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Chloride contamination in scrubber waters

I'm the plant chemist for an industrial chromium plating plant using a conventional hard chrome bath (about 250 g/l Cr03, 2.5 g/l H2S04). We have three "straight plating" tanks (4,5001 - 6,4001) and twelve "crank plating" tanks (3,0001 - 6,2001). "Straight plating" (SP) as we call it, is conventional hard chrome plating where the workpiece hangs within large anodes. "Crank plating" (CP), a specialized process for plating crankshafts, utilizes several small anodes, each dedicated to plating only one journal or area along the length of the workpiece. The two plating methods differ greatly in anode (area) to cathode (area) ratios.

We've been sequentially installing MACT standard scrubbers: The first scrubber went online two years ago and operates for the three SP tanks and one CP tank; the second scrubber went online a year ago and operates for our four large CP tanks (6,2001 each); we're currently in the process of installing the last two scrubbers which will serve the remainder of our CP tanks.

Our problem is that when we try to recycle scrubber rinse water in our CP tanks, the plating solutions are quickly overwhelmed by chloride contamination. ([Cl-] > 350 mg/l in extreme cases.) Since this isn't a problem for our SP tanks ([Cl- < 10 mg/l), it is assumed that the large anode to cathode ratios of the SP tanks makes the oxidation of chloride to chlorine a very efficient process. Also assumed, after a year and more of operation, the amount of chloride leaching from the PVC ductwork is negligible. So our conclusion is that the chloride in the scrubber recirculation tanks is concentrating by evaporation. We use tap water in the plating tank make up water and in the scrubber systems. The usual chloride background concentration is around 25 mg/l. I've found the scrubber recirculation rinse water chloride at times to be greater than 3,000 mg/l.

Presently we're paying quite a bit of money for the disposal of the excess scrubber rinse water. (The SP tanks don't take that much make up water.) Of course, when our third and fourth scrubbers go online, there'll be that much more waste water to dispose of. Also, we feel that we must re-use the scrubber rinse waters because they accumulate large quantities of costly chromium.

Our question then is will an R/O system be worth the effort and money to install at least for the scrubber systems? If the chloride is being concentrated over 100 times, will the recirculation water be clean enough to use in our CP tanks? Could anything else work (to substantially lower the chloride levels)? In reference to oxidizing chloride to chlorine, how time consuming would "dummying" the CP tanks be? How about chemical additions? I've tried (on a bench scale) using KMnO4 in the scrubber recirculation waters (to oxidize Cl-) with no conclusive results.

Peter H. Millican
power engineering - Denver, Colorado, USA
2001

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You may have another source of chlorides. Do you have any Hydrochloric acid pickling or stripping operations? If so, then it could be probable that the gassing of these solutions could be drawn through the scrubber and dissolved. Ira Donovan, M.S.F. Kansas City, Missouri 2001 2001 Peter, I don't know if I can be of much help ... but you mention tap water having a chloride content of 25 mgl/l and infer, too, that you are getting 'evaporation'. Maybe the key is in the tap water you use. That & evaporation. An efficient hard chrome scrubber [efficient in all 5 ways of a) cost b) actual mg/M3 efficiency c) low maintenance d) longevity and e) IMPORTANT ... capturing and returning the chromic emissions (to the tanks) BUT using, hopefully, LESS WATER to 'scrub' down the fumes than the tanks natural evaporative and drag-out losses.]will naturally or should naturally EVAPORATE any wash-down water UNLESS excessive amounts of wash-water are used. Nor did I know of people having to pay to dispose of rinse water when I thought that this was essentially a closed system. Early hard chrome scrubber Companies (back in the 70's} said that their equipment amortized in 2 years due to acid recovery and zero mention of dumping dilute wastes. Ignorance is bliss. I am not familiar with your problem. But then I was never a plater just a designer & supplier and perhaps others had similar problems to yours that were not mentioned to me at all. I wish I could be of more help. Cheers! Freeman Newton [deceased] (It is our sad duty to advise that Freeman passed away April 21, 2012. R.I.P. old friend). We can't really analyze the problem from this distance--only give you some "most likelies" that may help you. I don't think the voltage or current density has anything to do with the chloride problem. Use deionized water in the makeup (and rinse water, if any rinse water gets into the chrome tanks). I think you're concentrating the chlorides from the water supply. I'm a bit surprised that you've had success in returning the garbage from dry mesh pads to the chrome tank. My previous experience has been to return all the rinse water, then keep the freeboard as large as possible, maintain good fume suppressants, and forget the little chrome that has been mixed with 20,000 CFM of shop dirt. Ted Mooney, P.E. Striving to live Aloha finishing.com - Pine Beach, New Jersey 2001

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