Bauxite gibbsitic

Table 3.3. They are called gibbsitic, boehmitic, or diasporic bauxite. Gibbsitic bauxite is predominant. It is geologically the youngest and located in tropical or subtropical regions very close to the ground surface (i.e., laterites). The oldest deposits, which are found mainly in Europe (e.g., Gardanne in France and Patras in Greece) and in Asia, mainly contain boehmite and diaspora. They are usually underground deposits.

A commercial process which uses hydrothermal leaching on a large scale is the Bayer process for production of aluminum oxide (see Aluminum compounds). This process is used to extract and precipitate high grade alurninum hydroxide (gibbsite [14762-49-3]) from bauxite [1318-16-7] ore. The hydrothermal process step is the extraction step in which concentrated sodium hydroxide is used to form a soluble sodium aluminate complex ... 

Reaction conditions depend on the composition of the bauxite ore, and particularly on whether it contains primarily gibbsite, Al(OH)2, or boehmite [1318-23-6] AlOOH. The dissolution process is conducted in large, stirred vessels or alternatively in a tubular reactor. The process originated as a batch process, but has been converted to a continuous one, using a series of stirred tank reactors or a tubular reactor. 

The CAS registry Hsts 5,037 aluminum-containing compounds exclusive of alloys and intermetaUics. Some of these are Hsted in Table 1. Except for nepheline and alunite in the USSR and Poland, bauxite is the raw material for all manufactured aluminum compounds. The term bauxite is used for ores that contain economically recoverable quantities of the aluminum hydroxide mineral gibbsite or the oxide—hydroxide forms boehmite and diaspore. 

Unrefined bauxite [1318-16-7] is also used as a precursor to low cost activated alumina because bauxites can contain as much as 90% gibbsite (dry basis). These materials represent the low cost, low performance end of the activated alumina spectmm of products, but they are stiU used in significant quantities. 

Boehmite (OC-Aluminum Oxide-Hydroxide). Boehmite, the main constituent of bauxite deposits in Europe, is also found associated with gibbsite in tropical bauxites in Africa, Asia, and Austraha. Hydrothemial transformation of gibbsite at temperatures above 150 °C is a common method for the synthesis of weU-cry stalhzed boehmite. Higher temperatures and the presence of alkali increase the rate of transfomiation. Boehmite ciy stals of 5—10 ]liii size (Fig. 3) are produced by tliis method. Fibrous (acicular) boehmite is obtained under acidic hydrothemial conditions (6). Excess water, about 1% to 2% higher than the stoichiometric 15%, is usually found in hydrothemiaHy produced boehmite. 

Diaspore (P-Aluminum Oxide Hydroxide). Diaspore, found in bauxites of Greece, Cliina, and the USSR, can also be obtained by hydrothemial transfomiation of gibbsite and boehmite. Higher (>200°) temperatures and pressure (>15 AlPa-150bar) are needed for synthesis and the presence of diaspore seed cry stals helps to avoid boehmite fomiation. 

Alumina. A pure although not necessarily a refractory grade of alumina is obtained from bauxite by the Bayer process. In this process, the gibbsite from the bauxite is dissolved in a caustic soda solution and thus separated from the impurities. Alumina, calcined, sintered, or fused, is a stable and extremely versatile material used for a variety of heavy industrial, electronic, and technical appHcations. 

The possible content of hydrated alumina and iron. Hydrated alumina minerals like gibbsite [14762-49-3] Al(OH)2, boehmite [1318-23-6] AlOOH, and diaspore [14457-84-2] AlOOH, occur ia bauxitic clays. Bauxites grade chemically iato hydrated fermgiaous and manganiferous laterites. Hence, finely divided M2O2, usually hydrated, may be a significant constituent of a clay where M may be A1 or Fe. Hydrated colloidal s ica may play a role ia the sHppery and sticky properties of certain clays. 

High alumina clays refer ia the ceramic iadustries to nodular clays, budey-flint clay, budey and diaspore, gibbsitic or bauxitic kaolins (clays), abrasive clays, and others. Siace the depletion of diaspore varieties ia Missouri and Pennsylvania, most bauxitic kaolin and clay is produced ia Alabama and Arkansas. 

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 alkali process uses sodium hydroxide and is well known as Bayer s process. It involves relatively simple inorganic and physical chemistry and the entire flowsheet can be divided into caustic digestion, clarification, precipitation and calcination. Although mineral assemblage in bauxites is extensive, processing conditions are primarily influenced by the relative proportions of alumina minerals (gibbsite and boehmite), the iron minerals (goethite and hematite), and the silica minerals (quartz and clays-usually as kaolinite). 

The bauxite first undergoes a size reduction prior to dissolution by caustic. In the case of gibbsite, for example, the dissolution is described by the following equation ... 

Practically all world production of newly smelted aluminum was made from bauxite. The term bauxite is used for naturally occurring mixtures of aluminum monohydrate (boehmite or diaspore) and trihydrate gibbsite Al(OH)3, including impurities which are typically clay minerals, free silica, iron hydroxides and titania. The luminescence ofboehmite and diaspore and LIBS (Fig. 8.10) maybe used for the detection, sorting and commercial value evaluation of A1 minerals. 

The production of flame retardant quahty aluminium hydroxide has recently been reviewed [98]. Various crystal forms of aluminium hydroxide exist, but that used for polymer appHcations is Gibbsite. This occurs widely in nature, usually in the rock bauxite, but the natural form is usually not suitable for direct use and synthetic products are nearly always employed. Most aluminium hydroxide is manufactured through the Bayer process used to make alumina for refractory applications. 

