Rod mill equipment is very similar to ball mills, except they use long rods for grinding media. Those rods grind the ore material by tumbling within the mill, similar to the grinding balls in a ball mill. To prevent the conditions that lead to rod charge tangling, the length to diameter ratio is maintained at 1.4 to 1.6. Rod mill machines accept feed up to about 50 mm and produce a product in the size range of 3000 to 270 mm (usually –4 to –35 mesh). Grinding action is by line contact between the rods extending the length of the rod mill.
Of the three main types of rod mill, overflow, end peripheral discharge, and center peripheral discharge only the overflow rod mill is in common usage. Wet grinding rod mills are normally used in the mineral processing industry. Dry grinding is used in some areas, however, it is confronted with problems and should be avoided except where absolutely necessary. Rod mills operate at a slower speed than ball mill types of equipment since the rods are rolled and not cascaded. For an equivalent grind, a rod mill uses less steel than a ball mill because of the lower speed and better contact between the media and ore. The rod charge must be maintained in good working condition and broken and worn rods must be removed. Rod mills usually require greater operator attention.
Rod mills normally carry 35 to 65% rod charge by the volume. The limits on charge level are keeping the feed end trunnion open so that feed will get into the mill, and keeping the rod charge low so rods will not work their way into discharge openings where they can cause rod tangling.
