AU2013298188B2

AU2013298188B2 - Sintered alumina particle

Info

Publication number: AU2013298188B2
Application number: AU2013298188A
Authority: AU
Inventors: Bing Liu; Emmanuel Nonnet
Original assignee: Saint Gobain Centre de Recherche et dEtudes Europeen SAS
Current assignee: Saint Gobain Centre de Recherche et dEtudes Europeen SAS
Priority date: 2012-08-01
Filing date: 2013-07-29
Publication date: 2017-07-13
Anticipated expiration: 2033-07-29
Also published as: FR2994177B1; PT2879800T; AU2013298188A1; CN104684647B; FR2994177A1; ZA201500743B; EP2879800B1; WO2014020522A1; CN104684647A; KR20150037976A; EP2879800A1; KR102088077B1

Abstract

The invention relates to a sintered particle manufactured from a feedstock containing 4 wt % to 25 wt % of zircon relative to the weight of the dry feedstock, and having the following chemical composition, in weight percent for a total of 100%: 70 % ≤ Al

Description

WO 2014/020522 PCT/IB2013/056225

Sintered alumina particle

Technical field

The present invention relates to novel sintered alumina-based particles, especially in the form of beads, to a process for manufacturing these beads, and to the use of these particles as grinding agents, agents for dispersion in a wet medium, or for surface treatment.

Prior art

The mineral or mining industry uses particles for the fine grinding of materials that have optionally been pre-ground in the dry state using conventional processes, especially for the fine grinding of calcium carbonate, titanium oxide, gypsum, kaolin and ore containing metals in generally combined form (oxides, sulfides, silicates, etc.), which processes may also first involve methods of purification, for example by flotation.

Moreover, paint, ink, dye, magnetic lacquer and agrochemical compound industries use particles for dispersing and homogenizing liquid and solid constituents.

Finally, the surface treatment industry makes use of particles, notably for operations for cleaning metallic molds (for manufacturing bottles for example), deburring of components, descaling, preparation of a support for coating, shot peening or peen forming, etc.

All these particles are conventionally substantially spherical and have a size of from 0.005 to 10 mm in order to serve all the markets described above. So that they can be used in these three types of applications, they must in particular have good wear resistance.

Notably in the field of microgrinding, the following different types of particles are commercially available, in particular in the form of beads: sand with rounded grains, such as Ottawa sand for example, is a cheap natural product, but is unsuitable for modern pressurized, high-throughput grinding mills. This is because the sand is not very strong, has a low density, varies in quality and is abrasive to the equipment; glass beads, which are widely used, have a better strength, a lower abrasiveness and are available in a wider range of diameters, and metallic beads, especially ones made of steel, have low inertness with respect to the products treated, notably leading to contamination of mineral fillers, a greying of paints, a disruption of the steps of separation/purification by flotation, and an excessive density requiring special grinding mills. They notably cause high energy consumption, considerable heating and a high level of mechanical stressing of the equipment.

Beads made of a ceramic material are also known. These beads have better strength than glass beads, a higher density and excellent chemical inertness.

The following may be distinguished: 2013298188 16 Jun2017 fused ceramic beads, generally obtained by melting ceramic components, forming spherical droplets from the molten material, and then solidifying said droplets, and sintered ceramic beads, generally obtained by cold forming of a ceramic powder, then 5 consolidation by firing at high temperature.

In contrast to sintered particles, fused particles most often comprise a very abundant intergranular vitreous phase which will fill a network of crystalline grains. The problems that sintered particles and fused particles encounter in their respective applications, and the technical solutions adopted for solving them, are therefore generally different. Moreover, owing to substantial differences between 10 the methods of manufacture, a composition developed for manufacturing a fused particle is not a priori usable as such for manufacturing a sintered particle, and vice versa. WO 2005/075375 discloses grinding beads comprising mullite, stabilized zirconia and alumina, manufactured from a mixture of raw materials that may comprise zircon.

In order to increase the efficiency of grinding operations, i.e. the amount of ground product for a 15 specified cost, the grinding particles must be more and more resistant to wear.

The invention seeks to meet this need.

Throughout this specification and the claims which follow, unless the context requires otherwise, the word "comprise", and variations such as "comprises" and "comprising", will be understood to imply the inclusion of a stated integer or step or group of integers or steps but not the exclusion of any 20 other integer or step or group of integers or steps.

The reference in this specification to any prior publication (or information derived from it), or to any matter which is known, is not, and should not be taken as an acknowledgment or admission or any form of suggestion that that prior publication (or information derived from it) or known matter forms part of the common general knowledge in the field of endeavour to which this specification relates. 25

Summary of the invention

The invention relates to a sintered particle, preferably in the form of a bead, produced from a feedstock containing 4% to 25% of zircon, as a weight percentage based on the weight of the dry feedstock, said sintered particle having: 30 - the following chemical composition, as percentages by weight: 70% < Al203, AI2O3 constituting the balance to 100%; 3% < Zr02+ Hf02 2 18%, with Hf02 < 1%; 1%<SiO2<10%; 0.3% < CaO £ 2%, with CaO 0.6% if Al203 < 79%; 35 less than 5% of other constituents, and 2 - the following crystalline phases, as percentages by weight based on the total amount of crystalline phases: 2013298188 16 Jun2017 65% i corundum; the corundum constituting the balance to 100%; 5 3% < zirconia + hafnia, which are optionally stabilized < 21 %; mullite < 16%; less than 5% of other crystalline phases, and - a total porosity less than or equal to 5%.

2A PCT/IB2013/056225 WO 2014/020522

As will be seen in more detail later in the description, the inventors discovered, unexpectedly, that this combination of characteristics improves the wear resistance properties.

