Solid-state Diffusion and Crystal Growth: Two Important Steps for Gas-solid Reactions

The importance of solid-state diffusion and crystal growth of the solid product in a gas-solid reaction with solid product formation (GSRSPF) are discussed with direct sulfation of limestone as a representative example. A GSRSPF usually involves nucleation and crystal growth of the solid product, which are necessary processes for the formation of the new phase of the solid product. Solid-state diffusion is a process which is responsible for the transport of atoms or ions of the solid reactant and product during the reaction and crystal growth of the solid product. For a GSRSPF, the conversion of the solids may involve steps such as gas film diffusion, pore diffusion in product layer, chemical reaction, solid–state diffusion and crystal growth. The overall kinetics is usually significantly influenced by the relatively slow and highly activated solid–state diffusion. Due to the involvement of nucleation and crystal growth of the solid product, crystallographic properties of the solid reactant and product have a large influence on the overall kinetics as well since those properties determine how nucleation and crystal growth proceed.