BromicAcid - 26-4-2006 at 15:57
eBay offers a number of lots simply by searching 'gold scrap' or other similar phrases for cell phone boards, outdated processors, etc that are gold
plated or contain silver/palladium or other metals. Apparently there is some kind of market for it because they do in fact sell and there are CD's
available on eBay for $$$ that offer to tell you the secrets of recovering these metals at home.
With that background out of the way there are two questions that I pose here:
1) Is this cost effective on a home level?
Gold is at an incredibly high price at the moment that corresponds to something like $20 a gram and if you can get 450 grams of waste for $11
including shipping, you got to wonder if it is worth it. Although I am assuming you can with some practice pick out the lots that are going to give a
higher percentage of gold though I would think things are just gold plated with that gold leaf and therefore you'd get almost no gold.
2) Providing you would get some noticeable gold/silver/etc from the circuit boards/transistors/etc what would be the best way to obtain it?
I'm guessing the best way would be some sort of cyanide bath. Barring that I was thinking soak in HCl to get rid of the less noble metals, then
HNO<sub>3</sub> to pull out the silver and copper and such and finally some aqua regia to get gold but I'm sure there is a better way.
This is just something I was thinking about after noticing the auctions for these boards and how much some of them go for coupled with the high prices
of these metals as of late.
(I did some google sleuthing and found a few commerical sites that preform this service, I also found a small amount of information on doing it at
home, but nothing really about how economical it can be.)
Edit: I overlooked a similar though more focused thread:
https://sciencemadness.org/talk/viewthread.php?tid=5317
Edit again: Made the subject more expansive.
[Edited on 4/28/2006 by BromicAcid]
The_Davster - 26-4-2006 at 16:17
Have you seen http://webpages.charter.net/kwilliams00/bcftp/front.htm
?
12AX7 - 26-4-2006 at 16:28
Commercially they melt it all, treat the stack gasses (chlorinated plastics, eh?), add flux (if needed- fiberglass boards, being glass, melt well
enough already) and let the metal fall to the bottom. Electrorefine the blob.
These days, it's just barely econominal, with economy of scale and less-than-free scrap.
I know a fellow who was a refiner for many years, for instance he would clean up used crucibles and dishes and stuff from jewelers, etc. He says
electronics that aren't chock full brimming over with gold are *not* worth it.
FWIW, he used a lot of fluorospar, which you can imagine ate through his high-alumina furnace lining, requiring a replacement every so often.
Tim
BromicAcid - 26-4-2006 at 16:28
Yes, I saw that site some time ago though not relating to this. I knew something of the halide leaching but it slipped my mind. I might give it a
try, I have 1 old processor, too bad my scale doesn't go to the first decimal place. Still, how much gold would say 500 grams of scrap contain,
that's where the real quesion lies.
The_Davster - 26-4-2006 at 16:37
Well depending on the age of the electronics, you want them as old as possible from before they switched from pure gold to electroplating. Test the
flexibility of the pins I guess?
The other day I was thinking about this myself, so I ripped ~5 prongs off a p2 processor and dissolved it in 3:1 aqua regia. I got the charastic gold
colour within seconds. I had not done my research so I had forgotten how to recover gold from this so I just basified and got a blue ppt, must be all
the copper in the prongs. How would the HAuCl4 react with base anyway?
denatured - 26-4-2006 at 16:49
A while ago i watched a documentary about what you are talking about ... it was about a successful business in Japan (making platinum & gold out
of garbage) ... cause as you know in japan there are a lot of cell phones thrown in garbage every hour... they said it is profitable...
About the yield ...
The amount of metals in a metric ton (1,000 kilograms, or 2,205 pounds) of cellphones (about 16,670 handsets):
GOLD -- 0.4 pounds
SILVER -- 2.6 pounds
COPPER -- 255.7 pounds
PALLADIUM -- 0.2 pounds
STEEL -- 205 pounds
ALUMINUM -- 83.8 pounds
Edit:sorry .. link to article requires registeration ... i've attached it.
[Edited on 27-4-2006 by alnokta]
Attachment: article.zip (28kB)
This file has been downloaded 1077 times
BromicAcid - 26-4-2006 at 16:51
12AX7, I was under the impression they used cyanide as the main tool to get the gold due to:
http://www.cyanidedestruct.com/precious_metal.htm
It was one of the first pages that came up with respect to searching for precious metals and recovery from circuit boards. I decided to put my one
processor from 1995 into a beaker with some sodium hypochlorite and sodium bromide and let it sit for a day, if I get anything I will post about it
here for fun.
