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dragline
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Re: Fine Gold Recovery with Magnetic Mats or Magnets in Sluices ( 05:28:43 FriMar 10 2017 )

Geowizard,

From my experience working with Oregon beach gold it seems likely to me that there will be no simple black sand solution here. One major factor to consider is that when one is lucky enough to find a black sand lens with gold values worth working there will usually be a very high percentage of black sand and magnetite. In other words, if you happen upon some sand worth working the majority of that sand is black and a high percentage of that will be magnetite.

One advantage that I can foresee with an electromagnet strategy would be that one could periodically turn off the magnetic field flushing the retained magnetite. Even if you were to shut down the field for just 1 to 2 seconds that should be enough time to clear a 4 foot sluice and then resume accumulating black sand. A properly designed neodymium system could accomplish the same thing with the ability to have a thin barrier between the magnets and the slurry such that lifting the magnets would flush the magnetite to tails.

I can not possibly predict with certainty what the outcome is going to be here with a permanent magnet setup but I have decided to try designing an experiment to test that out. If it doesn't work or increase fine gold retention hopefully I can figure out why it didn't and make changes that might make it work. I have to say that I am not very optimistic here. I just don't see any innovative strategies here using magnets that seem to me to be guaranteed to work.

It seems likely that if it were this simple to use neodymium magnets to improve fine gold recovery someone else would have made it work years ago. Since there seems to be no evidence that anyone has made this work my best guess is that there is no simple solution here that will improve fine gold recovery with neodymium magnets. But that isn't to say that there might not be a more complex and as yet untried strategy that will work.

dragline

  
JOE_S_INDY
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Re: Fine Gold Recovery with Magnetic Mats or Magnets in Sluices ( 06:32:16 FriMar 10 2017 )

While I am a bit simplistic some times, I just hold dear the fact that both "Black Sands" minerals are a little over a Specific Gravity of 5, while Gold is almost 20.

That being said, keeping the particle size of Black Sand pieces less than 1/4 the size of the Gold pieces (through classifying) and in a water based sluice box (which has been set up "correctly"), the Gold remains behind in a vortex or catchment while the Black Sands wash through. Of course, Gold shape is a variable factor but it sure is easier in concept than some of these complex thoughts.

Don't get me wrong, the high level planning and engineering here is fascinating - but after a while it hurts my head.

Now, I do use Neo magnets in the cleaning of residual Gold concentrates? Yes I do. I pick up black sands from over top of settled Gold in my plastic pan and remove those Iron ores easily before the absolutely last cleanup of the Gold.

Joe

Joe



---
Wiser Mining Through Endless Personal Mistakes
 
 
WmA
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Re: Fine Gold Recovery with Magnetic Mats or Magnets in Sluices ( 07:52:52 FriMar 10 2017 )

The black sand on the Oregon beaches is all -80, some -100, and a lot -150...........

This make the gold recovery challenging to say the least.
Getting the gold out of the black sand is also a bit of a challenge since it is also -100 and much smaller even to the point of being invisible to the unaided eye.

A super concentrator and finishing pan are then the best tools for final separation.

the methods so far that have the best capture rates are....

1. A New Zealand Beach box .... aka .... Gravity well .... this one has not been used on the Oregon coast to my knowledge other than in the form of the Gold Cube.

2. sawtooth mat, and deep V matting of these two I believe the sawtooth to be the best performer.

3. A magnetic mat sluice .... This allows a carpet of magnetite that is loosely held on the magnetic sheeting which allows for the gold to work it's way down and into the magnetite.

4. A Gold cube.

All of the above systems are all fairly technique sensitive in the need for feed rate , water flow, and angle.

Also as you well know there is no power allowed on the Oregon beaches so #1 and #4 are not particularly viable options for us.

Gold hog mats are a limited option due to the no power regs.

  
dragline
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Re: Fine Gold Recovery with Magnetic Mats or Magnets in Sluices ( 18:06:03 FriMar 10 2017 )

Fleng,

You've brought up some very good points about the use of electromagnets and I appreciate your calling my attention to those door-lock electromagnets. There certainly does seem to be a potential strategy there for generating a whole lot of magnetic field strength for whatever purpose or ends one might have a design or plan to use them.

Strategies that one may want to consider as a possible means of leveraging magnetic braking in our sluices for increasing fine gold recovery involve the possibility for asymmetric braking effects. What I mean by this is that there may be a potential for magnetically breaking gold that is not directly proportional to the mass or spacial orientation of the particle of gold.

