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"Filtration of Hydrochloric Acid"

August 8, 2016

Q. I've been tasked with replacing a hydrochloric acid etch/dip tank. This tank is unique in that it was once used as a ferric chloride dip and then 'converted' to a acid etch/dip tank. I say etch/dip because the tank consists of 3 stations. The first station applies current (etch). The second and third are without power (dip).

What we are finding with the current tank is that there is significant build up of solids directly beneath the SS etch plates(This is partly due to the fact that operators hammer the plates to knock solids off instead of removing them and washing down). I'm not sure of the exact composition of the solids, but it is likely a combination of metal salts and SS.

Right now, we replace the acid and clean the tank at 2-3 week intervals depending on usage. Concentration is 25-32% HCL.

I propose installing a V-bottom tank with a pump suction manifold along the bottom of the V with the purpose of removing solids as they settle. These solids would then be pushed into a filter housing and solid-free solution returned to the tank.

My concern is that the solids might dissolve on the filter when exposed continuously to fresh acid cycling through the pump. Am I better off leaving these solids at the bottom of the tank and continuing as usual? Am I going to create more problems pumping these solids for filtration?

Jonathan Leonardi
Engineer - Atlantic, Iowa, USA

August 2016

A. Hi Jonathan. It is uncommon to filter HCl acid dips, but it is not unheard of. It is getting more common in this age of "acid extenders". If you clean the tank every 2-3 weeks out of necessity rather than habit, it sounds like filtration may be worthwhile.

To me the issue is that when you don't know what the precipitate is, and whether it will dissolve in concentrated HCl, you must do something -- whether it is an analysis of the precipitate, or putting it in a beaker of HCl with a magnetic spin bar to see if it dissolves -- before proceeding with a filtration installation based on guesses with no data :-)


pic of Ted Mooney
Ted Mooney, P.E. RET
Aloha -- an idea worth spreading
finishing.com - Pine Beach, New Jersey

August 9, 2016

A. Hi Jonathan

We don't know what you are etching so I can only comment that few if any grades of stainless steel would be expected to last long in hydrochloric acid. You may well be using more acid attacking the plates than etching the work.

geoff smith
Geoff Smith
Hampshire, England

simultaneous August 10, 2016


Thank you for your responses.

Ted, I'll be sure to do some investigation into whether or not these solids can be dissolved. We clean the tank out of habit, but I've heard some legends from our long time operators that solid build up was the main reason why the interval of 2-3 weeks was selected. I definitely have some work to do before making a decision.

Geoff, we are etching a CuPb23Sn2 alloy. The plates are 316SS and are usable for a few months.

Jonathan Leonardi [returning]
- Atlantic, Iowa, USA

August 18, 2016

A. Your electroetch is removing metals, especially lead, from your parts. This is being deposited as a loosely adherent plated sludge on the SS cathode plates.

This sludge contains all of the metals in your parts except maybe iron. It will slowly dissolve when not in contact with the cathodes, and can cause stray currents which could result in uneven etching.

It's best to keep the tank as clean as you can under the circumstances.

Lyle Kirman
consultant - Cleveland Heights, Ohio

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