HCl pickling agitation: Strong warning on compressed air

Summary for hcl pickling agitation Discussion

Air agitation of HCl pickling tanks is generally discouraged. In the thread, Ted Mooney says he would never recommend blowing air through an HCl bath because it greatly increases fumes for very little gain in pickling speed. He suggests that if agitation is truly needed, eductors are a better option, and notes that temperature and concentration are the main controls on pickling rate. He also points out that speeding up one step may not help if the rest of the plating line is slower, since it can just create work-in-process buildup. For the newer question about a 5,000-liter zinc electroplating line, the same caution applies to HCl, while the post also asks whether air agitation would help in water or alkaline cleaner tanks. The thread’s practical takeaway is to avoid compressed air in HCl and consider other methods only where process needs justify them.

[editor appended this entry to this thread which already addresses it in lieu of spawning a duplicative thread]
Q. Hi everyone.
We are into acidic zinc electroplating
Our process starts with degreasing the material with a particular chemical kept for 6-7 mins
Then we take it to HCl acid with titration between 8-18 (35% HCl)
1. I want to know if I use air agitation in the HCl tank will it be beneficial for me?
After HCl we rinse it with water.
2. If I use air agitation in water will it be beneficial for me. If yes, how?
Then we dip the material into alkaline cleaner
3. If I use air agitation in the alkaline cleaner tank will it be beneficial?
I have each tank of around 5k ltr.
Thank you.

Shubham
– Odisha, India
August 14, 2024

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A. Hi Shubham,

We added your inquiry to a thread where 6 experienced platers unanimously believe that adding air agitation to an HCl acid tank is a bad idea. But maybe eductors would be okay.

Air agitation in rinse tanks is almost always a good idea. Your plating tank should have air agitation, see thread 42671, “Air agitation for acid chloride zinc plating bath”. For more info on designing an air agitation system, please see thread 40886, “Agitation air sizing and calculations for plating & anodizing”.

Your alkaline cleaner should be an electrocleaner — this generates lots of gas bubbles at both the anode and the cathode, such that air agitation would hardly be worth it anyway.

Luck & Regards,

Ted Mooney, P.E. RET
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finishing.com – Pine Beach, New Jersey

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⇩ Similar, related, Q&As — oldest first ⇩

Q. Dear All:
Happy New Year,
Is it available to use a compressed air for HCl pickling agitation to accelerate the pickling process?
What are the advantages and what are the disadvantages?
Thank you, and best regards,

Salah Salman
– Cairo, Egypt
2007

A. Salah,
The disadvantages outweigh the advantages here. HCl pickling obviously emits HCl vapors. The escape of HCl is measured by a vapor pressure, the higher the pressure the more HCl escapes. If you add compressed air to the scenario the safety of workers will be greatly affected, the equipment around the pickling process will suffer, not to mention the structure of the building. The push air, fume exhaust, and fume scrubber will be far less efficient with the accelerated escape of the vapors. Sorry, I can’t give you any advantages.

Mark Baker
Process Engineer – Syracuse, New York

A. Poor idea. You will rust out every iron and steel part within 50 feet.Use a pump or a mixer.

James Watts
– Navarre, Florida

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A. Salah
I would never recommend blowing air through an HCl bath. It will greatly increase the fumes for very little increase in speed.
If you must increase the agitation consider using eductors ⇨

The primary controllers of speed are temperature and concentration.
But first, I would ask why you need to increase the speed. The overall speed of a process line is the slowest step. If you have a 30 minute plating process and a 2 minute pickle, you just produce a build up of work on the line.
It is also worth considering if HCl is the best acid. Dilute sulfuric is effective without the fumes and is easier to treat the effluent. It usually needs some heat, but in Egypt probably not much.

Geoff Smith
Hampshire, England

A. I would strongly recommend against using any form of air agitation with an HCl pickle, because of the considerable increase in fumes that would be generated from the process. HCl produces quite a lot of fumes already, and you don’t want to make it any worse. Consider mechanical agitation instead.

John Reid
– Brisbane, Queensland, Australia

A. Dear Salha
you can not use air agitation with any acids pickling tanks it is very dangers . you can use mechanical agitation for the work parts
best regards,

Ali Gomaa
– Cairo, Egypt

A. The best option is to use eductor pumped agitation which uses a chemical resistant pump to push the acid through an arrangement of eductor nozzles to achieve the required agitation without using air and without the nasty fumes associated with air agitation.

Mohamed El-Sharkawy
– Cairo, CA, Egypt
June 9, 2012

Ed. note: Thread 1820, “Eductor agitation benefits”, goes into a little more detail on this, or readers can search the site for dozens of references to eductors.