Alumina hydrates and Alumina


Alumina Hydrate (Coarse)

Aluminium hydroxide or alumina tri hydrate (ATH) is the hydrated oxide of aluminium. Alumina hydrate is separated from bauxite ore using the Bayer process, with average particle size ranging from 80-100 micron. The block crystals of alumina hydrate impart good chemical reactivity. Alumina hydrate can react with a base as well as an acid, and finds use in many applications as raw material.

Alumina hydrate (fine)

After drying, alumina hydrate is ground using mechanical mills and ceramic lined ball mills to obtain finer particle sizes. Hindalco manufactures ground hydrate with different particle size (5-15 micron) distribution. Surface-treated fine hydrate as well as super-ground fine hydrate (1-2.5 micron) are also available.

Calcined alumina (coarse)

Aluminium hydroxide, or alumina tri hydrate obtained in the Bayer process, is calcined at temperature above 1200°C and up to 1600°C to manufacture special grade alumina. During calcinations, alumina hydrate crystals lose bound moisture and recrystallise to form alumina crystals. The particle size of alumina remains at 85-100 micron. Special alumina contains predominantly alpha phase. The degree of calcination is a measure of the hardness of alumina – soft to hard. Coarse alumina is classified based on the soda (Na2O) content:

  • Low soda alumina - Na2O <0.1%
  • Medium soda alumina - 0.1% < Na2O <0.2%
  • Normal Soda alumina - 0.20% < Na2O < 0.45%

Fine alumina

Calcined alumina is ground in fluid energy mills or ceramic lined ball mills to meet the desired particle size required by the customers. Hindalco manufactures fine alumina with varying particle size (0.5 to 8 micron) and distribution. Low soda, medium soda and normal soda type are available in fine alumina also.

Reactive alumina

This alumina contains predominantly alpha phase with very fine particle size. High thermal reactivity and low water absorption are the special characteristics of reactive alumina. Upon sintering, reactive alumina will give a density close to the true density of alumina.

Product Applications

Alumina hydrate:

Aluminium hydroxide or alumina trihydrate is the hydrated oxide of aluminium. Alumina trihydrate is separated from ore bauxite using Bayer process with average particle size ranging from 80-100 micron. The blocky crystals of alumina trihydrate impart good reactivity. Alumina trihydrate can react with a base as well as an acid and finds many applications as raw material. Aluminium hydroxide or alumina hydrate is used in the manufacture of many inorganic chemicals like:

  • Non- ferric alum
  • Poly aluminium chloride
  • Aluminium fluoride
  • Sodium aluminate
  • Catalysts
  • Glass
  • Aluminium hydroxide gel

Alumina hydrate is available in wet as well as dry form.

Fine hydrate:

Alumina trihydrate (ATH) contain 3 molecules of water. On exposure to heat above 220°C, alumina hydrate decomposes into aluminium oxide (alumina) and water. This irreversible, endothermic reaction process makes alumina hydrate an effective flame retardant. Also, the smoke generated by decomposition is non-corrosive and non-poisonous. Ground alumina hydrate is used as fire retardant filler in applications like polymer composites, cable compounds, solid surface counter tops, etc.


Alumina used for applications other than aluminium metal extraction is classified as chemical grade alumina or special alumina. This type of alumina finds use in three major applications:

  • Refractory
    A refractory material is one which retains its strength when subjected to high temperatures. Refractory materials are used in linings for furnaces, kilns, incinerators and reactors. 70 per cent of the refractory produced in the world is consumed for the manufacture of iron/ steel. Refractory materials are selected based on the conditions which they face. Major characteristics required for refractory materials are chemically inert, excellent thermal shock resistance, withstands high temperature, low thermal conductivity and low coefficient of expansion. Alumina is used as refractory raw material in various forms – calcined fine alumina, reactive alumina, tabular alumina and white-fused alumina.
  • Ceramics
    Ceramic materials are inorganic, non-metallic material bonded with metallic or non-metallic elements primarily with ionic/ covalent bond. Ceramic materials are hard, brittle, strong in compression, weak in tension and stress. They have high resistance to chemical erosion when subjected to acidic or caustic environment and can withstand high temperatures 1000-1600°C. Ceramics is all around in our day to day life – from bricks used for construction to heat-resistant micro parts used in space shuttles. Ceramics are divided into four categories:
    • Traditional ceramics are naturally occurring clay based products. The most recent ones are alumina based for improved properties.
    • Industrial ceramics are alumina (normal & low soda) based materials. These include wear parts, spark plug, grinding media, etc.
    • Technical ceramics are alumina (low soda) based materials used for applications like thermocouple tubes, crucibles, etc.
    • Advanced ceramics include the most recent developments in the field of ceramics, which contain 99.8 per cent alumina based products also.
  • Polishing:
    Polishing is the process of preparing a smooth or shiny surface by rubbing or using a chemical reaction. Abrasive materials are used as polishing compounds. An unpolished surface contains mountains and valleys when looked at microscopic levels. By rubbing the surface with polishing compounds, mountains and valleys are reduced to small hills or plains. Alumina, being abrasive in nature, is used in many polishing compounds. Hard calcined alumina is used in stock removal and soft calcined fine alumina is used for imparting smooth mirror finish.

Hindalco alumina and their applications

Coarse alumina:

Calcined alumina is widely used in refractory and ceramics industry. Their specific properties include extreme hardness, refractoriness, high mechanical strength and resistance to abrasion, chemical attack and corrosion. Coarse alumina grades are white crystalline powders that are predominantly alpha crystals of high purity and consistent physical properties. These are used as feed material for fused alumina, refractory aggregates, etc.

Fine alumina:

This alumina finds application in ceramics – wear resistant liners, grinding media, HVAC/ HVDC insulators, in refractory – bricks, monolithic and gunning masses. Normal soda as well as low soda type fine alumina, are manufactured to meet specific demands.

Reactive alumina:

These grades are predominantly alpha alumina having wider particle size distribution. They yield a low water demand with very good flow properties when used in low–cement castables, ULCC and self-flow castable mixes. They are also recommended for usage in grinding media, wear resistant ceramic components and liners, owing to their superior packing characteristics.

Reactive ultra fine alumina:

These reactive alumina are produced by extensive grinding of the reactive alumina and are specially designed for technical ceramics and specialised refractory. These alumina are characterised by high thermal reactivity, low water absorption and controlled particle size distribution. Typical applications include LC and ULC castables, slide gates and refractory bricks, new generation ceramics and wear resistant ceramic parts.

Polishing alumina:

These grades are developed through controlled calcinations and processing in order to obtain consistent particle hardness, shape and size distribution. The particle size is controlled closely to optimise oil absorption. These alumina are recommended for use in solid bars, emulsions and pastes for metal and stone polishing applications.

In this section