ALUMINIUM PRODUCTION, ALLOYS AND APPLICATION
Aluminium is a chemical element in the boron group with symbol Al and atomic number 13. It is a silvery white, soft, ductile metal. Aluminium is the third most abundant element (after oxygen and silicon), and the most abundant metal in the Earth’s crust. It makes up about 8% by weight of the Earth’s solid surface. Aluminium metal is so chemically reactive that native specimens are rare and limited to extreme reducing environments. Instead, it is found combined in over 270 different minerals. The chief ore of aluminium is bauxite.
Aluminium is remarkable for the metal’s low density and for its ability to resist corrosion due to the phenomenon of passivation. Structural components made from aluminium and its alloys are vital to the aerospace industry and are important in other areas of transportation and structural materials. The most useful compounds of aluminium, at least on a weight basis, are the oxides and sulfates.
Despite its prevalence in the environment, no known form of life uses aluminium salts metabolically. In keeping with its pervasiveness, aluminium is well tolerated by plants and animals. Owing to their prevalence, potential beneficial (or otherwise) biological roles of aluminium compounds are of continuing interest.
Aluminium production starts with the raw material bauxite, a clay like soil type found in a belt around the equator. The bauxite is mined from a few meters below the ground. The bauxite is then transported to plants where the clay is washed off and the bauxite passes through a grinder. Alumina, or aluminium oxide, is extracted from the bauxite through refining. Alumina is separated from the bauxite by using a hot solution of caustic soda and lime. The mixture is heated and filtered, and the remaining alumina is dried to a white powder.
Alumina — or aluminium oxide (Al2O3) is produced from extracted ore. Despite its name, it has nothing to do with clay or black soil but resembles a flour or very white sand. Alumina is then transformed into aluminium through electrolytic reduction. One tonne of aluminium is produced from every two tonnes of alumina.
Bauxite consist of 40-60% alumina, as well as earth silicon, ferrous oxide, and titanium dioxide. To separate pure alumina, the Bayer process is applied. First, the ore is heated in an autoclave with caustic soda. It is then cooled and a solid residue — «red mud» — is separated from the liquid. Aluminium hydroxide is then extracted from this solution and calcined to produce pure alumina.
The final stage is the reduction of aluminium through the Hall-Heroult process. It is based on the following principle: when the alumina solution is electrolyzed in molten cryolite (Na3AlF6), pure aluminium is produced. The reduction cell bottom serves as a cathode, and coal bars immersed in cryolite serve as anodes. Molten aluminium is deposited under a cryolite solution with 3-5% alumina. During this process, temperatures reach 950°C, considerably higher than the melting point of the metal itself, which is 660°C.
Aluminium production technology applies pre-baked anodes, a method used at many European and American aluminium smelters, and characterised by less power consumption and a negative impact on the environment. The anodes are baked in huge gas furnaces and then, having been fixed into holders, are lowered into a furnace. Consumed electrodes are replaced with new ones and remaining ‘butts’ are sent away for recycling.
DIAGRAM OF ALUMINUIM MAKING PROCESS
Aluminium alloys are alloys in which aluminium (Al) is the predominant metal. The typical alloying elements are copper, magnesium, manganese, silicon, tin and zinc. There are two principal classifications, namely casting alloys and wrought alloys, both of which are further subdivided into the categories heat-treatable and non-heat-treatable. About 85% of aluminium is used for wrought products, for example rolled plate, foils and extrusions. Cast aluminium alloys yield cost-effective products due to the low melting point, although they generally have lower tensile strengths than wrought alloys. The most important cast aluminium alloy system is Al–Si, where the high levels of silicon (4.0–13%) contribute to give good casting characteristics. Aluminium alloys are widely used in engineering structures and components where light weight or corrosion resistance is required.
Alloys composed mostly of aluminium have been very important in aerospace manufacturing since the introduction of metal skinned aircraft. Aluminium-magnesium alloys are both lighter than other aluminium alloys and much less flammable than alloys that contain a very high percentage of magnesium.
