Basic Zirconium Sulfate in a 2.0 L continuous stirred tank with and without a draft tube
By
Dirksen, J.A.[1] and Ring, T.A.
Crystallization System
Reactor Geometry
Experimental Section
Crystallization Results
Abstract
The production of zirconium sulfate-a precursor to zirconia-in a continuous stirred tank reactor was studied as our model system because of its agglomeration tendencies. This system was systematically explored using 16 statistically designed experiments while concentrating on six key processing variables. The effects of (1) base concentration, (2) reaction pH, (3) reaction temperature, (4) mean residence time, (5) reaction mixing design and (6) mixing power on the resultant particle structure was quantified and related to measured levels of supersaturation. The results of this studied showed how the Zr/SO4, ratio could be controlled by monitoring the reaction pH, but impurity rejection by this reaction parameter was minimal. The reaction temperature was found to have a substantial affect on most measured results by increasing the aggregate size with increasing reaction temperature. The base concentration was found to change the inter-particle necking of the aggregates but the most significant contribution being towards reaction yield control. The mean residence time did not significantly affect the measured values monitored in this study. The mixing power was also found to have a large affect on particle size. The reactor mixing design played a significant role in generating inhomogeneities within the suspension. The two key processing affects which resulted from this study-reaction temperature and mixing power-signifies the dominance of shear aggregation in the particle growth mechanism.
This reaction was implemented on a mini-plant scale using a 2-liter reaction vessel operating on a continuous basis. This computer automated continuous stirred tank reactor is capable of producing 0.5 kg ZrO2/hr. For the powders prepared in this mini-plant, highly agglomerated structures were found to exist under some conditions, exhibiting fractal geometries; whereas, a more colloidally stable system resulted for others. The particle size distributions and morphologies of this ceramic precursor were successfully controlled with area average particle sizes ranging from 0.7 ± 0.8 microns to 8.5 ± 0.8 microns and BET specific surface areas of 20 to 1.5 m2/g, respectively, based upon varying reaction conditions. Yields were found to vary from 74 to 98% over the span of the experiments.
[1] Dirksen, J.A., The Precipitation of Basic Zirconium Sulfate in a Continuous Stirred Tank Reactor as a Precurson to Zirconia Ceramics” Ph.D. Thesis, Swiss Federal Institute of Technology Lausanne (EPFL), 1992.