A fundamental investigation of the hydrothermal dissolution and oxidation of manganese metal.

Abstract

The oxides of manganese display a number of allotropic structures, a number of which have significant industrial importance. In particular, mangano-manganic oxide (Mn30 4), is used as a raw material for the manufacture of soft ferrites. The preparation of Mn30 4 by hydrothermal dissolution and oxidation of manganese metal at elevated temperature and pressure yields a raw material that has unique chemical, morphological and structural characteristics. In this work experimental and theoretical investigations were conducted to determine the mechanisms of the dissolution and oxidation reactions as well as the influence that processing conditions have on the morphological and structural properties of the oxidation products. Simplified Pourbaix diagrams were generated to map the stability domains of relevant manganese oxides for hydrothermal processing conditions. These show that the domain of stability of manganous hydroxide, Mn(OHh(aq) may be larger than currently reported in literature. Hydrothermal dissolution experiments were conducted to investigate the influence of hydrogen partial pressure on reaction kinetics. An experimental and mathematical methodology was developed to facilitate the analysis of the non-isothermal dissolution kinetics with polydisperse shrinking particles of Mn metal. The rate of the dissolution reaction was found to be controlled by a heterogeneous chemical reaction occurring on the surface of the manganese metal particles but was not influenced by hydrogen partial pressure. The rate of oxidation of solid Mn(OHh formed after the dissolution of manganese metal by oxygen in a mechanically agitated, gas-sparged reactor was found to follow linear kinetics. Dissolution and reprecipitation processes were found to occur during the oxidation of Mn(OHh particles that results in the formation of a daughter population of particles. The hydrothermal oxidation of Mn(OHh occurs via a homogeneous oxidation mechanism. Mass transfer of oxygen from the gas phase to the liquid was found to be the rate-controlling step of the oxidation process. The changes in morphology and crystal structure of the hydroxide/oxide intermediates and products during hydrothermal oxidation were investigated using a number of different characterisation techniques. Mn30 4 made under hydrothermal conditions can be oxidised by heating in air in contrast to naturally occurring minerals or synthetic Mn304 made via high temperature processes. In addition, this material shows larger deviations from stoichiometry than materials prepared by high temperature synthesis. The crystal defect structures of oxidation intermediates and products were analysed using Rietveld structural refinement techniques and compared to a number of theoretical structural models. The reactivity of hydrothermal Mn30 4 is attributed to the cationic vacancies in tetrahedral and octahedral lattice sites.