The present disclosure relates to the field of metallurgy. Particularly, the present disclosure relates to a hydrometallurgical process for extraction of manganese values from high iron bearing low grade manganese ores by controlling iron values and dithionate concentration of less than 1 g/l.

BACKGROUND AND PRIOR ART OF THE DISCLOSURE
Lean manganese high iron bearing oxide ores are generally difficult to leach at lower pH values since reduction potential of Mn and Fe overlaps in that Eh-pH range as per Eh-pH diagram. Thus, it is generally known that manganese may be leached from the manganese dioxide ores using sulphur compounds especially sulphur dioxide. However, leaching of the manganese dioxide material using sulphur dioxide as reductant is also known to generate dithionate ions in solutions with levels of > 5g/l. Secondly, utilization of sulphur dioxide as lixiviant during leaching of manganese from high iron bearing ores (30-40 wt %) even tends to leach more than >8 g/l of iron along with manganese. Concentration of both these species other than Mn2+ is far higher depending on the amount of manganese being leached. Thus, economical leaching and purification of manganese leaching process from low grade high iron bearing manganese ore is a challenge worldwide.
Low grade manganese dioxide ores (< 40 wt % Mn) are presently uneconomical to process using conventional reduction roasting followed by magnetic separation due to its less recovery and low yield. Secondly, sulphuric acid based leaching process which employs reductants are not suitable for selective extraction of manganese as most of the iron comes into leach liquor. Utilization of other sulphur based lixiviants like sulphur dioxide gas seems to be a possible and viable option for selective leaching of manganese from these low-grade ores though all leaching processes involving sulphur dioxide as reductant leads to the formation of dithionate ions in the solution (> 5g/l) [US2176774A]. Thus, such high dithionate levels in the leach solution leads to increase the capital cost due to involvement of other process operations like “oxidation” and “aging”. Also, a longer residence time is required to oxidize the dithionate levels from >5 g/l to below 1 g/l which results in lower productivity and hence, the process becomes capital intensive. Therefore, it would be advantageous to design a process route to recover selective manganese value from low grade ferruginous manganese ores with control on level of dithionate below 5 g/l, and preferably less than 1 g/l in manganese sulphate solution. Advantage of this process is resulting in utilization of low-grade resources of manganese with subsequent avoidance of reduction roasting based process which employs carbon dioxide emissions and finally, a higher yield of manganese can be obtained employing hydrometallurgical processing of lean grade Mn ores to produce valuable manganese salts like manganese carbonate.
Manganese carbonate is a valuable material for many industrial products like pigments, for preparation of soft magnets, cattle feed and as food supplement. Other than this, manganese carbonate is a raw material for production of manganese-based salts. Secondly, utilization of high iron bearing low grade manganese ore for preparation of high purity manganese carbonate has not been explored. Few patents describe the method for preparation of manganese salt based on low grade manganese carbonate ores. For example, US7951282 discloses leaching of low-grade manganese ore from Australia region with Mn < 40 wt % with addition of sulphur dioxide as primary reducing agent. The patent describes the process of extraction with leaching at lower pH value of <1.5 at solid liquid ratio of 10 w/v during leaching operation. During the leaching process, dissolution of iron happens at pH less than 1.5 with reported value of 1.5 g/l and extraction time reported to be approximately 10 hours with 95% manganese extraction efficiency. The patent also refers to the process for further purification of leach liquor by removal of iron by jarosite precipitation and heavy metal removal by sulphide precipitation process. Finally, purified solution is processed to produce electrolytic manganese dioxide (EMD) and anolyte is sent back to leaching section. US8460631 discloses leaching of low-grade manganese ore with sulphur dioxide as primary reductant with formation of sodium sulphate crystals by evaporative crystallization. The patent also focuses on production of manganese carbonate via sodium sulphate precipitation process route and eventually even discusses the potential utilization of sodium sulphate stream to eliminate the dithionate anions via evaporative crystallization. EP1809777B1 describes the process for production of manganese carbonate by leaching of oxide ore using hydrochloric acid followed by formation of manganese sulphate by blowing sulphur dioxide gas in the solution with recovery of hydrochloric acid. US2176774A describes a process to process low grade manganese ore via sulphating method of roasting to produce high grade manganese sinter along with MgSO4, CaSO4, Fe2O3 and ferrous sulphate as by-products. Sized manganese ore is processed with sulphuric acid and SO2 gas at high temperature to form MnSO4 and FeSO4 which is subsequently leached with water to obtain manganese sulphate solution. The manganese sulphate solution is further processed to obtain high grade manganese sinter and other by-products. US5534234A describes the use of sulphurous acid as lixiviant for treatment of lean grade manganese ore. The patent discloses the method and condition for extraction of manganese as manganese carbonate and the mechanism has been explained using Eh-pH diagram to find the region for precipitation of iron as iron hydroxide. A method for preparation of manganese carbonate from low grade rhodochrosite ore has been disclosed in CN105152152A, wherein low grade rhodochrosite was leached with sulphuric acid followed by solution preparation to obtain primarily manganese carbonate along with other by-products. The specific process comprises the steps that (1) N and P compound fertilizer base materials are prepared; (2) orthophosphoric acid iron powder is recycled; (3) cobalt, nickel and copper concentrate is recycled; (4) aluminium hydroxide powder is recycled; (5) land plaster is recycled; (6) high-purity manganese carbonate powder is prepared; (7) magnesium ammonium phosphate slow-release compound fertilizer is prepared. CN104294038A describes preparation of manganese carbonate from silver-manganese ores. In their process, silver-manganese ore is used as a primary raw material for extraction of manganese by sulphuric acid route followed by removal of iron by addition of pure MnO2 or MnCO3 as oxidant. Subsequently, heavy metals are extracted from leach liquor by sulfuration process. Ammonium carbonate was used for precipitation of manganese as manganese carbonate. Silver was subsequently leached in second stage and extracted with recovery of 95%. A similar process was described by CN1101328A, where primary raw material used was rhodochrosite ore and leaching with sulphuric acid and finally, precipitation was carried out by ammonium carbonate. In CN104480314A, a method for recycling of waste residue from manganese residue for production of manganese carbonate has been elaborated. The method comprises of specific steps: (a) pre-processing of filter residue; (b) leaching the manganese and iron value with sulphuric acid; (c) recovery of iron value as haematite powder; (d) recovery of manganese carbonate concentrate; and (e) preparation of a nitrogen-phosphorus compound fertilizer base material. CN105084421A proposes a process for producing carbonates of manganese by reduction-roasting followed by acid leaching of low grade pyrolusite. The method comprises of reduction roasting followed by magnetic separation of iron value from the product to form ferric phosphate powder. Manganese based residue is leached with sulphuric acid to produce manganese carbonate in similar fashion as described by other patent literatures discussed above. Other than manganese carbonate, aluminium hydroxide, gypsum powder and N-P compound fertilizer is produced. A similar method for preparation of manganese carbonate has been described in CN105152153A, CN102660689A, CN102605186A and CN102220491A.

