Cell performance and anodic processes in aluminium smelting studied by product gas analysis
Aluminium smelting is an energy intensive process, and as a result there has been considerable
and ongoing research over a number of decades on the energy efficiency of various aspects of
the process. One of the most important measures is current efficiency, which has been shown
to have direct relationships with current density, cell temperature, electrolyte chemistry, and
anode-cathode distance. The effects of these variables on current efficiency are generally
accepted, however there remains debate over the influence of the alumina concentration in the
electrolyte on current efficiency.
This research relied upon the development of a laboratory scale aluminium smelting cell where
the current efficiency was measured via sampling of the product gases. A modified oxygen
balance was used, with gas analysis performed using online mass spectrometry.
The findings of this research agreed with the accepted current efficiency trends, showing a
current density influence of 17.25 %CE per A/cm2, over the range 0.3 and 1.1 A/cm2. The
influence of electrolyte chemistry was -7.8 %CE per unit cryolite molar ratio, between cryolite
ratios 1.99 and 3. The anode-cathode distance was shown to have no influence on current
efficiency in this cell, contradicting the established findings, however this was expected
because of the design of the cell with no metal pad at the cathode and therefore constant mass
transfer conditions at all the anode-cathode distances used.
The most significant finding concerning current efficiency is that the variation with alumina
concentration is so small, -0.0376 %CE/wt% Al2O3, that there is effectively no influence.
While in many other studies an influence was found, the values and direction of the relationship
varied. This suggests that in many cases the observed variation in current efficiency was
actually caused by a change in the level of stability in the cell, by processes such as dissolution
of sludge from the cathode or the thermal disturbance of alumina feeding, whereas in this
research the cell was stable under all operating conditions.