Cleaning and liquid contact with solids – Processes – For metallic – siliceous – or calcareous basework – including...
Reexamination Certificate
2000-04-28
2003-04-29
Gulakowski, Randy (Department: 1746)
Cleaning and liquid contact with solids
Processes
For metallic, siliceous, or calcareous basework, including...
C134S002000, C134S004000, C134S019000, C134S026000, C134S028000, C134S034000, C134S041000, C510S101000, C510S245000, C029S081010, C029S254000, C029S254000, C029S254000, C029S401100
Reexamination Certificate
active
06554908
ABSTRACT:
TECHNICAL PROBLEM
In the course of processes of manufacture of steel products that undergo hot-rolling operations or of intermediate products that undergo thermal treatment, such as annealing, it has long been known that the material becomes coated with a more or less thick layer of products of oxidation. It is therefore necessary, considering the need to obtain a degree of surface finish of the finished product that is smooth and shiny, to removing entirely such oxide layers. This is performed through the well-known processes of pickling, for which mixtures of inorganic mineral acids, such as hydrochloric, sulphuric, nitric, and hydrofluoric acids, either alone or in various ratios between them, are generally used.
In the field of stainless steels, on the basis of the knowledge of the industrial processes today in use, the most commonly employed pickling operation involves the use of a mixture of nitric acid and hydrofluoric acid, the reciprocal concentrations of which in the mixture vary according to the type of plant, the type of steel to be pickled, its surface characteristics, and the geometry of the product to be treated. The process is certainly economic and enables excellent results to be obtained; it presents, however, the very serious drawback of creating problems of an ecological nature that are very important and difficult to solve on account of the use of nitric acid. In fact, on the one hand, vapours of nitrogen oxides of the general formula NO
x
are emitted into the atmosphere, these vapours being extremely polluting and aggressive in regard to the metallic and non-metallic materials with which they come into contact, and, on the other hand, in the washing water and in the exhausted baths high nitrate contents are reached, which consequently must then be disposed of. The problems of purification both of the NO
x
present in the air and of the nitrates present in the baths entail major problems in terms of plants and systems, high management costs, and the uncertainty of achieving the results required by current standards and regulations in this connection. In the final analysis, then, the expenditure in terms of investments is difficult to sustain in the majority of industrial plants.
A pickling system that does not require the use of nitric acid is therefore called for by industry, and various proposals, above all in the last ten years, have been made in this connection throughout the world.
Processes Alternative to the Use of Nitric Acid
STATE OF THE ART
From a critical examination of the patents for cycles proposed as an alternative to the traditional processes for pickling stainless steel based upon HNO
3
+HF, which no longer contain nitric acid, and from a critical examination of the main technical literature on the subject, it has emerged that:
The British patent No. 2 000 196 of TOKAI Denka Kogyo envisages the use of a pickling bath consisting of ferric sulphate and hydrofluoric acid: to maintain an adequate concentration of ferric ions during the process, H
2
SO
4
and hydrogen peroxide in a molar ratio of 1:1 are continuously fed in. The patent claims the method for controlling the process by continuously measuring the redox potential of the system, which is to be maintained at values ≧300 mV by regulating introduction of H
2
SO
4
+H
2
O
2
.
The two European patents, which moreover are very similar to one another, EP 188975 and EP 236354 (=WO 87/01739), bearing as dates of priority Jan. 22, 1985 and Sep. 19, 1985, respectively, envisage the use of a pickling solution consisting of hydrofluoric acid (5-50 g/l) and trivalent iron ions introduced in the form of fluorinated complexes, into which air or oxygen is blown continuously. The treatment time is from 30 seconds to 5 minutes, and the temperature ranges from 10° C. to 70° C. A continuous monitoring of the redox potential is moreover recommended; this potential must be maintained, for the first patent, at between −200 and +800 mV, and, for the second patent, at between +100 and +300 mV, and a possible addition of an oxidant, such as potassium permanganate or hydrogen peroxide, is recommended if it is necessary to increase the potential value. All the tests carried out regard pickling of sheet steel alone.
A substantial step forward has been made with the pickling process described in the European patent 582 121 of the present applicant, which employs a bath of H
2
SO
4
+HF containing Fe
3+
and Fe
2+
ions and operating at a controlled redox potential where the re-oxidation of the Fe
2+
ions to ferric ions is obtained by periodic additions of H
2
O
2
to the pickling solution.
PROCESS ACCORDING TO THE INVENTION
The process that is the subject of the present patent application constitutes a technically valid, innovative, and from certain points of view, economically advantageous, evolution of the known processes mentioned above; in particular, it represents an important improvement of the previous patent EP 582 121 of the same applicant.
The process according to the present invention has proved particularly suitable for the pickling of stainless steels of the austenitic, ferritic and martensitic series, duplex steels, superaustenitic and superferritic steels, and Ni-based or Ni/Cr-based superalloys.
The process is based on the use of a pickling bath containing iron ions, HF, H
2
SO
4
, chloride anions, and conventional additives of the wetting, polishing and inhibiting-agent types, in which an oxidizing agent is continuously or periodically introduced, which is able to convert Fe
2+
ions that form in the pickling process into Fe
3+
ions, maintaining the redox potential of the pickling solution at the pre-established value.
The oxidizing agent can be introduced into the bath directly just as it is or in the form of an aqueous solution. The oxidation of Fe
2+
to Fe
3+
can be performed outside the pickling bath as a separate phase of operation, in particular with the method of electrolytic oxidation, such as the one described by WO.97/43.463; or else, it is possible to use air as oxidizing agent in the presence of a copper salt dissolved in the pickling solution as catalyst.
The basic characteristic of the process is the presence in the pickling bath of chloride anions in a ratio of 0.1 to 10 g/l, preferably 1 to 5 g/l.
The presence of chloride ions at the aforesaid concentration in HF, H
2
SO
4
and Fe
3+
ion-based pickling solutions increases macroscopically the rate of pickling of stainless steels. This property is all the more evident and advantageous from the industrial standpoint, the greater the difficulty in removing the oxidized layer from the surface of the material.
The properties of the oxide layer, and hence its ease of removal from a stainless-steel substrate, depend upon numerous variables, the most important of which being the composition of the alloy, the conditions of the forms of heat treatment to which the material is subjected, and the thickness and compactness of the oxide layer.
In industrial practice, the situations presenting greater difficulty are found in the treatment of duplex steels, austenitic steels with high chrome content, and chrome-nickel alloys.
In these cases, it may be found that the only way to achieve an effective pickling result in acceptable times is to carry out, prior to the pickling stage, a stage of mechanical pre-treatment (sand-blasting) or chemical pre-treatment (hot-oxidizing aqueous solutions, such as NaOH+KMnO
4
) or chemico-physical pre-treatment (oxidizing or reducing molten salts) so as to modify the compactness or nature of the oxide.
In addition, sometimes a preliminary chemical treatment is carried out with an aqueous solution containing H
2
SO
4
, HCl, HF and their mixtures.
It has been found, according to the present invention, that the addition of chloride ions to a pickling solution containing sulphuric acid, hydrofluoric acid and ferric ions enables, in addition to the acceleration of the pickling rate on each degree of material, also direct pick
Giordani Paolo Bruno
Negri Dario
Gulakowski Randy
Hedman & Costigan ,P.C.
Henkel Kommanditgesellschaft auf Aktien
Kornakov M.
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