Study on the liberation mechanism of middling coking coal based on the grinding kinetics and discrete element simulation

The intensive and precise separation of middlings is an important approach to alleviate the shortage of high-quality coking coal resources in China and improve resource utilization efficiency. This study investigates middlings from coal preparation as a research subject in view of the challenges associated with its complex coal-mineral structure, high gangue content, and poor washability. The influence of grinding concentration, mill speed, and grinding time on the dissociation effect is investigated by a crushing and grinding dissociation test. Results show that the theoretical vitrain yield of the crushed product increased from 3.08% to 50.56% when the theoretical ash content reached 11%. The organic matter and gangue minerals are accordingly effectively dissociated under the optimum conditions of 65% grinding concentration, 85 r/min mill speed, and 8 min grinding time. The proposed kinetic equation of the grinding process indicates that the grinding speed of each particle size material decreases gradually with the increase in grinding time, of which the coarse particles (−1 + 0.5 mm particle size) decrease obviously. However, the decrease in grinding speed of fine particle size material is relatively gentle. The discrete element simulation analysis shows that the medium throwing area is the main force area of grinding in the process of crushing and grinding, and the force and energy of the throwing area and the dropping area are greater than the grinding area, which is an effective crushing and grinding area. There is an inert area between the dropping area and the grinding area, which cannot crush and grind the ore.