BIOX®

Reasons to select BIOX® include:

• Lower capital and operating costs
• Environmentally sound
• Simplicity of operation
• Applicable to remote locations
• Established technology with 8 plants built worldwide, 2 in progress, 4 in design
• BIOX® first implemented in 1986 at Fairview mine in South Africa
• Anglogold-Ashanti’s Obuasi Mine have successfully operated a similar size BIOX® plant 90km away, processing similar ores from the Ashanti Trend.

BIOX® Flow Sheet

With our refractory ore, gold is encapsulated in a sulfide matrix and additional processing is required to unlock the gold to allow its recovery by cyanidation. The process we have selected as being the most economic, environmentally sound, and simple to operate is the BIOX® bio-oxidation process, licensed to Golden Star by Biomin, a subsidiary of Goldfields Limited. At full production, expected in late 2007, Bogoso/Prestea’s sulfide circuit will operate at a design capacity of 3.5 million tonnes per year of refractory sulfide ore into the crusher, equivalent to 34 tonnes per hour of concentrate through the BIOX circuit. Design recovery is estimated at 85% overall, resulting in annual gold production from the sulfide plant of between 230,000 and 260,000 ounces.

1. Gold concentrate from the standard flotation circuit is pumped into the Stock Tank and mixed with nutrients.
2. The mixture is transferred to the BIOX® stainless steel reactor tanks where the bacteria, with the help of oxygen, eat the sulfide minerals converting them to oxide minerals. Acid and heat are produced as a result of the process. The tanks are cooled to maintain the optimum temperature range.
3. The oxide minerals and the acidic bacterial solution are separated by decantation in a three-stage Counter Current Decantation (CCD) circuit; the minerals are washed and thickened to remove the acid solution from the gold-bearing material.
4. The waste acid is neutralized by the addition of limestone, then the bacterial solution is returned to the first reactor tanks.
5. The washed material is transferred to the carbon-in-leach (CIL) circuit. The cyanide solution dissolves the gold, which is subsequently precipitated onto activated carbon and later recovered in the elution circuit.

BIOX® Bacteria

The BIOX® process utilizes a combination of Acidithiobacillus ferrooxidans, Acidithiobacillis thiooxidans, and Leptospirillum ferrooxidans. Among other places, these bacteria are found naturally occurring at the hot springs in Yellowstone National Park. They feed on sulfurous materials, and in doing so, liberate the gold particles from the sulfide mineralization. Optimization of temperature (35 to 45 degrees Celsius) and pH (1.2 to 2.0) are the most important factors in keeping the bacteria active and vital.

Bio-oxidation Operations Worldwide