The Production of Aluminium from Bauxite

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Bauxite is a sedimentary rock compound that contains the element aluminium with traces of other minerals eg. Gibbsite, boehmite, diaspore, kaolinite and anatase.

The name bauxite derives from the small village Les Baux in southern France, where the aluminium ore was recognised as containing pure aluminium, the French geologist Pierre Berthier named it in 1821.

1.1 Description of Bauxite ores

There are two main types of bauxite ores:

Karst bauxite ores(carbonate bauxites) and lateritic bauxite ores(silicate bauxites)

The early discovered carbonate bauxites are found predominantly in Europe and Jamaica above carbonate rocks eg. Limestone and dolomite. These form through lateritic weathering and by clay dissolution residues of limestone.

On the other hand, silicate bauxites are found in more tropical and sub-tropical parts of the world. These were also formed by lateritization of various silicate rocks eg. Granite, basalt, shale etc.

The formation of bauxite can be very demanding as they require intense weathering conditions in locations that have very good drainage, so as to dissolve the kaolinite and allow the precipitation of the gibbsite (aluminium hydroxide).

1.2 Information on Bauxite Reserves

In 2007, Australia were the world's primary producers of bauxite, contributing to one-third of the world's production, followed by China, Brazil, Guinea and India. Known reserves of the bauxite ore are sufficient to meet worldwide demands for aluminium. Increasing aluminium recycling can help save on electrical power for further production of aluminium and ensure bauxite reserves are extended and preserved.

Below is a table of worldwide bauxite reserves:

Country

2007

2008

Reserves

Reserve base

Guinea

18000

18000

7,400,000

8,600,000

Australia

62400

63000

5,800,000

7,900,000

Vietnam

30

30

2,100,000

5,400,000

Jamaica

14,600

15,000

2,000,000

2,500,000

Brazil

24,800

25,000

1,900,000

2,500,000

Guyana

1,600

1,600

700,000

900,000

India

19,200

20,000

770,000

1,400,000

China

30,000

32,000

700,000

2,300,000

Greece

2,220

2,200

600,000

650,000

Iran

500

Surinam

4,900

4,500

580,000

600,000

Kazakhstan

4,800

4,800

360,000

450,000

Venezuela

5,900

5,900

320,000

350,000

Russia

6,400

6,400

200,000

250,000

United States

20,000

40,000

Other countries

7,150

6,800

3,200,000

3,800,000

World total

202,000

205,000

27,000,000

38,000,000

X1000 tonne, Numbers for 2008 estimated

Methods for Extraction of Aluminium Metal

Bauxite is usually extracted via strip mining (surface mining) as the ore is almost always found near the surface with little or no overburden.

Approximately 95% of the world's bauxite production is first processed into alumina, and then forming aluminium via electrolysis.

The extraction of aluminium is done in two stages:

2.1 The Bayer Process

The first stage is what's known as the Bayer process, in which the aluminium oxide is extracted from the bauxite.

Firstly, the ore must be purified before it can be refined to aluminium metal, so the bauxite is digested by washing with a hot solution of NaOH at 175oC. This forms aluminium hydroxide, and the alumina dissolves in the hydroxide solution, according to this chemical equation:

Al2O3 + 2OH- + 3H2O => 2[Al(OH)4]-

The other components in bauxite will not dissolve. Therefore the solution is filtered for impurities, which forms a solid impurity called red mud, which is known to be problematic in terms of disposal due to its high pH levels within a range of 10-13 and it is also known to take up excessive land areas and cannot be built on or farmed on.

Next step, the aluminium hydroxide is left to cool in the hydroxide solution and dissolve, which results in a precipitate eventually forming(white and fluffy solid texture).

This precipitate is then heated to a temperature of 1050oC (known as being calcined) and this decomposes to alumina and water vapour:

2 Al(OH)3 => Al2O3 + 3H2O

2.2 The Hall-Héroult Process

The second stage of the extraction is known as the Hall-Héroult process.

This process involves dissolving the alumina in molten cryolite and electrolysing the molten salt bath in order to obtain pure aluminium metal.

The work is carried out on electrolysis pots made of steel box (9.96m length, 3.97m breadth, 1.175m height) lined with a refractory, thermal insulator.

