cementation efficiency

[Swissgoldrefiner]

Hello,

Usually people who refine silver, they use nitric to dissove it and then cement out it to precipitate all metal above copper. Like Au, Pt, Pd,...
Then they use Silver cell to purify the silver and get the PM in the anode bag.

What about dissolving everything in nitric, precipitate silver with HCl and eventually dissolve other PM.
Then selectively precipitate each metal with SMB, DMG,...

Does the reason to avoid the second way is that if the concentration of PM is too low, DMG, SMB,...will miss some PM to precipitate?
Solubility of AgCl is around 0.5 mg/L. What is the selectivity of DMG for Pd?

 

[FrugalRefiner]

When dissolving silver in nitric, gold will not dissolve. Pd and some Pt can.

You can drop the silver as a chloride, but conversion back to metal can be a pain and generate a lot of waste. If Pd is involved, a lot of people will take this approach to separate the silver from the Pd. Note that using HCl will recreate nitric in the solution.

I believe DMG is quantitative for Pd.

Dave

 

[4metals]

Dimethylglyoxime is quantitative for Palladium
The dry salt is 31.68% Palladium by weight.

 

[Swissgoldrefiner]

Dimethylglyoxime is quantitative for Palladium
The dry salt is 31.68% Palladium by weight.

yes, but it dosent tell me how many mg it left behind/lost.
What would be nice is to find the equilibrium constant for the reaction of DMG and Pd for example...

 

[Ultrax]

yes, but it dosent tell me how many mg it left behind/lost.
What would be nice is to find the equilibrium constant for the reaction of DMG and Pd for example...

In your case, it is stability constant (also called formation or binding constant) is an equilibrium constant for the formation of a complex in solution. Like the nickel ion, the palladium (II) ion has a coordination number of 4 with respect to dimethylglyoxime and forms a flat square complex that is insoluble in water.

 

[Swissgoldrefiner]

In your case, it is stability constant (also called formation or binding constant) is an equilibrium constant for the formation of a complex in solution. Like the nickel ion, the palladium (II) ion has a coordination number of 4 with respect to dimethylglyoxime and forms a flat square complex that is insoluble in water.

Stability constant is an equilibrium constant.
We go closer...

 

[Ultrax]

https://chem.libretexts.org/Bookshe...ry/Complex_Ion_Equilibria/Complex_Equilibrium

 

[orvi]

Stability constant is an equilibrium constant.
We go closer...

You do not need to calculate this, even tho you can. You cannot do better with practically anything. It is even hard for cementation to leave less Pd behind, hence Pd cement in chloride and espetially copper chloride enviroment is quite soluble, despite Cu metal presence. DMG is miraculous scavenger for Pd, so easy to use on spent solutions, no need to prepare ultrafine cemented dust for days and days. You just dump it in, stir for few hours and filter. Easy.

However, with nitric acid residues or from plain acidic nitrate solutions it is somewhat less efficient, and some Pd can stay in solution - mostly when the solution to be scavenged is warm or worse, hot. Mainly if you let the DMG stirring for too long and DMG itself start to be decomposed in nitric enviroment. With nitric I found it is better to higher the pH a little bit over 1 and you are in the safe region.

 

[kurtak]

then cement out it to precipitate all metal above copper. Like Au, Pt, Pd

Per the bold print - I don't mean to be nit picky here but Au Ag & PGMs are NOT above copper in the reactive series - they are below copper

The reactive series starts with the least reactive metals (PGMs) at the bottom & then goes up as the metals get more reactive

So at the bottom & going up --- PGMs - Au - Ag - Cu - Fe - Zn - Al (generally speaking) Al being the most reactive & at the top & the other metals go down (below) Al

Kurt

 

[Ultrax]

Per the bold print - I don't mean to be nit picky here

Kurt

Strictly speaking, all PGMs, Au, and Ag are to the right from copper in the electrochemical series

Thus, copper can reduce all PGM, Au, and Ag ions from the solution. Theoretically simple, without taking into account the dissociation constants for complex metal compounds.

But copper has a lower reduction potential (+0.34) than Fe (-0.44), so Fe ions reduce PGM, Au, and Ag much faster than copper. Copper is used when time is not a problem and there is no desire to mess around with base metals, which are also reduced using other reducing agents.
But this is only theoretical; In practice, much also depends on the specific chemical conditions of reduction. Ions of many substances can strongly shift the conditions of reduction.

Try to reduce gold from sodium aurothiosulfate with copper or iron

And magnesium is more active than aluminum

 

[orvi]

Thus, copper can reduce all PGM, Au, and Ag ions from the solution. Theoretically simple, without taking into account the dissociation constants for complex metal compounds.

But this is only theoretical; In practice, much also depends on the specific chemical conditions of reduction. Ions of many substances can strongly shift the conditions of reduction.

Try to reduce gold from sodium aurothiosulfate with copper or iron

And magnesium is more active than aluminum

Very true.
Cementation of eg PGMs is quite tricky to do right. Exactly due to complex formation. It happened to me that in one conditions, copper started to reduce lead from the solution

In bulk, copper is very tedious metal for cementation. Try to raise the concentration to say 100-200g PGMs/L and stick copper plate with air bubbler in it likely form some strange equilibrium, and copper will be dissolved by air and HCL so much quicker than it cement PGMs... And you end up with 30-50% PGMs cemented, rest in solution, alongside with ton of copper and you are worse than you started.

I prefer iron, altough Fe is also nowhere easy and straightforward to use. But it does not take days and kilos of copper additional stirring to spend all base metals precipitated, but it will slowly get you where you want to be.

