Production of Aluminium

The production of aluminium is based on the electrolysis of molten alumina AI2O3 using the HaU-Herault process. Alumina is extracted in the Bayer process form the bauxite ore which contains 20 to 30 wt% Al. In 2002 the main producers of the ore are Australia (28%), Guinea (20%), Brazil (14%), Jamaica (7%), India (4%), and Guyana (3%) [1.9]. 

After milling, the ore is first broken down using an Al-containing NaOH solution which is seeded to precipitate aluminium hydroxide Al(OH)3. This is dehydrated at about 1100 °C according to 2Al(OH)3 — AI2O3 -I-3H2O. 

2Al +- -6e 2A1, and oxygen ions react with the carbon of the cathode to form CO2 (consumption of the cathode). A primary aluminium smelter requires on average 13 to 14 kW h per kg of Al. The main power used is hydro-electric (52.5% in 2001 [1.13]). In recent decades 

1 -5 Phase diagram of cryolithe NasAlFe-alumina AI2O3 [1.9] 

An electrolytic reduction is carried out in a cell as shown in Fig. 3.1-4. Cryolithe Na3AlFe is used as an additive to decrease the high melting point of pure AI2O3 (about 2050 °C) since the two compounds form a eutectic near 10wt% of AI2O3 in Na3AlFe, Fig. 3.1-5 [1.11,12]. The electrolysis is carried out in a cell lined with carbon, which serves as the cathode. Carbon anodes are suspended from above the cell into the electrolyte. Two main reactions occur  

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P. Ya. Zeltser, V. I. Chalykh, L. V. Chemyakhovskij, V. N. Smeta-nin, V. I. Kravchenko, and K. S. Elkin. Light plugging solution production—from mixture of Portland cement, silica-containing dust waste from production of aluminium alloys, sodium hydroxide and water. Patent SU 1728471-A, 1992. 

Carbon electrodes are widely used in electrochemistry both in the laboratory and on the industrial scale. The latter includes production of aluminium, fluorine, and chlorine, organic electrosynthesis, electrochemical power sources, etc. Besides the use of graphite (carbons) as a virtually inert electode material, the electrochemical intercalation deserves special attention. This topic will be treated in the next paragraph. 

Hudson, L. K., Production of Aluminium and Alumina, Burkin, A. R., Ed., John Wiley Sons, Chichester, England, 1987, 13. 

It consists in a deposition of ions from an electrolyte onto the cathode in an electrolytic cell, under the influence of an applied potential. Usually the process is accompanied by material dissolution from the anode. The electrowinning from aqueous solutions is an important commercial method for the production (and/or refinement) of many metals, including, for instance, chromium, nickel, copper, zinc. As for the electrodeposition from non-aqueous solutions, the primary production of aluminium, electrodeposited from a solution of A1203 in molten cryolite, is a typical example. Other metals which may be regularly reduced in a similar way are Li, Na, K, Mg, Ca, Nb, Ta, etc. 

Polynuclear aromatic hydrocarbons (PAH), of widespread occurrence in the environment, result from incomplete combustion of carbon- and hydrogen-containing substances. Many PAHs are well-recognized carcinogens and mutagens. Several industrial processes, such as fossil fuel conversion and production of aluminium and ferroalloys, can produce PAHs and result in their occurence in the working environment. 

The one exception to this picture is aluminium smelting, which, if not tied to electricity production, is exposed to large increases in marginal cost, with consequential migration of production outside the EU trading area. In all the scenarios, EU production of aluminium ceases. 

Although various modifications of aluminium trihydroxide, Al(OH)3, have been described in the literature, there are only three common forms gibbsite (originally also called hydrargillite) bayerite and nordstrandite. Gibbsite is die best known and most abundant. It is the main constituent of North and South American bauxite and is obtained as an intermediate product (i.e. Bayer Hydrate ) in the Bayer process for the production of aluminium from bauxite. 

A polymeric condensation product of aluminium oxide and aspirin which contains about 83% of total salicylates. 

An interesting illustration is found in the production of aluminium from the mineral bauxite. After grinding the ore. the AljOg Is dissolved by alkali to give aluminate. The insoluble ore particles left behind are flocculated by polymer and thus separated. Hien the aluminate is acidified again, and the colloidal AI2O3 is concentrated by flocculants and further used for the production of aluminium. 

A.R. Burkin, Production of Aluminium and Alumina. Society of Chemical Industry J. Wiley, Chichester, West Sussex, 1987. 

Phosgene has been formed at the graphite anode during the electrolysis of commercial calcium aluminates (for the production of aluminium-calcium alloys) when the aluminate content of the molten bath became low relative to the chloride-containing melt [233]. 

