Beneficiation of low concentration pre-reduced mno to produce mnso4

Abstract

A process for beneficiating a solid feedstock comprising manganese as pre-reduced manganese (II) oxide, to recover manganese comprised by the manganese (II) oxide as manganese (II) sulphate, comprises a step of forming a blend of the solid feedstock, a leaching agent comprising an acid, and water. The ratio of the solid feedstock to water, by mass, is no more than 1:1.8. Thus, a blended product mixture comprising solid hydrated manganese (II) sulphate is formed, from which manganese (II) sulphate is subsequently recovered as a manganese (II) sulphate solution and purified to form a manganese (II) sulphate pregnant liquor solution. Such recovery includes subjecting the blended product mixture to temperature treatment sufficient to dehydrate hydrated sulphates contained in it.

Description

[0001] BENEFICIATION OF LOW CONCENTRATION PRE-REDUCED MnO TO PRODUCE

[0002] MnSO4

[0003] Field of the invention

[0004] This invention relates to the production of manganese (II) sulphate (MnSO4). More specifically, the invention relates to the beneficiation of low-concentration pre-reduced manganese (II) oxide (MnO) to produce manganese (II) sulphate.

[0005] Background to the invention

[0006] The ferromanganese (FeMn) alloy industry is under pressure to deal with slags arising from the production of ferromanganese from manganese (Mn) ores, e.g. by smelting. Such slags are typically discarded in landfills or slag heaps which pose an environmental and health risk to surrounding ecosystems and communities. Disposal costs are also increasing. There is an estimated 53 Mt of slag in such slag dumps in South Africa alone.

[0007] On an elemental basis, ferromanganese alloys typically comprise manganese, iron (Fe), silicon (Si), and carbon (C). Manganese typically accounts for between 60% and 80 % by mass of the alloy.

[0008] A slag of a ferromanganese alloy will typically comprise manganese as pre-reduced manganese (II) oxide and silicon as silica (SiO2). Magnesium (Mg) may also be present, e.g. as magnesium oxide (MgO), calcium (Ca) as calcium oxide (CaO), and aluminium (Al) as alumina (AI2O3). Iron is usually absent, as all the iron is typically recovered to the alloy, although some trace amounts of iron could sometimes be present.

[0009] The term “pre-reduced”, e.g. in the sense of pre-reduced manganese (II) oxide referenced above, is in this specification used to mean originating from a higher oxidation state through an earlier chemical reduction in oxidation state. Preferably, such reduction resulted from an industrial process, i.e. not through natural environmental factors, although reduction as a result of natural environmental factors is not excluded from the scope of the invention.

[0010] For example, in the case of pre-reduced manganese (II) oxide, the term “pre-reduced” is meant to provide that the manganese (II) (Mn2+) of the manganese (II) oxide was reduced to manganese (II) by a prior industrial process, from higher manganese compounds such as manganese (II, III) oxide (MnsC ), manganese (III) oxide (M^ j), and manganese (IV) oxide (MnC>2), e.g. by subjecting a metalliferous, e.g. manganese, ore to smelting.

[0011] Manganese (II) oxide is usually present in low concentrations in such slags but nevertheless is readily available for recovery of manganese from the slag. This notwithstanding, the slag has very low value due to operating difficulties such as environmental hazards and special handling requirements associated with the recovery of manganese from the slag.

[0012] The manganese content of the pre-reduced manganese (II) oxide is nonetheless valuable, and it is known to process ferromanganese slags to recover manganese from such manganese (II) oxide.

[0013] More specifically, it is known that manganese can be leached from ferromanganese slags to recover at least some manganese from such slags. This is conventionally performed by vat leaching, wherein the slag is submerged in a leaching agent, typically comprising aqueous sulphuric acid (H2SO4).

[0014] To achieve economical yields and purities of manganese, a high slag to water (H2O) ratio is usually required of such conventional processes, typically in the region of 8 parts water to 1 part slag, particularly in the case of low concentration manganese (II) oxide ores, e.g. those which contain manganese (II) oxide in a proportion of no more than about 25% by mass. This presents a challenge from cost and operating perspectives.

[0015] It would be advantageous to provide a process for beneficiating slag containing pre-reduced manganese (II) oxide that uses less water while recovering economically attractive yields and purities of manganese.

[0016] Summary of the invention

[0017] In a first, broad, aspect, the present invention provides a process for beneficiating a solid feedstock comprising manganese as pre-reduced manganese (II) oxide, to recover manganese comprised by the manganese (II) oxide as manganese (II) sulphate, the process comprising a step of forming a blend of the solid feedstock, a leaching agent comprising an acid, and water, wherein the ratio of the solid feedstock to water, by mass, is no more than 1 :1.8, thus forming a blended product mixture comprising manganese (II) sulphate.

[0018] Preferably, the acid that is used is a sulphonating acid such that, in forming the blended product mixture, reactive components of the solid feedstock, typically comprising oxides and specifically metal oxides including the manganese (II) oxide, are sulphonated. Sulphuric acid is particularly preferred.

[0019] The amount of acid that is used is preferably in stoichiometric excess, e.g. up to 5% molar excess such as at 5% molar excess, to that which is required to sulphonate the oxide (O2') content, preferably the metal oxide content, of the solid feedstock. To determine the oxide content of the solid feedstock, e.g. using X-ray fluorescence (XRF), and to determine such a stoichiometric excess are within the skill of persons skilled in the art.

[0020] In a second, more specific, aspect, the present invention provides a process for beneficiating a solid feedstock comprising manganese as pre-reduced manganese (II) oxide, to recover manganese comprised by the manganese (II) oxide as manganese (II) sulphate, the process comprising

[0021] (a) forming a blended product mixture comprising manganese (II) sulphate according to the process of the first aspect of the invention, wherein the blended product mixture also comprises calcium sulphate (CaSC );

[0022] (b) subjecting the blended product mixture comprising manganese (II) sulphate and calcium sulphate to temperature treatment sufficient to produce a dry product mixture comprising manganese (II) sulphate and calcium sulphate;

[0023] (c) subjecting the dry product mixture comprising manganese (II) sulphate and calcium sulphate to dissolution in water to produce a dissolution product mixture comprising a manganese (II) sulphate solution and a first residue comprising calcium sulphate; and

[0024] (d) separating the manganese (II) sulphate solution from the first residue by solid-liquid separation.

