Aluminium oxide electrolysis

Aluminium is obtained on a large scale by the electrolysis of the oxide, dissolved in fused cryolite The oxide, occurring naturally as bauxite, AI2O3.2H2O, usually contains silica and iron(III) oxide as impurities. These must be removed first, since aluminium, once prepared, cannot be freed of other metals (which will be deposited on electrolysis) by refining it. The crude oxide is dissolved under pressure in caustic soda solution the aluminium oxide and silica dissolve and the ironflll) oxide is left ... 

Today we use aluminium in very large quantities. The annual production in the world is 19.5 million tonnes. The commercial extraction of aluminium has been made possible by two scientists, working independendy of each other, who discovered a method using electrolysis. The two scientists were Charles Martin Hall (USA), who discovered the process in 1886, and the French chemist Paul Heroult, who discovered the process independendy in the same year. The process they developed, often called the Hall-Heroult process, involves the electrolysis of aluminium oxide (alumina). The process involves the following stages. 

Aluminium oxide is an ionic compound. When it is melted the ions become mobile, as the strong electrostatic forces of attraction between them are broken by the input of heat energy. During electrolysis the negatively charged oxide ions are attracted to the anode (the positive electrode), where they lose electrons (oxidation). 

The electrolysis of aluminium oxide is a continuous process in which vast amounts of electricity are used. Approximately 15 kWh of electricity are used to produce 1 kg of aluminium. In order to make the process an economic one, a cheap form of electricity is required. Hydroelectric power (HEP) is usually used for this process. The plant shown in Figure 5.7 uses an HEP scheme to provide some of the electrical energy required for this process. Further details about HEP are given in Chapter 6, p. 94. 

Aluminium Bauxite Aluminium oxide AI203.2H20 Electrolysis of oxide dissolved in molten cryolite... 

Aluminium is one of the most abundant elements in the earth s crust, but, under feasible industrial conditions, can only be extracted by electrolysis. The process used is the electrolysis of aluminium hydroxide in molten cryolite (Na3AlF6) at 1030°C, pure aluminium hydroxide having been prepared from the mineral bauxite (hydrated aluminium oxide containing silica and some metal oxides such as iron) by the Bayer process. The cathode is carbon covered by molten aluminium metal and the anode is carbon, the total reaction being... 

The only reasonably successful advance in this sense is the Alcoa process, based on the electrolysis of aluminium trichloride in a 2-15 per cent concentration at 700°C in a 3 2 mixture of molten sodium chloride and potassium chloride using carbon electrodes. Aluminium oxide is previously converted into aluminium chloride using chlorine from electrolysis. The reactions are thus... 

Metallurgy. — The metals of most of the cerium group elements have been prepared, three general methods having been used t (1) fusion of the anhydrous halides with sodium, potassium, calcium, or aluminium (2) electrolysis of the fused chlorides or of a solution of the oxide in the molten fluoride (3) heating the oxides with magnesium, calcium, or silicon. Reduction with aluminium has also been tried, but it is not satisfactory except possibly for cerium itself. Electrolysis has been the most successful, the other methods usually giving at best an alloy. 

Some aluminium foundries melt material recovered from the electrolysis of aluminium. In this instance, inorganic pollutants such as hydrogen fluoride may be formed. These ean be eleaned from the flue-gas using a chemisorption step, which can be added onto the dust abatement system. In this treatment calcium hydrate or aluminium oxide ean be used as the adsorbant. 

The ore is first treated with caustic soda under pressure. The aluminium largely dissolves as the aluminate, the iron oxide is insoluble and the silica also remains in the form of a sodium aluminium silicate, which leads to a loss of aluminium. Hence the best bauxites are those low in silica. After filtration, the hydrated aluminium oxide is reprecipitated by seeding and the caustic soda solution may be re-used. The alumina is washed and then heated at 1200°C to remove water. The final step in the production of aluminium metal has to be electrolytic since the reduction of alumina with carbon is only possible at very high temperatures and the reverse reaction occurs on cooling. Moreover, because of the chemistry of aluminium, the electrolysis medium cannot be water in fact almost all commercial production of aluminium during the last ninety years has used an electrolysis in molten cryolite (Na3AlF5). 

Aluminium production is an intermediate case. It is made by electrolysis of a molten mixture of aluminium oxide and sodium hexafluoro-aluminate(III) (cryolite), but the carbon anodes waste away by reaction with the liberated oxygen and this provides thermal energy to melt the electrolyte, thus lowering the electrical energy input required ". ... 

The first attempts to isolate aluminium in its metallic form were performed in 1825 by Hans Christian Oersted, a Danish physicist and chemist, and fulfilled in 1827 by Friedrich Wohler, a German chemist, who isolated the pure metal from aluminium trichloride via reduction with potassium. The viable scaling-up of aluminium production was a rather long process that led, on the one hand, to the sodium aluminate process of Carl J. Bayer disclosed in 1887 producing pure aluminium oxide and, on the other hand, to the process of Charles M. Hall and Paul-Louis Toussaint Heroult" coinvented independentiy in 1886, in which aluminium oxide is dissolved in cryolite to yield via electrolysis pure metallic aluminium. 

The layer of aluminium oxide can be made thicker by electrolysis, to give even more protection. This process is called anodising. The aluminium is used as the anode of a cell in which dilute sulphuric acid is electrolysed. Oxygen forms at the anode and reacts with the aluminium, so the layer of oxide grows. 

Industrial production of aluminium is done by electrolysis according to the so-called Hall-process. Aluminium oxide AI2O3 is used as the raw material in this process. The electrolysis is carried out in a melt of cryolite NasAIFe at approximately 1000°C. At this temperature AI2O3 is partly soluble into Na3AlF6, and liquid, metallic A1 can be deposited from the melt due to the following cathode reaction... 

Manganese is the third most abundant transition metal, and is widely distributed in the earth s crust. The most important ore is pyrolusite, manganese(IV) oxide. Reduction of this ore by heating with aluminium gives an explosive reaction, and the oxide Mn304 must be used to obtain the metal. The latter is purified by distillation in vacuo just above its melting point (1517 K) the pure metal can also he obtained by electrolysis of aqueous manganese(II) sulphate. 

The layer is deposited with the saucepan immersed in a vat of dye solution (usually acidihed to pH 1 or 2), and made the positive terminal of a cell. As the electrolysis proceeds, so the aluminium on the surface of the saucepan is oxidized ... 

Preparation. Strontium metal can be obtained by electrolysis of the fused chlorides or by reduction of the oxide by aluminium. 

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