As can be seen from the prior art references, most of the disclosures have used rhodochrosite as manganese source for preparation of carbonate following sulphuric acid route. Secondly, low grade rhodochrosite ores doesn’t have iron and, thus iron removal and hence intermittent loss of manganese is not a problem. In another category of patents where sulphur dioxide or sulphurous acid has been used as lixiviant, the concentration of iron is considerably high in the leach solution which affects the overall economics of the process. Further, most of the processes describe the presence of dithionate ion > 5 g/l which is eventually controlled either by controlling the leaching conditions to regulate the generation of dithionate ions or by dissociation of dithionate ions to produce SO2 gas which is recycled back. In another patent, low grade pyrolusite ore was roasted followed by leaching with sulphuric acid to produce manganese carbonate. All the studies pertaining to manganese carbonate preparation was through sulphuric acid route. Since most of the manganese in low grade manganese ore is present in Mn (IV) oxidation state and in order to reduce the manganese, reductant needs to be added. Overall, manganese carbonate production through leaching in sulphuric acid of low grade rhodochrosite is an established process route, but no study has been published which deals with low grade manganese ores with stringent control of iron and dithionate concentration. Thus, the present disclosure provides for a simple, highly productive and a cost-effective process on selective leaching of manganese from low grade high iron bearing manganese ore along with minimum dissolution of iron as well as control of dithionate in the leach liquor.

