EP1525342B1 - Method and installation for heat treatment of carbon products containing sodium - Google Patents

Abstract

Carbon bodies are heated in an oven under low pressure and while being swept with an inert gas, gaseous effluent containing elemental or compound sodium in sublimed form being continuously extracted from the oven via an effluent exhaust pipe. At least one sodium-neutralizing agent is injected into the effluent exhaust pipe immediately downstream from the outlet for exhausting gaseous effluent from the oven. The sodium-neutralizing agent is selected from carbon dioxide and steam, and it can be injected continuously into the flow of gaseous effluent.

Description

Background of the invention

The invention relates to the high temperature heat treatment of carbon products containing sodium, and more particularly to the treatment of gaseous effluents produced during the heat treatment.

A particular field of application of the invention is the production of textures or preforms made of carbon fibers intended to constitute fibrous reinforcements for parts made of composite material such as carbon / resin composite, for example C / epoxy or C / phenolic composite, or thermostructural composite, for example carbon / carbon composite (C / C) or ceramic matrix composite and carbon reinforcement.

Such fibrous textures (cf. US-A-3,413,094 and US-A-4,388,299 ) are usually obtained from carbon precursor fibers that are better able to undergo the textile operations required for shaping these textures. Commonly used carbon precursor fibers are preoxidized polyacrylonitrile (PAN) fibers, pitch fibers, phenolic fibers, and rayon fibers.

At least for certain applications, it is necessary not only to transform the precursor into carbon, but also to perform a subsequent heat treatment at high temperature, typically above 1000 ° C and under reduced pressure, to remove metals or metal impurities especially sodium from the precursor, and / or to confer particular physicochemical properties on the fibers.

• une première étape de carbonisation proprement dite par transformation chimique du précurseur en carbone, cette première étape étant réalisée à l'échelle industrielle dans un four en élevant progressivement la température de chauffage du four jusqu'à environ 900°C, et
• une deuxième étape de traitement thermique à haute température visant notamment à éliminer par sublimation le sodium provenant du précurseur, cette deuxième étape étant également réalisée dans un four en élevant progressivement la température jusqu'à environ 1600°C, voire jusqu'à environ 2000°C à 2200°C, ou même 2500°C pour éliminer d'autres impuretés métalliques ou réaliser un traitement thermique à très haute température des fibres de carbone.

Thus, in the case, for example, of carbon products originating from the preoxidized PAN precursor, it is common to carry out two successive steps:

• a first step of carbonization proper by chemical conversion of the precursor to carbon, this first step being carried out on an industrial scale in an oven by gradually raising the heating temperature of the oven to about 900 ° C, and
• a second step of high temperature heat treatment aimed in particular to eliminate by sublimation sodium from the precursor, this second step also being carried out in an oven by gradually raising the temperature to about 1600 ° C, or up to about 2000 ° C at 2200 ° C, or even 2500 ° C to remove other metallic impurities or perform a very high temperature heat treatment of carbon fibers.

The second step is generally carried out under reduced pressure and under a purge of neutral gas such as nitrogen.

In the case of carbon products consisting of reinforcing fiber textures for composite material parts, the second step is usually carried out before densification of the fibrous texture by the resin, carbon or ceramic matrix of the composite material. In the case of thermostrutural composite material with a carbon and / or ceramic matrix, the densification can be carried out by a liquid route, that is to say impregnation with a liquid compound, such as a resin, precursor of the material of the matrix, and transforming this precursor by heat treatment. The densification can also be carried out by gaseous means, that is to say by chemical vapor infiltration, the two processes, the liquid route and the gaseous route being well known and possibly associable.

In existing plants, the cooling of gaseous effluents causes a sodium-containing deposit on the pipe walls downstream of the effluent outlet out of the heat treatment furnace. It is necessary to carry out a regular cleaning of these pipes, which is not easy because of the risk of violent reaction of the deposit containing sodium.

Object and summary of the invention

The object of the invention is to provide a method which avoids the aforementioned drawback by preventing the formation on the walls of gaseous effluent extraction pipes of potentially dangerous deposits at the stage of cleaning these pipes.

