Method for producing fused mayenite products

Abstract

The invention relates to a method for producing mayenite, comprising the following sequential steps: a) preparing a feed suitable for obtaining, at the end of step c), a product comprising more than 90% mayenite in percentage by weight based on the weight of the crystalline phase, said mayenite comprising calcium and / or strontium, aluminum and oxygen; b) melting the feedstock until a molten material is obtained; c) cooling the molten material in order to solidify the molten material and obtain a polycrystalline fused product; the melting is carried out using an electric arc furnace and in a reducing medium.

Description

Technical Field

[0001] The invention relates to a fused product of a mayenite electronic compound and to a method for producing such a product. Finally, the invention relates to a device comprising a fused product according to the invention or produced by a method according to the invention. Background Art

[0002] EP 1 717 217 describes mayenites or "mayenite electrides" in the form of electrides containing calcium and / or strontium, aluminum and oxygen, in particular having the formula (Ca 24 Al 28 O 64 ) 4+ )(4e - ) and ((Sr 24 Al 28 O 64 ) 4+ )(4e - ) mayenites. These mayenites are produced by melting a feed material, maintaining the molten material in an atmosphere having an oxygen partial pressure of less than or equal to 10 Pa, and then cooling.

[0003] The article "Mayenite-based electride C12A7e - :aninnovative synthetic method via plasma arc melting”, Materials Chemistry Frontiers, 2021, pp. 1301-1314, describes a method for making mayenite-based electronic compounds by melting using a plasma furnace.

[0004] Mayenite can also be obtained in the form of an electron compound by solid phase sintering followed by substitution of electrons for the oxygen ions present in the nanocages. This substitution can be caused, for example, by exposure to reducing metal vapors.

[0005] It is also known that the electrons of mayenite in the form of an electronic compound can be partially or completely replaced by anions arranged in the nanocage of mayenite. If the replacement is partial, the electrons keep the mayenite conductive, that is, keep the mayenite in the form of an electronic compound. Otherwise, the mayenite becomes electrically insulating and is therefore no longer an electronic compound.

[0006] The method for manufacturing gehlenite in the form of an electronic compound containing calcium and / or strontium, aluminum and oxygen is complex and expensive. These methods can only manufacture a small amount of gehlenite. Therefore, there is a continuous need for a simpler manufacturing method to manufacture gehlenite in large quantities, especially in the form of an electronic compound.

[0007] An object of the present invention is to respond to this need at least in part. Summary of the Invention

[0008] According to the present invention, this object is achieved by a method for manufacturing gehlenite in the form of an electronic compound, the method comprising the following sequential steps:

[0009] a) Prepare the feedstock;

[0010] b) Melt the feedstock until a molten material is obtained;

[0011] c) Cool the molten material so that it solidifies and a polycrystalline vitreous product is obtained, the feedstock in step a) being suitable for obtaining at the end of step c) a product containing more than 90% by mass of gehlenite based on the mass of the crystalline phase, the gehlenite containing calcium and / or strontium, aluminum and oxygen;

[0012] d) Optionally, grind the vitreous product into a powder form;

[0013] e) Optionally, perform particle size selection on the powder;

[0014] f) Optionally, replace only a part of the free electrons of the gehlenite of the vitreous product with anions arranged in the nanocages of gehlenite, the anion being preferably selected from F - , Cl - , OH - , H - , O2 - , O - , O2 (2-) , N 3- , NH 2- , NH2 - , C2 (2-) , S - , CN - , NO2 - , S2 - and mixtures thereof, the anions in the nanocages may vary depending on the nanocages considered, or preferably, regardless of the nanocages considered, the anions in the nanocages are the same,

[0015] The melting is carried out by an electric arc furnace and in a reducing medium.

[0016] In other words, the melting is carried out by the heat generated by the electric arc passing through the feedstock.

[0017] As will be seen in more detail in the remainder of the specification, the inventors have found that the above method is simple and inexpensive, since it only comprises a single melting step, making it possible to manufacture, at the end of step c), a vitrified product containing a very large amount of the ettringite electronic compound.

[0018] The manufacturing method according to the invention may also have one or more of the following optional features:

[0019] - In step b), the electrodes of the electric arc furnace are immersed in the molten material;

[0020] - In step a), based on the mass of the feedstock, the feedstock contains more than 0.5% of a reducing agent by mass percentage;

[0021] - In step a), based on the mass of the feedstock, the feedstock contains less than 7% of a reducing agent by mass percentage;

[0022] - The reducing agent is selected from carbon sources, metals and mixtures thereof;

[0023] - In step a), the feedstock contains less than 50% of ettringite containing:

[0024] - Calcium and / or strontium,

[0025] - Aluminium, and

[0026] - Oxygen;

[0027] - In step c), the cooling is adjusted so that the molten material solidifies completely in less than 3 minutes.

[0028] In one embodiment, step f) is replaced by step f’), which differs from step f) in that substantially all of the free electrons of the ettringite of the vitrified product are replaced by anions disposed in the nanocages of the ettringite, and the anion is preferably selected from F - , Cl - , OH - , H - , O2 - , O - , O2 (2-) , N 3- , NH 2- , NH2 - , C2 (2-) , S - , CN - , NO2 - , S2 -And mixtures thereof, the anions in the nanocages can vary depending on the nanocage considered, or preferably, regardless of the nanocage considered, the anions in the nanocages are the same. Thus, step f') renders the ettringite electrically insulating.

