Characteristics of particulate emissions from aluminium electrolysis cells
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Modern sampling and analysis techniques were applied to study the morphology and composition of pot exhaust particles from aluminium smelters. In this study the total spectrum of pot exhaust particles from prebake electrolysis cells was sampled on filters and in a standard cyclone. Application of a cascade impactor allowed for fractionation of raw gas dust into 12 fractions with particle diameters in the range from approximately 7 nm to 10 μm. The performed experiments demonstrated that the proposed methodology for sampling and analysis was useful to link changes in operational conditions to changes in particulate emissions. Alumina feeding proved to have a significant impact on emissions. The major part of the particulate emissions form the cells consisted of rather clean sodium fluoroaluminates in the sub-micron range, while a smaller amount of coarse particles contained the major part of the impurities (except S, K, and Pb). The sub-micron particles consisted mainly of quenched bath fumes typical for cryolite-based melts containing considerable amounts of NaAlF4. Sulphur, potassium, and lead were the only impurity elements accumulating in the fines with approximately 6.0, 0.6, and 0.1 wt%, respectively. Coarse particles represented the minor fraction in the pot exhaust but contained large amounts of impurities. The impurity content in particulate pot emissions increaseed rapidly with increasing particle diameters Di > 1 μm. The coarse particles originated most probably from the alumina feedstock, bath-cover material, electrolyte droplets and remains from anode consumption entrained into the off-gases by e.g. bubble burst due to gas evolution under alumina feeding and CO2 release. The ICP-MS element analysis as well as IR, XPS and XRD-analysis results indicated that scale samples (depositions found in gas treatment centres) consisted of considerable amounts of coarse pot exhaust particulates. Additions of AlF3-solutions to coarse pot exhaust particles at 110 °C changed the crystal structure as cryolite and NaAlF4 particles were recrystallized. The observations suggest hydrolysis and recrystallization as possible scale formation mechanisms. As proven in this work, impurities were concentrated to about 2.3 wt% (Ni, Fe, P, V, Ti, Co, Cu, Zn, and Ga) in pot exhaust particles with an EPA standard cyclone operated with a cut-size of 11 μm. Removing the coarse particles with diameters larger than approximately 10 μm (14 wt% of the total dust) from the raw gas reduced trace elements in the remaining particles by approximately 25 %, while carbon and sulphur were reduced by 47 % and 3.6 %, respectively. Based on these results it is claimed that removal of coarse particles (approximately 4-5 kg dust/metric ton Al) from the raw gas will prevent recycling of significant amounts of contaminants in the dry scrubbers, thereby improve current efficiency and metal quality. Still, the recycling of valuable fluoride from separated coarse fractions, the role of sulphur in the ultra-fines as well as the recrystallisation of particulate emissions have to be addressed in further work.