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[HAuCl4]

Republic isn't making 5-9's gold all the time. I believe it's only when requested.

Apparently their 5Ns process is somewhat different than their 4Ns process.

Can you give an educated guess on how they are doing it?.

 

[HAuCl4]

With appropriate wash procedures and expert handling it is entirely possible to get over 4N gold with a single precipitation.

The real secret to high purity gold is in fact dilution of solution and correct chelating ligands.

So basically one must rid the silver by the low temperature/optimal dilution procedure using ice. And filter most of it as silver chloride. Once optimized for temperature and dilution this should be straightforward and repeatable.

Then scavenge Pd, Pt, and the rest of the silver with some proprietary chelating chemicals/procedures. I guess ammonium chloride (or MagicFormulaNumber1) for the Pt and some Pd, DMG (or MagicFormulaNumber2) for the rest of the Pd that is left, and MagicFormulaNumber3 for the silver chloride that is still persisting in solution, with a pinch of corn starch/other magic flocculant to gather it all before filtering again?. Selecting a batch with low or no PGMs could be wise too.

That still leaves the copper contamination, which 4metals has suggested a workable way of dealing with it.

And finally thorough washing with HCl, ammonia, and nitric acid.

Interesting. Maybe that's all that they do to get 5Ns+consistently.

Certainly most of it are procedures that can be tried on a small scale. Thank you gentlemen. Any final ideas?. Is the hydrolisis before reduction a possibility or a definite no-way?.

8)

 

[HAuCl4]

run an ICP to check for gold (tech grade nitric acid sadly has some Cl- in there which will make a very weak aqua regia but will still dissolve very small portion of gold).

I have had similar issues using tech grade nitric on silver dissolves where gold was present but was not the major metal. I was successful in cementing out the gold digested thanks to the chloride impurity in the acid by using fine silver. It is not as fast as copper but it will change places in solution with gold.

I have also heard a technique used in dropping gold where the gassing of cold SO2 is stopped when about 90% of the gold is dropped. The gold is filtered out and the balance of the gold is gassed. This gold is washed and is the highest purity gold. The theory being that at the end of the reaction, the SO2 begins to react with the Copper and drop it as well. We have all seen the results of over-gassing gold in solution and the white copper crystals can be seen if you really over gassed. The copper is soluble in concentrated HCl but the purity will suffer. Well the copper apparently starts to come out of solution before all of the gold drops and while it is very little it is enough to be a problem when you are looking for .9999 purity or better. I have never had a client who needed the gold to be .99999 so I have yet to test this process. The gold dropped at the end of the precipitation is not as pure and has to be reprocessed.

This step may not be necessary if you are washing your precipitated gold with HCl as Lou suggests, as I said I have yet to test this procedure.

I like this 90% ish gassing procedure suggestion a lot. By trial and error one could determine the optimal % gassing point that still produces 99999+ gold, then get that 99999+ gold out (or not), re-oxidize the solution with a little sodium chlorate, then gas again and drop the 90%ish of the 10% ish that was left. It wouldn't take significantly longer. Thanks!. 8)

 

[HAuCl4]

Apparently this is a good chelating agent for silver. I don't know if it is the best.

http://www.springerlink.com/content/l007033wqx056142/

I'm going to spend some time doing experiments on high purity materials and visiting clients before I post again. Happy refining to all, and thanks again for the great ideas!. 8)