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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.
Dry pressing of ceramic materials requires homogeneously soft granules with good flowability to allow rapid die filling and to avoid packing defects. Spray-drying granulation appears to be the best method for obtaining granules with high flowability in industrial scale. But, strength reducing internal microstructural defects caused by spray-dried granules with hollow and hard shells are often observed using nano and/or multi-component starting powders.
Using the example of a ZTA composite, the potential of slurry optimization, ultrasound atomization and infrared drying for better granule properties and compaction behavior were investigated. Starting granules produced in a conventional spray dryer (Niro, Denmark) with a two fluid nozzle showed typical defects like large central pores and dimples.
The early step of slurry preparation already possesses an essential optimization possibility in the form of stability adjustments. Granule compaction was clearly improved upon a specific reduction in slurry stability. The second optimization opportunity to improve the granule quality was the atomization step. Implementation of an ultrasound atomizing unit into the conventional spray dryer positively affected granule size distribution and therefor flowability and as well granule yield. But, a combination of both process optimizations delivered the best sinter bodies with highest density and strength due to further reduction in maximum size and fraction of pores.
As last step of a spray drying process, the drying is the focus of further investigations. A current setup implying a spray dryer prototype utilizes stacked infrared heater in a countercurrent setup delivering a further increase in granule yield and enduring spraying process stability.