Undissolved metal in aqua regia

[inspector071]

I've been working with some cyanide gold plating solutions for a week or two now and am starting to see a bit of progress, but I'm still perplexed when I dissolve some of this metal in aqua regia. I had attempted to drop the gold out of the cyanide solutions using zinc, but all that happened was the zinc became coated with gold and ceased to react with the cyanide. I filtered that mess out and began to plate the gold out of solution. I suppose enough of the zinc had dissolved in the solution, because what is plating out seems to be a greyish gold spongy metal. It is untouched by hot dilute nitric or hydrochloric acid, leading me to believe it is of higher than 6K purity. As an aside, what karat gold ceases to react with nitric or hydrochloric acid by themselves? I read in Hoke that 6K is the preferred purity for quartation and dissolving the metal in a lone acid.

I weighed out 25 grams of this metal, which I believe to be a mix of gold and zinc (however, the plating solution, which was potassium gold cyanide based, also contained less than 1% of lead acetate. I believe this is insignificant) and created 120 mL of 4:1 aqua regia in a 500 mL round bottom flask. I set the flask into a heating mantle and placed an ice water chilled reflux condenser on top. I let the metal react for a while, then turned on the heat, letting it get to around 100 C. After bubbles ceased, I noticed a good bit of dark grey powder still was at the bottom. I added 25 mL more aqua regia but it didn't appear to react further. I filtered the metal, evaporated a good portion of the aqua regia to drive off the nitric acid, added more hydrochloric acid, boiled off more, and diluted with water. Upon addition of water, fine yellow-white crystals (presumably a gold chloride) fell out of solution. I added some concentrated hydrochloric acid and heated. Eventually the crystals redissolved. Did I do something wrong with the dilution step? Should I have added hot water instead? After the crystals redissolved, I then reduced the solution with sodium metabisulfite. 10 grams of gold were collected and melted down. The remaining 15 grams of metal from the initial 25 grams was the dark grey powder. Any idea what this stuff is, and why it doesn't dissolve in aqua regia? Could it still contain gold? I'm saving all of it for now.

 

[Harold_V]

I'm somewhat troubled by the description of the remaining powder. My mind goes to silver, although if it's finely divided, I fully expect that the treatment you provided should have converted such silver to silver chloride. If it's coarse enough, it would have been encapsulated by a thin layer of silver chloride, which would take on a strange gray/green cast. Once you've achieved that layer, even prolonged boiling in AR won't penetrate the crust, so even if there's gold present, it can't be extracted.

Gold does not dissolve in any single acid, which is what I carried away from your question. I expect you know that, and the wording wasn't exactly as it might have been.

The inquartation (quartering) is a reference point, so you can dissolve all of the base metals, without destroying the form of the alloy. It's very desirable for the remaining material to remain in one piece, so fine particles are not lost when the solution is decanted. There are those who will argue the point, however, and feel that it should be crumbled. That's not necessary if you inquart properly. You can achieve 100% extraction of base metals and silver, leaving the gold behind as a honeycomb.

Did you use zinc flour (dust), or did you try using a solid piece of zinc? You're much better served to introduce fine particles, which will be totally consumed in cementing, yielding a relatively high purity of gold (and silver, should it be present). Electrowinning isn't exactly my idea of doing a good job, as it will not affect a full recovery, very unlike zinc. It is known to recover gold to the point where it can't be detected.

Do not discount the possibility that the crystals you mentioned are not related to lead. I'm not suggesting that they are, but it's not beyond possibility, and you should go far out of your way to avoid allowing lead to follow your values. It's death on the ductility of gold.

You might try collecting the dark powder you speak of, and try melting it in a clean dish, one in which no other metal has been melted. Get the dish hot, then apply some borax and soda ash. Sprinkle the dark powder on the soda ash and borax, then cover with a little more of the same. Gently heat the material until it melts. If the powder is silver chloride, it should yield a small white button. This operation would be best performed in a furnace, with a small crucible, but it can be done with an oxy/acet torch as well. Work in a fume hood, or with a good fan at your side, blowing the fumes away. Do not put a fan at your back, as that can cause the fumes to curl towards your face.

Harold

 

[butcher]

I do not work with cyanide, and have no experience here.
So these are just my thought on the subject.

I assume you understand the dangers of acidifying cyanide compounds or metals that have seen cyanide, and are taking precaution of the deadly gas that would form from the reaction.


