Purification of MnSO4 solutions

Abstract

A method (10) for purifying a MnSO4 solution (34) comprising: (i) in the presence of a stoichiometric excess of fluorine anions (36), where the stoichiometric excess of fluorine anions is calculated based on the fluorine anions required to react with all Mg2 + and Ca2 + cations present as impurities in the MnSO4 solution to form CaF2 and MgF2, and (ii) precipitating (12) impurities from the MnSO4 solution in the presence of sulfide anions (52). The precipitation is carried out at a pH higher than 4, resulting in a slurry or suspension (54) comprising a purified MnSO4 solution as a carrier medium and a suspended precipitate comprising MnS, MnF2, CaF2 and MgF2. The slurry or suspension is separated (20) into a purified MnSO4 solution (56) and a precipitate (58), after which the precipitate (58) is reacted (24) with an SO4 salt (62) other than MnSO4 in a solid state reaction to produce recovered MnSO4 from MnS and MnF2. The recovered MnSO4 is recycled (76) into the MnSO4 solution (34) being purified, thereby reducing Mn loss and improving MnSO4 yield.

Description

[0001] The present invention relates to the purification of MnSO4 solution. In particular, the present invention relates to a method for purifying MnSO4 solution.

[0002] High purity MnSO4 solutions (i.e., with specified low levels of contaminants such as Ca, Co, Fe, Mg, Ni and Zn and a minimum Mn concentration) are commercially important, for example for use in electrochemical cells and batteries. Various methods exist for purifying MnSO4 solutions, but they tend to be cumbersome and expensive.

[0003] There is a need for a method for purifying MnSO 4 solution that is simpler and more economical than conventional methods for purifying MnSO 4 solution.

[0004] According to the present invention, a method for purifying a MnSO4 solution is provided, the method comprising:

[0005] (i) in the presence of a stoichiometric excess of fluoride anions, wherein the stoichiometric excess of fluoride anions is based on the total amount of Mg present as an impurity in the MnSO4 solution. 2+ and Ca 2+ The cationic reaction is calculated based on the fluoride anions required to form CaF2 and MgF2, and (ii) in the presence of sulphur anions,

[0006] precipitating impurities from a MnSO4 solution at a pH above 4 to produce a slurry or suspension comprising a purified MnSO4 solution as a support medium and a suspended precipitate comprising MnS, MnF2, CaF2 and MgF2;

[0007] separating the slurry or suspension into the purified MnSO4 solution and the precipitate;

[0008] reacting the precipitate with a SO4 salt (sulfate) other than MnSO4 in a solid state reaction to produce recovered MnSO4 from MnS and MnF2; and

[0009] The recovered MnSO4 is recycled to the MnSO4 solution being purified, thereby reducing Mn losses and improving MnSO4 yield.

[0010] The MnSO4 solution may be a pregnant leaching solution.

[0011] MnSO4 solution may be contaminated with Ca, Co, Fe, Mg, Ni and Zn as impurities. Other contaminants may include Cu, K, Na, Si, Al, Sr and Pb.

[0012] In one embodiment of the invention, the MnSO4 solution is a leachate obtained by a roasting and leaching process, such as the roasting and leaching process for extracting manganese from manganese-containing ores disclosed in South African Provisional Patent Application No. 2022 / 05305.

[0013] Typically, the process involves adding a base to the MnSO4 solution to ensure that the pH is above 4.

[0014] The base may be NH4OH, for example a 25% NH4OH solution. Advantageously, in the process of the invention, the ammonium salts formed as a result of the addition of NH4OH are decomposed downstream to form a treatable gas.

[0015] Preferably, the pH is controlled between about 5 and about 8, more preferably between about 5 and about 7, during precipitation of impurities from the MnSO4 solution.

[0016] A stoichiometric excess of fluoride anions can be produced in the MnSO4 solution by adding a stoichiometric excess of a fluoride anion source, such as a water-soluble fluoride salt or hydrofluoric acid. Preferably, a stoichiometric excess of fluoride anions is produced by adding a stoichiometric excess of a water-soluble fluoride salt, such as NH4F·HF, because it is typically less hazardous than hydrofluoric acid.

[0017] Typically, the method includes agitating the MnSO4 solution during precipitation of impurities from the MnSO4 solution.

