INTRODUCTION

This invention relates to a process for producing manganese sulphate monohydrate. In particular, but not exclusively, the invention relates to a process for producing a high purity manganese sulphate monohydrate from an ore.

BACKGROUND TO THE INVENTION

Previous methodologies of producing high purity manganese sulphate monohydrate involve sequential partial crystallisation steps and/or the precipitation of hard-to-remove alkali earth metals with fluoride. These processes are expensive and energy intensive.

It is an object of this invention to alleviate at least some of the problems experienced with existing processes for producing a high purity manganese sulphate monohydrate from an ore.

It is a further object of this invention to provide a process for producing a high purity manganese sulphate monohydrate from an ore that will be a useful alternative to existing processes for producing a high purity manganese sulphate monohydrate from an ore.

SUMMARY OF THE INVENTION

In accordance with a first aspect of the invention there is provided a process for producing manganese sulphate monohydrate comprising the steps of:

(a) Providing an ore containing Mn ²⁺

(b) Leaching the ore in sulphuric acid to produce a first manganese solution;

(c) Contacting the manganese solution with a source of bicarbonate, carbonate, or carbon dioxide and a source of hydroxide to precipitate manganese carbonate;

(d) Heating the manganese carbonate to produce a manganese oxide mixture;

(e) Washing the manganese oxide mixture to remove calcium;

(f) Contacting the washed manganese oxide mixture with hydrogen peroxide and sulphuric acid and/or with gaseous SO2 in acid to convert the manganese oxide to a second manganese solution;

(g) Crystallising manganese sulphate monohydrate from the second manganese solution;

(h) Dissolving the manganese sulphate monohydrate in water to produce a third manganese solution;

(i) Removing calcium from the third manganese solution; and

(j) Crystallising manganese sulphate monohydrate from the third manganese solution.

Preferably, the ore is previously calcined.

In step (b), the first manganese solution may be boiled and aerated, and the pH of the first manganese solution may be raised to above 5.

Preferably, the amount of heavy metals in the first manganese solution is less than about 5ppm. In step (c), the source of bicarbonate may be ammonium bicarbonate, alternatively the source of carbonate may be sodium carbonate, and the source of hydroxide may be ammonium hydroxide.

In step (c), the pH may be controlled at about 6 - 7 and the amount of magnesium in the manganese carbonate may be less than about 50ppm.

In step (d), preferably, manganese carbonate is heated in air at about 650°C to 950 °C for about 0.5-2 hours, and then cooled to ambient temperature.

Preferably, the manganese oxide mixture of step (e) is washed with deionised water, and preferably the deionised water is at a pH of about 7.

The second manganese solution of step (f) may be an ultra-pure manganese sulphate solution, having a calcium and magnesium content less than 20 ppm. Preferably, the content of the other metals in the second manganese solution is less than 5 ppm.

The second manganese solution may have a concentration of about 160 g/kg as manganese sulphate.

The crystallisation step (g) may further include the following steps:

(i) Adjusting the pH of the second manganese solution of step (f) to about 2 by adding sulphuric acid;

(ii) Evaporating the solution of step (i) to produce crystals;

(iii) filtering the solution of step (ii) to remove moisture from a crystal slurry; and

(iv) Drying the crystals.

Evaporation may take place at a temperature of about 80°C to 85°C, optionally under a vacuum until the specific gravity is about 1 .67. Preferably, the crystals are dried at a temperature of about 105°C to 150 °C, even more preferably at a temperature of about 105°C to 120°C.

The two-step crystallization process may include an intermediate calcium seeded precipitation process, which may or may not include aeration.

Preferably, the second manganese solution of step (f) undergoes a two-step crystallization process and intermediate calcium seeded precipitation process, before the manganese sulphate monohydrate dissolution of step (h).

Preferably, step (h) further includes dissolving the manganese sulphate monohydrate of step (g) in water and adding sulphuric acid to produce a third manganese solution.

The third manganese solution is manganese sulphate.

The manganese sulphate may be filtered and evaporated in order to produce crystals and a crystal slurry.

