Where theworld gathers for metal finishing
Q&As since 1989
IRON in a HCl pickle tank?
-----
I have customers that use HCl for pickling, in zinc plating lines.
They generally use the hydrochloric acid between 25 - 50%. I have been asked to tell them
how much iron is acceptable in the pickle, prior to dumping and making up a new solution?
I have read some of the previous comments on related topics, but have not found a fairly specific answer.
Thanks!
- Rockford, Illinois, U.S.A.
2006
2006
Sir:
The most concentrated available commercial hydrochloric acid is about 35% (weight/weight). HCl is commonly used at about 10% to 20% (weight/weight) for pickling steel. Lower concentrations are too slow for pickling and higher concentrations give off too much fume. The maximum (at room temperature) concentration for iron (Fe+2) in a residual 10% free HCl solution is about 12% Iron. Higher iron concentration will crystallize out as ferrous chloride at room temperature.
Interestingly the pickling rate (steel cleaning of mill scale) is faster as the iron +2 concentration increases. Thus what causes HCl to become "spent" is the lack of "free" HCl and too much water.
Regards,
Galvanizing Consultant - Hot Springs, South Dakota, USA
2006
Sir,
Normally a HCl bath for pickling steel starts with a conc. of around 24%. The discard limit would be typically when the Iron content reaches 10 to 12 gms/liter in the bath or in other words 10% to 12%.
HCl pickling should be supported by an efficient Fume Exhaust system.
- Mumbai, India
Q. At normal pickling temperatures i.e. around 80 to 85 °C, even Ferrous Chloride is soluble at concentrations as high as 150 g/l. For Pickling lines with discrete operation, the heating of acid is stopped at the end of the day, ferrous sulphate ferrous sulphate [affil links] crystals precipitate out as the temperature goes down, resulting in choking of acid piping and pumps suction/discharge. What possible solution is there for such problem? Draining out acid at the end of the day?
Arham Saleem- Karachi, Pakistan
November 7, 2012
A. Sirs:
10 to 12 grams/liter is NOT 10% to 12%.
There seems to be some confusion here in mentioning both HCl and H2SO4 pickling in the same question? Does this question relate to HCl OR H2SO4? One thing is certain the person asking the question is NOT familiar with acid inhibitors.
Regards,
Galvanizing Consultant - Hot Springs, South Dakota, USA
A. See I presume you are using HCl now. We discard acid somewhere around 100 gram/liter (it is silly to say 10 grams per liter, you are not galvanizing gold ) but people discard acids from 80 grams. Inhibitors are used but that also weakens the acid. This also depends on the quantity of work you are doing.
Nitesh Agarwal- Mumbai
February 23, 2013
Q, A, or Comment on THIS thread -or- Start a NEW Thread