Impurity Elements in Raw Gas Ultra-Fines from Aluminium Electrolysis Cells

The effect of alumina feeding and pot suction rate on the amount and composition of fine particles in raw gases has been investigated. Particle size distribution of ultra-fines (Di < 10 mu m) were measured in real time and the chemical composition of particle size groups were analyzed. Mass distribution of dust in fumes from cells under different operational condition were calculated from density estimates based on measured currents which again were converted to particle number impacts and compared to weighted samples. EDS analysis and HR ICP-MS revealed a significant increase in contaminant level for particles with a Di > 0.75 mu m. The findings indicate that particles with Di< 1.2 mu m consist mainly of quenched bath fumes, NaAlF4, Na5Al3F14, partly converted to Na3AlF6 and Al2O3. Particles with Di> 1.2 mu m exhibit significant levels of contaminants, presumably entrained particles from the combustion of anode carbon and alumina feed.