GB2051023A - Process for making manganous sulphate solution - Google Patents

Abstract

A method of making manganous sulphate solution with low level impurity of potassium from high potassium ores comprises adding the reduced ore to spent electrolyte containing ferric ions and digesting same at pH 1-2 and temperature of 60- 90 DEG C for 1-4 hours, and if desired thereafter again adding reduced ore to the digested product till pH is raised to 4-6 before separating the MnSO4 solution from the precipitated material.

Description

SPECIFICATION Process for making manganous sulphate solution This invention relates to a process of making manganous sulphate solution with low level impurity of potassium for manufacture of Electrolytic Manganese Dioxide.

Electrolytic Manganese Dioxide (EMD) is synthetically manufactured with naturally occurring manganese ore as the starting material. The most important property of EMD is its crystallographic structure.

It has to be essentially gamma phase and the other crystal phase like alpha and beta have to be kept bare minimum. Alpha phase MnO2 is characterised by the presence of potassium in the crystal lattice and can be loosely described as K Mn8O16. Therefore, the potassium content of EMD should be as low as possible, preferably within 0.3%.

The main source of potassium in EMD is the starting natural ore. In India, the chief deposits of MnO2 are pyrolusite/cryptomelane type and cryptomelane contains potassium in varying concentrations. The pure pyrolusite ore contains little or negligible potassium. But most of the MnO2 deposits in India, particularly in Central and Eastern India, are of cryptomelane type and potassium content is of the order of 1%. Some ore deposits contain potassium up to 2-3%.

Similar situation exists in many other countries.

If this ore is used as a raw material for manufacture of EMD using conventional process, the intermediate MnSO4 solution will contain potassium impurity more than acceptable and will ultimately give EMD contaminated with unacceptably higher alpha phase MnO2.

Hence there is a need to develop a method (which we have done and which forms the subject hereof) forthe production of EMD using high potassium MnO2 ores with the end product having essentially gamma phase (Potassium less than 0.3%). Since large proportion of available ores are of high potassium content, the method developed by us is of immense importance.

By the method of this invention the potassium impurity of MnSO4 solution obtained from manganese ores having potassium content more than 0.8 percent and for use in EMD manufacture can be substantially reduced or minimized.

The prior art on the use of high potassium ore reveals two methods: a) Aqueous leach process: In this process the ore is calcined with a reducing agent to MnO. The chemical bond between potassium and manganese dioxide in the crystal lattice of cryptomelane is thus broken and potassium is converted to soluble carbonate or inactive complex silicates. Such a calcine is leached with hot water to remove soluble potassium and separated and further dissolution of calcine so treated in acid yields an electrolyte solution sufficiently low in potassium to give gamma phase b) There is another method which uses powdered ferromanganese. Ferromanganese is added to the raw solution of MnO in H2SO4 or to spent electrolyte to produce a precipitate of ferric hydroxide at a pH of 6.0 to 6.5.Ferric hydroxide precipitate is claimed to occlude potassium and the filtered solution is said to contain low potassium.

The method proposed herein uses ferric iron compound to remove potassium impurity from MnSO4 solution during its preparation. It basically consists of digestion of partly neutralised solution of MnSO4 (spent electrolyte liquor of EMD cells) with soluble ferric ions at pH between 1-2 but preferably at 1.6 + 0.1 at temperature of 60-90 C - higher temperature being more effective-for a period between 1-4 hours. At the end of this period further neutralisation of free acid can be done using MnO, CaO or like base as required to completely precipitate iron. Reduced manganese ore (MnO) may be used advantageously.

This method differs from the first method (method "a" referred to above) in which intermediate stage digestion of partly neutralised MnSO4 solution as mentioned above at pH 1-2 is absent. The method invented by us is more effective in reducing potassium impurity as would be clear from the examples given later.

The present process for reducing potassium content of MnSO4 solution to be used for EMD manufacture prepared from manganese ore containing high potassium impurity has been arrived at by experimenting systematically on various factors like quantity of iron, pH, reaction time, temperature during preparation of MnSO4 solution and their effect on potassium content of this solution.

Accordingly this invention provides process of making manganous sulphate solution with low level impurity of potassium for manufacture of electrolytic manganese dioxide (EMD) from manganese ores containing high potassium impurity comprising adding reduced manganese ore (MnO) to spent electrolyte containing water soluble ferric ions till the pH of 1 to 2 is achieved, and digesting for a period of 1 to 4 hours maintaining the temperature of at least 60 to 90"C, thereafter again adding reduced manganese ore (MnO) till a pH of 4-6 is reached and separating the MnSO4solution obtained from the precipitated material.

The preferred temperature for digestion at pH 1-2 is 90"C and it should preferably be continued for at least 3 hours. Longer periods do not materially add to the improvement of the reaction process.

The invention is described below with reference to a preferred manner of carrying outthe process.

STEP-!. A source of iron is added to spent electrolyte such that ferric ion concentration in the above solution is between 0.2 to 6 gms per litre or more as required. (Iron concentration required depends upon quantity of potassium to be removed). Metallic iron dissolved in H2SO4 or FeSO4 itself can be used as source of iron and suitably oxidised to ferric state for the purpose, e.g. by MnO2, Mn2O3, O3 or H2O2.