[0025] The process may also include purifying the manganese (II) sulphate solution, to remove contaminants other than calcium, as calcium sulphate, from it. For example, such purification may be performed by means of jarosite precipitation. As noted in the background to the invention, in this specification the term “pre-reduced”, e.g. in the sense of pre-reduced manganese (II) oxide referenced above, is used to mean originating from a higher oxidation state through an earlier chemical reduction in oxidation state. Preferably, such reduction resulted from an industrial process, i.e. not through natural environmental factors, although reduction as a result of natural environmental factors is not excluded from the scope of the invention.

[0026] For example, in the case of pre-reduced manganese (II) oxide, the term “pre-reduced” is meant to provide that the manganese (II) of the manganese (II) oxide was reduced to manganese (II) by a prior industrial process, from higher manganese compounds such as manganese (II, III) oxide (MnsC ), manganese (III) oxide (M^ j), and manganese (IV) oxide (MnC>2), e.g. by subjecting a metalliferous, e.g. manganese, ore to smelting.

[0027] Brief description of the drawing

[0028] The invention is described in more detail below, including with reference to an embodiment of the invention that is illustrated in the accompanying drawing by way of non-limiting example only.

[0029] In the drawing:

[0030] Figure 1 shows an embodiment of the process of the more specific second aspect of the invention.

[0031] Detailed description of the invention

[0032] The inventor in the present invention has surprisingly found manganese can be recovered, as manganese (II) sulphate, at economically attractive yields and purities from solid feedstocks, and preferably metallurgical slags such as ferromanganese slags, comprising manganese as pre-reduced manganese (II) oxide, even when present in such feedstocks at lower concentrations, e.g. no more than 25% by mass, without using conventional vat leaching that typically requires about 8 parts water per 1 part feedstock.

[0033] More specifically, the inventor has surprisingly found that manganese can be so recovered from such a feedstock using a leaching agent, comprising an acid used in stoichiometric excess to that which is required to sulphonate the oxide content of the feedstock, and water, wherein the amount of water that is used is such that the overall ratio of feedstock to water is no more than 1 :1.8 (i.e. for every 1 part of solid feedstock, by mass, that is processed, no more than 1.8 parts of water, by mass, are used) to produce a blended product mixture comprising manganese (II) sulphate and other sulphate including metal sulphate impurities, from which the manganese (II) sulphate can be recovered at an economically attractive yield and purity as a manganese (II) sulphate solution.

[0034] It will be appreciated that the amount of water that is used in accordance with the invention is at least four times less than that which is typically used in the case of vat leaching, i.e. 1.8 parts according to the invention compared to 8 parts in conventional vat leaching.

[0035] Any suitable acid, i.e. any acid that is capable of producing the blended product mixture comprising manganese (II) sulphate, may be used. The preferred acid is a sulphonating acid, most preferably sulphuric acid, such that reactive components of the solid feedstock, including manganese (II) oxide in the solid feedstock, are sulphonated by the acid.

[0036] Thus, as noted above, the blended product mixture comprises manganese (II) sulphate and other sulphates including metal sulphate impurities. These may include, most prominently, calcium sulphate. In addition, magnesium sulphate (MgSC ), aluminium sulphate (Ah SC h), and iron sulphate (FeSC ) may also be present. This does not exclude other sulphates such as those of barium (Ba), potassium (K), sodium (Na), chromium (Cr), copper (Cu), nickel (Ni), lead (Pb), and zinc (Zn). The impurities present would depend on the composition of the solid feedstock.

[0037] When sulphuric acid is used, the sulphuric acid may be used at any available concentration.

[0038] In an embodiment of the invention, a preferred concentration of sulphuric acid, by mass, is from 90% to 100%, e.g. from 90% to below 100%, e.g. from 90% to 99%, more preferably above 95%, such as at 96%, with the difference being water. In other words, the preferred concentration of sulphuric acid preferably comprises no more than 10%, more preferably no more than 5%, e.g. 4%, water, by mass. Such water content is taken into account in observing the water limit that is applied in accordance with the invention.

[0039] Thus, the water component of the acid, when present, is preferably taken into account in observing the overall water limit, i.e. in observing a ratio of water to solid feedstock that is no more than 1 :1.8. In other words, the water of the blend of the solid feedstock, the leaching agent comprising the acid, and water, is preferably regarded as comprising water comprised by the leaching agent and / or free water that is combined with the solid feedstock and / or with the acid, in observing the water limit that is applied in accordance with the invention.

[0040] In addition, the inventor surprisingly found that the application of a temperature treatment to the blended product mixture, in the form of a calcium sulphate dead burning step, to dead burn hydrated calcium sulphate contained in the blended product mixture and dehydrate other hydrated sulphates comprised by the blended product mixture, including hydrated manganese (II) sulphate, contributes significantly to the recovery of manganese (II) sulphate at economically attractive yields and purities when using lower water volumes in accordance with the invention.

[0041] More specifically, surprisingly, in recovering manganese (II) sulphate in the blended product mixture through selective aqueous dissolution of the manganese (II) sulphate, many sulphate impurities that would otherwise have been recovered to a resulting manganese (II) sulphate solution are preferentially recovered to a residue of such dissolution. Without wishing to be bound by theory, the inventor understands that such impurities are unexpectedly rendered into highly insoluble sulphates by the dead burning step. This applies most prominently to calcium sulphate.

