Dvc for a smelting furnace lining

Abstract

The invention relates to a pulverulent mixture for producing a protective lining of a metal smelting furnace, wherein the mixture consists, for a total of 100% and as weight percentages on the basis of the weight of the mixture, of: - less than 1% by weight of water, and - a group of particles distributed between - an aggregate fraction and - a matrix fraction, the whole of the matrix fraction and of said liquid comprising a heat-activated binder, the heat-activated binder comprising, preferably consisting of, a phosphate selected from phosphates devoid of sodium, potassium and lithium, referred to as a "non-alkali metal phosphate", the chemical composition of the whole of the matrix fraction and of the liquid being such that 0.5% < P2O5 < 5% and Na2O + K2O + Li2O < 1.0%, as weight percentages based on the weight of said mixture.

Description

[0001] Description

[0002] Title: DVC for lining a foundry furnace

[0003] Technical field

[0004] The invention relates to a dry powdery mixture of the dry rammed earth type, hereinafter "DVC" (in English "dry vibratable mix" or "dry refractory"). A DVC according to the invention is in particular intended for the manufacture of a furnace lining, for example an induction furnace or an arc furnace, in particular for the melting of metals.

[0005] The invention also relates to a consolidated product obtained from a DVC according to the invention and to a method of manufacturing such a product.

[0006] State of the art

[0007] A protective coating is conventionally provided on the inner surface of the heating chamber of furnaces used for melting metals, at least in the part of this surface in contact with molten metal. It must in particular have:

[0008] - good corrosion resistance to molten metal;

[0009] - good resistance to cracking;

[0010] - good resistance to infiltration by molten metal; and

[0011] - good resistance to thermal shocks.

[0012] For this purpose, products obtained by thermal consolidation of a powdery mixture of the DVC type are used, as described for example in EP 1 224 153.

[0013] A DVC is a mixture which, unlike concrete, can be implemented "dry", that is to say without the addition of water or liquid binder, or, rarely, with a very small quantity of water or liquid binder, typically less than 5%, preferably less than 3%, or even less than 2%. This is why, unlike concrete, a DVC does not traditionally contain any binder capable of setting by the addition of water. Any lessons which could be learned from the study of documents relating to concrete are therefore not, a priori, transposable to a DVC.

[0014] The shaping of a DVC typically results from simple compaction at room temperature (20°C), with consolidation resulting from subsequent heat treatment.

[0015] Also conventionally, a DVC consists of refractory particles and particles of a heat-activated binder. The temperature of the consolidation heat treatment is between the melting temperature of the heat-activated binder and that of the refractory particles. During the consolidation heat treatment, the heat-activated binder can thus pass from the solid state to a viscous liquid state allowing adhesion to the refractory particles and a bond between them. The change from the solid state to this viscous liquid state is called "activation" of the binder. The product thus obtained is called "consolidated product". The heat-activated binder is also chosen so that it can be in this viscous liquid state at a temperature close to the operating temperature of the furnace, in particular during the first temperature increase.This viscous liquid state thus advantageously makes it possible to reduce the rigidity of the consolidated product, facilitating its deformation and thus increasing its capacity to adapt to local thermomechanical constraints. During subsequent temperature increases, the consolidated product can become rigid through enrichment in fine particles. EP 1 224 153 indicates that a heat-activatable binder such as boron oxide or boric acid is preferable because it is effective and inexpensive.

[0016] Conventionally, the lining of a furnace is obtained by a process comprising the following successive steps: a') preparation of a starting charge from a DVC; b') shaping of said starting charge, in particular by pressing, tamping or vibration, in the form of a layer; c') heat treatment of at least part of the shaped starting charge in order to

[0017] - to activate the heat-activatable binder in said part, and thus obtain a consolidated product in said part,

[0018] - or even when the temperature reached is sufficient, typically above 1000°C, or even above 1300°C, to decompose the hot-activatable binder and to sinter, by formation of a direct bond between the refractory particles (by partial fusion of the refractory particles) or formation of at least one ceramic binder phase by reaction of at least a portion of said refractory particles with each other.

[0019] Sintering can result in particular from the temperature in the furnace enclosure during use, the heat being provided by the furnace's heating system, or by an additional heat source, for example an auxiliary burner.

[0020] At the end of this process, the coating obtained thus comprises, successively, from a cold face opposite the hot face in contact with the heated enclosure of the furnace:

[0021] - a rear layer, unconsolidated, i.e. remaining in the initial powdery state, typically representing between 10% and 40% of the thickness of the coating;

[0022] - a layer consisting of consolidated product and / or sintered product, typically representing between 60% and 90% of the thickness of the coating.

[0023] In this last layer, we can distinguish a strongly bonded front layer, which will be exposed to the molten metal, and, behind this front layer, an intermediate layer with a weaker bond and less mechanical resistance.

[0024] The middle layer, however, is sufficiently bonded to be self-supporting (or "self-supporting").

[0025] If the front layer is sintered, it is substantially free of heat-activated binder, the heat-activated binder having been decomposed. The intermediate layer then typically has a structure consolidated by the action of the heat-activated binder, the temperature in this layer not having been high enough to destroy the bond formed by the heat-activated binder.

[0026] The article “Spinel formation in careless induction furnace linings”, Saikia et Al, Proceedings of the 4 thInternational Symposium on Advances in Refractories for the Metallurgical Industries, p 827-840 (2004) describes DVCs based on alumina and / or MgAhOr spinel. Sintered products obtained from these DVCs have good resistance to corrosion by molten metals. However, they have limited resistance to thermal shocks, particularly in the case of the melting of light alloys or aluminum alloys, which cause severe thermal shocks. However, thermal shocks create cracks that cause harmful infiltration phenomena by molten metals.

[0027] WO2018 / 00002068 A1 describes a DVC comprising a phosphatic binder and a feldspar. The feldspar provides a non-sintered back layer of sufficient thickness to stop any infiltration of molten metal. The presence of feldspar also facilitates dismantling of the furnace. The binder, preferably of the sodium or potassium phosphate type, works in synergy with the feldspar to improve resistance to infiltration and wear. Examples made with this DVC require a quartz filler, the fine fraction of which can pose health and safety problems. Tests have also shown that the lining made with such a DVC gradually erodes in service, which limits its lifespan. Finally, in the event of repair of the lining, it is difficult to regularly fix the repair material, which affects the lifespan of the repair.