This object is achieved by a process of the type in which the carbon products are heated in an oven, under a purge of neutral gas and under reduced pressure, with continuous extraction out of the effluent furnace. gas containing in particular sublimed sodium, by an effluent discharge pipe, in which process, in accordance with the invention, at least one sodium neutralization product is injected into the effluent discharge pipe, immediately downstream of the effluent gas outlet out of the oven.

In this way, the deposit that forms on the walls of the effluent discharge line or other devices downstream of the effluent outlet out of the oven can be easily and safely removed at a later stage. The Applicant has observed that not only elemental sodium is evacuated in sublimed form with the gaseous effluent, but also sodium compounds likely to form a potentially troublesome or even dangerous deposit, such as sodium oxide NaO ₂ . Sodium neutralization here means not only the neutralization of elemental sodium, but also the neutralization of compounds such as NaO ₂ .

By sodium neutralization product is meant a product to obtain a stable sodium compound and fairly easily removed. A relatively easy handling product, for example water vapor or preferably carbon dioxide, optionally mixed with water vapor, is preferably chosen.

The neutralization product may be injected at or downstream of a bend formed by the effluent gas discharge pipe out of the oven.

The injected neutralization product may further be diluted in a neutral gas such as nitrogen.

The neutralization product can be continuously injected into the gaseous effluent stream extracted from the furnace during the heat treatment, so as to form a stable and easily removable sodium compound and avoid the deposition of sodium on the pipe wall of the furnace. evacuation.

In another embodiment of the method, the neutralization product is injected into the evacuation pipe after the end of the heat treatment in order to neutralize the sodium deposited on the wall of the evacuation pipe, before cleaning it.

The invention also aims to provide an installation for implementing the method.

This object is achieved by means of a heat treatment plant for carbon products containing sodium, of the type comprising an oven, means for supplying the oven with neutral gas for scavenging, and a line for extracting gaseous effluent out of the oven, installation which further comprises, according to the invention means for injecting a sodium neutralization product in the extraction pipe immediately after the exit of the furnace.

Brief description of the drawings

• la figure 1 est une vue générale très schématique d'une installation selon un mode de réalisation de l'invention ;
• la figure 2 est une vue de détail montrant une partie d'un dispositif d'extraction d'effluent gazeux hors du four de l'installation de la figure 1 ; et
• la figure 3 est une vue de détail montrant une partie d'un dispositif d'extraction d'effluent gazeux hors du four de l'installation de la figure 1, selon un autre mode de réalisation de l'invention.

Other features and advantages of the heat treatment process and installation according to the invention will become apparent on reading the description given below, by way of indication but without limitation, with reference to the appended drawings in which:

• the figure 1 is a very schematic overview of an installation according to one embodiment of the invention;
• the figure 2 is a detailed view showing part of a device for extracting gaseous effluent out of the furnace of the installation of the figure 1 ; and
• the figure 3 is a detailed view showing part of a device for extracting gaseous effluent out of the furnace of the installation of the figure 1 according to another embodiment of the invention.

Detailed description of embodiments

Embodiments of the invention will be described hereinafter in the context of the application to the high temperature heat treatment of carbon fiber textures obtained by carbonization of carbon precursor fiber textures. High temperature heat treatment means a treatment at a temperature usually greater than that encountered by the textures during carbonization, that is to say a temperature greater than 1000 ° C., typically between 1400 ° C. and 2000 ° C. ° C at 2200 ° C or 2500 ° C. The heat treatment is carried out under reduced pressure, below atmospheric pressure, preferably below 50 kPa, typically between 0.1 kPa and 50 kPa, preferably below 5 kPa, and under a purge of neutral gas such as nitrogen or argon. The process according to the invention is applicable to the removal of sodium present at low levels in fibers, for example less than 80 ppm, or at a much higher content, greater than 3500 ppm.

The figure 1 shows very schematically an oven 10 comprising a susceptor 12 of cylindrical shape and vertical axis which laterally delimits a volume or enclosure 11 for loading products (not shown) of carbon.

