Secondary crushing is a professional noun phrase which means the part for medium-fine production. It is widely used in stone crushing production line, sand making production line, and recycled construction waste disposal. Crushing operation has single stage or multiple stage. We use single stage in crushing line. For example, some cement company produce that only use hammer break machine or ultrafine mill to crush limestone. Most of production line is multiple stages. We can use two stages crushing in a production line, if the raw material is less contained silicon. For example, in a common limestone production line, we can choose jaw crusher, ultrafine mill and vibrating screen to complete production line, due to the ultrafine mill equipment is ultrafine mill, and it has medium crush or fineness crush ability, also to shape material at the same time. Multiple stages crushing is common, and how to choose the secondary crushing equipment is important. With right machine, the production line will more effective and spend less money. In fact, two stages crusher is enough for low hardness materials. But for higher hardness material, we have to choose cone crusher for the secondary crushing and consider multiple stages crushing production line.