Grinding time is related to media diameter and agitator speed via:
T = KD2/N1/2
where T is the grinding time to reach a certain median particle size, K is a constant that depends upon the material being processed, the type of media and the particular mill being used, D is the diameter of the media, and N is the shaft rpm.
This equation shows that total grinding time is directly proportional to media or ball diameter and inversely proportional to the square root of shaft speed. Thus, increasing the media size increases the grinding time and vice versa.
Different types of mills suit different processing requirements. There are several key factors to consider when selecting grinding mill types and media.
Feed material. The nature of the material to be reduced is of utmost importance. Questions to ponder include:
1. Is the material friable, fibrous, heat sensitive or sticky?
2. What is the shape of the feed particle?
3. What is the hardness of the material and is it very abrasive?
4. What is the feed size and how fine a size is desired?
Generally the coarser the feed material the larger and denser the grinding media should be — because larger and heavier media generate greater impact forces. Final particle size also should be considered. The finer the required end particle size the smaller the grinding media should be — because small media make greater surface area available to perform grinding.
Some manufacturing processes benefit from combining complementary milling technologies. Such a two-stage process maximizes the efficiency of both. One example is using a stirred ball mill such as an Attritor to perform the first stage of grinding and then achieving the final polishing grind with a small media mill.
Material of construction is another crucial area to consider during mill and media selection. When processing chemicals it's often necessary for mill contact parts to be as inert and contamination-free as possible. In these instances, mill contact parts and grinding media can be manufactured from various types of stainless steel or ceramics. Some of the parts also can be lined or coated with different types of polymers.
Mills can be tailored to specific duties (Figure 3). Some feed materials pose special requirements. Those that easily oxidize must be milled under a blanket of an inert gas like argon or nitrogen. Materials such as plastics, etc., that aren't very friable require cryogenic milling (Figure 4). Sometimes this can be done by blanketing the material with liquid nitrogen; other times milling must be done in a liquid nitrogen slurry. When milling with solvents such as acetone that evaporate very easily, a water-cooled cover on the mill offers benefits.