A Laboratory Study of the HF Generation Potential of Particulate Fluorides from Cell Emissions

The injection type dry scrubber is the most commonly used technology in the aluminium smelting industry to manage HF emissions. One limitation of this technology is its sensitivity to temperature and humidity. Previous studies of smelter operation data and experimental work have demonstrated a clear link between the increase in temperature and/or humidity and increase in HF concentration in the treated off-gas at the dry scrubber stack. In this study, the generation of gaseous HF through a hydrolysis reaction between gas stream humidity and particulate fluorides has been tested under controlled laboratory conditions. The aim is to identify the HF generation potential of various particulate fluoride phases from cell emissions, and to understand the effect of temperature and humidity on the main reactive fluoride material(s). A range of particulate fluorides were tested, including purchased high purity fluorides and laboratory generated materials such as crushed bath and condensed fume. Results to date show the most significant HF generation bath phase to be the condensed fume phase NaAlF₄. However, two iron fluoride phases which were minor impurities in the lab generated fume were also found to be significant HF generation contributors.