Compare Conventional and Fluid Bed Jet Milling
Justin Klinger, Sep 18, 2020 9:46:14 AM
The process of jet milling (both fluidized bed and other methods) involves converting the potential energy of compressed gas into kinetic energy within a grinding chamber, where particles are introduced along with the compressed gas.
The particles’ movement within the air flow causes high-velocity collisions between them, and those collisions in turn cause the particles to fracture.
This process of particles grinding one another into a smaller size is called comminution. All jet milling uses comminution to reduce particle size. But that doesn’t mean all these processes are interchangeable.
So, what are the biggest differences between major jet milling methods, and how do you select the right process for your material and goal particle size? Let’s take a look.
Jet Milling Methods: A Quick Overview
In general, jet milling is the preferred method for superfine grinding or micronization of friable and abrasive materials, achieving particle sizes in the 1 to 10 micron (10 μm) range. Because it uses no moving mechanical parts, the process reduces the possibility of product contamination. Jet mills are useful for both grinding and blending materials.
Both conventional and fluid bed jet milling methods are appropriate for grinding friable, abrasive, brittle, dense, and hard materials, and because jet milling doesn’t generate heat, it’s a useful process for heat-sensitive ingredients. Raw feed with high moisture content and material that is sticky, fluffy, deforming, or elastic are poor candidates for jet milling.
Many products, particularly pharmaceuticals, require particle fineness to go beyond the micrometer range, achieving nano-range sizes as small as 200 nanometers. For products that require nanonization, and for very tight goal particle size distributions, fluid bed jet milling is the best solution.
Just as conventional jet milling achieves smaller particles and less loss of fines over mechanical milling methods, fluidized bed jet milling goes a step farther to achieve even smaller particle sizes, steeper size distributions, and less material loss in fines.
Compare Conventional & Fluid Bed Technologies
Spiral jet mills, the most commonly used conventional micronizing machines, have a built-in mechanism of air classification. Particles are sorted by size using the forces of air on them within the grinding chamber.
In a spiral jet mill, raw material is fed into the grinding chamber where high-speed, nozzle-fed gas accelerates the particles. The particles collide and fracture into smaller and smaller particles, eventually achieving a size range that is less affected by centrifugal force. Mill settings are adjusted to achieve a size that allows particles in that goal size range to migrate toward the middle of the grinding chamber and escape via a central outlet. This is called static air classification, and it is often sufficient for superfine grinding or micronization of many friable or abrasive materials.
But even greater fineness, or a tighter distribution of final particle size (or both), is a job better left to fluid bed jet milling.
Fluid bed jet mills (FBJMs) apply jets of compressed gas to particles inside a grinding chamber in which the particles have been fluidized. Fluidization is achieved by forcing gas through a membrane or perforated plate at the bottom of the grinding chamber. The upward force of the gas is adjusted to meet the downward force of gravity, suspending the particles and causing them to behave like a liquid or gas.
Fluidization quickly mobilizes fine particles upward into a deflector-wheel air classifier, preventing the creation of excess fines in the milling process. The classifier wheel transports particles in the desired size range out of the grinding chamber, and returns larger particles to the chamber. There, they recirculate until reaching the desired range, and again return to the classifier wheel. It is this dynamic air classifying step that enables steep particle size ranges, as well as nano-level milling.
Choosing the Right Milling Method for Your Product Needs
Determining the best process for your product goals begins with an examination of your raw material and your desired particle size (and morphology). Material characteristics are a critical factor, and it’s important to remember that not all toll processing companies are alike. A trusted toll processor will not only ask the important questions, a dedicated team of material scientists, R&D experts, and technical specialists should be able to answer your questions, too.
With our Milling Methods Comparison Guide, you can get a better understanding of the many material and equipment considerations that go into particle size reduction and toll processing. And if you have deeper questions about which milling processes can help you achieve the goal particle size and distribution for your application, just reach out and we’ll help you find the answer.