Sulfide recycling in manganese production

Abstract

A method of producing manganese metal or EMD by leaching a source of manganese with a solution comprising sulfuric acid to form a leach solution, adding one or more sulfides generated in a sulfide recycle stage to the leach solution in order to form sulfide precipitates comprising heavy metal sulfides, removing the sulfide precipitates from the leach solution, feeding the leach solution to one or more electrolytic cells, subjecting the purified leach solution to electrolysis so as to deposit manganese metal or EMD, reacting the sulfide precipitates with an acid to generate H.sub.2S, producing one or more sulfides from the H.sub.2S for recycle. Methods of producing manganese metal and a purified manganese sulfate solution are also provided.

Claims

1. A method of producing manganese metal or EMD, comprising: (a) leaching a source of manganese with a solution comprising sulfuric acid to form a leach solution; (b) adding the one or more sulfides produced in step (g) to said leach solution to form sulfide precipitates comprising one or more heavy metal sulfides; (c) removing said sulfide precipitates from said leach solution so as to provide a purified leach solution; (d) feeding said purified leach solution to one or more electrolytic cells; (e) subjecting the purified leach solution to electrolysis so as to deposit manganese metal or EMD; (f) reacting said sulfide precipitates with an acid to generate H.sub.2S; (g) producing one or more sulfides from said H.sub.2S; and (h) recycling said one or more sulfides produced in step (g) to step (b).

2. The method of claim 1, wherein producing said one or more sulfides from said H.sub.2S comprises reacting said H.sub.2S with a hydroxide solution or an Mn.sup.2+ solution.

3. The method of claim 2, wherein the step of reacting said H.sub.2S with a hydroxide solution or an Mn.sup.2+ solution forms at least one first sulfide, and wherein producing said one or more sulfides from said H.sub.2S further comprises the step of generating said one or more sulfides from said at least one first sulfide.

4. The method of claim 3, wherein said step of generating said one or more sulfides from said at least one first sulfide comprises reacting said at least one first sulfide with ammonium sulfate to generate (NH.sub.4)HS that is recycled back to step (b).

5. The method of claim 4, wherein in step (g) the H.sub.2S is reacted with a solution comprising Ca(OH).sub.2 and said at least one first sulfide comprises CaS.

6. The method of claim 1, wherein said sulfuric acid solution in step (a) comprises electrolyte solution removed from said one or more electrolytic cells.

7. The method of claim 2, wherein the method comprises the production of manganese metal and said electrolyte solution comprises anolyte removed from said one or more electrolytic cells.

8. The method of claim 1, wherein said one or more heavy metal sulfides comprise one or more sulfides of Ni, Co, Cu, Zn, Pb, Mo, Sb, As and Bi.

9. The method of claim 1, wherein in step (f) the sulfide precipitates are reacted with H.sub.2SO.sub.4.

10. The method of claim 2, wherein in step (g) the H.sub.2S is reacted with an Mn.sup.2+ solution in order to produce MnS.

11. The method of claim 2, wherein in step (g) the H.sub.2S is reacted with a solution comprising one or more hydroxides chosen from the group consisting of alkali hydroxide, alkaline earth hydroxide and ammonium hydroxide, and the sulfide precipitates formed in step (b) further comprise MnS.

12. The method of claim 1, wherein said one or more sulfides recycled in step (g) comprises (NH.sub.4)HS.

13. The method of claim 1, wherein in step (f) the sulfide precipitates are reacted with acid in an aqueous reaction solution, and further comprising the step of stripping said H.sub.2S from the aqueous reaction solution with air or steam.

14. The method of claim 2, wherein reacting said H.sub.2S with a hydroxide solution or an Mn.sup.2+ solution comprises absorbing the H.sub.2S in said hydroxide solution or said Mn.sup.2+ solution.

15. The method of claim 2, wherein step (g) comprises contacting the H.sub.2S gas and said hydroxide or Mn.sup.2+ solution in a tray column or a packed column.

16. The method of claim 2, wherein step (g) comprises bubbling the H.sub.2S gas into an agitated tank containing said hydroxide or Mn.sup.2+ solution.

17. A method of producing manganese metal: (a) leaching a source of manganese with a solution comprising sulfuric acid to form a leach solution; (b) adding metal sulfite solids generated in step (g) to said leach solution to form sulfide precipitates comprising one or more metal sulfides chosen from the group consisting of sulfides of Ni, Co, Cu, Zn, Pb, Mo, Sb, As and Bi; (c) removing said sulfide precipitates from said leach solution so as to provide a purified leach solution; (d) feeding said purified leach solution to one or more electrolytic cells; (e) subjecting the purified leach solution to electrolysis so as to deposit manganese metal on one or more cathodes of said one or more electrolytic cells; (f) reacting said sulfide precipitates removed from the leach solution in step (c) with an acid to generate H.sub.2S; (g) reacting said H.sub.2S generated in step (f) with an Mn.sup.2+ solution comprising at least one of: (a) a portion of said purified leach solution; and (b) catholyte withdrawn from said one or more electrolytic cells in order to produce metal sulfide solids comprising greater than 90% by weight MnS based on the total metal sulfide solids; and (h) recycling the metal sulfide solids produced in step (g) to step (b).

18. The method of claim 17, wherein the sulfide added to the mixed metal sulfate solution is chosen from the group consisting of MnS, Na.sub.2S and NaHS.

19. The method of claim 17, wherein the metal sulfide solids produced in step (g) comprises greater than 99.9% by weight MnS based on the total sulfide solids.

20. A method of producing a purified manganese sulfate solution, comprising: (a) leaching a source of manganese with a solution comprising sulfuric acid to form a leach solution; (b) adding the one or more sulfides produced in steps (g) and/or (j) to said leach solution to form sulfide precipitates comprising one or more heavy metal sulfides; (c) removing said sulfide precipitates from said leach solution so as to provide a purified leach solution; (d) adding the one or more sulfides produced in steps (g) and/or (j) to said purified leach solution to form MnS; (e) removing said MnS from said purified leach solution; (f) reacting said MnS sulfide precipitates removed in step (c) with an acid to generate H.sub.2S; (g) producing one or more sulfides from the H.sub.2S generated in step (f); (h) recycling said one or more sulfides produced in step (g) to step (b) and/or (d); (i) reacting said MnS removed in step (e) with an acid to generate H.sub.2S; (j) producing one or more sulfides from the H.sub.2S generated in step (i); and (k) recycling said one or more sulfides produced in step (j) to step (b) and/or (d).

