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Stripping Gold

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wylie

Fri Nov 20 2009, 04:40AM

Registered Member #882 Joined: Sat Jul 07 2007, 04:32AM
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Posts: 103

Decided to make a thread about this per my comment in the HV transistor heatsinking thread. Plus there just hasn't been enough posts in Chemistry lately. BoredChemist is getting bored i bet. Errr, more bored...i dunno.

So, lots of PGA chips with gold plated pins. I know it's not much gold, but i'm not trying to make a buck. It would be fun to have a vial of gold dust, or some gold i could use for whatever. Coat some of my of own connectors in gold or something, i don't know.

There's solution and there's heat. Both are going to affect the surrounding materials, but solution seems more segregated as long as i mount them well so the acid doesn't touch the package.

Brew up some Aqua Regia, and dip the pins in....and didn't i just dissolve the entire pin? Then i have to separate salts or something. Plenty of chem research before i get anywhere near trying that obviously.

Am I just considering an exercise in futility? Could heat be any more productive?

Any and All (potentially crazy) Ideas Appreciated.

(No need for "good god, don't do that!" If there's no supporting ideas, I'm already at the giving up stage. No worries there lol)

EDIT: How about subtractive dissolvography. I could snip the pins and dissolve out the base metal with plain HCL, leaving just the resistant gold plating. Thoughts?

Chris

Fri Nov 20 2009, 08:25AM

Registered Member #8 Joined: Thu Feb 02 2006, 04:34AM
Location: Harlowton, MT, United States
Posts: 214

It isn't that hard to precipitate gold highly selectively from aqua regia, but it is tedious. Once you have the mixed salts all filtered, you reduce the oxidizing agents (HNO3, Cl2, NOCl) in the acid with urea by adding until the pH is around 1. You then precipitate the gold out with sodium bisulfite, soak and rinse the precipitate with aqueous ammonia which will dissolve any CuCl that will co-precipitate if too much bisulfite is added. Since you have an unknown but small amount of gold, it will be impossible to know how much bisulfite to add; it is generally added just until SO2 begins to be released, or about 1g of dry metabisulfite per 1g of gold added as a dilute solution but the amount would be so small this would be difficult to detect via smell. You could deal with some excess and co-precipitation of CuCl as long as you thoroughly dissolve it with ammonia and rinse it several times. You could also use HCl + H2O2 in a primary or secondary purification step. The resulting gold is a fine brown dust, and is difficult to collect the last residues of from surfaces. The purity is easily 99.9% as long as everything is well filtered and kept clean during the process.

If you choose to use a selective digestion approach in the first place, nitric acid will be the most suitable agent. HCl + H2O2 could also work of course. I would remove and finely divide the pins before putting them in the acid. You will never achieve even close to the purity with such methods as you will with selective precipitation. Much of the alloyed metal at the boundary of the gold may not dissolve, and you would also be limited to the initial purity of the gold plating. I would probably use such a method just as a preliminary step to remove the bulk of unwanted metals before then dissolving the crude gold in aqua regia and proceeding from there. This would allow you to weigh the gold roughly before dissolving it, thus aiding in the precipitation procedure.

Yet a third method is to smelt (that terminology is a misnomer from a chemistry standpoint, but most people call this procedure smelting anyway because it involves using hot chemistry to extract a metal from raw material) the gold from the pins by melting it all together with an oxidizing flux. A common flux for this is a mixture of NaNO3 or KNO3 with boric acid/boric oxide or borax, but chlorates or perchlorates would likely work equally well as the oxidizing component. The less noble metals are oxidized in the process and the oxide slag is dissolved by the boric oxide or borax, leaving the insoluble gold (or other precious metal) easily separated. The traditional method of separating any dispersed pure gold from the resultant slag was to amalgamate it with mercury, and then distill off the mercury leaving a gold sponge. Of course some nicer chemical means could be used instead, perhaps a thorough wash in dilute nitric acid.

If you have never dissolved gold or other metals in aqua regia before, you may be surprised how vigorously it bubbles with noxious/choking/corrosive/toxic fumes and how very quickly and drastically the reaction rate responds to changes in temperature. I have had it overflow from a test tube when initially the tube was less than 10% full. Just add the pins or whatever you have to the aqua regia a few at a time, slowly and carefully. Just dipping the chip in as deep as the pins without getting the rest of the chip is not likely to work well as it may froth up over the chip, but fortunately a little bit of whatever from the rest of the chip is not going to hurt anything with the aqua regia process. It is also convenient that aqua regia is not nearly as aggressive toward human tissue and clothing as nitric acid on its own (you actually have a few seconds to wash it off), but still obviously not something you want to get on you!

johnf

Fri Nov 20 2009, 10:14AM

Registered Member #230 Joined: Tue Feb 21 2006, 08:01PM
Location: Gracefield lower Hutt
Posts: 284

If you want to dissolve the gold without too many other metals then mercury is the answer as long as the gold is on the surface -iron does not amalgamte with wercury --copper will albeit slowly. aluminium is to be avoided as this dissolves quicker than gold.
Use plenty of mercury and then distill recovering the mercury (distillation preferably under vacuum) distillate then can be processed back into gold in a reducing furnace.
Old gold prospectors used mercury to process but they boiled off the mercury into the environment --not acceptable in this modern age!!!

