Fluid Energy Mill Overview
Jet milling or fluid-energy grinding is used for comminution or pulverizing granular material into fine powder with a narrow size distribution. A fluidized bed jet mill has a cylindrical grinding chamber with air jets pointing at each other. The air jets form a fluidized bed of particles in the chamber and grinding occurs by projection of particles against one another. These particles are then carried upwards by the current of air to a classifier which only allows particles less than a certain size, determined by the speed of rotation, to leave the grinding chamber. Particles larger than the cut size are excluded by centrifugal force and return to the grinding section. Solids are fed continuously into the mill by a screw feeder, the product leaving the mill through the selector can be collected in a cyclone with a secondary reverse pulse filter 1
Whereas loop and pancake jet mills employ static classification of the ground powder, fluid bed air mills use an active air classifier to separator the ground material. The classifier wheel has variable speed to control the final powder size. This type of closed-circuit milling produces a more finely ground product and affords more precise control of the particle size distribution, especially on the top size.
The fluid bed jet mills are also different from the loop and pancake mills in that they have grinding nozzles that are directly opposed rather than tangential to the mill body. The direction of the air nozzles produces additional advantages outlined in the case studies of the Fluid Energy Roto-Jet mills described below.
Graphite Grinding System
The Model 30 Roto-Jet grinds natural and synthetic graphite at rates from 500 to 2,000 lbs/hr depending upon the particle size required. The Model 30 Roto-Jet uses up to 2,200 scfm of air compressed to 100 psig. The classifier rotor is powered by a 30 hp motor. The opposed nozzles on the Roto-Jet limit wall effects during the grinding process which makes the Roto-Jet well suited for abrasive materials. The grinding occurs in the center of the milling chamber, preventing direct impingement with mill surfaces. Without impinging the body walls, lining the mill is not needed to prevent wear on the metal.
A Model 8 Roto-Jet (picture at right) displays a sanitary polished classifier wheel as well as product and non-product contact surfaces. Units designed for the pharmaceutical, cosmetic or food industries include features like hinged access ports and motor locations allowing easy access for cleaning and inspections. With its advanced, dynamic classification it produces a narrower distribution with the finest top size.
Granular material that has a high aspect ratio (e.g. flakes or needles) and is difficult to classify is better suited for a fluid bed jet-mill. An example of such an application is toner powder. Toner manufacturers typically require a very specific top size (15-20 microns) while generating minimal fines (less than 4 microns) so a combination of air pressure and classifier wheel speed variations are used to meet the specifications.
Hard to Grind Materials
Any material that is difficult to fracture, such as polymers, alloys, or some ceramics, typically requires more residence time in the grinding stream than a loop or pancake mill can provide. In these situations, a fluid bed jet mill excels because the air classifier prevents the particles from exiting prematurely and rejects them back into the grinding zone.
If the feed material has a broad distribution of sizes (aka “poor quality raw feed”), the classifying rotor will consistently produce in-spec product regardless of fluctuations in the feed particle size. In mills with static classification of the material (loop or pancake mills), any variation in the aspect of the feed distribution (mean or top size) affect the final product unless other process parameters are changed, e.g. feed rate, air pressure, which typically lower the production rate. A fluid bed jet mill can adjust for these variations without slowing the productivity of the grinding process.
These case studies are just a few examples of granular solids that can be jet-milled in a fluidized bed jet mill. A list of additional product applications can be found here on the Roto-Jet equipment web page.