Edit: Thank you alnokta, that's one of the things that I wanted to know 
[Edited on 4/27/2006 by BromicAcid]
12AX7 - 26-4-2006 at 18:49
Yeah, but the thing is, you've got all those wires and stuff coated with epoxy (black or PCB) and ceramic (inside the sealed 486 chip and etc!), for
maximum yield (profit) you really have to break it all down and fire is a pretty cheap way of doing that.
Tim
BromicAcid - 27-4-2006 at 16:59
Following the procedure on the link Rogue Chemist provided, I preformed the halide leaching method that A.K Williams describes. Lets see, a Pentium
100 mhz chip was put in a beaker with a large pinch of NaBr, enough bleach to cover it, then just to be sure about two ml of elemental bromine. The
beaker was covered with a plastic bag and allowed to sit for 17 hours or so.
The result was the first image in the upper left-hand corner after it was agitated. Previous to that strange stalactites (the ones that grow down) of
what looked like rust grew off the pins of the processor. So, I removed the processor, washed it off into the beaker and set the processor in some
HCl, all of the rust looking material sure enough dissolved and it kept reacting, there are some silver colored pins under everything that refuse to
dissolve that might well be silver or something.
But onto the gold extraction. The second picture to the right shows me filtering the material using suction filtration and I was left with a deep red
solution that was without free particulates. I made a solution of KOH by dissolving maybe 5 grams of KOH in 50 ml of water and it is shown in the
bottom left-hand corner picture next to the filtered red crap.
Upending the pretty red solution into the KOH solution resulted in a immediate discoloration of the redness and a heavy black precipitate (Bottom
right-hand corner). You can still see bromine vapors in the other flask. So, I tried to filter out the black crap but it is too fine for the filter
paper for the most part, though some remains trapped.
So I have all of this black precipitate which may be gold or something else. The site says that it is gold which make sense in that the driving force
in Aqua Regia and indeed in this method is the formation of the AuCl<sub>4</sub><sup>-</sup> anion and once you add KOH you
destroy the ability for this to exist but what else is carried along with it I have to wonder? Copper likes to form a similar chloro anion as do a
number of other transition metals. I guess I will smelt what I have after I process a few more computer pieces and see if I get something that looks
like gold.
Twospoons - 27-4-2006 at 17:07
Did you expose the chip? There's probably as much gold in the bond wires as there is plated onto the pins.
BromicAcid - 27-4-2006 at 17:09
Yeah, I broke open the ceramic looking part of it to reveal the chip itself. I never thought it would look like it did, very shiny covered in glass
or something. The chip itself looks like it might be made of gold but I didn't break it open, the wires leading to it were thinner then hairs and
looked to be made of silver. If I honestly give this a go I will smash up the chip better first.
12AX7 - 27-4-2006 at 17:18
If you actually broke the chip, you'd see shiny silicon. If you desolder the bottom panel, you'll reveal the "modern cathedral" as it has been
called.
Here's some I opened up. 
zoomer - 27-4-2006 at 18:59
The "glassy" top to the chip is the passivation layer applied to prevent contamination during further processing.
The "silver" looking bond wires are actually aluminum. Nether silver or copper are used in the manufacture of ICs. Newer ICs (like 2005+) use copper
bond wires.
There are a few micrograms of gold used in the bond areas, but you will find far more gold plated on the pins. Total that with any exotics used in IC
manufacture is still probably less than 10 milligrams precious metal / chip. It's probably not worth the money you paid for the calories you had to
ingest to hold the hammer to smash them in the first place.
Z
[Edit] Nonetheless, a very interesting experiment, Bromic, thanks for the pics!
[Edited on 28-4-2006 by zoomer]
Fleaker - 27-4-2006 at 19:00
Nice to see you have an interest in this Bromic. Unfortunately, it really is not worth your time, even with the astounding prices of all precious
metals lately (market uncertainty or what eh?...actually, heard a very good lecture by a VP @ B&W about oil's influence on energy market a week
ago, but that's OT). Unless you can acquire material from specific year that assays for a known value in precious metals, it's a losing proposition,
trust me, I've looked into it. You also need a lot of it. If, however, you manage to get enough, don't bother with basified cyanide stripping baths,
or aqua regia for that matter. I wouldn't even use the late doctor's method (I have it on good info that the man who authored that website died
recently) of halide extraction, which in my honest opinion, is best applied to the extraction of PGMs from catalytic converters (that's a winning
proposition--if you have the right connections--you can make a LOT of money by doing that). I'll try and dig up a patent # for one such process.
