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Gold panning submersible bot

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Mention has been made, here and elsewhere, of gold in offshore sand and silt, particularly near large river outflows.

It seems likely that some outflows will tend to deposit small amounts of gold. Many if not all rivers carry gold particles, though often in amounts too small to be worth panning for. Over centuries, these particles will eventually be washed out to sea, and it seems plausible that they will tend to accumulate in particular regions near the outflow, based on flow rates, particle sizes and densities.

However, the concentrations of gold particles will still probably be too low make it worthwhile for humans to pan the offshore deposits.

But how about a gold-panning robot submersible? I'm thinking of something maybe a few feet in diameter, with solar panels on the top, some batteries, GPS and other gubbins.

Plonk the thing in the sea at a likely place. It floats until its batteries are fully charged, holding station (more or less) by using GPS. It then sinks a few metres to the ocean floor. Once there, it pans. Using a cunning system of propellors, it hoovers up silt or sand from one place, and ejects it at a constant rate, in a constant direction. The material will settle out according to size and density, falling at various distances from the point of ejection.

Once a large amount of stuff has been winnowed in this way, the submersible then moves to the place where it expects gold to have settled out, and repeats the winnowing. It can do this several times, each time enriching somewhat for gold particles.

The process will be horribly inefficient, but it's only the robot's time that's being wasted. Periodically, it resurfaces to recharge its batteries.

Once a few tons of sand or silt have been winnowed several times over, the robot hoovers up the most gold-rich region of the deposited sand, surfaces, and transmits a signal requesting collection.

MaxwellBuchanan, Nov 09 2012

International Seabed Authority http://en.wikipedia...al_Seabed_Authority
[MechE, Nov 09 2012]

Remote-control gold mining submersibles http://www.bloomber...COtH7ZyM&refer=home
Almost there. [Alterother, Nov 09 2012]

very affordable autonomous uav http://bordelon.net/ezstar.html
uses HAM, but you could imagine redundant cdma or 3g [fishboner, Nov 09 2012]

magnetic separator
[xaviergisz, Nov 09 2012]

Spherical animals http://youtu.be/alKPsxowZJo
to help visualise a spherical hamster [xaviergisz, Nov 12 2012]

River gold mining boats http://www.alibaba....ld-mining-boat.html
Rather than magnetic cleverness I think they use buckets of mercury. [bungston, May 17 2013]

How gold dredges work http://www.goldfeve...m/howdosugodrw.html
It looks like its just panning for gold scaled up. Weight based. [bungston, Sep 23 2013]

Retrieving gold from faeces http://www.theguard...ns-save-environment
[hippo, Mar 23 2015]

Explanation of Stevin's principle https://www.lhup.ed...work.htm#stevinprob
the rest of this site is very interesting too (perpetual motion machines) [notexactly, Mar 24 2015]

Seabed-Mining Robots Will Dig for Gold in Hydrothermal Vents http://spectrum.iee...-hydrothermal-vents
[Voice, Oct 10 2016]

[link]






       It's already regulated, either by the controlling nation (national waters) or by the ISA (see link).
MechE, Nov 09 2012
  

       Why solar? If it's near the mouth of a river, there's going to be current. Just drop an anchor and spin a turbine.
scad mientist, Nov 09 2012
  

       I love this! Maybe the process could be attached to some sort of automated dredge and kill 2 birds with 1 stone.   

       But what I really like is the prediction that subsea alluvial flumes should contain troveloads of gold nuggets. I believe!
bungston, Nov 09 2012
  

       // a while before your average joe can have one at a reasonable price//   

       I'm thinking just build one, as a fun project. We're not talking huge depths, nor the need for complex manipulations.   

       //It's already regulated// I'm pretty sure there's nothing to stop you panning for gold in the UK on any land (or in near-shore waters) that's not privately owned.   

       Re. the second link: that's dealing with conventional mining but done underwater (ie, collect gold-rich rocks), and usually in deep, deepty deep water. I'm thinking of something much simpler, with a lower yield.
MaxwellBuchanan, Nov 09 2012
  

       Hmm. Just thinking aloud.   

