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LOSS OF SILVER DURING ELECTROLYSIS
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SILVER EXTRACTION FROM PHOTOGRAPHIC WASTE
Background - During the photographic film processing, Silver leaches into the fixer solution (Sodium Thiosulphate) from the photographic film and forms Silver Thiosulphate complex ions. The fixer is considered exhausted after becoming silver-rich (~ 7 gm/l), as it can no more serve the purpose of fixing the film in effective manner. Such exhausted fixer can then be processed to recover the Silver from it. There are various methods for this. I read about this business in news paper and searched through the literature, internet, etc. and came to know the various techniques of Silver recovery from fixer.
I bought two 5 liter batches of fixer from a photography lab. and tried 2 methods with these two batches : 'Chemical Precipitation' as well as 'Electrolysis'. Estimated Silver content as shown by the Silver test strip (available in market) was ~ 6 gm/l (i.e. ~ 30 gm in each 5-liter volume). Considering loss during the recovery process, I was expecting at least 25 gm of Silver would be recovered.
Chemical Precipitation - As per literature, this process yields about 40-50% pure Silver. I added about 25 -30 gm of Sodium Sulfide (Na2S) in the 5 litre fixer solution slowly, till I reached the null point, i.e. when the Silver test strip showed absence of any Silver in the fixer solution. Then I filtered the precipitate (Silver Sulfide), which weighed about 25 gm. It was sent to a refiner. Refiner gave me 10 gm pure Silver. He charged heavily (Rs. 30/-) for this, as it was a small quantity to be refined. I incurred loss as the recovery was much less than expected (10 gm instead of 25 gm). 10 gm of pure silver could be recovered out of 25 gm of Silver sulfide, i.e. 40 % recovery, which is as expected for Chemical Precipitation technique (as noted in 1st Sentence of this para.). Hence, refiner might not have betrayed me, but the weight of the precipitate itself was much less than expected.
Electrolysis - As per literature, this process yields about 90-95 % pure Silver. I constructed an electrolytic cell in a plastic container out of two SS-316L electrodes (One cathode and one anode), an ammeter, a stirrer and two variable voltage supplies (one for stirrer and another for electrolysis). I passed a very small current of ~ 40 mA through the solution. (Limit on current was primarily due to the fear of Hydrogen evolution which would impair the process of Silver deposition on the cathode). Current decreased to about 30 mA after about 72 hours which I again restored to 40 mA by increasing the applied voltage. Over a period of about 90 hours, I got a very good deposition of Silver on the cathode. After that the silver test strip started showing 'No Silver'. Any further attempt of electrolysis was leading to deposition of black, inferior Silver Sulfide. Hence I stopped the electrolysis and scraped off the silver from the cathode. The silver flakes were weighed to be ~ 10.5 gm. After refining, refiner gave me 10 gm pure Silver. 10 gm of pure silver could be recovered out of 10.5 gm of Silver flakes, i.e. 95 % recovery, which is as expected for 'Electrolysis' technique (as noted in 1st Sentence of this para.). Hence, refiner must be genuine, but the weight of the deposit itself was much less than expected.
Conclusion - Thus, it can be seen that I got, in both the cases, ~ 10 gm of Silver as against the estimated ~ 25 gm. IS MY ESTIMATION PAPER FAKE? OR IS IT POSSIBLE THAT THE SUPPLIER IS ADDING SOME DYE IN THE FIXER TO DARKEN THE PAPER SO THAT HE WILL GET BETTER RETURNS OF HIS FIXER ? What is the solution to resolve this problem ?
VIRENDRA D [last name deleted for privacy by Editor]
- mumbai, maharashtra, india
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You have already demonstrated to yourself fairly conclusively that the refiner is treating you honestly, Virendra, and that there is only about 10-11 grams of silver in each 5-liter batch, rather than the 25 gms that you expected. My guess is that the test strips are inaccurate and the photo shop is using the same test strip to dilute the fixer to 5 gm/l before selling it. But I cannot see into the heart of the operator of the photo shop, i.e., whether he is honestly trying to standardize to 5 gm/l or he is trying to leave much less silver in it knowing that the test strips will back him up. letters 7293, 10011, 12102, 12944, 13040, 15133, 15559 is also about extraction of silver from photographic waste. Good luck.
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