Rod Mills can accept feed up to approximately 50 mm and are generally selected to grind products in the 4 mm to 0.8 mm range. Grinding action is by line contact between rods, which extend the length of the mill. Rod mills are usually operated in open circuit. Most rod mill applications involve wet grinding where materials are reduced in size from crusher product size to a size suitable for ball mill feed. To avoid breakage or tangling of rods the L/D ratio of rod mills is kept within the limits of 1.4:1 to 1.6:1. The reduction ratio of a rod mill for efficient grinding is usually limited to the range of 1:15 to 1:20. MBE supplies the three most common rod mill types, overflow, end peripheral and centre peripheral discharge.
An overflow mill is constructed with a feed opening at one end and a discharge opening at the other, both on the axis of the mill. Material leaves the mill by overflowing at the discharge end. An overflow mill is used for producing the finer end of the product range.
End Peripheral Discharge (EPD)
The end peripheral discharge mill’s steeper gradient produces a shorter retention time and thus a coarser product than the overflow type. Product is discharged through port openings equally spaced around the periphery of the shell rather than at the discharge end. End peripheral discharge is used to good advantage in damp grinding of coke on steelwork sinter plants.
Centre Peripheral Discharge (CPD)
Feed is from both ends and is discharged at the centre through discharge ports. The fact that the particles only travel half the mill length makes the CPD rod mill an outstanding unit for coarse grinding and rapid removal of material.
Ball Mills accept feed that ranges in size from 80% passing 20mm to fine feeds in regrind operations. Products may be as coarse as 0.5mm or as fine as 80% of 40 microns. In a ball mill the length may be less than, equal to, or greater than its diameter. A short length mill has a lower retention time and produces a coarser product. Longer mills have a longer retention practice and generally utilize closed circuit operation. Most ball mills operate with a reduction ration of 1:20 to 1:200.
This mill type embodies the principle of stoop pulp gradient, quick discharge operation. A grate and pulp lifter assembly at the discharge end of the mill enables a ball charge of 45%. The quick removal of ground material reduces over grinding, especially when operated in closed circuit with a classifier.
As the name implies, material is discharged from an overflow mill by overflowing the discharge opening. The overflow mill is simple to operate and maintain and is ideal for producing fine products such as in regrind operations