METHOD FOR DEMETALLIZING A REINFORCING ELEMENT FOR POLYMERIZED ARTICLES

Abstract

The invention relates to a process for demetallizing coated steel fibers intended for the reinforcement of 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 salt of pyrophosphate at a concentration 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.

Description

Title of the invention: METHOD FOR DEMETALLIZING A REINFORCING ELEMENT FOR POLYMERIZED ARTICLES Technical field of the invention

[0001] The present invention relates to the field of processes for treating metallic elements, in particular coated steel fibers intended for the reinforcement of polymerized articles, in particular rubber articles, and in particular to processes aimed at removing the coating present on these steel fibers in order to improve their recycling. Previous art

[0002] Increasing pressures on natural resources and the desire to minimize the amount of waste generated are leading manufacturers to develop an increased number of recycling solutions.

[0003] Steel recycling has been known for a long time. However, the proportion of recycled steel in a casting is hampered by the problem of managing residuals, that is, 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 a problem, particularly if one seeks to implement a so-called "circular" system in which used steel is recycled for reuse in the same process. Indeed, any closed-loop process leads to an accumulation of species that cannot be extracted, and this accumulation can ultimately limit the quantity of recycled steel, especially in the case of copper (Environ. Sci. Technol. 2017, 51, 6599-6606).

[0004] The steels used to manufacture wires for reinforcing polymerized articles, particularly rubber articles such as pneumatic or non-pneumatic tires, tracks, and conveyor belts, have very tight tolerances in terms of chemical composition. This is necessary both to allow for severe forming by wire drawing and to ensure sufficient mechanical strength to perform their reinforcing function. Therefore, the residual content in the steel composition must be as low as possible.

[0005] Implementing a circular system for these metal wires therefore requires very precise management of the residual content. These metal wires are typically coated with zinc, brass (a copper-zinc alloy), or bronze (a copper-tin alloy) to improve the adhesion of the rubber to their surface. Recycling them for reuse in the manufacture of pneumatic-grade steel requires therefore to develop solutions to effectively manage residual levels.

[0006] To achieve this, various solutions have been developed to demetallize steel, that is to say, to remove the surface coating from steel elements.

[0007] The document “Treatment of Coated Materials”, M1458, Engineering Techniques, describes numerous demetallization solutions, by chemical or electrolytic means, in particular based on alkaline solutions including sodium cyanide.

[0008] Document DE2233157 describes the decoping of 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. Furthermore, this method requires special handling of the treatment products, and in particular of the ammonia, which necessitates the implementation of specific protective measures.

[0009] Document CN103436899 describes the cleaning of the coating on steel cables for pneumatic tires. For this purpose, the cables are treated in a strongly alkaline bath of ammonia and sodium nitrate. The volatility of ammonia necessitates frequent top-ups and the implementation of special protective measures.

[0010] 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 are partially coated with rubber or not, thus opening the way for the recycling of these fibers to steelmaking processes for their reuse in the manufacture of pneumatic grade steel. Detailed description of the invention

[0011] The invention relates to a process 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, and the residence time of the steel fibers in the bath ranging from 1 min to 90 min. Definitions

[0012] In the present invention, unless expressly stated otherwise, all percentages (%) indicated are percentages (%) by mass.

[0013] 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 (that is to say, bounds a and b excluded) while any interval of values ​​designated by the expression "from a to b" means the domain of values ​​going from a to b (that is to say, including the strict bounds a and b).

[0014] By demetallization, we mean, as known to those skilled in the art, the removal of a layer of metallic coating.

[0015] The process according to the invention is a process for demetallizing steel fibers intended for reinforcing polymerized articles. By polymerized article is meant an article comprising a metallic reinforcing element and a polymeric matrix, such as a resin or a rubber composition. By rubber article is meant any rubber article reinforced with steel fibers. These articles are notably chosen from vehicle tires, whether pneumatic or non-pneumatic (i.e., supporting the weight of the vehicle by means other than a pressurized gas, for example by means of stays), conveyor belts, belts, and tracks, the term rubber being understood here as any polymeric compound exhibiting elastic behavior.

[0016] Steel fibers for reinforcing polymerized articles are defined as new steel fibers intended to be incorporated into polymerized articles, or steel fibers extracted from polymerized articles, whether these articles are in the so-called "raw" or non-crosslinked state, or in the "cured" state, i.e., crosslinked or vulcanized. The steel fibers are coated steel reinforcing elements cut to be processed in the method according to the invention.

