GENERAL INFORMATION FOR MILLS
Conventional mills most widely employed to grind metallic ores, industrial raw materials and coal - either wet or dry- are of two types as rod and ball mills.
Rod mills which are used in the first stage of grinding for products finer than 3mm are followed by ball mills that are used in the grinding of even finer products - for example those below 0.5mm. However, it would be more appropriate, in terms of circuit efficiency, to start primary grinding with ball mills in case the size of the final ground product is finer than 0.5mm.
It is possible to reduce the particle size of ores in mills down to 20-25 microns by using compression, impact, shear and attrition forces. Dominancy of forces changes depending on the type of grinding as to whether they are rod, ball, autogenous, pebble or semi-autogenous.
The calculation of parameters directly related to circuit efficiency -such as the diameter, length, rotation, motor power, shape and type of the linings- for a specific purpose definitely requires expertise. General information given below is to provide an idea on this topic. Although some empirical formulae are available, experience should also be required and blended with calculations in mill sizing and selection.
Empirical formulae given in this brochure are in nature of guideways to determine required mill power. Mills are to be designed to have long lasting efficiency and minimum maintenance requirements under heavy operating conditions.
The inner surfaces of mills are lined with abrasion resistant rubber. However, manganese liners are to be used in cases of inside temperatures higher than 70ºC and if there exist some chemical interactions. Prompt supply and quick and easy installation of liners are of paramount importance in terms of plant management. Tüfekçioğlu, therefore, is a reliable business partner being the committed producer to share its experience with the users since 1982.
Basic Calculations for Rod and Ball Mills
First step in determining mill size for desired fineness is to calculate energy consumption per ton of ore as kWh. For this purpose, an empirical relation known as “Bond Formula” is given below
Wi : Work index. (This is related to grindability of ores and it generally varies from 7-9 for soft or easy to grind ores and to
15-20 for tough ores)
P80 : Screen aperture through which 80% of ground ore (product) passes, micron
F80 : Screen aperture through which 80% of mill feed passes, micron
1.102: Conversion factor from short ton to metric ton
Above formula gives the required power at pinion shaft only. It is common practice that 10% is added to the calculated power at pinion shaft to cover possible losses in motor, gearbox and couplings, etc.
In dry grinding elastic/plastic deformations (cushioning effect) increase energy consumption by approximately 30%. For this reason, the calculated power is multiplied with 1.30.
Some Important Correction Factors:
Diameter: If mill diameter greater or smaller than 2.44m (8ft.) the following correction formula is applied:
D: Mill diameter from liner to liner, meter.
Fineness: If fineness of grind (P80) is smaller than 75 micron apply the
P80: Particle size, micron
There are some more additional correction factors may be applied depending on whether mills are run in an open or closed circuit, size reduction ratio, etc. In practice unless feed size are (F80) 25 and 10 mm for rod and ball mills respectively, above correction factors will suffice to determine the range of motor power.
W Values For Selected Ores
*Above Wi values may increase or decrease depending on geological processessuch as alteration, microtechtonics, metamorphism, etc.
First Charges of Rod and Ball
Unnecessarily greater rod or ball diameters significantly decrease grinding efficiency. For this reason correctly sized first charge is very important. The largest rod diameter to be used is calculated by the following formula:
Rod Charge: Ø(mm) =
Ø: The largest rod diameter, mm
B: Screen aperture through which 80% of feed passes.
ρ: Specific gravity of ore.
Cs: Critical mill speed as percentage
D : Rod mill diameter, meter
Wi : Work index (grindability) for rod milling.
Having calculated the largest rod diameter, other diameters in the charge are selected from the following table.