jet mill

Abstract

The invention relates to a jet mill comprising a grinding chamber (1) having a longitudinal axis (2), an inlet (3) at one end of the axis and an outlet (4) at the opposite end of the axis, the grinding chamber (1) comprising a plurality of pins (5) arranged in the free-flow cross section of the grinding chamber (3), wherein the pins (5) are arranged in at least two planes perpendicular to the longitudinal axis (2), the planes being longitudinally distanced from each other, and the pins (5) of one plane being laterally offset with respect to the pins (5) of a subsequent plane, wherein the grinding chamber (1) is divided into alternating pin sections and acceleration sections, each pin section having at least two pin planes (5) and the acceleration sections being free of pins. The invention further relates to a method of grinding solid particles comprising the steps of (a) injecting particles into a jet and (b) feeding the jet comprising the injected particles into the jet mill of the invention.

Description

[0001] The present invention relates to a jet mill comprising a grinding chamber having a longitudinal axis, an inlet at one end of the axis and an outlet at the opposite end of the axis. The present invention further relates to a method of grinding solid particles comprising the steps of injecting particles into a jet and feeding the jet comprising the injected particles into a jet mill according to the present invention.

[0002] Jet mills grind material by using a high velocity jet of compressed air, gas or steam to create impact between the particles. No mechanical tools such as high speed rotors are needed. The particles are pulverized by the energy introduced by the grinding gas. Compressed air is commonly used as grinding gas, more rarely inert gases such as argon or nitrogen can be used for grinding under inert conditions. Superheated steam can also be used and is used for special applications. The principle of jet milling is commonly used where a fine pulverization is needed. Jet mills grind dry materials to a fineness with a D90 value of 0.1 - 200 microns. Typically the working range is below 20 microns D90 value. The most common types of jet mills are opposed jet mills and spiral jet mills.

[0003] Opposed jet mills, also known as fluidized bed opposed jet mills, comprise a grinding bed, a transport zone and an air classifier zone. The particles to be ground are fed into the grinding chamber and form a fluidized grinding bed at the bottom of the jet mill. Gas jets are introduced into the grinding bed through nozzles arranged on the mill housing. The gas jets fluidize the grinding bed by accelerating the particles located at the bottom of the mill to high velocities. Along the gas jets and in the center of the grinding bed, the accelerated particles collide with each other and are thereby ground to smaller particles. The gas loaded with particles rises upwards through the transport zone to the center of the grinding chamber and transports the particles upwards to an air classifier installed at the top of the mill. Typically, the classifier comprises a classifier wheel driven by a variable speed motor. The air classifier separates the fine particles from the coarse particles. Too coarse particles are cut off by the classifier and fall back into the fluidized bed. The fine particles leave the mill together with the grinding gas and are separated from the grinding gas in a suitable separator or dust filter.

[0004] In recent years, many different devices and methods of operation have been developed to improve the performance of opposed jet mills. As examples, the documents US 2009 / 0236451 Al, US 2009 / 0261187 Al and US 2014 / 0021275 Al disclose methods and jet mill devices for producing fine particles by jet mills with integrated dynamic air classifiers.

[0005] Despite the fact that opposed jet mills are a mature technology for grinding particles, the technology also has some drawbacks. The performance of a jet mill in terms of fineness strongly depends on the velocity of the gas jet. The maximum jet velocity of air as grinding gas is about 330 m / s, which is the speed of sound of the gas. With a Laval nozzle, even supersonic speeds can be achieved at the outlet of the nozzle. However, recent studies have shown that the actual jet velocity within the mill is much lower. By means of particle image velocimetry measurements, it could be revealed that the maximum particle velocity is only about 40 m / s (Koeninger et al., Powder Technology 316 (2017) 49-58). Furthermore, the transfer of kinetic energy between particles in an opposed jet mill was measured and it was revealed that the energy transfer provided by each stress event is rather low (Koeninger et al., Powder Technology 327 (2018) 346-357). Since the kinetic energy of a particle increases with the square of the velocity, a particle of 300 m / s has about 56 times the energy of a particle of 40 m / s.

[0006] Another drawback is that the jets are only partially loaded with particles. The solids loading within the jet is very low, since most of the particles are accelerated at the outer circumference of the jet and cannot enter the interior of the jet. Thus, a large part of the high kinetic energy is wasted. Another drawback of opposed jet mills is that the jets spread sideways when they collide in the middle of the jet. Fine particles, which are easier to accelerate, can also more easily follow the spreading jets and thus avoid head-on collisions with each other. Larger particles, which have a higher moment of inertia, can collide with each other, but only at low velocities.

