Wolfgg
The higher the price of silver rises, the more the question arises as to whether it is possible to extract the silver from the fixer with a reasonable amount of effort.
Extracting silver from a fixer using electricity is considered a complex process. However, my experiment shows that a great deal can be achieved even with simple DIY methods. Anyone with a well-stocked DIY kit can manage with very little investment. Here is my method; you will need:
- A stainless steel container with a capacity of approx. 0.5 litres, e.g. from the ‘Everything for just 1 euro’ section of a department store. If the cashier asks what it’s for: ‘As a cathode’.
- A piece of graphite as an anode. My DIY box contained a graphite spray (Kontakt-Chemie); I used this to coat a copper-plated epoxy resin plate (electronic circuit board material) measuring 10x20cm in approx. 8 layers – i.e. I kept spraying it again and again after it dried to ensure the emulsion had sufficient density.
With this anode, I have desilvered approx. 5 litres of fixer to date and it shows no signs of damage so far. However, it is not yet possible to say anything about its lifespan. Perhaps someone knows of a flat piece of graphite, approx. 10x20cm or larger, that doesn’t cost too much. That would last forever.
- A power supply unit; it only needs to be capable of up to 2 volts and 0.5 A DC, but the voltage must be adjustable and the current readable (use a multimeter if necessary).
- A stirrer. The fixer needs to be stirred constantly; for this, I use a mini drill with a stand, a cut-up metal clothes hanger bent to fit as the stirring rod, a PVC tube slipped over it for chemical protection (stripped from computer cable insulation) and the lower end sealed with Uhu Plus. The drill must have adjustable speed, otherwise the fix will splash everywhere. So it’s best to use an adjustable power supply for this too, e.g. 0–12 volts. A standard hand drill with speed control will also work, but it’s noisy.
Now simply place the graphite plate at an angle into the container (it must protrude beyond the container for the power connection), clamp it to the container with two plastic clothes pegs, and first check whether you need to file away any graphite or copper where it rests against the container to prevent a short circuit. Then adjust the height of the stirrer so that it doesn’t streak anything. Now all that remains is to pour the fixer into the stainless steel vessel and set the stirrer’s speed so that nothing splashes out. Connect the negative terminal of the DC power supply to the vessel, the positive terminal to the graphite plate, and slowly increase the voltage until approximately 0.5 A flows. With 0.5 litres of spent fixer and a current setting of 0.5 A, it takes 3–4 hours for almost all the silver to have precipitated as a grey coating on the vessel. The best way to check whether there is still any significant amount of silver in the fixer is to use the copper wire method: Sand a piece of copper wire (e.g. 1 mm thick) down to 10 mm, hold it in the solution for 1 minute; if no more silver precipitates, the fixing bath can be considered de-silvered. Now carefully pour off the liquid; what remains is the silver, which can be easily brushed off the inner wall using, for example, an old toothbrush and placed into a collection vessel.
Well, what do you do with the silver now? Just collect it. Perhaps one day the silver will be worth as much as gold is today. Anyone who has collected a few kilograms can read on ‘over there’ (‘Silver Recovery’). There, Franz does his best to save a wildly determined silver bar caster from making a fool of himself.
Anyone with a similarly well-stocked DIY box can replicate the whole thing very cheaply (I only had to invest 1 euro); for those who have to buy everything from scratch (drill, power adapters), the effort won’t be worth it.
Regards, Wolfgang
cfb_de
Hello Wolfgang,
Thank you very much for your explanation. I’d like to add one more point: what should be done with the desilvered fixer?
It can still be used, provided one condition is met: if you’re not using flat-crystal film, you can safely use the desilvered fixer as the first fixer in a two-bath fixing process. (If you can
titrate for halides, even theoretically an unlimited number of times).
Best regards,
Franz
P.S: The 2V is rather arbitrary and chosen to be on the safe side. The overvoltage required for the electrolysis of water depends on several parameters (material of the cathode and anode, constituents of the solution). The voltage can safely be set high enough that neither visible gas evolution nor precipitation from the sulphate anion electrolysis occurs. I once worked with a fixing bath electrolysis unit that had a voltage adjustable in tenths of a volt and went up to 4.8V. The instructions specified the respective voltage for common laboratory fixers.
