The usual objective of reducing the size of run-of mine ore pieces is to separate the mineral of interest contained in the ore body from associated gangue minerals. As crushing only does not generally liberate a mineral, further size reduction is usually required. This is achieved by grinding the crushed ore in tubular mills or devices such as pan mills or roller-grinder mills. In tubular mills, a grinding media such as steel balls, rods or hard pebbles imparts the forces required for size reduction. On rotating a mill charged with rocks and grinding media, the entire charge rises against the perimeter of the mill in the direction of motion. On reaching a certain height, part of the charge cascades and falls to the bottom of the mill; the other part tends to slip down but soon travels in the direction of motion of the mill. During this process, the media drops repeatedly onto the rock breaking down its size. Some size reduction also takes place due to abrasive forces. As a result of the combined action of repeated impact and abrasion over time, size reduction takes place and given sufficient time the mineral of interest is liberated.
Some tubular mills are specially shaped mills, such as the Hardinge Mill, where only the central portion is cylindrical and the ends are shaped like the frustum of a cone. Straight cylindrical mills, however, are the more common. The grinding medium generally used is in the form of balls, rods or cylindrical media called cylpebs. Both steel and ceramic balls are in use depending on the hardness of the rock. For soft ores, pebbles are added or simply autogenously ground with no medium. Both wet and dry grinding is common. Figure 1 illustrates the grinding action inside a tubular mill.