EP3738673A1 - Grinding device for rounding particles - Google Patents

Abstract

The invention relates to a grinding device for rounding particles. The grinding device comprises a vortex chamber for treating the particles suspended in a fluid stream. The invention also relates to a corresponding method and optionally the associated use.

Description

The invention relates to a grinding device for rounding particles. The grinding device comprises a vortex chamber for treating the particles suspended in a fluid flow and is furthermore designed in accordance with the preamble of claim 1. The invention also relates to a corresponding method and optionally the associated use.

TECHNICAL BACKGROUND

Particles whose mean grain size is within a certain range and which have as few sharp edges as possible are required for a wide variety of technical applications.

A prominent application is the production of so-called permanent magnets.

A permanent magnet or permanent magnet consists of a magnetizable material, for example iron, cobalt or nickel. In many cases rare earth metals are added, especially neodymium, samarium, praseodymium, dysprosium, terbium or gadolinium. Rare earth magnets are characterized by the fact that they have a high magnetic remanent flux density and thus a high magnetic energy density.

Such permanent magnets are made from crystalline powder. The magnetic powder is pressed into a mold in the presence of a strong magnetic field. Under the influence of the magnetic field, the crystals align themselves with their preferred axis of magnetization in the direction along the magnetic field.

The compacts are then sintered. During sintering, the pulverized constituents of the powder are connected to one another or compressed by heating, but none or at least not all of the starting materials are melted. The compacts are heated - often under increased pressure - in such a way that the temperatures remain below the melting temperature of the main components, so that the shape of the workpiece is retained.

It is known for the production of a starting material, such as is required for the production of permanent magnets and in particular for the production of Nd-Fe-B (neodymium-iron-boron) magnets, to grind alloys comprising rare earth metals to an intermediate product. The regrind can be wholly or partly also a recyclate from old magnets.

Conventional comminution techniques are usually suitable for the production of intermediate products in powder form. The problem, however, is that the fine grinding of such rare earth magnet powders using conventional methods, for example in fluidized bed jet mills or similar grinding systems, produces powder particles that have sharp corners and edges. These sharp corners and edges are extremely undesirable for a variety of reasons.

For example, magnets made using such a sharp-edged powder show poorer magnetic values or lower magnetic energy densities than would be expected from calculations that assume largely round powder particles, i.e. particles that have essentially no sharp corners and edges.

THE TASK ON WHICH THE INVENTION IS BASED

The invention is based on the object of providing a device for producing smooth particles of a certain size, ie for producing particles with an at least reduced number of corners and edges.

THE SOLUTION ACCORDING TO THE INVENTION

In terms of device, the solution according to the invention is achieved by a grinding device for rounding particles in accordance with the first main claim.

The grinding device according to the invention has at least one vortex chamber for treating particles which are suspended in a fluid stream. “Being suspended” is understood to mean that the particles are at least predominantly carried by the fluid flow for the period of their processing.

Said fluid flow, which moves in the vortex chamber, is generated by fluid jets which are blown into the vortex chamber at different points by means of a blowing device. A fluid jet is understood as a locally flowing, bundled fluid. In this sense, a fluid jet is bundled not only, but at least when it has an absolute or mean diameter of less than or equal to 10 mm directly at its outlet opening.

The diameter of the fluid jet depends on the nozzle diameter and the nozzle shape (cylindrical or conical), but can also be significantly larger in individual cases.

The blowing device has a number of first blowing openings for blowing fluid jets into the vortex chamber, coming from the outside. These blow openings drive a fluid flow that tends to circle in a first main flow direction around the central axis of the vortex chamber.

According to the invention, the blowing device additionally comprises a number of second blowing openings. These are also used to inject fluid jets into the vortex chamber, coming from the outside. These second blow openings drive a fluid flow that tends to rotate in a second main flow direction around the central axis of the vortex chamber, which flow is opposite to the first main flow direction.

To clarify, it should be said that the said blowing in of the fluid jets preferably creates a turbulent, circling flow in the vortex chamber, not only, but especially if a rapidly rotating classifier wheel is also involved in the development of the flow in the vortex chamber. The term "main flow direction" therefore (roughly) denotes the resulting direction in which the local fluid flow or the vertically oriented layer that makes it up moves, generally with finite thickness, without prejudice to any superimposed local turbulence.

In terms of method, the solution according to the invention is achieved with the features of the corresponding, independent method claim.

OPTIONAL POSSIBILITIES FOR THE FURTHER DEVELOPMENT OF THE INVENTION

The blowing device is preferably designed as a guide ring, preferably separately exchangeable, with built-in blowing openings. This guide ring then forms a guide wall of the vortex chamber, d. H. a flow guide device. This influences the movement of the fluid flow in the circumferential direction. As a rule, this takes place in that the guide ring forces the fluid flow to rotate in the vortex chamber.

