Integrated sulfate recovery and value-added by-product generation in hydrometallurgical processes

Abstract

The present disclosure relates to an improved method for the recovery of sulfate in hydrometallurgical processes. In one aspect, the process involves reacting a sulfate, such as lithium sulfate (Li2SO4) or Na2SO4 with a calcium (Ca) source, such as calcium hydroxide (Ca(OH)2) or calcium oxide (CaO) in the presence of a chelating agent. The methods described herein may be used in, for example, mining and metallurgy, precursor cathode active material (pCAM) production, and battery recycling.

Description

INTEGRATED SULFATE RECOVERY AND VALUE-ADDED BY-PRODUCTGENERATION IN HYDROMETALLURGICAL PROCESSES

[0001] This application claims the benefit of U.S. Provisional Application No.63 / 603,252 filed on November 28, 2023, the entire contents of which are hereby incorporated by reference.FIELD

[0002] The present disclosure relates to the recovery of sulfate as a value-added product from hydrometallurgical processes. The methods described herein may be used in, for example, mining and metallurgy, precursor cathode active material (pCAM) production, and battery recycling processes.BACKGROUND

[0003] Sodium sulfate (Na2SO4) in either anhydrous form or hydrated form(Na2SO4 IOH2O, Glauber’s salt) is a by-product of many hydrometallurgical processes. While useful in certain applications, Na2SO4 has limited market value. Worldwide, Na2SO4 resources are expected to be sufficient to last hundreds of years at the present rate of world consumption. Consequently, the production of this salt leads to an excess supply, resulting in significant waste disposal challenges.

[0004] For example, large quantities of Na2SO4 are generated during pCAM production and may increase further in battery recycling processes. This poses a major environmental and economic issue and has been a bottleneck to date in pCAM production and battery recycling.

[0005] One approach to help solve this issue is the conversion of sulfates into calcium sulfate dihydrate (gypsum), a material with widespread applications in construction, agriculture, and environmental management. Gypsum is not only more valuable than Glauber's salt, but also offers enhanced utility in various sectors, including drywall manufacturing and soil conditioningSUMMARY

[0006] Disclosed herein are new, cost effective and efficient process that can be used to convert sulfates into industrially useful compounds, such as gypsum. In one aspect, the process involves reacting a sulfate, such as lithium sulfate (Li2SO4) or Na2SC>4 with a calcium (Ca) source, such as calcium hydroxide (Ca(OH)2) or calcium oxide (CaO) in the presence of a chelating agent (also referred to herein interchangeably as a complexing agent).

[0007] The processes described herein not only address environmental concerns associated with traditional by-products but also opens new avenues for revenue generation through the production of gypsum, positioning it as a valuable asset in the materials landscape. Sodium hydroxide (NaOH) and / or lithium hydroxide (LiOH) may be regenerated and reused in the process. The processes described herein may be employed in various hydrometallurgical processes such as, for example, mining and metallurgy, pCAM production, and battery recycling, and provide one or more of the following advantages.(1) Lower processing costs, by regenerating NaOH and / or LiOH, and reducing disposal costs due to the elimination of formation of Glauber’s salt;(2) Lower capital costs by eliminating the need for a Glauber’s salt production circuit;(3) Providing a lower carbon footprint by eliminating the need to dispose of Glauber's salt; and(4) Production of gypsum as value-added by product. Sulfate is removed from the system as gypsum instead of generating Glauber’s salt. Gypsum has a high market demand, unlike Glauber’s salt.

[0008] In the processes described herein, calcium exists primarily as an ionic species (Ca2+) in aqueous solution, thereby enhancing its reactivity with sulfate ions (SCO2’). The present inventors have achieved this by use of chelation / complexation. By forming stable complexes, the availability of Ca ions in solution is enhanced, thereby allowing for more effective interaction with sulfate ions and promoting the formation of gypsum. Chelating agents also help maintaincalcium in a soluble form, thereby increasing the overall efficiency of the reaction and ensuring optimal conditions for gypsum precipitation.

[0009] Any of the aspects of the present disclosure described herein may be combined.

[0010] In a first aspect, the present disclosure relates to a method of converting an alkali metal sulfate to calcium sulfate, the method including: reacting an aqueous solution of the alkali metal sulfate with an aqueous solution of a Ca source in the presence of a chelating agent.

