A rod mill is an ore grinding mechanism that uses several loose steel rods inside a rotating drum to extract metal.
A rod mill is an ore grinding mechanism that uses several loose steel rods within a rotating drum to provide its friction or grinding action. A charge of ore is added to the drum and as it rotates, the friction between the rotating rods breaks the ore into finer particles. Although similar in operation, a rod mill is many times more efficient than a ball mill as it requires lower rotational speeds and less steel to achieve the same results. However, it is limited to maximum rod and drum lengths of approximately 6 meters (20 feet) and is generally only used for wet milling processes. The rod mill also tends to suffer from accelerated barrel and elevator liner wear due to the increased weight of the rods.
Mills of various types have been used for centuries to break solids or coarse particles into finer finished products. From the humble mortar and pestle, through mills powered by animals, wind and water to the giant electrically powered versions common in modern industrial applications, they all share a common characteristic: mechanical friction or grinding. All types of mills use a grinding process of one description or another to gradually reduce the size of the initial charge of material. In older mills, for example, this action was achieved by placing coarse material between two millstones and turning them against each other to produce a finer end product.
Modern rotary mills apply the same principle, rotating the loose grinding elements in a closed drum to which the filler material is added. Common examples are rod mills and ball mills, both of which are rotary drum types that rely on internal grinding agents to perform their grinding action. Unlike the ball mill, which uses a large number of hardened steel balls to impart the grinding action, the rod mill uses steel rods placed inside the drum and parallel to its axis. When the drum rotates, these rods roll inside it, crushing the feed material between them.
The rod mill is generally more efficient than the ball mill because of its more effective cascading action and the larger bearing surface offered by the rods. This means it can operate at lower speeds and with less grinding agents and producing less unwanted sludge by-product. Rod mills, however, require more attention during operation to avoid rod entanglements and are generally ineffective in dry grinding operations. They are also limited to a maximum rod length of approximately 6 meters (20 feet), which means they are generally smaller than ball mills. Bar mills also exhibit more liner and elevator wear than other types of mills due to the comparatively high rod weights.