Study on the Technology of Preparing High-purity Manganese Sulfate with Low-Grade Manganese Carbonate Ores

Central South University, Lin Qingquan…

    Mn, an important metallic element, is widely used in producing steel, ferromanganese, non-ferrous alloy, dry cell, oil paint and other chemicals. Our country has abundant manganese ore resources and the reserves list top position in the world. But they have characteristics of unbalanced distribution and poor, thin, mixed and fine resource. It has been identified that most manganese ore resources centre on Guangxi, Hunan, Chongqing, Yunnan and other areas (Totally accounts for 87.6% of national reserves). Besides, most of them are lean ore and few are rich ore. National manganese average grade is only 21.5%. The majority of our manganese ore resources are manganese carbonate ore accounting for nearly 56%. Manganese oxide takes the second most accounting for nearly 25% and other types occupy 19%. With the rapid development of our industry, rich manganese ore reserve (manganese oxide ore whose manganese content is more than 30% and manganese carbonate ore whose manganese content is more than 25%) is very scarce only accounting for 6.4%. Rich ores which conform to international commercial grade (more than 48% of manganese content) is lacking. The reserve of lean manganese ore accounts for 93.6% of the national total reserves. Because of low grade and high impurity content, if adopts conventional beneficiation method (reselection, magnetic separation, flotation and so on), the improvement of manganese grade is limited (generally improve 3%-5%). We should adopt chemical beneficiating method to handle this kind of lean manganese ore. For example, getting manganese salt through sulfuric acid leaching and edulcoration is an effective way of rationally utilizing lean manganese ore. 
    Manganese sulfate is an important raw material for producing other manganites and manganese salt products. It is widely used in industry, agriculture, medicine and other fields. Seeing from economic angle, producing manganese sulfate through low grade manganese ore leaching is very important. In recent years, there are many researches which study adopting the reduction leaching of hydrogen peroxide, green vitriol, sawdust, sucrose and other substances and preparing high-purity manganese sulfate by using low grade pyrolusite as raw material. But there are few researches which are related to direct acid leaching to produce manganese sulfate by using low grade manganese carbonate ore. This is due to the high content of iron, calcium, magnesium and other impurities in manganese carbonate ore. Their separation and edulcoration are comparatively difficult, so the quality of manganese sulfate products is seriously influenced. Therefore, the researches which study direct acid leaching process of manganese carbonate ore, purification process of lixivium and crystallization and preparation of high-purity manganese sulfate are very important to fully develop and utilize our rich low grade manganese ore resources. 
    This research was using low grade manganese carbonate ores to react with sulfuric acid and preparing manganese sulfate after purifying the leaching solution. Under the conditions of liquid-to-solid ratio of 5:1, sulfuric acid concentration of 0.86 mol/L, agitation rate of 300 r/min and reaction time of 80 min, the kinetic study on the leaching process of manganese ore was made. As a result, the leaching process followed mainly the internal diffusion-controlled shrinking core model, and the apparent activation energy (Ea) was calculated to be 9.83 kJ/mol. Then, the leaching solution, which was obtained by extracting 91.4% from manganese ores at 70℃, was subjected to purification treatment. Firstly, adding manganese dioxide dosage of 1.5 times of the theoretical value to the lixivium, Fe2+ was oxidized to Fe3+. Then Fe3+ was removed through hydrolysis. At the pH value of 5.0, the iron-removal efficiency was 96.38%, and the loss ratio of manganese in iron-removal was merely 4.98%. Sodium Dimethyl Dithio Carbamate (S.D.D) was used to precipitate the heavy metals. Under the conditions of that the liquor temperature was about 60℃ , the pH value was 5.4~6.0, the dosage of S.D.D was 1.2 times of the theoretical value and the reaction time of 60min, the retained Ni2+ content in the purified solution could reach less than 15ppm and the efficiency of removing Ni2+ was 95.3%. Finally, carried out inspissations and crystallization under mesophilic conditions and got manganese sulfate product which had 98% of MnSO4•H2O.