COOK-IN AGENT

Abstract

The invention relates to a particulate mixture intended to serve as an annealing agent, - consisting of sintered ceramic particles having a porosity greater than 20%, - having a chemical composition such that, in mass percentages on the basis of oxides: Al2O3 + SiO2 > 70%, with SiO2 < 70% oxides other than SiO2 and Al2O3: complement to 100%; and - having a particle size distribution such that D90 < 5 millimeters and D10 > 0.5 millimeters, D10 and D90 denoting the particle sizes corresponding to the percentages equal respectively to 10% and 90%, by volume, on the particle size distribution curve of the particulate mixture respectively, said particle sizes being classified in ascending order.

Description

Title of the invention: RECYCLING AGENT technical field

[0001] The invention relates to an annealing agent and an annealing process, in particular for the manufacture of molten refractory blocks. Previous art

[0002] Refractory products are used in various industries, particularly for the manufacture of furnaces, especially in the form of blocks for the manufacture of glass melting furnaces. Among refractory blocks, a distinction is made between fused blocks and sintered blocks.

[0003] Fused blocks, often called "electro-fused" or "cast and melted," are obtained by melting a mixture of suitable raw materials in an electric arc furnace or by any other suitable technique. The molten material is then conventionally poured into a mold, typically made of sand, surrounded by a particulate annealing agent contained in a box. More annealing agent is then added to coat the mold.

[0004] Cooling leads to the solidification of the molten material, which transforms into a block. This cooling procedure is classically called "annealing".

[0005] The annealing agent serves to limit the cooling rate in order to reduce the risk of cracking in the manufactured block. It must also provide mechanical strength to help maintain the mold, and therefore the block, and thus limit deformation. It is known to use, as an annealing agent, alumina powder, sand, hollow beads, chamotte grains, or a mixture thereof. Annealing agents are described, for example, in FR2481691A1.

[0006] There is now a need for a annealing agent with a reduced environmental impact.

[0007] One object of the invention is to meet, at least partially, this need. Summary of the invention

[0008] The invention proposes a particulate mixture intended to serve as an annealing agent, - consisting of sintered ceramic particles having a porosity greater than 20% and a chemical composition such that, in mass percentages on the basis of oxides: Al₂O₃ + SiO₂ > 70%, with SiO₂ < 70% oxides other than SiO2 and Al2O3 (or "other species"): 100% supplement; and - presenting a particle size distribution such that D90 < 5 millimeters and Dio > 0.5 millimeters, Di0 and D90 denoting the particle sizes corresponding to the percentages equal to 10% and 90% respectively, by volume, on the particle size distribution curve of the particulate mixture respectively, said particle sizes being classified in ascending order.

[0009] Preferably, the particulate mixture has a density greater than 0.6, preferably greater than 0.7 and / or less than 1.5.

[0010] Preferably, the particles have an open porosity such that the median diameter of the open pores is less than or equal to one tenth of the Di0 of the particle size distribution.

[0011] The particles are preferably beads, preferably solid beads, preferably with microdispersed porosity.

[0012] The sphericity and the shape of the beads advantageously allow good flow and optimal arrangement of the particles when the particulate mixture is poured to constitute an annealing agent.

[0013] Such density and particle size distribution characteristics correspond to a volume and spatial distribution of the inter-particle spaces. The inventors have found that this combination improves the results obtained, and in particular the effective thermal conductivity at high temperatures, especially at 1300°C.

[0014] A particulate mixture according to the invention may further comprise one or more of the following optional characteristics: - the particulate mixture has an average sphericity greater than 0.85 and / or a density greater than 0.7 and less than 1.5, preferably greater than 1.0 and less than 1.3; - d90-dio < 4 millimeters, preferably D90-Di0 < 3 millimeters, preferably D90-Di0 < 2 millimeters; - D90 is less than 3 millimeters and / or D10 is greater than 0.7 millimeters, and / or D50 is between 1.0 and 2.5 millimeters; - the particulate mixture has a total SiO2 + Al2O3 content greater than 80%, expressed as mass percentages on the basis of oxides, preferably with a chemical composition such that, expressed as mass percentages on the basis of oxides and totaling 100%: SiO2: 25% - 70%; A12O3: 25% - 70%; oxides other than SiO2 and Al2O3: < 15%.

