Centrifugal air classifiers are used extensively to process aggregates, foods, metals, chemicals, minerals, fly ash, ceramics and other materials. Air classifiers are used for dry processing; to effectively remove fine powders, the surface moisture of the feed must be very low.
Air classifiers work effectively on granular coarse discharge with limited fines and dust, or product that is mostly fines and dust, and little coarse material. For dry materials of 100 mesh and smaller, air classification is the most effective and efficient means of separating a product from the feed stream for de-dusting or – when used in conjunction with grinding equipment – increasing productivity. It also eliminates the blinding and breakage issues associated with screens.
Air classifier advantages
The accuracy of mechanical centrifugal air classifiers makes them ideal for when the separation curve or cut point is too fine for screens (200 to 400 mesh or finer), when the capacity is too large for screens (up to 800 tonnes per hour) and when system flexibility is required to meet various product specifications.
One of the most significant advantages of a classifier is its dry process. For de-dusting 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 pulverisers 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, fly ash and ceramics.
Air classifiers can separate powders as coarse as 80 mesh (180 microns [μm]) and as fine as 2—3μm. The fineness of air- classified products is controlled by a precise balance between the quantity of rejector blades, the speed at which 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 pulveriser 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, and can also be used in a closed circuit with mills. In the latter case, use of the air classifier maximises the mill’s capacity and reduces its energy consumption because the mill does not have to serve as the sizing device.
Applications
Mechanical centrifugal air classifiers are useful in many applications, including the following:
• De-dusting undesirable 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 ilmenite ores.
• Upgrading the fineness of milled limestone, silica, feldspar, refractory slag, fused glass, kyanite, zirconia, alumina, lithium carbonate, copper oxide and others.
• Removing undesirable contaminants in fly ash, coal, kaolin, hydrated lime, diatomaceous earth and mica.
Seperation by density
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.
Effects of moisture
{{image2-a:r-w:300}}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 the 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 per cent.
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 air classifiers’ performance 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 surface moisture in stone sand limits the performance of air classifiers in aggregate plants or quarries. The drier the rock is (one to two per cent), 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 to three per cent), the fines stick to the rock and larger air classifiers are required, with more airflow than usual. Again, due to high surface moisture (3.5 to four per cent or more), the water will centrifuge out and clog up the equipment.
Pneumatic feeding
Air classifiers can be fed pneumatically and, in some cases, 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 rejector speed to stop these oversize particles. This can result in higher wear and lower efficiency in fines removal.
Cyclones or baghouses
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.
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 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.
Like internal fan air classifiers, the feed distribution plate in the external fan air classifier similarly imposes centrifugal force on the feed particles, delivering coarse particles to the inner cone and releasing 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.
Controlling particle size
The most common methods of controlling particle size in mechanical centrifugal air classifiers are rejector speed, cage aperture, size of rejector elements, airflow velocity and the ratio of feed rate to air.
Rejector speed controls the impact or collision force on the air-entrained particles as they attempt to exit the air classifier. Using a higher speed allows only the finest particles to pass through the rejector for collection, increasing the rejection of larger particles.
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 and cage aperture controls affect 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, allowing only the finest particles to pass through the rejector for collection, thus increasing the rejection of larger particles.
Measuring performance
{{image3-a:r-w:500}}The performance of mechanical centrifugal air classifiers can be evaluated by analysing the cut point, tolerance, yield and efficiency.
Cut point is simply the desired particle size that is intended to be classified. This value can be measured in millimetre 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 feed rate. Efficiency is the percentage of the desired particle size fraction recovered as product from the total amount available in the feed.
By understanding the different types of air classifiers, their advantages and how to evaluate their performance, operators can harness the use of mechanical centrifugal air classifiers to effectively separate fines.
Brolton Group is the distributor of Sturtevant air classifiers in Australia. Brolton is a multi- disciplined engineering solution provider that has worked with multiple industries over a broad range of projects, from conceptual studies through to major turnkey processing facilities.
Sturtevant is a family-owned manufacturer of material processing equipment, including three types of high performance air classifiers that separate fine and coarse particles with precision and accuracy. Sturtevant has sold more than 5000 air classifiers and has designed and manufactured a wide range of material processing equipment since 1888, including three generations of air classifiers: the Whirlwind, Sidedraft and Superfine classifiers.
Source: Brolton Group