Bacterial leaching, minerals

Bacterial leaching, minerals, 36 115-121 laboratory reactors, 36 116-117 monitoring organism growth, 36 117-119 nutrient effects, 36 119 pH control, 36 121 toxicity effects, 36 119 Bacterioferritin, see also Bacfer cluster, 43 362-363... 

Bacterial leaching is another example of oxidizing dissolution whereby specific bacteria either directiy attack the sulfide mineral or indirectiy enhance the regeneration of the oxidant. 

The bacterial leaching of uranium minerals is complex. This is because of the fact that uranium minerals are not sulfides and are not, therefore, directly attacked by the bacteria. However, the uranium sources usually have a substantial pyrite content which can be bac-terially oxidized to give an acidic ferric sulfate solution which is an effective leaching medium for uranium minerals. The reactions involved in the system can be shown in a simplified form as ... 

The only metals presently recovered on a commercial scale from their minerals using bacterial leaching as part of the process are copper (Z2) from mine waste dumps and uranium (M2) from old mine workings and mine waters. 

The existence of these chemical pathways for the oxidation of minerals during bacterial leaching does not exclude a direct role for bacteria. Both pathways may occur simultaneously, the relative importance of each depending upon the rate constants for the reactions and the concentration of Fe(III) present in solution. Experiments carried out on synthetic iron-free cobalt and nickel sulfides, using carefully washed cells of T. ferrooxidans to ensure the absence of Fe(III), showed consumption of oxygen and the solubilization of the metal as sulfate. Solubilization rates were appreciably increased on the addition of Fe(III). These results present support for the presence of both direct and indirect pathways (116). Recently, attempts have been made to compare the rates of these pathways for the oxidation of pyrite by T. ferrooxidans (74). 

Torma, A.E. and Subramanian, K.N., 1974. Selective bacterial leaching of a lead sulphide concentrate. Int. J. Miner. Process., 1 125—134. 

Corrans, I.J., 1970. Studies on the Bacterial Leaching of Natural and Synthetic Iron Copper Sulfide Minerals. M.Sc. Thesis, University of New South Wales, 117 pp. 

Khalid, A.M. and Ralph, B.J., 1977. The Leaching behaviour of various zinc sulphide minerals with three Thiobacillus species. Conference Bacterial Leaching. GBF Monograph Series, No. 4 pp. 165—173. 

Pings, W.B., 1968. Bacterial leaching of minerals. Colo. Sch. Mines, Miner. Ind. Bull., 2 1-19. 

Two other developments might be mentioned. One is the bacterial leaching of desired minerals, particularly copper and uranium. To a degree, naturally occurring bacteria are used, but more and more strains are being developed for specific applications. They are propagated by fermentation. 

It can be seen, therefore, that ferrous iron and chalcopyrite oxidation are acid-consuming reactions, while pyrite oxidation and iron hydrolysis are acid-producing reactions. Thus, whether the overall reaction in a dump is acid producing or acid-consuming depends on the relative proportions of chalcopyrite and pyrite and on the pH conditions. In practice, sulfuric acid additions to the leach solution applied to the dump are usually required to overcome the acid consuming reactions of the gangue minerals and to keep the pH in a suitable range, typically 2 to 2.4, to optimize bacterial activity and minimize iron hydrolysis. 

Holtum, D. A. Murray, D. M. Bacterial heap leaching of refractory gold sulfide ores. Miner. Eng. 1994, 7, 619-631. 

These authors observed that the leach solutions of chalcocite become more and more depleted in Cu and that this depletion is accompanied by a decrease of the Cu/S ratios of the solution from 2 to 1, which these authors ascribe to fractionation between diversely coordinated Cu in the different minerals. In contrast to chalcocite, chalcopyrite leaching produces no isotope fractionation. These authors also conclude from a comparison between columns seeded with bacteria and sterile columns that bacterial mediation had little if any influence on Cu isotopic fractionation in this specific experiment, which simply reflects that bacteria do not store signiflcant amounts of metal. 

SABA [Spherical Agglomeration-Bacterial Adsorption] A microbiological process for leaching iron pyrite from coal. The bacterium Thiobacillus ferrooxidans adsorbs on the surface of the pyrite crystals, oxidizing them with the formation of soluble ferrous sulfate. Developed by the Canadian Center for Mineral and Energy Technology, Ottawa. In 1990, the process had been developed only on the laboratory scale, using coal from eastern Canada. 

Evidence for direct bacterial attack upon the mineral surface comes from microscopy. The use of this technique has shown the attachment of bacteria to solid substrates, consistent with a direct role for bacteria in leaching. The adsorption of cells on suspended materials occurs within a few minutes. Detailed studies, illustrated by the following examples, have shown the attack of the bacteria to be selective with respect to the surface of the mineral, in notable contrast to attack by acidic solutions of Fe(III). 

Below the zone of leaching is a zone of illuviation, or zone of deposition, sometimes called the B horizon, in which dissolved organic matter and previously solubilized iron and aluminum are deposited. Deposition occurs when the organic acids and associated complexed metals are sorbed onto soil minerals or when the organic acid molecules themselves are mineralized by bacterial action, causing the previously complexed metals to precipitate. Beneath this depositional soil horizon is a relatively unweathered mineral material, often called the C horizon, or parent material, because it is the original material from which the soil profile developed. 

---