The chemical conversion of a massive solid

Consider the reaction of a solid reactant B with a reactant A present in a fluid phase, in a non-steady state situation. The chemical reaction proceeds only at the surface of the solid, i.e. the solid is not volatile or soluble. The reaction product P is volatile or soluble, and is carried away with the fluid phase. 

In this situation eq. (5.10) can be applied. However, since the solid reactant B is consumed, we have to consider non-steady state conditions. 

The following mass balance would hold for one spherical solid fi-particle with diameter d, density p and molecular mass M 

Let us first consider the case that the rate is completely determined by chemical kinetics. It is also assumed that A is present in large excess. 

Apparently, the diameter decreses linerarly with time, and after a critical time the particles are completely converted (dissolved). 

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In section 5.3.1.2 we discussed the case of the chemical conversion of a massive solid that is not soluble, so that the reaction takes place at the solid surface. This is a type of reaction of which there are very many examples, particularly in inorganic chemistry (e.g., in hydrometallurgy). In many cases, however, the solid is porous. This case will be treated here. It is to be distinguished from the case where the solid is slightly soluble and reacts in the fluid phase (see sections 53.4.2 and 5.4.2J, p. 153). 

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