Study of evaluation of economic rock-mixed ratio of longwall top-coal caving working face based on entropy weight-extension theory

The rock-mixed ratio of top-coal caving mining significantly affects the coal quality and economic benefits. This ratio at longwall top-coal caving (LTCC) faces is jointly determined by multiple factors, including geological conditions, extracting and caving technology, and mining technical level. For specific production geological conditions and coal caving parameters, there exists an optimal rock-mixed ratio for LTCC faces. When the rock-mixed ratio of raw coal mined from LTCC faces is controlled at this value, the coal mine achieves the maximum long-term economic benefits, and this optimal rock-mixed ratio is defined as the economic rock-mixed ratio. To scientifically determine the economic rock-mixed ratio of LTCC faces and identify the main controlling factors affecting the rock-mixed ratio, field investigations and theoretical analyses were conducted to examine the formation mechanism of gangue mixing in top-coal caving and the key factors influencing the rock-mixed ratio at LTCC faces. By exploring the relationships between the rock-mixed ratio and raw coal production, coal price, and production costs, a theoretical model for the rock-mixed ratio of LTCC faces was established. This model is designed to ascertain the economic rock-mixed ratio for a given LTCC face under its current production conditions. Based on the entropy weight-extension theory, an evaluation index system was proposed, incorporating three criterion layers: geological conditions, top-coal caving technology, and top-coal caving parameters. Subsequently, an evaluation method for the main control factors affecting the rock-mixed ratio of LTCC faces was established. This methodology was then applied to conduct a comprehensive evaluation of the LTCC face No.8222 at Tashan Coal Mine. The results indicate that: the economic rock-mixed ratio for face No.8222 at Tashan Mine under its current production conditions is 12.6%; geological conditions are the most dominant controlling factor influencing its rock-mixed ratio; and the thickness of gangue partings within the top-coal and the drawing interval have a particularly significant impact on the rock-mixed ratio for this LTCC face. The research findings provide a scientific basis for determining the economic rock-mixed ratio of FMTC faces, analyzing the primary controlling factors affecting rock-mixed ratio, and optimizing the top-coal drawing process and its associated parameters.