EP0766751B1 - Method for pickling metal materials - Google Patents

Abstract

PCT No. PCT/FR95/00744 Sec. 371 Date Jan. 7, 1997 Sec. 102(e) Date Jan. 7, 1997 PCT Filed Jun. 7, 1995 PCT Pub. No. WO95/34695 PCT Pub. Date Dec. 21, 1995Method in which the pickling solution is sprayed onto the metal part in a confined atmosphere with an oxygen supply by feeding into the spraying enclosure an oxygen-containing gas, the pickling solution being recycled in a closed pickling solution circuit. The device for pickling metal surfaces comprises a spraying enclosure (1) with a collecting tank (2), means for spraying (6) a pickling solution (S) in said enclosure, means (11) for recirculating said solution between said tank (2) and said spraying means (6) and means (8,9) for passing an oxygen-containing gas through said enclosure, and optionally means (13) for maintaining the temperature of said solution, and control means. The method of the invention is designed to improve the productivity of lines for pickling stainless or alloy steel components while economising on reagents.

Description

The present invention relates to a pickling process acid from metallic materials, especially alloy steel, steel stainless steel or titanium alloys.

According to a known method, in particular from FR-A-2,587,369, pickling of steel materials is done in a bath consisting of aqueous solution containing ferric ions and hydrofluoric acid. This bath has the advantage of not containing nitric acid and therefore of do not generate toxic compounds derived from nitric acid during pickling.

In substitution for hydrofluoric acid, which can be dangerous to handle, it has been proposed in FR-A-2 657 888 to use non-oxidizing organic acids of iron, in particular formic acid or acetic acid.

Other mineral acids such as sulfuric acid or phosphoric acid can also enter into the composition of such pickling baths.

It is known to pickle surfaces from metal parts by immersing the parts in baths of the type cited above. During pickling, the baths can be regenerated by introducing an oxidizing agent so as to increase their lifespan. For this can be done by bubbling air or an oxygen-containing gas in the bath, or introduce an oxidant stronger than gaseous oxygen such as ozone, hydrogen peroxide or a compound such as peroxide, peracid or parsley.

But, in particular in the case of stripping of strips in continuous, of metallic wires or parts, the pickling kinetics can be too slow and require too long and costly downtime of facilities.

In order to significantly increase the pickling kinetics, it is known to apply the pickling solution by spraying on the surface of said metal part: the surface is thus constantly renewed solution in contact with the surface to be stripped while eliminating very quickly pickling residue.

According to a conventional process, the pickling solution is sprayed on the part to be stripped, the solution is recovered after contact with the part and it is recycled in a closed circuit of solution of pickling. The solution is usually regenerated by performing a bubbling of air or an oxygen-containing gas at a point in the recirculation of the pickling solution.

In industrial pickling processes, these operations take place in a closed spray enclosure where the parts to be stripped. Under these conditions of etching in atmosphere confined, it was found that, if the pickling speed of said solution sprayed was much higher than that of pickling by immersion at first, this improvement disappeared after a few hours and could only be maintained for the life of said solution, despite the regeneration of the solution by air bubbling or oxygen.

The effectiveness of the process quickly becomes comparable again, or even less than that of an immersion process.

The object of the invention is to provide a pickling process acid of metal parts by spraying, in which the speed of pickling is significantly improved throughout the duration of using the same solution.

In order to maintain the pickling speed of said solution sprayed at a high level, it is possible to add an oxidant powerful, especially hydrogen peroxide, in the pickling solution.

However, such oxidizing agents, especially water oxygenated have the disadvantage of being expensive.

The invention also aims to improve in a way particularly economical the etching speed of the processes of the aforementioned type.

To this end, the invention relates to a process for pickling metallic materials, by a pickling solution S containing ferric ions in an acid medium comprising at least one acid chosen from a halogenated acid, a mineral acid and non-oxidizing organic acids from iron in which said solution is sprayed on the material, recovered and recycled in a closed circuit pickling solution and said spraying is carried out in an atmosphere confined in a spray enclosure with an oxygen supply by introduction into the spraying enclosure of a gas G containing oxygen.

Among the metallic materials likely to be pickled according to the claimed process more particularly, metallic materials can be cited alloy steel, stainless steel or titanium alloys.

The pickling method of the invention can also be applied to alloys based on zirconium, cobalt or nickel, or on non-alloy steels.

