# Unknown Contaminant



## Chondrule (Feb 5, 2015)

Hi folks,

I processed 395 grams of miscellaneous silver jewelry from the 60's and 70's (my mom cleaned out her jewelry box) using nitric and copper to do the precipitation. The shot I produced is silver in color like it should be and always is, however it's shiny, kind of mirror-like. The thing is, I then inquarted some gold with this contaminated silver, figuring the contaminant would remain in solution when I precipitated the gold with SMB, but I was wrong, the contaminant precipitated along with the gold, now it too has that shiny surface, although the color changed.




Concerning the gold shot, I tried a couple things to see what might happen to this mirror-like contaminant which seems to be mostly on the surface, but to no avail. I put some shot in nitric and nothing happened, I also tried melting some and adding sodium nitrate, again nothing. Strange that the contaminant would've been dissolved and in solution when I did the silver with nitric, then again in solution when I did the gold with aqua regia, both solutions having been filtered and crystal clear before precipitation. Now I get absolutely no reaction, yet the contaminant was in solution twice before, I don't get it..

There's a couple things which may or may not have meaning, both the gold shot and the silver are distinctly more difficult to melt than any I've done before, and they have a tendency somewhat to want to stick to my crucibles when fully molten and slipping and sliding around, in other words the molten beads get hung up in my ceramic crucibles, I never had these problems before and thought they might suggest contamination with palladium, yet from what I've read, SMB doesn't precipitate palladium out of aqua regia. Just to be clear, I never use my gold crucibles to melt silver, or for that matter, to melt any other metal other than gold, and the same goes for my crucibles designated for silver only, so contamination from the crucibles is out of the question.

Besides palladium, I considered platinum, but this didn't show up as a residue in any way shape or form when I originally dissolved the jewelry in nitric. I would think I'd see at least a trace of some sort of stubborn to dissolve metal in the sediment, whether it be palladium, platinum, or any other PGM, but there was absolutely nothing, all metal ended up in solution. Just to be clear, I didn't make shot out of the jewelry, I dissolved it as it was, simply cut it up as best I could and removed any iron such as springs, so I would think if one piece had managed to escape my notice and be made out of one of the PGM's, then I would expect that it would've been laying at the bottom whole, not affected by the straight nitric I was using, but nothing of the sort happened.

Just so you guys know, I use as little nitric as I possibly can, whether it be when working with silver and using only nitric, or when working with gold and using aqua regia at a proportion of 4 to 1. I always over calculate the amount of hydrochloric and under calculate the nitric. On top of that, if the under calculated amount of nitric ends up say for example to be 100ml, I use 40ml to start off, then go in increments of 50% of the last, thus the second addition is 20ml, always waiting till reactions subside before the next addition. Also, I never skip the addition of sulphuric, nor the evaporation step before precipitation, and I always filter and be sure make my solutions crystal clear. Just trying to give you guys some sort of idea that I've done this stuff before, with awesome results, until now.

It's interesting that whatever the contaminant is, it leaves the silver the same color as it should be, silver, yet the surface of the gold is turned orange, and by the way, the aqua regia was orange, not yellow or red, but right in between, orange. In the image you can see that the contaminant looks to be pretty much be on the surface, the biggest gold shot piece in the center has surfaces that are yellow as they should be, and surfaces that are orange covered. One last thing, when the gold was molten, there was no contaminants dancing around on the surface, the whole ball was exactly the same, like a mirror, they way it usually looks when I melt gold I've refined. Another indication that the contaminant is only superficial, thus only traces and not even 1% of the total weight, is that my original estimates concerning what I'd be ending up with where pretty much right on, the gold I got is pretty much exactly what I expected.

So I hope I've given a good enough idea of what I've encountered, does anyone have any idea what my contaminant might be and how to deal with it? Thanks.


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## Harold_V (Feb 6, 2015)

It would be my opinion that you are confusing a shiny surface with contamination. I do not expect there is anything of substance contaminating the silver, but may not be true of the gold. 

The color difference I see on the gold indicates to me that there is a small amount of some material that would be best removed. That it didn't show when it was melted, however, is a bit confusing to me. 

On one occasion I had a customer provide a small ingot of gold, marked 9999, which was contaminated, made obvious by the red hue of the surface. He wanted it combined with other gold, but I refused to do so unless it was properly refined. I found a small amount of platinum in the gold, although I'm not convinced that was the reason for the discolored surface. 

Try pickling the gold using a dilute sulfuric solution, heated. In fact, boil it for a few minutes, to see if there's a change in color. I'd suggest you try half the material, leaving the balance to use for comparison. That would likely be conclusive. 

Note that if you find a chance in color with the pickle operation, that will not have altered the general condition of the gold, as only that which is on the surface would have been removed. You can improve the quality with a second refining, simply by re-dissolving the precipitated gold powder, although doing so only after it has received a careful (proper) wash. Re-dissolving the shot you showed would work just as well, although it is somewhat slower to dissolve than powder. By doing so, you'll be able to test the solution for contamination after you have precipitated the gold. That's a good way to help make determinations of what you are leaving behind in the first refining. 