[0018] Precipitating impurities from the MnSO 4 solution may include first adding a source of fluoride anions and then adding a base to the MnSO 4 solution to ensure that the pH is above 4.

[0019] Typically, upon addition of the fluoride anion source, primarily Ca and Mg cations precipitate from the MnSO4 solution (i.e., as CaF2 and MgF2), in addition to some MnF2, with MnF2 forming the major component (i.e., the component with the highest concentration) of the precipitate. However, other cations that are present as impurities in the MnSO4 solution and form insoluble fluorides (e.g., Na forming insoluble Na2SiF6) may also precipitate.

[0020] Sulfide anions can be generated in the MnSO4 solution by adding a source of sulfide anions. The source of sulfide anions can be selected from MnS, (NH4)2S, H2S(g) and a mixture of two or three thereof.

[0021] In a preferred embodiment of the present invention, the source of sulfur anions is MnS.

[0022] Typically, the source of sulfur anions is added to the MnSO solution after the base is added to the MnSO solution. Thus, typically, in at least one embodiment of the present invention, precipitating impurities from the MnSO solution comprises first adding a source of fluoride anions to the MnSO solution, then adding a base to the MnSO solution to ensure that the pH is above 4, and then adding a source of sulfur anions to the MnSO solution.

[0023] MnS may be added to the MnSO4 solution at a concentration between about 2 and about 0.0001 mol MnS / liter of MnSO4 solution, preferably between about 1 and about 0.0005 mol MnS / liter of MnSO4 solution, and more preferably between about 0.1 and about 0.001 mol MnS / liter of MnSO4 solution (e.g., about 0.06 mol MnS / liter of MnSO4 solution).

[0024] The separation of the slurry or suspension into the purified MnSO4 solution and the precipitate can be performed using any conventional or suitable solid-liquid separation technique, such as filtration, gravity separation, cyclone separation, etc.

[0025] In a preferred embodiment of the present invention, the slurry or suspension is separated into the purified MnSO4 solution and the precipitate by filtering. Advantageously, MnS acts as a filter aid to filter small CaF2 and MgF2 particles from the MnSO4 solution, and acts as a precipitant to precipitate transition elements such as Co, Fe, Ni, Cu and Zn from the MnSO4 solution.

[0026] Typically, precipitation of impurities from MnSO4 solution in the presence of fluoride anions is carried out at ambient temperature and pressure.

[0027] In the presence of sulfide anions, precipitation of impurities from the MnSO4 solution can be carried out at a temperature in the range of about 5°C to about 100°C, preferably in the range of about 30°C to about 80°C, and more preferably in the range of about 50°C to about 70°C (e.g., about 60°C).

[0028] Thus, the method may include, after adding a base to the MnSO 4 solution, heating the MnSO 4 solution, and adding a source of sulfur anions (eg, MnS) to the heated MnSO 4 solution.

[0029] Based on all the Mg present in the MnSO4 solution 2+ and Ca 2+ The fluoride anions may be present in a stoichiometric excess of 6 times the stoichiometric amount, preferably at least 7 times the stoichiometric amount, based on the fluoride anions required for the cation reaction to form CaF2 and MgF2.

[0030] Based on all the Mg present in the MnSO4 solution 2+ and Ca 2+ The fluoride anions are typically present in a stoichiometric excess of no more than 9 times the stoichiometric amount, calculated on the basis of the fluoride anions required for the cation reaction to form CaF2 and MgF2.

[0031] Reacting the precipitate with a SO4 salt other than MnSO4 in a solid state reaction may include drying the precipitate.

[0032] Reacting the precipitate with a SO4 salt other than MnSO4 in a solid state reaction may include comminuting the precipitate to reduce particle size.

[0033] The precipitate may be pulverized to obtain a particle size distribution with a D90 of less than about 1000 μm, preferably less than about 500 μm, more preferably less than about 250 μm (eg, less than 106 μm).

[0034] Typically, the precipitate is comminuted to obtain a particle size distribution with a D10 of at least 10 μm.

[0035] Reacting the precipitate with a SO4 salt other than MnSO4 in a solid state reaction may include reacting the precipitate with (NH4)2SO4 or NH4HSO4.

[0036] The SO4 salt may have a particle size distribution with a D90 of less than about 1000 μm, preferably less than about 500 μm, more preferably less than about 250 μm (eg, less than 106 μm).

