Fosterville gold mine heated leach process

Abstract

The Fosterville Gold mine is located approximately 20 km to the East of Bendigo in central Victoria. The gold at Fosterville occurs as solid solution within disseminated arsenopyrite and pyrite. The Fosterville ore bodies contain various amounts of native carbon in the form of bituminous coal. This carbon (non-carbonate carbon or NCC) has been the predominant mechanism for gold loss from the processing facility through ‘preg-robbing’. Processing of the Fosterville ore is achieved initially through crushing, grinding and flotation to extract the sulphides. The sulphide concentrate is oxidised using bacterial oxidation, before being leached in a conventional CIL circuit. A high portion of the native carbon (NCC) in the mine ore is naturally hydrophobic in nature, and subsequently reports to the flotation concentrate stream, and ultimately onto the CIL circuit. This NCC has a notable preg-robbing ability. Treatment of black shale ores, which have elevated NCC levels, has historically resulted in CIL recoveries as low as 35%, with around 60–80% of the gold loss from the leach circuit attributed to preg-robbing. Standard technologies for the dealing with a carbonaceous leach feed have been trialled with limited success. The discovery of the significance of heat on leach recoveries, triggered extensive testwork on a range of leach feed and tails samples. Pilot plant testing demonstrated that ‘heated leaching’ of the CIL tails was the best process with an average recovery increase of 7·5% being achieved. Following a successful pilot study, an engineering feasibility study was completed by Minerva Engineering, and the project economics determined. With strong recovery gains evident and a predicted project payback of 1–1·5 years, approval for the installation of the full scale plant was granted. Detailed design commenced in September 2008 with installation commencing in January 2009. The circuit was successfully commissioned in April 2009. Following commissioning, the heated leach circuit has achieved recovery gains of 4–14%, and has proven itself to be a significant contributor to the overall plant performance.