Getting pure gold (shining)

[goldsilverpro]

Steve,

The gold powder forms a crust on the pieces you're dissolving just as silver chloride forms a crust in the aqua regia. This greatly slows down penetration of the nitric. Heat would help enormously. With strong heat, you could most probably get 100% dissolution quickly. I have used nitric, many times, on 10KT (40% gold) and have been left with nothing but gold powder.

Another possibility. I assume you torch melted when you inquarted. If so, you may have gotten incomplete mixing of the alloy. This could have created some pieces that were richer in gold when you poured the shot. It's easier to mix the melt in a crucible furnace. You simply stir with a carbon rod. You can buy these rods or scrounge them. They are used in arc lights. You could probably use these to stir the torch melt. If you preheat them to red, before stirring, no gold will stick to them. You can also use the rods to remove slag, although it won't stick to a red hot rod. To remove slag and not the gold, just heat the rod slightly. This is touchy. If it's too cold, it will pick up gold and slag. If it's too hot, it won't pick up anything. If it does pick up gold, just melt it back into the dish. I've never tried it on a torch melt but it should work. Probably, just holding the rod above the melt for a few seconds would heat it enough. Just wipe the rod quickly around the edge of the melt and remove it.

In the tutorial, you may have had less than 25% gold. I wouldn't recommend going much lower than 25%, though. When you do so, the gold can come out as a very fine slime. It seems to never settle and it is very hard to filter.

You need heat, unfortunately.

 

[Harold_V]

I'm sure I used enough nitric (70% Lab Grade). I even poured off the first batch and added more to be sure. I didn't get much more action. I think heating is the issue. Since I don't have a fume hood yet I do everything cold and just wait it out. As I've stated many times, I'm in no hurry.

There were a few batches I ran that were difficult to process. In my case it was a matter of not having added enough silver, so the end result is much as you describe. Even with a hard boil for a prolonged period of time, there were occasions when all action ceased, yet the core of the material being processed didn't have the base metals removed.

What I discovered in that instance is that you're more or less between a rock and a hard place. The only thing that works with certainty is to melt any un-dissolved pieces along with an ample amount of silver, to lower the gold content. Seems like if you run the first results in AR to exclude the gold, by the time it has been removed, the silver exposed will have formed a hard crust and makes running the balance of the material in nitric next to impossible. Re-melting eliminates all the headaches. However, in your case, where you're not in a hurry, could be that you'd have respectable results simply by allowing the remains to sit in some nitric. Do remember that nitric will be more active with some water included. I'm at a loss to explain the hydronium ion, but it, somehow, comes in to play and does wonders for dissolving silver. Reagent grade nitric is slow to attack silver, but does so with a vengeance with the slightest addition of water. I experienced that routinely when I made electrolyte for my parting cell. I think GSP could verify this information as well.

All of this tends to be troublesome and clumsy at first, Steve, but it gets to be routine and happens seamlessly as you progress. You arrive at the point where it's no big deal-----you just do it -------and it works. And when it yields problems, you'll understand why, and instinctively know what to do to solve them.

Keep us posted!

Harold

 

[goldsilverpro]

Reagent grade nitric is slow to attack silver, but does so with a vengeance with the slightest addition of water.

Very true, Harold. I think you hit the nail on the head. I've always read that a 50/50 blend of concentrated nitric and water works the best. In practice, I've found this is true and always end up cutting my nitric 50/50. Of course, I always use a lot of heat. When cold it will probably be different. Try about 10% - 20% distilled water. You might need a little more power, than normal, to cut through the gold barrier.

Another problem is that, in most cases, strong acids hold less metal, per volume, than weaker acids. Remember your working with solubility constants in chemistry. One exception is the solubility of silver in sulfuric, which works opposite - don't ask me why. When using concentrated nitric, the solution quickly becomes saturated with silver. Sometimes, you'll see silver nitrate crystals form on the chunks of metal. This can grind the reaction to a halt. Dilution increases the solubility of the silver.

I always dissolve big silver contact points in nitric. I cover the points with cold distilled water and start feeding in the nitric, a little at a time, to keep it from foaming over. A vigorous reaction starts almost immediately, even though the solution, to start, only contains about 10% nitric and the solution is cold. Soon, I have all the nitric in that I've calculated it needs, plus enough water to make 50%, by volume. The bucket is so hot you can hardly touch it. In the morning, everything is dissolved. The only thing left is the crud.