As regards the acid used according to the invention, preferably as halogenated acid, hydrofluoric acid, as mineral acid, sulfuric or phosphoric acid and as non-oxidizing organic acids in iron, formic acid or acetic acid.

The materials to be stripped can be in the form of strips, any wires, tubes, plates or metal parts.

The introduction into the confined atmosphere of a gas containing the oxygen makes it possible to maintain a speed of high pickling. The oxygen contained in said gas can be introduced, even partially, in the form of ozone.

This oxygen supply can advantageously be carried out continuously in maintaining the circulation in the spraying enclosure of a gas G containing oxygen, and very advantageously by uniformly distributing the continuous circulation of said gas in the enclosure for ensure a homogeneous composition of the atmosphere in the enclosure.

Preferably, the oxygen content of said gas G containing oxygen is more than 10%. The gas may for example be air.

Advantageously, the flow rate of said gas G containing the oxygen is at least equal to the spray rate of said solution S.

Preferably, the flow rate of said gas G makes it possible to renew the atmosphere in the spray enclosure on average at least all fifteen minutes, and preferably every five minutes.

It is also beneficial that the oxygen supply maintains an oxygen content of the atmosphere in the spray enclosure at least equal to 10%.

Furthermore, preferably, the ratio between the flow of spraying of said solution S on the metal surface expressed in liters / minute and the surface to be stripped per unit of time expressed in meters squares per minute is at least five.

In general, it is preferable that the solution of pickling be sprayed as small droplets which provide a large exchange surface between the sprayed liquid and the atmosphere prevailing in the spray enclosure. The size of droplets depend on the flow rates of liquid and especially gas in said pregnant and is advantageously in the range corresponding to atomization processes.

Preferably, in the process of the invention, a part at less of the pickling solution is introduced into said enclosure by atomization.

The pickling process can be discontinuous, the spraying parts to be stripped taking place in a closed enclosure provided with means for introducing oxygen.

However, it is generally preferred to operate continuously in an enclosure allowing the circulation of the parts to be stripped but creating a confined atmosphere around said parts. This enclosure can example be a spray tunnel. The enclosure also has means for introducing oxygen.

In an advantageous embodiment of the invention, an etching solution S is used, which contains hydrofluoric acid and Fe ³⁺ ions, present in the form of fluorinated complexes, the concentration of ferric ions Fe ³⁺ being included between 1 and 150 grams per liter, and which has a pH between 0 and 3; it also contains Fe ²⁺ ions and its redox potential measured with respect to a saturated calomel reference electrode is between -200 mV and + 800 mV; such a pickling solution is described in particular in European patent application No. 0 188 975 filed by the applicant.

Surprisingly, without adding an oxidizing agent powerful in the pickling solution in use especially hydrogen peroxide even after many hours of using the same solution in closed circuit recirculation and spraying of said solution on parts to be stripped in said confined atmosphere, by example in a tunnel, renewed thanks to the introduction of a gas containing oxygen, the etching speed remains permanently much higher that the pickling speed that would be obtained by immersion of said parts in said solution and oxidative regeneration by bubbling oxygen in said solution.

During the pickling, we can follow the consumption and aging of the pickling solution by measuring the redox potential and the pH of the solution in the recirculation circuit.

Thus, the pickling process can be implemented by checking the redox potential of the solution. We can maintain the value of the potential to a predetermined value by modifying the flow rate of gas G introduced, by adding an oxidizing agent to solution S in the circuit recirculation or by changing the spray rate of solution S.

Checking the redox potential of the pickling solution is particularly advantageous in the context of the present invention because it allows to use the recycled pickling solution for a very long lasting with good stripping efficiency while spacing out equipment maintenance interruptions.

The present inventors have been able to determine that one can spray pickling with a solution having a sufficiently high redox potential, to ensure good efficiency of pickling, but low enough to limit precipitation and crystallizations of ferric compounds and thus limit the risks of clogging in the installation, in particular at the level of the spray.

In particular, the measurement of the redox potential of the solution pickling allows, by comparison with the initial value of the potential of the solution at the start of pickling, to assess the aging of said solution or detect any anomalies. So although one of the goals of the invention either to avoid resorting to strong oxidants but expensive, depending on the value of the redox potential, you may need a strong oxidation means temporarily and / or locally completing the action oxygen carrier gas introduced into the spray enclosure to quickly return to an advantageous redox potential allowing a good pickling.

