Classifications

• • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
  • F27—FURNACES; KILNS; OVENS; RETORTS
  • F27D—DETAILS OR ACCESSORIES OF FURNACES, KILNS, OVENS OR RETORTS, IN SO FAR AS THEY ARE OF KINDS OCCURRING IN MORE THAN ONE KIND OF FURNACE
  • F27D5/00—Supports, screens or the like for the charge within the furnace
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  • C04B35/00—Shaped ceramic products characterised by their composition; Ceramics compositions; Processing powders of inorganic compounds preparatory to the manufacturing of ceramic products
  • C04B35/01—Shaped ceramic products characterised by their composition; Ceramics compositions; Processing powders of inorganic compounds preparatory to the manufacturing of ceramic products based on oxide ceramics
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  • C04B35/185—Mullite 3Al2O3-2SiO2
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  • C04B35/46—Shaped ceramic products characterised by their composition; Ceramics compositions; Processing powders of inorganic compounds preparatory to the manufacturing of ceramic products based on oxide ceramics based on titanium oxides or titanates
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Definitions

• the invention relates to a support for cooking structures of the honeycomb type and to its method of production.
• Such structures may be filtering and / or catalytic and are used in particular in an exhaust line of a diesel-type internal combustion engine.
• the invention relates more particularly to baking supports for filtering structures or catalyst supports based on oxide ceramics, and in particular aluminum titanate-based filters, and to their method of production.
• catalytic filters for the treatment of gases and the removal of soot from a diesel engine are well known in the prior art.
• These structures all most often have a honeycomb shape, one of the faces of the structure allowing the admission of the exhaust gas to be treated and the other side the evacuation of the treated exhaust gas.
• the structure comprises, between the intake and discharge faces, a set of adjacent ducts or channels of axes parallel to each other separated by porous walls.
• the ducts are closed at one or the other of their ends to delimit inlet chambers opening on the inlet face and outlet chambers opening along the discharge face.
• the channels are alternately closed in an order such that the exhaust gases, during the crossing of the honeycomb body, are forced to pass through the sidewalls of the inlet channels to join the outlet channels. In this way, particles or soot are deposited and accumulate on the porous walls of the filter body.
• the particulate filter is subjected to a succession of filtration (soot accumulation) and regeneration phases.
• soot particles emitted by the engine are retained and are deposited inside the filter.
• soot particles are burned inside the filter, in order to restore its filtration properties.
• the structures referred to hereafter as "asymmetrical" have a constant filter volume, a surface or a volume of the input channels different from that or of the output channels of said filter.
• asymmetrical it has been proposed in the patent application WO 05/016491 structures in which the wall elements succeed one another in cross section and following a horizontal and / or vertical row of channels, to define a sinusoidal shape or wave (wavy in English).
• the wall elements typically wave a half-period of sinusoid across the width of a channel.
• Such channel configurations provide a low pressure drop and a large soot storage volume.
• the filters are porous ceramic material, for example cordierite or silicon carbide or aluminum titanate.
• Silicon carbide filters made with these structures are for example described in patent applications EP 816 065, EP 1 142 619, EP 1 455 923 or WO 2004/090294 and WO 2004/065088, to which the skilled person For example, reference may be made for more details and details, both for the description of filters according to the present invention and for their method of production.
• These filters advantageously have a high chemical inertness with respect to soot and hot gases but a coefficient of thermal expansion a little high, which leads, for the production of large filters, the need to assemble several monolithic elements by a seal cement in a filter block, in order to reduce their thermomechanical stresses. Due to the high mechanical strength of the recrystallized SiC materials, it is possible to produce filters with thin filter walls and high porosity, with a very satisfactory filtration efficiency.
• Cordierite filters have also been used for a long time because of their low cost. Due to the very low coefficient of thermal expansion of this material, in the normal operating temperature range of a filter it is possible to produce monolithic filters of larger size.
