Sodium aluminate is an inorganic chemical that is used as an effective source of aluminium hydroxide for many industrial and technical applications. Pure sodium aluminate (anhydrous) is a white solid having a formula variously given as NaAlO2, NaAl(OH)4 (Hydrate),
Other related compounds, sometimes called sodium aluminate, prepared by reaction of sodium oxide and aluminium oxide 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.["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, ][Egon Wiberg, Arnold Frederick Holleman (2001) Inorganic Chemistry, Elsevier ]
Structure
Anhydrous sodium aluminate, NaAlO
2, contains a three-dimensional framework of corner linked AlO
4 tetrahedra. The hydrated form NaAlO
2·5/4H
2O has layers of AlO
4 tetrahedra joined into rings and the layers are held together by sodium ions and water molecules that hydrogen bond to O atoms in the AlO
4 tetrahedra.
Manufacturing
Sodium aluminate is manufactured by the dissolution of aluminium hydroxide (Al(OH)
3) 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% NaAlO
2 then forms. After being crushed, this product is dehydrated in a rotary oven. The resulting product contains 90% NaAlO
2 and 1% water, together with 1% free NaOH.
Reaction of aluminium and alkali
Sodium aluminate is also formed by the action of sodium hydroxide on elemental aluminium which is an
amphoteric metal. The reaction is highly exothermic once established and is accompanied by the rapid evolution of hydrogen gas. The reaction is sometimes written as:
- 2Al + 2NaOH + 2H2O → 2NaAlO2 + 3H2
However, the species produced in solution is likely to contain the Al(OH)
4− ion or perhaps the Al(H
2O)
2(OH)
4− ion.
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
.
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 chemicals, for fire brick production, alumina production and so forth.
Sodium aluminate solutions are intermediates in the production of zeolites.[Alan Dyer, (1994), Encyclopedia of Inorganic Chemistry, R. Bruce King (ed.), John Wiley & Sons, ]
See also
