Bauxite and Aluminas

Bauxite is the major source of aluminum sesquioxide alumina, Al Oj) worldwide. Bauxite is a soft and red clay, rich in alumina, and its name originates from Les Baux de Provence, a small vihage located in the region of Arles in southeastern France, where it was first discovered in 1821 by P. Berthier. From a geological point of view bauxite is defined as a residual sedimentary rock in the laterite family that results from in situ superficial weathering in 

Gibbsite, boehmite and diaspore Quartz, chalcedony, kaolinite Goethite, hematite and siderite Rutile and anatase Calcite, magnesite and dolomite 

According to the US Geological Survey, the world s bauxite resources are estimated to be 55 to 75 billion tonnes located mainly in South America (33%), Africa (27%), Asia (17%), Oceania (13%), and elsewhere (10%). Today, Australia supplies 35% of the demand worldwide for bauxite. South America 25%, and Africa 15%. The current reserves are estimated at being able to supply worldwide demand for more than two centuries. Note that about 95% of bauxite is of the metallurgical grade and hence used for the production of primary aluminum metal. 

Gibbsitic 2A10(0H).Hj0 + 2NaOH — 2NaA10j + 4H,0 Atmospheric pressure 

Boehmitic 2A10(0H) + 2NaOH — 2NaA10, + 2H,0 Diasporic 2A10(0H) + 2NaOH — 2NaA10, + 2H,0 

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M. F. Stevenson, Jr., Anodizing in ASM Handbook, Volume 5, Surface Engineering, p. 482-493. ASM International, Metals Handbook, Metals Park OH, 1996 P. A. Cox, The Elements on Earth, Oxford University Press, 1995 Patricia A. Plunkert, Bauxite and Alumina chapter in U.S. Geological Survey Mineral Commodity Summaries 2002, January 2002, p. 30-31, and Aluminum chapter in U.S. Geological Survey Minerals Yearbook 2002, Vol. I, Metals and Minerals, p. 5.1-5.10, http //minerals. usgs.gov/minerals/pubs/commodity/ aluminum... 

The more important groups of petroleum adsorbents are (1) fuller s earth, (2) bentonite, (3) various natural and treated clays, (4) bog iron ore, (5) bauxite and alumina, and (6) activated carbon. These are often... 

Sintered abrasives made from bauxite and calcined alumina are heavy-duty abrasives they are much too strong and tough for precision grinding. 

The quantity of feed materials required are 1—1.05 kg of metallic reductant, 5.4 kg of dolime, and 0.35 kg of calcined bauxite or alumina to produce 1 kg of magnesium. The quantity of slag produced depends on the feed material composition and may vary from 5.2 to 5.9 kg/kg of magnesium. 

Alternative Processes for Aluminum Production. In spite of its industrial dominance, the HaH-HAroult process has several inherent disadvantages. The most serious is the large capital investment requited resulting from the multiplicity of units (250 —1000 cells in a typical plant), the cost of the Bayer aluniina-puriftcation plant, and the cost of the carbon—anode plant (or paste plant for Soderberg anodes). Additionally, HaH-HAroult cells requite expensive electrical power rather than thermal energy, most producing countries must import alumina or bauxite, and petroleum coke for anodes is in limited supply. 

Calcium Aluminate Cements. Low purity calcium aluminate [12042-78-3] cements are obtained by sintering or fusing bauxite and lime in a rotary or shaft kiln. A high purity calcium aluminate cement, 2CaO 5AI2O2, capable of withstanding service temperatures of 1750°C can be prepared by the reaction of high purity lime with calcined or hydrated alumina (see Aluminum compounds). 

This reaction is cataly2ed by silica, bauxite, and various metal sulfides. The usual catalyst is activated alumina, which also cataly2es the reduction by methane (228). Molybdenum compounds on alumina are especially effective catalysts for the hydrogen sulfide reaction (229). 

Because calcium oxide comprises about 65% of Pordand cement, these plants are frequendy situated near the source of their calcareous material. The requisite silica and alumina may be derived from a clay, shale, or overburden from a limestone quarry. Such materials usually contain some of the required iron oxide, but many plants need to supplement the iron with mill scale, pyrite cinders, or iron ore. Silica may be supplemented by adding sand to the raw mix, whereas alumina can be furnished by bauxites and Al202-rich flint clays. 

Many metals are extracted from their compounds, as found in ores, by electrolytic processes. By far the most important is the Hall-Heroult process, invented in 1886, for producing aluminium from alumina, itself refined from bauxite ore. Alumina is dissolved in molten cryolite, Na3Alp6, and electrolysed, using carbon anodes and the aluminium itself as cathode. While various details are being steadily improved, the basic process is still the same today. 

