Enhanced leaching of manganese from zinc anode slime with a thiourea-assisted sulfuric acid solution

Abstract

Zinc anode slime (ZAS), a metallurgical byproduct generated during zinc electrowinning, contains Mn, Pb, Fe, and trace amounts of Ag. Hydrodynamic instabilities in the electrolyte circulation system cause ZAS particles to remain suspended, thereby compromising electrodeposition kinetics and the purity of cathodic zinc. Consequently, periodic removal and valorization of ZAS are necessary. This study proposes a novel hydrometallurgical route for recovering high-purity Mn₂O₃ from ZAS. The process integrates sulfuric acid leaching with thiourea (TU) reduction, followed by sequential purification, precipitation, and calcination. Under optimized conditions (ZAS:TU mass ratio of 10:3, 200 g/L H₂SO₄, solid-to-liquid ratio of 1:5, temperature of 60°C, duration of 2 hours, and 100-mesh ZAS particle size), a leaching efficiency of 91.39% for Mn is achieved. Subsequent precipitation recovers 96.21% of Mn as Mn(OH)₂, resulting in Mn₂O₃ after calcination. Kinetic analysis indicates that the reaction follows the unreacted shrinking core model, with mixed control by interfacial reactions and ion diffusion (E_a=35.87 kJ/mol). This work elucidates the reaction mechanisms and establishes a viable route for the valorization of industrial metallurgical residues.

Keywords

zinc anode slime / thiourea / acidic reduction leaching / leaching kinetics / high value recovery

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