Fluidized bed spray granulation can be defined as a particle-forming process by which a solid containing liquid is converted into non-dusty granular solids in one step by means of drying. Essentially, hot air is used to fluidize a bed of particles already formed by this process and identical in composition with the dissolved component. The liquid to be processed can be a solution, a suspension or even a melt and is continuously sprayed, usually by means of an atomizing nozzle, either onto the bed from the space above the upper bed surface or into the bed. The solvent is evaporated by the hot fluidizing, leaving behind the dissolved material on the surface of the seed particles.
Particle growth takes place continuously either by coalescence of two ore more particles, termed agglomeration, or by layering of solids onto the surface of single particles, termed surface layering. In the case of particle growth by agglomeration, the wetted particles are held together by liquid binding bridges which are converted into solid binding bridges when the solvent is evaporated. This particle growth mechanism results in fast particle growth and yields more irregular or cluster type agglomerates. In the case of particle growth by surface layering, the wetted particles dry sufficiently before collision and thus are prevented from agglomeration.
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