Biotechnology in gold extraction

The Hindu
February 21, 2002

THE HUTTI Gold Mines Company Limited , in Karnataka produces about three tonnes of the yellow metal. The Hutti gold deposits are located in the Hutti-Muski Precambrian greenstone belt and both free milling ores as well as gold-bearing sulphides are present.

One and a half century old gold cyanide leaching practice has been followed in the Hutti and Chitradurga plants of the Hutti Gold Mines. This suits free- milling gold ores where native gold is disseminated in a quartzite matrix. The newly found gold-ore deposits in Ajjanahalli, G.R. Halli, Hira Buddini and Gadag belts of Karnataka contain significant quantities of refractory sulphides in which finely divided gold and silver are encapsulated. Conventional fine grinding and cyanidation will not be efficient in the treatment of such 'refractory' gold ores, necessitating adaptation of newer and emerging technologies. An integrated biotechnological approach to gold processing holds great promise since it is cost-competitive, energy-efficient and environment-friendly.

Bioreactor technology for gold processing is being commercially practised in South Africa, Australia, Ghana and Brazil. The Hutti Gold Mines Company Limited established a research collaboration with the Indian Institute of Science at Bangalore to assess the amenability of bioprocessing of refractory gold ores of Karnataka. Refractory gold ores from the G.R. Halli mines were chosen for the research work since they contained both gold and silver besides the presence of base metals such as lead, zinc, copper and cobalt. A concentrate containing up to 30 gm/ton of gold and 800 gm/ton of silver was subjected to bacterial oxidation with Thiobacillus ferrooxidans under optimum conditions in different laboratory bioreactors. Bacteria indigenous and ubiquitous to the gold-bearing sulphide minerals namely, Thiobacillus ferrooxidans, isolated from the Hutti Gold Mines is a sulphur and iron oxidising, acidophilic autotroph exhibiting optimum growth at acidic pH of 2 at 350 deg C.

These organisms are capable of dissolving sulphides such as pyrite and arsenopyrite with which free gold particles are associated. The bacteria are thus capable of liberating the encapsulated gold particles from the sulphide minerals which after subsequent cyanidation will yield almost complete extraction of the precious metals.

Repeated laboratory results convincingly showed that with prior biotreatment as above, up to about 50 per cent enhancement in gold and silver recovery could be achieved. Scale up of the bioreactor system and engineering the demonstration assembly were undertaken by Indian Institute of Science, in consultation with Engineers India Ltd. The Hutti Gold Mines Company undertook the fabrication and construction work and established a demonstration series bioreactor assembly by the middle of 2001.

The nominal capacity of the plant is to treat 100 kg/day of flotation concentrate, which can be enhanced to 200 kg/day. There are three sections in the plant, namely, mixing, bioleaching and product separation. Refractory gold concentrates and process water are added to a mixing vessel to yield a slurry at 10-15 per cent pulp density which is then fed to reactors 1 and 2 on a continuous mode at acidic pH.

After the biooxidation is complete, the slurry is transferred to a settling tank and the bioleached residue collected, lime treated and cyanided. The demonstration bioreactor plant established at Hutti mines need be considered as a national facility and could be used to test and develop bioleaching processes for other metals such as copper, zinc, nickel and cobalt besides gold and silver.

K. A. Natarajan
Indian Institute of Science
Bangalore - 560 012