Praticle Size Reduction in the Ball Mill

Abstract

The size distribution produced by the ball milling of various crystalline and non-crystalline materials, showed that initially there was a fairly even distribution over the size range up to 355μm. However, as milling proceeded two distribution modes developed; one at about 90μm (the persistent mode) and one at about 250μm (the transitory mode). Unlike the work of Heywood (1) on coal, further grinding did not produce a gradual elimination of the coarse mode with corresponding increase in the persistent mode at 90μm. Less than 2%w/w of material was always found between the 170-180μm size, indicating that this was a critical dimension in the grinding process. A linear relationship was found between sample weight and milling time required to grind 50% of the sample down to 150μm. The slopes of these lines were steep for rock salt and sucrose (crystalline materials) indicating that milling was rapid, whilst with acacia the rate was half as fast and slower still for the other two non-crystalline materials, cinnamon bark and gentian root. In terms of milling efficiency defined as the amount of material milled to 150μm per minute, it was found that there was no optimum load size; doubling the sample size effectively doubled the milling time. However, the most critical factor affecting the milling process, was the weight and nature of the grinding medium. It was found that heavy porcelain was more efficient than glass spheres and that the optimum weight was 401.8g; the weight required to approximately half fill the ball mill.