Ok, coming to my point: I suggest you take all of the scrap, and either add it directly (or first burn to ashes), to molten copper. The copper will
readily absorb all precious metals and is then cast into an anode shaped ingot which is finally electrolyzed. Current density, operation temp, and
other conditions depend on cell size and target material. The more noble metals will fall to the bottom as a sludge, while pure copper will plate out
on the other electrode. Junk metals like iron and zinc will go into solution. Eventually empty the cell of the dense brownish powders, wash with
distilled water, then with 6M HCl and filter. After that you'll have a mixed bag of precious metals and you can put most of them into solution with
hot aqua regia (after simmering with HNO3 to remove silver and palladium) and then precipitate the gold and platinum at your leisure.
Copper is the easiest way to concentrate your values, and if you build a large reverbatory furnace (consult tim here) you can add the boards and
dental waste directly to the liquid metal. It's faster, just as effective, more fun, and less reagent intensive.
BTW, a good method for dropping out gold powder in an acidified, nitric acid free solution, is with SO2 which I can say from personal experience, is
fast and relatively cheap if you can find a supplier for the cylinder. Sodium sulfite works almost as well. Since AuCl3*3HOH (and chloroauric acid) is
a strong Lewis acid, it complexes readily. Also stains your hands a nice brown color for weeks on end. If you're into something more exotic, try
dibutyl carbitol (a glyme) as a selective reagent for the isolation of Au (III).
Any questions, message me privately or post further! I've done a fair bit of work with the precious metals, namely Pt, Pd, Ag, and Au.
jimwig - 27-4-2006 at 19:44
Correction that was supposed to be
The Recovery and Refining of Precious Metals.
by C.W.Ammen
(well i guess i didn't post that after all)
[Edited on 28-4-2006 by jimwig]
BromicAcid - 27-4-2006 at 21:08
Speaking of catalytic converters although a bit off topic (no longer off topic as I bordened the topic area), there is a seller on eBay that's selling books or CD's or something with instructions on how to extract precious metals from catalytic converters for $100 or
so plus exhorbrant shipping rates. And apparently they are actually selling them continuilly and additionally they have two seperate books on the
subject and encourage you to buy both at $100 each or so..... pitiful....
I considered recovery from such sources at one time when I needed some rhodium for a catalyst but abandoned it when I began to wonder exactly how I
would get a catalytic converter in my neighborhood.
[Edited on 4/28/2006 by BromicAcid]
jimwig - 27-4-2006 at 21:46
Don't have it in front of me right now But there is a government report (Google?) detailing catalytic converters. Not that impressive since the
technology for making CC's changed. They now use a different type of technology and consequently smaller amounts of PGM's. The older models were much
more viable for recovery.
From memory so don't quote me on this.
Fleaker - 28-4-2006 at 18:28
Considering the hundreds of thousands of used, older model cars, I'm not too worried about finding the proper type of converter. Irregardless, jimwig
has a good point. I do find that funny Bromic. Given our convenient command of chemistry (bit alliterative there 
), us amateur and professional chemists here could easily do what they claim, and do
it better! $100 for common sense? The claims put forth are bogus. The most net earnings I've *reliably* is about $50,000 from 710 mixed converters
(equivalent to a ton) via gas transport extraction. Still, considering that covers materials and purchase price, money can be made. The only problem:
acquisition of a steady supply.
Yes, I'm familiar with Ammen's book jimwig, it's quite nice; I prefer it over his "Metalcasting" which has a good bit of errors.
Metal Leaching
lordmagnus - 5-5-2006 at 16:39
Godz,
They do that sorta stuff in 3rd world nations, they recycle hard drives, and such, and soak the circuit boards in hot solvents, the filter out the
undisolvables, and runs them through an acid treatment, etc.. they dump the acid and solvent refuse in rivers and canals. It's really not worth it,
unless your collecting the stuff to export to them.