[0037] Typically, the SO4 salt has a particle size distribution with a D10 of at least 10 μm.

[0038] The solid state reaction between the precipitate and the SO4 salt may occur at a temperature in the range of about 375°C to about 525°C, preferably about 400°C to about 500°C, more preferably about 425°C to about 475°C (eg, about 450°C).

[0039] When the SO4 salt is (NH4)2SO4 or contains (NH4)2SO4, MnF2 present in the precipitate may react with (NH4)2SO4 according to the following reaction 1:

[0040] MnF2(s)+(NH4)2SO4(s)→MnSO4(s)+NH4F·HF(g)+NH3(g)…(1)

[0041] When the SO4 salt is (NH4)2SO4 or contains (NH4)2SO4, MnS present in the precipitate may react with (NH4)2SO4 according to the following reaction 2:

[0042] MnS(s)+4(NH4)2SO4(s)→MnSO4(s)+4SO2(g)+4H2O(g)+8NH3(g)…(2)

[0043] The method may include partially condensing the off-gas produced by the solid state reaction of the precipitate with the SO4 salt to condense and recover NH4F·HF from the off-gas and produce a cooled off-gas.

[0044] The off-gas may be partially condensed at a temperature below about 240°C, preferably below about 200°C, more preferably below about 150°C, such as about 100°C.

[0045] Typically, the exhaust gases are partially condensed at a temperature of at least about 25°C.

[0046] The method may include recycling NH4F·HF recovered from the exhaust gas to serve as a source of fluoride anions, thereby providing a stoichiometric excess of fluoride anions for precipitation of Ca and Mg impurities from the MnSO4 solution.

[0047] The method may include scrubbing the cooled off-gas with an absorbent, such as water, to remove NH3(g) from the cooled off-gas.

[0048] The method may include scrubbing the cooled flue gas in a SO2 scrubber to desulfurize the cooled flue gas. The NH3 scrubber for removing NH3(g) from the cooled flue gas may also serve as a SO2 scrubber.

[0049] The recovered MnSO4 can be recycled into the MnSO4 solution as recycled MnSO4 solution.

[0050] Thus, the process may include dissolving the recovered MnSO4 in water to form a recycled MnSO4 solution. An insoluble waste stream may be produced, which may be discharged from the process and discarded. The waste stream may act as a bleedstream to prevent or control the accumulation of Al, Ca, Co, Fe, K, Mg, Na, Ni and Zn concentrations in the process.

[0051] Dissolving the recovered MnSO4 in water to form a recycled MnSO4 solution can be accomplished using water at ambient temperature.

[0052] The recovered MnSO4 can be dissolved in water under stirring to form a recycled MnSO4 solution.

[0053] The invention will now be described by way of example, by way of example with reference to the following laboratory experiments and to the single accompanying drawing showing one embodiment of a method for purifying a MnSO4 solution according to the invention.

[0054] Laboratory experiments

[0055] Several laboratory experiments were conducted to investigate various aspects of the process of the present invention for purifying MnSO4 solutions.

[0056] Material

[0057] A 4-liter volume of MnSO4 enriched leach solution was used in the experiment and its analysis is shown in Table 1.

[0058] Table 1: Composition of MnSO4 enriched leaching solution (mg / l)

[0059] Al Ca Co Fe K Mg Mn Na Ni Zn 1.56 593 98.3 13.9 51.6 1755 127600 678 55.4 2.23

[0060] Fluoride precipitation

[0061] Fluoride precipitation is primarily used to precipitate calcium and magnesium cations from the pregnant leach solution. Other cations are also precipitated as insoluble fluoride precipitates, such as Na2SiF6. Unfortunately, MnF2 is also precipitated.

[0062] Although the solubility of MnF2 is about 100 times higher than that of CaF2 and MgF2, the stoichiometric addition of fluoride anions from a fluoride anion source such as HF or NH4F.HF surprisingly does not provide a MnSO4 solution that meets battery grade specifications. There is a balance, and it has been determined that to obtain an ultra-high purity MnSO4 solution, at least 6 times, preferably 7 times the stoichiometric amount of fluoride anions, based on the ratio of all Mg present in the MnSO4 solution, is adequately required. 2+ and Ca 2+ The cationic reaction is calculated with the fluoride anions required to form CaF2 and MgF2. Ammonium bifluoride (NH4F·HF) is selected as the fluoride anion source because it is less hazardous than HF solution. However, like HF, NH4F·HF is expensive, so the consumption and loss of NH4F·HF should be minimized. In addition, due to equilibrium, a portion of MnF2 is also precipitated as described above, which results in Mn loss and lower MnSO4 yield. The use of expensive reagents, the formation of hazardous fluoride wastes, and the loss of yield of Mn values ​​can all be mitigated by implementing at least some form of recycling.

