Method for demetallising a reinforcing element for polymerised articles

Abstract

The invention relates to a method for demetallising coated steel fibres intended for reinforcing polymerised articles, comprising at least one electrochemical treatment step in which the steel fibres are immersed in a treatment bath, comprising a solution of an alkali salt of pyrophosphate at a concentration of between 10 and 1900 g / l, by being subjected to an anodic current, the mass current intensity ranging from 1 to 2500 A / kg of steel fibres, the residence time of the steel fibres in the bath ranging from 1 min to 90 min.

Description

[0001] PROCESS FOR DEMETALLIZING A REINFORCING ELEMENT FOR POLYMERIZED ARTICLES

[0002] Technical field of the invention

[0003] The present invention relates to the field of methods for treating metallic elements, in particular coated steel fibers intended for the reinforcement of polymerized articles, in particular rubber articles, and in particular to methods aimed at removing the coating present on these steel fibers in order to improve their recycling.

[0004] Prior art

[0005] Increasing pressure on natural resources and the desire to reduce the quantity of waste generated as much as possible are leading manufacturers to develop an increasing number of recycling solutions.

[0006] Steel recycling has been around for a long time. However, the proportion of recycled steel in a casting faces the problem of managing residuals, i.e., all elements other than iron. While some residuals such as carbon, silicon, or manganese can be adjusted or eliminated during steel production, other impurities, such as copper or tin, are more difficult to extract and can pose problems, particularly if the aim is to establish a so-called "circular" system in which used steel is recycled with a view to being used again for the same purpose. Indeed, any closed-loop process results in an accumulation of species that cannot be extracted, which can ultimately limit the quantity of recycled steel, particularly in the case of copper (Environ. Sci.Technol. 2017, 51, 6599-6606).

[0007] The steels used to manufacture metal wires for reinforcing polymerized articles, in particular rubber articles such as pneumatic or non-pneumatic tires, tracks and conveyor belts, have very low tolerances in terms of chemical composition, on the one hand to allow severe shaping by wire drawing, and on the other hand to have sufficient mechanical strength to ensure their reinforcing function. Thus, the residual contents present in the steel composition must be as low as possible.

[0008] Implementing a circular system for these metal wires therefore requires very careful management of the residual content. However, these metal wires are traditionally coated with zinc, brass, a copper and zinc alloy, or bronze, a copper and tin alloy, to improve the adhesion of the rubber to their surface. Their recycling for reuse in the manufacture of tire-grade steels therefore requires the development of solutions to effectively manage residual content.

[0009] To do this, different solutions have been developed to demetallize steel, i.e. to remove the surface coating from steel elements.

[0010] The document "Treatment of Coated Materials", M1458, Engineering Techniques, describes numerous demetallization solutions, by chemical or electrolytic means, notably based on alkaline solutions containing sodium cyanide.

[0011] Document DE2233157 describes the stripping of copper from copper-coated steel sheets by electrolytic treatment using an ammonia bath of ammonium phosphate or ammonium borate. However, copper alone does not behave like brass, which is an alloy. In addition, this method requires special management of the treatment products, particularly ammonia, which requires the implementation of special protective measures.

[0012] CN 103436899 describes the cleaning of the coating of steel cords for tires. To this end, the cords are treated in a strongly alkaline bath of ammonia and sodium nitrate. The volatility of ammonia requires frequent topping up and special protective measures.

[0013] Continuing its research, the applicant discovered that a demetallization process implemented under specific conditions made it possible to effectively remove the metallic coating from steel fibers intended for the reinforcement of polymerized articles, in particular rubber articles, whether these fibers were partially coated with rubber or not, thus opening the way for the recycling of these fibers into steelmaking processes with a view to their reuse for the manufacture of tire-grade steel.

[0014] Detailed description of the invention

[0015] The invention relates to a method for demetallizing coated steel fibers intended for reinforcing polymerized articles comprising at least one electrochemical treatment step in which the steel fibers are immersed in a treatment bath comprising a solution of an alkali pyrophosphate salt at a concentration of between 10 and 1900 g / l while being subjected to an anodic current, the mass current intensity ranging from 1 to 2500 A / kg of steel fibers, the residence time of the steel fibers in the bath ranging from 1 min to 90 min. Definitions

[0016] In the present invention, unless expressly indicated otherwise, all percentages (%) indicated are percentages (%) by mass.

[0017] On the other hand, any interval of values ​​designated by the expression "between a and b" represents the domain of values ​​going from more than a to less than b (i.e., excluding the limits a and b) while any interval of values ​​designated by the expression "from a to b" means the domain of values ​​going from a to b (i.e., including the strict limits a and b).

[0018] Demetallization means, as known to those skilled in the art, the removal of a layer of metallic coating.

