Size Reduction

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. Each of these types of equipment is described with general information, equipment design, usage examples, and advantages/disadvantages.

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Crushers

Compression Crushers

In compression crushers, particles disintegrate due to opposing rigid forces. Compression crushers are mainly used in processing brittle materials.

General Information

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.

Equipment Design

Jaw Crushers

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, the 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.

Gyratory Crushers

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.

Rolls

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.

Usage Examples

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.

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Impact Crushers

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.

Usage Examples

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.

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Grinders

Pulverizers

Pulverizers, also known as fluid energy mills, are used for fine grinding and close particle size control. They are primarily used when contamination-free products are desired.

General Information

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. Equipment Design

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.

Usage Examples

Pulverizers are commonly used for chemicals, pigments, and food processing. The microscale air impact pulverizer is used in laboratories, where small samples are needed.

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Attrition Mills

Size reduction in attrition mills occurs through the scraping of one surface against another. Particles scrape against each other or against a rigid face. In general, the size of the resulting particles can get near the micrometer level, but in the pharmaceutical industry, demand has increased for particles on the nanometer level.

General Information

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.

Equipment Design

Sand Mill

The feed particles are suspended in a liquid to form a slurry that 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.

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.

Usage Examples

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)

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Cone Mills

General Information

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.

Equipment Design

Material is fed into the conical chamber by gravity or by 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.

Usage Examples

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 foods such as sugar, candy, and chocolate.

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Tumbling Mills

General Information

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.

Equipment Design

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.

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Vertical Spindle Mills

A popular type of vertical spindle mill is the ring-roller mill.

General Information

In ring-roller mills rollers feed solid coarse particles against a stationary grinding ring. The particles are broken apart by the resulting compression forces.

Equipment Design

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.

Usage Examples

Vertical spindle mills are used in the mineral industry to grind materials such as phosphate, limestone, magnesite, and bauxite.

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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.

General Information

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.

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.

Equipment Design

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 at 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.

Usage Examples

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 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.

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Roller Compactors

General Information

A roller compactor is used in dry processing to compact material into sheets or to reduce particle size.

Equipment Design

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.

Usage Examples

Roller compactors are used in the chemical, pharmaceutical, and food industries to increase the density of powders and for granulation.

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Cryogenic Grinders

General Information

Cryogenic grinders are used to attain extremely small particle sizes, primarily on the nanoscale. They also allow high moisture feeds to grind without caking. This is done by using grinding mechanisms at extremely low temperatures.

Equipment Design

The main cooling source for cryogenic grinding is liquid nitrogen. The material is brought from a hopper to the liquid-nitrogen-cooled area The material is cooled rapidly because of the extremely cold temperature of the liquid nitrogen. Once cooled, the material becomes very brittle and grinds efficiently. In addition, since the crystal size in the material is very small due to the rapid freezing, there is less damage done within the cells. This creates a high-quality product. The nitrogen gas is then liquefied as described in the cryogenics section. The system is described in the figure below.

Usage Examples

Cryogenic grinding can be used in any industry setting, but it’s primarily used in the food, pharmaceutical, plastics, explosives, and steel industries. Cryogenic cooling helps prevent volatile compounds and oils in spices from escaping, creating high-quality ground spices as shown in the video below. Extremely fine particle sizes are desired in the pharmaceutical industry because they allow for faster drug absorption. Plastics can be ground into smaller particle sizes. In the explosives industry, cryogenic grinding is advantageous because it allows explosives to be ground below their ignition temperatures, resulting in a safer process. Less heat is created in the grinding of steel because the cooled steel is easier to grind.

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Acknowledgements

• Aadvanced Machinery Inc., Clinton Township, MI
• Mendel Company, East Hanover, NJ
• Union Process Inc., Akron, OH
• Patterson Industries (Canada) Ltd, Scarborough, Ontario

References

• Ashurst, Phillip R. “Fruit Juices.” Kirk-Othmer Encyclopedia of Chemical Technology (2007): Wiley Online Library. Web.
• “Centrifugal Mill RM 1,5.” Voran Maschinen, www.voran.at/en/machinery/.
• Cone Mills. Bristol, PA: Kemutec. Web. kemutecusa.com.
• Junghare, H., Hamjade, M., Patil, C. K., Girase, S. B., & Lele, M. M. (2017, March). A Review on Cryogenic Grinding. International Journal of Current Engineering and Technology.
• LePree, J. (2018, February). Improving the Daily Grind. Chemical Engineering Essentials for the CPI Professional. Print.
• 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.
• Perry, Robert H., and Don W. Green. Perry’s Chemical Engineers’ Handbook. 7th ed. New      York: McGraw-Hill, 1997: 20-10 – 20-73.
• Prasher, C. L. Crushing and Grinding Process Handbook. New York: John Wiley & Sons, 1987. “Rigid Hammer Hammermill.” Machine & Process Design, www.mpd-inc.com/ .
• “Roll Compactor.” Prism. Prism, Web. www.drygranulationrollercompactor.com.

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