[0028] There is a continuing need to improve the service life of a metal melting furnace lining, particularly a foundry furnace, especially an induction furnace, especially in the event of repair.

[0029] One aim of the invention is to provide a DVC making it possible to meet, at least partially, this need.

[0030] Summary of the invention

[0031] The invention relates to a powder mixture, or DVC, intended for the manufacture of a protective coating for a metal foundry furnace, the DVC being made up, for a total of 100% and in mass percentages based on the mass of the DVC, of:

[0032] - less than 5% by mass of a liquid, and

[0033] - a set of particles, the maximum particle size of said set of particles being less than 10 mm, more than 70%, preferably more than 80%, preferably more than 90% of said particles being made of a refractory material, in mass percentage based on said set of particles, said particles being distributed between

[0034] - a fraction of aggregate consisting of said particles having a size greater than or equal to 200 micrometers, called “grains”, more than 90%, preferably more than 95%, preferably more than 99%, preferably 100% of the grains preferably being made of a refractory material, in mass percentage based on said grains, and

[0035] - a matrix fraction consisting of said particles having a size of less than 200 micrometers, called "fine particles", representing between 5% and 40% of the mass of said DVC, the whole of the matrix fraction and of said liquid (optional) comprising a heat-activatable binder, the heat-activatable binder comprising, preferably consisting of a phosphate chosen from phosphates devoid of sodium, potassium and lithium, called "non-alkaline phosphate", said non-alkaline phosphate representing more than 0.5%, in mass percentages based on the mass of the DVC, and the chemical composition of the whole of the matrix fraction and of the liquid being such that

[0036] 0.5% < P2O5 < 5% and

[0037] Na2O + K2O + Li2O < 1.0%, in mass percentages based on the mass of the DVC.

[0038] The condition P2Os < 5% indirectly imposes a maximum content of so-called non-alkaline phosphate.

[0039] As will be seen in more detail in the remainder of the description, the inventors have discovered that a DVC according to the invention leads to a coating which has an improved service life, including in the event of repair. In particular, it is very resistant to corrosion and thermal shock.

[0040] The inventors noted, without being able to explain it theoretically, that the intermediate layer, between the front layer and the back layer, presented remarkable homogeneity, particularly in terms of mechanical resistance and pore size.

[0041] In particular, tests carried out by the present inventors have shown that the intermediate layer obtained with the mixture of phosphatic binder and feldspar described in WO2018 / 00002068 Al is much more heterogeneous than that of a consolidated product manufactured under the same conditions (in particular under identical vibration or tamping conditions and with the same batch of raw materials for the refractory particles), but with a DVC according to the invention. The coating obtained according to the teaching of WO2018 / 00002068 Al erodes more quickly in service depending on the stresses of the furnace in a context where the user is looking for maximum service life. The repairs carried out with a DVC according to the invention are also of better quality, thus extending the service life of the furnace. Advantageously, these performances are obtained without it being necessary to add feldspar, which is detrimental to corrosion resistance.

[0042] The specific homogeneity of a consolidated product according to the invention would therefore explain the results obtained.

[0043] In a preferred embodiment, more than 90%, more than 95%, preferably more than 99% of the heat-activatable binder, in mass percentage, is in the matrix fraction. Preferably, 100% of the heat-activatable binder is in the matrix fraction, i.e. the liquid does not contain heat-activatable binder. Preferably, the composition of the liquid does not include P2Os, Na2O, K2O, or Li2O. The content of said liquid, preferably water, is preferably less than 1%, preferably less than 0.5%, in mass percentage based on the mass of the DVC.

[0044] A DVC according to the invention may further comprise one or more of the following optional characteristics: - the DVC comprises, in mass percentage based on the mass of the DVC, less than 5%, preferably less than 4%, preferably less than 3%, preferably less than 2%, preferably less than 1.5%, preferably less than 1%, preferably less than 0.5%, by mass of liquid, which is particularly advantageous for avoiding detrimental reactions in applications to a foundry furnace;

[0045] - the DVC, preferably said heat-activatable binder comprises less than 5%, less than 3%, preferably less than 1%, preferably less than 0.9%, preferably less than 0.5%, as a mass percentage based on the DVC, preferably does not comprise a phosphate comprising sodium and / or potassium and / or lithium, called “alkaline phosphate”;

[0046] - the DVC comprises less than 5%, less than 3%, preferably less than 1%, preferably less than 0.9%, preferably less than 0.5%, preferably does not comprise any other heat-activatable binder containing phosphorus than said non-alkaline phosphate, preferably only comprises said non-alkaline phosphate as heat-activatable binder, in mass percentages based on the DVC;

[0047] - said non-alkaline phosphate is exclusively in particulate form, so that it does not provide liquid;

[0048] - said non-alkaline phosphate is a phosphate, preferably a monophosphate, of at least one element chosen from aluminum, zirconium, calcium, magnesium, strontium, barium, boron, zinc, and copper;

[0049] - the heat-activatable binder is a non-alkaline monophosphate of an element chosen from aluminum, zirconium, calcium, magnesium, strontium, barium, boron, zinc, and copper;

[0050] - the matrix fraction comprises, as its main constituent, particles of an oxide of a said element, the hot-activatable binder being a non-alkaline phosphate of said element;

[0051] - the mass content of free SiCh is less than 10%;

[0052] - preferably, the mass content of feldspar in the heat-activatable binder is less than 0.9%, preferably less than 0.5%, preferably less than 0.2%, as a mass percentage based on the mass of the heat-activatable binder, preferably zero; advantageously the risk of creating a glass with a low melting point is reduced;

[0053] - preferably, the mass content of feldspar in the DVC is preferably zero; advantageously the risk of creating a glass with a low melting point is reduced;

