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Membrane (NAFION) electrowinning

August 18, 2011

Hello,

My name is Pedro Torres from Bogota, Colombia. I made a small custom cell divided by a Nafion membrane to recover platinum from barren platinum chloride solution about100 ppm (100 mg/L) from a few liters of refining effluent (small scale !). The solution contains ammonium ions too. pH is adjusted to 1.5 prior electrowinning. The solution contains cupric chloride too, but platinum can be partially removed selectively. There are no organics in the solution.

The divider is a Nafion membrane N424. Anolyte is Na2SO4, 0.5M. Current density is about 400 A/mA. Voltage 3V. Metal plated on the cathode is dendrite free.

My only problem is that it seems that some chloride ions may pass through to the membrane to form chlorine Cl2 (anode is Ir/Ta coated). I know that a very small amount of chlorine smell (1 - 10 ppm) but I have no way to measure chloride concentration in the anolyte or air. I use DI water to prepare it and there shouldn't have chlorides. I read that Nafion membrane rejects 100% of chloride in the chlor alkali electrolysis and there is no leak in my cell.

Does anybody have the same experience with such solution and membrane? or a way to troubleshoot the problem?

Thanks for help,

Pedro T.

Pedro Torres
Metal technician - Bogota, Colombia

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Ion exchange membranes are highly selective, but not 100% effective in preventing the oppositely charged ions, in this case chlorides, from passing through.

If this is only a very small amount of chlorine gas, then just do this in a well ventilated place or in a fume hood.

Lyle Kirman
consultant - Cleveland Heights, Ohio
August 22, 2011

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August 22, 2011

Dear Mr Kirman,

Thank you for you suggestion.

At this point, I plan to titrate chloride ion in my anolyte by Voller method.

I discovered that nafion permeability to chlorides highly depends on electrolyte composition.

I suppose that some metal chloride complexes (cation) may go across the membrane and then dissociate in anolyte media to release chlorides.

I come back as soon as I know more about chloride concentration.

Thanks.

Could I put a second membrane over the first one? The space between the two membranes would be filled with a bit of anodic solution in order to have a good conductivity.

Pedro Torres
- Bogota, Colombia