometallurgical process liquor (also referred to as process mother liquor) produced by a lithium extraction process. In some embodiments, the lithium-containing aqueous liquid may be a process liquid produced by extracting lithium from recycled lithium batteries or other electronic waste. It is to be understood that the concentration of lithium in the lithium-containing solution will vary depending on its source, and that the lithium-containing solution may be subjected to one or more processes to increase its lithium content to a concentration suitable for performing the methods described herein.

The lithium-containing solution may include one or more impurities. The term "impurities" as used herein refers to metal values other than lithium that are co-dissolved in a lithium-containing solution. Examples of typical impurities include, but are not limited to, Na, Cs, Rb, Si, Al, Mg, Mn, and Fe. It is to be understood that the lithium-containing solution may be subjected to one or more processes to remove or eliminate one or more impurities of the lithium-containing solution prior to performing the method as described herein.

As used herein, the term "apatite" refers to one or more substances of the formula Ca₅(PO₄)₃(F, Cl, OH) (repeating units) and may include hydroxyapatite, fluorapatite, chlorapatite, or mixtures thereof. Method for recovering lithium phosphate and/or lithium sulfate

Calcium is often present in lithium-containing solutions in undesirable concentrations, particularly in process mother liquors, as the process mother liquor may have been previously treated with excess lime to precipitate metal impurities such as calcium aluminate and calcium silicate from solution. It is common practice to subsequently deplete (or "soften") the process mother liquor of calcium by adding sodium carbonate to precipitate calcium carbonate. However, the concentration of sodium also rises as a result, making it difficult to separate the valuable potassium by-product from the solution.

Some embodiments described in the present disclosure provide alternative methods for softening lithium-containing solutions to reduce their calcium content from about 500ppm to less than 25ppm, particularly less than 20 ppm. In certain embodiments, wherein the lithium-containing solution further comprises fluoride, the method may also be advantageously used to reduce the fluoride content to less than 5ppm, particularly to reduce the fluoride content to 1-3 ppm.

In various embodiments of the present disclosure, it should be understood that the concentration of lithium in the lithium-containing solution may be greater than 1g/L, particularly greater than 4 g/L.

In some embodiments, the step of reducing the calcium content of the lithium-containing solution comprises adding potassium carbonate thereto to produce a calcium precipitate (e.g., calcium carbonate) and magnesium carbonate. Potassium carbonate may be added to the lithium-containing solution at a temperature of from ambient temperature to 90 ℃, in particular from 50 ℃ to 60 ℃, in a 20% w/w solution. The amount of potassium carbonate added to the lithium-containing solution may be sufficient to eliminate the residual calcium content in the lithium-containing solution or at least reduce the calcium content in the lithium-containing solution to less than 25ppm, in particular 20 ppm.

In embodiments where the lithium-containing solution includes fluoride, management of fluoride is important because some fluoride may be contained in the lithium phosphate produced downstream, as described below. The inventors have found that potassium phosphate can be added to the lithium-containing solution to facilitate the softening step, reducing the calcium content therein by producing calcium phosphate. Furthermore, the step of adding potassium phosphate to the lithium-containing solution also produces fluorapatite (calcium fluorophosphate, Ca)₅(PO₄)₃F) In that respect The production of fluorapatite reduces the calcium content of the lithium-containing solution to less than 25ppm and also reduces the fluoride content to less than 5ppm, in particular 1-3 ppm.

The potassium phosphate can be 100 g/L-800 g/L K₃PO₄The solution is added to the lithium-containing solution in one or more portions, in an amount greater than 100% of the stoichiometric amount (relative to the formation of fluorapatite), in particular in an amount of between 200% and 500% of the stoichiometric amount (relative to the formation of fluorapatite). Addition of concentrated K₃PO₄The solution is advantageous because it reduces dilution of the process stream, maximizes lithium phosphate precipitation downstream, and minimizes residual lithium in the lean solution. Also, most of the fluoride in the lithium-containing solution may precipitate as fluorapatite, thereby reducing the fluoride concentration to less than 5 ppm. The inventors believe that calcium fluorophosphate is the major phase produced during the initial addition of potassium phosphate, producing both calcium phosphate and hydroxyapatite.

In embodiments where the lithium-containing solution comprises fluoride, softening the solution by adding greater than 100% of the stoichiometric amount of potassium phosphate thereto provides a simple processing scheme for removing calcium and fluoride without ion exchange. It is to be understood that in embodiments where the lithium-containing solution does not contain significant amounts of fluoride, potassium phosphate may be used as an alternative softener to potassium carbonate, as described above.

It will be appreciated that in embodiments where the lithium-containing solution has a high Na content, for example in brine, the "softening" step may be carried out by adding a similar amount of sodium phosphate as described above to the lithium-containing solution, instead of potassium phosphate, to produce calcium phosphate and fluorapatite (if fluoride is present in the lithium-containing solution). Alternatively, another alkali metal phosphate may be used in the softening step.