[0011] In a second aspect, the present disclosure relates to a method of converting an alkali metal sulfate generated during a hydrometallurgical process (such as, but not limited to, pCAM production) to calcium sulfate, the method including:(a) reacting the alkali metal sulfate generated during the process with an aqueous solution of a Ca source in the presence of a chelating agent to produce calcium sulfate and a basic solution including an alkali metal hydroxide.

[0012] In one aspect of the second aspect, the method further includes:(b) isolating the calcium sulfate (e.g., by filtration);(c) washing the calcium sulfate (e g., with water);(d) drying the calcium sulfate; and(e) optionally, recovering the alkali metal hydroxide and / or residual chelating agent from the basic solution.

[0013] In a third aspect, the present disclosure relates to a method of recovering sulfate produced in a hydrometallurgical process (such as, but not limited to, pCAM production) in the form of calcium sulfate, the process including:(a) providing an aqueous solution of an alkali metal sulfate (e.g., lithium sulfate, sodium sulfate);(b) providing an aqueous solution of a Ca source in the presence of a chelating agent;(c) reacting the aqueous solution of a sulfate and the aqueous solution of a Ca source in the presence of a chelating agent, to produce solid calcium sulfate and a basic solution including alkali metal hydroxide and residual chelating agent;(d) isolating the calcium sulfate by filtration;(e) washing the calcium sulfate;(f) drying the calcium sulfate; and(g) optionally, recovering the alkali metal hydroxide and / or the residual chelating agent from the basic solution.

[0014] In one aspect of any of the methods described herein, the Ca source is calcium hydroxide (Ca(OH)2), calcium oxide (CaO), or a combination thereof. In one aspect of any of the methods described herein, the alkali metal hydroxide and / or the residual chelating agent is recovered by gravity separation, centrifugation, electrodialysis, nanofiltration, and / or reverse osmosis. In one aspect of any of the methods described herein, the method is conducted at a temperature of between about 20° C and about 90° C, such as between about 30° C and about 70° C, or between about 40° C and about 60° C.

[0015] In one aspect of any of the methods described herein, the chelating agent is selected from the group consisting of diols, triols, polyols, monosaccharides, disaccharides, metaphosphates, and any combination of any of the foregoing. Suitable diols include, but are not limited to, glycols, such as, e.g., ethylene glycol, propylene glycol, 1,3 -butanediol, 1,4- butanediol, and any combination thereof. Suitable triols include, but are not limited to, glycerol.

[0016] Suitable polyols include, but are not limited to, sorbitol (e.g., D-sorbitol), mannitol (e.g., D-mannitol), galactitol (e.g., D-galactitol), glucitol, xylitol, and any combination thereof. Suitable monosaccharides include, but are not limited to, glucose, fructose, galactose, and any combination thereof. Suitable disaccharides include, but are not limited to, sucrose,lactose, maltose, and trehalose, and any combination thereof. Suitable metaphosphates include, but are not limited to, sodium trimetaphosphate, sodium hexametaphosphate, sodium-potassium hexametaphosphate, potassium metaphosphate, and any combination thereof.

[0017] In one aspect of any of the methods described herein, the concentration (aqueous concentration) of the chelating agent is between about 10 wt.% and about 90 wt.%, such as between about 20 wt.% and about 80 wt.%, between about 30 wt.% and about 70 wt.%, or between about 40 wt.% and about 60 wt.% by weight. In additional embodiments of any of the processes described herein, the concentration (aqueous concentration) of the chelating agent is about 10 wt.%, about 15 wt.%, about 20 wt.%, about 25 wt.%, about 30 wt.%, about 35 wt.%, about 40 wt.%, about 45 wt.%, about 50 wt.%, about 55 wt.% or about 60 wt.%.

[0018] In one aspect of any of the methods described herein, the chelating agent (i) forms a complex with the calcium ions (Ca2+) in the solution, (ii) enhances solubility of calcium ions (Ca21) in the solution, and / or (iii) enhances interaction of calcium ions (Ca21) in the solution with sulfate (SC>42') ions in the solution.BRIEF DESCRIPTION OF THE DRAWINGS

[0019] FIG. 1 is an exemplary process flow diagram showing a process for recovering sulfate from an effluent containing an alkali metal sulfate as anhydrous sodium sulfate, performed in accordance with current industry practice.

