Impurities aluminate

Freund [42] and Lowe et al. [43] in which the activity of the sihca powders was concluded to reside with aluminate impurities in the silica powders. 

Cement. Portland cement, a mixture of calcium siUcate and calcium aluminate minerals, is produduced by the calcination of argillaceous limestone or mixtures of limestone and clay (see Cement). Although other clays can be used, ka olin is preferred because of its alumina and siUca content and low level of impurities. It is especially desirable in the manufacture of white cement and other types requiring careful control of chemical composition. Air-floated ka olin, because of its low cost, is usually used. 

Aluminum is the most abundant metallic element in the Earth s crust and, after oxygen and silicon, the third most abundant element (see Fig. 14.1). However, the aluminum content in most minerals is low, and the commercial source of aluminum, bauxite, is a hydrated, impure oxide, Al203-xH20, where x can range from 1 to 3. Bauxite ore, which is red from the iron oxides that it contains (Fig. 14.23), is processed to obtain alumina, A1203, in the Bayer process. In this process, the ore is first treated with aqueous sodium hydroxide, which dissolves the amphoteric alumina as the aluminate ion, Al(OH)4 (aq). Carbon dioxide is then bubbled through the solution to remove OH ions as HCO and to convert some of the aluminate ions into aluminum hydroxide, which precipitates. The aluminum hydroxide is removed and dehydrated to the oxide by heating to 1200°C. 

The production of synthesis gas from natural gas and coal is the basis of the 33 000000 tpa methanol production and is also used in the production of ammonia. After removal of sulfur impurities, methane and water are reacted over a nickel oxide on calcium aluminate catalyst at 730 °C and 30 bar pressure. The reaction is highly endothermic (210 kJmol ) (Equation 6.6). 

Bayer A process for making pure alumina hydrate from bauxite, used principally as a raw material for the manufacture of aluminum metal. The ore is digested with hot sodium hydroxide solution, yielding a solution of sodium aluminate. Insoluble impurities are separated off in the form of red mud and the solution is then nucleated with alumina hydrate from a previous batch, causing alumina trihydrate to precipitate ... 

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 ... 

The major reactions are complicated by a number of minor or secondary reactions, and by impurities in the raw materials. As a result the black ash may contain 1 to 2 per cent, of sodium silicate to If per cent, of sodium aluminate 1 per cent, of sodium ferrous sulphide small proportions of sodium cyanide and thiocyanate derived from the nitrogen of the coal a relatively small amount of ultramarine etc. Proposals to use barium carbonate, etc., in place of limestone in the black-ash process are indicated in connection with the preparation of sodium carbonate from sodium sulphide. 

The carbide impurities tend to distill and are reoxidized in the upper, cooler region of the charge. This process forms crusts near the top of the charge or around the cooler part of the reaction cmcible and causes trouble in furnace operation. Large amounts of dissolved calcium silicate and aluminate may form a viscous melt and impede the tapping process. Ferrosilicon is commonly removed from the crushed carbide by electromagnets. 

The high stability of the aluminate ion allows the production of concentrated solutions of aluminum with the virtual exclusion of the main metallic impurity, viz. iron as an oxide residue. The resultant impure aluminate solution is clarified and its temperature reduced when the reverse of the above reaction occurs with the formation of A1203,3H20 by a slow crystallization procedure. The high-purity alumina trihydrate product is calcined and then reduced electrochemically in a molten fluoride bath by the well-known Hall-Heroult process. The major problems in the Bayer process have their origin in the coordination chemistry of aluminum in alkaline solutions. The... 

Sometimes, ores are concentrated by chemical treatment. In the Bayer process, for instance, the A1203 in bauxite (A1203 x H20) is separated from Fe203 impurities by treating the ore with hot aqueous NaOH. The amphoteric A1203 dissolves as the aluminate ion, Al(OH)4 , but the basic Fe203 does not ... 

The sodium carbonate is lixiviated with water to separate it from the calcium sulphide or alkali-waste, and from other impurities such as sodium chloride, sulphate, silicate, and aluminate calcium oxide, sulphite, and thiosulphate iron oxide and alumina. 

Both MgO and CaO have high melting temperatures and do not combine into compounds. The only eutectic in the system CaO — MgO melts at about 2300 C. In a pure binary system, very high temperatures are required for sintering. However, natural dolomite always contains impurities (FCiOs, AI2O3, Si02) which facilitate formation of clinker at lower temperatures. The impurities partly produce a liquid phase above 1300 °C, and form silicates, ferrites and aluminates of calcium either directly or by recrystallization from the melt. However, most of the CaO and MgO remain uncombined. 

During electrolysis, per mole of potassium permanganate one mole of potassium hydroxide is produced, which has to be recovered. This can be achieved, for example, by evaporating the mother liquor to 750 g KOH per L, whereupon the dissolved potassium manganate(Vl) and calcium hydroxide crystallize out and are removed. The potassium hydroxide can be returned to the dissolution step. Other dissolved impurities from the ores, such as silicates or aluminates, have to be removed from the alkali cycle. 

Lithium aluminate, LiAlsOg with the inverse spinel structure, is a material with possible applications in ceramic blankets for thermal control of fusion reactors. Li and Li NMR has been used to measure the spin-lattice relaxation of lithium in this compound (Stewart et al. 1995). The results indicate that Li relaxes most significantly through interactions with paramagnetic impurities, whereas Li relaxes much more strongly through dipole-dipole interactions. 

Iron oxide, clays, and most other impurities do not dissolve under alumina digestion conditions and are quite finely divided (1-10 p,m). By adding wash water to decrease the viscosity, it is possible to decant the still hot sodium aluminate solution from the slowly precipitating red muds. These muds that are red from the high iron content are then washed with water to minimize losses of alumina and sodium hydroxide on disposal of the muds. By using this wash water for dilution of the next digester output, alumina and base are not lost from the Bayer circuit. Newer facilities use pressure filtration for both removal and washing of red muds. 

The specific surface areas vary between 0.6 and 11.7 m /g. The main impurities of the AI2O3 samples are CaO, Si02, Na20 and MgO. The eventual combination of these elements on the alumina surface is not known. The values of 7, determined at 100°C are in the 90-100 mj/m range and there is no evident correlation between 7 whatever the impurities contents. However, it is known, particularly in catalysis, that the association of AI2O3 and Si02 leads to silico-aluminates of increased Lewis acidity. To check this, we evaluated an acid base parameter, which is simply the ratio of the retention volumes of a basic probe, i.e. diethyl ether, and of an acidic probe, i.e. dichloroethane ... 

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