Method for Ultra High Purity Gold

Gold Refining Forum

Help Support Gold Refining Forum:

This site may earn a commission from merchant affiliate links, including eBay, Amazon, and others.

Lou

Moderator
Staff member
Moderator
Joined
Nov 4, 2007
Messages
5,203
Location
Rhodia Drive
A discussion with a friend via gmail chat earlier today sparked my mind about high purity precious metals. Sprinkled throughout the literature are methods for obtaining metals and compounds of exceptional purity.

Here is one such method that reliably produces 6N+ pure gold.

As I find methods, I will continue to post them. At the very least, some of us here may have academic interests.

Method for UHP gold:


All acids and reagents used must be of trace metal grade. Recommended is the Trace Metal line of products from J.T. Baker. Under no circumstances should any metal containing object or apparatus contact the gold or solution; only aqua regia boiled quartz and PTFE apparatus should be used.

Gold of 99,995% purity (20 g.) well rinsed in ether and then in HCl is dissolved in aqua regia in an 800-ml. acid washed quartz jointed flask, and the solution is concentrated to a thick syrup at 80 degrees Centigrade, removing the most part of the nitric acid. It is then boiled 5 times further and periodically 20 ml. of hydrochloric acid (4:1) is added to expel nitrogen oxides. The residue is taken up in 650 ml. of hot 18.2 MΩ water and digested until all soluble material is dissolved. It is then allowed to settle for eight days in a dust-free atmosphere, preferably in a closed system. The precipitate consists of AgCl containing small amounts of Au, Pd, SiO2, etc. The gold solution is vacuum filtered through a micron quartz frit via decanting (do not disturb the precipitate). This and all later precipitates are not worked up further to obtain gold.

Gold refined by use of SO2 still contains some Pd, while that precipitated with oxalic acid contains Cu, Pb and other metals. Therefore both of these procedures must be used to obtain gold of the desired purity. Ultra high purity sulfur dioxide is passed through the warm gold solution (80°C) obtained above; the gold precipitates quantitatively on careful neutralization with ammonia (1:1). The product is allowed to settle overnight and the deposit of spongy gold is washed by decantation (7 times) with 200 ml hot 18.2 MΩ water; it is then heated for four hours on a steam bath with concentrated hydrochloric acid and washed free of acid with hot 18.2 MΩ water. Then it is redissolved in aqua regia in a quartz flask, the nitrogen oxides removed, filtered, and reduced. The entire procedure above is repeated seven times in order to remove all Ag, Cu, Ni, Zn and Pb. It is no longer necessary for the gold solution to sit. The precipitated gold product is then digested for 12 hours with ammonia (1:1), washed free of ammonia with 18.2 MΩ water, heated for four hours on a steam bath with hot concentrated nitric acid, and decanted. Ammonia (1:1) is again added and later removed by washing with 18.2 MΩ water . The gold sponge is dissolved in dilute aqua regia; after addition of HCl, the solution is concentrated by evaporation, diluted with UHP H2O, decanted and filtered.

The gold is precipitated by careful addition (there is a danger of foaming over) of small portions of powdered, UHP crystalline oxalic acid (ACS grade may be resublimed twice). If the solution retains a yellow colour, it is carefully neutralized with ammonia and more oxalic acid is added; the addition of the acid is continued until the solution remains colorless. The resultant gold sponge is again dissolved and reprecipitated with oxalic acid. It is then Pd-free. Finally the gold is redissolved, precipitated with sulfur dioxide and heated with concentrated hydrochloric acid, and washed with 18.2 MΩ water. The residual traces of HCl are removed with ammonia and it is again washed with UHP water. The product is transferred to a quartz dish and dried at 110°C. Yield 75-80%. The gold prepared in this manner is spectroscopically and analytically pure (free of metallic Cu, Ag, Ni, Pd, Fe, Zn and Pt) and is above 99,9999% in quality.
 
Dang - Thanks for the info, but for a novice backyard operation like I have, Lazer Steve, here I come.

Seriously, that is real good information for the professional and novice, I even learned a couple of things from reading that (this old dog can learn a few new tricks.)

But, what is 18.2 M Ohm water?

Luck....Jack
 
Lou,

Great post, it reads like it came right from the Organic Synthesis site that I love so much!

18 Mega Ohm water is water that has a resistance of 18 Million Ohms on your ohm meter. Pure water free of dissolved solids has a very high electrical resistance.

Steve
 
Water of that quality is very hard to make. It is really friggen pure water :). The machine to make that water costs (or used to) about $8k, oh and did I mention that it likes to break down and that you need to replace cartridges on it. Water that pure is a pain in the butt to make--multiply distilled deionized water that's been passed over all sorts of ion exchange resins, activated carbon, assorted membranes, and then repeated. Unfortunately, if you left such water in an untreated glass bottle or out in the open air, it would be ruined! It would be basic from reacting with terminal hydroxys on the glass surface, and if exposed long enough to the atmosphere, it will become acidic from absorbing CO2. Now you see why analytical chemistry is somewhat challenging--variables, and lots of them!

