Mixed Black PGM Powders

[freechemist]

@kadriver:

Judging some of the fotos taken from your very first precipitation experiments to prepare (NH₄)₂PdCl₆, I would say, that the rather brown precipitates in a more or less intensly brown coloured solution are actually crystallized (NH₄)₂PdCl₄. The ammonium salt of the complex anion PdCl₄^2-, like the potassium salt of the same anion, is not very soluble in concentrated ammonium- or potassium-salt solution, and can be crystallized (not quantitatively) easily by adding concentrated NH₄Cl- or KCl- solutions to solutions containing significant amounts of dissolved PdCl₄^2-. Both salts, (NH₄)₂PdCl₄ and K₂PdCl₄ form brown, needle-shaped crystals and brown aqueous solutions.

@lazersteve:

The abbreviated formula for the yellow precipitate obtained by adding dimethylglyoxime-solution to Pd containing solutions is Pd(HDMG)₂, not Pd₂DMG. Dimethylglyoxime, chemical formula C₄H₈N₂O₂, abbreviated H₂DMG, is an organic molecule with two acidic protons, which first forms a mono-anion, HDMG^- and then, in strongly alkaline solution, a dianion DMG^2-. The yellow insoluble complex Pd(HDMG)₂ is formally composed of a central Pd²⁺-cation, complexed by two HDMG^-mono-anions, it's chemical formula being Pd(C₄H₇N₂O₂)₂. This complex Pd-compound is uncharged and therefor behaves like an organic molecule insoluble in water.

Dissolution of Pd in HNO₃:

To dissolve Pd in nitric or nitric/sulfuric-acid mixtures in my practice I always assumed the following redox stoichiometry:
N(V) + Pd(0) ==> N(III) + Pd(II) or, only formally:
3 HNO₃ + Pd(metal) ==> Pd²⁺ + HNO₂ + 2 NO₃^- + H₂O.
Nitrous acid, HNO₂, the reduction product of nitric acid is a weaker acid, than HNO₃, and can dissociate into a proton and a nitrite anion:
HNO₂ <==> H⁺ + NO₂^-.
In presence of a much stronger acid HNO₂ can also act as a base, being protonated to the unstable cation H₂NO₂⁺, which rapidly decays into water and a nitrosyl-cation:
HNO₂ + H⁺ <==> H₂NO₂⁺ <==> NO⁺ (nitrosyl-cation) + H₂O.
Finally NO⁺ and NO₂^- can combine to N₂O₃ (nitrous acid anhydride, N(III)-oxide), which disproportionates into a mixture of gaseous NO and NO₂:
NO⁺ + NO₂^- ==> N₂O₃ (unstable)
N₂O₃ ==> NO + NO₂ (nitrous gasses; NO_x).

 

[kadriver]

@kadriver:

Judging some of the fotos taken from your very first precipitation experiments to prepare (NH₄)₂PdCl₆, I would say, that the rather brown precipitates in a more or less intensly brown coloured solution are actually crystallized (NH₄)₂PdCl₄. The ammonium salt of the complex anion PdCl₄^2-, like the potassium salt of the same anion, is not very soluble in concentrated ammonium- or potassium-salt solution, and can be crystallized (not quantitatively) easily by adding concentrated NH₄Cl- or KCl- solutions to solutions containing significant amounts of dissolved PdCl₄^2-. Both salts, (NH₄)₂PdCl₄ and K₂PdCl₄ form brown, needle-shaped crystals and brown aqueous solutions.

freechemist:

I am not a chemist, and I am trying to decipher what you have written.

I can see, from your description above, that by adding saturated NH4Cl to the concentrated/reduced pd/nitric solution, I made an ammonium Pd salt (II) instead of a Pd(IV) and the Pd(II) is not very soluble in concentrated ammonia.

This would explain why there is much pink-flesh colored material left in the reaction vessel after adding so much concentrated ammonia.

I used concentrated HNO3 to dissolve the Pd from the mixed black powders that I started with.

If what I have written above is correct, and the 6 salt is more soluable in concentrated ammonia, then is there a way to avoid the formation of the Pd(II) salt, and get it to form the Pd(IV) salt?

I have more black powder to process for Pd and this information would be helpful for that future operation.

I want to take some chemistry classes, I had nine math classes for my BS in management, but no chemistry.

I know I have the ability to understand chemisrty. What I lack, until now, is the desire to learn it.