[0019] The method according to the invention is a method for demetallizing steel fibers intended for reinforcing polymerized articles. By polymerized article is meant an article comprising a metallic reinforcing element and a polymer matrix, such as a resin or a rubber composition. By rubber article is meant any rubber article reinforced with steel fibers. These articles are in particular chosen from vehicle tires, whether these tires are pneumatic or non-pneumatic (i.e. supporting the weight of the vehicle by a means other than a pressurized gas, for example by means of stays), conveyor belts, belts, tracks, the term rubber being understood here as any polymer compound exhibiting elastic behavior.

[0020] Steel fibers for reinforcing polymerized articles are understood to mean new steel fibers intended to be incorporated into polymerized articles, or steel fibers extracted from polymerized articles, whether these are in the so-called "raw" or uncrosslinked state, or in the "cooked" state, i.e. crosslinked or vulcanized. The steel fibers are reinforcing elements made of coated steel and cut so as to be treated in the process according to the invention.

[0021] Preferably, the rubber articles are chosen from vehicle tires, tracks, conveyor belts, and very preferably chosen from vehicle tires.

[0022] Generally, a vehicle tire comprises a crown having two axial ends each extended, radially inwards, by a sidewall then by a bead intended to come into contact with a rim, the assembly delimiting an internal toric cavity. More precisely, the crown comprises, radially from the outside inwards, a tread, intended to come into contact with the ground via a rolling surface, a crown reinforcement and a portion of carcass reinforcement intended to ensure the reinforcement of the tire. The carcass reinforcement connects the two sidewalls together by extending in a radially inner portion of the crown and is anchored, in each bead, to a circumferential reinforcement element, most often of the bead wire type.

[0023] The polymerized articles, in particular rubber articles, are treated, in a manner known to those skilled in the art, by cutting, sorting and grinding. The ground materials containing the metal reinforcing elements are then granulated in granulators and cleaned so as to reduce their rubber content. At the end of this treatment, the steel fibers obtained preferably have an individual length of at most 10 cm. Preferably, the steel fibers have a diameter ranging from 0.1 mm to 2 mm.

[0024] The steel fibers treated in the process according to the invention preferably have a gum content ranging from 0 to 15% by weight of steel fibers, preferably ranging from 0 to 10% by weight of steel fibers and preferably ranging from 0 to 4% by weight of steel fibers.

[0025] The steel fibers treated in the process according to the invention are steel fibers coated with a metallic coating chosen from brass, zinc, bronze, the metallic coating content preferably ranging from 0.01 to 0.50% by weight relative to the mass of metal in the steel fibers. These fibers are preferably obtained from the crown reinforcement, the carcass reinforcement and the bead cores of vehicle tires when these elements are reinforced with metal wires.

[0026] Preferably, the coating of the steel fibers is brass, the copper content in the steel fibers ranging from 0.1 to 0.3% by weight and the zinc content in the steel fibers ranging from 0.05 to 0.20% by weight relative to the mass of metal of the steel fibers.

[0027] According to the invention, the steel fibers are immersed in a treatment bath comprising a solution of an alkali pyrophosphate salt at a concentration ranging from 10 to 1900 g / l, preferably ranging from 50 to 1000 g / l, more preferably ranging from 50 to 500 g / l and very preferably from 75 to 300 g / l, while being subjected to an anodic current. Above 1900 g / l, the alkali pyrophosphate salt risks precipitating, while below 10 g / l the concentration is too low to allow efficient conduction of the electric current. The residence time of the steel fibers in the bath ranges from 1 min to 90 min. Preferably, the residence time is adjusted so as to reduce the content of metal coating relative to the mass of metal of the steel fibers by at least 90%.Preferably, at the end of the demetallization process and when the metallic coating is brass, the overall mass content of copper and zinc in the steel fibers is less than 0.005% for copper and 0.005% for zinc relative to the mass of metal in the steel fibers.

[0028] Electrochemical treatment of steel fibers using a solution of an alkaline pyrophosphate salt makes it possible to demetallize the steel fibers even in the presence of a quantity of residual gum, with a reduced treatment time compared to a chemical treatment and excellent demetallization selectivity. Indeed, the chemical treatments known from the prior art make it possible to remove part of the metallic coating, but also extract part of the iron from the steel.

[0029] The process according to the invention is preferably operated at a temperature ranging from 10°C to 80°C, preferably ranging from 20°C to 60°C. These temperatures make it possible to operate with low risks of emissions due to evaporation, while maintaining excellent demetallization performance.

