EP0601991A1 - Process and apparatus for cleaning metal bands - Google Patents

Abstract

The invention provides a process for cleaning metal bands which involves treating the sheet metal by atomising at high pressure, and hence spraying with great force, a degreasing solution onto the surface of the metal at impact pressures of from 100 to 5000 g/cm<2>. The invention also concerns an apparatus suitable for carrying out this process. <IMAGE>

Description

Subject of the invention

The invention relates to a new technology suitable for an installation for continuous cleaning of metal strips intended to provide the surface cleanliness required by subsequent coating treatments of such strips. It covers both a cleaning process and an installation for this purpose.

Technological background

Tape cleaning processes and installations, the role of which is to prepare the surface before treatment, still result today from empirical criteria.

Most often, treatments are used which work according to different modes of action, and which are very often successive, as is usual for most of the treatments used in the steel industry.

• les procédés physico-chimiques agissant essentiellement par projection de solutions ou agissant par immersion
• les procédés mécaniques incorporant essentiellement une brosseuse et
• les procédés électro-chimiques agissant à l'aide d'une grille électrolytique.

We distinguish :

• physico-chemical processes acting essentially by spraying solutions or acting by immersion
• mechanical processes essentially incorporating a brushing machine and
• electro-chemical processes acting using an electrolytic grid.

For physicochemical treatment, basic degreasing solutions are generally used, possibly combined with surfactants. Their effectiveness is mainly conditioned by the chemistry of the solutions which cannot give convincing results, mainly for the elimination of solids (iron fines) without a complementary and in fact determining mechanical action.

Currently, a degreasing installation generally consists of a low pressure immersion or sprinkling tunnel (in order to allow the physicochemical action of the solution), a brushing machine (exercising the mechanical action) and d '' an electro-chemical degreasing cell low density.

The main drawbacks of this equipment reside first of all in their size. The different processing steps are physically separated and are successive. It is therefore necessary to have facilities which take up a considerable amount of space, in particular if one takes into account the need for a considerable time of stay or action, at least for part of the treatments.

In addition, their reliability is low because the mode of operation of the brushes, among others, and the wear of these brushes impose a level of maintenance and monitoring which make them in practice unreliable.

Finally, the maintenance and operating costs of such installations are high.

Currently, among the techniques used, only the mechanical brushing action appears really decisive, the practical use of which is very delicate and in fact always relatively empirical.

In order to remain fully operational, it involves an unacceptable level of maintenance due to the rapid wear of the brushes. In addition, this wear leads to pollution of the baths by the fragments of the brushes resulting from this wear.

The document PATENT ABSTRACTS OF JAPAN, Vol. II, No. 214 (C-434) (2661) of July 18, 1987 which refers to Japanese patent application JA-A 6230892 describes a method of cleaning metal strips by injecting a cleaning fluid under pressure using nozzles having an adequate inclination with respect to the surface of the strip. This document indicates that injection pressures from 5 to 1000 kg / cm² with flow rates from 5 to 500 l / min / m² can be used. Neither the document itself nor the text of the Japanese patent application indicate impact values for the sheet to be treated.

The higher pressure values indicated would at least pose serious technical difficulties of production and the document does not in fact provide practical information for the implementation of the technical.

The document EP-A-0050036 (PARAMEC CHEMICALS) is intended for threads for cleaning. It does not indicate any parameters that could lead to transposition to sheet metal processing.

GB-A-2 143 254 (SMS SCHLOEMANN-SIEMAG) describes a treatment 60 bar or more on a surface using nozzles. It is technically undesirable to work at such pressures because of the equipment (pumps) to be put into service and the operating conditions according to the present invention are not achieved.

The document AU-A-413 792 (BONAR) essentially describes a device for cleaning sheets but does not include any parametric indication for performing this cleaning operation under the advantageous economic and technical conditions envisaged by the present invention.

Aims pursued by the present invention

The present invention essentially aims to avoid the drawbacks inherent in the known solutions proposed according to the state of the art. It particularly aims to provide a technology capable of providing a determining mechanical action (efficient and repetitive and not subject to efficiency drifts) going hand in hand with the physicochemical action provided by contact with a generally basic treatment solution. .

Character-defining elements of the invention

According to the invention, a method is proposed for cleaning metal strips, in particular steel strips comprising the treatment of the sheet by a high pressure spraying action and therefore a high impact projection of degreasing solutions on the surface of the sheet.

In practice, the projection is carried out under a pressure of the order of 15 to 60 bars and preferably of the order of 15 to 40 bars, at a distance from the upper surface of the sheet as small as possible while allowing efficient operation of the installation.

In industrial practice, the process is applied in continuous line installations, which necessarily implies that the projection heads must allow rational exploitation of the line without major interference from these projection heads.

Values of the distance from the projection head to the sheet ranging from a few tens of millimeters to a few centimeters have been found to be particularly effective. In practice, a distance of 30 to 200 millimeters is recommended. It will be noted that the smaller the distance, the more spray heads will be required to cover the same surface and that consequently a material limit due to the bulk is quickly reached.

The lower pressure limit indicated is that necessary to obtain good treatment efficiency.

The upper pressure limit is essentially conditioned by economic reasons involving the use of simpler technical means, namely pumps of the centrifugal type, without having to resort to positive displacement pumps which would become necessary if higher pressures were envisaged.

The fact that within the indicated pressure range efficient results can be obtained, without the need for expensive equipment, contributes to the general economy of the process.

The invention is based on the fact that one avoids having to separate the modes of action of the projection of solutions, from the mechanical action proper which is carried out using the brushing machine, according to the state of the technical.

