Particle spheroidization by using the flame method is an effective process to produce spheroidized particles, which have high fluidity and filling fraction, from mechanically-pulverized materials. Although it has been reported that ultra fine particles attached on the surface of spheroidized particles after the spheroidization process, the physics underlying the formation of the ultra fine particles in the flame have not been clarified. In consequence, the techniques to control the amount of the attached ultra fine particles on spheroidized particles have not been established yet. The purpose of the present research is, therefore, to control the amount of the ultra fine particles attached on spheroidized particles after the spheroidization process and to clarify the effect of pressure of the combustion field on the amount of the attached ultra fine particles. In order to clarify the effect of pressure of the combustion field, the SEM photographs, particle size distribution and the specific surface area with BET method are measured. These results clearly show that the ultra fine particles attached on the spheroidized particles were reduced with the increase in pressure of the combustion field. This can be derived from inhibited vaporization of the particles because of the shortened flame length and the subsequent reduced heat quantity added to particles whereas the flame temperature rises with the increase in pressure of the combustion field.

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