[0042] Furthermore, it was unexpectedly found that, without the dead burning step, the presence of hydrates in a dried blended product mixture makes the blended product mixture difficult to wash in a selective dissolution step, resulting in entrainment and non-recovery of a substantial portion of manganese (II) sulphate. Since the temperature treatment in the form of calcium sulphate dead burning also dehydrates hydrated sulphates comprised by the blended product mixture in producing a dry product mixture, the dry product mixture becomes easier to wash, thus improving manganese (II) sulphate recovery through selective dissolution.

[0043] The employment of a jarosite precipitation step further conveniently allows for the removal of certain remaining contaminants, including potassium, iron and aluminium from a manganese (II) sulphate solution recovered by selective dissolution after dead burning.

[0044] Each aspect or embodiment of the invention as defined herein may be combined with any other aspect(s) or embodiment(s) unless clearly indicated to the contrary. In particular, any feature(s) indicated as being preferred or advantageous may be combined with any other feature(s) indicated as being preferred or advantageous. In a first, broad, aspect, the present invention provides a process for beneficiating a solid feedstock comprising manganese as pre-reduced manganese (II) oxide, to recover manganese comprised by the manganese (II) oxide as manganese (II) sulphate, the process comprising a step of forming a blend of the solid feedstock, a leaching agent comprising an acid, and water, wherein the ratio of the solid feedstock to water, by mass, is no more than 1 :1.8, thus forming a blended product mixture comprising manganese (II) sulphate.

[0045] Any suitable acid that is capable of producing the blended product mixture comprising manganese (II) sulphate may be used. Preferably, the acid that is used is a sulphonating acid such that, in forming the blended product mixture, reactive components of the solid feedstock, typically oxides including metal oxides such as the manganese (II) oxide, are sulphonated. Sulphuric acid is particularly preferred.

[0046] The manganese (II) sulphate would typically be present in the blended product mixture as solid manganese (II) sulphate. It would most typically be present as hydrated, or crystalline, manganese (II) sulphate (MnSC .xFW wherein x > 0, e.g. wherein the MnSC .xFW comprises one or more species in which x is 1 , 4, 5, and / or 7).

[0047] The blended product mixture may also comprise calcium sulphate. The calcium sulphate would typically be present as solid calcium sulphate. It would most typically be present as hydrated, or crystalline, calcium sulphate (CaSC .xFW wherein x > 0, e.g. wherein x = 2 or x = 3 ). The presence of calcium sulphate in the blended product mixture would typically arise from reaction between calcium oxide in the solid feedstock and the leaching agent, when the leaching agent is a sulphonating acid.

[0048] The blended product mixture may also comprise other sulphate species produced by the sulphonation of other impurities than calcium impurities such as calcium oxide. For example, the blended product mixture may include magnesium sulphate (MgSC ), aluminium sulphate (Ah SC h), and / or iron sulphate (FeSC ), depending on the composition of the solid feedstock. This does not exclude other sulphates such as those of barium (Ba), potassium (K), sodium (Na), chromium (Cr), copper (Cu), nickel (Ni), lead (Pb), and / or zinc (Zn) from being present, depending on the composition of the solid feedstock. Typically, each of these would be present in the blended product mixture as hydrates, or crystalline forms, thereof. The formation of such other sulphate species would typically also have resulted from sulphonation of oxides thereof that were contained in the solid feedstock.

[0049] The process may include subjecting the blended product mixture containing manganese (II) sulphate to temperature treatment sufficient to produce a dry product mixture comprising manganese (II) sulphate and calcium sulphate, and any other sulphates that may be present.

[0050] It would be appreciated that such manganese (II) sulphate and calcium sulphate, as well as any other sulphates that may be present, would be in solid form. The calcium sulphate comprised by the dry product mixture may, in particular, be anhydrous calcium sulphate, and more specifically dead-burnt calcium sulphate. Preferably, other sulphates that are present in the dry product mixture, including the manganese (II) sulphate, are also dehydrated through the temperature treatment.

[0051] The temperature treatment may be performed as hereinafter described.

[0052] The process may also include subjecting the dry product mixture comprising manganese (II) sulphate and calcium sulphate, as well as other sulphates that may be present, to dissolution in water to produce a dissolution product mixture comprising a manganese (II) sulphate solution and a first residue comprising calcium sulphate. The dissolution product mixture may be a dissolution product slurry. In this regard too, the calcium sulphate would, of course, be in solid form. It would be appreciated that such dissolution is selective dissolution.

[0053] The dissolution may be performed as hereinafter described.

[0054] The process may also include separating the manganese (II) sulphate solution from the first residue by solid-liquid separation.

[0055] The solid-liquid separation may be performed as hereinafter described.

[0056] The process may also include subjecting the manganese (II) sulphate solution to purification, e.g. by means of jarosite precipitation, to obtain a manganese (II) sulphate pregnant leach solution.

[0057] The purification by jarosite precipitation may be performed as hereinafter described. In a second, more specific, aspect, the present invention provides a process for beneficiating a solid feedstock comprising manganese as pre-reduced manganese (II) oxide, to recover manganese comprised by the manganese (II) oxide as manganese (II) sulphate, the process comprising

[0058] (a) forming a blended product mixture comprising manganese (II) sulphate according to the process of the first aspect of the invention, wherein the blended product mixture also comprises calcium sulphate;

[0059] (b) subjecting the blended product mixture comprising manganese (II) sulphate and calcium sulphate to temperature treatment sufficient to produce a dry product mixture comprising manganese (II) sulphate and calcium sulphate;

[0060] (c) subjecting the dry product mixture comprising manganese (II) sulphate and calcium sulphate to dissolution in water to produce a dissolution product mixture comprising a manganese (II) sulphate solution and a first residue comprising calcium sulphate; and

[0061] (d) separating the manganese (II) sulphate solution from the first residue by solid-liquid separation.

[0062] The following statements apply both to the first and second aspects of the invention, as appropriate with reference to the recited features thereof.

