The Use of screening to remove shale or other lightweight aggregate from river sand is normally unsuccessful. It is very difficult to control the raw feed rate, and as a result, it is practically impossible to maintain a uniform, shallow bed depth (the depth of the material passing over screens) on each screen.
As a result, the shale particles which are normally lighter than other sands in deeper parts of those uneven layers don’t rise, and they get trapped in the sand. Also, adding spray bars to direct a downward water spray prevents these particles from rising.
The preferred way to remove shale and other lightweight material is to use these particles of light weight relative to other sand to your advantage through classification. When using water as a sorting medium, the density difference between same-sized particles makes gravity an effective tool.
As raw sand feed containing both shale and sand settles in the water of a classifying tank, the water provides just enough resistance so that the heavier sand particles quickly drop to their own bin, and the lighter particles descend more slowly. The sideways motion of a water spray in the classifier directs the lighter particles to a collection bin located on one side of the tank.
If the shale quantity is to high for this approach, the best way to control shale or lignite is to use a specially designed machine for this purpose. There are several types of equipment, such as rising current classifiers, jigs, or even a hydro cyclone.
