Coal grinding with mixed ball sizes: coal type dependence and the role of impact loading

Grinding-ball size composition strongly affects coal comminution. This study examined the grinding of anthracite, bituminous coal, and coking coal using 10 and 5 mm balls at large-to-small ball mass ratios of 1:0, 0:1, 1:1, 2:1, and 1:2. Grinding performance was evaluated using the t10 index and a first-order kinetic model. Discrete element method simulations of bituminous coal quantified ball–coal contacts, forces, and equivalent contact stresses. After 30 min, anthracite and coking coal achieved their highest t10 values at the 2:1 ratio, reaching 48.92% and 17.54%, respectively, whereas bituminous coal achieved its highest value of 83.30% with only large balls. The all-small-ball system produced relatively low t10 values for all coal types. Increasing ball–coal contacts from 282 to 1,999 did not improve grinding because the normal force per contact and total tangential force remained low. Equivalent contact stress increased from 3.20 MPa with only small balls to 4.60 MPa with only large balls. Thus, contact frequency alone cannot characterize grinding performance. Large balls dominate normal impact loading, while small balls increase contact opportunities and tangential grinding. The favorable composition depends on coal characteristics and the balance between impact and tangential loading.