Milling is defined as the method of taking large particles and inducing particle breakage through mechanical means. Often engineers struggle with the selection of the proper mill for the specific material of interest.  The biggest challenge in effective milling is matching the breaking action generated in the mill to the material to be milled. For example, using a jaw crusher in an attempt to reduce the size of rubber particles would be futile. The reason this fails is that rubber requires excessive strain to reach the ultimate failure point. The jaw crusher mill simply does not induce sufficient strain to cause the rubber to yield. However, placing rubber in a cutting mill with counter rotating blades will induce significant strain in the rubber particles and cause them to yield in shear (i.e. cut). Conversely, placing a brittle material in the jaw crusher would result in very effective breakage because the action in the jaw crusher allows the generation of large forces with little strain which is very compatible with the yield stress/strain behavior of brittle materials.

Some particles have a tendency to break in half or in thirds due to fracture. Other particles are sensitive to abrasion events, which result in the breaking off of small bits from the particle surface. Yet other particles require multiple stress/strain events to induce breakage by fatigue. Some particles can only be broken if subjected to great enough shear. Particle breakage tests exist to quantify the magnitude of breakage due to impact conditions, stress/strain conditions, and cutting events. Ideally, a prospective milling candidate material should be characterized by each type of particle breakage test to determine its propensity to fracture, abrade, or shear. A complete analysis, though not always required, would include exposing a bulk material to repeated breakage events of a given type (impact, cutting, or abrasion). The rate of breakage relative to an ideal material can then be computed to provide a milling fingerprint for the material sample to each particle breakage mechanism. In cases where such a detailed analysis becomes difficult, then a composite breakage test causing or inducing all of the breakage event types can be used, in conjunction with a population balance model, to determine the extent of fracture, fatigue, and attrition that exist in the material. In this instance, a single breakage test where particle size data is collected as a function of time may be enough to determine the sensitivity of a given material to key breakage mechanisms. The relative breakage numbers can be used to generate breakage mechanism (fracture, abrasion, and cutting) rankings between 0 and 100 for each material.    

Some mills produce just a few high-pressure stress/strain events during the milling action. Other mills induce sliding along surfaces, maximizing abrasion events. Yet other mills rely on impact to break or fracture particles. Each mill can be analyzed and then ranked in terms of the type of events that lead to particle breakage. For example, consider an air jet mill. A CFD or experimental analysis of the particular air jet mill may show that, at a given operational velocity, the jet mill induces sliding or glancing blows of particles 70% of the time, while 30% of the collisions with the particles or walls are direct impacts. The direct impacts will likely lead to fracture, while glancing blows will lead to abrasion. There are few events in an air jet mill that lead to shear. Thus, an air jet mill or any other mill can be ranked in terms of its propensity to fracture, abrade or cut. A simple algebraic distance analysis can be done to reduce the three rankings to a single number which represents the average deviation between the mill and material rankings. The mill-material combination with the lowest algebraic distance number is the best choice of mill for the given material. A similar analysis can be done to determine the effect of fatigue and correlate this to the number of repeated events at a given stress level in a particular mill. One can then select, with confidence, the mill with the greatest ability to effectively create the desired particles and increase process reliability and productivity.

Kerry Johanson, PhD, is chief operations officer at Material Flow Solutions (Gainesville, FL). He received his BS and doctoral degrees in chemical engineering from Brigham Young University. A consultant and educator for more than 30 years in the storage and flow of bulk solid materials, he has lectured worldwide at scientific conferences on the topic of powder flow in industrial applications. He holds five patents, has published more than 30 papers in peer reviewed journals, and developed a course for teaching powder flow and technology to graduate students at the University of Florida. Dr. Johanson holds PE licenses in both Florida and Utah and is an active member of AAPS, AIChE, ASME, and ASTM.