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Gravity Concentration 3

Sluice Box Design and Operation

Sluice boxes are far and away the most used method of placer gravity concentration. Although virtually never seen in a hardrock mill, they have been the mainstay of placer miners the world over for thousands of years. Sluices mimic the natural processes of a stream bed. The gold from the mountainside gathered in the stream gravels because of the action of water, so it is natural to use this same process to concentrate the gold further.

Water flows down a channel and washes away the gravel. Gold being heavier will accumulate in the natural cracks and crevices in the bottom of the channel.

Now a lot of folklore is used in the design of the sluice. The length, width and height, the type, size, material and shape of riffling, type of mat, water flows, sluice angle, or whether to screen the feed and to what size. There are tradeoffs on all of these.

Length: Should be as long as possible, a sluice can never be long enough. Sluices over a mile long in California were found to pay for every inch of sluice in the extra gold caught. Every foot of a sluice will catch more gold and will pay for its construction and use.

Width: Depends on how many tonnes per hour are to be processed. I have never seen a production one less than 6 inches ( 150cm ) and I have seen them up to 100 feet ( 30m ) wide.

Height: high enough to keep the water in but not so high as to make it difficult to remove rocks.

Riffles: One of the areas where the testimonials and strong beliefs come into play. Anything will work as a riffle, from nothing but the bottom of the sluice to a sheepskin to a root to fancy shaped iron. I will discuss more on this later.

Mat: It is a good idea to provide a bottom layer to the riffling. It acts like a safe repository for the fine gold to be protected from extreme bouts of turbulence or air bubbles. It also makes it a little easier to clean up the sluice.

Water flow: This is where we see the inherent problem with sluice boxes. With no flow of water obviously nothing happens. With a very high flow of water everything gets flushed straight through and nothing is recovered. We have to find the optimum flow rate of water.

Low flows: One may be tempted to keep the flow of water as slow as possible. In this fashion there will be little or no turbulence, no entrained air, and the finest particles of gold will have a chance to settle to the bottom of the sluice. The problem here is that there is not enough energy to move the larger particles. There may also not be enough water to fully breakup and wash the gravel. The gravel sits like a lump in the bottom of the sluice and one has to rely on mechanical means (box tender, or screening the feed) to move the gravel along. The next problem with low flows is that the sluice riffles pack solid with gravel and leave no opportunity for the gold to be captured.

So we need enough flow to thoroughly wash the gravels and keep the spaces between the riffles stirred and semi fluid. If we are trying to wash 12 inch ( 300cm ) rocks through the sluice we will need a lot of water.

High flows: With lots of water it is much easier to handle large gravels as the water carries the gravel through the sluice at a good pace. Gravels and soils may travel through the sluice as a "slug" of material, not even having the chance to be properly washed. The water is very turbulent and has the reputation of scouring the riffles of material. Old hands warn about "white watering" the sluice because much of the already caught gold can be lost. Lots and lots of water will cause loss of the gold and very low recoveries will be the result.

Ideal flow: Somewhere between low flow and high flow there must be a magical spot where ideal flow occurs. This will be the goldilocks point where it is not too fast and not too slow, just right. Unfortunately in a sluice box this point never occurs. The optimum flow rate may make a great difference in recovery but large losses of the fine gold will still occur, as the velocity needed to transport gravel is in the turbulent flow regime.

Sluice angle: There is an old adage that when you have fine gold you increase the angle of the sluicebox. This actually makes sense. When the angle of the sluice is greater one can still move the larger gravel with less usage of water.

Feed screening: This is the single most important thing to do when placer mining. Gravity concentration is far easier to do when the feed is prepared correctly. If you have spent a lot of time panning for gold such as I have you will know that the best gold is always stuck to the large rocks. The best pans are found in the cracks of the bedrock, or from the cervices in a large rock. The large rocks must not be discarded unwashed. Build a wet grizzley and make sure that no rock goes to the tailings unless it is completely clean.

Riffles: Any abrasion resistant material may make a good riffle. The essential design rules are: to present a smooth enough surface so that large material does not hang up, enough pockets of sufficient depth for the gold to settle into and to be protected when it is collected, and designed to be easily taken apart and reassembled for cleanup. You will notice in the next photo that the gold will settle even before it gets to the riffles.