In the Bayer process, the bauxite is leached with hot sodium hydroxide, thus forming a solution of sodium aluminate. After purification this solution is seeded with crystals of gibbsite and cooled. The process steps are summarised in Eqs. (1) and (2) ... 

Iron(III) oxide or alumina is refined from bauxite. Approximately 175 million tons of bauxite are mined annually worldwide, with virtually all of this processed into alumina. Alumina is a white crystalline substance that resembles salt. Approximately 90% of all alumina is used for making aluminum, with the remainder used for abrasives and ceramics. Alumina is produced from bauxite using the Bayer process patented in 1887 by Austrian Karl Josef Bayer (1847-1904). The Bayer process begins by grinding the bauxite and mixing it with sodium hydroxide in a digester. The sodium hydroxide dissolves aluminum oxide components to produce aluminum hydroxide compounds. For gibbsite, the reaction is Al(OH)3 + NaOH —> Al(OH)4 + Na+. Insoluble impurities such as silicates, titanium oxides, and iron oxides are removed from the solution while sodium hydroxide is recovered and recycled. Reaction conditions are then... 

High Alumina Clay Minerals. Several hydrated alumina minerals should be grouped with the clay minerals because the two types may occur so intimately associated as to be almost inseparable. Diaspore (a-AIO(OH)) and bochmitc (y-AIO(OH)). both AFOj H 0(AI Oi. 85% H 0. 15%) are Ihe chief constituents of diaspore clay, which may contain over 75% AhOj on the raw basis. Gibbsite, AliOj 3 HyO (AljOi. 65.4% HjO. 34.6%). and cliachite. the so-called amorphous alumina hydrate (much cliachite is probably cryptocryslalline), as well as Ihe monohydrates, occur in hauxile, bauxitic kaolin, and bauxitic clays. 

Bauxite is a mixture of minerals and contains e.g. diaspore [a-AlOOH], gibbsite [y-Al(OH)3], iron hydroxides, clay minerals and quartz. 

Aluminium oxide is the oldest ceramic material used in medicine. Bauxite and corundum are the main natural sources of aluminium oxide. Bauxite is a mixture of diaspore, gibbsite, iron hydroxides, clay minerals and quartz. It is formed by the tropical weathering of silicate rocks during which quartz and the elements sodium, calcium, magnesium and potassium are largely washed away. This is the reason why the remaining material becomes richer in the resistant elements titanium, iron and aluminium. The latter is extracted from this mixture in the form of aluminium hydroxide. In its turn this compound is converted into aluminium oxide by heating the mixture to 1200-1300 °C, this is called calcination. The hydroxide is thus made anhydrous. 

Aluminum is present in many primary minerals. The weathering of these primary minerals over time results in the deposition of sedimentary clay minerals, such as the aluminosilicates kaolinite and montmorillonite. The weathering of soil results in the more rapid release of silicon, and aluminum precipitates as hydrated aluminum oxides such as gibbsite and boehmite, which are constituents of bauxites and laterites (Bodek et al. 1988). Aluminum is found in the soil complexed with other electron rich species such as fluoride, sulfate, and phosphate. 

There are two main types of alumina (bauxite) ores used as the primary sources for aluminum metal and aluminum chemicals aluminum hydroxide [Al(OH)3] (gibbsite) and a mixed aluminum oxide hydroxide [AIO(OH)] (boehmite). Thus, bauxite is a term for a family of ores rather than a substance of one definite composition. An average composition of the ores used by industry today would be alumina (A1203), 35 to 60% silica (Si02), 1 to 15% ferric oxide (Fe203), 5 to 40% and titanium dioxide (Ti02), 1 to 4%. 

Natural sources of ATH (Gibbsite extracted from Bauxite) and magnesium hydroxide (Brucite) are available but generally have large particle size as a result of grinding operations and contain significant amounts of impurities. In wire and cable applications, finer particles sizes are utilized for higher FOI values, improved mechanical properties, lower brittleness temperatures, and smoother surface characteristics despite the drawback of increased mixture viscosity.75... 

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. 

Bacteria associated with bauxite ore have been shown to reductively mobilize significant amounts of the Fe(III) oxides in the bauxite under anaerobic conditions without mobilizing significant amounts of the Al(III) oxides like gibbsite or boehmite as long as the pH of the reaction system was kept above 4.5. The bauxite residue was thus enriched in aluminium (Ehrlich et al., 1995 Ehrlich Wickert, 1997). 

Aluminum is the second most abundant metal on earth s crust. It is a common metal in tropical soils called laterites (red soils). It is extracted from bauxite that is a rich laterite by Bayer process that involves dissolution and separation of the oxide in caustic soda solution between 150 and 250°C and 20 atm of pressure. Though abundant and inexpensive, alumina based CBPCs are difficult to form because even in an acid solution the solubility of alumina is very low. This solubility, however, can be enhanced by a mUd thermal treatment and suitable CBPCs can be formed. Alumina is available commercially as calcined alumina called corundum, or as its hydrated forms such as aluminum hydroxide (Al(OH)3), as bohmite, (A1203-3H20), gibbsite (AI2O3 H2O) or in impure forms as in kaolin clay. These mineral forms and their use in ceramic formation are discussed in Chapter 11. 

Aluminum, the third most abundant element in the Earth s crust, is mined as bauxite, a mixure of gibbsite, boehmite, and diaspore. These ores occur dominantly in Cenozoic deposits formed by lateritic weathering of aluminous parent rock, which has broken down to leave a high proportion of aluminum-bearing minerals. These deposits are residual in nature, with the balance of the rock constituents removed by selective leaching. These deposits form most commonly in tropical and subtropical areas. 

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