Fundamental and applied studies on cyanide formation in the carbon liner of an industrial aluminium electrolytic cell

The objective of this study was to investigate the presence of cyanide in the industrial aluminium electrolytic cell. Therefore, it was an aim of this investigation to identify the favoured regions of cyanide formation in potlining, and conduct parallel laboratory studies to determine some of the chemical factors likely to influence cyanide formation. The study was conducted in three stages. The first stage involved autopsies of cells of various ages, design and construction in detail. This was followed by extensive sampling of sections of the trenched sidewalls and cathode combined with chemical analyses of the specimens, to enable mapping of the cyanide distribution profiles. Examination of these profiles revealed that major amounts of cyanide were generally concentrated in the carbon sidewall region near the collector bars and to a lesser extent in the cathode near the mid-section of the pot. In particular, carbon that was heavily impregnated with metallic sodium and easily accessible to air ingress, was found to contain high concentrations of cyanide. Thus, from the cyanide profiles it: was evident that air ingress through the collector bar seals of the cathode, and sodium penetration of the carbon governed the favoured region for cyanide formation during normal pot operation. In the second stage, an examination of the kinetics of sodium cyanide formation in carbon-mix was performed in the laboratory. Metallic sodium was reacted with carbon and gaseous nitrogen over a range of temperatures. Rate plots derived from a combination of thermogravimetric and integral analysis techniques indicated that there was increasing reactivity with increase in sodium content during the initial stages of reaction. Furthermore, rate data obtained appeared to fit a second-order reaction relative to reactant sodium after the initial burst of reaction. Further laboratory investigations were conducted to establish the temperatures required for cyanide formation, and determine the susceptibility of different forms of carbon to cyanide generation. The carbon types ranged from plant derived carbon-mix through commercial preformed cathode material to high purity graphite. Whilst cyanide formation was found to readily occur in the temperature range of 500-600°C in all the carbons tested, the less ordered carbon-mix was significantly more vulnerable. However, highly graphitised carbon was the least susceptible to cyanide formation within its structure. Moreover, the reaction was accelerated by the presence of small amounts of iron impurity. Laboratory tests also indicated that the predominant cyanide-containing species formed in potlinings was most likely uncomplexed sodium cyanide. The overall results obtained from laboratory studies were consistent with plant measurements and observations derived from the complex industrial situation. Conclusions drawn from this work may have implications in terms of future cell design, and the ultimate impact of spent potlinings on the surrounding environment.