Purification of MnSO4 Solutions

Abstract

The present disclosure describes a process for purifying a MnSO.sub.4 solution including precipitating impurities from the MnSO.sub.4 solution (i) in the presence of a stochiometric excess of fluoride anions, where said stochiometric excess of fluoride anions is calculated on the basis of the fluoride anions required to react with all Mg.sup.2+ and Ca.sup.2+ cations present in the MnSO.sub.4 solution as impurities to form CaF.sub.2 and MgF.sub.2, and (ii) in the presence of sulphide anions. The precipitation is effected at a pH higher than 4, producing a slurry or suspension comprising a purified MnSO.sub.4 solution as a carrier medium and a suspended precipitate which includes MnS, MnF.sub.2, CaF.sub.2 and MgF.sub.2. The slurry or suspension is separated into the purified MnSO.sub.4 solution and the precipitate, whereafter the precipitate is reacted with a SO.sub.4 salt other than MnSO.sub.4 in a solid-state reaction to produce recovered MnSO.sub.4 from the MnS and MnF.sub.2.

Claims

1. A process for purifying a MnSO.sub.4 solution, the process including precipitating impurities from the MnSO.sub.4 solution (i) in the presence of a stochiometric excess of fluoride anions, where said stochiometric excess of fluoride anions is calculated on the basis of the fluoride anions required to react with all Mg.sup.2+ and Ca.sup.2+ cations present in the MnSO.sub.4 solution as impurities to form CaF.sub.2 and MgF.sub.2, and (ii) in the presence of sulphide anions, at a pH higher than 4, producing a slurry or suspension comprising a purified MnSO.sub.4 solution as a carrier medium and a suspended precipitate which includes MnS, MnF.sub.2, CaF.sub.2 and MgF.sub.2; separating the slurry or suspension into said purified MnSO.sub.4 solution and said precipitate; reacting the precipitate with a SO.sub.4 salt (sulfate salt) other than MnSO.sub.4 in a solid-state reaction to produce recovered MnSO.sub.4 from the MnS and MnF.sub.2; and recycling the recovered MnSO.sub.4 to the MnSO.sub.4 solution which is being purified, thereby to reduce Mn losses and improve MnSO.sub.4 yield.

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

3. The process according to claim 1, which includes adding a base to the MnSO.sub.4 solution to ensure that the pH is higher than 4.

4. The process according to claim 1, wherein, during the precipitation of the impurities from the MnSO.sub.4 solution, the pH is controlled at between 5 and 8, or between 5 and 7.

5. The process according to claim 1, wherein the stochiometric excess of fluoride anions is generated by the addition of a stochiometric excess of a water-soluble fluoride salt or hydrofluoric acid to the MnSO.sub.4 solution.

6. The process according to claim 5, wherein the stochiometric excess of fluoride anions is generated by the addition of a stochiometric excess of a water-soluble fluoride salt.

7. The process according to claim 6, wherein the water-soluble fluoride salt is NH.sub.4F.Math.HF.

8. The process according to claim 1, in which the sulphide anions are generated in the MnSO.sub.4 solution by the addition of a source of sulphide anions selected from the group consisting of MnS, (NH.sub.4).sub.2S, H.sub.2S(g) and mixtures of two or three thereof.

9. The process according to claim 8, in which the source of sulphide anions is MnS.

10. The process according to claim 1, wherein precipitating impurities from the MnSO.sub.4 solution includes first adding a source of fluoride anions to the MnSO.sub.4 solution, thereafter adding a base to the MnSO.sub.4 solution to ensure that the pH is higher than 4, and thereafter adding a source of sulphide anions to the MnSO.sub.4 solution.

11. The process according to claim 1, wherein the fluoride anions are present in the MnSO.sub.4 solution in a stochiometric excess of at least 6 times the stochiometric amount, or at least 7 times the stochiometric amount, calculated on the basis of the fluoride anions required to react with all Mg.sup.2+ and Ca.sup.2+ cations present in the MnSO.sub.4 solution to form CaF.sub.2 and MgF.sub.2.

12. The process according to claim 1, wherein reacting the precipitate with a SO.sub.4 salt other than MnSO.sub.4 in a solid-state reaction includes reacting the precipitate with (NH.sub.4).sub.2SO.sub.4 or NH.sub.4HSO.sub.4.

13. The process according to claim 12, which includes partially condensing off-gas generated by the solid-state reaction of the precipitate with the SO.sub.4 salt to condense and recover NH.sub.4F.Math.HF from the off-gas, and recycling the NH.sub.4F.Math.HF recovered from the off-gas to act as a source of fluoride anions thereby to provide the stochiometric excess of fluoride anions for precipitating Ca and Mg impurities from the MnSO.sub.4 solution.

