Metal Extraction
Blast Furnace
This is an industrial process to reduce iron oxides to obtain iron. It is a continuous process where iron (III) oxide (Fe2O3), coke and limestone are fed into the top, and heated by having hot air blown in near the bottom. There is a whole series of reactions taking place in the blast furnace (for a diagram see here).
First of all, the coke reacts with the oxygen in an exothermic reaction to form carbon dioxide, which then reacts with further coke to make carbon monoxide which is the reducing agent.
C + O2 ® 2CO2
CO2 + C ® 2CO
Now at about 1200°C carbon monoxide reduces most of the iron (III) oxide as follows:
Fe2O3 + 3CO ® 2Fe(l) 3CO2
But then what is the purpose of putting the limestone in the blast furnace, since we haven't used it yet. It decomposes to form carbon dioxide (which can become carbon monoxide for the above reactions) and calcium oxide and this removes some sand impurities as slag, which is used to make construction materials.
CaCO3 ® CaO(s) + CO2
CaO + SiO2 ® CaSiO3 (l)
The problem with the iron that is produced from the blast furnace is that it is quite impure so it has to be treated to remove these impurities. Firstly, magnesium is added to the mixture which reacts with sulphur to form MgS
Next, oxygen is blown into the the molten mixture with lime added in the Basic Oxygen Process this forms oxides of phosphorous, manganese etc. which react with CaO to make a slag that floats to the top. However, not all carbon is removed, but this is good because pure iron is too soft and a bit of it makes it stronger but not so much that it is brittle.
It is not possible to extract all metals by this process because rather than form pure metal they react to form carbides. For example titanium which reacts with carbon to make carbode:
TiO2 + 3C ® TiC + 2CO
Electrolysis
Aluminium is extracted from Bauxite by electrolysis. This is done by dissolving it in molten cryolite, this lowers the required heat to melt Aluminium Oxide from 2000°C to 970°C. At the cathode, aluminium is reduced and at the cathode oxygen is oxidised in the following reactions.
Al3+ + 3e- ® Al
2O2- ® O2 + 4e-
In terms of the economics, this process requires alot of electricity which means it is only done in places where using large amounts of electricity is cheap.
The process also causes environmental damage because some the oxygen produced, reacts with the anode (which is carbon) to form carbon monoxide and carbon dioxide. And the waste cryolite could contaminate can cause fluoride pollution.
Titanium
Titanium is a very useful metal, but unlike iron, having any carbon in it ruins the strength of Titanium, therefore titanium is extracted from a chloride. But the Ore is Titanium Oxide (TiO2) also known as rutile. This is converted into titanium (IV) chloride using chlorine and coke at 900°C in the following reaction.
TiO2 + 2C + 2Cl2 ® TiCl4 + 2CO
Titanium Chloride is a colourless liquid and is then purified by fractional distilation. And now to extract the titanium it is reduced by sodium (or magnesium) at 500°C, but since the reaction is exotrhermic, the temperature eventually gets up to 1000°C. The reaction is also performed in an atmosphere of argon because it is a noble gas and doesn't react. This is to prevent contamination with oxygen or nitrogen.
TiCl4 + 4Na ® Ti + 4NaCl
It is called a batch process because it isn't continuous, and is very expensive because of the extra chemicals that have to be added, the high temperatures, the danger of TiCl4 and the argon atmosphere that has to be maintained. And it is for this reason that despite its properties being more desirable than steel and its larger abundance than iron, it is only used in circumstances when nothing else will do because of the cost.
Recycling
Especially in the UK where landfill sites have only a few years left but around the world as well, recycling is being used more and more to prevent wastage and carbon emissions. But here we look at the science and reality of recycling.
Iron and Alumnium are an example of two metals that are extensively recycled, your drinks cans are usually made of one of these.
Iron is recycled in a number of ways. A large amount is added to the BOS process before the impure iron is added, but this produced carbon dioxide. Another method, which is much easier and also doesn't directly produce any carbon dioxide is to simply melt the scrap.
Aluminium on the other hand is very often a more desirable metal to recycle, because of the expensive of extracting it from its ore, and melting cans uses only 5% of the energy of extracting the equivalent amount. And the quality of this metal is often very good. However, there is an energy cost of collecting and sorting all of the rubbish which also has to be considered before deciding whether recycling or extraction is more efficient.