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Ted Mooney, P.E. RET
Pine Beach, NJ
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for Metal Finishing since 1989
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Harmful Impurities in Liquid Caustic Soda
Q. Recently the price of caustic soda ⇦liquid caustic soda in bulk on Amazon [affil link] has risen beyond any expectation. Similarly, as a consultant, I have had to service more and more customers complaining about poor finishes after caustic soda etching of aluminium. I once read that a number of possible impurities (amongst which Cl seems to be the most harmful)contained in low grade caustic soda may be responsible of such problems. Anyone knows more about this and can send me some references I can refer to? Thanks
Richard Reed- Naples, Italy
2007
A. Since no one else has weighed in, I will give you the 5 cent sodium hydroxide spiel.
The best kind of caustic there is is mercury cell caustic. It's made by electrolyzing brine with mercury metal as the cathode, separating the Hg, then reacting it with water. Since the brine never directly contacts the product, it can be very low in chloride.
Lower grade is diaphragm cell caustic. In this process, the chlor-alkali cell is divided by a permeable divider that separates the chlorine part of the cell from the caustic part. The product is separated from the brine by "freezing out." Of course, it is loaded with chloride.
In between grade is membrane cell caustic, where the two compartments are separated by a cation permeable membrane, such as DuPont's Nafion. I think they may actually start out with some caustic soda as the catholyte, and it increases in concentration as the reaction proceeds. The sodium to make the NaOH comes through the membrane - the chloride from the brine is held back - mostly. There's still some in the product, but less than in the second kind I discussed above.
Chloride is a notorious troublemaker in many processes. It promotes pitting, etc. because it is a redox catalyst and initiator of corrosion reactions.
I hope this has been of some help.
Dave Wichern
Consultant - The Bronx, New York
2007
2007
A. The reason you are no longer getting reliable quality caustic is that mercury is subject to the ever increasing number of EU directives on chemicals.
Once again, the 'cure' is worse than the problem.
Geoff Smith
Hampshire, England
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 Ted Mooney, P.E. Striving to live Aloha finishing.com - Pine Beach, New Jersey 2007 2007 Ted Geoff Smith Hampshire, England |
Geoff, please don't tell me that when writing the new directives, EU forgot to repeal The Law of Unintended Consequences?! They can look to the USA for model legislation; here, I'm told, laws don't have unintended consequences so we should not worry about it.Q, A, or Comment on THIS thread -or- Start a NEW Thread