Comminution features in an impact hammer mill

A systematic study on the comminution of a soft (coal) and a hard (iron ore) material is carried out to investigate the bulk comminution features of the hammer mill. The effect of the feed rate and the rotor speed on the mill efficiency in terms of reduction ratio, energy expenditure, fines produced and median size is investigated. High reduction ratios are obtained under all conditions. The hammer mill products are observed to exhibit Rosin–Rammler type size distribution and are invariably self preserving. A high rotor speed with a low to moderate feed rate is found to give better performance of the mill. An exponential dependence of the median size on the quantity of fines generated is established for the mill product. The data indicated that the reduction ratio and the specific energy expenditure are linearly related, the slope of which is inversely related to the Bond Index of the material. The energy expenditure goes into creating new surfaces and overcoming the elastic strain energy of the cracked surface. The quantity of fines generated in the mill can reflect the degree of attrition in the mill which is an indirect measure of the mill wear. It is established that good performance of the mill can be achieved even for the hard materials by keeping the wear at a low level. It is suggested that an impact hammer mill can be used for the comminution of harder materials under controlled conditions.