METHOD FOR PRODUCING ABRASIVE PARTICLES

Abstract

A method for producing abrasive particles includes the following method steps: i. preparing a starting mixture containing at least aluminium hydroxide, which mixture can be converted at least into aluminium oxide by means of heat treatment; ii. extruding the starting mixture to form an extrudate; iii. separating the extrudate into intermediate particles; and iv. heat-treating the intermediate particles. The intermediate particles are converted into abrasive particles that contain aluminium oxide, and the extrudate and/or the intermediate particles is/are subjected to an input of energy that is asymmetrical with respect to the geometry of the extrudate and/or the intermediate particles.

Description

BACKGROUND OF THE INVENTION

The invention relates to a method for producing abrasive particles, and abrasive particles produced according to the method. The invention furthermore relates to a method for producing a grinding tool for machining metallic materials as well as the grinding tool produced according to this method.

Different methods for producing abrasive particles are known from the state of the art. For example, in the applicant's EP 3 342 839 A1 a method is disclosed in which abrasive particles with a non-uniform shape and/or size are produced by chipping an extrudate. The objective in this method is to produce abrasive particles with an irregular geometry.

There, a disadvantage is that only relatively few abrasive particles can be produced in a particular time.

Furthermore, such a method results in a relatively high wear, as the cutting edges used for the chipping are subjected to a high load and thus wear relatively quickly.

SUMMARY OF THE INVENTION

The object of the present invention is to specify a method for producing abrasive particles which avoids the above-named problems, the abrasive particles produced therewith, a method for producing a grinding tool for machining metallic materials in which the abrasive particles produced according to the invention are used, as well as a grinding tool produced by means of this method.

In a method according to the invention, it is thus provided that the extrudate and/or the intermediate particles is or are subjected to an energy input that is asymmetric with respect to the geometry of the extrudate and/or the intermediate particles.

Because of the asymmetric energy input, an asymmetric heating of the extrudate and/or the intermediate particles occurs. As the extrudate and/or the intermediate particles do not cool evenly because of the asymmetric, thus irregular, heating, stresses occur inside the extrudate and/or the intermediate particles. These stresses lead to a twisting of the extrudate and/or of the intermediate particles and thus to abrasive particles with an irregular geometry.

Compared with methods known from the state of the art, more abrasive particles can be produced in the same amount of time, since several extrudate strands can be provided for example. In addition, the wear is less in a method according to the invention than in the state of the art, as no chipping device is needed.

It may be pointed out that the technique of converting a starting mixture containing at least aluminum hydroxide at least into aluminum oxide by heat treatment has already been known for quite some time. In this connection, reference may be made to the so-called “sol-gel process”. There, a starting mixture which contains at least aluminum hydroxide is used. Aluminum hydroxide can be present in different modifications. In connection with the present invention, powdered boehmite (γ-AlOOH) is preferably used. Further preferably, the boehmite is subsequently converted into a clear sol by the addition of water and the admixture of a peptizer, e.g. nitric acid. Then, through the further addition of an acid, e.g. nitric acid, or a nitrate solution, a reaction to form the gel, i.e. a dehydration and polymerization, is preferably initiated. Because of the gel formation, the boehmite is present in a very homogeneous distribution. In a subsequent work step, water released can be evaporated. In the course of a following heat treatment at a temperature of between 400° C. and 1200° C., preferably at a temperature of between 800° C. and 1000° C., the aluminum hydroxide can be converted into an aluminum oxide of the transition phase γ-Al₂O₃. In the reaction of boehmite to form aluminum oxide, nitrogen is released as residue of the acid and water. This low-temperature combustion is also called calcination. In a last step, a further heat treatment in the form of, preferably pressureless, sintering can then be carried out. This step is preferably effected at a temperature of between 1200° C. and 1800° C., particularly preferably at a temperature of between 1200° C. and 1500° C. Depending on the starting mixture, it can happen that secondary phases, such as e.g. spinel, form in addition to aluminum oxide (typically as alpha-aluminum oxide). Allowance is made for this circumstance by the expression “at least into aluminum oxide”.

By “extrusion” is meant a process technology in which solid to viscous hardenable materials are continuously pressed out of a shaping opening under pressure. In the process, bodies with a cross section of the opening form, called extrudate.

