15 10月 Liquid Aluminum Refining System
Liquid Aluminum Refining System – Purification of aluminum from impurities
For purification from non-metallic and gaseous impurities, as well as from compounds of sodium, magnesium and calcium, molten aluminum is purged with chlorine and settled in electric furnaces. In this case, there is a reaction:
2Al + 3Cl2 à 2AlCl3
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the process goes at a temperature of 700 0C. Under these conditions, AlCl3 is in a gaseous state. When AlCl3 bubbles pass through molten aluminum, they are adsorbed on alumina particles, metal fluorides, and coal. Bubbles together with the solid phase rise up and form a loose gray layer on the surface, which is removed. In addition, the transmission of chlorine helps to remove gases from the metal. After purification, the Al content is 99.5 – 99.8%. It is suitable for most consumers, but sometimes more pure aluminum is required, and then it is subjected to deeper purification by zone melting or by distillation through subconnections.
Zone melting is based on the fact that the solubility of impurities in solid metal is lower than in molten. For zone melting, rods of diameter 15-20 mm and length 250-300 mm are cast from aluminum. One bar is placed in a boat of very pure graphite. The boat is placed in a quartz tube, in which a vacuum is created (residual pressure 10-4 mm Hg) to prevent aluminum oxidation.
A ring inductor connected to a high frequency electric current source is placed around a quartz tube. Under the inductor, the rod is melted, and a zone of liquid aluminum appears.
The inductor is moved along the bar at a speed of 1 mm / min. After 10-15 runs, the bar is removed and 80-100 mm cut. The remainder contains 99.9999% Aluminum.
Cleaning through the subconnects proceeds by passing vaporous aluminum chloride or fluoride over molten aluminum at a temperature of more than 10,000 ° C. Under these conditions, aluminum interacts with aluminum chloride by the reaction:
AlCl3 + 2Al ↔ 3AlCl
When the temperature drops to 700-8000С, the reverse reaction occurs, that is, AlCl decomposes into metallic Al and AlCl3. Impurities are not distilled. The aluminum content after cleaning by this method is 99.99999%.