A process for purifying a lithium salt

Abstract

A process (100) for purifying a lithium salt comprising: (a) obtaining an aqueous stream (113, 114, 115) containing a lithium salt and salts of impurity elements; (b) contacting the aqueous stream (113,14,115) containing a lithium salt and salts of impurity elements with a soluble phosphate (126a) to convert the lithium salt to lithium phosphate and the salts of impurity elements to phosphates of impurity elements; (c) precipitating lithium phosphate (127b) from step (b); and (d) separating the lithium phosphate precipitate (127b) from a solution (127a) containing said phosphates of said impurity elements.

Description

[0001] A Process for Purifying a Lithium Salt

[0002] TECHNICAL FIELD

[0003] The present invention relates to a process for purifying a lithium salt.

[0004] BACKGROUND ART

[0005] The following discussion of the background art is intended to facilitate an understanding of the present invention only. The discussion is not an acknowledgement or admission that any of the material referred to is or was part of the common general knowledge as at the priority date of the application.

[0006] Lithium salts - in particular lithium hydroxide, lithium carbonate and lithium phosphate - are currently in high demand for production of batteries to be used for electrical vehicles and other applications.

[0007] Lithium may be extracted from its ores by a range of hydrometallurgical processes to produce lithium salts. Typically, lithium is leached from a calcined material and, following a sequence of process steps, a crude lithium salt is produced. To be useful as battery grade material - in particular as battery grade lithium hydroxide monohydrate (LHM) or lithium carbonate - further purification is required. Purification typically requires formation of an aqueous solution containing both the crude lithium salt and impurities. Examples of purification processes have been disclosed as follows.

[0008] International Publication WO 2022 / 101549 (Fortum Oyj) discloses passing a lithium salt containing solution through an acid cation exchange resin. A lithium raffinate is collected and other elements are collected into an eluate.

[0009] Australian Patent Application No. 2021332926 (POSCO) discloses purification of a lithium sulphate containing solution with electrodialysis.

[0010] International Patent Publication WO 2021 / 099333 (BASF) discloses a process for the recovery of lithium from waste lithium ion batteries or parts thereof. The process comprises the steps of A) providing a crude lithium hydroxide as a solid, which contains fluoride; and B) dissolving the crude lithium hydroxide solid with a lower alcohol such as methanol or ethanol to provide separation of lithium in high purity.

[0011] It is against this background that the process of the present invention has been developed.

[0012] SUMMARY OF INVENTION

[0013] The present invention provides, in one aspect, a process for purifying a lithium salt comprising:

[0014] (a) obtaining an aqueous stream containing a lithium salt and salts of impurity elements;

[0015] (b) contacting the aqueous stream containing a lithium salt and salts of impurity elements with a soluble phosphate to convert the lithium salt to lithium phosphate and the salts of impurity elements to phosphates of impurity elements;

[0016] (c) precipitating lithium phosphate from step (b); and

[0017] (d) separating the lithium phosphate precipitate from a solution containing the phosphates of the impurity elements.

[0018] Desirably, the process further comprises step (e) degradation of the separated lithium phosphate to regenerate the soluble phosphate for use in step (b). Conveniently, such degradation comprises reacting the lithium phosphate precipitate with an acid to form phosphoric acid and a lithium salt precipitate, for example lithium sulphate. The acid is preferably selected from the group consisting of sulphuric acid and hydrochloric acid.

[0019] Preferably, substantially all impurity elements present in the aqueous stream are separated as the solution containing phosphates of impurity elements. This simplifies purification, a typical cause of complexity in lithium extraction, particularly for higher grade lithium salt feedstocks directed to step (a). The solution containing phosphates of the impurity elements, and expected to have negligible lithium content, is desirably treated in a zero liquid discharge (ZLD) unit.

