Aluminium smelting : the toxicity, scale and longevity of fluoride pollution in the environment

PhD ThesisThe release of fluoride from the production of aluminium, and the subsequent impacts of deposition in the environment, is well documented (Weinstein and Davison, 2004). However, changes are occurring within the industry, driven by technological advances and growth in global manufacture, which have the potential to alter the impact of the aluminium industry on the environment. There needs to be a better understanding of the toxicity of Sodium hexafluoroaluminate (cryolite, Na3AlF6), the impact of modern operating procedures on the release of particulates in to the environment, and finally, an investigation of the longevity of fluoride concentrations in vegetation from industrial pollution once emissions cease (EU, 2008). The toxicity of cryolite exposure to 1st and 3rd instar Diamondback moth larvae (Plutella xylostella (L), Lepidoptera: Plutellidae) was studied through a series of bioassays. There were inter-stage variations in both the lethal and sub-lethal responses to cryolite, whereby low mortality corresponded with high sub-lethal effects and vice versa. A dose-related response threshold was observed in both instars indicating that the physical nature of cryolite plays a role in the toxic mode of action. The comparative toxicity of cryolite from the pot-rooms of Lynemouth smelter and pesticide Kryocide© (Cerexagri, Inc) was investigated along with the role of particle size and the purity of the cryolite particulate in the mode of action. Although both physical and chemical factors played a role in toxicity, these factors formed a hierarchy and the mode of action had a significantly over-riding chemical grounding. Dust samples were collected from the vicinity of Lynemouth smelter for a period of 10 months in order to quantify the impact of modern aluminium production on the environment. Although the volumes of dust collected at any one site were lower than the European Community Short Term Ambient Guideline Concentration of Particulate Matter, dust deposition corresponded with aluminium production rates at the smelter showing that, despite Better Available Technology at the smelter, modern aluminium production still impacts on the environment. When Anglesey Aluminium Metals Ltd ended its operations in September 2009, it created a unique opportunity to assess the long-term impacts of fluoride pollution on the environment. Samples of soil, leaves and lichens were collected to monitor the changes in fluoride content over a 15 month period. The results showed that fluoride contamination of vegetation from industrial processes had a short-term impact once emissions ceased and that mineral-based soils had limited potential as contaminating sources for plant uptake. The mode of action of cryolite toxicity and the impact of waste disposal and fugitive release of cryolite in to the environment are discussed.Rio Tinto Alca