TOP OF PAGE and
metal extraction index
4b.
The future for copper
mining and other sources of metals?
How is copper extracted by phytomining and bioleaching?
-
Introduction to alternative
biological
methods of extracting metals:
-
Such methods are needed because
the Earth’s resources of metal ores are limited.
-
Rich high grade ores will be more
rapidly used up than low grade ores, limiting sustainability
- unless we find ways of using low grade ores or waste from mining
high grade ores.
-
Traditional large scale copper
mining is damaging to the environment and produces huge amounts of
waste, but these methods have less impact on the environment and can
make good use of waste containing residual copper, but the processes
of bioleaching and phytomining are unfortunately slow.
-
Finite reserves of copper ores
are becoming scarce and new ways of extracting copper from low-grade
ores include phytomining, and bioleaching.
-
These two methods avoid
traditional mining methods of digging, moving and disposing of large
amounts of rock - they make finite copper resources last longer and
reduce the impact on the environment.
-
Phytomining uses plants to
absorb metal compounds and the plants are then harvested and burned
to produce ash that contains a more concentrated raw material of the
metal compounds. From this ash the metal can then be extracted.
-
Bioleaching uses bacteria
to produce leachate solutions that contain metal compounds. The
metal compounds can be processed to obtain the metal e.g. copper can
be obtained from solutions of copper compounds by displacement using
scrap iron or by electrolysis (experiments, hopefully you will have
done).
-
Copper–rich ores are
being depleted and traditional mining and extraction have major
environmental impacts, so there are important issues involved with
the future exploitation of copper ore reserves.
-
Because of these issues,
new ways of extracting copper from low–grade ores (eg containing ~1%
copper) or waste material from the mining operations, are being researched to limit the environmental impact of
traditional mining.
-
For example as mentioned above, copper can
be extracted by phytomining, or by bioleaching.
-
Phytomining
Phytoextraction ('mining with plants',
-
Extracting copper in this way
is a commercial example of phytoextraction.
-
Phytomining uses growing plants in soil to absorb metal compounds.
-
Such plants cannot always get rid of the copper ions and they
build up in leaves.
-
These plants
naturally absorb copper compounds through their roots as they feed
on the nutrients around them and because they can't always get rid of the
excess of certain metals like copper, this results in higher concentrations of
these copper compounds in the plant tissues e.g. leaves.
-
The plants are then cropped (harvested),
dried and burned in a furnace
to produce an ash that contains the metal's soluble compounds which
can be extracted.
-
The combustion process
concentrates the metal in the ash.
-
The bigger the plant and the
faster it grows, the greater the yield of metals like copper.
-
The ash
is dissolved in hydrochloric acid or sulfuric acid and the copper can be
extracted by electrolysis
or more cheaply by displacement of the copper with scrap iron.
e.g.
-
iron + copper(II) sulfate ==>
iron(II) sulfate + copper
-
Fe(s) + CuSO4(aq) ===> FeSO4(aq) +
Cu(s)
-
In this extraction process,
the copper metal is precipitated out of the copper sulfate
solution, the iron dissolves in the reaction forming iron
sulfate solution.
-
In terms of oxidation and
reduction the ionic redox equation is:
-
You should be able to
work out the two half equation:
-
iron atoms are oxidised
by losing 2 electrons: Fe ==> Fe2+
+ 2e-
-
copper ions are reduced
by gaining 2 electrons: Cu2+ + 2e-
==> Cu
-
Bioleaching 'bacteria
extraction' (bacterial extraction with
'rock eating bacteria'!)
-
Apparently 10% of copper in the US comes
from bacteria which live off the surrounding rocks.
-
You can use low grade ores or
waste material from the higher ore grade mining operations.
-
Bioleaching uses
bacteria (bacterial microorganisms) to produce leachate solutions
that contain soluble copper compounds that can be processed to
extract the copper.
-
Some bacteria naturally
absorb copper compounds as they chemically interact with the
surrounding mineral rocks to as an energy source and free the metal
from the ore to form copper ions.
-
ie it is a copper leaching effect with respect to
the surrounding rock material as they break down ores like
chalcopyrite (CuFeS2).
-
From the bacterial discharges you can
produce solutions (blue leachates of copper ions), which contain soluble copper
compounds in commercially viable concentrations.
-
Again, the copper
can be extracted by
electrolysis or more cheaply by
displacement of
the copper with scrap iron - as described in phytomining
above.
-
In electrolysis, the copper
ions are reduced and deposited on a negative copper cathode
electrode.
-
Cu2+(aq)
+ 2e- ===> Cu(s)
-
Advantages and
disadvantages of phytomining and bioleaching to extract copper
-
Neither phytomining or
bioleaching is fast
so it isn't always economical to use this slow technique for extracting
copper.
-
In phytomining the plants are
slow to grow and in bioleaching the biochemistry is relatively
slow.
-
Less energy is needed -
smaller carbon footprint.
-
There is less damage to the
environment - lower impact as low grade ores/waste does not have
to be mined in the same way - wildlife habitats are less
affected.
-
These methods might be
useful in developing countries where the huge capital investment
required to build complex smelting furnaces would not be available.
-
'Advanced' technical
notes on bioleaching
-
The microorganisms
essentially catalyses processes that occurs naturally eg the copper
sulphide minerals like chalcopyrite (CuFeS2) are
oxidised to a solution of copper(II) Cu2+,
iron(II)/(III) Fe2+/Fe3+ and sulfate ions SO42–.
-
The optimum conditions
for these bacteria is pH 2–3 and 20oC to 55oC.
These bacteria occur naturally so it is possible to spray dilute
acid on low grade ores, OR, spray the dilute acid onto waste
material from the mining process to try to get any remaining copper
from copper bearing rocks.
-
The aerated acidified
water slowly percolates through the pieces of broken rock and the
colonies of the useful bacteria establish themselves quite naturally
in this acidic environment.
-
The bacterial leachings
are dilute and impure but, after filtration, the copper can be
recovered, usually by displacement with cheap scrap iron.
-
The supply of
copper–rich ores is limited so it is important to recycle
as much copper as possible especially as demand for copper is
growing as the economies of African countries, India, China and
Brazil etc.
-
These rapidly developing and becoming increasingly
industrialised countries will experience increased consumer demands for all the e.g.
electrical products that we in the West take for granted.
|
How
is copper extracted? how is copper purified? What is the state of copper
ore reserves?
COPPER,
Cu 
Copper is used in electrical wiring because it is a good conductor of
electricity but for safety it is insulated by using poorly electrical conductors like PVC
plastic.
gap–word–fill exercise on METAL EXTRACTION
Reactivity
of metals
: all the reactions of metals with oxygen (air), water and acids,
displacement reactions
Enter chemistry words e.g. topic, module, exam board, formula, compound, reaction,
structure, concept, equation, 'phrase', homework question! anything
of chemical interest!