Smelter Fluoride Balances: The Interplay Between Alumina Phases, Pore Size Distributions, and the Impacts of Weather

Abstract

Fluoride cycling within a smelter has been examined as a function of alumina properties. Using an experimental configuration in which the two ends of a single potline were operated with different aluminas, quantitative measurements were made of fluoride evolution from the pots, and hydrogen fluoride (HF) levels at the scrubber and stack. Correlations were then examined with specific alumina properties, once environmental factors and other baseline effects had been considered. Although the phase analysis, including both residual gibbsite and alumina structural hydroxide content, is correlated with HF generation, local weather conditions (primarily humidity) also have a major influence on fluoride evolution. Importantly, HF concentrations at the scrubber (measured in the gas phase at the outlet to the gas treatment center), as a proxy for the kinetics of the scrubbing process, support the importance of alumina pore size distribution as opposed to simply specific surface area. This is also found to be true in the independent analysis of bath acidity, indicating a more general but extremely important influence of alumina porosity on the entire fluoride cycle. The study provides insight into the impacts of a range of typically unreported alumina properties on smelter performance and strongly supports the presence of kinetically inaccessible porosity in pores narrower than ~3 nm in alumina.