Alumina-based fused grain

Abstract

The invention relates to a molten grain having - a chemical composition such that, in mass percentages on the basis of oxides: MgO: 4.8% to 7.2%; Cr2O3: < 0.2%; oxides other than MgO, Cr2O3 and Al2O3: ≤ 1.5%; Al2O3: complement to 100%; - a crystallographic composition characterized by an X-ray diffraction pattern showing a "reference peak" in the angular range 2θ between 18.1° and 20.1°, a first peak in the angular range 2θ between 12.3° and 13.8° and, optionally, a second peak in the angular range 2θ between 25.2° and 25.8°, or "corundum phase peak", each peak having a respective height, in number of counts, the ratio R1 of the height of the first peak to the height of the reference peak being greater than 0.2 and the ratio R2 of the height of the corundum phase peak to the height of the reference peak being less than 1. No abstract figure

Description

Title of the invention: Alumina-based fused grain Technical field

[0001] The present invention relates to a fused alumina-based grain, a mixture of said grains, as well as a method for manufacturing said mixture of grains and an abrasive tool comprising said mixture of grains. The invention also relates to uses of grains according to the invention for abrading a surface made of hard steel. Prior art

[0002] It is known that the choice of grains for abrading a surface depends closely on the nature of said surface. To abrade a surface made of hard steel, the use of MA88K grains marketed by the company Saint-Gobain Speciality Grains and Powders is known.

[0003] Abrasive tools are generally classified according to the packaging of their abrasive grains: loose abrasives (powders of grains not fixed to a support, used in projection or in suspension), applied abrasives (support of the cloth or paper type, on which the grains are arranged in several layers) and agglomerated abrasives (circular grinding wheels, sticks, etc.).

[0004] In bonded abrasives, the abrasive grains are pressed with an organic or vitreous binder, typically a binder consisting of oxides, essentially silicate. The abrasive grains must themselves have good mechanical abrasion properties, and have good mechanical cohesion with the binder, i.e. the interface with the binder must be solid.

[0005] Among abrasive grains, a distinction is made between fused grains and sintered grains, which have different microstructures. The problems posed by sintered grains and by fused grains, and the technical solutions adopted to resolve them, are therefore generally different. A composition developed to manufacture a fused grain is therefore not a priori usable for manufacturing a sintered grain having the same properties, and vice versa.

[0006] There is a continuing need to improve the performance of fused alumina-based grains used to abrade a surface of hard steel, in particular to increase efficiency while maintaining energy efficiency greater than or substantially equal to that of prior art grains.

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

[0008] According to the invention, this aim is achieved by means of a molten grain, having: - a chemical composition such that, in mass percentages based on the oxides: MgO: 4.8% to 7.2%; Cr2O3: < 0.2%; oxides other than MgO, Cr2O3 and A12O3: < 1.5%; A12O3: 100% complement; - a crystallographic composition characterized by an X-ray diffraction pattern having a “reference peak” in the angular range 20 between 18.1° and 20.1°, a first peak in the angular range 20 between 12.3° and 13.8° and, optionally, a second peak in the angular range 20 between 25.2° and 25.8°, or “corundum phase peak”, each peak having a respective height, in number of counts, the ratio RI of the height of the first peak to the height of the reference peak being greater than 0.2 and the ratio R2 of the height of the corundum phase peak to the height of the reference peak being less than 1.

[0009] As will be seen in more detail in the remainder of the description, the inventors have discovered that, for the abrasion of a surface made of hard steel, the above chemical and crystallographic compositions provide a higher yield and an energy efficiency at least equal to those of known alumina-based fused grains.

[0010] A melted grain according to the invention may also have one or more of the following optional characteristics: - MgO > 5.0%, preferably MgO > 5.8%; - MgO < 7.0%, preferably MgO < 6.5%; - Cr2O3: < 0.10%, preferably Cr2O3: < 0.03%; - the content of oxides other than MgO, Cr2O3 and Al2O3 is less than 1.0%, preferably less than 0.4%; - Na2O < 0.3%, preferably Na2O < 0.1%, in mass percentages based on oxides; - SiO2 < 0.3%, preferably SiO2 < 0.1%, in mass percentages based on oxides; - the oxide content is greater than 96%, in percentages by mass based on the mass of the molten grain; - the ratio R2 is less than 0.7, preferably less than 0.3, preferably less than 0.05; - the RI ratio is greater than 0.25, preferably greater than 0.3; - the RI ratio is less than 0.5; - the release of gaseous hydrogen by hot acid attack for a mixture made up of said grains is less than 200 cmVIOO g.

[0011] The invention also relates to a mixture of grains comprising, as a percentage mass, more than 80% of melted grains according to the invention.

