The lining is an important feature in a mill. Lining refers to a layer that protects the inner surface of the mill’s rotating section, the mill shell. The lining protects the mill shell from wear and optimizes the movement of the load in the mill. The lining may consist of either entirely of bars, or plates with a lifter bar attached either to the plate itself or between the plates. A flexible surface made of, for example, rubber, is placed on top of the lining. The feed end liners of a rod mill should be slanted so that new material can fit in through the feed opening. The discharge end liners should be vertical, as the rods naturally travel toward the discharge end and the vertical liner keeps the ends of the rods in the same plane, regardless of the position of the rod. The end liners are usually smooth.

In rod mills, the lining pattern prevents the rods from sliding down along the edge so that they drop and crush the ore. In ball mills, the lining does not have to prevent the grinders from sliding, so the most important thing is wear-resistance. If the lifter bars in the rod mill are worn, the bar mass will start to slide, which increases the wear on the rest of the lining. The lifter bars used to wear faster than any other part of the liner, but modern liners are optimized so that all parts wear at the same rate. When the lining wears out, it needs to be replaced and for that, the mill must be shut down. The less this needs to be done, the better the efficiency of the mill, as a shut-down mill produces nothing.

Recent computer simulations have helped optimize mill lining even more. In the old model, the lining beams were placed densely. Today, larger spacing is preferred. Nowadays, the lining is made as prefabricated units so that it is quicker to replace and thus reduces the shutdown period of the mill.