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Study of the reaction of pyrrhotite with sulphur dioxide Ogle, Iain Colquhoun Gibson

Abstract

The oxidation of thin rectangular plates of pyrrhotite by sulphur dioxide between 700 and 900°C has been studied at sulphur dioxide partial pressures ranging from 0.25 to 1.0 atm. As reaction proceeded, an outer layer of magnetite thickened at the expense of pyrrhotite, sample dimensions remaining constant. Observations of partially oxidized samples have indicated a linear reaction front and the lack of retained pyrrhotite. Weight loss curves have indicated two types of behaviour: constant rate with increasing porosity at high sample densities, and increasing rate with increasing porosity at lower sample densities. Analysis by a transport control model has shown that chemical processes have no effect on the reaction rate and that the rate is controlled by gaseous diffusion through a laminar gas film at onset of reaction of low density samples. As the magnetite layer thickens, control shifts to that of gaseous diffusion through the porous magnetite. By correlation of observations and effective diffusion coefficients, it is demonstrated that the structure of magnetite varies as a function of its thickness and is dependent on reaction temperature and pressure. The primary mode of diffusion in the porous magnetite layer is determined to be transitional for the range of pyrrhotite densities investigated. High density samples are subject to rupturing. This causes approximately linear reaction rates which are independent of sample density. It is proposed that reaction is cyclic in that it proceeds by the transport of oxide ions to the reaction front, subsequent rupturing, and gaseous diffusion until pore distributions prohibit Knudsen diffusion.

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