From: sbonds@jarthur.claremont.edu (007)
Subject: Re: Silver Recovery
Organization: Harvey Mudd College, Claremont, CA 91711
Date: Mon, 31 Aug 1992 18:56:04 GMT

jay@metran.UUCP (Jay Ts) writes:

>I am now wondering if there is an easier way to perform silver recovery.

>Someone on the net must have a method they can recommend, right?

But of course...

There are lots of ways to get silver back out of the fixer. The silver gets
into the fixer by dissolving as the halide, I'll use silver chloride as an
example. Silver chloride dissolves in a solution of thiosulfate ion (S2O3 2-).

Silver metal will also dissolve in thiosulfate, but considerably slower than
the chloride does. (One of the big reasons thiosulfate was chosen for a photo
fixer!) The fixer solution then contains a mix of silver ions, chloride ions,
and thiosulfate ions. (There are others, but they aren't important for this
discussion.) The trick is to get these ions out of the solution.

The least technical way to get the silver out is to stopper the bottle and
let it sit for about 10 years and let electrochemistry do its thing. The 
fixer will decompose into sulfur and the silver ions will also decompose to 
silver metal. 
Generally speaking, people don't want to wait this long though...

Another way to get the silver out is to replace it with another metal. Iron
is commonly used. The silver (Ag+) is replaced by an iron (Fe2+). The iron
is oxidized and the silver gets reduced. This usually leaves you with a nice
sludgy mixture of rust and silver. (The iron rusts because of the water and
other ions in solution, not due to its reaction with the silver.) This method
is detailed in Kodak's book, _Silver_Recovery_. Copper also works well, though
the silver concentration in the fixer must be fairly high.

It is also possible to electroplate the silver directly out of the fixer. I
tried this with some limited success using a graphite anode and a silver
cathode. If you try this, you will need a high-current source like a car
battery charger. Don't use batteries or those baby 250mA power supplies. We
want some KICK here! (A high voltage is not required. If the current seems
too low, add a bit of sulfuric acid. It'll work a lot better then ;-)

Yet another method is to precipitate the silver out as its chloride. By
adding enough laundry bleach (sodium hypochlorite) the silver will 
precipitate out as a white light-sensitive solid. One drawback to this is
that the fixer by nature is designed to hold as much silver chloride as
possible. Thus, some AgCl will remain in solution. This amount can be
minimized by raising the pH of the fixer solution. The liquid remaining
is decanted (the silver remaining in solution can be extracted via another 
method, if desired) and the AgCl solid is washed several times to purify it.
It is possible to then convert the chloride to the metal. I didn't do this
because I needed some Ag2O for another experiment, so I just converted the
chloride to the oxide.

  AgCl(s) + HNO3(aq) -- > AgNO3(aq) 
  2AgNO3(aq) + NaOH(s) -- > Ag2O(s) 

The easiest way to convert AgCl to Ag(s) is probably to dissolve it in a
small quantity of nitric acid and then run a metal-replacement reaction
with a stoichiometric quantity of copper.

  2AgNO3(aq) + Cu(s)  -- > 2Ag(s) + Cu2+(aq) + 2NO3-(aq)

The bad part about this is that there will be some copper left in your
silver. This reaction may also be done using iron. It would be easier to
separate the iron out from the silver by electrolytic refining, but I
didn't need my silver of such a high purity.

It might also be possible to "develop" the silver chloride in your normal
photo developer, and recover the silver that way. (I haven't tried it, but
it works in theory.) Be sure to develop for at least an hour for a "normal"
developer. We want every scrap of silver chloride converted!

If you really want to get your silver pure, the best way to do it is by
electrolytic refining. Since what you probably have is just a pile of
divided silver, you'll need to melt it into a solid lump first. Put a little
bit of Borax on top before you melt it. The Borax will absorb some of the
impurities that may have survived. It can be chipped off once the silver has
cooled. I have not actually run this refining technique, but it was 
recommended to me by a man who does silver recovery for a career.

To run the electrolytic refining, you need a power source (high-current),
a silver cathode (Actually, any ol' cathode will work, but then you get the
problem of removing the cathode from the silver...), a non-conducting basket
to put the anode in, and a piece of cloth to catch the impurities when the
anode dissolves. You need an electrolytic bath which will not dissolve
the silver metal until the current is applied. A suitable concentration
of nitric acid should work. When the current is applied, the silver anode
will dissolve, and pure silver metal will plate out on the cathode. (If 
your impurity is copper, it will plate out BEFORE the silver does, making
it difficult to remove it in this way. By adjusting the voltage applied
to the cell, it is possible to keep such impurities as iron in the solution
without having them plate out.)

After this refining, you should have a silver block that is 99+% pure. The
problem you run into now is that not everyone will believe you if you are
trying to sell it. So the silver must be assayed. This costs $$$, and will
render all but medium or large scale silver producers non cost-effective.
If you need the silver for other projects, you can save yourself the cost
of buying it on the open market, or you may be able to sell your silver
to such a person just under market value, and benefit both of you.

I think this is all sound information, but if you are in the know and
find a glaring error, E-mail me with a correction and I'll check it out.

  -- 007

P.S. No flames, please. I'm still a budding chemist, so I don't know 
everything yet.

-- 
 000   000  7777 | sbonds@jarthur.claremont.edu
0   0 0   0   7  |----------------------------------------------------------- 
0   0 0   0  7   |        Childhood is short...
 000   000   7   |                    ...but immaturity is forever.
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