Recycling of alumina-containing sweepings for use in primary aluminium production

Abstract

Aluminium is produced by electrolysis using a melt that consists of cryolite and alumina. Domestic aluminium producers are facing a shortage of alumina, which is the raw material for making primary aluminium. This gives relevance to the problem of processing industrial alumina-containing waste in order to get Al2O3 back into the electrolysis process. When doing maintenance for aluminium cells, primary aluminium producers that rely on Soderberg cells generate alumina-containing waste with a complex and varying composition. However, it contains a considerable amount of valuable components, such as Na3AlF6, Al2O3, AlF3. When the waste is collected, valuable alumina particles get mixed up with various materials (lumps of asphalt and concrete, sand). As a result, alumina gets contaminated with iron and silicon compounds (SiO2, Fe2O3) and can no longer be reused in the electrolysis process as the main source of metal. RUSAL Bratsk PJSC has developed a unit that helps reduce the concentration of these impurities in the alumina-containing waste. The focus of this paper is on examining the operation of the developed unit, as well as searching for contrast properties of the waste material which could help enhance the performance of the unit. Having analyzed the phase composition of the samples, the authors established that the waste material is mainly comprised of cryolite, chiolite, corundum, quartz, feldspar, carbonaceous matter and a process phase of the following composition: (NaF)·1,5CaF2·AlF3. High concentrations of Si (1.91 %) and Fe (0.62 %) were registered in the following size range –0.63+0.315 mm. A microscopic study helped establish the contrast properties of the material. It was found that, considering the registered distribution of impurities, reusing the maximum amount of waste in the electrolysis process would require adoption of optical and gravitational processing. The contributors to this research includes L. V. Gavrilenko, Candidate of Technical Sciences, manager at the Directorate for Aluminium Production Processes and Advances, Engineering and Technology Centre (RUSAL ETC) in Bratsk.This research was funded through Presidential Grant No. SP-306.2022.1.