Field of the invention

This invention relates to a rinsing apparatus for use in a metal strip processing line and to a method of using said rinsing apparatus.

Background of the invention

Conventional metal strip processing lines, such as a plating line, a pickling line or a cleaning line, in many cases also include a rinsing step. This rinsing step usually requires a complicated set-up involving tanks, pumps, sprayers, etc. and the associated flow and level controls.

The pickling of steel is a process for removing scale from the surface of the steel that is present after various forming operations. Continuous sheets of steel are typically carried through several acid baths by immersing the metal strip completely in the baths for enough time to remove the scale. It is necessary to remove the acid residue from the steel after the pickling step to prevent corrosion, staining or spotting of the surface.

Pickling plants for treating a continuous steel metal strip usually include a metal strip rinsing facility downstream of the acid pickling stage in order to remove any residual pickling acid still adhering to the surface of the metal strip as the metal strip exits the pickling section. This rinsing process can be implemented by spraying the rinsing liquid directly onto the metal strip.

Alternatively, the metal strip can be rinsed by pulling the metal strip through a bath of the rinsing liquid. In plants where the rinsing liquid is sprayed through nozzles directly onto the metal strip, the metal strip surface suffers serious discoloration due to oxidation if the air and rinsing water act on the metal strip simultaneously during prolonged stoppages of the metal strip movement through the rinse station.

This invention relates generally to metal strip processing lines of the kind utilised in steel-making plants and has to do particularly with processing lines in which the steel metal strip is at some point passed through a liquid in a process tank. Upon emergence from the process tank, the steel metal strip retains on its surface a layer of the liquid. This liquid layer usually must be removed from the strip. Conventionally, metal strip processing lines employ a wringer roll arrangement in which the metal strip is passed between the nip of one or more pairs of freely rotating rubber rolls.

A state-of-the-art rinsing apparatus in a continuous metal strip processing line as depicted in Figure 1 requires considerable amounts of space in terms of line length and space for the intermediate storage of rinse liquid and waste-water treatment. The equipment is extensive, and the system is also quite inflexible. The amount of waste water produced is large. Objectives of the invention

It is an object of the invention to provide a rinsing device that is mechanically simple and easy to control and maintain.

It is also an object of the invention to provide a rinsing device that uses less water than conventional rinsing devices.

It is also an object of the invention to provide a process and apparatus for rinsing a metal strip while reducing the risk of oxidation and staining of the metal strip.

Another object of the invention is to provide a process and apparatus for rinsing a metal strip with a rinse liquid using equipment that is economical and easy to assemble.

Description of the invention

One or more of the objects is reached with the device according to claim 1 and the method according to claim 9. Preferable embodiments are described in the dependent claims.

In the rinsing apparatus according to the invention the metal strip leaves the process tank and is directed upwardly into the rinsing apparatus in a substantially vertical direction. For its effect to be maximised the direction of travel of the metal strip must be vertical. Small deviations in verticality may be permitted, but because the device's functioning relies on the effect of gravity in combination with the rotation of roller pair(s) below the point of insertion of the rinse liquid, these deviations must be as small as possible. The rinsing apparatus comprises a plurality (N) of roller pairs, where N is at least 3. For the sake of this invention the lowest roller pair is identified as roller pair number 1 (indicated with "a" in figure 2) and the upper roller pair is roller pair number N. The roller pairs create a substantially equidistant slit through which the metal strip is led upwardly. The axes of the rollers of each roller pair are substantially horizontal and substantially parallel. Small deviations may be tolerable but if the deviations become too large then the flow of water and metal becomes disturbed leading to uneven rinsing or pinching of the strip.

Fresh rinse liquid is introduced in the upper part of the rinsing apparatus to benefit from its downwards cascading in the most efficient and effective way. The uppermost roller pair or roller pairs may act as drying rolls, and in that case the rinse liquid is introduced below the first pair of drying rolls. Rinse liquid may be introduced at one location, or at more than one location further down the rinsing apparatus.

The means to provide the rinse liquid towards the metal strip are located such to introduce the rinse liquid in the gap between two subsequent rolls. The rinse liquid must be introduced in such a way that an even rinsing over the width of the metal strip is obtained, e.g. by using a plurality of spray nozzles over the width of the device. Preferably the means to provide the rinse liquid are installed on both sides of the metal strip and preferably symmetrically. If so desired, additional means to provide rinse liquid to the metal strip may be installed in gaps between two subsequent rolls upstream of the metal strip (i.e. further down in the rinsing apparatus).

