How do metals dissolve in acids?

[bswartzwelder]

If you have dissolved base metals along with PM's in an AR solution, you want to precipitate out only the PM's. I know this is impossible or next to impossible. You can use selective precipitants like copperas, or oxalic acid, but some base metals will still be present.

So now you have a sludge containing base metals and PM. There is no way, that I'm aware of to get rid of the base metals. If you put everything into fresh HCl and start adding HNO3 drop by drop, will the metals redissolve in any particular sequence? If not, what is the best way to deal with this mess? Do a second or even a third drop each using a different precipitant? If so, what would be the best way to accomplish this? What precipitant would you use first, then second, next?

 

[rewalston]

Some base metals dissolve in just HCl by itself. As we all know gold will not dissolve in HCl, so with repeated use of HCl and water it will remove the base metals. If you incinerate your powders that did not dissolve (may contain gold, silver and pgms) to red hot. You can then use nitric acid to remove any other base metals. Then I would incinerate again, and then go on to AR to get the pms. Please note, I could be wrong with some of the sequence.

Rusty

 

[Anonymous]

There are a number of steps you can take to remove base metals from AR prior to dropping the gold. These steps are all over the forum in all kinds of threads.

Yes you can get drag down when precipitating, and yes sometimes you need to remove copper, but as far as tin, lead, and others are concerned the information is here.

 

[martyn111]

By removing as much basemetal as possible during your recovery phase, you will have less possible problems (contaminants) during your refining phase, follow this with Harolds powder washing process (use the search to find Harolds washing technique) and the resulting end product will be much cleaner.

 

[Lino1406]

The only 'base' metal copperas will precipitate is copper

 

[Geo]

I know that it's not good practice but I have dropped gold from solution that was black. There's really no problem as long as you understand that the precipitated powder will be contaminated and will most likely need to be refined again and maybe even a third time depending on how pure you want it to be.

 

[rickbb]

I've followed the advise I've learned here to do just that.

Dissolve PM's and base metals with AR, drop the PM with one precipitant, then re-dissolve the precipitated powder again and drop with a different precipitant the second time.

If you use a good washing method on the dropped gold each time, such has Harold's, with this twice refined method you can get near pure gold out of a nasty looking muck quite easily.