Chris

Sat Nov 21 2009, 05:08AM

Registered Member #8 Joined: Thu Feb 02 2006, 04:34AM
Location: Harlowton, MT, United States
Posts: 214

The leads probably do contain copper though, and that does amalgamate with mercury, and there is a lot more of it present than gold in the pins so it doesn't matter if it amalgamates slower or not. You would not be purifying it much to just dissolve the pins in the mercury. Hence the "smelting" process would need to be done beforehand. Of course distilling mercury may be impractical to some anyway, and the aqua regia precipitation process still gets much higher purity. Once you have the powder you just melt it down in a small crucible (with a lid so it won't get blown away).

Proud Mary

Sat Nov 21 2009, 05:39AM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

Chris wrote ...

The leads probably do contain copper though, and that does amalgamate with mercury, and there is a lot more of it present than gold in the pins so it doesn't matter if it amalgamates slower or not. You would not be purifying it much to just dissolve the pins in the mercury. Hence the "smelting" process would need to be done beforehand. Of course distilling mercury may be impractical to some anyway, and the aqua regia precipitation process still gets much higher purity. Once you have the powder you just melt it down in a small crucible (with a lid so it won't get blown away).

I would expect typical gold plating of average consumer parts to be of the order of 10% of one American micron, so I would say the entire matter was one of economic balance : how many kilos of electronic scrap are needed to recover 1g Au, the price of impure Au on the gold market at that time, and the cost of your cyanides etc and all the rest you will need to license them, pay for disposal of cyanide wastes, transport costs, insurance, site inspection certificates etc - a few thousand dollars per barrel perhaps, in lots of 100 x 200 litre drums.

As is well known, this is the kind of work usually 'outsourced' to third-world countries, where the disposal of hazardous waste costs very little, or nothing at all, if you are in with the generalissimo. and have no worries about dumping hazardous waste in the barrio of the poor.

Unless you are talking metric tons of electronic scrap delivered by shipping containers, you'll be a thousand times better off just buying yourself a few grammes of auriferous salts etc - to play with. You can buy gold chloride for about $30/gm, so why not give yourself a break and just buy some of that - it'll be much cheaper than even a one single empty hazardous waste drum!

Chris

Sat Nov 21 2009, 06:53AM

Registered Member #8 Joined: Thu Feb 02 2006, 04:34AM
Location: Harlowton, MT, United States
Posts: 214

There is no cyanide waste in the refining processes discussed; no cyanide is involved. Cyanide methods would not be practical in this case. That's mainly for electroplating or extraction from raw ores anyway. Otherwise, I have no idea how many of these chips he has, but I would definitely agree that it is unlikely to be economical. The gold I have refined with the aqua regia method was from scrap jewelry that I obtained for somewhat less than spot gold price, and with the reagents (most of which I synthesized myself from more easily available reagents) and especially the time I still did not come out ahead. None of the refining methods will even work with quantities on orders of magnitude less than grams; you will just have too little to even collect, it will be but a smattering and most will be inevitably lost as it cannot be collected from container walls, and would not even move if it could be melted where it sits. You will want to have at least a gram or so for this to be a worthwhile exercise in chemistry (which I gathered is the main goal here), and it cannot not be economical until you are perhaps an order of magnitude or two higher than that, even with the current high price of gold.

Let us remember that if economic gain was the goal of every endeavor on these forums, we would be unbelievably foolish. Most of what we do here is a money sink for our own entertainment and learning, not a money source. Only a lucky and skilled few have made a dollar for their efforts.

Proud Mary

Sat Nov 21 2009, 07:14AM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

Chris wrote ...

Cyanide methods would not be practical in this case. That's mainly for electroplating or extraction from raw ores anyway.

S. Kulandaisamy, J. Prabhakar Rethinaraj, P. Adaikkalam, G. N. Srinivasan& M. Raghavan The aqueous recovery of gold from electronic scrap Journal of the Minerals, Metals and Materials Society
Vol. 55, Number 8 / August, 2003, pp. 35-38

Chris

Sat Nov 21 2009, 09:48AM

Registered Member #8 Joined: Thu Feb 02 2006, 04:34AM
Location: Harlowton, MT, United States
Posts: 214

I'm going to guess that most home chemists would not find that method preferable because it involves the toxic and difficult to obtain cyanide salt, but to each their own.

Proud Mary

Sat Nov 21 2009, 10:34AM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

My concern was to place a question mark against the notion that "Cyanide methods [are] mainly for electroplating or extraction from raw ores anyway," lest readers should be mislead.

As a matter of interest, whilst cyanidization remains the chief means of gold recovery from electronic scrap at present,* a newer technique using 50% nitric acid alone is gaining ground.

The relative porosity of gold films allows the acid to attack the copper substrate, so the gold film becomes detached and can be recovered as an undissolved residue.

* Sometimes with catastrophic environmental results, see, for example:

Korte F., Spiteller M.; Coulston F. The Cyanide Leaching Gold Recovery Process Is a Nonsustainable Technology with Unacceptable Impacts on Ecosystems and Humans: The Disaster in Romania.Ecotoxicology and Environmental Safety, Volume 46, Number 3, July 2000 , pp. 241-245

MinorityCarrier

Mon Nov 23 2009, 07:31PM

Registered Member #2123 Joined: Sat May 16 2009, 03:10AM
Location: Bend, Oregon
Posts: 312

You can dissolve gold with a solution of Potassium Iodide and Iodine.

I have not tried recovery of the gold from this solution, but it supposedly involves weak acid (ascorbic acid) and buffers to precipitate the gold.

The gold will be very fine particles, brown in appearance.

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