Geometallurgical Controls on Cyanide-Free Leaching of Witwatersrand Tailings: Glycine and Thiosulfate Versus Cyanide

Abstract

Legacy gold tailings in the Witwatersrand Basin of South Africa represent both an environmental liability and a secondary resource. Cyanide remains the dominant lixiviant for gold extraction, but its toxicity and governance burden continue to motivate evaluation of safer alternatives. This study compares cyanide, glycine, and thiosulfate for gold recovery from four Witwatersrand tailings sources by integrating particle-size analysis, heavy liquid separation, geochemical and mineralogical characterization, automated mineralogy, and bottle-roll leach testing under controlled laboratory conditions. Triplicate or replicated test work, where material allowed, was used to assess reagent response and to relate recovery to liberation and mineral association. Under the baseline 24 h ambient bottle-roll conditions, cyanide produced the highest observed recovery, reaching 83% in the Ergo Mining Proprietary Limited East Rand sample. Glycine and thiosulfate achieved mean recoveries of 55% ± 5 and 56% ± 7, respectively, demonstrating technical promise but slower kinetics under the tested conditions. The Krugersdorp/Randfontein sample returned the poorest performance (26% under cyanidation), consistent with its pyrophyllite-rich composition, preg-robbing tendency, and stronger gold locking. The results confirm that reagent performance is site specific and is controlled primarily by liberation, mineralogical composition, and reagent operating conditions rather than by lixiviant choice alone. Glycine and thiosulfate should therefore, be regarded as credible cyanide-free options for further optimization, particularly where lower toxicity and alternative residue-management strategies are strategic priorities. Graphical Abstract