Formation characteristics of dithionate and sulfate ions in the pyrolusite leaching process with SO₂

Abstract

As a byproduct in the pyrolusite leaching process with SO₂, the existence of MnS₂O₆ will decrease the purity of MnSO₄ produced by leaching. For obtaining MnSO₄ with satisfactory quality, it is beneficial to decrease the formation of MnS₂O₆ during the leaching process. A study on the formation characteristics of dithionate and sulfate formed in pyrolusite leaching process with SO₂ was carried out. The results indicated that ${\text{S}}_{ 2} {\text{O}}_{ 6}^{ 2- }$ formation and mole ratio of ${{{\text{S}}_{ 2} {\text{O}}_{ 6}^{ 2- } } \mathord{\left/ {\vphantom {{{\text{S}}_{ 2} {\text{O}}_{ 6}^{ 2- } } {{\text{SO}}_{ 4}^{ 2- } }}} \right. \kern-0pt} {{\text{SO}}_{ 4}^{ 2- } }}$ increased with increasing initial pH, liquid–solid ratio and SO₂ concentration, but was independent of particle size and stirring speed when particle size was less than 200 μm and stirring speed exceeded 400 rpm. The suitable conditions for obtaining MnSO₄ product was 45  C and 0.4 mol L⁻¹ H⁺ concentration with ${\text{S}}_{ 2} {\text{O}}_{ 6}^{ 2- }$ concentration and ${\text{S}}_{ 2} {\text{O}}_{ 6}^{ 2- } /{\text{SO}}_{ 4}^{ 2- }$ ratio were lower than 0.7 g L⁻¹ and 0.02. Besides, the molar ratio of ${\text{S}}_{ 2} {\text{O}}_{ 6}^{ 2- }$ to ${\text{SO}}_{ 4}^{ 2- }$ (R) was put forward as a correlated parameter to evaluate the formation of reaction products, whose kinetics was analyzed in depth. The kinetic equation was related to reaction temperature, H⁺ and SO₂ concentration, and the calculated reaction orders for SO₂ concentration and initial H⁺ were 0.27 and − 0.17, the activation energy was 9696 J mol⁻¹.