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Water Treatment for zinc electroplating waste
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Our company paints and Zinc plates automotive parts. We usually segregate the waste water from both the departments. But by accident, all the water got mixed up, and now we are having problems with the precipitation of Zinc. By the way, we use hydroxide precipitation to remove metals. We have been able to get down the Zinc concentration to 4 ppm using various chemicals, but not any lower than that. Our discharge limits are 2.61 (daily) and 1.48 (monthly). Various chemicals we tried : Caustic soda, Lime, Magnesium oxide, Ferrous sulfate, Calcium chloride, Sodium borohydride, Calcium polysulfide, two proprietary organometallic polymers. Any suggestions will be highly appreciated. Thanks in advance.
prasad s [last name deleted for privacy by Editor]information service
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We assume these values are after filtration, Prasad? If not, your problem is easily solved -- you need to clarify or filter the wastewater. My personal experience is that sulphide chemistry is often the most powerful precipitating method for zinc. Even though you have already tried calcium polysulphide, you may obtain success with DTC (sodium diethyl dithiocarbamate) or Degussa's proprietary TMT15. For this one-time troublesome batch, I suggest trying to lower the pH to about 5 and loading it up with your ferric chloride before raising the pH for the hydroxide precipitation. Filter out the precipitated zinc. Now treat the filtrate with the proprietary co-precipitant and the DTC or TMT15. Good luck.
----- Dear Mr. Mooney : Thank you for your reply. I forgot to mention, but we tried the carbamate too. Yes, we are filtering the effluent. Since my last letter, we had successfully precipitated out zinc using Magnesium Oxide and a proprietary organometallic polymer 326-C. But now we are having problem removing Nickel. This is not caused by 326-c. We had the problem with Nickel to start with. One more chemical we tried in the last couple of days is Ferric Chloride at around pH 8.0. Please suggest any chemicals to remove Nickel and at what pH, they work best. Thanks again for your reply. -Prasad. prasad s [last name deleted for privacy by Editor]information service |
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Hi, Prasad.
1). When you describe all these things they you have tried, they hopefully were each tried on small samples of the waste. You should not attempt treating the whole batch without lab scale beaker tests to verify that they work, or you will be faced with a large volume of witches' brew that already contains large amounts of a dozen different precipitants, and it just gets harder and harder to treat.
2). After some metal has precipitated out, you must get rid of it by filtering it before going on to deal with the remaining dissolved metal. If the precipitated metal remains in the batch you are working with, constantly ready to redissolve into equilibrium, you may never get there. The strong sulfur based precipitant will removes a molecule of zinc, but if the waste was not filtered, a molecule of formerly hydroxide precipitated zinc will just go back into solution to replace it.
3). In general, nickel precipitates best at high pH, about 10.5. When magnesium hydroxide is used for the treatment of nickel it has to be 'topped off' with caustic or another stronger alkali because magnesium hydroxide doesn't get the pH high enough. See letter 565 for starters.
4). In general, you need a treatment sequence and strategy, not a hunt for a miracle precipitant. For example: lower the pH, add ferric chloride, raise the pH, add flocculant, and filter out most of the zinc. Now you take the clear effluent and raise the pH further to remove the nickel, and filter it. Now you are left with a few ppm of zinc or nickel still in solution, and you add a few ppm of a powerful sulfur based precipitant to precipitate it. It is unaffordable and environmentally hazardous to try to remove the bulk of the metal with strong sulphides.
Good luck!
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 Ted Mooney, P.E. finishing.com Brick, New Jersey |