Stepwise recycling of Fe, Cu, Zn and Ni from real electroplating sludge via coupled acidic leaching and hydrothermal and extraction routes

Fe/S-bearing erdite flocculant has been proven to be effective in the precipitation of heavy metals from real electroplating wastewater, with the only drawback being the huge production of sludge. This sludge was rich in Fe/S/Zn/Cu/Ni and refractory to be recycled due to the extractant pollution by free Fe and the dissolution of sulphide. Herein, a multistep separation method was developed to dissolve sulphide and separate Fe prior to Zn/Cu/Ni. Results showed that more than 92% sludge was dissolved as Fe/Zn/Cu/Ni-rich leachate after the sludge was leached by nitric acid, with the rest of the remaining undissolved elemental sulphurs. When the leachate was directly extracted by using commercially extractant Acorga M5640 and Di-(2-ethylhexyl) phosphoric acid (P204), Fe was complexed by the phosphate group of the extractant. The Fe was effectively removed prior to Zn/Cu/Ni to avoid the extractant pollution. The Fe removal efficiency was only 38.34% without sucrose, but it rose to 99.94% with the addition of 0.5 g sucrose. The added sucrose reacted with nitrate to consume H+, which showed a similar rate to the H+ release from Fe hydrolysis. Thereafter, the Fe hydrolysis was continued to remove, the Fe at a high level. The removed Fe was in the form of high-purified hematite nanorod with a diameter and length of 300-600 nm and 0.5-2.5 μm, respectively. After Fe removal, Cu/Zn/Ni was extracted by using Acorga M5640 and P204 to form three halite, including a mixture of copper sulphate hydrate and bonattite (96.8% CuSO4·H2O/CuSO4·3H2O), gunningite (97.5% ZnSO4·H2O) and dwornikite (97.9% NiSO4·H2O). The rest of the solution was neutralised by lime water to remove sulphate as gypsum (95.9% CaSO4) to meet the discharge standard of the electroplating industry. In summary, the recycling efficiency of Fe/Cu/Zn/Ni from the sludge reached 94.4%, 92.6%, 94.7% and 95.3%, which provided an alternative strategy to resource utilise Fe/S-bearing solid waste.