Solubilization of metals, using Thiobacillus spp, is being carried out on a commercial scale in the recovery of copper and uranium by heap, dump and in situ leach techniques. The ore leaching biotopes are not only colonized by autotrophic bacteria (Thiobacillus spp., Leptospirillum ferrooxidans and Sulfolobus spp.) but the heterotrophic microorganisms, including bacteria and fungi of various species are also found in these habitats and may influence the leaching process carried out by autotrophs. The present research work was carried out to study mutual effect of these autotrophic and heterotrophic bacteria in leaching of two carbonate bearing low grade ores; non sulphidic uranium ore and complex sulphidic ore of Cu, Fe and Zn.
A number of acidophilic, chemoautotrophic sulphur oxidizers resembling Thiobacillus rhiooxidans and T. ferrooxidans have been isolated from mining, processing, overburden and process waste of uranium ore. The best strain in respect of sulphur oxidation, T. rhiooxidans (S-V), was used for leaching studies. Carbonate bearing sandstone uranium ore found in Pakistan requires 12 - 13 g H2SO4 for every 100 g ore to solubilize above 90%. T. rhiooxidalls (S-V) could produce 25 g H2SO41-1 of the medium dropping the pH from 2.5 to 0.87 in 18 days. With 5% w/v ore slurry, 2 % w/v elemental sulphur, these bacteria showed a lag period of 44 hrs when pH decreased to 3 and completed at 72 hrs incubation. Initial addition of H2SO4, resulted in shortening of lag phase and the process was completed in 36 hrs. Production of acid by bacteria for pretreatment of ore, followed by addition of ore was found successful in reducing this lag time.
A suspension of 5% w/v slurry of uranium ore in mineral salt medium supplied with elemental sulphur (1 %) and sodium thiosulphate (1 %) resulted in a pH of 5.9. Using controlled conditions in a bioreactor (Biostat E), within three days after inoculation of the bacteria, the pH dropped to 2.1 and resulted in 90% solubilization of the uranium. Incremental increase of slurry raised the pH which was then lowered by the action of bacteria. In this way up to 20% ore slurry could be treated without inactivating the culture. At this stage, more than 85 % U3O8 uranium was solubilized. These bacteria could then be adapted to grow in a mineral medium containing 10% w/v ore slurry (pH 8.3) supplied with sulphur and sodium thiosulphate as the energy source. Acid production resulted in the pH dropping below 1.3 within 8 days. When the ore slurry was increased to 15, 20 and 30%, after the pH dropped below 2 in each case, it was found that the bacteria could decrease the pH to 1.2 and result in 85 % solubilization of uranium.
Uranium ecosystem contained bacteria and fungi, capable of producing organic add from glucose and molasses and these can be used as agent for pretreatment of ore before applying T. thiooxidans. A fungal strain (F-4) was used for acid production from molasses and on day 7 (when pH was below 3) uranium ore was added and continued another 5 days, T. thiooxidans (S-V) was inoculated and sulphur provided as energy source. Addition of chemolithotrophs Le., T. thiooxidans did not result in additional solubilization. Survival was noted but no growth or production of inorganic acid could be observed. This might be due to the presence of molasses and organic acid.
Carbonate bearing sulphidic ore from Rammelsberg mine in Goslar, Germany was found to contain 1.78% Cu, 13.68% Fe, and 22.87% Zn. T. thiooxidans (NBl0l) and two strains of T. ferrooxidans (F40 and F41) were inoculated to shake flask for 5 weeks at 130 rpm and 30°C individually and in combination with 1 % w/v slurry of ore in different media. Chemical reaction of heavy metals sulphide by this acid as well as ferric iron resulted in production of sulphur in the form of passive film/layer.
T. ferrooxidans (Strain F-40) reported to be non nitrogen-fixer and strain F-41, a nitrogen fixer were studied for leachability behaviour alone and in combination with T. thiooxidans (lacking nitrogen fixing ability) using media with and without added ammonium nitrogen. Methylobacterium sp. (alt-25) a nitrogen scavenger, found in association with the thiobacilli in one of the leaching biotope in Germany was also tested with the afore mentioned combinations.
Decrease in pH and a small solubilization of Cu (2.6 %) and Zn (11.3 %) was observed when T. thiooxidans (NB-10l) was inoculated along with T. ferrooxidans (F-41) possessing nitrogen fixing ability in nitrogen free mineral medium (NFMM) supplemented with elemental sulphur. Supplying ammonium nitrogen in the medium, helped drop in pH and metal solubilization. T. thiooxidans (NB10l) have the ability to solubilize heavy metals from carbonate bearing complex sulphidic ore more efficiently than T. ferrooxidans. The combination of T. thiooxidans and T. ferrooxidans resulted in solubilization of almost 100 % Zn and up to 30% Cu and more than 25% Fe. Provision of sulphur resulted in further pH drop by T. thiooxidans but the solubilization of metals was reduced which might be due to non utilization of passive film of sulphur.
Higher solubilization of Zn was observed when nitrogen scavenger Methylobacterium sp. (alt-25) was inoculated in combination with T. thiooxidans and T. ferrooxidans in NFMM supplied with ferrous sulphate. This increase was 15-20 % over and above -treatment without nitrogen scavenger. Thus, the advantage of using this nitrogen scavenger is proven but suitable conditions are to be provided. In presence of ammonium nitrogen and ferrous sulphate, T. ferrooxidans alone and in combination with T. thiooxidans and/or nitrogen scavenger Methylobacterium sp. (alt-25) reached almost 100% Zn solubilization and 30-35 % Cu. Iron solubilization by bacteria was found maximum (30%) in this combination. Lower pH also hindered the action of nitrogen scavenger. Presence of nitrogen also inhibited nitrogen fixation.
Thus, it is clear that presence of heterotrophs in leaching biotopes influence the leaching efficiency of thiobacilli. This ability can be exploited for recovery of uranium from carbonate bearing non sulphidic ore and copper and zinc from carbonate containing complex sulphidic ore.