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  • Writer's pictureLori Dee

Sluices, Long Toms, and Dredges

Sluices, Long Toms, and dredges are all basically the same thing with some minor differences. A sluice is a chute that water flows through. It is made with some type of obstructions down the length of the chute to create changes in the water currents to trap gold and other heavy minerals. These obstructions are known as riffles and there are many types and variations.

A Long Tom is a very long and narrow sluice built to allow multiple people to shovel dirt into it along its length. These were very useful for large placer mining camps that covered large areas and employed many workers.

A dredge is a floating platform with a sluice on top. A pump is used to suction material off the bottom of the river or lake. The material is forced through a hose up to the surface where it is discharged into a sluice.

Since the sluice is the underlying method of gold recovery in all three of these, it is good to have a working knowledge of how sluices work.

There have been numerous studies done on various sluice box designs. These studies were primarily concerned with large commercial mining operations. They looked at a variety of variable factors such as riffle designs, water flow, size and configuration of sluices, material feed rate, and so on.

Three reports that are often quoted in other studies and provide a great deal of information are listed below. The third report, by Weishaupt and Jacobson, was more concerned with the small mine operator and amateur than with the large commercial operations.

Fine Gold Recovery of Selected Sluicebox Configurations, University of British Colombia, Dept. of Mining and Mineral Process Engineering (Studied sluice box configurations in commercial operations in the Yukon Territory).

An Analysis of Sluicebox Riffle Performance (aka The Clarkson Riffle Test), by Randy Clarkson and Owen Peer (Report prepared for the Klondike Placer Miners Association, March 1990)

How To Build and Operate Sluice Boxes, Part III - Riffle Testing, by Gary Weishaupt and Chris Jacobson

How a sluice works.

There are many different types, but they all work the same way. Sluices have a wide flare at the head end to funnel moving water from a stream or river into the body of the sluice.

Dredges use pumps to supply water from below the floating platform up and into the sluice. A high banker is a sluice that is used on land and also uses a pump to pull water from a source and deliver it into the sluice. Dredges and high bankers do not have flares, but instead, have a header box that the water is pumped into.

Just past the flare is normally an area of just bare bottom known as a slick plate. As the material is added to the water flow, the slick plate allows the slurry to begin stratifying into its various layers: heavies drop to the bottom and lights float up to the top. If the material is not classified, the sluice should have a longer slick plate. If material is classified then less slick plate area is needed and that space can be used in gold recovery.

Just past the slick plate is usually a short section of rubber V-mat. This is the first "nugget trap" in that the heaviest pieces of gold will stop as soon as it hits this sticky rubber mat. Very often smaller pieces will also get caught on this mat, especially flat flakes. The black mat makes the gold easier to see and this allows the operator to tell if the material is rich or not.

Next, we travel down the sluice into the gold recovery area. Think of the water flowing in a sluice in three dimensions. You have the length of the sluice, the width of the sluice, and the depth of the water in the sluice. The water flows in what is called laminar flow, which is like layers from top to bottom.

The water flowing along the bottom pushes heavy materials on their way downstream.

In the middle layer, lighter sediments are carried along in the current and will try to drop into the bottom layer due to gravity.

The top layer is where mostly dissolved solids flow. These are very lightweight materials that will get carried down the full length of the sluice. Due to gravity, they will try to settle into a lower layer but usually will just get washed out of the sluice.

Think about what we learned about the water flow in river valleys. Fast water moves heavy materials, while the slowest water allows the light sediments to drop and settle to the bottom. Moving water gets its energy (kinetic energy) from its movement. The faster it moves, the more energy it has. Obstacles in the flow absorb some of this energy, causing the water to slow down and lose some of its force.

The purpose of the riffles along the bottom is to provide a place where gold can get trapped. Since gold is very heavy, it will try to remain on the bottom. Early sluice boxes used wooden strips or sticks for riffles. The first sluice I ever made used popsicle sticks glued into a vinyl rain gutter. These crude and simple techniques worked, just not very efficiently. They were fine for recovering large gold and the old-time miners were content with that. However, over time, the big stuff became more and more difficult to find. Miners needed to shift their attention to ways to catch fine gold.

Improved riffle designs used different size angle-iron as riffles. Some of the riffles were just flat pieces sticking straight up, some had L-shape angles, and some were more like flattened L-shapes, called Hungarian Riffles.

