Final refining gold, - brainstorming

[9kuuby9]

So,

if I have used SMB (gold (medium brown), source e-scrap, possibly contaminated with everything common to e-scrap)

washed with hot 36% HCl (Pb, Cu)
washed with boiling water (Pb)
dissolved all AgCl with NH4OH (then made the filtrate acidic again with HCl to avoid unstable silver compounds and precipitated AgCl)

will there be anything left I could remove with

warm sulfuric 70%

?

Possibly Pd, but it depends on how clean your feed-stock is and what it originated from.

I haven't fully read the thread thoroughly but I haven't seen the Karat Inquartation method? (I know it is supposed to be intended for karat scraps) Which is also a good alternative To almost fully digest all those base metals, Ag and Pd with hot 35% nitric acid.

From Their you can keep adding a fresh solution of nitric acid until it stops reacting or doesn't gain a color. (do keep those Nitric leaches for future treatments) dissolve Au in AR, drop with SMB or ferrous sulfate. Apply harold's Washes and then redissolve and precipitate with Oxalic. You should now have at least 999+ fine gold; Do also note that clean glassware and proper rinsing and washing also makes a great difference in the concluding purity of the material.

If you suspect other base metals, warm, 70% sulfuric acid is highly effective at removing those, including Pd.

Do you possible know the solution color of Palladium, dissolved in a halide-free, acidic medium, like aqueous Sulfuric acid? and any solubility data in water?

Any help is always appreciated! :mrgreen:

 

[solar_plasma]

Full strength HCl washes are starting to make sense to me: I just read even PbCl2 is soluble in conc. HCl :shock: so all basemetals are more or less soluble in it. ....yes I am it bit slow sometimes :lol:

At least as a solid Pd (II) SO4 is red-brown: http://de.wikipedia.org/wiki/Palladium(II)-sulfat

 

[9kuuby9]

Full strength HCl washes are starting to make sense to me: I just read even PbCl2 is soluble in conc. HCl :shock: so all basemetals are more or less soluble in it.

More or Less, yes.

....yes I am it bit slow sometimes :lol:

Well a least you know what you are reading. While some don't :lol:

 

[freechemist]

Hi 9kuuby9,

The colour of a Pd-solution in aqueous sulfuric acid, obtained by oxydation of Pd-black with HNO3, is similar to the colour of a solution in e.g. 1:1 HNO3/H2O, like shown in the pictures of your post "Refining Palladium Concentrates" http://goldrefiningforum.com/phpBB3/viewtopic.php?f=51&t=19352. Concentrated Pd-solutions with 100-200g Pd/liter can easily be obtained.

Regards, freechemist

 

[9kuuby9]

Hi 9kuuby9,

The colour of a Pd-solution in aqueous sulfuric acid, obtained by oxydation of Pd-black with HNO3, is similar to the colour of a solution in e.g. 1:1 HNO3/H2O, like shown in the pictures of your post "Refining Palladium Concentrates" http://goldrefiningforum.com/phpBB3/viewtopic.php?f=51&t=19352. Concentrated Pd-solutions with 100-200g Pd/liter can easily be obtained.

Regards, freechemist

Thank you for the reply, I'm really grateful for the information you provided.
Because I think their isn't any or almost any information around the forum regarding palladium dissolved in a aqueous medium like Sulfuric acid.

I dissolved Pd in sulfuric acid with Caro's Acid, and ended up with a very dark brown solution. I did test it with stannous chloride, It firstly gave a dark brown spot and then became dark green after a while, indicating a concentrated presence of Pd.

How do you suggest on recovering Pd from a sulfuric solution? Should I cement the Pd or is their an other process involved in directly refining Pd from that solution? The solution contains a very little amount of base metals (around 1-5%)

It would be very appreciated if you would like to elaborate a little further on the subject of Palladium dissolved in a sulfuric medium, because their is so little information around here on the forum.

Kind regards,

9kuuby9

 

[Lou]

If the solution is relatively clean (no Au or Ag which should be removed prior), you can perhaps concentrate as much as possible but not over much or you will see PdO form if it begins to fume. Then partially neutralize with caustic soda until the pH is ~1. At that point you can take it into c. aqueous ammonia and form [Pd(NH3)4]2+ and filter to remove any precipitated hydroxides of base metals and business as usual. One thing to consider is that the hydroxides produced, much like those during any hydrolytic purification procedure, WILL contain values and should be recycled back into the digestion circuit.

Perhaps you could make another very nice picture post under its own thread?

 

[9kuuby9]

Thank you Lou!

If I were to have some Silver sulfate in my solution, how should I proceed? Would be adding HCl in increments until the Silver has precipitated as AgCl be an option? But them the problem still Persists, since both Silver sulfate and Silver chloride have a very small solubility and is considered among the insoluble salts. Or won't be that very small amount of soluble Silver sulfate be a problem?
I know that the cleaner the stock material is the greater the final purity will be.

