# SMB and Sodium Thiosulfate finger strip problem



## crip53 (Nov 17, 2007)

Ok, here is a new one that should test the old noggin:

I thought I would try some sodium thiosulfate to see if it would dissolve gold off of some fingers.
It took a little over 24 hrs (with aeration from a bubbler I inserted to give it the 02 kick it needs), and low and behold it did the job without taking the copper with it.
I was impressed. Only a couple of pieces were incomplete, and I imagine if I had run it for another 24 hrs it would have gotten that too.
It was a mixed batch of fingers, on purpose, so that I could see if it mattered.
Again, est 99% stripped and dissolved clean.

Now the problem: SMB
I don't know if the ammonia salts that are mixed in with the thio is the culprit, but I keep having to add more and more SMB to drop the gold.
I get a nice little bit of brown powder, but keep testing the liquid and it keeps showing for gold still in solution and the water still has the golden tint to it.

So, how much is too much if ever a thing exists? Do I just keep adding more and more SMB until it finally tests clear? I am up to an entire cup of SMB dissolved and still waiting to see how much gets dropped overnite again.

No impurities showing. Fluid is almost crystal clear but has the golden tint, but no particles floating around, and of what has dropped is a very fine clean looking brown powder, so that looks ok.

My conclusion, so far, is that it is way cool that the thio could dissolve the gold clean without grabbing the copper too, but if it is such a pain to get the gold back, it may not be worth the effort or cost.

Any comments are welcome.


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## Noxx (Nov 18, 2007)

This is the first time I ear about Sodium Thiosulfate... But this is very interesting ! I'll do some research on this product...
But to answer your question, habitually, with Aqua Regia, we add the same amount of SMB that the estimated gold in solution.

If you get a light brown powder, you have very pure gold... Do this product is gold selective ??

Thanks and give us more information please !


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## Platdigger (Nov 18, 2007)

Did you try just adjusting the ph instead of the smb?
Randy


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## lazersteve (Nov 18, 2007)

From the sounds of your post you are using Ammonium Thiosulfate, since you mentioned ammonia. I have no hands on experience with this reagent so I'm going purely on speculation below.

I've heard of Sodium Thiosulfate used as a cyanide substitute, but I'm not sure of why you bubbled O2 thru the solution. With the traditional Copper Chloride AP solution the O2 reacts with copper chloride (I) to form Copper Chloride (II) which attacks the copper under the gold foils. 

As I understand the use of Sodium Thiosulfate it attacks the gold directly and (as you experienced) forms Au(S2O3)2 ions which make the gold difficult to recover from the solution. 

SMB normally works by displacing the gold from the auric chloride (AuCl3) molecule with SO2. Since your process does not introduce chlorine (some from tap water maybe), the displacment doesn't occur. The above gold (I) complex is very strong and difficult to break. 

Since thiosulfates oxidize with aqueous halogens forming sulfates and hydrogen, you could possibly recover your gold by mixing up some chlorine water and adding it to the pregnant solution. This *may* form the familiar auric chloride solution which can then be precipitated with SMB. If you already have enough free SO2 in the solution the addition of chlorine water may drop the remaining gold directly. Be sure the ammonia is removed before adding any chlorine as auric chloride and ammonia can form explosive compounds.

Thiosulfate ions are also decomposed to elemental Sulfur and SO3 (sulfites) by additon of acids.


Steve


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## crip53 (Nov 18, 2007)

Interesting idea about the ph....never thought about that...wanted to keep solution "pure" without any other intrusions. But, will check it just to see.



I am using sodium thiosulfate and not ammonia thiosulfate.
I got it from goldmineworld. I have flown out to PA and met with Mike.
Some of his stuff is actually quite excellent, others a tad suspect. He is quite knowledgable, almost "professor-like" in his speech.
The ammonia salts is a mixture of his that is added to the thio crystals at startup.
Actually, this whole thio approach to gold recycling is to be done as a dissolve and then electrowin type of process. I just thought I would see if a) it would, in fact, dissolve off the gold cleanly and b) I would be able to recover it without the electrowin side since I do not have that setup as of yet...lazy I guess. Need to do one of these days.