[0012] The invention also relates to a method for manufacturing a mixture of molten grains according to the invention, said method comprising the following successive steps: a) mixing raw materials so as to form a starting charge suitable for the manufacture of said mixture of grains, b) melting, in a reducing medium, said starting charge until a molten material is obtained, c) cooling said molten material so as to solidify it entirely in less than 3 minutes, and obtain a solid mass, f) calcination of the solid mass, before or after an optional particle size reduction, at a temperature greater than or equal to 750°C and less than or equal to 1225°C.

[0013] The inventors consider that the RI and R2 peaks of a grain according to the invention are, with the chemical composition, a signature of this process.

[0014] Conventionally, specialists in the abrasion of a surface made of hard steel consider as a general rule that it is necessary, to machine such a surface, to use a high-hardness grain, such as a fused corundum-based grain described in FR2853898A1.

[0015] Starting from FR2853898A1, the inventors tested calcination temperatures lower than those recommended in this document. They observed, on the X-ray diffraction diagrams, a sudden decrease in the height of the peaks of the corundum phase. However, and contrary to the general rule above, the inventors tested these grains to abrade a surface made of a hard steel. Surprisingly, they found that certain fused grains containing little or no corundum could lead to superior performances than those of fused grains based on corundum, provided that they have a magnesia content in a specific range and that their manufacture includes calcination in a particular temperature range.

[0016] This discovery is in particular illustrated by the comparison of examples 2 and 8, according to the invention, on the one hand, and 3, outside the invention and in accordance with the teaching of FR2853898A1, on the other hand. The grains according to the invention prove to be much more efficient for treating a surface made of hard steel.

[0017] A manufacturing method according to the invention may also have one or more of the following optional characteristics: - the solid mass resulting from step c) is ground during a step d), before or after step f), so as to be in the form of a particulate mixture, the process preferably comprising a step e) of particle size selection applied to the particulate mixture; - the method comprises a so-called step e), step d) and step e) being carried out before step f); - in step f), the calcination temperature is greater than or equal to 800°C and less than or equal to 1200°C, preferably greater than or equal to 950°C and less than or equal to 1150°C, and / or the calcination temperature is maintained for a period greater than or equal to 30 minutes, preferably greater than or equal to 2 hours.

[0018] The invention also relates to an abrasive tool comprising grains bound by a binder and agglomerated, for example in the form of a grinding wheel, or deposited on a support, for example a strip or a disc, this tool being remarkable in that at least a part, preferably more than 50%, preferably more than 70%, preferably more than 80%, preferably more than 90%, preferably more than 95%, preferably more than 99%, in mass percentage, preferably all of said grains are in accordance with the invention and / or manufactured according to a method according to the invention.

[0019] The abrasive tool may be a grinding wheel, and in particular a grinding wheel, a precision grinding wheel, a sharpening wheel, a cutting wheel, a grinding wheel in the mass, a deburring or roughing wheel, a driving wheel, a portable grinding wheel, a grinding wheel for foundries, a drill grinding wheel, a grinding wheel on a rod, a cylindrical grinding wheel, a grinding wheel with cones, discs or segments.

[0020] The invention finally relates to a method for treating a surface made of hard steel, said method comprising an operation of abrasion of said surface with a mixture of grains according to the invention or manufactured according to a manufacturing method according to the invention.

[0021] Preferably, a treatment method according to the invention comprises - a production of a mixture of grains according to the invention, preferably according to a production method according to the invention, then - an operation of abrasion of said surface with said mixture of grains, preferably after putting the mixture of grains in the form of an abrasive tool according to the invention. Definitions

[0022] In the present description, unless otherwise stated, all compositions of a grain or mixture of grains are given in mass percentages, based on the total mass of oxides of the grain or mixture of grains.

[0023] The oxide contents of a grain according to the invention, including for “oxides other than MgO, Cr2O3 and Al2O3”, relate to the overall contents for each of the corresponding chemical elements, expressed in the form of the most stable oxide, according to the usual industry convention; therefore included are sub-oxides and possibly nitrides, oxynitrides, carbides, oxycarbides, carbonitrides, or even the metallic species of the aforementioned elements.

[0024] By "impurities" we mean the unavoidable constituents, necessarily introduced with the raw materials. In particular, compounds belonging to the group of oxides, nitrides, oxynitrides, carbides, oxycarbides, carbonitrides and metallic species of silicon, sodium and other alkalis, iron, and vanadium are impurities. As examples, in the sources of Al and Mg, mention may be made of SiO2, Fe2O3 or Na2O.

[0025] By "precursor" of an oxide, we mean a constituent capable of providing said oxide during the manufacture of a grain or a mixture of grains according to the invention.

[0026] A “grain” is a particle all dimensions of which are less than 20 mm.

[0027] An “alumina-based grain” is a grain comprising more than 85% by mass of alumina, as a percentage based on oxides.

[0028] By "molten grain", or more broadly "molten product", we mean a solid grain (or product) obtained by solidification, by cooling, of a molten material.