The particles according to the invention, notably the beads, are thus particularly suitable for applications of dispersion in a wet medium, microgrinding and surface treatment. A sintered particle according to the invention may also have one or more of the following optional characteristics, in all possible combinations: - the CaO content is greater than (4.55 - 0.05.AI203(%))% if 79% < Al203 85% (in other words, the CaO content is greater than 4.55% minus 0.05 times the weight content of Al203, as a percentage if said weight content of Al203 is greater than 79% and less than or equal to 85%); - the sintered particle comprises more than 0.1% of Y203; preferably the Y203 content is greater than 0.5%, or even greater than 0.9%, and/or less than 3.0%, preferably less than 2.0%, or even less than 1.5%, as percentages by weight; - the sintered particle comprises more than 0.1% of Ce02; preferably the Ce02 content is greater than 0.7%, or even greater than 1.5%, and/or less than 3.0%, or even less than 2.5%, as percentages by weight; - the content of "other constituents" other than Y203 and Ce02 is less than 3.0%, preferably less than 2.5%, preferably less than 2.0%, or even less than 1.5%, as percentages by weight; - the "other constituents" other than Y203 and Ce02 are impurities; - the oxides of a sintered particle according to the invention represent more than 95%, preferably more than 97%, preferably more than 99%, or even more than 99.5%, or even substantially 100% of the total weight of said sintered particle, the balance to 100% preferably consisting of impurities; - at least a portion, or even all of the mullite is created during the sintering, i.e. was not present in the feedstock; - the content of "other crystalline phases", as a percentage by weight based on the total amount of crystalline phases, is less than 4%, or even less than 3%, or even less than 2%; - preferably, the "other crystalline phases" are, for more than 90%, more than 95%, or even substantially 100% by weight, zircon ZrSi04 and/or anorthite and/or quartz and/or cristobalite and/or tridymite; - the amount by weight of amorphous, i.e. vitreous, phase as a percentage by weight relative to the weight of the particle, is less than 20%, less than 15%, less than 10%, or even less than 7%, or even less than 5%; 3 PCT/IB2013/056225 WO 2014/020522 - the total porosity is less than 4.5%, or even less than 4%, or even less than 3%, or even less than 2%; - the sintered particle has a size of less than 10 mm and/or greater than 0.005 mm; - the sintered particle is a bead; - the sintered particle has a sphericity of greater than 0.7, preferably greater than 0.8, preferably greater than 0.85, or even greater than 0.9; - the density of the sintered particle is greater than 3.60 g/cm3, or even greater than 3.70 g/cm3, or even greater than 3.80 g/cm3, or even greater than 3.85 g/cm3, or even greater than 3.90 g/cm3, or even greater than 4.20 g/cm3, or even greater than 4.10 g/cm3.

The above optional characteristics may be, where appropriate, applied to the first and second particular embodiments described below.

In a first particular embodiment, the feedstock contains 15% to 25% of zircon, as a weight percentage based on the weight of the dry feedstock, and the sintered particle has: - the following chemical composition, as percentages by weight: 70% Al203, Al203 constituting the balance to 100%; 13% Zr02 + Hf02 18%, with Hf02 1%; 5% Si02 9%; 0.6% CaO 2%; less than 5% of other constituents, and - the following crystalline phases, as percentages by weight based on the total amount of crystalline phases: 65% corundum; the corundum constituting the balance to 100%; 10% zirconia + hafnia, which are optionally stabilized: 21%; 4% mullite 16%; less than 5% of other crystalline phases, and - a total porosity less than or equal to 5%.

Preferably, in this first particular embodiment, a sintered particle according to the invention also comprises one, and preferably several, of the following optional characteristics: - the feedstock contains 20% to 25% of zircon, as a weight percentage based on the weight of the dry feedstock; - the Al203 content is greater than 72% and/or less than or equal to 79%, less than 78%, preferably less than 76%, preferably less than 75%, as percentages by weight; - the Zr02 + Hf02 content is greater than 15% and/or less than 17%, as percentages by weight; 4 PCT/IB2013/056225 WO 2014/020522 - the Si02 content is greater than 6%, preferably greater than or equal to 7% and/or less than 8.5%, as percentages by weight; - the CaO content is greater than 0.7% and/or less than 1.8%, preferably less than 1.5%, preferably less than 1.3%, preferably less than 1.1%, as percentages by weight; - the corundum content (Al203 phase), as a percentage by weight based on the total amount of crystalline phases, is greater than 68%, preferably greater than 70% and/or less than or equal to 84%, less than 82%, preferably less than 80%, preferably less than 78%; - the content of zirconia (Zr02 phase) + hafnia (Hf02 phase), which are optionally stabilized, as a percentage by weight based on the total amount of crystalline phases, is greater than 11%, or even greater than 12% and/or less than 19%, or even less than 17%; - the mullite content, as a percentage by weight based on the total amount of crystalline phases, is greater than 5%, preferably greater than 7%, preferably greater than 8%, and/or less than 14%, preferably less than 12%, preferably less than 11 %.

In a second particular embodiment, the feedstock contains 4% to 15% of zircon, as a weight percentage based on the weight of the dry feedstock, and the sintered particle has: - the following chemical composition, as percentages by weight: 85% Al203, Al203 constituting the balance to 100%; 3% Zr02 + Hf02 13%, with Hf02 1%; 1% Si02 6%; 0.3% CaO 2%; less than 5% of other constituents, and - the following crystalline phases, as percentages by weight based on the total amount of crystalline phases: 86% corundum; the corundum constituting the balance to 100%; 3% zirconia + hafnia, which are optionally stabilized: 14%; mullite 6%; less than 5% of other crystalline phases, and - a total porosity less than or equal to 5%.

Preferably, in this second embodiment, a sintered particle according to the invention also comprises one, and preferably several, of the following optional characteristics: - the feedstock contains 5% to 15% of zircon; - the Al203 content is greater than 87%, preferably greater than 89% and/or less than or equal to 93%, or less than 91%, as percentages by weight; 5 PCT/IB2013/056225 WO 2014/020522 - the Zr02 + Hf02 content is greater than 5% and/or less than 10%, preferably less than 8%, as percentages by weight; - the Si02 content is less than 4%, preferably less than 3%, as percentages by weight; - the CaO content is greater than 0.5%, preferably greater than 0.7% and/or less than 1.8%, preferably less than 1.5%, preferably less than 1.3%, preferably less than 1.1%, as percentages by weight; - the corundum content (Al203 phase), as a percentage by weight based on the total amount of crystalline phases, is greater than 88% and/or less than or equal to 96%, less than 94%, preferably less than 92%; - the total content of zirconia (Zr02 phase) + hafnia (Hf02 phase), which are optionally stabilized, as a percentage by weight based on the total amount of crystalline phases, is greater than 4% and/or less than 12%, preferably less than 10%, or even less than 7%; - the mullite content, as a percentage by weight based on the total amount of crystalline phases, is less than 4%, preferably less than 2%, preferably substantially zero.

The invention also relates to a powder of particles comprising more than 90%, preferably more than 95%, preferably substantially 100%, as percentages by weight, of particles according to the invention.

The invention also relates to a method of manufacturing sintered particles according to the invention, notably sintered beads, comprising the following successive steps: a) if necessary, grinding one or more powders of raw materials, preferably by cogrinding, so that the mixing thereof, in step c), leads to a particulate mixture having a median size of less than 0.6 pm, b) optionally, drying said particulate mixture, c) preparing a feedstock from said optionally dried, particulate mixture, the composition of the feedstock being adjusted so as to obtain, at the end of step g), sintered particles having a composition conforming to that of a sintered particle according to the invention, said feedstock comprising from 4% to 25% of zircon particles, as a percentage by weight based on the weight of the dry feedstock, d) forming the feedstock in the form of green particles, e) optionally, washing, f) optionally, drying, g) sintering at a sintering temperature above 1450°C and below 1600°C so as to obtain sintered particles. 6 PCT/IB2013/056225 WO 2014/020522

The feedstock may contain less than 15%, less than 10%, less than 5%, less than 2%, or even substantially no mullite, as weight percentages based on the dry feedstock. In the first particular embodiment, a portion, or even all of the mullite may be formed during the sintering.