In addition, the method according to the invention can be used for pickling with solutions containing ferric ions in acidic media containing acids other than hydrofluoric acid, in particular organic acids, used in a closed circuit and therefore requiring regeneration by an oxidizing agent, for example by a gas containing oxygen or hydrogen peroxide, regeneration controlled by measuring the redox potential of the solution.

We have seen that, in the case where the redox potential by adding a powerful additional oxidant, in particular hydrogen peroxide, to the pickling solution, the process of the invention remains economical by comparison with an immersion process where performs the same redox potential regulation. Indeed, for equal efficiency, a smaller amount of makeup oxidizer is used in the process spray of the invention.

Advantageously, the pickling solution is maintained within a predetermined temperature range, preferably at a constant temperature within operating variations, in the circuit closed solution recirculation.

Advantageously, an operation of heating and / or heating at any point of the recirculation circuit, using any suitable technique. Preferably, we circulate in the solution a stream of hot gas containing oxygen, advantageously a current of hot air, so as to achieve at the same time reheating and regeneration of the pickling solution. In the circuit re-circulation, reheating preferably takes place just before spraying the solution.

The method according to the invention applies to pickling in continuous, in particular of metal strips, and discontinuously, in particular of crowns of metal wire or metal parts.

• une enceinte de pulvérisation munie d'une cuve réceptrice,
• des moyens de pulvérisation d'une solution de décapage dans ladite enceinte,
• des moyens de recirculation de ladite solution entre la cuve réceptrice et les moyens de pulvérisation,
• et des moyens de ventilation de ladite enceinte par un gaz contenant de l'oxygène.

Le dispositif selon l'invention peut comporter une ou plusieurs des caractéristiques suivantes :

• lesdits moyens de ventilation délivrent ledit gaz contenant de l'oxygène à un débit minimum représentant le volume de ladite enceinte au moins toutes les quinze minutes, et de préférence toutes les cinq minutes ;
• lesdits moyens de pulvérisation de ladite solution de décapage sont du type à atomisation pneumatique ;
• lesdits moyens de pulvérisation de ladite solution de décapage comprennent des moyens du type à aspersion classiques et du type à atomisation pneumatique.

The invention also relates to a device for pickling by spraying surfaces of metal parts which implements a method as described above. This device includes:

• a spray enclosure provided with a receiving tank,
• means for spraying a pickling solution into said enclosure,
• means for recirculating said solution between the receiving tank and the spraying means,
• and means for ventilating said enclosure with a gas containing oxygen.

The device according to the invention may include one or more of the following characteristics:

• said ventilation means deliver said oxygen-containing gas at a minimum flow rate representing the volume of said enclosure at least every fifteen minutes, and preferably every five minutes;
• said means for spraying said pickling solution are of the pneumatic atomization type;
• said means for spraying said pickling solution comprise means of the conventional spray type and of the pneumatic spray type.

The device according to the invention may further comprise means for maintaining the temperature of said solution, capable of ensuring a reheating or cooling.

Most often, the pickling processes employ solutions above room temperature that have tendency to cool down during use. The means of maintaining temperature are therefore preferably means for reheating the pickling solution.

Reheating means may include a tray conventional conduction heater or means of insufflation in the solution of a hot gas containing oxygen, in particular air hot, these two types of means possibly being associated. They are preferably arranged just upstream of the spraying means.

Advantageously, the device can further comprise liquid / solid separation means, in particular by decantation, for rid the pickling solution circulating in closed circuit of solid particles from pickling which are entrained with the solution in the receiving tank. In this embodiment, said means of separation can be provided with means for blowing hot gas containing oxygen in the area where the liquid resides.

The device can advantageously include control means comprising at least one measurement probe, in particular the temperature, pH or redox potential of the solution, located at any point on the solution recirculation circuit pickling.

These control means may include in particular a redox potential measurement probe for the solution of pickling.

In the latter case, the device may further comprise means for injecting an oxidizing agent, in particular hydrogen peroxide, in the recirculation circuit of said solution, slaved to said probe potential measurement.

Furthermore, said ventilation means can be equipped with a gas flow control valve controlled by said gas probe potential measurement.

Likewise, said spraying means can be equipped with a solution flow control valve controlled by said probe potential measurement.