[0029] A "molten material" is a mass made liquid by heating a starting charge, which may contain some solid particles, but in an insufficient quantity for them to be able to structure said mass. To retain its shape, a molten material must be contained in a container. The molten grains according to the invention are conventionally obtained by melting at more than 1900°C.

[0030] The term "median size" of a powder refers to the size dividing the particles into first and second populations of equal mass, these first and second populations comprising only particles having a size greater than or equal to, or less than, respectively, the median size. The median size of a powder can be determined using a particle size distribution produced using a laser particle size analyzer.

[0031] In the present description, by “hard steel” is meant a steel having a Rockwell hardness greater than or equal to 55 HRC. Detailed description

[0032] The following description is provided for illustrative purposes and does not limit the invention. Melted grain

[0033] The chemical composition of a fused grain according to the invention, and preferably of a mixture of grains according to the invention, preferably has one or more of the following optional and preferred characteristics: - MgO > 4.9%, preferably MgO > 5.0%, preferably MgO > 5.2%, preferably MgO > 5.5%, preferably MgO > 5.8%, and / or preferably MgO <7.1%, preferably MgO < 7.0%, preferably MgO < 6.8%, preferably MgO < 6.5%, in mass percentages based on the oxides; - Cr2O3 < 0.15%, preferably Cr2O3 < 0.10%, preferably Cr2O3 < 0.05%, preferably Cr2O3 < 0.03%, preferably Cr2O3 < 0.01%, in mass percentages based on the oxides; - Cr2O3 is not voluntarily added to the starting charge, i.e. it is preferably an impurity; - the content of oxides other than MgO, Cr2O3 and Al2O3 is less than 1.4%, preferably less than 1.3%, preferably less than 1.0%, preferably less than 0.9%, preferably less than 0.8%, preferably less than 0.7%, preferably less than 0.6%, preferably less than 0.5%, preferably less than 0.4%, in mass percentages based on the oxides; - oxides other than MgO and Al2O3 are preferably impurities; - the Na2O content is less than 0.3%, preferably less than 0.25%, preferably less than 0.2%, preferably less than 0.15%, preferably less than 0.1%, in mass percentages based on the oxides; - the SiO2 content is less than 0.3%, preferably less than 0.2%, preferably less than 0.15%, preferably less than 0.1%, preferably less than 0.08%, preferably less than 0.05%, in mass percentages based on the oxides; - the oxide content is greater than 96%, preferably greater than 97%, or even greater than 98%, or even greater than 99%, or even greater than 99.4%, or even greater than 99.5%, or even greater than 99.6%, or even greater than 99.7%, in mass percentages based on the mass of the molten grain; - the release of gaseous hydrogen by hot acid attack, expressed in volume of gas per 100 grams of grains, is less than 200 cmVIOO g, preferably less than 100 cmVIOO g, and preferably greater than 2 cmVIOO g. It is measured as described in detail in the examples.

[0034] The X-ray diffraction pattern of a molten grain according to the invention, and preferably of a mixture of grains according to the invention, preferably has one or more of the following optional characteristics: - the ratio R2 is less than 0.9, preferably less than 0.7, preferably less than 0.5, preferably less than 0.3, preferably less than 0.1, preferably less than 0.05; - the RI ratio is greater than 0.25, preferably greater than 0.3, and preferably less than 0.5, preferably less than 0.4. Grain mix

[0035] A mixture of grains according to the invention comprises, in mass percentages, preferably more than 85%, preferably more than 90%, preferably more than 95%, preferably more than 99%, preferably substantially 100% of melted grains according to the invention.

[0036] Preferably, a mixture of grains according to the invention respects a particle size distribution in accordance with those of the mixtures or "grits" provided by the FEPA Standard 43-GB-1984, R1993 and the FEPA Standard 42-GB-1984, R1993.

[0037] Preferably, a mixture of grains according to the invention has a mass rejection at the 16 mm sieve, preferably at the 9.51 mm sieve, measured using a Ro-Tap® sieve, of less than 1%, as a percentage by mass.

[0038] Method for manufacturing a mixture of molten grains according to the invention

[0039] Fused grains according to the invention can be manufactured according to steps a) to f) mentioned above. The parameters can for example take the values ​​of the process used for the examples below.

[0040] In step a), raw materials are conventionally dosed so as to obtain the desired composition, then mixed to form the starting charge.

[0041] The elements Al and Mg in the starting charge are found substantially entirely in the melted grains. The element Mg, in particular in the form of oxides, can however be subject to fly-off phenomena during melting. A person skilled in the art knows how to adapt the composition of the starting charge accordingly.

[0042] Choosing the raw materials of the starting charge so that the solid mass obtained at the end of step c) has a chemical analysis consistent with that of a grain according to the invention therefore poses no difficulty to those skilled in the art.

