Aluminium is produced from bauxite. Al2O3. This is referred to as Alumina, or aluminium oxide is extracted from the ore. After the extraction, electrolytic reduction process is used to transform Alumina to Aluminium. From every two tonnes of alumina, one tonne of aluminium is produced. The bauxite consists of ferrous oxide, titanium oxide, earth silicon, and alumina, approximately 40 to 60%. Bayer process is used to separate pure aluminium from bauxite (Habashi 16). This process involves heating the ore in an autoclave that contains caustic soda. Cooling then takes place, in order to separate the solid residue that is red in colour from the liquid. Extraction of Aluminium hydroxide from the solution is done, and then calcined to produce pure alumina. Hydrogen is electrochemically nobler than aluminium and thus inhibits the production of Aluminium through the aqueous electrolytic process. Liquid aluminium is produced through the process of electrolytic reduction of alumina.
2Al2O3 +3C(S) = 4Al(l) +3CO2(g)
Aluminium, which has a melting point of 6600C, is formed at about 900oC. The spare heat is used to recycle the metal that has been recycled. The recycled metal is, therefore, blended with the new metal.
Following the Bayer process, the component of Aluminium in the ore is dissolved in sodium hydroxide solution in order remove impurities from the solution, and the precipitates of alumina trihydrate. Alumina is precipitated at low temperature, whereas the red mud is separated at the intermediate level. Alumina is used in ceramics, refectory industries, and abrasive.
2NaOH + Al2O3.xH2O = 2NaAlO2 + (x+1)H2O
The first stage in the production is milling. In this stage, the bauxite is crushed after it has been washed. Crushing the particles reduces their size and thus increases the surface area for digestion stage. The bauxite is taken through the process of desilication, to remove SiO2, which causes scale formation, and affects the quality of Aluminium. In the digestion stage, NaOH is used in the dissolving of the aluminium bearing minerals in the bauxite (Habashi 17). The minerals include gibbsite, bohmite, and diaspore.
Al(OH)3 + Na+ + OH- → Al(OH)4- + Na+
AlO(OH) + Na+ + OH- + H2O → Al(OH)4- + Na+
The ore that has high gibbsite content is produced at 140oC, whereas the bohmitic bauxite is produced at temperatures ranging between 280 and 200oC. The slurry is cooled at around 106oC in a series of flash tanks. Steam produced from these tanks is used to preheat the spent liquor. Production is boosted through injection of higher quality bauxite. This process reduces the rate of energy use per ton.
In the clarification stage, bauxite residue is separated from the remains of sodium aluminate through the process of sedimentation. The bauxite residue settles at the bottom of the tanks, and afterwards it undergoes washing in the washing tank, in order to recover caustic soda. In the precipitation process, the liquor is cooled progressively. This results in the formation of crystals of aluminium trihydroxide (Al(OH)3.
Al(OH)4- + Na+ → Al(OH)3 + Na+ + OH-
After the precipitation stage, evaporation stage follows. A series of heat exchangers are used to heat the liquor, and after heating the liquor is subsequently cooled in flash tanks. In the heaters, a condensate is formed, which is re-used in the process for washing bauxite residue or as boiler feed water. The caustic soda that has remained is recycled back into the digestion process after it has been washed. In the classification stage, the gibbsite crystals are classified according to their sizes. Classification of the cyclones is done using gravity classification tanks or cyclones. The fine crystals are returned to the precipitation stage after being washed, whereas the coarse crystals undergo calcinations.
In the calciners, the filter cake is roasted at temperatures ranging 1100oC, to remove the chemically connected water and the free moisture.
2Al(OH)3 → Al2O3 + 3H2O
Recycling of Aluminium
The process of extracting Aluminium from bauxite is tedious and uneconomical. The current challenge of environmental pollution is also becoming a societal problem. Better ways are being devised to address the current challenges concerning sustainable production in order attain optimum economical value and conserve the already degraded environment. Recycling of aluminium is a sustainable process that is used in utilizing scrap metals of aluminium in order to sustain the available aluminum and to reduce the pollution effect of aluminium (Schlesinger, Mark 2006). The process involves re-melting the metal, which is far less expensive than extraction of aluminium from Bauxite.
Recycling Aluminium requires 5% of the energy required to extract new aluminium from the ore. Currently, due to cost benefit analysis effect, 31% of the total aluminium production in the United States is made from recycled aluminium metal (Harris, Chris; et al 88). The source of scrap metal includes used beverage containers, ageing auto craft, automobiles, gutters, cookware and other small sources. Industrially, most firms consider recycling of Aluminium in order to reduce production costs and hence attain a market bargaining power.
The recycling process is technically simple. The first process involves the collection of cans and other forms of materials from municipal waste. The collected can are normally mixed with other forms of wastes. These are separated via eddy current separator, and the cans shredded into small pieces. Mechanical and mechanical cleaning is then applied to eliminate any possible oxidation during the recycling process. The cleaned parts are arranged into blocks and loaded into a furnace, where they are heated to 750 degrees Celsius to produce molten Aluminium. Dissolved hydrogen is degassed and dross removed from smelted aluminium. Impurities are removed, and the smelted Aluminium molded into constituent products (Hwang, Huang 51).
The recycling process is not limited to industries but also individuals. Individuals are encouraged to dispose their aluminium cans in the appropriate manner. Currently, agents are hired to collect cans and other scrap metal from homes and fast food joints at a fee. This process not only keeps the environment tidy but also creates an environment. The ability of any organization to recycle used materials increases the efficiency of production in the given organization (Harris, Chris; et al. 48).
I would recommend that countries and organizations embrace this strategy because is it highly sustainable and economic friendly. In order to encourage people in Saudi Arabia to recycle Aluminium, I would develop a proposition in most industries and then establish a campaign to encourage people to dispose cans appropriately. A program to resell old automobiles into smelting companies could play a significant role of reducing the waste materials from the environment.
In conclusion, Aluminium is a very important, yet rare component in modern day life. Developing a sustainable and efficient production path is a very important approach of ensuring that there is sufficient supply of Aluminium at a considerably lower cost than then of newly extracted Aluminium. Additionally, recycling of Aluminium will keep mining within manageable limits.
Habashi, F.; "A short history of hydrometallurgy". Hydrometallurgy, (2005) 79: 15–22
Harris, Chris; et al. Micro reform – impacts on firms: aluminium case study. Melbourne: Industry Commission, 1988
Hwang, J.Y., Huang, X., Xu, Z., Recovery of Metals from Aluminum Dross and Salt cake, Journal of Minerals & Materials Characterization & Engineering. Vol. 5 (2006), No. 1, pp 47-62
Schlesinger, Mark (2006). Aluminum Recycling. CRC Press. p. 248