The Du Pont Waste Pickle Liquor Process
RALPH AARONS, Supervisor
ROBERT A. TAYLOR, Representative
Development and Service
Industrial and Biochemicals Department
E. I. du Pont de Nemours and Company
Wilmington, Delaware 19898
A problem that has long plagued the steel industry is the disposal of the waste
liquor produced when sulfuric acid is used to clean, or "pickle," steel. With
the increasing concern about pollution, the discharge of this spent pickle liquor
into a sewer or stream often is no longer permissible, and steel companies frequently find it costs almost as much to dispose of the waste as it does to buy the
acid.
The du Pont Company, as a leading supplier of sulfuric acid to the steel industry, was acutely aware of the problem confronting the mills. Recognizing the
growing need for a more satisfactory disposal method, du Pont in I960 undertook
a research program aimed at finding such a method. The result of the company's
extensive laboratory and pilot plant studies was a reliable, patented process which
essentiaUy eliminates pollution and in most cases simplifies disposal.
Called the du Pont Waste Pickle Liquor Process, it involves continuous neutralization with lime arid simultaneous oxidation of soluble iron to insoluble magnetic iron oxide. This produces high-density, low-volume granular solids which
are easily separated and disposed of.
Before going into detail about the du Pont process, however, it perhaps would
be best to discuss the problems associated with the neutralization and disposal of
spent sulfuric acid pickle liquor.
Waste pickle liquors typically contain five to 20 per cent ferrous sulfate and
two to 15 per cent sulfuric acid as free acid. They also usually contain additional
small amounts of other metal salts, inhibitors, lubricants ana misceUaneous impurities.
Some alternate disposal methods for pickle liquor are presented in Table I.
As noted, the first-named method, simple dumping, is declining because of the
poUution problem. Other methods for disposing of untreated pickle liquor include
barging of wastes to the sea and deep well disposal. These have some major
limitations,  the most prevalent being that they are not applicable in many areas.
Of the methods involving treatment of the pickle liquor, simple neutralization
with lime slurry is the most common. However, it yields a voluminous, hard-to-
dispose-of wet sludge. The volume of this sludge usually exceeds the volume of
pickle liquor and lime slurry needed for neutralization hecause water must be
added to permit the sludge to be agitated or pumped. The sludge never dries and
must be retained in permanent lagoons. About 10 acre-ft of lagoon volume is required for every 1000 tons of sulfuric acid used for pickling.
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