• la Figure 1 est une vue schématique d'un dispositif de décapage en continu de bandes métalliques par pulvérisation selon l'invention,
• la Figure 2 est une vue en coupe du dispositif de la Figure 1 représentant deux rampes de pulvérisation du dispositif de décapage selon l'invention,
• la Figure 3 est une vue de côté d'un dispositif de pulvérisation de couronnes de fil métallique, faisant partie d'un dispositif de décapage selon l'invention.

Examples of embodiments of the invention will now be described with reference to the appended drawings, in which:

• FIG. 1 is a schematic view of a device for continuously stripping metal strips by spraying according to the invention,
• FIG. 2 is a sectional view of the device of FIG. 1 showing two spraying booms of the pickling device according to the invention,
• Figure 3 is a side view of a device for spraying wire rings, forming part of a pickling device according to the invention.

The device shown in Figure 1 includes a spray enclosure, means for spraying a solution of pickling in said enclosure, means of ventilation of said enclosure, control means, recirculation means, processing means and means for maintaining said temperature solution.

Said spray enclosure comprises a tunnel 1 provided a receiving tank 2 and a cover 3, an airlock 4A and a 4B exit airlock for the strip to be stripped and transfer means said strip composed of rollers 5 which define a strip path or stripping plan P.

Conventionally, the spraying means include spray bars 6 parallel to the rollers 5, regularly spaced and arranged above and below said plane pickling P, as shown in detail in Figure 2; ramps 6 are rigid tubular conduits which open into a multitude of spray nozzles 7 arranged regularly along the boom 6.

Said spray nozzles 7 are of a type known from itself intended to produce a flat spray.

The distance between the orifices of the nozzles 7 from the plane of pickling P along the direction of the jet is between 5 cm and 70 cm, and preferably equal to approximately 30 cm.

The nozzles 7 of the same ramp 6 are oriented to create flat jets that span the width of the strip path and the distance between the nozzles 7 is less than the width of the jet at the level of the stripping plane P so that the flat jets of each nozzle of a same ramp partially overlap over the entire width of the path strip, as shown in Figure 2.

In general, the means of ventilation include a supply circuit and a gas exhaust circuit the enclosure. These circuits are preferably adapted in a known manner in itself to ensure a homogeneous composition of the atmosphere of the enclosure.

In the device of FIG. 1, the ventilation means of the tunnel include a fan 8 connected to one end of the tunnel by a gas supply pipe and a gas evacuation chimney 9 located at the other end of the tunnel.

The intake of fan 8 is connected to a mouth air intake not shown and the fan is fitted with a flow control not shown; the chimney 9 is connected to a device gas treatment not shown which allows the recovery of a part of the mist or droplets discharged into the gas stream.

Said recirculation means comprise a tank buffer 10 connected to the receiving tank 2 and a pump 11 for reinjecting under pressure the solution from the buffer tank 10 to each ramps 6.

The pump 11 has characteristics which make it possible to supply said ramps at a pressure of between 0.5 × 10 ⁵ and 7.10 ⁵ Pa, and preferably equal to 3.10 ⁵ Pa; the pump 11 is provided with a flow control valve not shown.

The buffer tank 10 is provided with a feed orifice, in particular to supplement the quantity of pickling solution S.

Said control means consist of a redox potential measurement located at the buffer tank 10, not represented.

Without departing from the invention, said probe for measuring redox potential can be installed at other points in the recirculation of the pickling solution.

Said processing means comprise a decanter 12.

Decanter 12 is installed on a recirculation circuit secondary connected to the buffer tank 10 whose flow rate is regulated by a 12 'pump; the decanter 12, known in itself, is not described here in detail and has a solid residue discharge R.

Said temperature maintenance means comprise a heater 13 which is installed on the pipe connecting the pump 11 to ramps 6; the heater 13, known in itself, is not described here in detail.

We will now describe the implementation of the pickling according to the invention using the device described above.

The method according to the invention is used here for pickling a stainless steel strip B of width Lb.

The buffer tank 10 is filled with a pickling solution S containing ferric ions in hydrofluoric acid medium.

The spraying and recirculation means are started, treatment and heating of the pickling solution as well as the ventilation means of the spray tunnel 1.

Band B is continuously passed through the tunnel spraying 1 on the rollers 5 at a running speed Vb, successively by the entry airlock 4A then the exit airlock 4B.

Using the adjustment valve of the pump 11, it is possible to regulate the rate of spraying of pickling solution so that, expressed in liters / minutes, it is at least equal to five times the area to be pickled, here (Vb x Lb) expressed in m ² / minutes.

