Annealing agent

Abstract

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

Description

[0001] Description

[0002] Title: ANNEALING AGENT

[0003] Technical field

[0004] The invention relates to an annealing agent and an annealing method, in particular for the manufacture of fused refractory blocks.

[0005] Prior art

[0006] Refractory products are used in various industries, including 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.

[0007] Fused blocks, often referred to as "electrocast" or "melted and cast," are produced by melting a mixture of suitable raw materials in an electric arc furnace or other suitable technique. The molten material is then conventionally poured into a mold, typically sand-based, surrounded by a particulate annealing agent placed in a box. Annealing agent is then added to cover the mold.

[0008] Cooling leads to the solidification of the molten material, which turns into a block. This cooling procedure is classically called "annealing".

[0009] The annealing agent's function is to limit the cooling rate in order to reduce the risk of cracking of the manufactured block. It must also be mechanically resistant to help maintain the mold, and therefore the block, and thus limit deformations. It is known to use, as annealing agents, alumina powder, sand, hollow beads, chamotte grains or a mixture thereof. Annealing agents are for example described in ER2481691A1.

[0010] There is now a need for an annealing agent with reduced environmental impact.

[0011] One aim of the invention is to meet, at least partially, this need.

[0012] Summary of the invention

[0013] The invention provides a particulate mixture for use as an annealing agent,

[0014] - consisting of sintered ceramic particles having a porosity greater than 20% and a chemical composition such that, in mass percentages based on the oxides: AI2O3 + SiC>2 > 70%, with S1O2 < 70% oxides other than S1O2 and AI2O3 (or “other species”): complement to 100%; and

[0015] - having a particle size distribution such that D90 < 5 millimeters and D10 > 0.5 millimeters, D10 and D90 designating 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.

[0016] The inventors found that this combination of characteristics leads to a remarkable effective thermal conductivity at over 1000°C, over 1100°C, over 1200°C, or over 1300°C, this effective thermal conductivity being quite different from the effective thermal conductivity at room temperature.

[0017] Preferably, the particulate mixture has a density greater than 0.6, preferably greater than 0.7 and / or less than 1.5.

[0018] 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 D10 of the particle size distribution.

[0019] The particles are preferably beads, preferably solid beads, preferably with microdispersed porosity.

[0020] The sphericity and ball shape advantageously allow good flow and optimal arrangement of the particles when the particulate mixture is poured to form an annealing agent.

[0021] Such density and particle size distribution characteristics correspond to a volume and spatial distribution of interstices between particles. The inventors found that this combination improves the results obtained, and in particular the effective thermal conductivity at high temperatures, especially at 1300°C.

[0022] A particulate mixture according to the invention may also comprise one or more of the following optional characteristics:

[0023] - 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;

[0024] - D90-D10 < 4 millimeters, preferably D90-D10 < 3 millimeters, preferably D90-D10 < 2 millimeters;

[0025] - 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;

[0026] - the particulate mixture has a total SiO2+ AI2O3 content greater than 80%, in mass percentages based on the oxides., preferably a chemical composition such that, in mass percentages based on the oxides and for a total of 100%:

[0027] SiO2: 25% - 70% ;

[0028] AI2O3: 25% - 70%; oxides other than SiO2 and AI2O3: < 15%.

[0029] The median diameter of the open pores P50 is preferably less than or equal to Dio / 10, preferably P50 < Dio / 20. This characteristic reflects a well-distributed porosity in the particles, which limits heat transfer and provides good resistance to compaction.

[0030] The invention also relates to an annealing device comprising

[0031] - a mold intended to receive a molten refractory material; and

[0032] - a particulate mixture according to the invention arranged so as to be in contact with the mold, preferably so as to surround the side 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.

[0033] The invention also relates to an annealing method comprising the following steps: a) preparation of an annealing device comprising

[0034] - a mold intended to receive a molten refractory material; and

[0035] - a particulate mixture or "initial particulate mixture" arranged so as to be in contact with the mold, preferably so as to surround the side 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 side faces, the mold being able in particular to be sand-based; b) pouring, into the mold, the molten material 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, conventionally in the form of a block; the initial particulate mixture being in accordance with the invention.