Preferably, the solution is evaporated at about 80°C to 85°C under a vacuum to produce crystals.

The removal or precipitation of calcium in step (i) may further include:

(i) centrifuging the crystal slurry to produce a centrate;

(ii) seeding the centrate with calcium sulphate hemi-hydrate seed, or alternatively calcium sulphate dihydrate seed, aerating the solution for 12 hours, filtering and returning the centrate to the first crystallisation feed; and

(iii) redissolving the dewatered crystals in deionised water.

Preferably, the crystals are a high purity manganese sulphate monohydrate having a calcium and magnesium content of less than about 50 g/t. Preferably, the crystals which are a high purity manganese sulphate monohydrate conform to the following maximum impurity limits (ppm):

Preferably, the manganese content in the crystals is greater than 32%.

In accordance with a second embodiment of the invention, the manganese carbonate of step (c) is not roasted or heated, but the manganese carbonate is redissolved in sulphuric acid and water, and the solution may undergo a two-step crystallization process.

In one embodiment of the present invention, there is provided a process for producing manganese sulphate monohydrate comprising the steps of: a) Providing an ore containing Mn ²⁺ ; b) Leaching the ore in sulphuric acid to produce a first manganese solution; c) Filtering the first manganese solution to produce a filtrate; d) Contacting the filtrate with a source of bicarbonate to precipitate manganese carbonate; e) Dissolving the manganese carbonate in sulphuric acid to provide a second manganese solution; f) Removing calcium from the second manganese solution; and g) Crystallising manganese sulphate monohydrate from the second manganese solution.

In this embodiment steps (h), (I) and (J) are the same as the steps of the first embodiment described above.

BRIEF DESCRIPTION OF THE DRAWINGS The invention will now be described in more detail, by way of examples only, with reference to the accompanying drawings in which:

Figure 1 shows a block diagram for a process of producing manganese sulphate monohydrate in accordance with the invention;

Figure 2 shows a process flowsheet for a leaching step B and precipitation step C in accordance with the invention of Figure 1 ;

Figure 3 shows a process flowsheet for a heating step C, washing step D and reductive leach step F in accordance with the invention of Figure 1 ;

Figure 4 shows a process flowsheet for a crystallization step G in accordance with the invention of Figure 1 ;

Figure 5 shows a process flowsheet for manganese carbonate dissolution, polishing leach filtration, residual iron oxidation and iron filtration in accordance with another embodiment of the invention; and

Figure 6 shows a process flowsheet for calcium sulphate removal, 1 ^st stage crystallisation, and 2 ^nd stage crystallisation in accordance with another embodiment of the invention.

DESCRIPTION OF PREFERRED EMBODIMENTS

Referring to the drawings, in which like numerals indicate like features, a nonlimiting example of a process for producing manganese sulphate monohydrate in accordance with the invention is generally indicated by reference numeral 10. The process 10 consists of the steps of (A) providing an ore 12 containing Mn ²⁺ , (B) leaching the ore 12 in sulphuric acid 14 to produce a first manganese solution 16; (C) contacting the first manganese solution 16 with a source of bicarbonate, carbonate or carbon dioxide 18 and a source of hydroxide 20 to precipitate manganese carbonate 22; (D) heating the manganese carbonate 22 to produce a manganese oxide mixture 24; (E) washing the manganese oxide mixture 24 with water 26 to remove calcium 28; (F) contacting the washed manganese oxide mixture 30 with hydrogen peroxide 32 and sulphuric acid 34 or with gaseous SO2 in acid to convert the manganese oxide to a second manganese solution 36; (G) crystallising manganese sulphate monohydrate 40 from the second manganese solution 36; (H) dissolving the manganese sulphate monohydrate 40 in water 42 to produce a third manganese solution 44; (I) removing calcium 46 from the third manganese solution 44 to produce a washed third manganese solution 48; and (J) crystallising manganese sulphate monohydrate 50 from the washed third manganese solution 48.

