Investigation on Friedel's salt precipitation method for chloride removal from flue gas desulfurization wastewater

Flue gas desulfurization wastewater (FGDW) contained high concentration of chloride, which made it difficult to recycle or achieve zero liquid discharge (ZLD). Friedel's salt precipitation (FSP) method could effectively dislodge chloride from FGDW and was expected to achieve ZLD of FGDW. Effects of reaction conditions, reagents dosage and types on chloride removal efficiency of FSP method were investigated systematically by single factor tests. Results showed that chloride removal by FSP method was fast and basically completed in 60 min. A higher stirring speed (>= 90 rpm) would enhance the dispersion and dissolution of reagents, which was beneficial to improve the chloride removal efficiency. Moreover, the dosage and types of reagents had significant influence on chloride removal and should be selected according to actual need. A two-stage FSP method was applied to avoid the negative effect caused by the presence of sulfate, by which the sulfate and chloride removal efficiency reached 98% and 60%, respectively, in the real FGDW treatment. Through SEM and XRD analysis, precipitates formed in Stage I and II were proved to be CaSO4 and friedel's salt, which belonged to Ca-Al-layered double hydroxides. The kinetics of FSP method removed chloride was attempted to study, and inferred the reaction as third order reaction. FSP method has enormous potential to be applied in industrial wastewater treatment.