The invention relates finally to the use of a powder of particles, notably of beads, according to the invention, in particular manufactured by a method according to the invention, as grinding agents; agents for dispersion in a wet medium; propping agents, notably for preventing the closure of the deep geological fractures created in the walls of an extraction well, in particular for petroleum; heat exchange agents, for example for fluidized bed; or for surface treatment.

Definitions - The term “particle” is understood to mean an individualized solid product in a powder. - A sintered particle is "produced from a feedstock" when it results from a sintering of a green particle obtained by forming this feedstock. - "Sintering" refers to the consolidation, by heat treatment at over 1100°C, of a green particle (granular agglomerate), optionally with partial or total melting of some of its constituents (but not all of its constituents, so that the green particle is not converted into a liquid mass). - The term “bead” is understood to mean a particle having a sphericity, that is to say a ratio between its smallest diameter and its largest diameter, of greater than 0.6, regardless of the way in which this sphericity was obtained. - The “size” of a bead (or of a particle) refers to the average of its largest dimension dM and its smallest dimension dm: (dM+dm)/2. - The "median size" of a powder of particles, generally denoted by D50, refers to the size that divides the particles of this powder into first and second populations of equal weight, these first and second populations only comprising particles having a size greater than or less than the median size, respectively. The median size may for example be evaluated using a laser particle size analyzer. - The expression "sintered bead", or more broadly "sintered particle" is understood to mean a solid bead (or particle) obtained by sintering a green particle. - The term “impurities” is understood to mean the inevitable constituents necessarily introduced with the raw materials. In particular, the compounds that belong to the group of oxides, nitrides, oxynitrides, carbides, oxycarbides, carbonitrides and metallic species of sodium and other alkali metals, iron, vanadium and chromium may be impurities. As examples, mention may be made of Fe203, Ti02 or Na20. Residual carbon forms part of the impurities of the composition of the particles according to the invention. 7 PCT/IB2013/056225 WO 2014/020522 - When reference is made to Zr02 or to (Zr02 + Hf02), it should be understood as Zr02 and traces of Hf02. This is because a small amount of Hf02, chemically inseparable from Zr02 and having similar properties, is always naturally present in sources of Zr02 at contents generally of less than 3%, as a weight percentage based on Zr02 + Hf02. Hafnium oxide is not regarded as an impurity. - The term “precursor” of an oxide is understood to mean a constituent capable of providing said oxide during the manufacture of a particle according to the invention.

For clarity, the terms "Zr02", "Hf02" and "Al203" are used to denote the contents of these oxides in the composition, and "zirconia", "hafnia" and "corundum" are used to denote the crystalline phases of these oxides consisting of Zr02, of Hf02 and of Al203, respectively. These oxides may however also be present in other phases, in particular in the form of zircon (ZrSi04), or of mullite.

All the percentages relating to the chemical compositions are percentages by weight based on the weight of the particle, unless otherwise indicated.

All the percentages relating to the crystalline phases are percentages by weight based on the total weight of the crystalline phases present in the particle, unless otherwise indicated.

The expressions "containing a", "comprising a" or "having a" are understood to mean "comprising at least one", unless otherwise indicated.

Detailed description

For manufacturing the sintered particles according to the invention, the procedure of steps a) to g) described above, and presented in detail below, may be followed.

In step a), the powders of raw materials may be ground individually or, preferably, coground, if the mixing thereof in proportions suitable for preparing the feedstock, in step c), does not lead to a particulate mixture having a median size of less than 0.6 pm. This grinding may be wet grinding.

Preferably, grinding or cogrinding is carried out so that the median size of said particulate mixture is less than 0.5 pm, preferably less than 0.4 pm.

Preferably, the powders used, notably the powders of zircon ZrSi04, of alumina Al203, of CaC03, of a glass containing CaO, preferably a silica glass containing CaO, of a lime feldspar, of Y203, of Ce02, of aluminosilicate such as a clay, of Zr02, and of mullite each have a median size of less than 5 pm, or even less than 3 pm, less than 1 pm, less than 0.7 pm, preferably less than 0.6 pm, preferably less than 0.5 pm, or even less than 0.4 pm. 8 PCT/IB2013/056225 WO 2014/020522

Advantageously, when each of these powders has a median size of less than 0.6 pm, preferably less than 0.5 pm, or even less than 0.4 pm, step a) is optional.

Preferably, the zircon powder used has a specific area before grinding, calculated by the BET method, of greater than 5 m2/g, preferably greater than 8 m2/g, preferably greater than 10 m2/g, and/or less than 30 m2/g. Advantageously, the grinding in step a), generally in suspension, is facilitated by this.

Preferably, the alumina powder used has a median size of less than 7 pm, preferably less than 6 pm, or even less than 3 pm, or even less than 2 pm, or even less than 1 pm, or even less than 0.5 pm. Advantageously, the formation of mullite from said alumina powder and from the silica present in the particulate mixture or created during step g) is facilitated by this, as is the sintering in step g).

In step b), which is optional, the ground powders of raw materials are dried, for example in an oven or by spray drying, in particular if they were obtained by wet grinding. Preferably, the temperature and/or the duration of the drying step are adjusted so that the residual moisture content of the powders of raw materials is less than 2%, or even less than 1.5%. The powders obtained may be deagglomerated.

In step c), a feedstock is prepared at ambient temperature that comprises powders of ZrSi04, of Al203, and optionally of CaC03 and/or of a glass containing CaO, preferably a silica glass containing CaO and/or of a lime feldspar and/or of Y203 and/or of aluminosilicate and/or of Zr02 and/or of mullite.

These powders may also be replaced, at least partially, with powders of precursors of these oxides, introduced in equivalent amounts.

In particular, a portion, or even all of the zircon and/or of the mullite of a particle according to the invention may result from the presence of these crystalline phases in the feedstock.

The powders supplying the oxides or the precursors are preferably selected so that, in the sintered particles, the total content of constituents other than Zr02, Hf02, Si02, Al203 and CaO is less than 5%, as a weight percentage.

The feedstock contains a zircon powder, i.e. particles of ZrSi04, in an amount of greater than or equal to 4%, preferably greater than 5% and less than 25%, by weight based on the weight of the dry feedstock. In the first particular embodiment, the feedstock contains a zircon powder in an amount of greater than or equal to 15%, preferably greater than 20% and less than 25%, based on the weight of the dry feedstock. In the second particular embodiment, the feedstock contains a zircon powder in an amount of greater than or equal to 9 PCT/IB2013/056225 WO 2014/020522 4%, preferably greater than 5% and less than or equal to 15%, based on the weight of the dry feedstock.

