Techno-economic application of modular air cooling units for deep level mining at Mponeng

Abstract

As mining explores greater depths beyond 4000m at Mponeng Mine the challenges of obtaining suitable working environments and complying with regulations becomes significant. These depths mean a considerable increase in the virgin rock temperature and the surrounding working environment. By law the mine is obligated to provide working temperatures not exceeding 32°C wet bulb. This then necessitates the need for an effective cooling medium that not only has a great efficiency but should also be sustainable in the current economical environment faced with power supply shortages. Currently the advantages of ice are explored on Mponeng Below 120 level (120L) project with great effect. Due to the energy in the latent heat of fusion process, savings arise from lower pumping costs and conventional refrigeration plants are already being phased out. When applied in closed loop (U - tube) water cooling systems, colder water temperatures can be obtained at the working face. Currently Mponeng Mine is using Conventional Cooling Cars (CWC) that are similar in design to radiators for extracting heat from the working areas. The system absorbs the heat at the cooling car and rejects it in the fridge plants. If water temperatures raise this system is ineffective and costly from the loss of production. A solution for this problem comes in the form of a modular Air Cooling Unit (ACU) based on heat pump technology that was designed for use as a localised cooling unit. It allows for high supply water temperatures and provides cooling in the region of 100 kW. The study focuses on simulating the ACU in the proposed closed loop system for the Carbon Leader (CL) project and evaluating it with conventional cooling methods. Five different configurations between the 500 kW conventional cooler and 300 kW ACU are looked at and economically evaluated for the total life cycle costing (LCC). The study indicates that in a closed loop system that uses ice as the cooling medium CWCs is the best economical option. However ACUs proved to be economically the best method for cooling in two cases. Firstly in development ends and secondly using the warm return water to do additional underground cooling between the settlers and hot return water pumps. Based on the results from the study Mponeng should look into installing ACUs between the settlers and hot water dams as this scenario had the best net present value cost of all the simulations done.