Causal Factors in the Variation of the Cell Voltage in Industrial Aluminium Smelting Cells

Abstract

The Hall-Héroult process is used for the production of the primary aluminium metal. This process consists of the electrochemical reduction of alumina (aluminium oxide) dissolved in a cryolite (sodium aluminium hexafluoride) based electrolyte. The electrochemical potential of the cell or the cell voltage in the parlance of the industry, and the current are the only two continuously measured parameters. The cell voltage in particular is used almost exclusively by the industry to control the cell process and it is directly proportional to the energy usage in aluminium production. The aim of this PhD project is to investigate the causes of the variation in the cell voltage in industrial smelting cells. Understanding the variation in cell voltage is critical for the smelters because the magnitude of the voltage determines the energy consumption in the process, and a voltage saving, while small for one cell is magnified hundreds of times once all the cells in the plant are considered. In this research, a specific period of time was spent on an industrial site and monitoring of a single cell was undertaken for a period of sixty four days. The operating cell voltage of the cell was recorded continuously at the rate of 1 per second for the entire monitored period. Other parameters like electrolyte temperature and xs-AlF3 concentration were also measured to understand the process state. Observations of variation in the cell voltage were recorded under the three categories namely: metal, changes in concentration of alumina and anode effect related variation. Altogether six causal factors were identified for the variation in the cell voltage. The list of the identified causes ranged from the material balance error like excessive addition of AlF3 to the control system error of overlap in the stability and alumina addition control. Finally the impact of the causal factors on the entire monitored period was also analysed. Two major effects were identified. Firstly 0.027 V of extra cell voltage corresponding to an excess energy expenditure of 0.08633 kWhr/kg. Secondly, cyclic behaviour of the cell thermal state principally defined by the electrolyte temperature and xs-AlF3 concentration in the electrolyte.