Conceptual study of open pit transition depth determination through comparative analysis of open pit and underground mine production scheduling at Kibali gold mine in DRC, Africa

Abstract

In recent years, a certain number of open pit mines have transitioned to underground. The open pit to underground transition problem is one of the hot topics in the mining industry that has not been mathematically solved yet. Currently there is no mathematical algorithm that can successfully optimize the transition depth by considering the life of the mine schedule of both open pit and underground all together. Due to the complexity of the problem and its size, often the transition depth is defined by considering the open pit and the underground separately. Usually the transition depth is defined by comparing the cost of mining using open pit vs underground methods. As the pit gets deeper the stripping ratio increases and the transition depth is defined when the cost of mining the open pit is equal to the underground mining cost. Defining the transition depth by comparing the costs of these two mining methods, the economics of the mining project may not be optimized in terms of the net present value of the project. The underground development work and the value of the underground mine are not appropriately considered; therefore the costs and the value of the overall project may not be correctly estimated. The transition depth when correctly defined can significantly improve the discounted net present value of the project. In this dissertation, the transition depth from open pit to underground is determined by an iterative process by optimizing the life of the mine production of both the open pit and underground combined. A series of pits are generated by constraining the Lerchs and Grossmann algorithm by the elevation using Whittle software. The underground mining lodes and stopes are sequenced using CAE's Studio 5D and EPS software. For each pit scenario a crown pillar is left between the open pit and the underground, and both the open pit and the underground mine are rescheduled using OptiMine® software developed at CSM. For each scenario, the open pit and underground mine reserves are updated. The open pit and underground are scheduled using Optimine® and the financial impact of each scenario is evaluated using discount cash flow analysis. The transition depth is defined by comparing the life of mine profits of each scenario before and after tax. This methodology showed that it may be possible to improve the overall project economics by expending the KCD pit for a potential economic gain of US$100M. It is recommended that the suggested transition depth option should be evaluated further using more accurate mine design and cost parameters to check its validity.