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Author: Subject: Electrorefining of Tin
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[*] posted on 13-3-2019 at 03:09
Electrorefining of Tin


I've been working on purifying tin from pewter and lead-free solder and wanted to share my set up and results: following operating parameters in the range of those suggested by the authors of this paper

Materials Science and Engineering, Volume 37/2. (2012), pp. 19–26

and their other papers I set up an electrolyte with 1 M HCl and 20 g/L Sn in the form of SnCl2, hooked up a power supply with a limit of 0.2 V, and made a stainless steel cathode. While it takes quite a bit of time to build up any amount of tin (the current is pretty low), on the other hand the tin winds up over 99.9% pure (had a friend with work access to XRF check it out) and the dendritic crystals are really neat looking (see images).

Just wanted to share with some folks who'll appreciate it!



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[*] posted on 13-3-2019 at 04:01


those are some cool looking metal crystals. if i have the patience to purify some lead-tin alloy i have, i'm definitely going to try this out




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[*] posted on 13-3-2019 at 04:11


Neat looking indeed: I first though your electrode was a metallic foam !
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[*] posted on 13-3-2019 at 06:50


I s the tin on the cathode coming from the anode (in which case was this pewter, and what was happening to the anode during electrolysis) or was this just the tin chloride already in solution forming the crystals?
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[*] posted on 13-3-2019 at 07:00


Quote: Originally posted by RedDwarf  
I s the tin on the cathode coming from the anode (in which case was this pewter, and what was happening to the anode during electrolysis) or was this just the tin chloride already in solution forming the crystals?


The general idea is that the tin already in solution gets reduced on the cathode while simultaneously tin from the anode undergoes oxidation and dissolves into solution, ideally maintaining a constant concentration of tin in solution.

The anode appears black but if you try to scrape it a lot of black gunk (anode slime) will come off - basically, unoxidized remainders from the anode.

This particular anode started as a pewter plate that was 94% tin, 5% antimony, and 1% copper. I find that the anode slime typically still contains 50-65% tin as well as the antimony and copper.
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[*] posted on 13-3-2019 at 07:29


It is good that you operated at low current density because when I was playing with similar https://www.youtube.com/watch?v=G1sq4hnrBgM&t=11s
I found that higher currents cause longer dendrites,
also, long thinner-than-hair threads reach out to the anode then on contact vapourise due to the power supply current.

[Edited on 13-3-2019 by Sulaiman]




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[*] posted on 13-3-2019 at 08:38


Is it pure enough to convert to β-tin?



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[*] posted on 13-3-2019 at 09:14


Quote: Originally posted by Fulmen  
Is it pure enough to convert to β-tin?


99.9% pure. sure it can convert to β-tin





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[*] posted on 13-3-2019 at 10:02


Quote: Originally posted by Ubya  
Quote: Originally posted by Fulmen  
Is it pure enough to convert to β-tin?


99.9% pure. sure it can convert to β-tin


It's currently β, but as you say, pure enough that I worry about its spontaneous conversion to α and take care to store it out of my unheated basement!

At some point I am planning on seeing how long it can stay β at a temp slightly below the conversion temperature. I'm sure throwing it in the freezer would do the trick but I wonder how long it can stay unconverted at, say, 10 or 12 °C?
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[*] posted on 13-3-2019 at 10:37


My bad, I was of course thinking of α-tin. Would be fun to see how fast it can convert as well, say within "normal cold winter temperatures".



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[*] posted on 13-3-2019 at 14:01


Quote: Originally posted by Sulaiman  
It is good that you operated at low current density because when I was playing with similar https://www.youtube.com/watch?v=G1sq4hnrBgM&t=11s
I found that higher currents cause longer dendrites,
also, long thinner-than-hair threads reach out to the anode then on contact vapourise due to the power supply current.



Nice video! Yep, my current density's quite low (nominally ~60 A/m2 on the cathode not considering the massive increase in surface area due to deposited tin); I was going for quantity rather than speed but one would die of boredom watching my reaction (pictures are after 4 days' reaction time).
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[*] posted on 14-3-2019 at 09:29


If you were to run the electroysis at near freezing-temperatures, would the tin form as the other allotrope?



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[*] posted on 15-3-2019 at 03:59


Quote: Originally posted by DraconicAcid  
If you were to run the electroysis at near freezing-temperatures, would the tin form as the other allotrope?


Now that's an interesting thought! The cathode's crystal structure may preferentially form one over the other, and I don't think the α form is conductive (so it might coat a cathode and then passivate it?) but on the other hand if it did work it'd be a neat way to avoid the growth of long tin dendritic crystals that short out the cell:



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[*] posted on 15-3-2019 at 16:34


Is it completely non-conductive, or just not as conductive as the metallic form?



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[*] posted on 17-3-2019 at 03:02


Quote: Originally posted by DraconicAcid  
Is it completely non-conductive, or just not as conductive as the metallic form?


Well, though Wikipedia describes it as having "no metallic properties" on the other hand the abstract of this 1954 paper

https://journals.aps.org/pr/abstract/10.1103/PhysRev.97.607

gives the conductivity at 0 °C as 2.09 x 105 S/m which seems to be comparable to graphite (though a factor of 100 less than most metals). Sounds like, it would actually be conductive enough for an electrochemical process.
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[*] posted on 17-3-2019 at 03:41


I believe but have not verified that periodically briefly reversing the polarity applied to the cell reduces dendritic growth.

When playing with a small tin electrolysis cell I noticed that reversing the polarity causes tin to migrate from the dendrites, (as expected)
the thinnest dendrites appear to 'suffer' the most,
so the short (1 second or so ... t.b.d.) reverse pulses remove long thin dendrites and 'blunten' the shorter fatter ones.

I remember long ago reading about reverse polarity pulses being used for electrolytic plating,
and even during wet Ni-Cd battery charging (I think ... memory fading)
probably based on a similar principle ?

You could try manualy reversing the polarity periodically, (e.g. 1 second of reversal per minute ?)
or use something like a 555 timer and a 2-pole changeover relay.
-----------------------
I also believe that long dendrites form because the metal concentraion in the electrolyte is depleted in the volume near the cathode occupied by the mass of dendrites,
this leaves more concentrated higher conductivity electrolyte between the tips of the dendrites and the anode.
Also, the voltage gradient is greater between the dendrite tips and anode as the tips of the dendrites are nearer to the anode.
I guess that vigorous stirring would help to significantly reduce dendritic growth mechanisms.

[Edited on 17-3-2019 by Sulaiman]




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