The use of known low energy techniques, namely low impact sputtering and low density electrolysis in practice required significant action times, according to known techniques.

Contrary to what seems to be a general prejudice existing in the case of the use of techniques for treating steel strips, it seems that the use of the technique according to the invention comprising a projection of degreasing solutions with high impact avoids the use of long residence times which were believed to be essential and which in fact were necessary in the case of the use of equipment comprising a low-pressure immersion or spray tunnel.

It has been found in fact that the impact effect which can be measured as the force exerted on a unitary surface of the strip, at a given height of the projection head, is directly proportional to the flow rate and the square root of the spray pressure.

Consequently, the use of very high pressures such as pressures higher than 40 bars is less significant than the effect of the flow.

The use of centrifugal pumps which is recommended according to the invention makes it possible to reconcile relatively high pressures with high flow rates resulting in increased efficiency of the process.

Impact effect values of the order of 100 to 5000 and preferably 150 to 500 g / cm² have been found necessary.

Of course, the use of complementary techniques, in particular electro-chemical or physico-chemical techniques, is not excluded, as a possible complement to the techniques specific to the present invention.

The solution recommended by the invention makes it possible to avoid the cost and the empiricism of operating the brushing cylinders. The disturbances caused by brush fragments resulting from the wear of the brushes are also avoided.

The invention will be described below by way of illustration with the aid of an exemplary embodiment, without limitation, of the invention.

Brief description of the drawing

Figure 1,

schématiquement une installation convenant pour la mise en oeuvre du procédé;

Figure 2,

le détail du tunnel de dégraissage haute pression (vue en coupe schématique);

Figure 3,

un diagramme de l'effet d'impact correspondant à une forme d'exécution de l'invention donnée à titre d'illustration.

In the accompanying drawings, there is shown:

Figure 1,

schematically an installation suitable for implementing the method;

Figure 2,

the detail of the high pressure degreasing tunnel (schematic sectional view);

Figure 3,

an impact effect diagram corresponding to a given embodiment of the invention for illustration.

Description of a preferred embodiment of the invention.

An installation suitable for implementing the technique of the invention consists of a sheet unwinder 1, an electrolytic installation or quench bath 2, a high pressure degreasing tunnel 3, a rinsing station 4, possible subsequent treatments 5 and a winder 6.

Such an installation may be suitable for treating a sheet up to 400 mm in width and a thickness of up to 1 mm at a line speed which can vary from a very low value to 400 m / min.

In the installation 3 for cleaning a steel strip, the strip bearing the general reference 13 circulates continuously and is continuously subjected to the action of spray jets of a degreasing solution comprising surfactants, originating from spray heads 15 distributed above and below, as well as over the entire width of the strip, for example at a distance of 70 mm from the latter.

A centrifugal pump 17 provides a pressure of 35 bar with a flow rate of 30 m³ / h. for a strip width of 300 mm, which corresponds to an impact value of 270 g / cm² on the strip. This combined effect of impact action and chemical action is sufficient to perfectly treat a band circulating at a speed of 200 m / min.

The alkaline solution is recovered and recycled with possible interposition of filtration equipment (retaining dust and iron fines) and / or regeneration.

The equipment described can be associated in appropriate cases with a high density electrolytic degreasing cell, or with other treatment processes, if desired.

In Figure 3, there is shown the impact profile of a jet taking as a function of the distance in mm from the projection head the impact pressure expressed in g / cm².

Such an impact profile makes it possible to study and develop the optimal operating conditions based on the sheet qualities to be treated.

The composition of the solutions used for high pressure spraying is in principle adapted to the operating conditions of operation.

In principle, alkaline solutions can be used which exert a saponification effect on the esters, optionally with a surfactant. Many of these compositions are known.

The addition of an anti-foaming agent is generally useful and those skilled in the art on the basis of simple tests can easily, depending on the circumstances, adjust the various parameters such as temperature, pressure, distance of the jets, flow rate. , type of jets and their directions, in particular by experimenting with the installations and studying the achievable jets profiles.

The technology of the invention has been described essentially as applying to steel sheets, however it is not limited thereto. Other materials, namely any metal sheet contaminated with fats and / or metallic particles, for example alloyed steel or not or aluminum in particular, could be treated in this way.

In Table I which follows, we have, without being limiting, indicated on the one hand the various operating parameters with the most common operating ranges by way of example in the right column of the specific values which have been used in practice for the execution of a comparative example.

The results of the measurements carried out on an untreated sheet, on a degreased high pressure sheet according to this example and on a sheet treated according to commonly known techniques are indicated therein.

Claims (4)

Method for cleaning metal strips, in particular steel strips, characterized in that it comprises the treatment of the sheet metal by a high-pressure spraying action and therefore makes a high-impact spraying of degreasing solutions onto the surface of the sheet metal under a pressure of the order of 15 to 60 bars, and preferably 15 to 40 bars, at a distance from the upper surface of the sheet metal as small as possible while allowing efficient operation of the installation in order '' obtaining impact effect values of the order of 100 to 5000 and preferably 150 to 500 g / cm2 by adjusting the flow rates of the spray heads. Method according to Claim 1, characterized in that the distance of the projection heads from the upper surface of the sheet is between 30 and 200 Process according to either of Claims 1 and 2, characterized in that the spraying is carried out using a centrifugal pump. Installation for cleaning steel strips for implementing the method according to any one of claims 1 to 3, characterized in that it comprises a closed circuit for circulation of a degreasing solution and spray heads projecting this solution under high pressure.

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