I love the uncanny resemblance this technique's rinse procedure has to that advocated by Harold, Chris, Steve, and used by many, many more on this site!

I recently purchased some gold and platinum of 6N5 quality at spot price. Some of you may be thinking that it's a rip off, but gold from a supply house refined by a chemist to that level of purity would command three times the market value per ounces in bulk. For small amounts like 5-20g it would be 8 or more times market value.


As you may have noticed, the starting material was already purer than the quality of commercially pure gold (as Chris and Harold have corrected me) at 99,995 so you can see the trouble that is required just to make a couple orders of magnitude purer. This procedure is one I typed by hand (I had it loose leaf) onto the forum and it's fairly old (20 years at the least). I'm willing to bet that the procedure will give gold of even higher purity than 6N, which was probably the limit of the most complicated analysis back then.

I also had/have procedures for rhodium, palladium, platinum, rhenium, osmium, ruthenium, and iridium. I suppose they're not useful from the aspect of recovery, but they are highly useful as far as refining goes.
I think these procedures are enlightening just from the standpoint of it being the pinnacle of refining let alone from practical value it has for us as refiners in improving our technique.

As I find them, I will post them. Moderators willing, perhaps we can move them into another thread and sticky it for future reference.



Lou
 
I suppose another moderator stickied this post, it's not me this time :p

You're right Lou. The distilled/filtered water we use at our school lab has a pH aroud 5.5 if I remember correctly. This is due to dissolve CO2 of course.

I'm glad you posted the process. In my never ending goal to reach to purest gold possible, this is very useful.

Cheers :D
 
It would be cool if somone was setup for the ultra purity process. Everyone could send in their gold and do one big batch of it. Unfortunately, it's beyond my technical ability.
 
Lou,

Great post! but what if I'm happy with 4n's plus only. Is there any way I could refine scrape gold without costing me too much money and time? Can I cut some corner with your procedure above? Can you post a procedure to get 4n's plus. thanks
 
Absolutely. Really, a good investment is dibutyl carbitol, a polyether that solvates gold. Diethyl ether also works, but you'll get one hell of a buzz working with it in an unventilated environment. Its flammability also makes gasoline look benign.

If you just want to precipitate the gold once and get 4N, extract with dibutyl carbitol (nicknamed diglyme). Wash the extract well with 3M HCl until it the acid is colorless. Then precipitate with the reductant of your choice. Filter and collect the gold, washing well with solvent. Take the gold powder and (if rinsed right, it's chloride free) boil it with 4M nitric acid for an hour. If the nitric is colorless (no green, no blue) then chances are good that you're doing well. Filter again (saving the nitric to use for some other purpose) and add the gold to concentrated ammonia and stir for an hour or so. Any blue will indicated copper. You don't really need to rinse the gold after this, any ammonia or ammonium nitrate will be lost to the melting process. The gold will assay 4N and it should take less than 6 hours from dissolution of crude material to casting the pure metal.


Lou
 
Lou said:
Wash the extract well with 3M HCl until it is colorless.

Lou

:?:

If you extracted gold, won't your solution be yellow ?

I know for sure that your solution will be «cloudy» this happens to me all the time. Slow vacuum filtration plus 3 times HCl scrubbing is what is needed.


Anyway, I really like how DBG works. It's clean and efficient, and it's reusable !
 
Of course Noxx! I miscommunicated--I've edited it so it reads better.

I meant wash with HCl until the HCl rinse no longer has any color. If the HCl isn't picking up any color, then it's not solvating (m)any base metals.

The cloudiness is due to the solubility of water in the ether. As an aside, don't use anything other than HCl or quite dilute sulfuric--ethers can be cleaved by strong acids (i.e. HBr or HI).

Hope this clears it up for you, no pun intended!

Lou
 
hi everyone , i was searching the forum and found an excelent article betwen HaroldV and GoldsilverPro is astonishing and will shore clear many minds on refining gold to 9.999

Please read especialy Goldsilverpro method

http://goldrefiningforum.com/phpBB3/viewtopic.php?t=325&postdays=0&postorder=asc&highlight=refine+jewelry&start=15

good reading...
 
So if someone were actually able to produce 6n or better gold, where or how would you sell it? I have not made any but I might pursue it in the future if it became a practical option, although very unlikely.
 
Semiconductors and specialty research purposes. It's not a commodity at that point by any measure.
 
Holy wow, this is like the Olympic Gold Medal of refining. Has anyone here ever performed the process?

Who came up with it? Does it really require all those steps? I'm sure the answer is, "Yes, if you want 6N fine gold". What I'm interested in knowing is the WHY for each step. For you experienced refiners each step probably makes sense, but for newbs like me I'm interested in knowing what the chemical reasons are for each step. If some has the time to summarize I'd appreciate that. Otherwise, I guess I'll learn in time.
 
Back
Top