The primary jaw crusher is the first stage of processing in most sand, gravel, and mining operations. Material that has been excavated from the ground (Pit run) or blasted (Shot Rock) is fed to a large crusher, typically a jaw crusher, where it is crushed down to a manageable size for further processing.
Material that exceeds the maximum feed size of the crusher, (typically 85% of the actual size of the jaw box) or fines material that is too small to crush can plug up the crusher and severely hamper crushing performance or worse case, seize the crusher up, bringing the entire operation to a sudden and abrupt halt. The best way to prevent this from happening is to ensure that it doesn’t happen in the first place.
The process of removing this material is referred to as scalping and depending on factors like production volumes, material size and pit conditions; there are several ways to accomplish it though some are more costly and labor intensive than others.
The most common methods of primary scalping utilize grizzly bars or grid to remove and prevent the oversize rock from entering the jaw and a grizzly feeder that removes the fines and allows them to bypass the crusher completely.
Grizzly bars are placed across the hopper and the spacing between the bars set to accommodate the maximum allowable feed size of the crusher. Oversize rock lands on top of the grizzly bars and can easily be removed by pushing them off with a loader or tilting the grizzly so the rock slides off. The rock can be used for rip rap, landscape material or broken down later for processing.
The same principle as grizzly bars, but instead of clearing the rock, the grid holds the stone in place so that it can be broken down to a passable size with a hydraulic hammer.
A vibrating grizzly feeder (VGF) is located at the bottom of the hopper and directly feeds into the jaw crusher. As its name would suggest, the VGF vibrates and this agitation causes the material to separate. This process, known as stratification, moves the small material to the bottom and larger to the top. As it moves closer to the jaw, the material passes over the fingers at the end of the VGF and the smaller material falls through the spacing and the stone that remains enters the crushing chamber.