SMB and Sodium Thiosulfate finger strip problem

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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
 
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
 
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
 
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
 
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
 
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.
 
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
 
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
 
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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