The Production of Alumina

Low grade bauxite typically contains 50% alumina and about 12% of silica. Limestone is used to convert the silica into an insoluble silicate, allowing the alumina to be extracted as a soluble aluminate. 

In the combination-Bayer process, the bauxite is finely ground with sodium carbonate and sufficient limestone to give a CaO -r SiOa ratio of 2.0 to 2.2 [11.6], The mixture is sintered in a rotary kiln at 1100 °C to calcine the sodium carbonate to sodium oxide (equation 11.6) and the limestone to quicklime (11.7). The sodium oxide reacts with the alumina to form sodium aluminate (11.8) and the quicklime reacts with the silica to produce dicalcium silicate (11.9). 

The cooled product is ground and extracted with water. The soluble sodium aluminate is removed as a filtrate from the dicalcium silicate and insoluble matter. Injection of carbon dioxide into the filtrate produces a precipitate of aluminium hydroxide and sodium carbonate liquor (11.10), which is recycled to the initial stage. The aluminium hydroxide is heated to above 300 °C to produce pure alumina which is dissolved in fused cryolite (NaAIp4) at about 900 °C and electrolysed between carbon electrodes to produce molten aluminium. 

---

The manufacture of cryoHte is commonly iategrated with the production of alumina hydrate and aluminum trifluoride. The iatermediate stream of sodium aluminate from the Bayer alumina hydrate process can be used along with aqueous hydrofluoric acid, hydrogen fluoride kiln gases, or hydrogen fluoride-rich effluent from dry-process aluminum trifluoride manufacture. 

Low Soda Hydroxide. Tlie Na20 content of nomial Bayer hydroxide is around 0.2—0.4%, 0.1% of wliich can be removed by thorough wasliing. Tlie remaining soda is trapped within the hydroxide crystal. Experience shows that the occluded soda content is reduced when cry staUization is carried out under low aluniina-supersaturation conditions and at relatively higher temperatures (80—95°C). Soda contents as low as 0.05% Na20 can be obtained by tliis procedure. However, these conditions also reduce hydroxide ield and thus increase the production cost. Low soda aluminum hydroxide is generally employed in the production of aluminas for the ceramics industries. 

In the late 1940s zeolites were synthesized according to the procedure shown in Fig. 3.24. First an amorphous alumino-silicate gel is formed. This process is completely analogous to the production of alumina and silica gels described before. Subsequently this gel is crystallized into zeolite. The preparation of zeolites has drawn tremendous attention of the scientific and industrial community. A wide variety of zeolites have been synthesized, and reproducible synthesis procedures have been reported (often in the patent literature). Natural zeolites also exist massive deposits have been discovered in many places in the world. 

The major raw materials used at present for the production of alumina are bauxites, which are found in the following mineral forms gibbsite (Al(OH)3), boehmite (AlO OH), and diaspore (AlO OH). The major impurities are the oxides of iron, silicon, and titanium, and organic compounds, all of which must be removed before alumina is suitable for aluminum production. The process objectives are, therefore, separation of impurities and compound production in the present case. Bauxite is first dried to facilitate grinding, destroy organic matter, and oxidize the associated ferrous minerals to the ferric state. The temperature of drying is not allowed to exceed 150 °C, because at higher temperature a part of the combined water is expelled and the solubility is affected adversely. 

The lime—soda process is practiced mainly in isolated areas in some process operations, in the Kraft recovery process, and in the production of alumina. It is not as efficient a route as electrolytic production. 

In the Kraft recovery process the green liquor, which is an aqueous solution of sodium carbonate, is heated with lime to produce white liquor or caustic soda, which is then returned to the pulp digestion operations. In the production of alumina, lime and soda are fed to bauxite digesters. The CaCO 3 produced during the course of the reaction is rebumed to lime and is recycled. The main difficulties associated with these processes include the extensive mechanical handling through the use of causticizers, settlers, and repulpers in order to produce caustic that is low in carbonate and the high fuel consumption needed to reconvert CaCO to lime. 

A few key physical properties important for most catalytic application should serve as an example of the flexibility which alkoxide technology can offer in the production of aluminas... 

The Bayer process for the production of alumina uses caustie soda to extraet aluminium oxide from bauxite. The resulting solution of sodium aluminate also contains dissolved sodium silicate. The silicate is normally removed by allowing the aluminate and silicate to react to form precipitates of the zeolite structure having a eomposition of approximately NagAl6Si6024(0H)2. Slaked lime is added, when very low levels of silicate are required, to precipitate the silicate as cancrinite, which is removed, prior to the formation of the zeolite structures [31.19]. 

The precipitation of alumina trihydrate (Bayer hydrate) from caustic (sodium aluminate) solution is an important step in the production of alumina from bauxite by the Bayer process. The reaction... 

The world demand for caustic soda is shown in Fig. 3.16, along with its growth in various countries. Table 3.2 depicts the caustic demand during the period 1995-2010 [6]. Its use in the manufacture of various segments is illustrated in Fig. 3.17. The global caustic demand is projected to increase by 2.3% per year through the year 2010, primarily because of its demand in the production of alumina, soaps, detergents and... 

The major end use of caustic in Eastern Europe is the production of rayon fibers, accounting for 10-12% of the region s caustic soda demand. This may be compared with the world average of 4%. Other applications include wood pulping, petroleum processing, and the production of alumina. 

The principle of sol-gel processing is summarized in Sec. 2.6. Sol-gel is used on a large scale in the production of alumina-based fibers Nextel by the 3M Co.) and more recently of silicon-carbide and silicon-nitride fibers. 

Using aluminum alkoxides as the starting mataial, the production of alumina by the colloidal gel route involves the following main steps ... 

The aluminium industry is a major consumer of electrical energy. By the production of alumina, the use of carbon anodes, and during electrolysis, large amounts of CO2 are emitted to the atmosphere. 

The production of alumina, which is extracted from bauxite ore, is a business with many hazards. Employees work in situations where they are at risk of burns from caustic chemicals, hot liquids, and extremely high heat as well as of injuries resulting from the traditional hazards common to most industrial settings, such as slips and trips. This particular plant, however, had more problems than most. It was... 

Long ago, the refractory layer was made from powdered alumina that was poured on a layer of heat irtsrrlatiort. After shutdown of the cell, the alumina, infiltrated by electrolyte, was used for the production of alumina. The main disadvantages of such a variant are high thermal conductivity and low resistance to electrolyte. This type of refractory lining is not used today. 

Stable dispersions of silicone carbide slips utilized in sintering are obtained in the presence of polyphenylene. " Polyvinylbutyral was used as a binder in the production of alumina slips. 

---