Mechanical Centrifugal Air Classifiers

November 1, 2010 | By Joseph Muscolino, Sturtevant Inc.

An improved understanding of air classifier systems and their application can help in selecting the right device

Mechanical centrifugal air classifiers are used extensively to process aggregates, ceramics, chemicals, foods, minerals, metals, plastics, flyash and other materials. They are normally employed when the particle size that you need to separate is too fine to screen. The air-classified product can be either the granular coarse discharge with very little fines and dust, or it can be the fines discharge with very little coarse material.

Air classifiers eliminate the blinding and breakage issues associated with screens. They work by balancing the physical principles of centrifugal force, drag force, collision and gravity to generate a high-precision method of classifying particles according to size and density. For dry materials of 100-mesh and smaller, air classification provides the most effective and efficient means for separating a product from the feed stream, for dedusting, or, when used in conjunction with grinding equipment, for increasing productivity.

Air classifiers can only be used for dry processing. In order to effectively remove fine powders, the surface moisture of the feed must be very low.

Mechanical centrifugal air classifiers are masters of accuracy. They are a good choice when the separation curve or cutpoint is too fine for screens (200–400 mesh or finer), when the capacity is too large for screens (up to 800 ton/h) and when easy adjustability is required to meet various product specifications.

One of the most significant advantages of a classifier is its dry process. For dedusting aggregates, dry processing eliminates the need for water or settling ponds, saving money and land, and benefiting the environment.

Air classifiers do not handle the more aggressive work that pulverizers do and they operate at much lower speeds, so the equipment is less susceptible to wear. With the addition of protective liners, air classifiers can be used to economically process even abrasive powders, such as silica, flyash and ceramics.

Air classifiers have the ability to separate powders as coarse as 80 mesh (180 μm), and as fine as 2–3 μm. The fineness of air-classified products is controlled by a precise balance among the quantity of rejector blades, the speed at which the rejector blades operate, the velocity of the airflow and the rate at which the material is fed. Even with fragile powders, air classifiers rarely fracture or degrade particles because they do not operate at pulverizer speeds and most of the feed never makes contact with the rotating parts.

Air classifiers can be used as a single sizing device in an open circuit where the feed is split into a fines discharge and a coarse discharge. This equipment can also be used in closed-circuit with mills. In this case, use of the air classifier maximizes the capacity of the mill and reduces the mill's energy consumption because the mill does not have to serve as the sizing device (Figure 1).

Figure 1. Mechanical centrifugal air classifiers can serve as sizing devices, where the feed is split into coarse discharge and fines

Mechanical centrifugal air classifiers are useful in many applications, including the following:

• Dedusting undesireable fines in many types of aggregate stone, such as limestone, granite, trap rock, sandstone, basalt, diabase rock, gabbro, sand and gravel

• Reducing the fines load on a water flotation system for extracting precious metals from many types of minerals, including iron, gold, phosphate or illmenite ores

• Upgrading the fineness of milled limestone, silica, feldspar, refractory slag, fused glass, kyanite, zirconia, alumina, lithium carbonate, copper oxide and others

• Removing undesireable contaminants in flyash, coal, kaolin, hydrated lime, diatomaceous earth and mica

• Increasing the protein content in wheat flour, chick-pea flour, poultry-meal animal feed and others

While density does play a role in air-classifier separation, the internal air currents are mostly affected by the overall mass and weight of the particles in the feed. Lighter and smaller particles are removed by the airflow, while heavier and larger particles are not entrained in the airflow. If the lower-density material also has a finer particle size, then air classifiers can be very effective. However, large particles with low density can have a similar mass and weight as some small particles with high density. This can reduce the effectiveness of an air classifier's density separation.

Moisture effects are limited by surface moisture, rather than inherent moisture. Inherent moisture is naturally found inside particles of ores, minerals or stone sand after natural drying occurs in air. Inherent moisture does not hinder an air classifier's ability to remove fine powder or fine dust from coarse particles. For example, crushed coal is successfully air classified with inherent moisture as high as 10%.

Surface moisture, on the other hand, is found on the surface of ores, minerals or stone sand and comes from rainfall or from spraying water in an aggregate plant or quarry during dust suppression. Surface moisture is detrimental to the performance of air classifiers because the fine particles stick to the large particles and airflow is not enough to remove them. When surface moisture is very high, the water also centrifuges out and results in equipment clogging.

