Abstract:The formation of manganese dithionate is the key scientific issue limiting the extensive industry application of flue gas desulfurization with pyrolusite. So far, the formation mechanism of manganese dithionate has not been clarified, which is difficult to provide effective theoretical guides for controlling manganese dithionate formation. The rate-controlling steps and dynamic process of manganese dithionate formation for pyrolusite leaching process with sulfur dioxide waste gas has been clarified in this paper based on the combination of theoretical analysis and experimental verification. Firstly, the reaction system was analyzed theoretically through literature studies, based on the free radical production mechanism of SO₂ oxidation and the reduction leaching kinetics model derived from surface complexation and electrochemical model were proposed. The formation mechanism of manganese dithionate can be proposed based on HSO₃ free radical formation mechanism, which has manifested that the formation rate of manganese dithionate was depended on the production rate of HSO₃ free radical in microscopic scales, and can be determined by H⁺ and $\text{HS}{\text{O}}_{\text{3}}^{{-}} $ concentration in macroscopic view. The derived theoretical equation for manganese dithionate formation could be reported as follows ${{R}}_{\text{Mn}{\text{S}}_{\text{2}}{\text{O}}_{{6}}}{=}{k}\cdot{[}{\text{H}}^{{+}}]\cdot[{\text{HSO}}_{\text{3}}^{{-}}] $, the reaction orders with respect to H⁺ and $\text{HS}{\text{O}}_{\text{3}}^{{-}} $ both were 1.0. Then, the effects of SO₂ concentration, pH and temperature on manganese dithionate formation and dynamical processes were explored through dynamic experiments, the results showed that the manganese dithionate formation rate was decreased rapidly firstly, and then decreased easing up with pH and temperature, while raised under higher SO₂ concentration conditions, the calculated apparent activation energies for manganese dithionate formation was 7 068.98 J/mol, the reaction orders with respect to H⁺ and SO₂ concentration were –0.059 and 1.014, respectively. Finally, in combination with dynamical results and dissolution equilibrium analysis of SO₂, the reaction orders of manganese dithionate with H⁺ and $\text{HS}{\text{O}}_{\text{3}}^{{-}} $ were calculated as 0.955 and 1.014, respectively, which was extremely close to the theoretical reaction order of derived theoretical equation and verified the accuracy of theoretical analysis. This study showed that the formation mechanism of manganese dithionate could be explained HSO₃ free radical mechanism, which could provide effective theoretical basis and approaches for ascertaining the formation characteristics of manganese dithionate and exploring corresponding inhibition methods.