OBJECTS OF THE DISCLOSURE

An object of the present disclosure is to design a hydrometallurgical process for treating low grade high iron bearing manganese ore for selective leaching of manganese with producing iron rich residue.

Another object of the present disclosure is to utilize sulphur dioxide as reductant with control of iron and dithionate below 1 g/l throughout the process.

A still further object of the present disclosure is to use an additive to control the dithionate formation during downstream operation of manganese sulphate solution.

Yet another object of present disclosure is to operate the leaching operation at higher pulp density in the leach liquor with control on filtration.

Yet another object of the present disclosure is to produce high iron value residue from the leaching section of the circuit.

Yet another object of the present disclosure is to produce sodium sulphate crystals from the solution after manganese carbonate precipitation.

STATEMENT OF THE DISCLOSURE

The present invention relates to a process for processing of high iron bearing low grade manganese ores and the manganese obtained by the process, wherein the process comprises of: crushing the low-grade manganese ores and making a slurry with process water of pulp density greater than 10% w/v, reductive leaching of the slurry with sulphur dioxide gas in a reactor until a pH of about 4.0 is achieved to obtain leach solution, subjecting the leach solution for solid liquid separation and filtration resulting in wash liquor and leach liquor, purifying the leach liquor by addition of hydrated lime slurry and increasing pH of the leach solution to 4.5 to 5.0 to obtain pregnant leach solution (PLS); and purifying the PLS and extracting manganese as manganese carbonate by increasing pH of the solution to about 8.0 by alkaline precipitation; wherein iron concentration and dithionate concentration is maintained at a level below 1 g/l during the process.

BRIEF DESCRIPTION OF THE ACCOMPANYING FIGURES

Figure 1 is a process flow diagram for extraction of manganese carbonate from lean grade manganese ores.

Figure 2 is a graphical representation of manganese and Eh of the “leach solution” over time and relative to SO2 during the leach.

DETAILED DESCRIPTION OF THE DISCLOSURE

With respect to the use of substantially any plural and/or singular terms herein, those having skill in the art can translate from the plural to the singular and/or from the singular to the plural as is appropriate to the context and/or application. The various singular/plural permutations may be expressly set forth herein for sake of clarity. Throughout the specification, the word “comprise”, or variations such as “comprises” or “comprising” or “containing” or “has” or “having” wherever used, will be understood to imply the inclusion of a stated element, integer or step, or group of elements, integers or steps, but not the exclusion of any other element, integer or step, or group of elements, integers or steps.

As used herein, the phrases ‘low grade high iron manganese ore’ or ‘manganese ore’ refer to any ore such as pyrolusite or rhodochrosite having < 30% Mn.

The present disclosure relates to a process for processing of high iron bearing low grade manganese ores comprising steps of:

crushing the low-grade manganese ores and making a slurry with process water of pulp density greater than 10% w/v;

reductive leaching of the slurry with sulphur dioxide gas in a reactor to obtain leach solution;

subjecting the leach solution for solid liquid separation and filtration resulting in wash liquor and leach liquor;

purifying the leach liquor by addition of hydrated lime slurry and increasing pH of the leach solution to obtain pregnant leach solution (PLS); and

purifying the PLS and extracting manganese as manganese carbonate by increasing pH of the solution by alkaline precipitation.

wherein iron concentration and dithionate concentration is maintained at a level below 1 g/l during the process.