Does the c. aqueous ammonia have to be warm to fully convert the palladium in aqueous sulfuric acid into [Pd(NH3)4]2+?

Thanks again for the consideration, I have stored those hydroxides just to be sure. Would it suffice bringing the metal hydroxides into an acidic medium forming a salt and recover any value from their or are their additional steps involved?

I will make another very nice picture post under my existing thread on refining palladium concentrates in the future, Platinum metals are really fun! :mrgreen:

 

[9kuuby9]

Some updates on the process, Well sorry for Hijacking this post but I guess all the good things are scattered all around in this forum. :mrgreen:

As per Lou's Suggestion, I increased the ph to 1 with caustic, Followed by additions of c. aqueous ammonia.

After reaching the ph of 4-5 the solution started to boil heavily, Which brings me to think that their is a lot of free sulfuric acid left in the solution?

So I decided to evaporate down the solution, A lot of white crystals formed; My guess would be Ammonium sulfate (NH4)2SO4 ?

Is their any method to see if their is any free sulfuric acid in the solution? or a plausible means of ridding it?

Thanks in advance!

 

[Lou]

It cannot be boiled off. The idea with partial neutralization is to keep your solution volume down during the ammonia addition (very easy to start with 200 g/L Pd and end up with 50 g/L).

 

[9kuuby9]

I ended up with a dark brown hydroxide at a ph of 8, I tested the solution and it gave a negative result. Which means the hydroxide is probably Pd? I've reintroduced it to an acidic environment and rested the solution and it now tested positive for Pd.

Palladium hydroxide precipitates at a ph of 11 of I'm not mistaken?

It was first added NaOH to a ph of 1, then heated the solution and added aqueous ammonia until the reaction became too violent.

after letting it cool off, the dark brown hydroxide precipitated.

I don't know what went wrong in the process but something did.


I'm going to cement the Pd with Al and see how it works out.

 

[Lou]

Palladium oxide starts precipitating around pH 4 and is largely done at pH 6.5.

The hydroxide will re-dissolve in HCl and you can try again.

 

[freechemist]

It seems to me, that a whole bunch of new questions has emerged. I will try, to answer some, even if they have nothing to do with the title subject ("Final Refining Gold Brainstorming"). What follows is based solely on our own laboratory experiences and observations.

9kuuby9 wrote:
I dissolved Pd in sulfuric acid with Caro's Acid, and ended up with a very dark brown solution. I did test it with stannous chloride, It firstly gave a dark brown spot and then became dark green after a while, indicating a concentrated presence of Pd.

Pd-black dissolves in sulfuric acid by oxidation with nitric acid, giving an intensely orangered-brown colored solution. Only a slight excess of HNO₃ (ca. 10%) has to be used, assuming quantitative reduction of N(V) in HNO₃ to N(III) in hypothetical HNO₂, and quantitative oxidation of Pd(0) to Pd(II). Most intriguing in these digestion reactions was the fact, that only small quantities of nitrogen oxides escape, even on prolonged, strong heating. Adding small portions of sulfamic acid to well stirred, hot samples leads to fizzing, and finally to a colour-change to a more brown and less orangered colored solution.

9kuuby9 wrote:
How do you suggest on recovering Pd from a sulfuric solution? Should I cement the Pd or is their an other process involved in directly refining Pd from that solution? The solution contains a very little amount of base metals (around 1-5%)

There are several ways: One, as Lou suggests, forming first [Pd(NH₃)₄]²⁺, followed by crystallization of (NH₃)₂PdCl₂ the known usual way. Another possibility is cementation with formic acid in excessive HSO₄^-/SO₄^2--buffered solution. Partially neutralize with NaOH to a pH of ~2 (maximum). Add a small excess over the stoichiometric amount of formic acid and boil under reflux until only metallic Pd and a colorless mother liquor remain. Usually base metals will remain in solution. A third possibility consists in the crystallization of K₂PdCl₆, which can be worked up further the "ammine-way", or, if pure enough, by first reducing to K₂PdCl₄, followed by cementation with formic acid in an acidic HSO₄^-/SO₄^2--buffer.

Lou wrote:
If the solution is relatively clean (no Au or Ag which should be removed prior), you can perhaps concentrate as much as possible but not over much or you will see PdO form if it begins to fume.

I made a similar observation by heating a solution obtained by oxidation of commercial Na₂[Pd(SO₃)₂]*2H₂O with H₂O₂ in excvessive aqueous sulfuric acid.

9kuuby9 wrote:
If I were to have some Silver sulfate in my solution, how should I proceed? Would be adding HCl in increments until the Silver has precipitated as AgCl be an option?

This would definitely not be an option. Both, Pd and Ag are strong competitors, Pd maybe even stronger, "fighting for every chloride ion entering the solution". The only way to attain a clean separation of Ag from Pd needs excessive chloride (e.g. as HCl), that means more than stoichiometrically required, to form [PdCl₄]^2- and AgCl quantitatively.

