How Jet Mills Use Air Power for Particle Size Reduction and Classification
KT Brickman, Oct 20, 2022 10:46:00 AM
Jet mills use compressed air or gas to facilitate particle to particle collisions that reduce particle size. While a common grinding method, a deeper understanding of the process is helpful in understanding why and how jet milling benefits industries and applications.
Compressed Air Powers Results
Neither spiral nor fluid bed jet mills have any moving parts in the grinding chambers, or use grinding media to break down raw materials into smaller pieces. Instead, streams of high-pressure compressed air are used to create high-velocity collisions between particles of raw material, fracture them, and reduce their size.
While compressed air is most often used, it isn’t the only option. Depending on the application and the characteristics of the raw material, compressed air may be replaced by:
- Superheated steam (392-980℉) may be used on non-heat-sensitive materials
- Inert gasses Nitrogen or Argon protect materials from moisture, oxidation, combustion, and/or flammability
- Helium promotes higher-velocity impact between particles
In and of themselves, these elements don’t contribute to the jet milling process. The benefits lie in using the compressed flow to generate kinetic energy to achieve particle reduction instead of depending on moving parts within the mill. As a result, jet mill methods:
- Create fewer contact points between the material and mill, and less wear and tear on the equipment
- Virtually eliminate the risk of cross contamination since the mills are easy to clean and sanitize compared to mechanical mills
Particle Classification Influences Jet Mill Selection
Generally, the type of raw material to be ground, starting particle size, and desired fine particle size dictate whether a spiral jet mill or a fluid bed jet mill is used. For example, spiral jet mills are ideal for materials with sticky feeds and fluid bed jet mills work well with abrasive materials or materials with larger starting particle sizes.
As for reduced particle sizes, finer results and greater surface area are typically achieved with fluid bed mills. This primarily has to do with how the mills process particles for classification.
Fluid bed jet mills have built-in classifiers whereas spiral mills do not. As such, in a spiral mill reduced particles drop out of the centrifugal rotation and migrate toward a central outlet in the milling chamber after a single pass of processing. Particles leaving the spiral jet mill essentially get one opportunity to fall into the size range.
In fluid bed jet mills, reduced particles travel upward toward the mill’s built-in centrifugal classifier. Right-sized particles are allowed to pass through, and larger particles are returned to the mill for further reduction. Large particles continue to recirculate until multiple rounds of particle collisions reduce their mass to the appropriate classified size.
Jet milling for particle size reduction and collection is a clean and efficient choice for grinding an array of raw feed materials. However, it’s important to remember that all materials behave differently — and not all are suited for the process.
The best options are materials that are:
- friable (or, easily crumble)
Materials that are less conducive to jet milling are:
- easily deformed
- shock absorbent
- difficult to accelerate
How Toll Processors Can Help Improve Jet Milling Cost Efficiencies
Jet mill technology is expensive. Purchasing the equipment may be cost-prohibitive, leaving some manufacturers to conclude that mechanical milling methods are the only options available to them.
However, experienced toll processors like Custom Processing Services already have jet mills in operation — that means you can access jet milling and micronization services without incurring capital investment costs, setup fees, or related expenses. Learn the value-adds in our eBook, Jet Milling Fundamentals: A Manufacturer’s Guide. Click the button below to download your copy now.