Granulation (Granulating) and Drying of Powders
Pressing is the dominating shaping technique for ceramics and powder metals. Submicron and/or composite powders require adequate preparation to achieve a pressing performance that ensures dense compaction into a homogeneous state.
Conventional powder granulation methods (spray drying, sieve granulation etc) show drawbacks. Migration phenomena cause inhomogeneities 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.
- 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. Excellent pressing performance with easy break down of the granules is achieved. This results in homogeneous compacts with enhanced sinterability and optimal material properties.
Nanopowders can be granulated and dried into a powder and then re-dispersed to nanopowders again avoiding agglomeration (aggregates) of nanoparticles, 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.
Freeze Granulation – Pros and Cons
Advantages of Freeze Granulation compared with Spray Drying
- The granule density can be controlled (solid content of the slip)
- No cavities (voids) in the granules
- No migration of small particles and/or binder gives a high degree of granule homogeneity
- Nanopowders can be granulated and dried into a powder and then re-dispersed to nanopowders again avoiding agglomeration (aggregates) of nanoparticles, which is not possible with other granulation methods like spray drying
- Low temperature granulation and drying of pharmaceuticals, drugs, biomaterials and fine chemicals
- A mild drying procedure minimises oxidation of non-oxides and metals
- Lower granule density and evenly distributed pressing aids gives softer granules and ensure that all granules are broken during compaction
- Low waste of material (high yield)
- Not only large quantities but also very small experimental batches 50–100 ml slip) can easily be processed
- The equipment is easy to clean – (allows the use of latex as binder)
- Possibility of recycling organic solvents.
Disadvantages of Freeze Granulation
- Two process steps: spray freezing and freeze drying
- Limitation in the choice of solvent, based on the freezing properties – a freezing point between –20 and +10 °C is recommended (water and cyclohexane)
- Lower tap density of the granules requires a larger fill volume (higher pressing die)
- Large-scale equipment with a capacity beyond 100 kg granules per day needs to be further developed.