EP4392388A1

EP4392388A1 - Method for producing granular zirconium oxide

Info

Publication number: EP4392388A1
Application number: EP22764395.4A
Authority: EP
Inventors: Alexander Engels; Bettina Schwendinger
Original assignee: Ivoclar Vivadent AG
Current assignee: Ivoclar Vivadent AG
Priority date: 2021-08-25
Filing date: 2022-08-09
Publication date: 2024-07-03
Also published as: WO2023025588A1; CN117858856A

Abstract

The invention relates to a method for producing granular zirconium oxide from powdered zirconium oxide, and to the use of the granular material for producing dental blanks and dental restorations.

Description

Process for the production of a zirconium oxide granulate

The present invention relates to a method for producing zirconium oxide granules with advantageous processing properties.

Due to their favorable properties, zirconium oxide ceramics are used in a wide range of applications, e.g. B. as a solid electrolyte in fuel cells and for the production of dental restorations.

Zirconium oxide ceramic products are usually manufactured from a powdery zirconium oxide starting material. Typically, a zirconium oxide powder is formed into the desired product and sintered. Preferred methods are (i) axial pressing or cold isostatic pressing (CIP) followed by conventional sintering, (ii) slip casting followed by conventional sintering and (iii) hot pressing (HP) or hot isostatic pressing (HIP).

It is desirable for all the processes mentioned that the zirconium oxide starting material used has advantageous processing properties. In particular, the starting material should be free-flowing and not contain any undesired compounds that have to be removed again in the course of the production of zirconium oxide products.

Methods for improving the processability, such as flowability, of zirconia powders are known.

The flowability can usually be improved by adding a flow aid, such as a pyrogenic silica (e.g. AEROSIL® from Evonik Industries AG).

DE 10 250 712 A1 also describes the possibility of further improving the flowability of the powder by surface treatment, such as silanization, of the silica used as a flow aid.

The processability of a zirconia powder can also be improved by first processing it into granules. In traditional processes, such as roller compaction, granulation occurs through the application of external pressure, which results in compaction of the powder.

In addition, conventional methods require the use of binders such as organic polymers to strengthen the cohesion of the powder particles. EP 3 659 574 contains an overview of customary binders for use in the production of zirconium oxide ceramic products. As a rule, however, the binder must be removed in the further manufacturing process of a ceramic product by heat treatment, the so-called debinding step.

Proceeding from the problems described above, the object of the invention is to provide a method with which a zirconium oxide granulate with advantageous processing properties can be produced.

This object is achieved according to the invention by the method for producing a zirconium oxide granulate according to claims 1 to 11. The subject matter of the invention is also the zirconium oxide granulate according to claim 12 and the use of the zirconium oxide granulate according to claims 13 to 18. The invention is also directed to the method for producing a blank for dental purposes according to claim 19 and to the method for producing a dental restoration according to claim 20.

The method according to the invention for producing a zirconium oxide granulate is characterized in that a zirconium oxide powder is agglomerated to form the granulate by build-up granulation.

Surprisingly, it was found that the zirconium oxide granules produced according to the invention have advantageous properties which enable the granules to be processed particularly quickly and easily. In particular, the zirconium oxide granules produced by the method according to the invention are characterized by very good pressability. The properties of the granules allow z. B. the use of automated presses and enable the elimination of a debinding step, which reduces the overall duration of the sintering process can be significantly shortened. In addition, the method according to the invention is also particularly cost-effective, since the advantageous processing properties lead to reduced material loss. Furthermore, the method according to the invention allows the use of particularly favorable zirconium oxide starting materials which contain little or no additives.

For the purposes of the invention, the term "granulate" refers to particles that consist of a plurality of particles that have been put together, i.e. agglomerated, such as powder particles.

The granules are usually free-flowing. A "flowable" granulate can also be referred to as a free-flowing granulate and, within the meaning of the invention, is a granulate that can be transported free-flowing under the influence of gravity and without the application of additional forces. It is preferred that the granulate according to the invention, when it is in a sealed measuring funnel made of plastic, in particular polyoxymethylene homopolymer (Delrin® by Dupont), after opening the funnel opening can flow out of the funnel solely due to gravity, the funnel having an inclination of 40° and an outlet opening with a diameter of 8 mm having.