I am also assuming you were not using zinc powders or shaving to try and replace gold from the cyanide complex, I am also assuming you were using large pieces of zinc which the gold may have coated and protected the reactive zinc inside, this gold coating may have formed a protective coating keeping the zinc inside from reacting more, if this was the case finer shavings or powdered zinc may work better, to replace the gold from the cyanide complex.

Zinc is a very reactive metal, in the reactivity series it is above lead, so zinc will replace lead from solution.

Zinc is a very reactive metal, you said you had a "greyish gold spongy metal. It is untouched by hot dilute nitric or hydrochloric acid", if zinc was in this spongy metal the acid would attack the zinc from the spongy metal, unless you melted it it would not be an alloy of metal (like karat gold),

You state after aqua regia you had "a good bit of dark grey powder still was at the bottom" that you tried to get into solution, then after de-noxing,and dilution you state you had "fine yellow-white crystals (presumably a gold chloride) fell out of solution", this is not gold or gold chloride , it could be a salt of metal like NaCl or PbCl2, or other metal chloride (silver chloride could also form at this stage if it was involved).

if you have not melted these " fine yellow-white crystals" try dissolving them in cold water, NaCl will dissolve out, if anything is left, try to dissolve that in boiling hot water, lead chloride will dissolve in boiling water (and crystallize out of the water when cooled as white crystals (many times forming long needles of lead chloride), if silver chloride is involved it would not dissolve in cold water or even boiling hot water, but when the acid is washed out it will darken in the sunlight, the sunlight on the outer portion of the crystals change some of the AgCl to silver metal, the white crystals of AgCl will dissolve in ammonium hydroxide if no other metal like copper was involved the solution would be clear, adding HCl acid would again precipitate silver chloride from the ammonium compound (note the last step of adding acid to the silver ammonium complex (ammine) is important for safety, as the silver ammine solution could become dangerous otherwise).

lead like silver are not soluble as chlorides, but you can get a little bit of them to dissolve in a strong acid chloride solution like aqua regia (especially hot and concentrated) (as chloride of the metal in a higher oxidation state ), upon cooling and dilution they can precipitate back out as white powders.

This is my guess at this point.

 

[inspector071]

Thanks for the help, guys. I'll take some photos tomorrow of the dark grey powder for reference.

Harold, I was asking at what karat gold the individual mineral acids cease to have an effect on the alloy. Since 6K is typically used for inquartation, what alloy is no longer attacked by acids? I was trying to gauge a reference point for what karat the plated out metal was.

I used 320 mesh zinc powder. Very fine stuff indeed. I have around 4.5 gallons or so of gold plating solution, which, from the individual bottles, I estimate contains 6 troy oz. With that roughly 1 mole of gold, I added a slight excess of zinc, around 70 grams. I stirred the solution and let it sit for several days. After filtering, I boiled some of the powder (which was not brown, but rather a grey/light brown mix) in dilute nitric acid. Nothing seemed to happen, so I melted a small button of it and was left with a shiny, silver metal.

I'll melt a sample of the grey powder this weekend and post results.

Butcher: Yes, I'm well aware of the dangers of working with cyanide compounds. As stated above, I did use 320 mesh zinc powder, not large pieces of zinc. I figured that plating the metal out could potentially thoroughly mix the metals, essentially creating an alloy. Now that you mention it, when I was repurifying some other chunks of gold from a previous zinc/gold experiment, I was left with fine white crystals that when filtered from the aqua regia quickly darkened in the sunlight! Why there would be any silver in these solutions is beyond me, as they are microelectronics grade gold plating solutions. I'll see if the now dark powder dissolves in nitric acid, and then add some salt and watch for a white precipitate.

 

[Harold_V]

Harold, I was asking at what karat gold the individual mineral acids cease to have an effect on the alloy. Since 6K is typically used for inquartation, what alloy is no longer attacked by acids? I was trying to gauge a reference point for what karat the plated out metal was.

If you're discussing a single acid (nitric), I processed some wire, 60% gold, 40% copper, by prolonged boiling in dilute nitric. In my opinion, that's about the upper limit, and is accomplished only with difficulty. Any remaining copper would not interfere with the dissolution of gold, although that would not be true if the alloy contained silver as well.

In regards to using the above as a test to help determine what you're recovering, I'm not convinced you can do so. The material is so finely divided (as cemented) that the rules most likely won't apply. You most likely aren't recovering anything as an alloy.

