FR3048428A1 - INDUCTION DEPHOSPHORATION FURNACE - Google Patents

Abstract

Material having the following average chemical composition, in percentages by mass on the basis of refractory oxides and for a total of 100%: - more than 50% of Al2O3, - more than 4% of Cr2O3, - less than 20% of MgO, less than 5% of CaO, less than 3% of SiO2, less than 5% of Fe2O3, less than 30% of ZrO2, less than 10% of TiO2, the constituents other than Al2O3, Cr2O3, MgO, CaO, SiO 2, Fe 2 O 3, ZrO 2 and TiO 2 constituting the 100% complement, the sum of the Al 2 O 3 + Cr 2 O 3 + Fe 2 O 3 + ZrO 2 + TiO 2 oxides representing more than 80% of the said chemical composition.

Description

Induction dephosphorization oven

Technical Field The invention relates to a material for coating a dephosphorizing induction furnace of a ferrous metal, in particular a steel and / or a cast iron. The invention also relates to such an oven.

State of the art

The purpose of the dephosphorization is to eliminate the phosphorus contained in ferrous metals. It is obtained by an oxidation-reduction, possibly associated with an oxygen insufflation via a lance and / or an insufflation of neutral or reducing gas via an insufflation device at the bottom of the oven, as described, for example, by CN102936638.

The implementation of an induction furnace for the dephosphorization, rather than a VOD ("Vacuum oxygen decarburization") or RH-OB ("Ruhrstahl-Heraeus Oxygen Blowing") furnace or an arc furnace , allows lower investment and higher flexibility. It also allows better control of the endothermic reaction related to dephosphorization. However, it requires higher performance refractory coatings to protect the area of the crucible that is in contact with the molten metal.

Figure 1 shows schematically, in cross section, a crucible of an induction furnace for dephosphorization. This crucible has a side wall 10 which extends, substantially vertically, from a bottom 12.

The side wall 10 of the crucible conventionally comprises, from the outside towards the inside of the crucible: a support layer 14, on which the inductor 16 is fixed; at least one layer of thermal insulation 18; at least one layer of electrical insulation 20; - A refractory coating 22 itself comprising an unconsolidated region 24, preferably in the form of a compacted powder, and a consolidated region 26, defining the inner surface 28 of the crucible. The order of the layers of thermal insulation 18 and electrical insulation 20 is not limiting.

The layer of electrical insulation 20 made of a dielectric mineral material, for example mica, and has the function of electrically isolating the inductor and the refractory coating.

The thermal insulation layer 18 may in particular be a ceramic fiber mat, in particular an aluminosilicate. Advantageously, it makes it possible to compensate the effect of the thermal expansion of the refractory coating 22 during the heating and / or to protect the layer of electrical insulation.

The bottom 12 conventionally comprises, from the outside to the inside of the crucible: - sole 30, substantially horizontal; a refractory lining 22 'itself comprising an unconsolidated layer 24', preferably in the form of a compacted powder, and a consolidated layer 26 'defining the inner surface 28 of the crucible.

Conveniently, the manufacture of the coating is carried out after having manufactured the sole 30 and the support layer 14, and having fixed the layers of thermal and electrical insulation.

A powder of the material constituting the coating is first poured onto the upper surface of the hearth 30. The layer thus formed is then for example tamped or vibrated, until a thickness substantially corresponding to the thickness of the coating is obtained. above the sole. After a leveling operation which also eliminates the upper layer of the more weakly densified hearth, mold is provisionally placed on the layer thus obtained.

The mold is preferably shaped to extend substantially parallel to the surface defined by the layers of thermal and electrical insulation, remaining spaced apart by a distance corresponding to the thickness of the coating of the side wall of the crucible.

A powder of the material constituting the coating is then poured between the mold on the one hand and the surface defined by the thermal and electrical insulation layers on the other hand, preferably until the powder surrounds the mold over its entire length. height.

The powder is shaped by compacting, preferably by vibration, optionally after adding a temporary binder. All known means for compacting a powder are possible. The compaction can be carried out as and when the starting load is paid.

The temperature is then raised inside the crucible so as to consolidate at least a portion of the powder. If the powder contains a temporary binder or a heat activatable binder, raising the temperature will remove the temporary binder and activate the binder hot, without melting the refractory particles completely.

Preferably, the heating conditions make it possible to consolidate only part of the coating, which thus comprises a consolidated region, in the form of a layer on the interior side of the crucible, and an unconsolidated layer, in the form of a layer on the outside of the crucible.

To dephosphorize a ferrous metal, the induction furnace is loaded with metal and / or precursors, for example pre-reduced iron ore. The metal resulting from the melting of the filler is then brought into contact with a basic dephosphorising slag.

Corrosion gradually wears the consolidated region. This wear therefore increases the amount of energy transmitted to the unconsolidated region, in particular by the molten metal and the slag. This results in a progressive consolidation of the unconsolidated region, as a result of said wear. The consolidated region is thus permanently reconstituted, downstream of the thermal flow, at the expense of the unconsolidated region. After complete "consumption" of the unconsolidated region, the consolidated region is no longer reconstituted and its thickness is reduced. The coating must then be renewed.

Induction furnaces are known comprising a dry mud coating or "DVC", in English "dry vibratable mix" or "dry refractory", based on alumina-magnesia (spinel MgA104).

Such a coating is said to be "basic" as opposed to so-called "acid" coatings based on silica. A silica-based coating has very little corrosion resistance in a dephosphorization furnace. A slag called "dephosphorization" is indeed strongly basic. It typically comprises compounds based on calcium and / or barium, for example in the form of fluoride or aluminate, with in general an addition of FeO in the form of mill scale. The silica of the coating thus reacts strongly with the slag.