The endpoint can be identified from the current curve.
BTW: I have some doubts about the reaction time you’ve given. If I calculate with 4 hours, I arrive at 8.06 g of deposited silver. So, according to your figures, you’d have to push your fixer to a silver content of 16.1 g/l. But it hasn’t been fixing properly for quite some time at that concentration. (Faraday’s law: M/z * Q/F = m, F=96485C)
Wolfgg
Hi Franz,
Thanks a lot for your comments. You were quick off the mark – I suppose you’re always online on all the photography forums :).
My last fixers were from E6. It’s true, I always pushed it to the limit, only replacing them once the clearing time had increased significantly (after the stop bath, with E6 you can watch it so nicely and extend the time if necessary). And with the desilvering, it’s possible I exceeded the maximum time; towards the end, there was already a slight smell of SO₂ and H₂S in the air. The first test with the copper wire should probably be done after just 2 hours, and it’s best if everyone determines their own personal maximum time for their specific tank usage using this copper wire test.
Regards, Wolfgang
cfb_de
Hi Franz,
Thanks a lot for your comments. You were quick off the mark – I suppose you’re always online on all the hot forums :).
Hello Wolfgang,
Well, when I’m sitting at the computer, I do have a browse through a few forums now and then.
My last fixers were from E6. It’s true, I always pushed it to the limit, only replacing them once the clearing time had increased significantly (after the stop bath, with E6 you can watch it so nicely and extend the time if necessary). And with the desilvering, I may well have exceeded the maximum time; towards the end, there was already a bit of SO₂ and H₂S in the air, judging by the smell. The first test with the copper wire should probably be done after just 2 hours, and the best thing is for everyone to determine their own personal maximum time for their specific tank usage using this copper wire test.
That’s exactly what I’d thought :-)
And after 4 hours, no more 0.5A was flowing, was it? Otherwise, it wouldn’t just have been ‘a bit’ of SO₂ or H₂S that was noticeable.
In short: as you’ve described it, the setup is unsuitable for poorly ventilated rooms and completely unsuitable for unattended operation. (I’d be happy to put you in touch with the roofer who was kicked out of our lab building after suffering H₂S poisoning. He didn’t break any bones and survived with ‘just’ severe hydrogen sulphide poisoning. As a physiologically 100% cripple.)
If you stick to that, however, it is a wonderful description for a DIY electrolysis setup.
Placing a graphite rod (costs a few euros in shops; a sharpened 6B pencil will do the trick) in the centre makes the electric field more homogeneous and the deposition on the cathode surface more even.
Professional devices have a rotating cathode from which the silver is mechanically removed during operation. Advantage: You can run the device continuously (drain and filter the bottom occasionally; top up; the current adjusts accordingly).
Best regards,
Franz
Wolfgg
Hi Franz,
Yes, it’s worth mentioning that if you don’t keep a close eye on the timing, you’ll end up with dangerous gases once the silver ions run out, so you’ll need to ensure the area is well ventilated. And if you’re not sure how many silver ions are still present, it’s advisable to reduce the current at the end, for example to 0.25A. Fitting a gas sensor as a ‘ready indicator’ would be an obvious solution, but it’s too much hassle.
Regarding the graphite problem, I initially thought of pencil leads too, but I know from my youth that you rarely manage to pull the lead out of the wood in one piece, and it would need to be at least 10 cm long. However, I now recall from my school days that, before the invention of the fine-lead mechanical pencil (invented, to my knowledge, by Pentel), some pupils used a lead holder for which the pencil leads (which are not lead, but graphite) were available without the wood. When I look in the catalogue of an office supplies wholesaler, I am surprised to find that it still exists today. The thing is called the TK-Fallminenstift by Faber-Castell and the leads are 2 mm thick. To my delight, I find this holder (but no more leads) amongst my school stuff that I’ve kept, and I measure that the leads would need to be 10–12 cm long. So that would be the simplest solution for the anode.
Anyone with a knack for DIY could try using the tube itself as the anode, i.e. gluing pencil leads to it and supplying the current via a sliding contact (not via the drill, as that will ruin its ball bearings!).
