EP0100400B1 - Process for the electrolytical deposition of metals from aqueous solutions of metal-salts on steel sheets, and apparatus for carrying out the process - Google Patents

Abstract

1. Method for electrolytically separating metals from aqueous solutions of metal salts on a steel band, using high relative flow velocities between the electrolyte and the steel band, as well as anodes for achieving greater current densities at the lowest possible energy consumption, wherein a stream of electrolyte is passed parallel to the band surface from one of the ends of the electrolysis cell, in or against the direction of movement of the band, and wherein strong relative flow is obtained by subjecting the stream of electrolyte, directed along the plane of the steel band, to a turbulent state of flow, by partial streams of electrolyte from nozzles, arranged laterally of the edges of the band and spraying transversely of the direction of movement of the band and parallel to the band surface or in the direction towards the edges of the band.

Description

Process for the electrolytic deposition of metals from aqueous solutions of the metal salts on steel strip and device for carrying out the process.

Electrolytically refined, in particular electrolytically galvanized, steel sheet, which is produced on continuously operating systems, is increasingly being used for the manufacture of household and electrical appliances and in the automotive industry. Two-sided or only one side of electrolytically galvanized sheet steel is given an active corrosion protection by the coating and offers an excellent primer for subsequent painting or coating.

There are already a number of high-performance electrolytic broadband galvanizing systems which differ essentially in the design of the galvanizing cells (vertical, horizontal or radial strip guidance in the electrolysis area). The current densities common today are in a range between approximately 50 and 100 A / dm ² .

Through the development of a method and a device for electrolytic deposition with high cathodic current densities, either the electrolysis section required to produce a larger zinc coating and thus the actual treatment part can be shortened, or the belt speed, i. H. throughput can be increased.

If the electrolytic metal deposition is to be carried out with high current densities without dendritic crystal growth, usually referred to as burning, or a significant decrease in the current efficiency, the mass transfer to the cathode must be improved. It can be assumed that the metal ion concentration in the electrolyte and its temperature have already been largely optimized in the existing electrolytic strip galvanizing plants. The most important measure to achieve high current densities is therefore to reduce the diffusion layer thickness on the cathode, i.e. H. the steel strip while at the same time preventing an impermissibly large metal ion depletion of the electrolyte near the cathode.

The reduction of the flow limit and thus the diffusion layer thickness is preferably carried out hydrodynamically, which is to be understood as a targeted electrolyte movement.

However, increasing the current density only makes sense if it is also possible to reduce the voltage loss in the electrolysis cell by optimizing the design. The optimization must take into account the current transmission from the current rollers to the steel strip, the reduction and adaptation of the anode distances to the steel strip and the application of a directed electrolyte flow to the steel strip. The appropriate design and type of anode must be included in these considerations.

The implementation of one- and two-sided finishing must be guaranteed.

Numerous cell designs have been proposed for realizing metal deposition with high current densities.

The device known from DE-A) -30 17079 is characterized by horizontal band guidance in the finishing cell, line-contacting current rollers for current transmission to the steel band, insoluble anodes arranged under the band and soluble anodes arranged above the band, which can be tracked according to the processing which can be partially connected to the power source and the attachment of nozzles for adjusting the flow between the anodes or the anodes and the steel strip to be refined.

Such a device leads to problems in keeping the metal ion content constant during electrolysis. The different voltage drop between insoluble anodes and strip or soluble anodes and strip that occurs due to the construction leads to difficulties in keeping constant or adjusting the necessary distances between the respective anodes and the steel strip to be refined. A further disadvantage of this device is the gas evolution which occurs at the insoluble anode, which leads to gas bubbles settling on the surface of the strip under the steel strip to be refined and can lead to coating defects. A horizontal electrolysis cell construction usually has the defect that metal particles separating from the soluble anodes settle on the strip surface to be finished and can lead to strip surface defects there.

The further method with horizontal belt guidance known from DE-AI-2917 630 is characterized by a strong electrolyte flow opposite to the belt running direction, directed parallel to the belt, the steel belt to be refined being guided in the middle between insoluble anodes. The zinc is dissolved in order to keep the zinc ion content constant in the electrolyte by chemically dissolving preferably zinc slag in the bypass in appropriate facilities. The disadvantages of this method are that the metal ion content of the electrolyte is reduced over the refining section, which leads to increasing gas bubble loading for less than optimal use of the refining section.

The process according to DE-PS 1621 184 is characterized by a radial belt guide in the finishing area. In the tightened case, the current transfer to the belt also takes place through the deflection roller. With all processes with radial belt guidance, only one-sided belt finishing is possible in one cell. A double-sided finishing makes double Cell count required.

