Recovering Fine Gold

The sluice box is probably the most common gold recovery device after the gold pan. Every dredge, every high-banker, has one. Small-scale hand miners to commercial scale placer operators that run several hundred tons per hour, use it. They are reliable, inexpensive and simple to operate. They can recover a very high percentage of gold larger than 50 mesh, and when properly designed even recover a good percentage of the gold greater than 150 mesh in size.

Historically, there have been a huge number of designs with parallel riffles, cross riffles, stone lined bottoms and the modern Hungarian riffle types. If properly designed and operated, sluice boxes can provide much higher concentration ratios than most other gravity type concentrators, with very good overall gold recovery.

A sluice box is similar to the bed of a river, with some areas of the river having just the right conditions to concentrate considerable gold, while other areas in the river have all the wrong conditions and retain very little gold. Running your sluice box correctly will concentrate lots of gold, whereas if you do not run it right, it will allow much of the gold to run right through. So it’s worthwhile to take a little time to make sure your sluice box is operating properly and getting the reliable recovery you want to get.

The toughest gold to recover is the fines and they are the most likely to fail to settle and the most likely to be scoured out by turbulent waters. If you can run your box in away that it reliably catches 50 mesh gold, you won’t have problems with the coarse stuff. The actual shape of gold particles in the placer deposit can greatly affect recovery. Flat particles tend to lie down and resist the water’s flow, while round particles would have a tendency to roll, but unfortunately the placer miner cannot choose the shape and size of the particles in the dirt being run.

Reading the sluice, and interpreting the results, takes some understanding of how a sluice works. There is more than one mechanism at work in a sluice, and there have been a number of different opinions as to how these forces interact to capture the gold. In the past, the settling velocity of gold (and other high-density materials) was considered the chief factor, but more recent scientific research emphasizes the centrifugal force of the swirling water behind the riffles.

Researchers have stated that riffles work because “turbulent eddies are formed in the slurry as it flows over and around the flow obstructions that comprise the riffles.”

The sand/water slurry velocity (momentum) provides the energy that powers the vortex behind the riffle. If the velocity of the slurry is reduced through overloading with solids, insufficient waterflow or some other reason, a full size vortex may not be created, leading to riffle packing and loss of gold. Pay gravel that contains a high proportion of high specific gravity minerals, such as magnetite, or a high percentage of clay are more susceptible to riffle packing. These may require increased box tilt and/or water flow. On the other hand, if the sand/water slurry velocity is too high, extreme turbulence will occur and fine gold will not settle, or even after settling it maybe scoured out, causing excessive gold losses. The goal then is to find that happy medium where good flow keeps the material moving efficiently through the box, but high velocity and turbulence does not blow all the fine gold out as well.

Turbulence is a function of water speed. The more water you try to push through even a smooth bottomed trough (like a sluice), the more turbulent that water flow will be. Turbulence in the water flow of sluices is actually both good and bad. Behind the riffles where the turbulence helps sort the material in the slurry and accumulate the gold while sending the lighter materials downstream is a planned and limited turbulence exactly where you want it. On the other hand, turbulence is bad where it is large and unplanned—from excessively high water speeds around large rocks that get lodged in the system, etc. Random turbulence (mostly caused by high water speeds) is the enemy of good fine gold recovery. That really is the core “secret” of improving recovery of fine gold at commercial placer operations. If you can screen out the big rocks and small boulders to say 1 inch minus, then you only need a water speed sufficient to move a one-inch rock down the sluice. If you don’t remove the big stuff, you then need a stream of water sufficient to move an 8 or 10-inch rock through the sluice. The water speed needed to move that 10-inch rock (and the inherent turbulence it causes) is also sufficient to blow much of the 20 or smaller mesh gold right out of the box as well. That’s why frothing white water flying down a sluice box is a BAD thing.

Proper Angle and Water Flow

These are the adjustments and settings that concern us the most. Excessive water speeds and box tilts will allow gold to be flushed out of the sluice box. However, it is equally true that water that flows too slowly and tilts too flat will clog the riffles and also result in the loss of gold. Generally speaking, sluices should be run at the slowest possible speed without accumulating rocks in the sluice and without packing the space between the riffles. The slowest speed that will give a good movement of the slurry down the box. Over time you should accumulate some material in the box, but some matting should still show between the riffles.