If the guide ring can be replaced separately, possible, mostly abrasive wear on the guide ring can easily be compensated for by installing a new guide ring.

The separate interchangeability can be particularly interesting where a multifunctional device is to be implemented: as long as a guide ring with one of the unidirectional blow opening arrangements known from the prior art is installed, the device can be operated as a jet mill with aggressive blowing. As soon as a guide ring according to the invention with groups of blow openings oriented in opposite directions is installed, the device can be operated as a grinding device while blowing in at a lower speed. The particles are then - at least essentially - not smashed any further. Instead, only their edges are broken or rubbed off.

Ideally, the central longitudinal axes of the first blower openings running in the inflow direction lie in a first common plane which is arranged normal to the imaginary axis around which the fluid flow circles. The central longitudinal axes of the second blower openings running in the inflow direction then lie in a second common plane which is offset from said first common plane and which is normal to the imaginary axis around which the fluid flow circles. The offset is preferably 3 mm to 40 mm, more preferably 5 mm to 25 mm.

Ideally, the fluid jets graze one another essentially tangentially from two directly adjacent blow openings blowing in in opposite directions. The blow openings are positioned accordingly and / or equipped with jet-shaping nozzles.

In this way, a zone of high shear is created between two adjacent fluid jets, the Particles tend to rotate on their own. Such an intrinsic rotation is particularly advantageous for the grinding of particles coming into contact with one another.

The blow openings are not necessarily but ideally each formed by steel-forming nozzles. In this way, the injected fluid jets can be shaped in such a way that they only fan out later than without nozzle shaping. As a result, they retain the ability to drive the desired circular flow in the vortex chamber for longer, at least within the layer that they are responsible for influencing.

In some special cases, the jet-shaping nozzles can have a nozzle body which is raised with respect to the inner surface of the guide ring. In this way, the nozzle body protrudes into the circular flow of the suspension - usually more than just insignificantly, namely generally by at least 5 mm in a radially inward direction.

This creates a particularly characteristic turbulence. The additional turbulence created in this way is too mild to cause the particles to be broken up again. But it is energetic enough to keep the To intensify particles, which lead to the desired abrasion against each other, again significantly.

The nozzles preferably have a nozzle body which is cylindrical or preferably conical in the outlet area. As just described, appropriately designed nozzles release a conically widening fluid jet into the interior of the vortex chamber and thus produce a particularly effective grinding effect.

Ideally, the nozzle bodies are installed in bores in the guide ring.

Protection is also claimed for the use adapted to the invention (special operating mode and equipment) of a jet mill known per se as a grinding device for particles.

Finally, the contemplated claim of protection for a guide ring or "nozzle ring" equipped with an arrangement of blow openings or nozzles according to the invention as a replacement part or conversion part is reserved.

Further functions, advantages and further development opportunities result from the The following description of an embodiment based on the figures.

FIGURE LIST

• The Figure 1 shows the entire grinding device in a section perpendicular to the central longitudinal axis L.
• The Figure 2 shows the grinding device according to Figure 1 in a section parallel to the central longitudinal axis L.
• The Figure 3 shows an excerpt from the Figure 1 , but seen from behind.
• The Figure 4 shows an enlarged section Figure 3 .
• The Figure 4a shows another excerpt from that of Figure 3 area shown, but with the selection of a slightly different cutting plane.
• The Figure 4b shows a detail section that shows how directly adjacent nozzles, as shown by Figure 4 are represented are relative to each other.
• The Figure 4c shows schematically the swirl chamber of the grinding device and its additional outlet.
• The Figure 5 shows the same as that Figure 4 , perspective diagonally from behind, cut.
• The Figure 6 shows the same as that Figure 4 , seen in perspective from behind, uncut.
• The Figure 7 shows an RE M image of particles before grinding.
• The Figure 8 shows an RE M image of particles after grinding according to the invention.
• They show the same thing Figures 9 and 10 with regard to the satellite-like loading of abrasion particles.

PREFERRED EMBODIMENT

The best overall overview of the exemplary embodiment according to the invention is given by Figures 1 and 2 .

These two figures each show a meaningful axial section or longitudinal section through a grinding device 1 according to the invention.

The grinding device 1 comprises a housing part or a housing 2. The housing 2 forms a cavity in its interior. The cavity accommodates a guide ring 3, which is also called a "nozzle ring" on occasion, at least when it is optionally equipped with nozzles 12. The cavity is evidenced by the Fig. 2 preferably closed by an end cover 2a. This end cover 2a is ideally designed in such a way that it grants quick and easy access to the cavity when required.