The susceptor 12, for example graphite, is surmounted by a cover 14, is heated by inductive coupling with an inductor 16 which surrounds the susceptor with the interposition of a thermal insulator 18. The inductor is powered by a circuit 20 which delivers a current depending on the need for heating the oven.

The inductor can be divided into several sections on the height of the oven. Each section is powered separately with electric power to define different heating zones in the oven in which the temperature can be regulated independently.

The bottom of the furnace is formed of a thermal insulator 22 covered with a furnace hearth 24, for example made of graphite, on which the susceptor 12 rests.

The assembly is housed in an envelope 26, for example metal, sealed by a removable cover 28.

A pipe 30 provided with a valve 31 is connected to a source (not shown) of inert gas, for example nitrogen N ₂ . The pipe 30 supplies the oven 10 with inert scanning gas to the upper part thereof, possibly through several inlets 32 opening at different locations around the casing 26 of the oven.

An extraction device 40 is connected to an outlet duct 42 of the furnace passing through the bottom thereof, to extract the gaseous effluent produced during the heat treatment of the carbon products, in order to remove residual sodium in particular.

The device 40 is connected to the outlet duct 42 by an evacuation pipe 44 provided with a carbon dioxide injection inlet 46 CO ₂ . As shown in detail in figure 2 , The pipe 44 forms a bend 44 has at its extremity connected by a flange 45 to the outlet duct 42 of the furnace. The injection inlet 46 is connected to a pipe 48 connected to a source (not shown) of CO ₂ gas and provided with a valve 49. The pipe 48 is extended by a nozzle 50 which penetrates into the pipe 44 in order to inject the CO ₂ gas in this pipe towards the downstream end of the elbow 44a and to avoid an accidental injection of the CO ₂ gas inside the furnace by the exit pipe 42. Several points of injection of CO ₂ gas spaced apart from each other along the pipe 44 may be provided.

The CO ₂ injection is carried out as close as possible to the outlet of the furnace, at a level where the sodium contained in the effluent is always in sublimated form. The injection at a bend of the pipe 44 promotes a mixture by turbulence between the gaseous effluent and CO ₂ .

Two columns 52, 54 provided with trays 53, 55 imposing a tortuous path to the gases are connected in series between the pipe 44 and a pipe 56 provided with a valve 57.

A pump 58 is mounted on the pipe 56, between the valve 57 and a valve 59 in order to be able to switch on or isolate the pump 58. The pump 58 makes it possible to establish the desired reduced pressure level in the furnace. Although only one pump is shown, the presence of two pumps can be considered for the sake of redundancy. The gaseous effluent extracted by the pump 58 is fed to a burner 60 which supplies a stack 62.

The oven 10 is equipped with temperature sensors connected to the control circuit 20 in order to set the heating temperature to the desired value.

Two sensors 64, for example, is used, 64 b consist of pyrometers optical sighting, which are fitted in the lid 28 under 28a 28b windows made therein openings 14a and 14b formed in the cover 14 of the susceptor . The use of several pyrometric sensors is not a necessity, but makes it possible to measure at different levels and to eliminate any aberrant measurements by comparison. Preference is given to using bichromatic type pyrometers producing a continuously exploitable continuous signal.

The temperature measured by the sensors 64a 64b is transmitted to the control circuit 20 to supply the inductor to change this temperature according to a preset temperature rise profile.

From a temperature of about 1000 ° C, according to the pressure in the chamber, the sodium contained in the fibrous textures is released and is discharged with the gaseous effluent in a sublimated form, in the elemental state and optionally in the combined state, for example in the state of sodium oxide NaO ₂ . CO ₂ is injected into the pipe 44 with a controlled flow rate by opening the valve 49 in order to carry out a neutralization of Na (or NaO ₂ ) as soon as it leaves the furnace and to avoid its deposition on the walls of the pipe 44.

