Half a croissant, on a plate, with a sign in front of it saying '50c'
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Gold panning non-submersible car

  [vote for,

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.

MaxwellBuchanan, Mar 13 2015

Using this technique in recycling centres http://www.lbhf.gov...eet_tcm21-23390.pdf
[hippo, Mar 16 2015]


       So, fresh out of a job, now picking up aluminum cans?   

       Oh, and I will keep a careful distance from any Aston I see due to the "periodically dumped back on the roadside" feature.   

       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.
normzone, Mar 13 2015

       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.   

       And thanks for the encouragement. Better things are indeed on the horizon, or at least below the horizon but hypothetically there.
MaxwellBuchanan, Mar 13 2015

       Well if they were neither above nor below the horizon I would start worrying.   

       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.
pocmloc, Mar 13 2015

       " Objects on the horizon may be nearer than they appear "
normzone, Mar 13 2015

       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.   

       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.   

       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.
MechE, Mar 13 2015

       Oh, yes, like fractional distillation of metallic debris. Nice.
pocmloc, Mar 13 2015

       // retro-fit to municipal road-sweepers.//   

       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.   

       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.   

       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.   

       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.
MaxwellBuchanan, Mar 13 2015

       <5min later>   

       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).   

       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.   

       Result: the sovereign actually jumps (at least, it flips up and flops back down); the pound coin doesn't move.   

       Conclusion: my gold panning non-submersible car is probably within 2 orders of magnitude of feasible.
MaxwellBuchanan, Mar 13 2015

       <4min later still>   

       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).   

       Damn. If only I had a spare Aston.
MaxwellBuchanan, Mar 13 2015

       <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.
MaxwellBuchanan, Mar 13 2015

       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.   

       Also I imagine an a/c electromagnet would give different results from a fixed magnet.
pocmloc, Mar 13 2015

       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.
pocmloc, Mar 13 2015

       //I'm not sure sovereigns are typical of the kind of detrius you expect to find on the road//   

       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.   

       //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?
MaxwellBuchanan, Mar 13 2015

       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?   

       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?
MaxwellBuchanan, Mar 13 2015

       I am really looking forward to the answer to that. Let me try something... lurch lurch lurch
bungston, Mar 13 2015

       I get the impression that large flat expanses are needed for strong eddy currents. I am not sure that jewelry would fit the bill.   

       Here is a scheme for testing the strength of eddy currents.   

       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.   

       At the point beneath the pendulum nadir is a small clamp like the one on the microscope, to hold a coin or other item.   

       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)
bungston, Mar 13 2015

       //within 2 orders of magnitude of feasible//   

MechE, Mar 13 2015

       //large flat expanses are needed for strong eddy currents.//   

       Well, the 4cm diameter of a gold sovereign is enough...   

       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?
MaxwellBuchanan, Mar 13 2015

       //(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.//   

       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.
AusCan531, Mar 13 2015

       Ooops. OK, there goes a factor of 80.
MaxwellBuchanan, Mar 13 2015

       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.   

       At least this is how I remember it....
Ling, Mar 14 2015

       A thought just occurred to me.   

       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.
MaxwellBuchanan, Mar 15 2015

       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.   

       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.   

       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.
bungston, Mar 15 2015

       Yes, that's the basis of non-ferrous metal separators.
MaxwellBuchanan, Mar 15 2015

       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>
hippo, Mar 16 2015

       So Max, your prior idea along this line was "use a nonferrous metal separator to pan for gold"?
bungston, Mar 16 2015

       Yup. That's pretty much the present idea, also.
MaxwellBuchanan, Mar 16 2015

       There's a recent report of missing manhole covers within, and cattle grids without, several villages surrounding a certain tame Molecular Biologist's Estate.   

       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.
Ling, Mar 16 2015

       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).
MaxwellBuchanan, Mar 16 2015

       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?
Ling, Mar 16 2015

       /That's pretty much the present idea, also./   

       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.   

       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.
bungston, Mar 17 2015


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