[0015] The invention also relates to an annealing device comprising - a mold designed to receive molten refractory material; and - a particulate mixture according to the invention arranged so as to be in contact with the mold, preferably so as to surround the lateral faces of the mold, over more than 50%, preferably over more than 80%, preferably over more than 90% of their height, preferably over their entire height, preferably so as to completely cover the mold.

[0016] The invention also relates to an annealing process comprising the following steps: a) preparation of an annealing device comprising - a mold designed to receive molten refractory material; and - a particulate mixture or "initial particulate mixture" arranged so as to be in contact with the mold, preferably so as to surround the lateral faces of the mold, over more than 50%, preferably over more than 80%, preferably over more than 90% of their height, preferably over the entire height of said lateral faces, the mold being in particular made of sand; b) pouring the molten material into the mold at a temperature above 1200°C; c) cooling the molten material to a temperature below 100°C, so as to obtain a molten refractory product, classically in the form of a block; the initial particulate mixture being in accordance with the invention.

[0017] Optionally, the particulate mixture is vibrated between step a) and step b).

[0018] Optionally, some of the initial particulate mixture is added between step b) and step c) so as to cover the mold and the molten material during solidification.

[0019] The particulate mixture serves both as a heat-insulating material and as a support material for the mold.

[0020] The invention also relates to a recycling process comprising, after said steps a) to c), the following step: d) recovery and then particle size selection of the initial particulate mixture so as to obtain a final particulate mixture having a particle size distribution such that D90 < 5 millimeters and Di0 > 0.5 millimeters, and then use of the final particulate mixture as an annealing agent, preferably in an annealing process according to the invention.

[0021] Particle size selection can be carried out by any conventional means, for example by means of sieves.

[0022] As will be shown in more detail later in the description, the recovered initial particulate mixture can be handled without any particular risk and can be reused with no other operation than simple particle size selection, for example by sieving. This recycling capability limits the environmental impact and costs. Tests have shown that it has essentially the same effectiveness as the initial particulate mixture.

[0023] Furthermore, the low friability of a particulate mixture according to the invention allows for the generation of very few fine particles when the particulate mixture is used as an annealing agent, thus limiting dust problems during the handling of the annealing agent. Particle size selection therefore primarily serves to remove mold debris (essentially sand grains smaller than 0.5 millimeters and / or debris larger than 5 millimeters) that may be mixed with the annealing agent. The quantity of annealing agent thus remains essentially constant during recycling.

[0024] The invention finally relates to a method for manufacturing a molten refractory product, said method comprising the implementation of an annealing or recycling process according to the invention. Definitions

[0025] A “block” is an object of which at least one dimension, preferably each dimension, is greater than 5 cm, preferably greater than 10 cm, preferably greater than 20 cm, and preferably less than 100 cm.

[0026] Unless otherwise stated, the percentages used to define a composition are mass percentages based on oxides.

[0027] A total content of several oxides, for example SiO2 + Al2O3, does not imply that each of said oxides is present, even if, in one embodiment, each of said oxides is present.

[0028] When reference is made to zirconia or ZrO2, this should be understood to mean (ZrO2 + HfO2), with HfO2 < 5%, preferably HfO2 < 3%, preferably HfO2 < 2%. Indeed, a small amount of HfO2, which is chemically inseparable from ZrO2 and has similar properties, is always naturally present in zirconia sources. Hafnium oxide is therefore not considered an impurity.

[0029] By "impurities" is meant unavoidable constituents, introduced unintentionally and necessarily with the raw materials or resulting from reactions with these constituents. Impurities are not necessary constituents, but merely tolerated. Preferably, the mass quantity of impurities is less than 2%, less than 1%, less than 0.5%, and preferably substantially zero.