For safety, CO ₂ injection can be started at a temperature below 900 ° C. This injection is also preferably continued at least until the end of the process. The sodium carbonate product is collected in particular in the tray columns 52, 54. The purified gaseous effluent of sodium is fed to the burner 60.

It will be noted that the neutralization of sodium by CO ₂ results in a reduction in the content of cyanide ions (CN ^- ) in the deposit collected by the columns 52, 54 relative to what is observed in the absence of passivation, which adds to the security provided by the absence of Na deposition.

The extraction device 40, or at least a part thereof comprising the tray columns 52, 54 and optionally the pipe 44 is cleaned periodically to eliminate in particular the deposited sodium carbonate. The cleaning can be carried out by rinsing with water in situ or by washing with water in a washing container after at least partial disassembly of the extraction device.

According to another embodiment of the invention ( figure 3 ), the neutralization of sodium is carried out by hydration. For this purpose, the pipe 44 is provided with one or more injection devices 70 for example in the form of hollow rings 72 surrounding the pipe 44. The injection device 70 is placed immediately downstream of the bend 44a, with interposition an isolation valve 71 between the outlet 42 of the oven and the injection device 70. In the illustrated example, two rings are provided spaced apart from each other along the pipe 44. The rings injection 72 are fed in parallel through a pipe 74 connected on the one hand to a source of neutralizing agent, for example a source of water vapor via a pipe 76 provided with a valve 75 and, secondly, a source of neutral gas such as nitrogen or argon, via a pipe 78 provided with a valve 57.

Downstream of the injection device 70, in the flow direction of gaseous effluent, the pipe 44 has a purge port connected to a purge pipe 80 provided with a valve 81. Downstream of the connection with the pipe of purge, the pipe 44 can be connected directly to the pump 58 through the valve 57, the use of tray columns is not necessary here. The rest of the installation is identical to what was described above.

Each injection ring 72 forms an O-ring surrounding the pipe 44 and communicating with it through holes 74 formed in the wall of the pipe. The holes 74 may be inclined relative to the normal to the wall of the pipe 44 to direct the flow of neutralization agent downstream.

The injection of the mixture H ₂ O + N ₂ can be carried out during the heat treatment process, as described above with regard to the CO ₂ injection, or after the end of the heat treatment process to hydrate the sodium deposited on the wall of the pipe 44.

In both cases, in order to avoid a sodium deposit on the wall of the pipe 44 upstream of the injection device closest to the outlet of the furnace, the pipe 44 may be insulated along its portion connecting the pipe. output 42 to this injection device. Insulation 43 prevents premature sodium condensation on the wall of pipe 44 by too rapid cooling of the gaseous effluent. The insulation 43 may be replaced or supplemented by heating means for example by electrical resistors.

After the end of the heat treatment, when the hydration of the sodium contained in the gaseous effluent is carried out by continuous injection into the gaseous effluent stream, or after the hydration of the sodium deposition carried out as a result of the heat treatment, a purge or cleaning of the pipe 44 is carried out.

For this purpose, the valves 75 and 81 are open, the valves 71, 57 and 77 are closed and water in liquid form is admitted into the pipe 76 and thence into the injection device 70. Several rinses consecutive lines of the line 44 can be carried out to remove the soda produced by neutralization of sodium.

After rinsing, drying of the pipe 44 can be achieved by simply opening the valve 57 and turning on the pump 58, the valves 75 and 81 being closed.

Although only water vapor can be injected, in the embodiment of the figure 3 , the dilution with nitrogen is preferred to avoid a too violent reaction with sodium, the amount of sodium to be neutralized being low.

In the embodiment of figures 1 and 2 the injected CO ₂ can also be diluted by mixing with nitrogen.

Other alternative embodiments are possible, in particular by modifying the embodiment of the figures 1 and 2 in order to continuously inject not CO ₂ , but water vapor or a mixture of CO ₂ and steam, possibly with dilution by neutral gas.

It will be noted, however, that in comparison with H ₂ O, the neutralization of sodium with CO ₂ is advantageous in that it produces sodium carbonate which is easier to treat, less corrosive and less reactive than sodium hydroxide.