One question remains: the article in F&L only mentions the current density at the cathode, which must not exceed a certain ratio to the silver ion density, otherwise gas will form – which makes sense. But what about the current density at the anode? Does it generally play no role in the proper conduct of the electrolysis, meaning a single pencil lead is sufficient, even at >1A?
Regards, Wolfgang
orwograph
Blimey, you lot are getting up to some tricky business... it’s all a bit too hot for me and my knowledge of chemistry is a bit patchy, but I can help out with the graphite leads: art supply shops sell solid graphite pencils in all grades of hardness for drawing, at very reasonable prices. These are graphite rods wrapped in plastic film, about 6mm in diameter and 250mm long. The plastic film peels off very easily. There are even thicker, though shorter, pencils available:
http://produkte.boesner.com/shop/zeichenmaterial/k8971hb_grafitstift.html
http://produkte.boesner.com/shop/zeichenmaterial/k48652b_grafitmine.html
http://produkte.boesner.com/shop/zeichenmaterial/cc40602_grafitstaebchen.html
Wolfgg
Oh yes, that’s a great tip for the anode material, and it’s cheap too. Thanks very much for letting me know.
Best regards, Wolfgang
cfb_de
Hi Wolfgang,
The whole point is to keep the voltage high enough for water electrolysis. Unfortunately, though, that also depends to some extent on the current density at the electrodes :-)
I’d go for the thicker rods and use several of them at once. Better to have a slightly poorer cell than too high a current density. Then you can also use higher current rods and save time.
@orwograph: I don’t know what Wolfgang does. I’m a chemist and work in electrochemistry (though in analytics, so more in the millivolt and picoampere range).
Best regards,
Franz
Wolfgg
What do I do? At school, I was a cross between a physicist, a chemist and a mathematician. These days, depending on the project, I usually find myself somewhere between mechanical engineering, electrical engineering and computer science. So it’s always interdisciplinary. And the focus is different every time.
So, on the subject of fixed silver removal, all the essential facts have now been compiled in just a few hours, so that anyone interested can successfully replicate the process. Many thanks to everyone.
Best regards, Wolfgang
orwograph
I just mean – as long as you don’t start centrifuging anything out of your old Urantoner solution and then using the whole lot, graphite-moderated, for Duka lighting...
:blink:
peter.
ravebenni
I’m planning to build a setup like this myself.
The question for me is whether the cathode absolutely has to be made of stainless steel, or whether that’s more or less a matter of personal choice. It’s clear that an electrically conductive material is required. It also needs to have a relatively hard surface, otherwise you’d end up scraping off a mixture of silver and cathode material later on. But why stainless steel? By that I mean, of course, ‘stainless steel’, i.e. 1.4301 or 18-10 or ‘V2A’ (after all, almost anything can be called stainless steel :rolleyes:) I’m reminded of my materials science lecture: under certain conditions (electrochemical series), this steel can rust ‘very easily’. Then the two terms “hydrogen embrittlement” and “pitting corrosion” spring to mind. However, I can’t be bothered to dig out my lecture notes right now.
Never mind. The two things I mentioned take time (possibly several years) and are only really relevant when it comes to fatigue strength. In any case, I’m not worried that a stainless steel pot will suddenly collapse on me. Still, I’d like to know what else is available.
Is the silver actually easy to scrape off?
cfb_de
Hello Benjamin,
The pot in question isn’t so much about materials science for steel materials. It’s more about using electrode materials in which water electrolysis is inhibited by a suitably high overvoltage.
Hence the combination of graphite and stainless steel. Unfortunately, the surface of the steel also plays a significant role here. If you do everything correctly, ‘hydrogen embrittlement’ isn’t an issue – electrolysis isn’t usually carried out under high pressure in a photo lab....
Pitting corrosion will occur on the pot at some point anyway; the many nasty sulphur anions will see to that. Although even then, it takes quite a long time for the container’s structure to be compromised by corrosion. With the small amount of current acting on the container, the erosion rates are in the region of micrometres per week of current application. So: a plastic pot around the Mimik. Let me guess: you’re an engineer? Good at PowerPoint?
