Strategic mine planning optimization for the potential transition from open-pit to underground mining at Akyem Gold Mine, Ghana

Abstract

The transition from open-pit to underground mining operations is a frequently occurring procedure for mines around the world. The Akyem Gold Mine in Ghana is an example, where the transition is to be optimized with regard to the significant reduction in yearly throughput. The current processing plant is commissioned to efficiently process up to roughly 7.7 Mt of ore per year. The planned underground mine is likely to start operations in 2022 during the
final stages of the remaining open-pit operations, with an estimated yearly production of 1.6 Mt of ore. This implies that a major plant modification is recommended in order to prevent unnecessary high operating costs, once a full transition may occur. However, opportunities are seen to discard the plant modification and instead to add incremental open-pit material, which is currently considered as mineralized waste. The suggested option will be considered once the analyses prove that a certain incremental open-pit head grade will generate more value in combination with the underground activities. This analysis is based on financial modelling of the potential options, where a major trade-off is conducted between the plant modification for the planned option, and the incremental open-pit mining, processing and tailings storage for the suggested option. The analysis is tested by changing all different decision parameters to indicate the optimum solution. Prior to this head grade determination, blending optimization of the potential sources of ore is conducted, in order to determine the optimum period where the potential plant modification may occur. Results show that the open-pit mine will directly supply the processing plant until 2023, whereas its stockpiles will be depleted by the end of 2025. For this reason, the suggested plant modification is suggested to take place at the start of 2026. Subsequent results related to the option evaluations are in favor of the full transition to only underground mining. Despite any changes in the set of decision parameters and gold prices, the incremental open-pit head grade will not be possible to obtain. This results from the fact that the calculations indicate a deficiency of required gold ounces based on the remaining open-pit reserves. It may be concluded that the potential transition from open-pit to underground operations with a reduction in yearly throughput, is optimized through a full transition to underground mining. The planned option, with the cohering plant modification, is proven to be optimum and is currently considered as the solution for future mining at Akyem Gold Mine.