Experimental study on microwave propagation characteristics of different coal-gangue mixtures

The problem of coal-gangue identification is one of the technical problems that have not been effectively solved for a long time in the coal industry. By analyzing the characteristics and limitations of existing coal-gangue identification methods, the feasibility of coal-gangue identification based on microwave detection technology is discussed. Firstly, the electromagnetic parameters of different coals and gangues are measured and analyzed to provide a basis for the subsequent analysis of sample test results. Then, in order to explore the influence of coal gangue size parameters on microwave propagation, the propagation law of coal gangue dielectric samples with different thickness and cross-sectional area on different frequency bands is studied. Because the coal-gangue mixture is a multi-scale medium composed of coal, gangue and air, the volume and shape are different, and the spatial distribution and mixing form are complex and changeable. The scattering effect in the microwave irradiation area is very complex, resulting in obvious differences in the transmission characteristics of electromagnetic waves in different coal-gangue mixtures. Finally, in order to explore the variation law of microwave propagation characteristics in different coal-gangue media, some microwave detection experiments of coal-gangue mixture under different microwave frequency bands, different particle sizes and different gangue contents are carried out. The results show that: the electromagnetic parameters, thickness and sectional area have obvious influence on the propagation law of microwave in the media. Different particle size and gangue rate have certain influence on microwave propagation in coal-gangue mixture. When microwave with frequency greater than 4 GHz irradiates the coal gangue mixture, the increase of particle size will gradually increase the intensity values ofS₁₁ andS₂₁ and the amplitude ofO₂₁ signal. When the electromagnetic parameters of coal and gangue in the mixed medium are quite different, after the frequency is greater than 3.5 GHz, the increase of particle size reduces theS₂₁ intensity from −35.3 dB to −38.2 dB, and theO₂₁ signal amplitude from 1.6 mV to 1.26 mV, with certain time delay characteristics. Through the experimental analysis, the differences of the transmitted wave signal intensity value, the time-domain transmitted wave signal amplitude and the transmitted wave signal delay at the sensitive frequency points can be mastered, so as to provide a new idea and method for accurate identification of coal and gangue in top coal caving face.