Title of Invention

PROCESS AND A DEVICE FOR ELECTROLYTIC PICKLING OF METALLIC STRIP

Abstract

A process and device for electrolytic pickling of metallic strip (2), particularly stainless steel strip, and strip made of titanium, aluminium or nickel, where the electric current is conducted through the strip indirectly, i.e. without electrically conductive contact between the strip and the electrodes (15, 16), wherein the strip is run vertically through the electrolyte liquid fed in between the strip and the electrodes, at least one cathode (16) and at least one anode (15) being disposed on the same side of the strip and the spacing between anodes and cathodes being altered to suit the strip dimension.

Full Text

The invention relates to a process and a device for electrolytic pickling of metallic strip, particularly stainless steel strip, and strip made of titanium, aluminium or nickel, where the electric current is conducted through the strip indirectly, i.e. without electrically conductive contact between the strip and the electrodes. In conventional plants for pickling cold-rolled stainless steel strip, the so-called "neutral electrolyte" process is used. Here, the voltage is induced indirectly in the strip. This means that there are no points of contact between any conductor rolls and the strip. A further feature of this process is that the anodes and cathodes are completely covered with electrolyte and mounted horizontally, i.e. the cells used are flooded and arranged horizontally. Furthermore, a process for electrolytic galvanizing of strip is known from patent no. AT-PS 373 922. In this process, the electrodes are mounted vertically. The electrolytic liquid is fed into the gap between the anodes and the strip. The voltage is induced directly in the strip - the cathodes take the form of conductor rolls. In addition, pickling of stainless steel strip at higher current densities is known from patent no US 4363709. The patent mentions current densities of 40 -60 A/dm2, but does not give any details on the equipment with which this could be realized in a large-scale plant within a reasonable voltage range (= less than 40 volts). The present invention provides a process for electrolytic pickling of metallic strip, particularly stainless steel strip, and strip made of titanium, aluminium or nickel, where the electric current is conducted through the strip indirectly, i.e. without electrically conductive contact between the strip and the electrodes, -1A- wherein the strip is run vertically through the electrolyte liquid fed in between the strip and the electrodes, at least one cathode and at least one anode being disposed on the same side of the strip and the spacing between anodes and cathodes being altered to suit the strip dimension. The cell is space-saving due to its vertical arrangement. The space required is approximately 2 m as compared with 8 m needed for conventional plants. In addition, good strip guiding is achieved because there is no sag. An advantageous further development of the invention is characterized by the spacing between anodes and cathodes being altered to suit the strip dimension. This ensures that the maximum current yield is achieved at the lowest voltage for different strip dimensions. If the anode and cathode are mounted too close to each other a direct current flow exists. On the other hand, the voltage drop in the strip rises, the greater this spacing is. Naturally, this drop in voltage is linked to the strip cross-section. Thus, there is an optimum spacing for each strip dimension. A favorable configuration of the invention is characterized by the amount of electrolyte being fed into gap between the electrodes and the strip being controlled. In this way, the strip can be stabilized hydraulically exactly in the center between the electrodes. Thus, the spacing between the electrodes and the strip can be reduced to a minimum. An advantageous variant of the invention is characterized by the temperature of the electrolyte, concentration of the electrolyte and the loading density (coulomb per unit of area) being set specifically to suit the strip to be pickled. By doing so, the pickling conditions can be set at an optimum for each strip -2- dimension, plant speed, treatment time, type of scale, etc. In a preferred embodiment, the temperature of the electrolyte may be set between 20°C and 85°C, and preferably below 70°C. The concentration of Na2SO4 in the electrolyte may be between 100 and 350 g/I, and preferably around 150 g/I. The loading density may be set between 20 A/dm2 and 250 A/dm2, and preferably approximately 130 A/dm2 and approximately 180 A/dm2 for iridium anodes. An advantageous configuration of the invention is characterized by the gap between the electrodes and the strip being altered. By doing so, the spacing can easily be adapted to the waviness of the strip. A favorable further development of the invention is characterized by the strip waviness being measured and the electrodes moved away from the strip in order to avoid any contact between strip and electrode. This eliminates the risk of contact between the strip and electrodes, which would cause short-circuiting. The invention also relates to a device for electrolytic pickling of metallic strip, particularly stainless steel strip, and strip made of titanium, aluminium or nickel, by the method described above, where the electric current is conducted through the strip indirectly, i.e. without electrically conductive contact between the strip and the electrodes, wherein the strip runs vertically through the process, there being a vertical gap between the electrodes and the strip into which the electrolyte liquid is fed, at least one cathode and at least one anode being disposed on the same side of the strip and the spacing between anodes and cathodes being alterable to suit the strip dimension. The -3- cell is space-saving due to its vertical arrangement. The space required is approximately 2 m as compared with 8 m needed for conventional plants. In addition, good strip guiding is achieved because there is no