On Gaseous Emissions During Alumina Feeding

Abstract

Aluminium electrolysis involves feeding of alumina into a cryolite-based bath. Water originating from alumina as well as from air reacts with fluorides and results in HF evolution. Untreated HF gas is a significant environmental and economical issue. HF is, however, effectively adsorbed in (primary) alumina before being fed back to the cell as secondary alumina, thereby recycling the fluoride. As alumina is fed to the cell, it forms a raft, delaying the dissolution process and linked to several operational challenges. The goal of the current work is twofold; first, water content is investigated in a lab-scale setting, aiming to explain raft porosity, ultimately causing it to float rather than disperse in the bath. Secondly, the evolution of HF is investigated for different alumina in industrial measurements performed at Alcoa Mosjøen, aiming to identify correlations between gas evolution and alumina properties.