According to the invention, the term "build-up granulation" refers to processes in which, in particular, rolling movements or particle impacts of the powder or small particles of aggregated powder particles lead to the formation of larger particles of aggregated powder particles. In contrast, methods in which a granulate is formed by pressing the powder particles formed, as in roller compaction, is not understood as build-up granulation. Lomeration in the context of spray drying processes is not based on rolling movements or particle impacts of the powder or small particles, but on the evaporation or vaporization of a solvent.

A zirconium oxide powder is used as the starting material for agglomeration.

It is preferred that the powder has a primary particle size of 20 to 500 nm, preferably 80 to 300 nm, in particular 200 to 400 nm, particularly preferably 250 to 350 nm, measured as dso value and based on the volume of the particles . The particle size is determined in particular using the method of static laser diffraction (SLS) according to ISO 13320: 2009, e.g. B. using a particle analyzer LA-960 from Horiba, or dynamic light scattering (DLS) according to ISO 22412: 2017, e.g. B. using a Nanoflex particle measuring device from Colloid Metrix. The primary particle size can also be determined by scanning electron microscopy.

The zirconium oxide powder is in particular zirconium oxide based on polycrystalline tetragonal zirconium oxide (TZP). Preference is given to zirconium oxide which is stabilized with Y2O3, La2Oa, CeO2, MgO and/or CaO and in particular contains 2 to 14 mol %, preferably 2 to 10 mol % and particularly preferably 2 to 8 mol % of these oxides on the content of zirconium oxide, is stabilized.

In a preferred embodiment, the zirconium oxide contains 0 to 7 mol %, preferably 0 to 5 mol % and particularly preferably 0.1 to 5 mol % Y2O3.

The zirconium oxide powder can also be a mixture of zirconium oxide powders with different compositions, which in particular result in different coloring and/or translucency in the product ultimately made from the granules, such as a dental restoration. With the help of a mixture of differently colored zirconium oxide powders, the desired color for the manufactured product can be created easily and in a targeted manner. The degree of translucency can be controlled in particular by the amount of yttrium oxide in the zirconium oxide powders used.

In a further preferred embodiment, a suspension of the zirconium oxide powder is produced and dried before the agglomeration.

It is preferred that the suspension contains, as the liquid medium, water, linear alcohol, branched alcohol or a mixture thereof, in particular essentially linear alcohol, branched alcohol or a mixture thereof.

More preferably, the straight-chain alcohol or the branched-chain alcohol is a compound having the formula C _n H ₂ n+iOH, where n is an integer from 1-4.

In a further preferred embodiment, the liquid medium contains an organic component, preferably in an amount of not more than 4% by weight, in particular not more than 2% by weight, more preferably not more than 0.5% by weight. % based on the amount of solids in the suspension. In a particularly preferred embodiment, the liquid medium is essentially free of organic components.

Suitable organic components are, in particular, dispersants, agents for adjusting the pH, stabilizers and/or defoamers. The dispersant serves to prevent the agglomeration of suspended particles. The amount of dispersant in the liquid medium is in particular 0.01 to 3% by weight, preferably 0.1 to 2% by weight and particularly preferably 0.1 to 1% by weight, based on the amount of solid in the suspension.

Suitable dispersants are dispersants containing carboxylic acid or carboxylic acid salts, such as the ammonium salt of polymethacrylic acid (Dolapix CE64, Zschimmer & Schwarz Chemie GmbH).

Without the addition of a dispersant, the suspension can usually have a solids content of up to 50% by weight. By adding a dispersing agent to the suspension, higher solids contents, such as up to 85% by weight, can usually be obtained.

Acids and bases, such as carboxylic acids, e.g. 2-(2-methoxyethoxy)acetic acid and 2-[2-(2-methoxyethoxy)ethoxy]acetic acid, inorganic acids, e.g. hydrochloric acid, are particularly suitable as agents for adjusting the pH value and as stabilizers and nitric acid, as well as ammonium hydroxide and tetramethylammonium hydroxide. It is preferred that the liquid medium contains tetramethylammonium hydroxide.