In an embodiment of the present disclosure, the process employs low grade manganese ore having < 30% Mn

In another embodiment of the present disclosure, the process employs low grade manganese ore with Mn/Fe<1.0
In yet an embodiment of the present disclosure, the sulphur dioxide gas is passed through sparger for dispersion and mixing in the reactor.
In yet another embodiment of the present disclosure, the sulphur dioxide gas is passed either by burning sulphur flakes through sulphur burning step or by passing SO2 gas from the tonner.
In yet another embodiment of the present disclosure, the SO2 used for leaching is about 0.1lpm to about10 lpm.
In yet another embodiment of the present disclosure, the solid liquid separation involves washing of manganese values from the cake formed during leaching of the slurry.
In yet another embodiment of the present disclosure, pH of the solution during leaching is maintained below 4.0.

In yet another embodiment of the present disclosure, pH of the solution during leaching is maintained between 1.1-3.0.

In yet another embodiment of the present disclosure, the time for leaching is over a period of at least 2 hours
In yet another embodiment of the present disclosure, the leaching is carried either by batch or continuous mode.
In yet another embodiment of the present disclosure, the wash liquor is recycled for next batch.
In yet another embodiment of the present disclosure, the leach liquor is purified for elimination of dissolved iron and heavy metallic ions by precipitation.
In yet another embodiment of the present disclosure, the elimination of dissolved iron is carried out by oxidation of Fe (II) to Fe (III) by addition of oxidizing agent followed by its precipitation by addition of hydrated lime slurry.
In yet another embodiment of the present disclosure, the oxidizing agent is selected from a group comprising hydrogen peroxide, sodium perchlorate, oxygen gas, manganous oxide, potassium permanganate and combination thereof.
In yet another embodiment of the present disclosure, the addition of hydrated lime slurry precipitates Fe as its hydroxide along with the precipitation of Al and Zn.
In yet another embodiment of the present disclosure, the heavy metallic ions are selected from a group comprising Cu, Ni, Co and As.
In yet another embodiment of the present disclosure, the heavy metallic ions are precipitated by sulphidation.
In yet another embodiment of the present disclosure, the sulphidation is carried out using sodium sulphide or sodium hydrogen sulphide.
In yet another embodiment of the present disclosure, the pH is increased between 4.5 to 6.0 to obtain pregnant leach solution (PLS), preferably about 6.0.

In yet another embodiment of the present disclosure, the manganese is extracted as manganese carbonate by increasing pH of the solution between 6.0 to 8.5 by alkaline precipitation, preferably about 8.0.
In yet another embodiment of the present disclosure, the alkaline precipitation is carried out using sodium carbonate or ammonium carbonate.
In yet another embodiment of the present disclosure, the sodium carbonate or ammonium carbonate reacts with magnesium sulphate in the PLS resulting in sodium sulphate or ammonium sulphate recovered and cooled as crystals.

In yet another embodiment of the present disclosure, the iron value or concentration is maintained at a level below 1 g/l, preferably at a level below 500ppm by providing and controlling oxidation reduction potential (ORP) below 550 mV and by providing an excess or residual amount of manganese ions in the slurry.
In yet another embodiment of the present disclosure, the dithionate concentration is controlled below 1 g/l by addition of an acid-based additive.
In yet another embodiment of the present disclosure, the additive is a mixture of peroxydisulfuric acid, sulfuric acid and peroxymonosulfuric acid.
In yet another embodiment of the present disclosure, the cakes formed during the process are washed to recover any residual Mn.

In yet another embodiment of the present disclosure, concentration of metals is continuously checked during the process.
In yet another embodiment of the present disclosure, by-products formed during the process are sodium sulphate, manganese sulphate, recovered heavy metals, iron alumina cake, etc.
In yet another embodiment of the present disclosure, Manganese of up to 70-80 % is recovered by the process.
The present disclosure also relates to Manganese obtained by the above process.
The present disclosure relates a simple, highly productive and cost-effective process for processing low grade high iron bearing manganese ores. The process involves treating low grade high iron bearing pyrolusite ore for selective leaching of manganese from ore producing iron rich residue. The process also utilizes sulphur dioxide as reductant with control of iron and dithionate below 1 g/l. The leaching operation is carried at higher pulp density, with recovery of iron as iron aluminium hydroxide cake and removal of heavy metals as sulphides while producing good quality sodium sulphate crystals from the solution left after manganese carbonate precipitation.

