Raw materials undergo size reduction through different mechanisms: impact, compression, and attrition. One way to classify size reduction equipment is into crushers and grinders, where grinders produce finer particles than crushers.
In compression crushers particles disintegrate due to opposing rigid forces. Compression crushers are mainly used in processing brittle materials.
Compression crushers are used to break rocks and coarse particles larger than 3 cm in diameter. There are three main types of compression crushers: jaw crushers, gyratory crushers , and rolls.
Jaw crushers consist of a rectangular frame with a fixed jaw plate and a jaw stock carrying the moving jaw. The moving jaw swings toward the fixed jaw in the forward stroke, material is crushed, and discharge takes place during the backward or freeing stroke. The jaw crusher pictured below can be used to crush items from large objects such as demolition debris or broken concrete to smaller objects such as asphalt or river rock.
Material is fed through a feed plate that distributes the particles evenly into the chamber between the mantle and the concave casing. The mantle oscillates, causing a decrease in the volume between the mantle and the concave casing and crushing the material. The crushed material then exits freely from the apparatus.
During the crushing operation, the two rolls rotate toward each other, resulting in crushing. The feed particle size determines the type of rollers used. Rollers with teeth are able to grip large particles, making it possible to crush blocks of considerable size in rolls of moderate diameter. However, depending on what is being crushed and how fine it will be crushed, the rollers can be selected for best performance.
Jaw crushers and gyratory crushers are commonly used in mines and in quarries. Rolls are used to reduce crystalline or fibrous materials that fracture on compression, such as in the production of sponge metal, asbestos, bituminous coal and certain plastics.
- Energy efficient.
- Does not over-reduce materials.
- Variable capacity.
- Limited size reduction.
- Further classification needed.
General Information/Equipment Design
Size reduction in impact crushers occurs through particle concussion by rigid forces. The hammer crusher , also called the hammer mill, is an example of an impact crusher.
In hammer crushers, hammers swing freely on a disc mounted on the main shaft within the device . As the disc rotates, the feed material is struck and crushed by descending hammers until it is fine enough to pass through a screen or openings and exit the device .
The fixed knife mill is another example of an impact crusher. In a fixed knife mill, rotating pieces force the feed material against fixed knives within the device. The knives shred the material until it is fine enough to pass through a screen and exit the device.
Hammer crushers are commonly used to break down materials such as limestone, shale, phosphates, gypsum, barytes or asbestos rock. Additionally, hammer crushers and fixed knife mills are used for various food applications, such as fruit juice production.
Pulverizers, also know as fluid energy mills, are used for fine grinding and close particle size control. They are primarily used when contamination-free products are desired.
There are two main classes of pulverizers: air swept pulverizers and air impact pulverizers. Air swept pulverizers use air to transport particles to the pulverizing section of the apparatus. Air impact pulverizers use high speed air to accomplish pulverization. The products from both air swept and air impact pulverizers will be free of oversize particles and will need no further sieving or classifying.
Air Swept Pulverizer
The particle feed is mixed with air as it is fed into the mill inlet (1). The beater plates (2) support the hammers (3), and distribute the particles around the periphery of the grinding chamber. The hammers grind the solid against the liner (4) of the grinding chamber. The beater plates rotate between 1600 and 7000 rpm (revolutions per minute) to reduce the size of the incoming particles.
The classifier plate (5) separates the finely ground product for exit through the discharge outlet (6) while returning oversize material back to the mill inlet through the recycle housing (7).
Air Impact Pulverizer
In air impact pulverizers, superheated steam or compressed air produces the force that reduces the size of large particles. The jets accelerate the inlet material, causing the particles to smash into one another, resulting in smaller particles.
The P-jet fires the feed particles into the impact chamber, where they collide head-on with recycled particles moving in the opposite direction fired by the O-jet. After the collision, all the particles travel to the classification chamber.