[0017] Preferably, the rubber articles are chosen from vehicle tires, tracks, conveyor belts, and very preferably chosen from vehicle tires.

[0018] Generally, a vehicle tire comprises a crown having two axial ends, each extended radially inward by a sidewall and then by a bead intended to contact a rim, the whole assembly defining an internal toroidal cavity. More specifically, the crown comprises, radially from the outside in, a tread, intended to contact the ground via a rolling surface, a crown reinforcement, and a portion of the carcass reinforcement intended to strengthen the tire. The carcass reinforcement connects the two sidewalls by extending into a radially inner portion of the crown and is anchored, in each bead, to a circumferential reinforcing element, most often of the bead type.

[0019] Polymerized articles, in particular rubber articles, are processed, in a manner known to those skilled in the art, by cutting, sorting, and grinding. The ground material The materials containing the metallic reinforcing elements are then granulated in granulators and cleaned to reduce their gum content. Following this treatment, the resulting steel fibers preferably have an individual length of no more than 10 cm. Preferably, the steel fibers have a diameter ranging from 0.1 mm to 2 mm.

[0020] 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.

[0021] The steel fibers treated in the process according to the invention are steel fibers coated with a metallic coating selected from brass, zinc, and bronze, the metallic coating content preferably ranging from 0.01 to 0.50% by weight relative to the metal mass of the steel fibers. These fibers are preferably derived from the crown reinforcement, the carcass reinforcement, and tire bead wires for vehicles when these elements are reinforced with metallic wires.

[0022] 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 metal mass of the steel fibers.

[0023] 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 from 50 to 1000 g / L, most preferably from 50 to 500 g / L, and most preferably from 75 to 300 g / L, while being subjected to an anodic current. Above 1900 g / L, the alkali pyrophosphate salt is likely to precipitate, while below 10 g / L the concentration is too low to allow efficient conduction of the electric current.

[0024] The residence time of the steel fibers in the bath ranges from 1 min to 90 min. Preferably, the residence time is adjusted to reduce the metallic coating content relative to the metal mass 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 copper and zinc mass content of the steel fibers is less than 0.005% for copper and 0.005% for zinc relative to the metal mass of the steel fibers.

[0025] Electrochemical treatment of steel fibers with a solution of an alkaline pyrophosphate salt allows for the demetallization of steel fibers even in the presence of residual gum, with a reduced treatment time compared to chemical treatment and excellent demetallization selectivity. Indeed, known prior art chemical treatments allow for the removal of a portion of the metallic coating, but also extract some of the iron from the steel.

[0026] The process according to the invention is preferably carried out at a temperature ranging from 10°C to 80°C, preferably ranging from 20°C to 60°C. These temperatures allow operation with low risks of emissions due to evaporation, while maintaining excellent demetallization performance.

[0027] 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, and most preferably from 5 to 20 A / kg of steel fibers. These intensities, in conjunction with the other operating parameters, allow for a good compromise between the demetallization of the steel fibers and the demetallization treatment time.

[0028] The pH should preferably be maintained between 8 and 12, preferably between 10 and 12.

[0029] The electrolytic demetallization process according to the invention, based on an alkali pyrophosphate salt, makes it possible to obtain very good demetallization of 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.

[0030] The invention also relates to the use of steel fibers from the demetallization process according to the invention for the manufacture of reinforcing yarns for reinforced polymer articles.

[0031] 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, which is difficult to manage, opening up the possibility of recycling them at much higher levels in order to produce new steels for the reinforcement of reinforced polymer articles, in particular for the reinforcement of vehicle tires. Examples

[0032] In the following examples, different types of steel fibers are processed. A first batch, referred to hereafter as "bare fibers," consists of scraps of brass-coated metal wire from the manufacture of reinforcing wires for pneumatic tires. These fibers therefore do not contain rubber, as they have not been incorporated into a polymeric compound such as a rubber article.

[0033] A second batch is obtained from the processing of end-of-life pneumatic tires. These tires are processed, in a manner known to those skilled in the art, by cutting and sorting. and grinding. The ground material, which includes brass-coated cables, is then granulated in granulators and cleaned to obtain granules containing approximately 4% by weight of gum (scrap metal of category E51 according to AFNOR AF 08-821 standard).

