Mechanisms of Advanced Chemical Oxidation in Refractory Gold Ore Pretreatment and Cyanide-Free Leaching

Abstract

In gold hydrometallurgical leaching, oxidation plays a decisive role in pretreating refractory ores and cyanide-free leaching. For refractory sulfide ores, the liberation of encapsulated gold from the sulfide matrix under strong oxidative conditions is essential prior to leaching. Likewise, gold leaching by cyanide-free systems requires sufficient oxidizing environments for driving gold dissolution. Accordingly, this study focuses on the development of highly efficient advanced chemical oxidation processes, mainly based on free radical chemistry and coordination chemistry, to enhance gold liberation from refractory pyrite ore (Chapter III), and to achieve efficient and stable gold leaching by eco-friendly thiourea (Chapter IV), thiosulfate (Chapters V and VI), and iodide (Chapter VII). In this thesis, Chapter I briefly introduced the justification and objectives of this research. Chapter II comprehensively reviewed the previous research state of pretreating refractory sulfide ores and gold leaching in cyanide-free systems, namely, the antecedents. Chapter III develops a stepwise chemical oxidation process for pretreating a refractory pyrite concentrate by employing persulfate-activated hydroxyl radical (•OH), in which heat-activation at 55 °C initiated pyrite Step I oxidation (14.20%), and the pyrite dissolved Fe ions activation sustained pyrite Step II oxidation to 40.13%, thereby greatly increased Au and Ag liberation (Au 92.2% and Ag 88.6% by thiosulfate leaching). Chapter IV demonstrates that free radicals, generated from the Fenton reaction (Fe2+/H2O2), enable fast gold dissolution in acidic thiourea media, achieving complete gold leaching from a roasted concentrate within only 30 min. Moreover, using nitrilotriacetic acid (NTA) to complex Fe2+/Fe3+ ions can regulate the moderate superoxide radical anion (•O2⁻) as the reactive oxidizing species, which effectively suppresses harmful thiourea degradation. Chapter V focuses on using electrochemical methods to investigate the dissolution behavior of gold in the thiosulfate system by employing copper(II)-glycine complexes as the oxidant. The results show that Cu(C2H4NO2)3− in strongly alkaline media promotes gold dissolution more effectively than Cu(C2H4NO2)20 under near-neutral conditions, as evidenced by faster interfacial electron-transfer kinetics and reduced interface passivation. Chapter VI uses pentetic acid (DTPA) to stabilize copper(II) as the oxidant in thiosulfate gold leaching. Ammonia (NH3), released in-situ from (NH4)2S2O3, can couple with Cu(DTPA)3− to form a more reactive [Cu(DTPA)(NH3)]3− complex, which achieved gold leaching from a roasted concentrate as high as 97.8%, while thiosulfate consumption remained as low as 8.7 kg/t. Chapter VII presents a novel copper (III)/potassium iodide system for gold leaching under near-neutral conditions (pH 5.5), which achieves 94.1% from a gold ore within 60 min and 96.6% from e-waste within 20 min. In this system, copper(III) periodate acts as a catalyst to in-situ generate triiodide (I3−) from iodide (I−), which serves as the reactive oxidizing species driving gold dissolution.