ELECTROLYSIS APPARATUS HAVING LIQUID SQUEEZER OUT OF CONTACT WITH STRIP

Abstract: In an electrolytic apparatus that passes a strip between paired members of a liquid throttle unit provided on at least one of an inlet side and an outlet side of a treatment cell through which the strip is continuously passed, an electrolytic apparatus with strip non-contacting liquid throttle unit is provided which is characterized in that the spacing between the paired members of the liquid throttle unit is set very slightly larger than the thickness of the passed strip to maintain the surfaces of the strip and the liquid throttle unit members, e.g. a seal roll, nozzle device or wedge-shaped block, in a non-contacting state.

Description: This invention relates to an electrolytic apparatus with a liquid throttle unit that establishes non-contacting sealing between a strip and a liquid electrolyte during electrolytic plating, of the surface of a metal strip, with tin, zinc, chromium or other metal or during pickling or other surface treatment. Numerous methods and apparatuses have been proposed for electrolytic plating of the surface of a metal strip with tin, zinc, chromium or other metals. Recently, particular demand has arisen for high-efficiency, high-speed plating equipment that offers high performance in excess of 500m/min. For such high-speed plating, however, a specific requirement must be met, because, in the vertical type plating apparatus, the strip passes vertically and the running strip penetrates a portion of the cell body at its bottom end, while in the horizontal type plating apparatus the strip passes horizontally and the running strip laterally penetrates a center portion of the cell body. In order to conduct the plating (including pickling and other treatments) while continuously moving the metal strip to be plated, it is therefore necessary to seal the penetrated portion so as to prevent leakage of the treatment liquid. This is because the constant running state of the strip results in the plating treatment liquid also being leaked as an entrained flow along the running strip surface. Specifically, as shown in FIG. 1, the amount of plating treatment liquid leakage owing to entrained flow is proportional to strip running speed. It was found that at a strip running speed of around 200m/min, the amount of plating treatment liquid leakage (loss) rises to 20% or more of the fed treatment liquid, at a strip running speed of about 500m/min, it reaches 80% or higher, and at 1000m/min, the maximum strip running speed currently conceivable, the amount of leakage reaches nearly 100%. With such increasing leakage, the amount of treatment liquid fed must be increased to continue operation with the plating treatment cell kept constantly full. Sealing methods for preventing treatment liquid leakage include one, such as taught by JP-A-5-331695, in which a pair of damrolls are installed one on either side of the strip pass line to be rotatable in contact with the strip surface, the opposite axial ends of the damrolls are sealed by seal rings from the outside, and seal plates are installed for sealing by contact with the peripheral surfaces of the damrolls. This method, which is an improvement on the well-known rotating seal system, enables the sealing capability with respect to the strip surface to be increased substantially in proportion to the squeezing force between the damrolls. FIG. 8 illustrates a vertical type electrolytic apparatus disclosed by JP-A-5-171495. As shown, liquid electrolyte 103 is fed between a strip 100 and electrodes 101, 102 to impart an agitation effect between the strip and the electrodes. In addition, liquid seal devices 104a and 104b equipped with seal rolls 105a, 105b are installed at the lowermost portion of the vertical type electrolytic apparatus for preventing runoff of the liquid electrolyte 103, thereby obtaining a high current density while maintaining the level of the liquid electrolyte. As shown in FIG. 9, a vertical type electrolytic apparatus disclosed in JP-A-60-56092 (corresponding to U.S. Patent No 5,236,566) imparts an agitation effect between a strip 115 and a liquid electrolyte 110 by using liquid feed nozzles 113 and 114 to feed liquid electrolyte into spaces between electrodes 111 and electrodes 112 immersed in the liquid electrolyte 110. In the method of squeezing the strip with damrolls, however, the strip surface tends to be easily scratched. One reason for this is that the squeezing force of the rolls on the strip has to be maintained high in order to secure sealing pressure. Another is that contact scratches are produced between the strip and the roll surfaces owing to mismatching between the strip running speed and the circumferential speed of the rolls. What