The behaviour of phosphorus impurities in aluminium electrolysis cells

Abstract

Phosphorus is an important impurity element in the Hall-Heroult process, where it affects the current efficiency with respect to aluminium and the metal quality. The chemical and electrochemical behaviour of phosphorus compounds in molten cryolite-base electrolytes was studied in controlled laboratory experiments. Measurements were also carried out in industrial cells by analysis of bath and metal as a function of the time after additions of phosphorus compounds to the electrolyte. Phosphorus has a relatively long residence time in the electrolyte compared with metallic impurities such as iron, silicon and titanium. Phosphorus escapes from industrial cells through evaporation of elemental phosphorus or as a phosphorus compound attached to small carbon particles (carbon dust). It was found that only small amounts of phosphorus are primarily deposited at the cathode. A relatively minor increase of the normal operating temperature was found to cause a drastic decrease of the amount of phosphorus in aluminium.