Fused Alumina
Fused alumina is made in electric arc furnaces by passing a current between vertical carbon electrodes. The heat generated melts the alumina. The stove consists of a water-cooled steel shell, and 3-20 tonne batches of material are fused at any one time. The fused alumina has a high density, low porosity, low permeability, and high refractoriness. As a result of these characteristics, it is used in the manufacture of abrasives and refractories.
Calcined Alumina
If aluminum hydroxide is heated to a temperature over 1100℃, it passes through the transition phases of alumina.
The final product, if a high enough temperature is used, is α-alumina. The manufacturing process is commercially undertaken in long rotary kilns. Mineralizers are frequently added to catalyze the reaction and bring down the temperature at which the α-alumina phase forms; fluoride salts are the most commonly used mineralizers.
These calcined alumina products are used in a wide range of ceramic and refractory applications. The main impurity present is sodium oxide. Various grades are produced, which differ in crystallite size, morphology, and chemical impurities.
The calcined grades are often sub-divided into ordinary soda, medium soda (soda level 0.15-0.25% wt%), and low soda alumina.
