Bioleaching Provides Better Alternative to Copper Smelting

Micro-organisms have been interacting with minerals for as long as life has existed on earth. While some microbes dissolve minerals to feed off the energy released from the dissolution, others directly or indirectly aid in the precipitation of minerals through specific exothermic reactions. Certain strains of bacteria are even able to make insoluble minerals soluble. This latter reaction is the foundation for a process called bioleaching, which allows mining companies to treat insoluble sulfides and oxides via hydrometallurgy, as opposed to the more conventional methods. Since copper sulfide minerals are almost insoluble in acids, metal recovery from such ores requires flotation concentration followed by smelting at high temperatures and refinement by dissolution in acid and electrowinning. In smelting, sulfides are oxidized to SO2 and copper ions are reduced to the elemental metal. While smelting is limited by the amount of arsenic that base and precious metal concentrates can contain, and by a total tonnage limit that can be produced in a given year, bioleaching provides solutions to these challenges.

Bioleaching makes use of naturally occurring bacteria which are harmless to both humans and the environment, to liberate precious and base metals from ore, concentrates, and tailings that are otherwise difficult to treat. The bacteria are inoculated and allowed to grow within a dump, a heap, or a reactor, and then left to oxidize metal-containing sulfides. According to a study by the Pontificia Universidad Católica de Valparaíso, the advantages of bioleaching include the use of simple and low cost equipment, low energy requirements, zero atmospheric contamination, and the ability to treat low-grade or waste ores. On the other hand, its disadvantages include low reaction rates and productivities, requirement of extensive grounds, and an inadequacy for treating primary sulfides. Bioleaching can be particularly beneficial considering that the grade of copper ores is diminishing worldwide. Smelting of very low-grade ores, at 0.6% or less, is currently not economically viable. Moreover, arsenic trioxide gases are produced due to the heat from this process. Bioleaching, on the other hand, does not produce harmful gases and tolerates much higher levels of arsenic, converting arsenic into the environmentally benign ferric arsenate.

Though bioleaching of copper has always occurred naturally, the deliberate use of bacteria for modern extraction of metals dates back to the isolation and characterization of leaching bacteria done between 1947 and 1953. Early application of bioleaching of copper focused on the recovery from dumps, while currently the use of heaps proves to be more effective, as heaps are intentionally designed and erected to create ideal operational conditions that facilitate the complex reactions involved. Heaps are now widely used in the large-scale bioleaching of copper to process low-grade ores or as an alternative to smelting secondary copper minerals such as covellite and chalcosite. More recent installations use tank reactors for copper recovery as those designed and constructed by BacTech Environmental Corporation.

Though the process is relatively straightforward, the successful application of bioleaching depends on a series of variables and parameters, including the strain of microorganisms used for the operation. Since particle ores can differ considerably in their composition and component distribution, some of these materials may interfere with the bioleaching process, thus inhibiting microbial activity or modifying the acidity of the system. This may in turn affect the rate and yield of copper extraction. Moreover, particles must be small enough to facilitate bioleaching kinetics but not so small that they decrease bed permeability and impair the circulation of the leach liquor. Tank reactors, on the other hand, require particles to be of a certain size so that they may be kept suspended by means of an agitator. Other variables that must be controlled include temperature, pH, Eh, oxygen and CO2 content. Taking this in mind, a dump leach operation may take four or more years to produce results, while heap leaching may take between five and ten months, and a stirred tank reactor may take less than a month.

Bioleaching can also be applied to treat a variety of environmental problems. It can be used to treat acid mine drainage through the contained oxidation of sulfides, which are responsible for leaching toxic metals into surrounding areas. This method can also extract heavy metals from sewage sludge, making their reuse on land less hazardous, and to process tailings left behind by mining operations.

After decades of research and pilot programs in different countries around the globe, Industrias Peñoles set the stage for the use of bioleaching in Mexico. In 2001, the company contributed US$5 million to build a pilot plant in Monterrey and test the technology’s ability to treat dirty or complex base metal concentrates. This test proved that bioleaching was able to neutralize tramp elements in the concentrates and eliminate costly transportation of concentrates to smelters. Furthermore, Grupo Mexico has been funding a group of researchers from the Instituto Potosino de Investigación Científica y Tecnológica to identify and isolate bacteria that participate in the leaching of copper. Through such initiatives, bioleaching has the potential to become a trend in the Mexican mining industry as it has in other parts of the world.