[0030] The mass current intensity ranges from 1 to 2500 A / kg of steel fibers. Preferably, the mass current intensity ranges from 1 to 500 A / kg of steel fibers, preferably from 1 to 300 A / kg of steel fibers, preferably from 2 to 300 A / kg of steel fibers, very preferably from 5 to 20 A / kg of steel fibers. These intensities, in relation to the other operating parameters, make it possible to obtain a good compromise between the demetallization of the steel fibers and the duration of the demetallization treatment.

[0031] The pH should preferably be maintained between 8 and 12, preferably between 10 and 12.

[0032] The electrolytic demetallization process according to the invention, based on alkali pyrophosphate salt, makes it possible to obtain very good demetallization of the steel fibers while limiting the extraction of iron from the steel. Preferably, the alkali pyrophosphate salt is chosen from potassium pyrophosphate and sodium pyrophosphate, and is preferably potassium pyrophosphate.

[0033] The invention also relates to the use of steel fibers resulting from the demetallization process according to the invention for the manufacture of reinforcing wires for reinforced polymeric articles. When the metallic coating of the steel fibers is brass, the steel fibers treated according to the process according to the invention have a considerably reduced copper and zinc content, thus making it possible to push back the recycling limits linked to the presence of copper in recycled steel, the management of which is delicate, which opens up the possibility of their recycling at much higher contents in order to produce new steels for the reinforcement of reinforced polymeric articles, in particular for the reinforcement of vehicle tires.

[0034] Examples

[0035] In the following examples, different types of steel fibers are processed. A first batch, hereinafter referred to as "bare fibers," consists of scraps of brass-coated metal wires from the manufacture of reinforcing wires for tires. These fibers therefore do not contain any rubber, as they have not been incorporated into a polymeric article such as a rubber article.

[0036] A second batch comes from the processing of end-of-life tires. These tires are processed, in a manner known to those skilled in the art, by cutting, sorting and grinding. The shredded material, which includes brass-plated cables, is then granulated in granulators and cleaned to obtain aggregates comprising approximately 4% by weight of rubber (category E51 scrap metal according to AFNOR AF 08-821 standard).

[0037] Three samples from three tire processing campaigns are tested to account for the variability of the loads processed. These samples are hereinafter referred to as “Gummed Fiber A”, “Gummed Fiber B” and “Gummed Fiber C”.

[0038] A third batch, hereinafter referred to as "Bead Wire 1", consists of scrap zinc-coated metal wire from the manufacture of reinforcing wire for pneumatic tires intended for use as bead wire. These fibers therefore do not contain rubber, as they have not been incorporated into a polymeric article such as a rubber article.

[0039] A fourth batch, hereinafter referred to as "Bead Wire 2", consists of scraps of bronze-coated metal wires from the manufacture of reinforcing wires for pneumatic tires intended for use as bead wires. These fibers therefore do not contain rubber, as they have not been incorporated into a polymeric article such as a rubber article.

[0040] For each batch, the steel fibers individually have a length of no more than 10 cm.

[0041] The "bare fibers" obtained comprise 0.16% by weight of brass coating relative to the mass of metal of the steel fibers. An elemental analysis shows that the brass is composed, by weight, of 64.4% copper and 35.6% zinc and that the steel is composed, by weight, of 0.7% carbon, 0.5% manganese, 0.2% silicon and 98.6% iron, the other elements such as chromium, molybdenum, etc. being present in negligible quantities.

[0042] The resulting "Gumed Fibers A" comprise 0.15% by weight of brass coating relative to the mass of metal in the steel fibers. An elemental analysis shows that the brass is composed, by weight, of 63.6% copper and 36.4% zinc. The composition of the steel is identical to that of the "bare fibers".

[0043] The resulting "B-Gumed Fibers" comprise 0.23% by weight of brass coating relative to the mass of metal in the steel fibers. An elemental analysis shows that the brass is composed, by weight, of 62.8% copper and 37.2% zinc. The composition of the steel is identical to that of the "bare fibers."

[0044] The resulting "C-Gumed Fibers" comprise 0.16% by weight of brass coating relative to the mass of metal in the steel fibers. An elemental analysis shows that the brass is composed, by weight, of 64.4% copper and 35.6% zinc. The composition of the steel is identical to that of the "bare fibers".

[0045] The resulting “Rod 1 Fibers” contain 0.003% zinc coating by weight relative to the mass of steel fiber metal. The composition of the steel is identical to that of the “bare fibers.”

[0046] The resulting "Tringle 2 Fibers" contain 0.003% bronze coating by weight relative to the mass of steel fiber metal. An elemental analysis shows that the bronze is composed, by weight, of 99% copper and 2% tin. The composition of the steel is identical to that of the "bare fibers."

[0047] Percentage of demetallization To determine the percentage of demetallization, the procedure is as follows. The coating content (brass, zinc or bronze) T1 on the steel fibers is determined, expressed in mg / 100g of metal, before treatment. A sample of fibers is treated, then the coating content T2 on the steel fibers is determined again.