A dry rammed earth results from the thermal consolidation of a "dry rammed" powder, that is to say of the DVC type, as described for example in EP 1 224 153. This powder, unlike a concrete, can be implemented "dry", that is to say without adding water or liquid binder, or, rarely, with a very small amount of water or liquid binder (typically less than 3%) . Unlike a concrete, a dry mud powder conventionally comprises no hydraulic binder, that is to say, able to take in mass by adding water.

The possible lessons that could be drawn from the study of documents on concretes are therefore not, a priori, transferable to a powder of dry mud.

The shaping of a dry mud powder conventionally results from a simple compaction at ambient temperature, the consolidation resulting from a subsequent consolidation heat treatment. The implementation of a dry mud powder is therefore fast.

Also conventionally, a dry mud powder consists of refractory particles and particles of a heat-activatable binder. The consolidation heat treatment temperature is between the melting temperature of the heat-activatable binder and that of the refractory particles. During the consolidation heat treatment, the heat-activatable 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 heat-activatable binder is also chosen so that it can be in this viscous liquid state at a temperature close to the operating temperature of the oven, in particular during the first temperature rise. This viscous liquid state thus advantageously makes it possible to reduce the rigidity of the consolidated product, facilitating its deformation and thus increasing its ability to adapt to local thermomechanical stresses. During subsequent rise in temperature, this liquid state can be modified, and in particular become rigid by enrichment with fine particles.

The consolidated product consists of a refractory granulate hé by a matrix. Its thermomechanical behavior is superior to that of concrete. Furthermore, a dry mud coating can be advantageously free of joints, unlike a brick or block assembly. But the joints are privileged zones of infiltration of metal. The absence of joint is particularly advantageous in an oven provided with a water-cooled inductor. The article "Spinel Formation in Coreless Induction Furnace Linings", Saikia and Al, Proceedings of the International Symposium on Advances in Refractories for Metallurgical Industries, p 827-840 (2004) describes dry mud powders based on alumina and / or spinel MgAl 2 O 4. The sintered products obtained from these powders have good resistance to corrosion by molten metals. However, they have limited resistance to thermal shocks, especially in the case of melting light alloys or aluminum alloys, causing severe thermal shocks. However, thermal shocks create cracks at the origin of detrimental phenomena of infiltration by molten metals. JP 3 183 656 discloses dry mud powders based on crystalline silica. The sintered products obtained from these powders have the advantage of a boime resistance to thermal shocks, especially during repetitive temperature rise and fall. However, these sintered products exhibit limited resistance to corrosion and infiltration by molten metals. They also pose hygiene problems during their installation and disassembly, particularly because of the potential presence of crystalline silica dust. CN101717842 discloses a dephosphor induction furnace equipped with a refractory coating based on alumina-magnesia. This coating can be advantageously obtained by sintering in situ, at a temperature below 1600 ° C, a shaped starting charge. However, its resistance to corrosion is unsatisfactory.

The permanent thermal deformation is the non-reversible variation of the dimensions of the coating resulting from the consolidation of the shaped feedstock, for example by sintering, and a return to ambient temperature. This deformation can be measured according to the recommendation EN-993-10.

The ceramic material of CN101717842 has a very high permanent thermal deformation, from + 2% to + 6%. The initial operation of heating the oven coated with this type of material however leads to microfissurations and imperfect consolidation of the refractory lining.

Furthermore, a dephosphorization furnace operates classically discontinuous, typically with a weekly stop, or even daily. In particular, when the permanent thermal deformation is greater than 3%, the conduct of the heating operations is difficult. Indeed, the sinterings resulting from successive fusions lead to swelling in the not yet consolidated region of the coating. This swelling can strongly damage the previously consolidated area.

There is therefore a need for an induction dephosphorization furnace whose refractory coating has a longer life than current coatings.

There is also a need for an induction dephosphorization furnace whose refractory lining can be sintered in situ at a temperature of 1600 ° C or lower.

An object of the invention is to meet this need. SUMMARY OF THE INVENTION The invention proposes a material, in particular in the form of a powder, called "powder according to the invention", or of a consolidated product, called "consolidated product according to the invention", said material having the following average chemical composition, in percentages by weight based on the refractory oxides and for a total of 100%: - more than 50%, preferably more than 60%, preferably more than 70%, preferably more than 80% preferably more than 85% Al 2 O 3, more than 4%, preferably more than 5% or even more than 7% Cr 2 O 3, less than 20%, preferably less than 15%, preferably less than 10%. %, preferably less than 5% MgO, preferably less than 3.0%, preferably less than 2.0%, preferably less than 1.0%, preferably less than 0.5%, less than 5%, preferably less than 2.5%, preferably less than 1%, preferably less than 0.5% CaO, less than 3%, preferably less than 2%, preferably less than 1%, preferably less than 0.5% SiO 2, less than 5% Fe 2 O 3, less than 30% ZrO 2, less than 10% TiO 2, constituents other than Al 2 O 3, Cr 2 O 3, MgO , CaO, SiO 2, Fe 2 O 3, ZrO 2 and TiO 2 constituting the 100% complement, the sum of the Al 2 O 3 + Cr 2 O 3 + Fc 2 O 3 + ZrO 2 + TiO 2 oxides representing more than 80%, preferably more than 85%, preferably more than 90%, preferably more than 95%, preferably more than 97% of said chemical composition.

As will be seen in more detail in the following description, the inventors have discovered that, surprisingly, such a material is particularly advantageous for the manufacture of a dephosphorization furnace coating. Its composition, which contains little magnesia, but necessarily chromium oxide, considerably improves the life of the coating when in contact with a molten ferrous metal being dephosphorized.

