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Techniques for CollectorsGold: does it really evaporate?

22nd Feb 2007 11:27 UTCCarlos Calvet

Thank you Chris for your ample explanations about how to precipitate gold with oxalic acid and how to clean my gold-platinum from cupper and cupper oxyde rests. I have heated up the whole reactor to about 90ºC and hope the quite diluted HNO3 will dissolve all remaining non-precious metals.

I have a further question about a very confusing theme. I found that distinguished researchers warn from heating up gold more than 1000ºC because it could evaporate uo to 30 %. On the other hand, I have seen with my own eyes on TV, how brazilian gold-searchers heat gold barrens up with a conventional welding gun and produce barrens of raw gold.

So, my question is: is it true or not that gold evaporates at about 1000ºC when heated up, or is this rather a kind of myth?

A collegue who was searching stones at Sils told me, he collected gold from semiconductors and then he melted it, but that the final amount was always less than expected.

Hence, does gold evaporate more in reduced amounts and less in larger amounts, so that larger amounts can be heated up to fuse gold into barrens, while small quantities (some gramms only) will evaporate?

Carlos

22nd Feb 2007 12:09 UTCPeter Haas

Any liquid has a measurable vapour pressure at any temperature.

22nd Feb 2007 16:21 UTCJesse Fisher Expert

Liquid-state metals will behave like any other liquid, and thus some evaporation is expected. Small liquid beads will have greater surface area to volume ratio than large beads. As evaporation occurs at the gas-liquid interface (i.e.: the surface), smaller beads would be expected to evaporate more quickly than large ones.

24th Feb 2007 10:53 UTCCarlos Calvet

Well, the question is now: shall I heat up the residual gold-platinum with a welding gun, or better not? The amount for this sample is about a hand-full. Is it useful to cover the sample before heating, e.g., with a tin plate?

Thanks in advance!

Carlos

25th Feb 2007 09:48 UTCCarlos Calvet

Ok, I poured off about 1/2 of the liquid volume and poured in some more HNO3. The reaction began immediately, being released H2. I hope to get soon pure gold-platinum, will heat a sample and see what happens. If it does not shrink, I will make a small bar out of the whole to see whether it is more like platinum or if it ressembles gold. The extraction of precious metals from rocks is a very difficult and economically controversial theme at small scale. I will be happy to finish this and turn then to melt garnets.

Cheers

Carlos

26th Feb 2007 19:45 UTCPeter Haas

"The reaction began immediately, being released H2"

No. Protons do not oxidize copper, virtually no matter what the concentration is. In the reaction oc opper with HNO3, the anion is the active principle, and what forms in the reaction is not hydrogen, but nitrous oxides !

27th Feb 2007 15:25 UTCCarlos Calvet

Hallo Peter,

what you are commenting is in the case of concentrated HNO3. But with diluted HNO3, there will be released H2 since the Cuº is being dissolved by an acid that is no longer oxydant.

Carlos

28th Feb 2007 10:53 UTCPeter Haas

Nitrate ions are oxydants, no matter of the concentration. Protons do NOT react with copper at any concentration.

Simple test: if it isn't the nitrate, why not using, say, phosphoric acid ? According to your theory, it should attack copper ...

2nd Mar 2007 22:34 UTCDon Saathoff Expert

Carlos...in fire assay, the precious metals are "dissolved" in molten lead then "cupeled" in bone ash...the bone ash absorbs ~98% of the lead oxide leaving the precious metals as an alloy (bead) in the bottom of the cupel. This all occurs at temperatures starting at ~900degC and ends at ~1050degC. The melting point of gold is 1700degC, platinum is 1773degC, and silver is 960degC. In my furnaces I've done experiments to determine metal loss to volitization (evaporation). The ONLY detectable loss was silver at 1000degC. after being held at that temperature for 20 minutes, the loss was 2.9%. We do this procedure so as to be able to give an accurate result in a fire assay. Every assayer and every furnace will giv a slightly different result depending on accuracy of temperature measurement, oxidizing air-flow, etc. With gold and platinum, don't worry about it!!

2nd Mar 2007 23:10 UTCSteve Hardinger 🌟 Expert

"Protons do not oxidize copper, virtually no matter what the concentration is. In the reaction oc opper with HNO3, the anion is the active principle, and what forms in the reaction is not hydrogen, but nitrous oxides !"

This is not rigorously true, as redox reactions of this nature are equilibria. The equilibrium favoring oxidized copper may be small, but some oxidation does occur.

Steve

Chemist

5th Mar 2007 17:28 UTCPeter Haas

That's why I said "VIRTUALLY no matter" ...

15th Jan 2008 10:56 UTCAdde

Gold melts at 1065C

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