Juan Manuel Arcos Frank: I did not see anything about who first posted this. I did a Google search for the process and it came up. I copied it and gave the source in my post as I think it should be. I have tried to find the information on the GRF but without much luck. I do not wish to step on any toes or ruffle anyone. I only posted it in response to an existing post to try to help and I did ask for more input from others with more knowledge.
Mark
Below is the full post and yes it is accredited to the GRF and Juan Manuel Arcos Frank.
Re: X.ray films scrap
by Chris Owen » Sat Jun 12, 2010 7:21 am
The quote below is from goldrefiningforum.com. I have never used this method but it is claimed to work well. Keep the oxalic solution weak (the info below uses a 50 g/l solution) because if it is stronger, it is said that it might create sodium oxalate, which is explosive, although I have no idea where the sodium ion is coming from. Use a respirator and gloves when handling the oxalic acid powder, as it is toxic. After the solution is made up, it is supposedly not necessary to use a respirator. Also, it is said that the solution can be reused.
I have used about 4% sodium hydroxide for 1 hour at about 95C in an 40,000#/day film operation. Also, 10% sodium hydroxide, at about 60C or a bit less, will strip the film in about 10-15 min, although using it is more dangerous to use than the weaker solution. I have also used a proprietary enzyme/sodium bicarbonate solution, at about 40C, that will strip in about 10-15 minutes - this is about the safest solution to use although, since it ferments the emulsion, it really stinks. The problem with all these solutions is that the emulsion that is removed along with the silver must be incinerated before the silver is melted. Supposedly, this incineration isn't necessary with the oxalic acid solution.
No matter what chemical process is used, the films should be shredded before stripping, in order to get complete stripping and to keep the stripping time at an acceptable level. Unshredded films stick together when wet and this prevents the solution from getting in between them. I have seen both paper shredders and tree limb shredders used, but both tend to twist the film pieces and this reduces the bulk density tremendously. The best shredder is one that shears or cuts the film to 1/2", or smaller. That way, the pieces lay flat and you can get a lot more film in the tank. It is also better to use a tumbler. I once made one from a stainless tumbler out of an old clothes dryer that held about 150 pounds of film.
"There are many processes for silver recovery from B/W films,offset negatives and radiographies.Combustion of film,nitric acid leaching,caustic soda leaching and CLOROX treatment have been proposed in this forum.Except combustion,all of them produce a silver intermediate compound that needs more processing to obtain pure silver,besides,all these processes release toxic fumes to the environment and nitric acid leaching and caustic soda leaching can cause severe burns.
The new process uses a low concentration of oxalic acid to attack the gelatine of the film and the silver goes down as a grey/black mud that only needs drying and melting to obtain pure silver.Concentrated oxalic acid is poisonous,please,use NIOSH/MSHA approved respirator,NIOSH approved gloves,splash goggles for eye protection and wear appropriate clothing to prevent skin exposure. Read MSDS for oxalic acid before start the process.
The process is quite simple,just make a 5%(weight or volumen) solution of oxalic acid in tap water,heat this solution to 95 C,then submerge the films in the acid solution,in 20 minutes all the silver is removed.Once that all silver has been removed rinse the films and let them dry,you can sell the plastic base to a plastic scrap dealer.
Let the mud settle down,pour off the liquid solution,dry the mud and melt it,you will get 0.999 silver." Juan Manuel Arcos Frank, author