Mineral processing – Milling
Practical Action
Ball Mills
Description:
Ball mills are similar in concept to the rod mill but are charged with steel balls in place of the
rods. The mill consists of a cylindrical drum, sometimes tapered at one end, and usually has a
charge of steel balls (up to 40% by volume) ranging in size up to 125mm for larger mills.
Product size can be as small as 0.005mm, but product size is dependant upon the time the
charge spends in the grinding zone and therefore the reduction rate is a function of the
throughput. The lining material is of great importance as there is a significant amount of wear
taking place due to the action of the steel balls. The speed of rotation is optimum at about
75% of critical speed. Some mills are compartmentalised with each subsequent section
having a smaller ball size. The mineral can pass through to the proceeding section, but the
balls cannot. This ensures that the smaller particles are attacked by the smaller grinding
media.
Characteristics:
It is a versatile grinding mill and has a wide range of applications. The mill can vary in size
from small batch mills up to mills with outputs of hundreds of tonnes per hour. They are the
most widely used of all mills. Small hand operated ball mills are used in Bolivia for
preparation of ore, sand and gravel.
Iron ore
Limestone and lime
Cement
Gold ore
Table 6: Material Suitability for Ball Mills
Coal for combustion
Talc
Sand
Figure 3: A ball mill being used as part of a gold mining operation in Arequipa, Peru ©Practical Action
Roller Mills
Description:
There are two distinct types of roller mill. The first is a series of rollers which rotate around a
central axis within a drum. The reduction takes place between the rollers and the drum. The
second is where there are a series of fixed rollers and a rotating table. The milling takes place
between the rollers and the table. This type of mill is used for dry grinding only and accepts
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