Using a fluidized-bed jet mill for more than size reduction
By Gregg Shemanski and Becky Shemanski, Custom Processing Services
The fluidized-bed jet mill has long been known as an excellent machine for reducing a wide range of friable or crystalline materials down to micron-sized powders (called micronizing), grinding partly brittle but low-melt materials without damaging them, and grinding abrasive materials without the danger of cross-contamination from metal abraded from the mill’s housing. But the mill has more uses, as well.
There are many jet mill types, and all of them operate by using air (or other gas) jets to force particles to collide against each other at high speeds and break into smaller micron- and even submicron-sized pieces. In some jet mill types, the particles impact the grinding chamber walls as well as each other. The particles in the fluidized-bed jet mill don’t contact the walls; therefore, they produce finished products with high purity and little to no contamination. The fluidized-bed jet mill, in which the grinding action takes place in a fluidized bed in the mill, is the most versatile.
The fluidized-bed jet mill has a lower grinding area called the grinding chamber and an upper classification area called the classifier wheel. Several gas nozzles are arranged around the grinding chamber’s periphery and aimed toward the chamber’s center. Pressurized gas is forced through nozzles, forming high-velocity gas jets. Material enters the chamber by gravity and is entrained by the high-velocity gas, which produces a fluidized bed. The gas jets rapidly accelerate the particles and force them toward the bed center. Along the way, they impact each other and break into pieces. The gas expands, carrying the particles upward to the classifier wheel, which spins at a predetermined speed to produce the desired cut point, or particle size, and throws out the coarse particles, sending them back down to the fluidized bed. Fine particles entrained in the gas exit the classifier wheel, and the particles and gas are separated in a cyclone or dust collector.
The fluidized-bed jet mill is known for its ability to grind very abrasive materials down to precise micron sizes with little possibility of cross-contamination because the mill’s grinding action is particle-on-particle. The mill has no hammers, grinding discs, or grinding media that can wear away. In addition, the particle grinding occurs almost entirely in the fluidized bed’s center, while the bed’s outer portions act as a wear barrier, eliminating the danger of housing fragments wearing away and contaminating the material.
The fluidized-bed jet mill can also blend powders while grinding, coat particles with liquid, or dry a moist material by using jets of superheated steam rather than gas. Because the mill tightly controls all process conditions, it provides very stable and repeatable results within tight grinding specifications for particle shape, density, surface area, aspect ratio, particle diameter, and color.
To learn more about the jet mill’s grinding temperature and what materials are good candidates for reaction processing, read the article “Fluidized-bed jet milling: It’s not just for size reduction anymore” by Gregg Shemanski and Becky Shemanski, Custom Processing Services. You can also find articles listed under “Size reduction” in PBE‘s Article Archive.