How are sand and gravel washed?

Question

How do the huge plants wash and grade sand? I want to do it on a very small scale in a developing country by hand.

Answer 1

The sand & gravel mixture is dumped through a sequence of screens and conveyor belts, starting with the coarsest screen, and ending with the finest screen. Materials that pass through the first screen fall onto the conveyor belt and get dumped through the next screen. Washing is used to flush the captured materials off of the screens. If the screens are built at a steep angle so the materials roll off, the process can also be done without water - this type of screen is made of parallel peices of steel so that large peices roll down the channels between the rods (not a grid or mesh) - could be made with iron pipe, steel fence posts.

For a manual system, you could build a series of ramps and screens so you can push a wheelbarrow up the ramp, dump the materials through the screen, and catch the materials that fall through the screen in another wheelbarrow placed underneath. Then the wheelbarrow could be pushed up the next ramp, and so forth. You could also get by with just one ramp, and re-process the piles by switching to a finer screen.