Evaluation of transport options from KMS shaft to the mill at Obuasi mine, Anglogold Ashanti

Abstract

The underground mine at AngloGold Ashanti Obuasi Mine in Ghana currently transports about 175,000 tpm from four shafts to the mill. Two major shafts, Kwesi Mensah Shaft (KMS) and Kwesi Renner Shaft (KRS) handle about 85-90% of this total tonnage. Transfer of ore from all the shafts to the mill is largely by means of contractor trucks. The mill is approximately 2.8 km and 1.5 km by road from KMS and KRS shafts respectively. Earlier plans to introduce a surface conveyor to the mill were found not to be economically viable largely due to the lower production level prevailing at that time. The objectives at that time were to decrease cost, improve availability and security, and to limit the numerous problems associated with contractor trucking. The current Business Plan indicates a phased increase in the underground production from 175,000 tpm up to 220,000 tpm. The extra tonnage is to feed the new Tailings Sulphide Plant (TSP), located adjacent to the existing mill. With the plan to increase underground production, and the need to decrease cost and improve efficiency against the current financial downturn, this project serves to review the four ore transfer options that link the shafts to the mill. The options are the continued usage of contractor trucks, purchase mine-owned trucks, use surface or underground conveyors. The factors to be used in the selection process were categorised under economic, environmental and technical parameters. Secondly the options were multiple and consequently the appropriate selection method was the Analytic Hierarchy Process (AHP). This is a Multiple Criteria Decision Making (MCDM) methodology which is a widely used technique. The AHP is a very simple, structured and easily understandable method in which both non-numerical and numerical data are considered in the selection processes with multiple options, where the need for optimization is paramount. The overall observation by the experts’ favoured surface conveyors. However, the issue of capital in the present regime of the Obuasi mine, makes it quite difficult to obtain management approval in the immediate future. The surface conveyor however, has the potential to make adequate returns. From the financial analysis showing a six to ten year payback period in a mining environment, other forms of finance could be considered. Alternatively, the next highest ranking option, which is contractor trucking could be used but with two contractors. This would introduce competition which could improve cost and availability. This is the first time that a more structured approach has been used for equipment selection at Obuasi.