Oxides aluminum

Aluminum oxide (AI2O3), also known as alumina, is a highly stable compound with many industrial applications. It is readily obtained by CVD. Its characteristics and properties are summarized in Table 11.1. 

Chemical Resistance. Alumina is resistant to oxidation and has extremely low permeability to oxygen. It is not attacked by most chemical reagents at room temperature. 

A common reaction for the deposition of alumina is the hydrolysis of aluminum trichloride  

This reaction, based on the classic water-gas reaction, takes place in excess hydrogen, at an optimum temperature of 1050°C and at low pressure (ca. 1 Torr).bH3] It is the preferred reaction for tool coatings and electronic applications. The formation of alumina is 

Vickers Hardness 18.73 GPa Young s Modulus 378 GPa Flexural Strength 421 MPa Compressive Strength 3455 MPa 

Aluminum oxide (uh-LOO-min-um OK-side) is white crystalline powder that occurs in nature in a variety of minerals, including boehmite, bayerite, corundum, diaspore, and gibb-site. Corundum is second hardest naturally occurring mineral. Only diamond is harder. Aluminum oxide occurs in a variety of chemical forms in a variety of gemstones, including chryso-beryl, ruby, sapphire, and spinel. The color of these gemstones is a result of impurities, such as chromium (in the case of ruby) and iron and titanium (in the case of sapphire). The colors may also vary depending on the kind and amount of each impurity. 

Aluminum oxide s commercial uses depend not only on its hardness, but also on its high melting point and its low electrical conductivity. The compound is also non-combustible and resistant to attack by most solvents and other chemical agents. 

Aluminum oxide is produced by washing the rocky material bauxite with a hot solution of sodium hydroxide (NaOH). 

Aluminum oxide. Turquoise atoms are aluminum and orange atoms are oxygen. Gray sticks indicate double bonds. 

The aluminum hydroxide (A1(0H3)) that is produced in this reaction is then heated to drive off water, producing aluminum oxide. Wastes from coal mining operation are also treated to extract the aluminum sulfate (A12(S04)3) they contain. The aluminum sulfate is converted to aluminum hydroxide which, again, is heated to produce aluminum oxide. 

Aluminum oxide or alumina (AbOg) is also known as corundum. It is the most widely used inorganic chemical for ceramics and is produced from fhe bauxite mineral. The production method is called the Bayer process. Bauxite is hydrated aluminum oxide with iron oxide (Pe203), silica (Si02), and titania (Ti02) as impurities. It is the result of the weathering of igneous aluminous rocks under tropical conditions. 

A purity of 99.5% is obtained from the Bayer process, with the major impurity being NagO. The obtained crystal size range is 0.1-25 p. The Bayer process is a seven-step one. The first step is physical beneficiation. In this step, the bauxite is ground to get a particle size of 1 mm. Such small-sized particles reduce the time for the chemical reaction in the next step. 

In the reaction that takes place, hydrated alumina is converted into sodium aluminate. The reaction is represented by Equation 12.1. 

Filtration The impurities in bauxite do not dissolve in the previous step. Therefore, they are filtered out from the solution. 

Precipitation To the filtered sodium aluminate solution, fine gibbsite crystals are added. Gibbsite is a naturally occurring hydrated alumina [a-Al(OH)3], and its crystals act as seeds for the precipitation of A1(0H)3 from the solution. The pH of the solution is increased by bubbling carbon dioxide gas through it. This increases the precipitation. Washing The precipitate is separated from the solution by filtration. This is then washed to reduce its sodium content. 

Activated alumina activated aluminum oxide alpha aluminum oxide alumina alumina, activated alumina, calcined alumina, tabular aluminum oxide alumite aluminum trioxide. 

Aluminum oxide occurs naturally as the minerals bauxite, bayerite, boehmite, corundum, diaspore, and gibbsite. 

Aluminum oxide is used mainly in tablet formulations. It is used for decoloring powders and is particularly widely used in antibiotic formulations. It is also used in suppositories, pessaries, and urethral inserts. Hydrated aluminum oxide (see Section 18) is used in mordant dyeing to make lake pigments, in cosmetics, and therapeutically as an antacid. 

Aluminum oxide occurs as a white crystalline powder. Aluminum oxide occurs as two crystalline forms, a-aluminum oxide is composed of colorless hexagonal crystals, and y-aluminum oxide is composed of minute colorless cubic crystals that are transformed to the a-form at high temperatures. 

Solubility slowly soluble in aqueous alkaline solutions practically insoluble in nonpolar organic solvents, diethyl ether, ethanol (95%), and water. 