The ore 12 is a reduced ore which was previously calcined. The leaching process of step (b) includes addition of pre-wetted (ratio of about 1 :1), milled, calcined ore 12 to a dilute sulphuric acid solution 14 to produce a first manganese solution 16. The leach or first manganese solution 16 is held at a gentle boil during leaching for about 1 to 2 hours while being aerated and boiled (air is added to the mixture). The volume of the mixture is kept constant during the gentle boil by the addition of water. The pH of the first manganese solution 16 rises from about 1 to above 5. The first manganese solution 16 is then filtered, and the filter cake is discarded. The first manganese solution 16 has a manganese concentration of about 160 g/l to 180 g/l, and a heavy metal content including iron of below 5ppm.

The first manganese solution 16 is contacted with a source of bicarbonate, carbonate, or carbon dioxide 18 and a source of hydroxide 20 to obtain a precipitate of manganese carbonate 22. Preferably, the source of bicarbonate 18 is ammonium bicarbonate, the source of carbonate is sodium carbonate and the source of hydroxide 20 is preferably ammonium hydroxide. The source of bicarbonate 18 is a stoichiometric amount of ammonium bicarbonate and the pH is controlled with the addition of ammonium hydroxide 20 to a pH of about 6 to about 7, preferably a pH of about 6.5.

Manganese is precipitated as manganese carbonate 22 with the purpose of rejecting magnesium from the solution. Magnesium carbonate is much more soluble than manganese carbonate 22 near neutral pH and can therefore be rejected, as it stays in solution while manganese carbonate 22 is precipitated. Magnesium rejection from the precipitate 22 will be above 99% and yield of manganese carbonate 22 from the dosed ammonium bicarbonate will be above 85%. Preferably, the amount of magnesium in the manganese carbonate 22 is less than about 50 ppm.

The manganese carbonate 22 is heated in the presence of air at a temperature of about 650°C to 950°C in a heating apparatus (manganese carbonate 22 is roasted oxidatively). Preferably, the heating apparatus is a furnace. Manganese carbonate 22 is heated in order to convert all the carbonates to oxides, predominantly to MnaCU. The manganese carbonate 22 is roasted in air for about 0.5-2 hours to convert the manganese carbonate 22 to manganese oxide 24. Emissions are released during the heating process, which includes carbon dioxide. The manganese oxide 24 is allowed to cool to ambient temperature. Manganese oxides in the (III) oxidation state is insoluble in water, while calcium and magnesium oxides are soluble. The calcium (and any residual magnesium) oxides are not mixed in with manganese oxides, but exist as a separate phase that can be easily solubilised after the oxidative roast.

Calcium 28 is removed from manganese oxide 24, which is in the form of a powder, by washing the manganese oxide 24 with water 26. Preferably, the water 26 is deionised water with an initial pH of 7. Calcium oxide dissolves in the wash water. Two consecutive washes remove >90% of the calcium in the manganese oxides, as well as residual magnesium that may be present to produce a washed or purified MnO _x 30. The purified MnO _x 30 is filtered and the filtrate discarded. The purified MnO _x 30 is added to a dilute sulphuric acid solution 34, containing a slight excess of sulphuric acid to react with the manganese. An equimolar amount of hydrogen peroxide 32 is added to the mixture of purified MnOx 30 and dilute sulphuric acid solution 34 to react with the Mn(lll) in the MnOx (calculated as the amount in Mn ₃ C>4). Deionised water is also added to the mixture (not shown). The mixture is stirred until all the MnO _x dissolves to produce a second manganese solution 36. The resulting second manganese solution 36 is an ultra-pure manganese sulphate solution. The ultra-pure manganese sulphate solution 36 has a calcium and magnesium content of less than 20 ppm, and the content of all other metals is less than 5 ppm. The combination of sulphuric acid 34 and hydrogen peroxide 32 can also be substituted with SO2 addition in an acidic solution.

Manganese sulphate monohydrate is crystalized from the second manganese solution 36 by using the following steps:

(i) Adjusting the pH of the second manganese solution 36 to about 2 by adding sulphuric acid 38 to the solution 36;

(ii) Evaporating the solution of step (i) under a vacuum to produce crystals 40;

(iii) filtering the solution of step (ii) to remove moisture from a crystal slurry; and

(iv) Drying the crystals, preferably at a temperature of about 105°C to 120°C.