The guide ring 3 divides the cavity into a vortex chamber 4 and an annular channel 5 surrounding the vortex chamber 4.

The annular channel 5 is externally fed with fluid via the fluid inlet 5a from a pump or pressure source (not shown here). The annular channel 5 is used to continuously supply fluid during the grinding process.

This fluid is supplied to the swirl chamber 4 at different points in the form of fluid jets. For the sake of better visibility, the cut surface on the guide ring 3 is shown in FIG Figure 1 and the other figures are not shown hatched, but exceptionally drawn with fine dots. At Figure 4 is the cut surface in the area of the local outbreak, which is actually behind the plane of the drawing makes the lying area visible, instead of hatching with coarser dots.

The swirl chamber 4 is provided with a particle inlet 6. The latter is preferably designed as a vertical chute. Such an optional design allows the vortex chamber 4 to be loaded with the particles to be ground in batch operation with the aid of gravity. The loading then preferably takes place in such a way that at the beginning of a new grinding cycle approximately 25% to 50%, better approximately 30% to 40% of the volume of the vortex chamber 4 is filled with the particles to be ground.

In the case of batch operation, as is optionally practiced by this exemplary embodiment, the completely rounded particles can optionally be discharged via the classifier wheel, preferably by lowering the classifier wheel speed, which is accompanied by a reduction or removal of the locking effect of the classifier wheel. In this case, the grinding device 1 has a speed regulation or speed control for the classifier wheel, which is designed such that it reduces the speed during the discharge process or the unloading process.

Alternatively, the grinding device can have one or more additional product outlets 5b, via the or the the completely rounded particles can be drawn off, as a rule regardless of or without reducing the sifter speed. The Figure 4c shows schematically how such an additional product outlet is designed and arranged.

The additional product outlet 5b is expediently closed with a flap or a valve 5c. The product can then be suctioned off. The ring channel 5 is passed through a pipe coming from the vortex chamber and penetrating it, which leads all the way to the outside so that the vortex chamber 4 and the ring channel 5 do not come into direct contact with one another due to the additional product outlet.

In terms of process technology, there is also the possibility of working with an at least temporary overpressure in the grinding device 1.

As a result, the product is then pressed out of the completely rounded particles after opening the flap or valve from the vortex chamber.

The grinding device 1 according to the invention can instead also be equipped for continuous operation. In this case the product leaves the machine continuously through the classifier wheel, which is operated at a suitable speed.

This approach can be used, for example, in the so-called "disconnecting satellites", for example in the context of additive manufacturing. Satellites are understood to be small particles adhering to the actual useful particles or abrasion that has to be separated. Here it is sufficient to only briefly claim the product, for example (mostly abrasive in the sense of the invention). As soon as the satellites are separated, the particle size of the respective useful particle changes. The product from the useful particles can then pass through the classifier at a constant classifier speed.

The secondary outlet via the classifier wheel 7 can be seen in the exemplary embodiment shown here.

The classifier wheel is designed in a known manner. It consists of a rotating drum. This in turn typically consists of two flank-forming tires which are connected to one another via bars or blades which are spaced apart from one another and are generally parallel to the axis of rotation.

The very fine abrasion, which inevitably occurs during grinding and, as a rule, disruptive in the end product to be ground, is separated and removed via this classifier wheel 7.

At the same time, a fluid flow corresponding - at least essentially - to the currently newly blown-in fluid flow is withdrawn as a classifier fluid flow via the interior of the classifier wheel 7 and from there via its end face facing away from the vortex chamber. The sifter fluid flow flowing off in this way entrains the abrasion, which is too small and of little mass to be kept away from the sifter wheel located in the center by the centrifugal forces. The actual particles, which are supposed to be ground for a predetermined period of time, are too large. They circle so strongly in the vortex chamber that the centrifugal forces acting on them keep them away from the sifter wheel in the center. Otherwise they are rejected by the classifier wheel - back into the vortex chamber. This means that the actual particles cannot be removed from the vortex chamber via the classifier wheel. The operating speed of the classifier wheel is set accordingly, depending on the size of the particles to be processed or the desired classification quality.

The processing, ie the grinding, of the particles for the purpose of rounding their outer contour or outer surface takes place in that fluid jets are blown into the swirl chamber 4 via the blow openings 9 and 10. These entrain the particles in the vortex chamber 4 and float in the Vortex chamber 4 a fluid flow circulating mainly around the longitudinal axis L.