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) While the specification concludes with claims particularly pointing out and distinctly claiming the invention, it is believed that the invention will be better understood from the detailed description of certain embodiments thereof when read in conjunction with the accompanying drawings. Unless the context indicates otherwise, like numerals are used in the drawings to identify similar elements in the drawings. In addition, some of the figures may have been simplified by the omission of certain elements in order to more clearly show other elements. Such omissions are not necessarily indicative of the presence or absence of particular elements in any of the exemplary embodiments, except as may be explicitly stated in the corresponding detailed description.

(2) FIG. 1 depicts a schematic illustration of one embodiment of a manganese production process.

(3) FIG. 2 depicts a schematic illustration of another embodiment of a manganese production process.

(4) FIG. 3 depicts a schematic illustration of one embodiment of a process for producing a purified manganese sulfate solution.

(5) FIG. 4 depicts a schematic illustration of an alternative embodiment similar to FIG. 1, wherein a two step sulfide recycle stage is employed.

(6) The drawings are intended to illustrate rather than limit the scope of the present invention. Embodiments of the present invention may be carried out in ways not necessarily depicted in the drawings. Thus, the drawings are intended to merely aid in the explanation of the invention. Thus, the present invention is not limited to the precise arrangements shown in the drawings.

DETAILED DESCRIPTION

(7) The following detailed description describes examples of embodiments of the invention solely for the purpose of enabling one of ordinary skill in the relevant art to make and use the invention. As such, the detailed description and illustration of these embodiments are purely illustrative in nature and are in no way intended to limit the scope of the invention, or its protection, in any manner. It should also be understood that the drawings are not to scale and in certain instances details have been omitted, which are not necessary for an understanding of the present invention.

(8) Embodiments of the present disclosure provide a method of recovering and recycling sulfide from a heavy metal sulfide waste, wherein the sulfide is recovered as MnS that is then recycled back to a production process. A waste stream comprising a slurry of heavy metal sulfides is reacted with an acid in order to generate H.sub.2S. The H.sub.2S is then reacted with an Mn.sup.2+ solution to produce MnS that is recovered (e.g., by filtration) and then recycled back to a production process (e.g., a process for producing manganese or EMD). By way of example, following H.sub.2S generation, the H.sub.2S is removed and absorbed in a solution containing Mn.sup.2+. In some instances, the heavy metal sulfide waste stream is produced during the step of purifying a manganese-containing solution during the electrolytic production of manganese or EMD, and the recovered MnS is recycled back for use in this same purification step. The manganese-containing solution being purified contains manganese (e.g., MnSO.sub.4) as well as a plurality of other heavy metals. The acid reacted with the slurry of heavy metal sulfides comprises, for example, sulfuric acid.

(9) By way of one specific example, the heavy metal sulfide waste stream is produced during the pre-electrolysis purification of a leach solution in a process for the production of manganese metal or EMD, and the Mn.sup.2+ solution reacted with the generated H2S comprises electrolytic cell feed or electrolyte solution (e.g., catholyte) extracted from the electrolytic cell. By using cell feed or electrolyte solution extracted from the electrolytic cell, high purity MnS (>90%, >95%, >99%, >99.5%, >99.9%, >99.95%, >99.99%, or even >99.995% purity) can be produced and recycled back for use in purifying a manganese-containing solution. Applicant has discovered that the use of high purity recycled MnS for the pre-electrolysis purification of the leach solution (rather than adding conventionally used sulfides such as ammonium sulfide, alkali metal sulfide or alkaline earth metal sulfide) in order to provide the cell feed significantly reduces the amount of Mn in the heavy metal sulfide waste stream. In addition, this avoids the need to purchase other sulfides for purification (or significantly reduces the amount needed), and reduces the total amount of solid waste that is produced.

(10) Other embodiments of the present disclosure provide a method of recovering and recycling sulfide from a heavy metal sulfide waste stream, wherein the sulfide is recovered as one or more alkali metal sulfides, alkaline earth metal sulfides and/or ammonium sulfide (as (NH.sub.4)HS and/or (NH.sub.4).sub.2S) that are recycled back to a production process. A waste stream comprising a slurry of heavy metal sulfides is reacted with an acid in order to generate H.sub.2S. The H.sub.2S is then reacted with a solution, suspension or slurry containing one or more alkali, alkaline earth, or ammonium hydroxides and/or ammonia gas in order to produce the corresponding sulfide(s). These sulfides are then recycled back to a production process (e.g., a process for producing manganese or EMD). Alternatively, a two-step sulfide recycle stage is employed wherein the H.sub.2S is reacted with a solution, suspension or slurry containing one or more alkali or alkaline earth hydroxides to form the corresponding sulfide(s), and that sulfide(s) is then converted into another sulfide(s) (e.g., an ammonium sulfide) prior to being recycled back to the production process.

(11) By way of example, following H.sub.2S generation, the H.sub.2S is removed and absorbed in a solution, suspension or slurry containing one or more alkali, alkaline earth, or ammonium hydroxides. In some instances, the heavy metal sulfide waste stream is produced during the step of purifying a manganese-containing solution (e.g., pre-electrolysis purification of a leach solution) during the production of manganese or EMD, and the recovered sulfide is recycled back for use in this same purification step. The acid reacted with the slurry of heavy metal sulfides comprises, for example, sulfuric acid, and reaction of the slurry of heavy metal sulfides with H.sub.2SO.sub.4 generates not only H.sub.2S but also a metal sulfate solution. In some embodiments, the hydroxide(s) in the absorption solution, suspension or slurry comprise one or more of LiOH, NaOH, and KOH. In other embodiments, the hydroxide(s) in the absorption solution, suspension or slurry comprise one or more of Mg(OH).sub.2, Ca(OH).sub.2, and Ba(OH).sub.2. In still further embodiments, the hydroxide(s) in the absorption solution, suspension or slurry comprise one or more alkali or alkaline earth hydroxides (e.g., Mg(OH).sub.2, Ca(OH).sub.2, and/or Ba(OH).sub.2) such that the corresponding sulfide(s) is formed, which, in a second step, is reacted with ammonium sulfate ((NH.sub.4).sub.2SO.sub.4) to form an ammonium sulfide ((NH.sub.4)HS and/or (NH.sub.4).sub.2S) solution that is recycled back to the purification step. Alternatively, the H.sub.2S is absorbed into a solution containing ammonia or ammonium ion to form an ammonium sulfide solution. By way of one specific example, the heavy metal sulfide waste stream is produced during the pre-electrolysis purification of a leach solution in a process for the production of manganese metal or EMD.

