Material classification and separation of fine powders by particle size help ensure uniformity, leading to a more effective product or mixture. Various technologies help with classifying material, though identifying the best one for a specific application depends on the type of material and desired end product. Pneumatic technology is often used for classifying fine powders, but to understand why it’s the preferred method for such material classification, it’s important to understand the benefits of air classification milling for certain bulk powder end products.
Factors Involved in Fine Powder Material Classification
When bulk powder processers look at the most suitable technology for their application, it’s important to look at several aspects of the material. Particle size and shape are particularly important when it comes to material classification. The size and shape of particles can prevent them from going through classifying screens, for example, or otherwise slow processing of bulk powders. Particle size distribution affects the properties of bulk powders as well.
Size, shape, and distribution of materials can cause:
- Blocked screens and other mechanical difficulties.
- Changes in flow behavior and other properties of end products.
- Inordinate amounts of fine powders cause problems with equipment.
- Irregularly shaped or finer particles that increase viscosity of suspensions.
- Removal of material from the production process, increasing costs.
- Poorer quality end products.
- Wide distributions lead to particles' segregation by size, reducing flowability.
In addition to size and shape, the density of bulk powders must be considered. The weight of a material won’t necessarily correspond to its volume, with certain lighter powders taking up considerably more space. Bulk density affects size and shape, along with how cohesive individual particles are to each other.
Another factor in bulk powder material classification and processing involves its ability to absorb moisture. Called hygroscopicity, powders that readily absorb airborne water vapor result in greater particle cohesion and reductions in flowability. For this reason, processing bulk powders such as sugar in drier regions will result in a higher flow rate than if processed in a more humid region.
Determining the Best Material Classification System
A wide variety of factors go into determining which type of material classification is best for a specific bulk powder product. Aspects like moisture, temperature, or vibration, along with introducing powders with dissimilarly shaped particles into the mix, will affect the flow of a product, causing it to aerate, agglomerate, degrade or fluidize.
Softer powders like hydrated lime or flours tend to “rat hole” or bridge, which impedes their flow along a production line. Powders that are lighter and fluffier like talc or silica aerate, seeping into equipment cracks and even stopping up filters. Notorious for binding to or jamming moving parts, powders like titanium dioxide or carbon black often smear or become packed, causing a reduction in efficiency as they require time-consuming and labor-intensive efforts to keep equipment clean.
Milk and other dairy powders are high in fat, so cause buildups of this powder along production lines. This increases the risk of cross-contamination as well as causes problems with the flow of these types of powders. Reducing salts or sand into very fine particles can cause damage to powder handling equipment or conveyancing systems. Additionally, powder blends often are made up of contrarily sized and shaped particles with different densities that make particles more prone to separation, which diminishes product quality while producing unnecessary waste.
Material Classification with Air Classification Milling
Fine powder processing and reduction equipment use mechanical separation or air classification. Milling via purely mechanical means requires screens to allow smaller particles to pass through while larger, oversized particles are reprocessed. Air classification mills instead rely on airflow to separate larger, heavier particles from smaller, lighter ones. Bulk powder processers typically use air classification for separating very fine particles smaller than 20 microns. Generally, air classification mills fall under two types: those that utilize classifying wheels and those that don’t.
Types of Air Classification Milling
For those air classification mills that utilize a classifying wheel, the air moves against the centrifugal force outside the classifying chamber, moving towards its center. Featuring blades or fins arranged in a circular formation that extends parallel to the rotating axis, this classifying wheel spins on this vertically positioned axis at high speeds within the chamber. While the wheel spins, air flows through the moving wheel, carrying the finer particles with it. These fine particles are then discharged at the chamber’s top while coarser particles drop to the chamber’s floor.
The main advantage of air classification mills that don’t use a classifying wheel is simple. Any component tends to wear over time, including classifying wheels. This is especially useful for separating titanium powder employed in 3D printing, which requires air classifying equipment without a wheel to avoid contaminating or degrading the product. Using wheel-less air classification systems allows material classification to be achieved below one micron.
One type of air classification mill without a wheel is called an elbow jet air classifier. Containing no classifying wheel or rotating parts, it instead utilizes convex surfaces to control air movement. This is known as the Coandă effect after the Romanian inventor discovered its practical uses, using a concept similar to airplane designs in which flowing air is attracted to proximate surfaces. With the air moving like a fluid and jetting across the convex surface, elbow jet air classifying mills keep lighter and smaller particles close to the curved surface while larger and heavier particles are flung away from this surface. Typically, this process involves two or three separations.
Another method of air classification without a classifying wheel is cyclonic separation. Designed originally for dust collection, some manufacturers now use it to separate particles by size. Using a vortex instead of filters removes particles from gas or liquid streams and allows processors to recycle the separated coarse or fine material. Though not the most effective method for classifying with air, it’s inexpensive, takes up little space, and can be used with other cyclonic separators to enable better material classification.
Prater’s Air Classification Mill for Classifying Material
Prater Industries designs and manufactures air classification mills that provide narrow particle distributions and separation of very fine dry bulk powders, with ranges between 3-150 microns. Prater’s MAC Air Classifier and Mini-Split Air Classifier work well for conventional milling systems and closed-circuit grinding.
Prater’s air classifying milling systems offer the following features:
- Control over cut points through precision variations of rotor speed.
- Innovative rotor design that offers efficient and precise material classification and separation capabilities.
- Design enables low-power usage by utilizing low airflow resistance.
- Durable carbon or stainless steel fabrication.
- Options for working with abrasive materials with ceramic, polyurethane, rubber, or tungsten linings.
- Lower power consumption and lower temperature rise versus other machines on the market.
- Secondary air system that adjusts to enable capture of more near-size particles.
Prater’s air classification mills can adjust the centrifugal force used to separate particles by size. Conveying raw product to a primary air inlet, the drag forces within the classifier cause the moving air to act on individual particles, depending on the size and density of the material. With only minimal modifications, Prater’s air classifying mills can be incorporated into existing pneumatic systems.