plating, anodizing, & finishing Q&As since 1989
Water Filtering system for sodium dichromate
We have just recently set up a Chromate line per MIL-C-5541 [affil link] and a Sodium Dichromate/Sulfuric Acid etch primer preparation line. We have had people here to help us with a closed loop filtering system. The filtering system seemed to work for about a month as we were increasing our production. We are now having problems keeping the system clean for one day. The system consists of 2 banks of 3 each 8" (6 total) canisters of mixed bed (Anion/Cation) resin. Can someone help? Thank You,Vernon E. Harvey
aerospace - Beltsville, Maryland
First of three simultaneous responses--1999
Chromates are tough to remove with ion exchange. There tends to be a low level of leakage of them through a DI column under even optimum (low pH) conditions.
And, if you are using a mixed bed system, the pH within the column will tend to go up, assuming you are demineralizing, and the anionic resin is in the OH- form.
If you cannot live with the problem, you might consider installing a pre-column consisting solely of a strongly basic anion exchange resin in the *chloride* form. This will remove a large proportion of the chromate while the water is still fresh from the process, and the pH is still low. The chloride will then be removed by the main mixed bed columns, and away you go.
Consultant - The Bronx, New York
Second of three simultaneous responses--1999
You don't say whether you are getting any support from the equipment vendor. Also no word on how much waste you are loading onto the system. Ion exchange lifetime is directly related to concentration of contaminants. If you get sloppy or crank up production the life cycle goes down. It's not filtration, but a chemical reaction.
Mixed bed is not the best way to go for closing rinse loop. Cation concentration in typical waste streams is usually one-third to one-fourth of anion concentration. I think the best layout is 1 volume carbon, 1 volume cation, three volumes anion. If that isn't possible, then separate beds all same size but change anion three times as often. There are tests for determining which canister is exhausted.
microwave & cable assemblies - Mesa (what a place-a), Arizona
Third of three simultaneous responses--
Without seeing and testing your discharge it's hard to say. It may be that your columns aren't sized properly for the load you're putting on them. You may be getting some fouling of the resin beds by poor filtration. Improper flow can cause channeling through the beds. Backflushing of the columns is very important. If it isn't done right you don't get complete removal of the metals from the resin. Are the people that installed it helping you troubleshoot it? Maintaining an IX system at peak performance requires constant monitoring and maintenance. If it were me I'd sure be making my vendor earn the money I paid him for the system.
Mabank, Texas USA
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