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Advanced spray drying process by controlled slurry destabilization and ultrasonic atomization
(2017)
Spray drying of ceramic slurries aims for soft and free-flowing granules with homogenous microstructure suitable for uniaxial and isostatic pressing. A stable and continuous spray drying process of slurries with maximized solids content is a further development target, since a reduction of the energy intensive drying procedure is desirable.
First part of the investigation focuses on the development of an appropriate zirconia slurry for spray drying with optimized organic additive contents (dispersant + binder + pressing and lubricating agents). Characterization and improvement of slurries are based on zeta potential measurements and investigations of the sedimentation behavior in an optical centrifuge. Therefor the slurries were spray dried with a conventional spray dryer with a two stream nozzle run in fountain mode. The controlled destabilization of the slurries was introduced, since the unwanted formation of hard granules with donut-like shape could be circumvented. Spray drying of such modified slurries resulted in soft granules without voids and finally leaded to sintered bodies with improved microstructure, density and bending strength.
Expectedly, the destabilization process causes a significant increase in viscosity of the ceramic slurry. Hence, an alternative spraying concept utilizing an ultrasonic nozzle was tested. Indeed, the ultrasonic nozzle seems able to atomize slurries with viscosities beyond the capability of the regular two stream nozzles due to the slurry’s shear thinning behavior. In second part of the investigation the integration of the ultrasonic nozzle in a commercial spray dryer is shown and first results of the spraying tests are discussed. Continuous spraying processes of highly viscous alumina, zirconia and ZTA slurries were realized over an extended production period.
Spray drying of ceramic slurries aims for soft and free-flowing granules with homogenous microstructure neither containing voids nor hard shells suitable for uniaxial and isostatic pressing.
The controlled destabilization of the slurries was introduced, since the unwanted formation of hard granules with voids donut-like shape could be drastically reduced. Spray drying of such modified slurries resulted in soft granules and finally leaded to sintered bodies with improved microstructure, density and strength.
Optimized slurries were atomized in a laboratory spray dryer (Hi-Tec, Niro, Denmark) both with a commercial two stream nozzle and an adapted ultra sound atomization unit. Potential advantages of the ultra sound nebulization are investigated for model systems of alumina, zirconia and a ZTA composite while focusing on solids content of slurries, yield, and granule properties (size distribution, flowability, shape, microstructure, pressability) as well as the final sinter body properties (density, microstructure, strength).
Using ultrasound nozzles in spray drying of identical slurries, as before tested and optimized for a two stream nozzle atomization process, resulted in a more suitable size distribution for dry pressing (less particles below 20 µm), a better flowability and a higher yield. Furthermore the sinter bodies produced of the ultra sound granules have higher sintered density, more homogenous microstructure without large pores and higher strength.