 

[lazersteve]

You guys have a great point, but the reason I specified the concentration of the acid was to make clear that I wasn't using 'homemade nitric acid'. I diluted it as per my tutorial approximately 50/50 (1 part 70% HNO3 to 1 part distilled H2O) which would be close to 35% HNO3.

Steve

 

[goldsilverpro]

Back to square 1. Can't you heat it even a little? Maybe, run your heat gun over it a little? Maybe a little more strong nitric will help.

 

[lazersteve]

Sure I can , give me a few minutes and I'll let you know how it goes.

Steve

 

[lazersteve]

I heated the beaker until it was warm to the touch. No signs of a reaction, no bubbles, no vapors, nada. I added 10 mL more of 70% Nitric, nothing. The total solution size is about 100 mL.

The remaining pieces look silvery red with a moss of brown powder clinging to them. They are about the size of BB's. I see four of five of them amongst a 1/4" deep mossy brown mass. When swirled you can barely make out the BB's.

What next?


Steve

 

[Noxx]

Maybe you could try to break the gold «crust» covering the pieces with a glass rod...

 

[lazersteve]

It's not so much a crust as a loose moss. When I swirl the beaker it floats off of the BB's freely revealing the silvery red metal underneath.

 

[Harold_V]

It's not so much a crust as a loose moss. When I swirl the beaker it floats off of the BB's freely revealing the silvery red metal underneath.

Red flag!
Check the item with a magnet. It sounds like you may have a piece of iron that's precipitating copper----but that's a long shot. If it is copper, adding new acid should dissolve it-----but you say it doesn't dissolve.

When you inquart properly, the gold left behind, unless it breaks down to fine powder, which is often black, will be a dark reddish brown color, and will usually be the perfect image of what it was prior to dissolution of base metals, although it might be slightly smaller. There would be no reason for globules to grow (if they're not copper)------unless they are gold globules, being precipitated by some unknown element. None of your gold should have gone into solution with this process, so my money is still riding on the thing being magnetic. This is a strange one, Steve.

Harold

 

[lazersteve]

Harold,

Thank you for the reply, the material is not magnetic, I just checked it with a very powerful magnet and it didn't budge.

Here's a closeup of the stuff:

It's hard to see so I circled some of them in the photo.


Steve

 

[Harold_V]

Long shot, Steve. Could you have possibly included some aluminum in the lot? Something has to be responsible for the growth of which you speak, and it should be soluble in nitric. After all, what it grows from came from nitric solution. Aluminum would be a likely candidate at it is not soluble in nitric, but would easily precipitate copper (and silver).

I'm baffled!

If it was up to me, I'd boil it hard in nitric, then go to AR----and let the chips fall where they may. Once in a while you come across something that seemingly has no solution. I'm not suggesting this is one of them, but without being able to do to it what I would do to it, and make observations that may mean something to me, based on my past experiences, I'm afraid I'm out of guesses. Keep posting your results, and maybe we can get this thing resolved.

Were it not for growing nodules, I'd suggest all you need to do is re-melt with more silver, but this thing bothers me-----the growth does, anyway.

Can you tell me what you started with? If so, can you describe each piece to the best of your ability? There may be something in what you used that's key to this thing. Some newer jewelry, for example, has no precious metal content. Class rings are amongst them as I recall. Sure could be wrong, though. I've been away too long.

Harold

 

[lazersteve]

The source material came form the stripping cell.

I'm sure you've seen the pictures of it along the way here and there on the forum. I melted it without flux and it appeared to be mostly gold with a hazy or milkly (hard to describe) whitish red surface coloration weighing 11.5 grams. I was shocked at how impure the cell gold came out this time exactly the opposite of what I expected :?:

From here I parted the gold with the silver I showed you earlier :

I melted the silver and gold together in a brand new clean melting dish and cornflaked:

I put the cornflaked material in a beaker with diluted nitric acid. The results are as you saw in my previous post.

Steve

 

[Noxx]

Thanks for the great replies guys. I'll probably build a tutorial on this when I will get good results. Thanks to Harold for the quick How-to guide.

 

[Harold_V]

Steve,

We'll get this figured out now!

How I wish I'd have had a chance to talk to you about the material from the cell before you inquarted.

One of the purposes of inquartation is to expose gold so it can be dissolved. In your particular case, the gold was already very finely divided------which explains its color (colloidal), as you know. In this instance, there is nothing to gain from inquartation, although I figure you probably did so for a tutorial.

