Understanding oscillatory behaviour of gibbsite precipitation circuits

Abstract

Gibbsite precipitation plays a central role in the production of alumina, the feedstock for aluminium smelters. A mathematical model of a simplified precipitation circuit is used in this work to investigate the main process parameters affecting the operating stability of gibbsite precipitation circuits incorporating solids classification and seed recycle. The crystallization process in continuous precipitation tanks is modelled using a dynamic crystallization population balance equation incorporating crystal growth, nucleation, and agglomeration kinetics. The precipitation circuit model provides a detailed particle size distribution response, which is crucial for evaluating the circuit's precipitation yield and product properties. Transient responses of the precipitation circuit model were investigated for key process parameters. It is shown that a rectangular pulse disturbance in the feed flow rate can generate an oscillatory response of the precipitation circuit if the solids residence time in the classification section is comparable in magnitude to the total slurry residence time in the precipitation section. An oscillatory circuit response has a detrimental effect on the product particle size distribution, but only slightly affects the production rate. Oscillations in the product mean size are believed to be a result of the impact of the main precipitation process variables on the gibbsite agglomeration rate.