The defoamer is used to avoid air bubbles in the suspension. It is typically used in an amount of 0.001 to 1% by weight, preferably 0.001 to 0.5% by weight and more preferably 0.001 to 0.1% by weight, based on the amount of solid in the suspension in which liquid medium used. Examples of suitable defoamers are paraffin, silicone oils, alkylpolysiloxanes, higher alcohols, propylene glycol, ethylene oxide-propylene oxide adducts and, in particular, alkylpolyalkylene glycol ethers. Due to the low proportion of organic components, these can also be burned out within a short time from a blank that was obtained from the zirconium oxide granulate produced according to the invention. This process is also commonly referred to as debinding.

The suspension can also contain coloring elements, such as coloring oxides or solutions of coloring salts, in the solid and/or in the liquid medium. The d and f elements of the periodic table of elements can be used as coloring elements. Preferred coloring elements are Pr, Fe, Tb, Cr, Mn, V, Ti, Nd, Eu, Dy, Er and/or Yb.

In a preferred embodiment, before agglomeration, the suspension is dried by spray drying or to form a dry cake, in particular dried by spray drying.

In spray drying, the suspension is dried by spraying it in a heated drying medium.

Drying to a dry cake can be done, for example, in drying cabinets or by freeze drying. In a preferred embodiment, the suspension is dried in a drying cabinet for a period of 3 to 12 hours, preferably 6 to 10 hours, at a temperature of 50 to 200°C, preferably 100 to 180°C. Drying can also take place with the aid of microwave radiation.

The powder preferably has a solids content of at least 99% by weight, in particular at least 99.7% by weight and particularly preferably at least 99.9% by weight. Thus, it is also preferable that the powder such as a dry cake of the zirconia powder or one obtained by spray drying Powder, less than 1 wt .-%, in particular less than 0.3 wt .-%, and particularly preferably less than 0.1 wt .-% residual moisture.

In a preferred embodiment, the powder is sieved before agglomeration and/or subjected to a vibration treatment, with the sieving preferably being carried out through a sieve with a mesh size of 200 to 700 μm, in particular 300 to 600 μm, more preferably 400 to 500 μm and particularly preferably 450 gm done. This is intended to break up or dissolve any powder aggregates that may be present. Furthermore, it is particularly preferable to carry out the screening and the vibration treatment at the same time. The vibration treatment is preferably carried out with simultaneous screening for 10 to 30 minutes, in particular 15 to 20 minutes, at 10 to 50 Hz, in particular at 25 to 45 Hz.

For example, it can be advantageous to first mechanically comminute a dry cake into coarse pieces, e.g., smaller than about 10 cm, and then to subject it to a vibration treatment with simultaneous sieving.

The powders provided according to the above embodiments and optionally sieved and/or subjected to a vibration treatment are suitable for being agglomerated into granules.

In a preferred embodiment, the agglomeration takes place without adding and/or removing a liquid. "Removal" of liquid refers to any process to reduce the liquid content, such as evaporation or vaporization.

More preferably, no liquid is added and no liquid removed during agglomeration. In this version tion form, the liquid content remains essentially constant during agglomeration.

In another preferred embodiment, the powder is dry agglomerated. This means that the powder used for agglomeration is essentially dry and agglomeration also takes place without the addition of a liquid. Such conditions exist in particular when the agglomeration takes place without adding a liquid and, in addition, a powder with a residual moisture content of less than 1% by weight, in particular less than 0.3% by weight, and particularly preferably less than 0.1% by weight % residual moisture is used.

It has surprisingly been found that agglomeration is possible even with very little liquid and even under dry conditions. It is therefore not necessary to use a powder with a significant amount of liquid or to add liquid, such as a liquid binder, to the powder when agglomerating, as is the case with known methods.

In a preferred embodiment, the powder contains organic binder in an amount of 3% by weight or less, preferably 1% by weight or less, particularly preferably less than 1% by weight and more preferably 0.1% by weight or fewer. It is particularly preferred that the powder is essentially free of organic binders.

