Electrolytic silver recovery voltage & current?

Hi Gang!

 

I'm building an electrolytic silver recovery unit to help keep my

fixer clean and long lasting. Does anyone have a ballpark idea of the

power supply voltage & current requirements, so I can design the

power supply regulators accordingly? I don't want to put so much

energy into the tank that it literally "cooks" the fixer...

 

Thanks!

Dan

Aren't you reinventing the wheel here? I thought you could buy that kind of equipment.

Uhh, I'm an Electrical Engineer; and if I can't build it I should be shot! :)

I thought the normal goal of these units was to remove the silver from spent fixer to (1) recover it ($$$) and (2) keep it out of the sewer, since it's a regulated toxin in many places. I haven't heard of re-using the fixer after pulling the silver out.

 

I just keep a 2 liter soda bottle with a ball of steel wool in it. Just keeping the silver out of the sewer. I decant the fixer periodically.

Removing silver from hypo by the electrolytic process, as opposed to the steel wool, zinc, and dissimilar metals processes, is a delicate balance of power and time. The least of your concerns is harming the hypo, which is disposed of anyhow. The main problem is "burning" the "chip" as it's called. I'd research the specs VERY carefully and thoroughly.There are relatively inexpensive units on the market as well as non-electrical methods.

OK gang, the idea is to use electrolytic silver recovery to <i>keep</i> the fixer from being exhausted. Commercial labs do this all the time with silver removal from fixer, which cuts the replenishment rates in half. <br><br>

 

Plus, I already have a couple 5 gallon polyethylene tanks that are ideally suited for this purpose; and I also have plenty of spare electrical parts for that half...<br>

>> Uhh, I'm an Electrical Engineer; and if I can't build it I should be shot! :) <<

 

Hi Dan. There�s more to this than meets the eye; you might find it to be a humbling experience. I would second John�s suggestion to just buy a unit (used). Many mini-labs have used fairly small (~10-15 gallon tank) units with rotating cathodes. I am presuming from some of your other posts that you are planning to do a significant quantity of processing in the near future; so maybe one of these units is not too large? Also I would suggest don�t reuse the fixer unless you know what you�re doing with the chemistry.

 

If you still want to do this yourself, here is a brief discussion about it; I�m pulling this from way back in my memory, so there may be some factual errors. Probably not too many, I hope. This may be a little more technical than I usually post, but as you are an electrical engineer; hopefully you can make sense out of it.

 

You will be having both an anode and cathode in the solution. The silver is not all instantly available for electroplating; only the small proportion of �free� silver ions which are in the immediate vicinity of the cathode can be plated. This will set a limit on how much current you are allowed to run. If you exceed the limit, other reactions (unwanted) will occur; you will probably see this as darkening of the plate; thus you will know you need to reduce the current. The traditional method to keep silver ions available near the cathode is to use a larger surface area with lots of agitation; thus the popularity of the rotating cylinder cathodes in commercial units. If you don�t circulate the fixer, this will drastically reduce the amount of current you can run.

 

Just as a very rough idea for a starting point for fixer, with decent circulation and a starting silver content of 2 or 3 grams/liter (or higher), you might safely run a cathode current density of about 5 or 10 amps per square foot. As the silver content gets down below 1 g/l, or so, you�d have to reduce the current down to half this or less. Voltage to do this would be very roughly in the range of 1 or 2 or 3 volts with the cathode and anode several inches apart.

 

With fixer, you should be able to approach the theoretical rate of 1 silver atom per electron, which I think (fuzzy memory) is very nearly 4 grams silver per amp hour. It�s easy to verify this via calculation given the number of electrons per amp hour, the number of silver atoms in a mole and the molecular weight of silver (I�m too lazy to look them up). So very roughly, if you wanted to desilver, say, one liter of fixer containing about 4 grams/liter of silver, you would need to run about one amp-hour through it. With plain fixer, you can pretty much run the current as low as you want; if you get too high you�ll cause yourself plating problems. In an industrial setup, they have to keep the current high enough to keep up with the process.

 

Again, I suggest you do NOT try to reuse the fixer unless you understand the system as certain chemical corrections have to be made. PS: the reaction at the anode will mostly be seen as sulfite oxidizing to sulfate; roughly there will be about 1 gram as sodium sulfite consumed per 1 gram of silver plated. If you try too hard and run out of sulfite, you probably won�t have much success plating.

Bill,

 

Bingo!

 

That's EXACTLY what I'm looking for. I can easily set up a current-limiting power supply to do just this. Incidently, as you pour current through a conducting liquid, you'll also get I^2*R heating, which is something I'll need to recalculate, so I don't boil the fixer!

 

Most enlightening is the issue of the cathode size, which I can easily make larger than I originally anticipated. I'll be using 305 or 316SS for the electrodes...