       I have some stupidly strong rare-earth magnets, including a sphere about an inch across. If I drop it down an aluminium pipe, it falls sloooooowly at about 2 inches per second, because of eddy currents induced in the pipe.   

       So presumably, in a tube with a suitable arrangement of magnetic fields, the inverse situation would happen: conductive (non-ferrous) metals falling through the pipe would have eddy currents induced in them, and would fall more slowly than they otherwise would.   

       So, if you dropped seafloor silt down a water-filled pipe full of magnetic fields...   

       I have dipped into the literature on eddy current separation of non-ferrous metals, but it's not clear to me if a magnetic pipe would work on very small conductive particles.
MaxwellBuchanan, Nov 09 2012
  

       //The military have autonomous submersibles, but it will be a while before your average joe can have one at a reasonable price. Same for air drones.//   

       I thought cheap autonomous drones had been developed by hobbyists.
fishboner, Nov 09 2012
  

       Exaccerly.
MaxwellBuchanan, Nov 09 2012
  

       I am intrigued by the prediction of nonferrous but conductive metal falling slowly thru a coil of stacked ring magnets. Probably the coil must be of fairly small diameter and so to process bulk quantities of sediment there would need to be many in parallel. If it worked the sediment would need be delivered in quanta so water containing metal could be flushed into the recovery area after nonmetallic sediment exited the pipe.   

       I cannot think of a way to sort particles by velocity in a continuous moving stream.   

       A fun demonstration of this would be a streamside coil apparatus with pans of gravel poured into the top.
bungston, Nov 09 2012
  

       // I am intrigued by the prediction of nonferrous but conductive metal falling slowly//   

       Me too. I also suspect that this is well-trodden ground.   

       However, I am always amazed by the dropping- the-magnet- through-an-aluminium-pipe thing. It really is incredibly eerie.   

       So, what with orders of magnitude and suchlike, I presume that a suitable fixed field would cause a conductive non-ferrous object to fall equally slowly. I've just ordered a dozen powerful toroidal magnets, and will see what happens if arrange them along a clear plastic pipe and drop a copper nail down the pipe.   

       On the other hand, very fine particles of gold will fall slowly anyway due to air resistance, so any additional slowing will be less obvious. And eddy currents in a tiny conductor must be much less than in a large conductor. So maybe there'll be no significant effect.   

       Also, particle sizes (and hence fall rates, regardless of field) will vary randomly, so it's not as if all the gold will fall slowly while all the crud falls quickly.   

       Perhaps the solution is to use a 2-dimensional separation, cunningly arranged so that gravity or an air current is pulling in one direction, and eddy currents in another.
MaxwellBuchanan, Nov 09 2012
  

       Magnetic separation: Actually a "normal" magnetic pipe that is magnetized with a single pair of poles would have very little effect on a conductive object for most of the length since current is only induced when there is a change in the magnetic field. If it was a tube created out of Halbach arrays running lengthwise (as opposed to most Halbach cylinders designed to have a magnetic filed that is uniform in the lengthwise direction), it would be more effective. I have a potentially unfounded suspicion that the size of the metal object relative to the distance between changing N and P poles is going to make a difference in how effective this is. Unfortunately, the smaller the magnetic regions of your Halbach array, the weaker the field is going to be at a distance. Maybe, if you screen the silt to go through a very narrow magnetic sorter and the sand to go through a slightly larger one, this could be more effective.   

       Hey I was writing the following as you mentioned 2-dimensional separation. Good idea. I think dropping the silt through a vertical tube would not be too effective because denser particles generally fall faster through water, and the magnetic separation would be fighting that. However if water was flowing horizontally through a chamber and the silt is dropped through the stream of water, the denser material will hit the bottom closer to the starting point. If the chamber is made to be narrow, and Halbach arrays are added to the sides creating magnetic fields that change from left to right, but are relatively uniform top to bottom, that would slow the horizontal movement of conductive material, causing it to fall out even closer to the starting point.
scad mientist, Nov 09 2012
  

       Yes indeed - the problem is that size dispersal will outweigh magnetic susceptibility, in 1-D separation.   