[0036] Optionally, the particulate mixture is vibrated between step a) and step b). Optionally, initial particulate mixture is added between step b) and step c) so as to cover the mold and the solidifying molten material.

[0037] The particulate mixture serves both as a heat-insulating material and as a support material for the mold.

[0038] The invention also relates to a recycling process comprising, after said steps a) to c), the following step: d) recovery 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 D10 > 0.5 millimeters, then use of the final particulate mixture as an annealing agent, preferably in an annealing process according to the invention.

[0039] The particle size selection can be carried out by any conventional means, for example by means of sieves.

[0040] As will become clear in more detail in the following description, the initial recovered particulate mixture can be handled without any particular risk and can be reused without any operation other than simple particle size selection, for example by sieving. This recycling capacity limits the environmental impact and costs. Tests have shown that it has substantially the same efficiency as the initial particulate mixture.

[0041] Furthermore, the low friability of a particulate mixture according to the invention makes it possible to generate only very few fine particles when using the particulate mixture as an annealing agent and therefore limits dust problems when handling the annealing agent. The particle size selection therefore mainly leads to removing mold debris (essentially sand grains smaller than 0.5 millimeters and / or debris larger than 5 millimeters) which may be mixed with the annealing agent. The quantity of annealing agent thus remains substantially constant during recycling.

[0042] The invention finally relates to a method for manufacturing a molten refractory product, said method comprising the implementation of an annealing or recycling method according to the invention.

[0043] Definitions 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.

[0044] Unless otherwise stated, the percentages used to define a composition are mass percentages based on oxides.

[0045] A total content of several oxides, for example SiCL + AI2O3, does not imply that each of said oxides is present, even if, in one embodiment, each of said oxides is present.

[0046] When referring to zirconia or Z1O2, it is understood to mean (ZrCL+HfCL), with HfO2 < 5%, preferably HfCL < 3%, preferably HfCL < 2%. Indeed, a little HfO2, chemically inseparable from Z1O2 and having similar properties, is always naturally present in zirconia sources. Hafnium oxide is then not considered an impurity.

[0047] 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 only tolerated. Preferably the mass quantity of impurities is less than 2%, less than 1%, less than 0.5%, preferably substantially zero.

[0048] The 10th (denoted D10), 50th (denoted D50) and 90th (denoted D90) percentiles of the particulate mixture are 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 particle sizes of the particulate mixture (or "particle size distribution curve") respectively, said particle sizes being classified in ascending order. According to this definition, 90% by volume of the particles of the particulate mixture thus have a size greater than or equal to D90 and 10% of the particles, by volume, have a size less than D10.

[0049] Percentiles can be determined from a particle size distribution made using a laser particle size analyzer.

[0050] The fifty percentile is called the "median size" of the particle mixture.

[0051] The median diameter of open pores P50 is the pore diameter which divides, by volume, the population of open pores into two groups: one group representing 50% of the open pore volume and whose open pores have a diameter less than the median diameter and another group representing 50% of the open pore volume and whose open pores have a diameter greater than or equal to said median diameter. The distribution of open pore diameters is conventionally determined by mercury porosimetry.

[0052] The median diameter of the open pores is preferably less than 200 microns, preferably less than 150 microns, preferably less than 100 microns.

[0053] Porosity is the sum of open porosity and closed porosity.

[0054] The porosity of a particle is classically said to be "dispersed" when it is distributed in a substantially uniform manner throughout the particle. In particular, a particle with dispersed porosity does not have a central cavity, like a hollow bead.

[0055] 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 Dio 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 in the mass of the particle in question. The pores are therefore distributed throughout the particle, unlike hollow beads.

[0056] The SPHT sphericity of a particle is equal to the ratio 4.KA / U 2where A is the area of ​​the particle projection and U is the circumference of the particle projection.

[0057] A ball is a particle with a sphericity greater than 0.60.

[0058] 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.

[0059] The verbs "to understand" or "to behave" or "to present" should be interpreted in a non-restrictive manner, unless otherwise indicated.

