From Wikipedia,
the free encyclopedia.
Bioleaching is the
extraction of specific
metals from their
ores through the use of
bacteria.
Bioleaching is a new technique
used by the
mining industry to extract
minerals such as
gold and
copper from their
ores. Traditional extractions
involve many expensive steps such
as
roasting and
smelting, which requires
sufficient concentrations of
elements in ores. Low
concentrations are not a problem
for
bacteria because they simply
ignore the waste which surrounds
the metals, attaining extraction
yields of over 90% in some cases.
These
microorganisms actually gain
energy by breaking down minerals
into their constituent elements.
The company simply collects the
ions out of the solution after
the bacteria have finished.
Some advantages associated with
bioleaching are:
- economical: bioleaching is
generally simpler and therefore
cheaper to operate and maintain
than traditional processes,
since fewer specialists are
needed to operate complex
chemical plants.
- environmental: The process
is more environmentally friendly
than traditional extraction
methods. For the company this
can translate into profit, since
the necessary limiting of
sulfur dioxide emissions
during smelting is expensive.
Less landscape damage occurs,
since the
bacteria involved grow
naturally, and the mine and
surrounding area can be left
relatively untouched. As the
bacteria breed in the conditions
of the mine, they are easily
cultivated and recycled.
Some disadvantages associated
with bioleaching are:
- not economical: the
bacterial leaching process is
very slow compared to smelting.
This brings in less profit as
well as introducing a
significant delay in cash flow
for new plants.
- not environmental: Toxic
chemicals are sometimes produced
in the process.
Sulfuric acid and H+
ions which have been formed can
leak into the ground and surface
water turning it acidic, causing
environmental damage. Heavy ions
such as iron, zinc, and
arsenic leak during acid
mine drainage. When the pH of
this solution rises, as a result
of dilution by fresh water,
these ions precipitate, forming
"Yellow Boy" pollution. For
these reasons, a setup of
bioleaching must be carefully
planned, since the process can
lead to a
biosafety failure.
Currently it is more economical
to smelt copper ore rather than to
use bioleaching, since the
concentration of copper in its ore
is generally quite high. The
profit obtained from the speed and
yield of smelting justifies its
cost. However, the concentration
of gold in its ore is generally
very low. The cheaper cost of
bacterial leaching in this case
outweighs the time it takes to
extract the metal.
The process
The extraction of copper from
its ore involves two
bacteria,
Thiobacillus ferro-oxidans
and
Thiobacillus thio-oxidans.
In stage 1, bacteria
catalyse the breakdown of the
mineral
arsenopyrite (FeAsS) by
oxidising the
sulfur and metal (in this case
arsenic ions) to higher
oxidation states whilst reducing
dioxygen by H2 and
Fe3+. This allows the
soluble products to dissolve.
- FeAsS(s) ->
Fe2+(aq) +
As3+(aq) +
S6+(aq)
This process actually occurs at
the
cell membrane of the bacteria.
The electrons pass into the cells
and are used in biochemical
processes to produce energy for
the bacteria to reduce oxygen
molecules to water.
In stage 2, bacteria then
oxidise Fe2+ to Fe3+
(whilst reducing O2).
- Fe2+ ->
Fe3+
They then oxidise the metal to
a higher positive oxidation state.
With the electrons gained from
that, they reduce Fe3+
to Fe2+ to continue the
cycle.
- M3+ -> M5+
The
gold is now separated from the
ore and in solution.
The process for
copper is very similar. The
mineral
chalcopyrite (CuFeS2)
follows the two stages of being
dissolved and then further
oxidised, with Cu2+
ions being left.
Extraction from mixture
Copper (Cu2+) ions
are removed from the solution by
ligand exchange solvent
extraction which leaves other ions
in the solution. The copper is
removed by bonding to a ligand,
which is a large molecule
consisting of a number of smaller
groups each possessing a lone
pair. The ligand is dissolved in
an
organic solvent such as
kerosene and shaken with the
solution producing this reaction:
- Cu2+(aq)
+ 2LH(organic) -> CuL2(organic)
+ 2H+(aq)
The ligand donates electrons to
the copper, producing a complex -
a central metal atom (copper)
bonded to 2 molecules of the
ligand. Because this complex has
no charge, it is no longer
attracted to
polar water molecules and
dissolves in the kerosene, which
is then easily separated from the
solution. Because the initial
reaction is
reversible, and therefore not
a
displacement reaction, it is
determined by pH. Adding
concentrated acid reverses the
equation, and the copper ions go
back into an aqueous solution.
Then the copper is passed
through an electro-winning process
to increase its purity: an
electric
current is passed through the
resulting solution of copper ions.
Because copper ions have a 2+
charge, they are attracted to the
negative cathodes and collect
there.
The copper can also be
concentrated and separated by
displacing the copper with Fe from
scrap iron:
- Cu2+(aq)
+ Fe(s) ->
Cu(s) + Fe2+(aq)
The electrons lost by the iron
are taken up by the copper. Copper
is the oxidising agent (it accepts
electrons), and iron is the
reducing agent (it loses
electrons).
Traces of precious metals such
as gold may be left in the
original solution. Treating the
mixture with
sodium cyanide in the presence
of free oxygen dissolves the gold.
The gold is removed from the
solution by adsorbing (taking it
up on the surface) to
charcoal.
Further Reading
-
BHP Billiton -
[1]
- Bactech -
[2]
- T. A. Fowler and F. K.
Crundwell - 'Leaching of
zinc sulfide with Thiobacillus
ferrooxidans'