[0029] The present invention also relates to a vitreous product in polycrystalline form comprising more than 90% ettringite by mass percentage based on the crystalline phases, said ettringite comprising:

[0030] - calcium and / or strontium,

[0031] - aluminium, and

[0032] - oxygen,

[0033] By mass based on the mass of the vitreous product, the vitreous product comprises carbon in an amount greater than 15 ppm and less than or equal to 5%; said ettringite being:

[0034] - preferably, in the form of an electronic compound, or

[0035] - electrically insulating.

[0036] In the polycrystalline product, a carbon content greater than 15 ppm and less than or equal to 5% and an ettringite content greater than 90% by mass percentage based on the crystalline phases are considered to be characteristic of the step of melting in an electric arc furnace and in a reducing medium. Thus, the products according to the invention can be manufactured by the method according to the invention. In a preferred embodiment, the products are obtained by the method according to the invention.

[0037] Vitreous products obtained directly by the method according to the invention (i.e., without additional treatment) have a colour representative of their electrical conductivity. The colour of insulating ettringite is white. The colour of ettringite in the form of an electronic compound is between creamy yellow and black, preferably between green and black. Preferably, ettringite in the form of an electronic compound is dark green or black, preferably black. The closer the colour is to black, the higher the electrical conductivity.

[0038] As described in EP 3 489 197, a colour between creamy yellow and yellow corresponds to an electrical conductivity of less than 1·10 -4 S / cm, green corresponds to an electrical conductivity between 1·10 -4 S / cm and 1 S / cm, and black corresponds to an electrical conductivity greater than 1 S / cm.

[0039] In one embodiment of ettringite in the form of an electronic compound, the total amount of the elements Ca, Sr, Al, C and O, Ca+Sr+Al+O+C, is greater than 97% by mass percentage based on the mass of the vitreous product.

[0040] The polycrystalline fused product according to the invention may also include one or more of the following optional features:

[0041] - Based on the mass percentage of the crystalline phase, the product includes more than 95%, preferably more than 98%, of ettringite in the form of an electronic compound;

[0042] - The product has a carbon content greater than 300 ppm, preferably greater than 700 ppm, and / or less than 3%, preferably less than 1.5%;

[0043] - Based on the mass percentage by mass of the fused product, the product has a total amount of elements Ca, Sr, Al, C, and O, Ca + Sr + Al + O + C, greater than 97%, preferably greater than 98%;

[0044] - The product has a ratio of (CaO + SrO) / Al2O3 greater than 1.60, preferably greater than 1.68, and / or less than 2, preferably less than 1.80, where CaO, SrO, and Al2O3 are based on the molar content of oxides;

[0045] - The ettringite contains at least one element selected from Mg, K, Na, Li, Ba, Ga, Si, Ge, Sn, Ti, Zr, Cr, Mn, Fe, Co, Ni, Cu, Zn.

[0046] Finally, the invention relates to a device including the product according to the invention and / or a product manufactured by the method according to the invention.

[0047] When the product is in the form of an electronic compound, the device is selected from:

[0048] - A catalyst support, especially for the synthesis of NH3 or the decomposition of CO2;

[0049] - A solid oxide fuel cell (or SOFC), especially an electrode, especially a cathode;

[0050] - An organic light-emitting diode (or OLED), especially the cathode of such a diode;

[0051] - A Hall effect thruster, especially the cathode of such a thruster;

[0052] - A grid ion thruster (or ion thruster), especially the cathode of such a thruster;

[0053] - A proton-conducting ceramic fuel cell (or PCFC).

[0054] When the product is electrically insulating, the device is selected from:

[0055] - A solid oxide fuel cell (or SOFC), especially the electrolyte of such a cell;

[0056] - Solid oxide electrolyzer (or SOEC);

[0057] - CO2 capture device.

[0058] The device may also be a hydrogen storage and / or transportation device,

[0059] The product is in the form of an electron compound or is electrically insulating.

[0060] Definition

[0061] "Electron compound" is generally an ionic compound in which the trapped electrons act as negatively charged ions.

[0062] The term "gehlenite" is generally understood to refer to a compound with the chemical formula 12CaO·7Al2O3 or 12SrO·7Al2O3 or 12(Ca,Sr)O·7Al2O3, in which part of Ca and / or Sr and / or Al can optionally be replaced by another element, especially by Mg, K, Na, Li, Ba, Ga, Si, Ge, Sn, Ti, Zr, Cr, Mn, Fe, Co, Ni, Cu, Ir, V, P, Er, Yb, Eu, Au, Tb or Zn. The gehlenite crystal has a lattice that forms nano-cages. Gehlenite is electrically neutral.

[0063] Electrically insulating gehlenite can generally be considered to contain 12 nano-cages per unit cell and two O 2- ions in said nano-cages, and said O 2- ions are generally referred to as "free oxygen". The electrically insulating gehlenite can be represented by the formula ((Ca 24 Al 28 O 64 )) 4+ )(O 2- )2.

[0064] As is well known, all or part of the free oxygen can generally be called "free electrons" and be considered to be replaced by electrons located within the nano-cages: initially electrically insulating gehlenite then becomes conductive, that is, in the form of an electron compound. In a specific case where all the free oxygen is replaced by free electrons, gehlenite is obtained in the form of an electron compound, which is generally represented by the formula ((Ca 24 Al 28 O 64 )) 4+ )(4e - )).

[0065] It is also well known that all or part of the free oxygen and / or free electrons can be replaced by anions (such as F - ) (F -The number of anions ensures electrical neutrality).

