Cleaning and liquid contact with solids – Processes – For metallic – siliceous – or calcareous basework – including...
Reexamination Certificate
1999-07-15
2001-06-26
Carrillo, Sharidan (Department: 1746)
Cleaning and liquid contact with solids
Processes
For metallic, siliceous, or calcareous basework, including...
C134S002000, C134S015000, C134S026000, C134S028000, C134S029000, C134S030000, C134S032000, C134S036000, C134S040000, C134S041000, C134S12200P, C134S06400P
Reexamination Certificate
active
06250314
ABSTRACT:
FIELD OF THE INVENTION
The present invention is directed to a process a pickling stainless steel. More particularly, the invention is directed to a process for pickling stainless steel in an acid bath.
BACKGROUND OF THE INVENTION
The process of pickling carbon steel with hydrochloric acid has established itself world-wide over the past few years as an effective pickling process and is used in virtually all countries and steel works. In stainless steel pickling, however, there was always the technical belief that conventional steel pickling methods could not be used for stainless steel because of the different levels and type of scale formation on stainless steel. Surprisingly, however, it was shown that a pickling effect most certainly could be achieved with hydrochloric acid. However, the pickling times required to obtain a scale-free surface on stainless steel using hydrochloric acid are very long compared with carbon steel. A steel strip of grade St 12, for example, is pickled over a maximum of 30 seconds at 80° C., whereas a comparable strip of grade AISI 409 stainless steel takes up to 10 minutes at 90° C. These extremely long pickling times for stainless steel using hydrochloric acid are unsuitable for practical operations because the number and length of tanks needed for pickling would be too great.
During the formation of stainless steel sheet products, a layer of scale is typically produced on the surface of the steel sheet. The scale must be removed, typically by pickling, before the steel can be used or further processed. Many stainless steel pickling baths use a mixture of hydrofluoric acid and nitric acid. The mixture of these acids causes numerous problems associated by the sludge formation of these acids in the tanks. In addition, acid pickling of stainless steel by the conventional process is a very slow process and requires complex treating baths to provide adequate retention time of the stainless steel in the acid bath. The sequential pickling steps result in increased operating costs.
Accordingly, there is a continuing need in the industry for improved methods of pickling stainless steel.
SUMMARY OF THE INVENTION
The present invention is directed to a process and apparatus for pickling stainless steel. More particularly, the invention is directed to a continuous process for pickling a continuous strip of stainless steel in a hydrochloric acid bath in an efficient and economical fashion.
Accordingly, a primary aspect of the invention is to provide a process and apparatus for efficiently pickling stainless steel in an acid bath with reduced pickling times compared to prior processes.
Another object of the invention is to provide a process and apparatus for pickling stainless steel in a hydrochloric acid solution where the solution contains an oxidant.
Another object of the invention is to provide a process and apparatus for pickling stainless steel in a hydrochloric acid solution where an oxidant is added to the acid to oxidize at least a portion of the dissolved iron to ferric chloride.
Another object of the invention is to provide a process and apparatus for pickling stainless steel in an aqueous hydrochloric acid solution where an oxidant is added to the hydrochloric acid solution in an amount based on the calculated dissolved iron content in the acid solution.
Still another object of the invention is to provide a process and apparatus for pickling stainless steel in an aqueous hydrochloric acid solution where at least one third of the soluble iron content in the acid solution is present in the form of ferric chloride.
Another object of the present invention is to provide a process and apparatus for pickling stainless steel in an aqueous hydrochloric acid solution where at least one half of the soluble iron content in the acid solution is present in the form of ferric chloride.
A further object of the invention is to provide a process for pickling stainless steel in an aqueous hydrochloric acid solution at a temperature of at least 40° C.
A further object of the invention is to provide a process for pickling stainless steel in an aqueous hydrochloric acid solution at a temperature at or below the boiling point of an azeotropic hydrochloric acid solution.
Another object of the invention is to provide a process of pickling stainless steel in an aqueous hydrochloric acid solution where the ferric chloride content in the acid solution is measured and an oxidant is added to the solution in an amount based on the measured ferric chloride content
Still another object of the invention is to provide a process and apparatus for pickling stainless steel in an aqueous hydrochloric acid solution where an oxidant is added to the solution in an amount based on the weight or surface area of the stainless steel strip being pickled.
The objects of the invention are basically obtained by providing a process for pickling stainless steel comprising the steps of feeding a stainless steel article into a bath of an aqueous hydrochloric acid solution and continuously adding an oxidant to the hydrochloric acid solution in an amount in proportion to the surface area to the stainless steel article to maintain at least about one third of the dissolved iron content as ferric chloride.
The objects of the invention are further obtained by providing a process for pickling a stainless steel strip comprising the steps of providing a plurality of pickling tanks arranged in series where each tank contains an aqueous hydrochloric acid solution, feeding a continuous stainless steel strip through the hydrochloric acid solution in each of said tanks to pickle the surface of the strip, and continuously adding an oxidant to the hydrochloric acid solution in each of the tanks in an amount to oxidize Fe
2+
to Fe
3+
and to maintain a ferric chloride content of at least about one third based on the total dissolved iron content of the acid solution.
Other objects, advantages and salient features of the invention will become apparent from the following detailed description which in conjunction with the annexed drawings disclose a preferred embodiment of the present invention.
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Braun Edgar
Maresch Gerald
Wasserbauer Brigitta
Andritz-Patentverwaltungs-Gesellschaft m.b.H.
Carrillo Sharidan
Roylance Abrams Berdo & Goodman L.L.P.
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