[0020] The soluble phosphate for step (a) may be phosphoric acid (preferably in combination with an alkali as described below) but is desirably an alkali metal phosphate, preferably sodium phosphate or potassium phosphate, which on contacting with the aqueous stream containing lithium allows precipitation of lithium phosphate. Sodium phosphate has a natural pH of about 10.5 and may be generated in a separate unit or in situ by addition of sodium hydroxide to phosphoric acid until effective pH, most preferably about 10.5, for precipitation of lithium phosphate is achieved. The soluble phosphate may be provided in alkaline solution, conveniently in combination with an alkali, for example sodium or potassium hydroxide.

[0021] The aqueous stream may comprise a solution of a range of lithium salts, for example a lithium salt selected from the group consisting of lithium chloride, lithium sulphate, lithium carbonate, lithium hydroxide, lithium phosphate, lithium nitrate, lithium acetate, lithium chlorate, lithium formate, lithium hydrogen phosphite, lithium dihydrogen phosphate, lithium oxalate, lithium perchlorate, lithium tartrate and lithium thiocyanate. The process may be used to purify these lithium salts regardless of their form and grade though preferred grade of a lithium salt feedstock is selected from the group consisting of industrial grade, technical grade and battery grade lithium salt. The lithium salts may be chemically characterised as above, e.g. as a lithium hydroxide, but be low grade. By “low grade”, for example in the case of lithium hydroxide or lithium carbonate, is intended a grade lower than battery grade, typically 99 wt% lithium salt with the balance impurities.

[0022] Lithium salts may be obtained from a range of lithium containing material sources and the process allows treatment of a plurality of lithium salt feedstocks, whether in solid or solution form. Thus, the process advantageously allows treatment of lithium salts obtained from different sources, for example waste battery recycling, brines and lithium salts obtained from leaching of a lithium ore, lithium containing residue or concentrate. Each lithium salt feedstock - some of which may be in too low a volume to otherwise justify lithium recovery - may contain a different lithium salt, a different form (e.g solid or liquid) of the same lithium salt and / or a different grade of the same lithium salt. This provides a single integrated plant with capacity to treat lithium containing materials from a range of sources, thus reducing space requirements for lithium plants. Where a first lithium salt feedstock is in too low a volume to justify dedicated extraction, the process may enable economic recovery by bulking the first lithium salt feedstock with further lithium salt feedstock(s).

[0023] Where the lithium salt is available in solid form, an aqueous solution of the lithium salt is prepared prior to step (a).

[0024] The aqueous stream is unsaturated with lithium salt though in some embodiments has a substantial lithium concentration, for example greater than 1 g / L, greater than 2g / L, greater than 5 g / L lithium, or greater than 7g / L and below the solubility limit of lithium in the aqueous solution.

[0025] In some embodiments, the aqueous stream containing lithium and impurities may be, or may include, a bleed stream. Bleed streams allow impurities to be removed from a lithium process circuit as described in the Applicant’s co-pending International Publication No. W02024000013, the contents of which are hereby incorporated herein by reference for all purposes.

[0026] Lithium phosphate precipitate from step (b) may be directed to processing to produce a lithium salt, in particular for battery manufacture. Preferably, such processing follows degradation step (e) where the precipitate containing lithium phosphate is conveniently dissolved, as described above with an acid, to produce a lithium salt precipitate which may be further processed to a higher grade, e.g. battery grade, lithium salt such as lithium hydroxide monohydrate or lithium carbonate. The lithium salt precipitate is conveniently dissolved to form a lithium containing stream which is then treated in a neutralisation step - with an alkaline salt for example hydroxide or carbonate - to produce a precipitate of the selected lithium salt, typically in slurry form. The lithium salt can be recovered by a solid / liquid separation process such as filtration.