[0043] The element Mg is preferably introduced into the starting charge in the form of the oxide MgO. It can also be conventionally introduced in the form of precursors of this oxide, for example in the form of MgCO3.

[0044] The element Al is preferably at least partly introduced into the starting charge in the form Al2O3 and / or in the form of precursors of this oxide, for example in the form of aluminum hydroxide and / or boehmite. Preferably, the element Al is introduced into the starting charge partially in the form Al2O3 and partially in a metallic form.

[0045] In a preferred embodiment, the starting charge comprises at least one compound creating a reducing medium during melting.

[0046] Preferably, said compound is selected from a carbon source, a metal, and mixtures thereof. Preferably, the carbon source is selected from carbon, petroleum coke, pitch, coal and mixtures thereof, preferably petroleum coke. Preferably, the metal is aluminum.

[0047] More preferably, said at least one compound creating a reducing medium during melting and used in the starting charge is chosen from petroleum coke, aluminum and their mixtures.

[0048] A person skilled in the art knows how to determine the quantity of compound creating a reducing medium during fusion, in the starting charge, to obtain, in step b), fusion in a reducing medium.

[0049] Preferably, the starting charge contains an amount of compound creating a reducing medium during melting greater than 1%, preferably greater than 1.5% and, preferably less than 5%, preferably less than 4%, as a percentage by mass based on the starting charge.

[0050] In step b), an electric arc furnace is preferably used, preferably of the Héroult type with graphite electrodes, but all known furnaces are conceivable, such as an induction furnace or a plasma furnace, provided that they allow the starting charge to be melted in a reducing medium.

[0051] Melting in a reducing medium is preferably obtained by the presence, in the starting charge, of compounds creating a reducing medium during melting and / or by the fact that the electrodes are immersed in the bath of molten material.

[0052] Preferably, the starting charge contains elements creating a reducing medium during melting.

[0053] Preferably, the raw materials are melted at atmospheric pressure.

[0054] Preferably, an electric arc furnace is used, comprising a 70 liter tank, with a pre-casting melting energy of more than 2 kWh per kg of raw materials for a power of more than 220 kW, or an electric arc furnace of different capacity operated under equivalent conditions. A person skilled in the art knows how to determine such equivalent conditions.

[0055] In step c), the cooling must be rapid, that is to say so that the molten material is completely solidified in less than 3 minutes. For example, it may result from casting into molds such as described in US 3,993,119 or from quenching.

[0056] Preferably, the molten material is completely solidified in less than 2 minutes, preferably in less than 1 minute, preferably in less than 40 seconds, preferably in less than 30 seconds.

[0057] In one embodiment, step c) results in a solid mass in the form of a particulate mixture.

[0058] If step c) does not make it possible to directly obtain a mixture of grains, or if these grains do not have a particle size suitable for the intended application, grinding of the solid mass (step d)) can be carried out, according to conventional techniques.

[0059] A particle size selection (step e)), for example by sieving or cycloning, can be implemented to obtain a mixture of grains having a particle size adapted to the intended application.

[0060] Step d) and / or step e) may be carried out before or after the calcination step f). It / they are preferably carried out before step f).

[0061] Preferably, the molten grains obtained at the end of step c) and / or d) and / or e) have a carbon content greater than 15 ppm and less than 1000 ppm, said content being measured using a carbon-sulfur analyzer model CS744, marketed by the LECO company.

[0062] In step f), the solid mass obtained at the end of step c), preferably after reduction in the form of a particulate mixture during step d) if the process includes such a step, or preferably after step e) if the process includes such a step, is calcined.

[0063] If the calcination is carried out on the solid mass directly obtained at the end of step c), a step d) and preferably a step e) is / are then carried out after step f).

[0064] The calcination is carried out at a temperature greater than or equal to 750°C, preferably greater than or equal to 800°C, preferably greater than or equal to 900°C, preferably greater than or equal to 950°C, and less than or equal to 1225°C, preferably less than or equal to 1200°C, preferably less than or equal to 1150°C, the calcination temperature preferably being maintained for a time greater than or equal to 30 minutes, preferably greater than or equal to 1 hour, preferably greater than or equal to 2 hours, preferably greater than or equal to 3 hours, preferably greater than or equal to 4 hours, and preferably less than 100 hours, preferably less than 50 hours, preferably less than 20 hours, preferably less than 15 hours, preferably less than 10 hours.

[0065] Preferably, step f) is carried out at atmospheric pressure.

[0066] Step f) can be carried out in a reducing, neutral or oxidizing atmosphere.

[0067] Preferably step f) is carried out in an oxidizing atmosphere, preferably under air.