(12) In the above-described embodiments for the production of manganese metal or EMD, when sulfuric acid is reacted with the slurry of heavy metal sulfides to generate the H.sub.2S, a mixed metal sulfate solution is also generated. In some embodiments sulfide is added to the mixed metal sulfate solution in order to convert at least a portion of the metal sulfates into their corresponding metal sulfides.

(13) Embodiments described herein also include methods for electrolytically producing manganese metal or EMD wherein sulfide removed from the leach solution prior to electrolysis (i.e., as mixed metal sulfides) is recovered and recycled back to the pre-electrolysis purification step rather than being discarded (e.g., as mixed sulfide solids filter cake). In addition, heavy metals such as Ni, Co, Cu, Zn, Pb, Mo, Sb, As and Bi (hereinafter, “Secondary Metals”) can be recovered. It will be understood that not all of these Secondary Metals are necessarily present in the process, depending, in part, on the Mn-containing starting material.

(14) The mixed metal sulfide solids removed from the leach solution in the pre-electrolysis purification step are reacted with an acid (e.g., H.sub.2SO.sub.4) to generate H.sub.2S on site. When the acid used is H.sub.2SO.sub.4, the metals of the mixed sulfide solids (i.e., Mn and one or more of the Secondary Metals) form mixed metal sulfates that remain in solution, and the metals can be recovered therefrom in one or more subsequent steps (as further described herein). The H.sub.2S, generated by reacting the mixed metal sulfides with acid is the vehicle used to recycle sulfide back to the pre-electrolysis purification step.

(15) In particular, the generated H.sub.2S is reacted in order to generate sulfide(s) that is recycled back to the pre-electrolysis purification step. In some embodiments, the generated H.sub.2S is reacted with a solution containing Mn.sup.2+ ions (e.g., a solution containing MnSO.sub.4) in order to generate MnS that is then recycled back to the pre-electrolysis purification step. In one particular embodiment, the Mn.sup.2+ containing solution reacted with the H.sub.2S comprises electrolysis cell feed and/or catholyte extracted from the electrolysis cell. Since cell feed and catholyte have been purified to remove Secondary Metals, relatively pure MnS can be generated in this manner. For example, the MnS recycled back to the pre-electrolysis purification step is generally pink/orange in color, indicating that minimal amounts of Secondary Metal sulfides are present. Thus, the recycled sulfide in these embodiments is primarily MnS, with less than 10%, less than 5%, less than 1%, less than 0.5%, less than 0.1%, less than 0.05%, less than 0.01%, or even less than 0.005% by weight Secondary Metal sulfides (based on the total sulfide solids present in the recycle stream).

(16) In still further embodiments for electrolytically producing manganese metal or EMD wherein sulfide removed from the leach solution prior to electrolysis, the generated H.sub.2S is reacted with one or more alkali, alkaline earth or ammonium hydroxides and/or ammonia gas in order to generate the corresponding alkali metal sulfide(s), alkaline earth metal sulfide(s) and/or ammonium sulfide. The sulfide(s) is then recycled back to the pre-electrolysis purification step.

(17) Yet another embodiment of the present disclosure provides a method of purifying an MnSO.sub.4 solution containing one or more heavy metal impurities chosen from the group consisting of Ni, Co, Cu, Zn, Pb, Mo, Sb, As and Bi. This method comprises reacting the MnSO.sub.4 solution with MnS, without adding any additional sulfides, such that the heavy metal impurities form their respective sulfide precipitates (NiS, CoS, etc.). Applicant has discovered that by reacting the MnSO.sub.4 solution with high purity MnS (>90%, >95%, >99%, >99.5%, >99.9%, >99.95%, >99.99%, or even >99.995% purity), the heavy metal impurities are precipitated as their respective sulfides while the Mn remains in solution (as soluble MnSO.sub.4). The MnS used to extract the heavy metal impurities comprises high purity MnS containing less than 10%, less than 5%, less than 1%, less than 0.5%, less than 0.1%, less than 0.05%, less than 0.01%, or less than 0.005% by weight of other metal sulfides (based on total sulfide solids).

(18) FIGS. 1 and 2 and their discussion below describe the production of manganese metal (or, alternatively, EMD) using a source of manganese. By way of example, as is well-known to those skilled in the art, naturally-occurring manganese-containing material is reduction roasted prior to leaching in order to reduce higher oxides of manganese (e.g., MnO.sub.2, Mn.sub.2O.sub.3 and Mn.sub.3O.sub.4) to manganese oxide (MnO) (i.e., reduced Mn ore). It will be understood, however, that other sources of manganese may be employed as feedstock, including sources of MnO that do not require reduction, as well as Mn.sub.3O.sub.4-containing and/or manganese carbonate-containing materials. For example, the processes described in U.S. Pat. No. 5,932,086 and PCT Pub. No. WO 99/14403—sintering manganese ore in order to convert MnO.sub.2 to Mn.sub.3O.sub.4, then adding a reducing agent (e.g., sulfur dioxide, activated carbon, hydrogen peroxide, hydrogen sulfide, reducing sugars and/or molasses) to the leach solution—can be used to provide the manganese-containing material used in the processes described herein.

(19) For producing manganese, an MnSO.sub.4 leach solution, following purification to remove certain impurities, is added to the cathode side of an electrolysis cell(s). One or more electrolytic cells are employed, each having an anolyte chamber and a catholyte chamber, typically separated by a membrane or diaphragm. While operating conditions can vary, cell temperature is typically around 30 to 40° C. and the pH on the cathode side of the cell is typically about 7 to 9. Ammonium sulfate ((NH.sub.4).sub.2SO.sub.4) is also typically present in the cell, and acts as a buffer on the cathode side for maintaining the proper pH. The membrane/diaphragm ensures that the catholyte pH is significantly higher than the acidic pH of the anolyte, since acid (H.sub.2SO.sub.4) (along with water) is generated at the anode. When an electrical potential is applied between the cathode(s) and anode(s), pure manganese metal is deposited onto the cathode(s), from which it can be recovered by conventional means known to those skilled in the art.

