Sodium aluminate

Sodium aluminate
Other names Sodium aluminium oxide, Sodium metaaluminate Aluminate, (AlO21-), sodium
Identifiers
CAS number	1302-42-7 Y
PubChem	14766
Jmol-3D images	Image 1
SMILES O=[Al-]=O.[Na+]
InChI InChI=InChI=1S/Al.Na.2O/q-1;+1;; Key: IYJYQHRNMMNLRH-UHFFFAOYSA-N
Properties
Molecular formula	NaAlO2
Molar mass	81.97 g/mol
Appearance	white powder
Density	1.5 g/cm3
Melting point	1650 °C
Solubility in water	soluble
Solubility in Alcohol	soluble
Refractive index (nD)	1.58
Y aluminate (verify) (what is: Y/N?) Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa)
Infobox references

Sodium aluminate is an important commercial inorganic chemical. It works as an effective source of aluminium hydroxide for many industrial and technical applications. Pure sodium aluminate (anhydrous) is a white crystalline solid having a formula variously given as NaAlO₂, NaAl(OH)₄ (hydrated),^[1] Na₂O·Al₂O₃, or Na₂Al₂O₄. Commercial sodium aluminate is available as a solution or a solid.
Other related compounds, sometimes called sodium aluminate, prepared by reaction of Na₂O and Al₂O₃ are Na₅AlO₄ which contains discrete AlO₄⁵⁻ anions, Na₇Al₃O₈ and Na₁₇Al₅O₁₆ which contain complex polymeric anions, and NaAl₁₁O₁₇, once mistakenly believed to be β-alumina, a phase of aluminium oxide.^[2][3]

Structure

Anhydrous sodium aluminate, NaAlO₂, contains a three dimensional framework of corner linked AlO₄ tetrahedra. The hydrated form NaAlO₂·5/4H₂O has layers of AlO₄ tetrahedra joined into rings and the layers are held together by sodium ions and water molecules that hydrogen bond to O atoms in the AlO₄ tetrahedra.^[4]

Manufacturing

Sodium aluminate is manufactured by the dissolution of- aluminium hydroxide in a caustic soda (NaOH) solution. Aluminium hydroxide (gibbsite) can be dissolved in 20–25% aqueous NaOH solution at a temperature near the boiling point. The use of more concentrated NaOH solutions leads to a semi-solid product. The process must be carried out in steam-heated vessels of nickel or steel, and the aluminium hydroxide should be boiled with approximately 50% aqueous caustic soda until a pulp forms. The final mixture has to be poured into a tank and cooled; a solid mass containing about 70% NaAlO₂ then forms. After being crushed, this product is dehydrated in a rotary oven heated either directly or indirectly by burning hydrogen. The resulting product contains 90% NaAlO₂ and 1% water, together with 1% free NaOH.

Reaction of aluminium metal and alkali

Sodium aluminate is also formed by the action of sodium hydroxide on elemental aluminium. The reaction is highly exothermic once established and is accompanied by the rapid evolution of hydrogen gas. The reaction is sometimes written as:

2 Al + 2 NaOH + 2 H₂O → 2 NaAlO₂ + 3 H₂

however the species produced in solution is likely to contain the [Al(OH)₄]⁻ ion or perhaps the [Al(H₂O)₂(OH)₄]⁻.^[5]

This reaction has been proposed as a potential source of fuel for hydrogen powered cars.

Uses

In water treatment it is used as an adjunct to water softening systems, as a coagulant aid to improve flocculation, and for removing dissolved silica and phosphates.

In construction technology, sodium aluminate is employed to accelerate the solidification of concrete, mainly when working during frost.

Sodium aluminate is also used in the paper industry, for fire brick production, alumina production and so forth.

Sodium aluminate solutions are intermediates in the production of zeolites.^[6]

References

1. ^ "Aluminium". chemguide.co.uk. http://www.chemguide.co.uk/inorganic/extraction/aluminium.html. 
2. ^ Identification and characterisation of three novel compounds in the sodium–aluminium–oxygen system, Marten G. Barker, Paul G. Gadd and Michael J. Begley, J. Chem. Soc., Dalton Trans., 1984, 1139–1146, doi:10.1039/DT9840001139
3. ^ Egon Wiberg, Arnold Frederick Holleman (2001) Inorganic Chemistry, Elsevier ISBN 0-12-352651-5
4. ^ The Crystal Structure of Hydrated Sodium Aluminate, NaAlO₂·5/4H₂O, and Its Dehydration Product, James A. Kaduk, Shiyou Pei, Journal of Solid State Chemistry, 115, 1, 1995, 126–139, doi:10.1006/jssc.1995.1111
5. ^ Greenwood, Norman N.; Earnshaw, Alan (1997). Chemistry of the Elements (2nd ed.). Oxford: Butterworth-Heinemann. ISBN 0080379419. 
6. ^ Alan Dyer, (1994), Encyclopedia of Inorganic Chemistry, R. Bruce King (ed.), John Wiley & Sons, ISBN 0-471-93620-0

See also

• Aluminium hydroxide