Experimental research on the comprehensive utilization of valuable components and centralized removal of harmful elements in waste acid

Currently in China, the waste acid generated from large-scale smelting plants is treated as “wastewater with high concentration of heavy metals”, which leads to high cost and many wastewater treatment residues (hazardous wastes). In this paper, based on the main components of waste acid and zinc oxide dust, the adoption of a cyclic leaching process was proposed, in which zinc oxide dust is leached by waste acid, thus enabling the recovery of copper (Cu) and zinc (Zn) from the circulating leaching solution and the central treatment of arsenic (As). The main factors affecting the first and second cyclic leaching processes were investigated, including the final pH, leaching temperature, and leaching time. After leaching was completed, several factors in the first and second As removal processes were investigated, including the H₂O₂ dosage, Na₂S dosage, removal temperature, and removal time. The following optimal conditions were identified: the optimal final pH, leaching temperature, and leaching time for the first leaching are 1.5, 85 ℃, and 5 h, respectively. The optimal final pH and leaching temperature for the second leaching are 4 and 85 ℃, respectively. The optimal H₂O₂ dosage, removal temperature, and removal time for the first As removal are 0.067 mL per 1 mL of the secondary circulation leaching solution, 40 ℃, and 1.5 h, respectively. The optimal Na₂S dosage for the second arsenic removal is 0.02 mL per 1 mL of the second circulation leaching solution, and the removal temperature and removal time were determined to be 35 ℃ and 2 h, respectively. Under these conditions, the concentrations of As, Cu, and Zn can be reduced to 3.26, 2.63, and 50.63 mg·L⁻¹, respectively. The pH of the wastewater after processing was neutral, which meets the integrated wastewater discharge standard with minor treatment. In this way, valuable components in the waste acid can be comprehensively recovered, and the harmful element As centrally treated, thus reducing the production of hazardous wastes, saving energy, and reducing emissions.