Characterization of inorganic fraction of spent potliners: evaluation of the cyanides and fluorides content.

Spent potliner (SPL) is a solid waste generated by the aluminum industry during the manufacture of aluminum metal in electrolytic cells. Initially the electrolityc cell liners comprise of graphite and carbonaceous materials, but after several years of operation, the liner materials deteriorate and must be removed from the cells. Because of the presence of fluoride and cyanide in the SPL, the US Environmental Protection Agency (USEPA) has listed the materials as a hazardous waste. The purpose of this work was to characterize the extent of leaching of cyanides and fluorides from SPL, as a function of the number of years the material was present in an operating electrolytic cell. At Alumínio Brasileiro S.A. (ALBRAS) plant, SPL was separated into two fractions: a carbon component and an inorganic part. Inorganic materials from nine pots, about 28 tonnes per pot, were examined in this study. When placed in water at a ratio of 20 g solid to 20 ml of water, the pH for all samples varied from 10 to 11.8. The total measured fluoride content of the solid samples varied from 5.13 to 11.41%. However, when leached at a pH of 5, the dissolved fluoride was equivalent to only 0.26-3.46%. With a pH of 12 in the leachate solution, the dissolved fluoride was equivalent to 6.45-9.39%. The data show that the fluorides of the waste are more soluble in basic solutions, and when leached at a pH of 12 are much closer to the actual fluorides content. For the same samples, the dissolved quantity of cyanide was equivalent to 4.34-27.33 ppm, with an average of 13.26 ppm. For all the samples studied, there did not appear to be a correlation between the fractions of fluoride and cyanide leached from the samples and the operating life of the potliner materials.