Aluminium Hydroxide

Aluminium hydroxide is an inorganic compound of considerable industrial, environmental and pharmaceutical importance. Occurring naturally as the mineral gibbsite, it forms a major constituent of bauxite, the principal ore of aluminium. Several rarer polymorphs—bayerite, doyleite and nordstrandite—also exist, and all exhibit layered structures characteristic of hydroxide minerals. The compound displays amphoteric behaviour, reacting both as an acid and a base, and is closely related to aluminium oxide hydroxide (AlOOH) and aluminium oxide (alumina), both of which share the same amphoteric tendencies.

Mineral Forms and Structural Characteristics

Gibbsite is the most common naturally occurring form of aluminium hydroxide. Historically, it was referred to variously as hydrargillite or hydrargyllite, terms now known to have been applied mistakenly to an aluminium phosphate mineral, though they continue to be used synonymously with gibbsite in some regions. All recognised polymorphs of aluminium hydroxide possess similar fundamental structures, comprising layers of octahedrally coordinated aluminium ions bonded to hydroxyl groups. These layers are held together by hydrogen bonding, and distinctions between the polymorphs arise primarily from differences in how the layers stack.
In crystallographic terms, the hydroxide minerals adopt hexagonal crystal habits. The layered arrangement also explains why freshly precipitated aluminium hydroxide forms a gelatinous mass in aqueous solutions. This characteristic gel gradually crystallises over time, reflecting the compound’s tendency to organise into its layered structure when conditions permit.

Chemical Properties

A defining feature of aluminium hydroxide is its amphoterism. As a Brønsted–Lowry base, it neutralises acids to produce aluminium salts. Conversely, in strongly basic solutions, it behaves as a Lewis acid by accepting additional hydroxide ions to form aluminate species. This duality influences its behaviour in natural waters, industrial processes and biological systems. The compound is sparingly soluble in neutral conditions but becomes more reactive under strongly acidic or strongly alkaline environments, enabling a wide range of applications.

Industrial Production

Nearly all commercially significant aluminium hydroxide is produced using the Bayer process, the primary method for refining bauxite into alumina. In this process, bauxite is digested in concentrated sodium hydroxide at elevated temperatures, yielding a solution of sodium aluminate. Insoluble residues—known as bauxite tailings or red mud—are removed, and aluminium hydroxide is precipitated by controlled cooling or seeding. Calcination of the resulting hydroxide yields alumina, which is the starting point for aluminium metal production.
The caustic tailings left over from refining must be handled carefully due to their high pH. Historically, these residues were stored in lagoons, a practice associated with environmental hazards. The Ajka alumina plant disaster in Hungary in 2010 illustrated these risks when a containment failure released highly alkaline mud, contaminating land and waterways and causing several fatalities.

Applications as Fire Retardant and Filler

Aluminium hydroxide is widely used as a flame retardant and smoke suppressant in polymer manufacturing. Its effectiveness derives from its endothermic decomposition: when heated, it releases water vapour and absorbs heat, slowing combustion processes. Decomposition typically begins at a few hundred degrees Celsius, with maximum effectiveness near 300°C. These properties make it compatible with materials such as polyesters, poly(methyl methacrylate), ethylene–vinyl acetate, epoxies, PVC, natural rubber and engineered wood products.
Because the substance is white and inexpensive, it is also employed as a filler in artificial stone and acrylic composite materials. In combination with other minerals such as magnesium hydroxide, huntite or hydromagnesite, it provides enhanced fire resistance for specialised applications.

Precursor to Aluminium Compounds

Aluminium hydroxide serves as a key intermediate in producing numerous aluminium-based chemicals. These include calcined aluminas, aluminium sulfate, aluminium chloride, polyaluminium chloride, sodium aluminate, activated alumina and alumina-based catalysts. Freshly precipitated aluminium hydroxide forms gels useful in water purification, where aluminium salts act as flocculants to remove particulate impurities. Dehydration of such gels via water-miscible solvents produces amorphous hydroxide powders that dissolve readily in acids. Heating transforms the material into activated alumina, a highly porous adsorbent used in gas purification, drying and catalytic processes.

Pharmaceutical and Medical Uses

Under the generic name algeldrate, aluminium hydroxide is widely used as an antacid for both human and veterinary applications. It neutralises excess gastric acid without raising stomach pH above neutral levels, making it gentler than highly soluble alternatives. Trade names include Alu-Cap, Aludrox, Gaviscon and Pepsamar. Side effects may include constipation, as aluminium ions can slow gastrointestinal peristalsis. This is sometimes counterbalanced by combining aluminium hydroxide with magnesium-based compounds.
The compound also plays a role in managing hyperphosphataemia in individuals with kidney failure. By binding dietary phosphate in the gastrointestinal tract, it reduces absorption and helps maintain appropriate blood phosphorus levels.
As a vaccine adjuvant, aluminium hydroxide is valued for its protein-binding capacity, facilitating antigen presentation and enhancing antibody responses. Commercial preparations such as Alhydrogel are used in vaccines including the anthrax vaccine. The compound helps stabilise antigens during storage and promotes immune activation by triggering the release of uric acid, which attracts monocytes that develop into dendritic cells. These cells, in turn, stimulate the adaptive immune system. Aluminium hydroxide, however, is less effective at promoting certain T-helper cell (Th1) responses and is not particularly suitable for peptide-based antigens.

Safety Considerations

Speculation during the mid-twentieth century suggested a link between aluminium exposure and neurological conditions such as Alzheimer’s disease. Subsequent epidemiological studies did not support a relationship between environmental or ingested aluminium and neurodegenerative disorders. Experimental results in animal models have shown neurological changes following injected aluminium hydroxide under specific conditions, although these findings do not directly translate to typical human exposure. Ongoing research continues to evaluate the safety of aluminium-containing compounds used in medical and industrial settings.