As water flows down the sluice it has the energy that it uses to push materials along the length of the chute. It takes more energy to push heavy materials than it does to carry light materials that float along the top. Thus, gold will try to travel in a straight line. As the water flows over a riffle, it uses energy to get up and over it. It uses even more energy if it is pushing rocks and heavy chunks of gold up and over the riffles. Consequently, the water loses energy (force) and slows as it moves further down the sluice.

If a riffle is angled too steeply, the water flow hits it like a wall and shoots up and over. Any gold that it was carrying could also be launched into the upper layer and carried out of the sluice. By angling the riffles, like the Hungarian riffles, the water flows more smoothly over them without as much turbulence. This also allows the water to maintain more of its energy further down the sluice.

In their report, Weishaupt and Jacobson state: "Flow velocity is far more important than flow volume." The energy is in how fast the water is moving, not how much water is in the box. Remember what we learned about river water flow? Fast water erodes (pushes material) and slow water allows materials to deposit. We don't want the riffles to fill up with light sand or else there would be no room for the gold. If the riffles do not get cleaned out (called scour), any other heavies will just roll over the riffle and continue without being captured. The velocity must be just right so that it carries the light stuff out of the way while leaving the gold trapped in the riffle.

Let's look at this again. As the water flows up and over the riffle, it then comes down behind the next riffle, flows back up toward the first riffle then up and out, and over the next riffle. This circular motion creates a vortex in the riffle. Gravity pulls heavy material down with the flow of the water. As the water comes down into the vortex, it creates a small suction that pulls water and materials in behind it. The water then pushes the material back against the riffle and holds it there with centrifugal force. There is not enough energy left to lift heavy materials out of the riffle so they remain trapped. Lighter materials that require less energy, are boiled out and cast back up into the middle layer to be carried downstream. The area between riffles where all of this happens is called an exchange zone. This is where the riffles do their work capturing heavies and washing out the lights.

Think of each riffle as a ramp, and you are rolling a heavy stone up and over each riffle. It will require quite a bit of energy and sluices get all of their energy from moving water. So if you increase the water flow, (thus increasing energy and force), it is possible to make the heavy rocks bounce right on over each riffle and out of the sluice. But the smaller gold sediments that were in the middle layer never got a chance to settle and were washed right out. Likewise, with the increase in force and energy, the vortex becomes overpowered and starts washing heavies out of the riffles. A similar issue occurs if the riffles are too close together. Instead of the flow moving down the back of the next riffle, it hits the top of it and no vortex is created to trap the gold. So we need to compromise. We need enough water to make the riffles work to hold the small gold, have enough flow to work riffles at the end of the sluice, and not be so powerful that we lose fine gold.

From the Poling report:

"The scour condition that exists in the sluice is the most significant factor in predicting recovery. Each riffle type has a characteristic scour condition where gold recovery is optimal."

The easiest way to make that happen is to make sure we are sizing the material that goes into the sluice. This is why we classify our materials.

In their report, Weishaupt and Jacobson state:

"Classification is critical and the increase in fine gold recovery is almost directly proportional to the extent of classification used on the source materials in relation to the general distribution of pay materials at any particular site."

Remember the Golden Rule: When all things are equal, gold rules.

If everything in your sluice is the same size, then gold will be the heaviest thing there. If gold is the heaviest, it gets trapped in the riffles, and the other lighter stuff gets washed away. The key is to get your material down to the size of your largest riffles so it can be trapped there without needing to overpower the sluice with too much water velocity. You want a nice laminar flow going over the riffles, not a bunch of turbulence.

I have studied many reports and there are a few things that everyone seems to agree upon:

1. Riffles are a bad way to capture fine gold, but a great way to capture big gold and even nuggets.

2. The best way to capture fine gold is without riffles. Using expanded metal over matting (carpet), or even just matting alone will capture the most amount of small gold. BUT large gold will just roll on out of the sluice.

The conundrum here is that riffles require faster water to get the riffles to work and scour correctly. That much water velocity would be too fast for a fine gold recovery system using mats and/or expanded metal.

Since we want to capture ALL of the gold, we will need a system that uses both methods. Most of the gold we will capture will be fine gold, so most of the length of our sluice should be designed for that purpose. We don't want to ignore the possibility of finding a nugget, so we want to include places that have "nugget traps" that will capture any that we find. But we also don't want so many riffles that it interferes with our fine gold recovery.

A long and narrow sluice is better than a short/wide sluice. Remember what we learned about water flow. Water flows faster in a narrow channel and slows in a wider channel. A narrow sluice is easier to control the water velocity than a wide sluice. We can do this easily by raising and lowering the head-end to change the sluice angle. Sluices are heavy when they are full of water. A wide sluice is more difficult to adjust on the fly. The longer your sluice is, the more opportunity the sediments have to settle from the middle layer to the bottom where they can be captured. With a short sluice, the middle and top layers wash out of the sluice too quickly.

