The impact of Jarosites in Biox® product on CIL processes - a case study of AngloGold Ashanti (Obuasi mine)

AngloGold Ashanti Limited (Obuasi Mine), commissioned its Sulphide Treatment Plant (STP) in February 1994 to process 180,000 t/month of double refractory gold ore mainly, pyrite, arsenopyrite and pyrrhotite. The refractoriness requested a suitable pre-treatment process, which led to the design of the then world’s largest commercial scale bacterial oxidation plant which uses Gencor’s BIOX® technology. This technology was chosen based on its simplicity and environmental friendliness. Gold is recovered by gravity and cyanidation. Gravity recoverable gold includes all visible gold; free gold and partly liberated gold, whereas sulphide minerals encapsulating the microscopic and sub microscopic gold particles are concentrated by flotation, pre-treated by the bio-oxidation process (BIOX®) and recovered by cyanidation process (CIL Circuit). During sulphide mineral decomposition, iron in pyrite goes into solution as ferrous ion (Fe2+), oxidizes to ferric ion (Fe3+), and then hydrolyses and precipitates as hematite, jarosite [XFe3+3(SO4)2(OH)6] or basic iron sulphates, depending on the prevailing conditions in the reactors. Sulphuric acid and arsenic acid are generated as by-product of the process. The stability of hematite is high. However the less stable jarosite and basic sulphates favour the formation of complexes with the iron sulphide mineral making gold recovery difficult, uneconomical in the upstream process and may pose environmental issues if allowed to get into the environment. Jarosite/ basic iron sulphate is stable under acidic conditions (from pH values 1 to 7), but breaks down at higher pH (Babcan, 1971). In this study, composite samples from, sulphide concentrate feeding the BIOX plant, CIL feed, the conditioned material at the CIL, material in the first leach tank, CIL tails and the feed to the TSF, were taken, filtered and bottle ` iii rolled for 72 hours. There was an increase in recoveries, from 93.47% to 94.78%, when the pH was raised from pH 2 to pH 7 and then from pH 7 to pH 11 (2 stage conditioning) with slake lime before cyanidation, than raising the pH from pH 2 to pH 11 (single stage conditioning) with slake lime before cyanidation.
A thesis submitted to the Department of Materials Engineering, Kwame Nkrumah University of Science and Technology, Kumasi in partial fulfillment of the requirement for the the degree of Master of Science (Engineering),