US2314369A - Continuous steel strip pickling - Google Patents

Description

March 23, 1943'. 1'. F. REED 2,314,369

CONTINUOUS STEEL STRIP PICK'LING l Filed Aug. 22, 1941 Patented Mar. 23, 1943 CONTINUOUS STEEL STB/IP PIOKLING Thomas F. Reed, Dormont, Pa., 'assignor to United States Steel Corporation of Delaware, a corporation 'of Delaware Application August 22, 1941, Serial No. 407,967

2 Claims.

This invention is a steel pickling method, one

of the objects being to pickle continuously moving steel, such as steel strip. in a truly continuous manner so as to Aattain the maximum possible use from the pickling solution used.

The accompanying drawing schematically illustrates oxidized steel strip being pickled in accordance with this method.

In the drawing the strip S continuouslyr travels consecutively through vats number I through 4 (the vat I being arranged at a lower level than vats 2 through 4, and these last-named vats all being arranged at the same level). The arrangement is such that the pickling solution flows from vat 4 to vat I, from which the solution goes to the sewer, thus producing a counterflow of pickling solution respecting the traveling direction of the strip S. Guiding and sinker rolls are arranged in an appropriate manner to lead the strip S from vat to vat with the strip immersed in the solution in each vat.

As the strip leaves vat 4, it is sprayed with water by the water sprays illustrated, thus washing the major part of the pickling solution carried from vat 4 by the strip back into this vat. This continuously introduces water to vat 4. Also, as shown, strong acid of known concentration, Which will normally be sulphuric aci-d, is continuously introduced to vat 4, this mixing with the water from the water sprays and resulting in the maintenance of a supply of pickling solution in vat 4. The strength of this solution is constantly maintained at a known concentration by properly proportioning the ratio between the water and acid introduced in the manner described. It is obvious that the pickling solution in this vat 4 will be the strongest or most active in the system, since it flows from this vat through the other vats counter the travel of the oxidized steel strip, the acid concentration decreasing and the iron sulphate increasing as the solution ows toward vat I..

y In the interests of economy, when the ipickling solution leaves vat I and goes to the sewer, it should have the minimum acid. concentration and the highest iron sulphate concentration that is practical. This desirable end is attained by properly correlating the acidity of the solution in vat 4 with the rate of ow of the solution respective to the traveling rate of the steel strip S and to the amount of oxide which must be removed from its surface. This naturally requires some sort of control, which is provided in a very sensitive mannerby the present invention, as will presently be realized.

Elteairi is introduced to each of the vats in the manner illustrated by the drawing, this being for the purpose of maintaining the solution in the various vats at temperatures above atmosphere, as is common in the steel pickling art. The strip S is relatively cold when it enters vat I, and in this vat its temperature does not rise suiiiciently high to result in a very rapid pickling rate. However, by the time the strip reaches vat 2, its temperature has been increased to a fpoint where very rapid pickling action takes place. particularly since the acidity of the solution in vat 2 is greater than it is in vat I. Since at this time the greater proportion of the oxide is removed from the strip, the pickling rate is slower in vats 3 and 4, in spite of the higher -activity of the solutions in these vats.

Although in the setup illustrated, the greatest pickling rate is attained in vat 2, it will be found that in any lsystem where oxidized steel is continuously moved through a counterlow of heated pickling solution, a zone will 'be encountered where the pickling proceeds at a maximum rate. The :pickling action will be slower prior to the steel encountering this zone because the steel will not have a sufllciently high temperature and because the activity of the solution will be relatively lower, and after the steel leaves this zone, the pickling action will be slower in spite of the greater activity due to the higher acid concentration of the solution and in spite of the steel having arrived at a higher temperature, because the greater proportion of the oxide will have been pickled` from the steel in the zone of maximuni activity.

According to the present invention, this zone of maximum pickling activity is located -by suitable tests which are within the skill of a competent chemist. Since the oxide on the steel is normally uniform and since the traveling rate of strip and the solution temperature can :be easily fixed, it is possible to maintain the ratio between the acid and the iron sulphate in the solution leaving the system from vat I at a value representing most economical operation, by determining the amount of acid required in the zone of maximum pickling activity to produce this result and then maintaining this degree of acidity in this zone. Although the pickling rate throughout the system varies proportionately with the acid concentration and the temperature at all locations, the selection of the zone of maximum activity for effecting the acidity control of the system, results in an extremely sensitive control, since the acid concentration in this zone varies much more than it does in those zones Where the reaction rate is slower.

Knowing the acidity that should be maintained in the zone of maximum activity, which is vat 2 in the illustrated arrangement, it becomes necessary to maintain this acidity in some manner. According to this invention, this is done by mantaining the acidity in vat 4 at as high a value asis possible without harming the steel strip, this being done by the introduction of acid in the manner illustrated as required to maintain this high degree of acidity. By testing the acidity of the solution in vat 2, it is possible to determine whether this acidity has dropped below the value set as being necessary for most economical pickling, and if it develops that the acidity in vat 2 is too low, then the water introduced vto vat 4 by the water sprays Working on the exiting strip is increased so as to wash the more strongly acid pickling solution from vat 4 through vat 3 and into vat 2 at an increased rate as required to maintain the acidity in Vat 2.