[0063] The solid feedstock preferably comprises, and more preferably consists of, slag, i.e. a metallurgical slag, that was formed in the production of a metal or metal alloy such as a ferroalloy. Such alloys are typically produced by smelting a solid metalliferous feedstock, such as a metalliferous ore or a feedstock comprising metalliferous ore agglomerates, in the presence of a reductant such as carbon and other components such as fluxes.

[0064] Persons skilled in the art would know that, in the production of metals or metal alloys, liquid slags are initially formed as molten by-products which float on top of a smelted metal, allowing for the slag to be separated and thus recovered from the metal and for the metal to be recovered substantially free of the slag. When cooled, such liquid slags form solid slags.

[0065] In an embodiment of the invention in which the solid feedstock comprises a slag, the slag is in one embodiment of the invention a ferromanganese slag, i.e. slag formed in the production of ferromanganese. In another embodiment of the invention, comprises pre-reduced metalliferous ore, i.e. metalliferous ore that comprises pre-reduced manganese (II) oxide that was pre-reduced in, or optionally otherwise than in, the production of metals or metal alloys.

[0066] In addition to the manganese (II) oxide, the solid feedstock may include pre-reduced impurities such as one or combinations of any two or more of iron, silica, magnesium oxide, calcium oxide, and alumina. Impurities comprising barium (Ba), potassium (K), sodium (Na), chromium (Cr), copper (Cu), nickel (Ni), lead (Pb), and zinc (Zn) may also be present. Such impurities would also typically be present as oxides thereof.

[0067] In the case of ferromanganese slags, iron would typically be absent, or would only be present in a trace amounts, e.g. in a concentration of less than 5% by mass, more typically less than 2% by mass, e.g. about 1.8% by mass.

[0068] The solid feedstock is preferably in particulate form.

[0069] In an embodiment of the invention, the feedstock may comprise particles having a particle diameter of about 75pm. In an embodiment of the invention, at least about 80%, more preferably at least about 85% of particles in the feedstock may may have a particle diameter of about 75pm. Smaller than 75pm would also be acceptable.

[0070] The process may therefore include milling the feedstock to produce a milled feedstock, that is used as the feedstock in the invention. In the milled feedstock, it is preferred that at least about 80%, more preferably at least about 85% of particles have a particle diameter of about 75pm. Smaller than 75pm would also be acceptable. It is possible that the milled feedstock may be in the form of a slurry or suspension if milling is performed with water. The overall water limit must still be observed, however, in performing the invention.

[0071] In an embodiment of the invention, the concentration of pre-reduced manganese (II) oxide in the solid feedstock is no more than 25% by mass. For example, the concentration of prereduced manganese (II) oxide in the feedstock may be from 20% to 25% by mass. When the feedstock comprises or consists of a slag, these percentages would apply to the slag.

[0072] As has been noted above, in forming the blended product mixture, reactive components of the feedstock, including manganese (II) oxide and, typically, calcium oxide, as well as other impurities that may be present and are herein identified, would be sulphonated, i.e. formed into sulphates thereof. Such sulphates would most typically be hydrated, i.e. crystalline, sulphates.

[0073] Thus, as has also been noted above, the manganese (II) sulphate would typically be present in the blended product mixture as solid manganese (II) sulphate. It would most typically be present as hydrated, or crystalline, manganese (II) sulphate (MnSC .xHhO wherein x > 0, e.g. wherein the MnSC .xHhO comprises one or more species in which x is 1 , 4, 5, and / or 7).

[0074] As has also been noted above, calcium sulphate would typically be present in the blended product mixture as solid calcium sulphate. It would most typically be present as hydrated, or crystalline, calcium sulphate (CaSC .xFW wherein x > 0, e.g. wherein x = 2 or x = 3 ). The presence of calcium sulphate in the blended product mixture would typically arise from reaction between calcium oxide in the feedstock and the leaching agent, when the leaching agent is a sulphonating acid.

[0075] As has also been noted above, when the acid that is used is a sulphonating acid such as sulphuric acid then, in forming the blended product mixture, reactive components of the feedstock, including the manganese (II) oxide and calcium oxide, are sulphonated to form sulphates thereof, including the manganese (II) sulphate and the calcium sulphate.

[0076] As has also been indicated above, the blended product mixture may therefore also comprise other sulphate species produced by the sulphonation of other impurities than calcium impurities such as calcium oxide. For example, the blended product mixture may include magnesium sulphate (MgSC ), aluminium sulphate (Ah SC ), and / or iron sulphate (FeSC ), depending on the composition of the feedstock. This does not exclude other sulphates such as those of barium (Ba), potassium (K), sodium (Na), chromium (Cr), copper (Cu), nickel (Ni), lead (Pb), and / or zinc (Zn) from being present, depending on the composition of the feedstock. Typically, each of these would be present in the blended product mixture as hydrates, or crystalline forms, thereof. The formation of such other sulphate species would typically also have resulted from sulphonation of oxides thereof that were contained in the feedstock.

[0077] A stoichiometric excess of the acid is preferably used. Such an excess would be in excess to that which is stoichiometrically required to sulphonate the oxide content, typically the metal oxide content, of the feedstock. For example, the stoichiometric excess may in an embodiment of the invention be up to 5%, e.g. 5% molar excess. As has been noted above, when sulphuric acid is used, the sulphuric acid may be used at any available concentration. In an embodiment of the invention, a preferred concentration of sulphuric acid, by mass, is from 90% to 100%, e.g. from 90% to below 100%, e.g. from 90% to 99%, more preferably above 95%, such as at 96%, with the difference being water. In other words, the preferred concentration of sulphuric acid preferably comprises no more than 10%, more preferably no more than 5%, e.g. 4%, water, by mass.

[0078] As has also been noted above, the water component of the acid, when present, is preferably taken into account in observing the overall water limit, i.e. in observing the ratio of water to feedstock, preferably slag, as being no more than 1 :1.8. In other words, the water content of the blend of the feedstock, the leaching agent comprising the acid at the available or desired concentration, and water, is preferably calculated to include water comprised by the leaching agent in observing the ratio of water to feedstock, preferably slag, as being no more than 1 :1.8.