14. The process according to claim 1, wherein the solid-state reaction between the precipitate and the SO.sub.4 salt takes place 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 process according to claim 1, wherein the recovered MnSO.sub.4 is recycled to the MnSO.sub.4 solution as a recycle MnSO.sub.4 solution.

Description

EXPERIMENTAL

[0063] The 4 litres of the pregnant leach solution was first transferred into a 5-litre plastic polypropene beaker. To this 138.93 g NH.sub.4F.Math.HF was added while stirring. The pH dropped from about 6 or 7 to about 3. At this pH, no precipitation occurred. To this solution, 195 ml of NH.sub.4OH (25%) solution was slowly added while stirring until the pH reached 5, with a suspension forming. After 2 hours of stirring, the suspension was filtered. A resulting precipitate or filter cake was washed to recycle MnSO.sub.4 trapped in the moist filter cake. The washed filter cake was then dried at 150 C. and yielded 78.5 g fluoride residue. The analysis of the washed fluoride precipitate or filter cake, i.e. residue, is provided in Table 2.

TABLE-US-00002 TABLE 2 Fluoride precipitate composition (wt %) Si Al Ca Fe Mg Mn Sr Pb 2.91 0.095 2.49 0.008 8.27 37.7 0.019 0.007

[0064] Although the fluoride precipitation removed a significant portion of the Ca and Mg from the pregnant leach solution of MnSO.sub.4, it was found that, after filtration, the remaining pregnant leach solution (i.e. filtrate) still contains Ca and Mg levels at more than 50 ppm so that the filtrate does not comply with battery grade specifications, which require Ca and Mg at less than 50 ppm, with a minimum Mn concentration of 32%. The reason for this is that some of the MgF.sub.2 and CaF.sub.2 precipitated particles are too small to filter under normal laboratory conditions. The solution to this problem proposed as part of the present invention is the addition of MnS which acts as a filter aid to filter the small CaF.sub.2 and MgF.sub.2 particles, and also acts as a precipitation agent for the precipitation of transition elements such as Co, Fe, Ni, Cu, and Zn.

Sulphide Precipitation

[0065] Just like the fluoride precipitate forms an equilibrium, the sulphide precipitate also forms an equilibrium. Experimentally, it was found that about 0.06 mol of MnS/litre of pregnant leach solution of MnSO.sub.4 is enough to act as a filter aid to assist with the removal of the remaining Ca and Mg. It is also enough to precipitate all the transition elements as can be seen in the calculations below.

Calculations of Transition Elements:

[0066] The mols transition elements in the 4 litre pregnant leach solution: [0067] Co=0.0983 g=0.0017 mol/litre [0068] Fe=0.0139 g=0.00025 mol/litre [0069] Ni=0.0554 g=0.00095 mol/litre [0070] Zn=0.0023 g=0.000034 mol/litre [0071] Total mols of transition elements per 4 litres of pregnant leach solution=0.011736 mols

[0072] Freshly precipitated MnS was prepared via the reaction of Na.sub.2S (60%) and battery grade MnSO.sub.4 to produce a suspension of MnS precipitate and Na.sub.2SO.sub.4(aq). The MnS precipitate was filtered from the suspension and washed from any Na.sub.2SO.sub.4 contamination.

[0073] A total of 0.24 mol of the moist freshly precipitated MnS was added to the remainder of the 4 litres of the pregnant leach solution and heated to 60 C. After 2 hours of stirring at this temperature, a black precipitate was filtered to form a filter cake. The filter cake was washed to remove and recycle trapped MnSO.sub.4, and then dried at 150 C., yielding 54 g sulphide residue. Table 3 provides the analysis of the sulphide precipitate. It is believed that the amount of MnS per litre pregnant leach solution can be further optimised to reduce the amount.

TABLE-US-00003 TABLE 3 Sulphide Precipitate composition (wt %) 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

[0074] The final, purified MnSO.sub.4 solution had the analysis as set out in Table 4.

TABLE-US-00004 TABLE 4 Final MnSO.sub.4 solution after fluoride and sulphide precipitation (mg/l) Ca Co Fe Mg Mn Ni Zn 3.91 <1 <1 3.76 97843 <1 <1 12.8 ppm <1 ppm <1 ppm 12.3 ppm 32% <1 ppm <1 ppm <50 ppm <10 ppm <10 ppm <50 ppm 32% <10 ppm <10 ppm

[0075] The last row in Table 4 is the maximum allowable impurities levels for battery grade MnSO.sub.4.Math.H.sub.2O, for Ca, Co, Fe, Mg, Ni and Zn. The third row in Table 4 is the level of Ca, Co, Fe, Mg, Ni and Zn impurities after fluoride and sulphide precipitation.

