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Stripping Galvanize

February 21, 2009

Have noticed that galvanizers strip their rejects and racks in their pickling tanks. Would it not be advantageous to strip racks and rejects in a separate tank and neutralize remaining HCl with ammonia, oxidize to remove iron then use to top up flux.

An alternate question is would it be cheaper to do the stripping in sulfuric acid?

Geoffrey Whitelaw
- Port Melbourne, Australia

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February 27, 2009

Sir:
My article, "Making Hot Dip Galvanizing Preflux from Spent Hydrochloric Stripping Acid,"METAL FINISHING, 39-41, April 1981, can be obtained from a large library through inter-library loan.
Regards,

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

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February 28, 2009

The question requires info on relative acid costs.
Is HCl cheaper than sulfuric for you? In some places it is, and so is used preferentially, in other places the other way about.

It is generally better to strip in a separate tank. If using HCl, you'll end up with Zinc chloride in that tank without iron chloride. Zinc chloride is saleable. Similarly Iron chloride is saleable, but not when contaminated with zinc.
Pickle acid is deteriorated by stripping, so slowing the process.

But practicality sometimes over-rules theory, and many galvanizers strip in their pickle tanks.

Geoff Crowley
Crithwood Ltd.
Westfield, Scotland, UK