The metal Aluminium comes third after oxygen and silicon on importance on this planet. The aluminum industry in Australia is its economic backbone as it is one of the major products produced in the country and is a direct source of income to over 16,000 Australians. That is why I have chosen to discuss about Alcoa Australia, which is an important industry in supplying aluminium to the world. Alcoa functions as one of the renown integrated “bauxite mining, alumina refining, smelting and rolling system in Australia.” The manufacturing industry is a means of livelihood for over 6000 people (Alcoa in Australia, 2009).
Alcoa of Australia functions in the mines, refineries and smelters, while its sister company Alcoa Australia Rolled Products functions in the rolled products industry. The process of manufacturing aluminium commences in Western Australia where the company has “Huntly and Willowdale bauxite mines in the Darling Range south of Perth. “ These bauxite mines sites provide the raw material for manufacturing aluminium, bauxite, to Kwinana, Pinjara, and Wagerup alumina refineries. Bauxite is composed of about 45-60% aluminium oxide, 12-30% water, and other contaminants (Metals Advisor, n.d, para.1).
They refine bauxite to give yield to alumina (aluminum oxide) before it is turned into aluminum through the process of electrolysis. This involves incorporation of the Bayer Chemical process in the refineries. The alumina is extracted from the other impurities in bauxite in a solution of caustic soda that is then sieved to get rid of all insoluble components (European Aluminium Association, 2009). The alumina then precipitates from the soda solution, is cleaned in water and allowed to dry. Recycling of the caustic soda takes place. After calcination, a fine-grained white powder of alumina is formed.
Alcoa Australia mines bauxite from open pits and transported via road to the neighbouring refineries, and then transported to be smelted in Victoria, at Point Henry in Geelong and Portland. The open pits are a major environmental hazard as they clear the local vegetation, influence plants and animal life, and facilitate the process of soil erosion. Alcoa Australia has put in place special precautionary measures to overcome these hazards. The company employs water drainage practices in the course of extraction to ensure that soil erosion is minimized and flora is taken back to the virgin land. Either the top soil is re-used to fill the pits. The cost of rehabilitating the land constitutes part of the overall cost of mining. As a proof that Alcoa Australia is committed to environmental sustainability, it became the first mining industry to receive the coveted Global 500 Roll of Honour by the United Nations Environmental Program.
The Victorian Aluminium smelters of Alcoa constitute about 30% of the total aluminium manufactured in the country. The process of smelting alumina (aluminium oxide) that divides it into its constituent components of aluminium metal and oxygen gas by electrolytic reduction is referred to as the Hall-Heroult smelting continuous process (Alcoa of Australia Limited, 1998, p.3). During the process, “aluminium oxide is dissolved in cryolite bath material (sodium aluminium fluoride) in electrolytic cells referred to as pots and with oxidation of the carbon anodes.” A high electric current maintains the cryolite bath in the molten state since it resists current flow. The pot generally operates at temperatures of 920°-980°.
Aluminium formed is taken out through electrolysis. It is then frequently taken out for consequent casting process. In every pot, direct electric current flows following the following sequence: it passes from carbon anodes, through cryolite bath, to carbon cathode, and then to the carbon anodes of the subsequent pot. The gaps between the anodes in the pot are filled with the electrolyte that is made up of molten cryolite having dissolved aluminium oxide. Cryolite (Na3AlF6) is the best electrolyte in this process because its melting point is satisfactorily low, it has low vapour pressure, lower density than that of molten aluminium, and it is more resistant to reaction with either carbon or aluminium.
As the reaction continues, a solid crust constantly forms on top of the electrolyte. The solid crust is routinely severed and the aluminium oxide is constantly swirled to sustain its concentration. Aluminium is a little bit denser than the cryolite bath, hence it constantly forms deposits in a metal pool at the underneath the pot. Carbon dioxide is vigorously produced as oxygen reacts with the carbon anodes. The anodes are consequently being depleted in the process, therefore they are constantly lowered deeper into the bath and ultimately replaced on routine basis. The quantity of alumina lowers as the process of electrolysis continues. To maintain its desired level, alumina is added intermittently. Failure to do this may result in a condition called the “anode effect” whereby a gas film is formed around the anode. This film increases the current resistance and leads to high consumption of electricity. Correction mechanisms for this effect either consist of automatic or manual techniques that raise aluminium concentration in the cryolite bath. This practice is important as it results in process benefits and lowers the production of perfluorocarbons green house gases into the atmosphere.
The control bath composition is vital in the manufacture of aluminium. Pure cryolite melts at 1009°C, fluorspar, excess aluminium fluoride, and the dissolved aluminum oxide, lower the melting temperature to enable the reaction to take place at 920°C to 980°C as it enhances the workability of the pot. In the process of electrolysis, the cryolite takes part in the reactions but it is not depleted in the process. Therefore, the overall reaction can be equated as follows:
2Al2O3(dissolved) + 3C(s) → 4Al(l) + 3CO2(g)
In estimating of the daily cell production, the following equation is used:
Production per cell per day: (Current × Current Efficiency/100 × 0.008054) kilograms.
The major product in this reaction is Aluminium metal that deposits underneath the series of pots. The aluminium may be cast into ingots for sale or withheld in the furnaces for casting into various aluminium products. The process of manufacture of aluminium gives yield to byproducts such as wastes of fluoride in form of perfluorocarbons and hydrogen fluoride gases, and sodium and aluminium fluorides and the leftover cryolite. “The volatilized fluorides and gaseous hydrogen fluorides are accumulated with the other gaseous compounds produced from the series of posts by gas-collecting hoods or manifolds and are then channeled through ducts to central gas treatment and recovery facilities.” This technique protects the atmosphere from pollution and preserves important materials for recycling.
The fluoride gases are separated from the main product by channeling them through a bed of aluminium oxide where fluoride is soaked up. The particulate material is then placed in a fabric filter bag-house. The product (alumina having fluoride) is taken back to the main processing line of manufacturing aluminium. The fluoride gases are used for making uranium, and other fluorochemicals. According Alcoa’s website, it manufactures about 47% and 30% of Australia’s alumina and aluminium respectively. The company’s alumina production represents 11% of the worldwide demand.
The people working in the pot rooms are usually exposed to various hazards. Volatile chemical compounds from baking anodes in the Soderberg process, exposure to the fluorides gases, and other dusts and gases including carbon monoxide and sulphur dioxide are potential health hazards. Physical hazards include prolonged exposure to magnetic fields, noise pollution, and exposure to radiant energy. Carrying out efficient work and transportation practices reduces the hazards.
References
Alcoa in Australia, 2009. About Alcoa. Web.
Alcoa of Australia Limited, 1998. Smelting students notes. Web.
European Aluminium Association,2009. Production Process. Web.
Metals Advisor, n.d. Bauxite mining. Web.