[0054] - more than 90% of said particles and / or fine particles and / or grains are made of a refractory material, in mass percentage based on said set of particles, fine particles or grains, respectively, said refractory material being selected from the group formed by alumina, magnesia, chromium oxide, silica, bauxite, mullite, zirconia, partially stabilized zirconia, stabilized zirconia, mullite-zirconia, alumina-zirconia, magnesia-alumina spinel, zircon, cordierite, aluminum titanate, clayey chamottes containing between 30 and 50% alumina, wollastonite, alumina-zirconia-silica materials, alumina-zirconia-silica-chromium oxide materials, bauxite, zirconias reinforced by alumina, alumina-titanium oxide-zirconia materials, and mixtures thereof;- the DVC comprises a consolidation additive chosen from the group formed by borates, in particular fluoroborates, cryolite, fluoride salts, silicate compounds, magnesium chlorides, colemanite, clay, kaolin, amorphous silica, in particular silica fume, phenolic resins, furanic resins, ceramic frits, and mixtures thereof;

[0055] - the matrix fraction comprises a mullite precursor chosen from cyanite, andalusite, clay, silica in the presence of alumina;

[0056] - at least 50% by mass of the particles of the aggregate fraction are made of a material identical to that of the particles of the matrix fraction;

[0057] - the DVC does not contain any hydraulic binder or temporary organic binder;

[0058] - the total quantity of refractory grains, refractory fine particles, heat-activatable binder particles, anti-dust agent particles and anti-wetting agent particles is greater than 95%, preferably greater than 98%, as a mass percentage based on the mixture.

[0059] The invention also relates to a method for manufacturing a consolidated product according to the invention comprising the following successive steps: a) preparation of a starting charge from a DVC according to the invention; b) shaping said starting charge, in particular by pressing, tamping or vibration; c) heat treatment of at least part of the shaped starting charge at a temperature suitable for activating the heat-activatable binder, or even for sintering the refractory particles (i.e. into a refractory material).

[0060] The heat treatment is preferably carried out at a temperature between 400°C and 1600°C, preferably between 500°C and 1500°C.

[0061] The invention also relates to a consolidated product in which the refractory particles are bound by said heat-activatable binder, and to a sintered product in which the refractory particles are bound by sintering, the consolidated product and the sintered product preferably being obtained by a method according to the invention.

[0062] The invention also relates to the use of a consolidated or sintered product according to the invention in an application in which at least part of said product is subjected to a service temperature lower than the maximum temperature of said heat treatment.

[0063] The invention also relates to a metal melting furnace comprising at least one region consisting, at least in part, of a consolidated product according to the invention and / or of a sintered product according to the invention, in particular manufactured according to a method according to the invention, in particular a region intended to be in contact with a molten metal.

[0064] The furnace may in particular be a furnace for melting aluminum, aluminum alloys, such as for example the reference alloy 42200 according to standard NF EN 1706, magnesium alloys, zinc alloys or copper alloys. Of course, the nature of the refractory grains, preferably the nature of the refractory particles of the DVD, is determined so that the melting temperature of said grains, preferably of said particles respectively, is higher than the temperature to which it is intended to subject them during the manufacture and operation of the furnace.

[0065] The invention finally relates to a method for manufacturing a furnace, in particular a metal melting furnace, in which a crucible is manufactured comprising a product according to the invention, in particular in a region intended to come into contact with molten metal.

[0066] Brief description of the figures

[0067] Other characteristics and advantages of the invention will become apparent upon reading the detailed description which follows and upon examining the attached drawing in which:

[0068] - [Fig 1] illustrates a mounting on an oven door used in the examples;

[0069] - [Fig 2] represents the sectional view of an oven equipped with a door and a casing as described in the examples; and

[0070] - [Fig 3] illustrates a manufacturing method according to the invention.

[0071] This setup allows the sintering of a refractory lining under a thermal gradient representative of the operating conditions of a melting furnace. It allows the level of heterogeneity of a lining to be assessed in a much more precise manner than in a foundry furnace.

[0072] Definitions

[0073] “DVC” means a dry particulate mixture, or “powder mixture”. “Dry” means that the liquid content is less than 5%. Preferably, the water content, as measured by a moisture meter, is less than 1%.

[0074] By "heat-activatable binder" we mean a constituent which, under the effect of an increase in temperature, will form a phase capable of binding or agglomerating the particles of a DVC, i.e. capable of leading to a structured (self-supporting) product.

[0075] The solid elements of a DVC are called "particles". The particles of the matrix fraction are called "fine particles" and the particles of the aggregate are called "grains".

[0076] The "size" of a particle is conventionally given by a particle size distribution characterization. To determine the particle size distribution of a DVC according to the invention, it can be sieved through a square mesh sieve adapted to retain only grains, for example 1 mm square mesh. The distribution of the fraction having passed through the sieve can be determined with a laser particle size analyzer. A laser particle size analyzer allows the measurement of sizes less than or equal to 5 mm.

[0077] The 10 (Dio), 50 (D50), 90 (D90) and 99.5 (099.5) percentiles or "percentiles" of all particles in a DVC are the particle sizes corresponding to the percentages, by mass, of 10%, 50%, 90% and 99.5% respectively, on the cumulative particle size distribution curve of the DVC, with the particle sizes listed in ascending order. For example, 10%, by mass, of the particles in the DVC are smaller than Dio and 90% of the particles by mass are larger than Dio. The percentiles can be determined using a particle size distribution performed using a laser particle size analyzer.

[0078] The 99.5th percentile (099.5) of the DVC is called the "maximum size".

[0079] The D50 percentile is called the "median size", that is to say the size dividing the particles into first and second populations of equal mass, these first and second populations containing only particles having a size greater than, or less than, respectively, the median size.

[0080] Conventional techniques known to those skilled in the art make it possible to determine chemical compositions, typically by X-ray fluorescence analysis or by ICP, and crystallographic compositions, typically by X-ray diffraction analysis.

[0081] "Free" silica classically refers to a phase of SiO2 whose molecules are not associated with other molecules, for example in the form of refractory silicate, notably zircon or mullite. It can be crystallized silica in the form of quartz, for example. The measurement of free silica is carried out according to well-known methods.

