Removing black from anodizing dye rinse

A. Hi again, Hassan.

Dragging hexavalent chrome (which is a very powerful oxidizing agent) into a dye tank could certainly be a serious operational problem, but it could also affect the RoHS compliance of your product. Although requiring that the operators allow sufficient drip time is the first corrective measure, modern metal finishing facilities should not be arranged such that this is a problem. Please see our library article: "Plating Shops for the New Millennium". Also, if you are referring to plastisol covered plating racks, remember the importance of maintaining the rack coatings and of never standing a plating rack on the floor but only suspended from its hooks.

Luck & Regards,

A. Hi, Randy. You claim no experience and want to learn, so I do have an important thought for you. Do your experiments on a small beaker [beakers on eBay or Amazon] of the solution under a lab hood, a "jar test"; don't experimenting with the 300-gallon batch :-)

After you've proven the feasibility of a treatment sequence with a small beaker of the waste, only then should you scale it up.

But I think I would address the chrome first, and the black color secondarily. In other words, I would keep the pH low (about 3.0 - 4.0) and add the metabisulphite. Then I would raise the pH to 8.0-9.0 to precipitate the chrome. Then I would filter or allow settling time and decant. We can't do alchemy and destroy chrome, we can only precipitate it and remove it.

Then I would deal with the colored, chrome-free water by trying to oxidize the black dye. Peroxide is worth a shot, but you may find that bleach [adv: bleach/sodium hypochlorite in bulk on eBay or Amazon] is more effective in removing the color. Best of luck.

Regards,

A. I had this problem, once. Here's how I solved it.

First I lowered the pH to about 4.5. Then, some ferrous sulphate ferrous sulphate [affil links], added as a solution and mixed in well. Then, 35% hydrogen peroxide. You'll have to do some bench experiments to find out exactly how much. It will take about 12 hours till you see significant bleaching of the color. This chemical treatment is called "Fenton's Reaction".

Now bring the pH up with caustic soda [affil links] to about 9. If you see a lot of foaming, wait till it subsides. Then add an anionic flocculant to settle the solids.

I have heard of chlorination being recommended for organic wastes. My humble opinion is: this is a terrible idea. It could result in the formation of all kinds of chlorinated organic byproducts, some of which may be regulated and many of which are impossible to remove.

Hi, Dave. Thanks for the warning to be thoughtful before using chlorination on organic wastes. You certainly may be right that it could prove problematic, and that 27.5 gallons of it is a lot! Chlorinated amines are a problem.

But every housewife uses Clorox to remove dyes from fabrics; so I think in the case of most dyes it's probably de minimus and many metal finishing wastewater treatment systems are already putting chlorine in the wastewater treatment for cyanide treatment anyway. I've done wastewater treatment at a number of jobshops that did anodizing (although not black) and I think the residual chlorine did bleach out the anodizing dyes because we never had noticeable color in the discharge. There seems little doubt that, in terms of possible persistent molecules in the effluent, peroxide is a safer route though! Thanks again.

Regards,

A. Hi Drew. We added your question to a thread where Dave Wichern had already offered a promising answer.

Luck & Regards,