Preferably, evaporation of step (ii) takes place at a temperature of about 80°C to 85°C under a vacuum until a specific gravity of about 1 .67 is achieved.

Preferably, the crystals are dried at a temperature of about 105°C to 150 °C, even more preferably at a temperature of about 105°C to 120°C.

The purified crystals will conform to the following maximum impurity limits (PPm):

The manganese content is greater than 32% in the crystals.

In another embodiment of the invention, the manganese carbonate 22 of step (c) is not heated. The manganese carbonate 22 is redissolved in sulphuric acid (H2SO4), and the manganese solution is sent through a two-step crystallisation process. Calcium is removed through an intermediate calcium- seeded precipitation process, which may or may not include aeration.

In this embodiment, there is provided a process for producing manganese sulphate monohydrate comprising the steps of: a) Providing an ore containing Mn ²⁺ ; b) Leaching the ore in sulphuric acid to produce a first manganese solution; c) Filtering the first manganese solution to produce a filtrate; d) Contacting the filtrate with a source of bicarbonate to precipitate manganese carbonate; e) Dissolving the manganese carbonate in sulphuric acid to provide a second manganese solution; f) Removing calcium from the second manganese solution; and g) Crystallising manganese sulphate monohydrate from the second manganese solution.

Steps (h), (I) and (J) are the same as steps of the first embodiment described above.

The manganese carbonate 22 of step (c) is repulped into a slurry using water. The manganese carbonate 22, which is now in a form of a slurry, is redissolved by the addition of sulphuric acid (H2SO4) to obtain a solution with a concentration of about 120 g/kg Mn (as manganese sulphate). The product solution is filtered to remove any insoluble components before feeding the manganese sulphate to a crystallizer. Crystallization is conducted under a slight vacuum at a temperature of about 80°C to 85°C. A combination of a small bleed and a calcium removal step (described below) will keep the calcium concentration in the mother liquor below 400 mg/kg.

Calcium removal

• Manganese sulphate monohydrate slurry from the 1 ^st crude crystallisation is centrifuged.

• The centrate is seeded with calcium sulphate hemi-hydrate seed or alternatively calcium sulphate dihydrate seed, aerated for 12 hours, filtered, and returned to the crude crystallisation feed.

• The dewatered crystals were redissolved with deionised water and sent for final crystallisation.

Final - Second crystallisation

• The crystal slurry from the 1 ^st crude crystallisation step is redissolved with deionised water and brought back to about 80°C.

• A small bleed is used to ensure that calcium and magnesium content in the final crystal product is below the target of 50 g/t.

The resulting crystal slurry is centrifuged and washed to remove a portion of the entrained impurities, before it is air dried to produce the final high purity manganese sulphate monohydrate.

The process for producing a manganese sulphate monohydrate will now be further described more fully with reference to the below non-limiting embodiments.

DESCRIPTION OF SPECIFIC EMBODIMENTS

In the example shown in Figure 2, a pre-wetted, milled calcined ore 212 is fed to an oxidising leach container or tank B1 , typically an Oil Fired Kiln. A dilute sulphuric acid (H2SO4) solution 214 is added to the ore 212 in the container B1 to produce a first manganese solution 216. The first manganese solution 216 is initially at a pH of about 1 , and at a temperature of about 80°C. The first manganese solution 216 is held at a gentle boil for about 1 to 2 hours during leaching, while adding air 213 to the container B1 . The volume of the mixture or first manganese solution 216 is kept constant during the gentle boil by the addition of process water 215 and wash water 217. The pH of the first manganese solution 216 rises to about 5-6 while the temperature drops to about 60°C in the leach container B1 after about 1 to 2 hours of leaching. The first manganese solution 216 is filtered at a pressure of about 1800 KPa using a filtering apparatus B2 to produce a filter cake 219 and filtrate 219b. The filter cake 219 is discarded. Deionised water 223 is added to the filtering apparatus B2 during the filtration process. MSMH ML Recycle 441 , containing about 160 g/l of manganese is added to the filtered first manganese solution or filtrate 219b before the precipitation step (see Figure 4 for the composition of MSMH ML Recycle).