A material according to the invention may also comprise one or more of the following optional and preferred characteristics: the mass content of Cr 2 O 3 is less than 60%, preferably less than 50%, preferably less than 25%, preferably less than 20% preferably less than 15%, preferably less than 12%, preferably less than 10%; the Fe 2 O 3 content is less than 3%, preferably less than 2%, preferably less than 1%, preferably less than 0.5%; the TiO 2 content is less than 7%, preferably less than 5%, preferably less than 3%; the TiO 2 content is greater than 0.5% greater than 1%, greater than 2%, which promotes the formation of the alumina-chromium phase and imparts improved corrosion resistance; the content of ZrO 2 is less than 20%, preferably less than 10%, preferably less than 5%, preferably less than 2%, preferably less than 1%, preferably less than 0.5%; the material comprises a hot-activatable binder selected from the group consisting of boron oxide, boric acid, borates, cryolite, fluoride salts, silicate compounds, phosphate compounds, feldspars, magnesium chlorides, colemanite, clay, kaolin, crystalline silica, amorphous silica, in particular silica, organic resins, in particular phenolic resins, furan resins, ceramic frits, and mixtures thereof; the material consists, for more than 95% by mass percentage, of refractory oxides, hot-activatable binder, optionally anti-dust agent, optionally anti-wetting agent chosen from silicon carbide, SiAlON, and nitrides, and impurities; the refractory oxides represent more than 80%, preferably more than 85%, preferably more than 90% of the mass of the material; preferably, the amount of impurities is less than 2%, less than 1%, less than 0.5%, or even substantially zero, in percentages by weight; in one embodiment, the material has no hydraulic binder or organic binder; the material comprises; between 2% and 15% of an anti-wetting agent selected from silicon carbide, SiAlON, nitrides, and / or between 0.1% and 1% of an anti-dust agent selected from the group consisting of oils, in particular mineral oils, kerosene, organic polymers and mixtures thereof; the material is in the form of a powder, all the refractory particles representing more than 70%, preferably more than 80%, more than 90% of the mass of the material; the material is in the form of a consolidated product obtained by a consolidation heat treatment of a powder according to the invention, in particular in the form of a block or a coating, preferably in the form of a coating; the consolidated product has an open porosity preferably greater than 10%, preferably greater than 10%, preferably greater than 10%, preferably greater than 15%, and / or less than 30%, preferably less than 25%; preferably less than 20%. The invention also relates to an induction furnace for dephosphorizing a molten ferrous metal, said furnace comprising a refractory coating of which at least one region is made of material according to the invention.

A furnace according to the invention may also comprise one or more of the following optimum characteristics: the material is preferably in the form of a consolidated product obtained by consolidation of a powder according to the invention, in particular when it is in a region intended to be in contact with said molten ferrous metal; preferably, the coating covers, at least in part, a furnace crucible; the coating comprises a layer of consolidated material, in contact with the molten ferrous metal and / or with a dephosphorizing slag, and / or a powder material, said consolidated and powdered materials being in accordance with the invention; preferably, the material covers the region of the coating in contact or likely to be in contact with the dephosphorizing slag; the coating is free of joints.

BRIEF DESCRIPTION OF THE FIGURES Other features and advantages of the invention will become apparent on examining the description which follows and with reference to the appended drawing in which FIG. 1 schematically represents, in cross-section, the structure of a furnace with induction of dephosphorization. Definitions

By "powder" is meant a dry particulate mixture.

The solid elements constituting a powder are called "particles". The particles of the matrix fraction are called "fine particles" and the granular particles are called "grains".

The "size" of a particle is conventionally given by a particle size distribution characterization. For example, a laser granulometer allows the measurement of sizes less than or equal to 5 mm.

The percentiles or "percentiles" (Dio), 50 (D50), 90 (D90) and 99.5 (D995) of a powder are the particle sizes corresponding to the percentages, by weight, of 10%, 50% , 90% and 99.5% respectively, on the cumulative particle size distribution curve of the particles of the powder, the particle sizes being ranked in ascending order. For example, 10%, by weight, of the particles of the powder have a size smaller than Dio and 90% of the particles by mass have a size greater than Dio. The percentiles can be determined using a particle size distribution using a laser granulometer.

The "maximum size" is the 99.5 percentile (D99 5) of said powder.

The so-called "median size" is the percentile D50, that is to say the size dividing the particles into first and second populations equal in mass, these first and second populations comprising only particles having a larger size, or smaller respectively, at the median size.

By "impurities" is meant the inevitable constituents introduced involuntarily and necessarily with the raw materials or resulting from reactions with these constituents. Impurities are not necessary constituents, but only tolerated.

By "refractory" is meant "having a melting temperature greater than 1500 ° C." This definition is commonly used by those skilled in the art and cited in "Refractory materials and technical ceramics (ceramics and technology elements)", G. Aliprandi, Editions Septima Paris, 1979. This book also gives pages 297 to 301 of examples of refractory materials, in particular oxides, carbides and nitrides.

By "consolidation" is meant a heat treatment, for example sintering and / or activation of a hot-activatable binder, leading to a stiffening of a feedstock, typically a shaped powder.

The term "heat-activatable binder" means a component which, under the effect of an increase in temperature, will liquefy in a viscous form suitable for agglomerating the particles of a refractory powder in which it is mixed. By way of example, boric acid, potassium tetrafluoroborate and ciyolite melt at melting temperatures between 150 and 300 ° C, about 530 ° C and 1010 ° C, respectively, giving phases viscous capable of agglomerating the particles of a powder according to the invention, that is to say able to lead to a structured product (self-supporting).

By "borate" is meant a compound of oxoanions based on boron and electropositive elements. The boron oxoanion can be oxoanion borate BO2 ', oxoanion diborate 6205 ^^, oxoanion triborate BsOs', oxoanion tetraborate Β4θ7 ^', sodium tetraborate Na2B40v, lithium triborate LiBsOs magnesium diborate Mg 2 B 2 O 5 are examples of borates, and the borates can also be hydrated, such as, for example, borax Na 2 B 4 OH.

