Modified Predominance Diagrams for Gas-Solid Reactions

A new type of diagram is proposed to describe gas-solid reactions. These diagrams, called modified predominance diagrams, unite thermodynamics and kinetics into a new tool that should prove useful in many metallurgical and chemical systems. They are created from predominance diagrams by applying a mathematical transformation derived from basic transport and chemical rate phenomena. The diagrams predict the reaction product for given kinetic conditions and bulk atmospheric compositions. They consider the reaction from a single solid reactant to any solid product contained in the corresponding predominance diagram. The diagrams are similar to predominance diagrams except that some lines are curved and binary regions are created to satisfy both the equilibrium and kinetic requirements. The curvature results from the presence of gaseous reaction products at the solid reactant surface. The diagrams are developed in this article using the oxidation of a metal sulfide as an example. They show that the reaction path can be influenced even if the system is rate-limited by chemical kinetics. For a fixed bulk oxygen concentration, another diagram can be created, which can predict kinetic conditions under which mass transport no longer influences the reaction path predicted by thermodynamics.