What Is Jet Milling and How Does It Work?
Justin Klinger, Jun 18, 2020 9:20:00 AM
Jet milling is vital to the toll processing industry. As a primary particle size reduction method, jet milling uses high-pressure, compressed air to create high-velocity collisions between the material’s particles.
How small can jet mills grind? The grinding generally results in particle sizes in the 1- to 10-micron average particle size range. Some products require smaller particles, as small as 200 nanometers, while some particles need to be larger than 10 microns, which is accomplished by reducing the power to the mill or increasing the feed rate.
Jet milling finely grinds friable materials (polymers, chemicals, ceramics, etc.) as well as abrasive and temperature-sensitive products without contamination. Since jet milling is a one-step process for milling and classifying, toll processors have expanded their use throughout many new markets.
That’s the short story. This article explores how a jet mill works and some of the advantages of jet milling.
How Jet Mills Work
The jet mill’s multiple jets of air/steam are typically created by different compressed gases (approximately 50-120 psig). The most common is commercially compressed air, yet superheated steam (100-220 psig and 392-980°F.) can be used in specific applications where feeds are not heat-sensitive. Gases range from nitrogen (to protect a material from oxidation and fire) to argon (more expensive) to helium (higher velocity impacts).
High energy is required by a jet mill to create enough momentum to cause breakage during particle impact. The compressor and nozzles transform the high pressure into kinetic energy within a jet mill. Large particles are recirculated until the high-velocity collisions progressively reduce the particles’ mass.
As they’re reduced in size, particles naturally leave the bed and travel upward, toward the mill’s central discharge port. At this point, a centrifugal classifier automatically lets the right-sized particles through while returning the others for further reduction.
Two Categories Of Jet Milling
Based on how they control the final particle size, jet mills are designed in one of two ways:
- Fluid bed jet mills (FBJMs) have built-in air classifiers that can be tightly adjusted to the desired particle size and shape
- Those without have internal classification action within the grinding chamber
Neither type of jet mill has any moving parts. That significantly reduces wear and tear, makes thorough cleaning and sanitization easy, and generates no heat, so softer powders and sensitive materials can be jet milled.
Let’s explore more advantages that jet milling provides.
The Advantages Of Jet Milling
When it comes to particle size reduction technology, there are multiple advantages of using a jet mill:
- Jet milling achieves the finest and tightest particle size distribution when compared to other reduction methods
- Jet milling’s absence of blades/knives, media, and screens results in extremely low contamination possibilities
- Jet mills experience no temperature changes due to grinding, which is important for heat-sensitive materials; cosmetics, for example
Jet Milling Questions To Ask; Best Materials
To help determine the entire jet milling process, including mill type, speed, feed rate, etc., some information is needed. These questions (and more) will help jet milling be as effective as possible.
- What is the particle size of the feed material?
- What is the target particle size of the product?
- Is the material a combustible powder?
- Is the material abrasive?
- How will the product be collected?
Different materials react differently to the jet milling process, some better than others. The best materials are ones that are abrasive, brittle, dense, hard, and/or very friable. Materials that respond poorly to the jet milling process include ones that are elastic, deforming, wet, sticky, and/or light and fluffy, or ones that absorb impact or are difficult to accelerate.
Jet mills may be used to blend some powders, as two or more streams of material can be fed to the mill, resulting in a homogeneous blend.
We hope this gave you a good, brief overview of jet milling. No matter what milling style you use or what you need, nothing beats the support of an experienced toll processor for high-quality jet milling. Plus, continue your exploration into milling by reading our Milling Methods Comparison Guide.