"Temporary" means "removed from the product during consolidation heat treatment".

By "+", one classically means "and / or". For example, "Mg0 + Cr203" means "MgO and / or Cr203".

A phase is called "minority" when another phase with a higher mass content is present.

All percentages are by mass of the material unless otherwise indicated. Conventionally, the characteristics relating to a state of the material (melting temperature, viscosity, etc.) are supplied at a pressure of 1 bar, unless otherwise indicated.

detailed description

Powder according to the invention

A powder according to the invention preferably comprises more than 70%, preferably more than 80%, more than 90%, preferably substantially 100% of refractory particles, a particle of said powder being classified in the fraction called "matrix fraction" or in the fraction called "granulate" according to whether it has a size less than or equal to 200 μm, or greater than 200 μm, respectively.

A powder according to the invention can be manufactured by mixing raw materials having granulometries and suitable compositions.

The refractory particles may in particular be sintered or melted particles. • Granulometry

Preferably, the matrix fraction represents more than 10%, more than 15%, preferably more than 20%, and / or less than 40%, preferably less than 35% of the mass of the powder, the 100% complement. being, by definition, constituted by the aggregate.

Preferably, the powder comprises more than 10%, more than 20%, or even more than 30% and / or less than 50%, or even less than 40% of particles having a size greater than 1 mm.

More preferably, the powder comprises more than 10%, more than 20% or even more than 30%, and / or less than 50%, or even less than 40% of particles having a size of between 0.1 mm and 1 mm. .

In a preferred embodiment, the powder has the following characteristics: the maximum particle size is less than 5 mm, and the quantity of particles larger than 300 μm is greater than 50% and less than 70%, and the amount of matrix fraction is greater than 15%, preferably greater than 20%, and less than 40%, and the amount of particles smaller than 75 μm is greater than 10% and less than 35%.

Preferably, the particle size distribution of a powder according to the invention is adapted to promote its compaction. Compaction models such as the Fuller-Bolomey model or the Andreassen model can be used to determine the most suitable particle size distribution. • Composition

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%, even more than 95%, or even more than 98% of the mass. powder. Preferably, a powder according to the invention is formed by the single mixture of refractory particles and hot-activatable binder particles.

All the refractory particles used according to the prior art can be envisaged, for the matrix fraction as for the granulate, provided that the powder has a chemical composition according to the invention.

In a preferred embodiment, the material, preferably in powder form, comprises: - more than 85% Al 2 O 3, and - less than 15% Cr 2 O 3, and - less than 2% MgO, and - less than 2% of Fe 2 O 3, and less than 5% of TiO 2, and less than 10% of ZrO 2, and less than 0.5% of P2O 5, and less than 0.5% of alkaline oxides K 2 O + Na 2 O.

In one embodiment, more than 80%, more than 90%, preferably substantially 100% by number, of the refractory particles are substantially pure, that is, each is more than 80% %, more than 95%, more than 99%, or even substantially 100% of their mass, of the same refractory oxide.

Preferably, to limit the risk of pollution of the molten metal bath, the content of P2O5 and / or alkali oxides K2O + Na2O is less than 1%, preferably less than 0.5%. Preferably these oxides are impurities.

The heat-activatable binder must be chosen to be activatable at a temperature below the operating temperature.

The heat-activatable binder must also be selected so as to have a lower melting temperature than the refractory particles. During the consolidation heat treatment, the heat-activatable binder can thus pass from the solid state to a viscous liquid state allowing adhesion to the refractory particles and a bond between them.

In one embodiment, the powder comprises more than 0.1%, and / or less than 5%, preferably less than 3%, or even less than 1% of heat-activatable binder.

Preferably, more than 80%, more than 90%, or even substantially 100% of the particles of the heat-activatable binder are in the matrix fraction, in weight percent based on the mass of the heat-activatable binder.

In one embodiment, the hot-activated binder has a melting temperature of greater than 5 ° C., preferably greater than 15 ° C., preferably greater than 20 ° C., or even greater than 30 ° C., greater than 50 ° C. , greater than 70 ° C, greater than 80 ° C, greater than 150 ° C, and / or less than 1500 ° C, less than 1300 ° C, or less than 1200 ° C.

Preferably, the heat-activatable binder has a melting temperature of from 150 ° C to 1200 ° C.

Preferably, the heat-activatable binder is not a metal.

The heat-activatable binder is preferably selected from the group consisting of boron oxide, boric acid, borates, cryolite, fluoride salts, silicate compounds, phosphate compounds, feldspars, magnesium chlorides, colemanite, clay, kaolin, crystalline silica, amorphous silica, in particular silica fimo, resins, ceramic frits, and mixtures thereof. The resin may in particular be 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 convertible into a polymer during the consolidation heat treatment.

Preferably, the hot-activatable binder is selected from the group consisting of sodium borates, potassium borates, calcium borates, boric acid, boron oxide, clay, fluoride salts, and their mixtures. Preferably, the heat-activatable binder is selected from the group consisting of boric acid, boron oxide, clay, fluoride salts, and mixtures thereof.

Preferably, the heat-activatable binder is inorganic.

The powder may in particular comprise more than 0.1%, and / or less than 5%, preferably less than 3%, or even less than 1% of heat activatable binder, and in particular clay and / or boric acid.

In one embodiment, the powder contains an agent that makes it possible to reduce, or even eliminate, the dust during the setting up of the powder. This "anti-dust agent" can be chosen from the group formed by oils, in particular mineral oils, kerosene, organic polymers and their mixtures. Preferably this agent is kerosene. Preferably, the amount of anti-dust agent is between 0.1% and 1%.