[0060] Detailed description

[0061] Manufacturing process of the particulate mixture

[0062] To manufacture a particulate mixture according to the invention, it is possible, for example, to proceed according to a method 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 said starting charge to obtain a preform; iii) sintering said preform so as to obtain sintered ceramic particles.

[0063] In step i), powders of constituents and / or precursors are mixed so as to constitute a substantially homogeneous mixture. A person skilled in the art knows how to adjust the composition of the starting charge so as to obtain, at the end of step iii), particles having the desired composition.

[0064] The starting charge can typically include, in addition to ceramic raw materials,

[0065] - 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 "PVA"), binders including organic temporary 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;

[0066] - a pore-forming agent, the nature and quantity of which are adapted to the characteristics of the ceramic particles expected at the end of step iii); the pore-forming agent may in particular be chosen from sawdust, rice husk, coal powder or nut shells.

[0067] In step ii), any conventional shaping process known for the manufacture of porous ceramic particles may be implemented. Among these processes, mention may be made, for example, of granulation processes, for example using granulators, in particular intensive mixer-type granulators, fluidized bed granulators or granulation discs.

[0068] Shaping can in particular result from a granulation process, in particular using an intensive mixer.

[0069] In step iii), the preform is sintered, preferably in air, preferably at atmospheric pressure and at a temperature between 1200°C and 1500°C, preferably above 1250°C and / or below 1450°C. To obtain the desired porosity, the person skilled in the art knows how to adjust the particle size of the raw materials, the amount of porogenic agent, the shaping operation and the sintering conditions.

[0070] Particulate mixture

[0071] Preferably, more than 80%, preferably more than 85%, preferably more than 90%, preferably more than 95%, preferably more than 97% by mass of the ceramic particles of the particulate mixture each have a sphericity greater than 0.60, preferably greater than 0.70, 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.

[0072] 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.

[0073] The flowability of the particles and the arrangement of the particles when pouring the particulate mixture into the box containing the mold are improved. The substantially spherical shape of the particles also promotes homogeneity of the spatial distribution of the interstices between the particles, reduced dimensions of these interstices as well as uniformity of the dimensions of these interstices. It helps to limit heat transfer.

[0074] The particles of the particulate mixture may in particular have one or more of the following optional characteristics:

[0075] - a porosity greater than 25%, preferably greater than 30%, preferably greater than 35% and / or less than 60%, preferably less than 55%, or even less than 50%, or even less than 45%, or even less than 40%;

[0076] - closed porosity less than 10%, or even less than 8%, or even less than 5%, or even less than 3%;

[0077] - a substantially smooth surface;

[0078] - a total mass content of oxides greater than 90%, preferably greater than 95%, preferably greater than 99%;

[0079] - a total mass content of SiCh + AI2O3 greater than 80%, preferably greater than 85.0%, preferably greater than 90.0%, in mass percentages based on the oxides;

[0080] - a mass content of SiCh 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%;

[0081] - a mass content of AI2O3 of less than 70.0%, preferably less than 65.0%, preferably less than 50.0%, preferably less than 48.0%, preferably less than 45.0%, preferably less than 43.5%, preferably less than 43.0%, preferably less than 42.0%; according to a particular embodiment, a mass content of AI2O3 of less than 40.0%, preferably less than 35.0%, preferably less than 32.0%, preferably less than 30.0%, preferably less than 29.0%;

[0082] - a mass content of AI2O3 greater than 20.0%, preferably greater than 24.0%, preferably greater than 26.0%; according to a particular embodiment, a mass content of AI2O3 greater than 40.0%, preferably greater than 42.0%;

[0083] - 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%, preferably less than 3.0%, preferably less than 1.0%;

[0084] - a mass content of Z1O2 + HfCh 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%;

[0085] - a mass content of CaO + BaO + SrO + MgO + Fe2O3 + TiCh + Na2 <D + K2O supérieure à 0,5%, de préférence supérieure à 1,0%, de préférence supérieure à 1,5%, de préférence supérieure à 2,0% ;