In the present case, the cross section of the extrudate depends on a nozzle body used and is preferably rectangular, square, trapezoidal, parallelogram-shaped, triangular, drop-shaped, propeller-shaped or star-shaped and/or has at least one convex side or at least one concave side.

Not only is the method according to the invention for producing abrasive particles characterized by its simplicity and the lower maintenance requirement and wear compared with the state of the art, but it also makes it possible to vary the shape and/or size of the intermediate particles or of the abrasive particles present after the sintering easily and flexibly by replacement of a nozzle body and/or alterations during the separation.

One possibility for influencing or controlling the dimensions of the abrasive particles is to supply the extrudate to the method step of separation with an alterable infeed speed and/or in an oscillating motion. In the case of an oscillating motion, a particular length of the extrudate to be separated arises.

Furthermore, it can also be provided that the intermediate particles generated by the separation are comminuted before the heat treatment in a further method step, preferably by means of a cutting device. Instead of a cutting device, other comminution devices which, for example, also bring about a breaking and/or chopping of the intermediate particles can also be used.

A further possibility for influencing the shape and/or size of the abrasive particles is obtained by altering the consistency of the starting mixture. For this, it can be provided that during the provision of the starting mixture and/or during the extrusion of the starting mixture water, a peptizer, preferably nitric acid, and/or additives, for example an acid, which can likewise be nitric acid, and/or cobalt nitrate, are added.

Particularly preferably, the extrudate and/or the intermediate particles have a longitudinal direction, and the asymmetric energy input is effected transverse to the longitudinal direction.

On the one hand this favors a twisting of the extrudate and/or of the intermediate particles and on the other hand it makes a simple realization of an asymmetric energy input possible.

Advantageous embodiments of the method for producing abrasive particles furthermore consist in that in the course of the heat treatment the intermediate particles generated by the separation are calcined, preferably at a temperature of between 400° C. and 1200° C., particularly preferably at a temperature of between 800° C. and 1000° C., and/or are sintered, preferably at a temperature of between 1200° C. and 1800° C., particularly preferably at a temperature of between 1200° C. and 1500° C. As a supplement, it can be provided that in the course of the heat treatment the intermediate particles generated by the separation are pre-dried before the calcination and/or sintering, preferably at a temperature of between 50° C. and 350° C., particularly preferably at a temperature of between 80° C. and 100° C.

As previously stated, protection is also sought for a method for producing a grinding tool for machining metallic materials, wherein abrasive particles which were produced according to the method according to the invention for producing the abrasive particles are incorporated in a bond, for example in a ceramic bond or in a resinoid bond. A grinding tool with a porosity of from 2 to 50% and/or a density of from 1.5 to 4.5 g/cm³ advantageously results thereby.

BRIEF DESCRIPTION OF THE DRAWINGS

Further details and advantages of the present invention are explained in more detail below with the aid of the description of the figures with reference to the drawings. There are shown in:

FIG. 1 shows a first embodiment of the method according to the invention for producing abrasive particles,

FIG. 2 shows a second embodiment of the method according to the invention for producing abrasive particles,

FIG. 3 shows an embodiment of a nozzle body in a sectional representation,

FIGS. 4a-l are schematic representations of outlet openings of nozzle channels of a nozzle body according to the invention,

FIGS. 5a/5b are photographs of abrasive particles which were produced according to a preferred embodiment example of the method according to the invention for producing abrasive particels,

FIG. 6a is a photograph of abrasive particles which were produced according to an embodiment example of the method according to the invention for producing abrasive particles, and

FIG. 6b is a photograph, in a front view, of an abrasive particle which was produced according to an embodiment example of the method according to the invention for producing abrasive particles.

DETAILED DESCRIPTION OF THE INVENTION

In the first embodiment example, represented in FIG. 1, of the method according to the invention for producing abrasive particles, a starting mixture 2 is provided by introducing boehmite 13, water 14, nitric acid 15 and additives 16, for example cobalt nitrate, into a mixer 17, wherein the mixer 17 substantially consists of a mixing tank 17a and a rotating unit 17b arranged therein.