In a further preferred embodiment, organic binder is added to the powder during agglomeration in an amount of 3% by weight or less, preferably 1% by weight or less, more preferably less than 1% by weight and more preferably 0.1% % by weight or less. Particularly preferably, no organic binder is added during agglomeration. In addition, it was surprisingly found that the zirconium oxide powder can also be agglomerated with a small amount of binder or even without the use of organic binders to form granules which are excellently suited for various applications and in particular for the production of dental restorations.

It is further preferred that the agglomeration takes place by shaking, vibrating, oscillating, acoustically mixing and/or rotating the powder or a powder bed consisting of the powder. Suitable devices are e.g. B. Vibrating screens, rotating granulating machines (e.g. from Maschinenfabrik Gustav Eirich GmbH & Co KG, Germany), resonant acoustic mixers (e.g. from Resodyn Acoustic Mixers, USA) and pelletizers. The agglomeration takes place in particular without the use of mixer elements such. B. wings or leaves.

In a preferred embodiment, the powder is placed on a sieve, such as. B. agglomerated on a perflux screen from Siebtechnik GmbH, the screen preferably having a mesh size of 30 to 60 gm, in particular 40 to 50 gm, particularly preferably 45 gm. In this way, individual powder particles and small particles of aggregated powder particles can be sorted out through the mesh.

In a further preferred embodiment, the agglomeration can take place on a surface made of silicone polymer. It can e.g. B. a sieve can be coated with a silicone polymer in such a way that the meshes are closed and a flat surface is created. When using a closed surface, individual powder particles and small particles of aggregated powder particles can also be embedded in the granulate. be worked. A suitable silicone polymer is eg

Elastosil Vario 40 from Wacker Chemie AG.

It is further preferred that a surface in contact with the powder during agglomeration is non-conductive. Most preferably, none of the surfaces in contact with the powder during agglomeration is conductive. Suitable surfaces are e.g. LUS polyurethane coated metal surfaces.

Agglomeration preferably takes place for a period of 5 to 120 minutes, in particular 10 to 60 minutes, particularly preferably 10 to 20 minutes.

In a preferred embodiment, the agglomeration takes place at a temperature of less than 80°C, in particular less than 50°C and very particularly preferably less than 30°C.

It was surprisingly found that a high solids content of the zirconia powder can be advantageous in order to shorten the time required for agglomeration.

In a preferred embodiment, powders with a solids content of at least 99% by weight, preferably at least 99.9% by weight, are in particular 10 to 50 Hz, preferably 25 to 45 Hz, agglomerated. It can be advantageous to select the vibration frequency, if necessary depending on the filling level of the vibrating screen, in such a way that dust formation is avoided.

If the powder is sieved and/or subjected to a vibration treatment before agglomeration, it can be advantageous for an efficient process to pass the sieve in one suitable device , e.g. B. a vibrating screen to catch and agglomerate directly in it.

In a preferred embodiment, the granules have an average particle size of 10 to 600 μm and in particular 150 to 500 μm, measured as the d 50 value and based on the volume of the particles.

In a further preferred embodiment, the granules have an average particle size of less than 50 μm, measured as the dso value and based on the volume of the particles.

In a further preferred embodiment, at least 90%, in particular at least 95%, more preferably at least 99% of the particles of the granulate have a size of 10 to 600 μm, preferably 150 to 500 μm.

It has also been shown that typically larger particles can be achieved through a longer agglomeration time. By a duration in the range of several hours z. B. Particles are obtained which have a diameter of more than 2 mm.

In addition, it can be advantageous to screen the granules after agglomeration in order to obtain a more uniform particle size distribution. For example, particles with a diameter greater than 600 μm can be separated off by sieving.

The invention also relates to a method for producing a blank for dental purposes, in which zirconium oxide granulate is produced according to the method according to the invention and then formed into a blank for dental purposes. The zirconium oxide granules can be shaped into a blank for dental purposes using the methods known for this.

In a preferred embodiment, the shaping of the zirconium oxide granulate into a blank for dental purposes includes the zirconium oxide granulate being placed in a mold and then being pressed, in particular uniaxially and/or isostatically. It is particularly preferred that the zirconium oxide granules are placed in the mold with the aid of an air stream.

