How they are handled depends on the operation. In a hand operation, some cobbles may be picked up and washed (often by dipping them into a pail of water), but all operations have some size limit above which the only processing a piece gets is being shifted out of the way (and maybe being washed off).
In a machine-digging operation (with an excavator), a heavy grizzly is used - a heavy-duty grate at a fairly steep angle (30 to 50 degrees) with gaps between the bars ranging from maybe 2 to 6 inches. Pieces that are too large to pass between the bars roll or slide off. One or more spray bars may be used to wash fine material off oversize pieces and to help the sliding. Railway rails (upside down) can be used as bars in a heavy grizzly.
When using a grizzly, from time to time it is necessary for feeding to stop and for someone to climb up there with a hammer and remove pieces of rock that are jammed between the bars. This might happen multiple times per day to multiple times per hour, depending on the material.
The most common is a trommel. The blue one to the right is fairly small; a medium sized trommel might have a barrel 3 feet across and 10 to 15 feet long. Material that passes through the grizzly enters the upper end of a rotating steel cylinder (called a barrel or drum) at a moderate angle. The barrel is perforated for part of its length - holes might be one half to one inch across. A serious spray bar runs down the center of the barrel. Steel plate a few inches wide runs lengthwise inside the barrel to lift and then drop material as the barrel rotates (like in a clothes dryer). Because the barrel is at an angle, material is dropped a little lower than where it was picked up. Anything that makes its way through the holes is directed into a sluice box or jigs. Material that makes it to the end of the barrel is discarded. Because the contents are dropped repeatedly with water, trommels are good at breaking up clay.
An alternative to a trommel is a screening plant that uses screens at a moderate angle that shake or vibrate with water sprayed from above. Often, multiple screens are used, each finer than the one above it (which prevents large material from damaging a light-duty screen). The bottom screen might have spaces that are one quarter to one inch across. Anything that rolls or slides off the screens is discarded. Anything that makes it through the screens is directed to a sluice box or jigs.
Screening does not work well if the material contains much clay. In addition, screens get "blinded" (all the holes filled with something that won't go through), which isn't much of a problem with trommels (because of all the dropping and hammering that goes on inside the trommel barrel).
A sluice box should not be rigidly attached to a trommel or screening plant; ideally it should not be physically attached at all. You don't want vibrations from boulders hitting the grizzly or cobbles tumbling inside a trommel to shake gold out from where it has been captured in the sluice.
Black sand generally includes magnetite, which is magnetic. Using a magnet directly is frustrating because the particles are difficult to remove from the magnet. Inexpensive devices can be used that move a magnet to and away from the end so you can pick up the magnetic sand and then drop it elsewhere. You can accomplish the same thing with a magnet and plastic from a plastic bag.
If concentrates are high in magnetic sand, some gold can be trapped between grains of magnetic sand and be removed as well. This can be seen if the magnetic sand is released in a jar of water. The solution to the problem is to do the magnetic separation twice, ideally from concentrates that are in water.
A special purpose Sluice Box sluice box can be used to separate gold from black sand. This sort of sluice should have no regular riffles. Only ribbed rubber floor matting should be on the bottom, preferably of a color that makes it easy to see black sand and gold. The ribs run across the box. Doing the separation is a matter of adjusting angle and water flow to separate the black sand from the much denser gold.
Another type of sluice for concentrates is made from a piece of corrugated plastic drain pipe 4 to 6 inches across and 6 to 8 feet long. The pipe is cut in half length-wise and one half is screwed to a plank open side up. At the top, an arrangement for a garden hose might be used to supply a gentle flow of water. If the flow and angle are just right, almost all the gold will be caught in the first few corrugations. This sort of sluice may also be used in the field with a small (probably 12-volt) pump to supply water. Concentrates might be fed into the top end of the sluice with a spoon.
Gold spirals, are effective and they are available in a wide range of sizes, from about 18 inches across to 6 feet across. Details vary, but the general idea is that concentrates are fed onto a flat, tilted, slowly rotating disk near the top. The disk has a ridge or groove that spirals around many times, ending up at a hole in the center. Water flows over the disk, carrying away lighter material, while the gold is carried by the spiral, eventually falling into the hole through the center.
There are a variety of bowl separators of different sizes. I believe these devices are mostly for small amounts of concentrates - that is, up to cups, not pails. They swirl the material around the inside of a bowl using a flow of water. The lightest material moves furthest from the heavy gold. The "Blue Bowl" has a good reputation.
Shaker tables or shaking tables are very effective and are also available in a wide range of sizes. They are generally rectangular and tilted so that one long side is lower than the other. Small ridges run across part or all of the table. Water flows from the top edge of the table down over the ridges and off the lower edge. A slurry of the material in water steadily flows onto the top right corner of the table. The table repeatedly moves a little to the left and then snaps back to the starting position. Lighter material tends to be carried down the table, over the ridges and off the lower edge by the flow of water. Black sand, being heavier, doesn't move over the ridges as much and tends to move more diagonally towards and off the lower left corner. Gold moves mostly horizontally due to the ridges and the snap-back motion, and moves off the top half of the left side of the table.
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