Open Pit Optimisation and Design : A Stepwise Approach

Abstract

 The erstwhile Nkroful Mining Limited (NML) had a concession with an approximate area of 47.84 km² located at Nkroful in the Nzema East District of the Western Region of Ghana. The exploration data, assembled from 1997 to 2003, revealed a potential gold resource and indicated that the deposit could be mined by using open pit mining method. This paper demonstrates a stepwise approach to design an optimal open pit to exploit the deposit by using Surpac and Whittle software. Using the exploration data as the primary input, Surpac is used to create a block model of the deposit. The block model is imported into Whittle for the open pit optimisation, based on geotechnical and economic parameters derived from calculations or assembled from mines operating in Ghana. The optimal pit is then imported back to Surpac for detailed pit design incorporating a ramp and berms. The designed pit contains 5.03 Mt of ore at an average grade of 1.7063 g/t. However, it has 19.58 % more tonnes of waste, 16.05 % less tonnes of ore and 0.34 %  lower average grade of ore than the respective figures in the Whittle optimal pit. The differences in the figures are due to the widening of the pit bottom and the creation of a ramp and berms in the designed pit, which resulted in the addition of some waste, ore loss and dilution.