[0066] To represent the "oxide-based" composition or content, according to common industry practice, all elements except carbon are converted to the form of their most stable oxides. For example, the contents of the elements calcium, strontium, and aluminum are expressed as the contents after these elements are converted to the forms of CaO, SrO, and Al2O3, respectively. The content of the elements except carbon thus converted is expressed based on all the elements except carbon thus converted.

[0067] In particular, the contents of CaO, SrO, and Al2O3 relate to the total contents of the respective chemical elements Ca, Sr, and Al. Therefore, the contents of CaO, SrO, and Al2O3 mentioned in this specification express not only the contents of the elements Ca, Sr, and Al in the forms of the CaO, SrO, and Al2O3 phases, but also the contents of the elements Ca, Sr, and Al in other forms. Thus, this particularly includes the contents of the elements Ca, Sr, and Al in the forms of lower oxides, optionally nitrides, oxynitrides, carbides, carbon oxides, carbonitrides, or even metals.

[0068] When referring to an oxide phase, such as a CaO, SrO, or Al2O3 phase, this is specified in a way that indicates the CaO, SrO, or Al2O3 "phase".

[0069] The total amounts of the elements Ca, Sr, Al, C, and O, Ca + Sr + Al + O + C, are expressed as a percentage by mass based on the mass of the fused product, without converting the metals to the form of their most stable oxides.

[0070] The term "impurity" is understood to mean an inevitable component that is necessarily introduced together with the starting materials. In particular, compounds belonging to the group of oxides, nitrides, oxynitrides, carbides, carbon oxides, carbonitrides, and metal substances of silicon, sodium, and other alkali metals, iron, and vanadium are impurities, especially when not seeking to replace a part of the elements Ca and / or Sr and / or Al of gehlenite.

[0071] The term "precursor" of an oxide is understood to mean a component that can provide the said oxide during the manufacture of the fused product according to the invention.

[0072] The term "fused product" is understood to mean a solid product obtained by the solidification and cooling of melted materials.

[0073] The "melted material" is a substance that has been made liquid by heating the feed. The melted material may contain some solid particles, but the amount is not sufficient for them to form the said agglomerates. The melted material must be contained in a container in order to maintain its shape.

[0074] An electric arc furnace is a well-known furnace, typically used for manufacturing fused products, particularly refractory blocks or abrasive grains. It includes electrodes between which an electric arc is generated. This electric arc emits a large amount of heat, enabling the feedstock to be melted.

[0075] An electric arc furnace typically generates an electric arc in open air between electrodes, usually made of graphite, which passes through the feedstock to melt it. An electric arc furnace (EAF) is particularly different from a plasma arc furnace (PAF), in which a plasma melts the feedstock, the plasma being generated by using an electric arc to excite a gas (so the electric arc is not used to directly melt the feedstock). The heating kinetics are also very different, which can alter the phases of the products manufactured.

[0076] Typically, when the electrodes are arranged such that their free ends are embedded in a liquid bath during the melting process, the electrodes are said to be "immersed in the bath of the molten material".

[0077] A "reducing medium" is a medium that causes oxygen atoms to be removed from the molten material. Melting in a reducing medium does not pose any difficulties. In particular, a reducing agent can be added to the feedstock, or in an electric arc furnace, the process conditions can be adjusted to ensure reduction, especially by bringing the electrodes closer to the bath of the molten material, or even by immersing the electrodes in the bath.

[0078] When referring to "all free electrons" or "substantially all free electrons", this is understood to mean all free electrons, except for a few exceptions that do not change the measurable properties of gehlenite. When referring to "a portion of free electrons", this does not include "substantially all free electrons".

[0079] The term "median particle size" of a powder refers to the size that divides the particles into a first population and a second population of equal mass, these first and second populations containing only particles with sizes greater than or equal to, or less than, the median particle size, respectively. The median particle size of a powder can be determined using the particle size distribution generated by a laser particle size analyzer.

[0080] The terms "comprising", "including" and "having" should be interpreted broadly and without limitation. Detailed Description

[0081] The following description is given for illustrative purposes and does not limit the present invention.

[0082] Method for manufacturing gehlenite in the form of an electronic compound

[0083] The method according to the present invention can be the method described in the following examples. It includes steps a) to f).

[0084] In step a), the starting materials are generally metered to obtain a feedstock having the desired composition. Preferably, the starting materials are mixed.

[0085] It is not difficult for a person skilled in the art to select the starting materials of the feedstock such that the vitrified product, i.e., the solid mass obtained at the end of step c), has a composition that meets the expectations. Specifically, a person skilled in the art knows how to adjust the feedstock, especially in view of the volatilization of some starting materials of the feedstock during the melting process.

[0086] The feedstock is preferably in the form of a particulate mixture. The median particle size of at least one starting material of the particulate mixture, preferably of each starting material, is preferably less than 1 mm, preferably less than 0.5 mm.

[0087] Preferably, the feedstock contains less than 70%, preferably less than 50%, preferably less than 30%, preferably less than 10% of gehlenite containing calcium and / or strontium, aluminum and oxygen.

[0088] More preferably, the feedstock does not contain any gehlenite containing calcium and / or strontium, aluminum and oxygen. Advantageously, the method is simplified.