sag. A favorable further development of the invention is characterized by the anodes and cathodes being located in an offset arrangement in the strip running direction, i.e. the strip either runs between two anodes mounted opposite one another or two cathodes mounted opposite one another. This results in an even flow of current at the strip end and thus, achieves even pickling. An advantageous configuration of the invention is characterized by the surface of the anodes or the entire anodes being made preferably of lead or lead alloys, iridium oxide or graphite. The material the anodes are made of results in chemical passivation in the face of anodic attack by the electrolyte anions. Preferably, lead is used with sulfate ions (SO42-) and iridium oxide with sulfate and/or chloride ions (CI). In addition, graphite can also be used with different anions or mixtures thereof. An advantageous variant of the invention is characterized by a control device being provided for the electrolyte feed, where a separate control device can be provided for each liquid channel between strip and electrode. With this arrangement, the flow can be adapted to suit the strip width on the one hand, and it can also be set to an optimum level for strip varying widths. With the resulting hydraulic strip guiding, the strip can be guided carefully between the electrodes. Conventional plants usually -3A- require two rectifiers for each cell - one for the lower and one for the upper side of the strip. In the cell according to the invention, one rectifier can be installed for one or for several cells. Thanks to the hydraulic strip guiding made possible by controlling the amount of electrolyte fed in, the upper side of the strip, for example, can be pushed closer to the electrodes if it has more scale than the lower side and thus, pickled more intensively. A favorable configuration of the invention is characterized by a device being provided to set and control the spacing between anodes and cathodes. Adjustable spacing between anode and cathode permit adjustment of the current flow and as a result, a reduction in energy costs. An advantageous further development of the invention is characterized by a device being provided to set the gap between the strip and the electrode. With this device, the spacing can easily be adapted to the waviness of the strip. A favorable variant of the invention is characterized by a device which is connected to the setting device for the electrodes being provided to detect strip waviness. This avoids contact between the strip and the electrodes, which could lead to short-circuiting. Working Example: A pilot plant was built in order to test the process. The plant arrangement contained one decoiler (a) and one recoiler (b) which are able to pull the strip through the treatment plant at a speed of up to 60 m/min. The treatment plant comprises chemical degreasing (c) to clean the lubricated strip and an electrolytic cell (d). The cell is connected to 4 rectifiers (e). Each rectifier has a maximum rating of 3000 A/32V. The arrangement of the electrodes was selected such that only one anode (f) and cathode (g) pair 4 is connected to one rectifier. The spacing between the electrodes was set manually. A coil of annealed, cold-rolled stainless steel strip was treated with this plant arrangement. Material AISI 304 Thickness : 0.5 mm Width 320 mm Coil weight : 1000 kg The rectifier output was increased at a constant plant speed of approx. 50 m/min. At a power input of 6000 A (4 x 1500 A), the strip was completely free of scale. At a power input of 8000 A, the strip surface showed improved brightness. At a current density of approx. 200 A/dm2 here, there were no problems with the electrolyte overheating in the cell, nor with the strip overheating in places or inadequate gas removal. In the analysis of the tests the uniformity of the power input was also determined. This was achieved by measuring the brightness and color. The tests showed that the fluctuations were no greater than in the initial material. The strip edges with heavier scaling were also completely de-scaled. The invention is described below in examples and with reference to the accompanying drawings, where Fig. 1 shows a conventional pickling plant with neutral electrolyte, Fig. 2 shows a plant for the process according to the invention, and Fig. 3 illustrates a cell according to the invention. -5- Figure 1 shows a pickling tank 1 according to the state of the art. The metal strip 2 is guided through the electrolyte 3, Na2SO4 for example, between the cathodes 4 and anodes 5. The spacing between the electrodes and the strip are usually between approximately 70 and 150 mm, with the strip usually showing a certain amount of sag which can be reduced by inserting supporting rolls, for example in the center of the plant. A pump 7 feeds the electrolyte 3 through a pipe 6 into the pickling tank 1 and it is then drained off through a pipe 8 and into a storage tank 9 for example, from where the electrolyte 3 is then recirculated. Fig. 2 shows a plant with a decoiler 11 and a recoiler 18 which make it possible to pull the strip through the treatment plant at a speed of up to 60 m/min. The treatment plant comprises chemical degreasing 12 to clean the lubricated strip and an electrolytic cell 13. The cell is connected to 4 rectifiers. Each rectifier has a maximum rating of 3000 A/32V. The arrangement of the electrodes 15, 16 was selected such that only one anode (15) and cathode (16) pair is connected to one rectifier. Here, the strip 2 is deflected over rolls 14, 14'. Fig. 3 shows an electrolytic cell according to the invention. The metal strip 2, stainless steel for example, is fed into the gap between the electrodes 15, 16. Each pairing of one anode (15) and one cathode (16) is connected to one rectifier 19. The electrodes, e.g. the cathode 16, can be moved in the direction marked 20, thus the spacing between the anode 15 and 16 is adjustable. This permits optimum use of the electric power. A pump 7 also transports the electrolyte through a pipe 6, with pipes 21, 