In one embodiment, the powder contains at least one agent for reducing the wettability to the molten metals of said powder and / or the consolidated product obtained from said powder. Preferably, this "anti-wetting agent" is selected from silicon carbide, SiAlON, nitrogens. Preferably, this agent is chosen from silicon carbide. Preferably, the anti-wetting agent is introduced in the form of particles having a size less than or equal to 100 μm. Preferably, the amount of anti-wetting agent is between 5% and 15%, preferably between 5% and 10%.

Preferably, the powder consists of more than 95%, or even more than 98%, or even substantially 100%, of refractory particles, particles of heat-activatable binder, optionally particles of anti-dust agent and optionally particles of anti-wetting agent, the optional supplement to 100% being impurities, for example iron from a grinding stage.

Preferably, the powder does not contain a hydraulic binder or organic binder except, optionally, a resin. In one embodiment, the powder contains neither hydraulic binder nor organic binder. In a first embodiment, the powder contains no hydraulic binder or organic binder except, optionally, a resin. In a second embodiment, the powder contains neither hydraulic binder nor organic binder.

The refractory particles are preferably selected so that they can be consolidated, preferably sintered, to a temperature of at least 1000 ° C in the absence of a heat activatable binder. • Specificities of the aggregate

The granulate, the particles of which are called "grains", may also include one or more of the following optional features:

The amount of grain larger than 300 μm is preferably greater than 30%, greater than 40%, even greater than 50%, and / or less than 70%.

More preferably, the granulate comprises, or even consists of, refractory grains of a material selected from the group formed by aluminas, magnesia, chromium oxide, magnesia-chromites, bauxite, zirconia, partially zirconia stabilized zirconia, alumina-zirconia, magnesia-alumina spinel, zircon, cordierite, aluminum titanate, alumina-zirconia-silica-chromium oxide (or "AZS-Cr") materials, bauxite, zirconia reinforced with alumina, and mixtures thereof.

Preferably, said refractory grains are of a material selected from the group consisting of white corundum, tabular alumina, brown corundum and black corundum, preferably from the group consisting of white corundum and tabular alumina.

The maximum grain size is less than or equal to 10 mm, preferably less than or equal to 8 mm, preferably less than or equal to 6 mm, or even less than or equal to 4 mm.

The granulate preferably comprises less than 20%, less than 10%, or even less than 5%, or substantially no particles of said hot-activatable binder, called "binder grains", in percentages by weight on the basis of the powder. • Specificities of the matrix fraction

The matrix fraction, the particles of which are called "fine particles", may also comprise one or more of the following optional characteristics:

The amount of fine particles having a size of less than 75 μm is between 10% and 35%, based on the mass of the material.

The matrix fraction contains refractory fine particles of a refractory material selected from the group consisting of alumina, magnesia, chromium oxide, bauxite, zirconia, partially stabilized zirconia, stabilized zirconia, mullite-zirconia , alumina-zirconia, magnesia-alumina spinel, zircon, aluminum titanate, alumina-zirconia-silica-chromium oxide (or "AZS-Cr") materials, bauxite, alumina-reinforced zirconia and their mixtures. Preferably, said refractory fine particles are made of a refractory material chosen from the group formed by alumina, mullite and clay chamottes containing between 30% and 50% of alumina, preferably between 35% and 45% of alumina. , bauxite, and mixtures thereof. More preferably, said refractory fine particles are each of alumina or chromium. Preferably, the above-mentioned refractory fine particles together represent more than 80%, more than 90%, or even substantially 100% of the refractory fine particles of the matrix fraction. More than 80%, or even more than 90% or even substantially 100% of the heat-activatable binder belongs to the matrix fraction, as a percentage based on the mass of the heat-activatable binder.

The amount of heat-activatable binder fines is preferably greater than 0.1%, and / or preferably less than 5%, preferably less than 3%, preferably less than 1%, based on mass. of the material.

The invention also relates to a method of manufacturing a consolidated product according to the invention comprising the following successive stages: a) preparation of a starting charge from a powder according to the invention; invention; (1) the starting feedstock comprising a powder according to the invention optionally comprising a hot-activatable binder, the liquid phase in said feedstock representing less than 3%, preferably less than 2%, more preferably substantially 0% of the mass of the starting load; or (2) the feedstock consisting of a mixture comprising a powder according to the invention optionally comprising a hot-activatable binder, and water and / or a temporary binder, as described below; b) shaping said feedstock, in particular by pressing, tamping, or vibrating; c) heat treatment for consolidating the shaped feedstock at a temperature above the melting temperature of the heat-activatable binder and below the melting temperature of the refractory particles.

This method can be implemented for the manufacture of a coating of the side wall and / or the bottom of a crucible of an induction furnace.

In the case (1), the water, or even the liquid phase in said feedstock, preferably represents less than 2% of the mass of the feedstock. More preferably, and in particular for a crucible induction furnace coating, the feedstock does not have water or temporary binder. In step b), the shaping results preferably from the implementation of a conventional method as described in the introduction to the present description. In particular, a powder according to the invention is preferably poured between a mold on the one hand and the surface defined by layers of thermal and electrical insulation and on the surface of the sole on the other hand.

A rise in temperature then allows activation of the hot-activatable binder, that is to say allows the particles of hot-activatable binder to melt in a viscous form, and thus to ensure the cohesion of the refractory particles. In step c), the consolidation heat treatment is preferably carried out at a temperature above the operating temperature. The duration of the plateau at the maximum temperature reached during the consolidation heat treatment is preferably greater than 30 minutes, preferably greater than 1 hour, and / or less than 10 hours, preferably less than 3 hours.

The consolidation heat treatment can lead to sintering of the refractory particles.

