Fine grinding of brittle minerals and materials by jet mill
Lek Sikong, Kalayanee Kooptarnond, Noparit Morasut, and Thammasak Pongprasert
pp. 377 - 384
Abstract
Various variables affecting grinding, such as air pressure, minerals or materials hardness, feed size were investigated. The limitations of grinding of gypsum, barite, ilmenite, quartz and ferrosilicon were also elucidated by means of particle fineness size distribution and morphology of ground products. It was found that:
1) The density of particles, which are in the grinding zone affects the product fineness, i.e. higher feed rate results in a larger product size. The appropriate feed rate is suggested to be 0.2~0.5 g/s. Moreover, the density and hardness of minerals or materials tend to have an effect on the product fineness. Heavy minerals, such as barite or ilmenite, exhibit a finer product size than lighter minerals, like quartz. However, for quartz, the higher hardness also results in a larger d50.
2) Air pressure is the most vital variable which affects the grinding by a jet mill. The d50 seems to relate to the applied air pressure as a power law equation expressed as following:
d50 = aP b ; as P ≠ 0
The a-value and b-value have been found to correlate to the feed size. The higher the air pressure applied the finer the product size attained. Moreover, air pressure has a greater effect on hard minerals than on softer ones.
3) Feed size seems to have a small effect on ground the product fineness of soft materials, such as gypsum and barite, but a significant effect on that of hard materials, such as ferrosilicon and quartz, in particularly by milling at low air pressures of 2~3 kg/cm2.
4) For the breakage behavior and morphology of ground materials, it was also found that the minerals having cleavages, such as gypsum and barite, tend to be broken along their cleavage planes. Thus, the particle size distribution of these products becomes narrower. While quartz, ilmenite, and ferrosilicon have shattering and chipping breakage mechanisms,
grinding results in angular shapes of the ground products and a wider size distribution. Blocks or platelets and agglomerations may occur during grinding of soft minerals, like gypsum, especially at lower and higher air pressures, respectively.
