Recycling of high-purity simonkolleite from electroplating wastewater via a coupled PAC coagulation and hydrothermal separation route

Polyaluminium chloride (PAC) is a common flocculant that precipitates heavy metals from electroplating wastewater but is involved in the mass production of hazardous sludge. The conventional method of recovering heavy metals from sludge is tedious and heavily consumes extractant. Herein, we developed a facile strategy to effectively recycle heavy metals from electroplating wastewater effluent. The strategy included five steps. Firstly, wastewater with 87.4 mg/L Zn was collected from the effluent of an electroplating workplace and subjected to PAC coagulation to produce a brownish sludge. Secondly, the sludge was leached by 2.4 M nitric acid and 2.2 M hydrochloric acid to produce an acid solution with 18.4 g/L Ca, 2.7 g/L Al and 6.3 g/L Zn, where Ca and Al were the major impurities from the commercial PAC product. Thirdly, more than 97% Ca was precipitated as gypsum prior to the separation of Al and Zn by adding 0.6 M Na2SO4 with Al/Zn loss