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Induction heating gold

rjamjb

Jan 19, 2013
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Hi I've just started a project to extract gold from scrap electronic components using a chemical method. It is too labor demanding and costly to do on a micro scale (couple of lbs of components may give rise to 1-2 gs of gold for example) so if you are looking to get rich quick - don't follow my lead.

However, this is more an exercise for my satisfaction and to follow up, when I've a gram or two of gold power I would like to melt this into a small button. I can go down two routes 1) use mys sister's clay kiln to heat the power (and a little borax) - this should work as her kiln is hot enough, or 2) melt it in an induction heater.

2) is my preferred option if I can satisfy myself with two further things 1) the safety aspects of a home made induction heater and 2) whether I can actually melt gold in such a heater.

Question 1) Before I start, I know that gold has little magnetic resonance (but it does have some) so can an induction heater generate enogh heat in gold to melt it? (gold melts at over 1000C.

Question 2) what power draw from teh UK mains (which has approx 240V ac) would it take approx - my house is on a 100A fuse and that would seem ample but want to work it through.

Question 3) Safety safety safety - I am no newbie to electronics but this circuit has water running through copper pipes and very large powerful caps and fast switching high amp mosfets - these are BAD combinations !!! = I would like to run a thread with safety input to each stage to help others so would like any and all inputs to safety on this which I will cover in my posts also. Any initial inputs?

Russell
 

rjamjb

Jan 19, 2013
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By the way - my initial idea for a circuit contains some half bridge IGBTs which should be fast and at least 600+V and 300+A - these are very expensive from what I've looked around to see and will kill the project for me. Does anyone have an idea of any scrap/ex componentry that I could salvage these from or get at a reasonable price?
 

Arouse1973

Adam
Dec 18, 2013
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You have to note that copper melts at 1085 deg c and tin around 300. You will most likely get contamination whilst melting gold. I dont think you will melt gold in an induction heater but you might be able to melt it in an iron container which melts at about 1500 deg c or steel at 2700. But this would be very dangerous.
Adam
 

Martaine2005

May 12, 2015
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Also, the gas or electricity cost will far out way the gold found on pcb's and connecters.
Countries that rely on this for a pitiful living use free natural sources like fire.
I wish you well in your project.
Better to buy a metal detector and sweep the beaches at sun set.
Just my honest opinion. I do!!!

Martin
 

rickselectricalprojects

Feb 1, 2015
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it would be a lot safer and cheaper just to use a blow torch. to melt metal (apart from low melting temperature metals like aluminum and tin) in an induction heater you would need a very powerful induction heater.
 

rjamjb

Jan 19, 2013
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Thanks Minder, I'd seen that, there are also some really good examples using ferric materials that will actually stay within the copper coils just due to the magetic fields. When these heat up you end up with a molten mass suspended in mid air in the middle of the coils. Quite impressive.

Arouse1973, The copper remains cooled as water is pumped through it and the material being melted does not touch the copper. I've a graphite crucible and that's what I'd hold the gold in, inside the coils. It would be really cool if the gold would suspend in the air like the ferric metals, but I fear that it is too dense and also it is not that magnetic so won't be able to suspend in that way.

I'd love to make one of these but some of the components (as I've noted above) are $100-200 each and so I fear this is too much for me to follow this up.

Regards

Russell
 

Arouse1973

Adam
Dec 18, 2013
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Hi Russell
What I was talking about was the contamination from what else was on the components. I wasn't sure where the gold was that you were planning to salvage.
Adam
 

duke37

Jan 9, 2011
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Ordinary iron or steel is magnetic at low temperature but changes phase from body centred cubic to face centered cubic as the temperature is raised and loses its magnetism. The heating effect or levitation effect is therefore greatly reduced.

This phase change can be seen if some iron wire is fixed between a couple of posts and heated by passing a current through it. As the temperature rises, the wire will expand and sag. Furthur heating will cause the phase to change and the wire will tighten.

