Particle fractionation by air classification

In this article, we introduce Air classification particle separation. Powders and granular materials usually contain particles with varying sizes, shapes, and densities. In many applications, achieving a uniform collection of particles is essential for improving material performance. An air classifier effectively separates materials by combining differences in particle size, shape, and density. Consequently, this process enhances the uniformity of the collected particles. As a result, it leads to better overall performance in various applications.

Air classification is a method that separates powdery, granular, or fibrous materials based on settling air velocity, particle size, density, and shape. The ideal separation effect of an air classifier ensures that all particles exceeding the cut point are transported to the coarse fraction. Meanwhile, smaller particles are directed to the fines fraction.

This method is particularly valuable because it enables the dry separation of small particles. Achieving this separation is difficult with traditional sieving methods, especially for particles below 50 micrometers.

The typical operational process for an air classifier begins with inlet air mixing with the feed material. This action subjects the particles to centrifugal forces generated by a rotor and drag forces created by the air. Larger and denser particles experience mass-dependent centrifugal forces, causing them to move toward the outer edge of the chamber. In contrast, smaller and lighter particles are more influenced by the frictional forces of the air. Consequently, these lighter particles move toward the center of the chamber and are separated from the airstream by a cyclone.

You can adjust the relative magnitudes of these two forces by changing the rotational speed of the disc and the inlet air velocity. This adjustment allows for optimal separation of particles based on their characteristics.

In our field testing area, we use a Hosokawa multiplex classifier with a particle size separation range of 2 micrometers to 80 micrometers. This equipment enables us to obtain more uniform materials in the sub-100 micrometer range. Customers can then evaluate these materials for improved performance.

We characterize the separation of the air classifier using the concept of a cut point. Ideally, all particles below the cut point should end up in the fine stream, while all particles above the cut point should follow the coarse stream. However, some misplaced material will inevitably occur. This means a small number of particles smaller than the cut point may appear in the coarse stream, while a small proportion of particles larger than the cut point may end up in the fine stream.

To determine the extent of misplaced material, we measure the particle size distributions of both streams. If we identify misplaced materials, we can repeat the classification process. This repetition continues until the majority of particles are correctly sorted into their respective fractions.