Extractive metallurgy in future

Extractive metallurgy in future

The rate of the leaching of sulfides in sulfuric acid media is significantly less than that of oxides, but the intervention of bacteria accelerates the process. Bacterial leaching is being developed to operate much more effectively than that occurring naturally. Effective bacteria in sulfide leaching include Thiobacillus ferrooxidans, Thiobacillus thiooxidans, and Leptospirillum ferrooxidans. Mixed cultures of these bacteria solubilize metal sulfides more effectively than do pure cultures (that is, cultures containing only one species of bacteria).
The bacteria do not alter the electrochemistry of the leaching process, but rather catalyze individual reactions. For example, oxygen passes through the cell of the microorganism and is reduced in the cell matrix. The electrons released in this cathodic process are transferred via the cell cytochromes and through the cell wall to oxidize double- charged iron cations in solution to triple-charged iron cations. The triple-charged iron then chemically attacks the metal sulfides and converts them to dissolved metal ions. The hydrogen ions consumed by oxygen reduction maintain a high pH in the intercellular space to allow the bacteria to function while residing in a strong acid environment (pH 1-3). The electron transfer reactions involving iron and oxygen provide the bacterium with the energy required for life. The action of the bacteria maintain the triple-charged iron concentration in solution at a higher level than that existing under sterile conditions and hence the rate of leaching of sulfides is enhanced. The above-mentioned bacteria also oxidize sulfur to sulfate and this can also accelerate leaching through the removal of inhibiting surface layers. Thus, insoluble metal sulfides are oxidized to soluble metal sulfates.

Biochemical leaching is now a well-established commercial process for treating gold ores in which the gold is enclosed within sulfide minerals. Biochemical oxidation of the sulfide minerals releases the gold and allows it to be leached by the chemicals used in conventional gold leaching.

Biochemical processes are being developed for the treatment of copper, nickel and cobalt sulfide ores and are expected to become the major method for recovery of these metals in the future. The aim is to eventually replace smelting processes that generate sulfur dioxide.

The ultimate advance in recovering metals from sulfide ores would be to eliminate the mining stage and leach the ore-body where it is in the ground. This would involve pumping a leach solution underground and pumping up the reacted solution containing dissolved valuable metal species. Such in-place leaching has already been practiced commercially in uranium and copper mines. The best results have been obtained in the recovery of metals from residual ore in mines in which the high-grade ore has been mined out since mine workings provide access of the solution to the ore. The major problem with new ore-bodies is in breaking the ore underground to make it accessible to the leach solution without having physical access to it.