Particle size reduction varies widely from shredding to fine grind and ultra fine grinding of different materials. To accomplish various particle size requirements, there are different types of size reduction or grinding systems that can be employed.
Selecting the right grinding mill for the application depends on several factors. Typically, the two primary concerns in a fine grind situation are temperature and hardness limitations. Grinding systems are suited to some tasks and material more than others. As such, there is no single grinding system which can do it all; instead the capacities and limitations of each system in terms of the materials it can process and the fine grind range it can manage must be evaluated.
There is no single grinding system which can do it all; instead the capacities and limitations of each system in terms of the materials it can process and the fine grind range it can manage must be evaluated.
Choosing the appropriate system is based on the material itself, particle size requirement, heat and moisture sensitivity, abrasiveness, etc. Here are three major categories of fine grind systems, each with their unique characteristics and uses:
Air Classifying Mill
Being an air-swept grinding mill, it largely relies on the integral air-classification system as well as high-speed impact for processing materials. The system has an independent classifier wheel wherein speed and air flow can be adjusted to control and change particle size, from coarse to fine grind. The particle size which is typically processed by this mill ranges from 140-400 mesh. Its performance can be further enhanced by using heated or chilled air.
High Speed Impact Mills
There are many variations included in this type such as hammer mills, turbo mills, cage mills, pin mills, and more. They are used for grinding materials from coarse to fine products. They rely on high rotating speed and integral retaining screen to control particle size of materials. A good variation developed is the counter-rotating pin mill that has a two rotor assembly, spinning in opposite directions. This mill works on the principle of a much higher impact action or tip speed, resulting in the ability to process fine grind materials.
Attrition or Disc Mills
This type of milling machine is usually used in the rotational molding industry for reducing polyethylene to its powder form. The mills are configured with one rotating and one stationary disc. Their grinding surface have machined teeth. In order to change the final particle size, the distance between the discs is adjusted, as well as the shape, depth and number of the grinding teeth. The grinding process is more of a cutting action or attrition of the disc mills. In a large production system, double grinding chambers of hundreds of horsepower are utilized to achieve fine grind results.