Economic consequences of hole deviations in mining operations

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Kangwa, Sam
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In hard rock mining, drilling and blasting still remain the main method of fragmenting rock. In mining methods involving the use of long holes, extra costs are incurred as a result of consequences associated with hole deviations.The consequences include build-ups, hang-ups and poor rock fragmentation.These consequences usually lead to extra drilling, loss of drill strings, ore dilution, ore loss, increased explosive consumption, time wastage and delays in the chain of production operations, accelerated wear of equipment, extra crushing, extra mineral dressing work, and increased labour costs.Many studies do exist on hole deviations. Whereas sources and effects of the deviations have been investigated, no work has been done on quantification of extra costs due to hole deviations. The objectives, therefore, of this study were to identify consequences associated with hole deviations in mining operations and to quantify the consequences in economic terms. For the study to be meaningful, it was decided to carry out a field investigation at an operating mine. The investigation was carried out at South ore-body shaft of Zambia Consolidated Copper Mines, Nkana Division. Nkana Division operates an underground mine and produces copper and cobalt.The detailed studies of hole deviations and their consequences involved two selected operations, namely stope drilling and raiseboring of Vertical Crater Retreat (VCR) mining method. The studies involved examination of drilling plans, observation of drilling operations, measurement of hole deviations, observation of blasting activities, examination of fragmentation and handling of ore. The studies showed that almost all the holes observed had some degree of deviation. The linear deviation ranged from 0.2m to 12.0m for hole lengths of 30m to 105m. It was subsequently established that there is a strong link between hole deviations and many problems faced in the chain of mining operations.The consequences were identified and correspondingly quantified in economic terms. The cost estimations were done using both rudimental and computer aided calculations. From the estimates, the VCR stope studied accrued approximately US$274,000 extra cost, representing 62% of the total operational cost, while the raise pilot hole accrued approximately US$6,000 extra cost representing 44% of the total operational cost. These extra costs were as a result of hole deviations.These results clearly show that hole deviations have a significant negative effect on the mining operations and that they lead to poor mining economy.If the quality of production and service holes can be improved, a saving of up to 70% may be achieved in the VCR stope. For an underground mine such as Nkana Division, it may become possible for the mine to compete favourably with other underground and surface mines. For this to be achieved, it is recommended that management should:a) Set up a steering group to look into minimising hole deviations b) Rehabilitate or purchase drilling machines and accessories to improve machine productivity c) Train operators and supervisors incharge of drilling on the principles of achieving improved quality of holes, and d) Reinforce the use of computer program for planning blasting sequences to improve fragmentation.
Mining engineering