Methods for Increasing the Steepness of the Particle Size Distribution in the Grinding of Chemicals and Pharmaceuticals

Publication Reference: 
Author Last Name: 
Richard R Klimpel
Report Type: 
Research Area: 
Size Reduction
Publication Year: 
Publication Month: 
United States

Executive Summary

This paper summarizes a variety of this author’s industrial experiences with various conditions in size reduction processes which can either positively or negatively influence the narrowness of the product size distributions that can be produced. There are a relatively large number of operating factors, which if not properly selected, can give non first order breakage which unnecessarily broadens the final product size distributions produced by most industrial grinding devices. Key among these types of factors are dry powder aggregation. slurries that exhibit a rheological yield value, mill underfilling or overfilling, non-optimal rotating speeds for certain types of mills, media sizes that are too small for the material size being ground, and performing too large a size reduction ratio with a single grinding device. It is important in any size reduction operation. that these potentially negative factors be evaluated (checked) so as to give their best performance. In other word, the best place to start in narrowing particle size distributions at the industrial level is to avoid using non optimal operating conditions.

On the other hand, there are a number of factors which, when aggressively optimized, can give steeper (narrower) size distributions in a more proactive manner, Chief among these factors are the proper selection of staged grinding devices, the incorporation of appropriate classification, and, when possible, deliberate material selection (including modification in some cases) so as to promote the ability to produce steeper size distribution. The typical response of closed circuits involving classifiers are described in some detail. Also. a useful limiting case analysis method is presented which indicates the most narrow possible size distribution that can be produced with any material when all conditions are ideal, From an experimental viewpoint, the author also strongly recommends and describes a standard test sequence to be routinely run on each new material that an engineer might have to work with in a size reduction process. This rather minimal level of simple laboratory testing can give information which really helps to signal what types of operating responses might be possible on larger equipment with the material in hand.