[0082] To determine the chemical composition of the matrix fraction, the person skilled in the art knows how to extract by sieving the fraction passing through 200 pm on which to carry out the analyses mentioned above.

[0083] 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%, or even substantially zero.

[0084] By "refractory material" is meant a material having a melting temperature above 1500°C. This definition is commonly used by those skilled in the art and cited in "Refractory materials and technical ceramics (elements of ceramurgical and technology)", G. Aliprandi, Septima Paris, 1979. This work also gives on pages 297 to 301 examples of refractory materials, in particular oxides, carbides and nitrides, possible for a DVC according to the invention. For the present invention, carbon C and silicon carbide SiC are however not considered as refractory materials.

[0085] By “temporary” we mean “removed from the product during the consolidation heat treatment”.

[0086] Unless otherwise stated, all mass percentages are expressed relative to the mass of the DVC.

[0087] A sum of oxides, e.g., Na2O+K2O+Li2O, represents the total content of these oxides, but does not imply that every single oxide is present.

[0088] Conventionally, the characteristics relating to a state of matter (melting temperature, viscosity, etc.) are measured at a pressure of 1 bar, unless otherwise indicated. Detailed description

[0089] DVC

[0090] A DVC consists of a set of particles and optionally a liquid. It preferably comprises less than 5%, preferably less than 4%, preferably less than 3%, preferably less than 2%, preferably less than 1.5%, preferably less than 1%, preferably less than 0.5%, preferably less than 0.1%, of liquid, in mass percentage.

[0091] The liquid, and in particular water, typically comes from additives, or even from the heat-activatable binder.

[0092] In particular, it comprises less than 1%, preferably less than 0.5%, preferably less than 0.1%, of water, as a percentage by mass.

[0093] Preferably, the particle assembly preferably consists of only the mixture of refractory particles, heat-activatable binder particles and, optionally, optional additives, such as a consolidation additive, an anti-dusting agent or an anti-wetting agent.

[0094] Preferably, the DVC consists of more than 95%, or even more than 98%, or even substantially 100%, of refractory particles, heat-activatable binder particles, anti-dust agent particles and anti-wetting agent particles, the possible remainder to 100% being made up of impurities, for example iron from a grinding step, or even traces of moisture. All the refractory particles used according to the prior art can be considered, for the matrix fraction as well as for the aggregate fraction. The refractory particles can in particular be made of a fused ceramic product or a sintered ceramic product.

[0095] The nature of the refractory particles is preferably identical in the matrix fraction and in the aggregate fraction.

[0096] However, the mass content of free SiCL is preferably less than 10%, preferably less than 5%, more preferably less than 2% by mass percentage based on the DVC, which promotes resistance to corrosion by molten metals.

[0097] In one embodiment, the DVC comprises more than 0.1% and / or less than 2%, or even less than 1.5% of potassium fluoroborate (KBF4). KBF4 advantageously acts as a consolidating agent for consolidating the material at low temperature.

[0098] These impurities preferably represent less than 3%, less than 2%, or even less than 1%, as a mass percentage based on the DVC. Preferably the mass content of iron and / or the mass content of titanium is / are less than 2%, preferably less than 1%, preferably less than 0.5%.

[0099] Preferably, the DVC does not contain a hydraulic binder or an organic binder except, optionally, a resin. In a first embodiment, the DVC contains neither a hydraulic binder nor an organic binder except, optionally, a resin; in a second embodiment, the DVC contains neither a hydraulic binder nor an organic binder.

[0100] Preferably, the particle size distribution of a DVC according to the invention is adapted to promote its compaction. Compaction models such as the Fuller-Bolomey model or the Andreasen model can be used to determine the most suitable particle size distribution.

[0101] Preferably, the maximum particle size 099.5 of said set of particles is less than or equal to 10 mm, preferably less than or equal to 8 mm, preferably less than or equal to 6 mm, preferably less than 5 mm, preferably less than 4 mm, preferably less than 2 mm, preferably less than 1 mm.

[0102] The amount of particles larger than 300 in is preferably greater than 50% and / or less than 70%, as a mass percentage based on all the particles of the DVC.

[0103] Preferably, the refractory particles and the particles of the heat-activatable binder together represent more than 81%, more than 85%, more than 90%, more than 93%, or even more than 95%, or even more than 98% of the mass of said DVC.

[0104] Matrix fraction

[0105] Preferably, the matrix fraction represents more than 15%, preferably more than 20%, preferably more than 20% and / or less than 38%, preferably less than 35% of the mass of said set of particles.

[0106] Particles smaller than 75 μm preferably represent more than 10% and less than 35% of the mass of said set of particles.

[0107] Preferably, more than 80%, preferably more than 90% of the fine particles are made of a refractory material. Preferably, all the fine particles, with the exception of the fine particles of heat-activatable binder and the fine particles of optional additive, are refractory particles (i.e., made of a refractory material). The amount of refractory fine particles is preferably greater than 10%, preferably greater than 15%, and / or less than 27%, less than 25%, as a mass percentage based on the DVC.

[0108] Said refractory material is selected from the group consisting of alumina, magnesia, chromium oxide, silica, bauxite, mullite, zirconia, partially stabilized zirconia, stabilized zirconia, mullite-zirconia, alumina-zirconia, magnesia-alumina spinel, zircon, cordierite, aluminum titanate, clayey chamottes containing between 30% and 50% alumina, preferably between 35% and 45% alumina, wollastonite, alumina-zirconia-silica (or "AZS") materials, alumina-zirconia-silica-chromium oxide (or "AZS-Cr") materials, bauxite, zirconias reinforced with alumina, alumina-titanium oxide-zirconia materials, and their mixtures.

[0109] Preferably, said refractory material is selected from the group consisting of alumina, mullite, clayey chamottes containing between 30% and 50% alumina, preferably between 35% and 45% alumina, bauxite, and mixtures thereof. More preferably, said refractory material is selected from the group consisting of alumina, mullite, and mixtures thereof.

[0110] Preferably, the mass content of AI2O3 in the matrix fraction is greater than 50%.

