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"Removing black from anodizing dye rinse"

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Current question and answers:

February 14, 2022

Q. HI GUYS IT'S BEEN A WHILE ...
MY QUESTION IS REGARDING HEX-CHROME CONTAMINATION OF OTHER CHEMICALS; IN THIS CASE IN DYES. WE ARE CURRENTLY HAVING ISSUES OF POOR ADDITION WITH OUR BLACK DYE ON TYPE II AND III ANODIZE.
I HAVE BEEN CHECKING THE THICKNESS ON THE SUBJECTS AND ARE FROM 0.0001 TO 0.0003 TY II RANGES AND THE CONCENTRATION IN OUR BATHS ARE AS FOLLOW:
TYPE II 190g/L TOTAL H2SO4 AND 18g/L OF AL TEMP 65 °F +-2 °F
TYPE III 200g/L TOTAL H2SO4 AND 9.3g/L OF AL TEMP 43 °+-2 °F
BLACK DYE IS AT 10g/L IAW VENDOR SPEC AND THE TEMP IS 130 °F
LOOKS LIKE EVERYTHING IS WITHIN RANGE BUT IN THE PROCESS LINE I FOUND OUT THAT OUR OPERATORS PASS HEX-CHROME WORK OVER THE BLACK DYE TANK WITHOUT LETTING THE PARTS DRIP FIRST, AND THAT IS MY CONCERN.
CAN HEX-CHROME DAMAGE THE BLACK DYE ADDITION ON ANODIZE?
AND IF THAT'S THE CASE WHAT WOULD YOU RECOMMEND ME TO DO?

BEST REGARDS.

Hassan Tello
- Anaheim California
^


February 2022

A. Hi again, Hassan.

Yes, it has been a while, so thank you for presenting me with the opportunity for a periodic reminder to the readers :-)
This is NOT some weird sort of free consultancy, where a small handful of readers pretend to be experts on every possible finishing topic, while thousands of others visit only when they have a problem and expect it to function unidirectionally :-(
... This is a place of camaraderie and aloha which we want people in the industry to visit frequently, and where they are expected to offer help, not just ask for it. Everyone in this industry knows something that I don't know, and that very few other readers know :-)

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,

pic of Ted Mooney
Ted Mooney, P.E. RET
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
^




Previous Q&A's on this subject starting in 2000:

2000

Q. I just started waste treatment at a big company. I have no experience. I want to learn as much as possible. I am trying to treat Black Dye. I mixed 300 gallons of black dye with about 27.5 gallons of chlorine after I raised the pH to 8.5. I then mixed for about two hours. I lowered the pH to 3.0 with Sulfuric acid. Then I added metabisulfate to the ORP reading of 2.7. I brought the pH up to 9.0. I then added a flocculant. After mixing for about 4 hours, I brought a sample to the lab. The results came back with about 161 ppm of chrome. The lab suggested to use Hydrogen Peroxide. Will this drop the chrome out? Do you have any suggestions for future treatment of black dye?

Randy Mattson
- Sumner, Washington
^


2000

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 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 is more effective in removing the color. Best of luck.

Regards,

Ted Mooney, finishing.com
Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
^


2000

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

First I lowered the pH to about 4.5. Then, some ferrous sulfate, 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 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.

dave wichern
Dave Wichern
Consultant - The Bronx, New York
^


2000

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,

Ted Mooney, finishing.com
Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
^


April 5, 2012 -- this entry appended to this thread by editor in lieu of spawning a duplicative thread

Q. Bk Excel dye Disposal: Is there a chemical treatment to remove the color in a dye for neutralization and disposal?

Robert Galligan
- Melbourne, Florida
^



December 9, 2020

Q. To conserve on black dye cost we use a spray rinse after dying and use that rinse to feed the dye tank when water is needed. We follow it with a CFR but need to find a way to clean that first rinse of the CFR before it goes to the drain. Any idea as to what to use to remove the color? I don't think carbon filtering will work.

drew nosti
Drew Nosti, CEF
Anodize USA
supporting advertiser
Ladson, South Carolina
anodizeusa1
^


December 2020

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

Luck & Regards,

pic of Ted Mooney
Ted Mooney, P.E. RET
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
^

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