Article as featured in the October/November Pick & Shovel Gazette.
By Kevin Bell
Aurora Mining Products
The use of water to perform tasks such as grinding or milling has been practiced for thousands of years. Chinese, Roman and Middle Eastern countries each contributed to increasing the efficient use of water as a tool. Inventors such as Archimedes
of Ancient Greece and Bernoulli (the inventor of the principle that drives our modern dredge nozzles) used innovative scientific principles and techniques to advance the use of water in everyday life.
How can we use water more effectively in gold recovery?
One of the most important and overlooked aspects in mining is the dynamics of water and how it affects gold recovery. A significant portion of gold is lost due to failures in three major areas:
•Incorrect water velocity, depth and lack of agitation;
•Improper classification and material handling; and
•Using equipment with limiting
We can easily demonstrate water dynamics and its effects on gold using the classic inside bend example. When stream flow is straight and velocities are at their highest most recoverable or “flood” 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 can 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.
Today’s miners are lured into a false sense of security by using longer sluices or adding special riffled matting which has little or no effect and can even reduce capture by negating or altering built in flow characteristics.
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.
Finally, most capture devices provide little or no material agitation to break up the classified material and provide the necessary change in flow that facilitates gold separation. This lack of agitation allows smaller pieces to be carried through the machine.
Another common mining problem is improper classification. There is a direct relationship between material classification and water speed, which many miners overlook. 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.
Many miners compound the problem by not handling material correctly. Running material dry 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 also use a scoop and drop their material into the machine in large clumps or clods. This also allows particles to suspend instead of being processed.
Randy Clarkson describes this phenomenon in his 1990 study and research paper: “ ... If the slurry velocity is reduced through overloading with solids, insufficient water flow or shallow gradients it may not sustain a vortex ... and gold efficiency will be reduced” (Clarkson 1990:4, An Analysis of Sluicebox Riffle Performance). Material should always be liquefied or slurried prior to processing and material should cast side to side across the machine when either cast by hand, bucket or when using a scoop.
Another issue is using machines with limiting capabilities. Modern riffle sluice designs are long and heavy and are set up to maximize attempts at gold capture, not recovery. Riffles are evenly spaced making them too far apart to impart the “vortex” or reverse underflow that is talked about in mining circles. What actually happens is a 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 the 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.
Most modern riffle (standard and drop designs) sluices are susceptible to improper setup angle which can either clog the machine with material or blow it out completely. Either problem has the same result—loss of gold.
Finally, one of the biggest mistakes 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 could not be further from the truth.
This problem is compounded by dropping your material into the center of the slick plate. This forces your material into the center of the sluice which is quickly overloaded by the volume of material and causes the first series of riffles to lose their effectiveness.
Removing the flare from your sluice will eliminate the center concentration and will 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 increase water flow or speed. Casting your material across the width of the sluice will eliminate overloading of the riffles.
Boost your gold returns
So, how do we overcome these issues and increase our recovery?
First, you need to understand how water affects your individual machine, or in other words how water height and speed changes your recovery. You can either use measured weights of gold or go to an area where you are familiar with gold concentrations. Work with one variable at a time.
• Water height: My recommendation is to start with 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 drops. Make sure to use the correct method of dropping or 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.
• Water velocity: The next test is water speed. Set your sluice water height for maximum 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 or increase the quality of material processing.
• Classification: The final test is to verify how changes in classification affect material processing and ultimately gold recovery. A common misconception is that taking time to classify down reduces the amount of material you run, which in turn has a negative effect on the amount of gold you recover. Classifying actually increases the concentration of gold in your bucket and eliminates unnecessary materials from processing. You will be able to process material faster through your sluice. This coupled with further reduction in water speed will almost certainly increase gold recovery.
Fine-tuning your sluice
Finally, here are some thoughts on fine-tuning your sluice and other sluice options:
• How things like expanded metal and matting changes affect recovery. Are they really worth the extra effort and do they work?
• Can I easily add some sort of material agitation which helps in stratifying material?
Once you’ve determined the best water height and speed for your machine coupled with the best classification size to maximize recovery in your sluice, you may want to add these additional items. Be sure to follow the process of introducing one variable and verify how recovery is affected. You can also use things in tandem with your current sluice. For instance, run material through a hopper with a rubber flap on the slick plate that helps liquefy and add agitation to your material.
Another thing to consider is changing to a high recovery or what I call super sluices. These sluices have no riffles, expanded metal or matting of any kind and therefore can be used with several water heights and speeds.
Fluid bed machines use a combination of negative pressure and fluidization within their material box to increase collection and recovery of both large mesh and very fine flour gold.
Other examples include a magnetic sluice that captures gold from classified material using black sand. Either of these examples take material processing and gold recovery to a much higher level.
Take some time to explore water
dynamics. I’ll bet you’ll get more gold!
Kevin Bell is an experienced prospector and inventor of fluid bed sluices. He is the owner of Aurora Mining Products. He can be reached through the AMP website: www.auroraminingproducts.com.