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Gold Refining / Induction Heat Rings

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LutzH

Thu Jan 07 2010, 12:56AM

Registered Member #1721 Joined: Sat Sept 27 2008, 08:44PM
Location:
Posts: 136

Hello:

I figured I would post this in part as a favor to all the spouses, and significant others of our members:)

I refined almost an ounce of gold last year from old jewelry scraps purchased from friends at work. I found that you do not even need nitric acid to make aqua regia. If you use HCL (Muriatic Acid) with some KNO3 (Stump Remover) added, with a little heat, it works just like aqua regia. You do not want big chunks of gold, because they take a long time to be consumed.

For 10 grams of 14K gold I used about 150-200ml of HCL(30%) + about 40-50g KNO3, dissolve this in the HCL, add gold, and heat very gently to get the reaction going. Some chlorine is evolved so do this outside!!! and avoid the fumes period.

Once the gold was disolved I filtered the green solution, and precipitated the gold over night with NaSO3, which can be found in an almost pure state in some rust stain removers. Plain iron sulfate will work also. After washing the dark powder with some water, it can be dried and simply melted. I also saved the mother liquor to see if any more Au could be precipitated.

Another method to try would be to melt the gold with an induction heater, and pass some chlorine gas through the gold with a silica tube. All the Cu, and Ag in the carat gold will form a chloride salt slag leaving a button of about 23K gold behind. Commercialy they use chlorine but perhaps maybe Iodine vapor, could be used as a safer subtitute.

My reason for mentioning this is that many of us have some type of IGBT inverter already, which could be modified into an induction heater without to much effort. One company even sells a mini metal melting funace, which runs inside of your microwave, off the RF.

Once you get the gold to 23-24K, it is so easy to work and shape. Earing posts, and pendant loops ect, can be purchased. Or alloy the gold with 1/4 Cu/Ag and you have 18K yellow, for red gold now just use copper.

Imagine your girlfriend, or wife, watching you making, then casting the gold with an IGBT induction heater setup. Now you can have a ring, or a set of rings, with some real meaning behind it, plus the enjoyment of having your significant other appreciating your hobby for a change.

An easy ring mold can be made from a sheet, or chunk, of thick graphite. Just cut the ring shape with a small hole saw, or with a dremel tool, then pre-heat the graphite with a propane torch, and simply pour in the liquid gold. Yes lost wax / vacuum casting is much better, but its also a lot more complicated. The rest of the finish work can be done with small files, sand paper ect :)

GeordieBoy

Thu Jan 07 2010, 04:00PM

Registered Member #1232 Joined: Wed Jan 16 2008, 10:53PM
Location: Doon tha Toon!
Posts: 881

Indirectly heating it via a lossy crucible in the microwave is probably your best bet. Gold is non-magnetic and highly conductive electrically (low eddy current losses.) Rings are also physically quite small (poor magnetic coupling), so you may find it challanging to heat them directly via induction.

You can make good conductors like gold more lossy by increasing the operating frequency to reduce the skin-depth and drive up the AC resistance. You'll almost certainly need something in the MHz, so IGBTs wouldn't be the devices of choice!

-Richie,

LutzH

Thu Jan 07 2010, 07:15PM

Registered Member #1721 Joined: Sat Sept 27 2008, 08:44PM
Location:
Posts: 136

Hello:

So for induction heating you would recommend higher frequency operation for small amounts of gold then. The whole IGBT thing came from a large induction melting furnace I saw at a refinery a few years ago. They were running in the tens of KHz range, they did however have something like 10-20KW of power.

Thanks for this tip, I will not waste my time then trying this at the measly power levels which I can achieve with my HF driver. In my case I simply melted the metal with an oxy-propane torch flame. Oxyhydrogen would be ideal but I do not have H2, oxyacetylene is not good because it has to much carbon, which is why jewelers avoid it. This does not make much sense to me because you would think the carbon is pretty well consumed, but there must be a reason or something I simply do not know.

My reason for suggesting the induction approach is, I was thinking that it would be cool to make the melting crucible, the mold at the same time in the case of something simple like a ring. So do do this then microwaves, or an induction setup using MOSFET's in the MHz range would be the best bet then with limited power.

I was even thinking of a simple resistance setup. Lets say you start with a 3cm x 10 cm x 1cm thick sheet of graphite, then in the center you dig out the shape of the ring that you want. Then you narrow the center section as much as possible, leaving just enough material to support the assembly. Now if you pass a very low voltage, high amperage current through it the thinned section, it should heat up much more than the bulk material. The problem here would be getting enough amps.

I suppose that you could even try a welder with a carbon, or tungsten electrode, evening out the heat by hand. With all of the methods possible, the torch is looking mighty good in my mind. I suppose some of the fun would be in trying to use whatever you may have lying around, and getting creative. The correct method will be the one that you use :)

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