Cyanide inhibitors for the carbon cathode materials in aluminum reduction cells
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I.
INTRODUCTION
IN aluminum reduction cells, the electrolysis leads to the formation of metallic sodium in solution in the liquid aluminum. This metallic sodium diffuses in the carbon cathode materials and may react with the nitrogen from the infiltrated air to form sodium cyanide according to the following reaction: tL21 2 Na (in C) = 2 C (s) + N 2 (g) = 2 NaCN
[1]
Molten bath penetrates the carbon cathode materials during the operation of a cell at a rate that is limited by the diffusion of the metallic sodium, t2,31 However, after saturation of the carbon materials by the sodium, which may take place after 1000 days of operation, the rates of bath infiltration would be increased, since it is only controlled by the liquid penetration process. During infiltration, the bath reacts with the carbon, the metallic sodium, and the surrounding atmosphere to form NaF, t3m thus increasing the NaF/AIF3 weight ratio that characterizes the bath composition. Before the sodium saturation, the time to achieve this reaction would be higher than after saturation, because of the difference in the infiltration rate. The ratio of the infiltrated bath in the carbon cathode materials would thus be higher before, rather than after, sodium saturation. Since the cyanide formation in a cell increases with the molten bath ratio, t21 it could more generally be a function of the ratio of the infiltrated bath in the carbon lining of the cell. In such a case, the cyanide content in the carbon materials should be less after the sodium saturation. Moreover, any additive in the carbon lining of a cell which would reduce the infiltrated bath ratio would prevent the formation of sodium cyanide during the operation of aluminum reduction cells. The present study was initiated to verify the previous CLAUDE ALLAIRE, Professor of Ceramics and Refractories, is with the CIREP-NSERC Chair on refractory materials, I~cole Polytechnique, University of Montrral, Montrral (Qurbec), Canada, H3C 3A7. PAUL DESCLAUX, Consultant in Electrochemistry, is with Alcan International Limited, Jonquibre (Qurbec), Canada, G7S 4K8. Manuscript submitted June 7, 1993. METALLURGICAL AND MATERIALS TRANSACTIONS B
hypothesis with an emphasis put on the elaboration of cyanide inhibitors.
II.
EXPERIMENTAL PROCEDURES
To verify the effect of the ratio of the infiltrated bath in the carbon lining of aluminum reduction cells on cyanide formation, different types of carbon material were used in the Alcan Standard Electrolysis Bath T e s t . HI These carbon materials consisted of - 4 8 Tyler Mesh powder from either carbon-spent potlining or anthracite, with or without the addition of 20 wt pet cryolite (Na3AIF6) (Table I). The spent carbon from the potlining materials came from two different locations of the same cell (approximately 2500-days old): from the monolithic mix below the carbon side blocks and from the cathode blocks. As shown in Figure l, the 0.08-m-diameter x 0.08m-high carbon sample was placed in a cathodic inner graphite crucible. The latter was immersed in a molten c
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