Minerals Beneficiation - Rate of Dissolution of Manganese Dioxide in Sulfurous Acid

Manganese dioxide reacts with sulfur dioxide solution in the absence of oxygen to form manganous ion and dithionate ion, together with a small amount of sulfate ion. The effects of SO, concentration, hydrogen ion concentration, stirring speed, and temperature on the rate of the reaction were studied. If it is assumed that undissociated SO2 molecules react rapidly with MnO, at active surface sites and that bisulfite ions react slowly, an equation can be derived that satisfactorily represents the observed reaction rates. For solutions in which the reaction with undissociated SO2 predominates, reaction rates and activation energies estimated from diffusion theory agree well with those found experimentally. Manganese dioxide dissolves readily in aqueous solutions of sulfur dioxide. The nature of the dissolution reaction was first investigated by Gay-Lussac and welter1 in 1819; they identified a new oxysulfur radical, the dithionate ion, S2O6, as one of the reaction products. Since then, the reaction has become a standard method of preparing dithionates in the laboratory and one of the more promising methods of leaching manganese from low-grade ores. In the "dithionate process" developed by the U.S. Bureau of Mines2,3, manganese-dioxide ore is suspended in a solution of calcium dithionate and leached with gas containing sulfur dioxide. The manganese is dissolved and the sulfur dioxide is oxidized according to the reactions: