The Chemical Process
To refine aluminium from alumina which contains both aluminium and oxygen, electrolysis is used, in a process known as the Hall-Heroult process. Alumina has to be liquefied (it is mixed with cryolite to reduce melting temperatures) and each cell is supplied with four to six volts and 150,000 amps of electricity. The sequence of creation is:
All pots are covered so that exhaust gases are drawn into a large fume duct (will be explored later). As this involves consumable anodes, the pots cannot be sealed and every time a pot is opened, a small quantity of volatile gases escapes. For the safety and comfort of workers, the potroom needs to be well ventilated and the workers need breathing protection equipment. So much electricity is needed in this process that many smelters are located near hydro-electric plants or have their own power stations such as Alcoa which mines brown coal to generate electricity that meets 50% of its energy requirements. Approximately 16 kWh of energy is needed to produce one kg of molten metal in the Hall-Heroult Process compared to a theoretical energy of 6.34 kWh per kg of molten metal. Two main causes of this are the reoxidation of aluminium metal by carbon dioxide and high temperature, as well as side reactions wasting energy. Computers are now being used to monitor each pot continuously and set a best voltage for any particular condition reducing energy consumption. Covering the top of the anodes with an insulator to prevent them burning off and improved quality of carbon anodes reduces energy loss. Australia’s comparatively has a low consumption of power for each kilogram of aluminium produced. Research is currently being done to find an alternative extraction process and the two main ways are through carbon reduction and electrolysis of aluminium chloride.
The Product
Aluminium, Al a Group III element is the product that results. Its properties and uses were discussed above. Other than the pure substance, alloys are also made containing copper, magnesium, manganese, chromium, silicon, nickel, iron and zinc. Quality control issues include the issue that normal refining processes do not remove all the impurities from aluminium, so most commonly used industrial aluminium already has small amounts of impurities alloyed with it. Fortunately, this makes aluminium stronger but remains easy to bend. Some alloys are less suitable for extrusion than others, requiring higher pressures, allowing only low extrusion speeds and/or having less than acceptable surface finish and section complexity. Aluminium sheets which are rolled from ingots are not flat when produced from the rolling mills. To flatten it, they are stretched between heavy-duty hydraulically-operated grips. A Micro Alignment Telescope with sweep optical square is used to check the flatness within specified tolerance to ensure the quality. One random sample for each batch of the aluminium and its alloy ingots undergo an optical spectrometry analysis to ensure that the results lie within the limits determined by Australian specification. The by-products and waste products of this product are carbon dioxide and fluoride gases as well as sulphur and nitrogen oxides. Carbon dioxide gas is a greenhouse gas and large amounts of fluorides are toxic. Sulphur and nitrogen oxides are acid rain gases. Therefore these need to be controlled and this will be investigated below in Environmental Factors. The aluminium and its alloy ingots or its processed equivalents are packed and distributed through shipping. Approximately 78% of all aluminium produced in Victoria is exported.
Location
Australia has six aluminium smelters located in Tasmania (one), Victoria (two), New South Wales (two) and Queensland (one). However Bauxite mines are in Queensland, Northern Territory and Western Australia. This is because the energy costs are too high to set up a plant near the mines but in Tasmania, Victoria and New South Wales electricity is cheap. As lots of electricity is used in the electrolysis, it is cheaper to transport the bauxite to the smelters than to set up a smelter near the mines and pay higher costs for electricity. Other factors that are taken into account include the labour force available (generally higher near cities), transportation of the aluminium produced (so the smelter has to be near ports, railway stations or highways), geographic factors (smelters are built in residential areas where the land is cheap) and legal factors (whether the sounds and the activities such as the fumes released and the transportation of aluminium using trucks would affect nearby residential properties who could sue).
Environmental
By recycling aluminium, approximately 95% of the energy (approximately 2 billion kWh of electricity) otherwise required to produce the primary metal aluminium can be saved. This makes the aluminium cheaper to manufacture as well as reduce the rapid depletion of non-renewable fossil fuels to produce electricity. When land is mined for Bauxite, active reforestation must be carried out to ensure the stability of the environment as well as ensuring that the soil left over doesn’t erode and cause mud slides. Extreme care must be taken with the handling and disposal of red mud from the refineries. This is usually pumped into dams which are sealed with impervious material to prevent pollution of surrounding countryside. The manufacture of aluminium produces carbon dioxide, a greenhouse gas, fluoride gas, a toxic gas and other exhaust gases (such as the sulphur and nitrate oxides) that can potentially be harmful. To combat their pollution, all pots are covered and the fumes are drawn into a fume duct where the gases pass through beds of alumina which adsorbs over 99%of the fluoride. All cryolite needs to be manufactured synthetically and suitable fluorides are expensive and so they need to be recovered. The gases then pass through dust filter bags and a dry scrubber and treatment facilities to remove the greenhouse and other gases and only clean air is released. Although manufacture of aluminium uses a lot of energy and other options to mine aluminium are being investigated (as shown above), the use of aluminium itself has saved a large amount of energy because they are strong and light. It is estimated that 1,230 litres of petrol was saved in cars that used 64 kg of aluminium instead of other metals and this saved more than five times the energy required to produce each kilogram of aluminium used in the car.