The filtrate or filtered first manganese solution 219b has a pH of about 5.5 and is at ambient temperature. Ammonium bicarbonate, sodium carbonate or alternatively carbon dioxide gas 218 and ammonium hydroxide 220 are added to the first manganese solution 219b in order to precipitate manganese carbonate (MnCOs) 220. During the precipitation process, the pH of the mixture and of the precipitate rises to about 7. Manganese carbonate 220 is filtered at atmospheric pressure and ambient temperature using a filtering apparatus C2 to produce a manganese carbonate (MnCOs) wet solids 220a and filtrate 221. Deionised water 223 and recycled condensate 443 are added to the filtering apparatus C2 during filtration (see Figure 4 for the composition of the recycled condensate). A portion of the filtrate 221 is recycled back to the oxidising leach container B1 and the remainder is purged. The Manganese carbonate wet solids 220a proceeds to the heating stage.

In the example shown in Figure 3, manganese carbonate wet solids 220a is introduced to a heating or roasting apparatus D. The manganese carbonate is heated to a temperature of about 650°C-950°C in the presence of air 323(air is added to the heating apparatus), and at atmospheric pressure. The manganese carbonate 220a is preferably heated at 900 ^e C. During the heating process, the carbonates are converted to oxides 324 and emissions are also produced, including carbon dioxide 325. The oxides undergo a first wash E1 at a temperature of about 80°C and at atmospheric pressure. The wash may be an ultrasonic wash. Deionised water 326 is added to the oxides during the first wash. Optionally but not necessary, sulphuric acid 327 may be added to oxides 324 during the wash thus lowering the pH of the mixture to about 1 . The washed oxides 330 which are now at a temperature of greater than 60°C undergo a first filtration process E2. The oxides are filtered at an atmospheric pressure using a filtering apparatus. The wash water 217 from the filtration process is recycled back to the oxidising leach container of Figure 2. The oxides are subjected to a second wash E3 and filtration process E4 using the same conditions as the first wash E1 and filtration process E2 to produce a purified Manganese Oxide (Mn _x O _y ) 331 .

The purified Mn _x O _y 331 undergoes a reductive leach in a leaching container F1 . The purified Mn _x O _y 331 is added to a dilute sulphuric acid solution 334. Sulphur dioxide (SO2) 333 and deionised water 326 are also added to the leaching container F1 . Reductive leaching is conducted at a temperature of about 65°C, at a pH of about 1 and at atmospheric pressure in order to produce a leached solution. The leached solution is filtered at a temperature of about 65°C, pH 2.5 and at atmospheric pressure in a filtration apparatus F2 to produce waste 343 and a filtered leached solution. The filtered leach solution is then polished in F3 with hydrogen peroxide 335, air 337 and ammonium hydroxide 339 to produce a polished leached solution 336. Alternatively the Mn _x O _y 331 is contacted with hydrogen peroxide (H2O2) and sulphuric acid (lines not shown) in order to produce a leached solution. This solution is then contacted with more hydrogen peroxide (H2O2) 335, air 337 and ammonium hydroxide 339 in order to produce a polished manganese sulphate solution 336. This reaction takes place at a temperature of about 70°C, pH of about 4 to 5 and at atmospheric pressure. The manganese sulphate solution 336 is filtered to produce a filtrate 336a and waster 341. Waste 341 (filter cake) discarded.

In the example shown in Figure 4, manganese sulphate solution (filtrate) 336a which is now at a pH of about 4.6 is subjected to a crystallisation process G1 at a temperature of about 80°C and pressure of about -46.7 KPa. During crystallization, deionised water 438 is added to the manganese sulphate solution 336a, and the mixture consisting of deionised water 438 and manganese sulphate solution 336a is evaporated to produce manganese sulphate monohydrate crystals 440, MSMH ML recycle 441 , containing approximately 160 g/l manganese, and a condensate of pure water 443. MSMH ML Recycle 441 is added to the filtered first manganese solution 219b or filtrate, in the example of Figure 2, before the precipitation step. The manganese sulphate monohydrate crystals 440 are dried at about 150°C and at atmospheric pressure using a drying apparatus G2.