I can see acutely where having the knowledge would be helpful in refining - especially platinum group metals.

In college, the math professors taught on a chalk board with a piece of chalk in one hand, and an eraser in the other.

Pretty soon I learned to just tune the professor out, and work the example problems in the text book during class.

Essentially, I taught myself advanced mathematics including Algebra, Stats, and Trig.

I aced the Trig mid-term and the professor said, "The final is just more of the same" and he exempted me from the final and gave me an "A" for the class - true story. He thought I was a "Nuc" from one of the subs down on the waterfront - not.

Can anyone recommend a good chemistry text book that would facilitate the type of learning I described above - I know some texts are better than others.

You see, now the forum has driven me to improve my education - this is a great place to be!

kadriver


edit by Lou: kadriver, I changed the valence of the Pd as appropriate just so others are confused and there's more continuity. I changed 4 to Pd(II) and 6 to Pd(IV).

 

[freechemist]

Please let me add some last comments.

@samuel-a:

All Pd-compounds mentioned in this thread are coordination compounds. A coordination compound is composed of a central atom/ion, surrounded by a certain number of nearest neighbours, the ligands. The center, in this thread Pd, can exist in different oxidation states, namely +2 (Pd(II)) and/or +4 (Pd(IV)). The number of ligands around the center determines the coordination number. Most Pd(II)-coordination-compounds are composed by a central, formal Pd²⁺-ion, symmetrically surrounded by 4 ligating atoms/ions in a square-planar arrangement. The coordination number of the central Pd(II) is 4. Examples: Pd(NH₃)₄²⁺, (NH₃)₂PdCl₂, PdCl₄^2-. Pd(IV)-coordination-compounds usually have a formal Pd⁴⁺-center, symmetrically surrounded by 6 ligating atoms/ions in an octahedral arrangement. The oxidation-number is +4, the coordination-number is 6. Example: PdCl₆^2- in the salt (NH₄)₂PdCl₆.

Formation of Pd-ammine-complexes:

Simple ammine-complexes of Pd(II) are formed by mixing Pd(II)-solutions with aqueous ammonia. These can be solutions of Pd(II) in diluted HCl, HNO₃, H₂SO₄ and others. In excessive ammonia the tetrammine-palladium(II) ion, Pd(NH₃)₄²⁺ is finally formed. In case of HCl a pink coloured, only sparingly soluble salt, composed of Pd(NH₃)₄²⁺-cations and PdCl₄^2--anions, precipitates intermediately, whose elemental composition is exactly the same as that of the also sparingly soluble, yellow diammine-palladium(II)-chloride (NH₃)₂PdCl₂. This same yellow compound is also formed on lowering the pH-value of a [Pd(NH₃)₄]Cl₂-solution to about 3-4 by means of diluted HCl. In this reaction two of the totally 4 ligating NH₃-molecules are protonated to NH₄⁺ and replaced by chloride anions, leading to the non-electrolyte (NH₃)₂PdCl₂:
Pd(NH₃)₄²⁺ + 2 H⁺ + 2 Cl^- <==> (NH₃)₂PdCl₂ + 2 NH₄⁺.
On treating solid, red (NH₄)₂PdCl₆ with excessive aqueous ammonia, Pd(IV) is reduced to Pd(II), followed by final formation of a tetrammine-palladium(II)-solution, probably according to the following redox-stoichiometry:
3 Pd(IV) + 8 NH₃ ==> 3 Pd(II) + N₂ + 6 NH₄⁺
Hence also the fumes observed by kadriver.

 

[samuel-a]

Thank you freechemist.
Very informative !

 

[RaoOvious]

If what I have written above is correct, and the 6 salt is more soluable in concentrated ammonia, then is there a way to avoid the formation of the Pd(II) salt, and get it to form the Pd(IV) salt?

Kad Here is an additional procedure to the method(Lou's) employed by you specially for avoiding the formation of sparingly soluble brown colored (NH)4PdCl4(Ammonium terachloropalladate) as elucidated chemically by freechemist.

1)Get the palladium dissolved from mixed Blacks by nitric washes until it doesn,t change color.Use minimum nitric and when each nitric wash gets brown or less brownish or yellowish,as washes proceed,add distill water and let it sit for a while to settle the remaining blacks,then filter each nitric wash into a separate beaker(B),any unsettled blacks would get trapped into the filter,and u can rinse them with wash bottle into the original beaker(A) containing mixed blacks.

