The process doesn’t create heat like mechanical milling. So, softer powders and sensitive materials work well. Jet milling is a great choice when precision is important. It achieves uniform particle sizes, which is very appealing.

In circular and fluidized bed mills, jets of air or steam form from compressed gas. This gas has a gauge pressure of 50 to 120 psig. The most common gas used is commercially compressed air.

Superheated steam (392–980°F) can be compressed to 100–220 psig. This steam is useful for certain raw feed materials that don’t easily heat up. 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 change high air pressure into kinetic energy in 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, small particles are less affected by centrifugal force. As they’re reduced in size, they migrate toward a central outlet in the milling chamber.

In the fluid bed mill, smaller particles move up to the central discharge port. A centrifugal classifier now lets the right-sized particles pass through the port. Larger particles return to the bed for further reduction.