Aluminium Smeltering

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Most people take many things around them for granted. They get up in the morning, turn on the light, open their can of peaches then eat the peaches with a spoon. The light goes on magically, the peaches are still fresh in their can to no surprise and it takes little to no effort to lift the light spoon to the mouth. All these things happen or are there thanks to Aluminium. Amazingly enough, aluminium does not magically appear and to be produced lots of raw materials, chemical reactions, money and time are needed.

The process in which aluminium is made involves three complex steps; the mining of bauxite, the refining of aluminia and the smeltering of aluminium.

The first step, mining Bauxite, is planned five years in advance so the areas that are to be mined can be developed. Once mined the Bauxite has to be processed before being shipped to the refineries. In a beneficication plant, fine particles are removed from the Bauxite by screening and washing it. It is then placed on stockpiles before being shipped away.

In the second step aluminium oxide, which is Alumina, is removed from the Bauxite using the Bayer Refining Process. Firstly the bauxite is ground up into fine particles and is mixed with hot, caustic soda (NaOH) solution to dissolve the alumina and some forms of silica. Not dissolved in this process are other elements such as silica, iron and titanium. These are then filtered off.

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The alumina and caustic soda solution is then put into rows of thickener tanks. In the tanks a fine red mud sinks to the bottom as well as the silica, iron and titanium. Remaining alumina trihydrate is then filtered to clean it even more. The solution is then cooled, concentrated and stirred in open top tanks until pure alumina tyhydrate crystals form. To speed up the process pure alumina is added to the mixture to act as a growth site for the crystals. This process can take several days. The alumina trihydrate crystals are then washed and filtered before being heated to 1100 C in kilns, to remove the water molecules. A fine dry powder, alumina is formed, which is then cooled down and stored.

The third step in the process of manufacturing aluminium is smeltering, which is the part of the process, which is done at Tiwai Point in New Zealand at the Comalco Smelter.

To produce aluminium the Aluminium and Oxygen in the Alumina need to be separated. For this reaction to take place the alumina is put into a large, steel furnace, lined with carbon (cathode). These furnaces are known as reduction cells. In the cells the alumina is dissolved in molten cryolite, (NaAl ), which also conducts electricity at approximately 70 C. Electricity is then put into the cells through anodes which are carbon blocks made by smelters. An electric current of about 170 000 amperes is passed through all the cells and the temperature stays at nearly 1000 C. The current flows from the anode, through the alumina/cryolite mixture, the cathode lining and to the anodes in the next cell. The current causes the carbon anode to react with the alumina, forming aluminium and carbon dioxide. The aluminium sinks to the bottom of the cell and sticks to the carbon cathode. It is then siphoned out in a process called tapping and then put in holding furnaces to be cast into products such as pure aluminium or into aluminium alloys.

CROSS SECTION OF POT OF ELECTRIC FURNACE

The diagram below shows a cross section of one of the reduction cells.

It is evident that this process requires many raw materials, which are obtained from many different places from all over the world.

The raw materials required to produce the alumina are Bauxite, Caustic soda, lime, water and electricity. Bauxite, which is ore formed from weathered sedimentary rocks, which contain a large percentage of minerals containing aluminium. The Bauxite is mined in Weipa, on the Cape York Peninsula, Northern Queensland. Here the Bauxite is small, red pebbles or piscolites, approximately 5mm in diameter.

The caustic soda, which is used in the Bayer Process is obtained from the United States, the Middle East and Asia and the electricity, coal, lime and water are found locally. In the refining of the alumina and smeltering of aluminium the raw materials needed are Alumina, Alaska crude oil, coke, pitch cryolite and electricity. The pure Alumina is obtained from Queensland. The crude oil from America and Coal from China and Korea are used to make the coke and pitch required to make the anodes. The Cryolite used to dissolve the alumina is synthetically produces and electricity used for the reduction of alumina to Al is sourced from Lake Manapouri in Fiordland.

