When fine grinding (or simultaneously grinding and drying) a variety of materials, the grinding process can result in heat that can degrade the particles, alter their flow characteristics or result in the plugging of mills that use small classifying screens.
What is an Air Swept Mill?
Air swept mills finely grind the material with or without screens, typically producing diverse particle sizes during processing. Also often referred to as impact mills, they’re ideal for pulverizing a wide array of materials up to 50 microns. Air swept mills work best for heat-sensitive materials, dry to moist materials, and those with medium to high friability.
Typically, air swept mills feature:
- Dust-free operation using a negative pressure design that contains fine particles in the milling process, mitigating health and other hazards, along with ensuring cleaner surroundings during production.
- Easy to maintain, clean, service, and repair components and machinery.
- Efficient processing delivers products in a timely and cost-effectively while reducing energy consumption.
Air swept mills use a powerful stream of air to pull products through the mill.
How Air Swept Milling Works
In air swept mills, material feeds from a perpendicular plane across the mill’s hammers and retainer plate, with grinding occurring between the hammers and serrated liners or screens. Air sweeping the mill carries the ground product to a collection point, such as a cyclone or filter receiver, where air and material are separated then discharged from the system thru a rotary airlock. A fan at the end of the system generates the air movement that accelerates and transports the particles within the system. Depending on the material and grinding requirements, some mills even have water circulation jackets to aid in cooling the process.
Along with grinding, air swept milling offers drying capabilities when heat is added. Moist powders and slurries can be metered into the mill, where the convey air is heated to flash dry material during the grinding process. Because of the high surface-to-volume ratios achieved, flash drying during grinding can be extremely efficient compared to drying larger particles. These same mills can be outfitted with various types of cutting plates and a classifier to meet a wide variety of particle size requirements.
Here are the concepts of air swept milling:
- Material is metered into an airstream at mill inlet.
- The rotor accelerates the particles outward.
- Material distributed along the edge of the grinding zone.
- Hammers or blades impact grind the product against grinding plates, screens or corrugated liners, depending on mill type.
- Collisions occur at velocities of about 220-385 miles per hour (mph) for fine particle reduction.
- In certain models, a classifier separates product that’s been ground finely, allowing finer particles to exit the grinding machine.
- Oversized material is returned to the grinding zone, where it undergoes further reduction.
Some air swept mills can even achieve particle sizes as small as five microns through maximizing the mill’s speed and number of grinding plates. By making simple adjustments, air swept mills are able to achieve narrow particle distribution ranges for material that’s dry, fibrous, heat-sensitive, or moist material.
Air Swept Milling Applications
There are numerous products and applications for which the air swept milling process works well.
Products and materials that can be processed with air swept mills include:
- Adhesives
- Aquaculture food
- Asphalt
- Barley, bran, oats, rice & wheat (including wheat germ)
- Bentonite & other clays
- Botanicals & cosmetics
- Calcium carbonate
- Carbon black
- Cheese whey
- Cocoa
- Dextrose, lactose & other sugars
- Dyes & pigments
- Gums
- Herbs, salt & spices
- Inorganic chemicals
- Limestone, talc, zinc oxide & other minerals
- Pharmaceuticals
- Plant proteins
- Polyesters, polypropylene, PVC (polyvinyl chloride) & other plastics
- Powdered products
- Resins & other polymers
- Sugar
- Wood flour
Air swept milling also doubles as a conveying system to transport products.
Types of Air Swept Mills by Prater
Several different types of air swept mills are manufactured by Prater Industries, each with nuances that make it better suited for specific purposes.
Prater’s air swept mills and features include:
- Air classifying mills combine two stages of grinding and milling with an internal air classifier as a single unit, with the air classifier continually recirculating larger particles back to the grinding zone until they reach optimal particle sizes. They are used to achieve narrower particle distributions, and Prater’s design allows for easier cleaning, inspection and maintenance.
- Fine grinding mills reduce raw material as it is fed through the mill’s center, impacting rotor blades at high speed, and accelerating outwards to create additional impact and shear as the material meets the mill’s stationary screens and jaws. These unmoving surfaces decelerate particles, causing them to rebound back into the path of the rotor blade, only swept via air through the screen once particles reach the appropriate size.
- Rotordryers work with wet or moist material fed into the section of the dryer’s main body, where a grinding rotor disperses wet material until its particles become very fine and heated air fluidizes it within the grinding chamber. High temperatures then break apart the wet material, increasing surface area to facilitate evaporation.
- Rotormills allow varying airflows and use a series of grinding plates to expediate several stages of grinding within the upper section, pushing air and particles along the grooved lining of the mill’s interior, where small pockets of air create turbulence as they rotate at high speed during the mill’s regular operation. This causes particles to collide with each other, pulverizing the material, with the continuous airflow reducing internal heat.
Prater’s unique designs reduce a wide array of materials while enabling easy access for cleaning, inspection or repairs. Depending on the application, Prater’s technical team can also customize hammermills so that they work as air swept mills.
