Grinding is usually performed wet, although in certain applications dry grinding is used. When the mill is rotated, the mixture of medium, ore, and water, known as the mill charge, is intimately mixed; the medium comminuting the particles depends on the speed of rotation of the mill and the shell liner structure. Generally, grinding in mineral processing is a continuous process, material being fed at a controlled rate from storage bins into one end of the mill and overflowing at the other end after a suitable dwell time. Control of product size is exercised by the type of medium used, the speed of rotation of the mill, the nature of the ore feed, and the type of circuit used.
Due to the rotation and friction of the mill shell, the grinding medium is lifted along the rising side of the mill until a position of dynamic equilibrium is reached, when the bodies cascade and cataract down the free surface of the other bodies, about a dead zone where little movement occurs, down to the toe of the mill charge.