Tumbling mills action

Action in tumbling mills. The load of balls in a ball or tube mill is normally such that when the mill is stopped, the balls occupy about one-half the volume of the mill. The void fraction in the mass of balls, when at rest, is typically 0.40. The grinding may be done with dry solids, but more commonly the feed is a suspension of the particles in water. This increases both the capacity and the efficiency of the mill. Discharge openings at appropriate positions control the liquid level in the mill, which should be such that the suspension just fills the void space in the mass of balls. 

The diversity of designs of the above-described machinery implies that their operating variables differ quite considerably. While energy requirements are generally governed by the comminution laws previously discussed, as illustrated in Example 4.1, some other features such as capacity and rotational velocity are particular for each imit and may be related, in some way, to the predominant force performing the reduction action. Two types of equipment with critical particularities of operation would be roller mills and tumbling mills. 

Another type of miU commonly used in studies of mechanochemical processing is the planetary mUl. As implied by the name, the milling container is rotated about two separate paraUel axes in a manner analogous to the rotation of a planet around the sun. The miUing action of a planetary mill is similar to that of a conventional horizontal tumbling mUl. However, the velocity of the grinding media is not limited by centrifugal forces. 

The ball or pebble mill is considered to be a hybrid. The cascading action of the grinding media causes the pigment to be both impacted and intensively sheared by the tumbling balls. 

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