CROSS REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority to U.S. Provisional Patent Application No.

62/302,648, filed March 2, 2016, entitled "Sulfide Recycling in Manganese Production." The entire disclosure of the foregoing application is incorporated by reference herein.

BACKGROUND

[0002] High purity manganese and electrolytic manganese dioxide ("EMD") are typically produced by an electrolytic process (electrowinning, also known as electroextraction). For example, a manganese-containing material is leached with a sulfuric acid solution to provide a manganese sulfate (MnS0 ₄ ) solution. This leach solution is then subjected to electrolysis in an electrolytic cell, such that, depending on cell operating conditions, manganese is deposited on the cathode or EMD is deposited on the anode. Typically, spent electrolyte solution comprising sulfuric acid, manganese sulfate and ammonium sulfate ((NH ₄ ) ₂ S0 ₄ ) is withdrawn from the electrolytic cell, and provides most of the sulfuric acid solution for leaching. After the spent electrolyte solution is combined with the manganese ore (or other manganese source), the resulting leach solution containing manganese sulfate (as well as other sulfates, particularly (NH ₄ ) ₂ S0 ₄ )) is purified and thereafter returned to the electrolytic cell as the cell feed.

[0003] In the electrolytic production of high purity manganese, the spent electrolyte solution used for leaching comprises anolyte withdrawn from the electrolytic cell, and the cell feed (i.e., the purified leach solution) is introduced into the cathode side of the cell (i.e., as the catholyte). Pure manganese is deposited on the cathode(s) within the cathode chamber(s). For the electrolytic production of EMD (Mn0 ₂ ), spent electrolyte solution withdrawn from the cell (which is typically an undivided cell) is similarly used for leaching purposes, and the purified leach solution is used as the cell feed. Pure EMD is deposited on the anode(s) within the electrolytic cell.

[0004] The manganese-containing material is typically roasted prior to leaching in order to reduce higher oxides of manganese (e.g., Mn0 ₂ , Mn ₂ 0 ₃ and Mn ₃ 0 ₄ ) to manganese oxide (MnO). Alternatively, and as described in U.S. Pat. No. 5,932,086, issued August 3, 1999, titled "Process for Making Manganese," and International (PCT) Pub. No. WO 99/14403, published March 25, 1999, titled "Process for Making Manganese" (both of which are incorporated by reference herein), the manganese ore can be sintered in order to convert Mn0 ₂ to Mn ₃ 0 ₄ , and thereafter the Mn ₃ 0 ₄ leached with a sulfuric acid solution along with a reducing agent (e.g., sulfur dioxide, activated carbon, hydrogen peroxide, hydrogen sulfide, reducing sugars and/or molasses) to provide a manganese sulfate solution (i.e., the cell feed).

[0005] Purification of the leach solution is generally necessary prior to feeding the leach solution into the electrolytic cell (as the cell feed). In particular, the leach solution should have very low concentrations of Fe, Al, Si, Ni, Co, Cu, Zn, Pb, Mo, etc. These impurities are deleterious to electrolysis operation, causing low current efficiency, and can also reduce the purity level of the manganese or EMD product. Typically, iron, aluminum and silica are removed from the leach solution by increasing the pH of the leach solution from about 3 (or lower) to about 4 to 7 (e.g., 6 to 7) and adding an oxidizing agent. The pH is increased by adding a base such as ammonia gas and/or lime to the leach solution, and typical oxidizing agents used for this purpose include Mn0 ₂ and/or air. Iron, aluminum and silica (when present) will precipitate from the leach solution and can be removed by filtration or other conventional means.

