Suspensions flash spray drying

Flash spray drying of suspensions was examined by Monse et cd. (2001). Suspensions of BaS04 particles with a primary particle size of dv.so =1-7 pm were sprayed. The suspension was heated to temperatures within the range 140 °C < T< 160°C and flashed through a swirl nozzle into the tower with an outlet air temperature of about 160 °C. Figure 6.9 shows the morphology of the particles obtained from superheated and flashed (Fig. 6.8b), as well as from non-flashed (60 °C, Fig. 6.8a) suspensions. 

The resulting suspension of PCC is screened at about 50 pm and is either used as an aqueous suspension containing 20 to 50 % of solids, or de-watered using rotary vacuum filters, pressure filters or centrifuges. The resulting moist cake is dried in rotary, spray or flash driers. 

As mentioned by Devahastin [4], more recently flash dryers consisting of inert media have been employed at pilot scales to dry slurries and suspensions, which are sprayed onto than. The partides are coated thinly by the slurry and dried rapidly as a thin film. Due to particle-particle interactions, particle collisions, and shrinkage of the film in the drying process, the powda from the slurry is produced. 

For the first group the following types of dryers are generally used convective (tray, band, fluid bed, flash dryers, and their modifications) or contact (vacuum dryers such as double-cone dryer-blender, conical dryer with rotating helical mixer, paddle dryer). Pastelike materials are dried in tray dryers, band dryers equipped with extruding devices, and spin-flash dryers. Finally, thin pastes can be dried in spray dryers or on fluid beds or spouted beds of inert particles. Small amounts of solutions and suspensions are generally freeze-dried, especially if the product is thermolabile. 

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