[0068] Preferably, step f) is carried out in an oxidizing atmosphere, preferably in air, preferably at atmospheric pressure, at a temperature greater than or equal to 750°C, preferably greater than or equal to 800°C, preferably greater than or equal to 900°C, preferably greater than or equal to 950°C, and less than or equal to 1225°C, preferably less than or equal to 1200°C, preferably less than or equal to 1150°C, the calcination temperature preferably being maintained for a period greater than or equal to 30 minutes, preferably greater than or equal to 1 hour, preferably greater than or equal to 2 hours, preferably greater than or equal to 3 hours, preferably greater than or equal to 4 hours, and preferably less than 100 hours, preferably less than 50 hours, preferably less than 20 hours, preferably less than 15 hours, preferably less than 10 hours.

[0069] Method of manufacturing an abrasive tool according to the invention

[0070] Methods of manufacturing abrasive tools are well known and can be used to manufacture an abrasive tool according to the invention.

[0071] The abrasive tools may in particular be formed by agglomeration of grains according to the invention by means of a binder, in particular in the form of a grinding wheel, for example by pressing, or be formed by fixing grains according to the invention on a support, for example a tape or a disc, by means of a binder.

[0072] The binder may be inorganic, in particular a glass (for example, a binder consisting of oxides, substantially consisting of silicate(s) may be used) or organic.

[0073] An organic binder is well suited. The binder may in particular be a thermosetting resin. It is preferably chosen from the group consisting of phenolic, epoxy, acrylate, polyester, polyamide, polybenzimidazole, polyurethane, phenoxy, phenol-furfural, analine-formaldehyde, urea-formaldehyde, cresol-aldehyde, resorcinol-aldehyde, urea-aldehyde, melamine-formaldehyde resins, and mixtures thereof.

[0074] The binder may also incorporate organic or inorganic fillers, such as hydrated (e.g., aluminum trihydrate or boehmite) or non-hydrated (e.g., molybdenum oxide) inorganic fillers, cryolite, halogen, fluorspar, iron sulfide, zinc sulfide, magnesia, silicon carbide, silicon chloride, potassium chloride, manganese dichloride, potassium or zinc fluoroborate, potassium fluoroaluminate, calcium oxide, potassium sulfate, a copolymer of vinylidene chloride and vinyl chloride, polyvinylidene chloride, polyvinyl chloride, and mixtures thereof. The binder may also contain reinforcing fibers such as glass fibers.

[0075] Conventionally, a mixture of grains according to the invention is mixed with a said binder optionally containing organic or inorganic fillers. The mixture obtained, in which the binder conventionally represents between 2% and 60%, preferably between 20% and 40% by volume, is shaped, for example placed in a mold or deposited on a support. Then the binder is activated, for example by heating, to bind the grains together and / or with the optional support. After hardening of the binder and optionally demolding, an abrasive tool according to the invention is obtained. Examples

[0076] The following non-limiting examples are given for the purpose of illustrating the invention. Measurement protocols

[0077] The following measurement protocols were used to determine certain properties of melted grain mixtures. They allow an excellent simulation of the real behavior of the grains when they are implemented for abrasion.

[0078] In order to evaluate the abrasive performance of a mixture of grains of the examples, a monolayer of 1.02 grams of this mixture is applied to a metal grinding wheel with a diameter of 12.7 cm, said grains being bonded using a phenolic resin.

[0079] The surface of a 52100 hard steel plate with a Rockwell hardness of between 60 and 63 HRC, measuring 20.5 cm x 7.6 cm x 6.0 cm, is then machined with the grinding wheel obtained, under water spraying, with a back and forth movement at constant speed, maintaining a constant cutting depth of 20 pm and a grinding wheel rotation speed of 3600 rpm. The total energy developed by the grinding wheel during machining, Etot, is recorded.

[0080] After complete wear of the grinding wheel, the mass of machined steel (i.e. the mass of steel removed by the grinding operation) “Ma” and the volume of steel removed by the grinding operation “Va” are measured.

[0081] To evaluate the yield, the ratio S of the mass of machined steel divided by the mass of grains consumed during said machining is conventionally calculated, here equal to 1.02 grams.

[0082] To evaluate energy efficiency, the specific machining energy, Es, is conventionally calculated, equal to the energy required to remove a unit volume of steel (Es = Etot / Va).

[0083] To determine the composition of the fused grains, a bead of a mixture of these grains is made by melting the mixture, then chemical analysis is carried out by X-ray fluorescence, except for the measurement of the carbon content.

[0084] The median size of a powder is conventionally measured using a laser granulometer model LA950V2 marketed by the company Horiba.

[0085] The quantity of hydrogen gas released by hot acid attack is measured after a hot attack of the grains with a mixture of hydrochloric and hydrofluoric acid. This evaluates the reoxidation of under-oxidized species (sub-oxides, up to the metal).

