What Is Jet Milling and How Does It Work?

Jet milling is one of the most important services provided by toll processing companies. It’s a particle size reduction process that’s in high demand for industries including pharmaceuticals, cosmetics, paints and pigments, industrial chemicals, and many more.

This milling method uses high-pressure, compressed gas to create high-velocity collisions between particles of raw material. These collisions fracture the particles into smaller pieces.

The process of jet milling can achieve both particle size reduction and air classification in a single step. These mills can finely grind friable materials including polymers, chemicals, and ceramics. Abrasive materials can be processed on this equipment without contamination.

With the right expert at the controls and the right raw feed, even much smaller particle size ranges can be achieved. Temperature-sensitive products can also do well. 

Jet mills can even be used to blend some powders, as two or more streams of material can be fed into the mill, resulting in a homogenous blend.

In this article, we’ll take a look at types of mills used to achieve fine particle sizes, how they work, and some of their key advantages.

Two Types of Jet Mills and How They Work

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
• Circular jet mills (also called spiral jet mills) have internal classification action within the grinding chamber

Both mills are free of moving parts in the grinding chamber. This significantly reduces wear and tear on the equipment. It also makes for easy cleaning and sanitization, which virtually eliminates risk of cross contamination. 

The process doesn’t generate heat like mechanical milling methods do, so softer powders and sensitive materials are good candidates. The uniform particle sizes jet milling can achieve make it an attractive choice when precision is needed.

In both circular and fluidized bed mills, jets of air or steam are created by gas that’s compressed to a gauge pressure ranging between 50 and 120 psig. The most common gas used is commercially compressed air.

Superheated steam (392–980°F) compressed to 100–220 psig can also be used in specific applications on raw feed materials that are not heat-sensitive. Some of the other gases used include:

• Nitrogen, which can protect materials from oxidation and/or fire 
• Argon, another inert option, though more expensive than nitrogen
• Helium, used to achieve higher-velocity impact between particles

A great deal of energy is needed to create enough momentum to cause particles to break on impact. The compressor and nozzles transform the high air pressure into kinetic energy within the mill. Large particles recirculate, and multiple high-velocity collisions progressively reduce their mass.

At this point, the differences between circular and fluidized bed mills become clear. In a circular mill, the small particles are less affected by centrifugal force, so as they’re reduced in size, they migrate toward a central outlet in the milling chamber.

In the fluid bed mill, reduced particles travel upward toward the mill’s central discharge port. At this point, a centrifugal classifier automatically allows the right-sized particles through, and returns larger particles to the bed for further reduction.

How Small Can Jet Mills Grind? 

The process generally results in particle sizes in the 1- to 10-micron average particle size range, which is also referred to as micronization.

Some product formulations require much smaller particles, as small as 200 nanometers. Depending on the material, these extremely small sizes can be achieved by increasing the power to the mill and time the material spends in the milling chamber.

On the other hand, some products require particles larger than 10 microns, which can be accomplished by reducing the power to the mill or increasing the feed rate to the equipment.

Advantages and Disadvantages of Jet Milling Services

Among particle size reduction technologies, jet milling’s advantages include:

• The capability to achieve very fine particle sizes and tight distributions, compared with other milling methods
• No knives, blades, hammers, milling media, or screens mean minimal risk of contamination from worn mill parts
• The simple chamber design is easy to clean, reducing risk of cross contamination
• Unlike mechanical mills, there’s no temperature change due to grinding, which is important for heat-sensitive materials like those used in cosmetics

Different materials act differently on the jet mill. The best materials are: 

• Abrasive
• Brittle
• Dense
• Hard
• Very friable (in other words, easy to crumble)

Jet mills also have their disadvantages, though they’re few. For one, they can be expensive to buy — which is why working with a toll processor can be a great way to get the benefits of this technology. The process can also require high energy demands, the need for specialty gases, and other specialized accessories to achieve the desired specifications.

Materials that may respond poorly to the jet milling process are often: 

• Elastic
• Wet
• Sticky 
• Light and fluffy
• Easily deformed
• Shock absorbent
• Difficult to accelerate

Of course, an experienced toll processor has been achieving tough particle specifications for years and has already acquired not only the equipment, but also the expertise needed to optimize its use and achieve the best possible outcome.

Answers to Common Jet Milling-Related Questions

When you work with a tolling partner’s technical team to determine the ideal mill type, speed, feed rate, and any special considerations for your process, you should expect to be able to answer a few questions (in addition to providing a safety data sheet for every material that will be milled). They’ll want to know:

• 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 and returned or forward to the next process step?

Milling Methods and Toll Processing

Jet milling is an essential capability to look for in a toll manufacturer, but it’s certainly not the only one. And if you’re not certain which milling method is the best one for your material and particle size needs, look to the experts on your technical team to help you evaluate your options. Get a head start on the conversation with our free Milling Methods Comparison Guide. Just click the link below to download your own copy today.