Sodium aluminates

The sodium aluminates are a group of different chemical compounds that contain both sodium and aluminum ions .

The most important sodium aluminates include mixed oxides of sodium and aluminum Na _x Al _y O _z , which are also called anhydrous aluminates and are solids, and the mixed hydroxides, e.g. B. NaAl (OH) ₄ , which can also occur in aqueous solution.

Representation of the most important types of sodium aluminate

The aqueous sodium aluminates are formed when sodium hydroxide solution acts on aluminum or on aluminum salts, the sodium hydroxide solution being used in excess. A typical reaction is the dissolving ( pickling ) of metallic aluminum and the synthetic production from aluminum hydroxide by heating with caustic soda:

{\ displaystyle \ mathrm {2 \ Al + 2 \ NaOH + 6 \ H_ {2} O \ longrightarrow 2 \ NaAl (OH) _ {4} +3 \ H_ {2}}}

{\ displaystyle \ mathrm {Al (OH) _ {3} + NaOH \ longrightarrow NaAl (OH) _ {4}}}

A sodium aluminate solution is thus obtained .

The oxidic sodium aluminates are obtained by heating sodium carbonate Na ₂ CO ₃ or sodium hydroxide NaOH with aluminum oxide Al ₂ O ₃ or aluminum hydroxide Al (OH) ₃ to high temperatures, e.g. B. 1500 ° C. They are also formed in the case of sharp drying of hydrous sodium aluminates. Important examples of such oxidic sodium aluminates are solid sodium metaluminate and sodium β-aluminate (NaAl ₁₁ O ₁₇ ).

Sodium aluminates which are mixed oxides and hydroxides can also be isolated, e.g. B. Na ₃ [Al ₃ O ₂ (OH) ₈ ].

Sodium metaluminate

Sodium metaluminate (NaAlO ₂ ) is a white, water-soluble salt with a melting point of 1800 ° C and a molar mass of 81.97 g · mol ⁻¹ . It is used as a construction chemical (quick hardener for concrete ) and for the production of paints and soaps.

Sodium aluminate solution

Sodium aluminate solutions are of great technical importance. They arise as an intermediate product in the bauxite digestion using the Bayer process and as aluminate liquor when pickling metallic aluminum in anodizing plants. Using suitable cleaning and processing methods (e.g. Lippewerk Lünen; BK Giulini Chemie Ludwigshafen), the aluminate liquors from anodising plants can be cleaned ( recycling ). Commercially available sodium aluminate solutions are water-clear and contain 6–10% dissolved aluminum and approx. 20% NaOH.

Applications

Sodium aluminate solutions are used for drinking water treatment, wastewater treatment or as a raw material for zeolite production, for calcium aluminate and calcium aluminate sulphate extraction or for titanium dioxide coating. The use as a precipitant or flocculant in wastewater treatment is of particular importance because the various hydrolysis products achieve effects that are not possible with other precipitants (for example iron salts or milk of lime).

Aluminum ions form a hexaqua complex in aqueous solution , ie six water molecules are deposited according to its coordinative hexavalence, which can be gradually replaced by hydroxide ions .

{\ displaystyle \ mathrm {Al ^ {3 +} \ longrightarrow [Al (OH)] ^ {2 +} \ longrightarrow [Al (OH) _ {2}] ^ {+} \ longrightarrow Al (OH) _ {3 } \ longrightarrow [Al (OH) _ {4}] ^ {-}}}

Basic salts can form during precipitation and neutralization processes:

{\ displaystyle \ mathrm {2 \ HCl + NaAl (OH) _ {4} \ longrightarrow Al (OH) Cl_ {2} + NaOH + 2 \ H_ {2} O}}

{\ displaystyle \ mathrm {HCl + NaAl (OH) _ {4} \ longrightarrow Al (OH) _ {2} Cl + NaOH + H_ {2} O}}

In this way, basic sulphates or phosphates can also form, the latter especially when sodium aluminate is used for phosphate precipitation ( phosphorus elimination ) in wastewater treatment. Sodium aluminate has a special status in wastewater treatment because the precipitation process with sodium aluminate increases the pH value at the same time and, unlike acidic precipitants based on chlorides or sulfates (FeCl ₃ , AlCl _3, etc.), no additional anions are introduced.

Sodium β-aluminate

Sodium β-aluminate is an essential component of the ceramic membrane used as a solid electrolyte in sodium-nickel chloride batteries and sodium-sulfur batteries , because from a temperature of 270 ° C the sodium ions become so mobile that there is sufficient electrical conductivity. The idealized formula of sodium β-aluminate is NaAl ₁₁ O ₁₇ . However, the crystals typically contain more sodium than indicated by this formula, which is compensated for by aluminum vacancies, so that a real composition of Na _1.3 Al _10.9 O ₁₇ results.

literature

Entry to sodium aluminates. In: Römpp Online . Georg Thieme Verlag, accessed on May 2, 2014.

Individual evidence

^ AF Holleman , E. Wiberg , N. Wiberg : Textbook of Inorganic Chemistry . 102nd edition. Walter de Gruyter, Berlin 2007, ISBN 978-3-11-017770-1 , p. 1163.
^ AF Holleman , E. Wiberg , N. Wiberg : Textbook of Inorganic Chemistry . 102nd edition. Walter de Gruyter, Berlin 2007, ISBN 978-3-11-017770-1 , p. 1159.
^ AF Holleman , E. Wiberg , N. Wiberg : Textbook of Inorganic Chemistry . 102nd edition. Walter de Gruyter, Berlin 2007, ISBN 978-3-11-017770-1 , pp. 1163-1164.

[1] AF Holleman , E. Wiberg , N. Wiberg : Textbook of Inorganic Chemistry . 102nd edition. Walter de Gruyter, Berlin 2007, ISBN 978-3-11-017770-1 , p. 1163.

[2] AF Holleman , E. Wiberg , N. Wiberg : Textbook of Inorganic Chemistry . 102nd edition. Walter de Gruyter, Berlin 2007, ISBN 978-3-11-017770-1 , p. 1159.

[3] AF Holleman , E. Wiberg , N. Wiberg : Textbook of Inorganic Chemistry . 102nd edition. Walter de Gruyter, Berlin 2007, ISBN 978-3-11-017770-1 , pp. 1163-1164.