The heat treatment leads to activation of the heat-activatable binder when present and / or sintering which generates (s) a binder phase between the refractory particles. This produces a so-called "consolidated" product. Typically, the open porosity of such a consolidated product is between 10% and 30%.

If the activation temperature has not been reached over the entire thickness of the side wall and bottom of the crucible, a portion of the crucible, particularly near the furnace inductor, may not be consolidated.

After the consolidation heat treatment step, the mold can be removed or melted. The crucible is then ready to serve for the melting of metal.

This method of manufacturing a consolidated product can also be implemented for the manufacture of prefabricated or preformed unit blocks, in particular intended to be used as a consolidated region or to be assembled with other blocks.

In one embodiment, such a unitary block is a preformed crucible.

For the manufacture of blocks, it is preferable, according to option (2), to add to the powder a temporary binder, and / or a hydraulic binder and water. This gives a mixture that can be poured into a mold and, after step b), to obtain a preform having, at room temperature, a holding, so-called "cold". The invention also relates to such a mixture.

Preferably, the amount of temporary binder in this mixture is greater than 0.5% and / or less than 6%, and the amount of water is greater than 2% and / or less than 6%, or even less than 5% , in percentages by weight relative to the mass of the powder before addition of the temporary binder and water.

Temporary binders conventionally used for the manufacture of sintered ceramic blocks may be used, for example dextrin, calcium lignosulfonate, CMC, or PEG.

Consolidated product

Preferably, in a consolidated product according to the invention, the mass content of Cr 2 O 3 is less than 60%, even less than 50%, or even less than 20%, or even less than 15%; and / or the Fe 2 O 3 content is less than 5%, less than 2%, less than 1%, or even less than 0.5%; and / or the TiO 2 content is less than 10%, less than 5%; and / or the ZrO 2 content is less than 30%, less than 10%, less than 5%, or even less than 2%.

Advantageously, the dilatometric variations are reduced.

The principal phases (that is to say greater volume) present observed are corundum (alpha alumina) and spinel (MgAl 2 O 4; MgCr 2 O 4; FeCr 2 O 4; FeAl 2 O 4) phases. The chromium oxide is preferably predominantly present as a solid solution (Al, Cr) 203.

An aluminum titanate phase may be present. Anatase or rutile phase was only observed in a region where the material was not unbound.

A consolidated product according to the invention preferably comprises more than 0.1%, and / or less than 5%, less than 3%, less than 1%, heat-activatable binder and / or transformation product of said activatable binder. hot resulting from said consolidation heat treatment.

A consolidated product according to the invention preferably comprises more than 80%, more than 85%, more than 90% of oxides.

The density of a consolidated product according to the invention is preferably greater than 3.0 g / cc and / or less than 3.7 g / cm 2, preferably less than 3.5 g / cm 3.

The permanent thermal deformation, measured as in the examples below, of a consolidated product according to the invention is preferably less than 3.0%, preferably less than 2.5%, preferably less than 2.0%. preferably less than 1.5%.

The cold compressive strength, measured as in the examples below, of a consolidated product according to the invention is preferably greater than 18 MPa, preferably greater than 25 MPa, preferably greater than 30 MPa, preferably greater than 40 MPa, preferably greater than 50 MPa.

Melting furnace

Preferably, the induction furnace according to the invention is of the "crucible" or "coreless" type in English, as described in the introduction to the present description.

The ratio of the upper diameter of the crucible to the height of the molten metal is preferably greater than 0.4, or even greater than 0.5 or even greater than 0.6, to maximize dephosphorization.

Preferably, the unconsolidated layer has the same chemical composition as the consolidated layer. The total thickness of the refractory lining is preferably greater than 5 cm, or even greater than 6 cm, or greater than 8 cm, and / or less than 30 cm, or even less than 25 cm.

The consolidated layer defines the surface of the crucible in contact with the molten metal. The thickness of the consolidated layer is preferably greater than 5 mm, or even greater than 1 cm, or even greater than 2 cm. The thickness of the consolidated layer is preferably less than half that of the refractory lining.

The composition and / or the structure of the refractory lining may be variable in the vertical direction.

Preferably, the lower part of the refractory lining of the crucible, which extends from the bottom of the crucible to the height greater than one-third of the height of the crucible, or even up to the median height of the crucible, is made of a refractory material of DVC or rammed type, for example based on alumina-magnesium spinel.

Preferably, the intermediate portion of the refractory lining of the crucible, which extends from the bottom to a height greater than the level of the dephosphoric slag, and which is in contact with the molten metal and / or with the slag, is in a consolidated product according to the invention.

Preferably, the upper part of the refractory lining of the crucible, which extends from the intermediate part to the top of the crucible, is made of a monolithic material of concrete type, DVC or plastic mass. The consolidation of this material may advantageously be carried out at a temperature lower than that of the other zones which are in permanent contact with the metal and the molten slag.

Method of manufacturing the coating

The manufacture of a coating of a crucible of a dephosphor induction induction furnace according to the invention is conventional, as described in the preamble of the present description.

Preferably, before introducing the non-ferrous metal, the furnace is heated to a temperature greater than 1450C, preferably greater than 1500 ° C, to ensure sintering of the consolidated region.

Preferably, the refractory lining extends to define an inner surface 28 of the crucible without a seal.

The molten ferrous metal in the crucible may be a steel or a cast iron. The melt can be obtained in particular by direct reduction or from pre-reduced iron ore.

