FR2562562A1 - Process and device for continuous cleaning of a metal strip - Google Patents

Abstract

Process for continuous cleaning of a metal strip, characterised in that the said metal strip continuously enters, at room temperature, a first zone where it is subjected to the intensive radiation from heating elements, exhibiting a temperature of the order of 850 to 1100 DEG C, thus resulting in an instantaneous evaporation, called flash evaporation, and a gasification of the residues present at the surface of the strip, in that the products of this gasification are oxidised by the surrounding air entrained as a limiting layer by the strip itself, and in that, in a second zone, the said strip travels countercurrentwise to an atmospheric gas, so as to obtain a progressive oxidation of the carbonaceous residues originating from the flash evaporation and an at least partial reduction of the metal oxides present on the surface of the strip.

Description

 The present invention relates to a process for cleaning a metal strip in combination with a thermal effect and a reducing atmosphere gas. It also relates to a device for the implementation of such a method.

 It is known that, in many applications, the metal strips must be cleaned, in order to have a surface appearance such that neither oils or greases deposited on the surface, nor more or less homogeneous oxide films, prevent the coating of these strips or does not modify the structure of the metal during a subsequent heat treatment cycle.

 In installations according to the prior art, such as continuous annealing treatment units, followed, where appropriate, by a device depositing a metallic coating on the strip, by immersion of the latter in a liquid bath (for example , zinc, aluminum-zinc alloy, etc.), the cleaning of the strip thus treated is carried out upstream of the annealing furnace, using physicochemical processes which are generally carried out in liquid phases. , and often supplemented by mechanical treatments, in particular brushing, and drying. This known solution makes it possible to obtain a strip which is both degreased and free from deposits of metallic fines from cold rolling. However, this solution has the disadvantage of being expensive.

 In other known installations, degreasing of the metal strip is carried out at the entrance to the heat treatment unit, in a zone called "degreasing zone", by gas effect, this zone preceding the annealing furnace. The rise in temperature of the strip, which is subjected to the radiation of flames and to the action of the combustion gases in the heat treatment unit, makes it possible to obtain evaporation and elimination of deposits on the strip, without however allow elimination of fines and oxides. According to a variant of this known process, a calorific contribution is made using other means than an open flame, for example radiant tubes or electrical resistances, the atmosphere of combustion (generally having the following composition: N2 + C02 + CO + H2 + H20) then being generated by an annex unit or by recycling the fumes from the radiant tubes.

Such a system is described in French patent No. 81 12 289, belonging to the present holder. The thermal efficiency of such degreasing by the effect of atmospheric gas has the disadvantage of being limited, taking into account the
temperature levels necessary for the fumes, during preheating, the volume of the fumes thus conveyed, and the rejection temperature which is generally greater than 400 "C, or even 600" C.

 The process which is the subject of the present invention is intended to replace both the physicochemical treatments and the degreasing by sweeping of an atmosphere resulting from the combustion of fossil or waste fuels (for example, blast furnace gas or gas steelworks). This process is designed so as to suppress any supply of smoke into the treatment enclosure, this smoke being replaced by an atmospheric gas composed essentially of nitrogen and hydrogen, this process being able to allow a heat supply either by tubes radiant, either by electrical resistances, the treatment enclosure being maintained at slight overpressure.

 Consequently, the method for continuously cleaning a metal strip according to the present invention is essentially characterized in that said metal strip penetrates continuously, at room temperature, into a first zone, where it is subjected to the intensive radiation of elements. heaters, having a temperature of the order of 850 to 1100 C, thus causing instant evaporation ("flash evaporation") and gasification of the residues present on the surface of the strip, in that the products of this gasification are oxidized by ambient air entrained as a boundary layer by the strip itself, and in that, in a second zone, said strip circulates against the flow of atmospheric gas, so as to obtain a progressive oxidation of the carbonaceous residues, resulting from flash evaporation, and a reduction, at least partial, of the metal oxides present on the surface of the strip.

