Thermodynamic Insights Into Metallic Iron-Assisted Reduction of Spinel Ferrites in Zinc Hydrometallurgical Residues

The thermodynamic feasibility of reduction-assisted dissolution of spinel-type ferrites (ZnFe₂O₄, CuFe₂O₄, and CdFe₂O₄) in zinc hydrometallurgical residues was investigated using metallic iron as a reducing agent. Gibbs free energy calculations revealed that the reduction of ZnFe₂O₄ and CdFe₂O₄ is spontaneous over the entire temperature range (298–1288 K), whereas CuFe₂O₄ reduction requires elevated temperatures (~580–600 K) to become thermodynamically favorable, identifying it as the rate-limiting reaction. An operational window of 600–650 K is recommended to achieve effective ferrite reduction while minimizing energy consumption and avoiding adverse solid-state effects. The formation of Fe²⁺ and divalent metal oxides improves subsequent acid leaching of zinc, copper, and cadmium, supporting efficient recovery of valuable metals from industrial residues. The study provides a thermodynamic basis for developing integrated hydrometallurgical processes for sustainable metal recovery and waste management.