The larger particles are forced to the outer perimeter by centrifugal force, travel down the downstack to the O-jet and are recycled. Fine particles move towards the center of the classification chamber and are discharged.
Pulverizers are commonly used for chemicals, pigments and food processing. The microscale air impact pulverizer is used in laboratories, where small samples are needed.
- Air needed is free.
- Large range of sizes available.
- Homogeneous blend.
- Energy consuming.
Size reduction in attrition mills occurs through the scraping of one surface against another. Particles scrape against each other or against a rigid face.
The most commonly used attrition mills are sand mills and tower mills. In both of these a rotating rod/screw induces attrition between the feed particles. Other types of attrition mills include buhrstone, planetary, and USBM attrition mills.
The feed particles are suspended in a liquid to form a slurry which enters the chamber. Here, rotating paddles help induce attrition between the particles. Smaller reduced particles move up the chamber with the flow, pass through a screen and exit. Larger particles remain at the bottom.
(Copyright Union Process Inc., Akron, OH)
Tower Mill System
Feed enters the grinding chamber through the rotary feeder. The particles fall down a chute into the grinding chamber. In the grinding chamber a double helix screw rotates on a shaft, inducing the particles to grind against each other. The double helix screw and the grinding chamber are coated with rubber to make them wear resistant.
The ground product is then channeled through a series of two cyclone separators. The fine ground product exits through the discharge outlet, while any oversized material is recycled.
(Prasher, C. L. Crushing and Grinding Process Handbook. New York: John Wiley & Sons, 1987. Copyright John Wiley & Sons Limited, Reproduced with permission)
Attrition mills are used for fine grinding operations in the production of spices (pepper, cinnamon, paprika), food (peanuts, grain, cereal), fibers (chips, cork, cellulose) and blendings (face powders, insecticides). The pictures below show pepper and cinnamon, finished products from attrition milling.
(Pictures Copyright Chemical Engineering Department, University of Michigan, Ann Arbor, MI)
- Finely ground products.
- Large range of sizes available.
- Energy consuming.
- Needs specific input size.
Unlike most types of mills, cone mills can be used for hard to grind products while using less energy than other types of mills. Cone mills are preferred in some industries because they produce less noise, dust, and heat than traditional milling equipment.
Material is fed into the conical chamber by gravity or conveying it. Inside the chamber is a rotor that spins at a low velocity and forces the material against the wall. The rotor has two paddles that pass over the material on the wall, inducing a shear force on it. This shear force breaks apart the material and when the particles are small enough they pass through the holes in the wall and fall into a collection container. Since the rotor is spinning at a low velocity the particles that pass through the wall tend to have a uniform size and the rotor generates little heat. This system is completely enclosed so that little noise and dust are generated.
(Copyright Mendel Company,
East Hanover, NJ)
(Copyright Aadvanced Machinery Inc.,
Clinton Township, MI)
Cone milling is used in the pharmaceutical , food, and chemical industries. It is widely used in pharmaceuticals for wet and dry granulation. In the food industry it is many used for grinding of foods such as sugar, candy, and chocolate.
- High efficiency
- Low heat generation
- Low noise and dust emissions
- Can mill sticky materials
- Easy to clean
- Small volume
Types of tumbling mills include rod mills, ball mills, and tube mills. Pronounced preferential breakage of the coarser particles is responsible for the "screening effects" that occur in rod mills.
(Copyright Patterson Industries (Canada) Ltd, Scarborough, Ontario)
A rod mill may be regarded as containing a series of grinding zones in which both breakage and screening occur, such that the coarse size ranges are progressively eliminated. The main breakage mechanism is shatter breakage, resulting from particles striking against each other.
Vertical Spindle Mills
A popular type of vertical spindle mill is the ring-roller mill.
In ring-roller mills rollers feed solid coarse particles against a stationary grinding ring. The particles are broken apart by the resulting compression forces.
