Biohydro Metallurgy Essay

1240 words - 5 pages

Biohydro-metallurgy is an application used in mining to extract
certain metals from their ores using specialised bacteria such as
Thiobacillus ferro-oxidans which metabolise certain substances in the
ore to gain the energy they need to live, which breaks it down and
releases the metal ions into solution from which they can be
extracted. The bacteria require very little else to perform this task,
as they require no organic nourishment, apart from a slightly acidic
environment, access to carbon dioxide and oxygen from the air and an
ambient temperature of around 30°c for optimum performance.

Thiobacillus ferro-oxidans oxidise both Fe² and S² ions allowing the
insoluble minerals of copper, zinc and lead to be broken down as they
also contain S² ions. The process can be demonstrated by the oxidation
of arsenopyrite seen in the equation below

2FeAsS + 7O + 4H + 2H O ® 2Fe + 2H AsO + 2HSO The bacterial oxidation
occurs in two stages, the first involving reactions at the boundary
between the surface of the bacterial cell and the asenopyrite, where
the bacteria catalyse the formation of soluble compounds of iron(II),
arsenic (III) and sulphur(VI)

The second stage consists of two separate reactions in whish iron(II)
and arsenic (III) are oxidised.

A prime example of biohydro-metallurgy is the extraction of copper,
often used as a recovery technique, extracting the metal from
low-grade ores that are the waste product of any conventional mining.
The ore is piled up in an area where the ground has been made
impermeable and sprayed with an acidic leaching solution containing
Thiobacillus ferro-oxidans and Thiobacillus thio-oxidans. The result
of the process is conversion of the sulphide minerals in to a solution
containing Cu , Fe , Fe , and SO ions. The impermeable base allows the
solution to drain away where the copper is removed using another
solvent containing ligand compounds which bind with the copper ions,
to form copper molecules which are then extracted as sheets by the
process of electro-wining where an electric current is passed through
the copper ion solution and the metal collects at the negative
electrodes. The remainder of the leaching solution then flows into a
pond where Thiobacillus ferro-oxidans catalyse the oxidation of the Fe
and the Fe ions. This recharges the leaching solution, which can then
be used again.

This is a representation of the ligand reaction;

Cu (aq) + 2LH (organic) CuL (organic) + 2H (aq)

Another example of biohydro-metallurgy is the extraction of gold;
using a thermophillic bacterium called Sulpholobus acidocalderius in
reactions at 70°c has increased the recovery of gold from its ore from
10% to 100%. The only problem with this form of bio hydrometallurgy is
that the...

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