Jet Mill

Abstract

The invention relates to a jet mill comprising a milling chamber with a longitudinal axis, an inlet at one end of the axis and an outlet at the opposite end of the axis, the milling chamber comprising a multitude of pins arranged in the free flow cross-section of the milling chamber, wherein the pins are arranged in at least two planes perpendicular to the longitudinal axis, the planes being distant to each other in the longitudinal direction, and the pins of one plane being laterally offset to the pins of the subsequent plane, wherein the milling chamber is divided into alternate pin segments and acceleration segments, the pin segments each having at least two planes of pins, and the acceleration segments having no pins.

Claims

1. A jet mill comprising a milling chamber with a longitudinal axis, an inlet at one end of the axis and an outlet at the opposite end of the axis, the milling chamber comprising a multitude of pins arranged in the free flow cross-section of the milling chamber, wherein the pins are arranged in at least two planes perpendicular to the longitudinal axis, the planes being distant to each other in the longitudinal direction, and the pins of one plane being laterally offset to the pins of the subsequent plane, wherein the milling chamber is divided into alternate pin segments and acceleration segments, the pin segments each having at least two planes of pins, and the acceleration segments having no pins.

2. The jet mill according to claim 1, wherein the surface of the pins facing the inlet is convex.

3. The jet mill according to claim 1, wherein the pins are removably attached inside the milling chamber.

4. The jet mill according to claim 1, wherein at least some of the pins comprise a sensor capable of detecting a measure for abrasion of the respective pin.

5. The jet mill according to claim 1, wherein the pin segments each have two to five planes of pins.

6. The jet mill according to claim 1, wherein the length of the acceleration segments is larger than the longitudinal distance between planes in the pin segments.

7. The jet mill according to claim 1, wherein an acceleration chamber with an inlet and an outlet is connected to the milling chamber, the outlet of the acceleration chamber being the inlet of the milling chamber.

8. The jet mill according to claim 7, wherein the inlet of the acceleration chamber has a smaller cross-sectional area than its outlet.

9. The jet mill according to claim 1, wherein the height of the milling chamber is from 3 mm to 10 mm.

10. The jet mill according to claim 1, wherein the pins are made from a material selected from the group of wear resistant steel or wear resistant ceramics.

11. The jet mill according to claim 1, wherein the outlet of the milling chamber is coupled to the inlet of a classifier capable of separating fine particles from coarse particles.

12. The jet mill according to claim 11, wherein the outlet for the coarse fraction is recycled to the inlet of the milling chamber or to the inlet of the acceleration chamber.

13. A process for milling solid particles comprising the steps of (a) injecting the particles in a jet, and (b) feeding the jet including the injected particles into a jet mill according to claim 1.

14. The process according to claim 13, wherein the outlet of the milling chamber is coupled to the inlet of a classifier capable of separating fine particles from coarse particles, and separating at least one fine particle fraction from the classifier feed material.

Description

[0066] The invention is explained in more detail below with reference to the drawings. The drawings are to be interpreted as in-principle presentation. They do not constitute any restriction of the invention, for example with regards to specific dimensions or design variants. In the figures:

[0067] FIG. 1 shows a longitudinal top view of a first embodiment of a jet mill according to the invention.

[0068] FIG. 2 shows a longitudinal top view of a second embodiment of a jet mill according to the invention.

[0069] FIG. 3 shows a top view of a segment of a milling chamber with four planes of pins.

[0070] FIG. 4 shows a top view of a segment of a milling chamber with three planes of pins.

[0071] FIG. 5 shows a schematic view of a first embodiment of a jet mill coupled to a Coanda classifier.

[0072] FIG. 6 shows a schematic view of a second embodiment of a jet mill coupled to a Coanda classifier.

[0073] FIG. 7 shows a longitudinal top view of a jet mill according to a comparative example.

[0074] FIG. 8 shows a longitudinal top view of a second embodiment of a jet mill according to the invention.

[0075] FIG. 9 shows a comparison of particle size distributions obtained by milling particles in jet mills according to FIGS. 7 and 8.