21', 21" and 21'" being provided for this purpose and which then feed the electrolyte 3 into the gap 24, 24', 24" and 24'" between the electrodes 15, 16 and the strip 2. The amount of electrolyte fed in can be adjusted to the conditions required by control devices 22, 22', 22" and 22'". After passing 6 between the electrodes, the electrolyte 3 is collected in the lower section 23 of the electrolytic cell 13 and then fed back to the pump 7. The new cell yields higher performance than conventional ones. At the same electrical energy input, more current can be conducted through the strip since there is less drop in voltage. At the same time, however, the flow mechanics of the new cell have been designed such as to achieve very high mass and heat transfer figures. This results from the high degree of turbulence, caused by the small gap between the electrodes and the strip, as well as by the strip movement, i.e. the dissolved metals and the heat from the reaction zone are carried off very effectively. The main advantage of the cell according to the invention over the flooded cell which is currently state of the art (see Fig. 1) is better strip guiding and the higher level of mass and heat transfer, which results in better pickling performance. The rectifiers in conventional cells have a rating of 11000 A (2 x 5500). Depending on the spacing between the electrodes and the strip (approx. 50-150 mm), the drop in voltage is between 25 and 40 V. The cell according to the invention can transfer 50000 A to the strip at approximately 17 V. The invention is not limited to the configurations described. On the contrary, all known variants of electrode wiring and arrangement, such as appropriate polarization, or shorter anodes and longer cathodes to boost chemical treatment, can be used in the same way. 7 WE CLAIM : 1. Process for electrolytic pickling of metallic strip, particularly stainless steel strip, and strip made of titanium, aluminium or nickel, where • the electric current is conducted through the strip indirectly, i.e. without electrically conductive contact between the strip and the electrodes, wherein the strip is run vertically through the electrolyte liquid fed in between the strip and the electrodes, at least one cathode and at least one anode being disposed on the same side of the strip and the spacing between anodes and cathodes being altered to suit the strip dimension. 2. Process as claimed in claim 1, wherein the amount of electrolyte being fed into the gap between the electrodes and the strip is controlled. 3. Process as claimed in claim 1, wherein the temperature of the electrolyte is set between 20 and 85°C, preferably below 70°C. 4. Process as claimed in claim 1, wherein the concentration of NA2SO4 in the electrolyte is between 100 and 350 g/l, preferably around 150 g/i. 5. Process as claimed in claim 1, wherein the loading densities (coulomb per unit of area) is between 20 A/dm2 and 250 A/dm2, with approximately 130 A/dm2 for lead anodes and approximately 180 A/dm2 for iridium anodes, for example. -8- 6. Process as claimed in claim 1, wherein the gap between the electrodes and the strip is altered. 7. Process as claimed in claim 6, wherein the strip waviness is measured and the electrodes are moved away from the strip in order to avoid any contact between the strip and the electrode. 8. Device for electrolytic pickling of metallic strip, particularly stainless steel strip, and strip made of titanium, aluminium or nickel, by the method as claimed in any of claims 1 to 7, where the electric current is conducted through the strip indirectly, i.e. without electrically conductive contact between the strip and the electrodes, wherein the strip runs vertically through the process, there being a vertical gap between the electrodes and the strip into which the electrolyte liquid is fed, at least one cathode and at least one anode being disposed on the same side of the strip and the spacing between anodes and cathodes being altered to suit the strip dimension. 9. Device as claimed in claim 8, wherein the anodes and cathodes are located in an offset arrangement to one another. 10. Device as claimed in claim 8 or 9, wherein the surface of the anodes or the entire anodes is made preferably of lead or lead alloys, iridium oxide or graphite. -9- 11. Device as claimed in claim 8, wherein a control device is provided for the electrolyte feed. 12. Device as claimed in claim 11, wherein a separate control device is provided for each liquid channel between the strip and electrode. 13. Device as claimed in claim 8, wherein a device is provided to set and control the spacing between anodes and cathodes. 14. Device as claimed in claim 8, wherein a device is provided to set the gap between the strip and the electrode. 15. Device as claimed in claim 14, wherein a device which is connected to the setting device for the electrodes is provided to detect strip waviness. 16. Device as claimed in claim 8, wherein the electrodes and the strip are spaced at a gap of between 5 and 15 mm. 17. A process for electrolytic pickling of metallic strip, substantially as herein described, particularly with reference to and as illustrated in the accompanying drawings. -10- 18. A device for electrolytic pickling of metallic strip, substantially as herein described, particularly with reference to and as illustrated in the accompanying drawings. Dated this 6th day of October, 1997. (S. CHAKRABORTY) of D.P.AHUJA & CO. Applicants' Agent A process and device for electrolytic pickling of metallic strip (2), particularly stainless steel strip, and strip made of titanium, aluminium or nickel, where the electric current is conducted through the strip indirectly, i.e. without electrically conductive contact between the strip and the electrodes (15, 16), wherein the strip is run vertically through the electrolyte liquid fed in between the strip and the electrodes, at least one cathode (16) and at least one anode (15) being disposed on the same side of the strip and the spacing between anodes and cathodes being altered to suit the strip dimension.

