Attrition mills can operate in either batch or continuous mode and suit harder-to-grind material such as metal powders, metal carbides and glass frits. Their shaft speed runs from 75–500 rpm and media generally range in size from 5–13 mm. Feed material can be as coarse as 1/2 in., while end product size can be as fine as 2–3 microns if the mill operates in a batch mode.
Dry grind mills also are used to make dispersion strengthened metal (DSM). In this process (known as mechanical alloying or cold welding) the grinding media break the metals and additives into small particles first, and then beat them together to form agglomerates. Repeating the process evenly mixes and disperses the various metals to form the DSM.
Pigment makers also use these mills to develop color in pigments.
High-speed attrition mills rely on small (2–3-mm) media and operate at a much higher speed, generally from 400–2,000 rpm. Proprietary design features such as shaft/arm configuration and side discharge screens allow these mills to continuously produce fine powders, which are discharged by centrifugal force. However, the small media size used limits feed materials to 40 mesh and finer. The end products from these continuous mills generally are in the 2–5 micron range.
Dry grind mills can be used in conjunction with air classifiers or screeners to form a closed grinding process loop (Figure 5). By continuously classifying out fines and returning oversize material to the mill, such systems can very efficiently provide sharp particle-size-distribution grinds.
As a rule of thumb, dry grinding generally will achieve particle sizes of 3–5 microns. To mill to sizes below that range requires wet milling. Today, the trend clearly is to produce nanoparticles.
Wet grind processing can be done in batch, continuous or circulation modes. In recent years, many paint and mill manufacturers have focused much of their attention on a "new" type of "high circulation rate grinding" to achieve superior dispersions. In actuality, this type of grinding has been used for many years.
These units combine a grinding mill with a large holding tank equipped with both a high-speed disperser and a low-speed sweep blade. The entire contents of the holding tank pass through the milling chamber at least once every 7.5 minutes or about 8 times per hour. This high circulation rate results in a uniform dispersion, narrow particle-size distribution and faster grinding.
There are two types of high circulation mills — one uses 3–10-mm media to process material down to sizes of a few microns, the other uses 0.1-2-mm media to achieve sub-micron and nano-size products.
Choice of grinding media depends upon several factors, some of which are interrelated.
• Specific gravity. In general high-density media give better results. The media should be denser than the material to be ground. When grinding some slurries, media with higher density may be required to prevent floating.
• Initial feed size. Smaller media can't easily break up large particles.
• Final particle size. Smaller media are more efficient when ultrafine particles are desired.
• Hardness. The harder the media the better the grinding efficiency and, consequently, the longer the life.
• pH. Some strongly acidic or basic material may react with certain metallic media.
• Discoloration. Certain applications require, for instance, white material to remain white.
• Contamination. Material resulting from media wear shouldn't affect the product or should be removed by a magnetic separator, chemically or in a sintering process.
• Cost. Media that may be two-to-three times more expensive may wear better, sometimes lasting five-to-six times longer — and therefore may justify their extra cost in the long run.