Measuring the voltage drop across the cathode in electrolysis for the production of primary aluminum is a known control procedure to measure the state of graphite cathode blocks.
The measurement of the cathode voltage drop provides valuable status information of the cathode during operation. Accurate measurement of this parameter allows optimizing the operating voltage and organizing continuous monitoring of deviations from the normal course of the process, for example, an excessive rise of the ledge on cathode blocks and formation of sludge. With the control of the cathode voltage drop the behavior of the various operational types of cathodes blocks may be compared.
Until now, there was no unified method for the determination of this parameter and each smelter used its own measurement technology. Usually a steel rod was immersed to the bottom of the bath and connected to a voltmeter. The second terminal of the voltmeter was connected to the supply bar cell. This method is very inconvenient to implement, has low accuracy and poor reproducibility due to the instability of the freezing of Cryolite on the steel bar and its subsequent melting on contact with molten aluminium.
To eliminate the above-mentioned disadvantages Heraeus Electro-Nite has developed a combined probe C-V-Therm™ for measuring the temperature Cryolite bath and the cathode voltage drop. This probe includes a Pt-PtRh thermocouple and an external electrode of a thin molybdenum wire connected to one of the thermocouple terminals. A molybdenum wire makes electrical contact with the liquid aluminium, which in turn is in contact with the cathode without a significant voltage drop. Due to the fact that the accumulating ability of molybdenum wire is extremely small, only a little amount of Cryolite freezes onto it, which rapidly melts upon contact with molten aluminium.