(20) For producing EMD, a divided cell is not necessary since EMD is produced under acidic conditions. While operating conditions can once again vary, cell temperature for the production of EMD is typically around 90 to 100° C. and the pH throughout the cell is highly acidic (e.g., less than 2). Ammonium sulfate is also not needed under these operating conditions. When an electrical potential is applied between the cathode(s) and anode(s), EMD is deposited onto the anode(s), from which it can be recovered by conventional means known to those skilled in the art. Acid is also generated at the anode.

(21) FIG. 1 is a schematic representation of one embodiment of a process for producing manganese according to the present disclosure, wherein mixed metal sulfide solids (MnS+Secondary Metal sulfides) removed from the leach solution (e.g., by filtration) in a pre-electrolysis purification step (14) are reacted with H.sub.2SO.sub.4 to generate H.sub.2S. The H.sub.2S is then reacted with one or more alkali hydroxides, alkaline earth hydroxides or ammonium hydroxide and/or ammonia gas to generate the corresponding alkali metal sulfide(s), alkaline earth metal sulfide(s) and/or ammonium sulfide that is returned (i.e., recycled) to the pre-electrolysis purification step.

(22) Alternatively, as depicted in FIG. 4, the H.sub.2S is reacted with one or more alkali hydroxides or alkaline earth hydroxides (e.g., Ca(OH).sub.2) to form a corresponding first sulfide(s) (e.g., CaS), and that first sulfide is then reacted to form another sulfide(s) (e.g., ammonium sulfide) that is returned (i.e., recycled) to the pre-electrolysis purification step. By way of example, the H.sub.2S is reacted with Ca(OH).sub.2 to form CaS, which is then reacted (e.g., in a separate reactor) with an ammonium sulfate ((NH.sub.4).sub.2SO.sub.4) solution to form an ammonium sulfide ((NH.sub.4)HS and/or (NH.sub.4).sub.2S) solution that is recycled back to the pre-electrolysis purification step. The nature of the ammonium sulfide formed ((NH.sub.4)HS and/or (NH.sub.4).sub.2S) will depend, in part, on the acidity of the reaction solution when the first sulfide is reacted with ammonium sulfate to form ammonium sulfide.

(23) It will be understood that various conventional processing steps are not depicted in FIG. 1. In addition, the process of FIG. 1 can be modified in order to produced EMD rather than manganese, as described above.

(24) In leaching step (10), a source of manganese such as reduced manganese ore, primarily comprising MnO, is leached with a sulfuric acid solution in order to convert the MnO (or other manganese source) to manganese (II) sulfate (MnSO.sub.4). The sulfuric acid solution used for leaching comprises spent electrolyte solution, i.e., anolyte, withdrawn from the electrolysis cell(s). In addition to H.sub.2SO.sub.4, the spent electrolyte solution also contains MnSO.sub.4, and (NH4).sub.2SO.sub.4. (In the production of EMD, ammonium sulfate is not present in the electrolyte solution withdrawn from the cell(s) for leaching.) The Mn ore and sulfuric acid solution are combined in a suitable vessel, such as an open stirred tank. Of course, other types of conventional equipment can be employed for this purpose. Additional sulfuric acid and (NH4).sub.2SO.sub.4 are periodically added to the process, as needed, typically by an addition to the leach tank.

(25) The reduced Mn ore (or other feedstock) not only contains MnO (or other manganese source), but also one or more impurities such as Fe, Al, Si, as well as some or all of the Secondary Metals (Ni, Co, Cu, Zn, Pb, Mo, Sb, As and Bi). These impurities are removed prior to electrolysis. First, iron, aluminum and silica are removed from the leach solution by increasing the pH of the leach solution and adding an oxidizing agent. For example, NH.sub.3, lime and/or MnO is added to the leach solution in order to increase the pH (from about 3 or less) to about 4 to 9, about 4 to 7, or about 6 to 7. Suitable oxidizing agents include, for example, MnO.sub.2, oxygen (typically as air), O.sub.3 or H.sub.2O.sub.2. MnO.sub.2 and/or air are typically used for this purpose for cost savings. When used, air is bubbled into a vessel containing the leach solution. Following the pH adjustment and the addition of MnO.sub.2 and/or air as oxidizing agents, iron, aluminum and silica will precipitate from the leach solution and are removed by filtration in step (12) (or by other conventional means for removing solids).

(26) Following removal of Fe, Al and Si in step 12, the leach solution is subjected to pre-electrolysis purification step (14) in order to remove heavy metal impurities, i.e., the Secondary Metals. In this step, one or more sulfides are added to the leach solution, causing the heavy metals to be converted into their respective insoluble sulfides. In particular, an aqueous sulfide solution comprising one or more alkali metal, alkaline earth metal and/or ammonium sulfides is added to the leach solution, wherein the sulfides are obtained from the sulfide recovery loop described below. Suitable sulfides include, for example, Li.sub.2S, Na.sub.2S, NaHS, K.sub.2S, KHS, MgS, CaS, BaS and/or NH.sub.4HS, and their concentration in the sulfide solution used in step (14) preferably does not exceed their respective solubility limits (i.e., they are preferably in solution). The leach solution and sulfide solution are combined in a suitable vessel, such as an open stirred tank; however, other types of conventional equipment can be employed for this purpose.

(27) Following the addition of the sulfide solution, the heavy metal impurities are converted from their sulfates into their respective insoluble sulfides (e.g., NiS, CoS, etc.). A portion of the MnSO.sub.4 is also converted into insoluble MnS. The resulting sulfide precipitates are removed from the leach solution by filtration step (16) (or by other conventional means for removing solids), resulting in mixed sulfide solids (“MS.sub.x”) comprising sulfides of Mn, as well as sulfides of some or all of the Secondary Metals. (It will be understood that “MS.sub.x” is intended to refer generally to the various sulfides of these metals, rather than a precise chemical formula.) In general, the mixed sulfide solids resulting from step (16) in FIG. 1 comprise about 50 to about 95% MnS by weight (on a dry basis), along with varying amounts of other heavy metal sulfides depending on, among other things, the impurities present in the Mn-containing feedstock.