LIST OF REFERENCE NUMERALS USED

[0076] 1 . . . milling chamber [0077] 2 . . . longitudinal axis [0078] 3 . . . inlet of the milling chamber [0079] 4 . . . outlet of the milling chamber [0080] 5 . . . pin [0081] 6 . . . acceleration chamber [0082] 7 . . . inlet of the acceleration chamber [0083] 11 . . . gas supply [0084] 12 . . . additional gas [0085] 13 . . . additional gas [0086] 14 . . . fine particles outlet [0087] 15 . . . medium particles outlet [0088] 16 . . . coarse particles outlet [0089] 17 . . . particle feed [0090] 18 . . . separation unit [0091] 19 . . . off gas [0092] A1, A2 . . . axial distance [0093] L1, L2 . . . lateral distance [0094] P1, P2, P3, P4 . . . planes of pins

[0095] FIG. 1 shows a longitudinal cut-away top view into a jet mill as a first embodiment according to the invention. The jet mill comprises a milling chamber 1 with a longitudinal axis 2, an inlet 3 at one end of the axis and an outlet 4 at the opposite end of the axis. Inside the milling chamber 1 there are fifteen pins 5 arranged in three pin segments with five pins 5 each. Between the pin segments there is one acceleration segment each with no pins. Counting from the inlet 3 to the outlet 4, the first pin segment comprises three planes of pins. In the first plane, two pins 5 are symmetrically arranged with respect to the longitudinal axis 2. The second plane comprises one pin 5 which is arranged in the center of the milling chamber on the longitudinal axis 2. The third plane comprises two pins 5 attached to the left wall and the right wall of the milling chamber 1 respectively. The second pin segment comprises two planes of pins 5. The arrangement of pins in the first plane of the second pin segment is identical to that of the first plane in the first pin segment. The second plane of the second pin segment comprises three pins 5. One pin is arranged in the center of the milling chamber on the longitudinal axis 2. The two other pins are attached to the left wall and the right wall of the milling chamber respectively. The arrangement of pins in the third pin segment is identical to that of the second pin segment. All planes are distant to each other in the longitudinal direction. The pins of one plane are laterally offset to the pins of the adjacent planes in that the center of the axis of a pin in one plane and the center of the axis of a pin in a subsequent plane lie on different lines parallel to the longitudinal axis of the mixing chamber.

[0096] An acceleration chamber 6 with an inlet 7 and an outlet is connected to the milling chamber 1, the outlet of the acceleration chamber being the inlet 3 of the milling chamber. The cross-sectional areas of the milling chamber 1 and the acceleration chamber 6 are rectangular with the cross-sectional area of the inlet 7 of the acceleration chamber being smaller than its outlet.

[0097] All pins 5 of this example have the same cylindrical form. Their cross-section is circular, thus the surface of the pins facing the inlet 3 is convex.

[0098] FIG. 2 shows a longitudinal cut-away top view into a jet mill as a second embodiment according to the invention. The jet mill comprises a milling chamber 1 with a longitudinal axis 2, an inlet 3 at one end of the axis and an outlet 4 at the opposite end of the axis. Inside the milling chamber 1 there are twenty-four pins 5 arranged in two pin segments with twelve pins 5 in four planes each. Between the pin segments there is an acceleration segment with no pins. Counting from the inlet 3 to the outlet 4, the first plane of the first pin segment comprises three pins. One pin is attached to the right wall of the milling chamber whereas the other two pins are arranged with an equal lateral distance between the pins. The lateral distance of the left-most pin to the left wall is identical to the lateral distance between the pins in that plane. The pins in the second plane of the first pin segment are arranged in a similar manner as the pins in the first plane but are laterally offset to the pins of the first plane. The left-most pin is attached to the left wall of the milling chamber whereas the other two pins are arranged with an equal lateral distance between the pins. The lateral distance of the right-most pin to the right wall is identical to the lateral distance between the pins in that plane. The pins of the third plane are arranged like the pins in the first plane, and the pins of the fourth plane are arranged like the pins of the second plane. All pins in the first pin segment have the same cylindrical form. Their cross-section is circular, thus the surface of the pins facing the inlet 3 is convex.