[0020] FIG. 2 is an exemplary process flow diagram showing a process for recovering sulfate from an effluent containing an alkali metal sulfate as calcium sulfate (gypsum), according to one aspect as described herein. It will be understood that the process shown in FIG. 2 is only illustrative and is non-limiting. Additional process steps to those shown may be added (or removed) as required.DETAILED DESCRIPTION

[0021] The conversion of Na SO4 (or I 2SO4) to gypsum can be accomplished by reacting it with, for example, calcium chloride (CaCh) (Eq. 1) or calcium nitrate (Ca(NOa)2) (Eq.2). Owing to their high solubility of about 74.5 g and 121 g in 100 mL water at 20° C, respectively, these reagents can easily react with Na2SO4 to produce calcium sulfate.

[0022] However, for practical industrial applications, use of reagents such as CaCh and Ca(NO3)2 is not desirable due to their corrosivity, associated higher operational costs, limited availability, and environmental impact. In addition, the introduction of chloride and nitrate anions to the process can present significant challenges in their removal.

[0023] The reaction between an aqueous solution of Ca(OH)2 and Na2SO4 or Li2SO4 proceeds according to Eq. 3.M = Li or Na, (aq) = aqueous, (s) = solid

[0024] However, due to its low solubility of only 0.26 g per 100 mL water, the volume of calcium hydroxide solution necessary for this conversion is extremely large, which, in turn, presents costly challenges in the treatment of the resulting effluent. To overcome this, the solid form of Ca(OH)2 may be used as the reactant, but the reaction is not straightforward. The kinetics of the reaction are slow, which is made worse by the formation of NaOH (or LiOH) which hinders the reaction from proceeding.

[0025] U.S. Patent No. 8,273,181 describes a process of removing calcium and obtaining sulfate salts from an aqueous sugar solution. Bouzouaid et.al., Cement and Concrete Research, 149(2-3), 106563, 2021, describes the solubility of portlandite in the presence of gluconate, D- sorbitol, D-mannitol and D-galactitol.

[0026] There is a need for new methods that can be used to convert sulfates into industrially useful compounds, such as gypsum, that improve upon the drawbacks discussed herein. The present disclosure addresses these needs.

[0027] As used herein the term “about” means an acceptable error for a particular value as determined by one of ordinary skill in the art, which depends in part on how the value is measured or determined. In certain aspects, the term “about” means within 1, 2, 3, or 4 standard deviations. In certain aspects, the term “about” means within 15%, 10%, 9%, 8%, 7%, 6%, 5%, 4%, 3%, 2%, 1%, 0.5%, or 0.05% of a given value or range.

[0028] As used herein, the term “comprising” (and related terms such as “comprise” or “comprises” or “having” or “including”) includes, but is not limited to, those embodiments, for example, an embodiment of any composition of matter, composition, method, or process, or the like, that “consist of’ or “consist essentially of’ the described features.

[0029] All ranges disclosed herein are inclusive of the recited endpoint and independently combinable (for example, the range of “from 2 to 10” is inclusive of the endpoints 2 and 10, and all the intermediate values). The endpoints of the ranges and any values disclosed herein are not limited to the precise range or value; they are sufficiently imprecise to include values approximating these ranges and / or values. As used herein, approximating language may be applied to modify any quantitative representation that may vary without resulting in a change in the basic function to which it is related. Accordingly, a value modified by a term or terms, such as “about,” may not be limited to the precise value specified, in some cases. In at least some instances, the approximating language may correspond to the precision of an instrument for measuring the value. The modifier “about” should also be considered as disclosing the range defined by the absolute values of the two endpoints. For example, the expression “from about 2 to about 4” also discloses the range “from 2 to 4.” The term “about” may refer to plus or minus 10% of the indicated number. For example, “about 10%” may indicate a range of “9 to 11,” and “about 1” may mean from “0.9-1.1.” Other meanings of “about” may be apparent from the context, such as rounding off, so, for example “about 1” may also mean from “0.5 to 1.4.”

[0030] When amounts, concentrations, dimensions and other parameters are expressed in the form of a range, a preferable range, an upper limit value, a lower limit value or preferable upper and limit values, it should be understood that any ranges obtainable by combining any upper limit or preferable value with any lower limit or preferable value are also specifically disclosed, irrespective of whether the obtained ranges are clearly mentioned in the context.

[0031] FIG. 1 is an exemplary process flow diagram showing a process for recovering sulfate from an effluent containing an alkali metal sulfate as anhydrous sodium sulfate, performed in accordance with current industry practice.

