Grinders usually work together with classifiers to form grinding and classification units. The mill grinds the processed materials, and the classifier divides the grinded products into qualified products and unqualified products. Unqualified products are returned to the mill for grinding to improve the grinding process. The operation of classifier and the grinding operation of grading returning sand constitute a grinding section. The grading operation can be pre-grading, inspection grading, control grading or both. In two-stage grinding, the first grinding stage using a rod mill can also be carried out without satin separation.

The main factors affecting the determination of grinding stages are grindability of ore and disseminated characteristics of minerals, feed granularity of mill, granularity required for grinding mineral products, production scale of concentrator, necessity of separate treatment of ore and slime, and necessity of stage separation. Practice has proved that ore can be economically ground to any particle size required for separation by using one or two grinding processes without using more grinding stages. Increasing the number of grinding stages to more than two usually depends on the requirements of stage separation.

Metal concentrator mostly adopts wet grinding process, ball milling process or rod milling-ball milling process; In order to avoid excessive crushing, the rod mill process is usually used to treat ores containing fragile minerals, such as tungsten, tin and some lead ores in gravity separation plants.