[0086] For this, after magnetic separation, the grains are ground in a grinding bowl made of an oxidized material (for example, a fused alumina-zirconia-silica material) until a powder is obtained which passes through a square mesh sieve with an opening equal to 160 μm. 5 g of said powder are taken and placed in a polypropylene reactor with a volume of 100 cm3. 25 ml of the following acid mixture are then added: (for one liter) 250 ml of 40% HF, 375 ml of 37% HCl, and 375 ml of water. After closing the reactor, the attack is carried out at 85°C, in a water bath, for 25 minutes, stirring regularly. After cooling the reactor, approximately 0.5 ml is taken using a syringe through a septum and injected into a gas chromatograph with catharometer detection (with, for the separation column, a 5 angstrom molecular sieve and argon as the carrier gas).The result is expressed in volume of gas under normal conditions per 100 g of ground grains.

[0087] This release is called “release of gaseous hydrogen by hot acid attack”.

[0088] The X-ray diffraction diagram of the molten grains of the examples is produced on a powder of particles having passed through a sieve with an opening equal to 40 μm, obtained by grinding said grains.

[0089] The acquisition of the X-ray diffraction pattern is carried out using a Bruker D8 Endeavor device, over an angular range 20 between 5° and 100°, with a step of 0.01°, and a counting time of 0.34 s / step. The front optics comprise a 0.3° primary slit, a 2.5° Soller slit and a 0.0125 mm nickel filter. The sample is rotated on itself at a speed equal to 5 rpm, using the automatic knife. The rear optics comprise a 2.5° Soller slit, a 0.0125 mm nickel filter and a 1D detector with an aperture equal to 4°.

[0090] The diffraction patterns are then analyzed qualitatively using the DIFFRAC.EVA software marketed by the Bruker company and the PDF4 + 2023 database. Manufacturing protocol

[0091] The mixtures of the examples were prepared from the following raw materials - an alumina powder with a purity greater than 99.6% by mass, comprising the impurities Na2O, CaO, Fe2O3, MgO, TiO2, SiO2, and having a median size equal to 80 pm; - a magnesia powder, with a purity greater than 99% by mass, of which more than 85% of the grains, by mass, pass through the cloth of a 45 pm sieve.

[0092] Reference example 1 (“Ref”), outside the invention, is a mixture of fused grains marketed by the company Saint-Gobain Speciality Grains and Powders under the name MA88K.

[0093] The grain mixtures of Examples 2 to 11 were prepared according to the following manufacturing process: (a) mixing the raw materials so as to form a starting charge, said starting charge comprising 2% by mass of metallic aluminium shavings and 0.5% by mass of petroleum coke, b) melting in a reducing medium of the said starting charge in a single-phase electric arc furnace of the Héroult type with graphite electrodes, with a furnace tank 0.8 m in diameter, a voltage of 125 V, an intensity of 1040 A and a specific electrical energy supplied of 3 kWh / kg charged, c) sudden cooling of the molten material, so that said molten material is completely solidified in less than 3 minutes, by means of a casting device between thin metal plates such as that presented in patent US-A-3,993,119, so as to obtain a completely solid plate, constituting a solid mass, d) grinding said solid mass cooled in step c) so as to reduce it to the form of a mixture of grains, e) selection by sieving using a Ro-Tap® sieve of grains with a size between 500 and 600 pm, f) calcination carried out in air, at atmospheric pressure, at temperature T, the temperature T being maintained for a time t, the rate of rise to temperature T being equal to 80°C / h.

[0094] The following Table 1 provides the chemical composition of these grain mixtures, the crystallographic characteristics, the temperature T, the time t and the results obtained with said mixtures.

[0095] The percentage improvement of the S ratio is calculated by the following formula: 100x(S ratio of the mixture of the example considered - S ratio of the mixture of reference example 1) / S ratio of the mixture of reference example 1.

[0096] A positive and high value of the percentage improvement of the ratio S is sought, without significant increase in the specific energy, preferably with a decrease in the specific energy (positive value of the percentage reduction in the specific energy Es described below). The inventors consider significant a percentage improvement of the ratio S greater than 5%.

[0097] Preferably, the ratio S is improved by more than 10%, preferably by more than 20%, preferably by more than 30%, preferably by more than 40%, preferably by more than 50%, preferably by more than 60%.

[0098] The percentage reduction in specific energy, Es, is calculated by the following formula: 100x(Es with the mixture of reference example 1 - Es with the mixture of the example considered) / Es of the mixture of reference example 1.

[0099] The inventors consider a reduction of more than 5% in the specific energy Es to be significant. Preferably, the specific energy is reduced by more than 10%, preferably by more than 15% (compared to the reference).