Referring to the figures, the methods disclosed herein also include adding a phosphate to the softened lithium-containing solution to precipitate lithium phosphate (100).

The phosphate may be added in the form of an aqueous solution. The phosphate may be selected from phosphoric acid, potassium phosphate, sodium phosphate, or a combination thereof. It is understood that the concentration of the aqueous phosphate solution is actually limited by its solubility. For example, the concentration of the aqueous potassium phosphate solution may be 100g/L to 800 g/L. The phosphate may be added to the softened lithium-containing solution in stoichiometric excess to ensure that less than 100mg/L of soluble lithium remains in the solution and that more than 500mg/L, especially 500 to 3000mg/L, of P remains in the solution.

In embodiments where the phosphate solution comprises phosphoric acid, hydroxide ions (e.g., KOH) may be added simultaneously to the softened lithium-containing solution in an amount sufficient to maintain the pH of the solution above a threshold pH at which lithium phosphate may re-dissolve and increase the soluble lithium in the solution to greater than 100 mg/L.

The addition of phosphate to the softened lithium-containing solution to precipitate lithium phosphate may be carried out at a temperature of from 50 ℃ to below the boiling point of the solution, in particular greater than 90 ℃.

The lithium phosphate precipitate can be separated from the solution by conventional separation techniques and washed in several stages. Suitable separation techniques include, but are not limited to, filtration, gravity separation, centrifugation, decantation, and the like. The mother liquor and wash filtrate may be combined and may be subjected to a dephosphorylation step (160), as described below.

The separated lithium phosphate precipitate may then optionally be dried and shipped for sale. Alternatively or additionally, in some embodiments, the lithium phosphate precipitate may be subsequently treated to reprecipitate lithium phosphate, thereby reducing major impurities, such as K, Na, and S. The specificationThe treating step (105) comprises at least partially dissolving the lithium phosphate precipitate in phosphoric acid according to formulas (1) and (2) to form dilithium hydrogen phosphate (Li)₂HPO₄)。：

(1)Li₃PO₄+2H₃PO₄→3LiH₂PO₄

(2)2Li₃PO₄+H₃PO₄→3Li₂HPO₄

The inventors believe that although dilithium phosphate is the primary aqueous species and precipitates upon reaching saturation, it is thermodynamically unstable and rapidly converts to lithium phosphate, thereby regenerating phosphoric acid.

Advantageously, the refining step (105) may result in K, Na and S being reduced by at least an order of magnitude. The following table shows one particular example of reducing impurities in a test run:

	K(％w/w)	Na(％w/w)	S(％w/w)
				lithium phosphate	0.25-0.35	0.15-0.25	0.51-0.57
Reprecipitated lithium phosphate	0.008	0.005	0.027

In some embodiments, the lithium phosphate precipitate may be mixed with phosphoric acid to produce a slurry having a percent solids of 15 to 40 weight percent. Completely "dissolved" as Li relative to lithium phosphate precipitation₂HPO₄The amount of phosphoric acid required may be sub-stoichiometric. For example, the amount of phosphoric acid required may be from 50kg/t to 250kg/t based on lithium phosphate precipitation.

The step of reprecipitating lithium phosphate may be performed at ambient temperature or about 30 ℃. The dissolution and reprecipitation of lithium phosphate may be performed for 4 to 24 hours. A residence time of about 24 hours may be beneficial to maximize impurity removal at lower stoichiometric phosphoric acid additions.

The lithium recovered as reprecipitated lithium phosphate may be greater than 95%. It will be appreciated that the amount of residual lithium phosphate that may be dissolved in the liquid of the refining step may depend on the pH and solids content of the process stream. In one embodiment, the pH may be in the range of pH 4 to pH 6, in particular pH 5 to pH 5.5.

The reprecipitated lithium phosphate precipitate can be separated from the solution by conventional separation techniques and washed in several stages. Suitable separation techniques include, but are not limited to, filtration, gravity separation, centrifugation, decantation, and the like. Potassium hydroxide may then be added to the separated liquor (115) to regenerate the potassium phosphate stream. At least a portion of the potassium phosphate stream may then be recycled for use in step a) as a source of phosphate or as an alternative "softening" agent as described above.

The dried, isolated lithium phosphate can be stored and subsequently transported for sale, or used as a feedstock for other processes. Alternatively or additionally, in some embodiments, at least some of the separated lithium phosphate may be further treated with sulfuric acid to produce lithium sulfate.

