Top Particle Separation Methods You May Need From Your Toll Processor

Justin Klinger, Jan 20, 2022 8:15:00 AM

scientist-holding-grains-after-particle-separation

Particle separation methods have been used since the earliest days of civilization. Consider, for example, how grain is separated from chaff in agricultural threshing, or how wheat germ is removed from flour after that grain has been milled.

Toll processors and manufacturers use several types of screening and separation methods, and a wide range of classifying and screening equipment, to remove unwanted particles from the final product. Particle size cut points can range from microns to millimeters.

Particles may need to be removed from a final bulk solid lot because they’re an unwanted material, or because they are too coarse or too fine. Processing industries that call for particle separation techniques include:

  • Food and beverage
  • Pharmaceuticals
  • Pigments
  • Chemicals
  • Ceramics
  • Industrial materials

Separation can be performed on the production line or as a second step. Separating components of liquids, like botanical extractions, calls for different technologies. In this article, we’ll focus on solid particle separation methods.

Processes include sieves and mesh screens with or without mechanical forces applied, built-in air classification in a jet mill, a classifier added on to a fluid bed jet mill, sedimentation in liquid, centrifugal force applied to a wet-milled slurry, and more.

In applications that require particularly 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 a few particle separation and classification services, but particle size cut points can vary by toll processors’ experience, technical expertise, and equipment capabilities.

Why Do Particle Size and Size Distribution Matter?

Particle sizes and ranges affect material properties in many ways. The same raw feed, milled to varying sizes or particle shapes using different milling and grinding processes, can behave in different ways. Smaller particles may be more heat sensitive or dissolve faster than larger particles.

Material characteristics like viscosity, flow rates, and stability can be affected by particle size, too. In high-capacity production environments, materials that are out of specified size ranges can clog equipment, separate out of solution, and result in subpar products.

In pharmaceuticals, taking an active pharmaceutical ingredient from a jet milled, micronized particle size to a nanoscale particle (usually via a wet milling process) increases the particle surface area. 

That greater surface area can increase the efficacy of active pharmaceutical ingredients, which allows manufacturers to use less of an expensive compound to achieve the desired effect, potentially decreasing the risk for negative side effects while also reducing overall costs.

In paints, pigments, and coatings, particle size can impact color, sheen, tint strength, transparency, weather resistance, durability, and more. Smaller particle sizes can allow a manufacturer to use extreme precision in formulation, which can translate to higher-performance products and controlled production costs.

A Range of Separation and Classification Methods

Mechanical separation refers to the application of force to separate solid particles. Classification most often uses gas (such as air). Screening uses separation equipment such as woven wire or a screen surface, with forces applied to move the material through.

Particle Separation in Jet Milling

Jet milling is a preferred particle size reduction process for milling fine powders in many industries. Some types of mechanical air classification methods utilize jet milling process parameter adjustments such as feed rate, rate of air flow, and rotor speed to affect centrifugal force and drag force within the milling chamber or classifier. 

In some jet milling equipment, separation technology is built into the milling machine. For example, a spiral jet mill uses built-in static air classification. As the mill produces very fine particles, the finest-ground material exits the milling chamber through a central outlet. 

This built-in classification works because particles’ response to the forces is determined by their sizes, shapes, and specific gravities. Generally speaking, coarser particles are more responsive to centrifugal force, and finer particles are more responsive to drag forces.

In a spiral jet mill, the finer particles migrate toward the center and exit through the central outlet. Coarser particles migrate outward.

Fluidized bed jet mills often use a dynamic, deflector-wheel classifier step to apply a more complex array of forces on milled particles as they leave the grinding chamber. This precision control enables the fluid bed jet mill to produce consistent particle sizes, and can efficiently separate particles in the sub-100 micron range with high yields.

In many cases, coarse particles can be remilled and returned to the classifier for separation.

Mechanical Screening

Mechanical screening describes a process of applying forces like gyration and vibration to a screen or sieve as dry material or a wet slurry flows through the sieve. While classification is affected by characteristics like particle shape and density, screening can separate the material into two or more fractions based solely on particle size.

The material is allowed to flow through a single screen or a stack of increasingly finer-mesh steel or nylon screens. The material that passes through the screen is referred to as under size, while the material that stays on top of the screen is called over size.

A screen’s mesh number refers to the number of openings in one linear inch of screen. The fineness of a screen is limited by the wire diameter used to make the mesh. A 100-mesh screen has 100 openings per inch. Generally, higher mesh numbers mean smaller particle sizes. But the openings in a screen are uniform, and particle shapes can vary. That can affect their ability to pass through the screen.

Screens may be set horizontally or at an incline, depending on the characteristics of the material. As particles pass through the series of finer screens, they can be classified into multiple lots by size range.

Vibratory screening is among the most commonly used particle separation methods. It can screen particles ranging from microns to millimeters. Applying vibration to a static bed of material causes particles to stratify by size. The smallest particles migrate to the bottom layer and the coarsest rise to the top.

Screening Challenges and Solutions

Agglomeration, or the tendency of very finely milled particles to clump together, can pose a challenge to mechanical separation. Moisture’s effects can also impact the screening process. Vibration or gyration forces can be applied to help break up agglomerations and help dry materials flow more freely through the screen.

Sometimes screen openings can become clogged by larger particles or agglomerations of smaller particles. 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 all used to address screen blinding in mechanical screening.

Achieve Your Particle Size & Distribution Specifications With the Help of the Right Processor

When it’s all said and done, your milling process might call for particle separation or classification in order to achieve the specific, precise range of particle sizes for your application. That’s why it’s so important to know your tolling partner has experts on hand to ensure your specifications can be met.

A specialized team at your toll processor can determine the best method to separate and classify 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? 

The fact is, particle size reduction can be an incredibly complex subject — but the right toll processing team can make any process simpler and easier for you from day one. In fact, you can get started learning more about particle technologies right now when you download our Particle Technology Glossary.  Just click the link below to claim your own free copy.

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Posted in:Toll Processing