[0100] [Tableauxl] ex 1 (Re f) ex 2 ex 3 (*) ex 4 (*) ex 5 (*) ex 6 ex 7 ex 8 ex 9 (*) ex 10 (*) ex 11 (*) Calcination step f) Temperature T (°C) - 120 0 125 0 100 0 120 0 100 0 100 0 100 0 100 0 100 0 100 0 Time t (h) - 2 2 2 1 4 4 4 4 4 4 Chemical analysis, in percentages by mass based on oxides A12O3 Complement to 100% MgO 0.0 3 6.3 6.3 3.6 3.6 4.8 5.1 6.3 8.2 9.6 7.6 Cr2O3 0.0 3 0.03 0.0 3 0.0 3 0.0 3 0.0 3 0.0 3 0.0 3 0.0 3 0.0 3 0.0 3 Oxides other than MgO, Cr2 O3 and A12O3 0.5 0.14 0.1 4 0.5 0.5 0.1 0 0.1 1 0.1 4 0.1 1 0.1 0 0.1 0 of which Na2O 0.0 5 0.03 0.0 3 0.2 5 0.2 5 0.0 1 0.0 1 0.0 3 0.0 1 0.0 1 0.0 1 of which SiO2 0.0 5 0.05 0.0 5 0.0 5 0.0 5 0.0 5 0.0 5 0.0 5 0.0 5 0.0 5 0.0 5 of which TiO2 0.3 0.01 0.0 1 0.1 0.1 0.0 1 0.0 1 0.0 1 0.0 1 0.0 1 0.0 1 of which Fe2O3 0.0 4 0.03 0.0 3 0.0 5 0.0 5 0.0 1 0.0 2 0.0 3 0.0 1 0.0 1 0.0 1 of which CaO 0.0 2 0.02 0.0 2 0.0 5 0.0 5 0.0 2 0.0 2 0.0 2 0.0 3 0.0 3 0.0 3 Other characteristics Hydrogen gas evolution by hot acid attack, in volume of gas per 100 grams of grains 4 1 ndnd 90 27 6 25 ndnd R2 ratio - 0.65 4.7 7 0.0 5 0.2 5 0.0 5 0.0 5 0.0 5 0.0 5 0.0 5 0.0 5 RI ratio - 0.42 0.1 5 0.2 9 0.4 1 0.3 8 0.4 1 0.3 3 0.1 1 0.0 5 0.1 6 % improvement in S - 30 -25 30 47 70 68 80 15 -10 47 % reduction in Es - 6 -20 -16 -27 0 6 22 -22 -35 -12

[0101] (*): excluding invention

[0102] nd: not determined

[0103] A comparison of examples 1 (reference example), 2 according to the invention and 3 outside the invention and in accordance with the teaching of FR2853898A1, shows the impact of the calcination temperature: at the same MgO content equal to 6.3%, the mixture of grains of example 2, calcined at a temperature T equal to 1200°C for a time t equal to 2 hours, has a percentage improvement in the positive S ratio of 30% and a reduced specific energy of 6%, whereas the same mixture of grains calcined at a temperature T equal to 1250°C for a time t equal to 2 hours, has a percentage improvement in the negative S ratio of 25% and an increase in the specific energy of 20%.

[0104] A comparison of Examples 1 (reference example), 4 and 5 outside the invention, and 6 according to the invention, shows the positive impact of the presence of 4.8% of MgO in the grain mixture of Example 6, calcined at a temperature T equal to 1000°C for a time t equal to 4 hours: the S ratio is improved by 70% without degradation of the specific energy. The grain mixtures of Examples 4 and 5, outside the invention, having an MgO content equal to 3.6% and calcined at a temperature T equal to 1000°C and 1200°C, respectively, for a time t equal to 2 hours and 1 hour respectively, have an improved S ratio of 30% and 47% respectively, and an increase in the specific energy of 16% and 27% respectively.

[0105] A comparison of reference examples 1, and 7 and 8 according to the invention, shows that the grain mixtures of Example 7 and Example 8, both calcined at a temperature T equal to 1000°C for a time t equal to 4 hours, having an MgO content equal to 5.1% and 6.3% respectively, exhibit an improved S ratio of 68% and 80% respectively, and a reduction in specific energy of 6% and 22% respectively.

[0106] A comparison of reference examples 1 and 9 outside the invention shows that the mixture of grains of example 9 calcined at a temperature T equal to 1000°C for a time t equal to 4 hours, having an MgO content equal to 8.2%, presents an improvement in the S ratio of 15% and an increase in the specific energy of 22%.

[0107] A comparison of examples 1, reference, and 10 outside the invention, shows that the mixture of grains of example 10 calcined at a temperature T equal to 1000°C for a time t equal to 4 hours, having an MgO content equal to 8.2%, presents a reduction in the S ratio of 10% and an increase in the specific energy of 35%.

[0108] A comparison of examples 1, reference, and 11 outside the invention, shows that the mixture of grains of example 11 calcined at a temperature T equal to 1000°C for a time t equal to 4 hours, having an MgO content equal to 7.6%, presents a reduction in the S ratio of 47% and an increase in the specific energy of 12%.

