Raw materials recovery from spent hydrochloric acid-based galvanizing wastewater

The composition of spent hydrochloric acid-based pickling liquors, deriving from the galvanizing industry, greatly depends on how long the bath has been used for pickling. We considered solutions containing 40-50 g L−1 of iron and zinc (as FeCl2 and ZnCl2) and 130-220 g L−1 of chloride, and used tri-butyl phosphate (TBP) and di-(2-ethylhexyl) phosphoric acid (D2EHPA) in kerosene as extraction solutions. From the spent liquors, zinc was selectively extracted and then recovered from the organic phase by using water or dilute hydrochloric acid. By using the kinetic results obtained for TBP and D2EHPA, recovery selectivity of zinc over iron (αZn/Fe) was estimated to be of 16.7 and 0.5, respectively. According to the McCabe & Thiele diagram for an organic to aqueous ratio of 2, three stages of extraction are sufficient to remove about 98% of the zinc from a real solution. The remaining solution of FeCl2 can be used to recover HCl and FeSO4 through the so-called “sulphuric acid distillation”. Finally, the recovered HCl can be used as a raw material, together with limestone, to produce CaCl2. Overall, an integrated process to recover the above-mentioned raw materials (Zn salts, FeSO4·7H2O and CaCl2) is presented: all products have been obtained in a ready-to-sell form, minimizing the waste according to the strategies of the “circular economy” and the “near-zero discharge” of hazardous chemicals.