Crusher Reduction Ratio
A great deal has been written, and said, about the limitations of the crushing rolls in the matter of reduction ratio, and there has been a tendency to pin the machine as a class down rather definitely to fixed maximums, regardless of any variables in conditions and characteristics of the materials to be crushed. For many years-apparently by virtue of general consensus-it has not been deemed advisable to exceed a ratio of reduction of 4: 1, and seldom was any exception noted in stating this rule.
The permissible or advisable-reduction ratio for crushing rolls is subject to variation, just as it is in the other types of crushers. Light rolls are not capable of handling reductions as large as those which can be successfully performed in heavy machines. For a given size of feed, the large-diameter rolls will successfully handle higher reductions than will 1'olls of smaller diameter. And, for any particular machine, the permissible reduction ratio will vary inversely with the hardness or toughness of the rock.
The quality of product required has an important bearing upon the advisable reduction ratio. The way in which the crushing rolls perform their work, Le., the continuous "follow-through," once they have gripped the material, tends to create a "choke" condition in the discharge zone-a condition which would obviously be accentuated as the reduction ratio is increased.
Inasmuch as such a condition promotes production of fines, it follows that a high reduction is undesirable if minimum fines are a requirement. For most commercial crushing plant applications it is advisable to hold the ratio within 3: 1, rather than 4: 1.
On the other hand, if the rolls are being used to prepare feed for fine grinding units, fines are helpful rather than harmful. For such applications the permissible reduction is established by the ability of the individual machine to handle the job.
Reductions of 6 or 7 to 1 are not uncommon for such operations utilizing heavy-duty rolls. Higher reductions than this are being performed in cIosed-circuit operations, running the rolls with a heavy choke-feed and a high circulating load. Such a performance, it should be stated, can only be economically performed on soft rock or ore.
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