Preferably, the feedstock contains an alumina powder, i.e. particles of Al203 phase, in an amount of greater than or equal to 70%, and preferably less than 94%, by weight based on the weight of the dry feedstock. Preferably, said alumina powder is a reactive alumina powder and/or a calcined alumina powder and/or a transition alumina powder. Preferably said alumina powder is a reactive alumina powder. In the first particular embodiment, the feedstock preferably contains an alumina powder in an amount of greater than 72%, preferably greater than 74% and less than or equal to 79%, preferably less than 77%, based on the weight of the dry feedstock. In the second particular embodiment, the feedstock preferably contains an alumina powder in an amount of greater than 85%, preferably greater than 87% and preferably less than or equal to 93%, based on the weight of the dry feedstock.

The feedstock may contain a powder comprising CaO, preferably selected from CaC03, a glass containing CaO, preferably a silica glass containing CaO or a lime feldspar, in a suitable amount so that the CaO content in the sintered particles is in accordance with the invention.

The feedstock may not contain powder comprising CaO, said oxide being, for example, provided by the electrolyte of a gelling bath which reacts with a gelling agent introduced into the feedstock, it being possible for said electrolyte to be, for example, a calcium salt solution. The feedstock may comprise less than 7%, less than 5%, less than 2% of zirconia, i.e. of particles of Zr02, as a weight percentage based on the crystalline phases, or even may not comprise any. A portion of the zirconia, or even all the zirconia may in fact originate from the dissociation of zircon during the sintering of step g).

The feedstock may comprise less than 10%, less than 5% of mullite, as a weight percentage based on the crystalline phases, or even may not comprise any mullite. A portion of the mullite, or even all of the mullite, may be generated in situ during the sintering of step g), preferably from alumina and silica originating from the dissociation of zircon ZrSi04 to zirconia Zr02 and to silica Si02.

The feedstock may comprise a powder of particles of Y203, in an amount of greater than 0.1%, preferably greater than 0.5%, or even greater than 0.9%, and/or less than 3%, preferably less than 2%, even less than 1.5% by weight based on the weight of the dry feedstock. 10 PCT/IB2013/056225 WO 2014/020522

The feedstock may comprise a powder of particles of Ce02, in an amount of greater than 0.1%, preferably greater than 0.7%, preferably greater than 1.5%, and/or less than 3%, or even less than 2.5% by weight based on the weight of the dry feedstock.

The feedstock may comprise a powder of aluminosilicate particles, preferably a clay powder, in an amount of greater than 1%, preferably greater than 1.5% and preferably less than 3%.

The feedstock may comprise a powder of silica, i.e. of particles of Si02, in an amount preferably of greater than 0.5%, preferably greater than 1%, and/or less than 5%, preferably less than 3% by weight based on the weight of the dry feedstock.

Preferably, no raw material other than those supplying Zr02 + Hf02, Si02, Al203, CaO and optionally Y203 and/or Ce02, and the precursors thereof is introduced deliberately into the feedstock, the other constituents present being impurities.

The feedstock may moreover comprise a solvent, preferably water, the amount of which is adapted to the forming method of step d).

As is well known by a person skilled in the art, the feedstock is adapted to the forming method of step d).

Forming may in particular result from a gelling method. For this purpose, a solvent, preferably water, is added to the feedstock so as to make a suspension.

The suspension preferably has a solids content by weight of between 50% and 70%.

The suspension may further comprise one or more of the following constituents: - a dispersant, in an amount of from 0 to 10%, as a weight percentage based on the solids; - a surface tension modifier, in an amount of from 0 to 3%, as a weight percentage based on the solids; - a gelling agent, or "gelation agent", in an amount of from 0 to 2%, as a weight percentage based on the solids.

The dispersants, surface tension modifiers and gelling agents are well known by a person skilled in the art. The same is true for the electrolytes suitable for reacting with a given gelling agent.

As examples, mention may be made, - as dispersants, of the family of sodium or ammonium polymethacrylates, the family of sodium or ammonium polyacrylates, the family of polyacrylic acids (sodium or ammonium salts), or other polyelectrolytes, the family of citrates, for example ammonium citrates, the family of sodium phosphates, and the family of carbonic acid esters; - as surface tension modifiers, of organic solvents such as aliphatic alcohols; 11 PCT/IB2013/056225 WO 2014/020522 - as gelling agents, of natural polysaccharides, forming insoluble compounds in contact with divalent cation electrolytes, such as for example Ca2+ ion electrolytes.

Most of these elements disappear during the subsequent manufacturing steps, although some traces of them may remain.

The powders of oxides and/or of precursors are preferably added to a mixture of water and of dispersants/deflocculants in a ball mill. After stirring, water is added, in which a gelling agent was dissolved beforehand so as to obtain a suspension.

If forming is the result of extrusion, thermoplastic polymers or thermosetting polymers may be added to the feedstock.

In step d), any conventional forming method used for manufacturing sintered particles, notably sintered beads, may be employed.

Among these methods, mention may be made of: - granulation methods, for example employing mixers, granulators, fluidized bed granulators, or granulating disks, - gelation methods, - injection molding or extrusion methods, and - pressing methods.

In a gelation method, drops of the suspension described above are obtained by flow of the suspension through a calibrated orifice. The drops leaving the orifice fall into a bath of a gelling solution (electrolyte suitable for reacting with the gelling agent) where they harden after regaining a substantially spherical shape.

In step e), which is optional, the green particles obtained during the preceding step are washed, for example with water.

In step f), which is optional, the green particles, optionally washed, are dried, for example in an oven.

In step g), the green particles, optionally washed and/or dried, are sintered. Preferably, sintering is carried out in air, preferably in an electric furnace, preferably at atmospheric pressure.

The sintering in step g) is carried out at a temperature above 1450°C, preferably above 1475°C, preferably above 1480°C, preferably above 1490°C, and below 1600°C, preferably below 1550°C, preferably below 1520°C. A sintering temperature equal to 1500°C is very suitable. A sintering temperature below 1450°C does not make it possible to obtain a particle that has a total porosity of less than or equal to 5%. 12 PCT/IB2013/056225 WO 2014/020522

Preferably, the sintering time is between 2 and 5 hours. A sintering time equal to 4 hours is very suitable.

The sintered particles obtained are preferably in the form of beads having a smallest diameter of between 0.005 mm and 10 mm.

The content of CaO in the product obtained at the end of step g) may in particular be adjusted by the concentration of gelling agent in the feedstock in step c) and/or by the residence time of the particle formed in the electrolyte in step d) and/or by the duration of the washing of the particles in step e).