       Maybe what's needed is some combination of size selection followed by 1D or 2D magnetic separation.   

       I have this fantasy in which I construct a device that sits bolted to the sea floor. Tidal motion and some cunning funnels waft silt through a series of baffles and slots designed to select only those particles which have a specific and tightly-defined rate of settling. Then further tidal currents recirculate these uniformly-settling particles repeatedly through an array of rare-earth magnets, each time enriching ever so slightly for gold particles.   

       The whole thing would take ages to recover a small percentage of the gold particles from a substantial amount of silt, but it needs no human input and could just be left somewhere for a year or two, before someone (hopefully me) comes and scoops out the few kilos of highly gold-enriched mud.   

       In other words, I'm trying to imagine a passive machine which pans for gold slowly but steadily. Given that particular bends in particular rivers are known to accumulate gold particles ever so gradually, I feel that a well-designed tide-powered device could do so much, much more efficiently.
MaxwellBuchanan, Nov 09 2012
  

       I think it's called "alluvial sorting", and I think you'll find that gold is at the bottom of the deposits, at least on a normal river delta. You might find it just upstream of the delta proper, in holes on the river bottom. Out on a flat seabed, the gold is still going to be at the bottom of whatever is there.   

       But as you say, you could spray all the loose stuff up into the water, and look for the gold closest to the machine. (I once toured a water-treatment plant that had a sand filter bed in it. They back-flushed that to clean it, just by shooting water up from the bottom, making a bubbling turbulent mess. They said it all sorted itself out automatically, with the big gravel falling to the bottom, the lighter gravel on that, coarse sand and fine sand, all exactly as they needed it.)
baconbrain, Nov 09 2012
  

       //the gold is still going to be at the bottom//   

       I guess that is true. But if you found a region with only shallow deposits over rock, you might be able to hoover your way down to the bottom layers.   

       //magnetic black sands// Yes, my reading reveals that they're a big problem in gold panning. But they're easily removed by a "normal" magnetic separator, and this is often done in gold panning.
MaxwellBuchanan, Nov 10 2012
  

       Actually, on the moon this would work fine.   

       You just need a flinger, to throw moondust horizontally at a constant velocity, and a magnetic array. Anything non-conductive would follow a perfect parabola and would all land in one pile. Anything falling short of that pile would be conductive.
MaxwellBuchanan, Nov 10 2012
  

       //perfect parabola// sp. ellipse; *approximately* a parabola over short horizontal distances.
spidermother, Nov 10 2012
  

       /I've just ordered a dozen powerful toroidal magnets/ - from ACME, I hope, and in wooden crates.   

       /on the moon/ Yes yes, just so. And in that thought experiment is the method to sort particles in a continuous moving stream. Instead of falling silt and gravel one accelerates a stream of sediment (all underwater of course) to shoot horizontally over a horizontal plane. Larger pieces have more kinetic energy than smaller and so travel farther before settling down onto the plane Magnets affixed along the path slow conductive nuggets so they fall sooner and sort with the smaller nonconductive sand. The underlying horizontal plane has small holes which allow the fine sand to pass thru while retaining the conductive nuggets.   

       Probably one could derive a formula relating to density, size and conductivity such that at any given place along the plane, one could predict the particle sizes of conductive and nonconductive particles settiling down. Nonconductive (unslowed)will be smaller than conductive (slowed)at each place along the plane, and the filter holes designed accordingly.   

       A camera should be positioned over the horizontal plane so the user can keep track of when it is so clogged with nuggets of gold as to prevent easy sifting of the sand.
bungston, Nov 10 2012
  

       //*approximately* a parabola over short horizontal distances.// Yes yes. However, unless we are getting this moondust up to near-oribital velocities, a parabola will do fine.   

       //And in that thought experiment is the method to sort particles in a continuous moving stream// That was the plan. However, I think it's doomed to failure underwater (or even in air), because the dispersion of sizes (and hence rates of frictional slowing) will be huge compared to the magnetic effects. However, in a vacuum...   

