Chemical Reactions and Physical Changes of the Cathode During Aluminium Electrolysis. Laboratory and Industrial Studies

 

Harald A. Øye

Department of Chemistry

Norwegian University of Science and Technology

N-7491 Trondheim, Norway

 

The phases present in cooled-down cathodes were determined quantitatively as function of time by X-ray diffraction. Sodium is the primary penetration agent which facilitates wetting and subsequent penetration of the electrolyte. The sodium absorption also results in expansion and slow graphitization. The electrolyte reacts with sodium, carbon and the inside atmosphere to give the following reaction products: NaF, NaAlO₂, AlN, NaCN and Al₄C₃. The reactions are understood from equilibrium equations.

 

The laboratory studies were short-time (up to 2 hours) while 16 industrial cathodes aged 10 to 2534 days were examined. The composition of the electrolyte in the industrial cathodes changed with time giving a Na₃AlF₆ deficient phase from 400 to 800 days. The changes in the industrial cathodes are understood from the model developed by the laboratory studies.