Studies on reduction of manganese dioxide by (NH 4) 2SO 3 in ammoniacal medium

Abstract

The reduction of MnO 2 by (NH 4)SO 3 has been studied. The reaction path has been described by the model 1−(1− α) 1/3= kt. The reaction was found to be first order with respect to sulfite concentration and 0.5 order with respect to MnO 2. Ammonia and ammonium sulphate concentrations did not affect the kinetics of reduction of MnO 2 in the studied range. The activation energy was estimated to be 62 kJ mol −1. The reduced products were mixtures of Mn 2+, Mn 3+ and Mn 4+. With the increase in the reaction temperature from 353 to 383 K, the ratio of Mn 2+/Mn 3+ 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 MnO 2 are governed by a shrinking core model, it is implied that the reduction of any intermediate Mn 3+ species to Mn 2+ 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.