BromicAcid - 5-5-2006 at 17:35
They should be recycling metals a lot more, I read somewhere that each American consumes 25 lbs of copper a year and by consumes I mean they throw it
away. It's one of the contributing factors to copper being on the rise to such an extent. I have to wonder how long it will be before mining
landfills will become profitable enough.
As it is, gold is still on the rise, going from $640/ounce when I wrote the first post to $680 an ounce now. And that's in less then two weeks. A
senator was on television stating that gold is not up, the American economy is down. He told people that they should not be investing in gold but in
the economy even though gold will continue to rise.
For fun I 'harvested' two computers from the trash and took out all the gold colored parts. I've had them soaking in HCl for two days and they are
still bubbling. Then I will soak in nitric. Then Aqua Regia. I don't think I will get much but it's pretty fun. Maybe I should cut the foreplay
and just open a gold mine though.
Edit: By the way, my local scrap yard buys circuit boards for a fairly substantial sum. On par with aluminum (a shocker for me) ... and here I
thought they were trash.
[Edited on 5/6/2006 by BromicAcid]
The_Davster - 5-5-2006 at 17:49
How much for circuit boards? I happen to have just seen a dumpster today pretty much full of them....
It is amazing the waste of precious metals, in my lab job I was the only one there for a little while today, I went into the 'empty' bottle disposal
bucket. There was still about half a gram of gold powder in one and about 3g of Pr in another, but Pr is not a precious metal. I also was at the
chemical recycling centre at my uni today talking to the guy running it. Apparently they get even gold and platinum there when noone needs them, they
are then free to faculty.
skippy - 5-5-2006 at 19:54
Where I could see a book on cat converters being worth it is if it gave good advice on the wheeling-dealing aspect. Its one thing to have technical
knowledge of processing the things, but unless you know what to pay and how to get the most raw material for your time, then you might just spin your
wheels.
REAGENT USED IN COMMERCIAL GOLD RECOVERY
Till_Eulenspiegel - 21-11-2006 at 19:04
(With precious metals values at or near all time highs, now seems like a good time to present the following information.)
This reagent quantitatively reduces gold from solutions of Au(CN)2 - sodium dithionite, CAS # 7775-14-6, also known as sodium hydrosulfite. It's
used routinely in large commercial gold recovery operations. Sources of gold for these systems include electronic scrap and other plated materials:
circuit boards, plated connectors, "fingers" (plated zones on circuit boards, non-gold portion mechanically removed), crushed integrated circuits,
plated wiring harnesses, pins, etc. Naturally the denser the plating, the greater the yield of gold. Mil spec items usually have the densest Au
plating.
The actual recovery process is as follows:
Solutions from the strip tanks are decanted and pumped into a stainless steel tank equipped with an efficient mechanical stirrer and submersible
heater. (You might get by using an external propane burner for smaller scale work.) The setup is located in or near a hood, since toxic gasses are
evolved, as is a rather considerable stench. The solution in the precipitation tank is heated to 180 degrees F, and with efficient stirring, the pH is
increased to 13 with predissolved concentrated sodium hydroxide. Sodium dithionite is then added in small portions. The solution transitions through
about 7 colour changes, depending on presence or absence of Cu, Ni, Ag, etc.
Course of the reduction reaction is followed by atomic absorption spectrometry. Even a single beam instrument like an old Perkin Elmer 101 will work
well. You have to matrix match standard and sample however. This is real important since the CN- anion suppresses the signal. Use the Au line at
242.8 nanometres. Just calculate the cyanide anion concentration initially present in the strip tank and prepare the AA standard with the same [CN}-
concentration.
Additions of sodium dithionite are made until the reaction is complete. Sodium dithionite will reduce the gold concentration in the precipitation
tank to 10 ppm or less. Minor amounts of Ni and Cu will also be present in the precipitate. When the reaction is deemed complete via atomic
absorption, stirring is stopped, and the tank allowed to cool to room temperature, usually overnight. Then the spent strip solution is very carefully
pumped out, and the precious metals sludge is recovered from the bottom of the tank. (Actually a turkey baster type suction apparatus works well
here). Using quantitative transfer technique, the gold sludge is transferred to a Buechner funnel & vacuum filter flask and rinsed with
deionised water prior to purification.