[0089] The elements Ca and / or Sr, and Al are preferably introduced into the feedstock in the form of CaO and / or SrO, and Al2O3 oxides. They can also be introduced conventionally in the form of precursors of these oxides, such as in the form of CaCO3 and / or SrCO3 phases. The element Al is preferably introduced into the feedstock at least partly in the form of the Al2O3 phase and / or in the form of a precursor of this oxide, such as in the form of aluminum hydroxide and / or boehmite. Preferably, the element Al is introduced into the feedstock partly in the form of the Al2O3 phase and partly in metallic form. In one embodiment, the element Al is introduced into the feedstock entirely in the form of the Al2O3 phase.

[0090] In a preferred embodiment, the feedstock includes a reducing agent, i.e., a reagent that generates a reducing medium during the melting process, preferably selected from carbon sources, metals, and mixtures thereof. Preferably, the reducing agent includes a carbon source, preferably consists of a carbon source, and the carbon source is preferably selected from carbon, petroleum coke, pitch, coal, and mixtures thereof, preferably petroleum coke. Preferably, the metal is aluminum.

[0091] A person skilled in the art knows how to determine the appropriate amount of the reducing agent. Preferably, the amount of the reducing agent in the feedstock is greater than 0.5%, preferably greater than 1%, preferably greater than 1.5%, and preferably less than 7%, preferably less than 6%, preferably less than 5%, preferably less than 4%, preferably less than 3.5% by mass percentage based on the feedstock.

[0092] In step b), an electric arc furnace is used, preferably a Heroult type electric arc furnace with graphite electrodes, but all known electric arc furnaces can be envisaged as long as they can melt the feedstock in a reducing medium.

[0093] Melting in a reducing medium is preferably achieved by the presence of a reducing agent in the feedstock and / or by the proximity of the graphite electrodes to the bath of the molten material.

[0094] The melting of the feedstock is carried out with electrodes arranged at a certain distance from the feedstock, which is suitable for achieving melting in a reducing medium, preferably in a skimming mode, the distance between the electrodes and the feedstock being preferably less than 2 cm, or in a mode where the electrodes are immersed in the bath of the molten material.

[0095] Preferably, the electrodes are in the skimming mode or immersed in the bath of the molten material, and preferably immersed in the bath of the molten material. Preferably, the feedstock contains a reducing agent. Preferably, the electrodes are immersed in the bath of the molten material and the feedstock contains a reducing agent.

[0096] Preferably, the starting material is melted at atmospheric pressure.

[0097] Preferably, the starting material is melted in an uncontrolled gas environment, preferably in air.

[0098] Preferably, an electric arc furnace is used which comprises a 160 - litre tank, the melting energy before casting being greater than 1 kWh per kg of starting material and preferably less than 6 kWh per kg of starting material, the power being preferably greater than 200 kW, or an electric arc furnace of a different capacity under equivalent conditions is used. A person skilled in the art knows how to determine such equivalent conditions.

[0099] In step c), the molten material is cooled to solidify it and obtain a vitrified product.

[0100] Preferably, the cooling is rapid, preferably causing the molten material to completely solidify in less than 3 minutes, preferably in less than 2 minutes, preferably in less than 1 minute, preferably in less than 40 seconds, preferably in less than 30 seconds, and preferably greater than 1 second. The rapid cooling may in particular be caused by casting into a mould as described in US 3,993,119.

[0101] Steps a) to c) can in particular produce a vitrified product containing a large amount of calcium aluminate electronic compound, which calcium aluminate electronic compound includes free electrons and possibly free oxygen located in nano - cages.

[0102] In optional step d), the vitrified product is ground to obtain a powder. The grinding can be achieved by any conventional technique.

[0103] In optional step e), the powder is size-selected so that the particle size of the powder is adapted to the target application. Size selection can be carried out, for example, by screening or cyclone.

[0104] In optional step f), at least a part of the free electrons (and any free oxygen) of the ettringite in the fused product (possibly in the form of a powder) is replaced by anions arranged in the nano-cages of the ettringite. The anion is preferably selected from F - , Cl - , OH - , H - , O2 - , O - , O2 (2-) , N 3- , NH 2- , NH2 - , C2 (2-) , S - , CN - , NO2 - , S2 - and mixtures thereof. The anion can vary depending on the nano-cage considered, or preferably, regardless of the nano-cage considered, the anion is the same.

[0105] The method of replacing the free electrons (and any free oxygen) present in the ettringite of the fused product with anions is well known to those skilled in the art. For example, H - anions can replace at least a part of the free electrons present in the ettringite of the fused product by heat treatment at 1250 °C for 2 hours in a 100% H2 atmosphere.

[0106] Method for manufacturing insulating ettringite

[0107] In a non-preferred embodiment, the method includes an optional step f') instead of step f). During the optional step f'), substantially all of the free electrons (and any free oxygen) of the ettringite in the fused product (possibly in the form of a powder) are replaced by anions arranged in the nano-cages of the ettringite, and the electrical neutrality of the ettringite is maintained. The anion is preferably selected from F - , Cl - , OH - , H - , O2 - , O - , O2 (2-) , N 3- , NH 2- , NH2 - , C2 (2-) , S - , CN - , NO2 - , S2- and mixtures thereof. The anion can vary depending on the nanocage under consideration, or preferably, the anion is the same regardless of the nanocage considered.

[0108] Thus, the gehlenite in the form of an electronic compound before step f’) becomes electrically insulating.