Thanks for the responses


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## Platdigger (Nov 18, 2007)

Did you try to drop with T6?
Randy


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## crip53 (Nov 19, 2007)

Randy,
I didn't try to drop with T-6 for a couple of reasons.
1) Like I said I wanted to keep the solution pure
2) Even though T-6 is a great collector, it always leaves me with the added efforts and costs of then having to smelt down with St silver with the flux, etc. I suppose that I could smelt with just fine silver, since I am not after rhodium. Or maybe this may be the one occasion that lead may be the better collector since it will vaporize in the furnace. Thanks for getting me thinking about that avenue 
I have used T-6 with some mil spec that I dissolved with HCL/peroxide - the strong stuff not the 3% store bought. It collected the gold just fine and it smelted out with the gold pretty much centered in my dore' ingot. The downside is that now I have to break down the dore' to be able to extract out the gold. 
I love T-6 (though no neighbors do...lol), but when it comes to just going after dissolved gold, it is a bit of overkill. It is awesome with grabbing gold and PGMs together, but it leaves a lot off future work to get all of the goodies separated out.

Thanks again for the suggestion.


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## lazersteve (Nov 19, 2007)

Crip said:


> I have used T-6 with some mil spec that I dissolved with HCL/peroxide - the strong stuff not the 3% store bought



Crip,

If you already had the gold in solution why would you use T6 to precipitate? I've never used T6, but from my understanding it precipitates *all* dissolved metals from a solution. 

If you already had the gold in solution why not just drop it with a cheap selective reagent like SMB or Copperas? By using T6 you are undoing the work you did with the Peroxide to get the gold into the solution. The main idea of precipitation in refining work is the selectivity of the reaction. Specifically to refine the gold to a purer state without contamination of the other metals. 

Steve


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## Platdigger (Nov 19, 2007)

You are right Steve. But, I think basically he said just that. In other words, he understands this, and, agrees with you.
I guess the reason I brought up the T6 route is he was having trouble getting the gold out of solution. Not that it was a good route for refining.
Wait, here is the other thing I was thinking. If, this thio is soo selective for just the gold, perhaps there are not many other metals in solution. Still, there will be further steps to purify..........but having a route to disolve mostly the gold and not all the base metals as a first step seems to me to be a step in the right direction. Espesially when what we are talking about is computer scrap......which, as we all know, is mostly base metals.
Randy


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## Irons (Nov 19, 2007)

Thiosulphate has a high affinity for Silver as well.

Here's a good site that shows a sequence of one-pot reactions using Silver as an example:

http://chemed.chem.purdue.edu/demos/main_pages/12.2.html

Note that using Sodium Sulphide as the last reactant precipitates everything. It works much like T6.


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## Irons (Nov 19, 2007)

http://everything2.com/index.pl?node=Hypo%20Clear

Hypo Clear (used in photographic development) converts thiosulfate to sulfite.

And that's a good thing as Martha would say.


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## lazersteve (Nov 19, 2007)

Platdigger,

The reference Crip made was related to his experience using T6 with HCl/Peroixde dissolutions, not with Thiosulfate dissolutions...

Steve


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## Irons (Nov 20, 2007)

After stripping the Gold using Thiosulphate, use Hypo Clear to convert the Thiosulphate to Sulphite. The Gold should precipitate out.

The whole point of using Sulfite to drop Gold from Chloride is that the Sulfite or Metabisulfite is a Chlorine scavenging agent. Adding Chlorine to the solution just adds another contaminant.

THis looks like a very eco-friendly process, since the spent solution is benign.

You can get Hypo Clear from a photo chemical supply house.

Use as little Thiosulphate as needed to strip the Gold. Any excess will be a waste of money and time.


