First of all, let me apologize for, perhaps, making a judgment too early. I presently have no lab to test this process. So, let's assume that I was too hasty and let's start from scratch.
I recognize the fact that you know more about silver recovery than I.
Don't sell yourself short. You know a lot more about using oxalic acid than I do.
(1) Even the very slightest possibility of forming silver oxalate would prevent me from using this process, especially since the caustic process works so well. I would have to absolutely know that the conditions would be such that it would be
impossible to form silver oxalate. I find it difficult to think in anything but large quantities. Melting would be done in about 4000 oz quantities. If any silver oxalate were initially present, the results could be disastrous. Here's a guy that has played with this: It's not very good, but it's a start.
http://www.powerlabs.org/chemlabs/silver_oxalate.htm
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.
(2) Are you suggesting using the respirator when just mixing the chemicals or, also, when operating the process? I hate those damned respirators when running a process. I know they're not necessary when using weak caustic soda.
(3) Concerning the costs of a caustic soda operation. About 200 pounds of caustic will process 10,000 pounds of film. At about $.25 per pound of caustic, that is a chemical cost of $.005/pound of film. Also, the silver bromide on virgin film was easily converted to Ag2O. What is the chemical cost of oxalic acid, per pound of film? When running film, you have to count your pennies.
(4) Concerning the mylar problem. On certain x-ray films (I can't remember which - it may be only Dryview), a very thin film of transparent mylar is applied on one side of the PET plastic - you can't see it. If this is not removed, the scrap value of the plastic is nil. When selling it, it may take a while for the buyer to catch on, but contamination of mylar will come back to haunt you. Been there - done that.
After stripping everything off and rinsing and drying the plastic chips, the presence of mylar (and dirt, in general) can be tested as follows: Fill a small, shallow, heatable container (we used those standard disposable aluminum weighing dishes - they are about 1/2" deep and 2" to 3" in diameter) about 1/2 full with the clean plastic chips. Heat slowly on a hot plate until the plastic is completely melted (about 500 F). Don't overheat or you will burn the plastic. The melt should be a uniform blue (most of the plastic has a blue color - not all) with no evidence of black particles (mylar, silver bisuthate flakes, or dirt).
NaOH process forms a black colloidal solution of silver which is impossible to filtrate and never will precipitate so you have got to break down the colloidal solution by heating,stirring and settling down for a couple of hours
(5) Who cares? That's not a problem, especially when running large quantities. Usually, the larger the volume, the longer it takes to fill the equipment "pipeline." We were running 50,000 to 80,000 pounds per day. A batch spent about 1-1/2 days in the system, from film to bullion. Our settling times fit easily into this timeframe.
(6) The medical x-ray film scrap industry is in chaos in the US. Dryview has made huge inroads. It is developed by heat instead of by chemicals. To test for Dryview, hold a lighter under a transparent area of a piece of film (the border, e.g.) - with Dryview, the heated spot will turn black. Also, purely digital x-ray equipment is becoming popular - the x-rays are viewed on a monitor - hard copies are only generated when desired. Regular rare earth film scrap is becoming rarer and rarer. In less developed countries, the expensive equipment needed for these other technologies makes their presence less probable - rare earth film is still widely used.
On the other hand, litho film scrap is still alive and kicking. When I ran film, litho made up about 35% of the total volume of all types of film. It comes in 2 thicknesses - 4 mil and 7 mil. About 98% of all litho film is 4 mil. Litho film (4 mil) contains about twice as much silver, per pound, as medical x-ray.
(7) Is the gelatin
totally dissolved? In my experience in working with other acids is that it never is totally dissolved. A pound of developed rare earth x-ray film only contains about 1/10 of an ounce of silver, on the average. That's about 1/3 of a cc. The bulk density of the silver powder would be a somewhat less. How much volume of stuff do you get from a pound of film? I repeat - with any residue of emulsion, the sludge will have to be incinerated before it is melted. I am talking about when running large amounts of film. It is a matter of the capacity of the furnaces and of the needed control of the horrible fumes produced when burning the emulsion. It's easier and cheaper to control this with incineration than it is with melting, Also, the carbonaceous emulsion provides a voluminous, variable, additional fuel, which is difficult to deal with in a melt.
there exists a tremendous profit potential for recovering silver versus other precious metals
I totally agree, Lou. The last time I looked, there was as much silver scrap out there as gold scrap, dollar-wise. Also, there are probably 50 times more gold refiners than silver refiners. I love silver. Not because of its beauty, but because it paid the bills for a big part of my life. If someone wants to start another thread, we can discuss this in more detail.
