I've been saving my spent fixer for the past 2 years, after learning how much silver it contains. Rather than sending valuable silver to the hazardous waste facility and gaining nothing from it, I decided to make an attempt at extracting the silver myself.
Whereas there are several methods for precipitating solid silver from the solution of silver thiosulfate, I opted for the cheapest method, the iron reduction reaction. I used 0000 steel wool in a flask of fixer solution on a magnetic stirrer to precipitate the silver.
Left: the flask of fixer with steel wool being added under stirring. Right: the flask after 2 minutes of stirring, with a noticeable amount of silver already having precipitated.
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| This is approx. how much steel wool I used for 1 L solution. The piece had a mass of ~1.5 g. |
After allowing the flask to stir overnight, I placed a magnet onto the side of the flask to collect any remaining iron filings that didn't react. Luckily there were very few, and I was able to decant the silver precipitate into a beaker for washing. I didn't take photos during this process, but I washed the silver several times until the water remained clear between rinses. At first the water was turning yellow during washing, but the yellow color went away after 3-4 rinses. I suspect there were still iron salts in solution.
I collected the dry powder and melted it in a crucible with a little borax for flux. My oxy-propane torch made quick work of it! The silver piece weighed in at 3.17 g.
I'm quite happy with this yield; it seems as though I extracted nearly all the silver from the exhausted fixer. I'm still curious what the theoretical yield is for a 1 L fixer solution, so I think I will make another post in the near future with some calculations to figure out those numbers.
