Abstract: In order to increase the pre-concentration process before grinding in the primary grinding and classification system composed of lattice ball mill and spiral classifier，Longqiao Mining Co.，Ltd. carried out a study on the transformation of the primary grinding and classification system in view of the high failure rate of the spiral feeder of the lattice ball mill，the poor classification efficiency of the spiral classifier and the low processing capacity of the system，which will lead to the problem that the pre-concentration system before grinding cannot reach the designed processing capacity after the transformation. By removing the traditional screw feeder of ball mill and using elbow pipe to feed，the failure rate of equipment is reduced. The cyclone is used to replace the double spiral classifier，which improves the classification efficiency， makes the production process smooth and achieves the design processing capacity. After the transformation， the cost of purchasing and consuming spare parts can be saved by more than 200，000 yuan，and the annual power cost can be saved by more than 100，000 yuan，with remarkable economic benefits.

Key words: preselection process, classifier, feeding elbow, technical transformation, benefit