Cathodic Wear by Delamination of the Al 4 C 3 Layer During Aluminium Electrolysis
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INTRODUCTION
WITH nearly 60 million tonnes per year, aluminium is currently the second most widely produced metal in the world. Aluminium production showed a very substantial increase from 2000 to 2018. To adapt to the continuously rising demand, aluminium primary producers work on designing higher amperage electrolysis cells (‡ 400 kA) working at higher current to increase their production. However, increasing the amperage also means increasing the probability of a rapid deterioration of the cells. Indeed, life expectancy of electrolysis cells is too often limited by the cathode blocks; a higher cathode current density accelerates the wear of the graphitized cathode blocks, which may lead to
AI¨MEN E. GHERIBI and PATRICE CHARTRAND are with the Department of Chemical Engineering, CRCT - Polytechnique Montre´al, Box 6079, Station Downtown, Montre´al, QC H3C 3A7, Canada. Contact e-mail: [email protected] MOJTABA FALLAH FINI and GERVAIS SOUCY are with the Department of Chemical Engineering, Universite´ de Sherbrooke, Boulevard de l’Universite´, Sherbrooke, J1K 2R1, Canada. LOIG RIVOALAND is with the Carbone Savoie, 30 Rue Louis Jouvet, 69200, Ve´nissieux, France. DIDIER LOMBARD is with the Solutions Technologiques Aluminium - LRF, Rio Tinto, 73302, Saint Jean de Maurienne, France Manuscript submitted June 4, 2019.
METALLURGICAL AND MATERIALS TRANSACTIONS B
premature death of the cells. The wear mechanism is said to be due to: particle detachment, physical abrasion by solid alumina particles and chemical wear due to the formation of an aluminium carbide (Al4C3) layer at the interface between the cathode and metal pad. Aluminium carbide can be formed either chemically (Chem.) by a direct reaction between the liquid metal and the cathode carbon according to: 4AlðlÞ þ 3CðsÞ ! Al4 C3ðsÞ
Chem.
½1
or via an electrochemical (E. Chem.) reaction between the cathode carbon and the electrolyte or bath ( Na3 AlF6 , 11 wt pctAlF3 , 3 wt pctAl2 O3 and 5 wt pctCaF2 ) used to dissolve the alumina. A film of partially molten electrolyte is present in electrolysis cells between the liquid metal pad and the cathodic carbon blocks due to falling electrolyte particles or droplets (from alumina feeding process) and interfacial phenomena. The electro-chemical reaction at the surface of this film is usually reported as: 4AlF3ðbathÞ þ 3CðsÞ þ 12e ! Al4 C3ðsÞ þ 12F bath E. Chem.
½2
When formed, Al4C3 is subsequently slowly dissolved in the electrolyte film to form the Al3 CF38 complex anions according to the reaction:
3 Al4 C3ðsÞ þ 5 AlF3ðbathÞ þ 9F ðbathÞ ! 3Al3 CF8ðbathÞ
½3
To better understand the wear of the cathode block, several experimental studies have been conducted recently characterizing and analysing the structure and mechanical stability of Al4C3 layers.[1–5] These studies are post mortem on both industrial and bench-scale Hall-He´roult cells. Although they are not really characterized in the electrolysis cell operating conditions, these studies bring valuable information to the carbide layer. In 2013, Øs
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