In addition, it is preferred that the blank is subjected to a heat treatment in order to produce a pre-sintered open-pored blank.

In a preferred embodiment, the heat treatment for pre-sintering is carried out at a temperature of 700 to 1200° C., preferably at a temperature of 800 to 1100° C., and for a period of 5 minutes to 100 hours, in particular 10 minutes to 12 minutes h and more preferably 30 min to 6 h, where the term "duration" refers to the holding time of the maximum temperature.

Embodiments which have been described as suitable or preferred in connection with the method for producing a zirconium oxide granulate are also correspondingly suitable or preferred for the production of a blank for dental purposes according to the invention.

The invention also relates to a method for producing a dental restoration, in which a blank for dental purposes is produced using the method according to the invention and

( i ) the blank is subjected to machining to give the blank the shape of a dental restoration, and (ii) the blank is sintered by at least one heat treatment.

In a preferred embodiment, the machining includes milling and/or grinding. It is preferred that the machining is done with computer controlled milling and/or grinding devices. Machining is particularly preferably carried out as part of a CAD/CAM process.

In step (i), the blank is preferably given the form of a bridge, an inlay, an onlay, a crown, a partial crown, an implant, a veneer, a shell or an abutment.

The sintering can take place at a temperature of 1050 to 1700°C, in particular 1200 to 1600°C, preferably 1300 to 1550°C, particularly preferably 1350 to 1500°C. In particular, the heat treatment for sintering is carried out for a period of 0 to 240 minutes, preferably 5 to 180 minutes, particularly preferably 30 to 120 minutes, the term "duration" referring to the holding time of the maximum temperature.

Typically, sintering serves to consolidate the zirconia and thereby impart excellent mechanical properties to the dental restoration. In this case, the sintering is usually also referred to as "dense sintering".

In a preferred embodiment, the density of the blank formed for the dental restoration after the sintering of step (ii) is more than 5.9 g/cm ³ , in particular more than 6.00 g/cm ³ and particularly preferably more than 6. 02 g/ ^cm3 . Embodiments which have been described as suitable or preferred in connection with the method for producing the zirconium oxide granulate and the method for producing a blank for dental purposes are also correspondingly suitable or preferred for the production of a dental restoration according to the invention.

The invention also relates to flowable zirconium oxide granules which can be obtained by the process according to the invention described above.

The zirconium oxide granules produced by the process according to the invention typically have a bulk density of 1.1 to 1.5 g/cm ³ , in particular 1.2 to 1.4 g/cm ³ . The bulk density can be determined, for example, in accordance with DIN EN ISO 60.

In a preferred embodiment, the zirconium oxide granulate contains organic binder in an amount of 3% by weight or less, preferably 1% by weight or less, more preferably 0.1% by weight or less. In a particularly preferred embodiment, the zirconium oxide granules are essentially free of organic binders.

In a further preferred embodiment, the zirconium oxide granulate contains organic components in an amount of 9% by weight or less, preferably 4% by weight or less, more preferably 1% by weight or less. In a particularly preferred embodiment, the zirconium oxide granules are essentially free of organic components. Zirconium oxide granules, which are essentially free of organic components, have the advantage that no debinding step is usually required during further processing. Suitable organic components are, in particular, dispersants, agents for adjusting the pH value, stabilizers and/or defoamers, as have already been described above.

In addition, the zirconium oxide granulate can contain a lubricant. The lubricant usually serves to further improve the processability of the granulate, in particular during compression, by reducing the friction between the particles and between the particles and the compression mold.

Preferred lubricants are stearates, in particular magnesium stearate and ammonium stearate. It is also preferred that the granules contain 0.01 to 5% by weight, in particular 0.05 to 2% by weight, particularly preferably 0.1 to 1% by weight, of lubricant. The lubricant, e.g., ammonium stearate, can be added to the suspension. It is also possible to add the lubricant, e.g. magnesium stearate, after agglomeration.

Finally, the invention also relates to the use of the zirconium oxide granules obtainable by the above process for the production of a dental blank or a dental restoration. Typically, a dental blank is first produced from the granules according to the invention, which can then in turn be subjected to shaping and densely sintered in order to produce a dental restoration.