[0027] The lithium salt may, particularly in the case of lithium salts derived from battery waste treatment, contain magnetic particles which are an important impurity. Thus, the process may include a magnetic particle separation step. Conveniently, the magnetic particle removal step is conducted prior to or following the neutralisation step, for example on the lithium salt containing slurry. A plurality of magnetic particle separation steps may be conducted on lithium containing streams if desired. A range of processes, as apparent to the person skilled in the art of lithium salt production, may be used to produce a lithium salt feedstock. Some preferred embodiments of the invention include purification of lithium salts containing lithium produced by leaching of a lithium mineral containing material whether ore or concentrate. Such lithium may be derived from lithium containing minerals including spodumene, lepidolite, petalite, jadarite as well as lithium containing micas or clays or lacustrine lithium containing materials. Other preferred embodiments of the invention include purification of lithium salts containing lithium recovered by treatment of battery materials and including black mass. Lithium salts may, in other embodiments, be extracted from brines whether salar brines, brines produced during lithium ion battery (LIB) recycling, geothermal brines or oil field brines.

[0028] Another aspect of the invention provides a process for extracting lithium comprising the steps of:

[0029] (a) extracting lithium in a process generating an aqueous stream containing a lithium salt and salts of impurity elements;

[0030] (b) contacting the aqueous stream containing a lithium salt and salts of impurity elements with a soluble phosphate to convert the lithium salt to lithium phosphate and the salts of impurity elements to phosphates of impurity elements; and

[0031] (c) treating the precipitate containing lithium phosphate with an aqueous reagent to produce the soluble phosphate for re-use in step (a) and a purified lithium salt.

[0032] Conveniently, extracting lithium comprises leaching of a lithium containing material as described above. The lithium containing material may, particularly where in mineral form, be calcined or otherwise heat treated prior to leaching. Leaching may involve acid leaching, for example with sulphuric acid; or alkaline leaching, for example with alkaline solutions of sodium sulphate.

[0033] The process allows purification of a lithium salt derived from a number of sources. The process also allows for a plurality of lithium salt containing feedstocks to be treated in a single process with a simplified purification step, involving a regenerable phosphate reagent, providing production flexibility. The process may also be tailored to production of various lithium salts or a balance of these dependent on economic demand. The process can also advantageously be used in conjunction with treatment of battery waste for recovery of lithium and other elements.

[0034] BRIEF DESCRIPTION OF THE DRAWINGS

[0035] Further features of the process for purifying a lithium salt of the present invention are more fully described in the following description of several non-limiting embodiments thereof. This description is included solely for the purposes of exemplifying the present invention. It should not be understood as a restriction on the broad summary, disclosure or description of the invention as set out above. The description will be made with reference to the accompanying drawings in which:

[0036] Figure 1 is a flowsheet for a process for purifying a lithium salt according to one embodiment of the present invention.

[0037] DESCRIPTION OF PREFERRED EMBODIMENTS

[0038] Lithium Salt Preparation Process

[0039] Referring to Figure 1 , there is shown a process 100 for producing a lithium salt 190 suitable for applications such as electric vehicle battery production.

[0040] Process 100 is provided with a plurality of lithium salt containing feedstocks provided to block 110. In the embodiment shown, the lithium salts are 1 ) lithium sulphate, 2) lithium chloride and 3) a mixture of lithium hydroxide and lithium carbonate. It will be understood that other lithium salts or mixtures of lithium salts could be provided to block 110. Further, lithium salt feedstocks could be made available to process 100 as aqueous solutions of selected feedstocks. For purposes of exemplification only, lithium salts may be selected from the group consisting of lithium chloride, lithium sulphate, lithium carbonate, lithium hydroxide, lithium phosphate, lithium nitrate, lithium acetate, lithium chlorate, lithium formate, lithium hydrogen phosphite, lithium dihydrogen phosphate, lithium oxalate, lithium perchlorate, lithium tartrate and lithium thiocyanate. The number of lithium salts provided to embodiments of the process described herein may be selected by the processor and may be one or more. The solid lithium salts are provided to solids unloading and feed bin 111 and are conveniently separately batched through in the embodiment shown. That is, the lithium sulphate forms one batch, the lithium chloride forms another batch and the mixture of lithium hydroxide and lithium carbonate forms another batch. The salts are likewise dissolved in general purpose dissolution tank 112 and directed in one embodiment, in sequence to lithium sulphate solution storage tank 115, lithium hydroxide / carbonate solution storage tank 114 and lithium chloride solution storage tank 113. Such a sequence may be selected by the operator of process 100 and need not be followed in other embodiments.

