Ball mills are also classified by the nature of the discharge. They may be simple trunnion over-flow mills, operated in open or closed circuit, or grate discharge (low-level discharge) mills. The latter type is fitted with discharge grates between the cylindrical mill body and the discharge trunnion. The pulp can flow freely through the openings in the grate and is then lifted up to the level of the discharge trunnion. These mills have a lower pulp level than overflow mills, thus reducing the dwell time of particles in the mill. Very little over grinding takes place and the product contains a large fraction of coarse material, which is returned to the mill by some form of classifying device. Closed-circuit grinding, with high circulating loads, produces a closely sized end product and a high output per unit volume compared with open circuit grinding. Grate discharge mills usually take a coarser feed than overflow mills and are not required to grind so finely, the main reason being that with many small balls forming the charge the grate open area plugs very quickly. The trunnion overflow mill is the simplest to operate and is used for most ball mill applications, especially for fine grinding and regrinding. Energy consumption is said to be about 15% less than that of a grate discharge mill of the same size, although the grinding efficiencies of the two mills are the same.