The total volume of the spray tunnel being V in m ³ , using the fan flow control valve 8, the air flow pushed by said fan 8 into said tunnel is adjusted to V / 15 m ³ / minutes, so that the atmosphere of said tunnel is renewed on average at least every fifteen minutes, and preferably every five minutes.

When passing through spray tunnel 1, and especially between the spray bars 6, the strip B is sprayed with homogeneously on both sides by the pickling solution S sprayed by the nozzles 7 of the booms 6.

The spent solution comprising solid residues of pickling in suspension pours by gravity into the receiving tank 2 then in the buffer tank 10.

The redox potential measurement probe allows to measure the redox potential of solution S in buffer tank 10.

The solid residues of solution S are separated by the decanter 12 at the time of the passage of said solution in the circuit of secondary recirculation connected to the buffer tank 10.

The solution S freed of solid residues in suspension towards the booms 6 and the spray nozzles 7, by reheating it with the heater 13.

The heater 13 advantageously maintains the temperature of the solution between 15 and 80 ° C, and preferably between 40 and 70 ° C.

As band B continues to run continuously in spray tunnel 1, solution S is continuously sprayed on the said strip in an atmosphere constantly renewed in oxygen and recirculates in a closed circuit in the pickling device.

In order to compensate for the consumption of pickling S and maintain the pH within a predetermined range of values, it is possible to add acid or new solution directly into the buffer tank through its supply port or by other suitable points in the installation.

Without departing from the process according to the invention, it is possible to maintain the value of the redox potential measured by the probe at a value predetermined constant by modifying the air flow rate of fan 8 or by adding in the buffer tank 10 an additional oxidant more powerful than air, for example a peroxide, parsley or ozone, or by changing the spray rate.

According to a variant of the device of the invention, the nozzles of spray 7 are replaced by pneumatic atomizing nozzles as previously supplied with liquid by the pump 11 and supplied with pressurized gas by an air or gas compressor containing oxygen.

Pneumatic atomization nozzles are known by themselves and are not described here; pump features 11 are adapted to the liquid supply specifications of said liquid nozzles.

Thus, for the implementation of the method, the solution S is advantageously sprayed on both sides of the strip pickling and the atmosphere of the pickling tunnel is advantageously renewed at the level of the spraying means themselves by atomization pneumatic itself.

According to another variant of the device of the invention, the spray tunnel is vertical; this arrangement can be advantageous for pickling metal wires or tubes.

According to another variant of the invention, the device is suitable for stripping crowns or coils of wire. he comprises a spray enclosure, provided with a support for the crown, means for spraying a pickling solution on the crown in said enclosure, control means, means of recirculation, treatment means and means for maintaining temperature of said solution.

Figure 3 shows a detail of such a device and illustrates spraying a crown of wire.

The device comprises a hook-shaped support 14 which supports on its lower part a crown of metallic wire C.

The spraying means comprise two booms of spray 6 arranged above and below the wire crown metallic C and fitted with spray nozzles 7 similar to the nozzles 7 shown in Figure 2, supplied with pickling solution S by a pipe connected to the recirculation circuit of said solution.

The ramps 6 are arranged horizontally in the enclosure one above the other, the nozzles 7 of the upper rail opposite the nozzles 7 of the lower ramp. They are enough spaced so that the coil C carried by the support 14 can be inserted in the intermediate space.

The power and orientation of the jets produced by the nozzles 7 are such that the entire crown is brought into contact with the pickling solution.

This spraying device can be integrated into a wire stripping device discontinuously, the support 14 being fixed relative to the closed and ventilated stripping chamber. he can also be integrated into a continuous pickling device wire crowns which includes several pairs of ramps 6 defining a horizontal space in which circulate mobile supports 14 carrying crowns C and into which the pickling solution S is sprayed on the succession of crowns C.

The following examples illustrate the invention.

Comparative example 1

Pickling of a steel plate is carried out by immersion in a pickling bath containing ferric ions and acid hydrofluoric.

We take a rectangular plate of 20 mm x 50 mm AISI 430 grade steel.

About 1 liter of an aqueous pickling solution is prepared containing 30 grams per liter of hydrofluoric acid (expressed as HF), 30 grams per liter of total iron in solution with potential 250 mV redox compared to a reference electrode at saturated calomel, measured at 60 ° C.