2)Once you have got all the nitric washes into the beaker B,cement the Palladium with zinc turnings(as per Steve) from Palladium Nitrate deep brown solution until it gets clear in color,make sure to test with SnCl2 or DMG.Once all the palladium black with possibly minute contaminants(below zinc's reactivity) gets cemented,decant the above solution just as u had did in initial cementation of mixed blacks.Now rinse the palladium blacks with water to get rid of free nitric.

3)Treat the washed pd blacks with dilute HCL or 20% NAOH to get rid of zinc,as you did earlier.

4)Once your palladium black gets rid off zinc,its preferred that you dry and weigh it then otherwise.Then add 5ml of AP per gram of blacks(9:1,HCL:H2O2).The palladium blacks would dissolve within few hours,boil down the palladium chloride solution to get rid off free chlorine and also concentrate it to syrupy form,Put concentrated solution of NH4CL,if any platinum is there it would precipitate as canary yellow (NH4)2PtCl6.Filter the canary yellow platinum salt and save it to add to your platinum batch later.Now bubble chlorine to the filtrate solution(lots of alternatives for chlorine generation) to get Ammonia soluble brick red [(NH4)2PdCl6 Ammonium hexachloropalladate(II)] palladium salt,You know the next steps...Its ammonia treatment would dissolve all the flesh colored palladium salt into the ammonia solution.Then filter and re acidify with HCL,get ur bright yellow palladium salt(Diammine Dichloro Palladium II ),Calcine it and.......

Rao

 

[freechemist]

@kadriver:

Please excuse me, if I wrote somewhat cryptically for you. By extracting some statements from your last post, I try to clarify things a little.

"I can see, from your description above, that by adding saturated NH4Cl to the concentrated/reduced pd/nitric solution, I made an ammonium Pd salt (II) instead of a Pd(IV)"

This is what I suppose you having done by adding saturated ammonium chloride solution to the aqueous nitric acid solution of palladium. The dissolved Pd²⁺ ions reacted with chloride to form the complex anion PdCl₄^2-, which, in presence of a fairly high ammonium concentration, crystallized partially as solid, needle-shaped, brown (NH₄)₂PdCl₄. Solely with nitric acid Pd(II) is not oxidized to Pd(IV). A stronger oxidant must be used (e.g. Cl₂, generated out of chlorate/hydrochloric acid). This is also evident by the fact, that dissolution of Pd in AR results in a solution of H₂PdCl₄, not H₂PdCl₆.

"Pd(II) is not very soluble in concentrated ammonia"

(NH₄)₂PdCl₄ reacts with ammonia as well as (NH₄)₂PdCl₆, albeit less vigorously, than the Pd(IV)-compound. The same is true also for the potassium salts K₂PdCl₄ and K₂PdCl₆.

"This would explain why there is much pink-flesh colored material left in the reaction vessel after adding so much concentrated ammonia".

Heat and stir the pink suspension constantly near it's boiling point by adding small amounts of concentrated ammonia solution from time to time until all pink solid has dissolved.

"If what I have written above is correct, and the 6 salt is more soluable in concentrated ammonia, then is there a way to avoid the formation of the Pd(II) salt, and get it to form the Pd(IV) salt?"

Formation of the Pd(II) salt has not to be avoided. It is a step on the usual way to refine palladium. However, Pd-refining may be more efficient by first precipitating Pd(IV) under oxidizing, acidic conditions, separating the solid hexachloro-palladate(IV) and refining Pd further by reacting it with aqueous ammonia to a solution of Pd(NH₃)₄Cl₂, which can be purified by filtration and then by slow acidification in order to precipitate the sparingly soluble non-electrolyte (NH₃)₂PdCl₂.

"I have more black powder to process for Pd and this information would be helpful for that future operation."

For future operation I recommend the following procedures:

1.) Collect and let dry more of the black powder. Before working it up further always weigh the material to be treated.
2.) Dissolve it in HCl/H₂O₂ or HCl/NaClO₃ by heating the material in the required amount of 32% HCl, diluted with half it's volume water, by adding the oxidant (e.g.35% H₂O₂) to the constantly stirred and moderately heated (ca. 70^oC) suspension in small increments.
3.) Try to hold the total metal-concentration (Pd+Pt+others) at 50-75 grams per liter.
4.) To precipitate a solid hexachloro-palladate(IV) use KCl in place of NH₄Cl.

