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Processing

Separating Gold from Sand, Gravel, etc.

The equipment on this page is from Backcountry Mining Supplies.
Processing - using a "wash plant" - extracts the gold from the material that is excavated (whether by hand or using an excavator).

All placer processing has some basic steps...

  1. Removing over-size material
    1. Larger cobbles and boulders
    2. Smaller cobbles oversize gravel
  2. Extracting Concentrates (gold and black sand)
  3. Separating gold from concentrates

Step 1 - Removing Oversize Material

Gold is almost always a very small proportion of the material that is handled. It is often in material that contains larger pieces because both tend to settle out where the water slows. The extraction of gold in Step 2 generally can't deal with material that contains gravel above a certain size.

Removing larger cobbles and boulders

A "cobble" is basically anything bigger than a piece of gravel and smaller than a boulder.

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 heavy 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.

Removing smaller cobbles and oversize gravel

In a hand operation, a light-duty grizzly is often used to remove oversize gravel. It might be made from a rack from a refrigerator or oven or a sturdy BBQ grill. A sluice box with attached grizzly and a fitting for a water hose is often called a "high banker" or "highbanker".

For machine-digging operations, there are primarily two kinds of equipment used to remove cobbles and gravel above some size.

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 (probably because of all the hammering that goes on inside the trommel barrel).

Step 2 - Extracting Concentrates (gold and black sand)

Once oversize material is removed and all clay is broken up, the gold must be separated from the far larger amount of silt, sand and remaining gravel. This is done by taking advantage of the fact that gold is very dense - lead will float in mercury, gold will sink.

Generally, any way of separating gold from the remaining material will also separate out some "black sand" and maybe a little regular sand. This mixture is called "concentrates".

Sluice Boxes

The most common way of extracting concentrates is with a Sluice Box. They are simple and inexpensive. Processing has to stop for periodic cleanup because of a buildup of black sand and/or regular sand. If it is regular sand, adjustments to the angle of the sluice and/or the water flow may help flush out the sand from behind riffles without loosing any gold. The same thing applies to black sand, but it is more difficult to do without losing gold. This is particularly true if the gold is fine - it is better to cleanup more often than to lose gold. In fact, a sample from the output of the sluice should be panned out multiple times per day to see if gold is being lost; if digging moves into an area with more fine gold, the sluice should be adjusted to capture it.

Using multiple sluice boxes can increase the recovery of fine gold.

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.

Jigs

Jigs can be used with or instead of sluice boxes. Water and gold-bearing material flow across a box containing "ragging" - usually small steel balls. An arrangement in the bottom part of the box alternately pushes up and sucks down the water in the jig. Compared to lighter material, gold and other heavy minerals tend to move down more, and up less, than lighter material. Lighter material tends to flow across and off the top of the jig. Concentrates work their way down and out the bottom. Jigs are often used in pairs, with the material that flows off the first one falling a short distance onto the top of the second. Jigs are used by some mines, but they are expensive and because they have moving parts, they can break down. They may have to be adjusted when the material size or flow of water changes. They are not good at catching very fine gold or nuggets, but a nugget sluice upstream of the jigs can fix the second problem.

Step 3 - Separating Gold from Concentrates

Extracting gold from concentrates (gold, black sand and some regular sand) can be done in the field or concentrates can be shipped home and processed during the cold months when mining can't be done.

Small amounts of concentrates can be separated by hand panning.

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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