Enhancing Cu-Zn-Cr-Ni Co-extraction from electroplating sludge in acid leaching process by optimizing Fe³⁺ addition and redox potential

Passivation leads to low leaching efficiency of heavy metals from electroplating sludge (EPS). The main objective of this study was to improve acid leaching of valuable metals from EPS by optimizing operational parameters (including process pH, dosage of Fe³⁺, and solution oxidation-reduction (redox) potential) and form an advanced hydrometallurgical process capable of high co-extraction efficiency, and compare the transformations of heavy metal speciation in feed sludge and leaching residue by using four-stage sequential Community Bureau of Reference. Results indicated that a low pH (≤2.2) was advantageous and accounted for most of the Cu, Zn, Cr, and Ni release. Acid leaching efficiency of the four metals was further increased (by 6.3%-9.1%) when 1 g/L Fe³⁺ was added, which mainly promoted the extraction of Cu, Zn, Cr, and Ni from residual and organic matter- and sulfide-bound fractions that resisted dissolution by H⁺. However, further increase in the initial Fe³⁺ dosage led to additional iron precipitation and inhibited the metal extraction. In addition, leaching of Cu, Zn, Cr, and Ni was more efficient when the solution redox potential was controlled at an appropriate level (≤500 mV). This study has important implication in enhancing the dissolution rate and final recovery of metals from the sludge, and the findings also have great significance in direct guidance for improving the recycling and re-utilization of electroplating waste.