Assessment of options for economic processing of preg-robbing gold ores

One of the challenges facing the gold industry in the twenty-first century is the continuing need to identify new reserves of economically treatable ores. Discoveries of large, metallurgically simple orebodies are becoming increasingly rarer, forcing companies to investigate options for the processing of ores which may present recovery, economic and/or environmental issues, requiring innovative approaches to their treatment. Ores which contain naturally occurring carbonaceous material ('pregrobbing ores') is an example of one such ore type. Newmont's Jundee operation contains zones of carbonaceous ore within the oxidised ore body. A program consisting of detailed laboratory work and extensive plant trials has been undertaken to assess the options of the processing of this material. Laboratory tests demonstrated the advantage of carbon-in-leach (CIL) over direct cyanide leaching and carbon-in-pulp (CIP) for treatment of these ores. The laboratory program identified several factors which would enhance overall gold recoveries when preg-robbing ore was being processed, including: keeping preg-robbing ore separate from non-graphitic ore, maximising gravity recovery and ensuring the plant operates in 'pure' CIL mode i.e. no leaching before first adsorption tank. Plant trials were undertaken to assess the economic sustainability of treating moderate preg-robbing ores through a modified gravity/leach/adsorption circuit. Results indicated that where good operational control of the circuit was maintained to ensure high gravity recovery, minimal leaching prior to carbon contact and maintenance of good, active carbon inventory, then acceptable and sustainable overall gold recoveries could be obtained. However, plant trials did not produce gold recovery as high as laboratory tests. Using these relatively simple plant modifications potentially allows a plant to treat ores with preg-robbing index (PRI) values up to 1. When PRI values rise to 1·3-1·6 leach recoveries can drop from.85 to <40%, indicating a more intensive approach may be required, including kerosene addition and higher carbon inventories and activities