[0109] The vitrified product of gehlenite in the form of an electronic compound

[0110] The vitrified product according to the invention comprising more than 90% of gehlenite in the form of an electronic compound preferably has one or more of the following optional features:

[0111] - The content of oxide compounds is greater than 95%, preferably greater than 96%, preferably greater than 97%, preferably greater than 98% by mass percentage based on the mass of the vitrified product;

[0112] - The total of carbon and oxide compounds accounts for more than 99%, preferably more than 99.5%, preferably 100% of the mass of the vitrified product;

[0113] - The carbon content is greater than 15 ppm, preferably greater than 20 ppm, preferably greater than 100 ppm, preferably greater than 300 ppm, preferably greater than 500 ppm, preferably greater than 700 ppm, and / or preferably less than 4%, preferably less than 3%, preferably less than 2%, preferably less than 1.5% by mass percentage based on the mass of the vitrified product;

[0114] - The amount of gehlenite is greater than 92%, preferably greater than 93%, preferably greater than 94%, preferably greater than 95%, preferably greater than 96%, preferably greater than 97%, preferably greater than 98%, preferably greater than 99% by mass percentage based on the mass of the crystalline phase;

[0115] - The amount of the amorphous phase measured as described in the examples is less than 30%, preferably less than 20%, preferably less than 15%, preferably less than 10% by mass percentage based on the mass of the vitrified product;

[0116] - In a preferred embodiment, preferably for gehlenite in the form of an electronic compound, the total amount of the elements Ca, Sr, Al, C and O, Ca + Sr + Al + O + C, is greater than 97%, preferably greater than 98%, preferably greater than 99%;

[0117] - The ratio of (CaO + SrO) / Al2O3 is greater than 1.60, preferably greater than 1.64, preferably greater than 1.68, and preferably less than 2, preferably less than 1.94, preferably less than 1.86, preferably less than 1.80, preferably less than 1.76, preferably less than 1.74, where CaO, SrO and Al2O3 are the molar contents based on oxides;

[0118] - In one embodiment, the ettringite contains at least one element selected from Mg, K, Na, Li, Ba, Ga, Si, Ge, Sn, Ti, Zr, Cr, Mn, Fe, Co, Ni, Cu, Zn, and the at least one element preferably replaces Ca and / or Sr and / or Al;

[0119] - The Raman spectrum of the fused product according to the present invention has a band at 1870 cm -1 and the Raman spectrum is measured using a monochromatic source with a wavelength equal to 532 nm;

[0120] - A part of the nanocages of the ettringite is occupied by anions, and the anions are preferably selected from: F - , Cl - , OH - , H - , O2 - , O - , O2 (2-) , N 3- , NH 2- , NH2 - , C2 (2-) , S - , CN - , NO2 - , S2 - and their mixtures, and the ettringite also contains free electrons;

[0121] - A part of the nanocages of the ettringite is occupied by O 2- , and a part of the nanocages of the ettringite is occupied by anions selected from F - , Cl - , OH - , H - , O2 - , O - , O2 (2-) , N 3- , NH 2- , NH2 - , C2 (2-) , S - , CN - , NO2 - , S2 - , and the anions can be the same or different according to the nanocages considered, and the ettringite also contains free electrons;

[0122] - Different anions can be accommodated in different nanocages of the ettringite, or preferably, all the nanocages occupied by anions are occupied by the same anion, and the ettringite also contains free electrons.

[0123] In a preferred main embodiment, the fused product according to the invention has:

[0124] - a content of oxide compounds greater than 95%, preferably greater than 96%, preferably greater than 97%, preferably greater than 98% by mass, based on the mass of the fused product, and

[0125] - a carbon content greater than 15 ppm, preferably greater than 20 ppm, preferably greater than 100 ppm, preferably greater than 300 ppm, preferably greater than 500 ppm, preferably greater than 700 ppm, and less than 4%, preferably less than 3%, preferably less than 2%, preferably less than 1.5% by mass, based on the mass of the fused product, and

[0126] - an amount of monetite phase greater than 90%, preferably greater than 92%, preferably greater than 93%, preferably greater than 94%, preferably greater than 95%, preferably greater than 96%, preferably greater than 97%, preferably greater than 98%, preferably greater than 99% by mass, based on the mass of the crystalline phase, and

[0127] - an amount of amorphous phase measured as described in the examples less than 30%, preferably less than 25%, preferably less than 20%, preferably less than 15%, preferably less than 10% by mass, based on the mass of the fused product, and

[0128] - a total amount of elements Ca, Sr, Al, C, and O, Ca + Sr + Al + O + C, greater than 97%, preferably greater than 98%, preferably greater than 99% by mass, based on the mass of the fused product, and

[0129] - a ratio of (CaO + SrO) / Al2O3 greater than 1.60, preferably greater than 1.64, preferably greater than 1.68, and preferably less than 2, preferably less than 1.94, preferably less than 1.86, preferably less than 1.80, preferably less than 1.76, preferably less than 1.74, where CaO, SrO, and Al2O3 are molar contents based on oxides.

[0130] Preferably, the color of the monetite of the fused product is between green and black, preferably dark green or black, preferably black.

[0131] In one embodiment, the fused product is in the form of a powder, preferably having a median particle size greater than 0.1 μm and less than 1 mm.

[0132] Fused product of electrically insulating monetite

[0133] In a non - preferred embodiment, the monetite is electrically insulating. The monetite contains substantially no free electrons, and a part of the nanocages is occupied by anions, preferably occupied by anions selected from the following: F -, Cl - , OH - , H - , O2 - , O - , O2 (2-) , N 3- , NH 2- , NH2 - , C2 (2-) , S - , CN - , NO2 - , S2 - , The anions in the nanocages are the same or different depending on the nanocages considered. Preferably, a part of the nanocages of gehlenite is occupied by free oxygen O 2- and a part of the nanocages of gehlenite is occupied by anions selected from F - , Cl - , OH - , H - , O2 - , O - , O2 (2-) , N 3- , NH 2- , NH2 - , C2 (2-) , S - , CN - , NO2 - , S2 - and the anions can be the same or different depending on the nanocages considered.

