Analytische Chemie
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Spray drying is a widely used process step in ceramic technology to
convert fine grained raw powders to free-flowing granules. Type and
amount of the organic additives (dispersants, binders and plasticizers)
define viscosity and stability of the slurry, the granulate structure and
the compaction behaviour.
Alumina and Zirconia powders were characterized with regard to
particle size, shape and specific surface area (BET). Zeta-potential
measurements by Stabino® were used to investigate the effect of
different dispersants on slurry stability. Unfortunately, this method is
restricted to suspensions with less than about 30% by weight of solid
content. The multi-sample analytical centrifuge LUMiSizer® overcomes
this problem. This device enables analysis of the separation behaviour
of suspensions over a wide range of solid content. On the basis of
position and time resolved photometric detection of transmitted light,
it is possible to determine the instability index, the sedimentation
velocity, and the sediment height in dependence of type and amount
of organic additives at various centrifugal accelerations. Additional
information is obtained from the shape of the transmission profiles.
Based on LUMiSizer® measurements, suitable additive concentrations
were derived. Thereby, the number of investigations by viscometry and
spray drying experiments can be significantly reduced.
Finally, it was possible to produce alumina, zirconia and ZTA aqueous
slurries with up to 80 weight-% of solid. The optimized suspensions
were successfully spray dried and the resulted granules show good
compaction behaviour.
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.