Abstract: The ratio of gangues discharge is one of important parameters,which affect the safety and economy of medi- um speed mill operation. The rotation clearance will increase by wear,which would reduce ring nozzle outlet air veloci- ty and increase gangues discharge. The influence of ring nozzle rotation clearance on gangues discharge was studied by numerical simulation based on actual mill structure and gangues physical property. The results showed that the effect of rotation clearance on ring nozzle outlet air velocity was obvious. The outlet air velocity would reduce 10． 2 m / s and the ratio of gangues deposition would increase 114% when rotation clearance increased from 10 mm to 20 mm at 65 t / h total air volume. One rotation clearance optimization scheme was proposed in order to reduce air escape when the rota- tion clearance was increased by wear. When the rotation clearance was increased from 10 mm to 20 mm,the gangues deposition would reduce 31% compared with the case before optimization. After the practical improvement,the ratio of gangues discharge was reduced 68. 7% and had met the standard requirements without any increasing trend in nearly two years. The problem of insufficient output caused by gangues discharge was also solved effectively.