Biomining and Bioleaching

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Biomining and Bioleaching

• wstafford_at_uwc.ac.za

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Biomining and Bioleaching ?

• Mining companies have become increasingly aware
  of the
• potential of microbiological approaches for
  recovering base
• and precious metals from low-grade ores, and for
• bioremediating acid mine drainage and mine
  trailings.
• There are two strategies
• Biomining and Biosorption where microorganisms
  are used to recover metals in solution (e.g acid
  mine drainage) by precipitation of the metal, or
  complexing or absorption with cellualr molecules.
  Pyrometallurgy then enables recovery of these
  metals with the microbial biomass contributing as
  fuel.
• Bioleaching where microorganisms are used to
  facilitate the mining of metals from their ores
  by facilitating their solotion. Either use low
  grade ores (re-work mine trailings) or to avoid
  the extraction of large amounts of ores to the
  surface

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The need for Biomining and Bioleaching

• Biomining will become more important as
  high-grade surface mineral deposits are worked
  out and become less viable, and mining companies
  will be forced to find other mineral sources.
• These will include the working of low-grade ore
  deposits, mine tailings, mine dumps, and
  worked-out mines. The extraction of metals using
  mechanical and chemical methods is difficult and
  expensive but biological methods are more
  cost-effective, use little energy, can function
  well at low concentration of metals, do not
  usually produce harmful emissions and reduce the
  pollution of metal-containing wastes.

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Bioleaching approaches

• In situ bioleaching. The leaching solution
  containing Thiobacillus ferrooxidans is pumped
  into the mine where it is injected into the ore.
  The leachate is recovered from lower down the
  mine, pumped to the surface where the metal is
  recovered.
• Mine tailings or dumps. A dump on a slope with a
  depth of 7-20 m of crushed ore or tailings is
  sprayed with water acidified with sulphuric acid
  to ensure that the pH is between 1.5 and 3, -
  encourage the growth of Thiobacillus spp. The
  metals are collected by precipitation from the
  leachate.
• Bioreactors used are the highly aerated
  stirred-tank designs where finely ground ore is
  treated. Often nutrients are added and the
  bioreactor operated in a continuous manner. The
  leaching can take days rather than the weeks
  required with at temperatures of 40-50C,
  although the ore loading is appox. 20. Ores
  such as chalcopyrite (CuFeS2) and energite
  (Cu3AsS4) require temperatures as high as 75-8
  0C for leaching which cannot be generated in
  dumps and therefore can only be carried out in
  bioreactors

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Bioleaching Microbiology

• Iron pyrites and copper sulphide could be
  oxidized by Thiobacillus spp. and the first
  patent was filed (Zimmerley et al., 1958). The
  ability of micro-organisms to solubilize metals
  from insoluble metals is known as 'bioleaching
• Successful commercial metal-leaching processes
  include the extraction of gold, copper, and
  uranium (Suzuki, 2001).

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Bioleaching microorganisms

• The most important mineral-decomposing
  micro-organisms are the iron- and
  sulphur-oxidizing chemolithotrophs
• Chemolithothrophs obtain energy from inorganic
  chemicals, use carbon dioxide as their carbon
  sources, and are represented by hydrogen-,
  sulphur-, and iron-reducing Bacteria and Archaea.
  
• The most important metal-leaching microorganisms
  use ferrous iron and reduced sulphur compounds as
  electron donors and fix carbon dioxide.
• Many of these microorganisms produce sulphuric
  acid (acidophiles).

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Some Metal-leaching microorganisms
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Bioleaching technology

• It has been shown that micro-organisms can
  extract cobalt, nickel, cadmium, antimony, zinc,
  lead, gallium, indium, manganese, copper, and tin
  from sulphur-based ores.
• The basis of microbial extraction is that the
  metal sulphides, the principal component in many
  ores, are not soluble but when oxidized to
  sulphate become soluble so that the metal salt
  can be extracted.

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Many ores of valuable metals contain sulphur and
iron. Thiobacillus is often needed
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Extraction of copper

• In 1991 the biological recovery of copper
  exceeded 1000 million and accounted for 25 of
  the world's copper production. The waste formed
  is generally that remaining after extraction of
  rock from a mine where the copper level is too
  low for it to be extracted economically
  (0.1-0.5).
• The waste material is formed into terraced dumps
  100 m wide and 5 m deep with an impermeable base.
  Dilute sulphuric acid is sprinkled or sprayed on
  to the dump so that as it percolates through the
  dump the pH is reduced to 2-3, which promotes the
  growth of T. ferrooxidans and other leaching
  microorganisms. The copper, upon oxidation to
  copper sulphate, is dissolved in the dilute acid
  and is collected at the bottom of the dump

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Extraction of uranium

• There are two possible processes.
• Direct leaching by T. ferrooxidans has been
  proposed in the following equation.
• 2UO2 O2 2H2SO4 2UO2SO4 2H2O
• However, in conditions where oxygen is limited
  this cannot operate, and the indirect bioleaching
  process occurs
• UO2 Fe2(SO4)3 UO2SO4
  2FeSO4
• UO3 H2SO4 UO2SO4
  H2O

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Extraction of gold

• The normal method of extracting gold is to treat
  it with cyanide and then extract the gold from
  the cyanide extract with carbon. The cyanide
  waste is a major pollutant and has to be treated
  before release into the environment (Akcil and
  Mudder, 2003). Cyanide can be destroyed by a
  sulphur dioxide and air mixture or a
  copper-catalysed hydrogen peroxide mixture.
  However, there are biological methods, both
  aerobic and anaerobic, for the treatment of
  cyanide.
• Some of the micro-organisms known to oxidize
  cyanide include species of the genera
  Actinomyces, Alcaligenes, Arthobacter, Bacillus,
  Micrococcus, Neisseria, Paracoccus, Thiobacillus,
  and Pseudomonas.

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Lost gold

• Some ores are resistant to cyanide treatment as
  the gold is enmeshed in pyrite (FeS2) and
  arsenopyrite (FeAsS) and only 50 of the gold can
  be extracted. The leaching is carried out in a
  sequence of bioreactors with the first step
  bioleaching the FeS2 and FeAsS so that the gold
  can subsequently b eextracted.
• The processes includes aerobic rotating
  biological contactors, bioreactors, and
  stimulated ponds

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The future of Bioleaching

• Isolate new bacterial strains from extreme
  environments, such as mine-drainage sites, hot
  springs, and waste sites, and use these to seed
  bioleaching processes.
• Improve isolates by conventional mutation and
  selection or by genetic engineering. One
  possibility would be to introduce arsenic
  resistance into some bioleaching organisms, which
  could then be used in gold bioleaching.
• Heterotrophic leaching is a solution for wastes
  and ores of high pH (5.5) where many of the
  acidophiles would not grow. Fungi likeTrichoderma
  horzianum have been shown to solubilize MnO2,
  Fe2O3, Zn, and calcium phosphate minerals.
• The population dynamics within the bioleaching
  dumps and the relative importance of various
  organisms and mechanisms needs to be understood.