HEAP BIOOXIDATION FOR THE TREATMENT OF LOW GRADE REFRACTORY ORES AT ASHANTI GOLDFIELDS' OBUASI OPERATIONS

摘要

Ashanti Goldfields Company (AGC) is the largest gold mining company in Western Africa. Their jewel operation is a highly integrated set of mines and plants situated in Obuasi, in the Ashanti region of Ghana. This operation is responsible for mining 5.5 million tonnes of ore at 4.6 g/t annually from a series of open pits and underground mines. The sulphide ore is highly refractory with untreated gold recoveries in the range of 30-40 percent. The milling facilities are quite varied and include an oxide treatment plant (OTP), a heap leach, a sulphide roaster circuit (PTP), a tailings treatment plant (TTP) and the most recent, a sulphide treatment plant (STP). The sulphide treatment plant, the world's largest stirred tank biooxidation circuit, was commissioned in early 1994. This plant treats approximately 800 tonnes of refractory gold concentrate per day with overall recoveries of 75-85 percent~(1). Biooxidation was selected for its ease of operation and less exotic maintenance requirements as well as its ability to achieve comparable gold recoveries. Unfortunately, despite stirred tank biooxidation being one of the lower cost methods of treating refractory ores, it is still too costly for lower cost methods of treating refractory ores, it is still too costly for low grade refractory materials. GeoBiotics, Inc. has developed a proprietary technology that allows the economic treatment of lower grade refractory gold ores through heap biooxidation. AGC has a significant resource of low grade refractory sulphide ore. This ore is too low grade to warrant BIOX~(TM) sulphide treatment and its oxide plant recovery is low at 35-45 percent. GeoBiotics and AGC have completed preliminary testing and economic analysis of heap biooxidation for this material. Additionally, the feasibility of producing a gravity concentrate from the tailings of the sulphide treatment plant has been explored. This concentrate would be coated onto the low grade sulphide and biooxidized concurrently. Gold recoveries for the biooxidized low grade sulphide have increased to 75 percent and the gravity recovery from the STP tailings has achieved 80 percent utilizing a Falcon concentrator. The preliminary economics of the proposed full scale plant are very favourable with a pre-tax IRR of 233 percent and a payback period of 6 months. This paper details the biooxidation testing, flow sheet development and preliminary economics of employing heap biooxidation for low grade refractory ore treatment at Ashanti Goldfields' Obuasi operation.