Selective Precipitation. continued

[necromancer]

i couldn't help the fact that my interest in the questions & answers of selective precipitation were strong enough to restart the thread.

the point of this thread will be "Selective Precipitation"

i hope we can go on with the questions & answers

"As we express our gratitude, we must never forget that the highest appreciation is not to utter words, but to live by them."
"John F. Kennedy"

thank you

Dave Clarke

 

[Lou]

Do you mean the selective hydrolytic precipitation of precious metals, which is I think what Aristo was after in the Hindenburg thread?

If so, refer to the work done by Gilchrist in his now famous and/or heavily used procedure that I've so very kindly attached

Now there are other methods besides hydroxides that can be used; sulfides, halides, organics, thiocyanates (I guess a psuedohalide). At this point it is where it becomes "what's in the solution and what's the best to use to get the results I want?" and for that you need an intimate knowledge of what's in solution, your target, where you're going with it, and what you need to leave behind.

It's the refiner's job to try and make the situation to apply a given method or procedure as close to the same as possible every time. Stray far afield from the familiar and the wolves come out! Hence we see the incineration, milling, melting into copper, selective dissolution and all the other reductive, "consolidative" technologies applied in the recovery before the refining

Iterative concentration, my friends!

 

[necromancer]

thank you Lou

 

[Gratilla]

What is less well-known is that selective precipitation can be effected not only by pH manipulation, but also by ORP (Oxidation Reduction Potential) manipulation. To fully appreciate this a Pourbaix Diagram (PD) is invaluable. A set of PDs covering gold and the PGMs can be found in Lashley's SSN PDF, available at a number of locations on the forum. PDs plot pH against Eh for target elements/compounds to produce areas of pH/Eh conditions where the target is stable. Moving outside of these conditions changes the equilibrium for the target. Looking at the gold PD, for example, will show Eh/pH values under which AuCl3 is stable and an Eh value under which gold will not go into solution.

While pH can be manipulated using acids and alkalis, Eh can be manipulated with oxidizing and reducing agents. Thus adding sufficient, say, oxalic acid (a reducing agent) to a solution of AuCl3 (obviously above the Eh threshold) will drop the Eh sufficiently to bring the gold (as a metal) out of solution as a precipitate.

 

[Lino1406]

Very instructive Lou (as always)