Cleaning and liquid contact with solids – Processes – For metallic – siliceous – or calcareous basework – including...
Reexamination Certificate
1999-12-21
2002-06-04
Carrillo, Sharidan (Department: 1746)
Cleaning and liquid contact with solids
Processes
For metallic, siliceous, or calcareous basework, including...
C134S002000, C134S022130, C134S022160, C134S022170, C134S023000, C134S026000, C134S027000, C134S028000, C134S029000, C134S032000, C134S036000, C134S041000, C134S042000
Reexamination Certificate
active
06398876
ABSTRACT:
FIELD OF THE INVENTION
The present invention is directed to a process for the chemical and/or electrochemical pickling of stainless steel in an acid solution substantially in the absence of nitric acid. More specifically, the invention is directed to a process for pickling stainless steel using the spent electrolyte solution from an electrolytic pickling tank.
BACKGROUND OF THE INVENTION
The surface layer of oxide on a steel strip is generally objectionable when the steel is to be further processed. Hot rolled steel typically has a layer of scale on the surface which must be removed prior to drawing, stamping or other deforming process. The scale produced by hot working of steel is formed of iron oxides and other metal oxides. The scale formed on stainless steel typically contains chromium oxides and manganese oxides in addition to the iron oxides. Pickling is the process of chemically removing metal oxides and scale from the surface of the metal by exposing the oxide layer to inorganic acids. The rate and extent of pickling depends on several variables including, for example, the temperature and acid concentration of the pickling bath. Numerous acid pickling processes and pickling tanks are known in the industry.
Stainless steel sheets or strips are often produced by a hot rolling process followed by an annealing process which produces a layer of scale on the steel strip. The layer of scale is often removed by a pickling process using an acid mixture of hydrofluoric and nitric acid. The concentration and ratio of the acids can vary, depending on the plant and type of steel being pickled. Acid pickling of stainless steel produces good results, but produces considerable environmental concerns. The pickling processes that use nitric acid generate nitrous vapors (NO
x
). The nitrous vapors must be contained and removed from the system to comply with environmental regulations. The nitrates also are carried through the wash water and in the spent acid baths, thereby increasing the expense for proper disposal.
As a result of the draw backs of nitric acid pickling solutions, several processes have been proposed to pickle stainless steel without nitric acid. These processes generally replace the nitric acid with a different mineral acid such as sulfuric acid or hydrochloric acid. The redox potential is adjusted by the content of the iron oxides in solution. In the pickling process, iron in the form of bivalent ions dissolves in solution, and the trivalent iron ion is produced either by partial oxidation of Fe
2+
or by adding Fe
3+
salts. Various processes are known where the trivalent iron ions are produced by the oxidation of Fe
2+
with hydrogen peroxide. Other processes add Fe
3+
salts to the pickling bath in the form of sulfates, chlorides, nitrates or fluorides.
One example of a process for descaling stainless steel is disclosed in U.S. Pat. No. 5,879,465. The disclosed method treats the stainless steel with hydrochloric acid followed by a mechanical abrasion.
The hot-rolled stainless steel strip after pickling is often rolled to a thinner thickness and subsequently annealed a second time. This process produces a thin layer of scale which must be removed gently without damaging the surface quality or shine of the steel. Electrochemical pickling processes have attained general world-wide acceptance for pickling cold rolled stainless steel strips. The electrochemical pickling process can include a sodium sulfate solution. This process produces a spent pickling solution containing large amounts of chromate ions (Cr
6+
). Since the chromate ions are highly toxic, the spent pickling solution must be treated to remove the chromate ions before the solution can be discarded. Spent pickling solution is generally detoxified by reducing the Cr
6+
to Cr
3+
in a separate treatment process. The detoxifying step must be performed before the spent solution can be neutralized and the pickling solution regenerated. Chromate detoxification is very costly and requires the addition of various reducing agents such as Na
2
S
2
O
5
or similar sulfur compounds. The process for reducing Cr
6+
to Cr
3+
is as follows:
4H
2
CrO
4
+3Na
2
S
2
O
5
+3H
2
SO
4
→2Cr
2
(SO
4
)
3
+3Na
2
SO
4
+7H
2
O
An alternative reduction process is as follows:
2H
2
CrO
4
+6FeSO
4
+6H
2
SO
4
→2Cr
2
(SO
4
)
3
+3Fe
2
(SO
4
)
3
+8H
2
O
Various electrolytic pickling processes are known in the art. These processes apply a voltage between the electrolyte and the steel being treated using a reference electrode. One example of a prior process for electrolytically pickling a stainless steel sheet uses an alternating current in a mixed acid bath as disclosed in U.S. Pat. No. 5,786,556. Another process for electrolytically pickling a stainless steel strip is disclosed in U.S. Pat. No. 5,804,056. This process passes the stainless steel through an acid bath containing a plurality of anodes and cathodes arranged in an alternating fashion. The electrolytic pickling of stainless steel results in toxic chromium in the electrolyte. One prior process disclosed in U.S. Pat. No. 4,851,092 adds an acid and reducing agent to produce a solution free of CrO
4
2−
.
The above-noted processes for treating stainless steel to remove the scale are generally effective in removing the oxide layer. However, due to the environmental concerns, these processes have required additional detoxification steps and the expense for the detoxification. Accordingly, there is a continuing need in the industry for improved processes for pickling stainless steel.
SUMMARY OF THE INVENTION
The present invention is directed to a process for the chemical and/or electrochemical pickling of stainless steel. More particularly, the invention is directed to a process for pickling stainless steel in an acid bath substantially in the absence of nitric acid.
Accordingly, a primary object of the invention is to provide a pickling process for stainless steel which overcomes the disadvantages of the prior processes by eliminating the need to contain and remove nitrous vapors from the pickling tank.
A further object of the invention is to provide a pickling process for stainless steel which eliminates the requirement for detoxification of the chromate ions in the spent pickling solution.
A further object of the invention is to provide an acid pickling process for stainless steel where a spent sulfate electrolyte pickling solution is added to the pickling acid in an amount to reduce Cr
6+
to Cr
3+
.
Another object of the invention is to provide an acid pickling solution that contains spent electrolyte solution from an electrochemical pickling tank.
Another object of the invention is to provide a chemical or electrochemical process for pickling stainless steel and particularly a cold-rolled stainless steel using an acid pickling solution containing spent sodium sulfate electrolyte solution from an electrochemical pickling bath.
Still another object of the invention is to provide a process for pickling stainless steel where the Fe
2+
can be oxidized to Fe
3+
without the addition of conventional oxidizing agents.
A further object of the invention is to add an acid to a spent electrolyte neutral salt solution to simultaneously oxidize Fe
2+
to Fe
3+
and reduce Cr
6+
to Cr
3+
.
Another object of the invention is to provide a process for pickling stainless steel in an acid pickling solution where the redox potential of the acid is adjusted by controlling the amount of a spent electrolyte solution from an electrochemical pickling bath.
A further object of the invention is to provide a process for pickling stainless steel using an acid pickling solution where bivalent and trivalent iron salts are added to the pickling acid.
Another object of the invention is to provide a process for pickling stainless steel by providing a neutral salt electrochemical pickling bath and an acid pickling bath in series where the spent electrochemical pickling solution i
Gamsriegler Dietfried
Starcevic Jovan
Andritz-Patentverwaltungs-Gesellschaft m.b.H.
Carrillo Sharidan
Roylance Abrams Berdo & Goodman L.L.P.
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