Method of Measuring Ohmic Resistance in Aluminium Reduction Cells

Abstract

The objective of this work was to develop a method of directly measuring the ohmic resistance in an industrial aluminium reduction cell. This requires that the voltage due to ohmic resistance be distinguished from the voltage due to the Nernst potential and polarisation. Electrochemical impedance spectroscopy can be used to directly measure ohmic resistance at the laboratory scale, but it is not suitable for an industrial aluminium reduction cell because an alternating current of the required magnitude and frequencies cannot be produced, and the system does not stay at steady state for the duration of the measurement. A measurement technique was developed based on the principle of electrochemical impedance spectroscopy. A single bipolar pulse is generated by two capacitors. The current and voltage are measured and transformed to the frequency domain using Fast Fourier Analysis, from which the impedance at a range of frequencies is calculated. The ohmic resistance is the impedance where the imaginary impedance is zero (i.e. where there is only a real component). Measurements were conducted on a physical electrical circuit designed to represent an industrial aluminium reduction cell. Inductance had a significant impact on the performance of the measurement technique, but the measurement parameters could be optimised such that the ohmic resistance of the circuit could be determined. Measurements were conducted on a 500 A laboratory copper electrowinning cell with geometry similar to that of an industrial aluminium reduction cell. Inductance again had a significant impact on the performance of the measurement technique, but the measurement parameters could again be optimised such that the ohmic resistance of the cell could be determined. This gives some confidence that the bipolar capacitor technique could be used to measure the ohmic resistance on an industrial aluminium reduction cell, and recommendations on how to conduct these measurements are provided.