Zinc waste treatment problems

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.

A. Dear Roger,

I assume you are already treating with hydrated lime [affil links] which facilitates wonderful settling. 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. Their catalog says they are represented in the USA through a company called EM Industries Inc.

Good luck

Trudy

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.

A. Hi Rodger,

I think you got enough replies for the first part of your problem. So let me throw some light upon the second part of your query.

I have a pretty good hand on an Atomic absorption (AA) Unit [on eBay or Amazon].

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).

Regards

A. Direct filtration through filter presses is possible, though problematic. It can be done if you have 2 (preferably 3) presses and can stage them, and if the volume of filter aid won't make the cost of waste treatment prohibitive. But most people prefer to use a clarifier, and then only have to dewater the precipitated sludge.

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: When analyzing a dilute jobshop waste stream containing a mix of metals, sometimes the best you get is a good correlation factor rather than readings that are on the money. I.e., if you read 1.6 ppm in your lab when the sewer authority gets 2.6 using AA, it's a fair bet that if you next get 3.2, they'll get about 4.0 or more. The relationship may not be linear, but it is usually correlatable.

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

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.

A. Hi Sam. That's a better question for a week-long training course than for a forum response :-)
-- even the simple step of pH adjustment can be done with caustic soda [affil links] , or lime, or magnesium hydroxide, some new proprietary precipitants, or some other chemicals :-)

But the on-line version of the Metal Finishing Guidebook has several good chapters on the subject and will get you started. I'd suggest the "Wastewater Treatment" chapter by Thomas J. Weber as step one. Best of luck.

Regards,

A. Hi Sam.

2. Sodium metabisulfite is a reducing agent which is used to reduce hexavalent chromium to trivalent chromium. If you are not using hexavalent chromates you do not need it; but if you are, you do. It will accomplish that reduction at a pH between about 2 and 5. It's faster acting at low pH; but if the pH is too low, toxic sulfur dioxide gas is generated. It's slow at pH 5 and above about 5 it won't work, but there is much less sulfur smell at pH 4.5 - 5 than at low pH.
3. Lime is a more effective precipitant than caustic soda [affil links] as well as cheaper and safer; it does generate more sludge and can clog things up, but if lime is not causing serious problems, that's what to use.
5. I was not aware that alum efficiently reduced pH. I'm not sure why you ever need to get below pH 8.5; I don't think I would.
6. Polyelectrolyte is fine, but the point of flocculation is so the solids will settle out separately from the liquid, and you haven't mentioned yet what you do after the solution flocculates.

Regards,

A. Hi again. Assuming your waste is translucent/transparent rather than opaque, the cheapest and easiest way to know how much sodium metabisulphite to add is from the color. Hexavalent chromium tints the water amber colored even at concentrations of a couple of ppm. When the hexavalent chromium has all been reduced to trivalent, the water will be a very light green color (some say blueish-green). Try some samples in a beaker [beakers on eBay or Amazon] to get a good feel for this obvious color change. You can test for hexavalent chromium concentration and/or control the addition of metabisulphite with an ORP controller if you wish, but I don't think it's necessary or the best use of always scarce resources. Ideally you should do the chrome reduction step before mixing in the other wastes if it's possible.

As for how to add the lime, whatever works works. If you can add it as a powder directly to the pit without making a mess or getting it airborne, then no need to complicate things.

Alum is a fine thing to add as it is both a coagulant and a co-precipitant (when you put aluminum into the water you drive a little of the zinc and other metals out). I was just surprised to hear that alum had substantial power to lower the pH.

Regards,

A. Hi again, Sam. Did you get a chance to do what I suggested -- repeatedly treating beakers of solution with sodium metabisulfite at controlled pH until you are very comfortable with how the color changes when hexavalent chromate has been reduced to trivalent? Please do your development of treatment strategy and your practice a beaker at a time rather than 3000 liters at a time. Only after you are comfortable with this operation will you be able to make an educated guess as to whether the yellow color is related to hexavalent chromium or some other thing. Good luck.

Regards,

A yellow tinge in the wastewater is probably - but not certainly - hexavalent chromium that has not been reduced. You can check this easily with the 1,5 diphenylcarbohydrazide [affil links] test, which will give a strong colorimetric reaction with hexavalent chromium. One description of this test is given in the US EPA's test 7196A which is available at https://www.epa.gov/sites/production/files/2015-12/documents/7196a.pdf
While the test describes a quantitative method, the same procedure can be used qualitatively. A reddish purple result is indicative of hexavalent chromium. This is a pretty robust test.