       So, here's my design for a lunar gold panner:   

       Imagine a tube, with a little scoop and a cup at one end. The other end bifurcates. Now imagine several of those tubes arranged like spokes on wheel, with the scoopy end outward and the bifurcation near the hub. Now imagine four such wheels on a lunar rover.   

       The bifurcations are arranged such that each tube has one of its bifurcations pointed leftward (from the vehicles perspective) and the other pointed rightward.   

       Now we place magnets around each tube, with their polarities such that susceptible (ie, conductive) particles tend to be deflected rightwards whilst non-conductive particles tend to fall under gravity alone through the leftward bifurcation.   

       So, as the rover roves, the end of each "spoke" picks up a smidge of moondust as it turns. When the wheel has turned 180°, the dust falls out of its little cup, and is magnetically sorted to fall either leftwards or rightwards.   

       Each pass would only achieve a little discrimination but, with the rover slowly plodding back and forth over the same ground, it would eventually create furrows which were significantly enriched for gold.
MaxwellBuchanan, Nov 10 2012
  

       I once proposed a robot that would patrol the sewer systems of England, looking for the tell tale flushings of those who were in contact with the base chemicals of explosives. It would automatically provide position updates of areas of greater than expected concentrations of target substances. I did not become wealthy.
xenzag, Nov 10 2012
  

       //I did not become wealthy.// Ah, but that may be because the market for sewage-contaminated explosive residues is not bouyant.
MaxwellBuchanan, Nov 10 2012
  

       Successfully sniffing out just one bomb would save billions. MOD.... no imagination - mutter, mutter.
xenzag, Nov 10 2012
  

       //Successfully sniffing out just one bomb would save billions.// But not bullions. (tshh boom, etc).   

       Perhaps the problem is that large numbers of people come into contact with chemicals that are, very plausibly, the starting materials for bombs. I'm thinking of things like diesel, fertilizer, acetone and the like. Farmers with nail-polish would soon get tired of the dawn raids.
MaxwellBuchanan, Nov 10 2012
  

       For that matter, I have a sneaking suspicion that trying to detect ammonium nitrate in a sewer, which is generally chock full of ammonia and nitrates, might prove a little difficult
MechE, Nov 10 2012
  

       Maybe not with a neutron activation detector. That would also filter out those with commonly available chemicals on their hands or clothing, because it only looks for certain combinations and ratios. I'm not an expert on such things, I just know they exist; any airport security experts or nuclear physicists in the audience are welcome to call me out.
Alterother, Nov 10 2012
  

       More than bombs or moon, I want to think about a map where razor keen deduction predicts that heaps of gold nuggets lay gleaming in the mud-diffracted light. All the gold the 49ers could not get because it had already washed out to sea. That primal untapped gold.   

       And now is the time! Gold is worth a lot!
bungston, Nov 10 2012
  

       About 1 in 300-500 bags which are screened by the airport swab-and-spec machines comes up positive, usually for innocent reasons. And that's despite the fact that most people don't dispose of drain cleaner, plant-food, nail polish remover, wood glue, cellophane, paint-brush cleaner, hair wax, hairspray, furniture polish, unwanted medicines, engine de-greasers, or a thousand other common substances by pouring them into their bags.   

       Simple explosive ingredients are widespread. Complex explosive ingredients are generally organic molecules which would not last long in a sewer. Even if a terrorist were using some stable and distinctive explosive, only small amounts might find their way into his (or her) drains. If you could detect it in his/her house's drain, how about after it had been mixed with the outflows of a thousand other houses, plus roadwater? If it's still detectable, you'd need 60,000 such robots patrolling the sewers of the UK alone.   

       But hey. It's actually not a daft idea and I'm just being grouchy and arsey. If you build it, they will come.
MaxwellBuchanan, Nov 10 2012
  

       Well, you did say //perfect// parabola. A ballistic path is never a parabola , but (in a vacuum) always an ellipse.   

       However, the moon sorter seems more straightforward than the under-water one. In water, you have two types of drag (electromagnetic and hydrodynamic) which would interact in messy ways, and could not be made orthogonal, confound it.   