[0079] Forming the blended product mixture comprising manganese (II) sulphate may be a dry leaching step. This means that a reduced amount of water compared to that demanded by conventional vat leaching processes is used. More specifically, it means that a feedstock to water ratio of no more than 1 :1.8 by mass is used, in accordance with the invention.

[0080] The limited water usage enabled by the invention is advantageous at least in that it avoids the need to use expensive evaporating and / or crystallising equipment that negatively impact on process operation and economics.

[0081] In an embodiment of the invention, forming the blended product mixture comprising manganese (II) sulphate may comprise separate steps of forming a leaching mixture comprising the leaching agent; and forming a feedstock slurry, wherein the leaching mixture is subsequently blended with the feedstock slurry.

[0082] The overall water content of the leaching mixture and the feedstock slurry is preferably such that the ratio of feedstock to water, by mass, is no more than 1 :1.8, in accordance with the invention.

[0083] Forming the leaching mixture may comprise combining the leaching agent, i.e. the acid at the available or preferred concentration, with water. It will be appreciated that the water with which the leaching agent is combined would be in addition to water that may already be comprised by the leaching agent.

[0084] In an embodiment of the invention, such combination may be performed to produce a leaching mixture comprising, overall, from about 60% to about 70% of the acid, e.g. about 64% or about 65% of the acid. In this sense, the term “overall” is used to provide that, in calculating the acid content of the leaching mixture, the water content of the acid at the available or preferred concentration is accounted for along with the water with which the acid was combined.

[0085] In a case in which 96% sulphuric acid is used, for example, the 96% sulphuric acid may be combined with water in a ratio of approximately two-thirds of the 96% sulphuric acid to one- third water, by mass. It will be appreciated that, in such a case, the resulting leaching mixture would comprise, overall, about 64% by mass of the acid and about 36% water. A leaching mixture comprising about 65% by mass of the acid and about 35% by mass water is also suitable, in an embodiment of the invention.

[0086] The leaching mixture is preferably provided at a temperature of from 3°C to 8°C. Thus, in an embodiment of the invention, the leaching mixture may be a cooled or chilled mixture. Cooling or chilling may therefore be applied to the leaching mixture, or in forming the leaching mixture. In an embodiment of the invention, cooling or chilling may be supplied by the water component of the leaching mixture, i.e. the water with which the acid is combined to form the leaching mixture, e.g. by providing the water as ice.

[0087] Forming the feedstock slurry may include combining the feedstock with water.

[0088] The amount of water to be used in forming the feedstock slurry would typically be dictated by handling and blending considerations while observing the overall water limit that the invention imposes in providing the blended product mixture, i.e. that the overall water quantity that is used amounts to no more than 1 .8 parts water to 1 part feedstock, by mass.

[0089] In an embodiment of the invention, the feedstock slurry may comprise about 62% solids. For example, this may be the case when the leaching mixture comprises 64% or 65% of the acid and 36% or 35% water, by mass.

[0090] As noted above, forming the blended product mixture comprising manganese (II) sulphate may include blending the leaching mixture with the feedstock slurry. In forming the blended product mixture, an exothermic sulphonation reaction takes place and water that is formed as a result of the reaction is largely removed as steam that is generated as a result of the exothermic reaction.

[0091] The blended product mixture may be in the form of a paste. In other words, it may not be free flowing in contrast to, for example, the leaching mixture and the feedstock slurry. This may be as a result of the evaporation of water from the blended product mixture due to the exothermic nature of the sulphonation of the feedstock.

[0092] The blended product mixture is preferably a homogenous mixture.

[0093] In a continuous process, the blending of the leaching mixture with the feedstock slurry may be performed using suitable blending equipment. Preferably, such equipment comprises a pug mill.

[0094] The blended product mixture is then subjected to temperature treatment in accordance with the invention.

[0095] Preferably, the temperature treatment is preferably a so-called “dead burning” temperature treatment, meaning that it is performed such that the hydrated calcium sulphate is dehydrated to produce anhydrous calcium sulphate. This may include performing the temperature treatment at a suitable temperature and for a suitable duration for such dehydration.

[0096] In this regard it should be noted that while hydrated calcium sulphate, e.g. as its dihydrate CaSC>4.2H2O, dehydrates at around 170°C to 200°C, its anhydrous form produced under such conditions easily rehydrates when mixed with water. By dead-burning the hydrated calcium sulphate, which comprises heating it beyond 200°C, e.g. 200°C to 500°C, preferably 450°C, the anhydrous calcium sulphate that is produced becomes highly insoluble compared to that which is produced below 200°C. Such calcium sulphate is typically referred to as dead-burnt anhydrous calcium sulphate, which is what the invention prefers to be produced in the temperature treatment of the blended product mixture. The dry product mixture may therefore comprise solid anhydrous calcium sulphate, preferably dead-burnt anhydrous calcium sulphate. The temperature treatment may also be sufficient to dehydrate other, and preferably all other, hydrated sulphates contained in the blended product mixture, including the manganese (II) sulphate, such that anhydrous forms of such sulphates are comprised by the dry product mixture. The dry product mixture may therefore also comprise anhydrous forms of other sulphates previously present in hydrated form in the blended product mixture, including the manganese (II) sulphate. Essentially, the temperature treatment therefore results in substantially all of the hydrate mantles of hydrates contained in the blended product mixture being removed.

[0097] The temperature treatment would also preferably remove free water from the blended product mixture such that the blended product mixture is fully dried out in producing the dry product mixture.

[0098] Preferably, the temperature treatment is performed at a temperature of 200°C to 500°C, e.g. 450°C.

[0099] Preferably, the temperature treatment is performed for a treatment period of from 2 hours to 6 hours, e.g. 3 hours.

