On the AlF/sub 3/ and temperature control of an aluminum electrolysis cell

We propose a control strategy for excess AlF/sub 3/ and bath temperature of a prebake aluminum electrolysis cell based on analyses of measured data and studies of a simple dynamic model. Model validation indicates that there are dynamics that the model does not capture, and hence, we use real data of excess AlF/sub 3/ and bath temperature to estimate AlF/sub 3/ and energy disturbances, respectively. The estimation results show that the energy disturbance is the dominating disturbance. Hence, the proposed control strategy is based on an almost constant AlF/sub 3/ input close to average consumption and energy manipulations to compensate for the disturbances. This requires the possibility for the resistance reference to be reduced without decreasing the current efficiency. Compared to present control strategies, this introduces an additional degree of freedom in the controller. The proposed control strategy may imply a significant economic potential through increased and stabilized current efficiency, reduced and stabilized energy consumption, reduced consumption of expensive additives, and prolongation of cell life.