The compound is prepared by heating aluminum hydroxide or a-AlaOg to above 1100°C. Diaspore undergoes rapid conversion above 500°C. 

The )3 form crystallizes from molten aluminum oxide containing up to 5% of an alkali oxide. The transition takes place at a temperature not much below the melting point of cryolite (1020°C) when a mixture of cryolite and a-AlgOg is annealed. The a form is converted into yS-AlgOg after annealing a mixture containing 15-20% cryolite for 20 hours. 

Always contains a small amount of alkali. It is therefore occasionally considered to be a very aluminum-rich alkali alumi-nate. Hexagonal crystals. 

Recent investigations have established that annealing of alunai-num hydroxides (with the exception of diaspore) at temperatures between 400 and 1000°C yields not only y-AlgOg, but a series of various phases (x, rf, x, 6-AI3O3), all of which probably contain some residual water to stabilize the respective lattices. As far as range of existence and preparative conditions for these modifications is concerned, the original papers should be consulted. 

Teritan, Compt. Rend. Hebd. Seances Acad. Sci. 230. 1677 (1950). 

Bauxite is not used as such in aluminum production. First pure aluminum oxide has to be prepared from the ore. 

The finely ground bauxite is mixed with water to a slurry that is pumped into a pressure vessel, in which it is digested with a hot sodium hydroxide solution at 200-240°C and a pressure of about 30 atm. The alumina in the bauxite reacts with the caustic soda, forming a saturated solution of sodium aluminate NaAl(OH),j. The insoluble substances, mainly iron hydroxide, are separated from the solution and the red mud residue is thickened in filter presses and washed. The clear solution is passed into large, silo-like precipitators. Seed crystals of Al Oj 3HjO are added, which start the precipitation of Al(OH)3 that is filtered off and heated in rotary kilns at about 1100°C. The final product of this process is aluminum oxide Al Oj, white and pure. 

A relatively early work came from Sarikaya and Akinc [175] who used a mineral oil with Arlacel 83, a non-ionic surfactant (Sorbitan sesquioleate) as the support solvent (continuous phase) and an aqueous solution of aluminum nitrate as the water phase to form the emulsion by mechanical stirring. The most stable emulsions were obtained with W/O ratio of about 0.45-0.55 (with 5% emulsifier). The emulsion thus produced was added dropwise to a hot (240 C) mineral oil to obtain precipitates of an alumina precursor. 

The brown colored precursor particles of alumina were washed in toluene, dried up to 240 C and finally calcined at 1000 C/4h. The mean diameter of the synthesized white, hollow alumina particles increased from about 3pm to 9pm with increase in the concentration of aluminum nitrate ( - 0.25-2.0 mol/1) in the water phase. 

In a more recent work, Ray et al. [34] used A1(N03)3.9H20 for preparing an aqueous sol. The salt was dissolved in water maintaining the required solution concentration, and the solution heated to 70 -80 C under stirring and dropwise addition of ammonium hydroxide solution (1 1) to cause polymerization. The sol was dispersed in 1,1,1 trichloroethane with Span 80 as emulsifier. Externally gelled droplets were washed with acetone and dried/calcined at 70°-l 300 C. The spherical particles thus produced ranged in size between 4 and 30 pm. The phase development as a function of increasing temperature (Ih soak) was Amorphous y-f 5 —  

Siladitya et al, [42] used 1,1,1 trichloroethane as the oil phase with 0.5-15 vol% of Span 80 as emulsifier, and an aluminum-containing sol prepared by the same method as followed by Ray et al, [34] the sol organics volume ratio was generally 1 4. Gelation of the water phase droplets was effected by controlled addition of triethylamine. The gathered particles were washed with acetone, dried at 200 C and calcined at 1200 C /I h. Only mechanical agitation produced micronsized (1-50 pm) particles, while simultaneous sonication could yield around 10-650 nm particles under different conditions. 

A detailed work of Chatterjee et al. [57] describes the role of different organic 

It was mentioned above that the cluster modeling of the surface sites of highly coordinated oxide lattices faces certain difficulties. This is probably the reason why only a few computations were performed for such systems. The aluminum oxide structure is just of this type. 

The aluminum oxide surfaces exhibit only a weak Bronsted acidity. Therefore, studies of its surface hydroxyls have usually dealt with their structure. IR spectroscopy appears the most informative and appropriate method for these purposes. The IR spectra cover a wide variety of hydroxyl stretching vibrations, and their assignment is frequently quite a problem. Primarily, it was based on electrostatic concepts and on the electrostatic theory of valency (138). 