[0099] The pins of the second pin segment are larger in diameter than the pins of the first pin segment. Counting from the inlet 3 to the outlet 4, the first plane of the second pin segment comprises three pins. One pin is attached to the left wall of the milling chamber whereas the other two pins are arranged with an equal lateral distance between the pins. The cross-section of the left-most pin is half-circular whereas the cross-section of the other two pins is circular. The lateral distance of the right-most pin to the right wall is identical to the lateral extension of the left-most pin attached to the wall. The pins in the second plane of the second pin segment are arranged in a similar manner as the pins in the first plane but are laterally offset to the pins of the first plane. The right-most pin is attached to the right wall of the milling chamber whereas the other two pins are arranged with an equal lateral distance between the pins. The cross-section of the right-most pin is half-circular whereas the cross-section of the other two pins is circular. The lateral distance of the left-most pin to the left wall is identical to the lateral extension of the right-most pin attached to the wall. The pins of the third plane are arranged like the pins in the first plane, and the pins of the fourth plane are arranged like the pins of the second plane. The cross-section of all pins in the second pin segment is circular or half-circular, thus the surface of the pins facing the inlet 3 is convex.

[0100] All planes are distant to each other in the longitudinal direction. The pins of one plane are laterally offset to the pins of the adjacent planes.

[0101] An acceleration chamber 6 with an inlet 7 and an outlet is connected to the milling chamber 1, the outlet of the acceleration chamber being the inlet 3 of the milling chamber. The cross-sectional areas of the milling chamber 1 and the acceleration chamber 6 are rectangular with the cross-sectional area of the inlet 7 of the acceleration chamber being smaller than its outlet.

[0102] FIG. 3 shows a longitudinal cut-away top view of a pin segment of a milling chamber as a further exemplary embodiment according to the invention. The pin segment comprises four planes of pins. Counting from left to right in the direction of the jet, the first plane P1 and the third plane P3 comprise three pins each. One pin is attached to the right wall of the milling chamber, one pin is attached to the left wall of the milling chamber, and one pin is arranged in the lateral center of the milling chamber on the longitudinal axis (the axis not being shown in FIG. 3). The second plane P2 and the fourth plane P4 comprise two pins each which are arranged symmetrically to the longitudinal axis between this axis and the channel walls. All pins in this segment have the same cylindrical form with a circular cross-section.

[0103] In the lateral direction, i.e. perpendicularly to the longitudinal axis, the pins of a subsequent plane are arranged in the middle of the free passageway between two pins of the preceding plane. The ratio of the lateral distance L1 between two neighboring pins in a plane and the diameter of the respective pins is preferably from 0.8 to 1.5. In the example shown in FIG. 3 this ratio is 1.25.

[0104] The axial distance A1 between two neighboring planes is defined via the envelope of the pins in the respective planes. In the example shown in FIG. 3 the envelope of a plane is a line tangential to the outermost surface of the pins in that plane, as indicated by the dashed line in plane P3. It is preferred that the ratio of the axial distance A1 between two neighboring planes and the diameter of the pins in the respective planes is from 0.8 to 1.5. In the example shown in FIG. 3 this ratio is 1.15.

[0105] FIG. 4 shows a longitudinal cut-away top view of a pin segment of a milling chamber as a further exemplary embodiment according to the invention. The pin segment comprises three planes of pins. Counting from left to right in the direction of the jet, the first plane P1 comprises two pins that are arranged to the left and to the right of the longitudinal axis of the milling chamber (the axis not being shown in FIG. 4). The second plane P2 comprises one pin that is arranged in the center of the milling chamber on the longitudinal axis. The third plane P3 comprises two pins that are attached to the left wall and to the right wall of the milling chamber. All pins in this segment have the same cylindrical form with a circular cross-section.

[0106] The pins of the third plane P3 are laterally offset to the pins of the second plane P2 and to the pins of the first plane P1. In the lateral direction, the pin of plane P2 is arranged in the middle of the free passageway between the two pins of plane P1.

[0107] The ratio of the lateral distance L1 between the two pins in plane P1 and the diameter of the respective pins is preferably from 0.8 to 1.5. In the example shown in FIG. 4 this ratio is 0.95. The axial distance A1 between plane P1 and plane P2 is defined via the envelope of the pins in the respective planes. In the example shown in FIG. 4 the envelope of a plane is a line tangential to the outermost surface of the pins in that plane, as indicated by the dashed line in plane P1. It is preferred that the ratio of the axial distance A1 between the envelopes of the first plane P1 and the second plane P2 and the axial diameter of the pins in the respective planes is from 0.8 to 1.5. In the example shown in FIG. 4 this ratio is 1.2.