• the aluminum Titanate material may also have a low coefficient of thermal expansion and exhibits refractoriness and corrosion resistance superior to that of cordierite. It thus makes it possible to produce monolithic filters of large size provided, however, to control the thermal stability of the titanate of aluminum, especially during the regeneration phases of the filter.
• Monolithic filters are thus described in the patent application WO 2004/011124, which proposes structures based on aluminum titanate for 60 to 90% by weight, reinforced with mullite, present at a level of 10 to 40% by weight. According to the authors, the filter thus obtained has improved durability.
• the patent application EP 1 741 684 describes a filter having a low coefficient of expansion and whose main phase of aluminum titanate is stabilized firstly by the substitution of a fraction of the Al atoms by Mg atoms in the lattice Al 2 TiO 5 in a solid solution and on the other hand by substitution of a fraction of the atoms Al on the surface of said solid solution by Si atoms, brought into the structure by a phase additional intergranular type of potassium aluminosilicate and sodium, especially feldspar.
• the deformation is all the more marked that the extruded structure 2 is large.
• large size is meant in particular structures of diameter greater than 100 mm or section greater than 75 cm 2 .
• the problem becomes even critical for structures of very great length, for example longer than 150 mm and / or very large diameter, for example greater than 125 mm in diameter or large section, that is to say greater than or equal to 120 cm 2 .
• obtaining structures is problematic if the withdrawal of the structure after cooking, according to its largest dimension, is greater than or equal to 5%.
• shrinkage it is understood in the sense of the present description the difference, in percentage, between a characteristic dimension of the structure before and after firing, referred to said dimension before firing.
• the shrinkage can be measured either on the length or on the diameter of said structure.
• structure before firing is meant the structure in the dry green state, that is to say having a residual moisture of less than 1%.
• the shrinkage is measured on a section of the filter substantially parallel to the plane formed by the bearing surface of the structure in contact with its cooking support, sufficiently far from said surface of the support to overcome the phenomenon of "elephant leg” mentioned above. Practically, the shrinkage is measured ideally on the upper third of the height of the structure.
• an initial solution consists in extruding a raw structure of greater length than necessary. The part of the structure most deformed at its base, that is to say, that located in the portion near its face of support on the cooking support, is then cut after cooking. Finally, the structure is plugged and then optionally annealed in order to sinter the material constituting the plugs.
• the patent application EP 0 234 887 proposes a cooking support formed by a cooked or raw honeycomb having at least, at its face in contact with the structure to be cooked, a width less than the width of said structure to cook. For example, this cooking support may have a chamfer or different forms of course.
• the present invention relates in a first aspect to a method of cooking a honeycomb structure possibly filtering, using a new type of cooking support, which can effectively meet all the needs previously exposed , in particular :
• the present invention relates to a method of cooking a porous ceramic structure of the honeycomb type on a baking support, said structure comprising a plurality of longitudinal through channels. ending on two ends of the structure, the end bearing on the support having a maximum width L 3 before firing, according to a longitudinal sectional plane passing through the main axis of the structure.
• said support has according to said longitudinal sectional plane: a first level corresponding to a first face of the support serving as a bearing surface of the structure to be fired, said first level having a maximum width Li, a second level, spaced from said first face of the support by a thickness Ei 2 , said second level having a maximum width L 2 ,
• the main axis of the structure of the honeycomb type in a conventional manner, is defined according to the invention as the main axis of symmetry of the structure, parallel to the through channels . It is of course defined according to the shape of the structure and in particular of its section. By way of example, it is the axis of revolution in the case of a cylindrical structure, the central axis of a parallelepiped or ovoid structure.
• the main axis is in particular represented in the diagrammatic views of FIGS. 3 and 6.
• the support used according to the invention preferably has a generally similar overall shape, that is to say, respectively of substantially oval, round or square shape, in particular whose dimensions are such that Li is between L 3 and 1.1 L 3 , in any longitudinal plane passing through the main axis of the structure to be cooked.
• the width Li is between L 3 and 1.1 L 3 .