[0100] In an embodiment of the invention, the blended product mixture is subjected to the temperature treatment at 450°C for 3 hours.

[0101] The temperature treatment may be performed using any suitable equipment. For example, the temperature treatment may be performed using a rotary kiln.

[0102] During the temperature treatment, in addition to hydrated calcium sulphate being converted to anhydrous calcium sulphate, preferably dead-burnt anhydrous calcium sulphate, some of the impurities identified above may advantageously be converted to basic sulphates thereof. Such sulphates typically have a low solubility in water, thus making it easier to remove the impurities in downstream processing, e.g. by separating them from soluble compounds through selective dissolution of the soluble compounds in water.

[0103] Furthermore, the absence of hydrates facilitates washing to achieve selective dissolution, thus also improving recovery through such selective dissolution by preventing entrainment. In subjecting the dry product mixture comprising solid manganese (II) sulphate and anhydrous calcium sulphate to dissolution in water to produce the dissolution product mixture, water may be combined with the dry product mixture. In an embodiment of the invention, water may be combined with the dry product mixture at a dry product mixture to water ratio, by mass, of 1 :1.5.

[0104] The dissolution product mixture may, in an embodiment of the invention, be in the form of a dissolution product slurry.

[0105] The dissolution may comprise a washing step according to an embodiment of the invention, in which the dry product mixture is subjected to washing with water to dissolve manganese (II) sulphate contained in it. Successive washing steps may be employed.

[0106] The process may also include subjecting the dissolution product mixture to temperature treatment. For example, the dissolution product mixture may be heated to a temperature of about 60°C for about one hour.

[0107] Recovering the manganese (II) sulphate solution from the first residue would typically be performed by filtration, in which case the manganese (II) sulphate solution would be recovered as a filtrate and the first residue as a retentate.

[0108] The first residue would comprise high levels of solid calcium sulphate. Advantageously, it may also comprise the bulk of other impurities. The first residue may be processed further to recover the calcium sulphate as a by-product of the process. In this regard, the first residue may be washed one or more times and dried to remove water from it.

[0109] The manganese (II) sulphate solution separated from the first residue may still include some residual impurities rendering it unsuitable for downstream processing, e.g. processing to produce a manganese (II) sulphate electrolyte for a battery. Accordingly, the process may include subjecting the manganese (II) sulphate solution to a purification step.

[0110] Subjecting the manganese (II) sulphate solution separated from the first residue to a purification step may comprise a jarosite precipitation step. Persons skilled in the art would understand that jarosite precipitation is a known process that is used to remove iron and other impurities from sulphate solutions, in hydrometallurgical processes. It involves the precipitation of jarosite, a group of iron-hydroxysulphate minerals, from a hot, acidic solution. This is typically achieved by adding alkali metal or ammonium ions to the solution, causing the iron to precipitate out as jarosite

[0111] Subjecting the manganese (II) sulphate solution to a jarosite precipitation step may therefore produce a suspension of solid jarosite in a purified manganese (II) sulphate solution, from which the jarosite can be recovered to produce a manganese (II) sulphate pregnant liquor solution and a second residue comprising the jarosite.

[0112] The jarosite precipitation step may include subjecting the manganese (II) sulphate solution separated from the first residue to an oxidation step comprising contacting it with H2O2.

[0113] The jarosite precipitation step may also include adjusting the pH of the manganese (II) sulphate solution, to obtain a pH-adjusted solution. Preferably, a target pH of between 3.5 and 4 should be achieved.

[0114] Adjusting the pH may include adding an acid to the manganese (II) sulphate solution separated from the first residue. The preferred acid is sulphuric acid. The resultant mixture of the manganese (II) sulphate solution and the acid may be heated. For example, it may be heated to about 90°C.

[0115] Adjusting the pH may include adding a base to the manganese (II) sulphate solution separated from the first residue. For example, calcium carbonate may be added to the manganese (II) sulphate solution. This may be in addition to the addition of the acid to the manganese (II) sulphate solution.

[0116] Thus, a pH-adjusted solution may be obtained.

[0117] The resulting pH-adjusted solution may be subjected to temperature treatment. Preferably, such temperature treatment comprises cooling. Preferably, the pH-adjusted solution is cooled to about 60°C.

[0118] The pH adjustment may be followed by the addition of an alkali metal or ammonium ions to the pH-adjusted solution. For example, ammonium hydroxide (NH4OH) may be added. Addition of the alkali metal or ammonium ions may be subject to stirring. The addition of an alkali metal or ammonium ions may be followed by further temperature treatment. Preferably, such further temperature treatment comprises heating. Heating may, for example, be to 90°C. Performing such temperature treatment may serve to effect hydrolysis of residual ferric iron and aluminium in pH-adjusted solution and precipitation or the iron as jarosite, thus producing a suspension of solid jarosite in a purified manganese (II) sulphate solution.

[0119] Recovering the jarosite from the suspension produces a manganese (II) sulphate pregnant liquor solution and a second residue comprising the jarosite, which can be separated by filtration.

[0120] The manganese (II) sulphate pregnant liquor solution may be subjected to one or more beneficiation steps to obtain battery grade MnSC . For example, it may be subjected to the purification process described in WO 2023 / 175443.

[0121] As another aspect thereof, the invention extends to a manganese (II) sulphate solution produced according to the invention.

[0122] As another aspect thereof, the invention extends to a manganese (II) sulphate pregnant liquor solution produced according to the invention.

[0123] Referring to the drawing, reference numeral 10 generally indicates an embodiment of a process according to the second aspect of the invention described by way of non-limiting example only.

[0124] The process 10 includes - a leaching mixture preparation stage 12; a feedstock slurry preparation stage 14; a dry leaching stage 16; a temperature treatment stage 18; a dissolution stage 20; a filtration stage 22; and a jarosite precipitation stage 24.