[0063] Calculation of fluoride addition

[0064] The first step is to calculate the molar amounts of Mg and Ca in 4 liters of MnSO4 solution as follows:

[0065] Ca=0.593g=0.014825mol

[0066] Mg=1.755g=0.072mol

[0067] Total mol (Ca+Mg) = 0.08705 mol / liter PLS.

[0068] Based on a 7-fold stoichiometric excess, the amount of NH4F·HF required is calculated as follows:

[0069] NH4F·HF to be added = 7 × 0.08705 mol / liter × 57 g / mol NH4F·HF × 4 liters = 138.93 g

[0070] experiment

[0071] First, 4 liters of the enriched leaching solution was transferred to a 5 liter plastic polypropylene beaker. 138.93 g of NH4F·HF was added thereto while stirring. The pH value dropped from about 6 or 7 to about 3. No precipitate appeared at this pH. 195 ml of NH4OH (25%) solution was slowly added to this solution while stirring until the pH reached 5 to form a suspension. After stirring for 2 hours, the suspension was filtered. The resulting precipitate or filter cake was washed to recycle the MnSO4 trapped in the wet filter cake. The washed filter cake was then dried at 150°C to obtain 78.5 grams of fluoride residue. The analysis of the washed fluoride precipitate or filter cake (i.e., residue) is provided in Table 2.

[0072] Table 2: Fluoride precipitate composition (weight %)

[0073]

[0074]

[0075] Although the fluoride precipitate removes most of the Ca and Mg from the enriched leach solution of MnSO4, it was found that after filtration, the remaining enriched leach solution (i.e., the filtrate) still contained more than 50 ppm of Ca and Mg, making the filtrate non-compliant with battery grade specifications (which require Ca and Mg to be less than 50 ppm and a minimum Mn concentration of 32%). The reason for this is that some of the MgF2 and CaF2 precipitated particles are too small to be filtered under normal laboratory conditions. The solution to this problem proposed as part of the present invention is to add MnS, which acts as a filter aid to filter the small CaF2 and MgF2 particles, and also acts as a precipitant for precipitating transition elements such as Co, Fe, Ni, Cu and Zn.

[0076] Sulfide precipitates

[0077] Just as the fluoride precipitate forms an equilibrium, the sulfide precipitate also forms an equilibrium. Experimentally, it has been found that a rich leach solution of about 0.06 mol MnS / liter MnSO4 is sufficient to act as a filter aid to help remove the remaining Ca and Mg. It can be seen from the following calculations that it is also sufficient to precipitate all the transition elements.

[0078] Calculation of transition elements:

[0079] Transition elements in 4L enriched leaching solution (mol):

[0080] Co=0.0983g=0.0017mol / liter

[0081] Fe = 0.0139 g = 0.00025 mol / liter

[0082] Ni=0.0554g=0.00095mol / liter

[0083] Zn=0.0023g=0.000034mol / liter

[0084] Total moles of transition elements per 4 liters of enriched leaching solution = 0.011736 mol

[0085] Freshly precipitated MnS was prepared by the reaction of Na2S (60%) and battery grade MnSO4 to produce a suspension of MnS precipitate and Na2SO4 (aq). The MnS precipitate was filtered from the suspension and washed to remove any Na2SO4 contamination.

[0086] A total of 0.24 mol of moist freshly precipitated MnS was added to the remaining 4 liters of pregnant leach solution and heated to 60°C. After stirring at this temperature for 2 hours, the black precipitate was filtered out to form a filter cake. The filter cake was washed to remove and recycle the trapped MnSO4 and then dried at 150°C to produce 54 g of sulfide residue. Table 3 provides an analysis of the sulfide precipitate. It is believed that the amount of MnS per liter of pregnant leach solution can be further optimized to reduce this amount.