Aluminium alloy surfaces will formulate a white, protective layer of corrosion aluminium oxide if left unprotected by anodizing and/or correct painting procedures. In a wet environment, galvanic corrosion can occur when an aluminium alloy is placed in electrical contact with other metals with more negative corrosion potentials than aluminium, and an electrolyte is present that allows ion exchange. Referred to as dissimilar metal corrosion this process can occur as exfoliation or intergranular corrosion. Aluminium alloys can be improperly heat treated. This causes internal element separation and the metal corrodes from the inside out. Aircraft mechanics deal daily with aluminium alloy corrosion.
Aluminium alloy compositions are registered with The Aluminum Association. Many organizations publish more specific standards for the manufacture of aluminium alloy, including the Society of Automotive Engineers standards organization, specifically its aerospace standards subgroups, and ASTM International.
Aluminium alloys with a wide range of properties are used in engineering structures. Alloy systems are classified by a number system (ANSI) or by names indicating their main alloying constituents (DIN and ISO).
The strength and durability of aluminium alloys vary widely, not only as a result of the components of the specific alloy, but also as a result of heat treatments and manufacturing processes. A lack of knowledge of these aspects has from time to time led to improperly designed structures and gained aluminium a bad reputation.
One important structural limitation of aluminium alloys is their fatigue strength. Unlike steels, aluminium alloys have no well-defined fatigue limit, meaning that fatigue failure eventually occurs, under even very small cyclic loadings. This implies that engineers must assess these loads and design for a fixed life rather than an infinite life.
Another important property of aluminium alloys is their sensitivity to heat. Workshop procedures involving heating are complicated by the fact that aluminium, unlike steel, melts without first glowing red. Forming operations where a blow torch is used therefore require some expertise, since no visual signs reveal how close the material is to melting. Aluminium alloys, like all structural alloys, also are subject to internal stresses following heating operations such as welding and casting. The problem with aluminium alloys in this regard is their low melting point, which make them more susceptible to distortions from thermally induced stress relief. Controlled stress relief can be done during manufacturing by heat-treating the parts in an oven, followed by gradual cooling—in effect annealing the stresses.
The low melting point of aluminium alloys has not precluded their use in rocketry; even for use in constructing combustion chambers where gases can reach 3500 K. The Agena upper stage engine used a regeneratively cooled aluminium design for some parts of the nozzle, including the thermally critical throat region.
Another alloy of some value is aluminium bronze (Cu-Al alloy).
The Bayer Process is the most economic means of obtaining alumina from bauxite. Other processes for obtaining alumina from metal ores are also in use in some refineries, particularly in China and Russia, although these make up a relatively small percentage of global production.
The process stages are:
The bauxite is washed and crushed, reducing the particle size and increasing the available surface area for the digestion stage. Lime and “spent liquor” (caustic soda returned from the precipitation stage) are added at the mills to make a pumpable slurry.
Bauxites that have high levels of silica (SiO2) go through a process to remove this impurity. Silica can cause problems with scale formation and quality of the final product.
A hot caustic soda (NaOH) solution is used to dissolve the aluminium-bearing minerals in the bauxite (gibbsite, böhmite and diaspore) to form a sodium aluminate supersaturated solution or “pregnant liquor”.
Al(OH)3 + Na+ + OH- → Al(OH)4- + Na+
Böhmite and Diaspore:
AlO(OH) + Na+ + OH- + H2O → Al(OH)4- + Na+
Conditions within the digester (caustic concentration, temperature and pressure) are set according to the properties of the bauxite ore. Ores with a high gibbsite content can be processed at 140°C, while böhmitic bauxites require temperatures between 200 and 280°C. The pressure is not important for the process as such, but is defined by the steam saturation pressure of the process. At 240°C the pressure is approximately 3.5 MPa.
The slurry is then cooled in a series of flash tanks to around 106°C at atmospheric pressure and by flashing off steam. This steam is used to preheat spent liquor. In some high temperature digestion refineries, higher quality bauxite (trihydrate) is injected into the flash train to boost production. This “sweetening ” process also reduces the energy usage per tonne of production.