[0111] Preferably, the mass content of P2C>5 in the matrix fraction is greater than 0.5%, preferably greater than 1%, and / or less than 15%, preferably less than 10%, preferably less than 5%, more preferably less than 4%.

[0112] More preferably, the mass ratio P2O5 / (AhCh+ZrC +CaO+MgO+SrO+BaO) of the matrix fraction is greater than 5%, preferably greater than 5.5% and / or less than 10%, preferably less than 7.5%.

[0113] In a particularly advantageous embodiment, the matrix fraction comprises, on the mass of the DVC, more than 0.1%, preferably more than 0.5%, preferably more than 1%, or even more than 2% and / or less than 20%, preferably less than 15%, more preferably less than 10%, or even less than 5% of a mullite precursor chosen from cyanite, preferably raw, andalusite, clay, silica in the presence of alumina. With said non-alkaline phosphate, preferably non-alkaline mono-element phosphate, such a precursor makes it possible, after heat treatment for consolidation of the DVC, to obtain a binder phase that is even better distributed in the consolidated product, in particular in said intermediate layer. Preferably the mass ratio of P2O5 / (AhCh+SiCh) in the matrix fraction is between 0.05 and 0.25, preferably between 0.05 and 0.15, or even less than 0.10 or less than 0.08.

[0114] Heat-activated binder

[0115] The DVC comprises more than 0.5%, preferably more than 1%, more than 1.5%, more than 1.8%, and less than 20%, preferably less than 10%, less than 5%, less than 4%, less than 3%, or even less than 2.5% of non-alkaline phosphate, as a mass percentage based on the DVC.

[0116] The DVC comprises more than 0.5%, preferably more than 1%, more than 1.5%, more than 1.8%, and less than 20%, preferably less than 10%, less than 5%, less than 3%, or even less than 2.5% of heat-activatable binder, as a mass percentage based on the DVC.

[0117] Preferably, the non-alkaline phosphate, preferably the heat-activatable binder, has a loss on ignition measured at 900°C of less than 20%.

[0118] The non-alkaline phosphate, preferably the heat-activatable binder, may be liquid or solid, preferably solid, in particulate form. Preferably, the non-alkaline phosphate, preferably the heat-activatable binder, is in the form of a powder of particles, preferably fine particles, i.e. a powder whose median particle diameter is less than 200 micrometers.

[0119] The aggregate fraction comprises less than 20%, less than 10%, or even less than 5% of particles of said hot-activatable binder, called "binder grains", as a mass percentage based on the total quantity of hot-activatable binder, or even substantially no binder grains. Preferably, the quantity of fine non-alkaline phosphate particles, preferably hot-activatable binder, is preferably greater than 1%, greater than 1.5%, and / or preferably less than 5%, preferably less than 4%, or even less than 3.5%, as a mass percentage based on the DVC.

[0120] According to the invention, the non-alkaline phosphate comprises, preferably is constituted by a phosphate of at least one non-alkaline element, preferably chosen from aluminum, zirconium, calcium, magnesium, strontium, barium, boron, zinc, and copper, preferably chosen from aluminum, zirconium, calcium, magnesium and strontium. Said non-alkaline phosphate comprises, preferably is a monophosphate of said element (among those mentioned above), that is to say that the phosphate chains are independent, that is to say not linked to each other.

[0121] In one embodiment, the matrix fraction comprises, as main constituent, i.e. the one with the highest mass content, particles of an oxide of an element, and the heat-activatable binder preferably comprises a phosphate, preferably a monophosphate, of said element. For example, if the matrix fraction consists mainly (i.e. for more than 50% by mass) of alumina particles and / or of a mineral compound containing mainly the element aluminum (apart from oxygen), such as mullite or cyanite for example, the heat-activatable binder is preferably aluminum phosphate, preferably an aluminum monophosphate.

[0122] Additives

[0123] DVC may contain a consolidation additive, depending in particular on the furnace's operating temperature (temperature during operation). For example, cryolite may be used for an application in which the operating temperature is above 950°C.

[0124] The content of consolidating additive is preferably greater than 0.1%, preferably greater than 0.3%, more preferably greater than 0.5% and / or less than 1.5%, preferably less than 1.3%, preferably less than 1.1%, in mass percentage based on the DVC.

[0125] Preferably it has a particulate form and, preferably, it consists of more than 90%, more than 95%, preferably substantially 100% by mass of fine particles.

[0126] The consolidation additive is chosen so as to have a melting temperature lower than that of the refractory particles. During the consolidation heat treatment, the heat-activatable binder can thus form a phase binding the refractory particles at a lower temperature.

[0127] The consolidation additive is preferably chosen from the group formed by cryolite, borates, in particular fluoroborate, more particularly potassium fluoborate, fluoride salts, silicate compounds, magnesium chlorides, colemanite, clay, kaolin, amorphous silica, in particular silica fume, resins, ceramic frits, and mixtures thereof.

[0128] Preferably, the consolidation additive comprises a resin chosen from phenolic resins, furan resins, acrylic resins, polyester resins, epoxy resins, silicone resins, siloxane resins, alkyd resins, polyvinyl resins, and mixtures thereof. In a particular embodiment, the resin is chosen from particulate products that can be transformed into a polymer during the consolidation heat treatment.

[0129] The DVC preferably contains a dust-control agent to reduce or even eliminate dust during installation of the DVC. The dust-control agent is preferably selected from the group consisting of oils, in particular mineral oils, kerosene, organic polymers and mixtures thereof. Preferably, the dust-control agent is kerosene.

[0130] Preferably, the amount of anti-dust agent is between 0.1% and 1%, as a mass percentage based on the DVC.

[0131] The DVC preferably contains an "anti-wetting" agent for reducing the wettability to molten metals of the sintered product and / or the consolidated product obtained from the DVC. Preferably, the anti-wetting agent is chosen from silicon carbide, barium sulfate, SiAlON, nitrides. Preferably, the anti-wetting agent is chosen from silicon carbide and barium sulfate. More preferably, the anti-wetting agent is barium sulfate.