(28) Following removal of the mixed sulfide solids (e.g., as a slurry), the purified leach solution is the cell feed for electrolysis step (20). The level of impurities remaining in the cell feed will vary depending on the feedstock and the purification parameters (e.g., amount of sulfide solution added). For example, the level of heavy metals (Fe and the Secondary Metals) can be less than about 5 mg/L, or even less than about 1 mg/L. The cell feed is introduced into the cathode side of the electrolysis cell(s), thereby supplying Mn.sup.2+ to the catholyte (the solution on the cathode side of the electrolysis cell(s)). In some embodiments the cell feed will generally comprise less than 1 mg/L of heavy metal impurities (Fe and the Secondary Metals) and at least about 30 g/L Mn.sup.2+. One or more electrolytic cells are employed, each having an anolyte chamber (22) and a catholyte chamber (24), typically separated by a diaphragm or membrane such as a cloth membrane. When an electrical potential is applied between the cathode(s) and anode(s), pure manganese metal is deposited onto the cathode, from which it can be recovered by conventional means known to those skilled in the art.

(29) The sulfide recovery loop in the embodiment of FIG. 1 comprises an H.sub.2S generation stage (30) and a sulfide recycle stage (32). In the H.sub.2S generation stage (30), the mixed sulfide solids (MS.sub.x) slurry recovered from the leach solution is reacted with an acid, e.g., an aqueous solution of H.sub.2SO.sub.4. This reaction may be take place in any suitable vessel, such as an agitated tank. The mixed sulfide solids react with H.sub.2SO.sub.4 according to the reaction:
MS.sub.x+H.sub.2SO.sub.4.fwdarw.H.sub.2S+M(SO.sub.4).sub.x
wherein M is Mn as well as some or all of the Secondary Metals (i.e., Ni, Co, Cu, etc.), depending on the composition of the Mn-containing feedstock. The generated H.sub.2S is then stripped from the reaction solution using, for example, a packed column and air or other gas suitable for stripping H.sub.2S. Alternatively, the reaction vessel can be heated to boiling, with the steam carrying the H.sub.2S from the reaction vessel. Of course, a variety of apparatus can be employed for the H.sub.2S generation and removal, such as those commonly used for contacting a gas and a liquid. In addition, various other acids besides H.sub.2SO.sub.4 can be used, including HCl and H.sub.3PO.sub.4.

(30) In addition to stripping H.sub.2S from the reaction solution, the air, steam or other gas used in the H.sub.2S generation stage (30) facilitates the transfer of H.sub.2S from generation stage (30) to the sulfide recycle stage (32). The solution remaining in the H.sub.2S generation stage (30) comprises a solution of mixed metal sulfates (M(SO.sub.4).sub.x). As further described herein, the metals can be recovered therefrom in one or more subsequent steps.

(31) In the sulfide recycle stage (32), the H.sub.2S from generation stage (30) is absorbed in (i.e., reacted with) a solution, suspension or slurry of one or more alkali metal, alkaline earth metal and/or ammonium hydroxides. In particular, the H2S is put through a column such as a tray column or packed column, or other device commonly used for contacting gas and liquid, along with an aqueous hydroxide solution such as a solution of NaOH. Alternatively, the H.sub.2S can be bubbled into an agitated tank containing an aqueous hydroxide solution/suspension/slurry. The H.sub.2S reacts with, for example, NaOH according to the reactions:
H.sub.2S+2NaOH.fwdarw.Na.sub.2S+2H.sub.2O
H.sub.2S+2NaOH.fwdarw.NaHS+H.sub.2O
Other hydroxides react with H.sub.2S in a similar manner to generate the corresponding sulfide(s). For example,
Ba(OH).sub.2+H.sub.2S.fwdarw.BaS+2H.sub.2O
As yet another alternative, the H.sub.2S can be reacted with ammonia gas in order to generate NH.sub.4HS.

(32) As a further alternative, and as depicted in FIG. 4, in a first step (32) of the sulfide recycle stage an alkali metal or alkaline earth metal hydroxide is reacted with H2S to generate the corresponding sulfide(s). For example,
Ca(OH).sub.2+H.sub.2S.fwdarw.CaS+2H.sub.2O.
In a second step (33) the resulting sulfide solution (e.g., CaS) is then converted into a different sulfide that is then recycled to the purification step. For example, when CaS is formed in a first step (32) of the sulfide recycle stage, in a second step (33) the CaS solution is reacted with an ammonium sulfate ((NH.sub.4).sub.2SO.sub.4) solution to form ammonium sulfide (as (NH.sub.4)HS and/or (NH.sub.4).sub.2S) This second step can be performed in any suitable vessel, such as an agitated tank (e.g., a vessel separate from that used for generating the first sulfide from H.sub.2S).

(33) In either the embodiment of FIG. 1 or that of FIG. 2, the resulting sulfide solution (e.g., Na.sub.2S/NaHS, BaS, (NH.sub.4)HS and/or other sulfides) is then returned to purification step (14) described above in order to convert heavy metals in the leach solution into their respective insoluble sulfides (which are thereafter removed from the leach solution prior to electrolysis). In general, particularly since the processes described herein are typically performed on a batch basis, more and more hydroxide is added in order to generate a higher concentration sulfide solution, thereby reducing storage costs and maintaining a better water balance in the circuit. In addition, for the pre-electrolysis purification step (14), a stoichiometric excess (e.g., 5× to 10×) is typically used in order to ensure nearly complete precipitation of the Secondary Metals (as their respective sulfides).

(34) It will be understood that any alkali metal, alkaline earth metal and/or ammonium hydroxide can be used in sulfide recycle stage (32), including one or more of LiOH, NaOH, KOH, Mg(OH).sub.2, Ca(OH).sub.2, Ba(OH).sub.2 and/or NH.sub.4OH. The use of alkali metal and/or alkaline earth metal hydroxides is advantageous at sites where ammonia cannot be obtained or utilized, whether because of logistical reasons or prohibitions on its use. Similarly, in the embodiment wherein a first sulfide is converted into a second sulfide (e.g., (NH.sub.4)HS) this conversion is advantageous in that it allows sulfide to be recycled as ammonium sulfide without requiring the use of ammonia gas (e.g., at sites where ammonia cannot be obtained or utilized). Instead, solid ammonium sulfate ((NH.sub.4).sub.2SO.sub.4) can be used in the second step of forming ammonium sulfide(s) as the vehicle for recycling sulfide to the purification stage (14).

