The dynamic simulation and control of aluminium smelting cells
The Hall-Heroult process for the electrolytic production of aluminium from alumina is a costly and difficult to control process that has remained little changed since the early 1900's. A decreasing trend in the real profitability of the process since 1930 has made it necessary for aluminium smelting companies to reduce the expenditure in all aspects of the process in order to remain viable. The most significant proportion of the costs of production is the utilisation of the electrical energy required to produce the aluminium but improvements such as the rebuilding of the cell superstructure in modern low resistance materials are precluded by the capital costs involved. It is generally only possible to make improvements by changes in operational procedure and the control strategy. The introduction of computer control and data logging systems has reduced the manual involvement in cell operation and has allowed control strategies to be standardised on all cells within a smelter. Although the data logging facilities have increased the amount of data that can be collected and improved the understanding of the operation of aluminium smelting cells, the control of remains difficult due to the lack of data that can be continuously monitored, in particular the alumina concentration in the electrolyte. In this work, a mathematical model for the dynamic simulation of aluminium smelting cells is developed. A simulation program is then written incorporating the exact control algorithms from Anglesey Aluminium's GEL TROl computer control system for half-break cells. The aim of the simulation is to study the effects of different operating conditions and control strategies upon the operation of the cell. The simulation is developed to be modular in nature allowing different control systems and cell models to be easily incorporated and tested. The model is evaluated against data from Anglesey Aluminium's halfbreak cells and is successful in predicting the behaviour of these cells. The simulation is also used in plant trials to investigate the effects of different metal tapping schedules.