AGGLOMERATION DURING PRECIPITATION - AGGLOMERATION MECHANISM IDENTIFICATION FOR AL(OH)(3) CRYSTALS IN STIRRED CAUSTIC ALUMINATE SOLUTIONS

Reported here are the results of a study on the mechanism for Al(OH)(3) agglomeration during precipitation in caustic aluminate solutions. Two independent techniques are used to identify the agglomeration mechanism. Both identification techniques show that Al(OH)(3) agglomeration may be described by a size-independent agglomeration mechanism. The residuals between the experimental data and estimates from the proposed model are normally distributed, with a mean of zero and a standard deviation of 1. The size-independent model conflicts with the current view that there exists an upper limit, imposed by the hydrodynamics, to the aggregate size. Tracer crystal experiments, using Zn-doped Al(OH)(3) crystals, as well as the observed evolution of the experimental size distribution with time both demonstrate that crystals (or crystal aggregates) were agglomerating at sizes larger than the expected maximum aggregate size. This supports the size-independent model for the range of conditions studied. The analysis of the experimental Al(OH)(3) precipitation data also confirms that the growth rate is size independent and that there is no growth dispersion.