The following rules, well known by a person skilled in the art, may be applied so as to obtain a sintered particle according to the invention: - in order to increase the amount of corundum in the sintered particle: increase the amount of corundum in the feedstock and/or lower the sintering temperature in order to limit the formation of mullite in situ·, - in order to decrease the amount of corundum in the sintered particle: decrease the amount of corundum in the feedstock and/or increase the sintering temperature; - in order to increase the amount of zirconia in the sintered particle: increase the amount of zirconia in the feedstock and/or increase the amount of zircon in the feedstock; - in order to increase the amount of mullite in the sintered particle: increase the amount of mullite in the feedstock and/or increase the amount of zircon and/or increase the sintering temperature in order to promote the formation of mullite in situ; - in order to decrease the amount of mullite in the sintered particle: decrease the amount of mullite in the feedstock and/or decrease the amount of zircon and/or lower the sintering temperature in order to limit the formation of mullite in situ; - in order to decrease the total porosity of the sintered particle of defined feedstock: bring the sintering temperature closer to the preferred range of sintering temperatures and/or increase the amount of silica powder in the feedstock and/or increase the amount of aluminosilicate powder in the feedstock; - in order to increase the amount of CaO in the sintered particle: increase the amount of raw material providing CaO in the feedstock and/or increase the amount of gelling agent in the feedstock and/or increase the residence time of the particle in the electrolyte in step d) and/or reduce the duration of the washing in step e). 13 PCT/IB2013/056225 WO 2014/020522

The sintered particles according to the invention are particularly suitable as grinding agents or as agents for dispersion in a wet medium, and also for surface treatment. The invention therefore also relates to the use of a plurality of particles, notably of beads according to the invention, in particular manufactured by a method according to the invention, as grinding agents, or agents for dispersion in a wet medium.

The wear resistance of the particles according to the invention is all the more remarkable since it is obtained owing to the use of raw materials, especially zircon, that enable a cost reduction.

It should however be noted that the properties of the particles according to the invention, notably their mechanical strength, their density, and also their ease of production, may make them suitable for other applications, notably as propping agents or heat exchange agents, or else for surface treatment (in particular by blasting with the particles according to the invention).

The invention therefore also relates to a device selected from a suspension, a grinding mill, an apparatus for surface treatment and a heat exchanger, said device comprising a powder of particles according to the invention.

Examples

The following nonlimiting examples are given for the purpose of illustrating the invention. Measurement protocols

The following methods were used for determining certain properties of various mixtures of sintered beads. They enable an excellent simulation of the actual behavior, in operation, in the microgrinding application.

In order to determine the wear resistance known as "planetary" wear resistance, 20 ml (volume measured using a graduated cylinder) of beads to be tested having a size between 1.2 and 1.4 mm are weighed (mass m0) and introduced into one of 4 bowls coated with dense sintered alumina, having a capacity of 125 ml, of a RETSCH PM400 high-speed planetary mill. Added to the same bowl that already contains the beads are 2.2 g of Presi silicon carbide (having a median size D50 of 23 pm) and 40 ml of water. The bowl is sealed and set rotating (planetary motion) at 400 rpm with reversal of the direction of rotation at one minute intervals for 1 h 30 min. The contents of the bowl are then washed over a 100 pm screen so as to remove the residual silicon carbide and also the material stripped due to wear during the grinding operation. After screening over a 100 pm screen, the beads are dried in an oven at 100°C for 3 h, and then weighed (mass m^. Said beads (mass are again put in one of the bowls with a suspension of SiC (same concentration and amount as before) and undergo a new grinding cycle, identical to the preceding cycle. The contents of 14 PCT/IB2013/056225 WO 2014/020522 the bowl are then washed over a 100 μηι screen so as to remove the residual silicon carbide and also the material stripped due to wear during the grinding operation. After screening over a 100 pm screen, the beads are dried in an oven at 100°C for 3 h, and then weighed (mass m2). Said beads (mass m2) are again put in one of the bowls with a suspension of SiC (same concentration and amount as before) and undergo a new grinding cycle, identical to the preceding cycle. The contents of the bowl are then washed over a 100 pm screen so as to remove the residual silicon carbide and also the material stripped due to wear during the grinding operation. After screening over a 100 pm screen, the beads are dried in an oven at 100°C for 3 h, and then weighed (mass m3).

The planetary wear (PW) is expressed as a percentage (%) and is equal to the loss of mass of the beads relative to the initial mass of the beads, namely: 100(m0-m3) / (m0); the result PW is given in Table 1.

It is considered that the results are particularly satisfactory if the products have: - an improvement in the planetary wear (PW) resistance of at least 10% relative to that of the Reference 1 example for the particles according to the invention having an Al203 content of less than or equal to 79%, and - a planetary wear (PW) resistance at least equivalent to that of the Reference 1 example for the particles according to the invention having an Al203 content of greater than 79%.

The crystalline phases present in the sintered particles according to the invention are measured by X-ray diffraction, for example by means of an X’Pert PRO diffractometer device from the company Panalytical provided with a copper XRD tube. The diffraction pattern is acquired using this equipment, over an angular range 2 of between 5° and 80°, with a step of 0.017°, and a counting time of 60 s/step. The front lens comprises a programmable divergence slit, used fixed, of 1/4°, Soller slits of 0.04 rad, a mask equal to 10 mm and a fixed anti-scattering slit of 1/2°. The sample rotates about its own axis in order to limit the preferred orientations. The rear lens comprises a programmable anti-scattering slit, used fixed, of 1/4°, Soller slits of 0.04 rad and a Ni filter. The pattern obtained is processed with the software High Score Plus with Rietveld refinement, the mullite file being that of AI4.5Sh.5O9.74 described in "Crystal structure and compressibility of 3:2 mullite", Balzar et al., American Mineralogist; December 1993; v. 78; no. 11-12; p. 1192-1196. The various parameters are refined in order to reduce the "R Bragg" value as much as possible.

The amount of amorphous phase present in the sintered particles according to the invention is measured by X-ray diffraction, for example by means of an X’Pert PRO diffractometer device from the company Panalytical provided with a copper XRD tube. The diffraction 15 PCT/IB2013/056225 WO 2014/020522 pattern is acquired using this equipment, in the same way as for determining the crystalline phases present in the particles. The method applied consists of adding a known amount of a completely crystalline standard, in the present case of zinc oxide ZnO in a weight amount equal to 20%, based on the weight of zinc oxide and of sample of sintered particles according to the invention.

The total porosity, in %, is evaluated with the following formula:

Total porosity = 100.(1 -(dbeads/dground beads)), where - dbeads is the density of beads before grinding obtained using a helium pycnometer (AccuPyc 1330 from the company Micromeritics®), according to a method based on measuring the volume of gas displaced (in the present case helium), and - dground beads is the density of powder obtained by grinding the beads in an Aurec annular dry grinding mill for 40 s, followed by screening so that only powder passing through a 160 pm screen is used for the measurement.

Manufacturing protocol

Sintered beads were prepared from a zircon powder having a specific area of the order of 8 m 2/g and a median size equal to 1.5 pm, an alumina powder with a purity equal to 99.5% and a median size of less than 5 pm, and, depending on the examples carried out, a clay powder with a median size of less than 53 pm, having a loss on ignition performed at 1000°C of between 10% and 15% and having a total Si02+ Al203 content of greater than 82%, and a powder of Y203, having a purity based on rare earth oxides of greater than 99.9% and a median size of less than 10 pm, and a powder of Ce02 having a purity of greater than 99.5% and a median size of less than 10 pm.

