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I have previously mooted the possibility of using eddy
currents induced by magnetic fields to winnow tiny gold
particles from seafloor sediments.
However, there is probably more gold to be found on the
roads - dropped rings, broken jewellery and the like. And
these items, being bigger, would
sustain a larger induced
eddy current.
MaxCo. is therefore about to start cruising the streets in its
latest gold-panning roadster.
Mounted crosswise beneath the body of the otherwise
normal-looking Aston (did I mention it was an Aston? No
matter.) is a drum, set at a height to travel a couple of
inches
above the road surface. Stupidly powerful magnets are
fixed
inside the drum, with their poles outwardmost, and an
electric motor spins this drum at a couple of thousand RPM,
in the same direction as the wheel rotation.
A few inches behind the drum is a scoop (which, as a bonus,
produces extra downforce, which is always handy in an
Aston).
Meanwhile, up the front end of the car, a powerful fixed
magnet is, ah, fixed. This (like the roller) spans the full
width of the car. A motorised blade, activated every few
seconds, can scrape from right to left along this magnet.
With the system activated, we set off on the streets of a
prosperous district.
Most of the metallic road debris will be ferrous. This is
picked up by the front, fixed magnet and is periodically
scraped off by the blade and dumped back on the roadside.
Non-ferrous debris, however, is passed over by the fixed
magnet. A moment later, the fast-spinning roller passes
over
said non-ferrous debris and, due to the speed of the
spinning
magnets, substantial eddy currents are induced in the
debris.
These, in turn, create reciprocal magnetic fields with the
result that the non-ferrous debris is lifted and flung
backwards, to be collected by the scoop.
At the end of a long drive, we inspect the haul. Most of it
will be aluminium ring-pulls or squashed cans (themselves
worth something). However, any pieces of gold, silver or
platinum jewellery will also have been harvested.
What sort of yield can we expect? Well, let's assume that
the average adult (say, 40 million people in the UK) loses
two pieces of jewellery in a lifetime (a conservative
estimate). That's 80 million items lost per year.
Assume also that 1% of these items are lost on roads. That's
800,000 items landing on the roads every year.
Assume also that each item sits on the road for 5 days
(many will sit for longer, being squashed and unnoticed;
some will be spotted and picked up sooner; very few will be
washed or blown away, so 5 days is a fair average). That
means that there are on average 11,000 items of jewellery
on Britain's roads at any one time.
There are roughly 100,000 miles of road in Britain, meaning
that on average there is one item of jewellery lying in the
road every 10 miles or so, at any given time.
If we assume that we can harvest just 10% of these items
(for instance, because the other 90% have been squashed
and wedged into the road surface), that gives us one item
of jewellery for every 100 miles driven. If the average item
contains just 1 gram of gold (conservatively - a wedding
ring, for instance, will weigh several grams), we are
recovering about £25-worth of gold for every 100 miles
driven.
Using this technique in recycling centres
http://www.lbhf.gov...eet_tcm21-23390.pdf [hippo, Mar 16 2015]
[link]
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So, fresh out of a job, now picking up aluminum cans? |
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Oh, and I will keep a careful distance from any Aston I see due to the "periodically dumped back on the roadside" feature. |
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I know it seems trite, but when I recently became a free agent I was told that it meant better things were on the horizon. It turned out to be true, shockingly enough. |
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Norm - you'll note that the front scraper wipes from
right to left, and I live in the UK, meaning that the
ferrous scrap is dumped in neat piles at the road's
edge. |
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And thanks for the encouragement. Better things are
indeed on the horizon, or at least below the horizon
but hypothetically there. |
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Well if they were neither above nor below the horizon I would start worrying. |
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This idea would be a good retro-fit to municipal road-sweepers. Actually, the council road team might also quite enjoy getting standard-issue Astons as well. |
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" Objects on the horizon may be nearer than they appear " |
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Honestly you'll probably do pretty well from the non-
precious metals. Admittedly steel only seems to be
running ~70 pounds/tonne (I don't have a pound currency
symbol on my keyboard), but it's still probably worth
saving. Aluminum is closer to 500, copper ~3000, so
those add up quickly. |
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The other thing you'll get a lot of is coins. It's amazing
how many I see on the road while cycling. Most of them
will only be a few pence, but the pound coins will add up
quickly. |
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The other thing is that it would probably be possible to
presort (to some extent) by how high the bits jump, so
you set up multiple scoops. |
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Oh, yes, like fractional distillation of metallic debris. Nice. |
