The bacterial oxidation process is the latest processing technique developed for pre-treating double refractory gold ores. The technology which was commercialized in 1986 under the name, BIOX, employs a mixed population of chemolithotrophic bacteria to break down the sulfide mineral matrix, thereby liberating the occluded gold for subsequent cyanidation. The carbonaceous matter on the other hand, is only partially oxidized and therefore not rendered totally inactive. Bacterial oxidation of gold ores is differentiated from bacterial leaching in that the bacteria used do not dissolve the gold. 

Several types of bacteria are known to oxidize sulfides but the most common ones in biomining are Acidithiobacillus thiooxidans, Acidithiobacillus ferrooxidans and Leptospirillum ferrooxidans (Livesey-Goldblatt et al., 1983; Hutchins et al., 1987; Rawlings, 1997). 

These bacteria are acidophilic mesophiles and perform optimally within the pH range of 1.2-1.8 and temperature range of 35 and 45oC. The oxidation reactions of sulfide minerals are exothermic. The process is cooled with water to maintain the temperature within the optimum range. The oxygen demand for sulfide oxidation is high and thus large volumes of air has to be injected and dispersed into the slurry and bioreactors are designed to provide efficient and rapid oxygen uptake. Dissolved oxygen levels are kept 2 ppm and above 28.

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 Biooxidation of gold ores may take place in continuously stirred tank reactors (CSTR) or in heaps and dumps but the use of stirred tank bioreactors is more prominent. Feed to bioreactors is mainly flotation concentrate and the processing time is between 3 and 5 days if high sulfide oxidation levels are expected. During biooxidation, the bacteria require nutrients to sustain growth. Essential elements such as nitrogen; phosphorus and potassium are added into the primary reactors as solution of fertilizer grade ammonium sulfate and potassium phosphate or phosphoric acid (Rawlings, 1997; Hackl, 1997).

Thermophiles, which are micro-organisms which have growth activity between 45 to 55oC and and also between 50 to 80oC have been widely researched. Sulphobulus species which grow between temperatures of 50 to 80oC (extreme thermophiles) have been put to use for pre-treatment of both sulphidic as well as carbonaceous gold ores, have resulted in 80% recovery rates. The use of this same organism, Sulphobulus, for treating various carbonaceous ores and blanking the carbon before cyanidation, have yielded about of 84 to 94% recovery rates (Hutchison et al., 1988). 

Biooxidation process is efficient and cost effective making it an attractive alternative to the conventional roasting and pressure oxidation. The bacterial culture is robust; the plants are simple to operate and have proven high scale up potential. More so the process is environmentally friendly as neutralization of plant effluents produces precipitates that meet the most stringent environmental regulations.

The reactions for the bio-oxidation of arsenopyrite, pyrrhotite and prite are as follows:
Direct bacterial oxidation:
Arsenopyrite:
2FeAsS + 7O2 + H2SO4 + H2O  2H3AsO4 + Fe2(SO4)3……………………………..2.29

Pyrrhotite:
4Fe7S8 + 69O2 + 10H2SO4  14Fe2(SO4)3 + 10H2O………………………………….2.30 

Pyrite:
4FeS + 15O2 + 2H2O  2Fe2(SO4)3 + 2H2SO4……………………………………….2.31

A combination of direct and indirect attack can occur in a bio-oxidation process. Below are examples of the indirect attacks;

Ferric Generation:
4FeSO4 + 2H2SO4 + O2  2Fe2(SO4)3 + 2H2O………………………………………2.32

Ferric Attack:
FeS2 + Fe2(SO4)3  3FeSO4 + 2S…………………………………………………….2.33
Fe7S8 + 7Fe2(SO4)3  21FeSO4 + 8S…………………………………………………2.34

S Attack:
2S + 3O2 + 2H2O  2H2SO4………………………………………………………….2.35

For sustainable growth of the bacteria, they require some nutrients. Essential nutrients such as Nitrogen, phosphorus and potassium are added to the pulp as ammonium sulphate, potassium hydroxide and phosphoric acid (biomin, 2013)

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