• 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.
For attrition milling, media size ranges from 1/8 in. to 3/8 in., with smaller media generally resulting in faster particle reduction — because for a given volume there'll be more impact and surface contact. As media becomes smaller than 1/8 in., mass is significantly reduced, resulting in less impact force and thus longer grind times. When ultrafine grinding isn't needed, larger diameter media may prove to be faster and more efficient because of their greater mass. Here is a summary of the advantages and limitations of attrition milling:
Attrition Milling in Perspective
1. Fast, efficient and reliable fine grinding
2. Versatility of the grinding process
3. Low power consumption
4. Mills come jacketed for heating or cooling
5. Easy and safe to operate with minimal operator attention
6. Low maintenance
7. Compact design requiring small plant area
1. Maximum feed material size of 13 mm generally
2. Heat generation from dry milling
3. Multiple mills likely needed for large quantity production
Keep These Hints in Mind
Here're a few practical pointers to help you succeed with attrition milling. First, the smaller the feed size and the more uniform the feed size the higher the efficiency. Beyond that, the type of milling raises a number of specific issues.
Dry Milling —
1. Minimize moisture content of feed material; high moisture content (over 2%) can cause material to cake inside the milling chamber.
2. Continously add grinding aids or additives, whether in powder or liquid form, while size reduction is taking place. Most dry milling processes require such materials. Their function is to do one or more of the following:
a. minimize the effects of moisture inherent to the material being processed;
b. change the electrical charge on the surface of particles;
c. reduce the negative effects of static charge that may develop;
d. function as a lubricant between particles; and
e. act as a partitioning agent between particles to prevent agglomeration.
1. Avoid over-grinding material that's already smaller than desired size. Remove this fine material as soon as possible by some form of classification. This will increase process throughput rate and efficiency.
Wet Milling —
1. If needed, use wetting agents or surfactants. They serve several purposes:
a. helping to neutralize electrostatic charge;
b. lowering surface tension; and
c. assisting in improving the solution's rheological properties.
1. Ensure material is properly dispersed before adding to the mill. This will help to prevent agglomeration and clogging of the discharge screen.
2. Don't choke the mill by starting with solids content that's higher than the mill can process. A rule of thumb is to start with 50% solids and adjust as needed. Remember that as particle size is reduced surface area and viscosity will increase, which may require lowering the percent solids.
Keep in mind that no single mill will grind large particles to a very fine size efficiently by repeating or prolonging the process in the same unit. A specific type of mill (with fixed media, operating conditions or equipment parameters) is most efficient in grinding a particular material with a certain feed-size range. A ratio of feed size to desired particle size of greater than 100:1 requires a two-step milling process. The first step should rely on larger media to reduce particle size to a level that can be handled in step two with smaller media. Whenever possible run a laboratory test at the mill manufacturer's facility. This will demonstrate the feasibility of the milling system and provide valuable scaleup information for larger throughputs.
ROBERT E. SCHILLING, P.E., is national sales manager for Union Process Inc., Akron, Ohio. E-mail him at firstname.lastname@example.org.
1. Moir, D. N., "Size Reduction," p. 54, Chem. Eng. (Apr. 16, 1984).