[0086] - a mass content of iron oxide and titanium oxide Fe2O3 + TiCF of less than 5.0%, preferably less than 4.0%, preferably less than 3.0%;

[0087] - 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%;

[0088] - a mass content of magnesium oxide, 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%;

[0089] - a chemical composition such that, in mass percentages based on oxides and for a total of 100%:

[0090] SiO2: 25% - 70% ;

[0091] AI2O3: 25% - 70%; oxides other than SiO2 and AI2O3: < 15%; or such that

[0092] SiO2: 25% - 55%;

[0093] AI2O3: 40% - 70%; oxides other than SiO2 and AI2O3: < 15%; or such that

[0094] SiO2: 40% - 70% ;

[0095] AI2O3: 25% - 60%; oxides other than SiO2 and AI2O3: < 15%; or such that

[0096] SiO2: 63% - 68% ;

[0097] AI2O3: 27% - 32%; oxides other than SiO2 and AI2O3: < 9%; the oxides other than SiO2 and AI2O3 preferably being FC2O3, in a mass content preferably of between 3 and 5%, K2O, in a mass content preferably of between 2 and 3%, and Na2O, in a mass content preferably of less than 1%;

[0098] - for a species other than SiO2, AI2O3, Z1O2, HfO2, CaO, BaO, SrO, MgO, Na2O, K2O, FC2O3 and TiO2, preferably for any species other than SiO2, AI2O3, Z1O2, HfO2, CaO, BaO, SrO, MgO, Na2O, K2O, FC2O3 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%;

[0099] - a micro dispersion of open pores, preferably of all pores, the diameter of which 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. Preferably, the species other than S1O2, AI2O3, Z1O2, HfCL, CaO, BaO, SrO, MgO, Na2<3, K2O, FC2O3 and T1O2, consist only of impurities.

[0100] In one embodiment, for more than 90%, more than 95%, preferably substantially 100% of their mass, the oxides other than SiCl and AI2O3 consist of one or more oxide(s) chosen from the group formed by Fe2O3, K2O, CaO, TiO2, MgO, P2O5, Na2O, Z1O2, &2O3, MnO and SrO, preferably chosen from the group formed by Fe2O3, K2O, CaO. In one embodiment, for more than 90%, more than 95%, preferably substantially 100% of their mass, the oxides other than SiO2 and AI2O3 consist of FC2O3.

[0101] In one embodiment, AI2O3 and SiCL are in the form of mullite and silica, for more than 90%, more than 95%, preferably substantially 100% of their mass.

[0102] The particulate mixture may in particular have one or more of the following optional characteristics:

[0103] - a particle size distribution such that the 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, and / or greater than

[0104] 2.5 millimeters;

[0105] - 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 millimeter, preferably greater than 1.5 millimeter, preferably greater than 1.8 millimeter, preferably greater than 2.0 millimeter and / or less than 3.0 millimeter, preferably less than 2.5 millimeter, preferably less than 2.2 millimeter;

[0106] - a particle size distribution such that the D10 is greater than 0.6 millimeters, preferably greater than 0.7 millimeters, preferably greater than 0.8 millimeters, preferably greater than 1.0 millimeters, and / or less than 1.6 millimeters, preferably less than 1.4 millimeters;

[0107] - a particle size distribution such that the difference D90 - D10 is less than 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;

[0108] - a chemical composition having one or more of the characteristics relating to the composition of the ceramic particles mentioned above; - a density greater than 0.6, preferably greater than 0.65, preferably greater than 0.7, preferably greater than 0.75, preferably greater than 0.8, and / or less than 1.5, less than 1.4, less than 1.3, less than 1.2;

[0109] - 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;

[0110] - 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.

[0111] 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, the amplitude of which is at least 5 or 10 times greater than that of the other peaks (unimodal distribution). It can thus be more easily recycled.

[0112] Examples

[0113] The following non-limiting examples are given for the purpose of illustrating the invention.

[0114] Measurement protocols

[0115] The chemical composition of the particulate mixtures was determined by X-ray fluorescence.

[0116] The particle size distribution was conventionally carried out using a Camsizer XT laser granulometer marketed by Horiba, according to the ISO 13322-2 standard.