[0030] The 10th (denoted Di0), 50th (denoted D50) and 90th (denoted D90) percentiles of the particulate mixture are defined as the particle sizes corresponding to the percentages equal to 10%, 50% and 90%, respectively, by volume, on the cumulative particle size distribution curve of the particulate mixture (or "particle size distribution curve"), respectively. particles being classified in ascending order. According to this definition, 90% by volume of the particles in the particulate mixture have a size greater than or equal to D90 and 10% of the particles, by volume, have a size less than Di0.

[0031] The percentiles can be determined from a particle size distribution obtained using a laser particle size analyzer.

[0032] The fifty percentile is called the "median size" of the particulate mixture.

[0033] The median diameter of the open pores P50 is the pore diameter that divides, in The open pore population is divided into two groups: one group representing 50% of the open pore volume, with open pores having a diameter smaller than the median diameter, and another group representing 50% of the open pore volume, with open pores having a diameter greater than or equal to the median diameter. The distribution of open pore diameters is conventionally determined by mercury porosimetry.

[0034] The median diameter of the open pores is preferably less than 200 microns, preferably less than 150 microns, preferably less than 100 microns.

[0035] Porosity is the sum of open porosity and closed porosity.

[0036] The porosity of a particle is classically said to be "dispersed" when it is distributed in a substantially uniform manner within the particle. In particular, a particle exhibiting dispersed porosity does not have a central cavity, like a hollow bead.

[0037] Preferably, the median diameter of the open pores is less than or equal to one-tenth, or even one-twentieth, or even one-fiftieth, or even one-hundredth of the Di0 of the particle size distribution and / or less than 200 microns, preferably less than 150 microns, preferably less than 100 microns. Preferably, these pores, called "micropores," are dispersed throughout the mass of the particle in question. The pores are therefore distributed throughout the entire particle, unlike hollow beads.

[0038] The SPHT sphericity of a particle is equal to the ratio 4.ir.A / U2 where A is the surface area of ​​the projection of the particle and U is the circumference of the projection of the particle.

[0039] A ball is a particle having a sphericity greater than 0.60.

[0040] A “precursor” of a constituent of a particulate mixture is a raw material which is transformed into said constituent during the manufacture of the refractory mixture.

[0041] The verbs "comprendre" or "comporter" or "présenter" must be interpreted in a non-restrictive manner, unless otherwise indicated. Detailed description Process for manufacturing the particulate mixture

[0042] To manufacture a particulate mixture according to the invention, one can, for example, proceed according to a process comprising the following steps: i) preparation of a starting charge comprising constituents of the ceramic particles of the particulate mixture to be manufactured and / or precursors of these constituents; ii) shaping of said starting charge to obtain a preform; iii) sintering of said preform to obtain sintered ceramic particles.

[0043] In step i), powders of constituents and / or precursors are mixed to form a substantially homogeneous mixture. Those skilled in the art know how to adjust the composition of the initial feed to obtain, at the end of step iii), particles with the desired composition.

[0044] The starting charge may conventionally include, in addition to ceramic raw materials, - an additive which may in particular be chosen from the group consisting of: solvents, preferably water, plasticizers such as polyethylene glycol (or "PEG") or polyvinyl alcohol (or "PAV"), binders including temporary organic binders such as resins, lignosulfonates, carboxymethylcellulose or dextrin, deflocculants, such as alkali metal polyphosphates, alkali metal polyacrylates, polycarboxylates and mixtures of these products the quantity of which is adapted to the shaping method of step ii); preferably, the shaping additive is chosen from the group consisting of plasticizers; - a porogenous agent, the nature and quantity of which are adapted to the characteristics of the ceramic particles expected at the end of step iii); the porogenous agent may in particular be chosen from sawdust, rice hulls, charcoal powder or nut shells.

