Times are a changin’! You hear about it every day from the world around you: performance, efficiency, justification, ROI, etc. While these terms are nothing new, one thing is certain: Today’s companies are demanding a lot more from their processing equipment than they did previously. More specifically, companies are seeking competitive advantages through improved product characteristics, higher process efficiencies, and less energy usage. In terms of particle reduction, today’s roller mills achieve all of these objectives.
In many particle-reduction applications, the most important objective is to maximize particle uniformity. Take coffee, for example. To achieve a maximum yield, U.S. producers typically grind coffee to a size range of 16 × 40 U.S. mesh (1170 to 590 µm) with minimal fines (dust). In this case, fines are those particles below 40 mesh. After grinding, the coffee is not classified; rather, it proceeds directly to packaging. This type of streamlined process demands both precise grind uniformity and ultimate product consistency. If the grind size varies or additional fines are created, the coffee overextracts during the brewing process. Overextraction increases coffee bitterness (a characteristic that all coffee producers try to minimize).
In other particle-reduction applications, product classification is required to meet strict particle-size requirements. In such cases, fines are often thrown away, reprocessed, or used in secondary markets, resulting in lost money and wasted energy.
Because people and companies are increasing their awareness of process improvements, roller mill technology has grown across the board in the food, chemical, mineral, and pharmaceutical sectors. In each market segment, the story is similar: improve product characteristics, achieve higher process efficiencies, and use less energy.
In general, roller mills should not be used for particle reduction when the product is not friable or fragile and breakable, or when there is no value in maintaining a uniform particle size with minimal fines. Otherwise, roller mills provide better particle reduction results than any other grinding method, especially in applications in which the desired average particle sizes range from 100 to 1500 µm.
If an application meets these criteria, users must choose from three main roller mill design elements: roll cut (teeth) shape and size, roll speed ratio, and roll gap distance. Experience, lab testing, and individual application requirements determine the optimal roller mill configuration for any given application.
Roller mill technology has evolved and become much more refined over the past 50 years. Some may still think of roller technology in its crudest form as being composed of “crushers” that haphazardly pulverize friable materials down to a reduced size. But modern engineered roller mills are designed to produce “controlled explosions,” which achieve highly engineered particle reductions. Friable materials, or materials that can break, as opposed to materials that flatten under pressure, can be reduced with fewer fines and greater uniformity.
Roller mill technology has advantages over other grinding methods. For example, hammer mills grind using impact at high speeds, relying on a perforated sizing screen to control particle size. In contrast, roller mills typically create 50 to 75% fewer fines and improve particle uniformity by 50 to 100%.
Since hammer mills and other attrition milling methods involve multiple impacts, they typically require relatively higher energy use. Energy savings is a primary reason to choose roller mills over traditional particle-reduction methods. Because of their efficient reduction action, roller mill grinders produce 15 to 40% more material per hour at a given horsepower than comparable hammer mills.
Roller mill technology is not necessarily the best technology for every application. However, if a manufacturer works with friable materials and is looking for superior controlled reduction for distribution targets ranging from 100 to 1500 µm, roller mill technology is the best option. While traditional crushers and other imprecise methods literally gather dust, roller mills will continue to expand their role in this ever-changing world.
Scott Will is engineering manager for Modern Process Equipment Corp. (MPE), a manufacturer of food-, chemical-, and mineral-grinding equipment. With more than 10 years of equipment experience, Will has in-depth knowledge of size-reduction technology and its latest trends. A mechanical engineer, he received an MBA from Northwestern University in Evanston, IL.