The purified crystals will conform to the following maximum impurity limits (PPm):

The manganese content is greater than 32% in the crystals.

In the example shown in Figure 5, manganese carbonate wet solids 220a is redissolved in sulphuric acid (H ₂ SO ₄ ) 514 using a leaching container H1. Hydrogen Peroxide (H2O2) 516 and deionised water 518 are added to the leaching container H1 during leaching. Leaching is conducted at a temperature of about 65°C, pH of 2 to 5.5 and at atmospheric pressure to produce a manganese sulphate solution 520. The manganese sulphate solution 520 is polished and filtered using a filtering apparatus H2 to produce waste 522 and filtered solution 524. The filtered solution 524 is treated with hydrogen peroxide 526, ammonium hydroxide 528 and air 530 in order to remove residual iron from the filtered solution. The residual iron oxidation step H3 which is followed by filtration in a filtration apparatus H4 is conducted at a temperature of about 70°C, pH of about 4.5 to 5.5 and at atmospheric pressure to produce waste 534 and an even purer manganese sulphate solution 536.

In the example shown in Figure 6, the purer manganese sulphate solution 536 of Figure 5 undergoes a first crystallisation process J1 to produce manganese sulphate monohydrate 614. Crystallisation is conducted under a slight vacuum at a temperature of about 80°C, pH of about 4.6 and at a pressure of about -46.66 KPa. Deionised water 612 is also added to the vacuum during crystallisation. A portion of the manganese sulphate monohydrate 614 is bled off, and sent to a calcium removal step in order to keep the concentration of calcium below 400 mg/kg in the mother liquor. Calcium sulphate (CaSO4) 616 seed reacts with the manganese sulphate monohydrate 614 during the calcium removal step. The more refined manganese sulphate monohydrate 618 is fed to the first crystallisation process in a continuous loop, until the target concentration of calcium is achieved in the manganese sulphate monohydrate 620 produced by crystallisation. A calcium sulphate waste 617 is discarded.

The refined manganese sulphate monohydrate 620 undergoes dissolution J3 with deionised water 622 at a temperature of about 60°C, pH of 5.5. and at atmospheric pressure. The product from dissolution 624 undergoes a pH adjustment stage J4 wherein sulphuric acid 626 and ammonium hydroxide 628 are added to the product of dissolution 624 to produce a product 630 which is at a lower pH of about 4.9. The product 630 undergoes a second crystallisation and filtration process J2 at a temperature of about 80°C, pH of about 4.9 and pressure of about -46.66 KPa. The resulting crystal slurry 632 is centrifuged and washed to remove a portion of the entrained impurities, and then dried at about 150°C in a drying apparatus J5 to produce a final high purity manganese sulphate monohydrate 636. A portion of the crystal slurry 638 is also sent back to the first crystallisation step J1 for further processing.

The purified crystals will conform to the following maximum impurity limits (PPm):

The manganese content is greater than 32% in the crystals.

The present invention will now be further described more fully with reference to the below non-limiting examples.

Example 1 : Preparing 2.5 litre of leach solution or first manganese solution

1 . 1.5 litres of water was added to a large Slitre glass beaker;

2. 400 mis of 98% sulphuric acid was slowly added to the water;

3. 900 grams of water was added to 900g of calcined ore to produce a slurry, the slurry was added to the 5 litre glass beaker and stirred at 500 rpm;

4. The solution was aerated at 10-201/min through a very fine sparger (10pm);

5. The solution was kept at a gentle boil, and the volume kept constant with the addition of water;

6. The leaching process was completed after about 1 to 2 hours when the pH of the leach solution was about 5 to 6; and

7. The leached solution was filtered at a minimum of 5 bar and up to 18 bar to produce a filtrate and filter cake, the filtrate was retained for subsequent processes.

