Direct determination of the NaF/AlF3 molar ratio by Raman spectroscopy in NaF-AlF3-CaF2 melts at 1000 °C.
For the last 40 years, Raman spectroscopy has been very useful in investigating the structure of corrosive molten salts, such as the cryolite-based melts widely used as electrolyte in the Hall-Heroult process. Even if this process remains the most economically efficient for metallic aluminum electro-production, it suffers from a high energy loss, which is dependent on the melt composition. Therefore, controlling the chemical composition of the electrolyte is essential. The present paper proposes to apply Raman spectroscopy for the direct determination of the NaF-AlF3 molar ratio in NaF-AlF3-CaF2-based melts. Despite the experimental difficulties, a calibration curve based on equilibria taking place in the melt has been developed and the procedure has been successfully compared to industrial samples of known compositions. The possible exportation of the laboratory scale procedure to an industrial environment application for the control of the Hall-Heroult process is finally discussed.
Solids and Liquids in the NaF–AlF3–Al2O3 System: A High‐Temperature NMR Study
Aluminofluoride and oxyfluoride melts are essential in the electrolytic smelting of aluminum, as the precursors of fluoride glasses, and as analogue systems for molten silicates. In these systems, the quantification of the abundances of structural species is crucial in the understanding of both thermodynamic and transport properties. 27Al NMR spectroscopy at high temperatures can provide important constraints on speciation and coordination number in aluminofluoride liquids, as well as structural and dynamical changes in the crystalline fluorides. We present results on a series of solids and liquids in the NaF–AIF3–Al2O3 system. These illustrate the dramatic effects of composition and temperature on liquid structure and suggest that most Al cations are four- or six-coordinated, with a relatively low abundance of five-coordinated sites.
THE AUTOMATED CONTROL OF BATH COMPOSITION ON HIGH AMPERAGE CELLS
ABSTRACT Recent improvements in the process control technology for aluminium electrolysis pots have made possible thedevelopment of high amperage pots that provide high technical results and excellent conditions of operation. A major step is the automatic control of the bath composition owing to which the aluminium trifluoride contentand the electrolysis bath temperature are kept close to the target values. The control system manages the aluminium trifluoride shots from a special feeder and the modifications broughtto the setpoint resistance as a function of the thermal condition of the pot and of its cathode voltage drop. The mastery of this control system enables to consider new developments of electrolysis bathcomposition.