[0108] The pins in the third plane P3 are arranged such that the shortest distance between a pin in the second plane P2 and a neighboring pin in the third plane P3 has an axial component A2 and a lateral component L2. It is preferred that the ratio of the lateral component L2 of the pin-to-pin distance and the lateral diameter of the respective pin in the third plane P3 is from 0.8 to 1.5.In the example shown in FIG. 4 this ratio is 1.25. It is further preferred that the ratio of the axial component A2 of the pin-to-pin distance and the lateral component L2 of the pin-to-pin distance is from 0 to 2. In the example shown in FIG. 4 this ratio is 1.

[0109] FIG. 5 shows a schematic view of a first embodiment of a jet mill coupled to a Coanda classifier. Particles to be milled are fed into a gaseous medium and are injected into the inlet of the jet mill in a gas supply 11. The outlet of the jet mill is directly coupled to the inlet of the Coanda classifier. Coanda classifiers are known in the art (for example: Heinrich Schubert (Ed.): Handbuch der Mechanischen Verfahrenstechnik, chapter 7, page 608, Wiley-VCH Verlag GmbH & Co. KGaA, 2012).

[0110] In the example shown the Coanda classifier is able to separate the milled particles into three fractions, a fine particle fraction, a medium particle fraction and a coarse particle fraction. Additional gas streams 12 and 13 without particle load are available to influence the separation into the three fractions. The fine particle fraction is removed from the classifier via a fine particle outlet 14. The medium particle fraction is removed from the classifier via a medium particle outlet 15. The coarse particle fraction is removed from the classifier via a coarse particle outlet 16 and is recycled to the gas supply 11 and thus the inlet of the milling chamber of the jet mill. Fresh particles to be milled can be fed into the gas supply 11 directly and/or into the recycle stream from the coarse particle outlet 16 back to the gas supply 11. This option is shown in FIG. 5 in stream 17.

[0111] FIG. 6 shows a schematic view of a second embodiment of a jet mill coupled to a Coanda classifier. The difference between the first embodiment shown in FIG. 5 and the second embodiment shown in FIG. 6 is that the coarse particles removed from the classifier are fed to a further separation unit 18 before they are recycled to the inlet of the jet mill. The separation unit 18 may contain any suitable separation means, in particular a filter, a cyclone or a combination of a filter and a cyclone. The coarse solids fraction separated in the separation unit is preferably hermetically separated from the gas stream, e.g. via a rotary cell valve or similar devices such as screw conveyors. By these means a decoupling between the underpressure of the injector of the gas supply 11 and the underpressure in the Coanda classifier can be realized. This is beneficial to individually adjust the feed rate of the mill and the separating conditions in the Coanda classifier. A cleaned off gas 19 is withdrawn from the separation unit 18.

EXAMPLES

[0112] A jet mill according to the invention was compared with an opposed jet mill and a spiral jet mill according to the prior art. A limestone powder (Juraperle 150-300 by company Omya Gmbh, Kln, Germany) was milled in each of the three mills. The grain size parameters of the powder were as follows:

TABLE-US-00001 D10 [m] 4.6 D50 [m] 119.0 D90 [m] 225.0

[0113] In each case the milling pressure for the respective mill was set as high as possible as this leads to the highest product fineness. All mills were then loaded until the particle size distribution of the milled product became significantly coarser or the mill reached a critical operating state.

Comparative Example 1

[0114] As comparative example 1 an opposed jet mill (type AFG 100 by company Hosokawa Alpine AG, Augsburg, Germany) was used. The mill was operated with three nozzles with a diameter 10 of 1.9 mm each to supply the milling gas with a pressure of 7 bar. The material was fed into the milling chamber with a screw feeder at a feed rate of 4 kg/h. The diameter of the deflecting wheel classifier in the mill was 50 mm. The air classifier was operated at 12,500 rpm resulting in a circumferential speed of 33 m/s.

