More Questions and Answers

Q: How do I choose which type of coarse grinding mill is right for my application?

A: Rob Vorhees, Hosokawa Micron Powder Systems, says:

Coarse grinding mills for dry bulk materials operate based on one or more of four size reduction principles:

• In shearing, a particle is cut or cleaved in two with a sharp blade; this action is often required for reducing elastic, pliable, or fibrous materials.
• 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 particle; this action is used with materials that don’t fracture easily by compression or impact.
• In compression, a particle is pressed between two solid surfaces to reduce it; this action is best for reducing soft or friable materials to particles with a narrow size distribution and limited fines.
• In impact, a solid object moving at high velocity strikes a particle and breaks it; this action is used to achieve the finest possible particles, especially with hard crystalline and amorphous particles.

I will cover some common types of coarse grinding equipment. These machines are available from multiple manufacturers in various configurations and sizes to handle a wide range of applications.

Crushers. 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.

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.

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 nonhazardous 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.

Rotary granulators. A rotary granulator (sometimes called a rotary knife cutter) is typically used in recycling applications for reducing elastic, pliable, or fibrous bulk solids and films that are difficult to process in impact mills such as hammermills and pin mills. Materials commonly reduced in rotary granulators have a Mohs hardness of 3 or less and include nonferrous scrap, bark and wood scrap, and glass fiber scrap; gypsum board and roofing materials; and insulated copper assembly blades. Material is gravity-fed into the granulator’s top inlet, and the rotor assembly’s rotation causes the blades to shear the particles against the stator. Material discharges by gravity from below the stator to an outlet screen, and the screen retains oversize particles to control the final product’s particle size and limit fines generation.

The rotor assembly typically has low to medium (200- to 500-rpm) tip speeds, and the granulator is available in sizes from 10 to 650 horsepower. The unit can handle a wide throughput range, depending on the feed material, with the largest granulators handling up to about 13,000 lb/h. For high-capacity applications, the granulator can be equipped with a fan or blower that provides an airswept discharge for conveying reduced particles away from the discharge.

Conical screenmills and flake crushers. Conical screen mills and flake crushers are both screen mills. They’re used with friable and soft materials (primarily pharmaceutical active ingredients and excipients) that have a Mohs hardness up to 4 and reduce these materials to granules with narrow particle size distributions, thus improving the materials’ flow and solubility. While these mills have similar applications, their configurations and operating principles differ.

The conical screen mill (the most common version of a cone-type screen mill) primarily applies compression to reduce particles up to 20 millimeters to coarse particles and granules between 5 millimeters and 150 microns with minimal fines. The mill typically reduces pharmaceuticals after they’ve been compacted or before they’re moved to a tableting operation. The mill’s low energy input also allows it to handle heat-sensitive and friable materials.

The flake crusher also uses compression, but its lower rotor speed produces coarser particles — typically only down to 200-micron granules — than the conical screen mill. It also has low energy input, allowing the flake crusher to handle heat-sensitive and friable materials. The flake crusher is commonly used to reduce pharmaceutical materials that have been compressed into sheets or flakes in a roller compactor.

Attrition mills. An attrition mill uses a rubbing action in opposite parallel planes between a particle and a surface to reduce materials with a Mohs hardness of 3 or less to a narrow particle size range from less than 300 microns to about 2 millimeters. This mill is commonly used to reduce plastics. In in-house recycling applications, the mill fine-grinds PVC scrap (like precut pipe and electrical channels) to particles between 600 and 1,200 microns, and for rotation molding applications, the mill reduces PE granules to powders ranging from less than 300 to about 700 microns. The attrition mill can also reduce softer materials, including foamed plastics like expanded polystyrene, and difficult-to-grind foods such as hard, fibrous grains with shells or hulls.

Some selection advice. Before selecting a coarse size reduction machine for your application, you need to know some basics about your feed material and your final product’s requirements. These include:

• The feed material’s particle size.
• The feed material’s characteristics, such as particle shape, bulk density, cohesiveness, Mohs hardness, friability, and softening and melting points.
• The final product’s required particle size distribution.

Sharing this information with the equipment supplier will help the supplier guide you toward the coarse size reduction technology best suited to your application. Along the way, expect to run pilot-plant tests of your feed material in various mills. These tests will not only help narrow your equipment choices, but provide operating guidance to help you achieve top grinding performance once the mill is installed in your plant. With complete information about your feed, required final product, and other application details in hand, the supplier can ensure that the mill you choose will provide economical, trouble-free size reduction of your material over many years to come.