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It is not within the province of this work to discuss methods of drilling and blasting of rock or ore other than to point out that the system employed at any particular operation may have some influence upon the choice of the primary crusher, specifically, upon the size of the receiving opening. The foregoing discussion of primary crusher selection, for open-pit workings, has been predicated upon normal present-day practice in primary drilling and blasting, which in most cases involves either the familiar well drill or some system of tunnel shooting.
Admittedly, any kind of rock, regardless of physical structure or geological formation, can be blasted to suit any size of crusher receiving opening. During the early years of the application of power shovels to quarry operations in this country it was not uncommon to see crushers as small as the No. 6 (12-in. receiving opening) operating in conjunction with shovels of 1½ or 2 cu yd dipper capacity; crushers such as the No. 8 (18 in.) and No. 9 (21 in.) were considered adequate for practically any primary crushing application within the ranges of their capacities.
Such operations almost invariably called for extensive secondary shooting in the quarry, even where the old tripod drills were employed for the primary drilling operation. They also necessitated the employment of from two to foul' men at the crusher to hand-feed the rock to the machine.
Thus the labor and explosives costs were, in the light of present day practice, quite high, and with present day labor costs the expense of such a system of quarrying and crushing would be prohibitive.
Practically any open-pit operation will, over a period of years, justify the installation of a primary crusher of large enough proportions to permit carrying on the quarry operation with no more than a reasonable amount of secondary shooting. Quarrymen know that much can be done to minimize secondary shooting by adjusting primary drilling and blasting methods to suit the nature of the rock, but such expedients always involve an addition to the unit cost of operation-an expense that goes on and on through the life of the operation. The cost of providing an adequate receiving opening is a one-time expenditure, one that pays dividends over and over through the years.
The same argument may be carried on into the crusher house. If it costs $2000 per year to maintain a man on the feed platform, simple arithmetic indicates that the maintenance of this man for a period of twenty years (a reasonable life-expectation for a crusher) will cost the operator some $40,000. Usually it wiII be found that the difference in first cost, between an inadequate and an adequate crusher, wiII be considerably less than this figure. The well designed modern primary crusher house should run with one man on the feed platform, with possibly a second man available on call to assist in emergencies.
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