An experimental study of fluidized-bed coating: influence of operating conditions on growth rate and mechanism

This study investigates the influence of fluidizing gas velocity, atomizing air, and liquid flow rates, liquid concentration, initial bed mass, and particle size on the mechanism of growth of sand particles in a batch fluidized-bed coater. An aqueous solution of NaCl was used as the coating liquid and sprayed in the bed by means of a pneumatic atomizer. The results showed that for a given particle size, the fluidizing air velocity was the most important factor affecting the coating kinetics and stability. The dominant mechanism was the onion-ring layering, especially at excess gas velocities higher than 0.27 m/s. For a fixed value of the mass ratio of solute introduced in the bed to the initial particle mass, binder concentration, liquid flow rate, and the initial bed weight had no effect on the growth mechanism. The deposition quality was found to be affected by the droplet size. A decrease of droplet size resulting from increasing the atomizing air flow rate permitted homogenous coating of the solid surface.