In the present disclosure, a process for utilization of lean high iron bearing manganese ore as feedstock for production of high purity manganese salts as primary product has been described using hydrometallurgical processing. The process doesn’t employ any sintering or reduction and purely utilize the selective reductive leaching of lean manganese dioxide ore at higher pulp density using sulphur dioxide as reductant. In the process, level of dithionate ion is controlled below 1 g/l in the final solution during and after leaching process. Still preferably, pH of the solution during leaching is maintained between 1.1-3.0. In this specific process, low grade ferrogenious manganese ore with Mn< 30% and Mn/Fe<1.0 is chosen as feed stock and is subject to crushing and sizing to particular mesh size fraction followed by leaching using water as lixiviant and sulphur dioxide as reductant. In this process, leach comprises of a slurry of manganese dioxide containing material at more than 10% (wt %) solids, less than about 120 g/l of manganese sulphate in solution, a temperature of less than 100oC and operating pH between 1.1-3.0. Preferably, the leach solution has initial soluble iron concentration of less than 0.2 g/l. The iron in the leach solution is in the form of ferrous sulphate. The iron concentration in the leach solution is preferably maintained at a level below 0.2 g/l by providing and controlling the oxidation reduction potential (ORP) below 550 mV (vs Ag/AgCl reference electrode) and by providing an excess or residual amount of manganese ions in the slurry. The time for leaching is over a period of at least 2 hours whereby up to 80 % of manganese is dissolved. Conditions were optimized based on series of experiments for different samples of feed material. The sulphur dioxide quantity and addition rate are controlled as per target pH and Eh to achieve 80% dissolution of manganese. Iron dissolution during reaction is almost nil or negligible. The leach solution is first treated with an oxidizing agent like hydrogen peroxide to convert ferrous iron to ferric which is then precipitated as ferric hydroxide by adjusting the pH using lime slurry. During this stage, dissolved aluminium is also precipitated as aluminium hydroxide. An additive of acidic nature such as a mixture of peroxydisulfuric acid, sulfuric acid and peroxymonosulfuric acid is added to control dithionate level below 1 g/l which is quantified by testing the free SO2 in the leach solution. Heavy metals in the leach solution especially zinc-nickel-cobalt are removed from leach solution by treatment with sodium sulphide (Na2S) or sodium hydrogen sulphide (NaHS) at a pH between 5.5-7.0. Preferably, after whole treatment process with purification steps, the manganese in the solution is as manganese sulphate with pH of the purified solution between 6.0-7.0 which can be further processed to produce manganese salts especially manganese carbonate.
The object of the present disclosure is to provide a direct utilization of low-grade oxide based ores of manganese for preparing high purity manganese carbonate from the leach liquor subject to utilization of sulphur dioxide as reductant as per Figure 1. Another by product while producing manganese carbonate is iron hydroxide cake and secondly beneficiation enrichment without any high temperature reduction. Thus, the process proposed achieves clean and environmental friendly waste water recycle with minimal discharge of off gas and water to the green belt.
The described process can be achieved by the following specific process steps while preparing high purity manganese salts from lean grade manganese ores: (1) reductive leaching of manganese ore using SO2 as reductant in lixiviant as water; (2) recovery of iron value as hydroxide cake; (3) preparation of high purity manganese carbonate from purified leach solution.
The lean grade manganese ore with Mn<30 % and Mn/Fe:0.9 is subject to initial crushing in primary crusher like jaw crusher and subsequent size reduction in ball mill or roll pulveriser fitted with cyclone and pulse jet filter. The ground manganese ore is mixed with recycle process water at a target pulp density. The sulphur dioxide gas is then sparged into the leach slurry. The sulphur dioxide gas is generated by either burning sulphur flakes through sulphur burning step or by passing SO2 gas from the tonner. The solids percentage in the leach slurry is more than 10 (wt %). The reaction of sulphur dioxide and manganese dioxide is exothermic as a result of which the temperature of slurry increased to max 100 oC. The concentration of manganese in the leach solution is <120 g/l with maintaining the temperature of leach solution to be less than 90oC and subsequently pH of the solution is maintained below 3.0. The process parameters during leaching were controlled in such a manner that total iron value after subsequent leaching is less than 0.2 g/l and most iron shall be present in the form of ferrous sulphate (FeSO4). Leaching process is conducted either in batch or continuous. After completion of leaching process, the slurry mass is filtered and the leach solution is called as “Pregnant Leach Solution (PLS)”. The iron rich leach cake is obtained as by product of the process. Volume of sulphur dioxide during leaching is governed by two factors: total iron in the PLS shall be less than 0.2 g/l and dithionate ion less than 1 g/l. During the leaching process, the ORP value is precisely controlled by changing the gas flow rate into the reactor. During leaching of the manganese dioxide from the lean ore, following reactions take place:

