A Grinding is a unit operation in which solid material breaks into smaller pieces. There are many types of grinding mills and many types of materials processed in them. Historically mills were powered by hand, working animal, wind, water but now they are powered by Electricity. When a solid matter exposed to any mechanical force that break the structure by overcoming the bonding forces.After grinding the state of the solid changes inits grain size and in its grain shape. Grinding mills can be classified as Ball mills,Rodmills,Autogenous mills, SAG mills,Pebblemills,High pressure grinding rolls,Towermills,Vertical shaft impacter mills.
When a material is to be milled there are certain characteristics which have to be taken into account. These include the following
• Softening and melting temperature
• Structure (e.g. close grained or cellular)
• Specific gravity
• Free moisture content
• Chemical stability
Rod mill is a rotating drum causes friction and attrition between steel rods and ore particles. The rods used in the mill, usually a high carbon steel, can vary in both the length and the diameter. However, the smaller the rods, the larger is the total surface area and hence, the greater the grinding efficiency. It is a another tumbling mill but having a large percentage of its volume (30-40%) loaded with steel rods. The rods are placed axially in the mill and are loose and free to move within the mill. The internal lining of the drum has a series of lifters which raise the rods and drop them at predetermined point. The mineral is fed in at one end with a maximum size about 25mm. The rods crush the rock as the charge passes through the mill it is reduced in size to approximately 2mm to 0.1mm.The mill can be fed from one end with the product removed from the other end or, alternatively, the mill can be from the both ends with the discharge at the center. The process can be wet or dry but more commonly carried out wet. Maximum rod length is about 6 to 7 meters; otherwise there is a risk of the rods bowing. The drum diameter is limited to 0.6 or 0.7 times the length of the mill.
Questions and Answers:
(1) What is grinding?
Ans: Grinding is a unit operation designed to break solid material into smaller pieces.
(2)What are the uses of Grinding?
Ans: Increase of the surface area of the solid, manufacturing of a solid with desired grain size, pulping of resources.
(3)What are the different types of grinding mills?
Ans: Ball mill, Rod mill, Autogenous mill, SAG mill, Pebble mill, High pressure grinding rolls, Tower mill, Vertical shaft impactor mill etc.,
(4) Mention some of the characteristics which have to be taken into account, while the material to be milled?
Ans: Hardness, Brittleness, specific gravity, Purity, Chemical stability, Homogenity., etc.,
(5)What is the approximate crushing size of the material in a Rod mill?
Ans: 2mm to 0.1mm.
Mineral processing Technology by B.A.Wills