[0125] In the leaching mixture preparation stage 12, a leaching mixture comprising sulphuric acid at an available or preferred concentration, e.g. 96%, along feed line 26 is combined with water along feed line 28, thus producing a leaching mixture preferably comprising about 64% or about 65% sulphuric acid by mass and the difference water. The leaching mixture is prepared at a chilled temperature, e.g. in a range of about 3°C to about 8°C. To this effect, the water that is supplied along feed line 28 may for example be ice. Other industrial chilling methods may, however, also be used.

[0126] In the feedstock slurry preparation stage 14, feedstock comprising ferromanganese slag in particulate form in which at least 80%, more preferably at least 85%, is at most 75pm, along feed line 30, is combined with water, along feed line 32, to produce a feedstock slurry of about 62% solids.

[0127] The leaching mixture, along transfer line 34, is combined and blended with the feedstock slurry, along transfer line 36, in the dry leaching stage 16, thus producing a blended product mixture comprising solid hydrated manganese (II) sulphate and solid hydrated calcium sulphate. The dry leaching stage 16 preferably comprises a pug mill.

[0128] An overall quantity of water used in forming the leaching mixture, the feedstock slurry, and the blended product mixture, observes a maximum mass ratio of 1 part solid feedstock to 1.8 parts water, i.e. no more than 1.8 parts water to each 1 part solid feedstock.

[0129] The blended product mixture is passed, along transfer line 38, to the temperature treatment stage 18. The temperature treatment stage 18 preferably comprises a rotary kiln.

[0130] In the temperature treatment stage 18 the blended product mixture is subjected to temperature treatment sufficient to dead burn at least the calcium sulphate, but preferably also the manganese (II) sulphate and any other hydrated sulphates present in the blended product mixture, to produce a dry product mixture comprising anhydrous sulphates including dead- burnt anhydrous calcium sulphate and anhydrous manganese (II) sulphate.

[0131] The dry product mixture is passed, along transfer line 40, to the dissolution stage 18, in which it is combined with water along feed line 41 at a ratio of 1 part dry product mixture to 1 .5 parts water, thus producing a dissolution product mixture comprising a manganese (II) sulphate solution and a first residue comprising calcium sulphate. The dissolution product mixture may be subjected to temperature treatment in accordance with the invention (not illustrated). The dissolution product mixture is passed, along transfer line 42, to the filtration stage 22 in which the manganese (II) sulphate solution is separated from the first residue to recover the manganese (II) sulphate solution as a filtrate along transfer line 44 and the first residue as a calcium sulphate-rich byproduct along product line 46.

[0132] The manganese (II) sulphate solution is passed, along transfer line 44, to the jarosite precipitation stage for further purification according to the jarosite precipitation step of the invention, to produce a manganese (II) sulphate pregnant liquor solution along product line 48 and a jarosite residue along product line 50.

[0133] The manganese (II) sulphate pregnant liquor solution along product line 48 may then be directed for further purification in downstream processes.

[0134] Example

[0135] An embodiment of the invention is now described by way of illustrative non-limiting example only, with reference to the following worked example.

[0136] In the example slag from a slag dump in the Mpumalanga province in South Africa was subjected to beneficiation according to the invention on an experimental scale.

[0137] The slag was determined to have the following composition by X-ray fluorescence (XRF):

[0138] Table 1 : XRF analysis of the slag

[0139] The slag was formed into a blended product mixture comprising manganese (II) sulphate and calcium sulphate by a dry leaching step according to the invention, as follows:

[0140] To 450g ice in a 5-litre plastic beaker, 300g 96% H2SO4 was slowly added to form an acid and water mixture, as a leaching agent.

[0141] The acid and water mixture was gently stirred with an overhead stirrer until most of the ice was dissolved. The resulting temperature of this mixture (i.e. leaching agent) was about 7 °C.

[0142] 300g of milled slag (80% -75pm) was formed into a slurry with water to form a feedstock slurry at 62% solids The feedstock slurry was rapidly added to the leaching agent, while strong stirring was applied. An exothermic reaction took place and steam was released.

[0143] Within 5 minutes, the temperature of the mixture of the feedstock slurry and the leaching agent rose to 99°C. A homogeneous blended product in the form of a paste was obtained, being the blended product mixture comprising manganese (II) sulphate according to the invention.

[0144] After cooling for 30 minutes, 970g of the blended product mixture was removed from the beaker.

[0145] It will be appreciated, based on the quantities described above, i.e. 300g 96% sulphuric acid, 450g water (as ice), and 300g slag as a slurry at 62% solids, that the overall ratio of solid feedstock to water is 1 part of solids for every 1 .54 parts of water, in accordance with the ratio required by the invention.

[0146] The blended product mixture was then subjected to temperature treatment in accordance with the invention to effect dead burning of the calcium sulphate content thereof.

[0147] As discussed in relation to the invention, the reason for the temperature treatment is to ensure that the calcium sulphate (gypsum) is made anhydrous and that some other impurities are converted to basic sulphates which are less soluble in the upcoming dissolution step.

[0148] More specifically, after sampling of the blended product mixture, 856g of the blended product mixture was placed into a dish into an oven and heated to 450°C and soaked for 3 hours at this temperature.

[0149] The resulting dry product mixture was cooled, and after cooling 496g thereof was recovered.

[0150] The dry product mixture was then subjected to dissolution in water according to the invention, to produce a dissolution product mixture.

[0151] More specifically, to 484g of the dry product mixture, 726g water was added, i.e. at a ratio of 1 :1.5 to form a dissolution product mixture.

[0152] The dissolution product mixture was heated to 60°C while stirring it, for 1 hour. Subsequently, the dissolution product mixture was filtered to recover a filtrate comprising a manganese (II) sulphate solution and a first residue.

