Fundamental Chemistry of Precipitation and Mineral Scale Formation, 2005 Engineering, Pulping & Environmental Conference

The mineral scale that deposits in digesters and bleach plants is formed by a chemical precipitation process. As such, it is accurately described or modeled using the solubility product equilibrium constant. Although solubility product identifies the primary conditions that need to be met for a scale problem to exist, the acid base equilibria of the scaling anions often control where in the process scale will occur and are the primary control method to minimize or prevent scale in a bleach plant. As equilibrium processes, both the acid/base equilibrium and solubility product are influenced by temperature and ionic strength. In this paper, the chemistry of precipitation and acid/base equilibria will be reviewed. The use of the Gibbs Free Energy expression that connects equilibrium constants to enthalpy entropy and temperature will be discussed. The affect of ionic strength on ion activity and how it influences these processes will also be presented.