Experimental investigation of the particle growth mechanism and influencing factors during granulation process by pyrohydrolysis in fluidized bed

Abstract

Particle growth kinetics during the pyrohydrolysis of pickling waste liquor in fluidized bed has an important influence on the metal oxide recovery. This study innovatively investigates the particle growth mechanism by combining granulation characteristics in fluidized bed and collision behavior between droplet and heated spherical particle, and then explores the effect of operating parameters on particle growth and powder properties adhered on the surface in the fluidized bed at 800 ℃. The results indicated that the particle growth mechanism was highly dependent on the bed temperature. When T < 100 ℃, it was mainly based on the direct droplet deposition on the surface; at 100 ℃< T < 300 ℃, droplet deposition and powder collision adhesion together promoted particle growth; when T > 300 ℃, the powder adhesion by inertia collision was dominant. The change in particle growth mechanisms was attributed to the liquid-solid interactions. The increasing particle temperature promoted rebound and breakup of droplets after impacting with the particles and then led to fine powder formation in the bed gaps. However, the increase of liquid viscosity was beneficial for the direct adhesion of droplets to the particle surface and accelerated particle growth. The results also found that operating parameters had a significant effect on the granulation characteristics. The coating layer thickness, powder size and porosity all increased with bed temperature. When the bed temperature is 800 ℃, the size of powder and bed particles both increased with the concentration of Fe3+ but decreased with the atomized gas velocity. However, the increase in liquid flow rate led to slower particle growth because of the increase in superheated vapor volume.