Recycling of Reagents

[0076] From Table 1 the total Mn amount in the pregnant leach solution can be calculated as follows:

[00001]$4\mathrm{litres}127.6g/l=510.4g\mathrm{Mn}.$

[0077] The Mn units lost to the fluoride and sulphide precipitates can also be calculated as follows: [0078] Fluoride precipitate: 78.5 g37.7%=29.6 g=5.8% of the starting MnSO.sub.4 solution. [0079] Sulphide precipitate: 54 g49.5%=27 g=5.3% of the starting MnSO.sub.4 solution.

[0080] The fluoride precipitate and the sulphide precipitate were separately dry milled to 106 m. To 67.2 g of the fluoride precipitate, 110 g (NH.sub.4).sub.2SO.sub.4106 m was added, blended, and fired in a 3Cr12 stainless steel tube at 450 C. Calcined product from the tube was soaked for 3 hours at temperature, and after cooling to room temperature yielded 101.6 g white cake. It was found that almost all of this product is water soluble.

[0081] The MnF.sub.2 and the (NH.sub.4).sub.2SO.sub.4 reacted in the fired tube according to the following solid-state reaction:

MnF.sub.2(s)+(NH.sub.4).sub.2SO.sub.4(s)=MnSO.sub.4(s)+NH.sub.4F.Math.HF(g)+NH.sub.3(g)

[0082] The NH.sub.4F.Math.HF sublimes and can be condensed below about 240 C., e.g. below 100 C. to collect approximately 90-99% of the available fluorides.

[0083] The NH.sub.3(g) can be scrubbed and almost all the MnSO.sub.4(s) can be recycled back to the pregnant leach solution of MnSO.sub.4.

[0084] To 20 g of the sulphide precipitate, 130 g (NH.sub.4).sub.2SO.sub.4106 m was added, blended, and fired in a 3Cr12 stainless steel tube at 450 C. Calcined product from the tube was soaked for 3 hours at temperature, and after cooling to room temperature yielded 29.6 g white cake. It was found that almost all of this product is water soluble.

[0085] The MnS and the (NH.sub.4).sub.2SO.sub.4 reacted in the fired tube according to the following solid-state reaction:

MnS(s)+4(NH.sub.4).sub.2SO.sub.4(s)=MnSO.sub.4(s)+4SO.sub.2(g)+4H.sub.2O(g)+8NH.sub.3(g)

[0086] 10 g samples of the white cakes from both calcination products were dissolved in 50 ml water, filtered, and analysed. The analysis is provided in Table 5.

TABLE-US-00005 TABLE 5 MnSO.sub.4 solutions obtained after dissolution of calcination products (mg/l) Al Ca Co Fe Mg Mn Ni Zn 10 g <1 776 62 239 7702 54910 103 4.36 calcined fluoride filter cake 10 g 127 338 1189 832 479 75010 900 51.8 calcined sulphide filter cake

[0087] As will be noted, based on the analysis set out in Table 5, the MnSO.sub.4 solutions obtained from the solid-state reaction of the fluoride precipitate and the sulphide precipitate with (NH.sub.4).sub.2SO.sub.4 are suitable for recycling to the pregnant leach solution of MnSO.sub.4 used at the start of the experiment, to recover Mn values.

[0088] Referring to the single drawing, reference numeral 10 generally indicates a process in accordance with the invention for purifying a MnSO.sub.4 solution. The process 10 is a continuous process and includes a precipitation stage 12 comprising a fluoride precipitation vessel 14, a pH adjustment vessel 16 and a sulphide precipitation vessel 18. The process 10 further includes a first filter 20, a drying and comminuting stage 22, a rotary kiln 24, a dissolution vessel 26, a second filter 28, a cooler condenser 30 and an off-gas scrubber 32.

[0089] A MnSO.sub.4 solution feed line 34 leads to the fluoride precipitation vessel 14, which is also joined by an NH.sub.4F.Math.HF feed line 36. The fluoride precipitation vessel 14 is provided with a stirrer 38.

