A mixed surface reaction kinetic model for the reductive leaching of manganese dioxide with acidic sulfur dioxide

Previous researchers have rationalised the kinetics of dissolution of manganese dioxide in acidic sulfur dioxide solutions on the basis of either the adsorption of reactants followed by reactions with HSO3− and SO2 or a two-electron surface electrochemical reaction with H+ and HSO3−, the species involved in the equilibrium: SO2(aq)+H2O=H++HSO3−. This paper revisits the reported rate data for the dissolution of MnO2 in the form of plates [Herring and Ravitz, Trans. SME/AIME 231 (1965) 191] or particles [Miller and Wan, Hydrometallurgy 10 (1983) 219] to show that the rate of dissolution can be rationalised on the basis of a fast redox reaction : (i) MnO2(s)+SO2(aq)+H2O=MnOOH(s)+HSO3(aq) followed by one of the two rate determining redox reactions: (ii) MnOOH(s)+HSO3(aq)=MnSO40(aq)+H2O or Mn(OH)HSO4(s), (iii) MnOOH(s)+SO2(aq)=MnO(s)+HSO3(aq) depending on the pH of the medium. Whilst the rate equation for (iii) agrees with the electrochemical kinetic model reported previously, the dissolution of the solid Mn(OH)HSO4 or the direct reaction: (iv) MnO2(s)+SO2(aq)=MnSO40(aq) also appear to be rate controlling. The two rate constants k1 and k2 based on a shrinking sphere kinetic model: 1−(1−X)1/3=k1K0.5[SO2]0.5(t/rρ)+k2[SO2](t/rρ), for the leaching of monosized pyrolusite and electrolytically prepared MnO2 particles, are in reasonable agreement with k1 and k2 based on the dissolution kinetics of MnO2 plates.