SMB and Sodium Thiosulfate finger strip problem

[crip53]

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.

 

[Noxx]

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 !

 

[Platdigger]

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

 

[lazersteve]

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

 

[crip53]

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

 

[Platdigger]

Did you try to drop with T6?
Randy

 

[crip53]

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.

 

[lazersteve]

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

 

[Platdigger]

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

 

[Irons]

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.

 

[Irons]

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.

 

[lazersteve]

Platdigger,

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

Steve

 

[Irons]

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.

 

[goldsilverpro]

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

Water 750.0 ml
Sodium sulfite, dessicated 100.0 grams
Sodium bisulfite 15.0 grams
EDTA tetra-sodium salt 5.0 grams
Citric anhydride 5.0 grams
Water to make 1.0 liter

Dilute 1 part stock to 4 parts water for use.

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

 

[lazersteve]

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

 

[Joe]

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

 

[Irons]

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

Water 750.0 ml
Sodium sulfite, dessicated 100.0 grams
Sodium bisulfite 15.0 grams
EDTA tetra-sodium salt 5.0 grams
Citric anhydride 5.0 grams
Water to make 1.0 liter

Dilute 1 part stock to 4 parts water for use.

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

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

 

[goldsilverpro]

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.

 

[Irons]

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.

 

[goldsilverpro]

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.