Monitoring alumina content in cryolite-alumina melt

Abstract

Alumina content in electrolysis cells for aluminum production is one of the most important and poorly controlled parameters. The alumina is delivered to the cell by means of an alumina automatic feeding system, adjusted to the amperage or to the voltage alteration. However, a change in the alumina content in the cryolite-alumina melt depends not only on the current; and the voltage is not a parameter that responds only to the alumina amount in the melt. In order to check the current value of alumina content as well as the dissolution of alumina in industrial electrolytes (NaF-AlF3-CaF2-Al2O3), a novel electrochemical sensor was proposed. It was comprised of a carbon working electrode and a counter electrode interacting with aluminum. The sensor was easy to manufacture, and it allowed reducing the measurement error associated with back reactions at the working electrode. The novel approach was considered on an ex-ample of dissolving the alumina in the NaF-AlF3-(5 wt%)CaF2 melt ([NaF]/[AlF3] = 2.1 mol/mol) containing alumina (Al2O3) in amount of 0.69-4.51 wt% at 970 °C in conditions of natural and forced convection. It was found that the alumina solubility in the studied melt was 4.51 wt%. Depending on the initial content of alumina in the melt and convection conditions, its dissolution rate varied up to 0.36 mol/s·m3.