Vertical-Pass Electrotreating Cell

This invention relates to an apparatus for electrotreating of metal strip and more particularly for electrotreating such strip in vertical passes.

In the electrotreating (e.g. plating, cleaning, pickling) of metal strip, the most widely used system employs what may be termed a "conventional" vertical pass method in which the metal strip enters a tank by passing over a roll, is fed downward through the bottom of the tank where another roll is located, is then wrapped around this bottom roll or sink roll, and fed vertically upward until it exits from the tank over a roll in the same manner as it entered. The geometry employed in such conventional vertical systems is such that a relatively great distance between the strip and the electrodes is required, thus necessitating high voltages for relatively small current densities. This, in turn, requires either extremely expensive direct current power supplies or a reduction in the amount of current utilized, consequently limiting the speed and productivity of the electrotreating process. In addition to the spacing employed, the maximum currents which can be applied are also limited by the relatively small amount of turbulence in the electrolyte, resulting in the inhibition (concentration polarization) of the rate at which the electrotreating process can be effected. To overcome these limitations of the conventional vertical cell, the art has resorted to what may be termed horizontal plating cells, see for example U.S. Pat. Nos. 3,471,375; 3,616,426 and 3,718,547, wherein the strip is passed horizontally between a pair of closely spaced electrodes housed in the tube-like conduit through which electrolyte is pumped at a high turbulence to overcome concentration polarization limitations. Such horizontal systems have overcome the above-mentioned difficulties inherent in the conventional vertical systems. Nevertheless, since such horizontal systems require a rather radical departure from the conventional vertical tanks, and require significant capital expenditures in removing the vertical tanks and installing completely new apparatus, most facilities still employ such conventional vertical pass systems. It has now been discovered that the efficiency and high production rates of the horizontal pass systems can also be achieved in a vertical pass system, somewhat analogous to that shown in U.S. Pat. Nos. 2,317,242 and 2,673,836, by a modification of the apparatus shown therein to enable the use of insoluble anodes which (i) may be accurately and closely spaced from the strip surface (e.g. about 1/4 to 11/2 inches) to increase the efficiency of the electrotreating process and (ii) may readily be removed, reconditioned and reinserted, so as to maintain such requisite close spacing.

The advantages of the instant invention will become more apparent from a reading of the following description when read in conjunction with the appended claims and the drawings in which:

FIG. 1 is a cross-section of the vertical electrotreating cell of this invention, showing the basic elements thereof,

FIG. 2 is an enlarged perspective drawing of the T-shaped anodes shown in FIG. 1, and

FIG. 3 is a cross-sectional illustration of the electrodes shown in FIG. 2, showing one means by which such electrodes may be mounted and sealably inserted into the cell wall.

As noted above, a basic deficiency of the conventional-type vertical pass electrotreating apparatus is the inability of such systems to support high current densities. This inability is the result of comparatively (i) poor electrolyte turbulence, resulting in concentration polarization--in which the regions adjacent the electrode and strip surfaces become depleted of the ions requisite for achieving the desired electrotreatment, and (ii) large spacings employed between the electrodes and the strip (necessitated by variations in the pass line of the strip, thus resulting in difficulties in controlling electrode to strip distances), thereby substantially increasing resistance of the electric path and decreasing the efficiency of process. It is well known that the limiting diffusion current density of an electrotreating reaction may be increased by increases in the temperature, concentration and solution velocity of the electrolyte. Since for any specific electrotreating process the concentration and temperature ranges are generally fixed, the most practical method for overcoming concentration polarization is by increasing solution velocity, i.e. turbulence of the electrolyte. One means for achieving such high velocities, in a vertical pass system, is by forcing the electrolyte through a restricted corridor such as shown in U.S. Pat. Nos. 2,317,242 and 2,673,836. It was found, however, while the problem of concentration polarization could be overcome by a flow system analogous to that shown in the aforementioned patents, that the means for mounting electrodes shown therein were inappropriate for achieving the close electrode-to-strip spacings requisite in processes designed for the application of high current densities. In accord with the instant invention, such vertical-pass apparatus has therefore been modified to permit the insertion of electrodes from outside the walls of the corridors through which the electrolyte passes, requiring an electrolyte overflow system which prevents contact of the electrolyte with the outside of such walls.