[0109] As now clearly appears, the invention provides a mixture of fused alumina grains having a better yield and energy efficiency than those of the known fused alumina grains of the reference example for the abrasion of a surface of a hard steel.

[0110] Of course, the present invention is not limited to the embodiments described provided as illustrative and non-limiting examples.

[0111] In particular, the fused grains according to the invention are not limited to particular shapes or dimensions. They could be used in applications other than the abrasion of a surface made of a hard steel.

Claims

Claims

1. Fused grain having - a chemical composition such that, in mass percentages based on the oxides: MgO: 4.8% to 7.2%; Cr2O3: < 0.2%; oxides other than MgO, Cr2O3 and A12O3: < 1.5%; A12O3: balance to 100%; - a crystallographic composition characterized by an X-ray diffraction pattern having a “reference peak” in the angular range 20 between 18.1° and 20.1°, a first peak in the angular range 20 between 12.3° and 13.8° and, optionally, a second peak in the angular range 20 between 25.2° and 25.8°, or “corundum phase peak”, each peak having a respective height, in number of counts, the ratio RI of the height of the first peak to the height of the reference peak being greater than 0.2 and the ratio R2 of the height of the corundum phase peak to the height of the reference peak being less than 1.

2. Fused grain according to the preceding claim, in which - MgO > 5.0%, and / or - MgO < 7.0%, and / or - Cr2O3: < 0.10%, and / or - the content of oxides other than MgO, Cr2O3 and Al2O3 is less than 1.0%.

3. Fused grain according to the immediately preceding claim, in which - MgO > 5.8%, and / or - MgO < 6.5%, and / or - Cr2O3: < 0.03%, and / or - the content of oxides other than MgO, Cr2O3 and Al2O3 is less than 0.4%.

4. Fused grain according to any one of the preceding claims, in which - Na2O < 0.3%, in mass percentages based on the oxides, and / or - SiO2 < 0.3%, in mass percentages based on the oxides.

5. A fused grain according to the immediately preceding claim, wherein - Na2O <0.1%, in mass percentages based on oxides, and / or - SiO2 <0.1%, in mass percentages based on oxides.

6. A fused grain according to any preceding claim, wherein the oxide content is greater than 96%, as mass percentages based on the mass of the fused grain.

7. Fused grain according to any one of the preceding claims, wherein - the ratio R2 is less than 0.7, and / or - the ratio RI is greater than 0.

25.

8. Fused grain according to the immediately preceding claim, in which - the ratio R2 is less than 0.3, and / or - the ratio RI is greater than 0.

3.

9. A fused grain according to the immediately preceding claim, wherein the ratio R2 is less than 0.

05.

10. A fused grain according to any preceding claim, wherein the RI ratio is less than 0.

5.

11. A fused grain according to any preceding claim, wherein the release of hydrogen gas by hot acid attack for a mixture consisting of said grains is less than 200 cm3 / 100 g.

12. A mixture of grains comprising, in mass percentage, more than 80% of molten grains according to any one of the preceding claims.

13. A method of manufacturing a mixture of molten grains according to the immediately preceding claim, said method comprising the following successive steps: a) mixing raw materials so as to form a starting charge suitable for the manufacture of said mixture of grains, b) melting in a reducing medium of said starting charge until a molten material is obtained, c) cooling said molten material so as to solidify it entirely in less than 3 minutes, and obtain a solid mass, f) calcining the solid mass at a temperature greater than or equal to 750°C and less than or equal to 1225°C.

14. Method according to the immediately preceding claim, in which the solid mass resulting from step c) is ground during a step d), before or after step f), so as to be in the form of a particulate mixture, the method preferably comprising a step e) of particle size selection applied to the particulate mixture.

15. Method according to the immediately preceding claim, comprising a step e), step d) and step e) being carried out before step f).

16. Method according to any one of the three immediately preceding claims, in which in step f), - the calcination temperature is greater than or equal to 800°C and less than or equal to 1200°C, and / or - the calcination temperature is maintained for a period greater than or equal to 30 minutes.

17. Method according to the preceding claim, in which in step f), - the calcination temperature is greater than or equal to 950°C and less than or equal to 1150°C, and / or - the calcination temperature is maintained for a period greater than or equal to 2 hours.

18. An abrasive tool comprising grains bound by a binder and agglomerated or deposited on a support, at least a portion of said grains being in accordance with any one of claims 1 to 11.

19. An abrasive tool according to the immediately preceding claim, comprising more than 80% of grains according to any one of claims 1 to 11.

20. An abrasive tool according to any one of the two immediately preceding claims, in the form of a grinding wheel, belt or disc.

21. A method of treating a surface of a hard steel, said method comprising an operation of abrading said surface with a mixture of grains according to claim 12 or manufactured according to a method according to any one of claims 13 to 17.