Apparatus for treatment of metal strip with a liquid

Abstract

This invention relates to an apparatus for the treatment of metal strip with a liquid, comprising, at least one tank provided with an inlet and an outlet for the strip, which inlet and outlet are below the level of the liquid surface in operation, which tank comprises at least one treatment chamber through which the strip is adapted to pass, a reserve chamber, means for transporting liquid from the reserve chamber to the treatment chamber, and two bell chamber means, one close to the inlet and the other close to the outlet, each bell chamber means having an overflow means for liquid to the reserve chamber and each bell chamber means being so constructed that, in operation, the space above the liquid surface is isolated from the ambient atmosphere and is in communication with pressure-reducing means. The invention also includes a novel process for the treatment of metal strip.

Description

This invention relates to a process and apparatus for the treatment of metal strip with a liquid, in particular electrochemical treatment.

Electrochemical treatment of metals may be carried out in order to erode the metal and thereby modify the surface, in particular to roughen it; the treatment also may be carried out in order to product an oxide layer or electrochemically to remove a metallic layer. A chemical or physical treatment of the surface with a liquid is effected, e.g. in the case of etching or pickling and degreasing.

If the metal being treated is in the form of a continuous element, e.g. a wire or strip, the treatment is preferably carried out by continuously conveying the shaped article through the treatment bath.

It has been proposed, for example in German Offenlegungschift No. 1,621,115, to convey an aluminum strip successively through a series of various electrochemical treatment baths in which the strip is first electrolytically roughened and then anodically oxidized. The strip is conveyed by means of guide rollers.

French Patent No. 1,424,643, discloses a device for the anodic treatment of aluminum strip in which the strip passes horizontally through the treatment bath without the use of guide rollers, and is conveyed through inlet and outlet apertures in each of the side walls of the bath container, these apertures being fitted with sealing means to prevent the escape of liquid. The bath liquid is circulated by means of a pump in such a manner that very turbulent flow is maintained between the strip and the electrode. This guarantees a rapid exchange of electrolyte at the surface of the strip and a rapid dissipation of heat. Under these conditons, it is possible to operate at high current densities and thus at high throughput rates of the strip. The apparatus has the advantage that guide rollers and their supports, which are susceptible to corrosion, are not required.

A disadvantage of this apparatus resides in the use of rubber sealing elements which have an abrasive effect at the points where the strip enters and leaves the bath. This easily can lead to scratching of the surface of the strip, particularly when it has been electrochemically treated. Furthermore, the method of circulating the electrolyte described in this patent does not guarantee an optimum mixing of the entire bath liquid, nor an absolutely uniform flow over the entire width of the strip to be treated. Consequently, flow patterns may appear on the treated surface. Furthermore, the degassing of the electrolyte, i.e. the removal of the hydrogen evolved in the case of most electrochemical processes, is difficult in this apparatus.

In the apparatus according to the German Offenlegungsschrift No. 1,521,068, which describes a device for the electrochemical treatment of continuous metal strips, no sealing elements are needed; furthermore a better mixing of the electrolyte in the bath is ensured. In this case the strip is conveyed horizontally through an electrolyte trough which at its ends contains weirs over which the electrolyte flows. The inflow of the electrolyte is through a slot in the bottom of the trough which extends over the entire width of the same. The strip is here conveyed closely below the surface of the liquid (about 3 to 6 mm below). In this apparatus the main problem is always to keep the strip uniformly covered with liquid, since the quantity of liquid flowing over the weirs cannot be directly controlled and the level of the weirs is of necessity below the path of the strip. Since the quantity of liquid above the strip can be maintained or supplemented only by the lateral flow of electrolyte around the strip, the flow of the electrolyte between the strip and the trough bottom acting as a counterelectrode always has a longitudinal and a transverse component in a changing proportion. This may result in flow patterns forming on the treated surface. It is almost impossible with the supply slot used here, which is in the bottom of the trough and is made from a relatively narrow tube by means of extension, to achieve in practice a uniform distribution of flow rate over the width of the strip.