The granules according to the invention can be used in an advantageous manner in order to produce dental blanks or dental restorations from them. The advantageous properties of the granules make it particularly easy to handle the zirconium oxide granules, which means that material loss can be reduced and the process time can be shortened. Unlike the use of conventional zirconium oxide powder, the inventive proper granules z. B. used in automatic presses.

In a preferred embodiment of the use, the granules are introduced into a mold with the aid of an air stream. The direction of the air flow is irrelevant, i . H . the granules can be sucked or blown into the mold. It is particularly preferred that the mold is a press mold.

Furthermore, an embodiment is preferred in which the granules are introduced into a compression mold and pressed into a blank. The granules can be in dry or wet or. are pressed into a blank while wet. The granules are preferably pressed in the dry state to form a blank.

In a preferred embodiment of the use according to the invention, the pressing is carried out uniaxially at a pressure of 5 to 90 MPa, preferably 10 to 60 MPa, particularly preferably 10 to 40 MPa.

It can be advantageous to subject the blank to post-compaction by cold isostatic pressing, preferably at a pressure of 100 to 600 MPa, in particular 150 to 500 MPa and particularly preferably 150 to 400 MPa.

It is also preferred that the blank is pre-sintered, preferably at a temperature of 600 to 1300° C., in particular 700 to 1200° C., particularly preferably 800 to 1100° C., for a period of in particular 1 h to 100 h 6 hours to 80 hours and particularly preferably 20 hours to 40 hours.

After pre-sintering, the blank is typically in an open-pore state and can be used to cut from it to fabricate a dental restoration. When manufacturing the dental restoration, the blank is usually first given the shape of a dental restoration, eg in a CAD/CAM process, and then the shaped blank is densely sintered to form a dental restoration.

It is preferable that a green body is given the shape of a dental restoration and the formed green body is sintered at 1200 to 1600°C, more preferably 1300 to 1550°C, particularly preferably 1350 to 1500°C.

It is further preferred that the time for heating the blank from room temperature to the sintering temperature, holding it at the sintering temperature and cooling it to the final temperature is 120 minutes or less, preferably 60 minutes or less, in particular 40 minutes or less and particularly preferably 30 min or less. "Final temperature" is understood to mean a temperature at which the sample can be picked up, and it is in particular 15 to 80° C., preferably 25 to 60° C. and particularly preferably about 50° C. "Room temperature" is a temperature of in particular 15 to 30°C, preferably 20 to 25°C and particularly preferably about 25°C.

In a further preferred embodiment of the use according to the invention, the dental restoration is a bridge, an inlay, an onlay, a crown, a partial crown, an implant, a veneer, a shell or an abutment.

The dental restoration obtained after the dense sintering can optionally be provided with a veneer, polished and/or glazed. Dental restorations with very good optical and mechanical properties can be produced by the use according to the invention.

It was found that the solids content of the suspension produced to provide the powder can have an influence on the flexural strength of a dental restoration produced from the granules. Suspensions with a solids content of less than 70% by weight -% can e.g. B. used to produce dental restorations with a biaxial flexural strength of over 1000 MPa, determined according to ISO 6872:2015.

Since the use according to the invention and the methods according to the invention are different aspects of the same invention, the embodiments disclosed in connection with the use according to the invention and their advantages also apply correspondingly to the methods according to the invention and vice versa.

The invention is explained in more detail below using examples.

examples

Example 1: Production of zirconium oxide granules

A zirconium oxide granulate was produced using the method according to the invention. For this purpose, a suspension with a solids content of 45 wt. -% manufactured . The suspension was dried in a drying cabinet at 180° C. for 9 hours. The resulting dry cake was first coarsely mechanically comminuted and then subjected to a vibration treatment with simultaneous sieving on a sieve with a mesh size of 450 μm for 20 minutes at 40 Hz.

The powder produced in this way was agglomerated for 20 minutes at 45 Hz with the aid of a vibrating screen (Perf lux-Sieb from Siebtechnik GmbH) without the use of binders. The vibrating screen was coated with a silicone polymer (Elastosil Vario 40 from Wacker Chemie AG) so that it had a flat, closed surface.