Use of sodium dithionite in gold recovery is essentially an industry trade secret, as far as I can tell. For example, you won't find any mention of
it in C.W. Ammen's "Recovery and Refining of Precious Metals" or George Gajda's "Gold Refining" (ISBN # 0-686-17797-8). Both texts are to be
recommended very highly, however. Zinc precipitation is the ancient traditional method usually taught to precipitate Au from CN- solutions- overall a
tremendous hassle compared to the use of sodium dithionite.
More details on purification, as well as how to set up the strip tanks, in my next post.
evil_lurker - 21-11-2006 at 19:50
The only scrap I can see worth recycling is catalytic converters.
Each is supposed to contain 1-3 grams of platinum per cat which nowadays at around $40 a gram makes them very attractive targets for recycling.
Any ideas as how to get it out?
Fleaker - 22-11-2006 at 09:15
I mentioned earlier that it can most cheaply/efficiently done with a gas phase transport, wherein the operator fills a stainless steel tube with the
crushed, pre treated converter and then passes a hot halogen gas (400+C) through. The halogen oxidizes the Rh, Pt, Pd, and other metals into their
volatile halide salts, which then can be removed from the system through a cold finger at one end of the apparatus (where they deposit as a brown red
crust).
I have done some preliminary, small scale (1-3 converters) runs and can say it is better in yield than the hot aqua regia technique which inevitably
leaves some Pt and a Rh.
Feel free to U2U!
YT2095 - 22-11-2006 at 10:54
but what material is the substrate for this honeycomb catalyst?
if it`s an "ordinary metal" wouldn`t making cracks in it then dissolving it in dillute nitric acid leaving the precious metals to fall off to the
bottom be just as good?
Jdurg - 22-11-2006 at 11:02
I think the honeycomb is a ceramic material and not a metal.
YT2095 - 22-11-2006 at 11:10
aha, thnx !
in that case forget my idea 
Fleaker - 22-11-2006 at 16:30
It's actually called gamma alumina-silica (if you're referring to what the
honeycomb is made of). Some type of mullite derivative...
RECOVERY OF Pt & Pd FROM CATALYTIC CONVERTERS
Till_Eulenspiegel - 22-11-2006 at 17:01
Here's what C.W. Ammen has to say about this:
"The metallic composition of catalytic converters varies, some are palladium, some are platinum, and some are platinum-palladium. Some operators
advocate simply crushing the material up, boiling it in 1:3 hydrochloric acid: distilled water, washing the residue, soaking it in aqua regia for 72
hours, filtering the solution, boiling off the nitric acid, diluting the thickened solution with water, precipitating metallic platinum (with ammonium
chloride) and palladium (with potassium iodide or sodium chlorate) and converting each metal to sponge.
This method is widely circulated, but is sketchy to say the least and much too costly to undertake without more definite information. Your first
move should be to assay the material to determine exactly what you have. You basically have 3 Pt group metals to contend with: platinum,
palladium and rhodium.
Catalytic converters usually take one of two forms: pellets that carry a deposit of Pt, Pd, or Rh on a substrate of gamma alumina; or a monolith of
some combination of Pt, Rh, and Pd deposited on a honeycomb (usually made of a ceramic material) with a wash coat of gamma alumina.
Some converters are 2-way (those that oxidize carbon monoxide and hydrocarbons), some are 3-way (those that aside from oxidizing carbon monoxide and
hydrocarbons, also remove nitrous oxides, NOx).
The effort required to get a good representative assay depends on the type of catalyst, whether it is fresh or spent, and other factors. Fresh
catalysts can usually be decomposed with hot HCl and chlorine, hydrogen peroxide, or sulfuric acid and phosphoric acid. Because the catalysts in the
support media contain some silica, you have to use hydrofluoric acid to dissolve the silica. (!)
Platinum - rhenium catalysts are usually decomposed by heating the crushed material in dilute sulfuric acid that contains some sodium formate. The Pt
being insoluble in sulfuric acid and sodium formate, is filtered off, while the rhenium remains in solution in the filtrate. The platinum is then
dissolved in aqua regia and precipitated with ammonium chloride.
The bulk of the catalytic converters that you are likely to process are spent converters and this material, whether in honeycomb or in pellet form --
is usually about 95% by weight gangue (reaction by-products, carbon, organics, ceramics).