[0132] Preferably, the anti-wetting agent is introduced in the form of particles having a size less than or equal to 100 μm.

[0133] Preferably, the amount of anti-wetting agent is between 3 and 15%, preferably between 4% and 10%, in mass percentage based on the DVC.

[0134] Aggregate fraction

[0135] Preferably more than 80%, preferably more than 90%, preferably more than 95%, preferably substantially 100% of the particles of the aggregate fraction, called "grains", are made of a refractory material.

[0136] Preferably, more than 50%, more than 70%, more than 90%, more than 95%, or even substantially 100% by mass of the refractory grains (i.e. of a refractory material) are of a material identical to that of the refractory particles of the matrix fraction.

[0137] The refractory grains are preferably made of a material selected from the group consisting of alumina, magnesia, chromium oxide, silica, bauxite, mullite, zirconia, partially stabilized zirconia, stabilized zirconia, mullite-zirconia, alumina-zirconia, magnesia-alumina spinel, zircon, cordierite, aluminum titanate, clayey chamottes containing between 30 and 50% alumina, preferably between 35% and 45% alumina, wollastonite, alumina-zirconia-silica (or "AZS") materials, alumina-zirconia-silica-chromium oxide (or "AZS-Cr") materials, bauxite, zirconias reinforced with alumina, alumina-titanium oxide-zirconia materials, and mixtures thereof. Preferably, the refractory grains are made of a material chosen from the group formed by alumina, mullite, clayey chamottes containing between 30 and 50% alumina, preferably between 35% and 45% alumina, bauxite and mixtures thereof.More preferably, the refractory grains are made of a material selected from the group consisting of alumina, mullite, and mixtures thereof. Preferably, at least 50% by mass of the particles of the aggregate fraction consists of alumina (A12O3).

[0138] PVC manufacturing process

[0139] A DVC according to the invention can be manufactured by simply mixing raw materials with suitable particle sizes, crystallizations and compositions.

[0140] Method of manufacturing the product according to the invention

[0141] The manufacturing process of the product comprises the above-mentioned steps a) to c).

[0142] This method is described below, in a non-limiting manner, for the manufacture of a coating for the side wall and / or the bottom of a crucible, in particular a crucible of an induction furnace.

[0143] In step a), a starting charge is prepared with a DVC according to the invention.

[0144] Preferably, in step a), no water or temporary binder is added to the DVC, in order to limit the creation of porosity during step c).

[0145] Preferably, the DVC is used as is, i.e. step a) is optional, the starting charge being the DVC.

[0146] In step b), the load is shaped.

[0147] The bottom of the crucible, or "floor", is conventionally made by pouring the starting charge, preferably consisting of a DVC according to the invention, onto the floor of the furnace enclosure. The layer thus formed is then compacted.

[0148] All known means for compacting a DVC according to the invention are possible, in particular pressing, tamping, pouring or vibration.

[0149] Preferably, the compacted DVC is then leveled. This leveling also removes the lower-density upper part of the compacted or vibrated layer. A mold for the manufacture of the side wall is then temporarily placed on the leveled layer.

[0150] To produce the side wall of the crucible, the starting charge is then placed between the furnace enclosure, for example made of insulating materials, and the mold placed in said enclosure, preferably until the starting charge surrounds the mold over its entire height.

[0151] Preferably, the starting charge is compacted by vibration. Compaction can be carried out as the starting charge is poured.

[0152] In step c), a rise in temperature allows activation of the heat-activatable binder, and thus to form a phase binding the refractory particles together, so as to obtain a “consolidated” product according to the invention. The heat treatment can lead to total or partial sintering of the refractory particles of the DVC, so as to obtain a “sintered” product according to the invention in which said binding phase has been decomposed.

[0153] The heat treatment is preferably carried out at a temperature higher than the operating temperature at which the furnace operates in operation. The duration of the hold at the maximum temperature reached during the heat treatment is preferably greater than 30 minutes, preferably greater than 1 hour and / or less than 10 hours, preferably less than 3 hours.

[0154] The product consolidated by activation of the hot-activatable or sintered binder obtained at the end of step c) typically has an open porosity of between 10% and 30%, preferably between 15% and 25%.

[0155] In step c), the activation temperature is not always reached over the entire thickness of the DVC intended for the side wall and bottom of the crucible, so that part of the crucible, in particular near the furnace inductor, may not be consolidated or sintered (back layer).

[0156] The oven may thus have a coating comprising, successively, from the cold face opposite the hot face in contact with the heated enclosure of the oven:

[0157] - a rear layer, unconsolidated, remaining in its initial powdery state, representing between 10% and 40% of the thickness of the coating;

[0158] - a layer consisting of consolidated product and / or sintered product, representing between 60% and 90% of the thickness of the coating.

[0159] This last layer is preferably made up of:

[0160] - a front layer (which will be exposed to the molten metal) which is strongly bonded, even sintered and substantially devoid of heat-activatable binder, and,

[0161] - an intermediate layer, located behind this front layer in relation to the molten metal, which has a weaker and less mechanically resistant bond, sufficiently bonded to be self-supporting, within which the temperature has not been high enough to destroy the bond formed by the heat-activatable binder.

[0162] Preferably after the heat treatment step, the mold is removed or disposed of and the crucible is ready for use in melting metal. According to one possible embodiment, the mold or formwork is removed after shaping the powder.

[0163] Examples

[0164] The invention is not limited to the embodiments provided as examples and described below.

[0165] Preparation of DVCs

[0166] The following refractory particle powders and additives were used:

[0167] - sintered mullite powder of grade 3-5 mm comprising 70-73% AhCh and 22-27% SiGL.

[0168] - grade l-3mm sintered mullite powder comprising 70-73% AbCL and 22-27% SiGL.

[0169] - sintered mullite powder of grade 0-lmm comprising 70-73% of AI2O3 and 22-27% of SiGL.; - sintered mullite powder grade 0-0.09 mm comprising 70-73% AhCh and 22-27% SiCF. ;

[0170] - calcined alumina powder with an average size of approximately 4.5 pm and an alumina content greater than 99%;

[0171] - raw kyanite powder KYANITE® -100 mesh;

[0172] - boric acid in powder form with an H3BO3 equivalent greater than 99.9% and a median size D50 equal to 100 pm;

[0173] - a potassium fluoroborate powder typically containing 96% by mass of KBF4 and having a size less than 100 pm.

