Investigation of cryolite ratio measurement by potentiometric methods : a thesis presented in partial fulfilment of the requirements for the degree of Master of Technology in Chemical Technology at Massey University, Palmerston North, New Zealand
The cryolite ratio of an industrial aluminium electrolyte can effect many variables within the process including the overall energy efficiency. Careful control of the cryolite ratio is therefore very important. Currently no online measurements of the cryolite ratio are used in industry. The use of potential measurements between a sodium alloy electrode and an aluminium electrode, as a means of cryolite ratio measurement was investigated. The observed potential changes during cryolite ratio step changes were typically of the correct direction and of similar magnitude to the predicted potential changes calculated from activity data. The cryolite ratio of the electrolyte was found to change continuously during experiments. This was mainly due to the evaporation of NaAIF4
, which was identified by X-ray diffraction. This change in the cryolite ratio meant that long term stability of the electrodes was difficult to accurately quantify. Electrode damage was found to occur after several hours of use and was contributed to the intrusion of electrolyte into the porous boron nitride used as the electrode body. Furthermore the activity of the sodium alloy electrode was found to change significantly during experiments due to the loss of sodium into the electrolyte. This damage to the electrodes is thought to have effected the stability of measurements considerably. Dropping metal electrodes were investigated as a possible means of overcoming the changes in electrode activity. The overall stability of the potential measured between a dropping Pb-Na electrode and an aluminium electrode was somewhat better than the stability of potentials measured between two static pool electrodes. Operating difficulties with the dropping Pb-Na electrode combined with the short lifetime of these electrodes makes these electrodes unsuitable for industrial use. Some suggestions have been made for future electrode designs with single use designs recommended as the most viable option especially for industrial use.