These powders were mixed then co-ground in a wet medium until a particulate mixture having a median size of greater than 0.4 pm was obtained, with the aid of yttriated zirconia beads, of which the weight content of Y203 was equal to 5.2%. This grinding may be a source of zirconia and of yttrium oxide, due to the wear of said beads during the grinding of the powders. The particulate mixture was then dried. A feedstock consisting of an aqueous suspension comprising, as percentages by weight based on the solids, 1% of a dispersant of carboxylic acid type, 0.6% of a dispersant of sodium phosphate type and 0.4% of a gelling agent, namely a polysaccharide from the family of alginates, was then prepared from this particulate mixture. A ball mill was used for this preparation so as to obtain good homogeneity of the feedstock: A solution containing the gelling agent was firstly prepared. Next, the particulate mixture and the dispersants were added to water. The solution containing the gelling agent was then 16 PCT/IB2013/056225 WO 2014/020522 added. The resultant mixture was stirred for 8 hours. The size of the particles was monitored by the sedigraph method using a Sedigraph 5100 sedigraph sold by the company Micromeritics® (median size < 0.4 pm), and water was added in a given amount to obtain an aqueous suspension containing 66% solids and with a viscosity, measured with the 5 Brookfield viscometer using the LV3 spindle at a speed of 20 rpm, of less than 5000 centipoise. The pH of the suspension was then about 8.

The suspension was forced through a calibrated hole at a flow rate that makes it possible to obtain, after sintering, beads of around 1.2 mm to 1.4 mm. The drops of suspension fell into a gelling bath based on an electrolyte (Ca2+ divalent cation salt), which reacts with the gelling to agent. The green beads were collected, washed, and then dried at 80°C to remove the moisture. The beads were then transferred to a sintering furnace where they were brought, at a rate of 100°C/h, up to the desired sintering temperature Ts. After a hold of 4 hours at the temperature Ts, the temperature was allowed to fall by natural cooling.

Results 15 The results obtained are summarized in Table 1 below. 17 PCT/IB2013/056225

Examples Ref.l (*) K*) 2( *) 3(*) 4( *) 5 6 7 8 9 10(*) IK*) 12(*) 13(*) 14 15 16 17(*) 18 19 Sintering temperature Ts (°C) - 1500 1600 1500 1500 1500 1500 1500 1500 1500 1600 1500 1600 1500 1500 1500 1500 1450 1500 1600 Composition of the particulate mixture (weight percentages) Zircon powder _ 21.9 22.3 22.5 22.3 22 22.5 22.2 22 22.5 22.3 22.5 22 22.2 9.8 9.8 4.9 21.9 5 5 Alumina powder - 74.1 75.6 76.2 76.3 74.7 76.2 76.6 74.7 76.2 75.6 76.2 74.7 74.5 88.2 87.6 92.8 74 94.7 94.7 Y203 powder _ _ _ 1.3 1.4 1.3 1.3 1.2 1.3 1.3 _ 1.3 1.3 1.3 _ 0.6 0.3 0.5 0.3 0.3 Ce02 powder - 4.1 2.1 - - - - - - - 2.1 - - - - - - 3.6 - - Clay powder _ _ _ _ _ 2 _ _ _ _ _ - 2 _ 2 2 2 _ _ _ Chemical analysis after sintering (weight percentages) ai2o, (%) 75.8 73.9 72.3 78.1 74.7 73 73.1 72.8 71.3 73.1 71.4 75.5 72.7 74.7 85.2 84.7 89.2 72.8 90.3 90.3 Zr02 + Hf02 (%) 15.3 14.1 16.5 13.4 15.2 16.1 16.7 16 16.4 16.6 16.3 13.1 16.1 14.7 8.7 9.1 6 14.1 6.3 6.3 Si02 (%) 7.3 7.1 7.3 5.8 7 8.2 7.2 7.8 8.1 7.5 7.3 5.7 8.3 6.5 4.2 3.9 2.2 7 1.8 1.8 CaO (%) 0.58 0.2 0.35 0.37 0.5 0.74 0.85 1.02 1.7 0.61 1.01 3.59 0.74 2.27 1.29 0.84 1.22 1.43 0.38 0.38 Other constituents (%) including Y203 (%) including Ce02 (%) 1.02 4.7 3.55 2.33 2.6 1.96 2.15 2.38 2.5 2.19 3.99 2.11 2.16 1.83 0.61 1.46 1.38 4.67 1.22 1.22 0 0 0 1.15 1.42 1.4 1.49 1.43 1.37 1.49 0 1.11 1.4 1.31 0.2 0.72 0.41 0.54 0.47 0.47 0 4.1 2.04 0 0 0 0 0 0 0 2.02 0 0 0 0 0 0 3.4 0 0 Crystalline )hases, in wt% based on the weight of the crystalline phases Corundum (%) 66 71 68 78 75 76 75 74 72 67 75 85 66 77 92 94 90 68 94 92 Zirconia + hafnia (%) 15 18 20 12 15 12 15 16 19 17 23 15 17 17 8 6 10 17 6 8 Mullite (%) 19 10 10 10 10 11 10 10 9 15 2 0 17 6 0 0 0 14 0 0 Other crystalline phases (%) 0 1 2 0 0 1 0 0 0 1 0 0 0 0 0 0 0 1 0 0 Amorphous phase, in wt% based on the weight of the particle % of amorphous phase nd nd nd nd nd nd nd nd nd 5 nd nd nd nd nd 6 nd nd nd nd

Characteristics after sintering

Total porosity (%) 2.7 4.3 0.3 1.5 2.2 4.6 1.1 1.5 1.6 1 0.3 3.8 3.6 2.5 1.7 3.8 2.2 8.7 4.9 2.1 Bead density 3.8 nd 3.99 3.75 nd 3.7 3.75 nd 3.83 3.89 3.95 3.9 3.76 3.74 3.73 3.85 3.83 3.88 3.92 3.91 Planetary wear PW (in %) 2.3 4.97 4.9 3.12 2.4 1.72 1.84 1.8 2.02 2.1 3.15 4.32 2.89 2.4 1.9 2.1 2.15 8 2.3 1.9 PW% Reference 1 - -116 -113 -36 -4 25 20 22 13 9 -36 -88 -26 -4 17 9 7 -248 0 17 WO 2014/020522 (*): outside the invention nd: not determined Table 1 18 PCT/IB2013/056225 WO 2014/020522

The reference beads of the "Ref. 1" example, outside the invention, are sintered beads commonly used in grinding applications.