During the process a number of chemical reactions take place to transform the raw materials into other things such as Alumina then aluminium.

The key chemical reactions that take place are the dissolving of crushed bauxite in caustic soda, the filtering of Al(OH) then the decomposition . Also the two parallel reactions in which Aluminium is formed.

The filtering of the iron oxide, titania and silica are filtered off using the equations

·Al O + 6NaOH + H O - Na Al(OH)and

·SiO + 4NaOH - Na SiO4 + H O

The insoluble Fe O is insoluble and is filtered out.

Na Al(OH) decomposes into Al(OH) over time which is insoluble and is filtered out.

·Na Al(OH) + H O - NaOH + Al(OH) H O

Decomposed under extreme temperatures to produce alumina.

·Al(OH) HO - Al O + H O

Aluminium is produced using these two parallel reactions.

·Al O + C - Al + CO

·Al O + C - 4Al +CO

The process in which aluminium is made is the same in smelters around the world, although the scrubbing system, which is used at the New Zealand smelter, is currently the best one in the world.

Various waste materials are also produced during the process and they are dealt with in different ways.

In the second step of the process of manufacturing aluminium, during clarification, a red mud is formed which is a waste product. It is washed with water then put into ponds called tailings dams. Also in the process of smeltering the aluminium, carbon anodes, which are used, are recycled by being reformed and used again. Offgasses from the reduction step are cleaned to remove contaminants and are then released into the atmosphere. Primary aluminium, itself, is also recycled by being melted down and forming new products.

Aluminium is a very light and durable metal, which is a very good conductor of heat and electricity. It is easily shaped through moulding and extruding. Its main advantages are that compared with other metals it has a low density and it forms an oxide layer, which prevents it from reacting with oxygen in the air so a surface protection coating is not necessary. Therefore because of its many beneficial characteristics, aluminium is used for many things including uses in the transport, construction, packaging and electrical industries. Aluminium is used in the transport industry because of its lightness, which enables more metal to be used. In the packaging industry it is used for cans and tops, foil, drink cartons and as packaging for pharmaceuticals. It is used for electrical conductors, heat exchangers and cooking utensils as well as roofing and wall cladding. . Aluminium is also made into alloys, which improve its properties. Many alloys are lighter and corrosion resistant.

The aluminium, which is produced at the smelter, is a very economical and environmentally friendly product. It is easily recycled, by being melted and casted into new products, without loosing any of its properties, which means it is very economical. The recycled or secondary aluminium only uses five per cent of the energy used to produce the primary aluminium from Bauxite. Alumimium cans are one of the most important products made by secondary aluminium. Also because of aluminiums light, strong and pliable characteristics it is easily transported and requires less energy to manufacture it into products. This means significant energy cost savings for the industry. When costs of production decrease, supply increases (a little economic theory) and therefore more aluminium products are produced. Another environmental benefit is that lighter cars can be produced from aluminium, which consume less fuel and put out less pollution into the atmosphere.

To produce aluminium a lot people, time, money, raw materials and equipment are needed. In 17 the Tiwai smelter produced approximately 08,000 tonnes of aluminium. The smelter gets shipped over 750,000 tonnes of raw materials to a special wharf opposite Bluff Harbour. More than 700 megawatts of electricity is used at the smelter, which is 5% of New Zealands total generating capacity. There are 00 employees working at the smelter. 660 reduction cells are continuously operating with some 48 cells using technology. The technology is the most efficient in the world.

From all the information above it is clear that aluminium is made in a very complex process involving many peoples’ time and the efficiency of machinery.

Next time you get up in the morning, turn on your light and eat your can of peaches, remember that the electricity for your light is passing through aluminium wires, your peaches are sitting in one of the most important aluminium products and your spoon is probably light thanks to an alloy of aluminium.

REFERENCES

·VIII-Metals-D-Aluminium Production

·www.comalco.com.au

·Beginning Chemistry- Anne wignall & Terry Wales

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