Hydrazine‑Assisted Reductive Leaching of Zinc Cake: Thermodynamic Assessment, RSM Optimization, and Microstructural Evolution of Refractory Residues

Ferrite–silicate zinc leach residues are notoriously difficult to process hydrometallurgically—largely because ZnFe2O4 remains stable even under aggressive sulfuric acid conditions. This work presents, for the first time, a systematic study of hydrazine (N2H4) as a selective reductant for treating zinc cake generated at the Almalyk Mining and Metallurgical Complex (AMMC), Uzbekistan. Thermodynamic calculations over 25–120 °C confirmed that hydrazine readily reduces Fe(III) to Fe(II) within the ferrite lattice: ΔG ° values reached −860.8 and −608.7 kJ mol−1 for zinc and copper ferrites at 25 °C, and became more negative as the temperature rose. This Fe(III) → Fe(II) conversion disrupts the spinel structure, rendering it soluble in dilute H2SO4. Four leaching variables—acid concentration, hydrazine dosage, temperature, and time—were systematically evaluated using central composite design (CCD) coupled with response surface methodology (RSM). All four fitted models showed R2 > 0.98 and p < 0.05, with temperature and hydrazine consumption identified as the dominant factors for Zn recovery. Under the RSM-derived optimum (1.0 M H2SO4, 6.3 wt.% N2H4, 85 °C, 2 h), extraction efficiencies of 91.7% Zn, 98.1% Cu, 91.2% Cd, and 85.6% Fe were achieved experimentally, alongside a 55–60% reduction in solid residue mass. Post-leach X-ray diffraction (XRD) showed that readily soluble zinc phases were almost fully consumed, whereas anglesite (PbSO4), willemite (Zn2SiO4), and residual ZnFe2O4 survived selectively in the solid. High-resolution TEM and STEM-EDS analyses revealed dramatic particle fragmentation: the original ~74 µm feed agglomerates broke down to 0.9–2.0 µm fragments through preferential dissolution along ferrite–silicate grain boundaries. Importantly, hydrazine oxidized cleanly to N2 and H2O only—no CO2 or problematic solid byproducts were detected. Residual hydrazine in process effluents was neutralized below regulatory discharge.