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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?
Engineer - Atlantic, Iowa, USA
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 :-)
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.
August 10, 2016
- 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.
consultant - Cleveland Heights, Ohio