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// retro-fit to municipal road-sweepers.// |
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Several councils are considering (or perhaps
implementing) processing road dust to recover
platinum and other precious metals (largely from
catalytic convertors). However, the yield is quite low
(a very few grams of platinum per ton of dust), and
only worth recovering if you've already got to process
the dirt. |
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My method, in contrast, recovers the smaller
amounts of precious metal (smaller, that is, in terms
of mass per unit length of road), but in a more
worthwhile way. |
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As for coins - probably worthwhile too. Low-value
coins (including "coppers" - 1 and 2p) are now
ferromagnetic and would be thrown out with the
scrap. But pound coins are non-ferromagnetic, so
they might be recovered. |
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However, I've just tested a pound coin past a killer
magnet, and there's not much induced force; I
suspect pound coin metal is not as conductive as gold
or aluminium. I might try this on a gold sovereign for
comparison. |
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OK. We have some experimental results. Place one
gold sovereign (>99% pure; about 4cm diameter and
about 1.5-2mm thick; weight 31g) on a carpet next to
a pound coin (about 2cm x 3mm; weight about 10g). |
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Now, take a dangerously-strong (100kg holding force)
rare-earth magnet. Sweep magnet (poles pointing
up/down) over both coins, as low and as fast as
possible without hitting the coins. Speed estimated
at ?3m/s?, and distance between magnet and coins
estimated at about 1cm. |
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Result: the sovereign actually jumps (at least, it flips
up and flops back down); the pound coin doesn't
move. |
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Conclusion: my gold panning non-submersible car is
probably within 2 orders of magnitude of feasible. |
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Further result. Repeating the experiment on a
smooth surface (wooden floor) shows that the
sovereign is slid sideways by the passing field, with
little uplift. (I think the up-flip seen on carpet may
be because of the edge of the sovereign digging into
the carpet). |
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Damn. If only I had a spare Aston. |
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<several minutes later yet> A quick Google and
Wikipede suggests that the nickel-brass used to make
pound coins is only about 1/3rd as conductive as
gold. |
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Well seeing as most roads aren't wooden, and those that are, aren't polished, the flipping might still be useful. Try taking your sovereigns out into the street and trying this. |
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Also I imagine an a/c electromagnet would give different results from a fixed magnet. |
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Also, I'm not sure sovereigns are typical of the kind of detrius you expect to find on the road. Though a device that scraped sovereigns off the road and deposited them in a receptacle as you drove along would be quite useful. |
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//I'm not sure sovereigns are typical of the kind of
detrius you expect to find on the road// |
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Depends on the neighbourhood, surely? But - agreed.
Best case: the force applied (by a given magnet,
speed, distance) is proportional to the mass of the
gold object, in which case all will be well. Worst
case: force varies as some power (>1) of the mass, in
which case smaller objects will be less moved. |
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//Metal detectors// Yes, possible. I wonder if a
detector can get a decent signal from a small object
(and discriminate it from ferrous material) at the
necessary speed? |
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So anyway, here's a question for any proper physicists
here. Is there a magnetic geometry which can apply
a lifting force (rather than a sideways or downward
force) to a non-ferromagnetic metal resting on a flat
surface? |
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I can imagine, for example, two opposing magnetic
poles either side of the object to be lifted (so the
field lines run horizontally through the object). If
the magnets were lifted rapidly, would they produce
a lifting force? |
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I am really looking forward to the answer to that.
Let me try something... lurch lurch lurch |
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I get the impression that large flat expanses are
needed for strong eddy currents. I am not sure
that jewelry would fit the bill. |
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Here is a scheme for testing the strength of eddy
currents. |
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Affix (ah fix?) magnet to lateral swings, allowing it
to swing as a pendulum along a straight line. Like
those clicking ball desk toys, but just one. |
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At the point beneath the pendulum nadir is a
small clamp like the one on the microscope, to
hold a coin or other item. |
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The stronger the eddy current produced, the
more it will slow the passing pendulum magnet.
