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Metal Extraction
Metal Ores Metals are usually found in ores
containing minerals They are combined with oxygen or
sulfur to form oxides or sulfides Sulfide ores are converted to oxides
by heating them in the air, which is called ‘roasting’, e.g. zinc sulfide to zinc oxide
ZnS(s) + 1½O2(g) ZnO(s) + SO2(g)zinc
sulfideoxygen zinc
oxidesulfur
dioxide
Environmental Problems When roasting sulfide ores, sulfur
dioxide is produced as a waste product This is a contributor to acid rain if it is
allowed to escape into the atmosphere as it reacts in the air to form sulfuric acid
However it can be converted to sulfuric acid in controlled conditions which can then be sold for a variety of purposes
SO2(g) + H2O(g) + ½O2(g) H2SO4(l)sulfur dioxide
oxygenwater sulfuric acid
Extraction of Metals To obtain the metal its ore, a reducing
agent is used to remove the oxygen, in a reduction reaction.
Examples of some reducing agents; Coke (an impure form of carbon) Hydrogen (made from methane and
water) A more reactive metal may be used. Electrolysis can also be used for more
reactive metals.
Reduction of Metal Ores For the extraction of iron, manganese
and copper the reducing agents are carbon and carbon monoxide.
These are cheap because coke can be used which is cheaply produced by heating coal in the absence of air.
The extraction of iron is done in a blast furnace which reaches temperatures of 2000K.
Fe2O3(s) + 3CO(g) 2Fe(l) + 3CO2(g)Haematit
e
Carbon Monoxide
Iron Carbon Dioxide
Reduction of Metals Manganese is produced by the reduction of
its oxide with carbon. 2MnO(s) + C(s) 2Mn(l) + CO2(g)
Copper used to be produced this way but nowadays some ores are converted in to solutions containing Cu2+ ions. The copper is extracted by reducing with scrap iron.
Cu2+(aq) + Fe(s) Cu(s) + Fe2+
(aq)
This is ‘greener’ as no CO2 is produced and scrap iron is cheap and readily available.
Also, the solution can be made from low grade ore so it is readily available
Extracting Other Metals Some metals can’t be extracted
using carbon reduction, such as aluminium, titanium and tungsten.
Aluminium can’t because it is more reactive than carbon.
Titanium and tungsten can’t because it would form a metal carbide which would make the metal brittle.
Extraction of Aluminium Aluminium is extracted from purified
bauxite ore (mainly Al2O3) The oxide is dissolved in molten
cryolite, which forms a solution which melts at around 1240K (Al2O3 melts at 2435K therefore less energy is required
The solution is electrolysed so the main cost of the process is electricity which makes economic sense where cheap electricity is available
Extraction of Aluminium Aluminium is produced at negative
electrode 2Al3+ +6e- 2Al Oxygen is produced at the positive
electrode 3O2- 1½O2 + 6e-
This gives an overall equation of: Al2O3(l) 2Al(l) + 1½O2(g)
Extraction of Titanium As titanium can’t be reduced with carbon,
it is reduced by sodium or magnesium This is an expensive process as the
titanium oxide is first converted to titanium chloride by reacting it with coke and chlorine at 1173K
TiO2(s) + 2C(s) + 2Cl2(g) TiCl4(l) + 2CO(g)
The titanium chloride is then reduced by sodium or magnesium into titanium under an inert argon atmosphere at 1300K
TiCl4(l) + 4Na(l) Ti(l) + 4NaCl(l)
Extraction of Tungsten Tungsten is extracted from its oxide,
WO3, by reduction with hydrogen at high temperatures
WO3 + 3H2 W + 3H2O However, there is some risk as
hydrogen is a flammable gas, so using it as a reducing agent is a last resort.
Effects on the Environment Recycling scrap metals has many
environmental and economic advantages It reduces the amount of scrap metal in
landfill and it has already been extracted from its ore so energy isn’t required again
Melting the scrap metal does not in itself produce carbon dioxide unlike extracting metals from their ores however the energy needed to melt the metal will