       //susceptible (ie, conductive) particles tend to be deflected rightwards// Are you sure that's possible? I thought these eddy currents simply opposed motion, just like other forms of drag.
spidermother, Nov 10 2012
  

       //A ballistic path is never a parabola , but (in a vacuum) always an ellipse. //   

       <roots around in cupboard. finds the picky hat. puts it on.> OK, so I find a moon that nobody is using and I go and get my Dremel. After a long afternoon, I have carved the moon into a torus. Standing on one side of the torus, I pick up my catapult and a convenient hamster, and fire it at an angle of 45° (relative to the plane of the torus) across the torus's diameter. I do so at a velocity just sufficiently adequate to ensure that the rodent will land exactly opposite me on the torus.   

       Assuming that the hamster does not pass wind during its flight, just how elliptical will its journey be?   

       You may assume a spherical hamster.   

       // I thought these eddy currents simply opposed motion, just like other forms of drag.// Yes and no. I have seen eddy-current separators consisting of a wedge-shaped gap between pole pieces, which allegedly deflect non-ferrous metals sideways. I believe it's due to the transverse field gradient. This makes intuitive sense: if I were to drop a pebble through a tank of liquid which was more viscous on the left than on the right, then the centre of drag would not coincide with the centre of mass, and the pebble would move (I think) toward the viscouser fluid.
MaxwellBuchanan, Nov 10 2012
  

       Still an ellipse, as it happens. You should have fired your hamster through the hole in the torus, but off centre. Then you would have picked a pickier picky hat and avoided ellipticity.
spidermother, Nov 10 2012
  

       (Wikipedia) "Eddy current separators may use a rotating drum with permanent magnets, or may use an electromagnet."   

       So it seems you need a non-stationary magnetic field.
spidermother, Nov 10 2012
  

       //Still an ellipse, as it happens// wanna bet?   

       //So it seems you need a non-stationary magnetic field// No, you need a field which moves relative to the non-ferrous metal.   

       I have just conducted some experiments using the largest magnet I have. It's a rare-earth (N45, I think) magnet, about 2 inches by 2 inches by 1 inch. It's meant to have a holding force of about 100kg on thick steel. All I know is that when I put two of them together, I had to use a big pointy screwdriver to prise them apart.   

       If I move the magnet back and forth (poles facing up/down) over a heavy copper tray and about an inch from it, the drag is very, very noticeable and I can make the tray itself move. There is no detectable force between the magnet and the tray when stationary.   

       I have also just done a curious experiment with the same magnet and a pound coin (which is not attracted at all by the magnet). I stand the magnet on a flat wooden surface, and try to roll the coin past one of the faces (poles). It is impossible to roll the coin within 1cm of the face - the rolling coin flips over as it passes the face. It's the top of the coin that flips outwards (away from the magnet), regardless of the direction of the roll or which pole of the magnet it's passing. I suspect that the rotatiness has something to do with it, but the magnitude of the effect is surprising.
MaxwellBuchanan, Nov 10 2012
  

       Your hamster + torus system reduces to a classic two body problem - that is, it can be precisely modelled by two point masses - unless one of the bodies passes inside the other to any extent. The paths will therefore be elliptical.   

       Your coin seems to be exhibiting precession. The behaviour you describe follows conservation of angular momentum, assuming that the drag is greater on the side of the coin nearest the magnet.
spidermother, Nov 10 2012
  

       //No, you need a field which moves relative to the non-ferrous metal. //   

       I meant that to produce a force orthogonal to the direction of motion of the non-ferrous metal you need a moving (or changing, which is equivalent) magnetic field. If the object is moving in the X direction, through a stationary magnetic field, the force will be in the -X direction. To produce a force in the Y direction, you need (AFAIK) a magnetic field whose vector strength changes (over time) in the Y direction. Hence the need for rotating drums of permanent magnets, or (adjustable) electromagnets.
spidermother, Nov 10 2012
  

       // meant that to produce a force orthogonal to the direction of motion//   

       That sounds plausible. However, suppose I drop a non-ferrous (and non-rotating) sphere down the face of a magnet.   

