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Feature

Using crushers for coarse grinding
Rob Voorhees, Hosokawa Micron Powder Systemsphoto

Size reduction equipment for dry bulk materials can yield coarse, medium, fine, or ultrafine particles. In coarse grinding, the particles produced are usually in the range of 150 microns (0.75 millimeter) to 0.5 inch (12.70 millimeters) or larger.

Coarse grinding mills for dry bulk materials operate based on one or more of four size reduction principles: shearing, attrition, compression, and impact. In shearing, a particle is cut or cleaved in two with a sharp blade. In attrition (also called autogenous grinding), a rubbing action in opposite parallel planes between two particles or between a particle and a surface abrades each. In compression, a particle is pressed between two solid surfaces to reduce it. In impact, a solid object moving at high velocity strikes a particle and breaks it. This article will discuss crushers, which employ the final kind of grinding (impact) to achieve the finest possible particles, especially with hard crystalline and amorphous particles.

Crushers are available in many configurations and sizes to reduce a range of materials to coarse particles. Common types include horizontal-shaft impact crushers, which can have one or two rotating shafts; another common variation is the jaw crusher. We'll focus on the horizontal shaft impact crusher with two shafts.

The horizontal-shaft impact crusher with two shafts typically reduces materials before a subsequent process stage, usually further size reduction. Common applications are reducing up to 10-inch chunks or lumps of material with a Mohs hardness of up to 3, including caked powder and filter press cake, resin, wax, ammonium nitrate, pigments, and other materials. By applying shearing and compression forces, the crusher typically reduces the material to 0.5-inch or larger particles.

In a typical horizontal-shaft impact crusher, as seen in the image at the top of this page, two counter-rotating shafts are mounted with hook-, star-, teeth-, or cam-type tools. Material is gravity-fed between the shafts, which rotate at low speeds up to about 120 rpm. As the material is drawn down between the rotating shafts, the tools shear and compress the particles. The reduced particles discharge by gravity through an outlet screen below the shafts, which retains oversize particles to control the final product's particle size.

The horizontal-shaft impact crusher can handle large throughputs. For instance, the relatively small unit seen above, with a feed inlet of 12 by 18 inches, can reduce 8- inch resin blocks to 2-inch particles at 9,000 lb/h, 4-inch wax blocks to 0.75-inch particles at 5,000 lb/h, or 3- to 4- inch pigment blocks to 0.5-inch particles at 10,000 lb/h. These crushers typically come in models ranging from 0.5 to 10 horsepower.

An advantage of the horizontal-shaft impact crusher is that, unlike some other mills, it doesn't require ancillary equipment. Because gravity pulls material through its crushing zone, the crusher doesn't require a fan or blower at the discharge. The crusher produces few fines, so it doesn’t require a dust collector when processing non-hazardous materials. Because of the typically large feed particle size, in many applications an operator can manually feed the crusher without sacrificing crushing performance or throughput, eliminating the need for a feeder.

For more information on coarse grinding and the various mills available, you can read the article "Coarse grinding: An equipment overview" by Rob Voorhees, Hosokawa Micron Powder Systems. You can also find articles listed under "Size Reduction" in PBE's Article Index.

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