The present invention provides apparatus for treatment of metal strip with a liquid, comprising at least one tank provided with an inlet and an outlet for the strip, which inlet and outlet are at a level of levels below the level of the liquid surface in operation, which tank comprises

one or more treatment chambers through which the strip may be passed,

a reserve chamber,

means for transporting liquid from the reserve chamber to the treatment chamber(s), and

two bell chambers, one close to the inlet and the other close to the outlet, each bell chamber having an overflow device for liquid to the reserve chamber and each chamber being so constructed that, in operation, the space above the liquid surface is isolated from the ambient atmosphere and is in communication with pressure-reducing means.

The reserve chamber is preferably situated below the treatment chamber(s).

In the apparatus of the invention, neither guide rollers nor abrasive seals are required and a constant covering of liquid over the strip in the treatment bath is ensured. When the apparatus is used for electrochemical treatment, for which purpose it is primarily intended, a rapid and uniform flow of electrolyte between the strip and counter-electrode is achieved and, if necessary, effective degassing of the liquid may be effected.

In accordance with the invention, a process is furthermore proposed for the treatment of metal strip with a liquid wherein the strip is conveyed through a liquid contained in a treatment chamber having an inlet and an outlet for the strip below the surface of the liquid, wherein flow of the liquid out of the inlet and outlet is hindered by a reduced pressure above the surface of the liquid.

The inlet and outlet for the metal strip are preferably disposed at the same level so that the metal strip can be conveyed substantially horizontally through the liquid. The apertures preferably are slots in the outer walls of the bell chambers of such width and length that the strip can be satisfactorily conveyed through them contact-free. If only one side of the strip is to be treated, it is advantageously conveyed at the smallest possible distance from the bottom of the treatment tank. Air is continuously drawn out of the gas space of the bell chamber. In this manner a diminished pressure is produced in the gas space of the bell chamber which hinders flow of the bath liquid out of the inlet and outlet. As a result of the difference in pressure between the bell chamber and the main liquid treatment chamber, treatment liquid flows out of the latter into the bell chamber and out of this through the overflow into reserve chamber. The liquid in the main liquid chamber may be supplemented from the reserve container by return pumps.

The process and apparatus are primarily intended and particularly suitable for electrochemical treatment, in which an electrolyte is used as the treatment liquid, and in which there is at least one electrode, preferably arranged in the electrolyte above the path of travel of the metal strip. In this case, it is particularly important to produce in the gap between the electrode and the metal strip the most rapid and uniform possible flow of electrolyte.

The apparatus according to the invention also may be used for other continuous treatments of metal strip with liquids, wherein the strip must be conveyed friction-free through the liquid bath, and a rapid flow of liquid on the surface of the strip must be maintained. Treatments of this type are, for example, degreasing, pickling, descaling, and sealing of, for example, anodized aluminum.

The apparatus is described below with reference to its primary intended application, continuous electrochemical treatment.

Advantageously, the apparatus according to the invention is so constructed that at least one of the bell chambers is joined to or integral with an electrode on the wall facing the inside of the treatment tank, and that this liquid content is joined to that of the remaining tank only by the gap between the electrode and the transport path of the metal strip. Preferably, both bell chambers are joined to or integral with an electrode in this manner.

This construction guarantees that the flowing of the electrolyte liquid from the main liquid chamber into the bell-shaped chamber is effected exclusively through the space between the electrode and the path of travel of the strip, and that the liquid flow is conveyed over the entire width of this space parallel to the path of travel of the strip and at practically uniform strength.

In the apparatus according to the invention metal strip may undergo various different electrochemical treatments. A strip, for example, may be cleaned, descaled, roughened or the like on its surface by either direct or alternating current treatment. It is furthermore possible to carry out an anodic oxidation or to galvanize the strip. The treatment tank may be divided in the middle, in a manner known per se, by a vertical intermediate wall arranged perpendicular to the direction of movement of the strip and ending close above the strip. An electrode is then provided in each section of the treatment tank, and both electrodes may be joined to the poles of a current source. Here the current flows from one part to the other part of the bath substantially through the strip acting as neutral.

This type of circuit is chiefly used in alternating current treatment, but also may be used for direct current treatment. In this case the strip acts in one half of the treatment bath as an anode, and in the other half as a cathode.

The process and apparatus of the invention make it possible to work with a very high current density which may be - depending on the type of treatment - between approximately 2 and 150 A/dm.sup.2. In the case of electrochemical etching it is preferable to work in the range of approximately 25 to 120 A/dm.sup.2. Treatment times of approximetaly 10 seconds to 2 minutes are generally achieved here.