Advantageously, the production of lithium sulfate may remove residual fluoride or other contaminants. In these particular embodiments, the separated lithium phosphate precipitate may be digested in sulfuric acid (110) according to formula (3):

(3)2Li₃PO₄+3H₂SO₄→3Li₂SO₄+H₃PO₄

the pH of the resulting digestion mixture may be less than 3, particularly about 1.5. The digestion mixture may contain up to 50% by weight, particularly 10-30% by weight, lithium phosphate solids in solution with 30-35g/L lithium. Surprisingly, the extent of digestion of lithium phosphate and conversion of lithium sulfate does not appear to be affected by the increase in solids content, and some crystallization of lithium sulfate may occur during digestion.

It will be appreciated that the rate of digestion will depend on the temperature and concentration of the lithium phosphate solids in the digestion mixture. Complete digestion can take place within 1h-4h (especially 1h-2h), from ambient temperature to a temperature range of 80 ℃. Typically, digestion can occur within 2 hours at 50 ℃.

Although it is understood that some lithium sulfate crystallization may be present during the digestion step, in various embodiments, the isolation of lithium sulfate may be performed by an evaporative crystallization step. The resulting digest may be concentrated (120) by evaporation or vacuum pressure to provide up to 70 wt%, particularly 25-65 wt% H₃PO₄And (4) concentration. When H is present₃PO₄At concentrations of > 60 wt.%, the resulting mixture is extremely viscous and the inventors have noted 55-60 wt.% of H₃PO₄There may be a practical upper limit where about 80% of lithium sulfate crystallization is achieved. The lithium sulfate crystallization liquid may contain at least 5% of lithium. However, this may be recovered via the production of potassium phosphate as described below and returned to the production step of lithium phosphate (100).

The lithium sulfate precipitate may be separated from the concentrated digest by conventional separation techniques (130). Suitable separation techniques include, but are not limited to, filtration, gravity separation, centrifugation, decantation, and the like.

The remaining filtrate (or supernatant) can be present in the phosphoric acid in an amount of up to 70% by weight, in particular from 25 to 65% by weight. This particular stream can then be recycled upstream as a source of phosphate to precipitate lithium phosphate. Alternatively, the remaining filtrate (or supernatant) may be neutralized (140) by adding potassium hydroxide or a mixture of potassium carbonate and potassium hydroxide. The resulting potassium phosphate solution can in turn be recycled upstream (150) as a source of phosphate to precipitate lithium phosphate from the lithium-containing solution.

In some embodiments, the filtrate and/or supernatant from which the lithium phosphate precipitate is separated may be subjected to a dephosphorylation process (160), wherein soluble phosphate remaining in the filtrate or supernatant is recovered in the form of tricalcium phosphate and/or apatite.

The dephosphorylation process (160) includes adding calcium hydroxide to the filtrate or supernatant to produce tricalcium phosphate and/or apatite precipitates. The calcium hydroxide may be selected from hydrated lime (hydrated lime), quick lime (quick lime), hydrated lime (slaked lime), and mixtures thereof.

The tricalcium phosphate and/or apatite precipitate may be separated from the resulting liquid by conventional separation techniques. Suitable separation techniques include, but are not limited to, filtration, gravity separation, centrifugation, decantation, and the like. It should be understood that in some embodiments, calcium carbonate may be co-precipitated with tricalcium phosphate and/or apatite.

Although the liquor remaining after recovery of tricalcium phosphate and/or apatite may be potassium-rich liquor, it may also contain small amounts of sodium (less than 20g/L Na). Potassium sulfate may be recovered from the liquid by concentrating (170) the liquid to promote crystallization of potassium sulfate and then separating. The liquid can be concentrated to at most 10% of its original volume by evaporating the liquid at a temperature from ambient temperature to below 120 ℃. Alternatively or additionally, the crystal growth or particle size of the potassium sulfate may be promoted by cooling the resulting concentrate to about 10 ℃.

The potassium sulfate precipitate may be separated from the concentrate by conventional separation techniques. Suitable separation techniques include, but are not limited to, filtration, gravity separation, centrifugation, decantation, and the like.

In some embodiments, where the liquid remaining after recovery of tricalcium phosphate and/or apatite is rich in potassium and also contains a higher level of sodium, then glaserite (NaK) may be possible₃(SO₄)₂) Precipitation rather than potassium sulfate precipitation. While it is generally undesirable to deposit glaserite, it may be separated and subjected to further processing to isolate potassium sulfate.

It will be appreciated by persons skilled in the art that numerous variations and/or modifications may be made to the above-described embodiments without departing from the broad general scope of the disclosure. The present embodiments are, therefore, to be considered in all respects as illustrative and not restrictive.

In the claims which follow and in the preceding description of the invention, unless the context requires otherwise due to express language or necessary implication, the word "comprise" is used in a non-exclusive sense, i.e. to specify the presence of the stated features but not to preclude the presence or addition of further features in various embodiments of the invention.