Higher volume (larger sized) dredges often need an undercurrent sluice design to get decent fine gold recovery. High flows create turbulence that prevents small gold from settling out, keeping it in suspension until it eventually just goes out the back of the sluice. The smaller the gold size the more easily and longer it is kept in suspension, and the more easily it is lost.

If, after running a while, you have very little material accumulating behind the riffle, slow the flow down or decrease the tilt. If you run awhile and your box is accumulating sand and rock or burying the riffles, then you need more tilt or water flow. A “normal” sluice box tilt will typically run between 1.5 and 2.5 inches per foot of length, with high-banker sluices often having a bit higher tilt angles.

Gold Mattings

One of the best sluice box improvements made in the last 20 years are the spaghetti loop mattings. They are known by various trade names such as nomad or wayfarer, and are used under the riffles to catch fine gold and prevent it from eventually being washed down the box and out. These mattings can capture and hold considerable gold. They work by allowing the gold to fall down into the space between the loops and get out of the main flows of water. The fine gold is captured and protected from the scouring effects of the water flow. Based on a number of tests, the unbacked versions are preferred and appear to be the best box matting now in common use. There are a number of benefits to using these mattings. 1) They do not interfere with vortex formation behind the riffle; 2) most of its volume is available for gold storage; 3) it does not release trapped gold particles during sluice box operations; and 4) it is easy to clean. As a result, these mattings greatly improve the recovery of fine gold and are well worth the cost of adding to your sluice if you work in an area that has significant fine gold. After repeated use, a few small pebble grains can get lodged in the loops, but usually a good stretching of the matting will allow these to pop out.

Expanded Metal

Testing has shown that expanded metal actually recovers fine gold a bit better than the standard Hungarian or angle-type riffles. The greater height of the tall “standard” riffles makes more turbulence, which tends to flush out some of the smallest fine gold. The lower height of the expanded metal allows lower turbulence, therefore better recovery of fines. Researchers suggest that for best recovery, sluices should contain sections of short height expanded metal and sections of taller Hungarian riffles.

Material Sizing and Undercurrent Sluices

Separating out the fine gravels and processing them in a slower flow of water will result in a higher overall gold recovery. The design is called an undercurrent sluice and is also sometimes known as an over and under style sluice. The standard method is to pass the gravel slurry over a screening surface such as a punch plate or other similar device, allowing the small gold and finer sands to pass downward where they are diverted to a separate sluice area where water flows are slower, and fine gold particles can be recovered. The trick in designing these systems is to allow sufficient screening area so that the smaller material can settle down and pass through the holes. Where the fines are held up in suspension in the slurry, they cannot drop down through the screen. These then go to the “coarse” area of the sluice where fines recovery is not as good. Some commercial setups simply process the gravels by wet screening and only process the smaller material that passes through the screen. Another problem with these systems is that the screening unit, if it is a non-moving type such as a punch plate, will often plug after enough material has run over it, small rocks will lodge in the punch plate holes. So the system needs to be designed in such a way that this plate can be removed and cleaned.

Feed Rates

One very common problem with sluice operation is surge feeding through waves of material. This overloads the sluice, temporarily clogging the riffles. When the riffles get clogged the normal sluice processes are not working and gold may roll over the top and be flushed back out. This can be especially common in some commercial operations that are directly fed by a dozer. For hand-fed operations its easy to avoid this condition and worthwhile to do so. It is also important to note that when there is no feed coming into the sluice, and only clear water is running, the water flows have a tendency to scour out the fine gold and push it out the box. The spaghetti loop mattings do a great job of letting the gold drop down into the matting and preventing scour during periods when only clear water is entering the sluice.

High feed rates, which produce high-density slurries, are different than surge feeding. They are often found at commercial operations. As long as the other parameters of the sluice are properly designed and operated, good recoveries can be achieved with fairly high feed rates.

Dampers

Dampers are flat sheets of rubber that ride on top of the slurry. They spread out flow and even out distribution of the feed, helping to reduce excess turbulence. Dampers can drop any floating fine gold off the surface, but gold rarely floats in the normal turbulence of a sluice box. Dampers are well worth adding to a sluice box that does not have them.

Slick Plates

Some research reports done on sluice operations recommend slick plates in front of the riffles. This is to allow the gold to settle and be riding along the bottom of the sluice when it hits the riffles. This is the optimum point where one would want to have the gold located when it enters the riffle area. Some of the research reports recommend slick plates of around 4 feet long for large commercial dozer-fed operations, however for smaller scale individual operators with dredges or hand-fed sluices, much shorter slick plates are used.

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