The document discusses the ball mill, which is a type of grinder used to grind materials into fine powder. It works on the principle of impact and attrition, where balls drop from near the top of the shell as it rotates to grind materials placed inside. A ball mill consists of a hollow cylindrical shell that rotates about its axis, with balls occupying 30-50% of the volume to facilitate grinding as the shell rotates. Applications of ball mills include grinding metallic powders, carbon, coal, cement, fertilizers and other materials.