Extraction of aluminium from bauxite

What is bauxite ?

Bauxite(above) is an aluminium ore and is the main source of aluminium.This form of rock consists mostly of the minerals gibbsite (aluminium hydroxide), 2 aluminium oxide hydroxides (boehmite and diaspore) in a mixture with the two iron oxides (goethite and hematite), the clay mineral kaolinite, and small amounts of anatase(one of the mineral forms of titanium dioxide).

Extraction process 
 

Bayer-process

Firstly, crushed bauxite is treated with moderately concentrated sodium hydroxide solution. The concentration, temperature and pressure used differs based on the source of the bauxite and the form of aluminium oxide it contains. Temperatures are generally range from 140°C to 240°C; pressures can be up to about 35 atmospheres.

High pressures are required to keep the water in the sodium hydroxide solution liquid at temperatures above 100°C. (The higher the temperature, the higher the pressure needed.)

With hot concentrated sodium hydroxide solution, aluminium oxide reacts to give a solution of sodium tetrahydroxoaluminate(as shown below).

Al2O3 + 2 NaOH + 3 H2O → 2 NaAl(OH)4

The impurities in the bauxite remain as solids. For example, the other metal oxides present tend not to react with the sodium hydroxide solution and so remain unchanged. All of these solids are separated from the sodium tetrahydroxoaluminate solution by filtration. They(the residue) form red mud which is just stored in huge lagoons.

The sodium tetrahydroxoaluminate solution is cooled with the addition of some previously produced aluminium hydroxide being added. This provides something for the new aluminium hydroxide to precipitate around as a white, fluffy solid.

NaAl(OH)4 → Al(OH)3 + NaOH

Aluminium oxide (also known as alumina) is made by heating the aluminium hydroxide to a temperature of about 1100 - 1200°C.

2 Al(OH)3 → Al2O3 + 3 H2O

Hall–Héroult process

Secondly, in the Hall–Héroult process alumina, Al2O3, is dissolved in an industrial carbon-lined vat of molten cryolite, Na3AlF6,sodium hexafluoroaluminate, called a "cell". 
-Aluminium oxide has a melting point of over 2,000 °C (3,630 °F) 
-Pure cryolite has a melting point of1,012 °C (1,854 °F). 
-With a small percentage of alumina dissolved in it, cryolite has a melting point of about 1,000 °C (1,830 °F). 
-Some aluminium fluoride, AlF3 is also added into the process to reduce the melting point of the cryolite-alumina mixture.
(Note that Aluminium cannot be produced by the electrolysis of an aluminium salt dissolved in water due to the high reactivity of aluminium with the protons of water and the subsequent formation of hydrogen. )

The mixture then undergoes electrolysis. Direct electric current is passed through the mixture. the electrochemical reaction causes liquid aluminium metal to be deposited at the cathode as a precipitate. Oxygen from the alumina combines with the carbon from the anode to produce carbon dioxide. The carbon electrodes are then continuously replaced due to the oxidation of carbon.

The liquid aluminium is taken out with the help of a siphon operating with a vacuum(in order to avoid having to use extremely high temperature valves and pumps). It is then cast into aluminium ingots. 


 Impact on the environment 

Water pollution

Red mud, a result of the extraction, cannot be disposed of easily. Hence,  it is usually pumped into holding ponds as red mud takes up land area and can neither be built on nor farmed, even when dry. Red mud is a strong alkali with a pH ranging from 10 to 13. If accidents happen and the red mud leaks into rivers, animals and plants will not be able to survive in the extreme alkali conditions and die.


Loss of landscape

Due to the area a chemical plant needs and in the production and transport of the electricity, much land has been used up to build the factories and roads. In the process of clearing the land for usage, deforestation occurs which can lead to global warming from the burning of trees to clear forested areas to produce lots of carbon dioxide. Also, many habitats will be destroyed in the process(loss of plants) which may lead to the loss of animal species.

Using up natural resources

To produce the electricity needed for the chemical plants, fossil fuels are used as they are energy sources. However, fossil fuels such as coal, oil and natural gas are non-renewable resources. They take millions of years to be formed in the crust of the earth by natural processes. Once burned to produce electricity, they are loss forever.

Atmospheric pollution

Carbon dioxide which is a greenhouse gas is produced from the oxidation of the anode, carbon. Increase of carbon dioxide leads to global warming causing the temperatures to increase, and leading to the melting of icebergs, causing natural disasters such as floods to occur more often.

Solutions to impact 
  
Recycling aluminium as it only 5% of the energy used to extract aluminium and reduces the greenhouse gas (C02) being produced.

Planting of trees in the forest back after strip mining to encourage growth ,survival of animals
and reformation of habitat for animals.

Use non-fossil-fuel sources, such as nuclear, geothermal, hydroelectric, or solar to produce energy needed for electrolysis.