Choose the Right Grinding Mill

Consider the feed material's nature and the milling's objective.

By Robert E. Schilling, Union Process Inc.

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In most industries size reduction falls into two general categories: crushing and milling. Crushing typically means reducing large chunks to sizes about 1/2 in. to 3/4 in. in diameter or smaller. Milling usually means reducing material to sizes in the low micron or even nano-size range. Crushing mostly is done dry while attrition milling may be done wet or dry [1].

In this article we'll discuss attrition milling technologies as they relate to chemical processing applications. Many chemical operations frequently require finer materials; media milling is one common way to achieve the desired results.

Media milling technology plays a significant role in three major areas of chemical processing:
1. particle size reduction of chemicals;
2. mixing and milling of several chemicals to form new chemical compounds; and
3. activation or liberation of chemical raw materials.

Proper selection of media milling equipment is vital for success in all three areas.

There are many different types of grinding mills (Table 1). Some devices, such as ball mills, are more suitable for coarse materials. These mills use "large" media, ranging in size from 20 mm in diameter and up, to produce material from about ten microns to mesh sizes. Attrition mills are more appropriate for "mid-range" size particles. Such mills utilize 3–10-mm media to produce material ranging in size from approximately 1 to 10 microns. The first "small media" mill was introduced about 60 years ago. It was named a "sand mill" because it used 1/64–1/8 in. Ottawa silica from Canada. The latest advancements in milling technology target applications that require ultrafine grinding. These high-performance small media mills produce sub-micron particle sizes by employing beads ranging in size from 0.1 to 1 mm.

Comparison of Grinding Mills
Type of Mill
Media Size, in.
Tip Speed, ft./min.
Ball mill
1/2 and larger
Attrition mill
1/8 – 3/8
600 – 1,000
Sand mill
1/64 – 1/8
2,000– 3,000
Small media mill
0.1 mm – 3 mm
1,000 – 3,000

Attrition milling is simple and effective. Feed material is placed in a stationary tank with the grinding media. A rotating shaft with arms or discs then agitates the material and media. Both impact and shearing action result in size reduction as well as homogeneous particle dispersion with very little wear on the tank walls. These efficient forces (Figure 1) must be present for the most effective grinding action.

The configuration of the attrition mill's agitator system causes constant motion of material around the tank. The area of greatest media agitation is located approximately two-thirds the radius from the center shaft (Figure 2). In production-sized units for wet milling, a pumping circulation system augments the movement. As can be seen in Figure 2, grinding doesn't take place against the tank walls. There's actually very little wear on the walls, which leads to longer vessel service life. In addition, it means the tank walls can be thinner, thus enhancing heat transfer and temperature control.

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