[0041] The lithium salts are dissolved in general purpose dissolution tank 112 in water or an aqueous solution 112a. The aqueous solution 112a used for dissolution may conveniently comprise process liquors such as process washates, or process waters. Such streams may be derived from around the plant in which process 100 is conducted and may themselves contain a small quantity of lithium. The amount of aqueous phase delivered to general purpose dissolution tank 112 is determined by the pumps utilised for such delivery. The amount of aqueous phase may vary, as is known to practitioners in the field. In any event process waters are added until practical dissolution of each lithium salt feedstock is achieved.

[0042] Following dissolution, the solutions are directed to solution storage tanks 113 to 115 as described above.

[0043] Phosphate Purification

[0044] Solutions from each of solution storage tanks 113 to 115 may be delivered separately, or together, to line 121 for transfer to the phosphate purification block 120.

[0045] In the embodiment shown, a bleed stream 122 is combined with solutions from each of solution storage tanks 113 to 115. The bleed stream 122 is removed from an upstream lithium extraction process, for example as described in the Applicant’s International Publication No. W02024000013 incorporated herein by reference though not limited to such a lithium extraction process. Bleed stream 122 may contain, for example, 2.5 g / L lithium or less and may be saturated or close to saturated with salts such as, but not limited to, potassium sulphate or sodium sulphate. In stage 125, for example comprising one tank, pH is adjusted to a value determined by calculating the amount of required sodium hydroxide solution 125a, considering a pH of 10.5 is required after stoichiometric phosphoric acid addition in stage 126.

[0046] Liquor 125b from stage 125 is directed to stage 126 where phosphoric acid 126a from phosphoric acid storage 135 is added at solution temperatures above ambient, but preferably above 80 °C to precipitate lithium phosphate which is highly insoluble unlike phosphates of a range of impurity elements including sodium and potassium which may thus conveniently be separated in solution. The pH, in the tank(s) of stage 126, is controlled to above 10, but preferably to 10.5 through the previous addition of sodium hydroxide solution 125a. In other embodiments, phosphoric acid and sodium hydroxide could be added together to the lithium containing solution in stage 126. In further embodiments, a sodium phosphate solution may be used instead of phosphoric acid in combination with sodium hydroxide. Other alkalis could be used.

[0047] The lithium phosphate slurry 126b is separated by solid / liquid separation stage 127. Step 127 may conveniently involve filtration, for example using filters such as candle filters or plate and frame filters. The lithium phosphate depleted filtrate 127a is fed to a zero-liquor discharge (ZLD) bleed treatment unit (not shown) or alternative bleed treatment system. A ZLD unit produces anhydrous salts, for sale or disposal. Condensate from the ZLD is recycled to the process for washing duty or for slurrying duty.

[0048] The wet lithium phosphate filter cake 127b is, as shown in Figure 1 , directed to lithium phosphate storage 128 where it may be dried and / or combined with lithium phosphate solids 151 unloaded into lithium phosphate receival area 150. In alternative embodiments, the wet lithium phosphate filter cake 127b may be fed directly from solid / liquid separation stage 127 to lithium phosphate degradation stage 129.

[0049] Lithium phosphate degradation stage 129 involves treatment of lithium phosphate from solid / liquid separation stage 127 or lithium phosphate storage stage 128 in tank(s) with sulphuric acid 129a. Sulphuric acid for example of concentrations ranging from 10 wt% to 98 wt% acid is added in excess, for example at a 2.5% or higher stoichiometric excess of sulphuric acid. The selection of acid concentration is determined by the subsequent required solids density of the slurry in various embodiments. Treatment with sulphuric acid is at temperatures higher than 20 °C, preferably 40 °C or higher, to produce a slurry 129b consisting of lithium sulphate, unreacted sulphuric acid and phosphoric acid. The reaction scheme, which allows both lithium recovery and regeneration of phosphoric acid, is as follows:

[0050] The total residence time in the tank(s) of lithium degradation stage 129 is more than 30 min, preferably 80 min, to complete the required process cycles, including precipitation of lithium sulphate and cooling of the slurry 129b to enable filtration.

