A-level Chemistry/AQA/Module 2/Extraction of Metals

Reduction of Metal Oxides Using Carbon

Extraction of Iron: Iron, Fe, is extracted from its metal oxide in the Blast Furnace using Carbon as a reducing agent. To the Blast Furnace, the following 'ingredients' are added: Hematite (Fe₂O₃), Limestone (CaCO₃) and Coke (C). Hot air is also blown into the furnace which reacts with the Carbon in a highly exothermic reaction, producing much of the heat required to keep the mixture molten.

In the first stage of the process, the coke reacts with oxygen in the air to form carbon dioxide. This carbon dioxide reacts further with more coke, producing carbon monoxide as another reducing agent. Finally, the Haematite compound reacts with the carbon monoxide, producing iron and carbon dioxide.

$C+O_{2}\rightarrow CO_{2}$

$CO_{2}+C\rightarrow 2CO$

$Fe_{2}O_{3}+3CO\rightarrow 2Fe+3CO_{2}$

The Hematite compound contains a number of impurities, including silicon dioxide, and this is where the use of calcium carbonate comes in. Calcium carbonate decomposes in the high temperatures into calcium oxide and carbon dioxide. The calcium oxide reacts with the silicon oxide impurities forming a calcium silicate, more commonly known as slag.

$CaCO_{3}\rightarrow CaO+CO_{2}$

$CaO+SiO_{2}\rightarrow CaSiO_{3}$

This slag floats on top of the molten iron due to its lower density, and can be scraped off the top and used in the construction industry.

Iron Impurities The iron produced directly from the Blast Furnace still contains a number of impurities which render the iron unfit for some uses. These impurities can be removed in the Basic Oxygen Converter as follows:

Phosphorus impurities are removed by blowing air through the molten iron
$P_{4}+5O_{2}\rightarrow P_{4}O_{10}$

Lime (CaO) then reacts with the phosphorus oxides.

Sulphur impurities are removed by adding Mg. Magnesium is used instead of oxygen to prevent the formation of SO₂, which is a highly toxic gas and leads to the formation of acid rain.
$S+Mg\rightarrow MgS$

Reduction Using Electrolysis

Some metals are above carbon in the reactivity series, and therefore can not be extracted from their ores using carbon reduction methods.

Despite Aluminium being the most abundant metal on earth, it is a relatively expensive material, owing to the fact that the extraction of aluminium from its ore, Bauxite, is a costly process.

First, the Bauxite ore is purified to produce aluminium oxide as a white powder. In order to electrolyse the aluminium oxide the mixture must be melted so that the ions are free to move and carry electrical charge. Aluminium oxide has a very high melting point (over $2000^{o}C$), and therefore melting this compound would prove extremely costly - instead, an aluminium-containing compound is added, cryolite, which lowers its melting point and thus reduces some of the energy costs involved with the process.

Once the solution is melted the aluminium cations ($Al^{3+}$) and the oxygen anions ($O^{2-}$) are free to move to the electrodes. At the Anode (+ve charge) the oxygen ions each lose 2 electrons to become molecules of oxygen gas. At the Cathode (-ve charge) the aluminium ions each gain 3 electrons to become atoms of aluminium with no charge - they fall to the base where molten aluminium metal is collected.

Anode:
$2O^{2-}\rightarrow O_{2}+4e^{-}$

(Here, the oxygen slowly reacts with the carbon electrodes to form carbon dioxide, which causes the carbon to slowly burn away. Consequently the anodes need replacing periodically.)

Cathode:
$Al^{3+}+3e^{-}\rightarrow Al$

Extraction of Titanium

Titanium is one of the most abundant metals in the earth's crust and has some extremely useful applications and properties. Despite this, it is highly expensive due to the great costs of extraction, which can be explained by looking at the process by which titanium is extracted.

The first stage of this process involves the reaction between titanium oxide, TiO₂, chlorine, Cl₂ and coke, C at $900^{o}C$:

$TiO_{2}+2Cl_{2}+2C\rightarrow TiCl_{4}+2CO$

The titanium chloride produced is then reacted with either sodium or magnesium under an inert argon atmosphere at $500^{o}C$:

$TiCl_{4}+4Na\rightarrow Ti+4NaCl$
(The reaction is exothermic and thus the temperature increases to approx. $1000^{o}C$)

Once the reaction is complete and the mixture left to cool (for a few days, an obvious inefficiency), the mixture is washed with HCl to remove the sodium chloride.

Why is it so expensive? The above process is very costly. First of all it is carried out in a batch process, which renders it much more expensive compared to a continuous process. In both stages of extraction there is also a need for relatively high temperatures which result in high energy costs, and in stage 2 an argon atmosphere must be maintained to prevent oxidation of the titanium-containing product. Sodium/magnesium is also employed which must be initially produced, adding further costs to the process.

Some economies can be made by recycling the products from the reactions. The $NaCl$ produced in the final stage can be separated by electrolysis into sodium and chlorine, used in the first and second stages respectively.

Economic Issues & Recycling

Both iron and aluminium are readily recycled. Recycling these metals prevents the waste of raw materials and also saves on the high costs of extraction, particularly the high costs associated with aluminium extraction. Recycling also cuts down on the waste we produce and dump, thereby preventing unsightly scrap metal piles etc. Conversely there are some other financial issues that are associated with the recycling of metals; the waste must be transported, sometimes over long distances, to reach a site where the metal can be melted down and re-used. There are also costs associated with the melting of the scrap material etc.

The choice of which reductant method used is a balance between the reductant cost, the energy requirements for the particular process and the required purity of the metal.