Custom Processing Services Blog

Particle Size Distribution and Separation

Justin Klinger, Jan 31, 2025 10:15:00 PM

particle size

Particle distribution and separation techniques have been used since the dawn of civilization. For example, early agricultural societies separated grain from chaff during threshing or removed wheat germ from flour after milling.

Today, toll processors and manufacturers rely on various screening and separation methods, along with specialized classifying and screening equipment, to achieve precise particle distribution and remove unwanted materials. These processes can target particle size cut points ranging 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:

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

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.

For applications requiring ultra-fine particle sizes and tight size distributions, screening and classification are essential—whether performed during milling, afterward, or as a standalone service.

While most toll processors provide some level of particle separation and classification services, the achievable particle size cut points depend on their experience, technical expertise, and equipment capabilities.

Why Particle size Distribution and separation 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.

Particle size can also impact material characteristics such as viscosity, flow rates, and stability. In high-capacity production environments, materials that fall outside the specified size range can clog equipment, separate from the solution, and lead to lower-quality products.

In pharmaceuticals, converting an active pharmaceutical ingredient from a jet-milled, micronized particle size to a nanoscale particle—typically through a wet milling process—boosts the particle's 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 involves applying force to separate solid particles. Classification typically uses gas, such as air, while screening employs equipment like woven wire or a screen surface, with forces moving the material through.

air classification 

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.

Screening

Mechanical screening involves applying forces such as gyration and vibration to a screen or sieve as dry material or wet slurry passes through. While classification takes into account factors like particle shape and density, screening focuses on separating the material into two or more fractions based solely on particle size. Additionally, large particle separation is a key aspect of this process, ensuring effective fractionation for specific applications.

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. In general, higher mesh numbers correspond to smaller particle sizes. However, since the openings in a screen are uniform, the shape of the particles can influence how easily they pass through.

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 a widely used method for particle separation, handling sizes from microns to millimeters. Vibration causes particles to stratify by size, with the smallest settling at the bottom and the coarsest rising 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.

Screen openings can sometimes become clogged by larger particles or agglomerations of smaller ones, a phenomenon known as blinding. This can be mitigated by adding a deck beneath the screen, where objects (such as balls) bounce or vibrate to strike the screen from below.

Airflow, brushes, and ultrasonic vibration are all used to address screen blinding in mechanical screening.

achieve precise particle size and distribution with expert processing

When it comes down to it, your milling process may require particle separation or classification to achieve the exact particle size range needed for your application. That's why it's crucial to work with a tolling partner who has experts on hand to ensure your specifications are 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 truth is, particle size reduction can be quite complex. But with the right toll processing team, the process becomes simpler and easier for you from the very start. In fact, you can get started learning more about particle technologies right now when you download our Particle Technology Glossary. Simply click the link below to download your free copy.

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