The cluster approach opens the way for a direct quantum-chemical calculation of the influence of the nearest environment on OH vibration frequencies (12). In such a computation, the scheme of the neutral ionic cluster (12,144) is used that allows one to construct easily a consistent set of the required cluster structures. Two examples are clusters 7a and 7b. Cluster 7a models the hydroxyl group bonded with the A1 atom in tetrahedral coordination. Cluster 7b simulates the bridged hydroxyl group linking two 

Results of Calculations of Differently Coordinated OH Groups of Aluminum Oxidef 

The authors would like to express their profound gratitude to Drs. I. D. Mikheikin, A. G. Pelmenshchikov, and I. N. Senchenya for their invaluable assistance in preparing this review and to Dr. N. N. Weinberg for translating the manuscript. 

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. 

Colorless hexagonal crystal refractive index 1.768 density 3.965 g/cm (at 25°C) mp 2072°C bp 2980°C insoluble in water 

Colorless rhombic crystal mp between 2005 to 2025°C density 4.022 g/m hardness 9Moh white microscopic crystal 

Alumina exhibits amphoteric behavior. It is soluble both in acids and bases. With acids, it produces their corresponding salts. It froms Al2(S04)3, AlCNOsls and AICI3 upon reactions with H2SO4, HNO3, and HCl, respectively. In acid medium, it exists as a solvated aluminum ion, in which water molecules are hexacoordinated to trivalent AP+, as shown below  

Alumina forms hydroxide in aqueous alkaline solution. The reaction is slow. The products, aluminum hydroxides (hydrated aluminas), contain hexacoordinated aluminohydroxide anion  

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Using the data of Table X-2, estimate the contact angle for benzene on aluminum oxide and the corresponding adhesion tension. 

The monolayer amount adsorbed on an aluminum oxide sample was determined using a small molecule adsorbate and then molecular-weight polystyrenes (much as shown in Ref. 169). The results are shown in the table. Calculate the fractal dimension of the oxide. 

The compounds of greatest importance are aluminum oxide, the sulfate, and the soluble sulfate with potassium (alum). The oxide, alumina, occurs naturally as ruby, sapphire, corundum, and emery, and is used in glassmaking and refractories. Synthetic ruby and sapphire are used in lasers for producing coherent light. 

METHOD 1 [112l-5g phenol in dH20 is stirred 5 hours at 20 C with some ferric sulfate (Fe2(S04)3, an additional 7mLs dH20, 13mLs 6% H2O2 and a pinch of aluminum oxide (AI2O3). Yield of catechol is 2.5g (50%). 

Ethylene oxide Acids and bases, alcohols, air, 1,3-nitroaniline, aluminum chloride, aluminum oxide, ammonia, copper, iron chlorides and oxides, magnesium perchlorate, mercaptans, potassium, tin chlorides, alkane thiols... 

Microstructurc. Crystal size, porosity, and impurity phases play a major role in fixing the fracture characteristics and toughness of an abrasive grain. As an example, rapidly cooled fused aluminum oxide has a microcrystalline stmcture promoting toughness for heavy-duty grinding appHcations, whereas the same composition cooled slowly has a macrocrystalline stmcture more suitable for medium-duty grinding. 

Corundum. Comndum [1302-75-5] (see Aluminum compounds) is a naturally occurring massive crystalline mineral composed of aluminum oxide. It is an impure form of the gems mby and sapphke. Prior to 1900 comndum was an important abrasive for the production of grinding wheels. Today it is mainly employed as a loose abrasive for grinding and polishing optical lenses. Almost all the world s supply of comndum now comes from Africa, primarily from Zimbabwe. 

Sol—Gel Sintered Aluminum Oxide. A new and much more versatile sintered alumina abrasive is now produced from aluminum monohydrate, with or without small additions of modifiers such as magnesia, by the sol—gel process (see Sol-gel technology). The first modified sol—gel abrasive on the market, Cubitron, was patented (27) and produced by the 3M Corporation for products such as coated belts and disks. The success of this material promoted intensive research into sol—gel abrasives. 

Except for siUca and natural abrasives containing free siUca, the abrasive materials used today are classified by NIOSH as nuisance dust materials and have relatively high permissable dust levels (55). The OSHA TWA allowable total dust level for aluminum oxide, siUcon carbide, boron carbide, ceria, and other nuisance dusts is 10 mg/m. SiUca, in contrast, is quite toxic as a respkable dust for cristobaUte [14464-46-1] and tridymite [15468-32-3] the allowable TWA level drops to 0.05 mg/m and the TWA for quartz [14808-60-7] is set at 0.1 mg/m. Any abrasive that contains free siUca in excess of 1% should be treated as a potential health hazard if it is in the form of respkable dust. Dust masks are requked for those exposed to such materials (see Industrial hygene). 

