Selecting equipment for washing and classifying can seem like a daunting task.
There are countless solutions using a variety of equipment that can be put together based on a producer’s needs. Following is an overview of wet processing equipment available to producers.
Coarse material screw washers
Built primarily to wash crushed stone and gravel, coarse material screw washers effectively remove light, loamy clays, dirt, crusher dust and coatings that cannot be removed by wet screening alone.
Coarse material screw washer paddles are used in conjunction with screw flights to provide scouring and agitation. The turbulent washing action combined with rising current water, which is introduced at the feed end at the bottom of the box, results in separation of the lighter fraction from the sound aggregate.
Lighter fractions float to the surface due to water rising in the box and then overflow the weir located at the back of the box. Desired clean product is then scrubbed and conveyed by paddles and flights to the discharge end of the box.
Paddle configurations can vary based on the design and length of the coarse material screw washer. More paddles increase washing action but decrease capacity because paddles do not convey material up the washer box as fast as flights. When using additional paddles, it is necessary to lower the slope of the box and increase motor horsepower, which will help convey material to the discharge end. Additionally, reversing some of the paddles will retain material in the box longer.
Fine material screw washers
Fine material screw washers are used primarily to dewater, classify and wash minus-3/8-in. sand or other fine material. They are designed to accept feeds from sand classifying tanks, belt conveyors, other fine material screw washers or slurry feeds.
In a fine material screw washer, as material enters the feed box, heavier material sinks to the bottom of the box, while finer fractions float to the surface and over the back weir. Material that sinks is then conveyed from the pool area up an 18-degree slope toward the discharge end.
As material is conveyed, it enters the dry deck section of the washer box, at which point the water begins to separate from the material. Curvature of the rotating screw flights conform to the curved section of the washer box to efficiently convey, wash and dewater.
Sizing and selection of fine material screw washers is based on the type of material, feed gradation, desired product specification, capacity required and water volume, if in a feed slurry.
Dewatering screens
Dewatering screens have a variety of applications, including dewatering, desliming, degritting, rinsing, scrubbing and washing.
They are designed to produce a consistent drip-free product, which is typically significantly drier than other dewatering equipment, such as fine material screw washers. Materials such as sand, gravel, crushed aggregates, frac sand, industrial sands and mineral sands are all capable of being processed through a dewatering screen.
To dewater material effectively, slurry is fed onto a steep, downward-inclined screen surface at the feed end of the dewatering screen to achieve rapid drainage. A pool of water begins to form in the valley as material builds up on the slightly upward inclined surface.
Counter-rotating motors create a linear motion, driving solids uphill, while liquid drains through the screen media. The uphill slope of the screen, along with a discharge weir, creates a deep bed that acts as a filter medium, allowing retention of material much finer than the screen openings.
Information for this article derived from Pit & Quarry University, an educational resource of Portable Plants’ sister publication.