Aluminum oxide is found in the environment as the most common alumina; its chemical formula is AL2O3. In appearance, these are crystals that have no color, which at a temperature of 2044 ° C begin to melt, and when they reach the mark of 3530 ° C, they boil.
In the natural environment, the only stable modification of the substance is corundum, having a density of 3.99 g / cm3. This is a very solid sample belonging to the ninth level according to the Mohs table. The value of the refractive index is: for an ordinary ray - 1.765, and 1.759 for an extraordinary. In a natural environment, alumina often contains various metal oxides, therefore, the corundum mineral can acquire different shades of its color. For example, such are sapphires, rubies and other precious stones. In this form, alumina can be obtained by laboratory chemical methods. To do this, use metastable forms of A12O3 and decompose them thermally. Also, aluminum hydroxide is used as a laboratory source of aluminum oxide .
The standard modification of the compound is a tetragonic crystal lattice containing approximately 1-2% water in its composition. It is possible to obtain alumina that is amorphous in structure - alumogel, for which the gel-like AL (OH) 3 solution is dehydrated and the substance is obtained in the form of a porous transparent mass.
Aluminum oxide is completely insoluble in water, but can dissolve well in cryolite, heated to a high temperature. The substance is amphoteric. Such a property of synthesized aluminum oxide is characteristic as the inverse dependence of its formation temperature and chemical activity. Both artificial (that is, obtained at a temperature of more than 1200 ° C) and natural corundum in an ordinary environment exhibit almost one hundred percent chemical inertness and a complete absence of hygroscopicity.
Alumina begins to actively exhibit chemical properties at temperatures around 1000 ° C, when it begins to intensively interact with substances such as various alkalis, alkali metal carbonates . Aluminates are formed during this interaction. More slowly, the compound reacts with SiO2, as well as various kinds of acidic slags. As a result of these interactions, aluminosilicates are obtained.
Alumogels and alumina, which are obtained by firing any of the aluminum hydroxides at a temperature of at least 550 ° C, have very high hygroscopicity, excellently enter into chemical reactions and actively interact with acidic and alkaline solutions.
As a rule, bauxite, alunite, and also nepheline are used as raw materials for producing aluminum oxide. When the content of the substance in question is more than 6-7%, the production is carried out in the main way - the Bayer method, and with a lower content of the substance, the method of sintering ore with lime or soda is used. The Bayer method is the processing of crushed rock in ball mills, then bauxite is treated with alkaline solutions at a temperature of 225-250 ° C. The sodium aluminate composition thus obtained was diluted with an aqueous solution and filtered. In the filtration process, sludge containing aluminum oxide, the properties of which correspond to standard, is subjected to decomposition in centrifuges. This technology makes it possible to obtain a 50% yield of the substance. In addition, the use of this method allows you to save the mother liquor for use in subsequent operations to leach bauxite.
As a rule, synthesized aluminum oxide is used as an intermediate material to produce pure aluminum. In industry, it is used as a raw material for the manufacture of refractory materials, abrasive and ceramic cutting tools. Modern technologies are actively using single crystals of aluminum oxide in the manufacture of watches, printed circuit boards, and jewelry.