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topic 3392

Wastewater treatment of iron and zinc phosphate conversion coating processes


Q. I am searching for information concerning treatment of wastewater from a zinc phosphate conversion coating process. My contact is considering changing to zinc phosphate from iron phosphate.

Kirk K [last name deleted for privacy by Editor]
- Huntsville, Alabama

O&M of Surface Finishing Wastewater Treatment Systems
Clarence Roy


A. It's not hard to treat that type of water.

The rinse from the phosphating bath will have a pH of something like 5.5 - you need to adjust it to about 9, then add an anionic flocculant solution. Use sodium hydroxide - lime or mag hydroxide would likely add a lot to your sludge volume because of the phosphate. There should also be a large pH adjustment mix tank so that this buffered waste has a chance to equalize. Depending on conditions, your Zn residual will vary from 1 - 5 ppm.

If you need to do better than that, it may help to add some sodium sulfide, ferrous sulfide slurry, or DTC to the mixture after the pH adjust, but before the floc add.

Good luck.

dave wichern
Dave Wichern
Consultant - The Bronx, New York


Q. I own and operate an electrocoating plant and have waste treatment problems.

1. Our process uses a zinc phosphate stage as part of the pretreatment for the e-coat. Our waste water permit allows us to discharge a monthly average of 1.0 mg/Liter and a peak discharge (daily limit) of 2.6 mg/Liter. Our normal stream concentration is about 20-30 mg/Liter. I need an economical way to remove the zinc. I have a filter press, settling tower and several 2000 gallon poly tanks to use for storage. I have tried to precipitate out the zinc using a flocculating agent and raising then lowering the pH but when I send it through the filter press it clogs it up and also lets a lot of particles through the screens, and thus to drain.

2. Also, I am in need of an accurate and repeatable way to measure the zinc levels of our waste stream. Someone told me of some type of Atomic spectro something or other. Does anyone have any ideas on this? I have tried a colorimeter made by Hach Co. and it does not give accurate readings (as compared to the water department monitoring our discharge).

Thank you

Rodger S [last name deleted for privacy by Editor]
- Washington, Michigan


A. Rodger, your supplier of the proprietary zinc phosphate should be available to help you waste treat the waste stream. The vendor should be a full service company with waste treatment chemicals and experienced sales/technical service, including a laboratory.

Bill Hemp
Tech Svc. w/ chemical supplier - Grand Rapids, Michigan


A. Dear Roger,

1. I assume you are already treating with hydrated lime [linked by editor to product info at Amazon] which facilitates wonderful settling.

2. I am not too sure how sophisticated your laboratory is, but if you a need simple test for small quantities of zinc: we use the "Aquaquant" test from a company called Merck.

Good luck


Trudy Hattingh
- Durban, South Africa


A. Zinc phosphate, by itself, gives a very slimy sludge. You might want to look at using some magnesium hydroxide to precipitate it. This stuff is itself rather insoluble so is irritating to work with. Try reacting it with the acid to get some in the system and then finish off with lime or other caustic.

James Watts
- Navarre, Florida


A. Hi Rodger, let me throw some light upon the second part of your query.

I have a pretty good handle on an Atomic absorption unit or an (AA Unit).

This is a fine piece of accurate equipment which uses air and acetylene (or even nitrous oxide) and the sample of waste water is aspirated into this flame. The flame changes colour depending upon the concentration of the metal in it and this absorbency is decoded in the metal concentration present in the sample.

This is just a simple idea but the AA Unit is certainly a very reliable technique to detect any metal.

You use some hollow cathode lamps of the same metal (which is to be detected). Once you set up all calibration steps it takes you 5 seconds to detect a sample with 95% accuracy for sure. And you may proceed with any number of samples after calibration. So it is very fast too.

Ask your nearest Varian dealer. You may find a good deal in some used equipment ( say may be 5,000 USD).


Hemant Kumar
Hemant Kumar
- Florida, USA


A. Magnesium hydroxide is indeed a great precipitant, but it is a slurry of particles not a solution, and therefore the reaction time can run 6 hours and even more as the particles dissolve ever so slowly into the acidic waste.

Hemant tells you true. While I like the economy and simplicity of Hach type tests and colorimeters, your results are not atypical: the best you can usually expect from a relatively quick colorimeter test in a dilute mixed metal jobshop waste stream (because of so very many possible interferences) is a correlation factor. I.e., if you read 1.3 ppm in your lab when the sewer authority gets 2.6 using AA, it's a fair bet that if you get 2.0 next time, they'll get about 4.0. The relationship often correlates enough to at least tell you when things are safe or going south :-)

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


A. Perkin Elmer has a good model for Atomic Absorption. It's the AAnalyst 100. That's the one I personally run at work and I am within .1 ppm of what an independent lab runs our waste water at.

Vicki Harwood
- Wyoming, Michigan, USA


A. We also use an AA machine to detect the cadmium levels in the waste water test. I would suggest filter the solution before running it. Let the flame burn for five minutes after the test to clear the orifice.

Soda ash will cut the metals; floc will help the metal settle in solution. You will still need another chemical as a negative charge.

Dale May
mining - Bartlesville, Oklahoma


A. Dear sir,
I read about the problem you are having with your Zinc lines. I have read the other responses and I must say, some of those things I read made no sense to me.

Atomic Absorption or called Atomic Spectrophotometry is definitely an advantage, but for a simple process, it's overkill not to mention cost. A colorimeter or reagent test kit would suffice. I understand that you have tried a Hach test kit but were unable to achieve accurate results, I was curious as to why it wasn't working. I have used AA, Ultra-violet spectro, gas spectro and colorimeters; all of which were very accurate.

I could easily suggest some remedies that might solve your problem but would be wrong in doing so just for the simple fact, that to my ears there is too much missing information which can be vital in making any decision.

Anyone that has been involved in industrial chemistry should know this. I would be glad to help you in any way I can.

But first I need to know the following:

1. What kinds of chemicals do use in pre-process?
2. What type of processor operation are you supporting?
3. What is your rate of production?
4. How is your plating line set up? Concentration levels, rinse types, etc.
5. How many and what type of cleaners are you using?
6. How often do you dump and remake tanks?
7. What does your W.T. setup consist of?
7. What is your rate of discharge in waste treatment?
8. What are you using to control pH and what are the optimum pH levels you are using.
9. How knowledgeable are your technicians?
10. What is the climate like where you are at?

As I mentioned earlier, I would be happy to help. But all these questions I ask can mean everything ... why? Because that's chemistry for ya!

Currently I oversee a zinc phosphate line and waste treatment and to be honest with you we haven't had any problems at all.

I hope by now you have found a solution to your problem.

Eric F [last name deleted for privacy by Editor]
- Sanford, North Carolina

Ed. note: If you feel that some of the other responses made no sense, Eric, please try to be specific about them. Thanks.

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