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The simplest form of secondary crushing stage involves a single crusher, taking its feed by gravity flow direct from the primary with no interposed scalping separation. This is tantamount to making the over-all reduction in one machine, except of course that there will always be a certain amount of surge capacity between the two crushers. This type of installation has been used in a number of plants, starting with the advent of the power shovel and the large primary crusher, many of which were "tacked on" to existing plants during the conversion from hand-Ioading to shovel-Ioading operation. Probably the most notable of such installations were those of the first jaw crushers of the 84-in. class. These big machines were designed to operate at rather coarse discharge settings, usually from 10 to 12 in., and the usual practice was to follow them with gyratory crushers such as the No. 9 (21 in.) or No. 10 (24 in.), the over-all ratio-of-reduction for the two machines being 14 or 15 to 1.
The simple two-crusher, screenless arrangement works out fairly well for the large sizes of primary jaw crushers if the quarry-run rock is reasonably clean, 01' if any fine material which may be present in the feed is free flowing. The arrangement is not advisable un del' any circumstances if the setting of the secondary crusher is to be less than about 5 in.; for finer settings than this the fines produced through the two stages are apt to promote a sluggish action in the secondary machine, which might under certain conditions build up to a complete choke in this stage. So far as the commercial crushed stone plant is concerned the method violates the well established and very desirable principle of scalping off fines between all crushing stages in order to
minimize further production of fines by attrition. The safest practice in all cases is to scalp between the primary and secondary stages regardless of the sizes of crushers used 01' their discharge settings. A plain grizzly is better than nothing; a modern heavy duty vibrating screen is the ideal heavy-duty scalper.
Three factors are of prime importance in selecting the size and type of the secondary crusher. or crushers:
(1) capacity; (2) size of feed, (3) size of product. We might add to these the item of product characteristic, which embraces shape characteristic, and gradation. These are not always important, but they are sometimes very much so.
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