The number of swings of the pendulum before
stopping will be lower as the eddy current
produced is higher. A heavier pendulum will allow
greater sensitivity in discriminating between
similar objects (for example gold vs silver coins) |
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//within 2 orders of magnitude of feasible// |
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//large flat expanses are needed for strong eddy
currents.// |
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Well, the 4cm diameter of a gold sovereign is
enough... |
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What we need here is scaling factors - for a given
magnet passing at a given distance and speed, how
does the force generated vary with the dimensions of
the non-ferrous object? |
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//(say, 40 million people in the UK) loses two pieces of
jewellery in a lifetime (a conservative estimate). That's 80
million items lost per year.// |
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Wouldn't that be 40 million items lost per LIFETIME, not per
year? The idea is intriguing but I'd consider fitting it to a
4wd rather than an Aston and drive across the vast gold
nugget bearing plains of Western Australia. Lots of ferrous
stones to deal with though. |
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Ooops. OK, there goes a factor of 80. |
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I think frequency is another variable. Higher frequency will
work on smaller objects. This can be generated by moving
the magnet(s) faster.
An eddy current separator can be made with a drum full of
permanent magnets. Tuning of particle size can be done by
spinning the magnets at different speeds. |
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At least this is how I remember it.... |
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A thought just occurred to me. |
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The Earth's magnetic field flips every few hundred
thousand years. At this moment, non-ferrous metals
should all leap up out of the ground. |
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I am imagining a hopper of sand. In the bottom of
the hopper is a slot and the sand falls thru,
forming a falling sheet. Beside the sheet is Ling's
spinning drum of magnets. The falling sheet of
sand goes thru a wider slot in the bottom and
lands in the trash hopper. If done in an aqueous
sorter the sand will fall more slowly, which might
be for the good. In any case it will still work,
which is good because this river bottom stuff is
going to be wet. |
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Conductive materials mixed with the sand (i.e.
gold nuggets) are pushed backward out of the
sheet by current induced by the drum and so miss
the entrance to the trash hopper, landing instead
in the chest of loot. |
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Well, chest of loot and lost washers. Subsequent
analysis of each particle by conductivity or mass /
displacement ratio will allow your robot to sort
precious metals from other findings. |
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Yes, that's the basis of non-ferrous metal separators. |
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See link for a description of how automatic recycling sorting works, including the use of eddy currents to separate ferrous and non-ferrous metals. I wonder if they have look for high-value metals too? <obligatory Hitchhikers' Guide quote> "...eddies in the space-time continuum" - "Is he?" </oHGq> |
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So Max, your prior idea along this line was "use a nonferrous metal separator to pan for gold"? |
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Yup. That's pretty much the present idea, also. |
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There's a recent report of missing manhole covers within,
and cattle grids without, several villages surrounding a
certain tame Molecular Biologist's Estate. |
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Witnesses state that they were awakened one night by loud
clanging, and crunching of gears. Must be an Aston Eddy
current separator they reasoned, and went back to sleep. |
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It's worse than that. I hadn't realized, until driving
catastrophically close to a dairy herd, that there was
such a thing as a "cow magnet" (Google it - you'll be
surprised too). |
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Ah...
So an eddy current device can be made by tying a herd of
cows to the rear of the Aston, and driving rather quickly? |
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/That's pretty much the present idea, also./ |
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I never get tired of reading of eddy current magic. When I told my dad about moving the gold sovereign with a magnet he said that if he saw it he would suspect stage magic type stuff. So when you post video of this feat you will need to conclude by biting the coin to prove it is gold thru and thru. |
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My understanding is that current gold refining operations require loads of mercury. I have seen video of these dredge boats in the 3d world ripping up the riverbottoms and poisoning the dredge with mercury. If it could be done with an eddy current separator it would be cheaper (no mercury to buy) and environmentally sound. I can envision a pedal power magnet drum on the back of the boat, flinging out the conductives and dumping the rest back into the river. |
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