Types Of Hydrated Alumina
Hydrated alumina can be simply understood as alumina with crystal water. Usually, various hydrated aluminas are calcined to lose water to obtain corresponding alumina crystals. Therefore, it is a very important type of alumina precursor. The chemical formula of hydrated alumina is Al2O3·nH2O, which can be divided into two categories: crystal and gel (low crystalline hydrated alumina). The crystallinity of crystal is better than that of gel.
01 -- Crystal Form
Crystals can be divided into trihydrate alumina and monohydrate alumina according to the number of water molecules in their own structure.
Alumina trihydrate (also known as aluminium hydroxide) has three crystal forms: α-Al2O3 3H2O (English name Gibbsite), β-Al2O3 3H2O (English name Bayerite) , The new β-Al2O3 3H2O (English name Nordshandite).
Alumina monohydrate (also known as aluminium hydroxide) has two crystal forms: γ-AlOOH (English name Boehmite) and α-AlOOH (English name Diaspore).
02 -- Gel Type
The number of water molecules cannot be determined in the hydrated alumina gel structure, and there are usually two types: amorphous gel (Amorphous) and gel-type boehmite (also known as pseudo-boehmite, referred to as PB).
In recent decades, because boehmite and pseudo-boehmite have been studied at home and abroad, their properties are representative and their uses are very wide. The following two kinds of hydrated alumina are represented in detail.
Boehmite
Boehmite is a kind of hydrated alumina with incomplete crystallization, which belongs to the orthorhombic system and has a layered structure. In each single structural layer, it is composed of many AlO6 octahedrons, oxygen ions are arranged in cubic close-packed at the vertices of the octahedron, aluminum ions are located in the center of the octahedron to form a double-layer structure, and hydroxides are located on the surface of the layered structure. The layers are connected together by hydrogen bonds.
Boehmite has the following advantages: relatively high zero point charge and interfacial free energy, relatively large porosity and specific surface area, relatively good dispersibility and peptization properties, etc. It is mainly used as a raw material for the preparation of γ-Al2O3. In recent years, it has become a popular application in the field of new energy lithium battery separators.
Pseudo-Boehmite
Pseudo-boehmite is a kind of fine-grained hydrated alumina with thin folded lamella and incomplete crystallization. Its water-containing state is thixotropic gel with relatively large pore volume. and specific surface area. Pseudo-boehmite can be calcined at 400-700 ℃ for a period of time to obtain γ-Al2O3, while calcined at 1100-1200 ℃, nano-scale α-Al2O3 can be obtained.
Pseudo-boehmite and boehmite have similar structures, and there is no distinct boundary between them. There are two main differences between them. On the one hand, the two grain size and crystal integrity is different, boehmite grain is larger, the crystallization is more complete; On the other hand, the water content is different.
There are three different types of water in pseudo-diaspore, that is, surface water adsorbed on the surface of colloidal particles by van der Waals force, interlayer water bonded with colloidal particles by hydrogen bond, and the most firmly bonded crystalline water. However, boehmite has only interlayer structure water and crystal water, and no surface adsorption water. Therefore, pseudo-boehmite is also known as incomplete dehydration boehmite, often regarded as the poor crystallization of boehmite, it is because of the above differences, resulting in a great difference in the physical properties and application of the two.
