Studies on reduction of manganese dioxide by (NH4)(2)SO3 in ammoniacal medium

Abstract

The reduction of MnO2 by (NH4)SO3 has been studied. The reaction path has been described by the model 1 - (1 - alpha)(1/3) = kt. The reaction was found to be first order with respect to sulfite concentration and 0.5 order with respect to MnO2. Ammonia and ammonium sulphate concentrations did not affect the kinetics of reduction of MnO2 in the studied range. The activation energy was estimated to be 62 kJ mol(-1). The reduced products were mixtures of Mn2+, Mn3+ and Mn4+. With the increase in the reaction temperature from 353 to 383 K, the ratio of Mn2+/Mn3+ increased from 1.10 to 5.12 indicating preference of formation of lower valency states of manganese at higher temperatures. The obeyance of the shrinking core model in spite of formation of a variety of reduced products clearly indicates that none of the products form a protective layer on the manganese dioxide particles. Since the overall reduction kinetics of MnO2 are governed by a shrinking core model, it is implied that the reduction of any intermediate Mn3+ species to Mn2+ during the course of reaction follows a similar path. Depending on the experimental conditions, the dithionate content varied between 1.0 and 2.33 g l(-1), indicating predominant formation of sulphate over dithionate. (C) 1998 Elsevier Science B.V. All rights reserved.