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Help with Oxalic Acid and Hard Coating

In our Hard Coat anodizing process we have run into an issue with oxalic acid clogging an associated heat exchanger. Our finished product quality has not been effected, only the maintenance of the system.

The bath is approx. 1800 gal solution of oxalic acid and sulfuric acid at 32 degree F. The solution is removed from the tank and pumped to a heat exchanger where the temperature is approximately 24 degree F. The Oxalic acid concentration is specified at 1.9 to 2.1 (right around the saturation point...I have a feeling). About 50% oxalic acid recrystalized during a lab experiment simulating the bath and add conditions. And to add to the story, about 6-8 inches of precipitated oxalic is on the bottom of the tank.

After discussions with the Oxalic representatives, the best answers I have come up with is that we are bouncing on the saturation curve (which I have not seen at 32 degrees)and have supersaturated the bath with Oxalic acid. Consequently, a portion of the oxalic drops out of the added water-oxalic solution (52 °F) and an additional amount drops out at the heat exchanger when the temp drops again. This crystal growth provides additional surface area for further growth and eventually clogs the exchanger.

The only solution (no pun intended) that I have developed is to flush the exchanger as a O&M item and look for a substitute for the Oxalic.

Well all that being said...any ideas.

Chris Mullin
- Greater Hartford, Connecticut, USA
2000

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Put the heat exchanger in the anodizing tank, that should solve the problem(your anodizing tank is probably too small and of insufficient capacity). I mean make the tank bigger and run less parts per load. (Oh sure, if I could do that!)

How many amperes per gallon (of the solution in the anodizing tank) are you running?

  
Tom Pullizzi
Falls Township, Pennsylvania
2000

Why do you use oxalic acid? at 32 °F

Keith Tranmer
- England
2000