MnO2 + SO2 = MnSO4
2Fe2O3 + 4SO2 = 4FeSO4
MnO2 + 2SO2 = MnS2O6
However, it is predominant that while the process of the present disclosure ensures production of manganese sulphate, there is a formation of trace amount of dithionate. The production of dithionate is believed to proceed as a free radical combination reaction as follows:
SO3- + SO3- = S2O62-
Once the target quantity of sulphur dioxide has been fed through the leach solution and approximately 75-80% of manganese is recovered from the ore particles, the leaching reaction is stopped. Figure 2 provides the stoichiometric addition of SO2 with relative amount of manganese leached and Eh of the solution with respect to time at those respective values provide an accurate indication of completion of the manganese (IV) dissolution reaction.
On completion of leaching, slurry filtration is carried out using standard filtration technique. Once the filtration is completed, the PLS obtained is taken to purification section where an additive of acidic nature such as a mixture of peroxydisulfuric acid, sulfuric acid and peroxymonosulfuric acid is added and pH is controlled to a desired value of 0.2-1.5 and stabilized for 0.1-5 hours and during this, Eh of the solution is monitored continuously and eventually kept between 100- 300 mV (vs Ag/AgCl reference electrode). The iron value from the PLS which is present as ferrous (Fe2+) is converted to ferric (Fe3+) by adding oxidizing agent such as hydrogen peroxide, sodium perchlorate, oxygen gas, manganous oxide, potassium permanganate or a combination thereof and eventually iron is precipitated as hydroxide and reduced to <1 ppm level in solution through addition of hydrated limestone slurry. A solid-liquid separation step is then used for separation of iron hydroxide cake using filtration. Solids obtained is washed thoroughly to reclaim any residual manganese value and stored for subsequent processing for recovery of iron. Purified iron free PLS solution is passed to sulphidizing process for removal of heavy metals, including nickel, zinc, cobalt and copper as their respective sulphides. Following the sulphidizing step, the heavy metal precipitates are removed by fine cartridge filtration and subsequently the purified manganese sulphate solution is stored for preparation of manganese carbonate. Following reactions happen during the purification of leach liquor:

Fe2(SO4)3 + 3Ca(OH)2 = 3CaSO4 + 2Fe(OH)3
Al2(SO4)3 + 3Ca(OH)2 = 3CaSO4 + 2 Al(OH)3
CuSO4 + Ca(OH)2 =CaSO4 + Cu(OH)2
CoSO4 + 2NaHS = CoS + Na2SO4 + H2S
NiSO4 + 2NaHS = NiS + Na2SO4 + H2S
ZnSO4 + 2NaHS = ZnS + Na2SO4 + H2S
After purification of PLS, it is subjected to precipitation using sodium or ammonium carbonate. The precipitation is carried out at a controlled pH of 7.5 to 8.5 for 2-8 hrs at 40-80 oC. The manganese sulphate gets converted to manganese carbonate as per the following reaction:

MnSO4 + Na2CO3 = MnCO3 + Na2SO4

The carbonate slurry is then filtered and water washed for removal of sulphates. The cake washing is monitored online through measurement of conductivity of wash solution. The wet cake obtained is dried at 60-90oC for 2-8 hrs. The sodium sulphate solution generated during the process is sent to evaporation plant for preparation of sodium sulphate crystals.

