Answer: Use of a purely mechanical force, such as screening, to remove shale, lignite, or other lightweight aggregate from river sand is normally unsuccessful. In most sand plants it's very difficult to control the raw feed rate, and as a result, it's 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 or lignite 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' 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 quantity of minus 100 gradation in the raw feed to your plant's scalping isn't too high, you might add waterline with some adjustable nozzles to create an upward flow in the tank. The only problem is that you may increase the volume of overflow material and create a settling pond filling problem.
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 hydrocyclone.