The granules produced had an average particle size dso of approx. 200 pm, based on the volume of the particles, on .

Example 2 Use of zirconium oxide granules for the production of dental test specimens

The zirconium oxide granules produced according to Example 1 were uniaxially pressed at room temperature and a pressure of 50 MPa to give shaped bodies in the form of blocks or disks.

The moldings were then recompacted by cold isostatic pressing at room temperature and a pressure of 300 MPa. The moldings obtained were presintered at 1000° C. for 60 hours. The pre-sintered molded bodies were machined into the shape of specimens having a diameter of 12.5 ± 0.5 mm and a thickness of 1.2 ± 0.2 mm. The specimens were polished and densely sintered at 1550°C. The test specimens exhibited biaxial flexural strengths of around 1000 MPa, determined according to ISO 6872:2015.

Claims

Process for the production of a zirconium oxide granulate, in which a zirconium oxide powder is agglomerated to form the granulate by build-up granulation. A method according to claim 1, wherein the powder contains organic binder in an amount of 3% by weight or less, preferably 1% by weight or less, more preferably 0.1% by weight or less. Process according to Claim 1 or 2, in which the powder has a primary particle size of 20 to 500 nm, preferably 80 to 300 nm, measured as dso value and based on the volume of the particles. Process according to one of Claims 1 to 3, in which, before the agglomeration, a suspension of a zirconium oxide powder is produced and this is dried. Process according to Claim 4, in which the suspension contains water, straight-chain alcohol, branched-chain alcohol or a mixture thereof, in particular essentially straight-chain alcohol, branched-chain alcohol or a mixture thereof, as the liquid medium. Process according to Claim 4 or 5, in which the suspension is dried by spray drying or to a dry cake, in particular is dried by spray drying. Process according to one of Claims 1 to 6, in which the powder has a solids content of at least 99% by weight, in particular at least 99.7% by weight and particularly preferably at least 99.9% by weight. Method according to one of Claims 1 to 7, in which the agglomeration takes place by shaking, vibrating, swinging, acoustically mixing and/or rotating the powder or a powder bed consisting of the powder. A method according to any one of claims 1 to 8 in which a surface in contact with the powder during agglomeration is non-conductive. Process according to one of Claims 1 to 9, in which the agglomeration takes place for a period of 5 to 120 minutes, in particular 10 to 60 minutes, particularly preferably 10 to 20 minutes. Process according to one of Claims 1 to 10, in which the granules have an average particle size of 10 to 600 μm and in particular 150 to 500 μm, measured as the dso value and based on the volume of the particles. Zirconium oxide granules obtainable by a method according to any one of claims 1 to 11. Use of the zirconium oxide granules according to claim 12 for the production of a dental blank or a dental restoration. Use according to claim 13, in which the granules are introduced into a mold by means of an air current. Use according to Claim 13 or 14, in which the granules are introduced into a compression mold and pressed into a blank. Use according to any one of claims 13 to 15, in which the blank is pre-sintered, preferably at a temperature of 600 to 1300 °C, in particular 700 to 1200 °C, in particular ders preferably 800 to 1100 ° C, for a period of in particular 1 h to 100 h, preferably 6 h to 80 h and particularly preferably 20 h to 40 h. Use according to one of Claims 13 to 16 for the production of a dental restoration, in which a blank is given the shape of a dental restoration and the shaped blank is heated at 1200 to 1600 °C, in particular 1300 to 1550 °C, particularly preferably 1350 to 1500 °C. is sintered. Use according to one of Claims 13 to 17, in which the dental restoration is a bridge, an inlay, an onlay, a crown, a partial crown, an implant, a veneer, a shell or an abutment. A method for producing a blank for dental purposes, in which zirconium oxide granules are produced according to the method according to any one of claims 1 to 11 and then formed into a blank for dental purposes. Method for producing a dental restoration, in which a blank is produced according to the method according to claim 19 and

( i ) the blank is subjected to machining to give the blank the shape of a dental restoration, and

(ii) the blank is sintered by at least one heat treatment.