Similar dissapointment from time to time with cementation of Pt. In certain conditions Pt could resist practically any precipitating agent for days to this day I am not sure exactly why, but I assume it is easily reduced to Pt(II), and some Pt(II) complexes stay very stable no matter what. Mainly in neutral to basic conditions, where hydrolysis can occur and some hydroxo/aquacomplexes can form. You can still clearly see Pt on XRF of the solution - which is nearly colorless - stick Zn plate to the juice and wait 24h just to find that half of the original Pt is still there and sometimes it is done in 20 minutes. PGM chemistry is strange.

 

[Swissgoldrefiner]

https://chem.libretexts.org/Bookshe...ry/Complex_Ion_Equilibria/Complex_Equilibrium

Nice article, it could be nice to find some thing with DMG/Pd. Even some thing with Pd concentration and DMG concentration.

 

[Swissgoldrefiner]

You do not need to calculate this, even tho you can.

The matter is not that i want calculate it. I would like to know how efficienty it is, as it looks that if the solution is diluted it react not that well.
But as you mention, there is more parameter, like pH. How you increase it? NaOH, NH3, ...

 

[Swissgoldrefiner]

I remember my inorganic courses that we were using bivalent ligant (which i assume DMG is) to precipitate different metal cation.
It's strange that there is not better ligant as DMG...or there is but hard to get in hand?

 

[orvi]

I remember my inorganic courses that we were using bivalent ligant (which i assume DMG is) to precipitate different metal cation.
It's strange that there is not better ligant as DMG...or there is but hard to get in hand?

Exactly beta-oximes, pyridine aldoximes or other dioxime ligands exists. There is plentiful of them invented and described in literature. But oximes are espetially hard to make in bulk, as you need starting diketones to form them. And these are not very nice compounds to work with, aliphatic diketones are often unstable...

Rule of thumb is - bulkier hydrophobic alkyl chain or aromatic rings attached to dioxime backbone - generally lower solubility in water.
But this comes with the cost of being less selective. DMG is very, very selective for Pd. If you change methyl substituents for e.g. phenyl rings, or ethyl/isopropyl substituents, it has even grater "insolubility" as complex but start to complex also other metals.

Really, do not complicate it more. Gilchrist didn´t lost practically any Pd in his experiments proving efficiency of DMG. And I can relate - practically zero Pd stays in solution. you can go for more than 99,9% recovery based on his experiments, and also from my observation. If you fear loosing few mg of Pd per litre, you can catch the rest on some resin if you feel it would be economically viable.

 

[orvi]

The matter is not that i want calculate it. I would like to know how efficienty it is, as it looks that if the solution is diluted it react not that well.
But as you mention, there is more parameter, like pH. How you increase it? NaOH, NH3, ...

That is very general thing that in dilution reactions proceed slower. And with PGMs it is even more pronounced. But Pd and DMG is like a magnet. If the pH is acidic, somewhere above 1, it is one of the few scavenging agents for any PM that works very reliably, altough in chloride solutions it needs some time to create the complex due to very low solubility of DMG in HCL and chloride solutions.

You generally do not want ammonia present, as it always complicate waste treatment and making it nightmare due to all ammonia that evolve from it when treating with hydroxide. I use strong NaOH solution if BM content is high. To the end for fine-tuning I use bicarbonate.

 

[Ultrax]

I remember my inorganic courses that we were using bivalent ligant (which i assume DMG is) to precipitate different metal cation.
It's strange that there is not better ligant as DMG...or there is but hard to get in hand?

This is a funny question considering that dimethylglyoxime (DMG) was purposefully invented in 1905 by the Russian chemist Lev Chugaev (1873-1922) as a selective reagent for the determination and precipitation of nickel and palladium.
And no, no one has invented a better reagent for a hundred years

 

[orvi]

This is a funny question considering that dimethylglyoxime (DMG) was purposefully invented in 1905 by the Russian chemist Lev Chugaev (1873-1922) as a selective reagent for the determination and precipitation of nickel and palladium.
And no, no one has invented a better reagent for a hundred years

It is hard to invent new small molecules, which are relatively cheap to manufacture and possess benefits that outweigh bigger price tag. Lot of beta-oximes (most of the best remaining proprietary by big guys - they always say just "beta-oxime") is much more convenient in the long run for Pd recovery, since they does not decompose nearly as much as DMG, and are recyclable with ease to regenerate the ligand.

They even modify resins with beta-oximes, or dioximes to easen the separation phase by loading the columns with this modified resin - which is then in turn eluted with other liquid to free adsorbed Pd. Elegant, minimal employee demand for doing this, it even can be practically fully automated

Unfortunately, it is developed for many many years, whole processes which take truckloads of spent catalytic converter material in the beginning of the process and give you three vats of Pt, Pd and Rh respectively at the end. Big guys are making millions on it, altough it is noteworthy to say they also invested millions into the research. Sadly, they will keep it secret for their own understandable reasons :/

 

[Striker578]

I didn't want start a new thread for my question. Will boiling Cement Silver in HCL until no color change increase the purity to .999? If not how close will it get?
I know a Silver Cell will produce .9999 but a have yet to build one. I intend to reuse the HCL for AR production at a later date.

 

[Yggdrasil]

I didn't want start a new thread for my question. Will boiling Cement Silver in HCL until no color change increase the purity to .999? If not how close will it get?
I know a Silver Cell will produce .9999 but a have yet to build one. I intend to reuse the HCL for AR production at a later date.

That will create a thin layer of AgCl on each grain.
If done properly it will get to 999 by it self, maybe wash it a few times in Ammonia to remove Copper.
Wash until no more blue color can be detected.