Half a croissant, on a plate, with a sign in front of it saying '50c'
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Gold panning by density alone

  [vote for,

I can't believe this hasn't been tried (and perhaps found wanting), but Googling didn't turn anything up.

It is possible to create solutions that are denser than common minerals. The densest aqueous solution I know of is called Clerici solution, and it has a density of almost 4g/cm3, but it contains thallium and is not nice stuff. Sodium polytungstate solutions can exceed 3.5. Common minerals (including quartz, for instance) have densities below 3.

So, if you take a large tub of your dense solution, and dump in a bucketload of dry, gold-containing sand, silt or gravel, most things will float. The gold will sink. (You may need to add a detergent to ensure that small particles are properly wetted). You can then recover the gold (along with small amounts of other dense minerals) from the bottom.

You would need to recover your dense solute, by washing it out of the floating sludge and then drying the wash water to re-concentrate it. Perhaps the losses and costs would exceed the value of the gold recovered.

Yes, I know that gold panning relies on density as well, but only in an indirect and rather fuzzy way.

MaxwellBuchanan, Oct 08 2017

Sluice box with magnetic riffles. https://www.youtube...watch?v=oMjODJG5n8M
[2 fries shy of a happy meal, Oct 08 2017]

US 4557718 https://www.google....u/patents/US4557718
The separation of dissolved and/or undissolved materials having different densities or different buoyancy densities can be effected with the aid of agents comprising solutions of true metatungstates. [xaviergisz, Oct 08 2017]

Fluidized sand http://nerdist.com/...bble-fluidized-bed/
Bubble up gold down [mylodon, Oct 09 2017]

Cody's Lab did it, it's called https://www.youtube...watch?v=gwJyzsSjJrE
This guy did it, and also provides a great description of how it works [mace, Oct 09 2017]

Gold in Swiss Sewage http://money.cnn.co...ge-waste/index.html
wtf [theircompetitor, Oct 11 2017]


Why does detergent make small particles wetter?

       Small particles can float due to the surface tension of a liquid. If they are irregular, small amounts of gas can be enclosed, providing buoyancy.   

       Surfactants reduce surface tension, allowing the liquid to contact the surface of the particle ("wet" it).
8th of 7, Oct 08 2017

       So, to the economics.   

       I would guess that you could just about process one ton of dry silt/sand/gravel/grit with one cubic metre of dense solution. (Reasoning: that's very roughly 3 volumes of liquid to 1 volume of solids; wetting of the floating solid will mean that there's roughly 2 - 2.5 volumes of "free liquid" left after everything's settled down, which seems reasonable.)   

       A sediment considered rich in gold might contain 0.1 grams of gold in one ton of material. Value of gold at present puts this at $4.   

       After recovering the gold, you are left with about (guesstimating) 0.7 cubic metres of clean dense solution, and 0.6 cubic metres of floating mud - your dense solution absorbed in a mass of sand and silt. To recover the expensive solute from your of mud, you add (let's say) 3 cubic metres of water, let the solids settle out (they'll now sink), and take off 2.7 cubic metres of water-plus-solute. You then have to dry that 2.7 cubic metres of water back down to 0.3 cubic metres.   

       So, you need to evaporate 2.4 cubic metres of water to get back to your starting point. My trusty envelope-back tells me that that will take about 1500kWh of energy which, if you use gas, would cost you several dollars at domestic rates.   

       So, the process is marginal if you use energy at "domestic" rates. Of course, you could use sunlight and open ponds to evaporate the water, in which case you're winning.   

       But then there's the cost of the sodium polytungstate (if that's what you were using for your dense solution). It seems to cost about $1 per gram in small amounts, but assume $0.1/g for bulk buy. So your batch (one cubic metre of solution) is going to use about 3 tons at $0.1/g = $300,000 of sodium tungstate. Which means you can only afford to lose 0.001% of it in each recycle.   

       Hmm. What idiot proposed this method? What we need is a very cheap, dense, very soluble solute.
MaxwellBuchanan, Oct 08 2017

       What about a method using a linear motor ?   

       Reduce the ore to fine powder.   

       Drop the powder into the top of a tall column which has the air evacuated.   

       Arrange a linear induction motor part way down. This will push any metallic fragments off the centreline, somewhat like a Calutron.   

       The metal fragments fall to one side of a divider, the waste to another. Arrange the motor field so that all the metal is retained.   

       Repeat through cascaded columns. If each pass removes 20% of the waste, then after ten passes, the metal should be fairly well concentated.   

       Then chemically process the much reduced amount of metallic residue, which may well contain other useful metals.
8th of 7, Oct 08 2017

       You mean to use eddy currents to pull out metals (including non-ferrous ones) magnetically? Yes, I'd wondered about that (and I think I posted something once). What I'm not sure of is how effective it would be for very small gold particles, where you'd struggle to induce much of an eddy current.
MaxwellBuchanan, Oct 08 2017

       There is an existing sluice box system which uses magnets to ensure that gold remains in the riffles.   

       Can you link to it?   

       <edit> OK, a quick google shows that magnets are used either to take out ferrous sands, or to trap ferrous sands so that they in turn act as a trap for gold particles. I'm not seeing any convincing use of eddy currents to trap gold.
MaxwellBuchanan, Oct 08 2017

       Sure thing. [link]
The interesting stuff is after the 2:00 mark.

       Nice demonstration of eddy currents, and I see what he's trying to do.   

       What I'm not sure is how well it would work for small (sand-grain or smaller) pieces of gold. I'm not sure how eddy currents scale with particle size (in a given field), but I'm pretty sure they're worse than linear - ie, square, cube or fourth. That means that the force on the gold particles is going to be tiny. I would guess it's too tiny to have any significant effect, but I might be wrong.   