At the alumina plant, the bauxite ore is further crushed to the correct particle size for efficient extraction of the alumina through digestion by hot sodium hydroxide liquor. After removal of "red mud" (the insoluble part of the bauxite) and fine solids from the process liquor, aluminum trihydrate crystals are precipitated and calcined in rotary kilns or fluidized bed calciners to produce alumina (AljOj). Some alumina processes include a liquor purification step. 

High-alumina cement is a rapid-hardening cement made from bauxite and limestone. It is comparatively resistant to chemical attack. Milling retards the setting of aluminous cement [1582]. On the other hand, setting accelerators such as lithium carbonate increase their effect by this treatment. 

Pedersen A process for extracting aluminum from bauxite, which also yields metallic iron. The ore is first smelted in an electric furnace with limestone, iron ore, and coke at 1,350 to 1,400°C to produce a calcium aluminate slag and metallic iron. Aluminium is leached from the slag by sodium carbonate solution, and alumina is then precipitated from the leachate by carbon dioxide. The process requires cheap electricity and a market for the iron. It was invented by H. Pedersen in 1924 and operated at Hoy anger, Norway, from 1928 until the mid-1960s. British Patent 232,930. 

Townsend MC, Enterline PE, Sussman NB, et al Pulmonary function in relation to total dust exposure at a bauxite refinery and alumina-based chemical products plant. Am Rev Respir Dis 132 1174-1180, 1985... 

The hydrogen sulfide is then oxidized with air at 1000°C over a bauxite or alumina catalyst. The reactions taking place are given below. The Claus process is increasing in popularity and accounted for 24% of sulfur in 1973, 46% in 1980, 74% in 1991, and 87% in 1999. 

Various well-known industrial and municipal waste products particularly those from the base metal industry, contain appreciable amounts of Fe oxides which may make them suitable for remediation purposes. Two examples from industry are the residues from the alumina and the titanium industries. The extraction of either Al or Ti from the natural ores (bauxite and ilmenite/rutile, respectively) leaves behind an alkaline and acidic (sulphuric) residue, respectively, in which Fe oxides are enriched, as indicated by their names Red Mud and Red Gypsum . A sample of Red gypsum is reported to contain ca. 35% of Fe oxide consisting of goethite and hematite, half of which was oxalate soluble (Fauziah et al., 1996). As expected, this material had an appreciable adsorption capacity for phosphate and heavy metals and, if added to soils, could confer these properties on them (Peacock Rimmer, 2000),... 

Occurs in nature in abundance the principal forms are bauxites and lat-erites. The mineral corundum is used to produce precious gems, such as ruhy and sapphire. Activated aluminas are used extensively as adsorbents because of their affinity for water and other polar molecules and as catalysts because of their large surface area and appropriate pore sturcture. As adsorbents, they are used for drying gases and liquids and in adsorption chromatography. Catalytic properties may be attributed to the presence of surface active sites (primarily OFT, 02, and AF+ ions). Such catalytic applications include sulfur recovery from H2S (Clauss catalysis) dehydration of alcohols, isomerization of olefins and as a catalyst support in petroleum refining. 

The production of aluminum actually involves several steps. Bauxite is the ore that contains aluminum oxide (Al Oj) used to produce aluminum. Impurities of iron, sulfur, silicon and other elements are removed from bauxite using the Bayer process to produce purified alumina. The Bayer process, patented in 1887 by Austrian Karl Josef Bayer (1847-1904), involves pulverizing bauxite and treating it with a hot sodium hydroxide solution to produce sodium alu-minate (NaA102). Sodium aluminate is then placed in a reactor in which temperature and pressure can be varied to precipitate out impurities. The sodium aluminate solution is then hydrolyzed to produce purified alumina ... 

Bauxitic Kaolins andMullites. Deposits of bauxitic kaolins, kaolins having aluminous minerals, have been discovered that have alumina contents between 50 and 70%. These materials are made into refractory aggregates called calcines, grog, clinker, or grain. In addition to sdectivdy mined deposits, synthetic compositions can be prepared from kaolin and alumina and other minerals to produce compositions of desired alumina and mineralogical content. These synthetic mullites are readily available in the form of sintered and fused aggregates. 

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... 

Schrevelius asserts that in terms of the law of Vegard (1917) determining the lattice parameter a, we can derive the formula for a solid solution which is an abrasive material, e.g., (Ti0.oo6Al0.994.)203. Exactly the same can be done by an accurate measurement of hardness. Since all bauxite synthesized alumina contain up to some 3% Ti02, it is important to determine how much titanium is contained in the solid solution of the abrasive and how much is bonded into a soft aluminate CaO 6 [(Al, Ti)203] or is exuded as free rutile Ti02. 

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