[0006] Sulfides are used to remove heavy metals such as Ni, Co, Cu, Zn, Pb, and Mo as insoluble metal sulfides. In particular, after removal of Fe, Al and silica (and, in some instances, other impurities), one or more sulfides are added to the leach solution. Typically, the sulfides used for this purification step comprise one or more alkali metal or alkaline earth metal sulfides (e.g., NaHS and/or BaS) and/or ammonium sulfide, with about 5 to 10 times the stoichiometric amount of sulfide being necessary in order to reduce the heavy metal impurity level to below 1 mg/L (i.e., 1 ppm) in the leach solution/cell feed. Following the addition of the sulfide solution, the metal sulfide precipitates are removed from the leach solution, usually by filtration. However, not only are undesirable impurities such as Ni, Co, Cu and the like removed (as their insoluble sulfides), but also significant amounts of manganese (as MnS). In fact, these mixed sulfide solids (e.g., removed as a filter cake) can contain up to 90% MnS. In addition, the mixed sulfide solids are considered a hazardous waste material, and therefore must be disposed of in a controlled manner. As used herein, the terms "mixed sulfide solids" (or "mixed metal sulfide solids") and "mixed metal sulfate" refer to mixtures of two or more metal sulfides (e.g., MnS, S, CoS) and mixtures of two or more metal sulfates (e.g., MnS0 ₄ , NiS0 ₄ , CoS0 ₄ , etc.), respectively.

[0007] Thus, the above-described electrolytic production of high purity manganese or

EMD results in waste streams, particularly insoluble heavy metal sulfides of valuable metals such as Mn, Ni, Co, Cu and Mo, as well as sulfur (in the form of sulfides). In addition, many conventional processes for the electrolytic production of high purity manganese and/or EMD employ materials that can be difficult (or impossible) to obtain in sufficient quantities (or even in any quantity) at locations where manganese ore is typically processed. For example, ammonia, hydrogen sulfide and/or sodium sulfide are not always obtainable where manganese ore is processed, and in some instances, liquid and gaseous reactants are not permitted to be brought on site.

[0008] While a variety of devices and techniques may exist for producing manganese and

EMD, it is believed that no one prior to the inventor(s) has made or used an invention as described herein.

BRIEF DESCRIPTION OF THE DRAWINGS

[0009] 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. [0010] FIG. 1 depicts a schematic illustration of one embodiment of a manganese production process.

[0011] FIG. 2 depicts a schematic illustration of another embodiment of a manganese production process.

[0012] 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

[0013] 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.

[0014] 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 ₂ S. The H ₂ S is then reacted with an Mn 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 ₂ S generation, the H ₂ S is removed and absorbed in a solution containing Mn 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., MnS0 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.

[0015] 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 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.

[0016] 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 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 ₂ S. The H ₂ S 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). By way of example, following H ₂ S generation, the H ₂ S 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 ₂ S0 ₄ generates not only H ₂ S 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) ₂ , Ca(OH) ₂ , and Ba(OH) ₂ . In still further embodiments, the H ₂ S is absorbed into a solution containing ammonia or ammonium ion. 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.

[0017] 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 ₂ S, 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.

[0018] 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.

[0019] The mixed metal sulfide solids removed from the leach solution in the pre- electrolysis purification step are reacted with an acid (e.g., H ₂ S0 ₄ ) to generate H ₂ S on site. When the acid used is H ₂ S0 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 ₂ S, generated by reacting the mixed metal sufides with acid is the vehicle used to recycle sulfide back to the pre-electrolysis purification step. [0020] In particular, the generated H ₂ S is reacted in order to generate sulfide(s) that is recycled back to the pre-electrolysis purification step. In some embodiments, the generated H ₂ S is reacted with a solution containing Mn ²⁺ ions (e.g., a solution containing MnS0 ₄ ) in order to generate MnS that is then recycled back to the pre-electrolysis purification step. In one particular embodiment, the Mn ²⁺ containing solution reacted with the H ₂ S 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).

[0021] In still further embodiments for electrolytically producing manganese metal or

EMD wherein sulfide removed from the leach solution prior to electrolysis, the generated H ₂ S 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.

[0022] Yet another embodiment of the present disclosure provides a method of purifying an MnS0 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 MnS0 ₄ solution with MnS, without adding any additional sulfides, such that the heavy metal impurities form their respective sulfide precipitates (MS, CoS, etc.). Applicant has discovered that by reacting the MnS0 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 MnS0 ₄ ). The MnS used to extract the heavy metal impurities comprises high purity MnS containing less than 10%>, less than 5%o, 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). [0023] FIGURES 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., Mn0 ₂ , Mn ₂ 0 ₃ and Mn ₃ 0 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 ₃ 0 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 Mn0 ₂ to Mn ₃ 0 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.

