Non-Chemical Precipitation for newbs

[4metals]

This has been hashed out here many times before but repeating it can’t hurt. Silver chloride is soluble in aqua regia. But not highly soluble. And solubility is a function of temperature. The colder the solution is before you filter it, the more of the silver chloride will become insoluble. If the solution is even moderately warm silver chloride dissolved in the solution will pass through the best filter paper because it is in solution. Chill it, and it becomes insoluble and the filtration will catch it. I tell refiners to add ice before filtering and that usually eliminates high silver in the final product.

One of the pieces of equipment I advise refiners to acquire is an ice maker, they are often available used at auctions. A 5 gallon pail of aqua regia loaded with metal needs a gallon scoop of ice to do the deed.

 

[goldenchild]

I don't know why hydroxide would be used in Au precipitation (especially in dirty solutions) but I can speak on using urea. I also won't discourage anyone from trying/using it if they can make it work properly. That being said, I've used urea many many times. For me the key was keeping the solution cold. Adding ice to a crystal clear solution and then adding urea in small increments until there was no more reaction. I used urea in prill form and this worked in both dirty and clean solutions.

An observation I made is that you definitely don't want heat when using urea. I once, for one reason or another heated a solution that had the gold precipitated and contained urea. I either did it out of curiosity or to get the gold to clump up. Either way what happened was that AR was formed once more and digested the precipitated gold. This speaks to what others were saying throughout this thread about urea not eliminating nitric but just changing form. So theoretically you could use urea to actually denox. The same can be done with SMB but too much SMB creates other problems. But no need to denox if you use the urea properly. End rant.

 

[Martijn]

Thanks guys. I think thats it. And 4metals, it's always good to repeat basic information like that. As much as i've read, the fine details of refining have to be witnessed to understand and remember. Practice makes perfect. At least for me.
The silver must have come from the first button. It was my second one from some time ago when i did not cool with ice before filtering. So ruling out silver was my mistake. Ican remember another thread where someones button had the same triangle pattern. Palladium or platina was suggested as the culprit. Can silver cause that aswell?
Anyway, cold filtering left a beautiful clear dark yellow juice. The silver chloride turned dark in the sun. Now to washing and maybe a third AR bath. SMB drop went by the book. (As far as I know)

But theoretically: Can gold push out PGM'S like copper pushes out gold? Not that i have any....apparently.

Martijn.

 

[jamad46]

I have been putting some CPU chips in a dilute nitric bath to get the gold foils off. That is working fine but I also get a lot of mud ( at least that what it looks like ). Should I include this mud in aqua regia or try to separate it from the foils? Thanks for any help.
The Newb

 

[eshane211]

Raising the pH of Ar to around 2.8 (meter reading of 2.5 if I recall correctly) makes the SMB more selective for gold. I've tried it, it works and I mentioned it a few years ago on here.

Edit: It's important to note that I mean denoxxed AR and I don't use Urea.

That given It's not something I do as a rule because I can clean my gold well enough to avoid this step.

It takes one heck of a lot of base to raise a pH 1 solution to pH 2. That I know from pH balancing for Formate precipitation of Pd.

Can you please explain your formate process for precipitating palladium?

 

[Yggdrasil]

Can you please explain your formate process for precipitating palladium?

Anachronism is not here anymore, he left of his own free will a few years back.
There are formic acid reduction in Hokes book and there is several posts in the forum.
I can not remember where though.
Try a search for formate reduction and Palladium in the search parameters.