[0117] The average sphericity is measured on a batch of particles using a Camsizer XT marketed by the Horiba company.

[0118] The open porosity and the median diameter of open pores of P50 particles are determined, in a known manner, by mercury porosimetry, according to ISO 15901-1.2005 part 1.

[0119] Helium pycnometry measurements showed that, for example 3, the closed porosity of the particles is less than 5%.

[0120] Density is the bulk density evaluated by the ratio of the mass of a sample of particulate mixture divided by the volume occupied by this sample. Effective thermal conductivity. A cylindrical furnace is used, comprising a water-cooled metal tank and equipped with a central, cylindrical, graphite heating element. The heating element is protected by a sintered alumina tube. The useful volume of the furnace is thus a cylindrical crown 125 millimeters wide and 400 millimeters high. The useful volume is filled with a sample of the particulate mixture to be tested, between two layers of fibrous insulation in order to impose a radial flow on the heat flux ("radial flux"). The power of the heating element is 20 kVA, which makes it possible to reach 1600°C at the surface of the alumina tube.The radial flux is measured using two flux meters installed on the external surface of the useful volume, diametrically opposite and at mid-height of the useful volume. The thermal gradients are measured using thermocouples. The test includes a temperature rise phase at a rate of 100°C per hour, then 10-hour stages every 200°C, between 400°C and 1200°C, then a 10-hour stage at 1300°C. The test makes it possible to evaluate the performance of the annealing agent in service, i.e. at temperatures above 1000°C as encountered at the start of an annealing step and taking into account all the phenomena (conduction through the material, convection in the porosities of the particles and in the interstices between the particles and radiation). The effective thermal conductivity value at 400°C (Lambda E. @400) to 1300°C (Lambda E. @ 1300) are reported in the table.

[0121] 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 at an amplitude of approximately 300 microns on a vibrating table. The sample is then subjected to pressure from a 25 millimeter diameter piston moving at a constant speed of 10 millimeters per minute. Force sensors are placed on either side of the piston. A force of up to 60 kJ is applied to the piston and "C" is the coefficient of contraction 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@0.5MPa in the table below) or 5 MPa (value C@5MPa ​​in the table below) and the position corresponding to a stress of one atmosphere (0.1 MPa).

[0122] Friability test. To determine the recycling capacity of a particulate mixture, a sample of 600 grams (Mi) of particulate mixture is placed in 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 of testing, 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. Friability (F in the table below) is given by the formula (Mi-Mf) / Mi, as a percentage.

[0123] Particulate mixtures tested

[0124] The particulate mixture of Example 1 consists of chamotte (grog) particles.

[0125] The particulate mixture in example 2 is sand marketed by the company TADA.

[0126] The particulate mixtures of Examples 3 and 4 were obtained by conventional granulation (mixing of the ceramic raw materials with PEG and PVA binders and one or more pore-forming agent(s) (rice husk and / or carbon black and / or starch), granulation in an intensive granulator, then drying at 70-80°C and sintering at 1200-1300°C).

[0127] Examples 1 and 2 are intended to serve as comparison bases for Examples 3 and 4 according to the invention. Results

[0128] The results obtained are shown in the following table.

[0129] [Table 1]

[0130] *: example outside of invention

[0131] ND: Not Determined

[0132] Unexpectedly, the examples show that particulate mixtures with similar effective thermal conductivities at 400°C can have very different effective thermal conductivities at 1300°C. These high-temperature tests clearly highlight the remarkable performance of the particulate mixtures according to the invention.

[0133] 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 and therefore less generation of stresses, particularly at the start 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.

[0134] The size of the ceramic particles, their distribution (in particular a narrow distribution characterized by a D90 - D10 < 4.0 millimeters, or even D90 - D10 < 3.0 millimeters, preferably D90 - D10 < 2.0 millimeters), their spherical shape, their porosity as well as the density of the particulate mixture could make it possible, in service, to avoid the presence of large air-filled regions, whether inside the ceramic particles (pores) or between them (interstices between the particles) in the particulate mixture, and thus to limit the phenomena of convection, inter-particle radiation and conduction. This results in thermal effusivity and effective thermal conductivity such that the blocks have no or only few cracks at the end of the annealing operation.

