Title: [GreenYes] Re: Green Darkroom Parties
From: "Nancy Poh" <greenbeingnancy@no.address>
Subject: Re: Green Darkroom Parties (was [P2] Green cleaning parties)
Date: Sat, 28 Jun 2008 16:37:58 +0800
> I learned about extracting silver from films while watching Eco Tech on
> Discovery Channel. I have been researching the methods to extract silver
> from:
>
> 1) films
> 2) negatives
> 3) photo processing wastes
1 and 2 are essentially the same. Only black-and-white films,
negatives and radiographic films contain silver. Among these,
medical x-ray films contain the most silver, in order to
enhance the sensitivity to allow the minimum dose of x-ray
exposure. Properly processed color materials do not contain
silver.
Each liter of exhausted fixer solution contain about 5 to 30
grams of silver. As summarized in the article you mentioned,
there are various methods and there are trade-offs among
them. Well designed electrolysis system is the best in terms
of efficient silver recovery, but it requires a large
volume.
> Juan M. Arcos has this recovery method to share. He heats
> oxalic acid of 5% concentration to 100 degree Celsius, put
> the film into the acid for 20 minutes and all the silver
> will drop to the bottom. He said that there is no fumes,
> risk or environmental damage, just pure silver with this
> method.
This method by Syed et al. seems to work, and the silver is
collected as sludge. However, polyester films will vigorously
deform at 100C temperature and may splash the hot solution. In
terms of operator safety, a boiling oxalic acid solution
doesn't sound too safe for routine work, unless operated by an
automated plant or at least trained workers. It may be ideal
if another method that uses solutions of room temperature or
40C at most. However, oxalic acid is pretty cheap, and any
alternative methods would cost more.
--
Ryuji Suzuki
http://silvergrain.org
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