Although higher temperatures are often theoretically advantageous, there are several potential disadvantages, including the possibility of oxides other than alumina dissolving into the caustic liquor.
The first stage of clarification is to separate the solids (bauxite residue) from the pregnant liquor (sodium aluminate remains in solution) via sedimentation. Chemical additives (flocculants) are added to assist the sedimentation process. The bauxite residue sinks to the bottom of the settling tanks, then is transferred to the washing tanks, where it undergoes a series of washing stages to recover the caustic soda (which is reused in the digestion process).
Further separation of the pregnant liquor from the bauxite residue is performed utilising a series of security filters. The purpose of the security filters is to ensure that the final product is not contaminated with impurities present in the residue.
Depending on the requirements of the residue storage facility, further thickening, filtration and/or neutralisation stages are employed prior to it being pumped to the bauxite residue disposal area.
In this stage, the alumina is recovered by crystallisation from the pregnant liquor, which is supersaturated in sodium aluminate.
The crystalisation process is driven by progressive cooling of the pregnant liquor, resulting in the formation of small crystals of aluminium trihydroxite (Al(OH)3, commonly known as “hydrate”), which then grow and agglomerate to form larger crystals. The precipitation reaction is the reverse of the gibbsite dissolution reaction in the digestion stage:
Al(OH)4- + Na+ → Al(OH)3 + Na+ + OH-
The spent liquor is heated through a series of heat exchangers and subsequently cooled in a series of flash tanks. The condensate formed in the heaters is re-used in the process, for instance as boiler feed water or for washing bauxite residue. The remaining caustic soda is washed and recycled back into the digestion process.
The gibbsite crystals formed in precipitation are classified into size ranges. This is normally done using cyclones or gravity classification tanks (a series of thickeners utilising the same principles as settlers / washers on the clarification stage). The coarse size crystals are destined for calcination after being separated from spent liquor utilising vacuum filtration, where the solids are washed with hot water.
The fine crystals, after being washed to remove organic impurities, are returned to the precipitation stage as fine seed to be agglomerated.
The filter cake is fed into calciners where they are roasted at temperatures of up to 1100°C to drive off free moisture and chemically-connected water, producing alumina solids. There are different calcination technologies in use, including gas suspension calciners, fluidised bed calciners and rotary kilns.
The following equation describes the calcination reaction:
2Al(OH)3 → Al2O3 + 3H2O
Alumina, a white powder, is the product of this step and the final product of the Bayer Process, ready for shipment to aluminium smelters or the chemical industry.
Here, the refined alumina is transformed into aluminium.Three different raw materials are needed to make aluminium, aluminium oxide, electricity and carbon. Electricity is run between a negative cathode and a positive anode, both made of carbon. The anode reacts with the oxygen in the alumina and forms CO2. The result is liquid aluminium, which can now be tapped from the cells. The liquid aluminium is cast into extrusion ingots, sheet ingots or foundry alloys, all depending on what it will be used for. The aluminium is transformed into different products.
Aluminium production is very energy-intensive. It is for this reason that the most efficient place to construct aluminium smelters is in remote regions, where there is free access to power sources. Alumina is the most widespread metal in nature: making up about 8.8% of the earth’s crust. Due to its chemical reactivity, it almost doesn’t exist in free form. Contrary to popular opinion, aluminium mines do not exist, and only a few minerals and rocks containing aluminium are suitable for industrial production.
Aluminium is theoretically 100% recyclable without any loss of its natural qualities. According to the International Resource Panel’s Metal Stocks in Society report, the global per capita stock of aluminium in use in society (i.e. in cars, buildings, electronics etc.) is 80 kg. Much of this is in more-developed countries (350–500 kg per capita) rather than less-developed countries (35 kg per capita). Knowing the per capita stocks and their approximate lifespans is important for planning recycling.
1. Aluminium monoxide
2. Aluminium iodide
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