The roller pairs can be driven by external means such as a motor, or by the strip. The former requires careful control of the rotation to prevent scratching or pinching of the metal strip. The latter means that the strip is pulled through the rinsing apparatus which may, particularly for very thin and/or soft metal strip, require careful handling.

In the context of this invention it is important to note that the rotation of the roller pairs is such that at the point of contact with the metal strip the roll surface moves upwardly, substantially with the same speed circumferential speed as the speed of the strip moving upwardly, thus assisting the upward travel of the metal strip and avoiding scratches or other damage to the strip surface, and the roll surface moves downwardly at the point opposite of the point of contact with the metal strip, thus assisting the downward travel of the rinse liquid.

The rinse liquid is preferably water, or water-based. Additives may be provided to the rinse liquid if so desired.

The main purposes of the encapsulating shield are to control the flow of the rinse liquid in the rinsing apparatus and to prevent spillage of (used and potentially polluted) rinsing liquid because the shield catches any liquid flung away from the rollers and leads it back to the metal strip surface. In a preferable embodiment the encapsulating shield follows the contours of the roller pairs, and more preferably the distance between the encapsulating shield and the surface of the rollers is substantially constant. This allows the flow of rinsing liquid to be laminar through the device, rather than turbulent. However, if so desired, the encapsulating shield can also be designed such that the flow is turbulent, rather than laminar.

The flinging away of the rinse liquid from the roller pairs has several technical drawbacks. Firstly, the rinse liquid is no longer available for rinsing the metal strip further down the rinsing apparatus if the rinse liquid is flung out of the rinsing apparatus. Secondly, the effectivity of the rinsing process becomes lower and less controlled, for example because the amount of rinse liquid being flung out may not be constant, and thirdly, the used rinse liquid may scatter contaminants around if the unrinsed metal strip has picked up these contaminants from the process tanks. These contaminants may be harmful, and thus scattering these around must be prevented.

With the apparatus according to the invention the amount of ground space required in the line is reduced because the line goes up during rinsing. Only one rinse liquid collector may be needed, and the amount of rinse liquid is reduced due to a more efficient use of the rinse liquid because of the counterflow of rinse liquid and metal strip to be rinsed. The equipment is also relatively simple, and easier to control. In another embodiment the used rinse water is discarded into the process tank and dilutes the contents of the process tank. This may partly or completely compensate any evaporation that takes place from the contents in the process tank. Any excess rinse water can be collected and led away.

In this invention the rinse liquid is transported downwardly by the combined effect of gravity and by the rotating rolls in the direction of the rotation of the rolls. This means that the rinse liquid is carried downwardly by the rotating rolls during their rotation and transported to the metal strip. This outward movement of the rinse liquid and the transport to the metal strip is depicted in figure 2 by the arrows in the rinse liquid. It is unlikely that significant amounts of rinsing liquids seep downwardly through the slit between the roller pairs when a metal strip (with its adhering liquid layer) to be rinsed is travelling upwardly through the slit.

The rolls pairs do allow a small amount of liquid adhering to the metal strip to pass with metal strip upwardly. This adhering liquid is diluted each time by the rinse liquid in the gaps between subsequent rollers, so the result is that the liquid that is adhering to the metal strip becomes more diluted each time a pair of rollers is passed. If the last roll pair is a drying roll, then most, if not all, of the remaining liquid is removed from the metal strip just before exiting the rinsing apparatus. A drying step after the rinsing apparatus may be provided to completely dry the metal strip.

