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Bioleaching Enhanced recovery of Metals |
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Bioleaching
-
Enhanced
Recovery
of
Metals
Since
high
grade
ore
deposits
are
easily
accessible,
these
become
rapidly
depleted.
It
thus
becomes
necessary
to
recover
mineral
resources
from
low
grade
ore
deposits.
However,
no
appropriate
technology
is
still
available
for
recovery
of
metals
from
low-
grade
deposits.
It
is
encouraging
to
find
some
microorganisms
who
could
do
it
efficiently.
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This
potential
of
microbes
could
only
be
realised
recently
and
efforts
are
being
made
to
use
them
for
enhanced
recovery
of
mineral
resources
from
natural
deposits.
Microbes
have
been
used
for
recovery
of
two
important
natural
resources
-
metals
and
petroleum
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It
was
in
1957
that
a
relationship
between
the
presence
of
Thiobacillus
ferrooxidans
and
the
dissolution
of
metals
in
copper
leaching
operation
was
recognised
by
American
microbiologists.
T.
ferrooxidans
and
T.
thiooxidans
are
thermoacidophilic
archaebacteria.
They
are
autotrophs
and
grow
in
acidic
and
hot
environments.
It
has
been
demonstrated
that
these
Thiobacillus
spp.
can
be
used
for
extraction
of
copper
and
uranium
from
insoluble
minerals.
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This
implication
of
microbial
activity
in
weathering,
leaching
and
deposition
of
mineral
ores
could
develop
into
a
recent
field
of
biotechnology
-
biohydrometallurgy.
Biomineralisation
is
the
deposition
of
metals
as
insoluble
oxides
and
sulphides
due
to
microbial
activity
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Microbial
mining
is
the
process
of
bioleaching
recovers
metals
from
ores
that
are
not
suitable
for
direct
smelting
due
to
their
low
metal
content.
Bioleaching
uses
microbes
to
alter
the
physical
or
chemical
properties
of
a
metallic
ore
so
that
the
metal
can
be
extracted.
Metals
can
be
extracted
economically
from
low
grade
sulphide
or
sulphide
containing
ore
by
exploiting
metabolic
activities
of
thiobacilli,
particularly
T.
ferrooxidans.
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Under optimal conditions in the laboratory, as much as 97% of the copper in low-grade ores has been recovered by bioleaching, but such high yields are not achieved in actual mining operations. The process is at present commercially used for recovery of copper and uranium from low-grade ores.
Laboratory experiments could show that recovery of other metals such as Ni, Zn, Co, Sn, Cd, Mb, Pb, Sb, As and Se from their low- grade sulphide-containing ores is also possible through bioleaching. The leaching process can also be used to separate the insoluble lead sulphate (PbSO4) from other metals that occur in the same ore.
The general process carried out by T. ferrooxidans (TJ.) and related species can be shown by the following equation.
MS + SO2 - MSO4 where M is divalent metal. Because metal sulphide is insoluble and metal sulphate usually water soluble, this transformation produces a readily leachable form of the metal. T.f., a chemolithotroph derives energy through oxidation of either a reduced, sulphur compound or ferrous iron.
It exerts its bioleaching action by oxidising the metal sulphide being recovered either directly converting S2- Jo SO4 and or indirectly by oxidising the ferrous iron content of the ore to ferric ion. The ferric ion, in turn, chemically oxidises the metal to be recovered to a soluble form that can be leached from the ore. It is possible to leach the ore in situ without first mining it, if the ore formation is porous and overlays a water-impermeable stratum.
A pattern of boreholes is established with some of the holes used for injecting the leaching liquour and others for the recovery of leachate. More frequently, however, this bioleaching process is used after the ore is mined, broken up and piled in heaps on a water-impermeable formation or on a specially constructed apron. Water is then pumped to the top of ore heap and trickles down through the ore to the apron.
A continuous reactor leaching operation for recovery of copper from its low-grade sulphide ore . The leaching water and ore usually supply enough dissolved mineral nutrients required by T.f., but in some cases NH3 and PO4 may be added. The leached metal is extracted with an organic solvent and then removed from solvent by stripping. Both the leaching liquor and the solvent are recycled.
Copper is generally in short supply. Low-grade copper ore contains 0.1-0.4% Cu. The pregnant leaching solution may contain 1 to 3 g of Cull. In copper leaching operations, Thiobacillus involves both, direct oxidation of CuS and indirect oxidation of CuS via generation of ferric ions from ferrous sulphide, present in most of the important copper ores such as chalcopyrite (CuFeS2)' In the latter case, copper replaces iron i.e. CUSO4 + Fex -+ Cux + FeSO4' In 1980s various firms began to utilise bioleaching for extracting copper.
The recovery of uranium, a nuclear fuel, can also be enhanced by microbial activities, which should help overcoming global energy crisis. Moreover, the current controversies about nuclear plants may also be diluted solved, atleast from economics point of view, if not safety. Insoluble tetravalent uranium oxide (VO2) occurs in low-grade ores.
There is no evidence for direct oxidation. But VO2 can be indirectly converted to leachable hexavalent form (VO2SO4) by T.f., which oxidises ferrous iron in pyrite (FeS), that often accompanies uranium ores. The oxidised iron as an oxidant converts VO2 to VO2. SO4 chemically, which can be recovered by leaching. In Canada, bioleaching was first employed in 1970 for extraction of uranium.
Recovery of copper and uranium through bioleaching depends on several factors, such as type of the geological formations, ore characteristics and prevailing conditions under which the concerned microbe is to grow. Also, oxidative activity of Thiobacillus results into high temperatures, and other bacteria like Sulpholobus (obligate thermophile and acid tolerant) can be useful that can oxidise ferrous iron and sulphur in a manner similar to thiobacilli. Sulpholobus has been used for bioleaching of molybdenite (molybdenum sulphide), whereas Thiobacillus is intolerant of high concentrations of molybdenum, mercury and silver.
Besides
bioleaching,
some
microbes
including
fungi
are
able
to
accumulate
metals
in
their
cells
at
concentrations
higher
than
in
the
surrounding
media.
Such
bioconcentration
has
the
potential
for
extracting
rare
metal
ores
from
dilute
solutions
and
for
recovery
of
metals
(gold,
silver)
from
industrial
effluents.
Rhizopus
binds
uranium
from
low
grade
ores
and
nuclear
wastes.
Theoretically,
microbes
could
be
used
to
recover
gold
from
the
sea.
In
South
Africa
efforts
are
being
made
for
extraction
of
gold
through
bioleaching.
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