[0024] For producing manganese, an MnS0 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 ₄ ) ₂ S0 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 ₂ S0 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.

[0025] 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.

[0026] FIGURE 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 ₂ SO ₄ to generate H ₂ S. The H ₂ S 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. 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.

[0027] 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 (MnS0 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 ₂ S0 ₄ , the spent electrolyte solution also contains MnS0 ₄ , and ( H4) ₂ S0 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 ( H4) ₂ S0 ₄ are periodically added to the process, as needed, typically by an addition to the leach tank.

[0028] 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 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, Mn0 ₂ , oxygen (typically as air), 0 ₃ or H ₂ 0 ₂ . Mn0 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 Mn0 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).

[0029] 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 ₂ S, Na ₂ S, NaHS, K ₂ S, KHS, MgS, CaS, BaS and/or NH ₄ HS, 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.

[0030] Following the addition of the sulfide solution, the heavy metal impurities are converted from their sulfates into their respective insoluble sulfides (e.g., S, CoS, etc.). A portion of the MnS0 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 _X ") comprising sulfides of Mn, as well as sulfides of some or all of the Secondary Metals. (It will be understood that "MS _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. [0031] 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 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 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.

[0032] The sulfide recovery loop in the embodiment of FIG. 1 comprises an H ₂ S generation stage (30) and a sulfide recycle stage (32). In the H ₂ S generation stage (30), the mixed sulfide solids (MS _X ) slurry recovered from the leach solution is reacted with an acid, e.g., an aqueous solution of H ₂ S0 ₄ . This reaction may be take place in any suitable vessel, such as an agitated tank. The mixed sulfide solids react with H ₂ S0 ₄ according to the reaction:

MS _X + H ₂ S0 ₄ → H ₂ S + M(S0 ₄ ) _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 ₂ S is then stripped from the reaction solution using, for example, a packed column and air or other gas suitable for stripping H ₂ S. Alternatively, the reaction vessel can be heated to boiling, with the steam carrying the H ₂ S from the reaction vessel. Of course, a variety of apparatus can be employed for the H ₂ S generation and removal, such as those commonly used for contacting a gas and a liquid. In addition, various other acids besides H ₂ S0 ₄ can be used, including HC1 and H ₃ P0 ₄ . [0033] In addition to stripping H ₂ S from the reaction solution, the air, steam or other gas used in the H ₂ S generation stage (30) facilitates the transfer of H ₂ S from generation stage (30) to the sulfide recycle stage (32). The solution remaining in the H ₂ S generation stage (30) comprises a solution of mixed metal sulfates (M(S0 ₄ ) _X ). As further described herein, the metals can be recovered therefrom in one or more subsequent steps.

[0034] In the sulfide recycle stage (32), the H ₂ S 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 ₂ S can be bubbled into an agitated tank containing an aqueous hydroxide solution/ suspension/slurry. The H ₂ S reacts with, for example, NaOH according to the reactions:

H ₂ S + 2NaOH→ Na ₂ S + 2H ₂ 0

H ₂ S + 2NaOH→ NaHS + H ₂ 0

Other hydroxides react with H ₂ S in a similar manner to generate the corresponding sulfide(s). For example,

Ba(OH) ₂ + H ₂ S→ BaS + 2H ₂ 0

As yet another alternative, the H ₂ S can be reacted with ammonia gas in order to generate NH ₄ HS.

[0035] The resulting solution of sulfide solution (e.g., Na ₂ S/NaHS, BaS, 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., 5x to lOx) is typically used in order to ensure nearly complete precipitation of the Secondary Metals (as their respective sulfides). [0036] 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) ₂ , Ca(OH) ₂ , Ba(OH) ₂ and/or H ₄ OH. 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.

