Problems with SDT

[bogarasjoe]

Hello everybody! Please help me anybody, who knows!
I started gold refining 2 mounths ago from ceramic cpu-s. First time, I used AR. After filtering I lyed the solution with caustic soda, filtered. Auric hydroxide was baked on a hot plate. Then washed twice with nitric acid. Left for settle and cautiously sucked the clear solution from the top off with a syringe. The reddish brown, near colloidal gold was baked to dryness on a hotplate. It worked, but I found this too complicated, so I tried an other way: Sodium dithionite.
So cpus washed with HNO3, then soaked in AR, filtered, neutralized with urea and diluted. Than I added SDT. It seemed very very effective, few moments and a lot of gold praecipitated so fast, with yellowish brown colour. And it was easy to filtrate. I was so happy!
But... next day it dried. First, when it was dry, it was blackish. Hmmh, strange. Ok, I tried to smelt it (on graphite): it smooked, and smelled from sulphur-dioxide. And "gold" got a disgusting, black MUD. And of course, it sticked on the graphite... And I got really, really angry. Can anybody tell my why???? Why????
I've theory for it. Maybe, SDT was contaminated with Na2S, and maybe Au2S precipitated with gold. Am I right? It could make SO2 when smelting,and gold oxide leaves.
Maybe it could be contaminated with Ag, Pd, Cu, but I dont think, because I soaked cpus in nitric acid before soaking in AR.

 

[Irons2]

Sodium Dithionite will precipitate just about anything, besides, it acts as a chelating agent, so there are other side reactions that could cause problems.

Start over,your Gold is still there.

 

[goldsilverpro]

When you added the lye, what pH did you end up with? You likely precipitated the dissolved base metals as hydroxides.

If you really want to learn refining and are not just playing around, I would avoid both the lye and the sodium hydrosulfite (dithionite). In my experience, both cause more problems than they are worth. There are proven methods given on this forum that work. I would suggest you search and learn these first.

 

[bogarasjoe]

The PH was about 9-10 when I lyed it.
Ok, than should I use SMB? Or maybe copperas?
I think Oxalic acid is not good for me, because it mz solution could contain silver too, and silver oxalate can precipitate, its a sensitive explosive compound.
And what do you think about the Gold-sulfide theory? When I melted, it was smelling from SO2... and left a black mud (sulfides are often black).

 

[goldsilverpro]

The PH was about 9-10 when I lyed it.
Ok, than should I use SMB? Or maybe copperas?
I think Oxalic acid is not good for me, because it mz solution could contain silver too, and silver oxalate can precipitate, its a sensitive explosive compound.
And what do you think about the Gold-sulfide theory? When I melted, it was smelling from SO2... and left a black mud (sulfides are often black).

Finely divided reduced metals are often black also.

Were it me, I would use nitric first to dissolve the base metals and then I would separate the residue and dissolve it in AR, using a slight excess of HCl and a bare minimum of nitric, and then drop the gold with SMB. You could just dissolve everything in AR to start with but, with all those base metals in solution, dropping the gold becomes more difficult. In either case, I would not adjust the pH of the AR.

The trick in using AR is to only use enough nitric to get whatever you're dissolving dissolved, with no excess. That way, evaporation or the addition of urea is not necessary. This method has been covered many times on the forum. Basically, it consists of first covering the material with HCl and heating it somewhat. Then, the nitric is added in small increments, allowing the reaction to cease before making another addition. When an addition produces no reaction, stop adding nitric.

 

[bogarasjoe]

Thank yo so much everybody for so much excellent and useful information!I'll try the described method with the next CPUs, it sounds very nice. It's a nice forum!And gold refining is nice too .I'll recrystallize the contaminated gold too.
Abouut me a little: I'm a chemist student, I've worked with fullerene cocrystals about 2 years, and now we are starting at the institute a new project: metallic-organic polymers. And of course gold refining