The increase in the introduction of the water is, of course, regulated constantly as required to maintain the proper acidity in vat 2, it being preferableto use some sort of automatic control at vat 4, which automatically introduces more acid to this vat when its acid concentration drops, and a similar automatic control at vat 2, which automatically increases the water introduction rate at vat 4 when the acidity of the solution in vat 2 drops below the value xed as necessary to provide most economical'pickling. However, it is possible to carry out this method by manually making chemical tests at vats 2 and 4 at suiiiciently close intervals to assure reasonable control.

It is to be noted that the various vats interconneet at the proper level to maintain the solution constantly above the strip traveling through the vats, this likewise being true of the waste outlet for vat I. At the start of pickling operations, the acid concentration in all vats will be substantially the same, excepting that it might be more expedient to load vat I with solution of a relatively low acid concentration. As the process continues, the ac'dity of vat 2 rapidly drops, since this is the zone of maximum pickling activity, but this drop is balanced as soon as possible by the introduction of water to vat 4, which washes fresh acid into vat 2, it being remembered that pickling is proceeding more slowly in vats 3 and 4 than it is in vat 2. Acid fluctuations that are relatively small in other parts of the system are relatively greater in vat 2, and hence this invention aiords a means of easiervcontrol of the acidity of the system.

It is to be understood that a single vat could be used instead of the four vats illustrated, providing the outlet at the end of the vat where the strip is initially introduced was arranged at a sufliciently low level to provide for the necessary solution ilow. In 'such an instance, a certain portion of the solution rst encountered by the strip might be said to constitute a preheating zone which would merge with relative sharpness with a zone where the maximum reaction rate will exist, this rapid reaction zone, in turn, more or less sharply merging with what might be termed an acid storage zone which also performs the function of a pickling finishing zone. Control of the system would then be effected by sampling the acidity in the zone of maximum pickling rate and, in accordance with this sampling, Water would be introduced where the strip is exiting as required to wash more o! the strongly acid solution into this zone oi' maximum pickling action, this in turn requiring the addition of more acid to the storage zone as required to maintain the acidity of the same.

This invention has been practiced by the use oi equipment such as is schematically illustrated. In this instance, it was found that 40%' of the pickling action occurred in vat 2 with the remainder distributed relatively equally among the other vats. It was found that in the case of hot rolled strip, 9 to 10.5 grams of sulphuric acid per cc. of solution in No. 2 vat represented the acid concentration in this vat producing the most satisfactory and economical pickling. 'I'his acidity was maintained by maintaining the acidity in vat 4 at about 18 to 20 grains of sulphuric acid per 100 cc. o1' solution, and washing the acid into vat 2 by the addition of water to vat 4 as required. 'I'he acidity of vat 4 was maintained by introducing 60 B. sulphuric acid into the solution in this vat as required.

It might be supposed that the acidity Vof the system could be controlled directly by the acidity of the solution in vat 2, for instance, which could be done by introducing an acid solution to Vat 4 only' as required to maintain the proper degree of acidity in'vat 2. However, there is a small continuous loss of acid carried from the system by the exiting strip, in spite of attempts to prevent this by the water sprays. This acid loss cannot be easily predicted. Likewise, there is a, Water loss since the water is introduced by spraying the strip, this producing mist representing the water loss and being likewise unpredictable. The trouble incidental to these lossesis obviated by maintaining the acidity of the solution in vat 4 at a fixed value and Washing this solution into vat 2 when required.

I claim:

l. A continuous steel pickling method comprising producing a continuous iiow of pickling solution, heating said solution above atmospheric temperature, continuously passing said steel through said flow in an opposite direction, said steel being heated by said solution during its passage therethrough until it reaches a zone in said flow where pickling action of said solution is at a maximum rate, sampling said solution in said zone and adjusting the acidity of said ow in accordance with the result of said sampling to maintain a predetermined pickling rate in said zone. v

2. A continuous steel pickling method comprising producing a. continuous flow of pickling solution, heating said solution above atmospheric temperature, continuously passing said steel through said ilow in an opposite direction, said steel being heated by said solution during its passage therethrough to increasing temperatures until it reaches a zone in said flow where its temperature is such that it is pickled by said solution at a maximum rate, maintaining the acidity of said ilow in a preceding ilow zone at a higher concentration than in the first-named zone, sampling said solution in the first-named zone to determine the pickling rate, and varying the rate of said iiow from the second-named zone to the rst-named zone as required to maintain a predetermined pickling rate.

'THOMAS F. REED.

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