This solution is placed in a pickling tank provided a redox potential measurement probe and air bubbling means in the bottom of the tank to stir the solution in the tank.

Said plate is etched by immersion in the tank and bubbled air there at a rate of between 10 and 20 l / minute. During the pickling, the bath temperature is maintained between 50 and 60 ° C and the pH is maintained at its initial value by addition of hydrofluoric acid.

We measure the initial change in weight of the steel plate every thirty seconds for two minutes and then the average change after four hours and the evolution of the redox potential of the solution over time over four hours, indicative of aging of the solution.

• Variation initiale de poids : 30 g/m²/min.
• Variation moyenne en 4 h : 3,5 g/m²/min.
• Evolution du potentiel redox: baisse supérieure à 100 mV en 4 heures.

Si l'on désirait augmenter encore la durée de vie d'une telle solution, il serait nécessaire de compléter le bullage d'air par l'addition de faibles quantités d'agents oxydants, par exemple d'eau oxygénée.The following results are obtained:

• Initial weight variation: 30 g / m ² / min.
• Average change in 4 hours: 3.5 g / m ² / min.
• Evolution of the redox potential: drop greater than 100 mV in 4 hours.

If it was desired to further increase the life of such a solution, it would be necessary to supplement the bubbling of air with the addition of small amounts of oxidizing agents, for example hydrogen peroxide.

Example 1

The same rectangular plate 20 is stripped mm x 50 mm of AISI 430 grade steel, using a spray.

About 2l of the same pickling solution are prepared than in Comparative Example 1.

The spray device models a tunnel spray and includes an enclosure with a cover and a tank filled with pickling solution S and equipped with a measurement probe redox potential.

In this enclosure is installed a spray nozzle supplied with spray solution by a pump including the inlet pipe plunges into said tank.

The stripping plate is placed horizontally above from the tank, approximately 30 cm below the spray nozzle.

A fan ventilates the spray enclosure continuously.

The pickling solution S is sprayed homogeneously over the entire surface of the plate; the used solution is collected by the tank under the plate; the pump extracts the collected solution and returns it to the spray nozzle. The temperature of the solution is maintained between 50 and 60 ° C and the pH is maintained at its initial value by addition of acid hydrofluoric.

We measure, as for Comparative Example 1, the variation initial weight of the steel plate every thirty seconds for two minutes, then the average weight change after four hours and the evolution of the redox potential of the solution as a function of time over four hours.

• Variation initiale de poids : 50 g/m²/min.
• Variation moyenne en 4 h : 8,7 g/m²/min.
• Evolution du potentiel redox : baisse inférieure à 80 mV en 4 heures.

Par rapport à l'exemple comparatif 1, la solution S présente une durée de vie beaucoup plus elevee sans rajouter d'agent oxydant plus oxydant que l'air, comme de l'eau oxygénée.The following results are obtained:

• Initial weight variation: 50 g / m ² / min.
• Average change in 4 hours: 8.7 g / m ² / min.
• Evolution of the redox potential: drop less than 80 mV in 4 hours.

Compared to Comparative Example 1, solution S has a much longer lifespan without adding an oxidizing agent which is more oxidizing than air, such as hydrogen peroxide.

In addition, thanks to this spraying process, on the one hand an improvement in the pickling speed is obtained in the initial duration of use of the pickling solution (here: 2 minutes), and on the other hand this improvement in pickling speed remains significant even at after 4 hours.

Compared to the stripping by immersion of the example Comparative 1, the overall pickling kinetics is approximately multiplied by a factor of 2.5.

Comparative example 2

The conditions of this test are identical to those of Comparative Example 1, except that the plate is stripped for 3 hours while maintaining constant and at its initial value the value of the redox potential of the pickling solution; for this purpose, a solution of hydrogen peroxide at 30% by weight of H ₂ O ₂ is regularly added to the pickling solution.

After 3 hours, the total volume V _o of hydrogen peroxide added to the pickling solution is measured to keep its redox potential constant; the variation in weight of the steel plate is also measured.

The average change in weight over 3 hours is 10 g / m ² / min.

Example 2

The conditions of this test are identical to those of Comparative Example 2, except that the plate is stripped for 3 hours while maintaining constant and at its initial value, the value of the redox potential of the pickling solution; for this purpose, a solution of hydrogen peroxide at 30% by weight of H ₂ O ₂ is regularly added to the pickling solution.