happens most often, however, is that sludge carried in from the exterior and, particularly in the electrolytic cell, foreign matter such as electrolytic deposits, get into the treatment liquid and lodge between the strip surface and the damrolls to become sources of scratching. This lowers production yield, degrades quality, makes more frequent roll inspection and exchange necessary, and leads to a decline in production line operating rate. In a case where the strip passes between the seal rolls while running in a meandering state, moreover, if the strip should snake in the manner of weaving in the axial direction of the rolls, then, since the strip is squeezed between the rolls, the portions of the strip strongly squeezed by the rolls pass with no freedom in the thrust direction, thereby producing wrinkles in the strip. This, in conjunction with the aforesaid biting of foreign matter, further markedly degrades quality. In the aforesaid vertical type electrolytic apparatus, achievement of electrolytic plating at high current density during high-speed strip streaming of the strip requires efficient feeding of metallic ions to the plating surface and rapid removal the large quantity of gas produced by the high-current-density electrolysis from between the electrodes. The problems posed by these needs have not yet been solved. The vertical type electrolytic apparatus disclosed by JP-A-5-171495 (FIG. 8) still has the following problems: 1) Since the liquid electrolyte 103 is retained solely by electrode units formed by the electrodes 101 and 102 and, furthermore, prevention of liquid electrolyte runout is conducted by the pair of seal rolls 105a, 105b, the loads on the liquid seal devices 104a, 104b are excessive, making liquid retention difficult during high-speed strip streaming. 2) Scratching owing to slipping between the strip 100 and the seal rolls 105a, 105b is liable to occur during high-speed strip streaming and scratching is also produced by foreign matter pressed onto the strip after lodging between the strip and seal rolls. 3) Since the seal rolls themselves experience damage and wear that degrades their liquid seal performance and leads to increased liquid electrolyte leakage, the flow rate required at the electrodes for plating becomes hard to secure and defective plating therefore arises owing to uneven liquid electrolyte flow. On the other hand, the vertical type electrolytic apparatus disclosed by JP-A-60-56092 (FIG. 9) conducts plating with the electrodes 111 and 112 immersed in the liquid electrolyte 110 and can adequately handle currently used strip running speeds. However, if the strip running speed should be raised to a high level without implementing some measure such as installation of a liquid throttle device or the like, the loss owing to the entrained flow caused by movement of the strip 115 will, as shown in FIG. 1, increase with increasing running speed of the strip, namely, will accelerate up to and reach substantially 100% at around 500m/min. Even if the strip running speed is further increased to around 1000m/min, the loss by entrained flow will remain saturated. When this phenomenon occurs, the flow rate between the strip 115 and the electrodes 111, 112 becomes hard to secure and plating defects such as burnt deposits occur. US-A-4 162 955 discloses an electrolytic apparatus comprising liquid seals that prevent the escape of liquid coating solution from the treatment chamber, and liquid seals are each comprised of a pair of parallel 'nozzles for directing converging streams of liquid coating solution under pressure towards the travelling metal sheet, which provides a liquid bed for supporting the traveling metal sheet adjacent the end wall of the chamber. JP-A-07-207492 relates to providing a liquid moving device in which scattering and contact flows of liquid droplets on a plate passing at high speed are not caused and liquid associated with a belt body at a short distance just after a dip process is completely removed or is controlled to a prescribed film thickness. This device is constructed by liquid removing dies comprising at least a pair of members, and these dies heave a clearance gradually expanding backwards, and also have jet holes for jetting fluid. The present invention was made to overcome the foregoing problems. One of its objects is to provide a method for prevention of plating treatment liquid leakage and utmost avoidance of strip surface scratching and wrinkling. Another of its objects is to provide an electrolytic apparatus with a strip non-contacting liquid throttle unit that can facilitate inter-electrode liquid retention during high-speed strip streaming, prevent clinging of the strip to the electrodes, and enhance plated product quality and plating