Thermodynamic Analysis and Reduction of MnO₂ by Methane–Hydrogen Gas Mixture

Abstract

Methane and hydrogen reduction has emerged as promising technology for clean and efficient metallurgy. To understand the reduction process of MnO₂ by CH₄-H₂ gas mixture, the effects of various CH₄/(CH₄ + H₂) molar ratios (0–1) and temperatures (1100°C–1250°C) on the reduction of pure MnO₂ were researched. Characterization of the solid carbon derived from the decomposition of CH₄ gas was also determined. Thermodynamic analysis indicated that the MnO₂ was first reduced to MnO by CH₄-H₂ gas mixture, and then MnO was reduced to Mn₇C₃ according to this reaction, 7MnO_(s) + 10CH_4(g) = Mn₇C_3(s) + 7CO_(g) + 20H_2(g). Experimental results showed that under the CH₄/(CH₄ + H₂) molar ratio of 0.2, the reduction extent of MnO₂ increased with the rising of temperatures. Nevertheless, the deep reduction of MnO to Mn₇C₃ was retarded even by increasing the temperature and prolonging the time, which was a result of the formation of deposited solid carbon. The free carbon contents in the reduced samples increased with the increasing of the CH₄/(CH₄ + H₂) molar ratio and reduction time. The reduction of MnO₂ and formation of carbon can be regulated by adjusting the CH₄/(CH₄ + H₂) molar ratio in a gas mixture. For example, when MnO₂ was reduced by 50 vol.%CH₄-50%N₂, large amounts of MnO and Mn₇C₃ particles were wrapped by the platelike and rodlike carbon with nanoscale. Yet, the wrapping phenomenon cannot be observed under 10 vol.%CH₄-40 vol.%H₂-50 vol.%N₂.