Technology

BIOX® fact sheet [PDF - 1.5MB]

The BIOX® process, which pre-treats refractory sulphide gold ores such as pyrite, arsenopyrite and pyrrhotite, was developed to increase gold recovery rates during the metallurgical extraction process. The gold in these sulphide ores is encapsulated in sulphide minerals which prevent the gold from being leached by cyanide. The BIOX® process destroys the sulphide minerals and exposes the gold for subsequent cyanidation, increasing recovery rates.

The BIOX® process has many real advantages over conventional refractory processes such as roasting, pressure oxidation and nitric acid leaching. These include:

• Improved rates of gold recovery
• Significantly lower capital costs
• Low running costsRobust technology that is suited to remote areas
• Low level of skills required for operation
• Environmentally friendly
• Ongoing process development and improvement

Rights to the process, which has been available commercially for more than 15 years, are currently held by Biomin Technologies SA, a subsidiary of Gold Fields Limited. However, the initial research and development into the process was conducted by Gencor Process Research (now Billiton Process Research).

The BIOX® process

The process itself uses a combination of three bacteria that occur naturally, thiobacillus ferrooxidans, thiobacillus thiooxidans and leptospirillum ferroxidans, to break down the sulphide mineral matrix in the ore being treated, thus freeing the occluded gold for subsequent cyanidation. The bacteria attach themselves to the metal sulphide surfaces in the ore, resulting in the accelerated oxidation of the sulphides.

The BIOX® process involves the continuous feeding of the flotation concentrate slurry to a series of stirred reactors.

Low pH levels and a high slurry temperature enhance the efficiency of the process and it is important that these parameters are controlled within narrow ranges so as to maintain the right balance of bacteria in order to achieve the optimum rate of oxidation.

The reactors are aerated and the slurry temperature is maintained at the optimum level of 40-45ºC. As the oxidation reactions of sulphide minerals are exothermic, it is necessary to cool the tanks so as to maintain the slurry temperature within the optimum range. This is done by circulating cooling water and removing the excess heat via a cooling tower.

The pH level is controlled by adding limestone or sulphuric acid to the slurry. Since direct sulphide oxidation requires high levels of oxygen, large volumes of air have to be injected and dispersed in the slurry. This is one of the main engineering challenges in the design of a full-scale bio-reactor.

Furthermore, sufficient carbon dioxide is required for the bacteria to maintain cellular growth. This is obtained from the injected air as well as carbonate minerals. Should the latter be absent, limestone is added.

The bacteria also require nutrients to sustain growth. Nitrogen, phosphorous and potassium are added to the primary reactors in various forms and uantities,
depending on the composition of the concentrate being treated.

The overall residence time in the bio-oxidation reactors, which is mainly a function of the mineralogy, typically varies between four and six days. For an ore where the gold is locked mainly in arsenopyrite, a shorter residence time is expected to achieve optimum gold liberation than with an ore where most of the gold is occluded in pyrite. This is because the oxidation rate of arsenopyrite is faster than that of pyrite.

Some ores require only partial sulphide oxidation to liberate the gold. The circuit can be simplified for such ores and the residence time reduced to two days or less.

During the bacterial oxidation process, elements like iron, sulphur and arsenic are dissolved. After oxidation, the BIOX® product is washed in a counter-current decantation circuit and the solution is neutralised in a controlled two-stage process with limestone and/or lime. The precipitates formed meet environmental standards set in the United States and can be safely
deposited onto tailings dams. The BIOX® process is thus a non-polluting, environmentally clean means of treating refractory ore.

To save water, the neutralised effluent can be mixed with flotation tailings and thickened. The overflow solution can be recycled as dilution water in the milling, flotation and BIOX® sections of the plant. This makes the process ideally suited for arid regions.

The washed BIOX® product is treated in a conventional cyanidation plant from which the gold is finally recovered.

Operating parameters of the BIOX® process

Plants in operation

Four BIOX® plants are currently in operation – in South Africa, Ghana, Brazil and Australia – with the most successful of these being Ashanti’s Sansu plant near Obuasi in Ghana. The Tamboraque plant in Peru is currently being recommissioned.

Ashanti, Ghana

Ashanti Goldfields Company investigated several refractory treatment options including roasting, pressure oxidation and nitric acid leaching. The BIOX® technology was selected for, among other reasons, its lower capital and operating costs, reduced technical risk, relatively benign environmental impact and its ease of operation.

Designed with an initial capacity of treating 720 tonnes of concentrate per day, the Sansu plant has since been expanded and currently has four modules processing 960 tonnes per day in all. It is by far the largest biooxidation plant in the world and its modular design makes it possible to apply the technology to large refractory deposits. The simplicity of the process also makes it ideal for remote areas.

Furthermore, the metallurgical performance of the Sansu BIOX® plant has been highly satisfactory in coping with the local ore of which there are two types, which differ widely regarding their mineralogical and bio-oxidation characteristics. This demanded a plant design with sufficient flexibility to treat both concentrates, either individually or as a blend.

The capital cost of the plant totalled US$25 million (1994 terms) and the operating cost is currently US$17/tonne milled.

Fairview, South Africa

The BIOX® plant at the Fairview mine in Barberton, which was the initial pilot plant, has been fully operational for 15 years. It was originally designed to treat 10 tonnes a day but with the success of the project this has been increased over time to 55 tonnes of concentrate per day.

Much of the innovative research work on the biooxidation of refractory gold ores conducted in the late 1970s and early 1980s was driven by the need to replace the outdated Edward’s roasters at Fairview, which at the time were seriously contributing to atmospheric and water pollution in the environmentally sensitive Barberton area of South Africa.

This plant has played a vital role in the ongoing development of the BIOX® process as the scale of the operation has lent itself to the testing of new equipment, design modifications and process optimisation.

With the purchase of Fairview by Avgold Ltd’s Eastern Transvaal Consolidated, Avgold concluded a licensing agreement with Gold Fields entitling it to use the BIOX® technology to treat its concentrate. Gold Fields has retained access to the plant and site to enable it to continue its development work and train operators for new BIOX® plants.

The plant has been both a technical and commercial success, confirming the simplicity of the process and the robustness of the technology.

Sao Bento, Brazil

A BIOX® plant is also in operation in Brazil at the Sao Bento Mine where a pressure oxidation circuit is used to treat the refractory flotation concentrate. This plant has confirmed the viability of combining bio-oxidation with pressure oxidation as a cost-effective method of increasing capacity at an existing oxidation plant.

Wiluna, Australia

The fourth plant, in operation at Wiluna Gold Mine, Western Australia, was commissioned in 1993. Although originally designed to treat 115 tonnes per day of concentrate this has subsequently been increased to 158 tonnes per day. Since the cost of power initially amounted to 50% of the operating costs, a natural gas-fired power station was built to replace the diesel generation and limit costs.

Future developments

To date more than 250 concentrate and whole ore samples have been tested for compatibility with the BIOX® process and more than 15 integrated pilot plants to monitor and refine the design of commercial plants have been completed. One such pilot plant, together with a detailed commercial process design, was completed for Golden Star Resources Bogoso Mine in Ghana.

Currently, several projects are at the feasibility stage or awaiting financial backing. In addition, technology licence agreements have been signed with gold mining companies in Uzbekistan, Greece and Australia. Of these, the largest project is that in Uzbekistan where the agreement with the Navoi Mining and Metallurgical Combinat is for a BIOX® plant with an ultimate capacity of 2,055 tonnes of concentrate a day.

Project development framework