       The side nearest the magnet will be retarded more than the side away from the magnet (field gradient). Therefore, shirley, the effect will be similar to letting car drift off the road and into the soft earth next to the road - i.e. the sphere will be deflected toward the magnet, just as the car tends to be deflected further into the soft earth, n'est ce pas?
MaxwellBuchanan, Nov 11 2012
  

       You've re-discovered magnetic levitation, which is acting on the top side of the coin only, because it is moving.   

       I've been pondering why eddy currents work best on larger conductors, and I think it's because the vector of the magnetic field must vary throughout the conductor in order to generate different EMFs. Smaller objects tend to have the same field throughout. To improve this, the field needs a rapid change, probably achieved by higher frequency.
Ling, Nov 11 2012
  

       //You've re-discovered...//   

       That makes sense. I know this is a well-trodden ground, but it's still cool to see it working first-hand.
MaxwellBuchanan, Nov 11 2012
  

       Like the first superconducting levitation I saw, and played with, in real life...just pure witchcraft or black magic.
Ling, Nov 11 2012
  

       I may need to order one of those magnets. I am certain it would get put on my credit cards, though.   

       I am going to seek youtubery of nonsuperconductive magnet games. Anyone who knows good ones pls post link.
bungston, Nov 11 2012
  

       OK, latest experimental data (and I use the word 'data' quite wrongly).   

       If I take a gold Britannia coin (one ounce of gold) and a hefty cuboid rare-earth magnet (about 2 inches square by an inch thick, poles on the large faces, holding force maybe 100kg on thick steel), and I move the magnet at about 3-5m/s over the coin (flat face of coin parallel to flat pole of magnet) with a separation of about 1cm, I can create a lateral force on the order of 5-15 grams, which is huge.   

       Thus, if I had a spinning disc decorated with such magnets and spinning around a horizontal axis, and if I dropped the coin parallel to the disc and a centimetre from it, the coin would be massively deflected away from the vertical.   

       So, the question is how this would scale down to particles of gold maybe 50 microns across (which I'm guessing is a typical size for alluvial gold). Clearly the force imparted by the magnets will decrease _at least_ in proportion to the volume of the gold, but then again the force needed to deflect the particles will scale with their volume.   

       So the question is - by what power does the force scale with the volume (mass) of the gold object? If it were linear, there would clearly be no problem. But I presume it's a lot worse than linear.   

       (Experiments with the toroidal magnets are awaiting my finding some gold that I don't mind filing down into small enough bits to drop down the hole in the middle.)
MaxwellBuchanan, Nov 15 2012
  

       You could probably use copper for the experiments. Small bits of wire.   

       I am very much digging the fact that a magnet can move gold. I wonder what else is in the drawer where you got the magnet and the gold coin.
bungston, Nov 15 2012
  

       Cool idea [MB]. Do any of your less wealthy cousins, perhaps on the outer fringes of your family, have some smaller chunks of gold which they could lend you for experimental purposes?   

       The place to put this would be in-line on a gravel crusher in gold country. They are already digging, lifting, screening and processing thousands of tonnes of gravel every day anyway to make road gravel so your costs would be wildly reduced. (I had this idea when I was about 14 with conventional gold separation systems in mind only to be told by my Dad's boss that such things were already in place). A dry separation system like yours would be an enhancement.
AusCan531, Nov 15 2012
  

       Lead actually has very poor conductivity compared to gold (or copper), so it might not be a good model.
MaxwellBuchanan, Nov 17 2012
  

       I have never stopped pondering these matters. My proposal. 1: A mix of gravel and copper shot, copper here standing in for its downtable neighbor gold, and gravel representing gravel. 2: A plastic pipe. 3: A leafblower or shopvac. 4: Magnets!   

       Blower is positioned to blow thru pipe. Gravel / coper mix is dropped in thru hole at top of pipe. Magnets are positioned around exit of pipe. Pipe is pointed out over pavement.   

