26 Feb A New Direction for Biomining
Biomining, the biotechnology that uses microorganisms to extract metals from ores and concentrates, is currently used exclusively for processing reduced ores and mine wastes. Metals of economic value also occur extensively in oxidized ores, such as nickel laterites. While these are not amenable to oxidative dissolution, the ferric iron minerals they contain can, in theory, be disrupted by iron reduction, causing associated metals to be released. We have harnessed the ability of the facultatively anaerobic, acidophilic bacterium Acidithiobacillus ferroooxidans to couple the oxidation of elemental sulphur to the reduction of ferric iron in the goethite fraction of a limonitic nickel ore at 30 °C. Nickel and other metals (Co, Cr and Mn) were effectively solubilised and maintained in solution due to the low pH (1.8) of the leach liquor. The results highlight the potential for the bioprocessing of oxidized, iron-rich ores using an approach that is energy-saving and environmentally-benign compared with metallurgical processes currently applied to the extraction of Ni from lateritic ores.
Bioprocessing of metal ores and mineral concentrates has, over the past 50 years, developed from a low-key technology (“dump” leaching of waste rocks at copper mines in the USA) to far more controlled and sophisticated operations involving irrigated and aerated heaps (of up to 10 km2 at the Escondida mine in Chile) and temperature-, aeration- and pH-controlled stirred tanks, each typically ~1000 m3 in size. Global production of ~20% copper, ~5% gold and smaller amounts of other metals is currently achieved through bioprocessing. The metal-hosting minerals in ores and concentrates currently amenable to biomining are all reduced and mostly sulphidic. These contain either the target metal (such as copper in the mineral chalcopyrite, CuFeS2) or else enshroud precious metals (such as gold in refractory pyritic ores) and, therefore, need to be removed to allow chemical extraction (e.g., using cyanide in the case of gold). The “bio” components in biomining are some specialized microorganisms that are able to accelerate the oxidative dissolution of the sulphide minerals in extremely acidic liquors. The main process is the continuous regeneration by iron-oxidizing bacteria and archaea of ferric iron, which degrades sulphide minerals, while biological oxidation of the sulphur moiety in the minerals generates sulphuric acid, thereby maintaining acidic conditions that enhance both the solubilities of cationic metals and the activities of the acidophilic microorganisms involved. A third tier of acidophilic bacteria and archaea maintains the stabilities and robustness of mineral leaching consortia by metabolizing organic carbon originating from the primary producers. Since the energy required by the primary and secondary mineral-degrading microorganisms is the reduced iron and sulphur present in the minerals themselves and the dominant leaching bacteria and archaea are autotrophic, the only extraneous requirements for heap and tank operations are for some inorganic salts (agricultural fertilizer is often used for this), air (for both oxygen and carbon dioxide) and water.
Extraction of Metals by Reductive Dissolution of Oxidized Ores
Base and precious metals can also occur in ores that are partially or totally oxidized, such as nickel laterites, which represent the major nickel resource in the lithosphere, accounting for an estimated 72% of residual reserves. Nickel laterites typically comprise two distinct horizons: a limonite zone near the surface that contains mainly hydrated iron oxides and relatively little magnesium and a lower saprolite zone that is dominated by hydrous magnesium silicates. Limonites are formed during laterisation when weathering near the surface results in the oxidation and precipitation of iron. Nickel and cobalt, derived from the weathering process of parent rock material, are incorporated into the lattice structure of precipitated hydrated iron oxides, either by co-adsorption or substitution for iron, producing a modified goethite mineral of typical composition (Fe0.97, Ni0.03, Co0.003) O·OH. The intimate association of base metals with ferric iron minerals in limonites means that these ores are not amenable to processing by conventional oxidative mechanisms employed in biomining.
Hydrometallurgical or pyrometallurgical processes are currently used to break the iron-oxygen bond, thereby releasing the associated nickel, but these are typically high energy- and/or reagent-requiring processes. Although bioprocessing of nickel laterites has previously been explored (using either iron-chelating organic or strong mineral acids produced as by-products of microbial metabolism), slow rates and low yields of nickel extraction have precluded these approaches being developed as commercial operations. The past 25 years or so has seen a large increase in the number of bacteria that are known to respire on ferric iron in anoxic environments, including many that live in highly acidic environments. The acidophilic bacterium, Acidithiobacillus ferrooxidans, is well-known for its ability to oxidize ferrous iron in the presence of oxygen, though this prokaryote is a facultative anaerobe that can also grow anaerobically using ferric iron as its electron acceptor. Specific rates of iron reduction of this and related bacteria tend, however, to be much lower than those of specific iron oxidation. We have explored the possibility that iron reduction carried out at low pH by At. ferrooxidans could be used to mediate the dissolution of ferric iron minerals present in nickel laterites, thereby facilitating the solubilisation and recovery of the nickel and other metals of economic interest present in the ore. Such an approach represents, in essence, a reverse of the processes involved in conventional biomining.
Bacterially-catalyzed reductive dissolution of an oxidized (lateritic ore) at low pH resulted in greatly enhanced leaching of nickel compared to acid dissolution alone. The critical reaction was the reduction of ferric iron to ferrous, catalyzed under anaerobic conditions by the acidophilic bacterium Acidithiobacillus ferrooxidans, using elemental sulphur as the electron donor. This work has demonstrated a potential new direction for mineral bioprocessing, which could greatly expand the applications of biomining technologies.
This is an excerpt of the journal article: A New Direction for Biomining: Extraction of Metals by Reductive Dissolution of Oxidized Ores, by D. Barrie Johnson, Barry M. Grail and Kevin B. Hallberg. Published: January 30., 2013 in Minerals 3(1), 49-58, DOI: 10.3390/min3010049 under a Creative Commons Attribution License (CC BY NC ND 4.0).