[0174] The various particulate raw materials were introduced into a mixer and dry mixed for 5 minutes.

[0175] For comparative example 1, the refractory powders of mullite, raw cyanite and calcined alumina were mixed in the appropriate proportions to obtain the mineral chemical composition shown in Table 1. The heat-activatable binder is boric acid powder to which potassium tetrafluoborate has been added.

[0176] Comparative Example 2 was manufactured like Example 1, but the heat-activatable binder was a potassium dihydrogen orthophosphate in the form of a powder of the brand FFB393® supplied by Budenheim such as that used in WO2018 / 002068 AL

[0177] Example 3 according to the invention was manufactured like Example 1, but the heat-activatable binder was an aluminum monophosphate powder of the brand FFB716®, also supplied by Budenheim.

[0178] Comparative Example 4 was manufactured like Example 3 according to the invention but without the FFB716® brand aluminum monophosphate powder.

[0179] Comparative Example 5 was manufactured like Example 3 according to the invention but with twice the content of FFB716® brand aluminum monophosphate.

[0180] For each example, as illustrated in Figure 1, the DVC 10 was compacted by vibration (vertical arrow in the first diagram of Figure 1) in a formwork of aluminous refractory concrete 12 to which was added on the side intended to be exposed towards the interior of the furnace 14 a silicon carbide plate 16 in order to facilitate thermal conduction. The formwork had the following internal dimensions: 445 mm in length, 425 mm in height and 150 mm in thickness.

[0181] All examples contained less than 1% water by mass.

[0182] Oven door test

[0183] In order to request a DVC under thermal gradient, the formwork containing the compacted DVC was fixed to the door 18 of a laboratory furnace 14 equipped with a cooling device 15 (figure 2). Thermocouples 20 connected to a measuring device 22 were also placed every 2 cm in the direction of the thickness in order to follow the evolution of the temperature in different regions of the compacted DVC, in the direction of the thickness (from the hot face (on the side of the furnace resistance) to the cold face). The door 18 was then closed so that the compacted DVC was placed opposite vertical electrical resistances of the furnace. Then the temperature inside the furnace was increased to a holding temperature of 1500°C, with a ramp of 150°C / h. A thermal gradient then appears between the hot face of the compacted DVC exposed to the electrical resistances and the opposite cold face, on the side of the door cooled by water at 25°C (see figure 2).

[0184] The furnace was maintained at the holding temperature for 48 hours. Then the temperature was reduced, at a rate below 200°C / h, to room temperature (20°C).

[0185] The formwork was then dismantled and specimens of the consolidated material obtained were extracted in order to characterize regions of this material which had been subjected, during the landing, to temperatures

[0186] - below 400°C, the corresponding region being considered as the back layer,

[0187] - between 400°C and 800°C and between 800°C and 1200°C, the corresponding region being considered as the intermediate layer, and

[0188] - above 1200°C, the corresponding region being considered as the frontal layer (defining the hot face).

[0189] Characterization

[0190] The characterized DVCs did not contain water.

[0191] The particle size distribution of DVCs before shaping can be determined as described in the “Definitions” chapter above.

[0192] Chemical analysis is determined by X-ray fluorescence spectroscopy for elements with a content greater than 0.1% by mass. If the content of an element is less than 0.1% by mass, it is determined by ICP (Induction Coupled Plasma), on a Vista AX model (marketed by Varian).

[0193] The apparent density and open porosity after consolidation heat treatment are measured according to the following method:

[0194] Test specimens in the form of bars 100 mm long and 20 mm wide square sections were taken from the different layers of the consolidated product obtained after heat treatment. They were first dried at 110°C and then weighed to determine their dry mass Ms. They were then placed in a bell jar under vacuum for 30 minutes. The bell jar was then filled with water so that the specimens were completely immersed. After immersion, the vacuum was maintained for 30 minutes. Atmospheric pressure was then restored in the bell jar and the specimens were left to stand again for 30 minutes. The specimens were then subjected to hydrostatic weighing, giving a mass Mi. They were then wiped with a damp cloth and their wet mass Mh was measured. The apparent density is given by the ratio p.Ms / (Mh-Mi), in g / cm 3 , p being the density of water, taken equal to 1 g / cm 3. The open porosity is given by the ratio 100(Mh- Ms)Z(Mh-Mi), in %. The cold compressive strength was measured, according to standard EN 993-5, on cylindrical specimens of the consolidated product, 30 mm high and 30 mm in diameter, previously baked at 110°C for 12 hours.

[0195] The median pore diameter was measured using a mercury porosimeter in accordance with ISO 15901-1.2005 part 1.

[0196] The level of heterogeneity in the intermediate layer between 400 and 1200°C was assessed by visual observation of the material's appearance and in particular its macroscopic structure. A score between H0 (maximum homogeneity) and H4 (maximum heterogeneity) was assigned.

[0197] Results Table 1 below describes the mixtures. Table 2 summarizes the results obtained.

[0198] [Table 1]

[0199] [Table 2]

[0200] *outside the invention; NM = not measurable (powdery appearance); N.ME = not measured; ** in percentage, in the matrix fraction

[0201] As is now clearly apparent, a DVC according to the invention leads to remarkable homogeneity in the intermediate layer. In particular, a comparison of Example 3 according to the invention with Comparative Example 2 shows that the addition of a non-alkaline phosphate additive, in particular a non-alkaline monophosphate, makes it possible to obtain a more homogeneous intermediate layer and more regular properties that are relatively closer to those of the front layer. A coating obtained from a DVC according to the invention thus has more predictable behavior as the coating wears away from its hot face.