The performance of air classifiers in aggregate plants or quarries is limited by the surface moisture in the stone sand. The drier the rock is (1–2%), the more dust can be removed, often allowing air classifiers to replace water wash systems altogether. When higher surface moisture is present in stone sand (2.5–3.0%), the fines stick to the rock and larger air classifiers are required with more airflow than usual to be effective. When the surface moisture is very high (3.5–4.0% or more), the water centrifuges out and results in equipment clogging.

Air classifiers can be fed pneumatically and, in some cases, can be incorporated into a pneumatic conveying line. However, in a pneumatic feed process, particles enter the air classifier at a much higher velocity than gravity-fed particles. When these particles approach the classifier rejector blades at high velocities, they are more likely to pass through, which requires a higher rejecter speed to stop these oversize particles. Higher rejector speed can then result in higher wear and lower efficiency in fines-removal.

There are two categories of mechanical centrifugal air classifiers: Internal-fan models and external-fan models.

Internal-fan air classifiers recycle the air, and therefore, do not require airlocks, cyclones or baghouses to collect the separated fines. This design has a single shaft that controls three rotating elements — the feed distributing plate, particle-sizing selector blades and circulating fan (Figure 2).

The feed distribution plate imposes centrifugal force on the feed particles, moving them into the classification zone. Coarse particles fall down into the inner cone and exit at the coarse discharge. The circulating fan creates an upward draft of air that carries finer particles away from the feed and through the selector blades. Properly sized fine particles pass through the internal fan still entrained in air. Fixed vanes recycle the air back into the classifier, while the properly sized fine particles drop out of the airflow and slide down the fines cone, where they exit.

External-fan air classifiers (Figure 3) require cyclones or baghouses to collect the separated fines. This design uses a variable-speed rotor with multiple, closely spaced rejector blades for ultra-fine and ultra-efficient applications. The feed distribution plate imposes centrifugal force on the feed particles, moving them into the classification zone. Coarse particles fall down into the inner cone and exit at the coarse discharge point. The external fan creates a draft of air that carries finer particles away from the feed and through the rotor. Properly sized fine particles, entrained in air, pass through the rotor and exit the air classifier. A cyclone or baghouse is required to recover the classified particles out of the airflow.

The most common methods of controlling particle size in mechanical centrifugal air classifiers are rejector speed, cage aperature size of rejector elements, airflow velocity and the ratio of feedrate to air.

Rejector speed controls the impact or collision force on the air-entrained particles as they attempt to exit the air classifier. Higher speed allows only the finest particles to pass through the rejector for collection. This increases the rejection of larger particles (Figure 4).

Airflow velocity generated by a fan controls the drag force on particles as they enter the classification zone. Higher airflow allows larger particles to be removed from the feed, while lower airflow allows only the finest particles to pass through the rejector cage for collection.

Rejector elements / cage-aperture controls impact the collision force on the air-entrained particles as they attempt to exit the air classifier. A greater quantity of rejector elements (blades or rods) makes the cage aperture smaller and allows only the finest particles to pass through the rejector for collection. This increases the rejection of larger particles.

Feedrate-to-air ratio controls the entrainment of particles in the airflow. A higher feedrate allows only the finest particles to pass through the rejector cage for collection. This increases the rejection of larger particles.

The performance of mechanical centrifugal air classifiers can be evaluated by analyzing the cutpoint, tolerance, yield and efficiency.

Cutpoint is simply the desired particle size that is intended to be classified. This value can be measured in millimeter mesh or micron size. Tolerance is the percentage of oversized or undersized particles allowed in the finished product. Yield is the percentage of production rate per unit of feedrate. Efficiency is the percentage of the desired particle-size fraction recovered as product from the total amount available in the feed.

Edited by Scott Jenkins

Author

Joseph Muscolino is product manager for air classifiers at Sturtevant Inc. (348 Circuit St., Hanover, MA 02339; Phone: 800-992-0209; Email: [email protected]; Web: www.sturtevantinc.com). Muscolino has 26 years of industrial experience with air classifiers and mills. He is a member of various professional societies, including the National Stone, Sand and Gravel Assn., and is the author of several technical articles and case histories. He received a B.S. in mechanical engineering from Northeastern University in 1981.