[0034] Three samples from three pneumatic tire treatment campaigns are tested to account for the variability of the treated loads. These samples are referred to below as "Gummed Fiber A", "Gummed Fiber B" and "Gummed Fiber C".

[0035] A third batch, hereinafter referred to as "Tie wire 1", consists of scraps of zinc-coated metal wire from the manufacture of reinforcing wires for pneumatic tires intended for use as a tie wire. These fibers therefore do not contain gum, as they have not been incorporated into a polymeric compound such as a rubber article.

[0036] A fourth batch, hereinafter referred to as "Tie wire 2", consists of scraps of bronze-coated metal wire from the manufacture of reinforcing wires for pneumatic tires intended for use as a tie wire. These fibers therefore do not contain rubber, as they have not been incorporated into a polymeric compound such as a rubber article.

[0037] For each batch, the steel fibers individually have a length of no more than 10 cm.

[0038] The resulting "bare fibers" comprise 0.16% by weight of brass coating relative to the metal mass of the steel fibers. 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, with other elements such as chromium, molybdenum, etc., being present in negligible quantities.

[0039] The resulting "Gummed Fibers A" comprise 0.15% by weight of brass coating relative to the metal mass of the steel fibers. 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".

[0040] The resulting "Gummed Fibers B" comprise 0.23% by weight of brass coating relative to the metal mass of the steel fibers. 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".

[0041] The resulting "C-coated fibers" comprise 0.16% by weight of brass coating relative to the metal mass of the steel fibers. 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".

[0042] The resulting "Rod 1 Fibers" comprise 0.003% by weight of zinc coating relative to the metal mass of the steel fibers. The steel composition is identical to that of the "bare fibers".

[0043] The resulting "Rod Fibers 2" comprise 0.003% by weight of bronze coating relative to the metal mass of the steel fibers. 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". Percentage of demetallization

[0044] To determine the percentage of demetallization, the following procedure is used. The coating content (brass, zinc, or bronze) T1 on the steel fibers, expressed in mg / 100g of metal, is determined before treatment. A sample of fibers is treated, and then the coating content T2 on the steel fibers is determined again.

[0045] The extraction percentage is then calculated according to: %extraction =(T1-T2) / T 1x100.

[0046] To determine the coating content on the fibers, a method known to those skilled in the art is used, by chemically attacking the steel fibers and then measuring the elements copper, zinc and tin (depending on the coatings) in the solution used for the chemical attack. Steel attack

[0047] To assess the extent to which the steel is attacked by the treatment, the iron present in the chemical bath is measured after the treatment. The more iron 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 then compared to the amount of iron present in the steel fibers before treatment.

[0048] The analyses show that the non-electrochemical treatments evaluated, with the exception of treatment with soda, lead to the presence of a significant amount of iron in the treatment baths, with dissolved iron representing more than 20% by weight of the iron present in the steel fibers before treatment.

[0049] The evaluated electrochemical treatments and the non-electrochemical treatment with soda result in the extraction of very little iron from the steel fibers, with an amount of dissolved iron less than 0.05% by weight of the iron present in the steel fibers before treatment. Results

[0050] The conditions and results of the different treatments are presented in Table 1. Chemical treatment with ammonia diluted in a 30% by weight hydrogen peroxide solution provides good performance, but this solution is not deployable due to the strong constraints related to ammonia. It is observed that the process carried out under the conditions of the invention makes it possible to obtain excellent demetallization of the steel fibers while limiting the extraction of iron from the steel, in particular compared to known chemical treatments.

