What is Alumina Trihydrate?

Alumina trihydrate is the primary filler material made from alumina oxide and serves multiple functions, acting both as polymer additive and flame retardant. A highly microporous material, Alumina trihydrate releases large amounts of water molecules when exposed to heat – providing inherent fire and smoke suppression capabilities.

Al(OH)3 polymorphs can be distinguished by the geometry of their oxygen lattice, featuring aluminum cations that share edges or corners of octahedra (Figure 3.2), while others form corner-sharing oxygen tetrahedra. Furthermore, X-ray diffraction analysis confirms their distinct atomic structures.

What is ATH?

Alumina trihydrate (Al(OH)3) is the hydrated form of aluminium hydroxide (Al(OH)3) and is obtained during industrial production of boehmite by precipitating aluminum salts from an aqueous solution, often via precipitation with alkaline metal cations such as sodium. Once precipitated, however, pseudoboehmite formation occurs within it due to poor crystallization patterns of pseudoboehmite solids that form.

Alumina hydrates can be identified by their cubic, defective spinel structure (see image below). The spinel consists of two parallel layers of oxide anions containing both tetra- and hexacoordinated ions; studies on these substances indicate that hexa-coordinated ions dominate (145).

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Alumina hydrate has many uses, from fireproofing applications to filler in polymer composites and cable compounds – even solid surface counter tops! Alumina hydrate’s main use, though, is as a fire retardant. It can withstand temperatures of up to 220 degC before decomposing into aluminium oxide and water. Plus it’s non-corrosive and non-poisonous! It is an invaluable material which serves as filler material across a range of products including polymer composites cable compounds and solid surface counter tops!

Bauxite, an iron and aluminium hydroxides mixture, serves as the primary raw material for producing industrial-grade alumina. Bauxite undergoes transformation via Bayer process into sodium-contaminated gibbsite as the starting point for creating other aluminum compounds. Next step in production are acid or strong basic solutions redissolved alumina trihydrates with properties including high chemical reactivity and low water absorption during sintering; further transformation into grit through grinding processes to produce final granulated product alumina.

ATH Functions

ATH is used in numerous industrial applications as a filler and fire retardant, in both filler applications as well as fireproofing applications. Notable features include its excellent chemical resistance, thermal stability and de-icing agent properties. When added to polymers it serves as an effective flame retardant/smoke suppressant while being highly efficient slurry grindable using conventional as well as high speed mills.

Alumina trihydrate features a monoclinic crystal structure with aluminum in its center and three hydroxyl groups surrounding it (see Figure 3.1). It is one of four known polymorphs of alumina; others being gibbsite, boehmite and thorium dioxide; these differ depending on crystallinity, surface area and porosity.

Gibbsite (g-Al(OH)3) is the primary mineral form of alumina produced industrially. Produced by precipitating caustic aluminate solution in the Bayer process of commercial alumina production, its powder form consists of small plates and prisms alongside larger particles composed of pseudohexagonal tabular crystals that form plate-shaped crystals. After production, this powdery form is ground into fine particles using fluid energy mills or ceramic lined ball mills before being fused and sintered grades by calcination for subsequent fused and sintered grades to meet customer specifications before finally being fused and sintered for final fused and sintered grade production.

Corundum (a-Al2O3) can be obtained by high temperature treatment of gibbsite or boehmite at high temperatures, while transition aluminas (which have higher BET (N2) areas than their trihydrate counterpart) are produced through heat treating Al(OH)3 or AlOOH at intermediate temperatures to create less dense but porous structures known as transition aluminas with lower densities than their trihydrate counterparts.

As opposed to other polymorphs of alumina, alumina trihydrate stands out by having an easily identifiable mesoporosity that can be measured using techniques like X-ray diffraction and neutron diffraction. With average mesopore diameters ranging between one and three microns and an extremely high specific surface area. Furthermore, its low sintering temperature makes large-sized alumina slurries for use in refractories like wear resistant linings as well as acting as de-icing agents as well as being anticorrosive additives in cements and concretes.

ATH Applications

ATH is most often employed as an abrasive in applications like grinding and polishing, though it also finds use as a filler in fire retardant applications. When exposed to heat, its decomposition yields aluminium oxide and water as endothermic reactions which make for effective flame retardants with minimal smoke production during decomposition compared to traditional flame retardants; furthermore it’s non-toxic and corrosion proof which are features which make ATH an attractive choice in numerous applications where alumina is utilized.

Alumina hydrates (also referred to as alumosilicates) are crystalline polymorphs of aluminium hydroxide with the formula Al(OH)3, formed when water molecules get trapped between alumina crystals. A variety of methods exist for producing these polymorphs with very high purity results.

Gibbsite Alumina (g-Al2O3) is an intermediate phase in the Bayer process for producing commercial alumina. It is prepared by leaching bauxite ore with hot caustic aluminate solution (digestion) followed by seeded precipitation of purified gibbsite at lower temperatures; its structure resembles that of a spherical tabular or prismatic crystal with pseudohexagonal close-packed lattice structure.

Gibbsite of high purity is necessary to create catalytic applications of aluminas. Numerous synthesis protocols have been devised, featuring long aging times to avoid high sodium concentrations and final steps at acidic pHs. Sasol and Condea utilize one widely applied synthesis process referred to as the so-called sol-gel procedure as their method of choice for production.

Activated alumina is an abrasive alumina that has been subject to either heat or chemicals treatment to increase its surface area and adsorption capabilities, usually to produce microporous structures with greater surface areas and higher absorption capacities. Activated aluminas differ from other aluminas in that their nitrogen and hydrogen isotherms are reversible at 77 K, yet their adsorption capacities decrease with increasing surface area; nevertheless they still possess great potential for high temperature use.

ATH Specifications

The four Al(OH)3 polymorphs differ primarily in their oxide lattice structures. Aluminum atoms form edge-sharing octahedra that are arranged into planar pseudohexagonal patterns, connected by bridging hydroxyl groups that span both sides of a layer. Each structure may possess differing thermodynamic stabilities; however, such differences typically don’t matter for most applications; in reality kinetics rather than thermodynamics often distinguishes among them.

Alumina trihydrate has the chemical formula Al(OH)33H2O and has a molecular weight of 48.5 g/mol, making it highly airborne with an average BET surface area of 130m2g1. Alumina trihydrate is widely used as an antitracking agent, flame retardant, and smoke suppressant because of its ability to resist electrical arcing and tracking; additionally it exhibits low oil absorption properties while being non-corrosive.

Bauxite is the main source of industrial alumina production. Through the Bayer process, this raw material is transformed into sodium-contaminated gibbsite that is then leached with hot caustic aluminate solutions to form rarer polymorphs (bayerite, doyleite and nordstrandite) as well as trihydrate form of the mineral alumina. Each polymorph differs by virtue of different substitutions for aluminate ions within its crystal lattice; Alumina trihydrate has its characteristic crystal lattice formation with plates and prisms as its crystal habit.

At temperatures above 220degC, alumina trihydrate decomposes into aluminium oxide and water in an irreversible endothermic reaction that releases energy in the form of heat and smoke; making alumina trihydrate an effective flame retardant and smoke suppressant.

Hindalco offers various grades of ground alumina trihydrate to meet customer requirements. Each grade differs in its particle size distribution and degree of calcination achieved, which determines hardness and polishing characteristics of the material. Mild, medium and hard grades can be selected according to application; all three grades are suitable for use in castingables (LC and ULCC castables), technical ceramics and new generation refractories as well as grinding media, refractory bricks, slide gates and wear resistant ceramic components.

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