The guideline Hoke provided in her book (25% gold) provides for a reasonably easy digestion. I was never overly concerned about the exact ratio, as it is somewhat forgiving.

I took note that you were processing solutions from plating electronic apparatus, most likely eliminating any possibility of silver being present.

If you did not introduce a small amount of free cyanide, it's entirely possible that you wouldn't have achieved much of a recovery. It's also possible that the solution is proprietary, and may be extremely difficult to process. GSP is better suited to discuss this issue than I am, assuming you can attract his attention.

Harold

 

[bswartzwelder]

Have you tested any of the solutions with stannous chloride? I don't think it would work on a cyanide solution, but the acidic solutions where you introduced HCl and HNO3 should be able to detect very small amounts of gold. Also, are you sure the plating solutions still contain gold? A test is worth a thousand expert opinions.

 

[inspector071]

If you did not introduce a small amount of free cyanide, it's entirely possible that you wouldn't have achieved much of a recovery. It's also possible that the solution is proprietary, and may be extremely difficult to process. GSP is better suited to discuss this issue than I am, assuming you can attract his attention.

No, I did not add any free cyanide. I have a bottle of a solid gold etch, designed to be dissolved in hot water to strip off gold layers from microelectronics parts after photolithography stage. The etch contains <25 wt% ZnCN, <35% KCN, 5-10% KOH, and 30-40% sodium n-nitrobenzenesulfonate. The bottle is pretty old, and looks pretty lousy. Some of the powder is white, while some has turned orange. When dissolved in water, the solution becomes clear orange. I'd rather just use pure sodium or potassium cyanide, however. I'll see if I can get GSP in here and ask his advice.

 

[inspector071]

Have you tested any of the solutions with stannous chloride? I don't think it would work on a cyanide solution, but the acidic solutions where you introduced HCl and HNO3 should be able to detect very small amounts of gold. Also, are you sure the plating solutions still contain gold? A test is worth a thousand expert opinions.

Yes, the plating solutions definitely still contain gold. Many of the bottles of solution were opened by me when I started the reclamation process.

I tried to dissolve a small amount of the grey powder in aqua regia. It did not dissolve very well at all, but the solution did test positive for gold.

Let's say this dark powder is silver oxide that contains a little gold. Would the best course of action be to melt down all of it, reducing the oxide back into metallic silver, then dissolve the metal in nitric acid, leaving behind the gold and a solution of silver nitrate? Then I can just salt out the silver later as chloride.

 

[goldsilverpro]

For 6 oz of gold, it would take a at least an equal weight of zinc - 187g. I would probably add a little more than that. 70g is not nearly enough.

 

[inspector071]

For 6 oz of gold, it would take a at least an equal weight of zinc - 187g. I would probably add a little more than that. 70g is not nearly enough.

Ah, I figured it would have been stoichiometric. How much sodium or potassium cyanide should I add for the free cyanide that was mentioned earlier? Should the solution be agitated well, and does the reaction work just fine at room temperature? How long should the reaction take? Once I filter off the metal, would a wash of hydrochloric or nitric acid be enough to be left with relatively pure gold, or should I immediately dissolve everything in aqua regia? Thanks for the help!

 

[inspector071]

Here's a picture of the dark grey powder. I had my friend melt some of it today and dissolve it in nitric acid while I was out of town. He said some brown powder, seemingly gold, was left behind. I'll test the solution with some chloride and see if it is silver. I'm still not sure why silver is in the solution, though, but I suppose someone could have easily poured back a silver solution into a gold bottle after they were done with a plating session. After I find out more about the Merrill-Crowe Process, I'll try that again and worry about whatever is left in solution later.

 

[freechemist]

Hi inspector071,

What you write here, reminds me of an earlier post, concerning uncomplete recovery of gold from cyanide solution:

http://goldrefiningforum.com/phpBB3/viewtopic.php?f=49&t=18343&start=20
http://goldrefiningforum.com/phpBB3/viewtopic.php?f=49&t=18343&start=20#p191107

May be, these links can help you.

Regards, freechemist

 

[inspector071]

Thanks for the help, freechemist. I read through those threads, but there seems to be a good bit of speculation, and I'm not sure it applies to my situation very well. I'm hoping that GSP will pop in again and expound on some of his other thoughts.