One aspect of magnetic braking to consider is that it is also referred to as "Eddy Current Braking". There seems to be a tremendous amount of research recently into this phenomenon and one thing that the research has shown is that it is not so much the mass of the breaking object but instead it is the available surface area of the braking object owing to the fact that the electrical eddy currents responsible for the braking can potentially be much larger in diameter in a flat object with high surface area to mass relative to a more spherical object with a low surface area to mass ratio.

Potential strategies for employing magnetic breaking of fine placer gold in a way that might increase fine gold recovery include but are not limited to:

#1: The generally high surface area to mass ratio for gold flakes means that gold flakes will be decelerated or braked by the magnetic field to a greater degree than if that gold particle had been more spherical in shape.

#2: The orientation of a flat flake of gold relative to the magnetic field may effect the relative strength of the magnetic breaking.

#3. Given the first two effects above are factually correct there may be the possibility that thin gold flakes will tend to physically align themselves in the slurry such that they are held back (as kites) by magnetic fields (kite string) such that the flakes kite themselves down into the riffles when breaking against the slurry (flown in the wind).

dragline
[1 edits; Last edit by dragline at 02:54:41 Sat Mar 11 2017]

  
geowizard
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Re: Fine Gold Recovery with Magnetic Mats or Magnets in Sluices ( 01:25:54 FriMar 31 2017 )

dragline,

Have you given much thought to alternating current electromagnets and using different waveforms?

Alternating current might apply the brakes enough to slow gold AND black sand - then release the brakes so the black sand, having less density - continues on its way. This process could improve selection of GOLD. :smile:

- Geowizard

  
dragline
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Re: Fine Gold Recovery with Magnetic Mats or Magnets in Sluices ( 20:51:09 FriMar 31 2017 )

Geowizard,

No, while I had considered alternating current electromagnets, I had not thought about structured waveforms. That does sound interesting although being a mechanical engineer and understanding mechanical designs and assemblies fairly well, I have to admit that the safety requirements for electromagnets in saltwater environments concern me greatly.

My current approach to these experiments involve powerful N45 to N52 permanent neodymium magnets. Of course if my current efforts fail to achieve a cost effective benefit to fine gold recovery I might consider an electromagnet design.

Consider that my current neodymium strategy places the magnets above the slurry and riffles such that the magnetic attraction on the (magnetic) black sand would be the greatest at the top of the slurry and considerably less at the riffles. Likewise, the diamagnetic repulsion effects on the gold would be greatest at the top of the slurry becoming weaker at the riffles. The combination of these two opposing forces would tend to reduce black sand content in the riffles while also increasing gold content in the riffles. This is a win-win situation.

The problem with placing permanent neodymium magnets above the slurry is that these magnets will attract and retain black sand at the top of the slurry. While keeping black sand out of the riffles is a good thing, eventually, one would think that the black sand would accumulate above the slurry to the point that the flow of the slurry would become restricted thereby increasing the pressure necessary to force the slurry thru the sluice.

One approach to fixing this problem, as you correctly mention, would be to use electromagnets instead of permanent magnets. The electromagnets could be cycled off for a period of time thereby releasing the black sand retained above the slurry and allowing the black sand to travel further down the sluice towards the tails. When the electromagnets are cycled on then off repeatedly the black sand will travel down the sluice towards tails more slowly than the bulk of the non-magnetic slurry constituents but eventually the black sand might be managed so as to cause it to travel the length of the sluice to tails over time.

If the electromagnet duty-cycle were 50% one might assume that the (magnetic) black sands would transit the sluice at a speed that was 50% of the slurry velocity. Given enough magnetic field strength at the riffles one might also assume that the riffles might be reasonably free of (magnetic) black sand thereby freeing up a greater volume for the retention of fine gold. Employing electromagnets in this manner may indeed be efficacious.

Another approach that might achieve similar results with neodymium magnets would be to mechanically lift the neodymium magnets away from the slurry a sufficient distance (assuming a fixed barrier between magnets and slurry) so as to allow the (magnetic) black sand to drop down into the slurry for a period of time before the magnets are lowered to pick the black sands up again after the black sands had traveled some further distance towards tails.

I did order Qty 100 N45 neodymium magnets measuring 100mm x 20mm x 10mm which should hopefully be delivered in another 6 to 8 weeks. We shall see how those experiments turn out. Given the lack of public experimental results data on strong magnets in sluice sluices there is no telling whether any fine gold recovery benefits will pan out.

dragline

  

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