Analysis of the gas-solid two-phase flow characteristics and the impact of key structural parameters on the separation performance of medium-speed coal mills

Abstract

The EM-type medium-speed mill (EM mill) integrates the functions of crushing, conveying, drying, and separating. It is widely used for grinding coal powder and ores. The mill operates in a "black box" environment, where the internal conditions cannot be easily observed. Due to the limitations of the measurement technology and real-time monitoring, describing the complex particle motion within the mill is challenging. Furthermore, the mill has a complex structure, generating large eddies within the grinding chamber that are difficult to completely remove. This often results in non-compliant coal powder fineness and "over-grinding" phenomena, which significantly affect the production efficiency. This paper conducts numerical simulations using computational fluid dynamics (CFD) and powder classification methods, in order to study the particle motion characteristics and improve the internal flow field distribution of the mill. This allows to increase the particle transport and separation efficiency. These are then compared with experimental results, showing an error of less than 5 %, which demonstrates that the adopted model accurately predicts the flow characteristics and separation performance of the EM mill. Afterwards, the particle motion inside the mill is analyzed based on the coupling method of Fluent 2022R2 and EDEM 2022. Finally, the impacts of the height of the ash bucket cone and the number of separator blades on the internal flow field distribution and particle separation performance are studied. The obtained results show that the particle motion is significantly affected by the flow field. In addition, the increase of the number of separator blades results in reducing the vortex flow between them in a certain range, which significantly improves the coal powder fineness, at the expense of the high discharge of large particles. The reduction of the height of the ash bucket cone limits the generation of vortex flow in the secondary separation zone, which significantly increases the separation efficiency of the coal powder. This study provide valuable guidance for changing the ash bucket structure and adjusting the number of separator blades, and serves as a reference for improving the separation performance of particles and enhancing the internal flow field in EM mill.