Another method known from DE-AI-2714491 is characterized by a horizontal band guide, the band being wetted with electrolyte only from below and flowed from below with a nozzle. With this, only one-sided strip finishing is possible. The mass transfer in this process cannot be significantly increased compared to the conventional state, since the inflow does not occur at a sufficiently high speed to prevent electrolyte from reaching the back of the belt, which is not to be wetted, and there leading to an unwanted separation.

Furthermore, the method known from DE-AI-31 08 615 is characterized by a vertical strip guide in the finishing zone and an inflow of electrolyte onto the strip surface through insoluble anodes provided with corresponding openings. Despite the necessarily very high pumped-over amounts of electrolyte, flow dead spaces cannot be avoided due to the flow of the strip to be refined through the anodes. The complicated design of the anodes requires the use of insoluble anodes, which means that the known measures for the use of insoluble anodes are required.

Finally, DE-AI-31 08 615 shows a device for the electrolytic treatment of a metal strip, which is characterized by a container for defining an electrolytic treatment space for the metal strip, a plurality of conductive rollers which are arranged along a transport path of the metal strip which extends through the treatment space , at least a pair of electrode pads, each pair being located between the conductive rollers, spaced from the transport path of the metal strip, and the electrode pads facing each other, each electrode pad being provided with at least one slot through which the electrolyte is forced out to the surface of the metal strip means that a sufficiently high static pressure of the expelled electrolyte forms to hold the metal strip in the space between the electrode pad during the transport, a device for supplying the electrolyte to and through each slot Device for applying a voltage between at least one of the conductive rollers and the electrode pads. However, this known device has the disadvantage that the electrolyte is pumped completely through the electrode cushions against the metal strip to be treated, with the result that insoluble anodes have to be used in the electrolytic metal deposition. In addition, the escape of the treatment liquid from slots in the electrode cushions does not lead to a uniform flow state of the liquid over the entire surface of the electrode cushions, as a result of which zones with a strong and also with a weak relative flow are created.

It is therefore an object of the invention to enable electrolytic metal deposition with high current densities with the least possible use of energy for the flow and the smallest possible anode / cathode distance when preferably soluble anodes are used. Furthermore, care must be taken to ensure that the current is transferred to the steel strip as close as possible to the finishing section in order to minimize the loss of tension in the steel strip. In order to meet this requirement, measures must be taken to prevent the current rollers from being metallized disadvantageously. During the electrolysis process, the change in shape of the soluble anodes must be compensated for in order to keep the voltage constant.

This object is achieved by the method according to claim 1.

The device for carrying out the method in the form of at least one electrolysis cell, which encloses the steel strip, formed from the anodes and the flow shaft walls, is characterized in that electrolyte feed devices are attached to the narrow sides of the electrolysis cell, which are not covered by anodes, through which a subset of the total circulated The amount of electrolyte can be fed in the direction of the band edge, and that a further feed device is attached to one of the cell ends (band entry or exit), through which the remaining part of the total amount of electrolyte circulated can be supplied along the band running plane.

Advantageous embodiments of the device according to the invention are described in claims 3 to 10.

The method and device according to the invention enable electrolytic metal deposition with high current densities with the least possible use of energy for the flow and the smallest possible anode / cathode spacing with the use of preferably soluble anodes, the current being transferred to the steel strip as close as possible to the refining section in order to avoid the To minimize tension loss in the steel strip.

The invention is explained in more detail with reference to the drawings.

The electrolytic cell 1 (FIG. 6) is formed from a shaft, which is preferably made of plastic. Soluble anodes 8 are inserted into the shaft and can be tracked from the outside in accordance with their processing. The anodes 8 are preferably made from strips so that they can be easily adapted to the actual bandwidths. The electrolyte supply of the main stream takes place either from above, so that the electrolyte outlet speed is influenced by free fall, or from below, so that the electrolyte flow rate is generated by a pump, not shown.

When the electrolyte is fed into the top of the electrolytic cell 1, the electrolyte can be inclined sezelle 1 the exit speed can be set without changing the lower cell seal (Fig. 3).

The parallel flow to the steel strip 3 is influenced by partial electrolyte flows flowing in transversely to the strip running direction, preferably via nozzles 9, in order to generate a sufficiently uniform turbulence over the entire cell area. The direction of flow of the nozzles 9 can be changed so that an optimal degree of turbulence can be set. The flow rate in the electrolysis cell is regulated by a device (not shown) for changing the electrolyte outlet gap at the lower end of the electrolysis cell. This device can be of a mechanical and / or fluidic type (gas flow, liquid flow). In high-speed systems, it is preferable to work with a fluidic seal in order to avoid surface damage.