 

Thanks!

Dan

No lab around where I live would reuse fixer.. They do however use silver recovery. It's a free money maker..

<i>No lab around where I live would reuse fixer.. They do however use silver recovery. </i> <br><br>

 

You mean to tell me every lab in your area single-shots their fixer?!<br><br>

<b> I don't think so! </b>

<br><br>

>>�you'll also get I^2*R heating, which is something I'll need to recalculate, so I don't boil the fixer! <<

 

I think I can assure you that this will not be a problem. Your time might be better spent worrying about starting fires due to the friction of your shoes on the ground.

 

Very roughly, with a decent cell design, you�ll probably find that your 5-gallon tank has to run batches somwhere in the 4 to 8 hour range to fully desilver a batch of fixer. If you try to run it at a higher rate, the unwanted reactions will start (again, you�ll see the cathode darkening). Now, looking at the numbers, say you have 15 liters with silver load at 3 grams/liter; this is ~45 grams total silver. If each amp hour produes 4 grams of silver, this is ~11 amp hours total required. Now, spreading that out over maybe 6 hours, you can see that you�re only running about 2 amps. Given that this is at only a couple of volts, you can see that the heating effect on 15 liters of solution is pretty trivial.

 

Hopefully this will put your mind at ease about boiling the fixer, so you can get your mind on the things you really should worry about. Like designing the cell to be safe, figuring how you will keep the chemistry in good condition, etc, etc.

I meant for c-41. But I guess I should think before I type. duhhhh

<i>No lab around where I live would reuse fixer.. </i> <br><br>

You mean C41b (& up (?down?!)) bleach-fix, a/k/a "blix"

<br><br>

Cheers! <br>

Dan

OK, now my homebrew silver recovery unit taking shape...

<br><br>

A couple years ago, I bought cheapie Chinese 8 quart & 12 quart stainless stock pots for $15 total. I'll be using one as the vessel, which will also be the cathode -- I'll just let the silver continue to plate, then chisel it off when I need beer money.

<br><br>

I'll use a piece of stainless in the middle as the anode: It will be attached to a floating plastic lid.

<br><br>

I have both a benchtop JOBO ATL-3, as well as the console from a barely -cannibalized full boat console ATL-3 (complete with hot water heater, six chemistry pumps and a water panel). I'll grab one of the working chemical pumps, and use it to manually pump filtered fixer back up to the upper E6 fixer tank to top it off. <br><br>

 

If this works well, I'll build a second unit for B&W fixer with the other stock pot and another pump, since it appears that I need to segregate the *used* E6 & B&W fixer, despite me using the same fresh fixer/replenisher.<br><br>

 

So there you have it: My $20 inline silver recovery & fixer "cleaner." I'll post pictures to memorialize it for the Google crowd.

<br><br>

Cheers! <br>

Dan

"<i> Very roughly, with a decent cell design, you�ll probably find that your 5-gallon tank has to run batches somwhere in the 4 to 8 hour range to fully desilver a batch of fixer. If you try to run it at a higher rate, the unwanted reactions will start (again, you�ll see the cathode darkening). Now, looking at the numbers, say you have 15 liters with silver load at 3 grams/liter; this is ~45 grams total silver. If each amp hour produes 4 grams of silver, this is ~11 amp hours total required. Now, spreading that out over maybe 6 hours, you can see that you�re only running about 2 amps. Given that this is at only a couple of volts, you can see that the heating effect on 15 liters of solution is pretty trivial.</i>"

<p>

The difference of potential between a steel and a copper electrode would be about one volt; I have no clue what amperage it would produce, however. If one is in no hurry, is it possible to rely only on self generated power to do the silver recovery?

<p>

If not, I wonder what you use as a power source to provide 1-2 amperes on a continuous level? Batteries? Some kind of fancy transformer?

<i>If not, I wonder what you use as a power source to provide 1-2 amperes on a continuous level? Batteries? Some kind of fancy transformer?</i> <br><br>

Wall wart with DC output. Something like a 9 to 12 volt 1 amp wall wart should be plenty. If you have a current meter, simply start out with the electrodes far apart while watching the current. Then, move them closer together until the current is about 10% - 20% below the rating so you don't burn it out, i.e. if the rating is 1200 mA then pick a distance that yields about 1.0 amps.

<br><br>

 

In any case, look through your junk pile of computer, audio and telephone stuff; and after you untangle the cords (!) look at the ratings of the wall warts. First, it MUST output DC -- AC is useless. Next, pick out the ones with the highest milliamp ratings, and the ones with highest voltage ratings (as long as they can output at least 600 mA). Use alligator clips, an ammeter (your $9.99 Radio Shack meter can read milliamps); and then try different wall warts and electrode distances. Choose the one that gives you the most current (while staying at about 80 to 90% of the max current rating of that particular wall wart) -- No brain surgery is required!

