Sodium aluminate

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, Na₂O · Al₂O₃, or Na₂Al₂O₄.

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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.

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. In construction technology, sodium aluminate is employed to accelerate the solidification of concrete, mainly when working during frost. It is also used in the paper industry, for fire brick production, alumina production and so forth.

See also

• Aluminium hydroxide