STEP-2. Reduced manganese or (MnO) is added slowly to above solution of spent electrolyte containing ferric ions, kept agitated at temperature of 90"C. When pH of solution is 16 + 0.1,the addition of reduced ore is stopped.

STEP-3. The whole mass is kept agitated at same temperature (90"C) for a period of 3 hours.

STEP-4. Reduced ore (MnO) addition is started again slowly till pH of 4-6 is reached for complete precipitation of iron.

STEP- 5. The solution of MnSO4 so obtained which is low in potassium content is separated from the precipitate and other insoluble matter by e.g. filtration, decantation or centrifugation.

Following examples illustrative effectiveness of herein proposed method as compared to the ferromanganese method referred to above in removal of potassium impurity from MnSO4solution.

Example No. 7 Manganese ore analysing MnO2 - 74%, Mn-51 /O, Fe-5.5%, silicious matter5.4%, ALTO, - 2.5%, K - 0.9%, Ba - 1.2%, was reduced with fuel oil. Reduced ore contained about 3% MnO2, rest of manganese being present as MnO. This was used for making comparative study of MnSO4 solution (fresh electrolyte) prepared from spent electrolyte containing Mn-30 mg/litre, H2SO4-45 mg/litre and K - 660 mg/litre. Ferric sulfate was added to get 4 gm/litre ferric ion concentration. Temperature was maintained throughout the reactions at 90 ~ 2"C.

Rest of the processing method was similar to what is stated above.

In another experiment a method similar two ferromanganese processing method referred to above was adopted.

Percentage potassium removed by solid phase concentration of potassium, in spent and fresh electrolyte in the two experiments, is given below: %of total potassium inputrreduced Kinspent Kin fresh ore + spent electrolyte electrolyte electrolyte) removed mg per litre mg per litre Method of this invention 96 660 50 Method similar to the Ferromanganese method (referred to above) 45 660 635 Example - 2 Manganese ore, analysing MnO2 66.2% Mn 50.9% Fe 4.5% Silicious matter 3.7% Al2O3 0.54% K 2.91% Ba 1.1% was reduced using Fuel Oil. Reduced ore contained about 3% MnO2. This was used for preparing MnSO4 solution from spent Electrolyte containing Mn -32 gpl H2SO4-48.5 gpl, K-392 mg per litre.Weighed FeSO4. 7H2O crystals were added to the Spent Electrolyte so that iron concentration of 6 gpl was reached. This was further oxidised by addition of MnO2 ore.

Fresh Electrolyte from above materials was prepared using two methods.

- Method No. 1 wassimilarto Ferromanganese method (method "b" referred to hereinabove) in which reaction was carried outat 70"C.

- Method No. 2 is the method of the herein invention.

% of total Potassium input (Reduced ore + spent Potassium content mgilitre electrolyte) removed Spent electrolyte Fresh Electrolyte Method-1 39.2 392 1262 Method - 2 87.2 392 256 The above examples show effectiveness of the processing method invented by us in obtaining a manganous sulfate solution containing low potassium impurity (in acceptable amounts for manufacture of EMD) from ore containing potassium more than 0.8%.

Using above solution for plating, EMD of the following specification has been made: CHEMICAL ANALYSIS MnO2 90% Min Combined water 3% Min.

Sulfate 1.5% Max.

Potassium 0.3% Max.

Crystal phase Essentially Gamma

Claims (10)

1. A process of making manganous sulphate solution with low level impurity of potassium from manganese ores containing high potassium impurity characterised by adding reduced manganese ore (MnO) to spent electrolyte containing water soluble ferric ions until a pH of 1 to 2 is achieved, and digesting for a period of 1 to 4 hours at a temperature of at least 60 to 90 C. j

2. A process as claimed in claim 1 characterised in that afterthe said digestion for a period of 1 to 4 hours neutralization of free acid is effected by the addition of MnO, CaO or like substance which will precipitate iron.

3. A process as claimed in claim 1 or2, characterised in that reduced manganese ore is added until a pH of 4-6 is reached and thereafter separating the MnSO4 solution obtained from the precipitated material.

4. A process as claimed in claim 1, 2 or 3, characterised in that the temperature of digestion is 90"C.

5. A process as claimed in any one of claims 1 to 4, characterised in that the first mentioned pH is 1.6 + 0.1.

6. A process as claimed in any one of claims 1 to 5, characterised in that digestion is continued for 3 hours at pH 1.6 t 0.1.

7. A process as claimed in any one of claims 1 to 6, characterised in that the source of ferric ions is a solution of FeSO4 oxidised in situ by an oxidising agent.

8. Manganous sulphate solution whenever produced by a process as claimed in any one of claims 1 to7.

9. A process as claimed in any one of claims 1 to 7, substantially as hereinbefore described in Example 1 or2.

10. Manganous sulphate solution as claimed in claim 8 substantially as hereinbefore described in Example 1 or2.