Recovering precipitates from dechlorination process of saline wastewater as poly aluminum chloride

Chemical sludge massively generated from chloride removal of saline wastewater by the Friedel's salt precipitation method is difficult to handle and dispose. In this study, a novel process was proposed to recover dechlorination precipitate from saline wastewater as poly aluminum chloride (PAC). After Friedel's salt precipitation, 55.7% of Cl^– could be removed with residual Al³⁺ of 1.86 mmol/L, yielding precipitate (14.3 g DS/g Cl^– removed) with Al₂O₃ content of 22.6% utilized for PAC preparation. The results of leaching experiments showed that 97.2% of Al³⁺ in precipitate could be extracted at fixed HCl concentration of 210 g/L, temperature of 353.15 K and leaching time of 180 min. The atomic force microscope images and Fourier transform infrared spectroscopy spectra confirmed that high basicity of PAC could be achieved with addition of CaO, which was favorable for reducing residual Al³⁺ in treated water. The dechlorination precipitate is more suitable than other solid wastes as raw material for PAC preparation owing to its Ca-based frameworks. PAC recovery could efficiently compensate chemical cost for Cl^– removal by 63.9%–68.4%, which was economically beneficial and inspirable for utilization of Friedel's salt in other domains.