       What we need is an eddy currentologist.
MaxwellBuchanan, Oct 08 2017

       Use kiloTesla superconducting magnets. If you use ambient-temperature superconductors, it would be cheap and efficient enough to make the process economic.   

       For a small* fee, we'll be happy to provide your species with the necessary technology. Unlike those boring tightarses in the Federation, we don't bother with childish stuff like the Prime Directive ..   

       *Relatively small. Just think of all that cheap gold. Sign here ...
8th of 7, Oct 08 2017

       OK, a little more research reveals that the magnetic riffle box will not work. From a paper in Phys. Sep. Eng. Sci., 2004:   

       "Unfortunately, this force scales as the fifth power of the particle size for small particles [3], whereas the mass of a particle scales only with the third power. The result is that frictional forces dominate the separation for small particles."   

       which tells me there is no way on Earth that guy's setup will work as intended.
MaxwellBuchanan, Oct 08 2017

       Of course, if you used a dense emulsion in place of a dense solution, then your economic constraints could be addressed through floccinaucinihilipilification.
pertinax, Oct 08 2017

       How about gallium heated above its melting point (30ºC). It is fairly cheap $0.5 per gram, and I don't think gold dissolves in it (unlike mercury).
xaviergisz, Oct 08 2017

       Or what about grinding oar into dust, the fluidizing the oar itself with air? In a similar where boat can float through sand, sand could float above gold. or pure gold. I think maybe there would need to be smelting or alloying at some stage for purification.   

       But this would not work well for gold with well formed aerodynamic properties.
mylodon, Oct 09 2017

       Oar or ore?
pertinax, Oct 09 2017

       The other cost to take account of in your economic analysis of this method is, for every tonne of ore, the cost of disposing of slightly more than a tonne of detergent- and solute-contaminated ore.
hippo, Oct 09 2017

       That is what rivers are foar.
mylodon, Oct 09 2017

       // the cost of disposing of slightly more than a tonne of detergent- and solute-contaminated ore. //   

       If it's a third-world country, particularly one where officials are amenable to bribery, then [mydolon] is right. Dump it in the nearest watercourse and make it Someone Else's Problem.
8th of 7, Oct 09 2017

       Nice link [mace]. Super heavy salt water... plain salining.   

       If anyone knows of fluid bismuth molecules, "sodium polybismuthate" like, bismuth is cheap and heavier than gold so could make a fluid gradient.
beanangel, Oct 10 2017

       Why does the liquid have to be aqueous? Pure gallium is liquid on a hot day, relatively harmless, and has a density of about 6gm/cc   

       Yes, the stuff is kind of expensive, but it should recycle pretty easily, being a pure element.
Vernon, Oct 11 2017

       speaking of recycling, see link
theircompetitor, Oct 11 2017

       // Pure gallium//   

       Are gallium and gold intersoluble?
MaxwellBuchanan, Oct 11 2017

       Another option is do the London approach. Just like any tour guide will tell you, place a piece of gold oar in a window.   

       Over a hundred years - as oar is a fluid - the gold will sink to the bottom.   

       Then, shave it off with a sharp knife.
mylodon, Oct 12 2017

       There are at least three different ways that statement is wrong.
MaxwellBuchanan, Oct 12 2017

       [MaxwellBuchanan], Google offers no results for the phrase "gallium dissolves gold". It does dissolve aluminum, however. Gallium is very able to "wet" many substances, unlike mercury (which I think you know does dissolve gold, and has been used in gold-mining operations).   

       So far, then, gallium appears to be a valid candidate as a dense liquid for processing loose material for gold- separation.
Vernon, Oct 12 2017

       That's encouraging.
MaxwellBuchanan, Oct 12 2017

       Some freshwater fish are highly attracted to shiny objects, breed these to only respond to the gold color, and then teach them to poo in a litterbox.
beanangel, Oct 13 2017

       Family planning by destiny alone.
Wrongfellow, Oct 13 2017

       ^ I will thank you to leave my ancestors out of this.   

       Is settling time and energy to free up all the different constituent densities, a sticking point?
wjt, Oct 14 2017

       Not if you rely on density alone. You can shake the crap out of it to break up clumps, and leave it as long as you want for the gold to sink (and everything else to float).
MaxwellBuchanan, Oct 14 2017

       Crush the ore and form it all into exactly homogeneous, tiny spheres then fire them horizontally through the air. Those with higher densities (or destinies perhaps) will travel further in a nice statistical distribution pattern. Collect the 'winners' for further processing. Depending upon the day's cost of processing and market price of gold, you know whereabouts along the landing zones it's economic to begin collecting the spheres. Pure nuggets will obviously go the furthest.
AusCan531, Oct 14 2017

       Crush the ore down to individual atoms and run it through a calutron.   

       I like the gallium idea, though.
notexactly, Mar 14 2018

       // Crush the ore down to individual atoms //   

       Not easy to achieve mechanically.   

       Plasma ionization of a powder stream is possible, and then feed the resultant ion stream through a calutron (or indeed a quadrupole). You're going to want some serious vacuum pumps, though.   

       But in economic terms, chemical separation is probably better - the plasma method uses a lot of energy, almost all of it a total loss as low-grade heat.   

       Ion separation does give you very (isotopically) pure product, but is it necessary ?
8th of 7, Mar 14 2018

       It would be quite satisfying to pay someone (such as the Inland Revenue) in pure Gold-195.
MaxwellBuchanan, Mar 14 2018

       Wouldn’t a piece of uranium and some other less dense minerals do a reasonable impression of the amount of inertia, volume, and density in flight as a small gold nugget?
RayfordSteele, Mar 14 2018


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