Mining Mirror May 2018 - Page 19

The blasting process dictates muck-pile configuration, which affects shovel and loader fill factors. muck-pile control, muck cycle times, and environmental factors. Downstream impacts of drilling and blasting The drilling and blasting at a mine influence nearly every aspect and is the starting point for other processes. Poor drill and blast results lead to poor mine results and very expensive operating conditions. While many mines try to reduce these costs on their balance sheets, the ramifications for this can be an increase of expenses in all other departments. The basic ways a mine can begin to reduce their drill and blast costs are to drill larger boreholes, larger burdens, larger spacings, reduction of elimination subdrill, and increasing the drill diameter. By increasing the drill diameter, a mine can reduce blast costs significantly. With an increase in drill diameter (assuming bulk loading), there is an equal proportion increase in the blast burdens. For example, using a 250mm drill loaded with bulk emulsion explosives, a burden of about 7.31m (using Konya burden equation) would be calculated. An increase to a 300mm drill would be a 20% increase in the drill diameter. This leads to a 20% increase in the burden of the blast, to around 29′ (after rounding). Let’s assume an equilateral triangle pattern is used since the bench is around 50′ (15m) and the powder factor remains similar, and that, typically, the mine will not change the timing of the blast (this is an old-school design approach that is not correct, but is often done). What is the problem? First, the stiffness ratio of the blast has been decreased, reducing the explosive efficiency and resulting in cratering or uplift movement of the shot instead of proper flexural failure. This causes: • Increases in boulders and fines; • Decreased movement of the muck pile; • • • • Mining in focus Very hard toes; Severe backbreak; Increased ground vibration; and Air overpressure. Why will the drilling and blasting team be praised for this then when all other downstream processes (muck-haul, crushing, processing, slopes, and so on) will greatly suffer? Well, the bottom line is that the costs for blasting were decreased on the balance sheet and we have very poor methods at most operations to monitor any of these outcomes (besides the environmental aspects). Increasing blast burden In the case where a mine wants to decrease their drill and blast costs by using the same-diameter drill, we will assume a 10″ (250mm) drill bit is again being used with a 7.31m burden. Suppose the mine switches to a 7.92m MAY 2018 MINING MIRROR [17]