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

Abstract

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.