[0051] In filtration stage 130, slurry 129b is filtered to produce a lithium sulphate filter cake 130b and a combined phosphoric acid and sulphuric acid liquor 130a. Filtration stage 130 may involve using filters such as a vacuum filter, candle filter, or plate and frame filter. The lithium sulphate filter cake 130b may be subjected to a limited wash to displace remnant phosphoric acid and the wash may thus form part of slurry filtrate 130a.

[0052] The filtrate from the wash may be returned together with the slurry filtrate 130a to phosphoric acid storage 135. If required, make up phosphoric acid 136 is directed to phosphoric acid storage 135.

[0053] Lithium sulphate filter cake 130b from the lithium phosphate degradation stage 130 may , in one embodiment, be recovered as a product, dried, packed and sold, in another embodiment it may be introduced into a conventional lithium extraction plant, or in another embodiment be recycled to process 120. In the preferred embodiment it is dissolved in the tank(s) of the lithium sulphate dissolution stage 131 using process waters 131a sourced from elsewhere within the plant for conducting process 100. Such process waters 131a may include condensate, washates or bleeds.

[0054] The generated lithium sulphate solution 131 b is, advantageously, passed through a magnetic particle separation mechanism, such as in-stream magnetic pots, to remove magnetic particles. Such process may also be described as magnetic filtration.

[0055] Following magnetic particle separation, lithium sulphate solution 131 b is fed slowly to a lithium carbonate crystallisation stage 132. Lithium carbonate crystallisation stage 132 involves contacting lithium sulphate solution 131 b with soda ash (sodium carbonate) 132 for neutralisation. In one embodiment, tank(s) of lithium carbonate crystallisation stage contain a charge of dissolved soda ash at a suitable concentration to precipitate lithium carbonate, for example a concentration of about 300 g / L soda ash. The reaction produces lithium carbonate crystals. The temperature of the lithium carbonate production reaction is higher than ambient and the residence time ranges from 60 min to 180 min, with 90 min being preferred.

[0056] The obtained lithium carbonate slurry 132b may undergo a first or second magnetic particle separation step before being separated in lithium carbonate filtration stage 133. Filtration of lithium carbonate may involve a filter such as a centrifuge and the obtained wet cake is dried in any suitable dryer, such as a rotary dryer, and the dry lithium carbonate product 190 is packed for sale. In an alternative embodiment, lithium carbonate wet cake, or dry product 190, may be returned to process 120.

[0057] The magnetic particle separation step(s) described above enable production of battery grade lithium salts with exceptionally low levels (< 20ppb) of magnetic particles, below a typical specification for lithium salts of 100ppb maximum magnetic particles.

[0058] The filtrate 133a from lithium carbonate filtration stage 133 is returned to lithium bleed storage 123 which also collects a bleed stream 123 from a lithium extraction process, for example as described in the Applicant’s International Publication No. W02024000013 incorporated herein by reference. In alternative embodiments, filtrate 133 may be returned to the lithium salt solution storage tanks 113 to 115.

[0059] Although a lithium carbonate product 190 is described above, it will be understood that lithium hydroxide monohydrate could be reduced by use of a hydroxide for neutralisation. In other embodiments, other lithium salt products could be produced by processing as known in the art of lithium battery materials though use of lithium salts for other purposes is also possible.

