TABLE I BLASTS AT CHRISTMAS MINE
(Available in full paper)
The energy required to break rock by blasting can be evaluated on the same basis as that required in crushing, provided that the necessary information is available. This includes:
the tons of rock broken
the size that 80% of the broken rock passes
the pounds of explosive used
the energy output per pound of explosive
The work index (Bond, 1952) is the work input in kilowatt hours per ton required to break from theoretically infinite size to 80% passing 100 microns. Where W is the energy input in kilowatt hours per ton, F is the size 80% of the feed passes, and P is the size 80% of the product passes, then
Equation (1) (Available in full paper)
Equation (2) (Available in full paper)
Equation (3) (Available in full paper)
A pound of dynamite which is 45% nitroglycerine is considered to yield 0.67 kilowatts (Peele), so that W equals 0.67 times the pounds of dynamite used per ton. The 80% passing size P of the broken rock must be measured by size analysis or must be estimated to obtain the blasting work index.
Two blasting tests (Bond, 1954) on Taconite iron ore in Minnesota five years ago gave work index values of 14.5 and 13.5, or an average of 14.0, which compares with 14.7 recently obtained in crushing this ore in plant operations.
Three carefully controlled blasts in stopes were made recently in the underground Christmas mine of the Inspiration Copper Company in Arizona. Complete size analyses extending down to ¾ -inch were made on the entire lots of broken rock, with result as listed below in Table I. The dynamite used was 45% Amogel No. 3 with 3.5 sticks to the pound. The holes were 1.5-inch diameter spaced 3 feet apart and 6 to 10 feet deep, with eight or nine holes per blast.
The ore from each blast was dumped from one-ton cars onto a 15-inch grizzly above a nest of 4 x 6-foot square hole screens stacked1 2 inches apart. The screen openings were 12, 9, 6, 5, 4, 3, 2, 1, and to ¾ inches. Each car of ore was worked through the screens by hand, removed, and weighed, and the minus ¾-inch undersize was collected on an iron plate floor beneath the screens and weighed.
Blasts No. 2 and 3 broke to the end of the holes, but blast No. 1 with holes 6 feet deep broke to a depth of only 3.5 to 4 feet.
Third Theory plots (Bond, 1959) of the size distributions of the three blast products are shown in Figure1. All of these plots curve upward to the left, with evidence of a return to the straight tangent lines at ¾-inch, the smallest size tested. The exposure ratio values Er of blasts No. 2 and No. 3 are much larger than a primary crusher product would be.