The base of the pots are lined with prebaked carbon blocks and the sides with partially graphitized anthracite in coal-tar pitch. Two rows of 12 anodes were used in each pot. Each anode is made from coke and a pitch binder and each measure 154.94cm length, 72.07cm width and 61.91cm height. The gap between the anodes are 3.78cm and the width of the aisle:

End 33.97cm, side 21.91cm and center 16.51cm.

The cell itself functions at about 4.45V, 216kA and 960oC with a metal level of 10cm and a bath level of 19cm. The composition of the bath consisted of 11.8wt% excess AlF3, 5.5wt% CaF2, 3% Al2O3 and the rest of cryolite.

The process itself is monitored on a computer system using sophisticated controlled software.

The strategy involved includes the control of variables subject to short-term changes eg. Alumina concentration, cell voltage, inter-electrode separation; allowing for slowly changing variables but possessing an overriding system to prevent abnormalities eg. Sludging, anode effects, short-circuiting, accumulation of carbon dust in the bath.

The electric current is passed through the cell, causing an electrochemical reaction in which liquid aluminium is deposited at the cathode as a precipitate, while oxygen from the alumina combines with carbon from the anode to produce CO2. The current through each cell ranges from 220kA to 340kA in electrochemical cells for refining aluminium.

This very high current is supplied through heavy, low electrical resistance metal busbars made of pure aluminium or copper.

The cells are electrically heated to reach the operating temperature with this current, and the anode regulator system varies the current passing through the cell by raising or lowering the anodes and changing the cell's resistance.

The liquid aluminium is extracted with the help of a siphon operating with a vacuum. Then it may be transferred in batches or via a continuous hot flow line to a location where it is cast into aluminium ingots.

These ingots can be up to 9m long and weigh up to 32 tonnes - the weight of six elephants. The ingots can be either cast into the form of final cast-aluminium products, or they can be sent to rolling mills to be pressed into sheets or a wire-drawing mill for producing aluminium wires and cables.

By means of annealing, the ingot can be made ready for hot-rolling, heated to a maximum of 550oC to be cold-rolled later to 0.2mm thickness, with speeds up to 480m per minute. A coil may be up to 2.7m wide and can weigh up to 20 tonnes.

Practical Uses of Aluminium in the Economy

There are many uses for aluminium metal in economical and industrial sectors, as well as in the home.

Aluminium is widely used for window frames, door knobs, kitchen utensils, grills, refrigerators, toasters, kettles etc.

Aluminium is also an important metal in the production of many transportation vehicles, including air, water and road transport.

It is used on parts of aircraft carriers, as it is a lightweight metal. It is also used in making boats and ship construction.

Many car parts consist of aluminium also, eg. Wheels, engine blocks, suspension components, hoods, transmission housings etc.

Aluminium can also be used in the food industry in terms of packaging, eg. Drink cans, bottles, foils and trays are all made out of aluminium.

It is also an essential metal in the construction industry, as it possesses many uses in the sector.

Most construction materials involve aluminium eg. Casting, fabricating, pipes, sheets, tubing, wire, pins, railings etc.

Also, the properties of aluminium are quite unique as it is a lightweight metal, strong and durable, conductive metal, recyclable, ductile and less corrosive than other metals.

It has a melting point of 660oC and a density of 2.7 x 103kg/m3

In conclusion, by using both extraction methods of the Bayer process and the Hall-Héroult process, it is very useful and essential in obtaining pure aluminium metal from the raw bauxite ore.

Both methods are very useful and can extract the metal in an efficient manner, as the Bayer process helps to purify the overall solution first, before the Hall-Héroult process separates the mixture into its separate components via electrolysis in a cell pot.

As a result of the two stages in the experiment, aluminium is now renowned as a very common metal and can be found in large quantities worldwide, as there is according to worldwide statistics of bauxite reserves, there are approximately 38 million tonnes known worldwide.

Aluminium possesses a number of properties that make it an extremely useful engineering material. The advantages of the metal is that it has good corrosion resistance and low density, which are suitable for applications in the field of transportation - land, sea and air.

Aluminium can also be alloyed easily to help strengthen the overall metal. With its face centred cubic crystal structure, aluminium is very ductile and thus can be easily shaped into sheets or wires in the manufacturing industry.

Aluminium can also be a useful metal in the food industry, as it is used in fresh food packaging and manufacturing kitchen appliances.

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