The grinding device typically includes a controller. This limits the jet speed of the fluid jets blown in via the blow openings.

The limitation is such that the particles are not brought to collide with one another and / or against the housing 2 so violently that they break apart into pieces of comparable size and are thus smashed again and again, ie ground. The jet speed is preferably limited to values in the range between 150 m / sec and 300 m / sec, depending on the current material to be ground.

An essential part of the grinding device according to the invention is its specially designed blowing device.

How to get the most accurate information on the Figures 3 and 4 and 4a to c (but can also be partially understood from the other figures), the blower device comprises a number of first blower openings 9. The fluid flow emerging from each first blower opening 9 into the swirl chamber 4 and its jet expansion are shown schematically by a Fluid flow cone 8 illustrated. These fluid flows drive a fluid flow that is circling in a first main flow direction around the central axis of the vortex chamber. In addition, the blowing device has a number of second blowing openings 10. Such a blowing opening 10 is illustrated by the outbreak 11, which is only shown in FIG Fig. 4 is shown. The outbreak shows a section of that plane, actually behind the plane of the drawing, in which the center lines of the oppositely aligned blower openings 10 lie or the nozzles 12 that form them and act in opposite directions. The fluid flow emerging from every second blower opening 10 into the vortex chamber 4 and its jet expansion are also illustrated by a fluid flow cone 8 - albeit clearly emerging in the opposite direction. These fluid flows drive a fluid flow that circles around the central axis of the vortex chamber in a second main flow direction and is opposite to the first main flow direction. This can also be clearly understood with the help of the Figure 4a and 4b .

In this way, two fluid flows that tend to circle in opposite main flow directions are generated. A flow area with pronounced shear arises between these two fluid flows, cf. also again Figure 4b . The particles entrained in this area are affected by the shear apparently a pronounced rolling is imposed, which superimposes their turbulent-translational movement. At the same time, this tangential flow past one another leads to micro-eddies and corresponding turbulence, which increases the contact intensity and mixing.

Although in the present case the grinding machine according to the invention works with a kinetic energy that is significantly below the kinetic energy used in a jet mill, an unexpectedly strong grinding effect occurs when the particles rub against one another in the aforementioned manner .

However, the particles do not shatter.

If a break occurs, mostly only fragments are broken off, the mean diameter of which in many cases is at least a power of ten smaller than the mean diameter of the remaining particle. As soon as a particle no longer has any sharp edges, there is essentially no further breakoff.

The conditions are favorable when the first blower openings 9 all blow in in the same direction and the second blow openings 10 all blow in in the same direction and in opposite directions.

Ideally, the relevant first blower openings 9 are arranged at an angle A of approx. 15 ° to 45 ° and ideally from 25 ° to 35 ° relative to the tangent which is attached to the inner surface of the guide ring in the area of the opening of the relevant blower opening 9 in guide ring 3 is applied, cf. Fig. 4 .

The same applies analogously to the second blower openings 10. These are arranged at an opposite angle B of approx. 15 ° to 45 ° and ideally from 25 ° to 35 ° relative to the tangent that adjoins the inner surface of the guide ring 3 in the region of the mouth of the relevant blow opening 10 is created in the guide ring 3, cf. Likewise Fig. 4 .

Another preferably applicable dimensioning rule results from Figure 4a , which is essentially the Fig. 4 corresponds. Blow openings 9, 10 that are directly adjacent by the shortest path form, with the perpendiculars on their central longitudinal axes LL, a blow opening pair angle C between 45 ° and 90 °, better between 45 ° and 60 °.

During initial work with the solution according to the invention, it has been found that the number of pairs of blow openings 9 and 10 running in opposite directions should ideally be 4 to 12 and better just 4 to 8.

The whole thing is particularly effective when the blow openings 9 in the same direction and the blow openings 10 in the opposite direction with their center lines LL (the latter seen in the respective flow direction) are not in the same plane, but in different planes which are offset from one another by an offset V mentioned - viewed in the direction along the imaginary axis L around which the fluid flow in the swirl chamber 4 circles. The corresponding is for example in Figure 4b shown very clearly.

Easy to recognize from the figures and especially the Fig. 4 is that the blow openings 9 and 10 are preferably formed by nozzles 12, the nozzle body of which is a separate component that is fixed, preferably screwed, in the guide ring 3. For this purpose, the guide ring 3 carries a number of holes penetrating it obliquely in the radially inward direction. A nozzle body is fixed or screwed into each of these bores.

It is also worth mentioning that the grinding device according to the invention does not have to be a stand-alone device. Rather, it is possible to equip one of the known jet mills, for example of the Conjet type from the applicant, in such a way that it can be used for a purpose other than the grinding machine according to the invention.