This material would have been a perfect candidate for a direct AR process, no inquartation required, simply because you already had it finely divided. You can even reduce unwanted elements when in this state. The nice thing is, with proper pre-treatment, you very well could come up with 9995 (or better) gold without a second refining. The finely divided state of the material is ideal!

One of the great ways to eliminate an unwanted acid, so it won't affect the next operation, is to incinerate. That also oxidizes elements, so they might go into solution when they, otherwise may not have. Copper oxide, for example, is soluble in sulfuric, but clean copper is not. I've mentioned this because there are times when you must go from a nitric dissolve to an HCL wash before extracting gold. Jewelers floor sweeps are a good example. Incineration eliminates the nitric, so the HCL doesn't touch the gold, but cleans up the material so you don't have problems with filtering your gold chloride solution.

Anyway, I'd have incinerated what you had, then boiled it in HCL, which would have removed substances that make filtration of gold chloride difficult. That's a critically important step if the source of your gold is related to tin, which, in electronic scrap, is very likely.

After the HCL wash, I'd rinse the material with water, allowing everything to settle well before decanting. Hot water accelerates the settling time. If the solution was quite dirty, I might give it more than one rinse. Once I was satisfied, I'd go directly to AR.

Now that I understand the source of your material, can you tell me how much silver you used as compared to the weight of the gold? In this instance, you'd have to use roughly three times the weight of the gold, which you would assume to be pure for the sake of the calculation.

If you used less, and you didn't stir, you have probably landed on the source of your problem. The un-dissolved bits are likely high karat alloy---better than 10K. They won't break down in nitric once you've dissolved the surface away. The gold, which normally honeycombs, is too dense to permit penetration of the nitric. That condition is to be expected if you didn't use enough silver, or some of your gold didn't get well mixed with the silver you added. I still have no explanation for the growth you've experienced.

Harold

 

[lazersteve]

Harold the ratio of silver to gold was just shy of 3 parts sliver to 1 part gold. Lack of proper stirring must have been the problem.

 

[PhillipJ]

Trying to understand all that's going on in this topic is a little above myself. Although I did learn a lot. Just trying to figure out what evaporate means.

Anyhow, I took Harold's advice on washing the gold & got a lot of garbage out. On 2nd refinement I tried to control the nitric as best I could, but it still needed a little urea.

Before I precipitated, I diluted 3x water then added a good slug of sulfhuric acid like GSP says, and filtered it. I warmed the solution on a coffee plate and precipitated with sodium met. I boiled a little water and mixed the sodium met then dumped it in. It kinda boiled in a flash when I added it. The plastic spoon I used to stir it with came out coated with yellow shiny gold.

The gold that dropped was also a shiny gold color. As if it was gold dust. Not all of it dropped, so I added a little more sodium met. and the rest dropped a light brown like it normally does.

Anyone know what happened that it dropped all shiny? It's a real light tan / gold color and acts slippery like silica sand. Or is it really that clean and supposed to be that way?

In the pictures, the little white dots are reflections of light on the shiny gold mud.

 

[Harold_V]

Congratulations, Phillip!

That light color you referenced is exactly what the pure stuff looks like. When I used to discuss the second refining, I'd often comment on how
"blonde" the gold came down.

I'm not sure I know what I'm going to say is true, but my thoughts are that when there is little in the way of contamination present, the crystals of gold will grow larger, thus they begin to develop faces that reflect light directionally. Very finely divided gold doesn't do that, and eventually diffuses light in all directions, so you see it as black, or dark purple. In addition, because the gold has little in the way of contamination, it's far more eager to agglomerate. You'll notice that when you abrade the stuff, it will shine as if it's been melted. Dirty gold powder won't do that, or, if it does, it takes a lot more energy to make it do so.

There is no doubt in my mind, the color of the gold, as it precipitates, is a good indicator of the purity, but in no way should be construed as an assay.

I'm pleased to hear you found evidence of contamination upon washing---it's an important step that can't be stressed too much. Relying on flux to purify gold is a serious mistake. Sort of like wearing perfume instead of taking a shower!

I'd like to comment on the use of plastic when precipitating gold. I've already stated that I don't recommend it. It has a strong affinity for precipitated gold, so unless you recycle the containers after use, you'll lose values. Your gold plated spoon is exactly what I'd have expected.