The starting mixture 2 provided in this way is subsequently supplied to an extrusion device 18. It can be provided that the extrusion device 18 is arranged on a platform 19, which can be set in an oscillating motion. This oscillating motion is indicated schematically by means of a double arrow in FIG. 1.

The extrudate 3 leaving the extrusion device 18 has a particular cross-sectional shape which is determined by a nozzle body 6.

In this first embodiment example, a device for asymmetric energy input 8 is arranged directly after the nozzle body 6 and subjects the intermediate particles 4 to an asymmetric energy input. However, the device for asymmetric energy input 8 can also be arranged in other positions, for example in the region of a belt guiding device 20.

The asymmetric energy input by the device for asymmetric energy input 8 can be effected, among other things, by

• • contact with at least one heating device, preferably wherein the at least one heating device is formed plate-shaped at least in regions, and/or
  • introduction of an electric current into the extrudate 3 and/or the intermediate particles 4, wherein at least a part of an energy of the electric current is converted into heat by an electrical resistance of the extrudate 3 and/or the intermediate particles 4, and/or
  • convection, preferably by means of a fan heater device, and/or
  • action of an electromagnetic radiation, preferably wherein the electromagnetic radiation has at least a wavelength of between 780 nm and 1 mm or 380 nm and 100 nm, and/or is emitted by at least one laser or a radiant heater, and/or
  • by induction, wherein ferromagnetic particles are incorporated in the starting mixture 2 to be extruded.

It can also be provided that the device for asymmetric energy input 8 is formed as a drum or roller.

Furthermore, the device for asymmetric energy input 8 can in principle be arranged in any desired position between extrusion device 18 and sintering furnace 23.

The extrudate 3 is subsequently separated by a separator 10 formed as a rotating or oscillating blade. It can also be provided that the separation into intermediate particles is effected by means of at least one laser or at least one water jet cutter or at least one plasma cutter, preferably wherein the extrudate 3 to be separated by means of the at least one laser or the at least one water jet cutter or the at least one plasma cutter is deposited on a conveyor before the separation.

The intermediate particles 4 generated by the separation of the extrudate 3 are supplied to a pre-drying device 21 by means of a belt guiding device 20.

The pre-dried intermediate particles 4 are then transferred to a calcining furnace 22, in which a calcination of the intermediate particles 4 is effected.

After the calcination, a sintering furnace 23 follows, in which the intermediate particles 4 are sintered to form abrasive particles 5. The shape and/or size of the abrasive particles 5 produced in this way will be discussed in more detail with reference to FIGS. 5a and 5b.

Instead of three spatially separated, successive devices 21, 22 and 23 for heat treatment, one integrated device for heat treatment can also be used, for example a tunnel furnace, with temperature zones which are controllable independently of each other.

The sintered abrasive particles 5 are positioned on a belt guide 24. During the transport by means of this belt guiding device 24, the abrasive particles 5 generated by the sintering are cooled.

The finished abrasive particles 5 are then transferred to a storage device 25 and are available for a further processing, for example for a method for producing a grinding tool for machining metallic materials.

A second embodiment example of the method according to the invention is represented in FIG. 2. The embodiment examples differ substantially only by the position of the device for asymmetric energy input 8 and the separator 10.

It can be seen that the extrusion device 18 is rotated and the extrudate 3 exits from the nozzle body 6 in the direction of gravitational acceleration in the form of several extrudate strands 9. The device for asymmetric energy input 8 is arranged such that it subjects the extrudate strands 9 hanging downwards due to the weight force to an asymmetric heat input. The extrudate 3 is thus subjected to an asymmetric heat input, and the intermediate particles 4 are not.

The extrudate 3 subjected to an asymmetric heat input is then deposited on a belt guiding device 20 and separated by a separator 10.

The rest of the method according to the invention according to the second embodiment example proceeds analogously to the first embodiment example, shown in FIG. 1.

FIG. 3 shows an embodiment example of a nozzle body 6 in a sectional representation. It can be seen that the nozzle channels 7 are formed substantially cylindrical and have the same diameter as the inlet opening 7a.

In the case of a nozzle body 6 according to FIG. 3, a starting mixture 2 to be extruded thus enters the nozzle body 6 through the inlet openings 7a and, through the outlet opening 7b, undergoes an increase in its density and/or its speed.

