The use of moving air to separate and classify particles is a feature in many bulk material handling applications, with a key tool for such classification being the humble air classifier. The working principle for these machines involves separating a material’s particles according to density, shape, and size. Air classifiers are commonly employed for industrial processes involving bulk materials, especially in cases where particles can differ significantly in their physical characteristics. They’re utilized when particles within bulk materials require fast and efficient separation. Used for controlling air pollution and in the food processing industry, air classifiers are also used for manufacturing cement, chemicals, cosmetics, pharmaceuticals, pigments, and other materials. When using an air classifier, the working principles behind its operation allow manufacturers to classify more efficiently and separate particles for certain applications.
Main Air Classifier Working Principles
Though models may have different design features depending on their application, air classifier working principles always rely on two forces: a force associated with air drag and a force that depends on inertia, often referred to as centrifugal force. Air drag becomes more prevalent as particles are reduced en masse during processing. Separation occurs as particles with lower masses are more easily carried along the air stream. These finer particles are drawn up via moving air, which is then carried away from the zone in which the particles are classified.
Air classifier working principles are as follows:
- Material enters the classification area where airflow rises.
- Other particles with greater mass drop into the secondary air chamber.
- This secondary air inlet again separates coarse from fine particles.
- Material that doesn’t meet requirements is rejected and discharged at the air classifier’s base.
Air classifiers eliminate issues like blinding and wear often associated with machinery that utilizes screens. With an air classifier, working principles depend on physical actions that balance centrifugal force, drag forces, gravity, and the material’s particles as they collide. This provides a precise method for classifying particles by density and size. Air classification is most effective when used to separate dry materials with particles that are often smaller than 100 mesh (149 microns). Air classifiers are often used with grinding machinery to increase their productivity.
Air Classifier Working Principles for Separating Material by Particle Size
Air classifiers use various methods for managing particle size as a substitute for screens. Air classifier working principles involve airflow, classifier aperture, feed rate to air ratio, classifier wheel speed, and size and quantity of classifier blades. A fan generates air velocity and controls the drag force on particles entering this classification zone. Lower airflows permit only finer particles to go through the rejector cage, while higher airflows remove larger particles from the feed.
Two other basic air classifier working principles include the feed rate-to-air ratio and rejector speed. The particles within a pneumatic conveyancing system are controlled by this rate of feed, with higher rates allowing just the finest particles through the rejector cage, increasing the rejection rate of larger particles. The higher the rejector speed, the finer the particles that will pass through and the greater the chance that larger particles will be rejected, as it affects the force at which particles collide with each other as they move toward the air classifier’s egress.
Air Classifier Applications
Internal air classifier working principles for particle separation rely mostly on the weight and mass of particles within the feed, though density also plays a part. For the most part, smaller particles that weigh less are conveyed via moving air for collection, while larger particles that weigh more experience greater drag, so they drop out of the airstream. When low-density materials include finer particles, air classifiers offer an especially effective means for separating materials according to particle size. Yet if larger particles with lower densities have similar masses and weights to smaller particles with higher densities, this can reduce the air classifier’s effectiveness at separating by density.
Some applications in which air classifiers are used include:
- During food processing applications, augmenting protein content in various types of flour, certain animal feeds, and other materials.
- Dedusting fines are undesirable in certain aggregate stones, including basalt, diabase rock, gabbro, granite, gravel, limestone, and sandstone.
- Improving fineness when milling aluminum, copper oxide, feldspar, limestone, lithium carbonate, silica, zirconia, and other minerals.
- In water, flotation systems are used in precious metal extraction to reduce finer particles for a range of minerals, including gold, iron, and phosphate ores.
- Recycling centers are where various metals, paper, and plastics deliveries arrive together and require sorting before processing.
- Removing contaminants from coal, diatomaceous earth, fly ash, hydrated lime, and mica.
Though air classifiers can be incorporated into pneumatic conveying systems, doing so will considerably increase the velocity at which particles enter the air classifier. Working principles behind air classifiers within a pneumatic system require a higher rejector speed to keep oversized particles from passing through the classifying rejector blades. This results in more wear and greater inefficiency when removing fines than with gravity-fed air classifiers.
Air Classifiers from Prater Industries
Prater Industries manufactures two different types of air classifiers. Working principles for each are slightly different, though both kinds of air classifiers provide for narrow particle distributions with extremely fine particles separated. For each air classifier, working principles allow them to be used in standalone processes as both a feeding implement and a dust collection system. They’re also well-suited for integration into closed-circuit conventional grinding and milling systems.
For both types of Prater air classifier, working principles entail:
- Prater Mini-Split air classifier: Built originally for research purposes, these air classifiers are a good option for small workspaces. With lower noise output to decrease distractions, its smaller size makes it a more portable air classifier. The working principles of this design make it sufficiently flexible for testing applications and robust enough for use in production, though with lower throughputs.
- Prater MAC air classifier: Particle separation within pneumatically powered systems is achieved through an adjustable centrifugal force with this air classifier. Working principles subject individual particles to drag within the pneumatic airflow that separates them by size and density, causing particles to spiral toward the classifying rotor. Feed material that enters this vortex where classification occurs causes coarser portions to recirculate while a secondary stream of air helps collect near-sized particles. The MAC air classifier integrates easily into other pneumatic systems, requiring little alteration.
Prater’s air classifiers are designed to provide longer lifespans with better performance. Built to last, Prater air classifiers are designed for quick installation into existing pneumatic conveying systems. To learn more about air classifier working principles specific to our machines and our other material handling equipment, contact the experts at Prater today.