Hidrato de alúmina y trihidrato de alúmina

Alumina has a complex chemical makeup, including various polymorphs that can be converted to high surface area forms through thermal transformation processes such as those seen with boehmite and bayerite.

Thermal decomposition produces lamellar or fibrous g-Al2O3, with their respective morphologies depending on the starting material used in their creation.

Flame Retardant

Alumina hydrate (Al(OH)3) is a white, finely powdered substance with the chemical formula Al(OH)3. Produced through the Bayer process from bauxite ore, it has no smell or taste and low solubility and thermal stability making it an extremely versatile raw material with many applications in numerous industries as a source of Al2O3.

Gibbsite (-Al(OH)3), bayerite (-Al(OH)3), doyleite, and nordstrandite are the four polymorphs of Alumina Trihydroxide Al(OH)3 typically found in nature; all are related and share similar structures; however their morphologies differ significantly and impact their properties, with different viscosities depending on particle size distribution; however their morphologies can be controlled through heat treatment to achieve specific viscosities for specific applications.

Alumina hydrate’s flame retardant capabilities rely on its ability to release water vapor at elevated temperatures, thus cooling down materials and diluting flammable gases and slowing fire spread. This is accomplished by creating an oxygen and other flammable gas-trapping barrier, making it more difficult for these molecules to reach the surface and burn it off.

Alumina hydrates have very low oxygen uptake and high reactivity with various gases, such as sulphur dioxide, hydrogen sulfide, carbon monoxide and nitrogen oxides. Due to these characteristics they can serve as an ideal substitute for sulphur oxide in many applications such as pyrotechnics and gas discharge lamps.

Alumina hydrates are used as precursors for producing activated alumina (AA). Activated alumina is an industrial product made by thermal decomposition of hydroxides and oxyhydroxides of alumina. It finds applications across industries, particularly paper as a filler pigment or coating pigment and pharmaceuticals as an excipient; various preparation techniques result in different kinds of porous alumina with unique characteristics and applications potentials; for instance calcination of bayerite produces (111) spinel planes while calcination of Al(OH)3 produces hexa-coordinated Al3+ ions.

Anti-Smoke Suppressant

Due to its low melting point, alumina boasts excellent flame retardant properties. It can help prevent flame propagation in plastic materials or protect areas vulnerable to fire damage from spreading further. Furthermore, alumina’s oil absorption capabilities make it suitable as a scavenger against hydrocarbon-laden flames; additionally it has become invaluable as an additive in lubricants to avoid machinery degradation.

Alumina is an abundant natural mineral produced as the end product in the Bayer process of extracting aluminium from bauxite, typically by precipitating soluble aluminium hydroxides from water or reacting alumina trihydrate with alkali metal hydroxides to form boehmite, an extremely metastable and poorly crystallized substance with Al3 + ions surrounding microporous oxyhydroxides in an alkaline medium (31). 27Al MAS NMR shows multiple coordination levels between Al3 + and Al3 +, with several coordination patterns (31); BET-area of g-Al2O3 is approximately 275m2g-1 (41).

Thermally decomposed alumina can take the form of various polymorphs. Common examples are gibbsite (also hydrargillite) and bayerite, both produced via the Bayer process; nordstrandite occurs as part of North American bauxite deposits; gibbsite is often employed for ceramic glaze applications while nordstrandite may be found in enamels and stonewares.

Precipitation involves changing gibbsite and pseudoboehmite gels into boehmite using controlled water vapour pressure, when temperatures fall below 80 degC; then their form reverts back into their original form of alumina hydroxides that dissolve more readily in water; thus dissociating into melt to yield readily soluble alumina oxide which dissolves more readily with time or higher temperatures (60). At higher temperatures or under more rigorous aging conditions, this alumina oxide may eventually transform into well-crystallized boehmite (60).

Filler

Alumina trihydrate, commonly referred to as calcined alumina and aluminum hydroxide, is an extremely versatile filler. In plastics applications it serves to enhance flame retardancy as well as mechanical and thermal properties of various polymers; its versatility also lends itself well to glass, ceramics and paper applications as a filler material. Furthermore, paper manufacturers use it as coating pigment and to increase opacity and brightness levels in various papers; its alkaline nature also lends it a helping hand in some water treatment applications.

Hydrated alumina’s high reactivity makes it an excellent raw material for producing ceramic bodies and glazes, and often acts as an economical replacement to natural raw materials such as feldspar and silica. Available both wet and dry forms, the latter can be ground to produce particles of variable size distribution in fluid energy mills or ceramic lined ball mills.

Ground alumina hydrate added to glaze or glass melt quickly decomposes into aluminium oxide and water molecules via an exothermic endothermic reaction process, giving this material intrinsic flame retardancy properties and producing non-corrosive and non-poisonous smoke during this reaction.

For Alumina Trihydrate to function effectively as a fire retardant, it must be exposed to temperatures exceeding 220degC. When heated to this level, 3 molecules of water per molecule of Alumina evaporate into glaze melt as steam. This decomposition of Alumina Hydrate provides it with its distinctive level of flame retardancy not found elsewhere in fillers.

Addition of alumina hydrate to glazes and glasses can increase opacity by creating gas bubbles within the glaze melt, helping reduce firing shrinkage while producing glossy surfaces and providing low firing shrinkage rates. Furthermore, it is an ideal choice for making glazes that require low drying shrinkage rates.

Catalyst

Alumina hydrate is an excellent catalyst, creating gas bubbles in glazes and enamels to increase opacity by the Bayer process. Not only is it non-toxic and has low firing shrinkage but it’s also cost-effective, easy to handle, economical and has a large surface area – not bad qualities for an industrial material with annual production reaching approximately 100 million tons! Alumina trihydrate produced this way is ground into either its anhydrous or calcined forms for use as an integral ingredient.

There are various alumina polymorphs that each possess different properties due to differing stacking sequences, interlayer and intralayer hydrogen bonding geometry and substitution patterns of hydroxyl groups on Al(OH)6 edge-sharing octahedra. Yet their thermodynamic stability remains similar – rather, existence may depend more upon kinetics than thermodynamic properties of that material.

Microporous gibbsite alumina gels (pseudoboehmite and boehmite) can be created through careful management of gelation/flocculation, ageing and drying processes. Soaking gels in water leads to irreparable loss of BET-area and conversion to non-porous bayerite, however.

Flame-hydrolysis of alumina chloride at high temperatures results in a fine g-Al2O3 powder with an average particle size of 10nm and surface area of 130m2g-1. Alumina particles tend to have spinel lattices, although there may also be hexagonal or cubic close-packed crystallites present.

Alumina hydrate is one of the most stable and widely available alumina materials, boasting high surface area and low firing shrinkage rates that make it suitable for a range of applications. Furthermore, its anti-corrosive qualities and flame retardancy properties make it suitable as flame retardant agents; anti-corrosion research is underway and its particles have also proven successful as autocatalytic converters and fuel cell components thanks to discoveries that reveal how alumina particles react with water to generate hydrogen-rich streams of gas that can then be burned as fuel for cars and jets!