Comparative Example 2

[0115] As comparative example 2 a spiral jet mill with a diameter of the milling chamber of 170 mm and a milling chamber height of 15 mm was used. The spiral jet mill was equipped with ten cylindrical milling gas nozzles with a diameter of 1.5 mm each, equally distributed over the circumference of the milling chamber. The milling gas pressure was 3.6 bar. The diameter of the injector nozzle was 2.5 mm and the diameter of the booster nozzle was 8 mm. The injector nozzle pressure was 3.8 bar. The vortex finder of the mill had a circular shape with a diameter of 40 mm.

Example 1 According to the Invention

[0116] A jet mill similar to the embodiment shown in FIG. 1 was used as example according to the invention. The jet mill comprised a milling chamber with a longitudinal axis, an inlet at one end of the axis and an outlet at the opposite end of the axis. In the free flow cross-section inside the milling chamber there were fifteen pins arranged in three pin segments with five pins each. Each pin segment comprised two planes of pins, the planes being perpendicular to the longitudinal axis. Counting from the inlet to the outlet, in the first plane two pins were symmetrically arranged with respect to the longitudinal axis. The second plane comprised three pins. One pin was arranged in the center of the milling chamber on the longitudinal axis. The two other pins were attached to the left wall and the right wall of the milling chamber respectively. All planes were distant to each other. The distance in the longitudinal direction between the first planes and the respective second planes in each pin segment was 10 mm. The pins of one plane are laterally offset to the pins of the adjacent planes in that the center of the axis of a pin in one plane and the center of the axis of a pin in a subsequent plane lie on different lines parallel to the longitudinal axis of the mixing chamber. The pins were made of silicon carbide. All pins had the same cylindrical form. Their cross-section was circular with a diameter of 4 mm, thus the surface of the pins facing the inlet was convex. The distance in the longitudinal direction between the first planes and the respective second planes in each pin segment in terms of their envelopes was thus 6 mm. Between the pin segments there was one acceleration segment each with no pins. The length of both acceleration segments was 36 mm each.

[0117] An acceleration chamber with an inlet and an outlet was connected to the milling chamber, the outlet of the acceleration chamber being the inlet of the milling chamber. The length of the acceleration chamber was 50 mm. The length of the milling chamber was 165 mm. The cross-sectional areas of the milling chamber and the acceleration chamber were rectangular. The width of the milling chamber was 20 mm and its height 5 mm. The width of the inlet of the acceleration chamber was 9 mm.

[0118] The outlet of the milling chamber was coupled to the inlet of a classifier based on the Coanda effect. The overall setup is schematically shown in FIG. 6.

[0119] A screw conveyor was used to feed the feed material into the suction pipe of the injector. In the injector the solid feed material was dispersed in the milling gas jet. The dispersed material was accelerated in the acceleration chamber in a way that the particles reached a velocity similar to the milling gas velocity. Subsequently the particles hit the pins of the first pin segment and were crushed by the mechanical impact. Additionally, particle-particle contacts between particles reflected by the pin and particles dispersed in the milling gas led to high energy impacts and thus particle breakages. After the first pin segment the particles were re-accelerated until they hit the first pins of the second pin segment. After the third pin segment the particles were re-accelerated by the remaining milling gas pressure so that they could enter into the Coanda classifier at a speed similar to the milling gas. The main part of the milling gas entering into the Coanda classifier was forced into a bend motion along the bend shape of the Coanda inlet. Fine particles of a high specific surface followed the bend motion of the gas stream. Particles of a lower specific surface, e.g. medium sized or coarse particles, could only partially follow the motion of the bend gas and were less deviated from their initial straight motion. Thus, a parabolic distribution of fine to coarse particles could be realized inside the housing of the Coanda classifier. By adjusting the splitters in the paths of the particles of different size a split into a fine particle fraction and a coarse particle fraction was possible. To optimize the flight path of the particles in the Coanda classifier and thus to optimize the separation performance additional gas was sucked into the Coanda classifier. The split off fine particles were sucked into a filter to remove the solid phase from the gas phase. The coarse particles were sucked into a cyclone to remove the solid phase from the gas phase. The coarse particle fraction collected at the bottom of the cyclone was carried out of the cyclone by a screw conveyor and subsequently fed back into the suction pipe of the injector. Thus, the coarse particles were mixed with the fresh material and were re-fed to the mill.

