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 ZnFe 2 O 4 remains stable even under aggressive sulfuric acid conditions. This work presents, for the first time, a systematic study of hydrazine (N 2 H 4 ) 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: Δ G ° 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 H 2 SO 4 . 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 R 2 > 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 H 2 SO 4 , 6.3 wt.% N 2 H 4 , 85 °C, 2 h), extract ion 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 (PbSO 4 ), willemite (Zn 2 SiO 4 ), and residual ZnFe 2 O 4 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 N 2 and H 2 O only—no CO 2 or problematic solid byproducts were detected. Residual hydrazine in process effluents was neutralized below regulatory discharge limits (< 0.01 mg L −1 ) by H 2 O 2 treatment. Taken together, these results establish hydrazine-assisted reductive leaching as a thermodynamically grounded, statistically optimized, and environmentally viable route for recovering valuable metals from zinc metallurgical waste. Zinc cake valorization: Decomposition of ZnFe 2 O 4 using hydrazine, process optimization by RSM, and residue analysis