# AgPdCuSnPb alloy refining fail - lesson to learn for others



## orvi (Jan 1, 2023)

Hi PGM curious refiners

New refining story here. *Altough this time, I want to point not so sucessful refining venture* - due to *one wrong decision, and the later struggle with consequences.*

Material came from *old rotary switches, which had contacts from AgPd30 alloy*. This one time, I received used ones - so they were soldered of course. Afraid not to loose the Pd to the solder when de-soldering, I made a stupid mistake - I incinerated the whole lot (leads of AgPd wires soldered, bits of copper wires still attached) and smelted it down to metal dore. Removing the contact wires from the plastic holders is nearly impossible when dealing with hundreds of solitary contact wires, so I regularly incinerated the whole plastic carriers with contacts, pulverizing the ashes and sifting the contacts out of the filler dust. This time, *however, burnt tin/lead caked with the SiO2 and other stuff in the filler.* Also, I may "overcooked" the incineration, resulting in not-so-powdery residue as usual, and therefore contacts were still quite viciously attached to the caked filler/PbO/SnO2 matrix. *So I decided to smelt it as it was to obtain metallic dore.*

It went fine, filler from the plastic was little stubborn to slag (mixture of Al2O3 and SiO2), but eventual addition of CaO and bit of borax had done the trick at like 1400°C in induction furnance  I obtained the dore of this composition:


Slag was PM-free, so at least something. Smelting in graphite unfortunately predictably resulted in complete reduction of contained Pb and Sn to the dore - currently lacking clay crucibles for proper oxidative smelting.* If I had thought about it, I would proceed with slight cupellation/scorification in some old quartz dish with NaOH to remove at least bigger portion of tin and lead. My fault again.*

Assuring myself that 8,6% tin won´t be that bad during dissolution in nitric (bronze pins dissolving nicely even with 10% Sn),* I cleaned the dore and proceeded with nitric leaching, *using around 50% nitric to start. From the first glance, it went all okay. *But then solid tan-coloured crust of metastannic/tin oxide started to form* on the surface of the button and *reaction become very very slow*. When seeing this, I suddently realized the mistake and difference between bronze and this dore - *containing significant AgPd, which dissolve orders of magnitude slower than Cu, tin precipitates formation block the effective flow of liquid through the matrix of etched material.* Does not mean it won´t work, but as tin dissolve to either metastannic or SnO2 hydrates, it grows in volume, clogging the pores in the material...

*Dissolution was very very painful*, I replaced the leaching solution with fresh nitric couple of times - tho more importantly, despite this, leaching was *nowhere to complete.* After full day simmering in 65% HNO3, solid was still somewhat tan/dark in colour and XRF showing few% Pd and Ag in the material. Dissapointed by this, I broke the semi-solid lump to smallest possible pieces by glass rod - and when after one hour of leaching solution was no more dark then regular orange colour, I decided to move further and process this tin/low PdAg residue later.

I obtained dark brown liquid with relatively heavy metastannic/SnO2 residue (still containing Pd and Ag), but part of it was still pretty "colloidal" and filtration was as expected very tiring.
Then I got an idea - there is still lead in the solution. Adding *just enough H2SO4 should precipitate most of the lead, and this PbSO4 will re-drop on the surface of metastannic, hopefully blocking some of the pores and packing it down.* Apparently, this was a good idea. I proceeded with addition of H2SO4 - which resulted in nicely settling precipitate of all insoluble things, leaving relatively lead-free liquid of PdAgCu. *When filtering this time* - it gone order of magnitude better than before, whole filtration through small Buchner with PE mesh underneath filter paper* took like 2 minutes, washing the cake another 2 minutes. *At least something new discovered, when whole procedure was a bit more than slightly screwed up 


Silver was removed as silver chloride by pouring the pregnant liquid into excess of NaCl solution, silver chloride filtered and thoroughly washed three times with 3% HCL solution. Last filtrate was only yellowish in colour, no terrible retention of Pd in AgCl (I currently work on validation of better process of separating Ag and Pd, at least one where I end up with Pd contamined with Ag, rather than Ag contamined with Pd). I found out that Pd could be leached out of AgCl precipitate to the level of like 0,1-0,2% of Pd content (XRF). This last bit is only scavenge-able when the AgCl is converted back to Ag, redissolved in nitric and Pd is dropped by DMG addition. I won´t undergo this torturous procedure anymore...

Palladium was reduced by formic acid at pH close to 3 (to speed the process), obtaining very nice, grainy, heavy Pd precipitate, alongside some Pd mirror on the walls of the beaker. When scratched down, small leaves of Pd mirror flaked off. Very nice to see something like this  Purity was as expected lower, due to higher pH during the reduction and not so thorough washing - 98,8% Pd, rest Cu.





Liquid after addition of H2SO4 (after thorough mixing with stirrod) on the left and settled suspension of tin precipitates/PbSO4 after few minutes on the right.


Filtered solution and residue of AgPd containing tin/PbSO4 precipitates on the filter. Solution is crystal clear after single pass through filter paper.


Washing silver chloride with 3% HCL and filtering through glass frit. In the beaker is the second wash.


Onset of formate reduction.


Nearly complete Pd reduction. Greenish liquid suggesting small ammount of Pd is still left in solution. Notice palladium deposition on the walls of the beaker.

All in all, I obtained 8,25g of nice and heavy Pd powder, not so bad purity. 



From my crappy calculations, this should represent more than 92% of contained Pd in the material. Tin precipitate filtered was then suspended in fresh 65% nitric and left to simmer on hotplate for one whole day - again getting nearly brown solution. I will probably continue with this for like one more day, shot the precipitate with XRF - and if there isn´t appreciable Ag in the residue (less than 10%), I will smelt it down to metals and dissolve it in HCL or AR - hopefully to get the rest of Pd out. Not that much to be worried with, but I like the work to be done best as I could, no matter how badly I screwed it up 

At least, few conclusions could be made from this refining:
*1. Never attempt to dissolve alloys like this in nitric, if the tin content is not lowered down either by scorification or oxidative smelt. *My assumption is that with tin under 2-3%, this won´t happen, or at least retention won´t be so vicious.

*2. Precipitation of lead as PbSO4 from nitrate solution directly into the metastannic acid/hydrated tin oxides markedly ease the filtration,* with added benefit of removing lead from the feed in compacted form.

Lesson learned, moving forward to better techniques in near future 
I hope that you will find this at least educational in some point.


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