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Re: Chromate Conversion Coating



I worked with a chromium conversion coating operation ten years ago and we took their dilute rinse water effluent to zero, by the following process:  (1) replace each rinse tank with three such tanks.  (2) counterflow rinse water (parts go one way, rinse water the other) from one rinse tank to another; reduce rinse water flow to 0.1 % (or less) of the previous flow rate.  (3) (optional) replace tap water for rinses with RO water rinses -- this increases life of process tank contents dramatically and also improves quality of coating.  (4) use now more concentrated rinse water as make up water into tank preceding the rinse -- if you are using heated tanks, the amount of water should be approximately in balance or can be made so. 
 
We reduced rinse water to be treated/disposed to zero, reduced overall process water consumption by 99.8+ percent, improved quality and productivity (fewer rework or discarded parts due to poor quality coating).  The plant buys its RO water and has it hauled in -- less expensive for them than operating own RO system and paying for permits, etc., and is effectively a no-discharge operation.  Process tanks are changed out much less frequently (water contamination dragged into tanks was the cause of frequent changes), and the process tank contents are more readily recycled or recovered for reuse, since they lack the water-borne contaminants.
 
RC
 
Ralph E. Cooper, Ph.D.
Attorney and Mediator
San Antonio, TX 78230
 
----- Original Message -----
Sent: Monday, September 10, 2001 9:48 PM
Subject: Chromate Conversion Coating

Greetings P2Techies:
 
I would appreciate pollution prevention ideas for hexavalent chromium conversion coating. 
 
Are there any possible substitutes (trivalent chromium is not a possible substitute) or methods to reduce losses (no-rinse coating processes?).  I have never seen chromium recovered from the dilute rinsewaters of chromate conversion coating and reused in the process, but I have seen literature references.  Is this alternative practical and realistic?  If so, what is the best technology?
 
Also, Does the chromium rinsed from the coated metal contain significant quantities of trivalent chromium as well as hexavalent chromium?  In other words, during the conversion coating process does the hexavalent chromium at the surface of the coated metal get converted to trivalent chromium to any appreciable extent?
 
Thanks,
 
Rob Michalowicz
Chem Process & Environment Inc.
rob.michalowicz@sympatico.ca