jamthe3 said:
My understanding is that he eliminated quite a bit of potential hassle by sometimes incinerating again after a first run;
That's correct. When you process material that has a lot of "dirt' included, the end result can be, and often is, a solution that refuses to filter. I got baptism by fire in that regard, although my learning came not from dirty material, but material contaminated with dissolved tin. A liter of gold chloride took well over a day to filter, time that I could ill afford. From that I learned that I had to do something to eliminate contaminants that were troublesome. In this case, I don't recall that I read it in Hoke. It was something I tried on my own. Secure in the knowledge that anything I had already processed with nitric (which was pretty much my routine) would cause some of my gold to dissolve if I tried a wash with HCl, I tried incinerating the material a second time. The results were very acceptable.
The second incineration was also very revealing. What I learned is that in spite of my material being bone dry, it still contained a considerable amount of nitric, witnessed by the brown cloud coming off as the material was heated. It is for that reason I recommend all materials be incinerated if you wish to make a change of acids mid stream.
I continued to heat until the material no longer liberated signs of acid, then cooled and screened to insure there were no clumps. I followed that with a boil in HCl and tap water, resulting in a much improved material. Rinsing and decanting until the rinse water was free of color, I than dissolved the values, which filtered readily. That process became my routine, and saved far more time than it wasted, plus yielded a better quality product in the bargain. I recommend the process highly.
I then tried the same thing with floor sweeps and polishing wastes from a jeweler. I had previously boiled in nitric, in an attempt to recover traces of silver. By now, it was obvious to me that I was far better served to leave the traces behind, and pursue the material with HCl instead. Again, the end result was worth the effort. Filtration problems came to an abrupt end. Any silver that was left behind was recovered later, when the waste material was processed in an agitation tank with cyanide. By then, having been boiled in HCl, a great deal of it had been converted to silver chloride, making recovery very easy. In short, my routine became one of boiling in HCl only, never nitric, for floor sweeps and polishing wastes.
So then, if you are working with material that is troublesome, perhaps having particulate matter that may be of concern, try incinerating a second time, after the nitric digest (or the acceptable alternate method), then boil the material in HCl and water. Rinse, then pursue the values. I think you will be pleasantly surprised that the added effort will reduce handling time and reward you with an even better product.
and, I believe, he generally processed twice when refining Gold prior to melting (I don't think final products are smelted.) The in between incineration, I would assume, helped rid the powder of impurities and such. I doubt he ran into a whole lot of headaches accidentally producing AR at least.
To clear any confusion, I did not incinerate my precipitated gold. I incinerated only materials that were in process prior to dissolving the values for eventual recovery. I did this to enhance their ability to filter, and to rid them of unwanted substances that did not get eliminated in the preliminary digest. It worked both ways. I incinerated materials that had come from nitric, so they could be further processed with HCl before recovering values, but I also incinerated materials that had been exposed to HCl, when I wanted to do a second digest with nitric.
In both instances you can rinse well enough to reduce, or eliminate the risk of prematurely dissolving values, but it's a crap shoot. How many times is enough is not known, and is sure to vary, depending on the nature of the material in question. By sharp contrast, if you incinerate until the material has been well heated, you are assured of success with no regard to having values dissolved by the succeeding wash. The call is yours. I looked at incineration as my insurance. I have tried to help others understand the value of the process.
Harold
