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Steel Pickling: A Profile
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INTRODUCTION
Steel pickling is part of the finishing process in the
production of certain steel products in which oxide and scale
are removed from the surface of strip steel, steel wire, and
some other forms of steel, by dissolution in acid.
A solution
of either hydrogen chloride (HCl) or sulfuric acid is
generally used to treat carbon steel products, while a
combination of hydrofluoric and nitric acids is often used for
stainless steel.
Steel pickling and the associated process of
acid regeneration result in the emission of hazardous air
pollutants (HAPs).
Currently, the U.S. Environmental
Protection Agency (EPA) is preparing National Emission
Standards for Hazardous Air Pollutants (NESHAP) to apply to
existing and new steel pickling facilities and associated acid
regeneration plants under the authority of Section 112 of the
Clean Air Act.
The U.S. has 103 steel pickling facilities operating
currently.1 Many, but not all, are integrated into iron and
steel manufacturing plants. In an ancillary process, spent
HCl pickle liquor, which contains iron chloride plus HCl
solution is converted by a spray oxidation process into a
marketable iron oxide product plus HCl solution that can be
recycled for the pickling operation.
Ten facilities perform
acid regeneration in the U.S., including two independently
operated plants and eight process lines operating in
conjunction with steel pickling facilities.2
Emissions from
most existing pickling and regeneration facilities are
currently well controlled.
THE SUPPLY SIDE OF THE INDUSTRY
In this section, we describe the steel pickling process
and its relation to the entire steel manufacturing process.
This section also discusses the major by-products and
coproducts of steel pickling, mainly the regeneration of spent
acid.
The section also provides a discussion of steel
products as well as the costs of steel production and acid
pickling.
STEEL PRODUCTION
Figure 2-1 shows the entire set of operations involved in
producing finished steel, which includes five major groups of
activities:
coking, sintering, ironmaking, steelmaking, and
final rolling and finishing.
Coking involves heating coal in the absence of air and
results in the separation of the noncarbon constituents of the
coal from the product coke, which is principally carbon.
Coke
is used as a fuel and source of carbon monoxide in the blast
furnace during iron-making.
Sintering is the process that
agglomerates fine ore particles into a porous mass for input
to the blast furnace.
This process was developed to make use
of the coke fines, iron ore fines, and recovered blast furnace
flue dust.5
During ironmaking, molten iron is produced by reducing
iron ore in the blast furnace.
In this process, elemental
iron is separated by reduction from the oxygen with which it
is combined in iron ore.
Thus, the iron in molten form is
separated from the nonmetallic part of the ore and from
impurities such as sulfur and manganese.6
STEEL PICKLING
During the hot rolling or heat treating of steel, oxygen
from the atmosphere reacts with the iron in the surface of the
steel to form a crust that is made up of a mixture of iron
oxides.
The presence of oxide (or scale) on the surface of
the steel is objectionable when the steel is to be
subsequently shaped or cold-rolled and coated.
Numerous
methods have been used to remove iron oxides from metal
surfaces.
These methods include abrasive blasting, tubling,
brushing, acid pickling, salt bath descaling, alkaline
descaling, and acid cleaning. The preferred method in steel
production is steel pickling.
TYPES OF STEEL
All steels are classified as either carbon or alloy
steel.
Within the alloy classification, however, variations
in both the chemical composition of the product and the manner
in which it is processed yield steels with special properties.
As a result, steel products can be grouped into three grades:
carbon steels, alloy steels, and stainless steels. Table 2-2
provides information on U.S. shipments of steel mill products
by carbon, alloy, and stainless grades for 1992.