The examples show that, surprisingly, the test beads according to the invention display remarkable performance compared to the reference beads. A comparison of the products according to examples 1 to 8, 11 and 13 having substantially the same mullite content and substantially the same chemical anaylsis apart from CaO, with an Al203 content of less than 79%, show the influence of CaO on the wear resistance of the beads: the products according to examples 1,2, 3, 4, 11 and 13, outside the invention, which have a degraded wear resistance PW% relative to the product according to reference 1 contain 0.2%, 0.35%, 0.37%, 0.5%, 3.59% and 2.27% of CaO, respectively. The products according to examples 5, 6, 7 and 8, which have an improved wear resistance PW% relative to the product according to reference 1, contain 0.74%, 0.85%, 1.02% and 1.7% of CaO, respectively.

The product according to example 17, outside the invention, illustrates the fact that if the total porosity reaches 8.7%, the wear resistance PW% is significantly degraded relative to the product from reference 1.

The product according to example 10, outside the invention, shows that a zirconia + hafnia content of 23% degrades the wear resistance PW% by 36% relative to the product from reference 1. This product also illustrates the advantage of a minimum mullite content for alumina contents of less than 79%.

The products according to examples 5, 6 and 7 illustrate the fact that, for an Al203 content equal to 73%, 73.1% and 72.8%, respectively, and a CaO content equal to 0.74%, 0.85% and 1.02%, respectively, a mullite content equal to 10% or 11% significantly increases the wear resistance PW% relative to the product from reference 1. A comparison of the products from examples 9 and 5 illustrates the fact that the wear resistance PW% reaches a maximum value for a mullite content equal to 11%, for products having an Al203 content equal to 73.1% and 73%, respectively. The product according to example 12, outside the invention, has a mullite content equal to 17% and a wear resistance PW% that is degraded by 26% relative to the product from reference 1.

The products according to examples 14 to 16 and 19, according to the invention, having an Al203 content equal to 88.2%, 87.6%, 92.8% and 94.7%, respectively, display an improvement in the wear resistance PW(%) equal to 17%, 9%, 7% and 17%, respectively. 19 PCT/IB2013/056225 WO 2014/020522

The product from example 18, according to the invention, having an Al203 content equal to 94.7% and a total porosity equal to 4.9%, displays a wear PW(%) comparable to that of the product from reference 1.

The products from examples 5, 6 and 7 are the preferred products. 5 Of course, the invention is not limited to the examples and embodiments described above. In particular, other gelling systems are suitable for manufacturing a ceramic bead according to the invention. Thus, US 5,466,400, FR 2 84 24 38 and US 4,063,856 describe applicable sol-gel methods. FR 2 84 24 38 and US 4,063,856 use a gelling system similar to that described above (based on alginate) whereas US 5,466,400 describes a very different gelling system. 10 The method described in US 2009/0036291 and methods of forming beads by pressing or by granulation can also be envisaged.

The various embodiments may be combined. 20

Claims

1. A sintered particle, produced from a feedstock containing 4% to 25% of zircon, as a weight percentage based on the weight of the dry feedstock, said sintered particle having: - the following chemical composition, as percentages by weight: 70% < Al203, Al203 constituting the balance to 100%; 3% < Zr02+ Hf02 < 18%, with Hf02 < 1%; 1% < Si02 < 10%; 0.3% < CaO < 2%, with CaO > 0.6% if Al203 79 %; less than 5% of other constituents, and - the following crystalline phases, as percentages by weight based on the total amount of crystalline phases: 65% 2 corundum; the corundum constituting the balance to 100%; 3% < zirconia + hafnia, which are optionally stabilized < 21%; mullite < 16%; less than 5% of other crystalline phases, and - a total porosity less than or equal to 5%.

2. The sintered particle as claimed in claim 1, comprising more than 0.1 % of Y203.

3. The sintered particle as claimed in claim 2, in which the Y203 content is greater than 0.5% and less than 3.0%.

4. The sintered particle as claimed in any one of the preceding claims, comprising more than 0.1% of Ce02.

5. The sintered particle as claimed in claim 4, wherein the Ce02 content is greater than 0.7% and less than 3.0%.

6. The sintered particle as claimed in any one of the preceding claims, wherein the content of "other constituents" other than Y203 and Ce02 is less than 3.0%.

7. The sintered particle as claimed in claim 6, wherein the content of "other constituents" other than Y203 and Ce02 is less than 2.0%.

8. The sintered particle as claimed in any one of the preceding claims, wherein the oxides represent more than 95% of the total weight of said particle.

9. The sintered particle as claimed in claim 8, wherein the oxides represent more than 99% of the total weight of said particle.

10. The sintered particle as claimed in any one of the preceding claims, wherein a portion of the mullite is created during the sintering.

11. The sintered particle as claimed in claim 10, wherein all of the mullite is created during the sintering.

12. The sintered particle as claimed in any one of the preceding claims, having a content of "other crystalline phases" of less than 4%, as a percentage by weight based on the total amount of crystalline phases.

13. The sintered particle as claimed in claim 12, wherein the content of "other crystalline phases" is less than 3%, as a percentage by weight based on the total amount of crystalline phases.

14. The sintered particle as claimed in any one of the preceding claims, having an amount by weight of amorphous phase, as a percentage by weight relative to the weight of the particle, of less than 20%.

15. The sintered particle as claimed in claim 14, wherein the amount by weight of amorphous phase, as a percentage by weight relative to the weight of the particle, is less than 15%.

16. The sintered particle as claimed in claim 15, wherein the amount by weight of amorphous phase, as a percentage by weight relative to the weight of the particle, is less than 10%.

17. The sintered particle as claimed in any one of the preceding claims, having a total porosity of less than 4.5%.

18. The sintered particle as claimed in claim 17, having a total porosity of less than 4%.

19. The sintered particle as claimed in claim 18, having a total porosity of less than 3%.

20. The sintered particle as claimed in any one of the preceding claims, having a size of less than 10 mm and greater than 0.005 mm.

21. The sintered particle as claimed in any one of the preceding claims, having a sphericity, i.e. a ratio of its smallest diameter to its largest diameter, of greater than 0.7.

22. The sintered particle as claimed in claim 21, having a sphericity of greater than 0.8.

23. The sintered particle as claimed in claim 22, having a sphericity of greater than 0.85.

24. The sintered particle as claimed in any one of the preceding claims, having a density of greater than 3.6 g/cm3 and less than 4.20 g/cm3.

25. The sintered particle as claimed in any one of the preceding claims, produced from a feedstock containing 15% to 25% of zircon, as a weight percentage based on the weight of the dry feedstock, said sintered particle having: - the following chemical composition, as percentages by weight: 70% < Al203, Al203 constituting the balance to 100%; 13% < Zr02+ Hf02 < 18%, with Hf02 <1%; 5% < Si02 < 9%; 0.6% < CaO < 2%; less than 5% of other constituents, and - the following crystalline phases, as percentages by weight based on the total amount of crystalline phases: 65% S corundum; the corundum constituting the balance to 100%; 10% £ zirconia + hafnia, which are optionally stabilized: i 21%; 4% 2 mullite £ 16%; less than 5% of other crystalline phases, and - a total porosity less than or equal to 5%.