After 3 hours, the total volume V of water is measured oxygenated added in the pickling solution to keep its constant redox potential; we also measure the variation in weight of the plate of steel.

The average change in weight over 3 hours is 14 g / m ² / min.

To assess the difference in efficiency of the methods of Comparative Example 2 and of Example 2, the added volumes V _o , V of hydrogen peroxide are compared to the quantities of steel pickled, to calculate the gain of hydrogen peroxide. , which here stands at 54%.

So, in the case where an oxidant such as water is used oxygenated, the oxygenated jet process of the invention remains economical since it makes it possible to halve the quantity of this expensive reagent for achieve equal efficiency.

Comparative example 3

• Variation initiale de poids : 27 g/m²/min ;
• Variation moyenne en 4 h : 0,5 g/m²/min.

In this example, a 20 mm x 50 mm rectangular plate of AISI 304 grade steel is etched by immersion under conditions identical to those of Comparative Example 1. The following results are obtained:

• Initial weight variation: 27 g / m ² / min;
• Average change in 4 hours: 0.5 g / m ² / min.

Example 3

• Variation initiale de poids : 33 g/m²/min ;
• Variation moyenne en 4 h : 1 g/m²/min.

Par rapport au décapage par immersion de l'exemple comparatif 3, la cinétique moyenne de décapage sur plusieurs heures est multipliée par 2 avec le procédé de l'exemple 3.In this example, the same plate is etched by spraying under conditions identical to those of Example 1. The following results are obtained:

• Initial weight variation: 33 g / m ² / min;
• Average change in 4 hours: 1 g / m ² / min.

Compared with the pickling by immersion of Comparative Example 3, the average pickling kinetics over several hours is multiplied by 2 with the method of Example 3.

Example 4

The pickling solution used in the examples can be used for continuous pickling, in a tunnel spraying of a steel strip of AISI 304 grade.

The spray tunnel has a total volume of 50 m ³ . It is equipped at one end with a fan connected to an air intake vent blowing air into the tunnel at a rate of 3.5 m ³ / minute.

The steel strip, 1.3 m wide, runs in the tunnel at a speed of 25 m / min.

Common type spray bars are equipped with spray nozzles spaced 30 cm apart.

The pickling solution pumped into the receiving tank of the tunnel supplies the spraying booms under a pressure of 3.10 ⁵ Pa for a flow rate of 2 m ³ / min.

A heating device helps maintain the pickling solution temperature between 50 and 60 ° C.

This pickling device was able to operate for periods of several weeks while retaining a satisfactory redox potential with good pickling quality.

Example 5

An aqueous pickling solution containing 60 g / l hydrofluoric acid (expressed as HF), 70 g / l total iron in solution and having an oxidation-reduction potential of 220 mV compared to a saturated calomel reference electrode, measured at 75 ° C.

This solution is used to continuously strip cold-rolled AISI 304 stainless steel strips in a spray tunnel with a volume of approximately 31 m ³ .

This tunnel has three outlets connected to an air suction device and the suction air flow is 160 m ³ / min; the three mouths are arranged regularly along one of the side walls of the enclosure, parallel to the strip running path.

The tunnel has 32 spray bars arranged about 25 cm below and above the scroll strip and distributed regularly along the tunnel; each ramp has spray nozzles uniformly spaced 35 cm apart.

The ramps are supplied with pickling solution under a pressure of 3.10 ⁵ Pa (3 bar) and deliver a total of 2 m ³ / min of solution.

A heating device keeps the solution pickling temperature of around 75 ° C. We maintain the redox potential of 220 mV solution by regular addition of hydrogen peroxide. The steel strip, with a width of 1.24 m runs through the tunnel at a speed of 48 m / min.

We compared the results obtained with this installation to those of an immersion pickling installation of the same length with the same solution kept at the same temperature and at the same redox potential thanks to oxygenated water and applied to the same strips of steel moving at a lower speed of 42 m / min.

It can be seen that, thanks to the invention, 15% is consumed less hydrogen peroxide for a higher scrolling speed of 15 %.

So we gained 15% in pickling efficiency and saved 30% of the amount of hydrogen peroxide useful for pickling a given length of sheet.

