Bugs work cheap.
Brewery Creek in the Yukon was a failed bio-leach-gold mine. It failed for two reasons: low gold price in 1998 and very cold weather, which shut down their operation essentially over the winter months. It was however, a partial technical success. If they had followed my precepts of insulating the piles, and heating them with groundwater H/E, I believe they would have had a better result. Pile temperature are still above freezing. A successful experiment with heap leaching was carried out in Timmins Ontario. Most long term heap leaching of sulfides is assisted by bacteria. Leaching of ores by this method is quite dramatically successful, but wash out of metals by precipitation with ambient precipitation is a problem, as is rising Ph of circulating solution. I believe that SPM's and limestone filter beds may be a solution to rising Ph gradients. The limestone filter beds may be backwashed by organic acids and/or or ligands and hydrocarbons at a later date to recover the metals. There are aurophilic bacteria, but bio-leach gold mining is performed primarily by leaching the sulfide minerals. There are many refractory gold deposits around the world that will respond to bio-leaching. Northern Ontario, New Guinea, Solomons, Australia, South America and the SW United States all have copious quantities of such refractory gold ores at respectable grades.
Improvements in scale, throughput etc, will all yield to research over time. I would like to try bio-leaching in fine crushed residua stopes in situ and outdoors in Ontario. Agnew Lake U308 mine in Sudbury was a leaching in situ undergound mine (not specifically bioleach, but probably naturally assisted. It failed due to grade, ore size, problems at fairly low U prices way back when. It should be noted that U oxide is a very soluble metal as found in nature, and will respond to a number of alkalis and acids.
Thiobacillus ferrooxidans is a gram-negative, highly acidophilic (pH 1.5 to 2.0), autotrophic bacterium that obtains its energy through the oxidation of ferrous iron or reduced inorganic sulfur compounds. It is usually dominant in the mixed bacterial populations that are used industrially for the extraction of metals such as copper and uranium from their ores. More recently, these bacterial consortia have been used for the biooxidation of refractory gold-bearing arsenopyrite ores prior to the recovery of gold by cyanidation. The commercial use of T. ferrooxidans has led to an increasing interest in the genetics and molecular biology of the bacterium.
Sulfobacillus thermosulfidooxidans
Acidianus Brierleyi
Leptospirillum ferrooxidans
FeAsS(s) ? Fe2+(aq) + As3+(aq) + S6+(aq)
Fe2+ ? Fe3+
M3+ ? M5+
Fungi:
Aspergillus Niger
Penicillium simplicissimum
* Sulfide ore or concentrate ? binding or hosting the valuable metal; energy for bioleaching microbes
* Air (supplied actively or passively)
o O2 ? bioleaching microbes are aerobes and crave oxygen to extract energy from sulfide minerals.
o CO2 ? bioleaching microbes need the macro-nutrient carbon to build cell mass
* N, P, K, Mg ? nutrients for bioleaching microbes
* pH-regulators (to keep pH 1-2)
o H2SO4
o CaCO3, CaO
* Bioleaching microbes like T. ferrooxidans, T. thiooxidans & L. ferrooxidans
1. Cultivate (“amenability testing”, selection) or buy mixtures of bioleaching microbes for inoculation.
2. Inoculate – add to leaching reagents or spray ore before building heap (10^5 microbes/g ore for rapid Fe(II) oxidation)
* Control of temperature (affected by climate)
o Air regulation
o Cover
o Cooling tower
* Do a bleed in order to neutralize and precipitate metals (mostly iron)
* Distribution system, stirring (in tanks), sprinklers, airflow, tubes - "blood-circulation" of the heap.
* Reaction catalysts
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Efforts to establish bioleaching at the Rio Tinto mines were made in the beginning of the 1890's. Heaps (10 m in height) of low-grade ore (containing 0.75% Cu) were built and left for one to three years for "natural" decomposition. 20% to 25% of the copper left in the heaps was recovered annually. It was calculated that approximately 200'000 tons of rough ore could be treated in 1896. Although industrial leaching operations were conducted at Rio Tinto mines for several decades, the contribution of bacteria to metal solubilization was confirmed only in 1961, when Thiobacillus ferrooxidans (reclassified as Acidithiobacillus) was identified in the leachates.
Although metal leaching from mineral resources has a very long historical record and although the oxidation of reduced sulfur compounds and elemental sulfur resulting in the formation sulfuric acid was demonstrated already in the 1880's, the oxidation of metal sulfides was not reported until 1922 when the oxidation of pyrite and the mobilization of zinc from zinc sulfide was investigated. It was found that the transformation of zinc sulfide to zinc sulfate was microbially mediated. Based on these results, the economic recovery of zinc from zinc-containing ores by biological methods was proposed. In 1947, Thiobacillus ferrooxidans was identified as part of the microbial community found in acid mine drainage. A first patent was granted in 1958. The patent describes a cyclic process where a ferric sulfate sulfuric acid lixiviant solution is used for metal extraction, regenerated by aeration (ferrous iron oxidation by iron oxidizing organisms), and re-used in a next leaching stage. |
