Mineralogical Characterization and Acid Pretreatment of a Gold Calcine Leach Residue

Miners around Zimbabwe used to supply gold concentrates from sulphide flotation to the Kwekwe Roasting Plant for toll treatment. The concentrates were roasted in Edward’s roasters and the calcine product was leached by cyanidation. Due to inefficient roasting, overall gold recoveries of 75–80% left behind a rich calcine leach residue at the Kwekwe Roasting Plant. The characterization performed to establish a potential process route involved several techniques, such as X-ray diffraction (XRD), scanning electron microscopy (SEM), fire assaying and inductively coupled plasma (ICP). Assays conducted on samples from the 350,000 tons tailings dam residue gave an average Au grade of 5.58 g/t and 12.54 g/t for Ag. The base metals assayed included 0.11% Cu, 0.10% Pb, 0.17% Zn, and 26.05% Fe with major mineral phases of SiO2, Fe2O3, MgSi4O10(OH)2, NaAlSi3O8, and FeCO3. SEM gold scans showed negligible particulate gold, deducing from ultrafine milling and cyanidation that 68.53% was sub-microscopic. Diagnostic leaching released 54.80% of total gold after sequential acid treatment and cyanidation. The study highlights the refractory nature of the calcine leach residue and the effectiveness of acid pretreatment in enhancing gold recovery.