Referring to FIG. 1, the basic elements of the new system, analogous to the aforementioned vertical pass systems, comprise a piping system 2, which through piping from a reservoir (not shown), circulates the electrolyte in the direction of the arrows into the tank 3, up through the tube-like electrotreating corridors 4 and 4', through overflows 5 and 5', and into collector tank 6 for return to the reservoir--thereby maintaining requisite agitation and concentration of the electrolyte solution. Strip 7 enters the cell by initially being wrapped around conductor roll 8 and thereafter passing into the flow channel of corridor 4, the walls of which can be made of metal, plastic-type materials, or any other material compatible with the electrolyte being employed. On each side of the walls of the corridor is an opening wherein one or more T-shaped anodes 9 are placed, preferably in staggered relationship to those on the opposite wall. Such staggering is particularly desirable for electrodeposition processes so as to prevent one anode from plating onto the other anode. After entering tank 3, it is necessary to change the direction of the strip for passage through the next corridor. This is accomplished by sink roll 10. After its upward passage through corridor 4', any contaminants which might be dragged by the strip onto the conductor roll 8' are removed by spray headers 11. To prevent arcing from damaging the strip, hold-down roll 12 may be placed slightly below the tangent point at where the strip contacts the conductor roll.

It is well known that various alternatives are available for conducting electricity into and away from the strip. For example, if the apparatus were to be utilized solely for electrolytic cleaning or pickling, electrodes in the down-pass (or the up-pass) could be made either positive or negative with respect to the steel strip, depending on the polarity of the conductor roll which imparts the same polarity to the strip. Strip polarity can also be varied in either flow channel by varying the lead connections from the power supply. In an electroplating mode, the conductor roll and strip would be made cathodic (negative polarity) with respect to the electrodes, functioning as anodes. While the use of conductor rolls for making direct electrical contact with the strip is preferable for high current density electrotreating processes, i.e. current densities in excess of 500 amps/ft..sup.2, it should be recognized that the use of conductor rolls are not essential and that current transfer to the strip can be effected by what has been termed bi-polar electrolyzing (see for example U.S. Pat. No. 2,165,326) in which transfer may be effected from an electrode of one polarity, through the electrolyte to the strip and again through the electrolyte to an electrode of opposite polarity.

Details of a preferred design for the T-shaped electrodes (9 and 9') utilized in the instant apparatus are shown in FIGS. 2 and 3. Electrode 9 comprises two main portions: (i) inner portion 14 for insertion in liquid-tight engagement with the surfaces 15 of the hole in the wall of conduit 4, and (ii) an outer, flange portion 16 for exerting a sealing force against the outer wall surface 4.sub.o of the conduit wall. For more efficient current carrying ability, a bus bar, e.g. made from copper, may be integrally cast in the electrode body to enable electrical contact to be made from outside of the tank and away from possible contamination by the electrolyte. Additionally, such integral casting provides both better mechanical and electrical contact than would be achieved by the conventional manner of merely bolting the bus bar to the electrode. To prevent perturbation in the flow of the electrolyte through the flow channel of corridor 4, inner electrode face 14.sub.i desirably will be designed so as to fit flush with inner wall face 4.sub.i. To achieve desired liquid-tight sealing, bracket 18 may be employed in conjunction with anchoring screws 19 so that flange portion 16 may be exerted to bear either (a) directly against outer wall 4.sub.0 (not shown) or (b) against packing 20, both to seal and insulate the flange portion from cell wall. In addition to the improved sealing, and the ease of electricl connection permitted by use of the flange portion 16, the larger, external surface also permits enhanced electrode cooling; e.g. by natural convection, with or without the use of cooling fins, or by conductive cooling with a fluid heat transfer medium.