[0120] Parameters and results of the milling experiments are shown in the following table:

TABLE-US-00002 Comp. Ex. 1 Comp. Ex. 2 Inv. Ex. Feed rate [kg/h] 4 2 10 Milling pressure [bar] 7 3.6 7.8 Volume flow [Nm.sup.3/h] 62 86.6 36 Specific load [g/m.sup.3] 65 23 278 Specific energy [kWh/kg] 1.12 1.86 0.28 D10 [m] 0.4 0.6 0.3 D50 [m] 3.0 3.7 3.0 D90 [m] 5.7 9.7 6.5

[0121] As can be seen from the above table the product fineness as a result of milling the powder in the jet mill according to the invention is very similar to the product fineness achieved with the opposed jet mill. The product of the milling process in the spiral jet mill is coarser.

[0122] Due to its design, the jet mill according to the invention can be operated at a significantly higher specific load of particles in the milling gas (jet). Due to the high solids loading of the milling gas and the comparatively low volumetric flow, the specific energy consumption of the milling process according to the invention is much lower than those of the processes according to the prior art. In the examples above the specific energy consumption is lower by a factor of 4 compared to the opposed jet mill and by a factor of 6.6 compared to the spiral jet mill.

Comparative Example 3

[0123] In a further set of experiments the influence of the intermediate acceleration segments was investigated. The material to be milled was the same limestone powder (Juraperle 150-300 by company Omya Gmbh, Kln, Germany) as in the previous examples. In each case the milling pressure for the respective mill was set to 8 bars (abs) and the feed rate of the limestone particles was set to 18 kg/h.

[0124] As comparative example 3 a jet mill according to the embodiment shown in FIG. 7 was used. FIG. 7 shows a longitudinal top view of the jet mill in an in-principle presentation. The jet mill comprised a milling chamber 1 with a longitudinal axis 2, an inlet 3 at one end of the axis 2 and an outlet 4 at the opposite end of the axis. In the free flow cross-section inside the milling chamber 1 there were twenty-four pins 5 arranged in sixteen planes of pins, the planes being perpendicular to the longitudinal axis 2. Counting from the inlet 3 to the outlet 4, in the first plane two pins were symmetrically arranged with respect to the longitudinal axis 2. The second plane comprised one pin which was arranged in the center of the milling chamber on the longitudinal axis 2. The pattern of the first and second plane was repeated seven times. Thus, the arrangement of pins in the third, fifth, seventh, ninth, eleventh, thirteenth and fifteenth plane was identical to the arrangement of pins in the first plane, and the arrangement of pins in the second, fourth, sixth, eighth, tenth, twelfth, fourteenth and sixteenth plane was identical to the arrangement of pins in the second plane.

[0125] All planes were distant to each other. The distance in the longitudinal direction between the planes was 10 mm. The pins of one plane were laterally offset to the pins of the adjacent planes in that the center of the axis of a pin in one plane and the center of the axis of a pin in a subsequent plane lay on different lines parallel to the longitudinal axis of the mixing chamber. The pins were made of silicon carbide. All pins had the same cylindrical form. Their cross-section was circular with a diameter of 4 mm.

[0126] An acceleration chamber 6 with an inlet 7 and an outlet was connected to the milling chamber 1, the outlet of the acceleration chamber 6 being the inlet 3 of the milling chamber 1. The length of the acceleration chamber was 50 mm. The length of the milling chamber was 165 mm. The cross-sectional areas of the milling chamber and the acceleration chamber were rectangular. The width of the milling chamber was 20 mm and its height 5 mm. The width of the inlet of the acceleration chamber was 9 mm.

[0127] A screw conveyor was used to feed the feed material into the suction pipe of the injector. In the injector the solid feed material was dispersed in the milling gas jet. The dispersed material was accelerated in the acceleration chamber 6 in a way that the particles reached a velocity similar to the milling gas velocity. Subsequently the particles hit the pins 5 of the first pin segment and were crushed by the mechanical impact. Additionally, particle-particle contacts between particles reflected by the pin and particles dispersed in the milling gas led to high energy impacts and thus particle breakages. The limestone particles were collected at the outlet 4 of the jet mill and their particle size was determined.

Example 2 According to the Invention

[0128] A jet mill according to the embodiment shown in FIG. 8 was used as a further example according to the invention. FIG. 8 shows a longitudinal top view of the jet mill in an in-principle presentation.

