Filtrate evaporation

[Oz]

I have been putting gold foils and gold from inquarting and nitric into solution with HCl-Cl and dropping with SMB. I always test with fresh stannous to make sure I have a complete drop but have still saved the left over solution from filtration. I used triple coffee filters and it was always clear in color and of visable particulate. I had a couple of gallons of this set aside and now noticed some black gold precipitate on the bottom of the jars.

Does anyone know how sensitive stannous is in a HCl-Cl solution with SMB for detecting gold? I was surprised to find additional visable precipitation before evaporation.

I took 2 quarts of this solution and evaporated it down to about 1/5 the volume at just under a boil. It started off clear in color then at about half volume had a slight shade of green and stayed that color until I turned off the heat. I left it to set overnight to cool and in the morning it was a yellow color just like auric chloride and some clear crystals had formed in it interspersed with some precipitate that looked to be black gold. I tested the solution again with stannous and it showed clear. I even checked my stannous against a control solution of auric chloride. I am glad I saved my “waste” solution, there wasn’t much precipitation but enough to make it worthwhile.

I plan on adding a little water and heating to redissolve the crystals to filter out the black precipitation and save. I am curious about the chemistry of what happened when I evaporated this down if someone can explain it, such as what should have evaporated out of solution and what would have concentrated. I am assuming that I evaporated out all the free chlorine and that the clear crystals are probably sodium chloride and the green going to a yellow after cooling? I am not sure of this and it would be nice to understand the chemistry that is occurring with the evaporation of this solution a bit better.

Thanks,
Oz

 

[lazersteve]

Oz,

I seem to remember the accuracy of stannous to be in the neighborhood of 4 ppm (parts per million gold in solution).

The gold powder you are seeing is most likely appearing from settling after you filter through the coffee filters. Coffee filters never get the really fine stuff. This is why I don't filter my precipitated gold powder. Instead I let it settle over night in the container in which it is precipitated, then I siphon or carefully pour off the barren solution.

I rinse the powder several times with water to get all the SMB laden liquid out of it (until I can't detect the odor of SMB on the powder). Then, in the same container which it was precipitated in, I boil it in HCl for a while to remove any base metals present from drag down. Let settle again then siphon or pour off the dirty HCl. A few more water rinses then onto hot ammonium hydroxide (if silver was present in the original material), and finally a few more clean tap water rinses.

As for the chemistry of your stock pot I suspect the color changes are from residual copper in your solution reacting with both the chlorine and SMB. The salt is likely just table salt (sodium chloride) if crystalline, or copper I chloride if in the form of fine powder. The black is likely just gold that made it through your coffee filter set up.

One last note on the stannous test. When the gold concentration in a solution is very low, the stannous will have a delayed effect of showing the purple color (almost light gray at this level) of a positive gold test. I've seen this delay from several minutes to nearly an hour later for the barely visible purple spot to appear. I suspect this effect is due to the stannous drying out for the spot to appear.

Steve

 

[Oz]

Steve,

I would not have been surprised if it was all just very fine black particulate settling but there were some to large too pass the filters unless I somehow got sloppy. I do always let precipitations settle overnight and sometimes even days (life gets busy). I don’t siphon though but do decant carefully.

I also go through multiple wash procedures, it is simply amazing what you can get out. At 4 ppm with stannous I would be happy, I will just have to do the next couple batches extra carefully to try to see if I can find were it got through. I have seen the delayed stannous results as well, sometimes not seeing a reaction on the paper until the next morning.

Seeing the green made me think copper as well but turning yellow after cooling threw me. I don’t know about the sulphur surviving the approximately 100C evaporation temperature. The sodium from SMB and chloride from HCl made sense for sodium chloride. Not knowing the replacement reactions for these mixes or what I eliminated with 100C makes it tough.

Thanks again for you input.

 

[Lino1406]

Tried to prove directly it contains gold?

 

[Oz]

Not yet Lino, it is still sitting just as it was when I started this thread. Life has gotten too busy. But since you asked I will try to get to it this week. I have to do a test run of purifying some sulfur before the weekend anyway. My girlfriends kids are expressing interest in chemistry and it is something that gives instant gratification without using strong acids. Surely the next question will be what do you do with pure sulfur. Perhaps a little propellant is in order.

Oz

 

[Lino1406]

After and above any other test,
the ultimative test is dissolving
the suspected gold in cyanide
and - trying - to color a Zinc stick

 

[Oz]

I haven’t found the need to play with cyanide. I will just heat and dilute the solution a little then filter. I will then take any particulate into HCl-Cl and test with stannous.