[0087] Table 3: Composition of sulfide precipitate (weight %)

[0088] Si Ca Al Fe Mg Mn Ni Pb Zn S 0.24 0.12 0.015 0.01 0.29 49.5 0.356 0.01 0.017 8.17

[0089] The final purified MnSO4 solution had the analysis listed in Table 4.

[0090] Table 4: Final MnSO4 solution after fluoride and sulfide precipitation (mg / l)

[0091] Ca Co Fe Mg Mn Ni Zn 3.91 <1 <1 3.76 97843 <1 <1 12.8ppm <1ppm <1ppm 12.3ppm 32％ <1ppm <1ppm <50ppm <10ppm <10ppm <50ppm 32％ <10ppm <10ppm

[0092] The last row in Table 4 is the maximum allowed impurity levels for Ca, Co, Fe, Mg, Ni and Zn in battery grade MnSO4.H2O. The third row in Table 4 is the levels of Ca, Co, Fe, Mg, Ni and Zn impurities after fluoride and sulfide precipitation.

[0093] Recycling of reagents

[0094] From Table 1, the total Mn content in the pregnant leach solution can be calculated as follows:

[0095] 4 liters x 127.6 g / l = 510.4 g Mn.

[0096] The loss of Mn units to fluoride and sulfide precipitates can also be calculated as follows:

[0097] Fluoride precipitate: 78.5 g x 37.7% = 29.6 g = 5.8% of the starting MnSO4 solution.

[0098] Sulfide precipitate: 54 g x 49.5% = 27 g = 5.3% of the starting MnSO4 solution.

[0099] The fluoride precipitate and the sulfide precipitate were dry-ground to -106 μm, respectively. 110 g (NH4)2SO4 -106 μm was added to 67.2 g of the fluoride precipitate, blended, and calcined at 450°C in a 3Cr12 stainless steel tube. The calcined product from the tube was soaked at temperature for 3 hours, and after cooling to room temperature, 101.6 g of a white cake was obtained. It was found that almost all of this product was water-soluble.

[0100] MnF2 and (NH4)2SO4 react in a calcined tube according to the following solid-state reaction:

[0101] MnF2(s)+(NH4)2SO4(s)=MnSO4(s)+NH4F.HF(g)+NH3(g)

[0102] NH4F·HF sublimes and condenses below about 240°C (eg, below 100°C) to collect about 90-99% of the available fluoride.

[0103] NH3(g) can be scrubbed and nearly all of the MnSO4(s) can be recycled back into the MnSO4 rich leach solution.

[0104] 130 g of (NH4)2SO4 -106 μm was added to 20 g of the sulfide precipitate, blended, and calcined at 450° C. in a 3Cr12 stainless steel tube. The calcined product from the tube was soaked at temperature for 3 hours, and after cooling to room temperature, 29.6 g of a white cake was obtained. It was found that almost all of this product was water-soluble.

[0105] MnS and (NH4)2SO4 react in a calcined tube according to the following solid-state reaction: MnS(s)+4(NH4)2SO4(s)=MnSO4(s)+4SO2(g)+4H2O(g)+8NH3(g)

[0106] 10 g samples of the white cake from both calcined products were dissolved in 50 ml water, filtered and analyzed. The analysis is provided in Table 5.

[0107] Table 5: MnSO4 solution obtained after dissolution of calcined product (mg / l)

[0108] Al Ca Co Fe Mg Mn Ni Zn 10g calcined fluoride filter cake <1 776 62 239 7702 54910 103 4.36 10g calcined sulfide filter cake 127 338 1189 832 479 75010 900 51.8

[0109] As should be noted, based on the analyses listed in Table 5, the MnSO4 solution obtained from the solid-state reaction of the fluoride precipitate and the sulfide precipitate with (NH4)2SO4 is suitable for recycling to the MnSO4 pregnant leach solution used at the beginning of the experiment to recover Mn values.

[0110] Referring to the single drawing, reference numeral 10 generally indicates a method for purifying a MnSO4 solution according to the present invention. The method 10 is a continuous method comprising a precipitation stage 12, the precipitation stage comprising a fluoride precipitation vessel 14, a pH adjustment vessel 16, and a sulfide precipitation vessel 18. The method 10 further comprises a first filter 20, a drying and pulverizing stage 22, a rotary kiln 24, a dissolving vessel 26, a second filter 28, a cooling condenser 30, and an exhaust gas scrubber 32.