26. The sintered particle as claimed in claim 25, wherein the feedstock contains 20% to 25% of zircon, as a weight percentage based on the weight of the dry feedstock.

27. The sintered particle as claimed in claims 26 or 27, wherein the Al203 content is greater than 72% and/or less than 78%, as percentages by weight.

28. The sintered particle as claimed in claim 27, wherein the Al203 content is less than 76%, as a percentage by weight.

29. The sintered particle as claimed in claim 28, wherein the Al203 content is less than 75%, as a percentage by weight.

30. The sintered particle as claimed in any one of claims 25 to 29, wherein the Zr02 + Hf02 content is greater than 15% and/or less than 17%, as percentages by weight.

31. The sintered particle as claimed in any one of claims 25 to 30, wherein the Si02 content is greater than 6% and/or less than 8.5%, as percentages by weight.

32. The sintered particle as claimed in any one of claims 25 to 31, wherein the CaO content is greater than 0.7% and/or less than 1.8%, as percentages by weight.

33. The sintered particle as claimed in claim 32, wherein the CaO content is less than 1.5%, as a percentage by weight.

34. The sintered particle as claimed in claim 33, wherein the CaO content is less than 1.3%, as a percentage by weight.

35. The sintered particle as claimed in claim 34, wherein the CaO content is less than 1.1%, as a percentage by weight.

36. The sintered particle as claimed in any one of claims 25 to 35, having a corundum content of greater than 68% and/or less than 82%, as percentages by weight based on the total amount of crystalline phases.

37. The sintered particle as claimed in claim 36, having a corundum content of greater than 70% and/or less than 80%, as percentages by weight based on the total amount of crystalline phases.

38. The sintered particle as claimed in claim 37, having a corundum content of less than 78%, as a percentage by weight based on the total amount of crystalline phases.

39. The sintered particle as claimed in any one of claims 25 to 38, having a content of zirconia + hafnia, which are optionally stabilized, of greater than 11% and/or less than 19%, as percentages by weight based on the total amount of crystalline phases.

40. The sintered particle as claimed in claim 39, having a content of zirconia + hafnia, which are optionally stabilized, of greater than 12% and/or less than 17%, as percentages by weight based on the total amount of crystalline phases.

41. The sintered particle as claimed in any one of claims 25 to 40, having a mullite content of greater than 5% and/or less than 14%, as percentages by weight based on the total amount of crystalline phases.

42. The sintered particle as claimed in claim 41, wherein the mullite content is greater than 7% and/or less than 12%.

43. The sintered particle as claimed in claim 42, wherein the mullite content is greater than 8% and/or less than 11%.

44. The sintered particle as claimed in any one of claims 1 to 24, produced from a feedstock containing 4% to 15% of zircon, as a weight percentage based on the weight of the dry feedstock, the sintered particle having: - the following chemical composition, as percentages by weight: 85% £ Al203, AI2O3 constituting the balance to 100%; 3% < Zr02+ Hf02 < 13%, with Hf02 2 1%; 1%<Si02<6%; 0.3% < CaO < 2%; less than 5% of other constituents, and - the following crystalline phases, as percentages by weight based on the total amount of crystalline phases: 86% 2 corundum; the corundum constituting the balance to 100%; 3 % ^ zirconia + hafnia, which are optionally stabilized: i 14%; mullite ^ 6%; less than 5% of other crystalline phases, and - a total porosity less than or equal to 5%.

45. The sintered particle as claimed in claim 44, wherein the feedstock contains 5% to 15% of zircon, as a weight percentage based on the weight of the dry feedstock.

46. The sintered particle as claimed in claims 44 or 45, wherein the Al203 content is greater than 87% and/or less than 91%, as percentages by weight.

47. The sintered particle as claimed in claim 46, wherein the AI2O3 content is greater than 89%, as a percentage by weight.

48. The sintered particle as claimed in any one of claims 44 to 47, wherein the Zr02+Hf02 content is greater than 5% and/or less than 10%, as percentages by weight.

49. The sintered particle as claimed in any one of claims 44 to 48, wherein the Si02 content is less than 4%, as a percentage by weight.

50. The sintered particle as claimed in any one of claims 44 to 49, wherein the CaO content is greater than 0.5% and/or less than 1.8%, as percentages by weight.

51. The sintered particle as claimed in claim 50, wherein the CaO content is greater than 0.7% and/or less than 1.5%, as percentages by weight.

52. The sintered particle as claimed in claim 51, wherein the CaO content is less than 1.3 %, as a percentage by weight.

53. The sintered particle as claimed in claim 52, wherein the CaO content is less than 1.1%, as a percentage by weight.

54. The sintered particle as claimed in any one of claims 44 to 53, having a corundum content of greater than 88% and/or less than 94%, as percentages by weight based on the total amount of crystalline phases.

55. The sintered particle as claimed in claim 54, having a corundum content of less than 92%, as a percentage by weight based on the total amount of crystalline phases.

56. The sintered particle as claimed in any one of claims 44 to 55, having a content of zirconia + hafnia, which are optionally stabilized, of greater than 4% and/or less than 12%, as percentages by weight based on the total amount of crystalline phases.

57. The sintered particle as claimed in claim 56, having a content of zirconia + hafnia, which are optionally stabilized, of less than 10%, as a percentage by weight based on the total amount of crystalline phases.

58. The sintered particle as claimed in any one of claims 44 to 57, having a mullite content of less than 4%, as a percentage by weight based on the total amount of crystalline phases.

59. The sintered particle as claimed in claim 58, wherein the mullite content is less than 2%.

60. The sintered particle as claimed in claim 59, wherein the mullite content is substantially zero.

61. The sintered particle as claimed in any one of claims 1 to 24, wherein the CaO content is greater than (4.55 - 0.05.AI2O3)% if 79% < Al203 < 85%.

62. A powder comprising more than 90%, as a percentage by weight, of particles as claimed in any one of the preceding claims.

63. A method of manufacturing sintered particles as claimed in any one of claims 1 to 61, comprising the following successive steps: a) if necessary, grinding one or more powders of raw materials, preferably by cogrinding, so that the mixing thereof, in step c), leads to a particulate mixture having a median size of less than 0.6 pm, b) optionally, drying said particulate mixture, c) preparing a feedstock from said optionally dried, particulate mixture, the composition of the feedstock being adjusted so as to obtain, at the end of step g), sintered particles having a composition conforming to that of a sintered particle as claimed in any one of the preceding claims, said feedstock comprising from 4% to 25% of zircon particles, as a percentage by weight based on the weight of the dry feedstock, d) forming the feedstock in the form of green particles, e) optionally, washing, f) optionally, drying, g) sintering at a sintering temperature above 1450°C and below 1600°C.

64. A device selected from a suspension, a grinding mill, an apparatus for surface treatment and a heat exchanger, said device comprising a powder of particles as claimed in claim 62.