Synthesis and kinetic modeling of manganese carbonate precipitated from manganese sulfate solution

Manganese is one of the most difficult elements to be recovered from its respective solutions, due to its high solubility in both acid and neutral conditions. However, metal carbonate precipitation as manganese carbonate can be an effective way for its recovery. In this study, manganese carbonate (MnCO3) was precipitated by mixing an aqueous solution of manganese sulfate (MnSO4 containing 85% Mn2+) and sodium carbonate (Na2CO3) in different proportions. The maximum (∼72%) manganese was precipitated as MnCO3 at a pH value of 3 for 5 g Na2CO3 at 80 °C and 40-min reaction duration. X-ray diffraction of precipitated sample confirmed the formation of single-phase rhodochrosite (MnCO3). The observed microstructure comprised coarse grains composed of 15.69 wt.% C, 41.78 wt.% O, and 30.8 wt.% Mn along with Na and S as minor impurities. FTIR confirmed the presence of the characteristic MnCO3 band at 724 cm−1 corresponding to the C–O (CO32–) bond. The kinetic study of the process indicated that the reaction process followed the first-order kinetics. The reaction rate constants were observed to gradually increase with increasing reaction temperature, and the apparent activation energy was calculated to be 27.338 kJ.mol−1 demonstrating the existence of energy barrier between the reactants and products. A spontaneous and an endothermic nature of chemical reaction has been revealed by the thermodynamic parameters of activation (ΔH#, ΔS#, and ΔG#), accompanied by the entropical changes at solid solution interface during precipitation.