[0032] The current industry practice process of FIG.1 involves providing effluent containing alkali metal sulfates (10). The effluent then undergoes cooling crystallization (S100). The resulting product is then isolated by filtration and washed (S200) to produce Glauber’s salt (NazSCfi- IOH2O) (20). For a process directed to lithium battery (LiB) recycling, a Li recovery process may be performed (S300). The Glauber’s salt (20) is then dissolved (S400) in water and the solution is then subjected to evaporative crystallization (S500). The resulting solid is isolated by filtration (S600) and the sodium sulfate is dried (S700) to product anhydrous sodium sulfate (30).

[0033] FIG. 2 is an exemplary process flow diagram showing a process for recovering sulfate from an effluent containing an alkali metal sulfate as calcium sulfate (gypsum), according to one non-limiting aspect as described herein.

[0034] The non-limiting process of FIG. 2 involves providing effluent containing alkali metal sulfates (11). A solution containing a Ca source (such as Ca(OH)2) or CaO) and a chelating agent (21) is added and a solid is formed by precipitation (S101). The solid is isolated by filtration and washed (S201) to form wet gypsum (31). The filtrate, which is a basic solution containing alkali metal hydroxide and / or the residual chelating agent may be recovered and reused (S301). The wet gypsum (31) is dried (S401) to afford dry gypsum (41).EXAMPLESExample 1

[0035] The extent of dissolution of Ca(OH)2 in various solvents was experimentally determined by mixing the Ca(OH)2 with the solvent(s) in a beaker using a spin bar. The mixing was allowed to occur (residence time) for at least 1 hour. Then, the slurry was filtered using a Whatman 41 filter paper by gravity filtration to remove any undissolved Ca(OH)2. The filtratewas analyzed for Ca content using inductively coupled plasma optical emission spectroscopy (ICP-OES). The results are shown in Table 1 .Table 1 - Dissolution of Ca(0H)2

[0036] As can be seen, the solubility of Ca(OH)2 in water is limited. The addition of a chelating agent (such as those described herein) enhances the solubility and allows for higher dissolution of Ca(OH)2, thereby rendering the Ca ions readily available for the formation of CaSO4.Example 2

[0037] About 100 mb of sodium sulfate solution containing Na and S at a concentration of 78.3 g / L and 63.5 g / L, respectively, was added to about 180 mL of a Ca-sucrate solution containing 11.3 g Ca. The amount of Ca was equivalent to the stoichiometric requirement for the complete precipitation of S as CaSC . After 2 hours at a desired temperature (such as about 25° C to about 35° C), the slurry was filtered using a Whatman 41 filter paper fitted in a Buchner funnel via vacuum filtration. The filtrate was analyzed for Ca, Na and S using ICP-OES, and for free NaOH by acid-base titration. The results are shown in Table 2.Table 2 - Determination of the Extent of CaSC>4 Precipitation using 50 Wt. % Sucrose solution with Dissolved Ca

[0038] As can be seen, the precipitation of S as CaSO4 in the presence of the chelating agent sucrose is higher at lower temperatures. Without wishing to be bound by theory, the inventors theorize that this may be explained by the decomposition of sucrose into glucose and fructose at higher temperatures, of which the latter has very low reactivity with Ca.Example 3

[0039] In Example 2, a substantial amount of Ca remains unprecipitated. Owing to the alkalinity of the system and knowing that Ca(OH)2 has very poor solubility in water, it was theorized that the residual Ca was still in sucrate form. To test this theory, additional Na2SO4 was added to the solution. After 1 hour of reaction time at 250C, the slurry was filtered using a Whatman 41 filter paper fitted in a Buchner funnel via vacuum filtration. The filtrate was analyzed for Ca, Na and S using ICP-OES. The results are shown in Table 3.Table 3 - Test on Complete Reaction of Ca with Sulfate

[0040] The result of this experiment suggests that adding a sub-stoichiometric amount of Ca-sucrate based on the S content of the NazSCU solution pushes the reaction towards the complete reaction of Ca with the sulfate.

