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

Abstract

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 degrees C confirmed that hydrazine readily reduces Fe(III) to Fe(II) within the ferrite lattice: Delta G degrees values reached -860.8 and -608.7 kJ mol(-1) for zinc and copper ferrites at 25 degrees 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 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 H2SO4, 6.3 wt.% N2H4, 85 degrees 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 (PbSO4), willemite (Zn2SiO4), and residual ZnFe2O4 survived selectively in the solid. High-resolution TEM and STEM-EDS analyses revealed dramatic particle fragmentation: the original similar to 74 & micro;m feed agglomerates broke down to 0.9-2.0 & micro;m fragments through preferential dissolution along ferrite-silicate grain boundaries. Importantly, hydrazine oxidized cleanly to N-2 and H2O only-no CO2 or problematic solid byproducts were detected. Residual hydrazine in process effluents was neutralized below regulatory discharge limits (< 0.01 mg L-1) by H2O2 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. [GRAPHICS]