And on the subject of ‘scraping’: ideally, no force is required. Commercial devices therefore feature rotating electrodes with scrapers, operating at a speed matched to the deposition rate, along with filter technology.
With DIY devices, you need to pay attention to the current density and voltage. The former is a matter of trial and error; the latter is determined, among other things, by the overvoltages involved, which are also a factor in silver electrolysis. I wouldn’t work with more than 3.2V when using the combination of 18/10 cookware steel and high-purity graphite; above that, you’ll end up electrolysising the water as well. You can easily spot excessively high voltage by the gas development at one of the electrodes. Simply reduce the voltage slightly and you’re done. The appropriate current density then adjusts itself via the variable conductivity of the electrolyte (= fixer, continuously desilvered).
A discontinuous-operation unit with a capacity of approx. 5 litres can be built yourself for around €45. Provided you have the necessary technical know-how for a 2A power supply adjustable within the 1–4V range, without a scraper or filter.
Commercial devices operate using a mixed/by-pass current method and are therefore significantly more complex. The cost is then in the filter technology and the foolproof design. After all, the average amateur photographer is not a fully qualified chemical laboratory technician with additional training.
Best regards,
Franz (not an engineer. Chemist.)
piu58
Desilvering will work, but removing firmly adhered pieces of ore is quite a challenge in this solution. You must expect hydrogen sulphide to be produced by anodic reaction, which causes the silver to precipitate. The solution will then turn black due to the silver sulphide. You can allow this to settle and perform settling and filtration if necessary.
Wolfgg
I can add a quick note here:
When using graphite stubs from the graphic artist’s range, you need a slightly higher voltage because they have considerable ohmic resistance. Example: 4 graphite pencils with a 6mm diameter connected in parallel (Gioconda brand from eBay), immersed 6cm deep, require approx. 5V for a current of 0.5A; at 1A, approx. 7.5V is needed. At 1A, the pencils become noticeably warm, but not hot. The Fix cools them down, after all.
The graphite rods must absolutely be unvarnished, otherwise the current will only flow at the tip. And handle them with care, as the rods break easily. I have slipped a 1 cm long sleeve, bent from brass sheet, over the top end of each rod so that it clamps on, and then connect the positive terminal to this.
The silver deposits quite easily on the sides of the container and can be brushed off easily, for example with an old toothbrush. After desilvering, I pour off the Fix (which is reused), add some demineralised water, brush the silver into the water and tip everything into a glass container, where the water is then left to evaporate over the following days. Done.
Regards, Wolfgang
ravebenni
Would it be possible to use a piece of pure silver as the cathode? Not that I have anything like that, but then the problem with scraping it off wouldn’t exist. The whole thing would be in a non-conductive container; the cathode would accumulate more and more silver and grow larger and larger, and the silver would be in a very pure form. You’d only need a small piece to start with, from which an ever-larger one would grow. So you wouldn’t end up with silver sludge, but a solid piece.
That’s my theory, but does it actually work?
Wolfgg
Hello Benjamin,
No solid silver metal forms; instead, a silver sludge (tiny, finely dispersed silver particles) forms, which clings loosely to the inside of the vessel and can be easily brushed off without any effort. There is no need to scrape it off.
Best regards, Wolfgang
cfb_de
Hello Benjamin,
If you want to work with a silver cathode and allow the emulsion to grow, there are a few things you should bear in mind:
- The current density should be low to begin with
- Consequently, the time required increases dramatically
- Overvoltages (other sulphate-ion electrolyses, etc.) must be avoided at all costs
It will certainly work. But it is not practical for desilvering fixers. Voltage and current must be adjusted throughout the entire long electrolysis period. Overvoltages must be determined experimentally before each new attempt. To detect the endpoint, prior titration to the exact silver content is required.
This way, you eventually produce pure silver, but you cannot recycle fixers.
I therefore prepare my fixer (for now, at least – the next cheap 14/10 crucible is mine) by adding dithionite. The precipitate is deoxidised and smelted in a crucible using coke in the traditional way. Then 7.5% copper and other metals are added to make the mixture pourable.
Advantages: Simple, cheap. Equipment: An old bottle for the solution, a burner, a tripod, a clay triangle and a crucible.
Best regards,
Franz
