Metal finishing Q&As since 1989

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Can waste hydrochloric acid be used for hex chrome reduction

Q. I work in the environmental industry with several steel producing companies. We have found one of our clients, who typically uses sulfuric acid to complete hex chrome reduction on a waste, wondering if hydrochloric acid can be used. We are wanting to use waste hydrochloric acid from another facility but have been concerned that the reaction my not completely do the trick. Can we use waste hydrochloric acid to complete hex chrome reduction prior to final chemical neutralization of the current waste?

Matt Friday
Environmental Coordinator - Pittsburgh, Pennsylvania, USA
2004

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A. You may use the spent hydrochloric acid to lower the pH to the level required for chemical chromium reduction but it will not reduce the chromium by itself. You will still need to add a chromium reducing agent to the process. There may be some accidental reduction as the iron in the acid, catalyzed by the chloride ion, will cause some reduction. But don't expect it to finish the whole job. Paul Morkovsky - Shiner, Texas, USA 2004 A. There are two issues intertwined here. No mineral acid will reduce hexavalent chromate to trivalent--which is a necessary step in the treatment sequence. For that you need a reducing agent, sodium bisulphite being the most commonly used. But, yes, hydrochloric acid can replace sulfuric acid for the pH control. Most people don't use it because it is more corrosive, especially the fumes, but there is no chemical reaction reason you can't use waste HCl for this. Ted Mooney, P.E. Striving to live Aloha finishing.com - Pine Beach, New Jersey 2004 A. Any strong acid, including hydrochloric acid, can be used to reduce the pH in a chromium reduction reactor. However, fumes and extremely corrosive nature of hydrochloric acid can make it less desirable to use unless the tanks and the room are extremely well ventilated. As a small bonus, any ferrous iron in the waste acid will also reduce chromium and reduce the amount of bisulfite needed. Incorporated Lyle Kirman consultant - Cleveland Heights, Ohio 2004

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We are experiencing a possible masking of ORP & passed hex chrome out of our facility. What masks ORP and restricts the conversion from hex to tri-chrome? We had a demand for bisulfite that was apparently satisfied before our chrome was reduced. EMERGENCY Help needed.

Pete A. Zervos
- Gary, Indiana
2005

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Hello Pete,


This is only a public forum, which people may or may not read for a while, and may or may not decide to respond to. It's not the best place to get emergency help. Some consultants are listed at www.finishing.com/consultants/, or environmental consultants might be found in your local yellow pages.

An ORP meter reads oxidation-reduction potential and will see any reducing agent as having satisfied the need, and turn the reducing agent pump off. That is one reason why waste materials that need oxidation (like cyanide) must be kept separate from waste materials that require reduction (like hexavalent chromium). Usually the color is a good indication though; if there's a trace of yellow or amber to your chrome waste instead of light green-blue, there's probably still hex chrome in it. The light green-blue will precipitate out when you raise the pH. Good luck.

Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
2005

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Reducing trace amounts of hexavalent chromium on cupped parts

Q. I am currently working on a project that involves stains on Nickel Chrome electroplated parts. The parts have a blind hole and on the rack, it carries solution. We find that the part is carrying chromic acid from the plating tank through the rest of the process and is leaking out onto other parts after unracking causing stains. Since the line is old and changing the rack design or is extremely difficult, a more obtainable option is to reduce the hexavalent chrome to trivalent in the rinse tanks. Supposedly the trivalent chrome will be easier to remove and be less damaging to the part if staining occurs. Most reducers require a low pH, but this is allowing rust formation inside the blind hole where no plating is present, just bare steel. What would you recommend as a reducing make up to convert the chrome as well as protect the steel? We are experimenting with sodium metabisulfite as well as another make up containing hydrazine with varying pH's. Any information or insight will help. Thank you.

Mike Pliszka
Process Engineer - Buffalo, New York USA
December 29, 2014

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Q. There definitely seems to be many different options regarding reducing agents, but I am looking for the best in line option. Another plating company I spoke with uses sodium metabisulfite in line controlled with an oxidation-reduction potential meter. I am wondering if anyone has had any similar experiences. I appreciate your input Ted. I will be running laboratory experiments this week.

Lyle, would you have any other suggestions for agitation improvements? Deep blind holes are common here and are definitely not aided by the post style rack design. We currently have a pretty vigorous air agitation system, which probably cannot be increased without overflow or splattering. For some reason, whether it is the post in the blind hole, the angle of the part, or the threaded portion inside the blind hole, there always seems to be carry over after the chrome drag out and rinses. Once again any input you may have will be useful. Thanks.

Mike Pliszka [returning]
- Buffalo, New York USA
January 5, 2015