Biomining

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Biomining

• wstafford_at_uwc.ac.za

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Recovery of metals

• Micro-organisms are active in the formation and
  degradation of minerals in the Earth's crust.
  Biotechnology can harness and refine these
  techniques for enhanced recovery of metals.
• Iron pyrites and copper sulphide could be
  oxidized by Thiobacillus spp. (Bryner et al.1954)
  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'

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Biomining

• 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.
• Successful commercial metal-leaching processes
  include the extraction of gold, copper, and
  uranium (Suzuki, 2001).

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Bioleaching

• 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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Features o f Metal-leaching microorganisms

• The metabolism of metal sulphides produces
  sulphuric acid and not surprisingly almost all
  the micro-organisms are acidophiles growing below
  pH values of 3.0
• Mesophilic and highly acidophilic (pH 1.5-2.0)
  Thiobacillus ferrooxidans, Thiobacillus
  thiooxidans, and Leptosprillum ferroxidans, T.
  thiooxidans can only use reduced sulphur
  compounds and L. ferroxidans can only use the
  ferrous ions. Only together can they rapidly
  degrade pyrites (FeS2).
• Thermophilic bacteria, Thiobacillus TH-1 and
  Sulfolobus brieleyi, have been found to grow on
  chalcopyrite (CuFeS2). Most of these bacteria
  require some form of organic substrate.

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Bioleaching of iron

• Metal sulphides are the major mineral forms of
  many metals and iron sulphide (pyrite) is the
  most abundant sulphide.
• Their original role in bioleaching thought to be
  the re-oxidation of ferric (II) sulphate to
  ferrous (III) sulphate, after the ferric sulphate
  had leached the iron from the iron-ore pyrite
  (FeS2) in the following reactions.
• FeS2 8H2O 14Fe3 15Fe2 2SO42-
  16H
• 14Fe2 3.5O2 14H 14Fe3 7H2O
  
• Bacteria, fungi, and Archaea and have been
  isolated from natural and commercial bioleaching
  systems that are capable of degrading metal
  sulphides. They are mesophiles, the moderate
  thermophiles, and the extreme thermophiles,

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Bacteria
Bacteria in extracellular matrix
Indirect, and direct bioleaching
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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.
• There are three main methods of applying
  bioleaching in situ treatment, heaps or dumps,
  and bioreactors.

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• In situ bioleaching. The leaching solution
  containing T. 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, and the bacterial suspension aerated
  before pumping back to the mine.
• Mine tailings or dumps. set on a slope with a
  depth of 7-20 m of crushed ore or tailings. The
  dump is sprayed with water acidified with
  sulphuric acid to ensure that the pH is between
  1.5 and 3, - encourage the growth of
  Thiobacillus. The Thiobacillus spp. will leach
  metals into solution and this can be collected by
  precipitation from the leachate. Anaerobes have
  also been detected in the anoxic zones of dumps
  including the sulphate-reducing Desulfovibrio sp.
  oxidize sulphides at pH values of 4-7 to give
  sulphuric acid the dump represents a very
  diverse and dynamic microbial community of up to
  106 cells/g of rock.

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Bioreactors

• The bioreactors used are the highly aerated
  stirred-tank designs where finely ground ore is
  treated. Often nutrients such as ammonia and
  phosphate are added and the bioreactor operated
  in a continuous manner. The leaching can take
  days rather than the weeks required with
  dumpextraction as temperatures of 40-50C are
  used, although the ore loading is 20. (BIOX)
• 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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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 operates using pyrite. The ferric (ii)
  ion, which reacts with the uranium ore, and
  sulphuric acid, which also leaches uranium.
• 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.