operation efficiency. Such objects can be achieved by the features defined in the claims. The invention is described in detail in connection with the drawings, in which: FIG. 1 is a diagram showing the relationship between strip running speed and liquid electrolyte entrained flow, FIG. 2 is a diagram showing the relationship among strip thickness, liquid runout between liquid throttle unit members (seal rolls), and frequency of strip surface scratching, FIG. 3 is a conceptual diagram for explaining the configuration of an electrolytic apparatus using seal rolls that is an embodiment of the present invention, FIG. 4 is an enlarged explanatory diagram of an essential portion in FIG. 3, FIG. 5 is a conceptual diagram for explaining the configuration of a large electrolytic apparatus that is another embodiment of the present invention, FIG. 6 is a conceptual diagram for explaining the configuration of an electrolytic apparatus that is an electrolytic apparatus according to the present invention, showing a mode in the case of utilizing a single rotary drum, FIG. 7 is a conceptual diagram for explaining the configuration of a horizontal type electrolytic apparatus that is an electrolytic apparatus according to the present invention, FIG. 8 is a conceptual diagram for explaining the configuration of a conventional vertical type electrol tic apparatus, and FIG. 9 is a conceptual diagram for explaining the configuration of another example of a conventional vertical type electrolytic apparatus. The electrolytic apparatus based on the present invention offers a practical technology that is thoroughly compatible not only with current electrolytic apparatuses but also with electrolytic apparatuses with strip running speeds increased to 1000m/min or 1500m/min. The electrolytic apparatus further enables prevention of scratches to the strip surface while achieving a sealing effect able to keep pace with increasing strip running speed and, by establishing a suitable spacing between the strip surface and the liquid throttle unit, enables utmost prevention of entrained flow of liquid electrolyte owing to strip running. The inventors first made a study focused on the relationship between strip running speed and a decrease in liquid electrolyte by entrained flow. As a result, they obtained the data shown in FIG. 1. As can be seen from FIG. 1, a proportional relationship exists between the amount of liquid runout by entrained flow and the strip running speed. This is because treatment liquid (liquid electrolyte) used for the treatment has viscosity and due to this viscous action of the treatment liquid, which flows as a viscous fluid with passage of the strip through the treatment liquid, it is drawn along by contact with the strip. To overcome this problem, a liquid throttle unit comprising paired members is provided to sandwich the running strip in a strip non-contacting state, preferably with the spacing therebetween set very slightly larger than the thickness of the passed strip, and the liquid throttle unit is preferably constituted of a seal mechanism composed of a pair of seal rolls in the electrolytic cell. Specifically, the seal mechanisms are provided on at least one of the inlet side and the outlet side of the electrolytic cell through which the strip is continuously passed, thereby preventing excessive liquid electrolyte adherence and entrained flow while also avoiding occurrence of scratches on the passed strip surface because the liquid throttle unit is itself non-contacting. Tests showed that the aforesaid objects can be achieved if the spacing is made very slightly larger than the thickness of the passed strip, i.e., around 0.1mm-5mm, preferably 0.3mm-2mm. . In deciding the spacing between the strip surface and the liquid throttle unit members, the inventors conducted tests regarding the relationship among strip thickness, amount of liquid runout through the space between seal rolls and frequency of strip surface scratching. The data shown in FIG. 2 were obtained as a result. As can be seen from FIG. 2, even if the seal rolls are out of contact with the strip surface, so long as the spacing therebetween is set in the range of 0.1mm-5mm larger than the thickness of the passed strip, preferably in the range of 0.3mm-2mm larger, entrained flow produced by strip passage is throttled between the seal rolls owing to the diminishing space formed by the seal rolls in the direction of strip advance. Specifically, the flow path resistance increases to enable control of liquid electrolyte runout. The reason for limiting this spacing to 0.1mm-5mm is that, when using nozzle devices, 0.1mm is the minimum gap at which contact with the running strip can be avoided and is a sufficient spacing so long as a distance making