Free nitric problem?

[artart47]

Hi everyone !
I ran into a little problem. I think I figured out what I did wrong. I thought it thru and have a plan of action but, just wanted to get your comments before I start.
The source of this gold was from caret jewelry and some e-scrap. It was melted into two bottons of still impure gold. they had pipes but, were still in need of a second refining. total weight was 52 grams. I don't trust keeping AuCl or gold powder around too long at my place, that is why I melted it.
I covered the 52g. of gold with HCl and over about a week added small amounts of 67% nitric acid. It was kept in a water bathwith just a small amount of heat. The AR reacted withe the metalic gold very slowly. the last small piece of gold took about three days to go into solution.
I now had a beaker with about 300ml. of dark red AuCl. I was very carefull adding nitric and diden't add any more after the third day, but I thnk I may have gotten too much.
Ice cubes were added and brought the volume up to 800ml. It was filtered thru six coffee filters multiple times untill it was crystal clear. I added 51grams of metasodium bisulphite and the powder drop was immediate.
After about 36hrs. I syphoned off the solution, it was clear like water. I did a distilled water rinse and combined the water with the smb solution.
Now, the problem! When I covered the powder with 32% HCl to begin the "Harold wash", the solution began to look like AuCl and gave a very strong stannis chloride color. I tested the spent SMB/rinse water and got a negative result, but when I added a piece of copper to it, bingo! I believe it's using free nitric and is cementing black gold powder.
My plan is to heat the gold powder with the HCl/AuCl untill any free nitric or per-haps there are nitric salts in with the powder, are used up disolving some of the gold powder. Then decant the solution, do a distilled water wash ( boil ) and combine the two rinses and cement with copper.
From what I've learned over the years here, that should allow me to recover all of the gold that is in the solution and in the spent SMB and take care of the free nitric/ posible nitric salts.
Never had the process go like this time. Gold always vigorously reacted in AR ! Could the bottle of HNO3 be going bad?
If anyone has any advise or coments on my plan of action, I really respect your advise! What i'm enjoying now would not be posible without all of your time and help!
Thank you! artart47

 

[Harold_V]

Now, the problem! When I covered the powder with 32% HCl to begin the "Harold wash", the solution began to look like AuCl and gave a very strong stannis chloride color. I tested the spent SMB/rinse water and got a negative result, but when I added a piece of copper to it, bingo! I believe it's using free nitric and is cementing black gold powder.

I think I understand what you've intended to say, but lets make sure.

I got the impression that the solutions tested negative for values UNTIL you added HCl and started the wash, at which time the solution took on the appearance of gold chloride. Is that correct? If so, did your stannous chloride every yield a positive result? I ask because you may have been testing with a spent solution. That's the very reason why you should have a standard gold solution at your disposal, so you KNOW your stannous chloride is effective.

If you tested with stannous and got negative results, yet copper cements something---you have an issue with your stannous.

My plan is to heat the gold powder with the HCl/AuCl untill any free nitric or per-haps there are nitric salts in with the powder, are used up disolving some of the gold powder. Then decant the solution, do a distilled water wash ( boil ) and combine the two rinses and cement with copper.

Assuming you did have residual nitric, I heartily approve of your plan, although you're missing the boat on recovery. After you've driven the material to a low boil (keep it covered with a watch glass), all of the gold should have dissolved that's going to. You couldn't have had much free nitric present, otherwise you've have not achieved the rapid precipitation you experience.
Needless to say, you'll have to pour off the solution and rinse at least two times to remove residual gold chloride. Alternately, there's no reason why you can't simply precipitate in the same vessel. If you choose to do that, stir the gold well, so any trapped gold chloride will get exposed to SMB. You'd do this after the solution has cooled, by the way.

That being said, if you choose to not do the above, why don't you simply keep the wash and rinse solutions clean, then precipitate with more SMB in a separate vessel? There's absolutely NO reason for you to have to use copper, nor is there a benefit in doing so. In reality, it's taking a step backwards.

While it's not common, you can entrap some gold chloride in the resulting gold if you don't use enough precipitant initially. The resulting gold drags down a small amount of gold chloride, which may not rinse out until you've brought the solution to a boil. What you experienced may NOT be attributed to too much nitric, but I can't say with certainty that that is not the case.

You asked about nitric going bad. To my knowledge, no, it does not. I brought with me a small amount of nitric. I last refined in '94, so my at least 19 year old nitric is still very good. I expect yours is, too.

Harold

 

[artart47]

Hello Harold !
Thanks so much for your advise! Yes you are correct. That is what I was saying. When I did the first HNO3 addition and the first reaction came to a halt ( in the late evening ) I tested the AR solution at around five A.M. Stannis is about a week old but it turned deep purple almost black right away. So, I assume the test solution is good.
After all 52 grames of gold had disolved and I added the SMB, the gold dropped like a rock, two hours later everything was in the bottom of the beaker. The next morning I tested the solution. it was negative. I assume because of free nitric.
I decanted the liquid to a second beaker, added distilled water to the gold and heated it to a simmer for a half-hour or so and added that rinse water to the second beaker. then I covered the gold with HCl and was going to start heating it but it looked like it was becoming AuCl again. I left it for a few hours stirring it every so often and then I tested both the gold powder beaker and the second beaker. gold/HCl showed imediate dark purple but by the next morning the Q-tip had faded to white. The SMB/rinse water second beaker was negative. After sitting and thinking about it, I began thinking that I had too much nitric in the AR and and because I haden't yet thorouly washed the powder a little free nitric was left in the powder and that the bulk of it ended up in the second beaker.
I thought the nitric in the second beaker would explain the negative test and that if I added copper, the copper would use and free nitric and start cementing gold and would confirm my hypothesis. The copper did turn black and is shedding black flakes.
I'll follow your advise Harold! I can give the gold powder with the HCl a good long boil and then add SMB to it in the same beaker. When I get home I'm going to make some new test solution just in case, and retest.
Thanks again Harold! It's great to log in and see that you're still involved here on the forum!
artart47

 

[artart47]

Aditional observation this morning.

The second beaker has developed a pail blue color wich I assume would be copper nitrate in the solution. The piece of copper is black and has shedded a small amount of black flakes and powder.
artart47