Freeze granulation of powders
Pressing is the dominating shaping technique for ceramics and powder metals. Submicron and/or composite powders require adequate preparation to achieve pressing performance that ensures dense compaction into a homogeneous state prior to sintering.
Conventional powder granulation methods (spray drying, sieve granulation etc) show drawbacks. Migration phenomena cause inhomogeneities, cavities and hard granules that do not disintegrate properly during pressing.
The preparation of a powder suspension by applying colloidal processing combined with sufficient mechanical treatment provides optimal homogeneity. This homogeneity can be preserved by spray freezing and subsequent freeze drying resulting in homogeneous powder granules with superior performance made by Freeze Granulation.
Freeze granulation of organic/inorganic particles or powders is based on instant freezing of sprayed drops (granules) and subsequent freeze drying, a development of Spray Freeze Drying (SFD) or Spray Freezing into Liquid (SFL). This technology ensures high-quality granules with homogeneous distribution of particles, polymeric pressing aids and other additives. The solids content of the powder suspension (slip) and the processing parameters (pump speed and airflow) control the granule density and granule size.
Migration of the constituents that is common in other granulating processes such as spray drying is avoided in freeze granulation owing to the sublimation process where no moving liquid is present. Excellent pressing performance with easy break down of the granules is achieved. This results in homogeneous compacts with enhanced sintering ability and optimal material properties.
Owing to the weak inter-granular bonding, even nanopowders can be freeze granulated and then easily be re-dispersed into nanopowders again, avoiding agglomeration (aggregates) of nano-particles which is not possible with other granulation methods like spray drying.
Free-flowing granules with a wide granule size distribution (50–500 micron). Lower granule and tap density vs spray-dried granules. Spray-frozen and freeze-dried granules of a composite material, see photo below.
Summary and Developments
Freeze granulation has shown to be a competitive technique for the manufacture of granules for pressing owing to its ability to preserve the material homogeneity and enhance the pressing performance.
|Small granule quantities as well as larger ones can be produced with equal properties|
|The granule density can be controlled by the solid content of the powder suspension to be granulated|
|The mild drying provides a low degree of oxidation of non-oxide ceramics or metals and gives soft granules that are easily broken during pressing or re-dispersed to a suspension|
|Technical development of the first large-scale freeze granulator|
|Optimised system for processing in organic media.|