 

[kadriver]

Great info from everyone.

I'll print these instructions and add them to my reference library at my shop.

Lou - thanks for changing the valence numbers.

I took a picture of some of the red powder that was left in a small beaker.

I added distilled water to it expecting it to dissolve so I could add it to the Pd rinse water/stock pot.

It partially dissolved evidenced by the yellow color of the water.

But some of the red powder did not dissolve in the DH2O.

Some of this red powder - precipitated from nitric/Pd solution with saturated ammonium chloride - is insoluable in water.

One day I will understand what this material is - but for now it is a mystery.

kadriver

 

[kadriver]

Today I did another dilute nitric acid treatment on the remaining mixed black powder - this is the third time.

Before I started, I decanted the liquid to a seperate container (see in the back ground of the second photo).

The nitric acid turned yellow again - looks like more Pd removed - I will test with DMG tomorrow and then check for dissolved Pt as before.

kadriver

 

[RaoOvious]

Kevin,keep it going...

 

[Palladium]

That man has come a long way! Seems like yesterday he was trying to figure out the silver cell. Look at him today. My hat's off to you Kevin.

 

[kadriver]

Thank you palladium.

I have given a third and forth nitric treatment to the remaining black powders.

I have yet to get a clear nitric treatment - that is what I am trying to achieve so that I can move to the next step in Lou's abreviated process.

I belive the next step (once I get a clear nitric acid boil) is to remove the rhodium with concentrated sulfuric acid.

I hope I am not dissolving platinum with each of these nitric acid treatments.

I had to pause with this as I am cementing more mixed black powders from some HCl/Cl leach liquid - I've got about 6 or 8 more gallons to go.

Plus I had a batch of silver and gold to process that took some time away from the PGM work.

Then there is more leaching to do.

I'll post a picture of the 4th nitric treatment of the original 10 grams of mixed black powders that I started this post with - tomorrow.

kadriver

 

[kadriver]

Here is a photo of the 4th dilute nitric acid after a vigorous boil for an hour.

The color seems to be getting darker instead of lighter.

I drew some of the solution into a tube and added DMG - obviously still some Pd, but not much.

I allowed the Pd/DMG precipitate to settle, then drew off some of the clear liquid.

I put 5 drops of this clear liquid in well "A" on the spot plate, and 5 drops in well "B" on the spot plate.

I then added 3 drops of stannous chloride to well "B".

At first there was no change, then after a few seconds I could detect a faint orange color.

It looks like there is some platinum dissolving during these nitric acid boils.

I'm not getting the desired result - I'll just keep boiling in dilute nitric until I get a clear nitric solution (indicating there is no more Pd to dissolve).

Any input would be helpful - I don't know if I am on the right track with this remaining black powder.

kadriver

 

[lazersteve]

Kevin,

After reading back through the posts, I see you are using diluted nitric acid. Lou's instructions call for concentrated nitric acid, perhaps this will allow you to complete the Pd extraction so you can move on to the next step. Stirring may also assist in the removal of the remaining Pd. The color of the extracted solution is starting to look more like a Pt solution than a Pd one to me. This could be the result of trace chlorides in your water and nitric acid dissolving some of the Pt.

Steve

 

[Lou]

Very fine Pt can dissolve in nitric acid, so I've been told.


I do not know if it is a phenomenon of chloride traces or if it is a phenomenon of just the nitric acid. The engineer who works with me says he has seen it on Pt black.

 

[kadriver]

I concluded that the spot plate tests look like Platinum too, since the stannous test turned orange.

Just goes to show how these platinum group sister metals like to travel with each other.

I will do a concentrated nitric treatment on the remaining blacks to see if I can get the last bit of Pd out as per Steve's recommendation above - but it looks like a 100% extraction may not be possible.

So far I have about 400ml of dilute nitric decanted and stored for future use from these 4 nitric acid treaments.

On page 165 of Hoke, she describes an "alternative method" with nitric acid for the mixed black powders precipitated with zinc from the stock pot.

In the second paragraph on page 165, under the heading "ALTERNATIVE ACID METHOD FOR THE BLACK POWDER (STOCK POT CONCENTRATE)", she says to use diluted nitric, 1 acid with 2 water, and heat the mixed black powders "gently" for no longer than an hour to remove the palladium.

On page 166, she goes on to say that "a little platinum may dissolve with the palladium even under this gentle treatment".