[0090] A solution transfer line 40 leads from the fluoride precipitation vessel 14 to the pH adjustment vessel 16, which is also provided with a stirrer 42. An NH.sub.4OH solution feed line 44 leads into the pH adjustment vessel 16, whereas a suspension transfer line 46 leads from the pH adjustment vessel 16 to the sulphide precipitation vessel 18.

[0091] The sulphide precipitation vessel 18 is provided with a stirrer 48, a steam heating coil 50 and a MnS feed line 52.

[0092] A suspension transfer line 54 leads from the sulphide precipitation vessel 18 to the first filter 20, which is provided with a purified MnSO.sub.4 solution withdrawal line 56 and a filter cake transfer line 58. The filter cake transfer line 58 leads to the drying and comminuting stage 22, which is provided with a comminuted precipitate withdrawal line 60 which leads to the rotary kiln 24. The comminuted precipitate withdrawal line 60 is joined by a (NH.sub.4).sub.2SO.sub.4 feed line 62.

[0093] The rotary kiln 24 is a fired kiln which is provided with an off-gas withdrawal line 64 and a calcined precipitate withdrawal line 66, which leads to the dissolution vessel 26.

[0094] The dissolution vessel 26 is provided with a stirrer 68 and a water feed line 70.

[0095] A suspension withdrawal line 72 leads from the dissolution vessel 26 to the second filter 28, which is provided with a solids waste stream withdrawal line 74 and a MnSO.sub.4 solution recycle line 76 which joins the MnSO.sub.4 solution feed line 34.

[0096] The process 10 is used to purify a pregnant MnSO.sub.4 leach solution or other MnSO.sub.4 solution contaminated inter alia with Ca, Co, Fe, Mg, Ni and Zn as impurities.

[0097] The pregnant MnSO.sub.4 leach solution is fed by means of the MnSO.sub.4 solution feed line 34 into the fluoride precipitation vessel 14. A source of fluoride anions, namely NH.sub.4F.Math.HF is fed by means of the NH.sub.4F.Math.HF feed line 36 into the fluoride precipitation vessel 14 and the stirrer 38 is used to mix the two feed streams thoroughly, producing a well-mixed solution. Sufficient NH.sub.4F.Math.HF is fed by means of the NH.sub.4F.Math.HF feed line 36 to ensure a stochiometric excess of fluoride anions present in the fluoride precipitation vessel 14, equivalent to seven times the fluoride anions required to react with all Mg.sup.2+ and Ca.sup.2+ cations present as impurities in the pregnant MnSO.sub.4 leach solution.

[0098] Although no visible fluoride precipitation takes place 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 at a pH of about 3 is then transferred by means of the solution transfer line 40 to the pH adjustment vessel 16, where a base, namely sufficient NH.sub.4OH in the form of a 25% NH.sub.4OH aqueous solution, is fed into the pH adjustment vessel 16 by means of the NH.sub.4OH solution feed line 44 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, precipitation of fluorides such as CaF.sub.2, MgF.sub.2 and, undesirably, MnF.sub.2, commences. The stirrer 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, whereafter the agitated suspension is transferred from the pH adjustment vessel 16 to the sulphide precipitation vessel 18 by means of the suspension transfer line 46.

[0099] Particulate MnS is fed into the sulphide precipitation vessel 18 by means of the MnS feed line 52 and the suspension inside the sulphide precipitation vessel 18 is agitated by means of the stirrer 48, and heated to about 60 C. by means of the steam heating coil 50. In the sulphide precipitation vessel 18 sulphides precipitate, including sulphides of Ca, Mg, Co, Fe, Ni and Zn. The CaS and MgS precipitates are thus in addition to the CaF.sub.2 and MgF.sub.2 precipitates that formed in the pH adjustment vessel 16.

[0100] The particulate MnS is fed into the sulphide precipitation vessel 18 at a ratio of about 0.06 mol of MnS per litre of suspension fed into the sulphide precipitation vessel 18 by means of the suspension transfer line 46.

[0101] The sulphide precipitation vessel 18 is sized to provide an average residence time of about 2 hours, whereafter the agitated suspension, comprising a suspended black precipitate, is transferred by means of the suspension transfer line 54 to the first filter 20, where the suspension is separated into a purified MnSO.sub.4 solution or filtrate which is withdrawn by means of the purified MnSO.sub.4 withdrawal line 56 and a precipitate or filter cake, which includes MnF.sub.2, CaF.sub.2 and MgF.sub.2 as well as MnS, CaS and MgS and transition metal sulphides and/or fluorides, which is removed by means of the filter cake transfer line 58. Advantageously, the MnS acts as a filter aid to filter small precipitated CaF.sub.2 and MgF.sub.2 from the MnSO.sub.4 solution, whilst also acting as a precipitation agent for transition elements such as Co, Fe, Ni, Cu and Zn.

