Lou said:
Harold, please do post up a procedure. I too am interested in accumulating and processing dental wastes, along with other medical wastes.
I would like to see how you did it.
Only on one occasion did I pursue the method, because of the sheer number of items included, and lack of time to be able to sort by hand.
This is nothing that you wouldn't already be aware of, Lou. A basic dissolve and precipitate procedure, using only submitted material. The purpose is to reduce the amount of base metal so cleaner values can be precipitated.
After roasting all the items, and mechanically removing all teeth (hammer on an anvil of sorts), all of the items were combined in one vessel, to which I added a limited amount of AR & (tap) water. There is immediate action as the lot is heated. When action ceases, I insure that all the nitric is consumed by adding a trace of HCl. The material is boiled at a low heat, and observed. The remaining base metals slowly precipitate all of the values, reducing themselves in volume. When the solution tests barren, it is dilluted with tap water, and allowed to settle well, decanted, then the process is repeated. It's obvious when values are present (by color), although testing with stannous should be routine, to insure there are no losses. The solution, when barren, shifts from a dark brown color to blue/green color.
As the base metals are consumed, the solution will take longer to shift color, so the operation should be taken in small increments. That insures that the base metals can be eliminated instead of accumulated in solution. The final process will have dissolved all of the values, leaving only a small amount of base metal to be included.
The procedure is time consuming, but requires almost no attention, so it's a good way to eliminate the bulk of base metals instead of hand sorting. It has no other redeeming qualities, near as I could tell, aside from not removing the platinum group prematurely. When enough is present, it can be recovered directly after precipitating the gold. My logic is that gold is far easier to wash than the salts of the platinum group, plus gold is generally the predominant value.
Great care should be exercised when decanting, considering the values that are precipitated are simply tiny particles of black material and easily transfered when decanting.
I avoided including any amalgam. The tin that is found in amalgam complicates filtration, as you know. If, by chance, any is included, if the material is well oxidized prior to dissolution, my experience in handling amalgam indicates that you can improve filtration procedures immensely. I always oxidized amalgam after it was removed from the retort for that reason.
For the record, my normal procedure for processing dental waste was to inquart, after hand sorting. The majority of dental gold (that I experienced) was yellow, not white, and little of what was white was platinum or palladium, most of it being high temperature alloy, with no values included. I wanted to eliminate as much of the platinum group from the gold as I could, because of drag down. Palladium, when mixed with gold, is a particularly tough one to eliminate on the first refining. Considerable drag down is experienced, and it does not wash out well. It's presence is quite evident by the dark color of the gold powder, even after prolonged washing.
Inquartation removes the vast majority of the platinum metals when you use silver, which I did. Because it enables platinum to dissolve in only nitric, even that is almost totally eliminated in the initial process. Silver becomes an excellent carrier of those metals, and concentrates them such that you can effect a good recovery when the slimes from the silver cell are processed.
Rarely will traces of either platinum or palladium precipitate from solutions, and it's not worth the effort to evaporate to concentrate the solutions so they will. Nothing is lost---it is simply stored for the future.
I'd enjoy reading your comments, Lou. They need not be complimentary----just factual. This is a good opportunity for readers to learn a process that is normally discouraged, but can be used to great advantage under the right circumstances. All pitfalls should be discussed.
Harold