Claims (27)

1. Process for pickling metal materials by means of a pickling solution containing ferric ions in an acid medium comprising at least one acid selected from a halogenated acid, a mineral acid and organic acids that do not oxidise iron, wherein the solution is sprayed onto the material, recovered and recycled in a closed pickling solution circuit and the spraying is carried out in a confined atmosphere in a spraying enclosure with a supply of oxygen by introduction of an oxygen-containing gas into the enclosure.
2. Process according to claim 1, wherein the circulation of an oxygen-containing gas is maintained in the spraying enclosure.
3. Process according to claim 1 or 2, wherein the oxygen content of the oxygen-containing gas is greater than 10%.
4. Process according to any one of claims 1 to 3, wherein the flow rate of the oxygen-containing gas is at least equal to the solution-spraying rate.
5. Process according to any one of claims 1 to 4, wherein the circulation of the oxygen-containing gas enables the atmosphere in the spraying enclosure to be renewed on average at least every 15 minutes, and preferably every five minutes.
6. Process according to any one of claims 1 to 5, wherein the oxygen content of the atmosphere is maintained at a level higher than 10%.
7. Process according to any one of claims 1 to 6, wherein the ratio of the solution-spraying rate expressed in litres per minute to the surface area to be pickled per unit of time expressed in square metres per minute is at least five.
8. Process according to any one of claims 1 to 7, wherein at least some of the pickling solution is introduced into the enclosure by atomisation.
9. Process according to any one of claims 1 to 8, wherein the redox potential of the pickling solution is maintained at a predetermined value.
10. Process according to claim 9, wherein the redox potential is adjusted to the predetermined value by modifying the rate of introduction of the oxygen-containing gas.
11. Process according to claim 9 or 10, wherein the redox potential is adjusted to the predetermined value by adding an oxidising agent to the pickling solution.
12. Process according to any one of claims 9 to 11, wherein the redox potential is adjusted to the predetermined value by modifying the solution-spraying rate.
13. Process according to any one of claims 1 to 12, wherein a current of hot oxygen-containing air is blown into the pickling solution.
14. Process according to any one of the preceding claims, characterised in that the metal materials to be pickled are of alloyed steel, stainless steel or titanium alloys.
15. Process according to any one of the preceding claims, characterised in that the halogenated acid is hydrofluoric acid, the mineral acid is sulphuric acid or phosphoric acid, and the organic acids that do not oxidise iron are formic acid or acetic acid.
16. Device for pickling by spraying metal surfaces, which comprises a spraying enclosure (1) provided with a tank (2) which receives means (6) for spraying a pickling solution S in the enclosure, means (11) for recirculating the solution between the tank (2) and the spraying means (6), and means (8), (9) for ventilating the enclosure by means of an oxygen-containing gas G.
17. Device according to claim 16, characterised in that the ventilation means (8), (9) supply the oxygen-containing gas at a minimum rate representing the volume of the tunnel at least every fifteen minutes, and preferably every five minutes.
18. Device according to claim 16 or 17, wherein the means (6) for spraying the pickling solution are of the pneumatic atomisation type.
19. Device according to claim 16 or 17, wherein the means (6) for spraying the pickling solution comprise conventional means of the sprinkling type and means of the pneumatic atomisation type.
20. Device according to any one of claims 16 to 19, which also comprises means (13) for maintaining the temperature of the solution.
21. Device according to claim 20, wherein the means for maintaining the temperature comprise means for blowing a hot oxygen-containing gas into the solution.
22. Device according to any one of claims 16 to 21, which also comprises means for liquid/solid separation optionally provided with means for blowing a hot oxygen-containing gas into the region where the liquid is located.
23. Device according to any one of claims 16 to 22, which also comprises monitoring means comprising at least one measuring probe, especially for measuring the temperature, the pH or the oxido-reduction potential of the solution, which means are installed at any point in the solution-recirculation circuit.
24. Device according to claim 23, wherein the monitoring means comprise a probe for measuring the oxide-reduction potential of the pickling solution.
25. Device according to claim 24, which also comprises means for injecting an oxidising agent, especially oxygenated water, into the solution-recirculation circuit, which means are controlled by the potential-measuring probe.
26. Device according to either claim 24 or claim 25, wherein the ventilation means are provided with a valve for regulating the flow of gas G, which valve is controlled by the potential-measuring probe.
27. Device according to any one of claims 24 to 26, wherein the spraying means are provided with a valve for regulating the rate of fluw of solution S, which valve is controlled by the potential-measuring probe.

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