Harold


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## Chondrule (Feb 6, 2015)

Ok thanks Harold, I've read you reply at least half a dozen times and will do my best to follow exactly what you've said.

Concerning the silver shot I originally used for inquarting, interesting that it doesn't look to you to be something to worry about, so I won't. You've made me think, the mirror-like finish on both the silver shot and the gold, this shouldn't be considered any longer as an indication of the contaminant, the finish may just be a result of the conditions I'm producing shot under now, conditions which may for some reason be different than when I've done it in the past. I like your way of thinking, the mirror-like attributes of the shot should not have been linked to a contaminant and doing so only served to confuse matters.

So you've led me to focus on the orange discoloration of the gold, that that's the only reasonable concern I should have.

Question, what proportion of sulphuric to water do you suggest I use to do the test when I boil half my gold shot?


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## MarcoP (Feb 6, 2015)

I think 10% sulfuric would be sufficient.


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## Chondrule (Feb 6, 2015)

Hi Marco. I went ahead and did the sulphuric test without waiting for a response, figured if I was way off I could just start over and do it again. Amazingly I got it right, I had used 10% just like you're suggesting, I went with 10ml water to 1ml sulphuric, so thanks for the confirmation.

The result after boiling for 5 minutes, no reaction whatsoever, neither the gold nor the orange contaminant were affected.

So as of this moment I've moved on to what Harold suggested next, the gold is once again in aqua regia, this time without being inquarted, and once again the solution is already as orange as orange can be, so we'll see........


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## MarcoP (Feb 6, 2015)

My pleasure to be of help. I would personally use a different reducing agent for the second refine and then properly wash the gold powder and use an ammonia wash in between.


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## Chondrule (Feb 6, 2015)

Thanks for the reminders Marco, especially concerning the use of something other than SMB this time around. The thing is, SMB is all I've ever used and is all I've got to work with, so as soon as I'm done here I'm gonna go and order some sodium nitrite.

Question, is the reasoning behind not using the SMB again, is this because I'd be potentially re-precipitating the same exact contaminant I'm trying to eliminate, or does the contaminant have nothing to do with using a different reducing agent?

Question, what if I go ahead and use the SMB again (which I'm tempted to with maybe 1/4 of the solution simply out of curiosity), what might the consequences be?


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## Lou (Feb 6, 2015)

Nitrite can give fine gold, but very gassy, if I recall. That was a reason beside cost for my disliking it.


I would actually try HCl to pickle because the contaminant sounds like iron.

Good posts by the new guy!


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## jason_recliner (Feb 7, 2015)

Chondrule said:


> Thanks for the reminders Marco, especially concerning the use of something other than SMB this time around. The thing is, SMB is all I've ever used and is all I've got to work with, so as soon as I'm done here I'm gonna go and order some sodium nitrite.
> 
> Question, is the reasoning behind not using the SMB again, is this because I'd be potentially re-precipitating the same exact contaminant I'm trying to eliminate, or does the contaminant have nothing to do with using a different reducing agent?
> 
> Question, what if I go ahead and use the SMB again (which I'm tempted to with maybe 1/4 of the solution simply out of curiosity), what might the consequences be?



Q1: Yes. Each precipitant has its own characteristics and may co-precipitate, or drag down, different metals.
Q2: Just don't overdo it with the SMB. Start with less than you think you'll need. If you put in too much, you're more likely to precipitate the contaminants.


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## Chondrule (Feb 7, 2015)

Hi Lou. Interesting that you think iron is possibly my problem, I had considered that but given up on the idea since I'm just not good enough at this stuff. But now that you've suggested it, I've reconsidered iron and you just might have solved the mystery, although I have no way of proving it at this point since I'm still working on getting the metal into solution. It might be that I simply didn't do a good enough job at doing the water and hydrochloric washings before drying and making shot of the gold, which if that was the case, that's embarrassing since those are such simple steps.

Hi Jason. Thanks for your answers and input, you've given me ideas. No matter which precipitant I end up going with, I think I'll calculate the amount to use and then only use half for example, siphon off the solution and then use maybe 25%, siphon off the solution again and use the last of the precipitant. It sounds like a lot of extra work, but that's ok, maybe the contaminant, if any still exists after this second refining, will be isolated in the last precipitation.

Since I'm confronted at this point with having to use a different precipitant, that being sodium nitrite which I've never used, while reading up on what Hoke said about this precipitant, I also ran across what she says about only melting up a little of your material at first, if it looks like a good refining, then go ahead and melt it all. I've never done this before, but now I see why she suggests this, I could've realized I had a problem from simply melting up a tiny amount of my material.


