Aluminium oxide

Aluminum oxide is found in the natural environment in the form of the most common alumina, its chemical formula is AL2O3. In appearance, these are crystals that do not have color, which begin to melt at a temperature of 2044 ° C, 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 on the Mohs table. The magnitude of the refractive index is: for the ordinary ray - 1,765, and 1,759 for the extraordinary. In the natural environment, aluminum oxide often contains a variety of metal oxides, so a mineral of corundum can acquire different shades of its color. For example, such are sapphires, rubies and other precious stones. In this form, aluminum oxide can also be obtained by laboratory-chemical means. To this end, metastable forms of A12O3 are used and are decomposed by a thermal method. Aluminum hydroxide is also used as a source of aluminum oxide production by the laboratory method .

The standard modification of the compound is a tetragonal crystal lattice containing about 1-2% water in its composition. It is also possible to obtain amorphous alumina in its structure, for which the gel solution AL (OH) 3 is dehydrated and a 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. Characteristic is the property of synthesized aluminum oxide, as an inverse relationship between the temperature of its formation and chemical activity. As an artificial (that is, obtained at a temperature of more than 1200 ° C), and natural corundum in an ordinary medium, they exhibit practically one hundred percent chemical inertness and a complete absence of hygroscopicity.

Aluminum oxide chemical properties begin to actively manifest at temperatures around 1000 ° C, when it begins to interact intensively with substances such as various alkalis, alkali metal carbonates . In this interaction, aluminates are formed. More slowly, the compound reacts with SiO2, as well as various kinds of slags acidic. As a result of these interactions, aluminosilicates are obtained.

Aluminogels and aluminum oxide, which are obtained by roasting any of the aluminum hydroxides at a temperature of at least 550 ° C, have a very high hygroscopicity, excel in chemical reactions and actively interact with acidic and alkaline solutions.

As a rule, bauxites, alunites, and also nephelines serve as a raw material for the production of aluminum oxide. If the content of the substance in question is more than 6-7%, the production is carried out by the main method - the Bayer method, and with a lower content of the material, the method of sintering the ore with lime or soda is used. The Bayer method is the processing of crushed rock in ball mills, then the bauxite is treated with alkaline solutions at a temperature of 225-250 ° C. The sodium aluminate composition thus obtained is diluted with an aqueous solution and filtered. During the filtration process, the sludge containing alumina, whose properties correspond to the standard ones, are 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 the mother liquor to be stored for use in subsequent bauxite leaching operations.

As a rule, the oxide obtained by the synthetic aluminum method is used as an intermediate material for the production of pure aluminum. In industry, it is used as a raw material for manufacturing refractory materials, abrasive and ceramic cutting tools. Modern technologies actively use single crystals of aluminum oxide in the production of watches, printed circuit boards, jewelry.