• the relative difference (Li-L 2 ) / Li between the two widths Li and L 2 is greater than or equal to approximately 5%.
• the end of the structure and the face of the support which serves as a bearing face have a shape and / or a substantially identical geometry.
• the support is of the honeycomb type, and comprises a plurality of longitudinal through channels opening on two ends, the upper end of the support constituting said first level.
• the support has a porosity adapted to the porosity of the material constituting the walls of the filter.
• the support generally has between 20 and 65%, preferably between 30 and 50%, the average pore size being ideally between 10 and 20 microns.
• porosity that is too high leads to a level of mechanical resistance that is too low to support the filter. Too low a porosity can be harmful for the filter because the removal of the support may not be sufficient to accompany the removal of the filter during cooking.
• the thickness of the walls of the honeycomb support is advantageously between 0.2 to 1.0 mm, preferably 0.2 to 0.5 mm.
• the number of channels in the filter elements is preferably between 7.75 and 62 per cm 2 , said channels having a section of about 0.5 to 9 mm 2 .
• the first level corresponding to a first face of the support serving as support for the structure to be fired is a plane.
• the third level corresponding to the second face to the base of the support serving as support of the support on the cooking device is preferably a plane, such a plane for a better stability.
• the thermal expansion of the material constituting the support varies by at most 2% of the thermal expansion of the material constituting the structure, in the temperature range of the cooking.
• the firing shrinkage of the material constituting the support varies by at most 2% of the baking shrinkage of the material constituting the structure, in the temperature range of the firing.
• the support has a trapezoidal shape, said second level corresponding to the second face of the support.
• said support has a third level corresponding to the second face of the support, having a width L 3 , the width L 3 being greater than or equal to the width L 2 and at least equal to 3/5 of the width Ls.
• the cooking method as described above can in particular be used advantageously in a process for obtaining a honeycomb filtering structure, in which the longitudinal through channels of the structure are previously alternately plugged at their ends.
• the invention also relates to the support suitable for firing a porous ceramic structure of the honeycomb type as just described above, and comprising in particular, according to a section plane passing through its main axis:
• a second level spaced from said first face of the support by a thickness Ei 2 , said second level having a maximum width L 2 ,
• the width Li is between L 3 and 1.1 L 3 .
• the relative difference (Li-L 2 ) / Li between the two widths Li and L 2 is greater than or equal to approximately 5%.
• said cooking medium of the honeycomb type, comprises a plurality of longitudinal through channels opening on two ends, the upper end of the support constituting said first level.
• said support has a trapezoidal shape, said second level corresponding to its second face.
• said support has a third level corresponding to its second face, of maximum width L 3 , the width L 3 being greater than or equal to the width L 2 .
• the thermal expansion measured at a temperature T corresponds to the percentage of variation in length of a specimen of the material subjected to a variation of the temperature up to the temperature T (final temperature of the cooking), relative to its initial length at ambient temperature (20 ° C.), taken as a reference. It is conventionally measured by differential dilatometry according to the standard provided for this purpose NFB40-308.
• the expansion of the test piece of material constituting the support or the structure to be cooked is measured in a plane parallel to the plane formed by the bearing face 3 on the cooking support 4 as indicated previously in FIG.
• a thermal expansion of the support in the temperature range of the cooking is close to that of the structure within the meaning of the present invention if it is equal to that of the structure, plus or minus 2%, and of preferably at least 1%, regardless of the temperature in the range considered (20 0 C - T).
• the support is believed to best support the dimensional variations of the structure during cooking.
• the support material is chosen such that its shrinkage after the heat treatment for firing the structure is equal to that of the structure plus or minus 2%, preferably plus or minus 1%.
• the support according to the invention can be implemented according to different modes, some of which are illustrated below. Of course, the invention is not, in any of the described aspects, limited to these modes.
• the support according to the invention has a width Li ranging between L 3 and 1.1 L 3 .
