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Regardless of the number of stages of reduction in the plant, the primary crusher, at least in those plants designed to handle shovel-loaded rock, is usually set at or near the minimum safe discharge setting for which it is designed, although as we have stated, this setting may not be strictly maintained over a long period of time.
This practice of utilizing the full permissible reduction-ratio of the primary crusher is quite sound from several standpoints. The primary crusher represents a substantial investment, and the user is justified in his desire to obtain from it all the reduction of which it is capable.
Furthermore, the machine is likely to have some excess capacity as compared to the succeeding units in the plant, even at its minimum setting. When such is the case there is nothing to be gained by operating it at a coarser setting. Thirdly, if the primary is a medium or large size machine, its minimum setting will be wide enough to obviate any excessive creation of fines by attrition; in any event the difference in percentage of fines produced at the minimum setting as compared to any coarser setting will not seriously affect the percentage of fines in the finished plant product. Unless the material is very hard and tough. Or contains a viscid admixture which makes it difficult to get through the crusher everything is in favor of working the primary at its maximum safe ratio-of-reduction.
The arguments in the preceding paragraph do not always hold good for the secondary stage. Here the discharge settings are much finer and the ratio-of-reduction begins to have a significant influence upon the percentage of fines that will show up in the .plant product. Therefore where minimum fines are desired the amount of work done in the secondary stage should be held within conservative limits. The characteristics of non-choking are to be minimized. unless the setting of the secondary stage is to be quite coarse.
For the combination of large jaw crusher followed by a gyratory of the 30- or 36-in. size, there is no particular reason why the maximum reduction-ratio cannot be utilized through both stages especially if interstage scalping is employed. These secondaries are quite large; in fact such crushers serve as primaries in medium size crushing plants; and their minimum settings are wide enough to permit operating them in the manner suggested without excessive fines production.
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