[0007] Spiral jet mills are widely used in industry, with mill sizes ranging from small laboratory units for only a few grams of product sample to production machines producing several tons per hour. Spiral jet mills can also be divided into grinding, conveying and classification zones. In contrast to opposed jet mills, the air classification is achieved by a static vortex instead of a rotating wheel classifier. Another difference is that the jets do not face each other, but enter the mill tangentially. Several nozzles are arranged tangentially on the circumference of a typically circular and flat grinding chamber. The grinding gas flow that enters through the nozzles forms a spiral. The particles to be ground are fed into the grinding chamber via an injector. The working principle of this mill is that the particles to be ground perform a circular movement in the grinding chamber and thus have to collide with the incoming gas jets. The particles are ground by mutual particle collisions.

[0008] Due to the rotational movement of the particles, a centrifugal force acts on the particles in the grinding chamber. The gas introduced by the jet can leave the mill housing through a circular opening in the middle of the mill. Small particles can follow the gas stream and can thus be carried out of the mill. For larger particles, the relationship between the centrifugal force and the radial drag force component is in favor of the centrifugal force, so the larger particles stay in the grinding chamber until they are small enough to leave the mill. The fineness of the resulting ground particles can be influenced by the jet pressure and the specific gas load. A higher gas load leads to a higher statistical probability of particle contact. On the other hand, too high a gas load inhibits the correct functioning of the classification.

[0009] For opposed jet mills, a number of different devices and operating methods have been developed in recent years to improve the performance of the spiral jet mill. Examples are disclosed in documents US2004 / 0169098A1 and US2011 / 0049278A1.

[0010] Despite the widespread use of spiral jet mills in industry for a variety of applications, the technology also has some disadvantages. For opposed jet mills, the performance of the spiral jet mill in terms of fineness strongly depends on the velocity of the gas jet. Under normal operating conditions with air or nitrogen as operating gas, the maximum jet velocity is limited by the sonic velocity of the gas, for example, about 330 m / s for air or nitrogen. Supersonic gas velocities can be generated using Laval nozzles, but only for a limited distance. However, for opposed jet mills, a significant amount of jet energy is lost due to dissipation into the surrounding gas and expansion of the jet after it has left the nozzle.

[0011] A further disadvantage of both mills is that the probability of two particles colliding with each other decreases with the size of the particles. Even for very high gas jets, the probability of two particles colliding head-on with each other is low. More likely, they will collide tangentially, thus only utilizing part of the kinetic energy for grinding.

[0012] A further problem is that the collision intensity spectrum of the jet mills known in the art is very broad due to the long residence time in the grinding zone and the random movement of the particles. This leads to an unnecessarily large number of particle collisions that absorb energy without leading to particle breakage.

[0013] A further problem is that the specific gas load of opposed jet mills and spiral jet mills is limited by the classification system. For grinding purposes, a high specific gas load would be advantageous to generate more collisions. For air classification systems, too high a gas load leads to imprecise separation, thus reducing product quality. It is therefore not possible to operate these mills in their optimum operating window.

[0014] The document US 4059231 discloses an alternative construction of a jet mill. The jet mill comprises an air delivery system for carrying entrained particles of different mass, a Venturi tube for accelerating the air flow and the entrained particles, a duct for receiving the accelerated air flow and particles, and an impact bar mounted in the duct for selectively pulverizing the entrained particles. The impact bar is located in the accelerated air flow to establish a counter pressure field grading the entrained particles and to provide an impact surface for fragment particles larger than a predetermined mass, the remaining particles being deflected around the bar under the influence of the counter pressure field.

[0015] A further problem is that the existing jet mills can only produce one ground product. The used grading system cannot grade into various fractions with precisely defined particle sizes.

[0016] It is an object of the present invention to provide a jet mill which features a more efficient energy input of the jet, a higher throughput, a narrower particle size distribution and the ability to produce finer particles.

[0017] According to the invention, this task is solved by a jet mill according to claim 1. Furthermore, this task is solved by a method of grinding solid particles according to claim 13. Advantageous variants of the jet mill and the method are set out in claims 2-12 and 14.

[0018] The first subject of the invention is a jet mill comprising a grinding chamber having a longitudinal axis, an inlet at one end of the axis and an outlet at the opposite end of the axis, wherein the grinding chamber comprises a plurality of pins arranged in the free flow cross section of the grinding chamber. The pins are arranged in at least two planes perpendicular to the longitudinal axis, said planes being longitudinally distanced from each other and the pins of one plane being laterally offset with respect to the pins of a subsequent plane. The grinding chamber is divided into alternating pin sections and acceleration sections, each pin section having at least two pin planes, while the acceleration sections are free of pins.

[0019] The term "laterally offset" means that the centers of the pin axes in one plane and the centers of the pin axes in a subsequent plane are located on different lines parallel to the longitudinal axis of the mixing chamber.

[0020] The second subject of the invention is a method of grinding solid particles comprising the steps of (a) injecting the particles into a jet, and (b) feeding the jet comprising the injected particles into the jet mill of the invention.