[0134] Thus, the present invention relates to a vitreous product in the form of a polycrystal comprising more than 90% by mass of electrically insulating gehlenite based on the mass of the crystalline phase, said gehlenite comprising:

[0135] - calcium and / or strontium,

[0136] - aluminium, and

[0137] - oxygen,

[0138] The vitreous product contains carbon in an amount greater than 15 ppm and less than or equal to 5% by mass based on the mass of the vitreous product.

[0139] The vitreous product has a white color.

[0140] The other characteristics of the fused product of monetite in the form of an electronic compound, in particular those related to the content of oxide compounds, those related to the content of carbon and oxide compounds, those related to the content of carbon, those related to the amount of monetite, those related to the amount of amorphous phase, those related to the total amount of the elements Ca, Sr, Al, C and O, those related to the ratio (CaO + SrO) / Al2O3 (CaO, SrO and Al2O3 are based on the molar content of the oxides), those related to the elements that may be present in monetite (in particular elements that replace Ca and / or Sr and / or Al), and those related to the anions present in the nanocages, are optionally applicable to this embodiment.

[0141] Examples

[0142] The following non-limiting examples are given for the purpose of illustrating the invention.

[0143] Measurement protocol

[0144] To determine the composition of the fused product, beads are made by melting the powder of the fused product. The content of elements other than carbon is measured by X-ray fluorescence, and oxygen is considered as the mass balance up to 100%.

[0145] The carbon content of the fused product is measured using a CS744 carbon-sulfur analyzer sold by LECO.

[0146] The median particle size of the powder is usually measured using a LA950V2 laser particle size analyzer sold by Horiba.

[0147] The amount of various crystalline phases present in the fused product is measured on a sample ground in a dry state in a RS100 grinder sold by Retsch, which is equipped with a bowl and tungsten carbide wheels, such that the sample is in the form of a powder having less than 5% by mass of oversize material on a 40 μm sieve.

[0148] Obtained using a Bruker D8 Endeavor machine, in the 2θ angle range between 5° and 80°, with a step size of 0.01° and a counting time of 0.34 seconds / step. The front lens includes a primary slit of 0.3° and a Soller slit of 2.5°. Using an automatic knife, the sample rotates by itself at a speed equal to 15 rpm. The rear lens includes a Soller slit of 2.5°, a nickel filter of 0.0125 mm and a 1D detector with an aperture equal to 4°.

[0149] The diffraction pattern is then qualitatively analyzed using EVA software version 6.0 and the COD database.

[0150] The record COD 4308076 in the COD database (Crystallography Open Database) can identify the monetite phase. The peaks of the monetite phase present in the vitrified product may be slightly offset from the data record used, depending on the presence of the element Sr and the elements selected from Mg, Ga, Si, Ge, Sn, Ti, Zr, Cr, Mn, Fe, Co, Ni, Cu, Zn, and mixtures thereof.

[0151] Once the phases present have been identified, the amounts by mass of monetite and other crystalline phases can be evaluated by Rietveld refinement using HighScore Plus software.

[0152] The amount of the amorphous phase present in the vitrified product was measured by X-ray diffraction using a Bruker D8 Endeavor machine according to the following method. A diffraction pattern was obtained using this equipment, in the same manner as for the determination of the crystalline phases, and the sample analyzed was in the form of a powder having less than 5% by mass of oversize on a 40 μm sieve.

[0153] After opening the diffraction pattern obtained using EVA 6.0 software, click on the "define background" icon: the "define background" window appears on the screen.

[0154] In the "define background" window, select "original measurement" and "add background as scan", and then check the box "Auto Curvature and Threshold".

[0155] In the "Properties" window of the diffraction pattern, check the boxes "Calculate crystallinity" and "Display amorphous".

[0156] The value for the amount of the amorphous phase is the value of "% amorphous" in the "Properties" window of the diffraction pattern, in mass percentage based on the mass of the sample.

[0157] Manufacturing protocol

[0158] The examples were prepared from the following starting materials:

[0159] - Alumina powder with a purity greater than 99.8% by mass and a median particle size equal to 90 μm,

[0160] - Calcium carbonate powder with a purity greater than 99.3% by mass and a median particle size equal to 1.6 μm,

[0161] - Petroleum coke.

[0162] The fused product of Comparative Example 1* was prepared according to the teachings of EP 1 717 217 as follows: 37.3 g of alumina powder and 62.7 g of calcium carbonate powder were mixed in a jar mixer for 30 minutes, and then the mixture was placed in a carbon crucible, which was closed with a carbon lid. Finally, the filled crucible was placed in a quartz tube of a tube furnace, and an argon atmosphere was maintained in the quartz tube during the entire following heat treatment:

[0163] - Rising from ambient temperature at a rate equal to 400 °C / h until 1650 °C,

[0164] - Maintaining at 1650 °C for 9 hours and 30 minutes,

[0165] - Descending from 1650 °C to ambient temperature at a rate equal to 400 °C / h.

[0166] The fused product of Comparative Example 2* was prepared in the same manner as Example 1*, with the feed additionally containing 3.1 g of petroleum coke (3%).