The disclosure is further exemplified by the following example. However, the example should not be limited to construe the scope of the disclosure.

Example-1

The specific example for extraction of manganese carbonate as per the flowsheet shown in Figure 1 is described below.
1. Low grade manganese ore lumps with Mn:Fe: 1.5 (detailed chemical analysis shown in Table 1) is crushed to size distribution below <1mm using jaw crusher and pulveriser based system. The oversized particles are sent back to screen again for crushing.
2. Crushed manganese ore is made slurry with process water of desired pulp density of 20 wt.% and eventually the slurry is transferred to the leaching reactor for reductive leaching with sulphur dioxide as main reducing agent. 0.1-10 litre per minute (l/m) of gas is passed through sparger to make bubble size of below <1 mm for better dispersion and mixing. Eh and pH of the leaching slurry is continuously monitored and after achieving a pH value of below 4.0, the leaching is stopped. During the leaching, intermittent liquid solutions are tested for Mn (II), Fe (II) and free SO2 dissolved in the solution. Once the desired value is achieved, the gas to the leaching reactor is slowly stopped and allowed to stabilize.
3. The leach slurry is taken to filter press for solid liquid separation and cake washing is carried out to wash out manganese values from the solid cake. The wash liquor is stored separately and recycled to next batch. . The leach cake obtained after filtration is stored for further utilization. The leach liquor obtained is taken for purification to remove iron, aluminium, heavy metals etc. A recovery value of >70% manganese is obtained along with < 0.5 g/l Fe, <0.9 g/l dithionate ions, and >99% removal of impurities.
4. The leach solution obtained is subjected to purification for elimination of dissolved iron and other heavy metallic ions by subsequent precipitation. A mixture of peroxydisulfuric acid, sulfuric acid and peroxymonosulfuric acid is first added and solution is stabilized for 1-2 hours to control dithionate level below 1 g/l. Iron which is generally present as Fe (II) has to be oxidized to Fe (III) and is carried out by addition of hydrogen peroxide of 30-40 wt% in to the leach solution. Hydrated lime slurry of 20wt% is added in to the leach solution and subsequently pH of the leach solution is increased to 6.0 with precipitation of Fe, Al as hydroxide cake. The iron cake is removed from the PLS by filtration using filter press. Further purification of the leach solution is carried out to remove heavy metals from the leach solution such as Zn, Cu, Ni, Co, traces of As etc. using sulphidation by addition of sodium hydrosulphide of 30 wt% concentration with continuous monitoring of Eh and pH for 1-4 hours. The PLS obtained is stored for further processing to produce manganese carbonate powder.
5. The purified PLS solution is subject to manganese recovery as manganese carbonate. To extract manganese as manganese carbonate, pH of the solution is increased close to 8.0 by controlled addition of Na2CO3. Manganese carbonate obtained is washed and filtered from the solution to obtain high purity manganese carbonate as product. By product solution from manganese carbonate contains sodium sulphate which is recovered as crystals by evaporation and cooling, as pure sodium sulphate crystals.
Table 1. Example of Leaching of manganese ore as per Example 1

Chemical Analysis of Manganese Ore (ROM):
Mn (%) Fe (%) Cu (%) Ni (%) Co (%)
26.54 28.17 0.014 0.003 0.021
Pregnant Leach Solution (PLS):
Mn (g/l) Fe (ppm) Cu (ppm) Ni (ppm) Co (ppm)
75.5 92 3.9 9.0 45.6
Solution after Iron removal:
Mn (g/l) Fe (ppm) Cu (ppm) Ni (ppm) Co (ppm)
75.6 <1 <1 3.7 45.6
Solution after Sulphide Precipitation:
Mn (g/l) Fe (ppm) Cu (ppm) Ni (ppm) Co (ppm)
75.67 <1 <1 1.7 17

Manganese carbonate obtained as per the current process and example 1 comprises approx. Mn – 42.74%, Fe – = 0.05%, Sulphate (SO4 2-) - = 0.5%, Chloride (Cl-) = 0.05%.