Molten Na[AlCl4] catalyses the reaction at 600 C, in a three-phase system with gaseous COCl and solid y-AIjOj, to give very high purity aluminium(III) chloride [962], and this reaction (in the presence of carbon) has formed the basis of a patent for the industrial scale production of aluminium(III) chloride [909]. The chlorination of AljOj by phosgene has been studied in a LiCl-KCl eutectic melt at 470 C as a function of pO it is essentially a two-step process [1842] ... 

A detailed state-of-the-art report of the production of aluminium, via aluminium(III) chloride, from clay, and particularly kaolinitic clay, compares the effectiveness of phosgene with other chlorinating agents [1203a], In a similar manner, fly-ash (typically ca. 95% SiOj-AljOj in a 5 3 ratio) can be treated with phosgene, and the kinetics of this process between 450 and 800 "C have been investigated [8],... 

White bauxites are chiefly used for the production of aluminium sulphate and the alums. Red bauxites form the raw material for the preparation of alumina, and therefore of aluminium. Intermediate or refractory bauxites, fused in an electric furnace, give artificial corundum. 

Bauxite is treated either by Deville s method or by that of Bayer, the latter being almost exclusively employed. A third method depends upon the production of aluminium nitride. This is obtained by heating bauxite in air to 1800°-1900° in an electric furnace. It is then decomposed in an autoclave in presence of soda solution, giving (i) ammonia, used as a manure in the form of its sulphate, (ii) sodium aluminate, from which commercially pure alumina can be obtained. 

France, as is shown in the accompanying map (Fig. 3), is favourably situated for the production of aluminium. The close proximity of the bauxite beds, the alumina works, and the water power necessary for the electro-metallurgy, forms a unique combination, and, in addition, carbon can bo easily conveyed to the works. 

A number of aluminium aspirin preparations have also been described, to which different structures have been assigned . One of these compounds, aluminium aspirin NF, was found to be poorly absorbed compared with acetylsalicylic acid. Aloxiprin (Palaprin) is a different form of aluminium aspirin, being a polymeric condensation product of aluminium oxide and acetylsalicylic acid, with the formula Al302(C8H4(o-OCMe)-COO)s 9B. This compound is therapeutically as effective as acetylsalicylic acid it produces less gastric irritation and gastro-intestinal blood loss, but has a slight tendency to cause constipation. ... 

In connection with the problems of emissions of fluorine compounds from the electrolytic production of aluminium, as a further example, we can mention the exchange of electrolysers with self-sintering electrodes for those with preliminarily calcinated anodes, which enabled the hermetic sealing of electrolytic vats and the complete withdrawal of the produced exhalates into separators. 

Huang, P.M., Violante, A., 1986. Influence of organic acids on crystallization and surface properties of precipitation products of aluminium. In Huang, P.M., Schnitzer, M. (Eds.), Interactions of Soil Minerals with Natural Organics and Microbes. Special Publication 17, Soil Science Society of America, Madison, WI, pp. 159-221. 

The electrolytic production of aluminium is more complex. Hall and Heroult developed the current process independently in 1886. In this process, alumina, AI2O3, is dissolved in molten sodium aluminium fluoride (Na3AlF6) and electrolysed (Figure 9.22). Originally, the aluminium oxide was derived directly from the mineral bauxite, and the sodium aluminium fluoride was used in the form of the naturally occurring mineral cryolite. These days, synthetic cryolite is used, and the aluminium oxide is produced from a wider variety of mineral sources, consisting of aluminium oxide-hydroxides. 

Because of the expense of this process, schemes using electrolysis of molten titanium salts, similar to the production of aluminium, have been widely investigated. To date, none of these has worked well. Although metal can be produced in this way, it is often dendritic in form, and very reactive, oxidising on contact with air. The various valence states of titanium found in melts ( A+, 3- - and 2- -) lower the efficiency of the methods and contribute to unreliable results. 

The simplified equation for the production of aluminium by the HaU-Heroult process is ... 

Nonferrous metals (e.g., aluminum and zinc) are more corrosion resistant than iron and steel. They corrode in a different way and at a much slower rate. The corrosion products of aluminium and zinc are white salts which form on the surface. Aluminum and zinc are used to provide sacrifical protection to iron and steel in the form of galvanizing and metal sprays. 

The last phrase is particularly important since, as has already been observed, it is technically much easier to recycle metals, paper and glass than plastics and more energy is vested in the primary production of aluminium, steel, glass and paper than in plastics (Tables 2.1). Moreover, the first three have no value as fuels whereas polymers do (next section). It might be anticipated then that the proportion of plastics ultimately recycled will settle out at the lower end of the range. 

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