[0037] 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 _X ) are converted into their sulfates (M(S0 ₄ ) _x ), and the resulting M(S0 ₄ ) _x solution remaining after the H ₂ S generation stage (30) can be readily processed to recover not only Mn (e.g., as MnS0 ₄ , which can be returned to the leaching step (10)), but also the Secondary Metals.

[0038] By way of example, when the MnS0 ₄ concentration in the M(S0 ₄ ) _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(S0 ₄ ) _x solution can be neutralized with an alkaline or alkaline earth hydroxide or MnO, and separated from any solids such as BaS0 ₄ and/or CaS0 ₄ . By way of further example, if Ba(OH) ₂ is added in the sulfide recycle stage (32), the mixed sulfide solids reacted with acid in the H ₂ S generation stage (30) will contain insoluble BaS0 ₄ . Next, sulfide (e.g., a stoichiometric amount of sulfide such as MnS, Na ₂ S or NaHS) is added to the neutralized M(S0 ₄ ) _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 MnS0 ₄ solution, with small amounts of impurities, and can be returned to the leach solution where it will provide additional Mn ²⁺ for subsequent electrolysis or sold.

[0039] 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. [0040] 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 ₂ SO ₄ in order to generate H ₂ S. In this embodiment, however, the H ₂ S is then reacted with a solution containing Mn 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 ²⁺ solution reacted with the H ₂ S contains low levels of Secondary Metals, high purity MnS is produced. In the example of FIG. 2, the Mn ²⁺ solution reacted with the H ₂ S 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 ₂ S in step (132).)

[0041] 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'SC-4→ MnS0 ₄ + M'S _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.

[0042] 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 (MnS0 ₄ ). As before, the sulfuric acid solution used for leaching comprises anolyte (spent electrolyte) withdrawn from the electrolysis cell(s). Additional sulfuric acid and ( H ₄ ) ₂ S0 ₄ ) may be added, as needed. By way of example, the Mn 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 ²⁺ concentrations in the leach solution are also suitable for the process of FIG. 1. The Mn ²⁺ concentration of the cell feed is similar to that of the leach solution, as negligible amounts of Mn ²⁺ is lost to the mixed sulfide solids (as MnS) in step (116).

[0043] 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 5x to lOx) is used. The high purity MnS solids/slurry is obtained from the sulfide recovery loop described below.

[0044] Following the addition of the MnS solids/slurry, the Secondary Metal impurities are converted from sulfates into their respective insoluble sulfides (e.g., S, 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.

[0045] 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 ₂ S generation stage (130) and a sulfide recycle stage (132). The H ₂ S generation stage (130) is the same as that described previously for FIG. 1, with the generated H ₂ S stripped from the reaction solution and supplied to the recycle stage (132). The M(S0 ₄ ) _x solution generated in step (130) can be processed to recover Mn (e.g., as MnS0 ₄ for return to the leaching step (110)) and the Secondary Metals in the manner described previously.

[0046] In the sulfide recycle stage (132), the H ₂ S from generation stage (130) is reacted with a solution containing Mn 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 ²⁺ containing solutions can be used, the cell feed and/or catholyte provide a readily available Mn ²⁺ solution for this purpose, as each contains appreciable amounts of MnS0 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 ₂ S0 ₄ in the H ₂ S generation step (130), H ₂ S0 ₄ is already used in the process and can be inexpensively manufactured on site.

[0047] The H ₂ S is reacted with cell feed and/or catholyte solution such that the H ₂ S reacts with MnS0 ₄ according to the reaction:

H ₂ S + MnS0 ₄ → MnS + H ₂ S0 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 ₂ S into an agitated tank containing the Mn ²⁺ solution.

[0048] Assuming that sufficient MnS0 and H ₂ S 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 ₂ S will no longer react with Mn 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 ₂ S). 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 ₂ S 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.

[0049] 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 ²⁺ . [0050] 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, H4HS, 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.

[0051] 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 ²⁺ in the sulfide recycle stage (132) is either cell feed or electrolyte solution withdrawn from the electrolysis cell(s).