[0060] Those skilled in the art will appreciate that the process for purifying a lithium salt as described herein is susceptible to variations and modifications other than those specifically described. The invention includes all such variation and modifications. The invention also includes all of the steps and features referred to or indicated in the specification, individually or collectively and any and all combinations or any two or more of the steps or features. In particular, the process allows recovery of lithium from a bleed stream from a lithium extraction process allowing losses inherent in previous processes to be reduced or even eliminated. Throughout this specification, unless the context requires otherwise, the word "comprise" or variations such as "comprises" or "comprising", will be 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. The process of the present invention described herein may include one or more range of values (e.g. temperature, time, pressure etc). A range of values will be understood to include all values within the range, including the values defining the range, and values adjacent to the range which lead to the same or substantially the same outcome as the values immediately adjacent to that value which defines the boundary to the range. The process of the present invention is not to be limited in scope by any of the specific embodiments described herein. These embodiments are intended for the purpose of exemplification only. Functionally equivalent methods are clearly within the scope of the invention as described herein.

Claims

CLAIMS1 . A process for purifying a lithium salt comprising:(a) obtaining an aqueous stream containing a lithium salt and salts of impurity elements;(b) contacting the aqueous stream containing a lithium salt and salts of impurity elements with a soluble phosphate to convert the lithium salt to lithium phosphate and the salts of impurity elements to phosphates of impurity elements;(c) precipitating lithium phosphate from step (b); and(d) separating the lithium phosphate precipitate from a solution containing said phosphates of said impurity elements.

2. The process of claim 1 , further comprising step (e) degradation of said separated lithium phosphate to regenerate said soluble phosphate for use in step (b).

3. The process of claim 2, wherein separated lithium phosphate is reacted with an acid to form phosphoric acid and a lithium salt precipitate.

4. The process of claim 3, wherein said acid is selected from the group consisting of sulphuric acid and hydrochloric acid.

5. The process of any one of the preceding claims, wherein substantially all impurity elements present in the aqueous stream are separated as the solution containing phosphates of impurity elements.

6. The process of claim 5, wherein the solution containing phosphates is treated in a zero liquid discharge (ZLD) unit.

7. The process of any one of the preceding claims, wherein the soluble phosphate is phosphoric acid.

8. The process of any one of claims 1 to 6, wherein the soluble phosphate is an alkali metal phosphate.

9. The process of claim 8, wherein the soluble phosphate is provided in alkaline solution, optionally by a combination of phosphoric acid and alkali metal phosphate.

10. The process of claim 8 or 9, wherein said alkali metal phosphate is sodium phosphate or potassium phosphate.11 . The process of any one of the preceding claims, wherein the lithium salt is selected from the group consisting of industrial grade, technical grade and battery grade lithium salt.

12. The process of any one of the preceding claims, wherein said lithium salt is selected from the group consisting of lithium chloride, lithium sulphate, lithium carbonate, lithium hydroxide, lithium phosphate, lithium nitrate, lithium acetate, lithium chlorate, lithium formate, lithium hydrogen phosphite, lithium dihydrogen phosphate, lithium oxalate, lithium perchlorate, lithium tartrate and lithium thiocyanate.

13. The process of any one of the preceding claims, wherein said lithium salt is derived from a source selected from the group consisting of waste battery recycling, leaching of a lithium ore, lithium containing residue or concentrate, and a brine.

14. The process of any one of the preceding claims, wherein said lithium salt is a first lithium feedstock bulked with at least one further lithium salt feedstock.

15. The process of any one of the preceding claims, wherein lithium phosphate precipitate from step (b) is directed to processing, preferably to produce a battery grade lithium salt.

16. The process of claim 3 or 4, wherein the lithium salt precipitate is directed to processing, preferably to produce a battery grade lithium salt.

17. The process of claim 16, wherein said lithium salt precipitate is dissolved to form a lithium salt containing stream, said lithium salt containing stream being treated in a neutralisation step to produce a slurry of the selected lithium salt.

18. The process of claim 17, wherein said neutralisation step comprises treatment with a hydroxide or carbonate to respectively produce lithium hydroxide monohydrate or lithium carbonate.

19. The process of claim 17 or 18, wherein a magnetic particle removal step is conducted to remove magnetic particles from a lithium salt containing stream, preferably at least prior to neutralisation.

20. The process of any one of the preceding claims, comprising a plurality of magnetic particle removal steps.

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