The only practical way to deal with this melange of gangue is to incinerate it, burning the carbonous material to carbon dioxide. The incineration
prodict is then ground to a fine powder. The collected ash (residue) after grinding, should be blended (mixed) in a suitable mixer such as a "Y"
blender.
If the material is not incinerated properly, leaving behind carbonous materials that have not been converted into carbon dioxide, the excess carbon
can cause reprecipitation of the values when you get to the dissolving stage of your process.
The next problem is how to concentrate the values in a suitable solvent. One consequence of incineration is that some of the palladium is converted
to palladium oxide, a black, green. or amber solid that is resistant to chemical attack, but must be reduced.
Many reducing agents easily reduce palladium oxide to metallic palladium. Sodium formate in a carbonate-buffered aqueous solution is a good one to
use. The reduction is carried out at aprox. 190 deg. F. , with the solution at a pH of 10. The reduced palladium is washed & filtered, then the
residue from the incineration and the reduced palladium are extracted and refined in the usual wet chemical manner.
The above processes are carried out in the standard manner using expensive reagents. If your operation is not efficient enough to recover a very
high percentage of the values, you may find that catalytic converters are a losing proposition.
You should also be wary of auto wrecking firms, who frequently have a very inflated idea of the value of the converters. I have seen claims of from
one to three ounces each of palladium and platinum recovered from ten mixed converters.
The best policy here is to buy the converter guts by the pound, based on an assay."
garage chemist - 25-11-2006 at 04:34
I have not been able to reply to this thread due to my internet not working at the time.
Precious metal recovery from alloys and jewelry is my kind of thing.
I have successfully recovered 34g of pure gold from dental alloys and also the platinum and palladium.
After dissolution in aqua regia, the solution is diluted to make all the AgCl precipitate. The AgCl is boiled with dilute HCl to extract some
palladium which has coprecipitated.
The combined solutions are boiled down in a still and evaporated two times with concentrated HCl to ensure that no nitrate ions are left (important).
The gold is then quantitatively precipitated by heating the solution to 80°C and bubbling in sulfur dioxide until no more gold (yellow to brown,
crystalline, very dense precipitate) settles out. Don't mess with other reducing agents like dithionite, disulfite, FeSO4 or similar, they intruduce
sodium or Fe(III) ions into the solution which will be a nuisance later.
The gold is washed and redissolved in aqua regia, boiled down, evaporated two times with HCl and precipitated by adding oxalic acid in excess, boiling
and then slowly neutralizing the solution with ammonia until the solution is colorless.
This purification produces almost 999 fine gold. If gold of certainly 999 fineness is desired, it is redissolved, evaporated with HCl as before and
precipitated this time with FeSO4.
The solution from the precipitation of the gold is bubbled with chlorine at room temperature to reoxidise the platinum to IV.
The solution is then boiled for an hour to reduce the oxidised palladium.
Ammonium chloride solution is added, yellow ammonium hexachloroplatinate precipitates which is filtered.
To the solution from before is addedmore ammonium chloride, it is cooled to 10°C and chlorine is bubbled in. Brick-red ammonium hexachloropalladate
precipitates. It is rapidly filtered and washed with chlorine- containing NH4Cl solution (not with water!).
If the filtrate is still brown, the treatment with NH4Cl and chlorine and filtering of red precipitate is repeated.
STRIP TANKS -PRECIOUS METALS RECOVERY, CONTINUED
Till_Eulenspiegel - 25-11-2006 at 23:19
Hi:
Here are a couple of useful US patents on sodium dithionite and gold recovery 3, 271, 135 and 3,271,136 by Charles Davidoff.
Continuing with my post regarding Au recovery from scrap sources including electronic scrap:
Strip tanks often used in the industry usually are low-form solid polymer tanks of maybe 20 - 100 gals or so depending on the scrap availability.
Usually there are 3 tanks, one for stripping, one for rinsing, one for reserve. Suspended above the strip tank via hydraulic hoist or pulley is a
hexagonal or similar perforated drum, equipped with either a belt driven or direct drive electric motor. One of the panels on the drum opens for
access and can be secured.
The plated scrap is mechanically pre-cleaned as much as possible, boards are cut to remove components, etc. Since the reaction is oxygen dependent,
sometimes the tanks are gently aerated. Watch out for any mist.