The method and the installation which have just been described are particularly suitable for carbon products obtained from pre-oxidized PAN precursor, in particular for fibrous carbon textures intended for the manufacture of parts made of carbon / resin composite material C / C or C / ceramic, for example silicon carbide matrix (C / SiC) or ternary matrix silicon-boron-carbon (C / Si-BC).

The textures are made of fibers in the precursor state of carbon more capable of undergoing textile operations than carbon fibers. These textures may be one-dimensional such as wires or cables, or two-dimensional, such as fabrics or webs formed of parallel son or cables, or three-dimensional, such as preforms obtained by filament winding, or by stacking, winding or draping fabrics or plies in superimposed layers and possibly linked together by needling or stitching, for example. Examples of fibrous preforms are nozzle preforms or divergent nozzles or brake disc preforms.

The invention also applies to carbon products, obtained from carbon precursor materials other than preoxidized PAN, also containing sodium and optionally one or more other metals or metal impurities to be removed. Examples of such precursors are pitches, phenolic materials, or rayon.

The process according to the invention is advantageous in that it makes it possible to eliminate sodium which is present at a very low content in the fibers, for example less than 80 ppm, which sodium would be impossible to remove by another method such as rinsing with water. 'water. It also makes it possible to eliminate sodium present in larger amounts in the fibers, for example at more than 3500 ppm.

In addition to sodium, calcium and / or magnesium can be removed by sublimation.

In the case, especially of carbon products to have a high degree of purity, metals such as Fe, Ni and Cr may also have to be eliminated in addition to sodium. It is then necessary to perform the heat treatment to a temperature sufficient to ensure the sublimation of these metals, for example a temperature up to 2000 ° C or 2200 ° C, or 2500 ° C.

Claims (12)

1. A method of heat treating carbon bodies containing sodium, in which method the bodies are heated in an oven under low pressure while being swept with an inert gas, with gaseous effluent containing sodium in sublimed form being extracted continuously from the oven via an effluent exhaust pipe,
   characterized in that at least one sodium-neutralizing agent is injected into the effluent exhaust pipe immediately downstream from the outlet for extracting gaseous effluent from the oven.
2. A method according to claim 1, characterized in that the sodium-neutralizing agent is selected from carbon dioxide and steam.
3. A method according to any one of claims 1 and 2, characterized in that the sodium-neutralizing agent is injected at or downstream from a bend formed by the pipe for exhausting effluent from the oven.
4. A method according to any one of claims 2 and 3, characterized in that the injected sodium-neutralizing agent is diluted in an inert gas.
5. A method according to claim 4, characterized in that the inert gas is nitrogen or argon.
6. A method according to any one of claims 1 to 5, characterized in that the sodium-neutralizing agent is injected continuously into the stream of gaseous effluent extracted from the oven during heat treatment.
7. A method according to any one of claims 1 to 5, characterized in that the sodium-neutralizing agent is injected into the exhaust pipe after the end of heat treatment in order to neutralize sodium deposited on the wall of the exhaust pipe prior to cleaning it.
8. An installation for heat treating carbon bodies containing sodium, the installation comprising an oven (10), means (30) for feeding the oven with an inert gas for sweeping purposes, and a pipe (44) for exhausting gaseous effluent from the oven, characterized in that it further comprises means (50 ; 72) for injecting a sodium-neutralizing agent into the exhaust pipe (44) immediately after the outlet from the oven.
9. An installation according to claim 8, characterized in that it comprises at least one nozzle (50) for injecting the sodium-neutralizing agent, which nozzle penetrates into the exhaust pipe (44).
10. An installation according to claim 9, characterized in that the nozzle (50) penetrates into a bend (44a) of the exhaust pipe (44).
11. An installation according to any one of claims 8 to 10, characterized in that it comprises a plurality of points for injecting sodium-neutralizing agent, which points are spaced apart from one another along the exhaust pipe.
12. An installation according to any one of claims 8 to 10, characterized in that the exhaust pipe (44) is provided with a purge device (80, 81).

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