We Claim:

1. A process for processing of high iron bearing low grade manganese ores comprising steps of:
crushing the low-grade manganese ores and making a slurry with process water of pulp density greater than 10% w/v;

reductive leaching of the slurry with sulphur dioxide gas in a reactor to obtain leach solution;

subjecting the leach solution for solid liquid separation and filtration resulting in wash liquor and leach liquor;

purifying the leach liquor by addition of hydrated lime slurry and increasing pH of the leach solution to obtain pregnant leach solution (PLS); and

purifying the PLS and extracting manganese as manganese carbonate by increasing pH of the solution by alkaline precipitation.

wherein iron concentration and dithionate concentration is maintained at a level below 1 g/l during the process.

2. The process as claimed in claim 1, wherein the sulphur dioxide gas is passed through sparger for dispersion and mixing in the reactor.

3. The process as claimed in claim 1, wherein the reductive leaching of the slurry is carried out at a pH below 4.0, preferably between 1.1 to 3.0.

4. The process as claimed in claim 1, wherein the solid liquid separation involves washing of manganese values from the cake formed during leaching of the slurry.

5. The process as claimed in claim 1, wherein the wash liquor is recycled for next batch.

6. The process as claimed in claim 1, wherein the leach liquor is purified for elimination of dissolved iron and heavy metallic ions by precipitation.

7. The process as claimed in claim 6, wherein the elimination of dissolved iron is carried out by oxidation of Fe (II) to Fe (III) by addition of oxidizing agent followed by its precipitation by addition of hydrated lime slurry.

8. The process as claimed in claim 7, wherein the oxidizing agent is selected from a group comprising hydrogen peroxide, sodium perchlorate, oxygen gas, manganous oxide, potassium permanganate and combination thereof.

9. The process as claimed in claim 7, wherein the addition of hydrated lime slurry precipitates Fe as its hydroxide along with the precipitation of Al and Zn.

10. The process as claimed in claim 6, wherein the heavy metallic ions are selected from a group comprising Cu, Ni, Co and As.

11. The process as claimed in claim 10, wherein the heavy metallic ions are precipitated by sulphidation.

12. The process as claimed in claim 11, wherein the sulphidation is carried out using sodium sulphide or sodium hydrogen sulphide.

13. The process as claimed in claim 1, wherein the pH is increased between 4.5 to 6.0 to obtain pregnant leach solution (PLS), preferably about 6.0.

14. The process as claimed in claim 1, wherein the manganese is extracted as manganese carbonate by increasing pH of the solution between 6.0 to 8.5 by alkaline precipitation, preferably about 8.0.

15. The process as claimed in claim 1, wherein the alkaline precipitation is carried out using sodium carbonate or ammonium carbonate.

16. The process as claimed in claim 15, wherein the sodium carbonate or ammonium carbonate reacts with magnesium sulphate in the PLS resulting in sodium sulphate or ammonium sulphate recovered and cooled as crystals.

17. The process as claimed in claim 1, wherein the iron concentration is maintained at a level below 1 g/l, preferably at a level below 500ppm by providing and controlling oxidation reduction potential (ORP) below 550 mV and by providing an excess or residual amount of manganese ions in the slurry.

18. The process as claimed in claim 1, wherein the dithionate concentration is controlled below 1 g/l by addition of an acid-based additive.

19. The process as claimed in claim 18, wherein the additive is a mixture of peroxydisulfuric acid, sulfuric acid and peroxymonosulfuric acid.

20. Manganese obtained by the process as claimed in claim 1.