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Ion exchange for chrome recovery from "green chromating" effluents


 

I represent a company which implements effluent and waste treatment solutions. I am currently assessing the feasibility of implementing an ion exchange system for treating the effluent from a green chromating process for aluminium. One of the objectives of implementing such a system would be to recover quantities of chrome from particularly the rinse waters. Does anyone have any experience of implementing ion exchange systems on effluents associated with the green chromating process for aluminium? Are there any obvious "fatal flaws" which may preclude the use of ion exchange on these types of effluents? Furthermore any critical evaluation of the objective of recovering chrome from this effluent would be most welcome.

Neil Rein
- Johannessburg, Gauteng, South Africa


 

I haven't heard of green chromating, but maybe you are referring to what I would call olive drab chromating. Ion exchange should work fine. The chrome will be captured as a CrO4= ion. But returning it to the tank is a whole 'nother issue. Well, I shouldn't really say that returning it is hard--keeping the chromating bath functional with this garbage being returned to it is the hard part :-)

Ted Mooney, finishing.com Teds signature
Ted Mooney, P.E. RET
finishing.com
Pine Beach, New Jersey


 

From what I have been able to establish "green-chromating" is also referred to as phosphochromating and involves the deposition of a chrome phosphate layer on the aluminium surface. The green chromating process is dependent on the presence of hydrofluoric acid in the coating solution. Is the presence of highly corrosive hydrofluoric acid likely to affect the ion exchange process? Does anyone have any practical experience of implementing ion exchange systems treating hydrofluoric acid containing solutions?

Neil Rein
- Johannesburg, Gauteng, South Africa


 

Green chromating is a well-known process in Denmark for chromating of aluminium but step-by-step yellow chromating is taking over -- also because yellow chromating contains less chromate. Today the ratio is probably 50:50. A green chromating solution is a mixture of chromic acid (5-6 g/l), phosphoric acid (12-18 g/l) and hydrofluoric acid (3-5 g/l). I have done a lot of work concerning recovery of chromating baths using membrane electrolysis where you get the chromium+3 oxidized back to chromate and at the same time aluminium is removed (aluminium does not penetrate the membrane). It is technical possible to do so but it is a rather expensive and sensible process that is still not very common in Denmark.

By recycling the rinse water after the green chromating bath you will get a mixture of sodium salts of chromates, phosphates and fluorides in the an-ion eluate (using NaOH for regeneration) while aluminium should only be present in the cation eluate together with chromium+3. There will be no problems by recycling the water but it will not be that easy to recover the an-ion eluate because you have a lot of sodium ions in this solutions. I have herd about solutions where sodium afterwards is exchanged by H+ in another cat-ion exchanger but I have never tried or seen this solution in real life.

Flemming Dahl
- Copenhagen, Denmark



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