<br><br>

 

Nothing fancy is needed, i.e. the DC doesn't need to be filtered (smoothed) -- Just converted from AC to DC. This is why wall warts are so popular!

<br><br>

 

Cheers! <br>

Dan <br><br>

<hr width = "200"> <br>

OK, I see, I should be able to find something... I wonder what was the output of my two 2400 bps modems? Oh, too bad, they're AC... Oh well, I'll manage.

 

Now, how do you wire this? I suppose that rather than keeping the fancy plug, you separate the two wires and then send the positive to the steel electode and wire the negative on the copper electrode?

Use stainless steel electrodes; or in my case, a stainless steel 8 quart stock pot (a cheapie "made in China" Wal-Mart special) and a stainless steel cathode. This way, the plating occurs right on the walls of the pot.

 

Since it's stainless, I can simply disconnect the leads and dissolve the silver in nitric acid right in the same container...

Convenient... Yes and no. A large container means that you'll need a lot of nitric acid to dissolve the silver later. With a smaller, removable electrode, only the electrode gets plated and you can dissolve the silver in a smaller container. I admit that a copper electrode isn't a good idea if you have to dip it in nitric acid, however. But it all depends on the scale you work at, I suppose.

 

In your experience, is the silver really plated? Some sources suggest that it only loosely deposits on the electrode and that you can wash it away with distilled water.

>> In your experience, is the silver really plated? Some sources suggest that it only loosely deposits on the electrode and that you can wash it away with distilled water. <<

 

Of course it�s plated. What source did you get your info from?

 

If you�re doing bleach-fix, you will tend to get a crumbly plate which will vary with certain conditions. But no need to worry, you don�t have a prayer of being able to desilver bleach-fix with the kind of setup we�re talking about.

 

In the event of plain fixer, the plate will be much harder, although nothing like sterling silver (which is not pure silver). I would discourage anyone from using Dan�s cathode method if you ever want to remove the silver. I�d suggest a single flat sheet of SS about 2/3 the crossection of the tank (this is so you can get some circulation around the sides). For example, if the height and width of your tank is 10� by 10�, try a cathode about 7� or 8� square. You�ll be able to build up an appreciable thickness of silver in your lifetime (hopefully). Definitely use a pump for circulation; a small submersible pump like they use in garden ponds should be fine; aim the output at the center of your cathode. By the way, commercial units typically use carbon anodes (there are special varieties made for plating); but you�ll be just fine with another SS plate.

 

With the smaller cathode, you�ll be able to build up an appreciable thickness of silver in your lifetime (hopefully). If you can get your plate maybe 1/10 inch thick, you can break it off by flexing the the sheet of SS back and forth; the silver will break off rather than flex (the special type of surface SS has prevents really strong adhesion of the electroplated silver.

 

FWIW, I don�t know about that power supply method. I�d much rather have the ability to vary voltage than trying to screw around with cathode position. I won�t advise on the power supply, but one needs to be sure there is no danger of electrocution. However, commercial units have often used a an autotransformer feeding to a properly sized isolation transformer which then goes into a rectifier system to get the DC. Again, it falls on the designer to be certain the design is safe.

 

If someone is just playing around (Phillipe?), I would guess that you could probably plate successfully with a single flashlight battery. Just use a couple alligator clips to a couple pieces of metal (do you have a couple of old Ginsu steak knifes around?) and wiggle them around in your fixer for circulation. If they start to get dark, move a little farther apart. I don�t know how much a D-cell puts out (Dan?), but I wouldn�t be surprised if it could keep up an amp for 2 or 3 hours. 4 amp hours is enough to plate out about 15 grams of silver, which is roughly ½ troy oz, I believe.

 

>> If one is in no hurry, is it possible to rely only on self generated power to do the silver recovery? <<

 

Well, actually putting in steel wool is a self powered system, but you won�t get very pure silver. Copper alone, will also work, but copper in solution is probably worse environmentally than the silver. I don�t have any idea about the iron/copper electrodes. Even if it worked, once the silver covers one electrode, won�t things stop (one of the electrodes will be silver)?

Actually, recovery shouldn't be that big a pain once the cathode is plated. If my memory is correct silver melts at about 1100 farenheit. A simple adjustable bullet casting pot should be hot enough. Throw in a handful of your silver coated anodes and let the silver melt off. Skim the dross, pour the silver off into your molds, then reuse the stainless steel electrodes when they cool.

Hey guys it's nice to have such a partners like you helping ... i was looking for the same answers thrugh 3 months ago but i finally find it here

Thanks alot

i have two more questions .... I think it may be silly to ask but I have to do

1- the cathode charge shoud be (-) or (+) ?

2- Some kind of machine i found uses 3 anodes of carbon why they do so?

please help