Oz said:
The only difference in a cell that I can see that could mitigate this is that you are keeping a fine balance or equilibrium with your free nitric. Do you notice a significant difference as to how much of these are left in your anode bag compared to a regular nitric digestion?
While I never assayed my silver, it was re-dissolved for making new electrolyte, so if there was any contamination present, it was obvious. Silver nitrate that is pure and has been permitted to exhaust all traces of fumes, will be almost colorless. The slightest hint of green or brown is an excellent indicator that there is a trace element present. Generally, if I found anything, it was copper, but on one occasion, one whereby I had allowed way too much palladium and platinum to accumulate in my silver, I co-deposited a trace of palladium. It is obvious when it happens, if by no other means, by the color of the electrolyte. When it shifts to a dark green, you've gone well past an acceptable level of contamination.
Do keep in mind, I added NO copper to my electrolyte. I had more than enough experience in running without----and had proven that it is not essential, although it did provide for a crystal growth that was much easier to knock down. Silver from my cell, when it was first started after an electrolyte change, would deposit in a film instead of growing crystals. It didn't take long for the copper level to escalate, then crystals would develop instead. With low copper content (in the electrolyte) they were quite rigid----but as the copper concentration grew greater, they grew softer and softer, finally, about the time that copper would co-deposit, the silver grew as hair.
I ran my cell this way for years----and got consistent results every time. If you get familiar with the characteristics of any given cell, I'm of the opinion that you can fairly well judge the quality that is coming off, and know when it's time to change electrolyte.
Everything I read indicates industry doesn't do that, they, instead, change a portion of the electrolyte constantly, to keep the copper at an acceptable level. I found it much easier to simply make a wholesale change. That works fine for a small cell such as mine.
While discussing PGMs it is my understanding that Pd contamination of the electrolyte becomes critical at lower grams per liter concentration than the Cu. At what concentration of Pd would you become concerned with co-deposition?
I can't address that in terms that are meaningful-----but if you manage to co-deposit palladium, your electrolyte will have long before changed color to the point where you'd be concerned. I would not dwell on that issue unless you have a huge amount of palladium mixed with your silver.
As I do not have practical experience running a silver cell the above are assumptions on my part, by all means correct me.
One thing that is good to know is that it is not a good idea to allow too much gold, platinum and palladium to accumulate in your silver. When I processed the values that were recovered from my waste materials, I experienced that very thing.
I had used my stock pot as a method of saving traces of the pt. group metals, which were commonly found in small quantity in the waste materials from jeweler's benches. The amount was usually way too low to precipitate (remember, they must be concentrated to precipitate), so I just dumped all traces in my stock pot. I also allowed silver chloride to accumulate in my wastes, with the idea of the silver acting as a collector to recover the values mentioned. Turns out that worked exceedingly well, but the doré contained so much of the pt group that it was troublesome in parting in the silver cell. I had to literally scrape the sludge off the anode, often with considerable effort. That lead, in one instance, of depletion of my electrolyte, the silver no longer being able to leave the anode.
When you have a low level of concentration, your anode will shed the values easily, which can be easily scooped from the basket using an acid dipper. That was my procedure, but it requires that you use a filter that has good integrity. Too light of weight allows the fine particulate matter to go through the filter material. The product I used was made expressly as filter material, and had one side fuzzy, which trapped the fine particles. It was made of cotton, so it was easy to incinerate when it was changed. Being contaminated with silver nitrate, it burned readily, even when wet, and reduced to an ash that was easy to digest with nitric, to recover the residual silver and palladium. Most of the platinum and all of the gold would remain behind as a solid.
Harold