[0202] Comparison of Example 3 according to the invention with Comparative Example 4 shows the advantage of using non-alkaline phosphate in association with a mullite precursor such as kyanite. Indeed, the mixture of Example 4 comprising only kyanite without non-alkaline phosphate has very good properties in the front layer and part of the intermediate layer. But the intermediate layer is very heterogeneous, which in service means a shorter service life for the consolidated coating.

[0203] Furthermore, Example 3 according to the invention has a less porous structure in its front layer compared to Comparative Example 1.

[0204] Furthermore, Example 5, with a mass ratio P2O5 / (AhO3+ZrO2+CaO+MgO+SrO+BaO), in the matrix fraction, greater than 10%, has a mechanical resistance, in the intermediate layer exposed to 500°C, which is considered too high, likely to create stresses in the refractory lining. Finally, the consolidated product obtained from a DVC according to the invention has good mechanical properties and a stable microstructure between 500 and 1000°C, i.e. in the temperature range to which it is subjected at the start of the furnace in the intermediate layer.

Claims

CLAIMS 1. Powder mixture intended for the manufacture of a protective coating for a metal foundry furnace, the mixture being made up, for a total of 100% and in mass percentages based on the mass of the mixture, of: - less than 1% by mass of water, and - a set of particles, the maximum particle size of said set of particles being less than 10 mm, more than 70% of said particles being made of a refractory material, in mass percentage based on said set of particles, said particles being distributed between - a fraction of aggregate consisting of said particles having a size greater than or equal to 200 micrometers, called “grains”, and - a matrix fraction consisting of said particles having a size of less than 200 micrometers, called "fine particles", representing between 5% and 40% of the mass of said mixture, the whole of the matrix fraction and of said liquid comprising a heat-activatable binder, the heat-activatable binder comprising, preferably consisting of a phosphate chosen from phosphates devoid of sodium, potassium and lithium, called "non-alkaline phosphate", said non-alkaline phosphate representing more than 0.5%, in mass percentages based on the mass of the DVC, the chemical composition of the whole of the matrix fraction and of the liquid being such that: 0.5% < P2O5 < 5% and Na2O + K2O + Li2O < 1.0%, in mass percentages based on the mass of said mixture, and the mass ratio P2Os / (Al2O3+ZrO2+CaO+MgO+SrO+BaO), in the matrix fraction, is greater than 5% and less than 10%.

2. Mixture according to the preceding claim, in which the non-alkaline phosphate, preferably the heat-activatable binder, is a phosphate of at least one element chosen from aluminum, zirconium, calcium, magnesium, strontium, barium, boron, zinc, and copper.

3. Mixture according to any one of the preceding claims, in which the non-alkaline phosphate, preferably the heat-activatable binder, is a monophosphate of an element chosen from aluminum, zirconium, calcium, magnesium, strontium, barium, boron, zinc, and copper.

4. Mixture according to any one of claims 2 to 3, in which the matrix fraction comprises, as main constituent, particles of an oxide of a said element, and in which the non-alkaline phosphate is a phosphate of said element. Mixture according to any one of the preceding claims, the mass content of free SiCh of which is less than 10%.A mixture according to any one of the preceding claims, wherein more than 90% of said particles and / or fine particles and / or grains are of a refractory material, in mass percentage based on said set of particles, fine particles or grains, respectively, said refractory material being selected from the group consisting of alumina, magnesia, chromium oxide, silica, bauxite, mullite, zirconia, partially stabilized zirconia, stabilized zirconia, mullite-zirconia, alumina-zirconia, magnesia-alumina spinel, zircon, cordierite, aluminum titanate, clayey chamottes containing between 30 and 50% alumina, wollastonite, alumina-zirconia-silica materials, alumina-zirconia-silica-chromium oxide materials, bauxite, alumina-reinforced zirconia, alumina-titanium oxide-zirconia materials, and mixtures thereof.Mixture according to any one of the preceding claims, comprising a consolidation additive chosen from the group formed by borates, in particular fluoroborates, cryolite, fluoride salts, silicate compounds, magnesium chlorides, colemanite, clay, kaolin, amorphous silica, in particular silica fume, phenolic resins, furan resins, ceramic frits, and mixtures thereof. Mixture according to any one of the preceding claims, in which the matrix fraction comprises a mullite precursor chosen from kyanite, andalusite, clay, silica in the presence of alumina. Mixture according to any one of the preceding claims, in which at least 50% by mass of the particles of the aggregate fraction are made of a material identical to that of the particles of the matrix fraction.A mixture according to any preceding claim, comprising neither a hydraulic binder nor a temporary organic binder. A mixture according to any preceding claim, wherein the total amount of refractory grains, refractory fine particles, heat-activatable binder particles, anti-dusting agent particles and anti-wetting agent particles is greater than 95%, as a mass percentage based on the mixture. A mixture according to any preceding claim, wherein said non-alkaline phosphate is in the matrix fraction, the chemical composition of the matrix fraction being such that. 0.5% < P2O5 < 5% and Na2O + K2O + Li2O < 1.0%, in mass percentages based on the mass of said mixture.

13. Mixture according to any one of the preceding claims, in which the non-alkaline phosphate, preferably the heat-activatable binder, is in particulate form.

14. Mixture according to any one of the preceding claims, in which the heat-activatable binder does not comprise feldspar and / or comprises less than 1% of a phosphate comprising sodium and / or potassium and / or lithium, as a mass percentage based on the mixture.

15. Method for manufacturing a consolidated product comprising the following successive steps: b) compaction of a mixture according to any one of the preceding claims; c) heat treatment of at least part of the shaped starting charge at a temperature suitable for activating the heat-activatable binder, or even for sintering the particles into a refractory material.

16. Metal melting furnace comprising at least one region intended to be in contact with a molten metal, said region being constituted, at least in part, of a consolidated product and / or a sintered product manufactured according to a method according to the immediately preceding claim.

17. Oven according to the immediately preceding claim, comprising a coating comprising, successively, from a cold face opposite the hot face in contact with the heated enclosure of the oven: - a rear layer, unconsolidated, remaining in its initial powdery state, representing between 10% and 40% of the thickness of the coating; - a layer consisting of consolidated product and / or sintered product, representing between 60% and 90% of the thickness of the coating.

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