[0045] In step ii), any conventional shaping process known for the manufacture of porous ceramic particles can be implemented. Among these processes, examples include granulation processes, for example using granulators, in particular intensive mixer-type granulators, fluidized bed granulators, or granulation discs.

[0046] The shaping can in particular result from a granulation process, in particular using an intensive mixer.

[0047] In step iii), the preform is sintered, preferably under air, preferably at atmospheric pressure and at a temperature between 1200°C and 1500°C, preferably above 1250°C and / or below 1450°C.

[0048] To obtain the desired porosity, a person skilled in the art knows how to adjust the particle size of the raw materials, the quantity of porogenic agent, the shaping operation and the sintering conditions. Particulate mixture

[0049] Preferably, more than 80%, more than 85%, more than 90%, more than 95%, more than 97% by mass of the ceramic particles of the particulate mixture each have a sphericity greater than 0.60, more than 0.70, more than 0.80, more than 0.85, more than 0.90, more than 0.92, more than 0.94, more than 0.95.

[0050] The average sphericity of the particulate mixture is preferably greater than 0.80, preferably greater than 0.85, preferably greater than 0.90, preferably greater than 0.92, preferably greater than 0.94, preferably greater than 0.95, preferably greater than 0.97, preferably greater than 0.98.

[0051] The flowability of the particles and the arrangement of the particles when pouring the particulate mixture into the box containing the mold are improved.

[0052] The particles of the particulate mixture may in particular have one or more of the following optional characteristics: - a porosity greater than 25%, preferably greater than 30%, preferably greater than 35% and / or less than 60%, preferably less than 55%; - a closed porosity of less than 10%, or even less than 8%, or even less than 5%, or even less than 3%; - a virtually smooth surface; - a total mass content of oxides greater than 90%, preferably greater than 95%, preferably greater than 99%; - a total mass content of SiO2 + Al2O3 greater than 80%, preferably greater than 85.0%, preferably greater than 90.0%, in mass percentages on the basis of oxides; - a SiO2 mass content of less than 68.0%, preferably less than 66.0%, preferably less than 65.0%, preferably less than 60.0%, preferably less than 55.0%, preferably less than 45.0%, preferably less than 40.0% and / or greater than 5.0%, preferably greater than 10.0%, preferably greater than 14.0%, preferably greater than 18.0%, preferably greater than 20.0%, preferably greater than 25.0%, preferably greater than 28.0%; - a mass content of A12O3 less than 70.0%, preferably less than 65.0%, preferably less than 50.0%, preferably less than 45.0%, preferably less than 43.5%, preferably less than 43.0%, preferably less than 42.0% and / or greater than 20.0%, preferably greater than 24.0%, preferably greater than 26.0%; according to a particular embodiment, a mass content of A12O3 greater than 40.0%, preferably greater than 42.0%; - a total mass content of "other species" of less than 25.0%, preferably less than 20.0%, preferably less than 15.0%, preferably less than 10.0%, preferably less than 5.0%; - a mass content of ZrO2 + HfO2 of less than 20.0%, preferably less than 15.0%, preferably less than 10.0%, preferably less than 5.0%, preferably less than 2.0%; - a mass content of CaO + BaO + SrO + MgO + Fe2O3 + TiO2 + Na2O + K2O greater than 0.5%, preferably greater than 1.0%, preferably greater than 1.5%, preferably greater than 2.0%;

