MINING - Hard Rock Mining and Fragmentation

 N G NAIR, MALANJKAND, INDIA.

IMPROVED FRAGMENTATION CAN BE EXPECTED IF DUE ATTENTION IS PAID TO THE FOLLOWING DETAILS, VIZ:

(a)   The use of well coupled explosive.

(b)   The use of a small decked charge in the collar stemming of the hole.

(c)   The use of inclined drilling especially in so far as it affects the collar portion of the hole; an improvement in fragmentation can also be anticipated in the remaining portion of the hole.

(d)   By resorting to millisecond delay multirow blasting where holes are fired in a staggered pattern. The chevron pattern of hole initiation is especially recommended in this regard.

(e)   Fragmentation is very much a function of the burden and therefore it might be expedient to significantly increase the spacing at the expense of the burden yet maintaining constant the product of burden x spacing.

Back break and therefore the creation of more stable face conditions can be minimized by paying attention to the following:

(a)   The introduction of a controlled blasting technique which involves the extensive use of light decoupled charges. Holes are drilled relatively close together along the contour line and these techniques rely on the fact that the available explosives energy is usefully employed in shearing between holes assuming that they are fired simultaneously.

(b)   Light loading the upper half of holes, especially those adjacent to the contour line of the blast, with a low density / velocity explosive. Decoupling of charges would also be an advantage. The possible use of a low strength Anfo has attractive possibilities in this regard.

(c)   The use of deck loading as a means of cutting down on charge concentraion. Sand stemming between charges would appear attractive in broken ground where the explosives gases may have the effect of tearing the rock apart. However, if it is possible to divide the charges with air gaps this could have the great advantage that the available explosives energy is more evenly distributed along the length of the hole.

(d)   With multirow blasting it is advisable to increase the delay period between the second last and last row to at least 50 milliseconds as a means of reducing the shock transmitted to the solid.

(e)   An elongated drilling pattern, where the spacing is significantly greater than that of the burden as could be required for achieving improved fragmentation, would have added advantage of there being proportionately less holes to break back into the solid.

(f)     The use of inclined drilling is known to minimize the extent of back break in the vicinity of the collar of the hole, at the same time creating safer working conditions over the whole face.