[0111] The MnSO4 solution feed line 34 leads to the fluoride precipitation vessel 14, which is also connected to an NH4F·HF feed line 36. The fluoride precipitation vessel 14 is equipped with a stirrer 38.

[0112] A solution delivery line 40 leads from the fluoride precipitation vessel 14 to the pH adjustment vessel 16, which is also equipped with an agitator 42. An NH4OH solution feed line 44 leads to the pH adjustment vessel 16, while a suspension delivery line 46 leads from the pH adjustment vessel 16 to the sulfide precipitation vessel 18.

[0113] The sulfide precipitation vessel 18 is equipped with an agitator 48 , a steam heating coil 50 and a MnS feed line 52 .

[0114] The suspension delivery line 54 leads from the sulfide precipitation vessel 18 to the first filter 20, which is equipped with a purified MnSO4 solution discharge line 56 and a filter cake delivery line 58. The filter cake delivery line 58 leads to the drying and pulverizing workbench 22, which is equipped with a pulverized precipitate discharge line 60 leading to the rotary kiln 24. The pulverized precipitate discharge line 60 is connected to a (NH4)2SO4 feed line 62.

[0115] The rotary kiln 24 is a roasting kiln which is equipped with an exhaust gas discharge line 64 and a calcined precipitate discharge line 66 which lead to the dissolution vessel 26 .

[0116] The dissolution vessel 26 is equipped with an agitator 68 and a water feed line 70 .

[0117] A suspension discharge line 72 leads from the dissolution vessel 26 to the second filter 28 which is provided with a solid waste stream discharge line 74 and a MnSO 4 solution recycle line 76 which is connected to the MnSO 4 solution feed line 34 .

[0118] The method 10 is used to purify a leaching solution enriched in MnSO4 or other MnSO4 solution contaminated especially with Ca, Co, Fe, Mg, Ni and Zn etc. as impurities.

[0119] The enriched MnSO4 leach solution is fed to the fluoride precipitation vessel 14 via the MnSO4 solution feed line 34. The fluoride anion source (i.e., NH4F·HF) is fed to the fluoride precipitation vessel 14 via the NH4F·HF feed line 36, and an agitator 38 is used to thoroughly mix the two feed streams to produce a well-mixed solution. Sufficient NH4F·HF is fed via the NH4F·HF feed line 36 to ensure that a stoichiometric excess of fluoride anions is present in the fluoride precipitation vessel 14, equivalent to all the Mg present as impurities in the enriched MnSO4 leach solution. 2+ and Ca 2+ The amount of fluoride anion required for the cationic reaction is seven times that of the fluoride anion.

[0120] Although no visible fluoride precipitation occurs in the fluoride precipitation vessel 14, the pH of the well-mixed solution drops from about 6 or 7 to about 3. This well-mixed solution, having a pH of about 3, is then transferred via solution transfer line 40 to a pH adjustment vessel 16, wherein a base (i.e., sufficient NH4OH in the form of a 25% aqueous NH4OH solution) is transferred via NH4OH solution transfer line 44 to the pH adjustment vessel 16 to raise the pH of the solution in the pH adjustment vessel 16 to about 5. At the elevated pH in the pH adjustment vessel 16, fluorides such as CaF2, MgF2, and undesirable MnF2 begin to precipitate. Agitator 42 is used to ensure that a suspension is formed and maintained in the pH adjustment vessel 16. The pH adjustment vessel is sized to provide an average residence time of about 2 hours, and the stirred suspension is then transferred from the pH adjustment vessel 16 to the sulfide precipitation vessel 18 via suspension transfer line 46.

[0121] Particulate MnS is fed into the sulfide precipitation vessel 18 via the MnS feed line 52, and the suspension in the sulfide precipitation vessel 18 is stirred by the stirrer 48 and heated to about 60° C. by the steam heating coil 50. Sulfides are precipitated in the sulfide precipitation vessel 18, including sulfides of Ca, Mg, Co, Fe, Ni, and Zn. Thus, the CaS and MgS precipitates are in addition to the CaF2 and MgF2 precipitates formed in the pH adjustment vessel 16.

[0122] Particulate MnS is fed into the sulfide precipitation vessel 18 at a rate of approximately 0.06 mol MnS per liter of suspension fed into the sulfide precipitation vessel 18 via the suspension transfer line 46 .