Tight coupling is necessary and significant resistance, hence the graphite crucible.
 

Arouse1973

Adam
Dec 18, 2013
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Ordinary iron or steel is magnetic at low temperature but changes phase from body centred cubic to face centered cubic as the temperature is raised and loses its magnetism. The heating effect or levitation effect is therefore greatly reduced.

This phase change can be seen if some iron wire is fixed between a couple of posts and heated by passing a current through it. As the temperature rises, the wire will expand and sag. Furthur heating will cause the phase to change and the wire will tighten.

Tight coupling is necessary and significant resistance, hence the graphite crucible.

Good point Trevor. The curie temp of iron, it's about 700 deg c isn't it? Above the curie temperature the hysteric heating effect disappears and your just left with eddy current heating.
Adam
 

Minder

Apr 24, 2015
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if you wa

if you want to melt gold, you will probably need something a lot more powerful than a royer induction heater.

Well all you have to do is build one with the capacity, the optimum frequency may be something that would need to be researched.
As mentioned, it may be the crucible that heats up and melts the gold?
M.
 

rjamjb

Jan 19, 2013
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Hi, thanks Minder, thanks Arouse and Rick - all good points.

Arouse - you may be right about the crucible heating the gold. I hadn't considered that. There are videos showing ferric materials being held in the magnetic field and turning molten - no crucible, but I know ferric materials work well in an induction heater and gold is not very magnetic - however there should be eddy fields caused in a metal, even gold through this process, so, it won't matter which is heating which if I've got the gold in a crucible, providing it melts. I'll do a little more research on this point.

Minder, it is the resonance that is key to this, I've seen one example induction heater where the resonance was measured and re-calibrated real-time via an Arduino chipset feedback loop and also one where the resonance was supposedly self correcting (but I need a bit more time with that circuit to understand how it is working exactly - it might be only possible within a certain power/frequency band.)

Arouse, on your other point re the gold having contaminants, I'm refining from electrical components and should be achieving 99.5-99.9....+ % Karat gold - Contaminants are likely to be higher metals, silver or platinum (may be a tiny amount of Cu) - or very trace elements above all these, but negligible. So it really needs to be the gold that has eddy currents generated within it.

Thanks for all your feedback - it is good to have some testing on my thoughts.

Optimum frequency - now that, I think will be the key to this project!!!
 

rjamjb

Jan 19, 2013
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Ps - Rick may really of hit the nail on the head - this might be just a vanity project - a blow torch and a ceramic crucible - they would work perfectly and be much more simple!!!!

Russell
 

Alec_t

Jul 7, 2015
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So it really needs to be the gold that has eddy currents generated within it.
Even if you could generate eddy currents, since gold has excellent conductivity (low resistance) I think any heating effect would be small.
 

Minder

Apr 24, 2015
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Minder, it is the resonance that is key to this, I've seen one example induction heater where the resonance was measured and re-calibrated real-time via an Arduino chipset feedback loop and also one where the resonance was supposedly self correcting (but I need a bit more time with that circuit to understand how it is working exactly - it might be only possible within a certain power/frequency band.)

I have worked on two application, one was a steel ingot weighing about 150lbs and used 60hz, the other was silver soldering the filler pipe into gas tanks using silver solder and this ran around 455khz, the metal heating melted the S.S.
The frequency will changes somewhat with material size and shape and also as the part heats up.
M.
 

duke37

Jan 9, 2011
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It is many, many years since I used an induction heater. This was an adapted medium wave broadcast transmitter, the high tension fuses were about two feet (600mm) long. These would blow if the oscillation stopped.
The difficulty is getting the correct coupling. If the gold particles are small, I think it may fail. The skin effect depends on frequency and so small particles will need a high frequency. You could obviously use a few sovereigns or pieces of eight to start the system. Powdered iron cores are used for high frequency applications because the heat generated is low.
 
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