The chemical process industries (CPI) include many applications of impact comminution to process solid matter, such as minerals, chemical products, foods and pharmaceutical products. This one-page reference discusses conditions for particle fracture and feed characteristics in industrial milling equipment.
Particle stress
Comminution requires applying a level of stress to the particles that is sufficient to fracture them. This stress can be generated by pressure (compression between two solid components) or impact (contact with impact plates or other particles; Figure 1).
FIGURE 1. Fragmentation of glass beads is achieved by either impact or pressure (F = force; a = contact area; b = cracks; c = fines)
Impact comminution requires particles to exhibit brittle-elastic behavior. This refers to whether the deformation of the sample is initially proportional to the applied stress, causing the breakage to occur suddenly. In impact comminution, the kinetic energy of the particles generates the requisite degree of deformation. In the linear range, the deformation is elastic and reversible. As soon as higher stresses occur, the material strength is exceeded locally and cracks are triggered. The cracks grow extremely quickly and lead to destruction of the particle.
Single-particle impact tests suggest the following: A) a minimum fracture energy must be applied to the particles; B) the probability of fracture is dependent on the kinetic energy; and C) the resultant particle-size distribution is dependent on the properties of the material being processed [1].
Theory states that increasing particle speed by a factor of 5.6 would be necessary to double the stress in the particles. Impact speed is the only parameter for which it is possible to effectively change the stress condition in a particle, which is why impact mills usually run at high peripheral speeds (up to 150 m/s). Two rotors rotating in opposite directions, increases relative speeds. In jet mills, compressed gas is expanded in Laval nozzles. At common gas pressures and temperatures, the exit velocity of the gas jets is around 500 m/s [1].
A single fracture event leads to coarse fragments, but relatively little fine product. Because of this, the aim is to stress the particles multiple times in mills in order to obtain a fine end product. One can assume that a feed material is subjected to between 5 and 20 impact events in simple rotor impact mills [1]. For jet mills, up to 100 stressing events can be necessary to grind the feed material to high fineness values if it is of poor grindability. The particle-size distribution of the ground product is a function of the mill parameters, stressing frequency and stressing intensity, as well as the feed-material characteristics.
Feed material requirements
Economically viable comminution requires feed material with certain requirements, as follows.
Feed particle size. In fine impact mills, there is a maximum feed size that must not be exceeded if the mill is to remain undamaged by large lumps in view of the high peripheral speed. A particle size of 5 to 10 mm can be assumed as the maximum, whereby the faster the rotor rotates, the finer the feed material should be. Tramp material must be removed from the feed material.
Contamination. Mechanical impact grinding can be used on materials with Moh’s hardness of up to 3–4. But even small amounts of harder components in the feed material can lead to an uneconomical degree of wear. For example, when grinding soft natural gypsum, a few percent of quartz in the feed will make it necessary to exchange the wear plates in the mill frequently. The same or similar phenomenon applies to vegetable products that are contaminated with sand.
Heat-sensitivity. Mechanical impact mills can create issues with heat-sensitive feed material. Softening or melting points should be above 70°C if the material is to be ground in an impact mill, because the energy of the motor is converted almost completely into heat, and mills heat up to 50–60°C under full load. Heat-sensitive products can be processed only by cooling the feed material or the mill air (or both), which ultimately adds costs.
Dryness. Liquid components in the feed can be disruptive. The product should be dry, with the maximum moisture content of only a few percent. Liquid released during comminution frequently leads to buildup.n
Editor’s Note: The content for this column was adapted from the following articles: 1. Furchner, B., Fine Grinding with Impact Mills, Chem. Eng., August 2009; and 2. Lee, I., Sirota, E. and Moment, A., Milling in the Pharmaceutical Industry, Chem. Eng., October 2022, pp. 27–33.