Now, let's look at the most common sluices being produced for sale. We will ignore the gimmicky types that are new to the market and instead look at what is being produced by the big manufacturers, like Keene, Royal, etc. Many of them want to sell you a "mini-sluice" because they are small enough to take anywhere. Stay away from those. As we have just discussed, they are not a good way to sluice.

Next, we see that they lined the bottom with matting, topped that with expanded mesh, and then put Hungarian Riffles on top of that. They have all of our methods in one package. That's good, right? No, it does not work that way.

The riffles and expanded metal over the mat must be in separate areas of the sluice. The riffles cause so much turbulence that the expanded metal cannot do its job of getting the fine gold down into the mat. Riffles need fast water. Expanded metal and matting need slower water.

If you look carefully, there are gaps between the riffles, the sides, and the expanded metal where water can flow through. That means anything in that spot will get flushed. Many of these sluices only have one or two hold-downs to keep the expanded metal/riffles in place. If not held tightly, even the expanded metal will get pulled up in the water flow allowing materials to get flushed. I tried using silicone to seal the gaps to prevent this, but the turbulence from the riffles still worked against me. The mats filled with sand and captured no fine gold because there was no place for it to go. It just got washed out.

I finally ripped everything out and threw it away. I then invested in Gold Hog rubber matting and have had no problems ever since. Gold Hog offers a wide range of mats allowing you to customize how your sluice works under various conditions. I knew what size gold is in my area, and how the creek water flows in both high Spring runoff and in late Summer lows. I used that information to put together a sluice that works year-round. I have pics of the gold I have found in my Gold Gallery to show that it works.

If you are designing your own sluice, remember that you will want your riffles/nugget traps up near the head-end of the sluice where the water is running fastest. Then a short space to allow the turbulence to calm down before entering your fine gold recovery area.

Poling disagrees and feels that the nugget traps should be at the end of the sluice. That way, the turbulence from the riffles will not interfere with the fine gold recovery:

"Angle iron dredge riffles should be used somewhere in the fine gold recovery area to recover gold particles much coarser than 20 mesh and smaller than the upper feed size. Frequently referred to as a "nugget trap" when used in this manner, the dredge riffles would serve to capture gold particles too large to be retained in the relatively low profile expanded metal riffles. Except for extremely flat particles, which might be caught in the fine gold riffles, the recovery of the +10 mesh gold nuggets should be high in dredge riffles. An ideal location for such a 'nugget trap' would be at the discharge end of the sluice. This location would allow the fine gold riffles to process the well sorted slurry at the sluice entrance. In this manner the maximum amount of fine gold could be recovered in the more efficient riffles prior to passing over the less efficient, more turbulent, dredge riffles. The gradient of the 'nugget trap' portion of the sluice could also readily be changed to produce the appropriate scour without influencing conditions in the fine gold riffles."

The reason I disagree is as I have just discussed. Water velocity is lost as it travels down the sluice. Most of the water's force is at the head end. The water will be at its slowest at the discharge end and may not be sufficient to properly scour a set of riffles. The lower half of the sluice should be where fine gold is captured.

Some parting thoughts from the Weishaupt and Jacobson report:

"Sluices have to be brought up (or down) in flow velocity to the point where the riffles (or meshes) begin to ‘work’ and actually start concentrating heavy materials. Proper sluice setup and tuning can account for huge differences in collection efficiency so in this respect there is no substitute for owner experience with a particular piece of equipment. A novice running a particular sluice will never collect as much ‘good stuff’ as somebody well experienced with the same gear digging in the same spots. This is just one of ‘facts of life’ that we all have to get used to. There is no ‘magic’ box that makes all of us equal and experience will always trump technology.

"In a similar vein as the theme mentioned above there are no ‘good’ riffles or ‘bad’ riffles as they all work up to a point and an experienced user can get good results using a less than optimum riffle design compared to a newcomer who happens to be using state of art riffle designs. For this reason ‘testing’ of any type is kind of a subjective thing to begin with as over time we all learn how to maximize what it is that we have on hand at any particular moment in time. Don’t take ‘tests’ too seriously as the end user has almost total control over their own gear no matter what some ‘tests’ may indicate. The objective is to find and capture gold and how you end up doing that is entirely up to each individual. Miners are about the most independent bunch of people on this planet."


All content Copyright 2021- 2024 - Lori Dee



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