PhillipJ said:
On rare occasion, when I'd re-refine, I'd have a touch of nitric still left in solution. The gold came down fine, but when it came time to wash the gold (with HCL), I'd get the slightest of dissolution. Stop washing after allowing it to boil for a brief period (to consume the nitric with the precipitated gold) and then ice the solution
I had this happen to me, on my 2nd refinement, 1st rinse, with HCL and water. The solution turned a little yellow. I precipitated it in the normal fashion and did not put it on ice, because I didn't know.
The ice addition may not be needed in your case. I precipitated with SO2, and know from doing so that the precipitation process is exothermic in nature. It wasn't uncommon for my solution to get hot enough to slow down, or even stop the precipitation because the heat wasn't being dissipated. By cooling the solution, that didn't happen. Often times I'd add ice to a solution that had quit precipitating, and would achieve an almost instant reaction, which I attributed to the cooling. If you add your precipitant to your solution by mixing it with water, could be you provided enough of a heat sink to do the same thing. Hard to say. I've been away from refining for a long time now, and I used the same system for years, so the other methods are sort of a stranger to me now.
Is there any reason that the 1st rinse couldn't be done in plain water to avoid this?
My initial reaction is to say that it might be a waste of time, but maybe I'm wrong. The purpose of washing in acids is to remove undesired substances, which we both understand. Water in and of itself would be rather poor at doing that, but the fact that there's a trace of nitric remaining may be enough of an offset. I really don't know-------but my personal choice would be to stay the course and wash with HCL. I did that in practice, although I'm not saying it's the best way to go. It worked for me, so that was reason enough to do it.
Also. Does it really matter if the rinse is with HCL or nitric. As long as the end result is clean.
That's a reasonable question------one that will answer itself if you give it a go.
Talking about eliminating the chlorides is a lot easier than doing it. The slightest trace of HCL in your solution when you introduce nitric will be no different than a trace of nitric when you wash your precipitated gold
with HCL. In either case, you'll dissolve some of your gold.
Assuming you can fully eliminate the chlorides, from my vantage point it may be every bit as good as HCL-----until the drying process. If you don't force dry the gold, it will retain enough nitric to burn your skin. It's a lot less forgiving of being handled than is HCL.
The point I've tried to stress right along is that unless your thing is experimenting, there are tried and proven methods to achieve the desired end. As long as you have the necessary chemicals at your disposal, there's no good reason to deviate from accepted practice. If you're a curios type by nature, and like to experiment, one of the things that you should do before getting too creative is to read a Merck Index and understand the compounds of gold and silver. Getting creative with either of them can lead to an early death.
In Utah, where I lived, a person with a chemical degree managed to blow a wall out of a lab by experimenting with a gold solution. I don't recall the details now, for it was about 30 years ago, but that was reason enough for me to stick to the book. Not being educated, I simply don't think I'm smarter than those that went before me.
I've posted this picture before. It is a portion of the last lot of gold I re-refined just prior to selling my refining business. Note the color. Also note that it required no pickle, and was not melted under a flux covering. I know, beyond a shadow of a doubt, that my method worked----and worked very well.
No------I make no guarantee that it is 9999. That number is difficult to achieve----which is the point I've tried to make right along.
Harold
Edit: added words for clarity
