The development of an aqueous leaching process for the treatment of spent pot lining

© 2001 Dr. John BesidaSpent pot lining (SPL) is the failed cathode lining of the electrolytic cells used in the production of aluminium metal via the Hall-Heroult process. It consists of carbon, refractory brick material and various residual components of the electrolytic bath. SPL is classified as a hazardous waste because it contains significant quantities of leachable fluoride, predominantly present as the compounds sodium fluoride, cryolite and calcium fluoride, and cyanide, usually present as sodium cyanide and sodium ferrocyanide. As such, it cannot be disposed of in a conventional manner. Moreover, major restrictions are also imposed on the handling, transportation and storage of SPL. Although numerous processes for treating this solid have been investigated, few have proceeded beyond pilot plant stage. Of those that have, virtually all employ high temperatures to destroy cyanide by oxidation. However, only a small number of these attempt to recover the fluoride values for recycling. Currently, the safe treatment/disposal of SPL constitutes a major concern for the aluminium smelting industry because there is, as yet, no universally accepted technology for the treatment of this waste. The research presented in this thesis attempts to address the SPL problem. It is specifically aimed at the development of an efficient aqueous leaching process for the detoxification of SPL in a manner which ultimately enables the recovery and recycling of fluoride. “Wet de-lined” SPL, from a smelter operated by the Tomago Aluminium Company in New South Wales, Australia, was used throughout this study. Importantly, it was established early in the investigation that subjecting the SPL to a water leach, in the first instance, results in the dissolution of most of the cyanide and the complete extraction of sodium fluoride and sodium carbonate from the solid. The removal of sodium fluoride proved to be a significant result because that compound was found to constitute approximately 50% by weight of the fluoride initially present in the SPL. A thorough investigation on the feasibility of using aqueous fluoro-acids as extractive reagents for the remaining fluoride and recoverable aluminium values, present in water washed SPL, was subsequently undertaken. It was discovered that hydrofluoric acid, fluorosilicic acid, fluoroboric acid and monohydroxyfluoroboric acid each react with the oxidic compounds present in water washed SPL, such as Al(OH)3, NaAISiO4, SiO2 and CaCO3, to give fluoridated products, such as Na5Al3FI4, Na2SiF6 and CaF2, the precipitation of which are highly dependent upon the initial concentration of acid used. An in depth discussion on the possible mechanism of formation of these products is given in this thesis and it is concluded that the primary factors contributing to their precipitation are the overall fluoride ion concentration in solution and the presence of a large quantity of Na+ in the SPL. Significantly though, it was established that regardless of the initial concentration of fluoro-acid used, or the level of extraction achieved, the overall fluoride content of the water washed SPL increased with respect to that originally present prior to treatment with the acids. This result alone excludes the possible use of fluoro-acids as effective leachants for SPL. Consequently, it was decided to implement a totally different approach towards the treatment of water washed SPL. It was concluded that a more effective leaching strategy should focus solely on the total extraction of fluoride without attempting to recover the abundant aluminium values also present in the material. As such, an investigation on the leaching of fluoride from water washed SPL by aqueous solutions containing Al3+ was undertaken. Aqueous Al3+ proved to be a highly effective leachant in that it was demonstrated that approximately 95% of the fluoride in water washed SPL could be extracted by suitable control of reagent concentration and reaction temperature. Furthermore, it is hypothesized that optimization of other reaction parameters, such as SPL particle size, may lead to even higher levels of fluoride extraction. Once the efficacy of aqueous Al3+ to leach fluoride from water washed SPL was established, it was decided to investigate a possible means of generating Al3+ leachates from readily available materials. It was shown that a cheap and abundant mineral called nepheline syenite, which is used extensively in the glass manufacturing industry could be easily decomposed by dilute aqueous mineral acids to generate solutions of Al3+ which are capable of dissolving cryolite and calcium fluoride. These two fluorides are the main fluorides present in water washed SPL. Moreover, it was demonstrated from "model" compound studies that by suitable adjustment of the molar ratio of total fluoride to total aluminium, in leaching mixtures containing cryolite, calcium fluoride and aqueous Al3+, selective dissolution of the cryolite can be achieved. The option of selectively extracting cryolite from water washed SPL may simplify the procedure by which fluoride is ultimately recovered from the leachates. Finally, as a conclusion to the work presented in this thesis, a description of the unit operations of a conceptual process for the treatment of SPL is given. Included in the description are various chemical options which could be used to recover fluoride from the leachates in a form amenable towards recycling back into the aluminium smelting process