[0167] The product of Example 3 according to the present invention was prepared according to the following manufacturing method of the present invention:

[0168] a) By mixing, a feed was produced consisting of 36.2% alumina powder, 60.8% calcium carbonate powder and 3% petroleum coke by mass percentage based on the mass of the feed;

[0169] b) The feed was melted in a Heroult-type single-phase arc furnace with graphite electrodes in a reducing medium. The furnace vessel had a diameter of 0.8 m, a voltage of 80 V, and a current of 2250 A. The specific electrical energy provided was 3 kWh / kg charged, and the electrodes were immersed in the bath of the molten material;

[0170] c) The molten material was suddenly cooled by a device for casting between thin metal plates (such as the device described in patent US-A-3993119), causing the molten material to completely solidify in the form of plates within less than 3 minutes.

[0171] The product of Example 4 according to the present invention was prepared in the same manner as the product of Example 3, except for step c): The molten material was cast into a graphite mold with dimensions of 220*200*180 mm 3 to obtain a fused product. The cooling of the molten material was slower than that of Example 3.

[0172] The product of Example 5 according to the present invention was prepared according to the following manufacturing method of the present invention:

[0173] a) By mixing, a feedstock is produced that consists of 37.3% alumina powder and 62.7% calcium carbonate powder by mass based on the mass of the feedstock;

[0174] b) The feedstock is melted in a Heroult-type single-phase electric arc furnace with graphite electrodes in a reducing medium. The furnace vessel has a diameter of 0.8 m, a voltage of 88 V, and a current of 2250 A. The specific electrical energy provided is 6 kWh / kg charged, and the electrodes are immersed in the bath of the molten material;

[0175] c) The molten material is suddenly cooled by a device for casting between thin metal plates (such as the device described in patent US-A-3993119), causing the molten material to completely solidify in the form of plates in less than 3 minutes.

[0176] The following Table 1 summarizes the results obtained:

[0177] [Table 1]

[0178]

[0179] n.d.: Not determined *: Outside the present invention

[0180] The products of Example 1 and Example 2 contain almost no gehlenite.

[0181] The products of Example 3 and Example 4 are black, which is characteristic of gehlenite electronic compounds with high electrical conductivity.

[0182] The product of Example 5 is yellow, which is characteristic of gehlenite electronic compounds with low electrical conductivity.

[0183] The products of Example 3 to Example 5 exhibit an amount of amorphous phase less than 10% by mass based on the mass of the vitrified product.

[0184] In Example 1, the melting was carried out in a closed carbon crucible placed in a neutral argon atmosphere, and the feedstock contained no reducing agent. It was found that Example 1 did not contain any gehlenite, while the method according to the present invention resulted in an amount of gehlenite electronic compound greater than 90% by mass based on the mass of the crystalline phase. The almost complete absence of gehlenite is also contrary to the teachings of EP 1 717 217.

[0185] The comparison between Example 1* and Example 2* shows that under the same manufacturing conditions, adding 3% petroleum coke as a reducing agent to the feedstock (Example 2*) increases the amount of gehlenite, but is not sufficient to obtain an amount of gehlenite phase electronic compound greater than 90% by mass based on the mass of the crystalline phase.

[0186] The comparison between Example 2* and Example 3 shows that, surprisingly, with the same content of petroleum coke (reductant) in the feed (3%) and with melting carried out in a reducing medium, a vitrified product containing more than 90% of the calcium aluminate phase electronic compound can be obtained, in mass percentage based on the mass of the crystalline phases.

[0187] The significant increase in the amount of calcium aluminate (multiplied by 2.9) is not the result of the only additional carbon provided by the electrodes (the amount of which is negligible). Nor is it caused by different amounts of reductant, which are the same for both examples. Surprisingly and in the absence of a theoretical explanation for this at present, this increase is therefore attributed to the use of an electric arc furnace.

[0188] The comparison between Example 3 and Example 4 shows that rapid cooling is beneficial in terms of the amount of calcium aluminate.

[0189] Finally, Example 5 shows that the presence of a reductant is not necessary to obtain a high content of calcium aluminate electronic compound, provided that melting is carried out with the electric arc furnace adjusted to ensure melting in a reducing medium.

[0190] It is noted that the Raman spectrum according to Example 3 of the present invention has a band at 1870 cm -1 and the Raman spectrum was measured using a monochromatic source with a wavelength equal to 532 nm.

[0191] As shown by the examples, the method according to the present invention is simple and inexpensive because it only includes a single melting step, enabling the production of a vitrified product containing a very large amount of calcium aluminate electronic compound. In other words, several heating steps are not required before obtaining the desired amount of calcium aluminate phase.

[0192] Of course, the present invention is not limited to the described embodiments, which are provided by way of illustrative and non - limiting examples.

[0193] In particular, the vitrified product according to the present invention is not limited to a specific shape or size.

Claims

1. A method for manufacturing gehlenite in the form of an electronic compound, the method comprising the following sequential steps: a) Prepare a feedstock; b) Melt the feedstock until a molten material is obtained; c) Cool the molten material so that the molten material solidifies and a polycrystalline vitreous product is obtained, the feedstock in step a) being suitable for obtaining, at the end of step c), a product containing more than 90% by mass of gehlenite based on the mass of the crystalline phase, the gehlenite containing - calcium and / or strontium, - aluminum, and - oxygen; d) Optionally, grind the vitreous product into a powder form; e) Optionally, perform particle size selection on the powder; The melting is carried out using an electric arc furnace and in a reducing medium.