The mixture 2 to be extruded then exits from the nozzle body 6 through the outlet openings 7b as extrudate 3. The outlet openings 7b in this embodiment example resemble a three-blade rotor in terms of their shape.

A nozzle body 6 according to FIG. 3 can be produced using an additive manufacturing method or using at least a material removal manufacturing method.

In the case of a material removal manufacturing, it could be provided for example that blind holes are introduced into a metallic blank. Outlet openings 7b could then be cut into these blind holes by means of laser cutting. However, any other suitable manufacturing method can also be provided.

FIGS. 4a to 4l show schematic representations of outlet openings 7b of nozzle channels 7 of a nozzle body 6. It is apparent that the outlet openings 7b can have a wide variety of geometric shapes. The outlet openings 7b represented in FIGS. 4a to 4l are only to serve as examples, in principle all suitable geometric shapes are conceivable for the outlet openings 7b.

The shape of the outlet openings 7b also determines the cross-sectional shape of the extrudate 3 and therefore the cross-sectional shape of the intermediate particles 4 and abrasive particles 5.

FIGS. 5a and 5b show photographs of abrasive particles which were produced according to a method according to the invention for producing abrasive particles 5. With reference to the photographs, the size of the abrasive particles 5 on the one hand and the shape of the abrasive particles 5 on the other hand are apparent. It can be seen that a majority of the abrasive particles 5 from the photographed sample have a twist angle of from 90° to 180°. In particular, however, it can be provided that the abrasive particles 5 have a twist angle of up to 360°.

FIG. 6a shows a photograph of abrasive particles which were produced according to a method according to the invention for producing abrasive particles 5 with an embodiment of a nozzle body according to FIG. 3. With reference to the photograph, the size of the abrasive particles 5 on the one hand and the shape of the abrasive particles 5 on the other hand are apparent.

It can be seen that a majority of the abrasive particles 5 from the photographed sample have a twist angle of from 90° to 180°. In particular, however, it can be provided that the abrasive particles 5 have a twist angle of up to 360°.

FIG. 6b shows a photograph, in a front view, of an abrasive particle which was produced according to a method according to the invention for producing abrasive particles 5 with an embodiment of a nozzle body according to FIG. 3. With reference to the photograph, the size of an abrasive particle and its cross section can be seen.

LIST OF REFERENCE NUMBERS

• 1 method
• 2 starting mixture
• 3 extrudate
• 4 intermediate particles
• 5 abrasive particles
• 6 nozzle body
• 7 nozzle channels
  • 7a inlet opening
  • 7b outlet opening
  • 7c funnel-shaped section
  • 7d twisted section
• 8 device for asymmetric energy input
• 9 extrudate strand
• 10 separator
• 11 conveyor
• 12 grinding tool
• 13 boehmite
• 14 water
• 15 nitric acid
• 16 additives
• 17 mixer
  • 17a mixing tank
  • 17b rotating unit
• 18 extrusion device
• 19 platform
• 20 belt guiding device
• 21 pre-drying unit
• 22 calcining furnace
• 23 sintering furnace
• 24 belt guiding device
• 25 storage device

Claims

1. Method for producing abrasive particles, having the following method steps: providing a starting mixture, containing at least aluminum hydroxide, which can be converted at least into aluminum oxide by heat treatment,extruding the starting mixture to form an extrudate,separating the extrudate into intermediate particles, andheat-treating the intermediate particles, wherein the intermediate particles are converted into abrasive particles which contain aluminum oxide,wherein the extrudate and/or the intermediate particles is or are subjected to an energy input that is asymmetric with respect to the geometry of the extrudate and/or the intermediate particles.

2. The method according to claim 1, wherein the asymmetric energy input is effected at at least one outlet opening of at least one nozzle body of an extrusion device and/or on at least one belt guiding device and/or in at least one device for asymmetric energy input, preferably comprising at least one drum and/or roller.

3. The method according to claim 2, wherein the asymmetric energy input is effected at at least one outlet opening of at least one nozzle body of an extrusion device, wherein at least one extrudate strand hanging downwards under the influence of the weight force is subjected to the asymmetric energy input.

