Rod milling in the size range utilized is more efficient than ball milling in that the desired product is obtained at a lower cost per ton. This is due to several reasons:

 

Grinding rods do not require cascading as do ball charges, thus enabling rod mills to be operated at lower peripheral speeds than ball mills.

 

Less void spaces exist in a rod charge tan in a ball charge. Therefore, these is more grinding contact between metal an ore per surface are of media in a rod mill.

 

 

Lower steel consumption per ton of product occurs in a rod mill than in a ball mill due to better contact between steel and ore. The action within the rod mill causes the energy of rods to be directed to the largest sized pieces of ore. Often in a ball mill the balls grind on one another or against ore already ground fine enough.

 

Rods are more suited than balls to breaking larger pieces of ore due to their much higher weight. (Energy is equal to hall the product of the mass times the velocity squared). Larger media are capable of transmitting moiré energy per collision.

 

Grinding rods can be maintained in good working condition because broken and worn rods can be removed through the large open discharge end. Often broken pieces of rods will float out with the pulp thus avoiding entanglements.

 

In a rod mill it is important that the rods stay essentially parallel to one another. If rods become misaligned, grinding action is lost and more importantly, rod entanglements will occur. For this reason, rod mills are designed with their length roughly 1.3 to 2.0 or more times their diameter. Rod mill size depends on the feed and product size, capacity, type of grinding circuit and the power required to achieve the grind. The limitation on rod mill length is in the ability to make rods that will remain straight enough not to tangle. The current practical limit on rod length is about 20 feet. The diameter of a rod must increase as the length increases if the rods are not to sag appreciably under their own weight.