See Adsorption, LIQUID separation Aluminum compounds, aluminum oxide (alumina) Carbon, activated carbon Ion exchange Molecular sieves Silicon... 

Physical Properties. Physical properties of importance include particle size, density, volume fraction of intraparticle and extraparticle voids when packed into adsorbent beds, strength, attrition resistance, and dustiness. These properties can be varied intentionally to tailor adsorbents to specific apphcations (See Adsorption liquid separation Aluminum compounds, aluminum oxide (alumna) Carbon, activated carbon Ion exchange Molecular sieves and Silicon compounds, synthetic inorganic silicates). 

Traditional adsorbents such as sihca [7631 -86-9] Si02 activated alumina [1318-23-6] AI2O2 and activated carbon [7440-44-0], C, exhibit large surface areas and micropore volumes. The surface chemical properties of these adsorbents make them potentially useful for separations by molecular class. However, the micropore size distribution is fairly broad for these materials (45). This characteristic makes them unsuitable for use in separations in which steric hindrance can potentially be exploited (see Aluminum compounds, aluminum oxide (ALUMINA) Silicon compounds, synthetic inorganic silicates). 

Aluminum-containing propellants deflver less than the calculated impulse because of two-phase flow losses in the nozzle caused by aluminum oxide particles. Combustion of the aluminum must occur in the residence time in the chamber to meet impulse expectations. As the residence time increases, the unbumed metal decreases, and the specific impulse increases. The soHd reaction products also show a velocity lag during nozzle expansion, and may fail to attain thermal equiUbrium with the gas exhaust. An overall efficiency loss of 5 to 8% from theoretical may result from these phenomena. However, these losses are more than offset by the increase in energy produced by metal oxidation (85—87). 

There are several processes available for the manufacture of cryoHte. The choice is mainly dictated by the cost and quaUty of the available sources of soda, alumina, and fluoriae. Starting materials iaclude sodium aluminate from Bayer s alumina process hydrogen fluoride from kiln gases or aqueous hydrofluoric acid sodium fluoride ammonium bifluoride, fluorosiUcic acid, fluoroboric acid, sodium fluosiUcate, and aluminum fluorosiUcate aluminum oxide, aluminum sulfate, aluminum chloride, alumina hydrate and sodium hydroxide, sodium carbonate, sodium chloride, and sodium aluminate. 

Formamide decomposes thermally either to ammonia and carbon monoxide or to hydrocyanic acid and water. Temperatures around 100°C are critical for formamide, in order to maintain the quaUty requited. The lowest temperature range at which appreciable decomposition occurs is 180—190°C. Boiling formamide decomposes at atmospheric pressure at a rate of about 0.5%/min. In the absence of catalysts the reaction forming NH and CO predominates, whereas hydrocyanic acid formation is favored in the presence of suitable catalysts, eg, aluminum oxides, with yields in excess of 90% at temperatures between 400 and 600°C. 

Mag nesia. ndAlumina. Suspension. A mixture of salts, available as Maalox, Mylanta, Gelusil, and Aludrox, contains magnesium hydroxide [1309-42-8] Mg(OH)2, and variable amounts of aluminum oxide in the form of aluminum hydroxide and hydrated aluminum oxide, ie, 2.9—4.2% magnesium hydroxide and 2.0—2.4% aluminum oxide, Al O, for a mixture of 4.9—6.6% combined magnesium hydroxide and aluminum oxide. This mixture may contain a flavoring and antimicrobial agents in a total amount not to exceed 0.5% (see Aluminum compounds, aluminum oxide). 

The amount of herbicide sorbed by a given soil is influenced by properties of both the soil and the herbicide. Important properties related to the soil s retention abiHty include clay mineralogy, organic matter content, soil pH, and iron and aluminum oxide content. These properties, in turn, affect the... 

Hollow Fiber with Sorbent Walls. A cellulose sorbent and dialy2ing membrane hoUow fiber was reported in 1977 by Enka Glan2stoff AG (41). This hoUow fiber, with an inside diameter of about 300 p.m, has a double-layer waU. The inner waU consists of Cuprophan ceUulose and is very thin, approximately 8 p.m. The outer waU, which is ca 40-p.m thick, consists mainly of sorbent substance bonded by ceUulose. The advantage of such a fiber is that it combines the principles of hemodialysis with those of hemoperfusion. Two such fibers have been made one with activated carbon in the fiber waU, and one with aluminum oxide, which is a phosphate binder (also see Dialysis). 

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 ... 

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