• the support used according to the invention preferably has a similar general shape, that is to say, respectively oval, round or square, whose dimensions are such that Li is between L 3 and 1.1 L 3 , in any longitudinal plane passing through the main axis of the structure to be fired.
• the support has a width L 2 less than Li so as to accompany the horizontal deformation of the structure during cooking.
• the relative difference between the two widths ((Li-L 2 ) / Li), in percentage is greater than or equal to about 5%, preferably greater than or equal to about 10%, even more preferably greater than or equal to about 15%. In general, this difference remains less than 50%, preferably less than 40%, or even less than 30%.
• the support of FIG. 3 furthermore has a width L 3 measured on a second face or face of the support opposed to the face 3 horn indicated in FIG. 1, so as to ensure a sufficient stability of the support-structure assembly this second being in contact, during the cooking process for example the sole of the baking oven or the cooking device.
• the support has a sufficient thickness Ei_ 2 between the level Li in contact with the structure to be fired and the level L 2 of the support.
• the values of E 1 2 and the width L 2 according to the invention depend and vary according to the nature of the material constituting the support, of its internal geometry
• the conditions of the heat treatment envisaged in particular the final temperature level, the speed of the rise in temperature and the cooking time at the end of maximum temperature.
• the optimum thickness E 1 2 can be determined and adjusted experimentally.
• the width L 3 of the structure to be fired for example the external diameter of the structure if it is of cylindrical section,
• the thickness E 1 2 and the width L 2 are adjusted experimentally, for example by successive iterations, so as to have optimum resistance creep or subsidence under the load of the structure to be cooked.
• E 1 and Li 2 , L 2 are chosen so that the angle CC, as shown in Figure 3, is greater than 15 °, preferably greater than 45 °.
• E 1 and Li 2 , L 2 are chosen so that the angle CC is less than 85 °.
• the support has a constant thickness E 2 _ 3 between the level L 2 and the level L 3 at the base of the support which is in contact with the hearth of the oven or with the cooking device.
• E 2 _ 3 is preferably less than 5/3 of L 3 .
• the support according to the invention may also have other shapes between the levels L 2 and Li, provided that the distance L 2 remains lower than the distance Li, in particular rounded edges or concave or convex surface curves.
• the support according to the invention may also advantageously comprise one or more of the following optional features: a) the support preferably has a suitable roughness, for example as described in EP 1808423, but this is not necessary for the benefit of the support according to the present invention. Indeed, the deposition of a grain or a bed of powder, for example alumina or globular corundum can avoid the phenomena of bonding to the support while filling the surface irregularities of the support. b) the support is raw and its mineralogical and granulometric chemical composition is similar or identical to that of the structure to be cooked.
• the support is porous. In particular, it preferably has an open porosity close to that of the structure to be fired. After cooking, the porosity of the support is typically between 10% and 80%, and preferably between 30% and 70%. Especially in the particle filter application, too low porosity leads to a too high pressure drop.
• the median diameter d 5 in volume, pores constituting the porosity of the support after firing is preferably between 5 and 30 microns, preferably between 8 and 25 microns.
• the support itself is formed of channels, as shown in FIG. 6, so as to allow the gases emitted by or reacting with the structure to flow freely during debinding and cooking of said structure, oriented in the same direction when cooking.
• the support may have the same internal honeycomb macrostructure as the structure to be cooked.
• the support 4 is preferably of the honeycomb type in an arrangement as shown in FIG. 6, in which the plugs 6 are furthermore shown. shutting the channels.
• the dimensional characteristics of the support channels are then preferably close to those of the structure to be cooked, or even identical.
• the support can be adapted so that its channels are of the same open surface as those of the structure facing and at the level of the support face to be cooked.
• Such an embodiment can be particularly advantageous for optimal cooking of structures of the "asymmetrical" type, that is to say the surface of part of the inlet channels is different and preferably greater than that of part of the output channels, as illustrated by WO05 / 016491 previously cited.
• Such an arrangement allows an accelerated elimination of binders at the start of cooking, at the open channels of the support on the outside.
