Enrichment and recycling of Zn from electroplating wastewater as zinc phosphate via coupled coagulation and hydrothermal route

Electroplating is a common process of converting zinc ion in electrolyte as a micro level zinc layer on electroplating pieces. After electroplating, the remaining electrolyte on the surface of the pieces is washed with water, and accordingly, Zn-containing electroplating wastewater is generated. Hazardous Zn-containing wastewater is generated during the washing of electroplating pieces and plating tank. Herein, Zn was enriched from wastewater by commercial flocculant and then recycled as highly purified zinc phosphate via coupled acid extraction and hydrothermal treatment. Firstly, 98.4% Zn was recovered as sludge from wastewater by adding 0.2 g/L of flocculant. Then, the sludge was dissolved into an acid solution to produce a leachate with 31.2/10.8/19.3 g/L of Fe/Al/Zn and then hydrothermally treated to remove Fe/Al. The Fe removal rate was only 54.2% without phosphate and glucose but was increased to 98.6% after the addition of 0.5 g of glucose with Al/Zn loss < 2%. However, when 0.5 g of glucose and 15 g/L of phosphate were used, 99.8% Fe and 96.6% Al were synchronously removed as giniite with Zn loss < 2%. After Fe/Al separation, the remaining Zn was finally recycled as Zn phosphate particles with 98.1 wt% Zn₃(PO₄)₂·2H₂O. The added phosphate predominated the synergy removal of Fe/Al and especially lowered the Gibbs value of Al hydrolysis from 39.7 kJ/mol of boehmite to ?5.96 kJ/mol of giniite, thereby reducing start-up temperature and reaction time. The proposed method showed practical application in the enrichment and recycling of valuable metals from wastewater.