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## goldsilverpro (Nov 20, 2007)

I read up on Hypo Clear and, it seems that, it itself is mainly composed of sodium sulfite. This converts the hypo to a sulfate (not a sulfite), which is more water soluble and rinses out better than hypo. Here's a formula I found:




> The formula for Kodak Hypo Clearing Agent is proprietary but is apparently close to:
> 
> Sulfite Wash Aid Stock Solution
> 
> ...



http://www.freelists.org/archives/pure-silver/12-2005/msg00063.html


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## lazersteve (Nov 20, 2007)

Here's another excellent link with precipitation information:

Thiosulfate Precipitation

Looks like introduction of copper sulfate may be the key. The precipitation times are exceptionally fast.

I'm going to order some supplies today and give this method a whirl.

Steve


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## Joe (Nov 20, 2007)

Good work, guys. I'll be following this thread.


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## Irons (Nov 23, 2007)

goldsilverpro said:


> I read up on Hypo Clear and, it seems that, it itself is mainly composed of sodium sulfite. This converts the hypo to a sulfate (not a sulfite), which is more water soluble and rinses out better than hypo. Here's a formula I found:
> 
> 
> 
> ...



Sulfite breaks down to sulfate with the release of SO2 on contact with water.


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## goldsilverpro (Nov 24, 2007)

> Sulfite breaks down to sulfate with the release of SO2 on contact with water.



Under normal conditions, water alone won't do this. It needs to be acidified or it needs to contain something that can be reduced by the sulfite.


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## Irons (Nov 24, 2007)

goldsilverpro said:


> > Sulfite breaks down to sulfate with the release of SO2 on contact with water.
> 
> 
> 
> Under normal conditions, water alone won't do this. It needs to be acidified or it needs to contain something that can be reduced by the sulfite.



Dissolved oxygen or exposure of the solution to Air will cause the Sulfite to break down.

The Sodium Sulfite comes as an anhydrous powder to limit the decomposition.

Sodium Sulfite is used in Boiler feed water to reduce corrosion by scavenging dissolved Oxygen.

This is a good reason not to mix up more solution than you need when using it as a precipitant.


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## goldsilverpro (Nov 24, 2007)

> Sulfite breaks down to sulfate with the release of SO2 on contact with water.



Granted, concerning the dissolved oxygen. However, your original statement, above, made it sound like the water itself was what was decomposing the sulfite and, it would happen immediately, to all of the dissolved sulfite.

As I understand it, at room temp, water will only hold from 0 to 15 ppm of oxygen. In a liter of water, there would only be enough oxygen to decompose about .12 grams of sodium sulfite, max. A very minor amount, considering that the solubility of sodium sulfite in cold water is about 125 gm/liter. Actually, in practice, this discussion is moot.

The reason for my original post was to correct the following erroneous statement you made about Hypo Clear.



> Hypo Clear (used in photographic development) converts thiosulfate to sulfite.


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## Shecker (Jun 29, 2008)

Over the years I have used this reagent a great deal. As an advantage it will dissolve all naturally occurring salts of precious metals as well as many man made ones. As a disasdvantage it aggressively holds those values and if they are precipitated as fine metal it will re-dissolve them. But there is a way to get the gold (or silver) out. This method is to use sodium sulphide to generate sulphides of gold and silver. This has one additional value. If you use sodium sulfite with the sodium thiosulfate, the sodium sulphide reacts with the sodium sulfite and causes the insitu regeneration of sodium thiosulfate. The sulphide of silver is black and the sulphide of gold varies from black to brown. 

Randy in Gunnison


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## Shecker (Aug 4, 2008)

I use to work with a company that recovers major amounts of silver from sodium thiosulfate solutions. They would run the solution across clean copper plates. The silver would immediately plate out on the copper and come off in the form of large silver flakes. This was a purely chemical reaction -- no electricity was involved. For their purposes it had the additional advantage of chemically killing the thiosulfate solution so that there was no discharge problem. I've used this for recovery from silver thiosulfate solutions and it works very quickly. I didn't like the fact that it killed the chemistry, but that may not be a problem when working with gold
or other pm's. Sodium Thiosulfate will leach palladium if an oxidizer is supplied. Try a small amount of ferric sulfate as an oxidizer, as thiosulfater compounds like to work with copper(II) and iron(III) as oxidizers. Another thought is to recover the gold and other pm's on selective resins -- this allows for reuse of the solution.

