High pure aluminum can be obtained in three ways: by zone-melting, distillation through subgalogenidy and electrolysis. Of these methods of producing high pure aluminum, industrial utilization was a way to zone melting.
The principle of zone melting is in multiple passage of the molten zone along the ingot of aluminum.
It is necessary to prepare aluminum intended for zone melting, preparation consists in filtering, degassing and etching. Filtering is necessary for removal infusible and solid oxide pellicle, dispersed in the metal, from aluminum.
Preliminary degassing is performed before zone melting. This operation is intended to prevent spraying of metal on melting the zone in the case of carrying out the process under high vacuum. The last stage of the preparation of the aluminum to the zone melting is etching the surface with a mixture of concentrated hydrochloric and nitric acids.
Since the aluminum has a large chemical activity, the zone melting of aluminum is carried out in vacuum or in an inert gas atmosphere (argon, helium).
Zone melting in vacuum provides great purity of aluminum. This occurs in consequence of volatilization of impurities (magnesium, zinc, cadmium, and alkali and alkaline earth metals). Besides, this prevents pure aluminum from contamination. Pure aluminum smelting process takes place in vacuum-sealed quartz ampoules, of which the pre-evacuated to a residual pressure of about 1ּ10–3 Pа.
The essence of cascade zone melting method is that the purification of initial purity aluminum 999 is conducted sequentially repeating the cycles (stages) of zone melting. At the same time the starting material of each subsequent cascade is the average, the most pure part of aluminum ingot, produced in a previous cleaning cycle.