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Monday, November 16 2020
The use of water for grinding and milling has been in practice for thousands of years. From simple water wheels to inventors such as Archimedes of Ancient Greece and Bernoulli, a Swiss mathematician and physicist in the 1700s whose principles appear on modern day dredge nozzles, science has always advanced the use of water in our everyday lives. But have you considered just how much the dynamics of water can affect gold recovery?
Water Flow. The easiest example of how water dynamics affect gold is the classic inside bend in a waterway. When stream flow is straight and velocities are at their highest, gold will be suspended in the flow. As a bend in the stream causes flow to begin slowing down, the heaviest gold begins to drop out because the water no longer has enough velocity to suspend the material. The stream continues to lose power as the bend length increases and smaller and smaller gold continues to be deposited until there is no longer any in suspension, or the remaining water flow and speed cannot carry it farther downstream.
Water velocity and flow affect attempts at recovery in all capture devices in exactly the same way. Each piece of gold in classified material requires a certain flow and length of time to separate, drop out, and be captured based solely on its relative size and weight. If the velocity and flow are too great, the gold will remain suspended and will exit the machine regardless of machine length or method of recovery. This is why some miners are lured into a false sense of security just by using longer sluices.
Improper water depth in the sluice can create additional problems in recovery. Methods of material collection have much less effect if the gold is suspended in the upper part of the water flow. Most sluices provide little or no material agitation to break up the classified material and provide the necessary change in flow that facilitates gold separation. Lack of agitation allows smaller pieces to be carried through the equipment.
Classification. There is a direct relationship between material classification and water speed, which is often overlooked. It is common to classify for ease and speed of filling buckets rather than size of gold being targeted. Larger classification requires increased water speed to push the material through most recovery systems. The trade-off is forfeiting recovery of finer gold by increasing the length of drop time. This reduces the number of gold capture attempts and even machine effectiveness by having concentrations of larger material wedging into riffles, drops, etc.
Running dry material requires the sluice to liquefy the material while it is passing through. Gold particles will suspend in the dry material, reducing the chance of capture. Many miners use a scoop and drop their material into the machine in large clumps. This also allows particles to suspend instead of being processed. Material should always be wet prior to processing, and material should be cast (sprinkled) side to side across the machine when using a scoop.
Riffles. Some riffle designs are actually not that ideal for gold recovery. Some sluice riffles are evenly spaced, making them too far apart to impart the “vortex” or reverse underflow. What actually happens is an up-and-down wave motion as the water travels down the sluice. The only “action” imparted is the same negative flow that happens when water flows over a rock or some other obstacle in a stream. Material suspended at the bottom of the flow is deposited as the water climbs over the riffle and slows. Material that is heavy enough and can no longer be suspended drops. The rest of the material remains in the flow and continues to be carried down (and possibly out of) the sluice.
Flare. Most modern riffle sluices (standard and drop designs) are susceptible to improper setup angle, which can either clog the machine with material or blow it out completely. Either problem results in the loss of gold. Another big mistake in sluice design and operation is the “V” fallacy. The idea that channeling your water towards the center of the sluice using a wide-angle flare will increase its effectiveness and therefore increase gold recovery is false. On top of that, miners drop material into the center of the slick plate. This forces material into the center of the sluice, which is quickly overloaded by the volume of material and will cause the first series of riffles to lose their effectiveness.
Remove the flare from your sluice to eliminate the center concentration and create an even flow across the machine. You can create a V-shaped wing dam with a short straight section in front of your sluice if you need increased water flow or speed. Casting your material across the width of the sluice will eliminate overloading of the riffles.
Understanding how water height and speed changes your recovery can also make all the difference in boosting gold returns:
Water height. Begin with the water flowing just above the riffles or drops. Run small amounts of material and watch as the dirt passes through the sluice. Notice how the material enters the riffles or droops. Be sure to use the correct method of casting material evenly from side to side. Gradually increase the water height until processing suffers. This will determine the minimum and maximum water height for your sluice or highbanker.
Water velocity (speed). Set your sluice water height for max processing and a fairly fast flow, then gradually reduce water speed as you process small amounts of material. You may have to adjust your sluice to maintain proper water height through this process. Note how changes in water speed decrease the quality of material processing.
Spend some time exploring water dynamics. Introduce only one variable at a time, and verify how your gold recovery is affected before moving on. No doubt, with a little more understanding and experimentation, you’ll recover more gold with your sluice!
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