Aluminium

Aluminium compounds

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What is a chemical compound?

  • Aaluminium compound is a substance, which is formed of two or more chemical elements, one of which is aluminium.
  • These elements are interconnected chemically by covalent, ionic or metallic bonds.
  • Usually, each chemical compound has a fixed and constant chemical composition of the chemical formula.

Metallic aluminium

Pure aluminum is a silvery-white, malleable, ductile metal having an atomic number 13 and relative atomic mass 26,98. With few exceptions, it is in the form of chemical compounds in the Al+3.

This metal has amphoteric properties. It reacts with mineral acids and strong alkalis. Although aluminum is one of the most chemically active base metals, it has a very high resistance to corrosion. When contacting the aluminum surface with fresh oxygen, water or other oxidizing agents, a dense oxide film (Al2O3), metal which provides high resistance to corrosion. This oxide film dissolves in alkaline solutions with the evolution of hydrogen and the formation of soluble alkaline earth aluminates [1].

This oxide film is resistant to some acids (for example, nitric acid) and prevents further chemical attack on the metal. However, it dissolves in some acids (for example, in hydrochloric acid or hot sulfuric acid), as well as in alkaline solution, that it makes it possible to further chemical reactions on the aluminum surface.

At elevated temperatures, aluminum reacts:

  • with water (above 180 ºС) with the formation of hydroxide Al (OH)3 and hydrogen H2;
  • many metal oxides to form oxide Al2O3 and metal, reduced from its oxide.

The latter reaction is used in the production of certain metals (alumothermy), for example, manganese and some alloys, for example, ferrotitanium.

Aluminium alloys:

  • Are not chemical compounds.
  • Are homogeneous or heterogeneous mixtures of their constituent chemical elements or compounds.
  • Do not have a fixed chemical composition and do not have the chemical formula.

Classification of aluminium compounds

The modern classification of aluminium compounds from the Encyclopedia of Aluminum [4]:

Class 1 – Binary compounds with nonmetals:

  • Oxides: Al2O3, AlO2, AlO
  • Nitride: AlN
  • Carbide: Al4C3
  • Phosphide: AlP
  • Silicide: AlSi
  • Borides: AlB12, AlB31
  • Hydride: AlH3
  • Сhalcogenides: Al2S3, Al2Se3, Al2Tl3
  • Hallogenides: AlCl3, AlF3, AlCl, AlBr3, AlBr

Class 2 – Intermetallides:

  • CuAl2, Al2MgLi, Al2Mg2Cu

Class 3 – Hydroxides

  • Hydroxide: Al(OH)3
  • Oxide-hydroxide: AlOOH

Class 4 – Aluminium salts

  • Sulphate: Al2(SO4)3∙18H2O
  • Chloride: AlCl3∙6H2O
  • Phosphate: AlPO4
  • Potassium alum: KAl(SO4)2∙12H2O
  • Cryolite: Na(AlF3)

Class 5 – Aluminates

  • Mixed oxides: MgAl2O4, Y3Al5O12
  • Salt-like: NaAlO2, K3AlO2
  • Hydro-complexes: Ba3(Al(OH)6)2

Class 6 – Organometallic compounds:

(CH3)3Al

Natural compounds of aluminium

Minerals

The most important minerals in aluminum [4]

  • Corundum: Al2O3
  • Diaspore, boehmite: γ-AlO(OH)
  • Chrysoberyl, beryllium aluminate: BeAl2O4
  • Gibbsite: Al(OH)3
  • Kyanite: Al2∙SiO4
  • Kaolinite: Al2O3∙2SiO2∙2H2O
  • Muscovite: K2O∙3Al2O3∙6SiO2∙H2O
  • Alunite: (Na, K)Al3(OH)6(SO4)2
  • Anorthite: CaO∙Al2O3∙2SiO2
  • Nepheline: (Na, K)2O∙Al2O3∙2SiO2
  • Spodumene: Li2O∙Al2O3∙4SiO2
  • Cryolite: Na3AlF6
  • Albite: Na2O∙Al2O3∙6SiO2
  • Orthoclase: K2O∙Al2O3∙6SiO2
  • Beryl: 3BeO∙Al2O3∙6SiO2

Bauxite

Bauxite is not mineral and chemical compound. This name is bauxite (or, more, bauxite) – used to designate various types of aluminum ores, which contain aluminum compound, basically – different kinds of hydroxides.

Commercial bauxite contains three main types of hydroxide minerals:

  • gypsum
  • bogemite
  • diaspore

These types of hydroxides significantly differ in such physical properties, as:

  • water content,
  • crystal system
  • hardness,
  • density,
  • dehydration temperature
  • solubility in technological solutions.

Usually natural bauxite deposits consist of one of these types of hydroxides, In some cases the same bauxite may contain mixed hydroxides.

Bauxite vary in color from cream to dark brown with a high iron content.

Aluminium compounds - Typical bauxite

Figure 1 – Typical bauxite

Typical bauxite for industrial aluminum production includes the following compounds:

  • alumina – Al2O3: 40-60 %
  • silica – SiO2: 1-6 %
  • ferric oxide – Fe2O3: 2-25 %
  • titanium oxide – TiO2: 1-5 %
  • calcium and magnesium oxides – CaO + MgO: 0,2-0,6 %
  • oxides of other elements: from 0,01 to 0,4 % (everyone).

