forever dazed & confused....
forever dazed & confused....
For those of you that are nervous about aluminum, here's an interesting discussion:
I find the phrase "fear mongers" appropriate. The digi-crowd likes to use it against we who prefer traditional photographic processes because "they" have no impact on the waste stream. (Yeah, right; millions of discarded inkjet cartridges, cell phone cameras clogging landfills, etc, etc,...).
Photo Engineer and I are both chemists.
When you add a less noble metal to a solution of a silver salt, an oxidation-reduction reaction takes place. The less noble metal is oxidized into solution and the silver salt is reduced to silver metal, which appears as a black sludge.
I've done this many times by adding steel wool to fixer. After a day or so, the steel wool has partially dissolved and there is silver present as a black suspension or sometimes a "mirror" on the wall of the container. The overall reaction is 2Ag(I) + Fe -> 2Ag(0) + Fe(II). There may also be some Fe(III) present. The principle with aluminum is similar, in that case the reaction goes as 3Ag(I) + Al -> 3Ag(0) + Al(III). Depending on the pH of the solution, your Al(III) will precipitate spontaneously and mix into the Ag sludge.
It is not a great idea to ingest huge amounts of aluminum salts, but small amounts are okay. Aluminum hydroxide is one of the ingredients of Mylanta, and it dissolves in the stomach to produce soluble Al(III). Aluminum hydroxide flocculation is a major method of wastewater treatment.
There are two feasible ways to remove silver from hypo solution.
As I stated above, addiing solid or a concentrated solution of KI (Potassium Iodide) to hypo that is exhausted will precipitate yellow Silver Iodide which can be filtered out and reclaimed.
The second method involves either electrolysis or metal exchange with a metallic surface such as Iron. Often steel wool is suggested. Kits for this are available from several manufacturers.
There are many posts about this subject here on APUG and also on Photo Net among others.
Since the latter method I've mentioned is used commercially, it is the best method. It can be easily carried out at home.
A final mention might be made of a method used formerly at Kodak and by the military, which is no longer in use. The scrap film and used hypo were incinerated leaving behind silver residue in the ash. This is not in use currently and has not been AFAIK for about 50 years.
I mention this because the heat and oxidation has suggested to me that if you can decompose the hypo by strong acid or oxidant, then the Silver ions would probably precipitate from the exhausted solution. I have never tried this, but it probably would work.
Reclaiming Silver as Silver ions rather than Silver metal is not very economical.
Since the method of incineration can produce toxic byproducts, the incinerator must be very 'clean'. Kodak does incinerate many items, but AFAIK, they no longer incinerate scrap film. That is all I can speak to.
They do incinerate some chemical waste and vent the smoke through a very very efficient carbon scrubber to removed all toxic waste.
Suppose that I wanted to electroplate out the silver onto a cathode; could I put the used fixer solution in a plastic tub, attach metallic electrodes to either side of the container, and connect an appropriate DC current source?
What would be the 'best' electrode materials, considering both cost and efficiency; and what is the voltage needed?
Also, are there any gasses produced in the reaction that I would need to be concerned about, such as hydrogen?
Don't use any electricity, just throw in a piece of steel wool (Iron, not stainless). Silver will plate out. Read other posts on this to brush up.