Documents:

01867-cal-1997-abstract.pdf

01867-cal-1997-claims.pdf

01867-cal-1997-correspondence.pdf

01867-cal-1997-description(complete).pdf

01867-cal-1997-drawings.pdf

01867-cal-1997-form-1.pdf

01867-cal-1997-form-2.pdf

01867-cal-1997-form-3.pdf

01867-cal-1997-form-5.pdf

01867-cal-1997-gpa.pdf

01867-cal-1997-priority document(other).pdf

01867-cal-1997-priority document.pdf

1867-CAL-1997-FORM-27.pdf

1867-cal-1997-granted-abstract.pdf

1867-cal-1997-granted-claims.pdf

1867-cal-1997-granted-correspondence.pdf

1867-cal-1997-granted-description (complete).pdf

1867-cal-1997-granted-drawings.pdf

1867-cal-1997-granted-examination report.pdf

1867-cal-1997-granted-form 1.pdf

1867-cal-1997-granted-form 2.pdf

1867-cal-1997-granted-form 3.pdf

1867-cal-1997-granted-form 5.pdf

1867-cal-1997-granted-gpa.pdf

1867-cal-1997-granted-letter patent.pdf

1867-cal-1997-granted-priority document.pdf

1867-cal-1997-granted-reply to examination report.pdf

1867-cal-1997-granted-specification.pdf