[0123] The sulfide precipitation vessel 18 is sized to provide an average residence time of about 2 hours, and the agitated suspension containing the suspended black precipitate is then transferred via suspension transfer line 54 to a first filter 20 where the suspension is separated into a purified MnSO4 solution or filtrate which is discharged via a purified MnSO4 discharge line 56, and a precipitate or filter cake containing MnF2, CaF2 and MgF2 as well as MnS, CaS and MgS and transition metal sulfides and / or fluorides which is removed via a filter cake transfer line 58. Advantageously, the MnS acts as a filter aid to filter the small precipitated CaF2 and MgF2 from the MnSO4 solution while also acting as a precipitant for transition elements such as Co, Fe, Ni, Cu and Zn.

[0124] If desired or necessary, the filter cake may be washed with water (not shown) in a conventional manner, for example to remove entrapped MnSO 4 , and the filter cake wash water recycled to the fluoride precipitation vessel 14 or filter 20 , or to the dissolution vessel 26 .

[0125] In the drying and pulverizing station 22, the filter cake or precipitate is dried in a conventional manner (e.g., at about 150°C) and then pulverized (typically by dry grinding) to obtain a D90 particle size distribution of less than about 106 μm. The dried pulverized precipitate is discharged through the pulverized precipitate discharge pipeline 60 and fed to the roasting rotary kiln 24. Granular (NH4)2SO4, which also has a particle size distribution of D90 less than about 106 μm, is also fed to the rotary kiln 24 through the (NH4)2SO4 feed pipeline 62.

[0126] In the rotary kiln 24, the dried pulverized precipitate and particulate (NH4)2SO4 are thoroughly mixed and heated to an elevated temperature of about 450°C, resulting in a solid state reaction between the MnF2 and MnS present in the dried pulverized precipitate on the one hand and the (NH4)2SO4 on the other hand according to reactions (1) and (2) described above to produce MnSO4(s).

[0127] After a suitable long time (e.g., about 3 hours) at elevated temperature, the solid material (i.e., the calcined precipitate containing MnSO4(s)) is removed from the rotary kiln 24 through the calcined precipitate removal line 66 and fed to the dissolution vessel 26. Water is also supplied to the dissolution vessel 26 through the water supply line 70, whereby the suspension formed in the dissolution vessel 26 is stirred by the stirrer 68. As the MnSO4 dissolves in the water, a recycled MnSO4 solution is formed in the dissolution vessel 26.

[0128] The suspension containing the recycled MnSO4 solution is discharged from the dissolution vessel 26 through the suspension discharge line 72 and fed to the second filter 28, where insoluble impurities are separated from the recycled MnSO4 solution and discharged through the solid waste stream discharge line 74. The recycled MnSO4 solution is conveyed from the second filter 28 to the MnSO4 solution feed line 34 through the MnSO4 solution recirculation line 76.

[0129] The exhaust gas from the rotary kiln 24 is discharged through the exhaust gas discharge line 64, and is cooled and partially condensed in the cooling condenser 30. Plant cooling water is used in the cooling condenser 30 to partially condense the exhaust gas having a temperature of about 90° C. The condensed NH4F·HF from the cooling condenser 30 is conveyed into the NH4F·HF feed line 36 through the NH4F·HF recycle line 78. In this way, about 90% of the fluoride anions added to the process 10 are recovered and recycled to the fluoride precipitation vessel 14.

[0130] The cooled exhaust gas from the cooling condenser 30 is conveyed to the exhaust gas scrubber 32 through the cooled exhaust gas line 80, which scrubs NH3(g) and SO2(g) from the cooled exhaust gas using water fed through the water feed line 82. The scrubbed exhaust gas is exhausted from the exhaust gas scrubber 32 to the atmosphere through the exhaust line 84, and the excess ammonia-containing scrubbing liquid is discharged through the scrubber solution discharge line 86.

[0131] Although described as a continuous process, it should be understood that process 10 can be operated on a batch or semi-batch basis. For example, fluoride precipitation vessel 14, pH adjustment vessel 16, and sulfide precipitation vessel 18 can be combined into a single vessel operated on a batch basis.

