Mining Science and Technology (China)最新文献

With the increase in mining depth many mining areas in China have entered a period necessitating mining above aquifers. Production safety in coal mines in northern China is under serious threat from Ordovician karst water on coal seam floors. In order to analyze the destruction of water-resisting strata in floors of coal seams being mined and to achieve safe mining above deep aquifers, we established a numerical model of water-resisting strata, considering the structural characteristics and mechanical properties of a floor layered with hard and soft rock. We simulated the distribution characteristics of deformation, failure and seepage using the analytical module of fluid–structure interaction of FLAC. We also obtained the corresponding stress distribution, deformation and flow vectors. Our results indicate that: (1) the advance of the working face causes water-resisting strata in goaf floors to form a deep double-clamped beam, subject to homogeneous loading at the bottom; (2) the two sides of the rock beam are subject to shear failure; (3) both sides of the rock seam at the bottom of the water-resisting strata are subject to tension and the greater the working face advance, the more serious the failure; (4) the original balance of the stress and seepage fields are broken and redistributed due to mining activities, especially the interaction of the abutment pressure in both sides of the goaf; the lateral pressure on the goaf floor and the water pressure on the floor of the aquifer promote floor heave and shear failure on both sides of the floor, forming a water-inrush passage. Our study results can provide references for the mechanism of water-inrush on mine floors.

The exact shape and size of the gasification channel during underground coal gasification (UGC) are of vital importance for the safety and stability of the upper parts of the geological formation. In practice existing geological measurements are insufficient to obtain such information because the coal seam is typically deeply buried and the geological conditions are often complex. This paper introduces a cylindrical model for the gasification channel. The rock and soil masses are assumed to be homogeneous and isotropic and the effect of seepage on the temperature field was neglected. The theory of heat conduction was used to write the equation predicting the temperature field around the gasification channel. The idea of an excess temperature was introduced to solve the equations. Applying this model to UCG in the field for an influence radius, r, of 70 m gave the model parameters, u_1,2,3…, of 2.4, 5.5, 8.7… By adjusting the radius (2, 4, or 6 m) reasonable temperatures of the gasification channel were found for 4 m. The temperature distribution in the vertical direction, and the combustion volume, were also calculated. Comparison to field measurements shows that the results obtained from the proposed model are very close to practice.

This paper describes the influence of joint spacing and joint orientation on the penetration rate of a Tunnel Boring Machine (TBM) disc cutter as modeled by the Discrete Element Method (DEM). The input data for the simulations were obtained from the sandstone along the Alborz tunnel that is currently being excavated in Iran using a 5.2 m diameter open TBM. Three joint spacings, 150, 200, and 300 mm, were modeled together with seven values of joint orientation; 0°, 15°, 30°, 45°, 60°, 75°, and 90°. The results show that the penetration increases when joint orientation increases from 0° to 75°, but it decreases as the joint orientation increases further from 75° to 90°. This is true for each joint spacing. In addition, for a given joint orientation increasing the joint spacing causes the TBM penetration to decrease. The optimum joint orientation, from the viewpoint of TBM penetration, is about 60–75°.

Coal seam pyrolysis occurs during coal seam fires and during underground coal gasification. This is an important source of polycyclic aromatic hydrocarbon (PAH) emission in China. Pyrolysis in a coal seam was simulated in a tubular furnace. The 16 US Environmental Protection Agency priority controlled PAHs were analyzed by HPLC. The effects of temperature, heating rate, pyrolysis atmosphere, and coal size were investigated. The results indicate that the 3-ring PAHs AcP and AcPy are the main species in the pyrolysis gas. The 2-ring NaP and the 4-ring Pyr are also of concern. Increasing temperature caused the total PAH yield to go through a minimum. The lowest value was obtained at the temperature of 600 °C. Higher heating rates promote PAH formation, especially formation of the lower molecular weight PAHs. The typical heating rate in a coal seam, 5 °C/min, results in intermediate yields of PAHs. The total PAHs yield in an atmosphere of N₂ is about 1.81 times that seen without added N₂, which indicates that an air flow through the coal seam accelerates the formation of PAHs. An increase in coal particle size reduces the total PAHs emission but promotes the formation of 5- and 6-ring PAHs.