[0135] 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 particulate mixture is facilitated.

[0136] It is also found that the particulate mixture has an equivalent, preferably improved, resistance to compaction, in particular when the density is greater than 0.8, preferably greater than 0.9, preferably greater than 1.0. Its use as an annealing agent therefore makes it possible to limit the deformation of the mold and therefore of the block, and thus to reduce the machining requirements of the molten block obtained. The good resistance to compaction also makes it possible to limit the production of fine particles. The friability test also shows a remarkable advantage for the products of the invention: the handling of a particulate mixture according to the invention leads to a much lower generation of fine particles, or "dust", which limits the risks during loading or unloading operations in the box, transport or sieving of the particulate mixture.Furthermore, the quantity of particles lost in each cycle is very limited, which allows recycling.

[0137] The low content of particles smaller than 0.5 millimeters, particularly silica, after use limits the health risk associated with fine particles of crystalline silica (Example 2 has 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.

[0138] Of course, the invention is not limited to the examples and embodiments described above.

Claims

CLAIMS 1. Particulate mixture intended to serve as an annealing agent, - made of sintered ceramic particles with a porosity greater than 20%, - having a chemical composition such that, in mass percentages based on oxides: AI2O3 + SiO2 > 70%, with 5% < SiO2 < 70% oxides other than SiO2 and AI2O3: complement to 100%; and - having a particle size distribution such that D90 < 5 millimeters and D10 > 0.5 millimeters, D10 and D90 designating 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.

2. Particulate mixture according to the immediately preceding claim, in which 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, wherein P50 < Dio / 20.

4. A particulate mixture according to any preceding claim, 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 1.

5.

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

3.

8. A particulate mixture according to any preceding claim, wherein D90-D10 < 3 millimeters.

9. Particulate mixture according to the immediately preceding claim, wherein D90-D10 < 2 millimeters.

10. A particulate mixture according to any preceding claim, wherein D90 is less than 3 millimeters and / or D10 is greater than 0.7 millimeters.

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

12. Particulate mixture according to any one of the preceding claims, having a total SiO2+ AI2O3 content greater than 80%, in mass percentages based on the oxides.

13. Particulate mixture according to any one of the preceding claims, having a chemical composition such that, in mass percentages based on the oxides and for a total of 100%: SiO2: 25% - 70% ; AI2O3: 25% - 70%; Oxides other than SiO2 and AI2O3: < 15%.

14. Particulate mixture according to any one of claims 1 to 12, having a chemical composition such that, in mass percentage based on oxides, 20.0% < AI2O3 < 42.0%.

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

16. A particulate mixture according to any preceding claim, wherein the ceramic particles have been sintered at a temperature above 1200°C.

17. An annealing agent consisting of a particulate mixture according to any one of the preceding claims.

18. Annealing device comprising - a mold intended to receive a molten refractory material; and - a particulate mixture - made of sintered ceramic particles with a porosity greater than 20%, - having a chemical composition such that, in mass percentages based on oxides: AI2O3 + SiO2 > 70%, with SiCh < 70% other than SiCh and AI2O3: complement to 100%; and - having a particle size distribution such that D90 < 5 millimeters and D10 > 0.5 millimeters, D10 and D90 designating 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, said refractory mixture being arranged so as to be in contact with the mold, preferably so as to completely cover the mold.

19. A device according to any preceding claim, wherein the refractory mixture is in accordance with any one of claims 1 to 16.

20. Annealing process comprising the following steps: a) preparation of an annealing device comprising - a mold intended to receive a 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 side faces of the mold; b) pouring, into the mold, the molten material 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 being in accordance with any one of claims 1 to 16.

21. Recycling process comprising, after an annealing process according to the immediately preceding claim, the following step: d) recovery then granulometric selection of the initial particulate mixture so as to obtain a final particulate mixture having a distribution of the particle size such that D90 < 5 millimeters and D10 > 0.5 millimeters, then using the final particulate mixture as an annealing agent.

Images