[0132] As shown, a very large proportion of the fluoride anions are advantageously recycled in the process 10. As shown, the process 10 increases the yield of MnSO4 by including process steps for suppressing Mn loss and recycling Mn (e.g., via the rotary kiln 24, the dissolution vessel 26, and the second filter 28). Advantageously, the process of the present invention can produce a battery grade MnSO4 solution having less than 1 ppm Co, Fe, Ni, and Zn and less than 15 ppm Ca and Mg from an enriched MnSO4 leach solution.

Claims

1. Used for purification of MnSO 4 A method for a solution, the method comprising (i) in the presence of a stoichiometric excess of fluoride anions, wherein the stoichiometric excess of fluoride anions is based on the MnSO 4 All Mg present as impurities in the solution 2+ and Ca 2+ The cations react to form CaF 2 and MgF 2 The required fluoride anion is calculated, and (ii) in the presence of a sulfide anion, At a pH above 4, the MnSO 4 The impurities are precipitated from the solution to produce a purified MnSO containing as a carrier medium. 4 Solution and containing MnS, MnF 2 , CaF 2 and MgF 2 A slurry or suspension of suspended sediment; The slurry or suspension is separated into the purified MnSO 4 solution and said precipitate; Make the precipitate and remove MnSO 4 Other SO 4 The salt (sulfate) reacts in a solid state reaction to form MnS and MnF 2 Producing recycled MnSO 4 ;and The recovered MnSO 4 Recycle to the MnSO being purified 4 solution, thereby reducing Mn loss and improving MnSO 4 Yield.

2. The method according to claim 1, wherein the MnSO 4 The solution is a pregnant leach solution.

3. The method according to claim 1 or claim 2, comprising adding to the MnSO 4 Base is added to the solution to ensure that the pH is above 4.

4. The method according to any one of claims 1 to 3, in, In the MnSO 4 During precipitation of impurities in the solution, the pH is controlled between 5 and 8, or between 5 and 7.

5. The method according to any one of claims 1 to 4, wherein the MnSO 4 A stoichiometric excess of fluoride anions is produced by adding a stoichiometric excess of a water-soluble fluoride salt or hydrofluoric acid to the solution.

6. The method of claim 5, wherein the stoichiometric excess of fluoride anions is generated by adding a stoichiometric excess of a water-soluble fluoride salt.

7. The method of claim 6, wherein the water-soluble fluoride salt is NH 4 F. HF.

8. The method according to any one of claims 1 to 7, wherein the step of adding a 4 ) 2 S.H. 2 S(g) and a mixture of two or three thereof are preferably selected from the group consisting of the sulfur anion source and the MnSO 4 The sulfide anions are generated in the solution.

9. The method of claim 8, wherein the source of sulfur anions is MnS.

10. The method according to any one of claims 1 to 9, wherein the MnSO 4 Precipitating impurities from the solution includes first adding the MnSO 4 A fluoride anion source is added to the solution, and then the MnSO 4 A base was added to the solution to ensure that the pH was above 4, and then the MnSO 4 A source of sulfur anions is added to the solution.

11. The method according to any one of claims 1 to 10, wherein the MnSO 4 All the Mg in the solution 2+ and Ca 2+ The cations react to form CaF 2 and MgF 2 The fluoride anions are present in the MnSO in a stoichiometric excess of at least 6 times the stoichiometric amount or at least 7 times the stoichiometric amount, calculated based on the fluoride anions required. 4 in solution.

12. The method according to any one of claims 1 to 11, wherein the precipitate is mixed with a 4 Other SO 4 The salt is reacted in a solid state reaction comprising reacting the precipitate with (NH 4 ) 2 SO 4 or NH 4 HSO 4 reaction.

13. The method according to claim 12, comprising partially condensing the precipitate and the SO 4 The solid-state reaction of the salt produces an exhaust gas, and condenses and recovers NH 4 F·HF, and recycling NH recovered from the exhaust gas 4 F·HF acts as a fluoride anion source, thereby providing the stoichiometric excess of fluoride anions for the reaction of the MnSO 4 Ca and Mg impurities are precipitated in the solution.

14. The method according to any one of claims 1 to 13, wherein the precipitate and the SO 4 The solid state reaction between the salts occurs at a temperature in the range of 375°C to 525°C, or in the range of 400°C to 500°C, or in the range of 425°C to 475°C.

15. The method according to any one of claims 1 to 14, wherein the recovered MnSO 4 As recycled MnSO 4 The solution is recycled to the MnSO 4 in solution.

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