Miller Magazine Issue 137 May 2021

61 COVER STORY MILLER / may 2021 equal or they may be platelike or needlelike. For compact grains, the largest dimension or apparent diameter is generally taken as the particle size. For particles that are plakelike or needlelike, two dimensions should be given to characterize their sizes. Energy is an important expense in milling. During milling, the particles of feed material are first distorted and strained. The work necessary to strain them is stored temporarily in the mate- rial like mechanical energy of stress. Then, as additional stress added, they are distorted beyond their ultimate strength and suddenly rupture into fragments, and a new surface is gener- ated. So, a unit area of material has a definite amount of sur- face energy, the creation of a new surface requires work that is supplied by the release of energy, the creation of new surface requires work that is supplied by the release of energy of stress when particle breaks. By conservation of energy, all energy of stress in excess of the new surface energy created must ap- pear as heat. Therefore, the temperature of the mill surface during comminution should be cooled or controlled to prevent overheating. Also, the ratio of the surface energy created by milling to the energy absorbed by the solid is the crushing effi- ciency. The surface energy created by fracture is small in com- parison with the total mechanical energy stored in the material at the time of rupture and most of the latter is converted into heat. Milling efficiencies are therefore low. They should contin- uously be measured and controlled. The energy absorbed by solid is less than that fed to the machine. Part of the total energy is used to overcome friction in the bearings and other parts of the machine, and the rest is available for milling. The ratio of the energy absorbed to the energy input is mechanical efficiency. In size reduction or milling, Rittinger, Kick and Bond equation can be used to make the calculation. As a result, the milling performance depends on a lot of param- eters. Therefore, a high knowledge is required to fix all parame- ters to the maximum level. In general, milling systems do not use high knowledge therefore a lot of loss in performance can occur. Figure 2. Milling performance