Fine particle coating was conducted by the rapid expansion of supercritical fluid solution (RESS) in a fluidized bed for release control of some key component in core particles. The supercritical carbon dioxide solution of paraffin was jetted into the fluidized bed of the core particles. The rapid phase change of the fluid solution from supercritical state to gas results in a solute at high supersaturating state in the solvent

which forms a huge number of superfine nuclei in the jetting flow. The deposition of the superfine nuclei on the surface of the core particles leads to a thin layer coating of paraffin. The size of the superfine nuclei is in the order of 40 nm. A porous spherical particle was selected as the core particle

which carried a tracer component of a kind of dye. Coating level was examined by the tracers release concentration in a solvent over a certain time. The state of coating was analyzed by measuring the average mass of coated particles and a SEM observation on the surface of coated particles. The rapid expansion of the supercritical fluid solution causes a big temperature drop at the nozzle outlet. The low temperature of the nozzle outlet affects the phase of carbon dioxide and the properties of the superfine nuclei in the jetting flow

therefore it affects the particle coating process. The effect of temperature at nozzle inlet

an important parameter

on surface coating was investigated. Seal coating was formed on the core particle surface at higher temperature. Porous coating was formed on the core particle surface at lower temperature. The temperature of the nozzle inlet affects the nucleus size significantly. Higher temperature results in a bigger size of the superfine nuclei. By controlling the operation parameters

a satisfactory quality of coated particles was achieved.