(35) As a result of the above process, it is not necessary to continually add sulfide to the process, as the sulfide necessary for purification (i.e., the precipitation of the Secondary Metals) is recovered from the mixed metal sulfides and recycled back into the process. (Although it may be necessary to add additional sulfide from time to time in order to, for example, make up for lost sulfide.) In addition, the mixed metal sulfides (MS.sub.x) are converted into their sulfates (M(SO.sub.4).sub.x), and the resulting M(SO.sub.4).sub.x solution remaining after the H.sub.2S generation stage (30) can be readily processed to recover not only Mn (e.g., as MnSO.sub.4, which can be returned to the leaching step (10)), but also the Secondary Metals.

(36) By way of example, when the MnSO.sub.4 concentration in the M(SO.sub.4).sub.x solution generated in stage (30) reaches high levels (e.g., about 20 to 300 g/L), the impurity level will typically be about 0.1 to 10 g/L. At this point, the M(SO.sub.4).sub.x solution can be neutralized with an alkaline or alkaline earth hydroxide or MnO, and separated from any solids such as BaSO.sub.4 and/or CaSO.sub.4. By way of further example, if Ba(OH).sub.2 is added in the sulfide recycle stage (32), the mixed sulfide solids reacted with acid in the H.sub.2S generation stage (30) will contain insoluble BaSO.sub.4. Next, sulfide (e.g., a stoichiometric amount of sulfide such as MnS, Na.sub.2S or NaHS) is added to the neutralized M(SO.sub.4).sub.x solution, causing the Secondary Metals to precipitate as their respective sulfides. After filtering, the Secondary Metal sulfides can be, for example, sold for their metal value. The remaining liquid will mainly comprise an MnSO.sub.4 solution, with small amounts of impurities, and can be returned to the leach solution where it will provide additional Mn.sup.2+ for subsequent electrolysis or sold.

(37) FIG. 2 is a schematic representation of an alternative embodiment of a process for producing manganese according to the present disclosure. As before, the process of FIG. 2 can be modified in order to produced EMD rather than manganese, as previously described herein.

(38) In the process of FIG. 2, like that of FIG. 1, mixed metal sulfide solids removed from the leach solution following a pre-electrolysis purification step are reacted with an acid such as H.sub.2SO.sub.4 in order to generate H.sub.2S. In this embodiment, however, the H.sub.2S is then reacted with a solution containing Mn.sup.2+ ions in order to generate MnS that is recycled back to the pre-electrolysis purification step (e.g., in the form of a slurry). When the Mn.sup.2+ solution reacted with the H.sub.2S contains low levels of Secondary Metals, high purity MnS is produced. In the example of FIG. 2, the Mn.sup.2+ solution reacted with the H.sub.2S comprises a portion of the electrolysis cell feed and/or catholyte extracted from the electrolysis cell(s). (In the case of producing EMD using the process of FIG. 2, cell feed is reacted with the H.sub.2S in step (132).)

(39) The applicant has found that, when high purity MnS is recycled back to the pre-electrolysis purification step, the MnS will react with the metal sulfates in the leach solution according to the following reaction:
MnS+M′SO.sub.4.fwdarw.MnSO.sub.4+M′S.sub.x
wherein M′ is one or more of the Secondary Metals. In other words, the recycled MnS is used as the sulfide in the pre-electrolysis purification step.

(40) Accordingly, in the leaching step (110) of the process depicted in FIG. 2, reduced manganese ore (or other suitable manganese-containing feedstock) is leached with a sulfuric acid solution in order to convert the MnO (or other manganese source) to manganese (II) sulfate (MnSO.sub.4). As before, the sulfuric acid solution used for leaching comprises anolyte (spent electrolyte) withdrawn from the electrolysis cell(s). Additional sulfuric acid and (NH.sub.4).sub.2SO.sub.4) may be added, as needed. By way of example, the Mn.sup.2+ concentration of the leach solution can be about 12 to 70 g/L, about 30 to 40 g/L, or about 32 g/L. These same Mn.sup.2+ concentrations in the leach solution are also suitable for the process of FIG. 1. The Mn.sup.2+ concentration of the cell feed is similar to that of the leach solution, as negligible amounts of Mn.sup.2+ is lost to the mixed sulfide solids (as MnS) in step (116).

(41) Iron, aluminum and silica, to the extent present in the feedstock, are removed from the leach solution in the manner described previously with respect to FIG. 1, i.e., in filtration step (112) (or by other conventional means for removing solids). The leach solution is then subjected to pre-electrolysis purification step (114) in order to remove heavy metal impurities (i.e., the Secondary Metals). In this step, sulfide solids or a sulfide slurry primarily comprising high purity MnS is added to the leach solution, causing the Secondary Metals to be converted into their respective insoluble sulfides. As before, a stoichiometric excess (e.g., about 5× to 10×) is used. The high purity MnS solids/slurry is obtained from the sulfide recovery loop described below.

(42) Following the addition of the MnS solids/slurry, the Secondary Metal impurities are converted from sulfates into their respective insoluble sulfides (e.g., NiS, CoS, etc.). As in the previous embodiment, the resulting sulfide precipitates are removed from the leach solution by filtration step (116) (or by other conventional means for removing solids), resulting in mixed sulfide solids similar to those produced in the embodiment of FIG. 1.

(43) As in the embodiment of FIG. 1, the purified leach solution following filtration step (116) is the cell feed for electrolysis step (120). Likewise, the sulfide recovery loop in the embodiment of FIG. 2, like that of FIG. 1, comprises an H.sub.2S generation stage (130) and a sulfide recycle stage (132). The H.sub.2S generation stage (130) is the same as that described previously for FIG. 1, with the generated H.sub.2S stripped from the reaction solution and supplied to the recycle stage (132). The M(SO.sub.4).sub.x solution generated in step (130) can be processed to recover Mn (e.g., as MnSO.sub.4 for return to the leaching step (110)) and the Secondary Metals in the manner described previously.