[0041] The resulting filtrate after filtering off the gypsum may be processed via known and suitable methods such as, but not limited to, gravity separation, centrifugation, electrodialysis, nanofiltration, or reverse osmosis to preferentially recover the sucrose solution and alkali hydroxide solution separately for reuse and recycling.Example 4

[0042] The effluent from a hydrometallurgical process, such as pCAM production or battery recycling, containing mainly Na and / or Li sulfates, is reacted with lime (CaO) or Ca(OH)2 at about 20° C to about 90° C, in the presence of a chelating agent (RA), to convert it to a basic solution containing NaOH and / or LiOH, and gypsum (see Eq. 4 and Eq. 5).Na2SO4+ Ca(OH)2+ 2H2O + RA CaSO4.2H2O + NaOH + Na-A + ROH (4)Li2SO4+ Ca(OH)2+ 2H2O + RA CaSO42H2O + LiOH + Li-RA + ROH (5)

[0043] The slurry is then filtered, washed, and dried to produce the gypsum by-product. Ca is then removed from the solution containing residual Ca, NaOH or LiOH, and chelating agent using a process such as, but not limited to, ion exchange and chemical precipitation, prior to separation and recovery of the chelating agent and alkali hydroxides for reuse and recycling. The chelating agent and / or alkali hydroxides are recovered by, for example, gravity separation, centrifugation, electrodialysis, nanofiltration, or reverse osmosis.

[0044] All patents and publications cited herein are incorporated by reference in their entirety.

Claims

WHAT IS CLAIMED IS:

1. A method of converting an alkali metal sulfate to calcium sulfate, the method comprising: reacting an aqueous solution of the alkali metal sulfate with an aqueous solution of a calcium source in the presence of a chelating agent.

2. The method of claim 1, further comprising:(b) isolating the calcium sulfate;(c) washing the isolated calcium sulfate;(d) drying the isolated calcium sulfate; and(e) optionally, recovering alkali metal hydroxide and / or residual chelating agent from the solution.

3. A method of converting an alkali metal sulfate generated during a hydrometallurgical process to calcium sulfate, the method comprising:(a) reacting the alkali metal sulfate with an aqueous solution of a calcium source in the presence of a chelating agent to produce calcium sulfate and a basic solution comprising an alkali metal hydroxide.

4. The method of claim 3, further comprising:(b) isolating the calcium sulfate;(c) washing the isolated calcium sulfate;(d) drying the isolated calcium sulfate; and(e) optionally, recovering the alkali metal hydroxide and / or residual chelating agent from the basic solution.

5. A method of recovering sulfate produced in a hydrometallurgical process in the form of calcium sulfate, the process comprising:(a) providing an aqueous solution of an alkali metal sulfate;(b) providing an aqueous solution of a calcium source in the presence of a chelating agent;(c) reacting the aqueous solution of a sulfate and the aqueous solution of a calcium source in the presence of a chelating agent, to produce solid calcium sulfate and a basic solution comprising alkali metal hydroxide and residual chelating agent;(d) isolating the calcium sulfate;(e) washing the calcium sulfate;(f) drying the calcium sulfate; and(g) optionally, recovering the alkali metal hydroxide and / or the residual chelating agent from the basic solution.

6. The method of any one of claims 1-5, wherein the calcium source is calcium hydroxide (Ca(0H)2), calcium oxide (CaO), or a combination thereof.

7. The method of any one of claims 1-6, wherein the process is conducted at a temperature of between about 20° C and about 90° C.

8. The method of any one of claims 1-7, wherein the chelating agent is selected from the group consisting of a diol, a triol, a polyol, a monosaccharide, a disaccharide, a metaphosphate, and any combination of any of the foregoing.

9. The method of claim 8, wherein the diol is selected from the group consisting of ethylene glycol, propylene glycol, 1,3 -butanediol, 1,4-butanediol, and any combination thereof, the triol is selected from the group consisting of glycerol, the polyol is selected from the group consisting of sorbitol, mannitol, galactitol, glucitol, xylitol, and any combination thereof, the monosaccharide is selected from the group consisting of glucose, fructose, galactose, and anycombination thereof, the disaccharide is selected from the group consisting of sucrose, lactose, maltose, trehalose, and any combination thereof, and the metaphosphate is selected from the group consisting of sodium trimetaphosphate, sodium hexametaphosphate, sodium-potassium hexametaphosphate, potassium metaphosphate, and any combination thereof .

10. The method of any one of claims 1-9, wherein the concentration of the chelating agent is between about 10 wt.% and about 90 wt. %.

11. The method of any one of claims 1-10, wherein the chelating agent (i) forms a complex with calcium ions (Ca2+) in the solution, (ii) enhances solubility of calcium ions (Ca2+) in the solution, and / or (iii) enhances interaction of calcium ions (Ca2+) in solution with sulfate (SO42) ions in the solution.

12. The method of any one of claims, 2, 4 and 5, wherein the alkali metal hydroxide and / or the residual chelating agent is recovered by gravity separation, centrifugation, electrodialysis, nanofiltration, or reverse osmosis.

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