Chapter 2. Production and Processing of …

1 7 Chapter 2. Production and Processingof Extraction of AluminumThe extraction of aluminum from its ore and subsequent Processing into finished prod-ucts takes place in a series of successive operations, each largely independent of the the various processes are carried out at different plant sites. A summary ofproduction steps from the bauxite mine through casting is given in Fig. Bauxite miningAluminum comprises approximately 8% of the earth s crust, making it second only tosilicon ( ). Iron is third at about 5%.

2 Metallic aluminum is not found in nature; itoccurs in the form of hydrated oxides or silicates (clays). The principal ore from whichaluminum is extracted is called bauxite after the town of Les Baux in southern Francewhere the ore was originally discovered. Bauxite occurs mainly in the tropics and insome Mediterranean countries. Today, the main mining locations are in Latin America,Australia, India, and is a weathered rock containing two forms of hydrated aluminum oxide, eithermostly a monohydrate AlO(OH) in caustic bauxite, or mostly a trihydrate Al(OH)3 inlateric bauxite.

3 Besides these compounds, bauxite contains iron oxide, which usually givesit a reddish-brown colour, as well as silicates (clay, quartz) and titanium oxide. The crys-tal structure also contains 12 20% by weight of water. Tropical monohydrate bauxite gradesyielding 35 55% Al2O3 will no doubt continue to be the most favored aluminum ores formany rocks similar to bauxite, but with lower alumina content, are available in largequantities. Clays became a source of alumina to a limited extent in Germany during theFig. : Production steps for World War.

4 In addition, many other types of rock contain considerable amountsof alumina, such as kaolin, nepheline, andalusite, leucite, labradorite, and alunite. Theformer Soviet Union exploited such ores to maintain partial autonomy, but these oresplay no significant role in today s aluminum Production . Chapter 16 will show that theworld s bauxite supplies are guaranteed into the distant The alumina plantThe starting material for electrolytic smelting of aluminum is pure, anhydrous alumi-num oxide (Al2O3) called alumina.

5 In the Western World, the Bayer1 process, invented inthe 19th century, is by far the most important process used in the Production of alumi-num oxide from bauxite. The process has been refined and improved since its shows that the Production of alumina is a complex chemical process. The alu-mina content of bauxite ores varies from one deposit to another, and methods of treat-ment differ accordingly (see ). This means that each alumina plant is almost tailor-made to suit a particular bauxite. The processes are nevertheless basically similar, and ageneral description is given in the following.

6 The bauxite from the mine is crushed andground. It is then mixed with a solution of caustic soda and pumped into large auto-claves. There, under pressure and at a temperature of 110 270 C, the alumina containedin the ore is dissolved to form sodium aluminate. The silica in the bauxite reacts andprecipitates from solution as sodium- aluminum -silicate. Iron and titanium oxide and otherimpurities are not affected chemically, and being solid, settle out of solution. This wastematerial, known as red mud, is separated from the sodium aluminate solution, washedto recover the caustic soda, and then pumped to disposal disposal of red mud can present an environmental problem simply because there isso much of it.

7 From a few alumina plants, red mud is deposited on the sea bed understrictly controlled conditions. One very common method of disposal is to contain themud in an area surrounded by dikes. After an interval of some years, these ponded areascan be recultivated to eliminate visual pollution. Although a great deal of effort hasbeen expended on finding and developing various uses for red mud, no bulk applicationof commercial value has yet been the weak soda washed out of the red mud to the sodium aluminate solutiondilutes it and cools it to about 100 C.

8 With stirring and cooling to 60 C, aluminum hy-droxide Al(OH)3 (hydrargillite) precipitates. Seeding the liquor with crystals from a pre-vious cycle helps to control precipitation. Vacuum filters separate the hydroxide precipi-tate, which is then washed with pure water. Calcination in rotary kilns or in fluidizedbeds at 1100 C to 1300 C finally converts the hydroxide to a dry, white powder. Thispowder is technical purity alumina, containing as impurities at most SiO2, Fe2O3, and grade of the alumina (particle size, - and -Al2O3 content)

9 Can be influenced byprecipitation and calcining conditions, and it is usual to differentiate between two maingrades, floury alumina, which is highly calcined and contains mostly -Al2O3, and sandy alumina, which calcined to a lesser degree with mainly -Al2O3 in the hydrated1 Named after the Austrian The sandy alumina has a large, active surface area, which makes it suitable for usein dry scrubber systems for fluoride abatement at aluminum reduction plants. There is aclear trend toward the Production of increased quantities of sandy Russia, due to a lack of bauxite, a process using nepheline as feed-stock has been usedto produce alumina.

10 Essentially the technique consists of sintering a nepheline ore, orconcentrate, with limestone. The resultant sinter-cake consists of sodium and potassiumaluminates and dicalcium silicate. This material is crushed, ground, and leached. Afterleaching, the aluminate liquor is desilicated and decomposed by carbonation. Aluminahydrate is separated from the liquor and calcined to obtain alumina. After evaporationand crystallization, the carbonate liquor yields soda and potash. These are centrifuged,dried, and packed for shipment.