Thomas E. Warne

There are almost as many different methods of performing size reduction on materials as there are materials themselves. Consequently, choosing the best piece of equipment for any given product can be a daunting task. The nature of the material, initial size, required particle size after processing, tonnage, and available space must be considered in the selection process. Focusing on preliminary and secondary grinding, I will look at some common types of equipment used for size reduction with the goal of narrowing this selection process.

Impact crushing: equipment that contains apparatus designed to deliver a severe blow to the material, causing it to shatter. Typical equipment falling into this category includes hammer mills, impactors, and certain types of lump breakers. Materials that lend themselves to this type of machinery include friable products, such as agglomerated powders, stone, glass, ceramic, and other brittle materials. Impactors are generally used when the feed stock is large and the tonnage requirement high. They are limited in ability to produce a fine material and are generally used as an initial step in processing. Hammer mills can accept a mid-size feed stock and can reduce most materials to granular or powder consistency if necessary. Particle size can easily be controlled when using a hammer mill by changing the screen size and rotor speed. Lump breakers are generally used to reduce agglomerated materials in order to make them flowable when a specific particle size is not required. They are most commonly found with either a dual shaft rotor assembly, or with a single rotor assembly with the blades passing through a comb.

The advantages of impact crushers are generally lower acquisition costs, low cost of replacement parts, simplicity, high production rates, and greatest control over particle sizing. Disadvantages include higher horsepower requirements, somewhat louder operation, and, with the possible exception of the lump breaker, the requirement for manual or metered feeding.

Compression crushing: machines that reduce the size of materials by squeezing the product between two surfaces, such as jaw and cone crushers. Compression crushers are most commonly used in the quarry industry for preliminary crushing of rock, but can be found in many other industries as well. A compression crusher is an ideal selection when the material to be crushed is fairly large and abrasive, and when a fine particle size is not required.

The advantages of compression crushing machines are their ability to process abrasive materials at high rates of throughput with minimal wear. Disadvantages include weight and girth of the equipment and minimal particle-size control.

Shredding: machines that rip or shred typically nonfriable materials, such as rubber, plastics, and metals. The two most common styles of shredders are dual-, counter-revolving-shaft designs, or single-shaft, ram-fed machines. Dual-shafted machines pull the material down between the two counter-revolving shafts. Single-shafted shredders have cutter teeth affixed to a solid rotor. A hydraulic ram pushes the material in the hopper against the revolving shaft. The teeth cut and pull the material down through an anvil featuring a zigzag pattern. Often shredders are provided with a sizing screen to control the output particle size. These machines operate at low speed and with high torque.

The advantages of these shredders are their ability to accept a hopper of material, lower horsepower requirements, ability to operate unmanned, and generally quieter operation. Disadvantages include higher initial cost, more costly replacement parts, and, in the case of the ram-fed grinder, lower production capacities.

Many of the pieces of equipment listed above can be used outside of their mainstream design. Hammer mills can act as shredders on some materials, while jaw crushers can be used as lump breakers. Often a combination of machines is the best approach. Jaw crushers or shredders are often used as the initial stage, followed by hammer mills for finish grinding. A combination of machines may be more practical and economical than trying to get one machine to perform multiple tasks that do not suit its capabilities.

When looking to purchase a piece of size-reduction equipment, remember the adage, “there’s more than one way to skin a cat.” With a little homework, you can find the equipment that will not only work, but work optimally.

Thomas E. Warne is president, CEO, and co-owner of Schutte-Buffalo Hammermill LLC (Buffalo, NY), which manufacturers a wide range of size reduction equipment.