(44) In the sulfide recycle stage (132), the H.sub.2S from generation stage (130) is reacted with a solution containing Mn.sup.2+ ions in order to generate high purity MnS that is recycled back to the pre-electrolysis purification step (114), such as in the form of solids or a slurry. While other Mn.sup.2+ containing solutions can be used, the cell feed and/or catholyte provide a readily available Mn.sup.2+ solution for this purpose, as each contains appreciable amounts of MnSO.sub.4 in solution. Also, by using cell feed or catholyte to generate MnS for recycle back to the purification step (114), considerable cost savings can be achieved. For example, as compared to the process of FIG. 1, there is no need to purchase additional sulfide such as BaS for the pre-electrolysis purification step (114). While it is necessary to add acid, such as H.sub.2SO.sub.4 in the H.sub.2S generation step (130), H.sub.2SO.sub.4 is already used in the process and can be inexpensively manufactured on site.

(45) The H.sub.2S is reacted with cell feed and/or catholyte solution such that the H.sub.2S reacts with MnSO.sub.4 according to the reaction:
H.sub.2S+MnSO.sub.4.fwdarw.MnS+H.sub.2SO.sub.4
This reaction is carried out, for example, in the manner described above with respect to FIG. 1, such as using a tray column or packed column, or other device commonly used for contacting gas and liquid, or by bubbling the H.sub.2S into an agitated tank containing the Mn.sup.2+ solution.

(46) Assuming that sufficient MnSO.sub.4 and H.sub.2S are available in sulfide recycle stage (132), the above reaction will proceed until the pH of the reaction solution reaches about 3 to 4—at which point H.sub.2S will no longer react with Mn.sup.2+ to produce MnS. Thus, the pH of the reaction solution should be maintained above 4, or above about 4.5 in order to prevent excessive odor (from unreacted H.sub.2S). Also, since the catholyte typically has a higher pH (about 8.5) than the cell feed (pH about 7), more MnS can be produced from catholyte before the lower pH limit is reached. In addition, base can be added to the reaction solution in recycle stage (132) in order to maintain the pH at about 6 to 7, while adding sufficient H.sub.2S to precipitate all of the Mn (as MnS) in the reaction solution. Suitable bases include, for example, alkali, alkaline earth or ammonium hydroxides and/or ammonia gas, or even MnO.

(47) The reaction product from the recycle stage (132) is filtered (or otherwise removed) in step (136) and the recovered high purity MnS (as a solid or slurry) is returned to purification step (114) described above in order to convert the Secondary Metals in the leach solution into their respective insoluble sulfides (which are thereafter removed from the leach solution prior to electrolysis). The high purity MnS returned to purification step (114) contains less than 10%, less than 5%, less than 1%, less than 0.5%, less than 0.1%, less than 0.05%, less than 0.01%, or even less than 0.005% by weight of other metal sulfides (based on the total sulfide solids). The filtrate remaining after filtration step (136) can be used, for example, as additional cell feed, particularly when an excess of cell feed or catholyte is used in recycle stage (132) such that the filtrate contains unreacted Mn.sup.2+.

(48) Once again as a result of the process of FIG. 2, it is not necessary to continually add sulfide to the process, as the sulfide necessary for purification (i.e., precipitation of heavy metals) is recovered from the mixed metal sulfides and recycled back into the process. In addition, the pure MnS used for purification purposes is safer and easier to store than the sulfides used in conventional processes, does not introduce additional water into the system, has very little odor (especially compared to ammonium sulfide/bisulfide), and does not add unwanted elements such as Na to the cell feed. Furthermore, the cost of sulfides (e.g., NaHS, BaS, NH4HS, etc.) is nearly, if not entirely, eliminated, as are the safety hazards associated with the disposal of impurity sulfides. The process also facilitates the recovery of Mn, Ni, Co, and other valuable metals, while also producing very little solid waste material.

(49) FIG. 3 depicts a schematic representation of an alternative embodiment that provides a process for producing a purified manganese sulfate solution according to the present disclosure. In this embodiment, the stream identified as the “cell feed” in FIGS. 1 and 2 (“purified leach solution” in FIG. 3) is instead combined with a sulfide solution in order to cause the precipitation of high purity MnS. First and second sulfide recovery loops are provided in the embodiment of FIG. 3, with a first sulfide recovery loop (230/232) used to recycle sulfide to the purification step (214), and a second sulfide recovery loop (330/332) used to recycle sulfide used to cause the precipitation of high purity MnS. It will be understood that one or both of the sulfide recovery loops in FIG. 3 can be replaced by the sulfide recovery loop of FIG. 2, wherein sulfide is recovered and recycled as high purity MnS (as a solid or slurry) rather than a sulfide solution. Alternatively, one or both of the sulfide recovery loops in FIG. 3 can be replaced by the sulfide recovery loop of FIG. 4, wherein sulfide is recovered as ammonium sulfide(s) in a two-step process.

(50) Like the previously described embodiments, in leaching step (210), a source of manganese such as reduced manganese ore, primarily comprising MnO, is leached with a sulfuric acid solution (not shown in FIG. 3) in order to convert the MnO (or other manganese source) to manganese (II) sulfate (MnSO.sub.4). If necessary, iron, aluminum and/or silica are removed from the leach solution by increasing the pH of the leach solution and adding an oxidizing agent (e.g., NH.sub.3, lime and/or MnO is added to the leach solution in order to increase the pH). Suitable oxidizing agents include, for example, MnO.sub.2, oxygen (typically as air), O.sub.3 or H.sub.2O.sub.2. Following the pH adjustment and the addition of MnO.sub.2 and/or air as oxidizing agents, iron, aluminum and silica will precipitate from the leach solution and are removed by filtration in step (212) (or by other conventional means for removing solids).

(51) The leach solution is then subjected to a purification step (214) in order to remove heavy metal impurities, i.e., the Secondary Metals. As before, one or more sulfides are added to the leach solution, causing the heavy metals to be converted into their respective insoluble sulfides. In particular, an aqueous sulfide solution comprising one or more alkali metal, alkaline earth metal and/or ammonium sulfides (as (NH.sub.4)HS and/or (NH.sub.4).sub.2S) is added to the leach solution, wherein the sulfides are obtained from the first sulfide recovery loop (230/232). Following the addition of the sulfide solution, the heavy metal impurities are converted from their sulfates into their respective insoluble sulfides (e.g., NiS, CoS, etc.). A portion of the MnSO.sub.4 is also converted into insoluble MnS. The resulting sulfide precipitates are removed from the leach solution by filtration step (216) (or by other conventional means for removing solids), resulting in mixed sulfide solids (“MS.sub.x”) comprising sulfides of Mn, as well as sulfides of some or all of the Secondary Metals.