       If magnets selectively slow conductors, the gravel should travel farther on exiting the pipe. The copper should drop short of the gravel. This distribution should be evident on looking over the pavement. When the magnet is off (or removed) gravel and copper should travel together.   

       As a control one could also include bits of iron or steel, which should be directly slowed by the magnet.   

       Video please.
bungston, May 17 2013
  

       /So the question is - by what power does the force scale with the volume (mass) of the gold object? /   

       If the power depends on eddy currents induced within the gold, then it would not be mass but area amenable to forming the eddy current. So a very thin but flat piece of gold foil would be readily moved, while an irregular nugget with deep clefts might not.
bungston, Sep 23 2013
  

       Damn. There goes my next million.
MaxwellBuchanan, Sep 23 2013
  

       Mercury. Gold is soluble in mercury. Suck up the sand and pump it though a big tank of mercury. The metals (including gold) dissolve in the mercury. Pass the mercury through a reformer to separate the mercury in vapour form from the metals. Condense and re-use the mercury (recovering the heat), separate, refine and sell the metals.
8th of 7, Sep 23 2013
  

       Yes, I was wondering about that.   

       But there must be a finite rate of mercury loss, and I wonder what proportion of gold is needed to offset the cost of gradually lost mercury.
MaxwellBuchanan, Sep 23 2013
  

       No, just heating. You're thinking of cyanide-based gold extraction from the basic ore.   

       // a finite rate of mercury loss //   

       Still cheaper than gold.
8th of 7, Sep 23 2013
  

       //Still cheaper than gold.//   

       That very much depends. If I lose 1kg of mercury in the course of recovering 0.1g of gold from 20 tons of very low-grade material, I am not coming out ahead.
MaxwellBuchanan, Sep 23 2013
  

       I am thinking of that gold Brittania coin again, now crossbred from my recent reading of Ling's maglev gyroscope concept.   

       For eddy currents to be produced, the gold must move through the magnetic field (or vice versa). I wonder if rotational motion counts. Specifically 1. Gold coin lies flat atop magnet. Perhaps as part of a plastic toy top, to facilitate #2. 2. Gold coin is spun up to speed, maybe with a pull cord. 3. Spin produces eddy current in gold which repels magnet. 4. Coin lifts off and hovers!
bungston, Oct 07 2013
  

       My instinct says "no", but I'm highly unsure. My feeling is that the electric field (if not the magnetic) cannot "see" that the coin is spinning.
MaxwellBuchanan, Oct 07 2013
  

       Don't think of a spinning magnetic field! A magnetic field can change in strength and direction, so it can only seem to be spinning in some cases (induction motors have a so-called rotating magnetic field). If a conductor spins on the end of a magnet, each part of the conductors sees an unchanging field, so nothing happens. Also if the magnet is spinning on it's axis...same thing.
Ling, Oct 08 2013
  

       /Don't think of a spinning magnetic field! / Now I can only think of a spinning magnetic field. Just like the StayPuf Marshmallow Man.   

       But thank you for weighing in with that, Ling. It makes sense.
bungston, Oct 08 2013
  

       I saw this idea as God Planning Submersible Boat. Which makes a lot more sense.
theircompetitor, Oct 08 2013
  

       Pondering this one again. It occurred to me, Max, that a pawnshop might have scrap gold they would part with.   

       To induce a field in a conductive object the position of the object in the field must change. I wonder if, instead of permanent magnets, one used electromagnets with AC current the rapid variation in the field would produce a more dramatic change in the vicinity of the object and so a more forceful induced field? The goal is to oppose gravity in a falling gold particle and so the direction of the field is not important.   

       If this works, I am sure that the dozen powerful toroidal magnets have enough self esteem to get past being sacked.
bungston, Jan 01 2015
  

       [bungston] Now that's generating a gold field.
wjt, Jan 05 2015
  

       Ever seen film of a platypus hunting for food? That's what I'm picturing, only robotic. [+]
BunsenHoneydew, Jan 20 2015
  

       //Ever seen film of a platypus hunting for food? That's what I'm picturing, only robotic. [+] //   

       No no no. We want to find gold. Finding robotic platypus food would be a waste of time.
MaxwellBuchanan, Jan 20 2015
  

       I am thinking about magnetic or magnetizable debris. Permanent magnets would accumulate a coating of such, which would be unsightly and hard to scrape loose. I think for this application an electromagnet is better because it can be turned off. Plus you can make one out of wire instead of niobium.   