 According to a characteristic of the invention, the atmosphere gas consists of nitrogen and hydrogen.

 According to the invention, the oxidation of carbonaceous residues is obtained by the action of water vapor present in the atmosphere gas, this water vapor can be obtained by reduction, at least partially, of the metal oxides present on the surface of the strip, or trace oxygen in the nitrogen of the atmosphere gas. According to a variant, this water vapor can be added as an addition to the atmospheric gas, at a level which makes it possible to avoid oxidation of the strip by this same water vapor.

It is understood that the process which is the subject of the present invention operates in two stages 1 "- in the first stage, flash evaporation and gasification are obtained.
tion of the products present on the surface of the metal strip. The band
continuously enters a first zone, where it is subjected to the ray
intensive heating elements with higher temperature
at 850 "C, and usually kept below 1000l. The presence
departures, on the surface of the metal strip, increases its emissivity
apparent, and, given the temperature difference between
the radiant heating elements and the surface of the strip, these dice
pots very quickly absorb part of the heat flux emitted.
provides an entry airlock in the cleaning area, which is designed so
to reduce the turbulence of the gases at the level of the strip, and to favor
Training, during the introduction of this strip in the area of
cleaning, of a maximum limit layer of air after the first roll
of introduction into the airlock 20 - in a second stage, an at least partial reduction of the
metal oxides present on the surface of the strip, and oxidation
carbon residues from flash evaporation. To this end, the tape,
after passing through the zone of intensive radiation where it was carried out
flash evaporation, circulates, always continuously, against a current of
atmosphere gas which consists of a mixture of nitrogen and hydrogen
preferably from the annealing furnace. According to the invention, the speed of
band scanning by atmospheric gas can be increased by reducing
the space which separates the radiating heating element from the strip,
on the one hand, and by introducing an additional volume of gas from atmos
sphere generally containing less than 15% hydrogen in the area
cleaning, on the other hand. This make-up gas can be preheated to
to eliminate, at least partially, and upon entering the
cleaning, the traces of oxygen that are generally present in
nitrogen being part of the atmospheric gas.

Other characteristics and advantages of the present invention will emerge from the description given below with reference to the appended drawing, which illustrates an embodiment thereof devoid of any limiting character. On the drawing
- Figure 1 is a schematic view, in side elevation and axial section, of an installation according to the invention
- Figure 2 is an enlarged view showing the entrance airlock of the installation shown in Figure 1; and,
- Figure 3 is a curve representing the temperature variation, as a function of time, the surface of the sheet and the residues it contains.

 The installation shown by way of example in the drawing essentially comprises an entry airlock 10, in which the first support roller 12 (horizontal line) or return roller (vertical line) is provided. This airlock 10 is in communication with the atmosphere, so as to allow the reconstitution, after the first roll 12, on the face of the strip 14 which is in contact with this roll, of a boundary layer composed by the air of which circulation is controlled inside this airlock (induced air).

 Following this airlock 10 is arranged the flash evaporation zone 16, the passage from the airlock 10 to this zone 16 being effected by means of an orifice 18, the passage section of which can be adjusted so as to adjust the pressure drop to the values necessary to avoid a circulation of smoke from the flash evaporation zone 16 to the airlock 10. For this purpose, an articulated flap 20 can be provided.

 This flash evaporation zone 16 is produced in the form of a thermally insulated enclosure which communicates with the outside, on the one hand, through the orifice 18 for passage of the strip 14, and, on the other hand, by the smoke evacuation duct 22. The walls of said enclosure, facing the strip 14, are equipped with electrical resistors 24 whose temperature is regulated zone by zone. The other end of the flash evaporation zone 16 is in communication with the annealing zone 26, the passage from zone 16 to zone 26 being effected by means of a rectangular slot 28, which allows the passage of the strip 14. The section of this slot 28 is reduced, in order to allow better control of the circulation of atmospheric gases from the annealing zone 26 to the flash evaporation zone 16.

