Silver recovery with sodium dithionite

[schlger]

Hello,
I would like to recover silver from old bleach fix baths (E-6 and RA-4) by reducing it with sodium dithionite. For B&W fix I have read, the theoretical ammount of dithionite is 0,81g per 1g of silver, so I would be on the safe side to use 1,6g. How much should I use for bleach fix? And could it happen, that still active bleach will resolve the silver again? How can I proceed and does anyone has experience?

Thanks a lot
Gerhard

 

[AgX]

What do you plan to do with the result, as I assume it will be a mess.

 

[schlger]

Sell the silver after melting and dispose the rest at public waste disposal. So valuable silver will not be lost.

 

[KenS]

Hello,
I would like to recover silver from old bleach fix baths (E-6 and RA-4) by reducing it with sodium dithionite. For B&W fix I have read, the theoretical ammount of dithionite is 0,81g per 1g of silver, so I would be on the safe side to use 1,6g. How much should I use for bleach fix? And could it happen, that still active bleach will resolve the silver again? How can I proceed and does anyone has experience?

Thanks a lot
Gerhard

Gerhard,

I have 'posted' my 'silver recovery' on a couple of forums (fora?)
Save the used fixer in a large 'carboy'... add used , but not 'dead' developer.. shake and allow the pure 'black' silver to 'precipitate' syphon off the 'clear liquid'... add more used fixer, then more used but not 'dead developer' Shake and re-shake and 'repeat' when you have aa few inches of precipitate (which is the PUREST silver you will ever find).
Wash 'well' and ''melt" in a muffle furnace or 'gentle' blow torch using a non-metallic crucible
sell it to a jeweler.. and use the $$$ for more film (and or chemicals)

Ken

 

[schlger]

Ken, thanks for your advice,

I have some questions to your procedure:
1) How much of the sodium dithionite do you use per gram of silver in the fix?
2)What would happen, if I´d try to desilver oxidized fix with precipitated sulfur and black siler at the walls of the container, that smells for hydrogen sulfide? Is there a rest of solved silver in the liquid of dead fixer that falls out, when I use dithionite or has all the silver together with some of the sulfur precipitated and only has to be decanted and filtered and liquid has zero content of silver and can be discarded?
3) What about bleach fix, dead or alive? Can it be desilvered with dithionite, and if so, how much dithionite do I have to use?

Thanks, Gerhard

 

[celgazer]

When sodium dithionite is added to blix solution, it will first reduce Fe(III) to Fe(II), in this stage you will find that the brownish Fe(III) color becomes nearly colorless. After the reduction of Fe(III), then the silver can be reduced. As sodium dithionite isn't a stable substance, it can readily decompose in aqueous solution, so you may have to use more sodium dithionite than theoretical amount. In patent application CN1062042A it mentioned that you may use 2-4g sodium dithionite per gram silver. Excessive amount of sodium dithionite can increase the silver recovery efficiency, but it may cost you more. Another article in Chinese use 4g/g silver and have a silver recovery rate of 93%.

 

[Frank53]

I remember we used black manganese. It’s cheap. Some 40 years ago we thought we could earn some money that way. Collected about 100ltrs fix from all people we knew had a darkroom, put in the stuff, stirred a few times and waited some time. Then we got rid of the liquid, dried the black stuff at the bottom and gave it to a goldsmith we knew who melted it and we had a lump of pure silver. It was fun but a lot of work and we didn’t know what to do with 5 grams of silver. It got lost somehow
Regards,
Frank

 

[schlger]

Thaks, I will try it with 4 g/g Ag, when I´ll have the dithionite at hand, that will be mid of september.
But I don´t know yet, what to do with already oxidized fix or bleach fix.

regards, Gerhard

 

[AgX]

It was fun but a lot of work and we didn’t know what to do with 5 grams of silver. It got lost somehow
Regards.

After precipitation one must dry and then melt the silver, which again necessitates some tools. And then one must find some buyer for it. Meanwhile here every town has a shop buying gold, but I do not know if they easily would buy some home-molten lump of silver. Maybe a local goldsmith would.

For an amateur photographer with his typically low amounts of silver one may question the involved hassle. On the other hand one may argue that the above hassle has to be compared to the hassle of getting rid of silver containg baths.
I am curious to learn of Apuggers precipitating and melting their silver regularly.

 

[schlger]

I think it was ESG in Germany, where I sold my silver in 2007. That was the last time, I melted silver.
https://www.scheideanstalt.de/ankauf/
It was a large amount of about 11 kg, wich I made mainly from silver sulfide, that has formed in a defective desilvering machine. I still have the melting pot with a content of 1 litre.

 

[AgX]

11kg ... well, that is a different league... worth a lot of hassle.

 

[koraks]

I am curious to learn of Apuggers precipitating and melting their silver regularly.

Currently not, but it's one of those things I sometimes play with in my head. Not to sell the silver, but to recycle it into silver nitrate. But the latter is a slow process with the concentration of HNO3 I can get cheaply, and I'm not too enthusiastic about the fumes.

 

[Mr Bill]

I am curious to learn of Apuggers precipitating and melting their silver regularly.

Long time ago - beyond 25 years or so(?) - I, or at least my department, used to oversee the silver recovery operations for the processing labs of a large studio chain outfit. I actually "designed" the effluent-control strategies as well as the major design work for our bleach-fix electrolytic recovery system.

We never tried "melting down" our own, though. There didn't seem to be much benefit to doing so, and it raises the question of what to do with it. And a lot of other issues. So we always shipped our silver flake, etc., to refiners.

We collected the great majority of our silver as electrolytic "flake," which has the great advantage that you know pretty closely the value of what you have when you send it to the refiner. Iron wool systems mean that you really don't have much idea how much silver is in them, even if you monitor the outflow. The reality is that you will be paid based on what the refiner says was there, not what you think should be. For example, if it is one-fifth of what you expect, and you raise this issue, the refiner will say, "Well, this is all we got." With electrolytic flake this sort of thing doesn't happen. Well, at least not if you are a large customer.

We did try sodium hydrosulfite (aka dithionite) at one time as a pre-treatment for bleach-fix prior to electrolytic desilvering in the manner of celgazer's post #6 (first sentence). (As you attempt to desilver you are fighting against the bleaching power). As I recall it did work but we judged it more effort than it was worth.

 

[celgazer]

I've just carried out a small batch experiment with my blix. Here I used excessive sodium dithionite and heated the mixture to a higher temperature to accelerate the reduction speed. As shown in Fig. 2, you can see that at the surface between air and solution, some Fe(II) are oxidized to Fe(III). On the flask wall in Fig. 3, some silver is oxidized and redissolved into solution due to the air oxidation.So if you want to recover silver with Na2S2O4, you'd better carried out this procedure in an air-tight container or use excessive sodium dithionite.
To those oxidized blix and fixer, I'm not sure whether there is still some silver in it. You can test them with small batch experiment or use other analytical procedures (silver test strips, ISE, etc.) to determine the silver concentration. Oxidized fixer and blix can have lower pH value, which can weaken the reducibility of sodium ditihionite and cause it to decompose. So you'd better adjust pH to 6-7 in this process. Higher pH (>7) can promote the reducibility of sodium dithionite (theoretically), but to some fixer like F5 which contain potassium alum as a hardener, high pH can cause the precipitation of Al3+. To avoid decomposition of sodium dithionite, this compound should be added in batches and the temperature should not be too high.