[0051] [Tables 1] Lot Recoating imposed chemical Amoèragé Conceritreifon chemical compound mo:l / 1 T MM fs / nioll tancentrat ton D&rnp. chemical g / ! Time min fiber g Vdtime of solution ' il Concentration .ffbffiï g / t mass current intensity A / kg E^tractien deposition tiotSr Atta-gue steel Bare fiber Brass H2SO4 - 15 ambient 58 147 58 28 0.5 40 - 74 Bare fiber brass: H 2504 • 5 ambient 98. 147 80 20 8.5 -48 94 Strong bare fiber Brass: HNG3 - ambient 63 53 53 28 0.5 40 - 25 Strong Bare fiber brass mœ. 1 ambient 83 ■ 53 &ô ZO 8.5 48: 5S Strong Fiber bare The item Ammonia 4 water Oxygenated 1S.3 ambient 27 1S4 50 28 05 40 - 98 Very weak Fiber bare Brass Ammonia -1-*» 1 oxygenated 188 bringing 17 184 60 zo 8.5 sa Very weak bare fiber Lsitcn baZcOJ 10 os ambient 186 100 5 50 2 18 SCO 42 Very weak Fiber bare Brass M2CŒ IC- 3.9 ambient ISO iœ 18 2'3 2 10 5£® 79' 74s Weak Fiber bare Brass 20 OO amusing 538 100 5 20 .2. 23 580 91 Very low Bare fiber: Brass £49207 IC- 8;3 ambient.330 ICO 18 28 2 10 508 97 7,'és rarbre Gummed fiber C Brass Na3H 5 2.5 ambient 40 100 5 20 *- 18 .250 55 T <es Farbie Fibre gomméaC Laiton MaOH 5 2.5 ambiant 40 IGG' 18 28 7. 10 258 71 Très faîbre Fibre gomméeC Laiton MaQH s 2.5 ambiant AG ICC 15 20 2 10- 250 72 T'es ta-b:e 'Fibre gommée C Laiton NaÙH 5 2.5 ?0 40 itæ 5 22 2 20 :25à 79 Très faible Fibre gommée A Laiton £49207 LC 05 ambiant 530 ica 55 25 2 '10 500 94 Très. Faible Fibre gemmée A Laiton KZ?2O7 20 03- amb-ant 558 100 LS 20 ? 18. 1080 95 Très faible Fibre go<mmee£‘ Laion K 49207 S 2.3 ambiant 330 100 5 28 2 10 253 8S Très. îa> b:e Gem-coated fiber C Brass WZO7 5 03 ambient 535 200 5 30 ? .15; 167 91 Very weak Gum-coated fiber C Brass £49207 5 C-.3 ambient 330 100 5 48 2 ZC- 125 83 Very strong Gum-coated fiber C Brass 849207 5 0.5 ambient 538 100 S 20 2 18 250 89 Very weak Gum-coated fiber A Brass £49287 5 Q3 ambient 330 100 5 28 2. 10. 258 88 Very weak Gum-coated fiber 8 Brass S 0.3' amber 330 100 S zc 2 there 250 92 T^ês Fa-b:Ê ■ Gem-tipped fiber € Brass 1492 CO 5 0.3 ambient 338 100 ÎS 20 2 18 258 98 Very weak Gummed fiber £ Brass £49207 3 C.3 ambient 330 iœ 15 Z'3 2 10 250 96 7'és Fa-b=e Gem-tipped fiber A Brass K4P2O7 5 05 amusing 358 100 15 20 2. 23 25S 94 Very weak Gummed fiber O : Brass £49207 5 G5 ambient .330 100 15 28 2 10 253 91 T.'és itree Fii TringmA Zmc £40207 5 05.

Claims

Demands

1. A process for demetallizing coated steel fibers 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 salt of pyrophosphate 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. Demetallizing 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. A demetallizing process according to any one of the preceding claims wherein 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, most preferably from 5 to 20 A / kg of steel fibers.

4. A demetallization process according to any one of the preceding claims wherein the alkaline pyrophosphate salt is selected from potassium pyrophosphate and sodium pyrophosphate, and is preferably potassium pyrophosphate.

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

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

7. A demetallization process according to any one of the preceding claims wherein the steel fibers are steel fibers coated with a metallic coating selected from brass, zinc, bronze.

8. A demetallization process according to any one of the preceding claims, wherein the metallic coating content of the steel fibers ranges from 0.01 to 0.50% by weight relative to the mass of metal of the steel fibers.

9. Demetallizing process 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 metal mass of the steel fibers.

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

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

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

13. A demetallizing process according to any one of the preceding claims wherein the polymerized articles are rubber articles selected from vehicle tires, tracks, conveyor belts, and most preferably are made of vehicle tires.

14. A demetallization process according to any one of the preceding claims wherein the steel fibers are derived from the top reinforcement, the carcass reinforcement and the tire bead wires for vehicles when these elements are reinforced with metal wires.