The rinse liquid is sprayed towards the strip between two pairs of rollers. The term "towards" includes (i) the spraying directly onto the metal strip or (ii) the spraying into the gap between the rollers but not directly onto the metal strip or (iii) both. The spraying of the rinse liquid directly onto the strip after which the rinse liquid joins the water cushion is the most effective version because then the strip is directly rinsed with the cleanest rinse liquid. The rinse liquid forms a rinse liquid cushion in the gap (see e.g. figure 2) which dilutes the adhering liquid film on the metal strip. This may be between the top roller pair (N) and the one just below that (N-l) or lower down in the apparatus. The roller pair(s) above the point where the rinse liquid is sprayed onto the strip may serve as drying rollers. The minimum amount of roller pairs is 3, but it is preferable to have at least 4 or 5 roller pairs. The more roller pairs, the higher the rinsing potential (and optional drying potential when using drying rollers) of the device. However, at some stage the added complexity and upkeep no longer outweighs the benefits. A suitable maximum number of roller pairs is 10, preferably N is at most 9, more preferably at most 8. It should be noted that (pairs of) guiding rolls or pairs of turning rolls, which have no rinsing or drying contribution according to the invention are not counted in this invention. Turning rolls are not shown in the figures (see e.g. figure 2 where the strip changes direction from horizontal in the process tank to vertical in the rinsing apparatus. The fresh rinse liquid rinses the strip at the point where the rinse liquid is sprayed onto the strip and is subsequently transported down by the joint effect of gravity and the rotation of the roller pair below the point of insertion. The roller pair picks up the liquid and transports it to the gap between that roller pair and the roller pair below it, thereby again allowing the rinse liquid to contact the metal strip and clean it. Only minute amounts of rinse liquid cling to the strip and are transported upwardly. Most of the rinse liquid is transported again by the roller pair further down the rinsing apparatus, thereby rinsing the metal strip in counterflow. The rinse liquid travels downwardly, and the strip travels upwardly.

The rinse liquid that is provided to the metal strip tends to adhere to the rotating rollers. However, above a certain threshold rotational speed the rinse liquid is no longer able to cling to the roller's surface and will be flung away from the roller in all directions. The centrifugal force that is responsible for that can be calculated, so that the maximum rotational speed of the surface of the roller can be calculated before the rinse liquid is flung away from the roller. The rotational speed is dependent on the roll radius and the frequency of rotation.

To avoid the rinse liquid to be flung away the speed of rotation of the rollers can be limited. However, this requires a low line speed. To allow the invention to be used at high line speeds of up to about 1000 m/min an encapsulating shield must be used that prevents the rinse liquid to be flung away from the rolls and contains the rinse liquid near the rolls. The rolls are then able to transport the rinse liquid from the upper parts of the rinsing apparatus where the fresh rinse liquid is introduced to subsequently lower parts of the rinsing apparatus wherein the rinse liquid is brought into contact with the metal strip to be rinsed each time until the lowest pair of rollers is reached, after which the rinse liquid is collected and led away for waste rinse liquid treatment and possible re-use after said treatment, or proper disposal of the waste rinse liquid.

Drying may also be performed by subjecting the metal strip to a drying treatment, e.g. by convection drying or blowing (hot) air onto the surface.

The main variables for this theoretical assessment are (see also figure 4): v (m/s) : metal strip speed D (m) : roll diameter (R=radius) w (m): width of the metal strip F (m ³ /s) : amount of rinse liquid per side p (kg/m ³ ): density of rinse liquid co (rad/s): rotation of roll s (kg/s ² ) : surface tension of rinse liquid The two relevant forces are the centrifugal force and the surface tension of the rinse liquid. Because of the relatively high attraction of water molecules to each other through a web of hydrogen bonds, water has a higher surface tension (72.8 millinewtons per meter or 0.0728 kg/s ² at 20 °C) than most other liquids.

The centrifugal force is given by: F _K = mo ² R = pwdrd*ro ² R and the surface tension is given by F _s =(a/R)d*w.

Equalling these two forces F _K to Fa enables to calculate at which moment the rinse liquid starts to be flung from the roller.

If the centrifugal force F _K exceeds a, then the liquid will no longer cling to the roller.

Wringer rolls are known to pass 5 ml/m ² (or 5.10 ⁶ m ³ /m ² ) between the wringer roll and the strip. This means that for a 1 meter wide strip an amount of v.w.5.10 ⁶ m ³ of water is let through between the roller and the strip.

The thickness of the water layer clinging to the roller during its rotation is calculated by equalling the value of pd _r v ²

Depending on the size of the rollers this starts to happen at metal strip speeds of about 100 m/min if the rinse liquid is water. As the surface tension of a liquid is temperature dependent, the threshold value becomes lower at higher temperatures.

It is undesirable that the rinse liquid is flung from the rollers because the liquid can no longer have a rinsing effect after that, and the fresh or used rinse liquid also flies everywhere uncontrollably. The inventor found that it the application of an encapsulating shielding closely following the contours of the roller pairs enables to maximally benefit from the rinsing effect and the cascading-down effect of the subsequent roller pairs. The rinse liquid that would otherwise be flung from the rollers is encapsulated by the shielding and will be led back to the metal strip to rinse it upstream as seen from the travelling direction of the metal strip.