Example 2: Preparing 100g of manganese carbonate: 1 . A slurry consisting of 72g of food grade ammonium bicarbonate and 100 ml of deionised water was made;

2. About 500 mis of a first manganese solution at 100 g Mn/I and with a pH of about 5.4 was removed from the leached solution;

3. Ammonium bicarbonate slurry of step 1 was added to the first manganese solution of step 2 while stirring the mixture;

4. The pH of the mixture was controlled with the addition of 25% ammonium hydroxide (approximately 40 mis) to a target pH of about 6.5, and with a band of 6 to 7; the ammonium hydroxide was added slowly, drop by drop; and

5. The mixture was continuously stirred for 2 hours then filtered, and the filter cake was washed with deionised water.

Example 3: Preparing 100g of Manganese Oxide:

1 . 180g of manganese carbonate was placed in an oven, and roasted in the presence of air at 900°C for about 2 hours, which resulted in the conversion of manganese carbonate to manganese oxide; and

2. The manganese oxide was removed from the oven and allowed to cool to ambient conditions before the next step.

Example 4: Calcium Removal Process

1 . 800 ml of deionised water was added to a beaker;

2. The solution of step 1 was heated to a temperature of about 80 to 85°C;

3. 100g of fine manganese oxide was added to the hot deionised water, and the mixture stirred (300 rpm) for 45 minutes while the temperature was kept constant at about 80 to 85°C;

4. The mixture of step 3 was hot filtered after 45 minutes, the filter cake washed with deionised water and dried, and the filtrate was discarded;

5. Step 1 to 4 were repeated in order to produce MnO _x with a calcium rejection of above 90%. Example 5: Reductive leach Process

1 . The Mn content in 100g of MnO _x was measured;

2. Stochiometric amount (Mn:H ₂ SC>4) of 98% sulphuric acid, and 800ml of deionised water was added to MnO _x of step 1 , and 1 ~2 ml extra FhSC was also added;

3. The mixture of step 2 was stirred at 300 rpm, and drops of hydrogen peroxide were added to the mixture until the solution turned pink, showing that all the MnO _x had dissolved and the resulting solution was a pure manganese sulphate solution.

Example 6: Preparing 525kq of Manganese Sulphate Crystals (dry basis)

1 . 1700 kg of manganese solution at 136 g/kg manganese was seeded with 50 kg of MSM. In practice this stream is a blend of freshly dissolved manganese carbonate, the centrate bleed from the final crystallisation, and the calcium depleted solution.

2. Calcium level was at 134 mg/kg, and magnesium level was at 87 mg/kg.

3. The solution was evaporated at about 80°C until calcium content in the mother liquor reached 400 mg/kg.

4. The product from evaporation was 525 kg of manganese sulphate monohydrate crystals and 566 kg of entrained mother liquor.

Example 7: Calcium Removal Step

1 . 1 1 kg of calcium sulphate seed was added to 105 kg of centrate from the crude crystallisation step.

2. The centrate had a maximum amount of 400 mg/kg of calcium.

3. The mixture of manganese sulphate seed and centrate was aerated for 12 hours, , and filtered. Calcium content in the filtrate was reduced by up to 50%, and the filtrate was returned to the crude crystallisation feed. Advantages

• The invention involves progressive removal of the “hard to remove” alkali earth metals, calcium and magnesium, first by carbonate precipitation and then by progressive partial crystallisation under vacuum and calcium precipitation;

• The oxidative roasting and washing step has a high efficiency of removing calcium from the system;

• The crystallisation steps use calcium solubility limits to make sure that calcium does not precipitate together with manganese sulphate crystals, and the carbonate step uses ammonium hydroxide to maximise manganese carbonate yield;

• Other methodologies of producing high purity manganese sulphate monohydrate would involve precipitation of the hard-to-remove alkali earth metals with fluoride; and

• The proposed process has a very low reagent consumption since all the carbon is recycled, and employs a mechanical vapour recompression setup to minimise energy costs for the crystallisers.