Development of an analytical method to determine the cryolitic bath composition by Raman spectroscopy

Since the end of the nineteenth century, metallic aluminium is produced by electro-deposition from a solution of aluminium oxide in cryolitic melts around 1000°C (Hall-Héroult process). The industrial melt is composed mostly of cryolite (Na3AlF6) and AlF3 and is characterized by the molar NaF/AlF3 ratio, named cryolitic ratio (CR). It turns out that the bath composition is critical: for instance, it has been shown that a small change in the Al2O3 content leads to a great change in the overvoltage required for the electrolysis. Therefore controlling the melt composition is very important in order to reduce the energy lost. Unfortunately no in situ analytical method allows studying the composition of the melt yet.
Considering our experience in the study of such highly corrosive media by Raman spectroscopy and since the bath spectrum is function of both the CR and the Al2O3 content, we have proposed in the past to apply that technique to the direct determination of the melt composition. Despite the CR could be well evaluated in the lab, experimental problems however made the practical application difficult.
The purpose of this presentation will be to show the new results obtained on an updated instrument:
- Spectra are recorded in 20 s or less with a higher quality than before.
- The previously developed home-made software was adapted to the updated instrument and various spectra analysis procedures are under study.
- A procedure to prepare reference samples was also developed taking into account the homogeneity problems that have been met.
- The slopes of the alumina calibration curves are depending on the bath CR, in confirmation of our previous results.
- The new results are compared with the previous ones.
It will be concluded that Raman spectroscopy is indeed becoming a suitable technique for developing an analytical method to determine the composition of industrial cryolitic melts.