Process optimization to enhance utilization efficiency of precipitants for chloride removal from flue gas desulfurization wastewater via Friedel's salt precipitation

The treatment cost for Cl^- removal by Friedel's salt precipitation depended significantly on utilization rate of the precipitant aluminate. In this study, effects of Ca/Al molar ratio, reaction time, temperature and Al/Cl molar ratio were investigated to maximize Al utilization rate for Cl^- removal from flue gas desulfurization wastewater. Batch results showed that the maximum Al utilization rate of 55.8-60.3% was obtained at Ca/Al ratio of 3.00, reaction time of 90 min, temperature of 35 °C and Al/Cl ratio of 0.50 regardless of the initial Cl^- concentration. The precipitate obtained at the highest Al utilization rate had the highest interlayer spacing, the best crystal integrity, and the strongest binding energy of the Al-OH bond. The optimized condition made ion exchange between Cl^- and OH^- easier, and obtained more stable Friedel's salt structure to adsorb Cl^-. Pilot-scale results showed that maximizing Al utilization rate with low dosages of precipitants had insignificant effects on the removal of Mg²⁺, Ca²⁺ and sulfate compared to the strategy to maximize Cl^-, but enhanced Al utilization rate from 38.2% to 56.4%. Economic analysis showed that enhancing Al utilization rate greatly reduced treatment cost of the Friedel's salt precipitation method by 30.5%, and made the two-stage desalination process more feasible and worth popularizing.