Opportunities and challenges of deep mining

For coal mining, the concept of fluidized mining includes the following five main processes: (1) unmanned mining; ② Automatic beneficiation; (3) fluidization conversion of solid mineral resources; ④ Controlled filling; Power transmission, intelligent power control and power storage. For metal mining, the concept of fluid mining includes the following three steps: (1) unmanned mining; ② Fluid conversion of solid mineral resources; ③ Controlled filling [24].

There are four technologies to realize fluidized mining of deep underground solid mineral resources [24] : (1) Conversion of solid mineral resources into gas, such as underground gasification of coal; ② Conversion of solid mineral resources into fluid fuels, such as underground liquefaction of coal and high-temperature biological and chemical conversion of coal; ③ Conversion of solid mineral resources into mixtures, such as explosive coal dust and coal water slurry; ④ Solid mineral resources are converted into electricity in situ, such as underground in-situ power generation of coal. Fluid mining is indeed a disruptive mining technology innovation, especially for deep mining in the future.

6. Enhanced simulation facilities for deep mining

Over the past 20 years, a large number of laboratory test equipment and numerical simulation software have been developed around the world to simulate the properties of real rock masses under raw rock stress conditions. In Australia, for example, the CSIRO Rock Mechanics Laboratory is equipped to simulate the real rock mass properties of deep mining using the latest bespoke triaxial devices, core displacement laboratory equipment and indoor digital modeling tools. In the United Kingdom, the University of Portsmouth has developed a mesoscale rock deformation machine to determine the failure mechanics of rock mass in seismic environments. The Department of Earth Science and Engineering at Imperial College London has a reservoir-condition core displacement laboratory facility with X-ray imaging capabilities, gas-liquid mass spectrometry, chemical testing equipment and advanced laboratory modeling tools ranging from rock pore simulation to very large scale model simulation. The Department of Earth Sciences at the University of Cambridge has facilities that simulate the microstructure and geochemical characteristics of rock masses using optical, electronic, infrared, nuclear magnetic resonance and X-ray diffraction (XRD) analyses. In the United States, the University of Minnesota has a variety of closed-loop, hydraulically servo loaders for uniaxial, biaxial (plane-strain) and traditional triaxial compression tests, as well as related digital imaging and acoustic emission (AE) tests. In Canada, the Rock Fracture Dynamics Laboratory at the University of Toronto has a variety of advanced equipment, including a multi-axis servo-controlled rock deformation system and a true three-axis system with AE and three-dimensional (3D) speeds.

Among the excellent geotechnical research centers around the world, the DeepEarthEnergy Laboratory of Monash University in Australia has a variety of advanced research equipment, which can carry out intensive research on rock mass characterization and fragmentation (3GDeep; http://www.3gdeep.com). The mesoscale equipment includes customized high pressure and high temperature enhanced triaxial test machines. The large equipment is represented by enhanced core displacement experiments and shear devices, including a high-pressure triaxial testing machine, a high-pressure hydraulic-mechanical test chamber, and a three-dimensional compression and monitoring Hopkinson bar for testing rock mass properties during rock mass failure. Microscale equipment includes X-ray microscopy for 3D contrast imaging, CT scanning, scanning electron microscopy (SEM), and XRD.

These test sets are complemented by basic simulation tools such as finite element method (FEM)/Finite Difference Method (FDM)/particle flow model (PFC). In order to understand the microscopic results of the rock mass environment (including true fracture, leaching, fluid flow characteristics, visible pore areas, and bending of liquids as they flow through the pore structure), the response characteristics of the ore body/very large rock mass, and the time-dependent impact on the surrounding environment, 3GDeep uses a comprehensive 4-stage test and numerical simulation rule (Figure 3) to study the entire mining process from micro to very large scale. The four scale ranges are as follows: (1) Micro scale: 0.03μm~20mm; ② Mesoscale: 20~100mm; ③ Macroscopic scale: 100~1000mm; ④ Ultra-large scale: 1000m. Studying the relationship between these four scales will help to obtain the underlying geological characteristics that are needed to synthesize very large scale models under real field conditions.

The macroscopic triaxial testing machine [FIG. 4 (a)] and the true triaxial testing machine [FIG. 4 (b)] are particularly important for conducting reliable rock mass fracture tests in these devices, that is, simulating more realistic rock mass fracture tests with non-traditional large samples under typical field conditions of high pressure and high temperature. The macroscopic triaxial test machine performs mechanical tests on rock samples with a diameter of 500mm, which is almost two orders of magnitude larger than the rock samples of traditional advanced triaxial equipment. The device can also simulate the flow of multiphase fluids (liquid and gas) through rock samples, using fluid pressures up to 25MPa.