AGITATION IMPROVEMENTS IN PREDESILICATION TANKS AT THE EURALLUMINA REFINERY

The EurAllumina refinery (Italy) had a problem with predesilication tanks becoming filled with stagnant thick mud in a period of approximately six months. A new multiple impeller agitation system was installed in an attempt to extend the time needed between cleaning the tanks. This did not address the problem, with the tanks subsequently requiring cleaning within a period of one to two months. The design criteria were reviewed and measurements made of the rheological characteristics of the slurry. The slurry rheology was found to be highly time dependent and it is believed this behaviour contributed to the performance problems.

Operational procedures were also examined and were identified as having contributed to the problem. Further, these procedures were not considered in designing the multiple impeller agitation system. In particular, a tank being brought on-line would be filled over a 24 hour period, resulting in a significant period of time in which the predesilication slurry in the tank was not agitated nor pumped forward to the digesters. Rapid filling of the tank at start up, changing the position of the inlet and outlet pipes, varying tank levels regularly and increasing the agitation speed has resulted in the tanks being relatively clean after a period of three months operation. This project has highlighted the caution needed when implementing an experimentally tested design and the need to consider operational practice in modifications to a plant.