Examples

The following raw materials were used: white corundum powder having a median size of 1 mm, a minimum size of 75 μm and a maximum size of 4 mm, having an alumina content greater than 99.6%, the density of the material constituent of the particles being equal to 3.95 g / cm 2; white corundum powder having a size less than 200 μm and a median size of 30 μm; calcined alumina powder AC44B6, with a median size of about 6 μm and having an alumina content of greater than 99%, marketed by ALTEO; mid-size fine quartz sand of 20 μm provided by Sibelco; pigmentary chromium oxide typically containing 98% by weight Cr203, having a median D50 of 0.3 μm and a grain density of 5.0, marketed by BAYFERROX; magnesia powder (magnesium oxide) calcined, containing 97% by weight of MgO, the particle size being between 500 μm and 4 mm, marketed by MAF Magne site; calcined magnesia powder, containing 97% by weight of MgO, the maximum particle size being 200 μm, the median size being 90 μm, marketed by MAF Magne site.

The various particulate raw materials are introduced into a kneader and mixed dry for 5 minutes.

For Example 1, the magnesia was mixed with calcined alumina and quartz sand in proportions adapted to obtain the mineral chemical composition shown in Table 1. Example 2 is obtained by substituting white corundum a large part of the magnesia and all of the crystalline silica provided in the form of fine quartz sand used in the mixture of Example 1.

In Example 3, unlike the mixture of Example 2, a portion of the calcined alumina and fine calcined magnesia was substituted with an equivalent weight of chromium pigment oxide.

In Example 4 according to the invention, the level of chromium oxide is multiplied by more than 2.5 relative to Example 3 by replacing a portion of the calcined alumina.

In the example 5 according to the invention, by difference with Example 3, the magnesia was totally substituted by chromium oxide pigment. Comparative Example 6 has an even higher chromium oxide content. This was done by mixing a fraction of 0.2 to 2 mm obtained by crushing-grinding a coimu refractory product, called Zirchrom 90®, 18% alumina, 5% chromium oxide pigment and 3% of quartz sand.

After homogenization of the mineral powders, 1% of dextrin powder is also added to a mixer, this percentage being relative to the mass of said powder. After 5 minutes of mixing, 3% water is gradually added, in percentage relative to the mass of the initial powder (thus excluding dextrin powder), without stopping the mixer. After introduction of all the water, mixing is continued for 5 minutes. The addition of a temporary binder such as dextrin and water are necessary for the manufacture of small samples, but are not required for industrial applications, particularly for forming a coating of an oven.

The wet mixtures are then poured into suitable molds for the manufacture of the following parts: a cylinder with a diameter of 50 mm and a height of 60 mm drilled at its center with a hole of diameter 12 mm, intended for measuring the permanent thermal deformation, 3 cylinders of the same dimensions, without hole, for cold compressive strength measurements, a cylinder 70 mm in diameter and 380 mm in height for the corrosion test. For this purpose, the wet mixture poured into the molds undergoes uniaxial pressing at a pressure of 90 MPa. The preforms thus obtained are demolded and then dried for 24 hours at 110 ° C.

The cylinder for measuring permanent thermal deformation is not heat treated.

The other cylinders are consolidated according to the following thermal cycle: rise in temperature, from 20 ° C to 1600 ° C, to 150 ° C / h; 2 hours at 1600 ° C; down to 150 ° C / h up to 20 ° C.

Characterization Chemical analysis is performed 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 weight, it is determined by ICP ("Induction Coupled Plasma"), on a Vista AX model (marketed by Varian).

The measurements of the bulk density and the open porosity after consolidation heat treatment are carried out according to the following method: the cylinders mentioned above are first weighed dry to determine their dry mass. They are then placed in a bell under vacuum for 30 minutes. The bell is then filled with water, so that the bars are completely immersed. After immersion, the vacuum is maintained for 30 minutes.

Atmospheric pressure is then restored to the bell and allowed to stand for another 30 minutes. The samples are then subjected to hydrostatic weighing, giving a mass Mi. They are then wiped with a damp cloth and their wet mass Mh is measured.

The apparent density is given by the ratio p.Ms/(Mh-Mi), in g / cm.sup.2, with p being the density of the water, taken equal to 1 g / cm.sup.2.

The open porosity is given by the ratio 100 (Mh-Ms) / (Mh-Mi), in%.

The measurement of the permanent thermal deformation is carried out, according to the standard EN993-10, by comparison of the dimensions of the cylinder between the "raw dry" state, that is to say preceding a heat treatment, and that after said heat treatment at 1600 ° C for 2 hours and return to room temperature.

Measurements of mechanical resistance to cold compression are made according to EN 993-5.

Corrosion resistance is evaluated in a laboratory induction furnace with a capacity of 250 kg of ferrous metal. The cylinder, introduced into the furnace, is rotated axially at a linear speed of 10 cm per second (30 rpm) and then immersed for 20 minutes in a bath of molten ferrous metal at a temperature of 1580 ° C. at a depth of between 10 and 15 cm below the surface of the bath. The initial mass composition of the ferrous metal in molten metal is as follows: 0.2% C; 0.3% Si; 0.7% Mn and 0.08% P. The initial mass composition of the dephosphorizing slag is as follows: 58.8% CaO, 10.6% CaCl and 30.3% FeO. Results

The following table 1 summarizes the compositions and the results obtained.

Na: unmeasured *: comparative example

Alumina-magnesia type products such as those of Comparative Examples 1 to 3 have a permanent thermal deformation that is too high or a low resistance to corrosion. Products with a high content of chromium oxide, such as Example 6, have the disadvantage of a low mechanical strength.

The tests show that the permanent thermal deformation is much lower with the examples according to the invention. The risk of cracking is reduced and the life of the coating increased. The thickness of the coating can also be reduced.

The tests also show that the corrosion resistance for the intended application is improved with the examples according to the invention. Example 5 is the most preferred example. As now clearly apparent, the invention provides a material with good corrosion resistance, high mechanical strength and permanent thermal deformation of less than 5%, preferably less than 3%.

Of course, the invention is not limited to the embodiments provided as examples.