[0053] - a mass content of iron oxide and titanium oxide Fe2O3 + TiO2 less than 5.0%, preferably less than 4.0%, preferably less than 3.0%; - a mass content of calcium oxide, barium oxide, strontium oxide and magnesium oxide, CaO + BaO + SrO + MgO, of less than 5.0%, preferably less than 4.0%, preferably less than 3.0%, preferably less than 2.0%, preferably less than 1.5%; - a mass content of sodium oxide and potassium oxide Na2O + K2O of less than 4.0%, preferably less than 3.0%, preferably less than 2.0%, preferably less than 1.5%; - a chemical composition such that, in mass percentages based on oxides and for a total of 100%: SiO2: 25% - 70%; A12O3: 25% - 70%; oxides other than SiO2 and Al2O3: < 15%; or such SiO2: 25% - 55%; A12O3: 40% - 70%; oxides other than SiO2 and Al2O3: < 15%; or such SiO2: 40% - 70%; A12O3: 25% - 60%; oxides other than SiO2 and Al2O3: < 15%; - for a species other than SiO2, A12O3, ZrO2, HfO2, CaO, BaO, SrO, MgO, Na2O, K2O, Fe2O3 and TiO2, preferably for any species other than SiO2, A12O3, ZrO2, HfO2, CaO, BaO, SrO, MgO, Na2O, K2O, Fe2O3 and TiO2, a mass content of less than 1.0%, preferably less than 0.7%, preferably less than 0.5%, preferably less than 0.2%; - a micro dispersion of open pores, preferably of all pores, whose diameter is less than 200 microns, preferably less than 150 microns, preferably less than 100 microns, preferably less than 50 microns, preferably less than 25 microns, preferably less than 20 microns, preferably less than 15 microns.

[0054] Preferably, the species other than SiO2, Al2O3, ZrO2, HfO2, CaO, BaO, SrO, MgO, Na2O, K2O, Fe2O3 and TiO2, consist only of impurities.

[0055] The particulate mixture may in particular have one or more of the following optional characteristics: - a particle size distribution such that D90 is less than 4.5 millimeters, preferably less than 4.0 millimeters, preferably less than 3.5 millimeters, preferably less than 3.0 millimeters, preferably less than 2.8 millimeters; - a particle size distribution such that the median size D50 is greater than 1.0 millimeter, preferably greater than 1.2 millimeter, preferably greater than 1.3 millimeters, preferably greater than 1.5 millimeters, preferably greater than 1.8 millimeters, preferably greater than 2.0 millimeters and / or less than 3.0 millimeters, preferably less than 2.5 millimeters, preferably less than 2.2 millimeters; - a particle size distribution such that Di0 is greater than 0.6 millimeters, preferably greater than 0.7 millimeters, preferably greater than 0.8 millimeters, preferably greater than 1.0 millimeters; - a particle size distribution such that the difference D90 - Di0 is less than 3.5 millimeters, preferably less than 3.0 millimeters, preferably less than 2.8 millimeters, preferably less than 2.5 millimeters, preferably less than 2.0 millimeters, preferably less than 1.5 millimeters; - a chemical composition exhibiting one or more of the characteristics relating to the composition of ceramic particles mentioned above; - a density greater than 0.6, preferably greater than 0.65, preferably greater than 0.7, preferably greater than 0.8, and / or less than 1.5, less than 1.4, less than 1.3, less than 1.2; - an effective thermal conductivity of less than 1.0 W / (mK), preferably less than 0.9 W / (mK), preferably less than 0.8 W / (mK) at 1300°C; - a contraction coefficient of less than 5.0%, preferably less than 4.0%, preferably less than 3.0%, preferably less than 2.5% at 5 MPa, preferably less than 2.0% under a pressure of 5 MPa.

[0056] Preferably, the particulate mixture consists of a single population of ceramic particles, i.e., all the particles have substantially the same composition and the particle size distribution has a main peak whose amplitude is at least 5 or 10 times greater than that of the other peaks (unimodal distribution). It can thus be more easily recycled. Examples

[0057] The following non-limiting examples are given for the purpose of illustrating the invention. Measurement protocols

[0058] The chemical composition of the particulate mixtures was determined by X-ray fluorescence.

[0059] The particle size distribution was classically carried out using a Camsizer XT laser particle size analyzer marketed by the company Horiba, according to the ISO 13322-2 standard.

[0060] The average sphericity is measured on a batch of particles using a Camsizer XT marketed by the company Horiba.

[0061] The open porosity and the median diameter of open pores of the P50 particles are determined, in a known manner, by mercury porosimetry, according to ISO 15901-1.2005 part 1.

