Study Of The Preparation Of Micropowder By Combining Jet Milling With Electrostatic Dispersion

Jet milling is an indispensable technology of powder grinding method in modern area of powder preparation. Although jet milling has many advantages, the cohesive forces, especially the electrostatic attraction caused by friction of particles during jet milling process and reduction of the particle size, may lead to the formation of aggregates that offset the advantages of jet milling to a certain extent. Electrostatic dispersion is an innovative method to disperse the fine powder in air, which employs electrostatic effects to mitigate or eliminate aggregation, as particles carrying charges of the same sign repel each other. In this thesis, we developed a new method for the preparation of micropowder by combining jet milling with electrostatic dispersion(denoted J/E henceforth), the refined particles can be charged and dispersed timely after jet milling. By means of this method, we can achieve the dual goal of producing fine powders and maintaining the dispersiveness. The resist aggregation performance of particles can be improved effectively after jet milling. The main research contents and conclusions are listed as follows:(1) The particles size distribution of CaCO3 micropowder prepared by J/E method was investigated. The results indicate that when the air pressure is constant, the particle size distribution of micropowder prepared by the J/E is related to the charging voltage and the initial size of powder, the higher the voltage is and the larger the initial size is, the smaller the mean size of prepared micropowder is. The size distribution of micropowder prepared by J/E is better than the ball milling, for micropowder prepared by the latter is often agglomerate. As the losing of charges on the particles, the mean size of powder which varies with time satisfies the relationship of 0ktd ?d ?e after J/E treatment, and the particles present evenly dispersed state after J/E treatment in the charged aging, which is almost similar to the surface modification method.(2) The charging property of CaCO3 etc. micropowder under the coupling effect of airflow field and corona electric field was discussed. The results show that the q/m of powder increases with increased charging voltage until reach the maximum in the J/E process, after the particles reach the maximum charge, the q/m starts to decrease with further increase of the charging voltage when the air pressure is fixed. Powder with larger initial size has a higher extremum voltage. While when the charging voltage is fixed, the q/m of powder is determined by the competition between two conflicting factors, the refined particle size and the charging time. These two factors are both influenced by coupled effect of the air pressure and the initial particle size.(3) The non-contact measuring method was designed to study the charge losing law in air of the CaCO3 and barium ferrite powders prepared by J/E method. It turns out that the attenuation of q/m of charged particles presents exponential relationship in air, the decaying law of charged particles can be expressed as ? ?0q/ m(q/ m)(q/ m) exp( kt)(q/ m)? ??-- ?, the larger the charging voltage is, the larger the initial rate of decay is, and the q/m is larger at the same post-J/E time during the whole decaying process. The charge storage performance of the powder is related to its particle size and the relative permittivity, the smaller the particle size is and the larger the relative permittivity is, the better the charge storage performance is.(4) The flow dynamics of charged particles during the J/E process was simulated as well. The simulation consequences suggest that when the jet velocity and the charging voltage are both low, the dispersiveness of powder in the chamber is poor and the aggregation is obvious. Only increased charging voltage can’t improve the dispersion of the powder during its dynamic process. However, when the jet velocity is increased, the dispersion of the particles is more or less improved in the chamber. The dispersion of powder has been improved further and the particles have good dispersion in the J/E chamber throughout the whole process when the charging voltage is increased at this moment.