Claims (18)

1. Material presenting the following average chemical composition, in percentages by mass on the basis of the refactory oxides and for a total of 100% | - more than 50% of AlaOs, more than 4% of Cr203, less than 20% of MgO, less than 5% of CaO, less than 3% of SiO 2, less than 5% of FezOs, Less than 30% of ZrO 2, less than 10% of TiO 2, the constituents other than Al 2 O 3, Cr 2 O 3, MgO, CaO, SiO 2, FezO 3, ZrO 2 and TiO 2 constituting the 100% complement, the sum of the oxides AlzOa-tCuZOj + FezOs + ZtOz + TiOz representing more than 80% of said chemical composition. 2. Material according to the preceding claim, comprising: "more than 60% of AlzOa, and - more than 5% of CrzOs, and - less than 15% of MgO, and less than 2.5% of CaO, and less of 2% of SiO 2, and - more than 0.5% of TiO 2, and in which the sum of the oxides Alz 2 O + Cr 2 O + Fc 2 O + ZrO 2 + TiO 2 represents more than 90% of said chemical composition. 3. Material according to the immediately preceding claim, comprising; - more than 70% AlzOs, and ^ more than 7% CrzOs, and - less than 10% MgO, and less than 1% CaO, and ^ less than 1% of $ i02, and more than 1% TiO2, and wherein the sum of the AlzOs + CiiQs + FezOs + ZrOz + TiOz oxides accounts for more than 95% of said chemical composition. 4. Material according to any one of the preceding claims, comprising: more than 80% of AbOa, and less than 25% of CnCh, and less than 5% of MgO, and less than 0.5% of CaO, and less than 0.5% of SiCb, and less than 5% of Ρβ2θ3. and less than 10% TiO 2, and less than 30% Zt 2, and less than 1% P 2 O 5, and less than 1% alkali K 2 O + Na 2 O; and wherein the sum of the oxides Αΐ2θ3 + θΓ2θ3 + Ρβ2θ3 + Ζτθ2 + Τΐθ2 represents more than 97% of said chemical composition. 5. Material according to any one of the preceding claims, comprising; more than 85% Al 2 O 3, less than 20% CT 2 O 3, and less than 2% MgO, and less than 2% Pe 2 O 3. and less than 5% of ΉΟ2, and less than 10% of Z1O2, and less than 0.5% of P2O5, and less than 0.5% of K2O + Na2O alkalis. 6. Material according to the immediately preceding claim, comprising: less than 15% of Cr 2 O 3, and less than 1% of MgO, and less than 1% of Ρθ 2 O 3, and less than 3% of TiO 2, and less than 5% of % of Z1O2. 7. Material according to any one of the preceding claims, in the form of a powder, all the refractory particles of said powder representing more than 70%, preferably more than 80%, more than 90% of the mass of said powder. 8. Material according to the immediately preceding claim, wherein the aggregate consists of refractory grains of a material selected from the group consisting of aluminas, magnesia, chromium oxide, magnesia-chromites, bauxite, zirconia, partially stabilized zirconia, stabilized zirconia, alumina-zirconia, magnesia-alumina spinel, zircon, cordierite, aluminum titanate, alumina-zirconia-silica-chromium oxide materials, bauxite, zirconia reinforced with alumina, and mixtures thereof. 9. A material according to any one of the preceding claims, comprising a hot-activatable binder selected from the group consisting of boron oxide, boric acid, borates, ciyolite, fluoride salts, silicate compounds. , phosphate compounds, feldspars, magnesium chlorides, colemanite, clay, kaolin, crystalline silica, amorphous silica, in particular silica, phenolic resins, organic resins, especially furan resins, ceramic frits, and mixtures thereof. 10. Material according to any one of claims 1 to 8, comprising no heat activatable binder. 11. Material according to any one of the preceding claims, consisting, for more than 95% by weight, of refractory oxides, optionally hot-activatable binder, optionally anti-dust agent, optioimellement anti-wetting agent. selected from silicon carbide, SiAlON, and nitrides, and impurities. 12. Material according to any one of the preceding claims, comprising no hydraulic binder or organic binder. 13. Material according to any one of the preceding claims, comprising - between 2% and IS% of an anti-wetting agent selected from silicon carbide, SiAlON, nitrides, and / or - between 0.1% and 1% of an anti-dust agent selected from the group consisting of oils, especially mineral oils, kerosene, organic polymers and mixtures thereof. 14. A material according to any one of the preceding claims in the form of a consolidated powder or powder. 15. A furnace for induction of a molten ferrymix metal, said furnace comprising a refractory coating of which at least one region, in particular a region intended to be in contact with said ferrous metal in foil, is made of a material according to any one of the preceding claims. 16. Oven according to the immediately preceding claim, wherein the coating comprises a layer of a consolidated material, in contact with the molten ferrous metal and / or with a dephosphoric slag, and / or a powder material, said materials consolidated and powder according to any one of claims 1 to 14. 17. Oven according to any one of the two immediately preceding claims, wherein the coating is free of seals. 18. A process for producing a coating of a furnace according to any one of the three immediately preceding claims, said process comprising the following successive stages: a) preparation of a feedstock, (1) feedstock starting material comprising a powder material according to any one of claims 1 to 9 or 11 to 14 comprising a heat activatable binder, the liquid phase in said feedstock representing less than 3% of the mass of the feedstock; or (2) the feedstock consisting of a mixture comprising a powder material according to any one of claims 1 to 9 or 11 to 14 comprising a heat-activatable binder, on the one hand, and a temporary binder, an amount greater than 0.5% and less than 6%, and / or water in an amount of greater than 2% and less than 6%, and secondly in percentages by weight relative to the mass of the powder prior to addition of temporary binder and water; b) shaping of said feedstock by pressing, stirring or vibrating; c) heat treatment for consolidating the shaped feedstock at a temperature above the melting temperature of the heat-activatable binder.