Granulation of Metal Powders in Water
Freeze granulation can with advantage be used for granulation of various kinds of metal powders for the achievement of granules with homogenous distribution of metal particles and binder or pressing aid. However, large metal particles require normally an adaptation of the liquid medium in terms of viscosity to avoid critical settling during processing.
A specific advantage with freeze granulation regarding metals is that water in many cases can be used as the freeze drying ensures very limited oxidation versus conventional spray drying in air as no liquid water/air interface where oxidation take place is present.
There is no problem to process hard metal powders (WC/Co/Cu) in water with binder (PVA, PEG, PVP) with Freeze Granulation. Diamonds can be mixed in a slurry, but with larger sizes (>100 um) problems with segregation due to difference in density will increase.
As we use air atomization in Freeze Granulation, the size distribution of granules will be approximately 10-700 um, but can be adjusted by spray settings to the finer or coarser side.
We cannot guarantee one diamond in each granule since granule size varies. It will be random, but for a pressed green body, distribution of diamonds will be homogeneous.
For solvents we use Cyclo-hexane or Tert-Amyl alcohol. Ethanol and Acetone have too low melting point for FG. We only use organic solvents when powders actually react with water. Most metal powders can be processed in water. There is very limited oxidation since drying is made under vacuum. Paraffin wax can be used in Cyclo-hexane, but processing in water systems is always preferred. Dryer needs to be tray type with heated shelves.
Freeze Granulation offers a technique where a powders suspension is atomized, the droplets are instantly frozen and subsequently freeze dried resulting in highly homogeneous granules. This technique can with advantage be used for pre-preparation of fine powders prior to mix it into a PIM-formulation. Colloidal processing by ball milling in water or a solvent to properly deagglomerate the powder prior to Freeze Granulation ends up in homogeneous granules with weak interparticle bonds. Hence, the powder can easily be re-dispersed into primary particles within the PIM-formulation with preserved homogeneity, without heavy mechanical treatment, and, often to a higher powder loading than possible with direct mixing. Other compounds intended to be used in the PIM-formulation can also be added prior to Freeze Granulation for an effective formulation mixing. PowderPro AB provides equipment for Freeze Granulation as well as know-how of powder suspension processing.
Applications for Freeze Granulation