Separation of fluoride and chloride ions from ammonia-based flue gas desulfurization slurry using a two-stage electrodialysis

This study reports the use of electrodialysis (ED) to separate fluoride (F−) and chloride (Cl−) ions from an ammonia-based flue gas desulfurization (FGD) slurry. The optimal operation voltage, circular flow rate, and pH were determined based on the ion migration efficiency and energy consumption. An applied voltage of 15 V and a circular flow rate of 200 L/h were determined to be the optimum conditions, while solutions with pH values in the range of 4–6 were beneficial for the migration of F−. The field slurry was also subjected to ED after being pretreated with microfiltration and ion exchange. The selected monovalent-anion-selective ACS membrane showed superior performance towards F− and Cl− separation and SO42− retention compared with a homogeneous AMX membrane. To further separate F− and Cl− from the multicomponent ammonia-based FGD slurry, experiments were carried out in two stages. The variation of separation efficiency and specific ion selectivity during the experiment were analyzed and the appropriate operation duration was obtained. As a result, the Cl−-containing solution reached a maximum purity of 98.6%, whereas the F−-containing solution reached a purity of 51.4%. The two-stage ED treatment process investigated herein can potentially be used to separate F− and Cl− and simultaneously purify ammonia-based FGD slurry.