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Can we still recycle rinsewater after adding inhibitor to pickling acid?

April 18, 2009

Dear Sir,
We use HCl Conc. 20% for pickling in galvanising plant. After water rinse, the same water is recycled for use in general cleaning and gardening. Now we want to add acid inhibitor (1%) in our concentrated HCl.
We Want to know that after water rinse the same water can be recycled or not. The inhibitor is based on amines and fatty acids.
your response is highly appreciated.

Vikram Bhatia
buyer - India

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Recycled how?

If via ion exchange, that would depend on the type of resins used, and the pK(a)/pK(b) of the organic acid/amines.

If via RO, I am not sure. I believe that non-ionic compounds will go through the membrane, while ionic ones will not. Because amines are weak bases and organic acids are weak acids, it's likely that one or the other will be un-ionized whatever the pH.

Dave Wichern
Consultant - The Bronx, New York
April 24, 2009

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May 6, 2009

Sir,

The amount of inhibitor for HCl depends on HCl conc. and temperature. The concentration of 1% based on HCl conc. is much, much greater than what I used in my METAL FINISHING published article. In that article I used between 0.01% to 0.03% based on total tank volume (e.g. 1 to 3 liters per every 10,000 liters of tank volume). Most HCl inhibitors did not perform properly at these low concentrations and were discarded.

Most unfortunately most inhibitor companies recommend far too high of concentrations of their inhibitors so as to sell more product. I strongly recommend that you obtain a copy of my article "Selection and Use of Pickling Acid Inhibitors," so that you can properly test and determine the best inhibitor available in your area. You should also ignore the inhibitor companies' recommendation of a high concentration. The much lower inhibitor concentration will make it much more likely that you can reuse your rinse water.

Regards,

Dr. Thomas H. Cook
Galvanizing Consultant - Hot Springs, South Dakota, USA