       And it would offer a chance to test the idea that a rapidly varying magnetic field slows a falling item more than a steady magnet. You can use the same stack of coils and run a series of drops: first with DC then with AC.   

       When you are done you can have electric eyes turn on the coils individually to make a coilgun and see what sequence of triggering maximizes the distance of a pinball fired from the gun. Invariant gun angle, of course.
bungston, Mar 24 2015
  

       // niobium //   

       You mean neodymium?   

       -----   

       These two situations are equivalent, and I think they are both wrong about what the falling object will do:   

       // I have seen eddy-current separators consisting of a wedge- shaped gap between pole pieces, which allegedly deflect non- ferrous metals sideways. I believe it's due to the transverse field gradient. This makes intuitive sense: if I were to drop a pebble through a tank of liquid which was more viscous on the left than on the right, then the centre of drag would not coincide with the centre of mass, and the pebble would move (I think) toward the viscouser fluid. //   

       // […] suppose I drop a non-ferrous (and non-rotating) sphere down the face of a magnet. The side nearest the magnet will be retarded more than the side away from the magnet (field gradient). Therefore, shirley, the effect will be similar to letting car drift off the road and into the soft earth next to the road - i.e. the sphere will be deflected toward the magnet, just as the car tends to be deflected further into the soft earth, n'est ce pas? //   

       The claim is that because the center of drag is offset horizontally from the center of mass, the falling object will move toward the more draggy (either viscous or electromagnetic drag) direction.   

       If an object intrinsically has a center of mass and center of drag in different places, such as a rocket, and it moves through a region with non-spatially-varying drag, then it will rotate so that its center of mass is forward (in its direction of motion) of its center of drag—see anything on ballistic stability. However, if its centers of mass and drag are in the same location, as in the case of a rotationally symmetric falling object that might be made of gold, this will not happen (obviously). Now, with the addition of the varying viscosity or magnetic field strength, we can move its center of drag to the side of its center of mass. From a kinematic point of view, this would appear to cause it to rotate, as the center of drag, which tries to hold it in place, is to the side of the center of mass, which tries to pull it down. This would cause the object to swing down about the center of drag (or some point between it and the center of mass), right? But from a thermodynamic point of view, there's no reason for it to do that, because it doesn't gain anything entropically by doing so. Think about it like this: At any given moment, it could rotate about a point between the centers of the two forces acting on it (gravity and drag), but after rotating an infinitesimal amount, the forces' centers would still be where they were within the reference frame of the falling object, so nothing would be gained. See my link to an explanation of Stevin's principle.
notexactly, Mar 24 2015
  

       //So, if you dropped seafloor silt down a water- filled pipe full of magnetic fields...// Very strong magnetic fields within this submersible's collection and filtering apparatus would surely mean that the majority of stuff collected by this submersible would be ferrous material, which would be ironic...
hippo, Oct 10 2016
  

       I thought of an eco friendly gold prospecting technique There is a thing called crevicing, where crevices fill up with heavy things like gold, then prospectors dig out the crevices to find nuggets.   

       Instead, take a log, put thousands or tens of thousands of little crevices on it, then toss it near a river mouth. It gradually accumulates gold nuggets, then pull the log out and harvest the gold. if people forget about it, it is just a log. If the thing accumulates gold at a measurable rate, then the future value could be calculated and the person could get money at the present for the gold accumulating logs laying around
beanangel, Oct 10 2016
  

       This is already one of my favorite hb ideas but Bean your scheme puts it over the top. Especially because I picture a reedy mandolin-playing undergrad backed by a brush wielding drummer singing your words as a bluegrass song. The line where the log is forgotten becomes the bridge - downtempo, a little melancholy but ultimately ok. Then back to being paid for theoretical logs of gold.
bungston, Oct 10 2016
  
      
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