liquid electrolyte jetting possible can be secured and that at smaller values contact is made with the running strip to increase the frequency of strip surface scratching. It is clear from FIG. 2 that adopting this value lowers the amount of liquid electrolyte runout and enables a marked reduction in the frequency of strip surface scratching. On the other hand, the maximum spacing value of 5mm corresponds to the maximum thickness of the liquid film drawn along by the strip surface and it was experimentally determined that for obtaining further throttling effect it must be made 2.0mm, which is the mean value of the liquid film. A spacing greater than 5mm reduces the frequency of strip surface scratching but is not preferable because it increases the amount of liquid electrolyte runout. When these maximum and minimum values of the gap are set, a thin film can be formed at the gap where the space formed between the strip and the nearest portion of the seal roll surface. By utilizing this thin film, resistance can be imparted against leakage of the liquid electrolyte in the electrolytic cell. Moreover, the formation of the thin film on the seal roll surface can be promoted by rotating the seal roll. Even if foreign matter should get mixed into the liquid electrolyte, it is prevented from producing strip surface scratches because it is kept from lodging by the space between the strip and the seal rolls. In addition, wrinkles are not produced even if the strip weaves in its width direction because the seal rolls do not restrict the strip in the thrust direction. By driving the seal rolls to rotate at a circumferential speed identical to the strip running speed, moreover, the relative speed between the circumferential surface of the seal rolls and the strip surface can be made zero to prevent occurrence of strip surface scratches even if the seal rolls should contact the strip. An example of a vertical type electrolytic apparatus when seal rolls are provided as the seal mechanisms will now be explained with reference to FIGS. 3 and 4. As shown in FIGS. 3 and 4, a turn-back roll 10 is rotatably disposed in a lower tank 11 filled with liquid electrolyte 12. A liquid feeding unit 13 and a waste liquid unit 14 are provided to continue upward from the lower tank 11 and electrode units 17 and 18 are provided to continue upward from the liquid feeding unit 13 and the waste liquid unit 14, respectively. The electrode units 17 and 18 are respectively formed between a pair of electrodes 15 and a pair of electrodes 16. Like the lower tank 11, they are filled with liquid electrolyte 12. A waste liquid unit 19 similar to aforesaid waste liquid unit is disposed above the electrodes 15 and a liquid feeding unit 20 similar to the aforesaid liquid feeding unit is disposed above the electrodes 16. Like the lower tank 11, they are filled with liquid electrolyte 12. Conductor rolls 21 and 22 are installed above the waste liquid unit 19 and the liquid feeding unit 20, respectively. A strip 23 conveyed to the vertical type electrolytic apparatus having the foregoing configuration first wraps over the conductor roll 21 and then descends through the electrode unit 17, reverses direction at the turn-back roll 10, ascends through the electrode unit 18, wraps over the other conductor roll 22 and advances to the next processing step. Simultaneously with the running of the strip, liquid electrolyte 12 is fed to the electrode unit 17 from the liquid feeding unit 13 and forcibly imparted with a given flow rate, whereby electrolytic plating is conducted on the strip 23. The liquid electrolyte after electrolytic plating is recovered by the waste liquid unit 14. In the vertical type electrolytic apparatus provided witch seal rolls as the seal mechanisms according to this aspect of the invention, a liquid throttle unit 30 composed of a pair of seal rolls 32 and a liquid throttle unit 31 composed of a pair of seal rolls 33 are provided at the upper portion of the lower tank 11 filled with liquid electrolyte 12 at points below the liquid feeding unit 13 and the waste liquid unit 14, respectively, in a state immersed in liquid electrolyte 12. An enlarged view of this section is shown in FIG. 4. In FIG. 4 (which shows only the strip inlet side of the electrolytic apparatus, the outlet side being omitted because it has the same configuration), the pair of seal rolls 32 constituting the liquid throttle unit 30 are supported and held in place by upper partitions 35 and lower partitions 36 via interposed seal members 37 and 38. for preventing leakage of the liquid electrolyte 12 at the liquid throttle unit 30. The spacing (d) of the seal rolls 32 is such that the seal rolls 32 face each other separated by a distance that is 0.1-5mm, preferably 