The cleaned scrap is placed in the drum, and the drum half submerged and very slowly rotated in the solution until all the Au is removed, which is
very rapidly with fresh solutionsalso depending on plating density of course. The drum with spent scrap is rinsed with a spray of DI water over the
rinse tank and the drum reloaded. This is continued until all the scrap for that job is stripped.
The solutions are assayed via atomic absorption (again be sure to matrix match sample and standard. CN- anion can cause an attenuation of half an
order of magnitude.
Au standards are unstable, good for maybe 3 days max. 10 ppm is just about right concentration for older instruments to product a full-scale signal.
Make up the standard in the stripper /CN- solution to the exact same concentration as being run in the plant. Any extra CN- added to the strip tank
later will suppress the signal, causing an erroneously low value to be reported.
Anyhow if the volume of the strip solution is known, the {Au} can be reliably determined to within .1 oz or less.
Regarding the composition of the strip solutions, commercially available products work really well. Here’s the specs for one:
(Technistrip) (there are others):
Strip rate: .0001” in 5-10 minutes
Au stripped per gallon up to 2.5 troy ounces
“will strip gold by immersion, (no current), from nickel, nickel alloys, copper, copper alloys, silver and iron with no basis metal attack.”
(Experience has shown there is some basis metal attack, however –ed. )
“10 lbs of stripper product per 20 gals”
Anyhow the strip tanks don’t have to be heated unless it’s below 15 deg or so. . The reaction is oxygen dependent, so aeration can help. Monitor
the pH continuously. Keep above 10. Always keep the place ventilated. Use all proper safety equipment and observe all environmental requirements.
Safety first!.
For quantitative precipitation of the Au refer to my earlier post.
STRAIGHTFORWARD PURIFICATION OF GOLD
Till_Eulenspiegel - 26-11-2006 at 00:49
“Off the boards and into the tank, right from there –straight to the bank”.
No tedious triple boildowns , no nonsense here. Industrial method:
Depending on your source of gold, your strip technique and technologies, assuming you derive the aurium from plated scrap, as well as your cyanide
reduction conditions, the crude sludge at the bottom of the hydrolysis (reduction) unit contains: Au, Ag probably as Ag2O, Cu, and Ni as Ni(OH)2.
Collect in a Buechner funnel and wash thoroughly with DI water in situ with vacuum.
Perform all these operations in the hood. Use eye and respiratory protection throughout.
One prepares a 50:50 HNO3I water mixture; and places the crude sludge in a 4
liter beaker, (again subsequent to through DI wash to remove any trace of CN-.)
Also you better make sure that your nitric acid is clean reagent grade and contains no HCl! Otherwise you will lose some gold in this pre-leach
steep. Designed to remove the silver and base metals prior to purification of the gold to better than 999 purity.
After you have made up some 50% nitric, now add the dilute acid a few cc’s at a time –be alert to the impending exotherm and possible boilover
-- have a beaker of DI nearby or a squirt bottle. Always use a secondary containment vessel in case the primary vessel with the sludge cake boils
over or fractures.
Cover ½ of the sludge cake by volume at first with the dilute nitric and place on a warm hotplate. Be alert, use heat only to initiate the
reaction. You will see that it takes off by itself pretty quick. Don’t leave this reaction unattended! After the reaction subsides somewhat,
really boil the contents for at least 15 min. The function of the nitric pre-leach is of course to remove any nitric acid soluble base metals.
Unfortunately, any Ni(OH)2 is scarcely soluble in nitric, and seriously inhibits efficiencies timewise and purity wise in this procedure. It’s
tough to filter and mechanically retains the gold in its intractable green gel from hell clutches. Try adding celite or diatomaceous earth silica to
help filtration. Your best bet is to avoid any nickel like the plague, don’t let your stripping go so long as to dissolve the nickel in the first
place. It displaces useful solution solvation capacity that might just as well be utilised for gold or platinum atoms. One has to strike a balance
between getting all the gold off the substrates, and removing a lot of useless nickel. Some commercial strip products are worse than others in this
regard.
When you feel that all the base metals are in solution, cool, then filter in the Buechner thru small pore filter paper. Keep the solution off your
skin, acid gold chloride, HAuCl4 stains skin purple, is absorbed and may have immune system modulating properties.
Reason for using 50% instead of conc. nitric is that, depending on the initial sludge composition, conc. nitric acid will dissolve around 1000 ppm of
gold or more. You probably suffer sufficient losses already without any further ones. 50% nitric only dissolves about 10-20 ppm Au, as shown by
atomic absorption analysis.
