Capacity of Secondary Stage Crushing

The capacity of the secondary crushing stage need not necessarily match that of the primary crusher; more often than not it can be substantially less, for reasons outlined in the preceding section. It must of course be adequate to handle its share of the total plant capacity. How much its capacity should exceed the average plant capacity will depend upon ho\V the job is engineered.

If the installation is of the simple "one-two" variety we have just described, and very little surge capacity is provided between the two stages, the capacity of the secondary stage should at least equal that of the primary; otherwise the quarry equipment may be held up at times, waiting on the secondary crushing stage to clear itself. If, on the other hand, an adequate surge is provided to take care of the inevitable fluctuations in feed-rate to the primary, the capacity of the secondary stage may be predicated on requirements of the succeeding stages. This presupposes that the secondary machine 01' machines will be large enough to eliminate delays due to bridging 01' blocking of the feed from the primary.

If scalping between the two stages is resorted to, the rating of the secondary stage may be reduced by whatever amount of undersize material is to be removed from the product of the primary. This can be determined within reasonably close limits by means of the product-gradation curves we have presented. In making this calculation it should be remembered that the scalping screen, regardless of type or size, will not remove all of the undersize. From 85 to 90 percent efficiency is the usual range; therefore the 10 or 15 percent undersize which will be carried over must be added to the theoretical tonnage, as figured from the curves, in arriving at the estimated feed to the secondary stage.

Once the required capacity for the secondary stage has been determined, it may be checked against tabulated ratings, at the desired discharge setting, for the type of crusher being considered for the stage. In making this check it should be remembered that capacity tables, while they are prepared on a conservative basis, are predicated upon a uniform rate of feed-and upon the assumption that the crusher is in first-class condition.

If for any reason it is expected that the feed rate will not be uniform, adequate allowance should be made for whatever degree of fluctuation is anticipated, just as was suggested for the primary crusher. Also, inasmuch as it is to be expected that so me drop in efficiency of the crushers wiII occur from time to time because of mechanical conditions, some allowance should be made for this, regardless of whether or not the feed rate will be uniform. From 10 to 15 percent will be sufficient for this in a properly maintained plant.

Surge storage between the primary and secondary stages, while always the theoretically desirable, may not always be practicable or economical.

The product of the very large sizes of jaw crusher, for example, may be so coarse and slabby that it would be exceedingly difficult to handle through a surge bin or on mechanical feeders of reasonable size and cost. Therefore, where these large jaw crushers comprise the primary stage, it is generally best to consider the two stages as a unit and to match their capacities closely enough so that the material can be kept on the move-providing only a small open surge space between them. Then, if surge storage is desired it may be incorporated in the flow line after the secondary stage.

The product of the largest sizes of gyratory crushers can be handled satisfactorily through storage.