[0062] Helium pycnometry measurements have shown that, for example 3, the closed porosity of the particles is less than 5%.

[0063] The density is the bulk density evaluated by the ratio of the mass of a sample of particulate mixture divided by the volume occupied by that sample.

[0064] Effective thermal conductivity. A cylindrical furnace is used, comprising a water-cooled metal chamber and equipped with a central, cylindrical graphite heating element. The heating element is protected by a sintered alumina tube. The furnace's working volume is thus a cylindrical ring 125 millimeters wide and 400 millimeters high. The working volume is filled with a sample of the particulate mixture to be tested, sandwiched between two layers of fibrous insulation to impose a radial flow of heat ("radial flux"). The heating element has a power rating of 20 kVA, enabling a temperature of 1600°C to be reached at the surface of the alumina tube. The radial flux is measured using two heat flux meters installed on diametrically opposite surfaces of the working volume, at mid-height. Thermal gradients are measured using thermocouples.The test includes a temperature ramp-up phase at a rate of 100°C per hour, followed by 10-hour plateaus every 200°C, between 400°C and 1400°C. The test allows for the evaluation of the annealing agent's performance in service, i.e., at temperatures above 1000°C such as those encountered at the beginning of a stage of [processing / processing]. annealing and taking into account all phenomena (conduction through the material, convection in the pores of the particles and in the interstices between the particles, and radiation). The effective thermal conductivity values ​​at 400°C (Lambda E @ 400) and 1300°C (Lambda E @ 1300) are reported in the table.

[0065] Compaction test. A sample of the particulate mixture to be tested is poured into a cylindrical tube 25 millimeters in diameter to a height of at least 10 centimeters, then vibrated for 30 seconds at 60 Hz with an amplitude of approximately 300 microns on a vibrating table. The sample is then subjected to the pressure of a 25-millimeter-diameter piston moving at a constant speed of 10 millimeters per minute. Force sensors are positioned on either side of the piston. A force of up to 60 kJ is applied to the piston, and "C" is the contraction coefficient, equal to the piston stroke, given as a percentage of the initial height of the sample, between the position corresponding to a stress of 0.5 MPa (value C@0.5 MPa in the table below) or 5 MPa (value C@5 MPa in the table below) and the position corresponding to a stress of one atmosphere (0.1 MPa).

[0066] Friability test. To determine the recyclability of a particulate mixture, a sample consisting of 600 grams (Mi) of the particulate mixture is introduced into a cylinder 80 millimeters in diameter. The cylinder is rotated at 80 revolutions per minute for 4 hours. At the beginning of the test and at the end of each hour, the fraction of particles smaller than 1 millimeter is removed (using a sieve). At the end of the test, the sample is sieved, and the mass Mf of particles remaining on the sieve, i.e., those larger than 1 millimeter, is measured. The friability (F in the table below) is given by the formula (Mi-Mf) / Mi, expressed as a percentage. Particulate mixtures tested

[0067] The particulate mixture of example 1 consists of chamotte (grog) particles.

[0068] The particulate mixture of example 2 is sand marketed by the company TADA.

[0069] The particulate mixtures of examples 3 and 4 were obtained by conventional granulation (mixing of ceramic raw materials with PEG and PVA binders and one or more porogen agent(s) (rice hull and / or carbon black and / or starch), granulation in an intensive granulator, then drying at 70-80°C and sintering at 1200-1300°C).

[0070] Examples 1 and 2 are intended to serve as a basis for comparison for examples 3 and 4 according to the invention. Results

[0071] The results obtained are shown in the following table.