Randy in Gunnison


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## Lino1406 (Aug 5, 2008)

with slight acidification
can be tried


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## eagle2 (Aug 6, 2008)

It seems to me that Zinc dust would break the Thiosulfate Gold bond. 

It would bring down some Copper and all the PM`s at once, but then you would redissolve the cleaned precips. in HCl + H2O2 or Cl2 and have a much more workable solution to separate. 

I have used Zinc to bring down traces of the Gold and PM`s from heavy contaminated used AP solutions with excellent results. 

SMB is not necessarily the best precipitant for every job. There are many other methods.

Al


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## Shecker (Aug 6, 2008)

Zinc actually has little effect on thiosulfate solutions. That is why alternative recovery systems has been sought by big companies using thiosulfate systems. Most recovery from photographic fixer solutions uses iron because iron readily reacts with thiosulfate solutions. But it also chemically kills the chemistry. The introduction of one metal ion to recovery another metal ion always renders thiosulfate solutions dead in the water. For reuse of chemistry electrowinning is the mainstay in pm recovery, but resins are also becoming more and more applicable.

Randy in Gunnison


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## eagle2 (Aug 8, 2008)

Thanks for the info Randy.

I`m surprised that Zinc dosn`t affect Thiosulfate. Looking at the electomotive series, Zinc is way more active than Copper or Iron. You said earlier that Copper sheets are used to cement the PM`s from Thio solutions in commercial work. Must be something like Sulfur coats the Zinc and stops the cementing. 

Aluminum or Magnesium are even more active. 

On a small scale like I do, it wouldn`t matter if the solution was `killed`, just so long as the Gold would precip.

Al


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## Lino1406 (Aug 8, 2008)

Would you think acid
will convert gold thiosulphate
to gold sulphite + sulphur?


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## goldsilverpro (Aug 8, 2008)

I have seen zinc used to drop silver out of photo hypo (sodium thiosulfate), with no problems. One reason it is not used is that the excess zinc, combined with the silver, is difficult to remove. 

Most Ag from fixer is recovered electrolytically. With electrolytic cells, the only contaminate in the silver is from 2% to 25% sulfur, which is easily removed from the melt using rebar. 

Commercial steel wool canisters are widely used by the little guys and as a final polish from the electrolytic units, but they are inefficient (they tend to channel) and it is difficult to recover the silver from them, especially when unused steel wool still remains mixed with the silver.


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## Shecker (Aug 8, 2008)

Right on GSP. I was taking the information on zinc and iron from Kodak's manual on "Recovering Silver from Photographic Wastes". A good starting point for anyone who wishes to recover silver with thiosulfate. Steel wool does indeed channel and losses a great deal of reactivity. But it is used by photo shops because it is passive and it kills the solution and iron thiosulfate is not considered an environmental problem.

Randy in Gunnison


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## goldsilverpro (Aug 8, 2008)

I might mention a problem that occurs when trying to plate out the silver.

The silver complex in a thiosulfate solution is negative. Therefore, the negative cathode will repel it. If you tried to plate with no agitation, most of what will plate out will be sulfur. In most commercial electrolytic cells, a rotating SS, drum shaped, cathode is used. This constantly removes the silver depleted cathode film and replaces it with fresh solution. Even with this system, some sulfur will always deposit. Also, when the silver concentration in the solution gets low, a greater percentage of sulfur will deposit at the cathode.

I also think that systems using ion exchange resins are available for fixer solutions


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## Lino1406 (Aug 8, 2008)

by solid NaOH


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## Platdigger (Aug 8, 2008)

Lino,
Is that "solid" sodium hydroxide as a powder? Or, larger chunks somehow?
Randy


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## Lino1406 (Aug 9, 2008)

is being solid - activity enhanced.
I used both powder and flakes
and since no nitrate in there I
added formalin - all in wide hand,
and a while after addition - mix well


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## Lino1406 (Aug 9, 2008)

NH3 may evolve, so I used
a loose cover


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