In a ring-roller mill, the material to be milled is fed into the grinding section . The moving roller forces the coarse material against the stationary grinding ring. The particles emerge from the machine slightly finer than they entered.
Vertical spindle mills are used in the mineral industry to grind materials such as phosphate, limestone, magnesite, and bauxite.
- Easily cleaned.
- Dust-free operation.
- High capacity.
- Automatic operation.
- Rings and rollers wear easily.
Mechanical Impact Mills
Two of the most common mechanical impact mills are the colloid mill and the pin mill. The colloid mill shown here disperses a liquid feed into droplets.
(Copyright Chemicolloid Laboritories Inc., New Hyde Park, NY)
There are four different types of colloid mills: the hammer or turbine, smooth surface disk, rough surface disk, and valve or orifice type. Colloid mills are designed to break up agglomerates, particles that have bonded together, into droplets about 1 micrometer in diameter.
(Copyright Chemineer, Inc., Dayton, OH)
Pin mills are used to reduce the size of medium- to low-density materials such as talc and clay. Pin mills use centrifugal forces to smash large particles against pins.
A colloid mill rotor is an intermeshing rotor used for the size reduction of materials by multiple impacts with the maze of pins. High speed rotation applies fluid shear stress, inducing the breakdown of agglomerates. The speed of the rotation can also be reduced during mixing to induce a smoother impact and avoid material degradation while still producing enough turbulence to create a homogeneous blend.
The semi-liquid feed enters a grooved conical rotor that rotates with speeds of about 3600 rpm. The rotation produces currents inside each of the grooves, producing hydraulic shear, centrifugal action, and particle impact. After the agglomerates are broken apart, the liquid rises through the grooves until it reaches the outlet. The colloid mill is wrapped in a cooling jacket to counteract the high heat generated by the process.
In pin mills, solid material is fed through a hopper onto the rotor. The rotor rotates either clockwise or counterclockwise. This rotation causes the material to be broken apart into uniform-sized particles between the stationary upper pins and the lower rotating pins.
The standard-duty rotor has more pins than the heavy-duty rotor, resulting in smaller particles.
Colloid mills are used largely in asphalt production and grease manufacturing. They are also used in a wide variety of industries, such as paints, pigments, food and cosmetics, such as in the production of the lipstick . In the food processing industry, colloid mills are used in the production of mayonnaise, peanut butter, salad dressings, buttered syrups, and chocolate toppings.
Pin mills are commonly used to produce talc, clays, resins, flour and starch.
- Rugged and durable.
- Wide variety of uses.
- In colloid mills, the feed must be in a pumpable slurry.
- Pins in pin mills wear easily.
A roller compactor is used in dry processing to compact material into sheets or to reduce the particle size.
Powdered material is fed into a feed chute and is driven down to the rolls by a feed screw. It passes through the rolls and is compacted as the pressure increases till it reaches a maximum at the minimal distance between the rolls. The rolls come in different sizes and types depending on the desired granule or flake size. The pressure can also be controlled by varying the gap between the rolls and their speeds.
(Copyright Mendel Company, East Hanover, NJ)
Roller compactors are used in the chemical, pharmaceutical , and food industry to increase the density of powders and for granulation.
- No additional binders needed.
- Small size.
- Dust free.
- Does not work for all powders.
Aadvanced Machinery Inc. , Clinton Township, MI
Mendel Company , East Hanover, NJ
Union Process Inc. , Akron, OH
Patterson Industries (Canada) Ltd , Scarborough, Ontario
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Cone Mills. Bristol, PA: Kemutec. Web. kemutecusa.com.
Lynch, A.J., Mineral Crushing and Grinding Circuits . New York: Elsevier Scientific Publishing Company, 1977.
Miller, William T. W. Crushers for Stone and Ore . London: Mining Publications, Ltd, 1935.
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“Rigid Hammer Hammermill.” Machine & Process Design, www.mpd-inc.com/ .
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