 

[Palladium]

If no one has told you this freechemist let me be the first to say " Having you here on the board is a real asset."
Both you and Lou bring a great deal of knowledge to this form with your academic educations. Even though i only understand only part of what you say i can see the knowledge and wisdom in your educational field of studies. 8)

 

[Lou]

I agree that freechemist is very, very knowledgeable.

One thing to emphasize--a degree isn't very helpful in refining. I don't think there's any amount of school that can be substituted for reading and then doing it.

 

[Palladium]

I agree! The knowledge side comes from education. The practical side comes from doing.
I'm the monkey see, monkey do type. This might not make sense but i'm a better teacher than i am a learner. Teaching and relating comes easy for me, but first learning that knowledge so i can relate it is sometimes troublesome.

 

[9kuuby9]

Thank you freechemist, your help and contribution is appreciated as always. :mrgreen:

Now I have some more information regarding the refining of PGM's to add to my notes. 8)

Can you also elaborate for us the pro and cons of oxidizing Pd with nitric acid in a sulfuric buffer? I somehow feel that you prefer this method over the classical Nitric buffer, Their might be a sound reason behind this.
Most of the literature on this subject (That I know of) covers a very small part on how to process PGM's. The first thing I could say it to limit Noxx fumes and the use of nitric.

 

[freechemist]

9kuuby9 asks:
Can you also elaborate for us the pro and cons of oxidizing Pd with nitric acid in a sulfuric buffer? I somehow feel that you prefer this method over the classical Nitric buffer, Their might be a sound reason behind this.

There is, indeed, a very sound reason behind. Digestion in nitric acid alone leads to solutions, whose further working up needs evaporation of copious amounts of excessive (and unnecessary) nitric acid, like elaborated in my post to "Nitric and Palladium".
Nitric and Palladium

 

[9kuuby9]

Thank you for the proper elaboration freechemist, Yet other precious knowledge I need to take into account. 8)

Have you also came to the conclusion that Palladium sometimes tends to passivate in nitric acid? Especially when the solution is exhausted from Nitric acid? Because When their is still un-reacted Pd left in the reaction vessel and you then add more nitric acid to the vessel, their somehow is no visible reaction progressing.

I have just read your post and it's very self explanatory. (Edit)

However do you have any Idea why it happens? Maybe due to it's catalytic properties?

 

[goldsilverpro]

From links somewhere on this forum, I just watched 2 videos on using weak sulfuric (H2SO4) + hydrogen peroxide (H2O2) to dissolve copper and I am sold on it as a replacement for AP. I feel it can work safely. One I saw used straight 12% H2O2 and 37% sulfuric acid (battery acid) solution (he said, because of the foam, to use a container 10 times bigger than the solution volume). The reaction was vigorous and generated a lot of heat and foam. Another video used about the same mix but diluted it considerably. It worked, but slowly. Somewhere in the middle is maybe the right answer.

This is a weak piranha solution. The "real" piranha solution uses concentrated sulfuric (97%) - 3 parts, and 1 part of, say, 30% - 35%hydrogen peroxide. It's used for such things as super-cleaning glassware. If 50%, or higher, H2O2 were used, an explosion could occur (Wikipedia) when pouring the strong H2O2 into the 97% sulfuric. However, it seems that, if you use a dilute solution, these dangers fade away. Also, once you get it mixed, the dangers seem to fade.

Pros and cons of dilute H2SO4/ versus AP

Pros:

1 - Fumes are far, far less than AP or Nitric and I would bet it is a lot faster than with AP. The fizzing is bubbles. All bubbles are gases, coated with the solution from whence they came. These get into the air, burst, and fill the air with minute drops of H2SO4 solution. Weak sulfuric in the air will rust steel and, if it's strong enough, it will force you to vacate the area, pronto.

2 - You end up with a copper sulfate solution, which is a good copper compound to work with. For example, if you evaporate the solution, near boiling, until crystals barely start to form, allow the solution to cool, and then chill it, you can remove about 80% of the copper as cupric sulfate crystals. Then, you analyze the solution for sulfuric (easy) and, maybe for H2O2 (not too hard), add some chemicals, heat it up, and dissolve some more copper. I believe you could re-use the solution.
3 -

Cons:

I can't think of any, if it works well.

Someone really should play with this. Find the best mix. How fast? Etc, etc. Be safe.

 

[solar_plasma]

Find the best mix. How fast? Etc, etc. Be safe.

First time I used this on silver/copper, I wasn't prepared and was not aware of it's a weak piranha solution, I learned very quickly to have respect of it and even more when I short time after got aware of what it actually is and read about it. It is my impression, that it works fast also because it is getting quite hot and works slowlier, when it is cooled down again. So, maybe the most dilluted version could be fast, too; if it is heated. Just a possibility in order to find the best compromise between safety and effectivity....