Particle Separation Technologies in Toll Processing
Justin Klinger, Nov 12, 2020 6:45:00 AM
Particle separation methods have been used in processing materials since the earliest days of human civilization. Think about how, for example, grains are separated from chaff in agricultural threshing, or wheat germ is removed from flour before baking into bread.
Modern industrial applications that call for precision in particle separation and classification include:
- Food ingredients
- Industrial materials
- And more
In applications that require fine particle sizes and narrow size distributions, screening and classification is a necessary step — during, after, or as a separate service from milling. Most toll processors offer at least some separation and classification services, though particle size cut points can vary by toll processors’ experience and equipment capabilities.
Particle separation and classification ensure that all particles in a bulk solid lot conform to very fine and/or very narrow size distributions to meet specifications and end use requirements.
A Range of Separation and Classification Methods
Specialty particle separation services include methods such as screening (wet or dry) and mechanical air classification.
Many jet milling techniques include separation technology. Spiral jet mills, for example, have built-in static air classification, allowing fine-ground material to leave the chamber via a central outlet. Fluidized bed jet mills often have a deflector-wheel classifier step added as milled particles leave the grinding chamber.
Specialty particle separation services, separate from milling, include methods such as screening (wet or dry) and mechanical air classification.
Mechanical screening describes a process of applying mechanical forces, such as gyration or vibration, to a screen or sieve as dry material or a slurry flows through it. Material can be allowed to flow through a single screen or a stack of increasingly finer-mesh steel or nylon screens, which can be set horizontally or at an incline, depending on the characteristics of the material.
Vibratory screening, among the most commonly used particle separation methods, can screen particles ranging from microns to millimeters.
Applying vibration to a static bed of material causes particles to stratify, with the smallest particles migrating to the bottom layer and the coarsest to the top layer. The application of vibration or gyration can also help address agglomeration of dry materials, helping them flow more freely through the screen.
Sometimes screen openings can become clogged by larger particles in the screening process. This is called blinding, and it can be addressed by adding a deck below the screen, on which objects (such as balls) bounce or vibrate and strike the screen from below. Airflow, brushes, and ultrasonic vibration are also often used to address screen blinding.
Mechanical Air Classification
Mechanical air classification methods, described above as a separation step in many jet milling processes, use adjustments to process parameters such as feed rate, rate of air flow, and rotor speed to manipulate the centrifugal force and drag force to which particles are subjected within the classifier.
Particles’ behavior in response to these forces is in turn determined by their different sizes, shapes, and specific gravities. Generally speaking, coarser particles are more responsive to centrifugal force, and finer particles are more responsive to drag forces, so in an air classifier wheel, the two particle types migrate to different areas. Coarser particles migrate outward, and fines drift into the center.
Dynamic air classification uses an air classifier wheel to apply a more complex array of forces on particles, and can efficiently separate particles in the sub-100 micron range with high yields. In many cases, coarse particles can be remilled if necessary, and returned to the classifier for a second separation run.
Choosing the Right Process...and the Right Processor
When it’s all said and done, particle separation and classification can be a necessary step in processing a broad range of materials for many applications and industries, and the particle size cut points can range from microns to millimeters.
A specialized team at your toll processor can help you determine the best method of separation and classification for your material, whether air classification or screening, single or multiple deck, and whether screening is applied as a stand-alone process or part of a sizing system.
If that sounds like a lot to figure out, well...that’s because it is. Separation by cyclone or centrifuge? Measuring density or specific gravity? Millimeters or microns?
Particle technology can be complex, and industry terms can be confusing. You can strengthen your understanding of the fundamentals with our Particle Technology Glossary — and if, after you’ve had a chance to review it, you have questions in need of answers, just reach out to us here.