[0048] The extraction percentage is then calculated according to ■ %extraction = (Tl-T2) / Tlxl00.

[0049] To determine the coating content on the fibres, the procedure is known to those skilled in the art, by chemical attack of the steel fibres and then measuring the copper, zinc and tin elements (depending on the coatings) in the solution used for the chemical attack.

[0050] Attack of Steel

[0051] To assess the attack on the steel by the treatment carried out, the iron present in the chemical bath is measured at the end of the treatment. The more iron there is in the chemical bath, the more the steel in the steel fibers has been attacked by the treatment, which is detrimental. The amount of iron present in the treatment bath is reduced to the amount of iron present in the steel fibers before treatment.

[0052] The analyses show that the non-electrochemical treatments evaluated, with the exception of the soda treatment, lead to the presence of a significant quantity of iron in the treatment baths, with dissolved iron representing more than 20% by weight of the iron present in the steel fibres before treatment.

[0053] The evaluated electrochemical treatments and the non-electrochemical treatment with soda lead to the extraction of very little iron from the steel fibers, with a quantity of dissolved iron less than 0.05% by weight of the iron present in the steel fibers before treatment.

[0054] Results

[0055] The conditions and results of the different treatments are presented in Table 1. Chemical treatment with ammonia diluted in a 30% by weight solution of hydrogen peroxide allows good performance to be obtained, but this solution is not deployable due to the high constraints linked to ammonia. It is observed that the process operated under the conditions of the invention allows excellent demetallization of the steel fibers to be obtained while limiting the extraction of iron from the steel, in particular compared to known chemical treatments.

[0056] [Table 1]

Claims

CLAIMS

1. A method for demetallizing coated steel fibers intended for reinforcing polymerized articles comprising at least one electrochemical treatment step in which the steel fibers are immersed in a treatment bath comprising a solution of an alkali pyrophosphate salt at a concentration of between 10 and 1900 g / l while being subjected to an anodic current, the mass current intensity ranging from 1 to 2500 A / kg of steel fibers, the residence time of the steel fibers in the bath ranging from 1 min to 90 min.

2. Demetallization process according to the preceding claim in which the temperature of the treatment bath ranges from 10°C to 80°C, preferably from 20°C to 60°C.

3. Demetallization method according to any one of the preceding claims in which the mass current intensity ranges from 1 to 500 A / kg of steel fibers, preferably from 1 to 300 A / kg of steel fibers, preferably from 2 to 300 A / kg of steel fibers, very preferably from 5 to 20 A / kg of steel fibers.

4. A demetallization process according to any one of the preceding claims in which the alkali pyrophosphate salt is chosen from potassium pyrophosphate and sodium pyrophosphate, and is preferably potassium pyrophosphate.

5. Demetallization process according to any one of the preceding claims in which the concentration of alkali salt in the treatment bath ranges from 50 to 1000 g / l, preferably from 50 to 500 g / l and very preferably from 75 to 300 g / l.

6. A demetallization process according to any one of the preceding claims wherein the steel fibers have a gum content ranging from 0 to 15% by weight of steel fibers, preferably from 0 to 10% by weight of steel fibers and more preferably from 0 to 4% by weight of steel fibers.

7. Demetallization method according to any one of the preceding claims in which the steel fibers are steel fibers coated with a metallic coating chosen from brass, zinc, bronze.

8. A demetallization method according to any preceding claim wherein the metal coating content of the steel fibers is from 0.01 to 0.50% by weight relative to the mass of metal in the steel fibers.

9. A demetallization method according to the preceding claim wherein the coating of the steel fibers is brass, the copper content in the steel fibers ranging from 0.1 to 0.3% by weight and the zinc content in the steel fibers ranging from 0.05 to 0.20% by weight relative to the mass of metal in the steel fibers.

10. A demetallization method according to any one of the preceding claims wherein the steel fibers have a diameter ranging from 0.1 mm to 2 mm.

11. A demetallization method according to any one of the preceding claims wherein the steel fibers individually have a length of at most 10 cm.

12. A demetallization method according to any preceding claim wherein the residence time is adjusted so as to reduce the metal coating content relative to the mass of metal in the steel fibers by at least 90%.

13. Demetallization process according to any one of the preceding claims in which the polymerized articles are rubber articles chosen from vehicle tires, tracks, conveyor belts, and very preferably consist of vehicle tires.

14. A demetallization method according to any one of the preceding claims in which the steel fibers come from the crown reinforcement, the carcass reinforcement and the bead wires of vehicle tires when these elements are reinforced with metal wires.

15. Use of steel fibers from the demetallization process according to any one of the preceding claims for the manufacture of reinforcing wires for reinforced polymeric articles.

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