Investigation of the comminution process in pendular roller mills

• The gap size of pendular roller mills was measured for the first time.
• The power consumption of the mill motor is found to be independent of the product-PSD.
• A master curve for mill torque and gap size is obtained.
• The solids throughput is limited by air flow rate and not by the comminution process.
• A model for pendular roller mills relating throughput and airflow rate is introduced.

In the dry grinding industry the demand for very fine particle size distributions is increasing rapidly. At the same time a growing awareness for an economical management of our planet's resources has raised the demand for energy efficient processes.Roller mills in general and specifically pendular roller mills are best suited to meet these conflictive demands, since they can, up to this day, produce particle size distributions with a D97 < 10 μm while evidencing comparatively low power consumption values and high solids throughputs.The design of the pendular roller mill is thus far based on the experience gathered from industrially utilized mills. Due to the aforementioned development of roller mill applications this experience will no longer be sufficient for guaranteeing a successful scale-up process. In order to find valid scale-up rules for fine grinding pendular mills the operating behavior of such mills must first be fully understood.The grinding process in roller mills can be subdivided into the following steps: comminution between the rollers and the grinding ring, classification and solids transport between milling zone and classifier. In order to verify and improve the existing scale-up methods all process steps must be completely characterized.In order to characterize the comminution process a new measuring method was developed with which the milling gap between the rollers and the grinding ring and the rotation speed of the rollers can be determined simultaneously. This method was used on a pilot scale pendular roller mill during course and fine grinding processes to investigate the influence of process settings on the comminution conditions.As a result of measuring the milling gap, it is now possible for the first time to measure the relationships between parameters such as energy input, solids throughput, solids hold-up or pressure loss on the one hand and the milling gap on the other hand. These relationships can be used to establish a physical model of the comminution process, from which a scale-up rule for the comminution process can be derived.This publication will show and assess the relation between process parameters and the milling gap for course and fine grinding processes respectively.