Abstract

This research explores the hydrometallurgical extraction of zinc from sphalerite (ZnS) using a salted-out alkaline leaching system composed of ammonia and ammonium sulphate, with particular focus on the kinetic mechanisms controlling dissolution. Conventional routes for zinc recovery, including pyrometallurgical and acidic hydrometallurgical processes, remain limited by their high energy requirements, elevated operating temperatures, and significant gaseous emissions. In contrast, ammonia–ammonium systems offer an environmentally benign alternative, enabling zinc solubilization through stable ammine complex formation while minimizing the dissolution of impurity metals such as iron and lead. Sphalerite ore sourced from Zurak, Plateau State, Nigeria, was subjected to detailed physicochemical and mineralogical characterization using XRF, XRD, SEM–EDS, and AAS techniques. The sample showed a moderate loss on ignition (4.5%) and a near-neutral pH (7.5), indicating minimal volatile or carbonate content. Elemental composition revealed 5.88 wt% Zn and 16.51 wt% Fe, whereas SEM–EDS micrographs displayed a rough, porous morphology conducive to enhanced leaching. Batch leaching studies were conducted to examine the influence of reagent concentration, temperature (25–80 °C), stirring speed (200–500 rpm), and pulp density (5–20 g) on zinc extraction. Maximum dissolution of approximately 70% was achieved at 3.0 M NH₃–(NH₄)₂SO₄, 80 °C, 400 rpm, and 180 minutes. The dissolution rate increased with temperature and reagent concentration but declined at higher pulp densities. Kinetic interpretation using the shrinking-core model revealed that the reaction was governed by diffusion through a product layer, with an apparent activation energy of 2.445 J mol⁻¹ and an overall reaction order of nearly second order. These results highlight the ammonia–ammonium sulphate medium as an efficient, selective, and environmentally sustainable leaching system for zinc recovery from sphalerite under mild process conditions, offering valuable insight for industrial-scale hydrometallurgical applications.

Keywords: Zinc extraction,Sphalerite, Salted-out alkaline leaching, Ammonia–ammonium sulphate system, Dissolution kinetics, Shrinking-core model