HPGRs consist of a pair of horizontal cylindrical rollers through which material is passed. The two rollers rotate in opposite directions, "nipping" and crushing material between them. A similar type of intermediate crusher is the edge runner, which consists of a circular pan with two or more heavy wheels known as mullers rotating within it; material to be crushed is shoved underneath the wheels using attached plow blades.
Ball grinding mills have been in use since the late 1800's. They are rotating/tumbling mills in which comminution (size reduction) takes place with the aid of metallic balls. Since balls have a larger surface area per unit weight than rods, they are better suited for fine grinding. The length of the cylinder is typically 1 to 2 times longer than the diameter. Greater than 2 times the diameter is often called a Tube Mill. Ball mills typically have a ball charge of 40 to 45% with size reduction being achieved by a combination of impact, attrition and abrasion forces. Ore and balls are swept up one side of the mill and at a certain point it falls back to impact the toe of the charge (bottom of mill). Ore particles and balls in the body of the charge slide when moving to different heights and are subjected to attrition and abrasion forces.