Scaling prediction based on thermodynamic equilibrium calculation — scopes and limitations

A comprising thermodynamic approach is presented to investigate the behavior of saline solutions with respect to CaCO3, CaSO4 and SiO2 scaling in water treatment processes. Thr Pitzer activity coefficient model is used to describe the aqueous species activities and the corresponding equilibrium reactions are solved to determine the saline solution composition. A well evaluated parameter set for the system H-Na-Ca-Mg-OH-Cl-CO3-HCO3-CO2-SO4-HSO4-SiO2 at 25°C is compiled and applied for calculation of the pure scale solubilities as well as mixture effects such as CaCO3/CaSO4 coprecipitation and silica adsorption. New data on silica removal dependant on the saline solution composition are used to estimate the ratio of silicate formation and silica adsorption onto other precipitating salts. Whereas the saturation states for pure scales are found to be well predictable at varying conditions, only qualtitative estimations for mixed scale formation can be achieved. Here, the predictability by thermodynamic equilibrium calculation is shown to meet its present boundary and its valuable service for understanding the mechanisms of scaling and the species involved is highlighted.