Cleaning and liquid contact with solids – Processes – For metallic – siliceous – or calcareous basework – including...
Patent
1996-04-08
1998-11-17
Glass, Margaret W.
Cleaning and liquid contact with solids
Processes
For metallic, siliceous, or calcareous basework, including...
134 2, 134 28, 134 41, 510258, C23G 302
Patent
active
058370617
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
The invention relates to a process for the post-treatment of pickled, hot carbon steel strip by application of water in continuous rinsing systems.
It is known that steel material is pickled with acid in order to remove scale. Most modern pickling plants use hydrochloric acid as the pickling medium. After pickling, the residual pickling acid adhered to the material surface, for instance hot strip, must be removed. This is normally done in continuous rinsing systems, such as, for instance, multi-stage counter-current rinsing systems, which are designed as pressure spray systems in most cases.
The pickling acid entrained with the strip from the last pickling stage enters the rising system and is treated with rinsing water section by section counter-currently to the strip moving direction. This brings about a defined concentration gradient of hydrochloric acid and iron ions in the individual rinsing sections. The rinsing water essentially contains the following substances: Fe(II) ions, Cl ions and H+ions. The H+ion concentration determines the acidity of the rinsing water. This acidity is more appropriately stated as pH value, which is defined as negative decimal logarithm of the H+ion concentration.
As the acid concentration of the rinsing water is reduced, for instance, by progressive dilution, the pH value of the rinsing liquid rises. Starting from a certain critical pH value the iron ions are hydrolised, i.e. the following coupled reactions take place:
The bivalent iron hydroxide Fe(OH).sub.2, very easily oxidises with the atmospheric oxygen to form the trivalent iron hydroxide Fe(OH).sub.3.
The sequence of these hydrolysis reactions is to a great extent dependent on the pH value and the temperature. Apart from these physico-chemical influencing factors, the hydrolysis and oxidation reactions are time-dependent. Experience shows that the critical period of time for hydrolysis reactions to occur is about 30 seconds.
In a rinsing system following a pickling plant, the retention time of the strip under normal operating conditions is far shorter than the critical time after which the hydrolysis reactions might commence. Thus, it is impossible for hydrolysis products to be deposited on the strip surface.
However, if the strip is halted due to a malfunction of the pickling operation, depositing of hydrolysis products on the strip surface is inevitable if this failure extends beyond the critical time for hydrolysis reactions. This deposit mainly takes place in the rinsing system's last two rinsing sections, where due to progressive dilution the pH value has risen most. The depositing of hydrolysis products on a metal surface, also called hydrolysis stains, deteriorates the quality of the product. In many cases, material that is strongly contaminated by hydrolysis products is unsuitable for further use. Improvement of the post-treatment of pickled steel products with a view to ensuring advantageous, hydrolysis-restraining conditions constitutes an important technical problem with regard to economy of operations.
Several processes are known to prevent hydrolysis reactions in rinsing systems arranged after pickling plants for steel products:
One method to prevent hydrolysis reactions consists in cooling the rinsing liquid. Due to cooling and on account of the temperature-dependency of the hydrolysis reaction a slight increase of the critical hydrolysis time is achieved.
Experience shows that the factor for the increase of the critical hydrolysis time is about two, i.e. after about 60 seconds of the strip having stood still, hydrolysis products are formed. Operating malfunctions normally last much longer than 60 seconds, and it is thus not possible to achieve a substantial decrease of the hydrolysis stains using this method.
According to another method to prevent hydrolysis reactions, chemicals are added to the rinsing liquid, which inhibit the hydrolysis reactions. Addition of chemicals gives a far better effect than cooling of the rinsing water, but the additives cause problems in the
REFERENCES:
patent: 4536637 (1985-08-01), Horiuchi et al.
patent: 5616424 (1997-04-01), Carey, II et al.
JP 3020486 (abstract), Jan. 1991.
Jirenec Karl
Karner Wilhelm
Andritz-Patentverwaltungs-Gesellschaft m.b.H
Glass Margaret W.
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