Comminution and structural changes in a jet mill

Abstract

The energy of collision in jet mills which grind by means of high-velocity collisions results not only in highly intensive comminution, but certain changes take place in the structure of the material. Even the crystalline particles of so-called rigid materials are known to become microplastic and to accumulate a certain energy when their size is below 1 μm. The energy of collision may remove ions or atoms from the surface of the crystal lattice, producing thereby a mechanochemically activated state. Due to mechanical damage of the crystal structure and to plastic deformation the material may become partly or totally amorphous, and this too may be accompanied by an increase in free energy. Some mineral materials, such as dolomite, limestone, crystalline sulphur and, of the plastics, PVC powder, were ground in a jet mill type Fryma JMRS-80. Our work is part of the research aimed at the utilization of the rich sources of natural high pressure CO 2 occurring in Hungary. Changes in the structure of the material due to comminution were determined by means of differential thermal analysis and X-ray diffractometry. The results confirmed that in addition to producing intensive comminution, high-speed collision affects also the crystal structure of the materials and their internal energy content. If the ground products are destined for further processing, their more active state must be taken into consideration, that is, mechanochemical activation must be taken account of as forming part of the preparatory process. Cooling due to the expansion of the gas entering the milling zone over-compensates for the thermal energy generated there by the friction of the colliding particles. The high milling gas to solid ratio in the jet mill ensures the conduction of any eventual locally arising thermal energy, in contrast to systems in which milling is performed without a gas phase. If a structural change, i.e. disintegration of the pargicles, occurs in course of jet milling, the suggested mechanism is as follows: the mechanical energy introduced into the particles by their collision gives rise in the crystal lattice to oscillations which act by causing a mechanical strain on certain sites (lattice points, bonds) in such a way that local energy accumulation may lead to the disintegration of the particle or to other structural changes [1]. Accumulation of the oscillation energy is analogous to the accumulation process of thermal energy, which can be expressed as a temperature rise. The observation of sharp chipped surfaces on jet milled PVC particles, without any sign of local softening, disagrees with the possibility of a local temperature increase.