Some 20 years ago a bio-oxidation technology was being developed as an alternative to conventional refractory processes for gold recovery.
Unlike conventional gold ore, the gold contained in refractory (or complex) ores is encapsulated in sulphide minerals which prevent the gold from being leached by cyanide. The process developed and refined by Biomin Technologies, a subsidiary of Gold Fields Limited, pre-treats the ore by destroying the sulphide minerals and liberating the gold for subsequent cyanidation, the next step in the gold refining process.
Temperature must be constant
The technique, known as the BIOX® process, involves the introduction of three types of naturally occurring bacteria into concentrate slurry in a series of stirred reactors. The concentrate, which is the crushed and ground sulphide material, must be kept at specified levels of temperature and low pH in order to maintain the right environment for the bacteria. The reactors are aerated and agitated while the slurry is kept at a constant temperature of between 40 - 45º C by using water in the cooling pipes which pass through the reactors.
The bacteria attach themselves to the metal sulphide surfaces in the ore, resulting in the accelerated oxidation of the sulphides, thus freeing the occluded gold. It is important that the parameters are controlled within narrow ranges so as to maintain the right balance of bacteria in order to achieve the optimum rate of oxidation.
More gold
The BIOX® process has many real advantages over conventional refractory methods such as roasting, pressure oxidation and nitric acid leaching. These include lower capital and running costs, improved rates of gold recovery and a much kinder environmental impact.
Development of the process started at Gencor Process Research in the late 1970’s and resulted in the commissioning of a 10 tpd pilot plant at the Fairview mine in 1984. The process proved to be technically and commercially viable and the first commercial BIOX® plant was constructed and commissioned at the Fairview mine in 1986. Much of the initial research work at Fairview was driven by the need to replace outdated roaster technology which was seriously contributing to air and water pollution in the Barberton area. The plant has since 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.
Accurate and reliable drive
The search for a highly accurate and reliable drive to run a 20m³ test reactor agitator led to a recommendation by Fairview’s instrumentation technician to look at Danfoss. BIOX® Process Engineering Manager Rudolph van Deventer received favourable quotes and operating specifications from Danfoss and a 55 kW VLT® drive was installed for the test unit in August 2007.
This off-the shelf unit also comes with a basic version of MCT10 software package which allows live information from the drive to be visualised or recorded and then analysed on a laptop. Operating data like kilowatts, amps, speed and torque can be used on the scope function to see how the agitator performs in the process. The ongoing objective at Fairview is to reduce overall installed power and to cut down on capital costs.
Says van Deventer, "If we cannot run the process with precise accuracy the bacterial activity is compromised, with resulting commercial consequences. With Danfoss, we know we can rely on the kind of precision we require and we have been more than happy with the results".
Danfoss drives have also proved their worth at a mini-plant at the SGS Lakefield laboratories. Here, six smaller VSD’s have been installed to run the agitators on a series of mini-tanks which process test material from mines around the world. This testing is necessary as the BIOX® process has to be uniquely calculated for complex ores originating from mines in different regions. There are also plans to develop a portable mini-plant that can be taken to remote sites where testing is required.
The full scale, commercial process is currently in use at seven processing plants around the world, with the latest plants commissioned in 2007 at the Bogoso gold mine in Ghana and the Jinfeng gold mine in China. The Kokpatas BIOX® plant is due to be commissioned in Uzbekistan later this year and will be the largest BIOX® plant in the world.
