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How to produce 99.999%(5N) Tin

Q. Hi, I am involved in a research project in university in which one part is to prepare 99.999% (5N Tin) (i.e Pb <1ppm) by electrolysis. I have with me 99.9% (3N) tin. I want to purify it to 99.999% (5N). Can any one please suggest how to proceed? What are optimum conditions like electrolytic bath composition, Temperature, Current density, pH etc..
I am new to field of electroplating and haven't much experience.
I'll highly appreciate your responses.

Santosh Kumar
Student - Seoul, South Korea
November 5, 2010

A. You are not concerned with the criteria which affect platers for decorative or functional coatings, only the purity of deposit.
The simplest solution is probably stannous sulfate/ sulfuric acid. Concentration of both 50-100g/l The process operates at room temperature. 20-30 °C is OK but not over 40 °C. Avoid any organic additives which will contaminate the deposit.
Use your reasonably pure tin as an anode.
A current of 1-10 a/sq dm should be satisfactory.
Wash the deposit in DI water
I would suggest a piece of polished and passivated stainless steel as a cathode and you can probably peel the tin off.
Don't expect a smooth or shiny deposit but it should be high purity.

Geoff Smith
Hampshire, England

A. Hi Santosh Kumar

I don't know why and what for you try to achieve this, but I would start with electroplating the Sn from a Sulphate type electrolyte. The solubility of Pb sulphate is very low. If you use an electrolyte of sulfuric acid and Sn-sulphate (60 ml/l. sulfuric acid and 70g/l. SnSO4 at 20 - 25 °C and 4 A per dm2 cathodic current density) and you use a pure Sn-anode and continuously fine filter the electrolyte, you may have a change to reach the very pure Sn deposit at the cathode. Don't expect the Sn to be bright or good adherent in that case, commercial Sn electrolytes contain organics brightening, anti-treeing and wetting agents to improve the properties of the Sn. Maybe you can wash, dry and remelt the obtained deposit to make it look like Sn again. Don't be afraid if the electrolyte gets a milky appearance, part of the Sn(II) is oxidised to Sn(IV) and this causes the milky appearance. You can also buy a commercial bath and remelt the deposit to destroy the organics present. After this the next challenge is how to determine the 99,999% pureness......

Success,

Harry van der Zanden
consultant - Tilburg, Netherlands

Q. Dear Sir (Geoff Smith),
Many thanks for your response.Can I use some ion-selective membranes or others to further remove Pb and other impurity. My aim is to develop 5N tin (high purity solder) which has Pb, Bi etc. impurity less than 1 ppm. Basically these impurities are source of radioactivity and can introduce soft error in electronic components if less than 5N pure tin is used as solder.

Santosh Kumar [returning]
- Seoul, South Korea

A. I would be amazed it you could not buy 5 nines metal from some company somewhere. Even if you do plate it to that level, and I have some doubts, how are you going to prove that it is not .99994?

James Watts
- Navarre, Florida

A. Hi Santosh

I have never heard of radioactivity at those levels being an issue with electronics. I would be surprised if the radiation was above background levels.

Both tin (121 & 126) and bismuth have isotopes with long half life and I would expect lead to be self shielding.

Bismuth could possibly co-deposit but lead sulphate is highly insoluble and unlikely to interfere.

Geoff Smith
Hampshire, England

Q. Dear Harry,
How to continuously fine filter the electrolyte i.e how to set up this process. Maybe I use 10 µm polypropylene or dynel filter? Do I need to add any additives as I am not much concerned with appearance of deposit. I can dissolve the obtained metal at cathode and cast to make tin ingot. I want to use carbon adsorbent (to remove impurities) in bath; can it interfere with the final deposit purity or what precaution I need to take regarding this? I am using titanium as cathode as stainless steel cathode may introduce some Fe impurity in deposit.

Santosh Kumar [returning]
- Seoul, South Korea

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