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How could I separate Cd impurities from ZnO without contaminating it and without using overly expensive equipment or extremely dangerous solvents?

I don't want to contaminate it by introducing other heavy metals or being left with more than trace levels of undesired solvents.

I'd be fine losing some amount (say max 10-20%) of my ZnO in the process, if necessary.

Please ask me for any further clarity I could bring to this question.

I know nothing about this so I'd be happy with any contributions on how to easily separate any reasonable portion of $\ce{Cd}$ salts from $\ce{ZnO}$ in general (precipitating it out or solving it away from $\ce{ZnO}$ for instance). Cycling the zinc through a series of reaction and then back to ZnO would be fine also (if not leaving trace levels of anything biologically harmful).

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    $\begingroup$ Have you thought of using a calutron? $\endgroup$
    – A.K.
    Commented Jun 27, 2019 at 22:23
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    $\begingroup$ Purity of $\lt \pu{26 mg}/\pu{100 mL}$ of $\ce{Cd}$ salt in solution is okay? $\endgroup$
    – Mathew Mahindaratne
    Commented Jun 28, 2019 at 2:25
  • $\begingroup$ @A.K. Hello A.K! I had actually never heard of calutron. I'll read about it! (though I'm not sure I can get access to the necessary equipment) $\endgroup$
    – Veritas
    Commented Jun 28, 2019 at 4:52
  • $\begingroup$ @MathewMahindaratne 26 mg/100 mL may be a little bit too high. My target end result should at the very minimum be under 5ppm of dry product, preferably <0.5 ppm. Though this doesn't need to be a one step process, and I welcome any measure that can allow me to conveniently separate some amount of Cd salts from my ZnO samples (I can then look at all the separation possibilities and make up the right process based on the equipment I need and my requirements) $\endgroup$
    – Veritas
    Commented Jun 28, 2019 at 4:57
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    $\begingroup$ Think about treating it with caustic to form a zincate ... crystallise to remove Cd dwc.knaw.nl/DL/publications/PU00011954.pdf add the pure zincate to water to dissolve and adjust pH of slurry / solution to return zinc oxide. That's how I'd start, there are most probably cleverer ways of doing it. And if you keep your waste liquors it may be worth attempting a different purification of them. Hope this helps. $\endgroup$
    – DarrenRhodes
    Commented Jul 2, 2019 at 12:35

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Since you actually considered the use of a calutron, I assume that you are interested in separation of relatively small amounts of pure ZnO - on the order of a few grams.

Cadmium and zinc are similar in many respects, which makes me think of rare earths. One of the separation techniques used there is ion exchange chromatography. Resins with special affinity for many ions (either to catch it or reject it) are available. Some contaminants are not easy to remove by conventional ion exchange resins. In many cases, very specific resins have been developed for these contaminants. Selective resins from Rhom and Haas, www.lenntech.com, are available today for the removal of: • Boron • Cadmium, mercury and other heavy metals • Chromate • Lead • Nickel • Nitrate • Perchlorate and some other contaminants.

On the more researchy side, the stability constants of the tetra ammonium complexes of zinc and cadmium differ by a large factor, unlike their complexes with EDTA and similar chelators. For Zn(NH3)4++, the dissociation constant is 9.8 x 10^-10; for Cd(NH3)4++, it is 2.5 x 10^-7. (Reference Book of Inorganic Chemistry, Latimer and Hildebrand, 1951, 3rd ed., pp 135-6). The impure ZnO could be dissolved in NH4Cl (which will also dissolve CdO) and the solution run thru a column. Here's the research: a column of what? Perhaps a column of the impure ZnO. Since the zinc complex is more stable (more soluble), it should concentrate in the first eluate, leaving CdO behind. Solid ZnO would rip NH3 off the cadmium complex and become soluble, so the ZnO will turn into CdO. I have no idea how useful this scheme could be, but the big difference in stability constants is an eye-catcher.

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  • $\begingroup$ Thanks a lot for this answer. There's some very nice thinking in there! There's one point I don't understand though: Does ZnO + NH4Cl yield Zn(NH3)4++ and not ZnCl2? $\endgroup$
    – Veritas
    Commented Jul 6, 2019 at 7:16
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    $\begingroup$ NH4Cl is equivalent to NH3 + HCl. ZnO dissolves with 2 NH4Cl and goes to ZnCl2 + H2O + 2 NH3. Then the 2 NH3 complex with Zn++, giving Zn(H2O)2.(NH3)2++, or maybe 50% Zn(NH3)4++, or a mix, depending on how strong the ammonia bonding is. Probably fairly strong if you can dissolve insoluble Zn compounds by reducing the concentration of Zn++/Zn(H2O)4++ in solution. $\endgroup$
    – James Gaidis
    Commented Jul 6, 2019 at 13:57
  • $\begingroup$ I will set up to test your idea! It has a bit of elegance :) $\endgroup$
    – Veritas
    Commented Jul 6, 2019 at 15:25
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    $\begingroup$ I forgot to say that you can add more NH3 /NH4OH to get all the zinc complexed. But since zinc complexes NH3 more strongly than cadmium does, an insufficiency of NH3 would tend to leave the cadmium uncomplexed as Cd++ (or, probably hydrated). This might magnify the difference and make separation easier or more complete. $\endgroup$
    – James Gaidis
    Commented Jul 7, 2019 at 14:15

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