In order to study dynamic laws of surface movements over coal mines due to mining activities, a dynamic prediction model of surface movements was established, based on the theory of support vector machines (SVM) and times-series analysis. An engineering application was used to verify the correctness of the model. Measurements from observation stations were analyzed and processed to obtain equal-time interval surface movement data and subjected to tests of stationary, zero means and normality. Then the data were used to train the SVM model. A time series model was established to predict mining subsidence by rational choices of embedding dimensions and SVM parameters. MAPE and WIA were used as indicators to evaluate the accuracy of the model and for generalization performance. In the end, the model was used to predict future surface movements. Data from observation stations in Huaibei coal mining area were used as an example. The results show that the maximum absolute error of subsidence is 9 mm, the maximum relative error 1.5%, the maximum absolute error of displacement 7 mm and the maximum relative error 1.8%. The accuracy and reliability of the model meet the requirements of on-site engineering. The results of the study provide a new approach to investigate the dynamics of surface movements.

Reliability of braking systems is a key requirement to ensure the safety of in using downward belt conveyor brakes. By analyzing and comparing three commonly used braking velocity curves, we conclude that the Harrison curve is the best. Given the characteristics of a downward belt conveyor, we studied the control in a closed-loop velocity, a conventional PID method and an optimal PID control method. We used MATLAB/Simulink to simulate the three control methods. Our simulation results show that optimal PID control is especially suitable for disc braking systems. To verify the results from theoretical analysis and simulation, a multifunctional test-bed was developed to simulate the braking process of a disc brake system. Our experimental results demonstrate that the optimal PID control can make the output velocity to follow a preset velocity correctly with only small fluctuations, meeting the requirements of a flexible brake for a belt conveyor.

Two coal samples of similar rank were chosen from Australia and China to investigate the differences in Coal–Water Slurry (CWS) made from them. The effect of ash content and particle size gradation on these properties was also studied. Different grinding times were used when grinding the two coals and particle size analysis of these ground coals was used to select samples with a “double-peak” particle size distribution. All the “double-peak” samples were used to prepare a CWS. The concentration, viscosity, fluidity, and stability of each CWS were measured. The results show that the properties of a CWS prepared from a coal sample with a “double-peak” size distribution are better than those CWS prepared from samples with a mono-modal particle distribution. The ash content of Australian coal is 21.72% higher than the ash content of Shenhua coal. The highest coal concentration in slurry from the Australia coal is 11.01% higher than in CWS from the Shenhua coal. The fluidity and stability of the CWS prepared from the Australian coal are both better than the fluidity and stability of slurry prepared from Shenhua coal. High ash content in the Australian coal improves the pulping results of a CWS made from it.

A wet impact crusher was used to breakdown waste printed circuit boards (PCB’s) in a water medium. The relationship between the yield of crushed product and the operating parameters was established. The crushing mechanism was analyzed and the effects of hammerhead style, rotation speed, and inlet water volume on particle size distribution were investigated. The results show that the highest yield of −1 + 0.75 mm sized product was obtained with an inlet water volume flow rate of 5.97 m³/h and a smooth hammerhead turning at 1246.15 r/min. Cumulative undersize-product yield curves were fitted to a nonlinear function: the fitting correlation coefficient was greater than 0.998. These research results provide a theoretical basis for the highly effective wet crushing of PCB’s.

Asphaltene–ceramic composite membranes were fabricated from ceramic supports and an asphaltene component, which was obtained from the separation of coal to give a kind of new carbonaceous precursor material. Using SEM and thermogravimetric analysis to measure the microstructure and properties of the asphaltene component allowed the porosity, permeability, and retention ratios to be determined. The results show that the asphaltene component can be regarded as a good carbon membrane precursor material because of its high carbon content and strong bonding capacity. When ceramic supports are impregnated with asphaltene colloid the asphaltene easily combines with the support surface and forms a good carbonaceous film after carbonization. Little of the asphaltene component permeates into the internal pores of the ceramic support. Although the number of coats applied to the substrate had little affect on the porosity of the asphaltene–ceramic composite membranes the permeability varied depending upon the number of times the substrate was treated. The way bubbles escape from the film, and the phenomenon of coalescence, as affected by different film thicknesses also seem closely related to the number of coats. A composite membrane carbonized at a final temperature of 600 °C is relatively dense and the permeability of Fe(OH)₃ colloid through it is very low. A membrane fired at 800 °C is porous and its permeability and retention of Fe(OH)₃ colloid are 88 L/(m² h MPa) and 85.3%, respectively when the trans-membrane pressure is 0.22 MPa.