• Such an arrangement is ultimately favorable to the good performance of the filter structure and in particular prevents the appearance of cracks or deformation of the structure during cooking.
• An example of a method of manufacturing a honeycomb structure in which a support according to the invention can advantageously be used typically comprises the following main steps: a) preparation of a composition based on the material constituting the structure and shaped, in particular by extrusion through a die of said material and cutting to obtain a structure of honeycomb, b) preparing a composition of a sealing material and sealing said raw structure of part of the channels with said composition before and / or after drying before and / or after cooking according to step e) in order to to obtain a filtering structure after cooking. c) optionally drying in air according to a technique chosen from hot air drying, drying by microwave drying, drying by lyophilization at a temperature below 130 ° C. or a combination of said techniques, d) baking said structure on a support according to the invention, possibly comprising an initial debinding step.
• the firing step e) is carried out up to a temperature depending on the material constituting the structure.
• the support according to the invention was particularly advantageous for cooking previously clogged filter structures, which makes it possible to avoid an additional cooking step in order to sinter the plugs.
• the use of the supports according to the invention ultimately promotes the production of filter structures having improved cohesion between plugs and walls.
• the filtering structure is monolithic and the filtering walls are based on an inorganic oxide material, in particular based on titanate.
• Aluminum or Cordierite or Mullite or a composite from these materials By the term “based on”, it is understood that said walls comprise at least 50% by weight and preferably at least 70% by weight, or even at least 90 or even 98% by weight of said material.
• the porous walls of the filtering structure consist of a material based on aluminum titanate.
• the composition of the porous ceramic material based on aluminum titanate may have all known additions for stabilizing the aluminum titanate phase.
• High temperature stability means the ability of the aluminum titanate material not to decompose into two phases of TiO 2 titanium oxide and Al 2 O 3 aluminum oxide, under normal conditions of use. a particle filter.
• this property is measured according to the invention by a stability test consisting of determining the phases present in the material, typically by X-ray diffraction, then subjecting it to heat treatment at 1100 ° C.
• the porous walls of the filtering structure consist of an SiC-based material and a ceramic and / or vitreous binder matrix, said vitreous matrix optionally comprising SiO 2.
• ceramic binder matrix is meant a continuous structure between grains typically of size or average diameter of between 1 and 100 microns, preferably between 10 and 100 microns and obtained by baking or sintering so as to consolidate the material constituting said matrix.
• vitreous matrix is understood to mean, in particular, a matrix formed by a material having little or no crystallinity and comprising at least 30% silica (SiO 2 )
• the porous walls of the filtering structure consist of a material based on alumina.
• the porous walls of the filtering structure consist of a Cordierite-based material.
• the filter is constituted by the assembly of monolithic filtering elements, the section of a monolithic element constituting the assembled structure is preferably square, the width of the element being between 30 mm and 50 mm.
• the seal material is understood here as a moldable composition formed by a particulate and / or fibrous mix, dry or wet, capable of setting in mass able to have sufficient mechanical strength at ambient temperature or after drying and / or heat treatment of which the temperature will not exceed the softening or subsidence temperature which defines the refractoriness of the material (s) constituting the monolithic elements.
• the seal material preferably comprises particles and / or fibers of ceramic or refractory material, chosen from non-oxides, such as SiC, aluminum and / or silicon nitride, aluminum oxynitride, or from oxides, especially comprising Al 2 O 3 , SiO 2 , MgO, TiO 2 , ZrO 2 , Cr 2 O 3 or any of their mixtures.
• non-oxides such as SiC, aluminum and / or silicon nitride, aluminum oxynitride, or from oxides, especially comprising Al 2 O 3 , SiO 2 , MgO, TiO 2 , ZrO 2 , Cr 2 O 3 or any of their mixtures.
• the assembled or unassembled filter preferably has a coating cement integral with the assembled filter, in particular of the same mineral composition as the grouting material in order to reduce the thermomechanical stresses.
