Environmentally sound hydrometallurgical recovery of chemicals from aluminium industry spent potlining

Abstract

Spent Potlining (SPL) is a waste material generated by all types of electrolytic cells used in the smelting of alumina oxide to make aluminium metal. This material has been traditionally stored on site in many locations. Numerous processes have been investigated to deal with this problem. Some of the solutions involve the treatment of the waste, with chemicals and high temperature incineration, primarily aimed at eliminating cyanides and reducing leach rates of soluble fluorides prior to landfilling. Alcan has taken a different approach. SPL contains recoverable chemical elements (C, F, Na and Al) which are lost in stabilisation or immobilisation processes. Using hydro-metallurgical technology, we have developed a process which recycles the chemicals contained in the SPL. The process consists of grinding and classifying the SPL before digesting it in hot dilute caustic. The resulting slurry is filtered and the residue washed and used as fuel. The filtrate is autoclaved to decompose the cyanides then evaporated to precipitate the sodium fluoride. The sodium fluoride crystals are then filtered from the caustic liquor. This liquor is recycled to the alumina refinery. The sodium fluoride can be sold as is or be redissolved and converted to calcium or aluminium fluoride. This process is not only competitive with existing SPL treatments but is also environmentally superior. The process has been piloted.