[0129] The jet mill comprised a milling chamber 1 with a longitudinal axis 2, an inlet 3 at one end of the axis 2 and an outlet 4 at the opposite end of the axis 2. In the free flow cross-section inside the milling chamber 1 there were twenty pins 5 arranged in four pin segments with five pins each. Each pin segment comprised two planes of pins, the planes being perpendicular to the longitudinal axis 2. Counting from the inlet 3 to the outlet 4, in the first plane two pins 5 were symmetrically arranged with respect to the longitudinal axis. The second plane comprised three pins 5. One pin was arranged in the center of the milling chamber on the longitudinal axis. The two other pins were attached to the left wall and the right wall of the milling chamber respectively. The arrangement of pins in the second, third and fourth pin segment was identical to the arrangement of pins in the first pin segment.

[0130] All planes were distant to each other. The distance in the longitudinal direction between the first planes and the respective second planes in each pin segment was 10 mm. The pins of one plane were laterally offset to the pins of the adjacent planes in that the center of the axis of a pin in one plane and the center of the axis of a pin in a subsequent plane lay on different lines parallel to the longitudinal axis of the mixing chamber. The pins were made of silicon carbide. All pins had the same cylindrical form. Their cross-section was circular with a diameter of 4 mm, thus the surface of the pins facing the inlet was convex. The distance in the longitudinal direction between the first planes and the respective second planes in each pin segment in terms of their envelopes was thus 6 mm. Between the pin segments there was one acceleration segment each with no pins. The length of the three acceleration segments was 36 mm each.

[0131] An acceleration chamber 6 with an inlet 7 and an outlet was connected to the milling chamber 1, the outlet of the acceleration chamber 6 being the inlet 3 of the milling chamber 1. The length of the acceleration chamber was 50 mm. The length of the milling chamber was 165 mm. The cross-sectional areas of the milling chamber and the acceleration chamber were rectangular. The width of the milling chamber was 20 mm and its height 5 mm. The width of the inlet of the acceleration chamber was 9 mm.

[0132] A screw conveyor was used to feed the feed material into the suction pipe of the injector. In the injector the solid feed material was dispersed in the milling gas jet. The dispersed material was accelerated in the acceleration chamber 6 in a way that the particles reached a velocity similar to the milling gas velocity. Subsequently the particles hit the pins 5 of the first pin segment and were crushed by the mechanical impact. Additionally, particle-particle contacts between particles reflected by the pin and particles dispersed in the milling gas led to high energy impacts and thus particle breakages. After the first pin segment the particles were re-accelerated until they hit the first pins 5 of the second pin segment. After the second pin segment the particles were re-accelerated until they hit the first pins 5 of the third pin segment. After the third pin segment the particles were re-accelerated until they hit the first pins 5 of the fourth pin segment. The limestone particles were collected at the outlet 4 of the jet mill and their particle size was determined.

[0133] FIG. 9 shows a comparison of the particle size distributions obtained by milling particles in jet mills according to comparative example 3 (FIG. 7) and example 2 according to the invention (FIG. 8). On the abscissa, the particle size is given in micrometers (m). The ordinate shows the mass fraction in percent. The dotted line represents the feed material which was characterized by about 80% of the particles being larger than 50 m and about 60% of the particles being larger than 100 m.

[0134] The dash-dotted line shows the particle size distribution of the sample of particles obtained at the outlet of the jet mill of comparative example 3. About 54% of the particles of this sample were smaller than 50 m and about 30% of the particles were still larger than 100 m.

[0135] The solid line shows the particle size distribution of the sample of particles obtained at the outlet of the jet mill of example 2 according to the invention. About 62% of the particles of this sample were smaller than 50 m and about 14% of the particles were larger than 100 m.

[0136] The milling process in a jet mill with intermediate acceleration segments according to the invention yields smaller particles with a more homogeneous particle size distribution, which can also be deducted from the graph in FIG. 9 where the slope of the solid line in the range of from 20 to 100 m is much steeper than the slope of the dash-dotted line in the same range.

[0137] Further experiments with a triboluminescent material showed that the breakage of particles in a mill according to FIG. 7 mainly occurred at the first two planes of the mill. In a mill according to the invention as presented in FIG. 8, a constantly intense glowing could be observed over all pins of the mill clearly indicating the more intense milling process due to the acceleration segments.