In this exemplary embodiment, the atmospheric gas (N2 - H2) comes from the annealing zone 26. In FIG. 1, the conduits for introducing the make-up atmospheric gas (N ~ H2) have been diagrammed ) and water vapor, if any
2 2 slightly warmed up using a heat exchanger 30.

It is understood that, in the method according to the invention, the phenomena which govern the cleaning of the strip are as follows:
In the first stage, on the strip introduced at room temperature
Thanks to the heat flux absorbed by the surface deposits of the strip, there occurs in zone 16, flash evaporation of the formulation residues:
C + H + HO + metallic fines.

nC02 + m H20 +H1
H2O liquide

H2 O vapeur
2
Ces réactions de gazéification et d'évaporation ont lieu alors que la température de la bande est encore inférieure à 350/4000C, ce qui permet d'éviter l'oxydation superficielle du métal par CO2 ou H2O, ou 2' présents dans le milieu réactionnel, les cinétiques de ces réactions étant en effet très lentes en-dessous de ces températures (350/400"C), et le temps de séjour du métal dans ce milieu étant réduit,
Dans le deuxième stade
La bande 14, qui circule à contre-courant des gaz d'atmosphère (N , H
2 2 tend progressivement vers des températures de l'ordre de 600 à 800"C, selon les traitements ultérieurs prévus.nm 2
The turbulence created and maintained by flash evaporation promotes the entrainment, total or at least partial, of fine solid particles, mixed with organic and aqueous products which are carbonated. There is thus obtained, at the surface of the strip 14, a mist which acts as a thermal screen between the radiating elements 24 and the metal constituting the strip, whose emissivity is low. The flux emitted by the radiating elements 24 is partly absorbed by carbonated products and during gasification, which increases the temperature, and, when the energy level is sufficient, the oxygen in the air, entrained in the boundary layer, reacts and allows the oxidation of the compounds carbonated organic, which can result in the following general reactions CH + ~~~~
nm 2 2
2

nC02 + m H20 + H1
Liquid h2o

H2 O steam
2
These gasification and evaporation reactions take place while the temperature of the strip is still lower than 350 / 4000C, which makes it possible to avoid the surface oxidation of the metal by CO2 or H2O, or 2 'present in the reaction medium. , the kinetics of these reactions being very slow below these temperatures (350/400 "C), and the residence time of the metal in this medium being reduced,
In the second stage
Band 14, which circulates against the current of atmospheric gases (N, H
2 2 gradually tends towards temperatures of the order of 600 to 800 "C, depending on the subsequent treatments planned.

Soot particles may exist on the surface of the strip at the end of the flash evaporation phase, which can be accompanied by partial charring of heavy residues. The residual carbon, which constitutes the soot particles, reacts with the hydrogen in the atmospheric gases and with the H2 O molecules, resulting from the reduction of "polluting" oxygen from the nitrogen in the atmospheric gas. , on the one hand, and the reduction of metal oxides, on the other hand, to give gaseous products according to the following reactions

 The ratio of partial pressures H2 O - H2 promotes reactions in the direction "2".

 Where appropriate, in particular when there is a large proportion of grease on the surface of the strip 14, it is necessary to make up H2O in the form of vapor in the atmosphere gas, as has been specified above. -above. This top-up is carried out in the second part of the cleaning zone, in order to favor the oxidation of carbonaceous residues. However, this make-up must be limited enough not to cause oxidation of the surface of the cleaned metal. According to the invention, the H2O / H2 ratio is preferably maintained less than 0.1.

 It can be seen that, in the method according to the present invention, the adjustment of the power delivered to the terminals of the different radiating elements 24 makes it possible, on the one hand, to carry out a significant transfer at the very start of cleaning, in order to favor the phenomena flash evaporation, and, on the other hand, in the second phase of the process, to gradually and regularly raise the temperature of the metal of the strip, in order, / to obtain a progressive oxidation of the carbonaceous residues and a reduction, to less partial, metal oxides.

This gradual and regular increase in the temperature of the strip 14 is clear from the examination of FIG. 3.