For this purpose, a guide ring 3 equipped with the appropriate spraying is inserted into the existing machine and then it is ensured that the existing machine only works with a significantly reduced jet speed, as is absolutely necessary for the invention.

The high effectiveness of the grinding device according to the invention show Figures 7 and 8 .

The Figure 7 shows particles from a permanent magnetic material (rare earths) ground by means of a conventional jet mill in 5,000-fold SEM magnification. The inadequate rounding of the particles, which extends to sharp edges, is clearly visible.

In contrast, the Figure 8 the same material after grinding, preferably for several minutes, in a grinding device according to the invention. The rounding quality is significantly better.

The Figure 9 shows particles which have been ground round using another method not according to the invention. It is precisely the large particle in the center of the image that shows how heavily it is contaminated like a satellite with fine, dust-like abrasion particles that interfere with many applications. The Figure 10 shows particles that are on the grinding device according to the invention were ground round. Their contamination with satellite-like abrasion particles is practically zero.

REFERENCE LIST

1

Grinding device

2

casing

2a

cover

3

Guide ring or nozzle ring, if equipped with nozzles

4th

Vortex chamber

5

Ring channel

5a

Fluid inlet

5b

Fluid outlet or additional fluid outlet

5c

Valve

6

Particle inlet

7th

Classifier wheel

8th

schematically indicated fluid flow cone of the injected fluid with "jet expansion"

9

first blow hole

10

second blow hole

11

Breakout for a more detailed illustration of an oppositely oriented nozzle 12 with a second blow opening 10

12

jet

L.

Central longitudinal axis or longitudinal axis or axis

A.

angle

B.

angle

C.

Blow opening pair angle

V

Offset

LL

Center line of a blow hole

Claims (11)

Grinding device (1) for rounding particles,
with a vortex chamber (4) for the treatment of particles under their suspension in a fluid flow which is generated by fluid jets which are blown into the vortex chamber (4) at different points by means of a blowing device,
wherein the blowing device comprises a number of first blowing openings (9) which drive a fluid flow circulating in a first main flow direction about the central axis (L) of the swirl chamber (4),
characterized in that
the blower device comprises a number of second blower openings (10) which drive a fluid flow which is circular in a second main flow direction about the central axis (L) of the swirl chamber (4) and which is opposite to the first main flow direction. Grinding device (1) according to claim 1, characterized in that the blowing device as, preferably separately replaceable, guide ring (3) with built-in blow openings (9, 10) is formed, which forms a guide wall of the vortex chamber (4) which influences the movement of the fluid flow in the circumferential direction. Grinding device (1) according to one of the preceding claims, characterized in that the first blow openings (9) lie in a first common plane which is arranged normal to the imaginary axis (L) around which the fluid flow orbits, and the second blow openings ( 10) lie in a second common plane which is arranged with an offset (V) relative to the first common plane and which is arranged normal to the imaginary axis (L) around which the fluid flow circles. Grinding device (1) according to claim 3, characterized in that the fluid jets of two directly adjacent blow openings (9, 10) blowing in in opposite directions brush essentially tangentially. Grinding device (1) according to one of the preceding claims, characterized in that the blow openings (9, 10) are each formed by steel-forming nozzles (12). Grinding device (1) according to Claim 5, characterized in that the jet-shaping nozzles (12) have a nozzle body which is raised and protrudes into the circular flow of the suspension. Grinding device (1) according to claim 6, characterized in that the nozzles (12) have a nozzle body which is cylindrical or preferably conical in the outlet area. Grinding device (1) according to one of the preceding claims in conjunction with claim 2, characterized in that the nozzles or nozzle bodies are installed in bores in the guide ring (3). Grinding device (1) according to one of the preceding claims, characterized in that a classifier wheel (7) is arranged in the center of the swirl chamber (4), via which the abrasion occurring in the course of the grinding of the particles in the swirl chamber (4) can be removed or the rounded material can be discharged by reducing the speed. Method for grinding particles suspended in a fluid stream in a swirl chamber (4), at different points by means of a blower device Fluid jets are blown into the vortex chamber (4), characterized in that the fluid jets are injected in groups in such a way that they drive a fluid flow circling in a first main flow direction around the central axis (L) of the vortex chamber (4) and that they drive a fluid flow into a second Driving main flow direction around the central axis (L) of the vortex chamber (4) circulating fluid flow which is opposite to the first main flow direction. Use of a jet mill for grinding particles without further crushing them, using a jet speed that is reduced compared to the grinding operation and a blowing device with the features specified in this respect by one of claims 1 to 9.

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