Modelling of heterogeneous gas-solid reactions

Abstract

A general modelling is presented of the heterogeneous gas-solid reactions relevant to the field of extractive metallurgy. The unreacted core shrinking (topochemical) and the homogeneous model are presented as the limiting cases between a reaction taking place on a sharp interface and one taking place in a diffuse manner throughout a pellet-matrix. Analysis shows that under certain conditions of temperature, porosity, and grain size, the reaction interface may spread out over a definite width and thus make the topochemical model an unrealistic one. Also for cases where the spread factor is large and intragranular resistance is important, the homogeneous model becomes inadequate. Two models are discussed and developed. The first one which neglects intragranular resistance takes into consideration the diffuse character of the interface by incorporating spread and effectiveness factors into the reaction scheme. The second model, which assumes intragranular resistance, includes important variables such as porosity, grain size, solid reactivity, and gas-solid diffusivities in the overall reaction system; in this respect it gives a better insight on a quantitative basis as to the effects of each variable upon the rate of reaction.