(52) Following removal of the mixed sulfide solids (e.g., as a slurry), the purified leach solution is subjected to a manganese precipitation step (314) followed by a second filtration step (316). In the Mn precipitation step (314), an aqueous sulfide solution comprising one or more alkali metal, alkaline earth metal and/or ammonium sulfides (as (NH.sub.4)HS and/or (NH.sub.4).sub.2S) is added to the purified leach solution, wherein the sulfides are obtained from the second sulfide recovery loop (330/332). Following the addition of the sulfide solution, the MnSO.sub.4 in the purified leach solution is converted into insoluble, high purity MnS. The resulting high purity MnS precipitate (>90%, >95%, >99%, >99.5%, >99.9%, >99.95%, >99.99%, or even >99.995% purity) is removed from the purified leach solution by a second filtration step (316) (or by other conventional means for removing solids). The waste sulfate solution removed in filtration step (316) is discarded or further processed to recover components thereof, as desired.

(53) The high purity MnS is then reacted with acid (e.g., H.sub.2SO.sub.4) in the H.sub.2S generation stage (330) of the second sulfide recovery loop, thereby generating a high purity manganese sulfate solution along with H.sub.2S in the same manner as described above for H.sub.2S generation stage (30) (as well as H.sub.2S generation stage (230) in the embodiment of FIG. 3).

(54) In particular, the embodiment of FIG. 3 includes two sulfide recovery loops, each of which comprises an H.sub.2S generation stage (230, 330) and a sulfide recycle stage (232, 332). In the H.sub.2S generation stage (230), the mixed sulfide solids (MS.sub.x) slurry recovered from the leach solution is reacted with an acid, e.g., an aqueous solution of H.sub.2SO.sub.4, in order to generate H.sub.2S and M(SO.sub.4).sub.x (wherein M is Mn as well as some or all of the Secondary Metals (i.e., Ni, Co, Cu, etc.)). In the H.sub.2S generation stage (330), the high purity MnS slurry recovered from the purified leach solution is likewise reacted with an acid, e.g., H.sub.2SO.sub.4, in order to generate H.sub.2S and the high purity manganese sulfate solution. In both H.sub.2S generation stages (230, 330) the generated H.sub.2S is then stripped from the reaction solution using, for example, a packed column and air or other gas suitable for stripping H.sub.2S. Alternatively, the reaction vessel can be heated to boiling, with the steam carrying the H.sub.2S from the reaction vessel. Of course, a variety of apparatus can be employed for the H.sub.2S generation and removal, such as those commonly used for contacting a gas and a liquid. In addition, various other acids besides H.sub.2SO.sub.4 can be used, including HCl and H.sub.3PO.sub.4. In addition to stripping H.sub.2S from the reaction solution, the air, steam or other gas used in the H.sub.2S generation stage (230, 330) facilitates the transfer of H.sub.2S from generation stage (230, 330) to the sulfide recycle stage (232, 332).

(55) The solution remaining in the H.sub.2S generation stage (230) comprises a solution of mixed metal sulfates (M(SO.sub.4).sub.x) and can be further processed in the manner described above with respect to the embodiment of FIG. 1.

(56) As before, in the sulfide recycle stages (232, 332), the H.sub.2S from generation stage (230, 330) is absorbed in (i.e., reacted with) a solution, suspension or slurry of one or more alkali metal, alkaline earth metal and/or ammonium hydroxides. Alternatively, the H.sub.2S can be reacted with ammonia gas in order to generate NH.sub.4HS. As a further alternative, an alkali metal or alkaline earth metal hydroxide is reacted with H2S to generate the corresponding sulfide(s). For example,
Ca(OH).sub.2+H.sub.2S.fwdarw.CaS+2H.sub.2O.
The resulting sulfide solution is then converted into a different sulfide that is then recycled to the purification step. For example, when CaS is formed in a first step of the sulfide recycle stage (232, 332), in a second step the CaS solution is reacted with an ammonium sulfate ((NH.sub.4).sub.2SO.sub.4) solution to form ammonium sulfide ((NH.sub.4)HS and/or (NH.sub.4).sub.2S). This second step can be performed in any suitable vessel, such as an agitated tank (e.g., a vessel separate from that used for generating the first sulfide from H.sub.2S).

(57) The resulting sulfide solution (e.g., Na.sub.2S/NaHS, BaS, (NH.sub.4)HS and/or other sulfides) is then returned to purification step (214) or Mn precipitation step (314) described above in order to convert heavy metals in the leach solution and MnSO.sub.4 in the purified leach solution into their respective insoluble sulfides (which are thereafter removed from the leach solution or purified leach solution, as described above). Once again it will be understood that any alkali metal, alkaline earth metal and/or ammonium hydroxide can be used in sulfide recycle stages (232, 332), including one or more of LiOH, NaOH, KOH, Mg(OH).sub.2, Ca(OH).sub.2, Ba(OH).sub.2 and/or NH.sub.4OH. Similarly, in the embodiment wherein a first sulfide is converted into a second sulfide (e.g., (NH.sub.4)HS), this conversion is advantageous in that it allows sulfide to be recycled as ammonium sulfide without requiring the use of ammonia gas (e.g., at sites where ammonia cannot be obtained or utilized).

(58) While FIG. 3 depicts the sulfide solution from the first sulfide recovery loop (230/232) being recycled to the purification step (214) and the sulfide solution from the second sulfide recovery loop (330/332) being recycled to the Mn precipitation step (314), it will be understood that in some embodiments this may be reversed. Alternatively, some of the sulfide solution from either or both of the first and second sulfide recovery loops can be recycled to both the purification step (214) and the Mn precipitation step (314).

(59) As a result of the above process, a highly purified manganese sulfate solution is produced, wherein the solution contains less than 10%, less than 5%, less than 1%, less than 0.5%, less than 0.1%, less than 0.05%, less than 0.01%, or even less than 0.005% by weight of other metals.

(60) While various embodiments have been described in detail above, it will be understood that the processes, components, features and configurations described herein are not limited to the specific embodiments described above. For example, the processes described herein can be used in the production of EMD. In the case of EMD production, spent electrolyte solution is used in the leaching step. In addition, when the process of FIG. 2 is used in conjunction with EMD production, the most convenient source of Mn.sup.2+ in the sulfide recycle stage (132) is either cell feed or electrolyte solution withdrawn from the electrolysis cell(s).