0.3-2mm, larger than the thickness (t) of the strip 23, whereby the strip runs between the seal rolls in a non-contacting state. The entrained flow of the liquid electrolyte induced by the passage of the strip can be suppressed by this configuration because the gap through which the liquid electrolyte flows from the electrode unit to the lower tank is throttled to a small size by the liquid throttle unit, thereby increasing the flow path loss. Since a sufficient liquid electrolyte flow rate can therefore be obtained at the electrode unit, a uniform flow can be maintained and, as a result, excellent plating can be conducted. In the embodiment of the electrolytic apparatus according to the invention shown in FIGS. 3 and 4, owing to the provision of the liquid throttle units 24, 25 or 30, 31 between the lower tank 11 and the liquid feeding unit 13 or between the lower tank 11 and the waste liquid unit 14, a stable liquid electrolyte flow rate can be constantly secured between the electrodes at strip running speeds ranging broadly from low speed to high speed. Since the current density can therefore be increased, the plating operation can be conducted with high efficiency and the number of vertical type electrolytic apparatuses installed can be reduced. Particularly noteworthy is that during high-speed strip running at around, 1000m/min, strip passage between the electrodes stabilizes owing to the entrained flow accompanying passage. Since the distance between the electrodes can therefore be shortened, electrolysis can be conducted at a lower voltage to reduce plating power consumption. Further, as shown in FIG. 4, in the electrolytic apparatus according to this embodiment of the present invention, the seal rolls 32 are rotated by drive motors 34. Since the circumferential speed of the seal rolls 32 are set equal to the running speed of the strip, the seal rolls 32 and the strip 23 can be synchronously operated. therefore, even if the strip should contact a seal roll, the situation remains substantially the same as if the strip did not contact the seal roll because the strip and the seal roll move at the same speed. Specifically, lodging of foreign matter between the strip and the seal rolls can be ininimized and occurrence of harmful scratching owing to lodging of foreign matter can be made almost'nil to realize a large improvement in plating quality. the configuration of a vertical type electrolytic apparatus that is another embodiment of the invention will now be explained with reference to FIG. 5. The apparatus illustrated in FIG. 5 is a vertical type electrolytic apparatus using large, long cylindrical lower tank 39 in place of the lower tanks shown in FIG. 3 and having the constituent elements shown in FIG. 3, namely, the liquid feeding units, the waste liquid units, the electrodes and the liquid throttle units, immersed in the liquid electrolyte 12 in the lower tank 39 in the same layout. Owing to the installation of liquid throttle units at an upper portion of the lower thank, the vertical type electrolytic. apparatus of FIG. 5 achieves the same effects as the embodiment shown in FIG. 3. when the electrolytic apparatus according the present invention has only a single turn-back roll 10 immersed in the liquid electrolyte 12 charged into the lower tank 39, as shown in FIG. 5, the arrangement shown in FIG. 6, can be adopted. Specifically, as shown in FIG.6, a liquid feeding unit 13 and a waste liquid unit 14 are provided at laterally symmetrical positions relative to the center line of the turn-back roll 10 and the two are made into a unitary structure by installing a guide 48 provided along and spaced a prescribed distance from half the circumferential length of the turn-black roll 10. Liquid electrolyte 12 i's supplied from the liquid feeding unit 13 in the direction opposite to the running direction of the strip 23 (in the direction opposite to the rotating direction of the turn-back roll 10) and the liquid electrolyte 12 is discharged from the waste liquid unit 14. In this aspect of the invention, the liquid throttle unit constituted of a seal mechanism is provided at a location of the strip 23 apart from the turn-back roll 10, namely, directly above the liquid feeding unit 13, whereby entrained flow is suppressed, and a sufficient liquid electrolyte 12 flow rate can be obtained at the electrode unit so that a uniform flow can be maintained and, as a result, excellent plating can be conducted. The electrolytic apparatus according to the present invention can be a horizontal type electrolytic apparatus instead of a vertical type electrolytic apparatus. An example is shown in FIG. 7. As can be seen in FIG. 7 the strip 23 to be electrolytically plated wraps over a conductor roll 50 and then moves into