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The ideal feeding arrangement for jaw crushers of any size is the mechanical feeder. This device, which has been developed in several forms, provides regulation as smooth as is possible to achieve in the handling of coarse materials; it permits instant shut-off of the flow of feed when blockades occur; it allows quick dumping of cars 01' trucks; and it delivers the feed to the crusher in a ribbon, the width of which can be adapted to the width of the receiving opening. For those installations employing grizzlies ahead of the crusher the advantage of the uniform flow is obvious; very short grizzlies will hand die the coarse separation ordinarily required at this point, whereas grizzlies three or four times as long will generally be required if large amounts of material are dumped over them in a "one-shot" do se.
The mechanical feeder is of course an excellent device for feeding any type of primary crusher. Some type of feeder is used for such service in almost every gravel plant, at some point in the system-at the shovel, under the surge pile, or immediately ahead of the crusher, depending upon the methods utilized in excavating and transporting the pit-run gravel to the crushing plant. Feeders of the reciprocating plate type, steel apron type, and electromagnetic type are all popular for this duty. These types are also extensively used in conjunction with underground mining operations.
For heavy-duty quarry or open-pit mining operations the heavy-duty steel apron feeder is the most practical type. It can be made in any length to fit in with any type and size of transportation equipment and crusher house arrangement. It has been developed in sizes to suit any size of primary crusher, and of proportions suitable for handling any rock that the largest sizes of crushers will take.
Because of their heavy construction these heavy-duty apron feeders add materially to the initial cost of the primary crusher installation; naturally they add something to the overall cost of maintenance and power.
The question of whether or not this added expense is justified will depend upon conditions surrounding each particular installation. If the crusher has an ample margin of capacity over and above the plant requirements, as is often the case when it has been chosen from the standpoint of an amply large receiving opening, occasional delays due to bridging are not apt to be serious; and the shovel and transportation equipment, chosen to work with such a crusher, are usually capable of making up for any time losses thus incurred. For such operations it is questionable if the cost of the heavy feeder is justified. On the other hand, if the operation demands full-time, or nearly full-time, crushing to meet the plant requirements, the case for the feeder is clear cut from every standpoint, at least so far as the jaw crusher is concerned. For reasons which have been discussed, their use with gyratory crushers, especially the larger sizes, is debatable, unless, as is done in some large mining operations, the designer wishes to establish a surge 01' stockpile ahead of the primary crusher.
To revert to the gravel and underground mining operations, now that vibrating screens of extra-heavy construction have been developed to handle large pieces of rock over their top deck, it is very probable that such screens will come to be used more and more as combination scalpers and feeders a service for which they are admirably adapted. The advantage of being able to make a clean cut and definitely sized separation at this point will be of great value in many cases; and the high efficiency of the vibrating screen, as compared to the stationary bar grizzly, will appeal to those operators who want no undersize material in the feed to the primary crusher. The same idea in modified form, using smaller and lighter screens, can be applied to operations involving small rock and small crushers.
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