Sulfuric acid was historically the acid of choice; however, hydrochloric acid has displaced it. Hydrochloric acid, although more expensive, pickles much faster than sulfuric acid, with less base metal loss. New automatic high-speed steel mills require the integration of rapid systems, including pickling. Operating speeds of 600 ft/min are reported.
Sulfuric acid is fed to the pickling bath at a concentration of about 20% and it is considered spent when half its acid value is replaced by ferrous sulfate, FeSO4. Hydrochloric acid fed at about 20% concentration is almost completely consumed before it is "spent." The spent liquor is estimated as 35-45 lb of pickling acid per ton of steel, resulting in 8-15 gal of spent pickle liquor per ton of steel. For an annual steel production of 50 million tons, an estimated 500 million gal of spent liquor are produced annually.
The pickling area is followed by the rinsing-neutralization area. This may consist of several baths in series, with pickle liquor flowing cocurrent or countercurrent to the steel. Pickle bath heating may be performed by steam injection, which causes some dilution of the liquor, or the steel ware may be preheated. Heated tanks, maintained at temperatures as high as 200°F, tend to concentrate the liquor due to evaporation. Balancing the acid makeup and drag-out, and returning some drag-out to the pickle tank can control concentration.
During sulfuric acid pickling, hydrogen is released, which attacks the fresh surface and causes embrittlement. Chemicals to pacify the surface or inhibit the rate of hydrogen attack can be used. In addition, chemicals called accelerators—which promote the removal of scale without affecting the rate of base metal attack—are utilized. The spent sulfuric acid pickle liquor, half iron and half free acid, may also contain a variety of additional chemicals.
Hydrochloric acid does not cause hydrogen embrittle-ment and has a high intrinsic pickling rate with little base metal attack; additives are not usually needed. Additional advantages of hydrochloric acid include:
• no need for scale breaking
• better surface finish
• faster reaction rate than sulfuric acid
• prevention or reduction of overpickling due to slower attack rate on base metal
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