a plating apparatus provided with an electrode unit 52. Liquid electrolyte is supplied from a liquid feeding unit 53 provided immediately ahead of a conductor roll 51 of the plating apparatus in the direction opposite to the running direction of the strip 23 in the plating apparatus and is discharged from a waste liquid unit 54. The liquid throttle unit in this aspect of the invention is provided immediately after the liquid feeding unit on the side that the strip 23 exits from the plating apparatus, whereby the same effects are obtained as in the case of the foregoing vertical type electrolytic apparatuses. Specifically, entrained flow is suppressed and a sufficient liquid electrolyte 12 flow rate can be obtained at the electrode unit 52 so that a uniform flow can be maintained and, as a result, excellent plating can be conducted. Advantages realized by applying the invention to this horizontal type electrolytic apparatus are that the length of the electrolytic plating apparatus footprint can be shortened and installation at a relatively low equipment cost is possible. As explained in the foregoing, by providing a vertical type electrolytic apparatus with a liquid throttle unit of relatively simple structure, the present invention enables a stable liquid electrolyte flow rate to be constantly secured between the electrodes at strip running speeds ranging broadly from low speed to high speed. Since the current density can therefore be increased, the plating operation can be conducted with high efficiency and the number of vertical type electrolytic apparatuses installed can be reduced. Particularly noteworthy is that the strip passage between the electrodes is stabilized during high-speed strip running at around 1000m/minbecause liquid runout attributable to the entrained flow caused by the strip passage is suppressed to ensure uniform liquid flow between the electrodes. Since the distance between the electrodes can therefore be shortened, electrolysis can be conducted at a lower voltage to reduce plating power consumption.

Claims: An electrolytic apparatus with a strip non-contacting liquid throttle unit that, in a method of passing a strip between paired members of the liquid throttle unit provided on at least one of an inlet side and an outlet side of a treatment cell through which the strip is continuously passed, is characterized in that the paired members of the liquid throttle unit are seal mechanisms and the seal mechanisms comprise a pair of seal rolls, and a spacing between the paired members of the liquid throttle unit is set very slightly larger than the thickness of the passed strip to maintain the surfaces of the strip and the liquid throttle unit in a non-contacting state. An electrolytic apparatus according to claim 1, chazacterized in that the spacing between the pair of seal rolls is 0.1mm-5mm, preferably 0.3mm-2mm larger than the strip thickness. An electrolytic apparatus according to claim 1 or 2, wherein treatment liquid is throttled in spaces formed by the seal rolls to diminish in the direction of strip advance, and thin film layers of treatment liquid in the treatment cell are formed between the strip surfaces and circumferential surfaces of the seal rolls to produce sealing capability with respect to the treatment liquid. An electrolytic apparatus any of Claims 1 to 3, characterized in that a drive system for rotating the seal rolls is adopted that matches the direction of rotation with the passing direction of the strip and makes the circumferential speed of the seal rolls identical to the running speed of the strip to synchronize the operations of the strip and the seal rolls. An electrolytic apparatus according to any of Claims 1 to 4, wherein a strip is run through an electrode unit formed between electrodes disposed at a prescribed spacing, a liquid feeding unit provided on an outlet side of the electrode unit passes liquid electrolyte to the electrode unit to conduct electrolytic treatment, liquid electrolyte after electrolytic treatment is recovered by a waste liquid unit provided on an inlet side of the electrode unit and a liquid electrolyte tank is provided on the inlet side or the outlet side of the electrode unit to communicate and connect with the electrode unit through the liquid feedin unit or the waste liquid unit, and the liquid throttle unit is provided adjacent to the electrode unit and the liquid electrolyte tank filled with liquid electrolyte.

Inventors: SANADA MASAHARU, SHIMAMURA MICHIHIRO, SANADA, MASAHARU, SHIMAMURA, MICHIHIRO, SANADA, Masaharu, Nippon Steel Corp. Plant &amp;, SHIMAMURA, Michihiro, Nippon Steel Corp. Plant &amp;
Applicants: NIPPON STEEL CORP, NIPPON STEEL CORPORATION
Application year: 1997
Main classification: C25D7
IPC codes: C25D7