The rate of flow of the electrolyte liquid in the gap between the strip and the electrode is adjusted to the value adequate for the desired current density. Advantageously, as described in French Patent No. 1,424,643, the rate of flow should be such that with the given values, Reynolds numbers (Re) above approximately 2500 are yielded. The regulation of the rate of flow may be effected by alteration of the capacity of the pump means which pumps the electrolyte liquid from the reserve chamber back into the treatment chamber, and if necessary by alteration of the suction capacity in the bell chamber. Furthermore, the rate of flow may be altered by changing the width of the gap between the strip and the electrode. The width of the gap is normally in the order of approxmately 1 to 5 cm, but may be tapered at the junctions to the bell chamber to a fraction of this value.

The apparatus according to the invention is primarily intended for, and especially suitable for, essentially one-sided electrochemical treatment. In principle, a two-sided treatment is possible in which further electrodes are inserted in the bottom of the treatment tank and the strip is conveyed through the bath at a correspoding distance from the bottom of the tank. In the particularly preferred process, the strip is generally conveyed at so small a distance from the bottom that it just does not touch it. In this method of operation, the rear side of the strip is practically unchanged apart from narrow marginal zones. The strip also, however, may lie on the bottom of the tank without great disadvantage.

Depending on the suction capacity, greater or smaller quantities of air may be drawn through the electrolyte liquid into the bell chambers at the ends of the apparatus. In conjunction with the diminished pressure prevailing in these chambers, the air bubbling through produces an effective degassing of the electrolyte liquid, i.e., a substantial removal of the hydrogen formed when the liquid passes through the gap between the strip and the electrode. The degassed electrolyte liquid collects in the reverse container and is cooled there or in another suitable point in its circuit. Furthermore, it may be directed through a filter or sieve before being pumped back into the treatment tank in order to free it from dispersed impurities. Renewal of the electrolyte liquid may be effected by portion-by-portion removal of the spent electrolytes and addition of the corresonding quantity of fresh electrolytes in the reserve container.

The apparatus according to the invention advantageously may be composed of several of the above-described basic units comprising a treatment chamber and two bell chambers. Between these basic units, washing stations are advantageously arranged for freeing the strip from the liquid, from a treatment station, still adhering to it. Also, chemical etching or pickling steps may be arranged before, after, or between the electrochemical treatment stations. An example of the combination of several electrochemical treatment steps in a continuous plant is the electrochemical roughening and subsequent anodic oxidation of a strip of aluminum. Etching and galvanizing treatments of aluminum, copper, steel or other metal strips also may be carried out in several successive stations using the apparatus according to the invention.

The invention will be further illustrated by way of example only, with reference to the accompanying drawings, in which:

FIG. 1 is a longitudinal cross-section through a basic unit of apparatus according to the invention, and

FIGS. 2 and 3 are simplified longitudinal cross-sections through possible combinations of two basic units according to FIG. 1.

Referring now to FIG. 1, a treatment tank 1 has a non-conductive base 2 over which the metal strip 3 to be treated is conveyed. At the ends of the tank 1 there are the bell chambers 4' and 4", which have their outer walls 5' and 5" the inlet and outlet apertures 6' and 6" for the strip 3. Inner walls 7' and 7" of the bell chambers 4' and 4" are joined at their lower ends to the electrodes 8' and 8". Gas discharge pipes 9' and 9" lead out of the gas space of the chambers 4' and 4", which discharge pipes are connected with a suction pump 10.

In the liquid space of the chambers 4' and 4" there are overflow pipes 11' and 11" which terminate in a reverse chamber 12 disposed below the tank 1 and which, viewed in the plane of the drawing, pass in front of or behind the strip 3.

In the reserve container 12, the electrolyte liquid is maintained at the desired temperature by heat exchangers 13, by means of which the heat produced in the electrochemical treatment is normally drawn off. From the reserve container 12, a conduit 14 leads back via a pump 15 to the central part of the tank 1. Two discharge apertures 14' and 14" of the conduit 14 are disposed above the electrodes 8' and 8" near the walls 7' and 7". By means of this arrangement, the rough turbulence of the electrolyte liquid pumped back is calmed before the liquid flows into gap 16' or 16" between the electrode and the strip.