I am treating with dilute nitric, 1 acid with 1 water, and doing a rapid boil for about an hour.

I will continue to do nitric treatments and test with DMG until I get either no yellow precipitate or very little of the yellow precipitate, post a photo of the test, then continue with the rest of the process.

It is starting to look like I might not get a clear nitric acid - no matter how many times I treat the remaining black powders.

These processes are all new and exciting - I am gaining much valuable experience, plus contributing to the forum.

I look foreward to the day I get to refine the pure platinum from these mixed black powders.

Thanks for the help!

kadriver

 

[kadriver]

One other thing to mention, I am going to use distilled water to do the washings of the remaining mixed black powders.

I had been using tap water - but this may contribute to some chlorine being present and forming a small amount of aqua regia when the nitric acid is added and causing some Pt to dissolve.

Thank you - kadriver

 

[kadriver]

I performed a 5th nitric acid treatment today on the remainingn mixed black powders using hot concentrated nitric acid (with a few ml of distilled wash water left on the black powders).

After heating for about 1 hour, and once the solids settled (about 3 hours later), I drew off some of the nitric solution and added it to a test tube - about 2ml along with 3 or 4 ml of distilled water.

I heated my DMG test solution to dissolve all the DMG crystals that had formed and added about 3 or 4 ml of DMG test solution to the diluted nitric solution in the test tube.

There was absolutely no yellow precipitate formed - I concluded that all the palladium has finally been removed.

I did a spot plate test on this same solution and got a very very faint orange color - probably a little platinum dissolved from the hot nitric acid treatment.

I am going to decant this acid and and save it for later treatments of palladium removal.

I'll then wash the remaining black powders until I get a neutral pH to ensure all the acid has been rinsed and go to the next step in the process - treating the remaining black powders with concentrated sulfuric acid to remove the rhodium.

Comments and ctritique welcomed and invited - thanks for looking.

kadriver

 

[RaoOvious]

Good work as usual kad,I think for your next batch Lou's Methodology should be changed a bit as he himself explained that some platinum can dissolve when in black.

Logical change to Lou's Method would be to remove the Nitric washes' step and directly do hot concentrated sulfuric acid washes,which would remove both palladium and rhodium(plus BM) until it gets clear,u would have almost pure platinum left as blacks(Rest as usual),the dissolved Pd and Rh can be again cemented with zinc,then do nitric washes until all the palladium gets dissolved,I think u can handle rest of the art....

 

[kadriver]

RaoOvious - I have read through both of your suggested processes - several times.

They look straight forward and easy to follow.

But for now, I would like to stick to the plan and continue to follow the process that I have already started.

Freechemist has also offered some good suggestions - but I must resist the temptation to deviate to one of these new processes and stick to the original plan.

I have printed these instructions and added them to my reference library at my shop.

In the future, I may start a new thread and use one of these alternate processes.

Thank you - kadriver

 

[kadriver]

Here is a shot of the remaining black powders - should be rhodium and platinum - most of the palladium has been removed with nitric treatments.

The photo is of the 5th rinsing using distilled water - the pH is between 6 and 7.

Each time I rinsed the remaining black powders, I have had to wait for at least 2 days for the suspended black powders to settle.

I am trying to achieve neutral (pH of 7).

I have a small beaker of distilled water sitting nearby.

I take a pH test strip and tear it into two pieces.

I then dip one of the pieces of pH test strip into the distilled water sitting nearby, and the other piece into the beaker with the Rh, Pt, and rinse water.

Then I compare the two pieces of test strip with each other and with the color standards printed on the pH test strip package.

Once I get an absolute neutral (pH 7) I will decant the last bit of rinse water with a pipette (thanks for that tip butcher).

Once all the water has been decanted that can be removed with the pipette, I will evaporate the remaining moisture with low heat so the black powders will be completely dry.

then I'll add concentrated sulfuric acid to start the rhodium removal.

A question please; is there any reason or benefit associated with incinerating the remaining blacks to redness before I add the sulfuric acid?

Also, what would happen if there was some residual nitric acid present when I added the sulfuric? Would this cause a problem later in the process? Is it essential to remove every last bit of nitric acid with water washes before adding the sulfuric?

Hopefully I can get to the sulfuric acid treatments after two more distilled water washes - but at the rate of two days of settling per wash, I am looking at about 4 more days before I can begin the sulfuric acid treatments.

Any inputs to correct any errors I may be making would be greatly appreciated.

kadriver