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Neutralizing HCl after metal is dipped




Q. I am a gold extractor and use a locally assembled ball mill I got second hand galvanized steel that I used to assemble the ball mill however the crushed gold bearing stones seems to have mixed with the zinc coating in the ball mill which altered the mode of extraction, how do I clean the inside of the ball mill? What chemical is safe, cheap and best to get rid of the zinc coating in the ball mill?

David Boen
- Rift Valley, Kenya
February 12, 2023


A. Hi David. HCl is cheap. Slightly more expensive would be inhibited HCl (containing organic ingredients that tend to coat exposed steel to lessen the attack). But zinc is so much more aggressively attacked by HCl than steel is, that plain HCl (probably about 10-15%) will quickly remove the zinc and probably not severely affect the steel in the half minute or minute needed to remove the zinc. After treatment, rinsing with highly diluted sodium bicarbonate will neutralize it.

"Safe" is another matter though. Chemical removal of zinc and "safe" is probably an oxymoron ... but with reasonable care and personnel protective equipment neither people nor machinery should get hurt. Luck & Regards,

pic of Ted Mooney
Ted Mooney, P.E. RET
Striving to live Aloha
finishing.com - Pine Beach, New Jersey




⇩ Closely related postings, oldest first ⇩



Q. We clean paint from metal using a burn off oven and then power wash off loose ash before dipping in a HCl solution to loosen the rest of the ash. We then power wash again to clean the rest of ash as well as the HCl. The acid does not seem to rinse completely off and some is left on the sub part of the steel which then leaches out after the rust inhibitor is applied. We would like to be able to dip the parts in a solution to neutralize the acid and then rinse parts to be sure all acid is gone. Any help would greatly be appreciated.

Dale Healey
Paint removal - Muscatine, Iowa
2006


A. Anodizers process their parts in 10 percent sulfuric acid and, to deal with rinsing problems like this, they either neutralize in dilute sodium bicarbonate (baking soda [on eBay or Amazon affil links] ) or dip in dilute nitric acid to improve the rinsing, or alternate between hot and cold rinses. Good luck.

Ted Mooney, finishing.com
Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey




HCl After Pickling -- is there a residue left

Q. After pickling steel coils with Hydrochloric Acid and sending them through a water rinse and oil bath; is there a chance of "trace" residue of HCl left behind in the crevices of the material? Thank you,

Kathy Reed
- Lenoir City Tennessee USA
August 25, 2016


A. Hi Kathy. It is absolutely possible to do insufficient or even poor rinsing which leaves acid residue on the coils. It's probably best to look at it from the opposite direction, i.e., what is the evidence that the rinsing is sufficient.

Regards,

pic of Ted Mooney
Ted Mooney, P.E. RET
Striving to live Aloha
finishing.com - Pine Beach, New Jersey


Q. Thank you for your reply. It sort of answered my question but what I needed to know was, and maybe I did not make it clear: after going through the rinse no matter how thoroughly they are rinsed is there a "trace" of acid left in or on the coil either on the surface or in the porous edges of the material? From things I have read, HCl basically etches itself "into" the material and there is no way to remove "traces" of HCl? Again, thank you.

Kathy Reed [returning]
- Lenoir City Tennessee USA


A. Hi again. There are 6.02 x 10^23 molecules of anything in a mole, and no way you'll ever eliminate the last molecule, so there is probably no such thing as no trace of something in anything.

As a practical matter, it's probably not good practice to expose steel to HCl and leave it unneutralized. I'm not familiar with steel mill practices, but I would expect that in some way or other a neutralizing step would be applied, as flash rusting would be expected otherwise. It would not surprise me to hear that a dilute sodium bicarbonate dip is incorporated or that the rust inhibitor oil includes neutralizing agents. Regards,

pic of Ted Mooney
Ted Mooney, P.E. RET
Striving to live Aloha
finishing.com - Pine Beach, New Jersey




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