[0072] [Tables 1] 1* 2* 3 4 SiO2 (%) 71.8 96.8 63.8 60.1 A12O3 (%) 20.2 1.7 27.3 28.6 K2O (%) 2.8 0.9 2.1 0.6 Fe2O3 (%) 1.2 0.1 3.1 1.3 MgO (%) 0.6 / 0.6 2.6 TiO2(%) 1.1 / 1.0 0.7 Others (%) 2.3 0.5 2.1 6.1 Djo (mm) 6.0 0.1 1.5 0.8 D50 (mm) 9.7 0.4 2.1 1.4 D90 (mm) 17 0.7 2.5 2.5 D90 - D10 (mm) 11 0.6 1.0 1.7 Average sphericity 0.70 0.84 0.96 0.89 Open porosity (%) 44 <5 35 50 P50 of open pores (pm) 19 ND 0.8 14 Particle diodes / P50 316 ND 1875 57 Density (g / cm3) 0.6 1.6 1.2 0.6 Lambda E. @400 (W / (mK)) 0.20 0.30 0.17 0.20 Lambda E. @ 1300 (W / (mK)) 1.20 1.00 0.60 0.64 C@0.5MPa (%) 0.6 ND 0.1 0.4 C@5MPa ​​(%) 2.5 ND 0.2 2.4 F (%) 41 ND < 1 ND

[0073] * : example outside the invention ND: Not Determined

[0074] Unexpectedly, the examples show that particulate mixtures exhibiting similar effective thermal conductivities at 400°C can exhibit very different effective thermal conductivities at 1300°C. These high-temperature tests clearly demonstrate the remarkable performance of the particulate mixtures according to the invention.

[0075] In particular, the various examples appear to be equivalent at 400°C (or lower temperatures), and the inventors have discovered that the invention makes it possible to achieve a new range of effective thermal conductivities at 1300°C. This results in better control of the annealing process and therefore less stress generation, particularly at the beginning of the annealing operation, when the temperature of the molten material is still very high. This would explain the absence of cracks on the blocks annealed with the annealing agents according to the invention.

[0076] The size of the ceramic particles, their distribution (in particular a tight distribution characterized by a D90 - Di0 < 4.0 millimeters, or even D90 - Di0 < 3.0 millimeters, preferably D90 - Di0 < 2.0 millimeters), their spherical shape, their porosity, and the density of the particle mixture could, in service, prevent the presence of large air-filled regions, whether within the ceramic particles (pores) or between them (interstices between particles) in the particle mixture, and thus limit the phenomena of convection, inter-particle radiation, and conduction. This results in thermal effusivity and effective thermal conductivity such that the blocks exhibit few or no cracks after the annealing operation.

[0077] The results also show the advantage associated with particle sphericity. Preferably, the average sphericity is greater than 0.90, preferably greater than 0.95. The flow and therefore the handling of the particle mixture is facilitated.

[0078] It is also observed that the particulate mixture exhibits equivalent, preferably improved, resistance to compaction, particularly when the density is greater than 0.8, preferably greater than 0.9, and preferably greater than 1.0. Its use as an annealing agent therefore limits mold deformation and thus reduces the machining requirements of the resulting molten block. The good resistance to compaction also limits the production of fine particles. The friability test also demonstrates a significant advantage for the products of the invention: handling a particulate mixture according to the invention leads to a much lower generation of fine particles, or "dust," which reduces the risks during loading or unloading operations in the crate, transport, or sieving of the particulate mixture.Furthermore, the quantity of particles lost in each cycle is very limited, which allows for recycling.

[0079] The low content of particles smaller than 0.5 millimeters, particularly silica, after use limits the health risk associated with fine crystalline silica particles (Example 2 contains more than 90% crystalline silica or cristobalite; Example 4 consists of 30% crystalline silica and 43% mullite). The particulate mixtures according to the invention are therefore particularly well-suited for recycling.

[0080] Of course, the invention is not limited to the examples and embodiments described above.

