Characterisation of Biooxidation Feed and Products for Improved Understanding of Biooxidation and Gold Extraction Performance

Abstract

This paper presents a study on characterisation of refractory ore, biooxidation feed and product, and cyanidation tailings with the aim of understanding the causes of excessive continuous frothing, incomplete sulphide oxidation, high reagent consumption, high cyanidation residues and low overall recovery as encountered in biooxidation of refractory ores. Techniques involving carbon and sulphur speciation, Quantitative X-Ray Diffraction (QXRD), Scanning Electron Microscopy (SEM) and Optical Microscopy (OM) were used to characterise the ore samples, flotation concentrate (BIOX® feed), biooxidised product (BIOX® CIL Feed) and cyanidation tailings (BIOX® CIL Tails) from a biooxidation plant. The main minerals present in the ore were quartz (45%), chlorites (21%), plagioclase feldspar (13%), dolomite (5%), pyrite (2%) and mica group (2%). The flotation concentrate recorded 18% mica, and this was responsible for excessive frothing in the biooxidation circuit as confirmed by the QXRD analysis. The carry-over froth to the CIL circuit led to short-circuiting of poorly leached material into the cyanidation tailings, resulting in high cyanidation residues. Secondary refractory minerals; gypsum and jarosite, which were observed in the biooxidation product by the QXRD, have the potential to coat unreacted sulphide particles, leading to incomplete sulphide oxidation as observed here. Partially oxidised sulphides led to high consumption of reagents such as oxygen and cyanide during cyanidation. Gypsum and jarosite also encapsulated gold particles as observed in the BSED analysis. Coated gold particles had reduced access to lixiviants during the subsequent cyanidation process, leading to high leach residues. The biooxidised product (BIOX® CIL Feed) also recorded a high organic carbon content of 6.67, while analysis by BSED revealed the presence of graphitic carbon and coatings on gold surfaces; an indicator for high preg-robbing activities during cyanidation of the concentrate. Preg-robbing indices of 64.4% and 72.7% were recorded for the flotation concentrate (BIOX® feed) and BIOX® CIL feed respectively. The overarching effect of all the observations is a decrease in overall gold recovery.