Alumina

Almost all alumina produced from bauxite, which contain about 50 % oxide Al2O3 as hydroxides. This aluminum ore was treated under pressure in caustic soda solution, to dissolve the alumina in aluminate form, and separate it from the red sludge, containing oxides of iron and other major impurities. Then this solution was precipitated aluminate crystals of aluminum hydroxide.

At temperatures below 700 ºС the technological mass contains the following different types of aluminum compounds – its hydroxides:

  • giʙʙsit
  • bayerit
  • nordstrandid
  • diasporas
  • bogemit.

The final process step of manufacture of alumina is obtained by calcining at previous stages hydroxides mixture. Firing (calcination) is carried out at a temperature 1200 ° C to yield pure alumina with a content of Al oxide2O3 more 99 %.

Aluminium compounds - Alumina

Figure 2 – Alumina

For industrial production 1 alumina ODP requires about 2 tons of alumina.

aluminum compounds, which are most important for non-metallurgical industries – are it:

  • oxide;
  • sulfate,
  • silicates.

Aluminium oxide

Oxygen aluminum compound

  • Pure aluminium oxide is a white powder in the form of particles of various shapes and sizes.
  • Because of its amphoteric properties of the oxide is soluble in mineral acids and strong alkalis. It can have various modifications.
  • The most stable is its alpha modification “corundum” (alpha-Al2O3).

Anodic oxide

The anodic oxide obtained by electrochemical oxidation of aluminum. This compound is a nanostructured material having a unique structure. Anodic aluminum oxide consists of cylindrical pores, that provide him widely used in the art. It is thermally and mechanically resistant, an optically transparent and have high electrical insulation properties. The pore size and thickness of the anodic oxide layer can be easily adjusted parameters Technology, which enables its use not only as protective coatings for decorative aluminum products, but also as the basis for nanotechnology.

Aluminium compounds - The structure of anodic oxide film

Figure 3 – The structure of anodic oxide film [5]

Hydroxides

  • Aluminum hydroxides  may be in various forms. The most studied forms are trihydroxide Al(OH)3 and oxide-hydroxide AlO(OH).
  • Al(OH)3  is used in large volumes for sewage treatment, as well as the manufacture of other aluminum compounds, including its salts.

Aluminium sulphate

The structure and composition

  • Aluminum sulfate may exist with different proportions of water. A common form of this compound is Al2(SO4)3· 18H2O. He is almost insoluble in anhydrous alcohol, but it is highly soluble in water.
  • At temperatures above 770 ° C decomposes to alumina.

Applications

Finds application in the following industries and areas of life [1, 2]:

  • water purification and wastewater treatment systems;
  • paper production;
  • fire-fighting protective clothing;
  • refining oils and fats;
  • waterproofing concrete;
  • antiperspirant production;
  • dressing of leather;
  • production of paints;
  • in agricultural pesticides;
  • production of chemicals;
  • means for increasing the acidity of soils;
  • production of cosmetics and soap;
  • medical preparations.

Alum

  • Aluminum sulphate is included in combination with monovalent metal sulfates to form double salts, called alum.
  • The most important of these salts is aluminum potassium sulfate. This chemical compound is also known as potassium alum.
  • These alum are widely used since ancient times in leather production, drugs, fabrics and colors.

Clays

Clays are composed mainly of aluminosilicates.

Aluminium chloride

In the reaction of chlorine gas with the molten aluminum is formed of aluminum chloride. This compound is commonly used as a catalyst in the synthesis reactions of various organic compounds. Hydrated chloride AlCl3∙H2O, It is used as a deodorant or antiprespirant. This compound is one of several aluminum salts, that apply to the cosmetic industry.

The hexahydrate form of aluminum chloride is used:

  • for the protective treatment of wood,
  • as a disinfectant in animal husbandry and meat production;
  • crude oil refinery
  • papermaking

Aluminium hydride

Since aluminum forms a hydride with hydrogen AlH3, which is widely used in organic chemistry, including, as lithium aluminum hydride (LiAlH4). This compound was prepared by reacting aluminum chloride with lithium chloride.

Intermetallic compounds

An intermetallic compound (intermetallic compound) is a chemical compound of two or more metals. intermetallic compounds, as it should be in chemical compounds, have a fixed relationship between its components. Usually in the intermetallic compounds the bond between atoms – metal.

Table – Intermetallic compounds of aluminium [2]

Aluminium compounds - Intermetallic compounds of aluminium

Sources:

1. http://www.inchem.org/documents/ehc/ehc/ehc194.htm

2. Handbook of Aluminum: Vol. 1: Physical Metallurgy and Processes / ed. G. E. Totten, D. S. MacKenzie – 2003.

3. Aluminium and Aluminium Alloys /ed. J.R. Davis – ASM International, 1993.

4. Тринадцатый элемент: Энциклопедия /А. Дроздов, 2007

5. TALAT