2. The method according to the preceding claim, wherein, In step b), the electrodes of the electric arc furnace are immersed in the molten material.

3. The method according to any one of the preceding claims, wherein, In step a), based on the mass of the feedstock by mass percentage, the feedstock contains more than 0.5% reducing agent.

4. The method according to any one of the preceding claims, wherein, In step a), based on the mass of the feedstock by mass percentage, the feedstock contains less than 7% reducing agent.

5. The method according to any one of the two immediately preceding claims, wherein, The reducing agent is selected from carbon sources, metals, and mixtures thereof.

6. The method according to any one of the preceding claims, wherein, In step a), the feedstock contains less than 50% gehlenite containing calcium and / or strontium, aluminum, and oxygen.

7. The method according to any one of the preceding claims, wherein, The electric arc furnace includes graphite electrodes, and in step b), the melting of the feedstock is carried out with electrodes arranged at a certain distance from the feedstock, and the feedstock is suitable for melting in a reducing medium.

8. The method according to any one of the preceding claims, wherein, In step c), the cooling is adjusted so that the molten material solidifies completely in less than 3 minutes.

9. The method according to any one of the preceding claims, comprising step f) after step c), and / or when the method comprises step d) and / or step e), comprising step f) after step d) and / or step e), in which step f), only a part of the free electrons of the gehlenite of the vitreous product is replaced by anions, the anions preferably being all the same.

10. The method according to the immediately preceding claim, wherein, The anions are selected from F - , Cl - , OH - , H - , O2 - , O - , O2 (2-) , N 3- , NH 2- , NH2 - , C2 (2-) , S - , CN - , NO2 - , S2 - and mixtures thereof.

11. A method for manufacturing insulating gehlenite, the method comprising: - The method according to any one of claims 1 to 8, and - In step f') after step c), and / or when the method according to any one of claims 1 to 8 includes step d) and / or step e), in step d) and / or step e) after step f'), in step f'), substantially all free electrons of the gehlenite of the fused product are replaced by anions.

12. The method according to the immediately preceding claim, wherein, The anions are selected from F - , Cl - , OH - , H - , O2 - , O - , O2 (2-) , N 3- , NH 2- , NH2 - , C2 (2-) , S - , CN - , NO2 - , S2 - and their mixtures.

13. A polycrystalline fused product, the polycrystalline fused product comprising, by mass percentage based on the crystalline phase, more than 90% of gehlenite in the form of an electronic compound, the gehlenite comprising: - calcium and / or strontium, - aluminum, and - oxygen, The fused product comprises carbon in an amount greater than or equal to 15 ppm and less than or equal to 5% by mass based on the mass of the fused product.

14. The fused product according to the immediately preceding claim, - comprising, by mass percentage based on the crystalline phase, more than 95% of gehlenite in the form of an electronic compound; and / or - having a carbon content greater than 300 ppm and less than 3%; and / or - having a total amount of elements Ca, Sr, Al, C and O, Ca + Sr + Al + O + C, greater than 97% by mass percentage based on the mass of the fused product; and / or - having a ratio of (CaO + SrO) / Al2O3 greater than 1.60 and less than 2, where CaO, SrO and Al2O3 are based on the molar content of oxides; and / or wherein, The gehlenite contains at least one element selected from Mg, K, Na, Li, Ba, Ga, Si, Ge, Sn, Ti, Zr, Cr, Mn, Fe, Co, Ni, Cu, Zn.

15. The fused product according to the immediately preceding claim, - comprising, by mass percentage based on the crystalline phase, more than 98% of gehlenite in the form of an electronic compound; and / or - having a carbon content greater than 700 ppm and less than 1.5%; and / or - having a total amount of elements Ca, Sr, Al, C and O, Ca + Sr + Al + O + C, greater than 98% by mass percentage based on the mass of the fused product; and / or - having a ratio of (CaO + SrO) / Al2O3 greater than 1.68 and less than 1.80, where CaO, SrO and Al2O3 are based on the molar content of oxides.

16. The fused product according to any one of the immediately preceding three claims, having a conductivity between 1·10 -4 S / cm and 1 S / cm, or a conductivity greater than 1 S / cm.

17. The fused product according to any one of the immediately preceding four claims, having a Raman spectrum with a band at 1870 cm -1 measured using a monochromatic source with a wavelength equal to 532 nm.

18. The fused product according to any one of the immediately preceding four claims, which is manufactured by the method according to any one of claims 1 to 10.

19. A white polycrystalline fused product, the fused product containing, based on the mass percentage of the crystalline phase, more than 90% of calcium aluminate of the formula ((Ca 24 Al 28 O 64 ) 4+ )(O 2- )2, the calcium aluminate containing: - calcium and / or strontium, - aluminum, and - oxygen, The fused product contains carbon in an amount of greater than or equal to 15 ppm and less than or equal to 5% by mass based on the mass of the fused product.

20. The fused product according to the immediately preceding claim, which is manufactured by the method according to any one of claims 11 and 12.

21. A device, comprising: - The product according to any one of claims 13 to 18, the device is selected from: - Catalyst support; - Electrodes of solid oxide fuel cells; - Organic light-emitting diodes; - Hall effect thrusters; - Grid ion thrusters; - Proton-conducting ceramic fuel cells; - Hydrogen storage and / or transportation devices, or - The product according to any one of claims 19 and 20, the device is selected from: - Electrolytes of solid oxide fuel cells; - Solid oxide electrolyzers; - CO2 capture devices; - Hydrogen storage and / or transportation devices.

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