To press the rollers against the metal strip the roller pairs may be provided with means to open and close the slit between the roller pair. The means to open and close the slit between the roller pair may enable pressing the pairs of rolls together to exert pressure on the passing metal strip. The means may be provided on each roller of the roller pair, or the means to open and close the slit between the rolled pairs act upon one of the two rolls of a roller pair, and wherein the other roll of said rolled pair is fixed.

The invention is also embodied in a method of rinsing a metal strip using the apparatus according to the invention. Brief description of the drawings

The invention will now be explained by means of the following, non-limiting figures.

Figure 1 schematically shows a typical state-of-the-art rinsing apparatus in a continuous metal strip processing line. The process tank is representative for the type of processing performed in the line. It may be a pickling process, a plating process, a cleaning process, a passivation process, or the like. The processing as such is not particularly limited. After emerging from the process tank the metal strip may retain on its surface a layer of the liquid which it is desirable to remove. In this state-of-the-art apparatus the metal strip is subsequently led to a sequence of (in this example 3) rinsing tanks (Rinse 1, Rinse 2, Rinse 3). The rinse liquid, which is water in this example, is introduced in the circulation tanks in counterflow to the direction of travel of the metal strip, so that the metal strip emerging from the process tank is rinsed with water that has already been used in Rinse 2 and Rinse 3. Waste water is extracted from the circulation tank connected to Rinse 1. Fresh water is introduced into the circulation tank connected to Rinse 3. The system in this schematic drawing requires large rinsing tanks, circulation tanks, pumps, level controls, valves, pipes and is complicated to build, maintain and is therefore costly and susceptible to malfunction if not properly maintained.

Figure 2 shows a schematic drawing of the rinsing apparatus according to the invention. 7 pairs of rollers (N = 7) are shown, in this case divided in two groups consisting of two pairs of rollers at the upper part of the rinsing apparatus and the other group consisting of 5 pairs of rollers in the lower part of the rinsing apparatus. A cut out is shown at the right of the drawing showing more detail of the apparatus. Of the (N-l) ^th roller pair and the (N-2) ^nd and (N-3) ^rd roller pair only the left one is shown. The rotation direction of the rollers is also shown. The rotation of the rolls may be provided by external means, or it may be provided by the metal strip driven through the apparatus. The process tank 6 as part of the continuous metal processing line is shown Also the means for introducing the rinse liquid 4 are shown.

Fresh rinse liquid, in this case water, is introduced between the (N-l) ^th roller pair and the (N-2) ^nd roller pair. The amount of water introduced ensures that a cushion of water is formed on the metal strip. This water is subsequently transported by the (N- 2) ^nd roller pair to the metal strip between the (N-2) ^nd and (N-3) ^rd roller pair and rinses the metal strip. As the water has already been used to rinse the metal strip the water is not fresh water anymore, but still cleaner than the layer of liquid adhering to the metal strip surface. The waste water, having cascaded down the rinsing apparatus this way, and becoming more and more polluted can be collected by collection means and transported away for processing in a waste water facility. The collection means are also shown schematically in the drawing. Alternatively, if the spent waste liquid is compatible with the contents of the process tank, then the waste liquid may also (partly) be discarded in the process tank and (for instance) serve to reuse the washed-off electrolyte in case the process tank is a plating tank containing plating electrolyte, or dilute the liquid in the process tank to compensate for losses due to evaporation.

Figure 3 shows the set-up of the rinsing apparatus wherein the uppermost pair (only one of each pair or rollers is drawn for clarity) of rollers are drying rolls, in this example squeegee rolls, so that the metal strip exiting this pair of rollers is dry. The rinse liquid is sprayed onto the metal strip between the second and third roller. In this particular example the bottom two rolls allow the cascading down of the rinse liquid, in counter-current flow to the metal strip to be rinsed. In the left drawing the rotational speed of the rolls is such that the rinse liquid clings to the rolls. In the left picture an encapsulating shield 5 is provided that encapsulates the rinse liquid while it is transported down. Figure 4 shows the schematics of the set-up and shows the different variables that play a role in the functioning of this rinsing apparatus.