Disclaimer: what follows are just my personal opinions based on articles I have read, plus my own personal experiences. I could be dead wrong about either my facts or my conclusions. So be sure and check things out for yourself. Where possible, I have listed the URLs for the sites that contain data I have used. mdv
Have you ever heard of the Carlin Trend? It is located in Nevada and is the site of gold mines that have produced millions of ounces of gold every year since 1961.
It is, in fact one of the largest producing gold mines in North America and owned by Newmont Mining.
But here’s a little known fact: 97% of the gold they recover is so small it is invisible to the naked eye, measuring 1 micron in size…that’s one millionth of a meter…that’s .00003937 inches…and yet they recover millions of ounces a year! Wow!
I believe the majority of the large gold (4 mesh and larger) in the Eastern gold fields of the United States has been found. While there are surely plenty of “pockets” yet to be discovered, the larger gold you can “easily” recover with a pan, highbanker, or dredge has been found…and yes there is still a good bit of gold to be found that is smaller than 4 mesh.
However, the majority of the gold found on the East Coast is so small it is routinely called “fly poop”. In fact a lot of gold panners on the East Coast don’t even weigh their gold…because it is hardly worth the effort. It may take years to accumulate even an ounce of gold.
I believe 99% of all gold miners on the East Coast are missing 97% of the gold flowing through their equipment, why? Because it is too small to be captured in their sluices due to: Settling Velocity.
Settling velocity for gold is, in layman’s terms, the rate at which a sphere of gold will sink to the bottom of the stream, sluice, etc. under controlled conditions.
Scientifically, it is measured in the amount of time it takes to sink 100 millimeters (mm)-about 4 inches. Take a look at the chart on page F9 of the document on Hydraulic Equivalence
Notice the chart indicates that a sphere of approximately 140 mesh gold (0.1mm) will take around 4/10 of a second to sink 4 inches in clean water with no turbulence.
Think about this…how long does your material stay in your sluice before it is washed out the end as tailings? Try timing how long it takes your rubber ducky to go from one end of the sluice to the other…you may find 4/10 of a second isn’t really very much time at all. But even 4/10 of a second is in perfectly clean (no slurry) water, with zero current. Doesn’t sound like any sluice I’ve ever seen!
If nothing else, this would certainly make a case for longer sluices!
Conclusion: I am allowing the majority of the really fine gold I am putting through my trommel and sluices to flow right out the end, right back into the ground!
Enter the centrifuge…
The centrifuge is a device that spins the slurry and acts as a magnifier of gravity. A centrifuge typically accelerates at about 50 G’s. That is, the weight of an object spinning in the centrifuge is 50 times what it weighs when it is stationary.
This dramatically reduces the amount of time needed for a piece of gold to settle to the bottom-or in the case of a vertical centrifuge, to reach the side wall where it stays until the bowl stops spinning.
A centrifuge uses water to wash away the lighter materials up and out the top and the gold remains behind because it is heavier, and held in place by centripetal force.
In essence, a centrifuge is a Settling Velocity Magnifier…it makes gold settle faster by making it weigh more.
In order for a centrifuge to be effective, the material needs to be classified so the difference in specific gravity and density of the gold can be taken advantage of fully.
This also minimizes some of the hydraulic equivalence issues that would be encountered if the material contained different sized materials along with the gold.
See the Knelson Concentrator website to see a diagram of this at 00:45 (turn your sound down as the sound track is a bit cheesy)
Micron gold, I believe, is out of reach to most of us without resorting to sophisticated and expensive chemical processes which require running millions of tons of dirt to make it profitable.
However, even without a commercial grade centrifuge, we should be able to routinely recover gold down to 400 mesh with devices like the ‘Lil Spinner.
But, in order to do this, the output of your sluice will have to go directly into the centrifuge. Ideally, it will go into a crusher to keep the larger rocks and other material properly classified, but it can also be classified out with a grizzly or punch plate of some sort.
Unless you only run your tailings directly into the centrifuge, most of the really fine gold is not being captured in the mats and is therefore washing right out into your tailings…in other words, “You can’t get what ain’t there”.
The best concentrate device in the world, including a centrifuge, can’t recover 400- gold if the fine gold has already left the sluice box and washed out into your tailings, in which case we’re just leaving it for the next generation…come to think of it, I’m glad the old timers couldn’t get it all. That left some for me!
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