Steel rods are used as grinders in rod mills. The standard diameter of a rod is 3″ (7.62 cm). The rods are nominally 12’ (365.76 cm) long but actually always 10–20 cm shorter than the inside length of the mill. A 4″ diameter rod and a longer mill are used with difficult-to-grind ores.

The rods are usually made of high-carbon steel. If the rods are too soft they bend instead break in the mill as they wear, causing them to get stuck in the mill.

In rod mills, the lining plays an important role. The lining and rotation speed control the movement of the load. The lining is designed to lift the rods along the outer casing and then drop them onto the ore, crushing it.

Rod mills were put into operation at the old concentration plant in 1939 when production at the Mökkivaara shaft began.