Gold solubilisation by the cyanide producing bacteria Chromobacterium violaceum

The cyanogenic bacterium Chromobacterium volaceum was investigated for gold extraction from gold bearing ores, as part of the search for environmental ly friendly lixiviants to be used in heap leach situations. The bacteria co ntinuously produce small amounts of cyanide, which reaches a peak at the on set of the stationery phase of growth. It is considered that cyanide volatilises at the pH values most conducive t o bacterial growth, and the higher the pH the more cyanide will stay in sol ution. However, at pH values above 8, growth becomes inhibited. Thus leachi ng experiments, using the bacteria, were carried out at pH 8. The salts fer rous sulphate and di-sodium hydrogen orthophosphate improve cyanide product ion((1)) but have an inhibitory effect on gold extraction. Also, ores with a high iron content cannot be leached effectively by the bacteria. It was found that 53% of the gold could be extracted from low grade ore con taining 3.2 g/t Au after 20 days. On the other hand, 10 days leaching of a high grade ore with 58.5 g/t Au extracted only 16% gold as compared to the 44.5% achieved after 10 days leaching of low grade ore. It was concluded that the bacteria show potential for gold solubilisation f rom low grade ores under heap leach conditions where much longer leach peri ods are required.