4. The method according to claim 1, wherein the asymmetric energy input is effected by contact with at least one heating device, preferably wherein the at least one heating device is formed plate-shaped at least in regions, and/oris effected by introduction of an electric current into the extrudate and/or the intermediate particles, wherein at least a part of an energy of the electric current is converted into heat by an electrical resistance of the extrudate and/or the intermediate particles, and/oris effected by convection, preferably by means of a fan heater device, and/oris effected by action of an electromagnetic radiation, preferably wherein the electromagnetic radiation has at least a wavelength of between 780 nm and 1 mm or 380 nm and 100 nm, and/or is emitted by at least one laser or a radiant heater, and/oris effected by induction, wherein ferromagnetic particles are incorporated in the starting mixture to be extruded.

5. The method according to claim 1, wherein the extrudate and/or the intermediate particles have a longitudinal direction and the asymmetric energy input is effected transverse to the longitudinal direction.

6. The method according to claim 1, wherein in the course of the extrusion the starting mixture is pressed through at least one nozzle body with at least one nozzle channel, preferably a plurality of nozzle channels running substantially parallel, preferably wherein the at least one nozzle body was produced using an additive manufacturing method.

7. The method according to claim 6, wherein the at least one nozzle channel of the at least one nozzle body has a, preferably circular or elliptical, inlet opening, through which the starting mixture enters the at least one nozzle channel, and an outlet opening that is preferably rectangular, square, triangular, drop-shaped or star-shaped and/or has at least one convex side or at least one concave side, via which the extrudate exits from the at least one nozzle channel.

8. The method according to claim 6, wherein the at least one nozzle channel has a funnel-shaped section following the inlet opening with a diameter decreasing in the direction of the outlet opening, whereby the pressure, the density and/or the speed of the starting mixture to be extruded is increased.

9. The method according to claim 1, wherein the extrudate is separated into intermediate particles by a separator, preferably by a rotating or oscillating blade, and/or by means of at least one laser and/or at least one water jet cutter and/or at least one plasma cutter, preferably wherein the extrudate to be separated by means of the separator is deposited on a conveyor before the separation.

10. The method according to claim 1, wherein in the course of the heat treatment the intermediate particles generated by the separation are calcined, preferably at a temperature of between 400° C. and 1200° C., particularly preferably at a temperature of between 800° C. and 1000° C., and/orare sintered, preferably at a temperature of between 1200° C. and 1800° C., particularly preferably at a temperature of between 1200° C. and 1500° C.

11. The method according to claim 10, wherein in the course of the heat treatment the intermediate particles generated by the separation are pre-dried before the calcination and/or sintering, preferably at a temperature of between 50° C. and 350° C., particularly preferably at a temperature of between 80° C. and 100° C.

12. The method according to claim 1, wherein the abrasive particles present after the heat treatment are cooled.

13. The method according to claim 1, wherein during the provision of the starting mixture and/or during the extrusion of the starting mixture water, a peptizer, preferably nitric acid, and/or additives, for example an acid and/or cobalt nitrate, are added.

14. Abrasive particles produced according to the method according to claim 1, preferably wherein the abrasive particles are formed helical at least in sections.

15. The abrasive particles according to claim 14, wherein the abrasive particles have a base that is rectangular, square, trapezoidal, parallelogram-shaped, triangular, drop-shaped, propeller-shaped or star-shaped and/or has at least one convex side or at least one concave side.

16. The abrasive particles according to claim 14, wherein the abrasive particles have a length of from 0.5 mm to 4 mm, preferably between 1 mm and 2 mm.

17. The abrasive particles according to claim 14, wherein the abrasive particles have a width of from 200 μm to 800 μm, preferably between 500 μm and 700 μm.

18. The abrasive particles according to claim 14, wherein the abrasive particles have a thickness of from 50 μm to 400 μm, preferably 150 μm to 250 μm.

19. The abrasive particles according to claim 14, wherein the abrasive particles have a twist angle of between 0° and 360°, preferably between 180° and 360°.

20. A method for producing a grinding tool for machining metallic materials, wherein abrasive particles which were produced according to the method according to claim 1 are incorporated in a bond, for example in a ceramic bond or a resinoid bond.

21. A grinding tool produced according to the method according to claim 20, wherein the grinding tool has a porosity of from 2 to 50% and/or a density of from 1.5 to 4.5 g/cm3.