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## Lou (Feb 7, 2015)

Iron will orange up the gold. Pd/Pt give it a weird grayish hue and sometimes (with Pd) a certain peacock coloration from the formation of PdO on the surface of the shot.
I wish you'd a photo of your sponge so we can see what it looked like.

Fact of the matter is, following Harold's wash procedure (ought to be linked/posted all over) is the thing to do--even dirty gold less than 99 fine can be cleaned up.
I really think you can just use SMB again and get much higher purity gold. Probably skip the wash for the most part.


I dislike ammonia water washes to remove AgCl because there's often not that much AgCl to remove. I think strong muriatic is one of the best ways to remove Ag, Pb, Fe, Ni and other elements. AgCl complexes nicely with excess chloride.
If one does do an ammonia wash, it should be after the HCl. This does two things, removes any acidity so that you don't oxidize HCl to Cl2 and lose gold when torch melting with a slight lean flame (recommended for Au). 

Also good at cleaning up dirty gold is 60% warm sulfuric in the sonicator. You can pulp gold in situ in an hour or less from 3N to 4N just like that.


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## MarcoP (Feb 7, 2015)

Chondrule said:


> Hi Jason. Thanks for your answers and input, you've given me ideas. No matter which precipitant I end up going with, I think I'll calculate the amount to use and then only use half for example, siphon off the solution and then use maybe 25%, siphon off the solution again and use the last of the precipitant. It sounds like a lot of extra work, but that's ok, maybe the contaminant, if any still exists after this second refining, will be isolated in the last precipitation.


You need to simply add SMB and stir well, wait a minute or so and test with stannous for gold in solution, add more reducing agent if test is positive. You don't have to decant and siphon.



Lou said:


> I dislike ammonia water washes to remove AgCl because there's often not that much AgCl to remove. I think strong muriatic is one of the best ways to remove Ag, Pb, Fe, Ni and other elements. AgCl complexes nicely with excess chloride.
> If one does do an ammonia wash, it should be after the HCl. This does two things, removes any acidity so that you don't oxidize HCl to Cl2 and lose gold when torch melting with a slight lean flame (recommended for Au).
> 
> Also good at cleaning up dirty gold is 60% warm sulfuric in the sonicator. You can pulp gold in situ in an hour or less from 3N to 4N just like that.


Adding to my notes about ammonia and washings! I just looked up about sonicators, expensive little toy! I guess only already relatively pure gold can be used, 2N+ or even less? ...and gold must be in powder form right?


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## Lou (Feb 7, 2015)

Marco,

That is correct. 

Given where you live, you probably don't have a Harbor Freight (which has a sonicator that is like 80USD). Oddly enough, I find those to work best.


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## Chondrule (Feb 7, 2015)

Progress has been slow, all I had time for today was to take this image to share.


Just to be clear, in solution is the same exact 14.72g of orange colored shot shown in my initial post.
I used 60ml hydrochloric, 60ml distilled water, and 12ml nitric.
Maybe tomorrow I can get the evaporation done and nitric killed.

I know this isn't much of a report, but it's all I've got, hopefully next post will be more interesting.


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## Lou (Feb 8, 2015)

Reduce with sulfite and follow the wash procedure--your gold will be fine.

Unless it's an Se issue--that comes down with SO2.


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## Chondrule (Feb 10, 2015)

Well folks, I'm going to cut to the chase, the 2nd refining was a success. First things first though, I want to thank everybody for their input, without it I'd have been lost.

So here's the finished product.......



The weight of the material before digestion and refining for the 2nd time was 17.72g, a mere .02g difference, which is amazing to me that such a whisper of contaminant could be so outrageously obvious and turn the gold orange.
It must be pointed out that I elected to use SMB again during the 2nd refining, I did so because I wanted to duplicate the 1st refining and find out if I had done something wrong, which I must have done something wrong because the 2nd was successful.
I suspected, after helpful input from you guys, that my failure to adequately wash the sponge was why the gold turned out orange, I can't prove this but I feel strongly that this was the case.
Boiling the sponge in hydrochloric during the 2nd refining produced a slight yellowing of the acid, this must have been the orange contaminant, this must have been what I neglected to do a very good job of the 1st time around.

On a different note, I've also been working on what I think is platinum from a 1930's electrical item, something I've never done before, which means most likely I'll need some help with it also.
This platinum project though, would best be for another thread, should I start another one, I mean are you guys interested?


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## jason_recliner (Feb 10, 2015)

That looks great Scott. I'm really glad it's worked out so well.

Harold has famously stressed the importance of the washing and you've given us all a brilliant visual example of what 0.13% can do. In my first hot HCl wash I was also surprised how much yellow came off, so much that I wondered if I had gold redissolving. I'm still yet to melt anything.

I for one would love to follow your platinum project.


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## Chondrule (Feb 11, 2015)

Thanks Jason.
I'm going to put it in the platinum section with those articles, so check there.


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