Abstract
Miners around Zimbabwe used to supply gold concentrates from sulphide flotation to the Kwekwe Roasting Plant (Zimbabwe) 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, created over the operational years, gave an average Au grade of 8.58 g/t and 12.54 g/t for Ag. The base metals assayed—0.11% Cu, 0.10% Pb, 0.17% Zn and 26.05% Fe. SiO2 (36.1%), Fe2O3 (36.9%), Mg3Si4O10 (OH)2 (8.9%), NaAlSi3O8 (6.9%), and Fe3O4 (6.4%)—were the major mineral phases in the cyanide leach residue. SEM gold scans on 24 polished sections showed only 2 discrete gold particles of less than 5 µm, with one partially liberated and associated with quartz, while the other was fully liberated. Therefore, the particulate gold in the calcine leach residue was negligible. It was deduced from the analysis after ultrafine milling (P80 < 5 µm) followed by cyanidation that 68.53% of the gold was sub-microscopic. Direct cyanidation using bottle roll resulted in only 2.33% of the total gold being leachable, indicating that the calcine leach residue was highly refractory. Diagnostic leaching by sequential use of acids in order of their strength resulted in HCl leachable phases (CaCO3, CaMg(CO3)2, PbS, Fe1-XS, and Fe2O3) freeing 4.2% of the total Au during subsequent cyanidation leach. H2SO4 leachable phases (Cu–Zn sulphides, labile FeS2) released an additional 26.57% during cyanidation, whereas HNO3 leachable phases (FeS2, FeAsS) released a further 20.98% of Au. After acid treatment and subsequent cyanidation, hot caustic leach of the residue followed by carbon in pulp resulted 4.43% of the total gold being eluted. Therefore, 4.43% of the total gold was surface bound. From the analysis after diagnostic acid leaching, it was deduced that a total of 54.08% of the gold was in the acid-leachable phase. Due to cost and environmental considerations, H2SO4 was selected for the evaluation of acid digestion as a pretreatment stage followed by cyanidation. Increasing the H2SO4 strength for the pretreatment of the calcine leach residue increased gold recoveries during cyanidation.
Highlights
The mineralogical investigation of ores is an important tool and a critical aspect in the prefeasibility studies of metallurgical work and quality control of metallurgical processes
Microscopic gold and submicroscopic gold associated with As, S, silicates and talc in the Fairview calcine leach residue have been reported by Mutimutema [18]
The material used in the study was a calcine leach residue obtained from the Kwekwe Roasting Plant, Zimbabwe
Summary
The mineralogical investigation of ores is an important tool and a critical aspect in the prefeasibility studies of metallurgical work and quality control of metallurgical processes. Some of the minerals that associate with or carry gold make it inaccessible to lixiviants during leaching, and sometimes these can compete with gold for oxygen or cyanide [6] These types of ores are referred to as refractory. Douglas and Semenyna [17] reported the greater part of the gold in the Barric Goldstrike’s calcine leach residue as being submicroscopic gold locked up in iron oxides and the remainder being gold salts, gold locked up in quartz and surface adsorbed gold. Microscopic gold and submicroscopic gold associated with As, S, silicates and talc in the Fairview calcine leach residue have been reported by Mutimutema [18]. Corrans and Hayes [30] reported that acid digestion did not give satisfactory results when they tried it as a pretreatment method using a leached calcine residue
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