[0153] 500ml of the manganese (II) sulphate solution was collected. The characteristics of the manganese (II) sulphate solution were determined to be that it has a pH of 3 and an Eh of 360mV and the composition set out in Table 2, below, on an elemental basis as determined by XRF:

[0154] Table 2: XRF analysis of the manganese (II) sulphate solution

[0155] The first residue was washed five times through water displacement washes and was then dried at 80°C, yielding 270g thereof with a moisture content of 2.2% and an LOI of 11 %. The species in the first residue are set out in Table 3, below, as determined by XRF:

[0156] Table 3: XRF analysis of the third residue

[0157] Persons skilled in the art would understand that XRF analysis report calcium sulphate inter alia in terms of calcium oxide.

[0158] The manganese (II) sulphate solution was then subjected to a purification step comprising jarosite precipitation. In this regard, the following steps were performed:

[0159] While stirring, 500ml of the manganese (II) sulphate solution was first oxidised with 5 - 10ml of 30% H2O2. This served to ensure that all the iron is present as ferric iron. The pH of the solution was about 3.

[0160] Approximately 4 ml ^SC was used to reduce the pH of the solution to a pH of 1.5 and the solution was then heated to 90°C.

[0161] At 90°C, small amounts (2g) of CaCCh powder were added every 5 minutes while stirring to obtain a pH between 3.5 and 4. In total 15g CaCCh was added over a period of 3 hours.

[0162] The solution to which CaCCh was added was allowed to cool to 60°C and then 15ml of 25% NH4OH solution was added to it while stirring to form a suspension. Afterwards it was heated to 90°C to complete the hydrolysis of the remaining ferric and aluminium components.

[0163] After cooling to 60°C the suspension was filtered and washed to produce a manganese (II) sulphate pregnant leach solution and a second residue.

[0164] 330 ml of the pregnant liquor solution was recovered comprising manganese (II) sulphate and 84g of the second residue was obtained after drying.

[0165] The elemental analyses of the pregnant liquor solution (PLS), having a pH of 6.3, and the second residue, having 1.5% moisture and an LOI of 14.3 %, are as follows (Tables 4 and 5, respectively, determined by XRF):

[0166] Table 4: XRF analysis of the pregnant liquor solution comprising manganese (II) sulphate

[0167] Table 5: XRF analysis of the second residue

[0168] Looking at Tables 1 and 2, it will be seen, with reference to the manganese content, that significant yield of manganese is obtained while substantial removal of impurities including calcium, most prominently, is achieved. This removal is aided by the dead burning of the blended product mixture.

[0169] In Table 4, the purification achieved by the jarosite precipitation step is evident.

[0170] It must be understood that, in an industrial process, there would be recirculation of wash waters to optimise manganese sulphate yield.

[0171] It will be appreciated that the steps of dead-burning, dissolution, filtering and precipitating described herein may be substituted by one or more chemically equivalent steps without departing from the scope and spirit of the invention.

Claims

CLAIMS1. A process for beneficiating a solid feedstock comprising manganese as prereduced manganese (II) oxide, to recover manganese comprised by the manganese (II) oxide as manganese (II) sulphate, the process comprising a step of forming a blend of the solid feedstock, in particulate form, a leaching agent comprising sulphuric acid, and water, wherein the ratio of the solid feedstock to water, by mass, is no more than 1 :1.8, thus forming a blended product mixture comprising solid hydrated manganese (II) sulphate.

2. The process according to claim 1 , wherein the solid feedstock is a ferromanganese slag formed in the production of ferromanganese.

3. The process according to claim 1 or claim 2, wherein the pre-reduced manganese (II) oxide in the solid feedstock is no more than 25% by mass, preferably from 20% to 25% by mass of the solid feedstock.

4. The process according to any of claims 1 to 3, wherein the sulphuric acid is concentrated sulphuric acid at a concentration above 90%, more preferably above 95%, most preferably at 96%.

5. The process according to any one or claims 1 to 4, wherein the amount of sulphuric acid that is used is in stoichiometric excess to that which is required to sulphonate a metal oxide content of the solid feedstock.

6. The process according to any of claims 1 to 5, wherein forming the blended product mixture comprising manganese (II) sulphate comprises separate steps of forming a leaching mixture comprising combining the leaching agent with water; and forming a feedstock slurry by combining the solid feedstock with water, wherein the overall water content of the leaching mixture and the feedstock slurry is such that the ratio of solid feedstock to water, by mass, is no more than 1 :1.8, and wherein the leaching mixture is subsequently blended with the feedstock slurry to produce the blended product mixture.

7. A process for beneficiating a solid feedstock comprising manganese as prereduced manganese (II) oxide, to recover manganese comprised by the manganese (II) oxide as manganese (II) sulphate, the process comprising(a) forming a blended product mixture comprising solid hydrated manganese (II) sulphate according to the process of any one of claims 1 to 6, wherein the blended product mixture also comprises solid hydrated calcium sulphate;(b) subjecting the blended product mixture comprising solid hydrated manganese (II) sulphate and solid hydrated calcium sulphate to temperature treatment sufficient to produce a dry product mixture comprising solid anhydrous manganese (II) sulphate and solid dead-burnt anhydrous calcium sulphate;(c) subjecting the dry product mixture comprising solid anhydrous manganese (II) sulphate and solid dead-burnt anhydrous calcium sulphate to dissolution in water to produce a dissolution product mixture comprising a manganese (II) sulphate solution and a first residue comprising solid calcium sulphate; and(d) separating the manganese (II) sulphate solution from the first residue by solid-liquid separation.

8. The process according to claim 7, wherein the temperature treatment is applied at a temperature of 200°C to 500°C, preferably 450°C.

9. The process according to claim 7 or claim 8, wherein subjecting the dry product mixture comprising solid anhydrous manganese (II) sulphate and solid dead-burnt anhydrous calcium sulphate to dissolution in water to produce the dissolution product mixture comprises combining water with the dry product mixture at a dry product mixture to water ratio, by mass, of 1 :1.5.

10. The process according to any one of claims 7 to 9, which includes removing impurities from the manganese (II) sulphate solution by jarosite precipitation.

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