The ultra-high lime with aluminum process for removing chloride from recirculating cooling water

摘要

Chloride is a deleterious ionic species in cooling water systems because it is important in promoting corrosion. Chloride can be removed from cooling water by precipitation as calcium chloroaluminate using ultra-high lime with aluminum process (UHLA). The research program was conducted to study equilibrium characteristics and kinetics of chloride removal by UHLA process, study interactions between chloride and sulfate or silica, and develop a model for multicomponent removal by UHLA. Kinetics of chloride removal with UHLA was investigated. Chloride removal was found to be fast and therefore, removal kinetics should not be a limitation to applying the UHLA process. Equilibrium characteristics of chloride removal with UHLA were characterized. Good chloride removal was obtained at reasonable ranges of lime and aluminum doses. However, the stoichiometry of chloride removal with UHLA deviated from the theoretical stoichiometry of calcium chloroaluminate precipitation. Equilibrium modeling of experimental data and XRD analysis of precipitated solids indicated that this deviation was due to the formation of other solid phases such as tricalcium hydroxyaluminate and tetracalcium hydroxyaluminate. Effect of pH on chloride removal was characterized. Optimum pH for maximum chloride removal was pH 12 ? 0.2. Results of equilibrium experiments at different temperatures indicated that final chloride concentrations slightly increased when water temperature increased at temperatures below 40oC. However, at temperatures above 40oC, chloride concentration substantially increased with increasing water temperature.An equilibrium model was developed to describe chemical behavior of chloride removal from recycled cooling water using UHLA. Formation of a solid solution of calcium chloroaluminate, tricalcium hydroxyaluminate, and tetracalcium hydroxyaluminate was found to be the best mechanism to describe the chemical behavior of chloride removal with UHLA.Results of experiments that studied interactions between chloride and sulfate indicated that sulfate is preferentially removed over chloride. Final chloride concentration increased with increasing initial sulfate concentration. Silica was found to have only a small effect on chloride removal. The equilibrium model was modified in order to include sulfate and silica reactions along with chloride in UHLA process and it was able to accurately predict the chemical behavior of simultaneous removal of chloride, sulfate, and silica with UHLA.