Claims

Demands

1. Particulate mixture intended to serve as an annealing agent, - consisting of sintered ceramic particles having a porosity greater than 20%, - having a chemical composition such that, in mass percentages on the basis of oxides: A12O3 + SiO2 > 70%, with 5% < SiO2 < 70% oxides other than SiO2 and A12O3: complement to 100%; and - having a particle size distribution such that D90 < 5 millimeters and Di0 > 0.5 millimeters, Di0 and D90 denoting the particle sizes corresponding to the percentages equal to 10% and 90% respectively, by volume, on the particle size distribution curve of the particulate mixture respectively, said particle sizes being ranked in ascending order.

2. Particulate mixture according to the immediately preceding claim, wherein the median diameter of the open pores P50 is less than or equal to Dio / 10.

3. Particulate mixture according to the immediately preceding claim, in which P50 < Diq / 20.

4. Particulate mixture according to any one of the preceding claims, wherein the porosity is greater than 30% and less than 55%.

5. Particulate mixture according to any one of the preceding claims, having an average sphericity greater than 0.

85.

6. Particulate mixture according to any one of the preceding claims, having a density greater than 0.7 and less than

7. Particulate mixture according to the immediately preceding claim, having a density greater than 1.0 and less than 1.

3.

8. Particulate mixture according to any one of the preceding claims, wherein D90-Di0 < 3 millimeters.

9. Particulate mixture according to the immediately preceding claim, in which D90-Di0 < 2 millimeters.

10. Particulate mixture according to any one of the preceding claims, wherein D90 is less than 3 millimeters and / or Di0 is greater than 0.7 millimeters.

11. Particulate mixture according to any one of the preceding claims, having a median size D50 between 1.0 and 2.5 millimeters.

12. Particulate mixture according to any one of the preceding claims, having a total SiO2 + Al2O3 content greater than 80%, in mass percentages on the basis of oxides.

13. Particulate mixture according to any one of the preceding claims, having a chemical composition such that, in mass percentages on the basis of the oxides and for a total of 100%: SiO2: 25% - 70%; Al2O3: 25% - 70%; Oxides other than SiO2 and Al2O3: < 15%.

14. Particulate mixture according to any one of claims 1 to 12, having a chemical composition such that, as a mass percentage on the basis of oxides, 20.0% < Al2O3 < 42.0%.

15. Particulate mixture according to any one of the preceding claims consisting of particles having - an open porosity such that the median diameter of the open pores is less than 200 microns.

16. Particulate mixture according to any one of the preceding claims, wherein the ceramic particles have been sintered at a temperature between 1200°C and 1500°C.

17. Annealing device comprising: - a mold for receiving molten refractory material; and - a particulate mixture - consisting of sintered ceramic particles having a porosity greater than 20%, - having a chemical composition such that, in mass percentages based on oxides: Al₂O₃ + SiO₂ > 70%, with SiO₂ < 70% other than SiO₂ and Al₂O₃: making up 100%; and - having a particle size distribution such that D₁₀ < 5 millimeters and D₂₀ > 0.5 millimeters, D₂₀ and D₁₀ denoting the particle sizes corresponding to percentages equal to 10% and 90%, respectively, by volume, on the particle size distribution curve of the particulate mixture, said particle sizes being classified in ascending order, said refractory mixture being arranged so as to be in contact with the mold, preferably so as to completely cover the mold.

18. Device according to claim 17, wherein the refractory mixture conforms to any one of claims 1 to

19. 10. Annealing process comprising the following steps: a) preparation of an annealing device comprising - a mold for receiving molten refractory material; and - a particulate mixture or "initial particulate mixture" arranged so as to be in contact with the mold, preferably so as to surround the lateral faces of the mold; b) pouring the molten material into the mold at a temperature above 1200°C; c) cooling the molten material to a temperature below 100°C, so as to obtain a molten refractory product; the initial particulate mixture conforming to any one of claims 1 to 16.

20. Recycling process comprising, after an annealing process according to the immediately preceding claim, the following step: d) recovery and then particle size selection of the initial particulate mixture so as to obtain a final particulate mixture having a particle size distribution such that D90 < 5 millimeters and Di0 > 0.5 millimeters, and then use of the final particulate mixture as an annealing agent.