• the filter structure obtained according to the process according to the invention may further comprise a supported or preferably unsupported active catalytic phase, typically comprising at least one precious metal such as Pt and / or Rh and / or Pd and optionally an oxide such as Ce ⁇ 2, ZrO 2 , CeO 2 -ZrO 2 .
• Aluminum Titanate was prepared from the following raw materials:
• alumina approximately 40% by weight of alumina with a degree of purity in Al 2 ⁇ 3 of greater than 99.5% and a median diameter of 5 ⁇ m of 90 ⁇ m, sold under the reference AR75® by the company Pechiney, approximately 50% by weight rutile titanium oxide having more than 95% TiO2 and about 1% zirconia having a median diameter d 5 o of about 120 microns, marketed by the company Europe Minerais,
• magnesia powder with a MgO purity level of greater than 98% and of which more than 80% particles having a diameter of between 0.25 and 1 mm, sold by the company Nedmag.
• a particle size fraction is characterized by a median diameter d 5 o substantially equal to 50 microns, referred to as a large fraction according to the present invention, a particle size fraction characterized by a median diameter d 5 substantially equal to 1.5 microns, referred to as the fine fraction according to the present invention.
• the median diameter d 5 o denotes the diameter particles, measured by sedigraphy, below which is 50% by volume of the population. Microprobe analysis shows that all the grains of the melt phase thus obtained have the following composition, as a percentage by weight of the oxides (Table 1):
• the porosity characteristics were measured by high-pressure mercury porosimetry analyzes carried out with a Micromeritics 9500 porosimeter.
• the shrinkage is measured by the percentage ratio of the difference between the diameter (in mm) of the monolith after cooking. and that of the dry green monolith related to the diameter (in mm) of the dry raw monolith.
• the diameter is measured in the upper part of the filter, that is to say close to the face opposite to that directly in contact with the cooking support, so as not to take into account the possible deformation in elephant leg at the base. of the filter.
• the removal value reported in Table 2 corresponds to an average obtained by measurements made on a population of 10 monoliths.
• Example 1 the monoliths were cooked on a raw support in a nest bee with the same structure and the same mineralogical and granulometric chemical composition as the monoliths.
• the dimensional characteristics of the support are described in table 3.
• the baking support also has a cylindrical shape and a diameter identical to that of the structure to be fired.
• the monoliths obtained according to Example 2 were fired on a support of a geometry identical to that of FIG. 7 of patent EP 0 234 887.
• the dimensions of this are shown in Table 3.
• the attached FIG. 7 schematizes the shape of the support, the dimension a and the angle ⁇ characterizing the support described in this prior art.
• the monoliths of example 3, 4 and 5 according to the invention were cooked on a raw honeycomb support whose shape is illustrated in FIG. 5 and whose dimensions are shown in FIG. Table 3 below.
• the support is constituted this time by two circular levels of respective diameter Li (level 1) and L 2 (2nd level), the diameter L 2 being in accordance with the invention, less than the diameter Li.
• the width L 3 was measured on dry green structure at the base of the monolith.
• the dimensions Li, L 2 , L 3 , E 1 -2 , E 2 -3, a, H and the angles CC and ⁇ were measured on the dry green support.
• the horizontal deformation (elephant leg), that is to say parallel to the plane formed by the surface of the support in contact with the piece to be cooked was measured on each monolithic filter after cooking.
• the percentage of strain expressed in Table 3 is an average percentage measured on a sample of 10 filters. It is determined by measuring the difference in the outer diameter at the top and base of the filter in contact with the firing support and dividing by the average diameter of the filter and multiplying by 100. A horizontal strain greater than 2% is considered no satisfactory with respect to the application.
• the vertical deformation was measured on filter or monolith after cooking by the difference in length of the filter at the center and periphery of the filter, dividing by the length at the center of the filter and multiplying by 100.
• a vertical strain greater than 0.1% requires additional machining of the part obtained and is considered for this reason as not satisfactory.

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• 2010
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