I had a description (a report) on the various processes of silvery recovery some years ago and remember the recovered silver was described as a "relatively pure form". But I may have read it wrong and I trust your knowledge on the subject.
"Relatively pure form" is a slippery phrase. But depending on the silver content of the hypo and the type of hypo solution, yes it can be 'relatively pure'. The other ingredients are iron and iron salts primarily. And the silver is rather unusable until it undergoes a rather lengthy repurification such as described by Ian. This involves dissolving it again, electrolysis and then dissolution in nitric acid and recrystallization.
Ron, the smelted silver from iron sludge's is relatively pure after smelting, usually over 90% and up to about 96%, that is why there can be confusion, as we both know the sludges have a much lower silver content.
Thanks Ron and Ian for clearing that up!
Yes, but the sludge before smelting can vary from low to medium or high depending on a huge variety of factors, and that is all I was pointing out.
The smelted silver at 90% - 96% is sill too impure for photographic use due mainly to iron contamination. The photo grade silver ingot must analyze to about 99.999% pure with a very very tiny fraction of iron, lead, mercury, zinc, copper.. well you get the picture.
I should add a afterthought.
Since many trace heavy metals are added at 6x10^-6 moles / mole of silver, this means that some impurities must be present at well below ~0.00000x grams / 108 grams of silver. The x denotes 6 x the molecular weight of the material. This gives you an idea of how pure photo grade silver must be.
AFAIK you will have to use difficult to get *iron*wool rather than usual steelwool as you can buy easily in your local home improvement store. I have been told, using ironwool is a rather messy operation. The better way seems to be using sodium dithionite. Use 2 tablespoons per litre and put the (open!) containment in the garden far away from noses for some days. It will outgas hydrogen sulfide. After the reaction has finished the metallic silver particles sink down to the bottom of the containment and the liquid can be siphoned off with a small tube.
In the US, a product called Steel Wool comes in 8 grades. It is dull gray and is produced as fine threads in various sizes according to these grades. It is sold in hardware stores. All of the grades appear to work in desilvering hypo. Another product comes as shiny ribbons as a kitchen product for cleaning pots and pans. It will not (AFAIK) work in hypo. It is sold in grocery stores.
Ulrich, the cheapest wire wool* is used for silver recovery from fixer and has been for a great many years, the sludge is normally just left to dry before smelting. *From your local home improvement store.
When I worked in precious metal recovery our factory plated out as much silver as it could overnight then the residual silver was recovered by passing the fixer through normal wire wool. Commercial cartridges for bleach fix use a different grade because bleach fix rots the wire wool, it's this that can be very messy.
Ron, my comments were really an addition to yours to a previous poster.
Yes, I understood that. I felt that my comment needed further clarification in the following two posts of mine. Thanks. I would add that Wire Wool in the UK, would probably be what we call Steel Wool in the US.
I have found that acid fix rots the 'wool' as well as the silver replaces the iron and the whole thing is a real mess to me.