Energy management framework to reduce energy costs of deep-level gold mines nearing closure

Abstract

There has been a consistent decline in gold production in the South African mining industry. This is primarily due to available ore reserves getting deeper, thus increasing energy costs. This contributes to the increased cost of production, leading to the infeasibility of operating gold mines in South Africa. These increasing costs, along with ageing infrastructure, have led to the closure of multiple gold mines over the years, with approximately 77% of existing gold mines planning to close between 2023 and 2040. Compressed air systems and water reticulation systems form a significant portion of the underground infrastructure on conventional deep-level gold mines. These systems are also the most energy-intensive systems on these mines. The compressed air and water reticulation systems consist of vast underground reticulation networks of pipes that supply underground demands. Due to ageing infrastructure, a lack of maintenance, and inadequate downscale planning, the underground compressed air and water reticulation networks are prone to significant wastage. Therefore, there is a need to evaluate energy management strategies to reduce energy consumption on deep-level gold mines nearing closure. A systematic literature study was conducted to evaluate relevant studies addressing these issues. The literature was categorised into three domains namely, life-of-mine (LOM) planning, baseline development, and energy management. A gap in literature was found between LOM planning as well as baseline development and energy management. This study aims to address this gap by accomplishing the following objectives: identifying key characteristics of mines nearing closure, evaluating the LOM production plans to determine changes in production location and output of mines nearing closure, developing resource and energy demand baselines for mines nearing closure, outlining an energy management implementation strategy for mines nearing closure, and evaluating the impact of implementing energy management initiatives on mines nearing closure. The objectives were addressed by developing an energy management methodology based on concepts obtained from literature. Step 1 involved evaluating the LOM characteristics of mines nearing closure. These characteristics include declining production trends, low capital expenditure, and high energy intensity. The two case study mines were selected based on these criteria. The LOM production trends of these mines were then evaluated. It was found that Mine A and Mine B planned to decrease production by 3 062 m² and 617 m² respectively over a five-month downscaling period. These values were used to determine the zero-waste baselines presented in Step 2. The zero-waste baseline method determined a potential energy reduction of 55% on Mine A's compressed air system and a 73% reduction on Mine B's refrigeration system over the planned production downscaling period. These results were used to identify the potential sources of wastage in the system. A waste reduction and energy management initiative was outlined and implemented in Step 3. This involved closing inactive areas and reducing system wastage. This resulted in an energy reduction of 45% and 37% over the downscaling period on Mine A and Mine B respectively. The total energy and cost impact of implementing an energy management initiative on Mine A and Mine B was evaluated in Step 4. Mine A realised a LOM energy and cost reduction of 32 GWh and R32 million respectively. Mine B realised an energy and cost reduction of 0.89 GWh and R2.9 million respectively. The full extent of savings was not realised on Mine B due to the impact of a seismic activity and infrastructure breakdowns as the mine neared closure. Further savings could have been achieved on Mine B by conducting further investigations after the impact of the seismic event. However, the results obtained from the case studies validated the methodology and successfully addressed the study objectives.