In chemistry, an aluminate is a compound containing an oxyanion of aluminium, such as sodium aluminate. In the naming of inorganic compounds, it is a suffix that indicates a polyatomic ion with a central aluminium atom.[ Nomenclature of Inorganic Chemistry IUPAC Recommendations 2005 – Full text (PDF)]
Aluminate oxyanions
Aluminium oxide (alumina) is amphoteric: it dissolves in both bases and acids. When dissolved in bases it forms hydroxyaluminate ions in the same way as aluminium hydroxide or aluminium salts. The hydroxyaluminate or hydrated aluminate can be precipitated and then
calcination to produce anhydrous aluminates. Aluminates are often formulated as a combination of basic oxide and aluminium oxide, for example the formula of anhydrous sodium aluminate NaAlO
2 would be shown as Na
2O·Al
2O
3.
A number of aluminate oxyanions are known:
-
The simplest is the approximately tetrahedral found in the compound Na5AlO4,
-
framework ions in anhydrous sodium aluminate NaAlO2
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A ring anion, the cyclic anion, found in tricalcium aluminate, Ca3Al2O6, which can be considered to consist of 6 corner sharing {AlO4} tetrahedra.
-
A number of infinite chain anions in the compounds Na7Al3O8 which contains rings linked to form chains, Na7Al13O10 and Na17Al5O16 which contain discrete chain anions.
Mixed oxides containing aluminium
There are many
containing aluminium where there are no discrete or polymeric aluminate ions. The
with a generic formula that contain aluminium as Al
3+, such as the mineral
spinel itself, MgAl
2O
4 are mixed oxides with cubic close packed O atoms and aluminium Al
3+ in octahedral positions.
[Wells A.F. (1984) Structural Inorganic Chemistry 5th edition, Oxford Science Publications ]
BeAl2O4, chrysoberyl, isomorphous with olivine, has hexagonal close-packed oxygen atoms with aluminium in octahedral positions and beryllium in tetrahedral positions.["Refinement of the chysoberyl structure", E.F. Farrell, J.H. Fang, R.E. Newnham, The American Mineralogist, 1963, 48, 804]
Some oxides with the general formula of MAlO3 sometimes called aluminates or orthoaluminates such as YAlO3, Yttrium ortho-aluminate are mixed oxides and have the perovskite structure.["Crystal structure refinement of YAlO3, a promising laser material", R. Diehl, G. Brandt, Materials Research Bulletin (1975) Volume: 10, Issue: 3, Pages: 85–90]
Some oxides such as Y3Al5O12, usually called YAG, have the garnet structure.
Hydroxoaluminates
The anion is known in high pH solutions of Al(OH)
3.
Aluminate glasses
Alumina on its own cannot easily be made glassy with current techniques, however with the addition of a second compound many types of aluminate glasses can be formed. The glasses produced display a range of interesting and useful properties, such as high refractive index, good infrared transparency, and high melting point, as well as the ability to host laser active and
fluorescent ions. Aerodynamic levitation is a key method used to study and produce many aluminate glasses. Levitation allows high purity to be maintained in the melt at temperatures in excess of .
[Haliakova, A., Prnova, A., Klement, R.D. and Tuan, W.H. "Flame-spraying synthesis of aluminate glasses in the Al2O3-La2O3 system". webofknowledge.com. September 2012. Pages: 5543–5549. Accessed 2012-10-09.]
Some materials that are known to form glass in binary combination with aluminium oxide are: rare earth oxides, alkaline earth oxides (CaO, SrO, BaO), lead oxide, and silicon dioxide.
Also, the Al2O3 (aluminate) system has been discovered to form sapphire-like glass ceramics. Often, this ability is based upon compositions in which interplay between glass forming ability and glass stability is approximately balanced.
Applications of aluminates
Sodium aluminate, NaAlO
2, is used industrially in dyeing to form a
mordant and the hydrated forms are used in water purification, sizing of paper and in the manufacture of zeolites, ceramics and catalysts in the petrochemical industry. In the isomerization process of alkenes and amines
Calcium aluminates are important ingredients of cements.
Li5AlO4 is used in the nuclear power industry.[Allen W. Apblett, "Aluminium: Inorganic Chemistry", (1994), Encyclopedia of Inorganic Chemistry, ed. R. Bruce King, John Wiley & Sons, ]
Aluminate suffix used in the naming of inorganic compounds
Examples are:
-
Tetrachloroaluminate ion found in for example lithium tetrachloroaluminate.
-
Tetrahydroaluminate ion found in for example lithium aluminium hydride.
-
Hexafluoroaluminate ion found in for example sodium hexafluoroaluminate.
Aluminates made using new raw materials
Many recent research studies have focused on an effective solution for waste treatment. This has led some residues to be made into new raw materials for many industries. Such an achievement ensures a reduction in energy and natural resource usage, decreasing of the negative environmental impact and creating new fields of work.
A good example comes from the metals industry, particularly the aluminium industry. Aluminium recycling is a beneficial activity for the environment, since it recovers resources from both manufacturing and consumer waste. Slag and scrap which were previously considered as waste, are now the raw material for some highly profitable new industries. There is added-value in materials made using an aluminium residue which is currently considered as a hazardous waste. Current research is investigating the use of this waste to manufacture glass, glass-ceramic, boehmite and calcium aluminate.[López Delgado, A. and Tayibi, H. "Can hazardous waste become a raw material? The case study of an aluminum residue: a review". webofknowledge.com. May 2012. Pages: 474–484. Accessed 2012-10-09]
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