The process according to the invention makes it possible, compared with conventional thermal processes which use fumes, to significantly improve the thermal efficiency of the cleaning operation: the volume of atmospheric gas
3 used, which is indeed often less than 5 m per tonne of strip treated, is a "fatal" volume, since it comes from the annealing furnace and necessary for the operation of this furnace, unlike the fumes of the usual treatments , which are added to this "fatal" volume. The heat losses to the fumes are all the more reduced as part of the latent heat of evaporation of the residues is compensated by their own oxidation, in the first zone, by reaction with the only external atmosphere, entrained in boundary layer by the tape itself.

 The method according to the invention also has the advantage of allowing continuous treatment of metal strips, without the need to use a hydro-carbon, fossil or waste fuel.

 Finally, the absence of fumes, which always contain significant proportions of oxidizing C02 and H2O, at the target temperatures ((around 600 to 850 G), eliminates the possible risks of surface oxidation in conventional preheats, both during normal operation and in the event of accidental or voluntary shutdown of the installation.

 It remains to be understood that this invention is not limited to the examples of implementation or embodiment described here, but that it encompasses all variants thereof.

Claims (10)

 1 - A method of continuously cleaning a metal strip, characterized in that said metal strip penetrates continuously, at room temperature, into a first zone, where it is subjected to intensive radiation from heating elements, having a temperature of l 'around 850 to 1100 ° C, thus causing instant evaporation; said flash evaporation, and a gasification of the residues present on the surface of the strip, in that the products of this gasification are oxidized by ambient air entrained in a boundary layer by the strip itself, and in that, in a second zone, said strip circulates against the current of an atmospheric gas, so as to obtain a progressive oxidation of the carbonaceous residues resulting from flash evaporation, and a reduction, at least partial, of the metal oxides present on the strip surface.  2 - Process according to claim 1, characterized in that the turbulence of the gases is reduced, at the start of the cleaning zone, to promote the introduction of a maximum limit layer of air on the two faces of the metal strip .  3 - Method according to one of claims 1 or 2, characterized in that the scanning speed of the strip by the atmospheric gas is increased, on the one hand, by reducing the space between the radiating element and the strip, and, on the other hand, by introducing an additional volume of atmospheric gas, preferably containing less than 15% of hydrogen, into the cleaning zone.  4 - Method according to claim 3, characterized in that said additional volume of atmospheric gas can be preheated to eliminate, at least partially, and upon entering the cleaning zone, the traces of oxygen present in the nitrogen atmospheric gas.  5 - Method according to any one of the preceding claims, characterized in that the oxidation of carbonaceous residues is obtained by the action of water vapor present in the atmosphere gas.  6 - Process according to any one of claims 1 to 5, characterized in that said water vapor is obtained by reduction, at least partial, of the metal oxides present on the surface of the strip.  7 - Method according to any one of claims 1 to 5, characterized in that said water vapor is obtained by reduction of the oxygen present in trace amounts in the nitrogen of the atmosphere gas.  8 - Method according to any one of claims 5 to 7, characterized in that said water vapor can be added as an addition in the atmosphere gas sphere, at a level which prevents oxidation of the strip by this water vapor.  9 - Process according to claim 8, characterized in that said top-up of water vapor is carried out in the second part of the cleaning zone, and in that the H O / H ratio is less than 0.1.  2 2  10 - Device for the implementation of a method according to any one of claims 1 to 9, characterized in that it comprises: a thermally insulated enclosure, the walls located opposite the strip (14) are provided electric resistors (24), the temperature of which is regulated zone by zone, which communicates, on the one hand with the outside by an entry airlock (10), and, on the other hand, with an annealing zone ( 26), via a passage of reduced section (28), to ensure better control of the circulation of atmospheric gases (N, H), from the annealing zone (26) to the zone eva  2 2 flash poration (16); a flash evaporation zone (16), provided in said enclosure and communicating with said airlock by an orifice (18) whose passage section is adjustable, and pipes for introducing make-up atmosphere gas and steam of water, possibly preheated.