Carbon in gas-phase‒solid-phase processes restoration of ore materials

Abstract

The problems of the participation of carbon and carbon-containing gases in the processes of carbothermal reduction are discussed on the example of solid-phase reduction of chromite ores. A “gas-phase–solid-phase” reduction model has been proposed. Gaseous carbon-containing molecules are transport agents. Molecules diffuse to the surface and deep into the ore piece through pores and cracks. On a solid surface, gaseous molecules disintegrate releasing atomic carbon. This carbon is the main reducing agent. A thermodynamic assessment of the sequence of formation of metal products during the carbothermal reduction of iron chromite was carried out, taking into account the diffusion distribu-tion of carbon along the depth of a porous piece of ore. A thermodynamic assessment of the sequence of formation of metal products during the carbothermal reduction of iron chromite was carried out, taking into account the diffusion distribution of carbon along the depth of a porous piece of ore. The first portions of the reduced metal in the deep parts of the ore piece should be a ferrous low-carbon alloy. As it approaches the surface of the piece, the reduced metal be-comes enriched in chromium and carbon. In the surface parts, the metal corresponds in composition to carbon ferro-chrome. The porosity of chromite ores after heating under reducing conditions was determined experimentally. Open porosity in ores varies from 15 to 25 vol. % depending on the amount and type of host rock. The distribution of porosity by pore size has been established. A computational assessment of the transfer of carbon deep into a porous piece of ore by methane-containing gas has been carried out. The results were obtained by solving a system of differential equations for methane diffusion coupled with the reaction of thermal decomposition of methane. The degree of metal reduction corresponds to the total carbon flow into the ore piece. Experiments on reduction of chromite ores of different texture by graphite, coke and methane-containing gas were carried out. The reduction results are in agreement with the “gas-phase–solid-phase” model both in terms of the nature, amount and localization of the reduced metal, and in the chemical composition of the metal. The presence of carbon in the pores and in the reduced metal deep in the ore pieces is direct evidence of carbon transfer through the gas phase.