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Aluminum hydroxide

Aluminium hydroxide, Al(OH)3, ATH, sometimes erroneously called Hydrate of alumina, is found in nature as the mineral  (also known as hydrargillite) and its three, much more rare : bayerite, doyleite and nordstrandite. Closely related are , AlO(OH), and , Al2O3, differing only by loss of water. These compounds together are the major components of the aluminium  . Freshly precipitated aluminium hydroxide forms, which is the basis for application of aluminium salts as in water purification. This gel crystallizes with time. Aluminium hydroxide gels can be dehydrated (e.g., using water-miscible non-aqueous solvents like ethanol) to form an amorphous aluminium hydroxide powder, which is readily soluble in acids. Aluminium hydroxide powder which has been heated to an elevated temperature under carefully controlled conditions is known as activated alumina and is used as a desiccant, an adsorbent, in gas purification, as a Claus catalyst support, water purification, and an adsorbent for the catalyst during the manufacture of polyethylene by the Sclairtech process.

 

Contents

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[]Nomenclature

The naming for the different forms of aluminium hydroxide is ambiguous and there is no universal standard. All four  have a chemical composition of aluminium trihydroxide (an  atom attached to three  groups).

Gibbsite is also known as hydrargillite, named after the  words for water (hydra) and clay (argylles). The first compound named hydrargillite was thought to be aluminium hydroxide, but was later found to be ; despite this, both gibbsite and hydrargillite are used to refer to the same polymorphism of aluminium hydroxide, with gibbsite used most commonly in the  and hydrargillite used more often in . In 1930 it was referred to as -alumina trihydrate to contrast it with  which was called -alumina trihydrate (the alpha and beta designations were used to differentiate the more- and less-common forms respectively). In 1957 a symposia on nomenclature attempted to develop a universal standard, resulting in gibbsite being designated -Al(OH)3 and bayerite becoming α-Al(OH)3 and  being designated Al(OH)3. Based on their properties, a suggested nomenclature and designation is for gibbsite to be α-Al(OH)3, bayerite to be designated β-Al(OH)3 and both nordstrandite and  are designated Al(OH)3. Under this designation, the α and β  refer to hexagonal, close-packed structures and altered or  polymorphisms respectively, with no differentiation between nordstrandiate and doyleite.

 

[]Properties

Gibbsite has a typical metal hydroxide structure with . It is built up of double layers of hydroxyl groups with aluminium ions occupying two-thirds of the octahedral holes between the two layers.

Aluminium hydroxide is . It dissolves in , forming Al(H2O)63+ (hexaaquaaluminium(3+)) or its  products. It also dissolves in strong , forming Al(OH)4- (tetrahydroxidoaluminate(1-)).

 

[]Polymorphism

Four  of aluminium hydroxide exist, all based on the common combination of one  atom and three  molecules into different  arrangements that determine the appearance and properties of the compound. The four combinations are:

  • Nordstrandite
  • Doyleite

All polymorphs are composed of  layers of aluminium hydroxide with the aluminium  in the centre and the  groups on the sides, with  holding the layers together. The polymorphisms vary in how the layers stack together, with the arrangements of the molecules and layers determined by the , presence of  (including ) and the surface of the minerals the substance forms on. Under most conditions gibbsite is the most form of aluminium hydroxide. All forms of Al(OH)3crystals are .

 

[]Production

Virtually all the aluminium hydroxide used commercially is manufactured by the  which involves dissolving bauxite in sodium hydroxide at temperatures up to 270°C. The remaining solids, which is a , is separated and aluminium oxide is precipitated from the remaining solution. This  is damaging to the environment and highly toxic. It is usually stored in large artificial lakes, this is what led to the  in 2010 in Hungary, killing nine people and injuring 122. The dam holding back the  burst allowing it to contaminate large areas of land and waterways. The aluminium oxide that is produced can be converted to aluminium hydroxide through reaction with water.

 

[]Uses

Annual production is some 100 million tonnes,[] over 90% of which is converted to aluminium oxide[] (alumina) that is used in the manufacture of aluminium metal.[]

The major other uses of aluminium hydroxide is as a feedstock for the manufacture of other aluminium compounds: specialty calcined aluminas, aluminium sulfate, polyaluminium chloride, aluminium chloride, zeolites, sodium aluminate, activated alumina, aluminium nitrate.[]

 

[]Fire retardant

Aluminium hydroxide also finds use as a fire retardant filler for polymer applications in a similar way to and mixtures of  and . It decomposes at about 180 °C, absorbing a considerable amount of heat in the process and giving off water vapour. In addition to behaving as a fire retardant, it is very effective as a smoke suppressant in a wide range of polymers, most especially in polyesters, acrylics, ethylene vinyl acetate, epoxies, PVC and rubber.

 

[]Pharmaceutical

This compound is used as an  under names such as Alu-Cap, Aludrox or Pepsamar. The hydroxide reacts with excess acid in the stomach, reducing its acidity. This decrease of acidity of the contents of the stomach may in turn help to relieve the symptoms of ,  or . It can also cause constipation and is therefore often used with  or , which have counterbalancing effects. This compound is also used to control  (phosphorus) levels in the blood of people suffering from kidney failure.

 aluminium hydroxide is included as an  in some  (e.g. ). One of the well-known brands of aluminium hydroxide adjuvant is Alhydrogel, made by Brenntag. Since it absorbs protein well, it also functions to stabilize vaccines by preventing the proteins in the vaccine from precipitating or sticking to the walls of the container during storage. Aluminium hydroxide is often mis-called "" even by researchers; however, "alum" properly refers to  (alum).[] The aluminium hydroxide causes  of  made of , which slows the release of the antigen from the injection site (the "depot effect"), as well as causing a nonspecific irritation to the .Vaccine formulations containing aluminium hydroxide stimulates the immune system by inducing the release of, an immunological  signal. This strongly attracts certain types of  which differentiate into . The DCs pick up the antigen, carry it to , and stimulate and . It appears to contribute to induction of a good  response, so is useful for immunizing against pathogens that are blocked by antibodies. However, it has little capacity to stimulate cellular (Th1) immune responses, important for protection against many pathogens, nor is it useful when the antigen is-based.

 

[]Potential adverse effects

In the 60's and 70's it was speculated that aluminium was related to various  including. Since then, multiple  studies have found no connection between exposure to aluminium and neurological disorders.

The pathological persistence of aluminium hydroxide used in some vaccines has also been associated with, a rare muscle disease.

 

 

 
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