If you use a glass vessel that isn't scratched, you can use a plastic policeman to rid the glass of clinging gold. Scratched vessels become a real problem. Gold will firmly attach to the scratches, making removal almost impossible. My work habits revolved around using specific beakers for precipitation. As old beakers were retired, I'd promote the beakers, and start using the new ones strictly for precipitation. If you're using other vessels, the same concept would apply------so keep that in mind when you select your labware.

Evaporate can be taken literally. When you use too much nitric, one of the options to rid the solution is to evaporate it until it's expelled. You heat the solution (best done in an evaporating dish, which presents a very large surface for evaporation, but it can be carried out adequately in a beaker) at a low heat, allowing the contents to go into the atmosphere. If you do it at the proper speed, it happens with no losses of values. If you go too fast (too hot), you can often see a slight yellow tint to the gasses------a sure sign you're losing gold.

As you evaporate, your solution changes color. It borders on the impossible to tell you from what to what, because the color is heavily influenced by the metals contained within. Assuming you have only gold, it starts life somewhat honey colored, and darkens to a deep red as the solution evaporates. If you add HCL (which actually helps expel the nitric, but you have to carry the evaporation to the red stage before it does), the solution reverses color and gets lighter, and you can often see a brown cloud coming off (that's the nitric being expelled).

Hope the explanation helps. The best advice I can offer you is to get a copy of Hoke's book------it's totally outlined. Get the book, even if you can successfully process gold. There's one hell of a lot of useful information in it for the novice. I can't over state the value of that book.

Harold

 

[goldsilverpro]

PhillipJ,

Great looking gold. You've really come along fast. If you rinse and melt it well, you're going to have a very high purity ingot.

Harold,

I agree with you in most every aspect. You know my feelings in the plastic vs glass controversy, so I won't get into that. I would like to discuss the subject of cleaning up impure gold in the melt.

It's much more difficult to clean off purity gold in a melting dish than in a crucible furnace. However, it can be done in both. I'm talking about gold that still has base metals in it. I'm not talking about gold contaminated with Pt group metals. With Pt group contamination, you have to start over and redissolve it in aqua regia.

I try, as you do, to do everything possible to get the gold very pure before melting it. Sometimes, though, it is slightly off purity and needs a little flux work. At this point, I will play with it until it is pure. It takes, at the most, 10 extra minutes. Probably, 98% of the time, I am successful. If I understand your past posts, you see the need for fluxing as a bad thing and will put it right back through aqua regia. I don't understand your reasoning and maybe I have misunderstood you. I look at flux cleaning as only another gun in my arsenal. The end product is all that counts. It doesn't matter how I get there.

The blond gold is great when you get it, but it's not necessary for pure gold. At least, not that I've ever noticed. Most times, the darker gold is just as pure. The darker gold is more finely divided and, therefore, is harder to rinse. I never really paid much attention to color, since I had developed good rinsing techniques and I knew I could clean most anything off purity in the melt. We both knew what high purity gold looked like. The gold we both produced met these stringent visual requirements. You ran your gold through twice. I ran mine once and cleaned up problems in the melt. Both ways worked. Same end product. Same deep pipe, gorgeous color, and super well-defined crystal.

I worked for a guy that was the best refiner I've ever known. Along with silver cells, his main expertise was karat golds and he could do them faster than either of us could imagine. He didn't inquart but he cast a bar and ran it through a huge set of power rolls. He rolled it paper thin, then coiled it up and put it into aqua regia. If it was brittle, the rolls crumbled it into a jillion pieces. In both cases, it had much more surface area than any shot you've ever seen*. After aqua regia, he melted in a dedicated pure gold furnace and was a master at flux cleaning. I would guess that he could clean gold as low as 99.5%, or less. For gold he couldn't clean - a rarity - or gold with Pt group, he ran it back through aqua regia and used a different precipitating agent.

* I noticed that, in the link for the Italian refining machine that aflac gave, they use a metal atomizing gun to shot the gold. This would be the absolute best way to optimize the dissolving speed of the aqua regia.

 

[PhillipJ]

Harold,

Thank You for your advice here. I like to read what you have to say at this site because of your hands on experience.

Had it not been for this site and folks like yourself I would have continued to refine poor quality gold without knowing any better. Now I am going to reprocess what little I do have.

Nobody that I know, or ever heard of around my area does refining or even has a clue that there are precious metals in electronic scrap. That leaves me to do much by the seat of my pants. The supplier of the sodium nitrate only gives limited directions.

It's great that guys like you and the other experienced refiners here are willing to share their tricks so us newcomers don't have to guess and wonder.