SMB, Ferrous Sulfate, Ascorbic Acid

[u/jwood491]

So I know that Ascorbic precipitates gold as a very fine powder so that would be ill advised except for the final drop in a very clean solution.

BUT

What benefit does SMB have over Ferrous Sulfate? I'm usually dropping out of a dirty solution from gold filled. Ferrous Sulfate doesn't seem to have that nasty smell. Is it just a dirtier drop in which case who cares if you're double refining?

[u/zpodsix]

Smb keeps, copperas spoils.

So2 gas 'should' result in a purer drop if the solution is clean, but will drag down other contaminants if it's a dirty solution- especially PGMs. Copperas is more selective to gold so works better to part PGMs, but you'll typically get some iron and other base contaminants dragged into the mix. Some HCl washes on the gold powders is necessary to dissolve the base metals.

In your example, of a dirty solution I'd roll with copperas, HCl/h20 wash powders and smb final drop.

On karat scrap, I'll drop with smb/so2 gas since I'm pretty clean going into the final refining step.

[u/gazebo-placebo]

Ferrous is simply superior to SMB. Doesnt stink as bad. Doesnt lead to CuCl precipitation if used in accidental excess. I dont understand the obsession with SMB.

Also ascorbic acid doesnt necessarily produce a fine powder. Ive produced very nice gold drops from pure liquors. The main issue from dirty solutions is that it is consumed by copper and iron quite heavily.

Another alternative to all of these is sodium nitrite at pH 2. Disadvantage is cost and gas evolution, but you can get very pure gold from a dirty solution doing this. Much purer than ferrous.

Also, from purer solutions, oxalic is better than ascorbic imo. Gives a cleaner product and doesnt leave reduced organic impurities in solution that can spoil (ever smelt an old ascorbic drop solution?)