For a finishing plant, several such electrolysis cells 1 are preferably connected in series and the electrolyte reflux can either be carried out jointly for all electrolysis cells 1, as shown in FIG. 1, or respectively for electrolysis cells 1 arranged in pairs in accordance with FIG. 2 can be proceeded according to Fig. 3. Collection tanks 5 for the electrolyte 4 are arranged under the electrolysis cells 1. The necessary pumping devices are not shown.

The current is transmitted either by looping or by line contact with current rollers 2 in a vertical or inclined arrangement. The current rollers 2 are arranged both above and below the electrolysis cell 1 (Fig. 1-5).

4 and 5 show the use of line-contacting current rollers 2. FIG. 4 shows the use of two line-contacting current rollers 2 with opposing pressure rollers 10. The large deflection rollers 6 serve to guide the steel strip 3. FIG. 5 shows an arrangement of line-contacting current rollers 2 as counter-rollers for the deflection rollers 6. With this arrangement it is possible to reduce the overall height of the system.

In order to avoid metallization of the lower current rollers 2, suitable shields 7 are used.

FIG. 6 shows a sketch of the electrolytic cell 1, from which the arrangement of the anodes 8 and the nozzles 9 used to generate the cross flow can be seen.

example

The method according to the invention has been tested with a pilot plant for electrolytic strip galvanizing with an electrolytic cell 1 according to FIGS. 3 and 6. The steel strip 3 has been degreased, rinsed, pickled in dilute sulfuric acid, rinsed, then galvanized on both sides, rinsed and dried.

Zellenkennwerte :

Electrolysis parameters:

Cell characteristics:

The current densities shown in FIG. 7 could be achieved. The quality of the galvanizing is in accordance with the conventional electrolytic galvanizing. The voltage loss at the greatest achievable current density was i. M. 28 V. It can be seen that favorable flow conditions for electrolytic zinc deposition on steel strips are already evident in i. M 40% nozzle flow, based on the total flow rate, can be achieved.

Claims (10)

1. Method for electrolytically separating metals from aqueous solutions of metal salts on a steel band, using high relative flow velocities between the electrolyte and the steel band, as well as anodes for achieving greater current densities at the lowest possible energy consumption, wherein a stream of electrolyte is passed parallel to the band surface from one of the ends of the electrolysis cell, in or against the direction of movement of the band, and wherein strong relative flow is obtained by subjecting the stream of electrolyte, directed along the plane of the steel band, to a turbulent state of flow, by partial streams of electrolyte from nozzles, arranged laterally of the edges of the band and spraying transversely of the direction of movement of the band and parallel to the band surface or in the direction towards the edges of the band.

2. Apparatus for carrying out the method according to claim 1, in the form of at least one electrolysis cell, which surrounds the steel band. formed of anodes and flow duct walls, characterised in that, at the narrow sides of the electrolysis cell (1), not covered by the anodes (8), electrolyte supply devices are provided, through which a portion of the total quantity of electrolyte circulating can be supplied in the direction toward the edges of the band and that a further supply device is provided at one of the ends of the cell, through which the other part of the total quantity of electrolyte circulating is introduced along the plane of movement of the band.

3. Apparatus according to claim 2, characterised in that the supply devices at the narrow sides of the electrolysis cell (1) are nozzles (9), which can be adjusted at an angle to the direction of movement of the band.

4. Apparatus according to claim 2, characterised in that the duct-shaped electrolysis cell (1) can be set at any angle to the horizontal greater than 0°.

5. Apparatus according to claim 4, characterised in that introduction of the stream of electrolyte directed along the plane of the steel band (3) takes place at the upper end of the electrolysis cell (1) and the flow of electrolyte through the electrolysis cell (1) is produced by the force of gravity.

6. Apparatus according to claim 4, characterised in that introduction of the stream of electrolyte directed along the plane of the steel band (3) takes place at the lower end of the electrolysis cell (1) and the flow of electrolyte through the electrolysis cell (1) is produced by pumps against the force of gravity.

7. Apparatus according to claim 5, characterised in that the rate of discharge of the electrolyte (4) from the electrolysis cell (1) is controlled either by setting the angle of the electrolysis cell (1) to the horizontal or by modifying the discharge opening in a mechanical and/or pneumatic and/or hydraulic way.

8. Apparatus according to claim 4, characterised in that the duct-shaped electrolysis cell (1) is directed vertically.

9. Apparatus according to claim 2, characterised in that electrical contact of the steel band (3) (cathode) takes place through current rollers (2), which are arranged before and behind the electrolysis cell (1) in the direction of movement of the band.

10. Apparatus according to claim 9, characterised in that contact with the steel band (3) is effected by partial guidance around the current rollers (2) or by line contact with the current rollers (2) or by a combination of the two systems.

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