Effect of undersize misplacement on product size distribution of Bond’s ball mill test

Abstract

The standard Bond ball mill grindability test determines the ore grindability (g rev−1) and work index on an ore which gives the ball mill power consumption. The test is carried out on a standard sample and the number of revolutions per minute (rev min−1) required for the next cycle is determined after screening out and weighing the undersize fraction from a screen of a mesh the same size as the intended product passing size. The standard procedure assumes 100% efficiency in screening out the undersize fraction. However in plant grinding and classification circuits especially those based on dry grinding and classification, the efficiency of classifiers will be less than 100%. The misplacement of undersize particles in the oversize recirculating load is a practical reality which cannot be avoided. The aim of this study was to examine the effect of misplaced undersize particles on the P80 of product. The P80 values obtained from the standard Bond ball mill procedure and those of an inefficient ball mill grinding – classifier circuit were compared. The P80 values obtained with an inefficient classification were lower than the values obtained from the standard Bond ball mill procedure. The relationship between standard P80 values and those obtained with inefficient classification can be used as a guide to adjust operating parameters such as mill load and mill speed to produce the desired product size.