OK. After filtration and washing in situ in the Buechner, at least 3 times with DI water, test the combined filtrates for silver by diluting 1:1 again
with DI and adding sodium chloride to precipitate the silver chloride, which is extremely insoluble in aqueous solution. The solubility product
constant is 1.0 x 10 exp –23. (You knew that already). AgCl is a lot more soluble in HCl or aqua regia however. Collect the precipitated silver,
if any after settling overnight. Obtain pure silver metal by reducing the silver chloride with zinc and 5% sulfuric acid. (Add the acid first, a
volume sufficient to cover the silver chloride, then the zinc a little at a time, with major stirring, or you’ll be in for a surprise). (Hydrogen
gas evolves, which can ignite via its own heat of evolution.)
Now take the leached metals cake and quantitatively transfer it into another clean 4 l beaker (or one of appropriate size) Add filter papers and all.
Use a rubber policeman and your finest quant technique.
You’ll next digest the gold in aqua regia and remove the contaminants, and finally precipitate the gold as a fine pure powder. To carry on, you
need to add about 150cc AR per pure ounce troy of Au, in theory. In any event use 4:1 AR, estimate the number of oz. you have and add the requisite
amount of hydrochloric acid all at once. Cautiously, (fortune favours the brave) add the calculated amount of nitric slowly. In between additions
warm the unit on a hotplate to initiate the exothermic reaction. Beware boilovers, always use secondary containment.
The reaction exotherm is often delayed. Warm the stuff slightly and stir using magnetic stirring. Don't use your rubber policeman here. Remember you
want to minimise the amount of nitric you use since it delays the reduction reaction later on. When you think all your metal is dissolved, then add a
few drops of HBr and H2SO4 to your beaker. The first precipitates silver as silver bromide, and the sulfuric precipitates any lead as lead sulfate.
Presence of any silver, in form of AgCl is a major problem in gold refining. It’s one of the primary principal contaminants in gold bullion. It’s
a problem. Silver chloride is pretty soluble in concentrated Ar and HCl. Thus, dilute your gold in AR solution with DI water about 30% at this point.
Cool the beaker to zero degrees C, and vacuum filter through #42 Whatman, perhaps twice. To remove the precipitated silver salts, cellulose, nickel
hydroxide, lead salts, debris, etc.
Wash the filter cake of crap several times with DI water until it stops dripping yellow. HauCl4 is reddish orange-yellow when concentrated. When
dilute, it can resemble the colour of iron chlorides, sort of yellow.
Now for the final finale. Place a teflon coated stirbar in your beaker with the H AuCl4 solution in AR, and dilute the clear solution at least another
1:1 with DI. Don’t dilute your AR solution of Au down to less than 3 gms per liter or you may have a bit of trouble with the final precipitation.
Add slowly with mag stirring, small amounts of sodium meta bisulfite or sodium sulfite, about a table spoon at a time, Don’t stir with excessive
speed, The idea is to generate sulfur dioxide gas at a controlled rate. Take your time.
Dense red toxic fumes are evolved! You better do this in a hood, and wear a respirator too, especially if you are sensitive to sulfur dioxide. Keep
adding the powder until all the NOx is evolved and dissipated from the solution. Now the reduction of the gold begins. Remember, minimizing the
amount of nitric you use in dissolving the gold makes this reduction a snap. When to stop adding the sodium sulfite? When all the yellow color is
gone from the solution.
If your solution is contaminated by base metals, over addition can make the precipitate appear black. Normally it is light brown, the colour of a
blonde lebanese. Sometimes it is dark chocolate brown. Occasionally it’s brown, with a coating of yellow, bright yellow. The supernatant can be
various colors, depending on what base or trace metals are present. Just collect the precipitate and wash with:
1. DI water
2. Conc. or strong HCl
3. DI
4. 25% ammonia
5. DI water
6. Alcohol
7. Dry the gold powder prior to firing. Any trace impurities can be removed very effectively by adding a small pinch of potassium nitrate to the gold
powder during the firing process. This drives off the less dense impurities as volatile nitrates. Very pure gold can be distinguished by any of
several macroscopic characteristics . One salient example is the appearance of concentric rings on the surface of the refined gold.
You can actually fire your gold powder without an oven. More next time.