Recycling waste material for backfill coupled heat exchanger systems in underground stopes of mines

The continuous exploitation of mineral resources into the deep strata provides suitable conditions for the development of geothermal energy. Backfill technology can not only ensure the safety of mining operation and realize the recovery and utilization of solid waste, but also serve as artificial thermal reservoir. However, ordinary backfill materials have poor thermal properties and limit the geothermal energy extraction. In this paper, a novel backfill material with good thermal performance was proposed and tested to replace the traditional materials. The feasibility of horizontal backfill coupled heat exchanger system was evaluated by experimental method for the purpose of efficient cooperative development of mineral resources and geothermal resources. Natural flake graphite and other solid waste materials, such as copper slag, steel slag, aluminum slag and coal gasification slag were used to replace tailings of 2.5%, 5%, 7.5% and 10% by mass fraction to make the composite backfill material. The uniaxial compressive strength, mini-slump, thermal conductivity, specific heat capacity and density of the composite backfill material were tested to explore the best proportion. An experimental set-up of a backfill-coupled heat exchanger system was then constructed and filled with the backfill material to determine the heat storage and release processes on the thermal performance of the system. The influence of the composite backfill material and surrounding rock temperatures on the energy efficiency of the backfill coupled heat exchanger system was evaluated. The results showed that, with the increase of the tailings substitution rate, the uniaxial compressive strength, specific heat capacity and density of the composite backfill material were reduced compared with the traditional backfill material, while the thermal conductivity and thermal diffusion coefficient were increased. Finally, 10% natural flake graphite instead of tailings was used to make the backfill body for testing. Compared with traditional backfill coupled heat exchanger system, when the surrounding rock temperature was 55 degrees C, the heat discharging and the total efficiency of natural flake graphite -backfill coupled heat exchanger system increased by 579.1 kJ and 31.6 %, respectively. The research results provide a reference for the application of backfill coupled heat exchangers in engineering practice. (c) 2021 Elsevier B.V. All rights reserved.