The reserve containers 12 is provided simultaneously as a safety tank and a storage tank for the electrolyte liquid cntained in the tank 1. It contains sealable slots 17' and 17" for the strip 3 at the corresponding positions in its front and rear walls. When starting or stopping the treatment plant, or if there is a breakdown in the suction pump 10 or any other distrubances in operation, the slots 17' and 17" are closed so that none of the electrolyte liquid can escape from the container 12 to the outside.

The electrodes 8' and 8" are connected to the poles of an A.C. source. In the middle of the tank 1 there is a non-conductive separating wall 18 which ends closely above the strip 3 and divides the tank 1 into two essentially symmetrical halves. The flow of current is therefore from the electrode 8' via the gap 16' to the strip 3 and from there into the right-hand half via the gap 16" to the electrode 8". In this arrangement, the flow of current from the electrode 8' through the electrolytes in the gap between the wall 18 and the strip 3 to the electrode 8" is practically negligible.

In case small quantities of liquid escape out of the apertures 6' and 6" on to the strip during operation of the apparatus, in spite of the diminished pressure in the bell chamber, air nozzles 19' and 19" are provided which remove the liquid from the strip and cause it to flow back into the reserve container 12.

It is also possible to connect a separate power supply to the strip itself. In this case, a roller 20 is provided which is in conductive contact with the metal strip, one pole of the current source being applied to the roller 20 and the other pole to the electrodes. It is also possible here to use one electrode only; the dividing wall 18 is then also dispensable.

FIGS. 2 and 3 illustrate, in simplified form, two possible arrangements of two devices according to FIG. 1 (basic units) for carrying out various electrochemical treatment steps on a strip.

In FIG. 2, the strip 3 runs successively through two basic units A and B. Before, after and between the basic units A and B there are normally degreasing, pickling and washing stations which for the purpose of clarity have been omitted. In the two basic units the strip is inserted as the neutral between the electrodes 8' and 8", in which in the basic unit A it is inserted against the first electrode 8' as an anode and against the electrode 8" as a cathode. In the basic unit B the series is reversed. An A.C. source may also be supplied in the basic unit A, and in the basic unit B the strip 3 can be connected to the anode and two electrodes to the cathode of a D.C. source.

FIG. 3 shows the possibility of a commutation. Two independent D.C. sources U.sub.1 and U.sub.2 are used. The voltage U.sub.1 may have its poles reversed by shifting the change-over switch S. In the solid line representation of the switch S, the strip 3 rests against the cathode and all electrodes against the anode. If S is in the position shown by the dotted line, the electrodes 8" are negative and the electrodes 8' positive while the strip acts as the neutral.

Obviously many other variations are possible.

It will be obvious to those skilled in the art that many modifications may be made within the scope of the present invention without departing from the spirit thereof, and the invention includes all such modifications.

Claims

What is claimed is:

1. An apparatus for the treatment of metal strip with a liquid, comprising, at least one tank provided with an inlet and an outlet for the strip, which inlet and outlet are below the level of the liquid surface in operation, which tank comprises

at least one treatment chamber through which the strip is adapted to pass,

a reserve chamber,

means for transporting liquid from the reserve chamber to the treatment chamber, and

two bell chamber means, one close to the inlet and the other close to the outlet, each bell chamber means having an overflow means for liquid to the reserve chamber and each bell chamber means being so constructed that, in operation, the space above the liquid surface is isolated from the ambient atmosphere and is in communication with pressure-reducing means.

2. An apparatus as claimed in claim 1 including at least one electrode.

3. An apparatus as claimed in claim 2 wherein the electrode is arranged above the path of travel of the strip.

4. An apparatus as claimed in claim 3 including means for ensuring a rapid and uniform flow of liquid between the electrode and the path of the travel of the strip.

5. An apparatus as claimed in claim 4 wherein the inner wall of at least one of the bell chamber means is joined to an electrode, and the only means of communication between the treatment chamber and the bell chamber means is by gap means between the said electrode and the path of travel of the strip.

6. An apparatus as claimed in claim 5 wherein the inner wall of each bell chamber is joined to an electrode.

7. An apparatus as claimed in claim 6 where the treatment chamber is divided into two compartments by vertical wall means.

8. An apparatus as claimed in claim 1 wherein the reserve chamber is situated below the treatment chamber.

9. An apparatus as claimed in claim 1 wherein the inlet and outlet are at substantially the same level.