[0102] If desired or necessary, the filter cake may be washed (not shown) with water in conventional fashion, e.g. to remove trapped MnSO.sub.4, with filter cake wash water being recycled to the fluoride precipitation vessel 14 or to the filter 20 or to the dissolution vessel 26.

[0103] In the drying and comminuting stage 22, the filter cake or precipitate is dried in conventional fashion, e.g. at about 150 C. and then comminuted, typically by means of dry milling, to have a D90 particle size distribution of less than about 106 m. The dry, comminuted precipitate is withdrawn by means of the comminuted precipitate withdrawal line 60 and fed to the fired rotary kiln 24. Particulate (NH.sub.4).sub.2SO.sub.4, also with a particle size distribution with a D90 of less than about 106 m, is also fed to the rotary kiln 24 by means of the (NH.sub.4).sub.2SO.sub.4 feed line 62.

[0104] In the rotary kiln 24, the dried, comminuted precipitate and the particulate (NH.sub.4).sub.2SO.sub.4 are thoroughly admixed and heated to an elevated temperature of about 450 C., leading to solid-state reactions between the MnF.sub.2 and the MnS present in the dried, comminuted precipitate on the one hand, and the (NH.sub.4).sub.2SO.sub.4 on the other hand, according to Reactions (1) and (2) as hereinbefore described, to produce MnSO.sub.4(s).

[0105] After a suitably long time at the elevated temperature, e.g. about 3 hours, a solids material, i.e. calcined precipitate comprising MnSO.sub.4(s), is withdrawn from the rotary kiln 24 by means of the calcined precipitate withdrawal line 66 and fed to the dissolution vessel 26. Water is also fed into the dissolution vessel 26 by means of the water feed line 70 with a resultant suspension formed in the dissolution vessel 26 being agitated by means of the stirrer 68. A recycle MnSO.sub.4 solution is formed inside the dissolution vessel 26 as a result of the dissolving of the MnSO.sub.4(s) in the water.

[0106] A suspension comprising the recycle MnSO.sub.4 solution is withdrawn from the dissolution vessel 26 by means of the suspension withdrawal line 72 and fed to the second filter 28, where insoluble impurities are separated from the recycle MnSO.sub.4 solution and withdrawn by means of the solids waste stream withdrawal line 74. The recycle MnSO.sub.4 solution is transferred from the second filter 28 to the MnSO.sub.4 solution feed line 34, by means of the MnSO.sub.4 solution recycle line 76.

[0107] Off-gas from the rotary kiln 24 is withdrawn by means of the off-gas withdrawal line 64, and cooled and partially condensed in the cooler condenser 30. Plant cooling water is used in the cooler condenser 30 partially to condense the off-gas at a temperature of about 90 C. Condensed NH.sub.4F.Math.HF from the cooler condenser 30 is transferred by means of the NH.sub.4F.Math.HF recycle line 78 to join the NH.sub.4F.Math.HF feed line 36. In this way, about 90% of the fluoride anions added to the process 10 is recovered and recycled to the fluoride precipitation vessel 14.

[0108] Cooled off-gas from the cooler condenser 30 is transferred by means of the cooled off-gas line 80 to the off-gas scrubber 32, which employs water fed by means of the water feed line 82 to scrub NH.sub.3(g) and SO.sub.2(g) from the cooled off-gas. Scrubbed off-gas is vented from the off-gas scrubber 32 to the atmosphere by means of the vent line 84, whereas excess ammoniacal scrubbing solution is withdrawn by means of the scrubber solution withdrawal line 86.

[0109] Although described as a continuous process, as will be appreciated, the process 10 can be operated on a batch basis or on a semi-batch basis. For example, the fluoride precipitation vessel 14, the pH adjustment vessel 16 and the sulphide precipitation vessel 18 can be combined into a single vessel operated on a batch basis.

[0110] In the process 10, as illustrated, a very large proportion of the fluoride anions is advantageously recycled. By including process steps to inhibit Mn losses and to recycle Mn (e.g. by means of the rotary kiln 24, the dissolution vessel 26 and the second filter 28), the process 10, as illustrated, increases MnSO.sub.4 yield. Advantageously, the process of the invention can produce a battery grade MnSO.sub.4 solution with less than 1 ppm Co, Fe, Ni and Zn, and less than 15 ppm Ca and Mg, from a pregnant MnSO.sub.4 leach solution.