[0021] The jet mill of the invention shows several advantages compared to the jet mills known from the prior art:

[0022] The design of the grinding chamber as a channel having a longitudinal axis avoids dead zones which lead to dissipation of the energy introduced by the jet. Furthermore, this design avoids free expansion of the jet fed into the mill, which is common in known designs. Thus, the jet mill of the invention facilitates the use of a significantly larger portion of the kinetic energy of the jet fed into the mill.

[0023] The pins installed in the grinding chamber have several advantageous functions. Their first function is to act as an obstacle for the jet. The particles in the jet are forced to collide with the pins at maximum speed. This greatly increases the probability that the particles are crushed on the first collision. The kinetic energy is increased by about 60 times compared to existing mills. Depending on the substance used for the jet, even higher values can be obtained, for example if dry steam is used for operation. Their second function is to act as a nozzle. The cross section of the grinding chamber is reduced by the pins. This leads to an acceleration of the jet, which in turn accelerates the particles to be ground. The third function of the pins is that their position forces the jet to form a curved flow around the pins. The particles have to change their flow direction with the jet. Coarser particles cannot follow this change in direction due to their inertial momentum and will therefore collide with the next pin. Smaller particles, however, will follow the jet, thus reducing the likelihood of a collision with a subsequent pin. The design of the grinding chamber thus enables a selective grinding method in terms of particle size distribution. It also allows the position of the pins to be adjusted and optimized in such a way that very fine grinding can be carried out, for example by reducing the distance between the pins, thus increasing the curvature of the jet. The number of pins and the adjustable speed of the gas and particles allow the impact force and the number of impacts in the mill to be precisely adjusted. This allows products with a very precise particle size distribution to be produced. Another advantage of the mill is that there are no moving parts in direct contact with the gas loaded with solids. Difficult sealing of frequently easily worn moving parts is thus avoided. The simple design of the mill makes it easy to automatically clean the mill. This is beneficial for autonomous operation in a completely sealed space, for example for reasons of dust protection in the case of grinding hazardous materials.

[0024] In the acceleration section without any pins, the particles contained in the jet are accelerated compared to the pin sections. This leads to an increase in the impact energy of the particles on the pin surfaces in the subsequent pin sections. The overall efficiency of the mill is increased compared to mills that do not include intermediate acceleration sections.

[0025] The grinding chamber can have any form suitable for the jet loaded with solid particles to flow through. The length, width and height of the grinding chamber can be chosen to meet the requirements of the grinding task to be solved, for example in terms of throughput, available amount of grinding gas volume flow, maximum permissible pressure drop or required fineness of the ground particles.

[0026] The length of the grinding chamber is preferably 50-1000 mm, more preferably 100-400 mm. Relevant parameters for the appropriate selection of the length of the grinding chamber are the number of available pins and the maximum pressure drop in the grinding chamber. If the grinding chamber is too short, there is not enough space to provide enough pins with sufficient distance between the pins. The efficiency of the jet is therefore reduced. If the grinding chamber is too long, the pressure drop will be too high, thus leading to a reduction in the overall efficiency of the method.

[0027] In a preferred embodiment, the grinding chamber, perpendicular to the longitudinal axis, has a rectangular cross-section. The cross-sectional area can be constant or vary along the length of the grinding chamber. In one embodiment, the cross-sectional area is constant along the length of the grinding chamber. In another embodiment, the cross-sectional area at the grinding chamber inlet is smaller than the cross-sectional area at its outlet. In this case, the grinding chamber inlet functions as a nozzle. In yet another embodiment, the cross-sectional area at the grinding chamber outlet is smaller than the cross-sectional area at its inlet. In this case, the jet accelerates towards the outlet of the grinding chamber. Depending on the type of classifier used for separating particles, a higher-velocity jet entering the classifier can improve the efficiency of the classification process.

[0028] Preferably, the specific flow rate of the grinding gas jet at the maximum cross-sectional area is 25-450 m³ / h. 3 / m 2 / s. Given a specific jet flow rate, the height and width of the grinding chamber can be selected accordingly, thereby choosing its cross-sectional area.

[0029] In a preferred embodiment, the height of the grinding chamber is 3-10 mm, particularly 5-6 mm.

[0030] The width of the grinding chamber is preferably selected based on the amount of pressurized gas or steam to allow 10-250 cubic meters of gas per hour and per centimeter of chamber width (m³). 3 A jet of / h / cm.

[0031] Preferably, at least the inner wall of the grinding chamber is coated with or made of a wear-resistant material, such as ceramic materials like silicon carbide or wear-resistant steels like Hardox (a trademark of SSAB AB, Stockholm, Sweden). More preferably, the material is conductive to avoid the formation of excessive electrostatic charge.

[0032] The pin can have any form suitable for causing particle collisions in the jet of the impact pin.

[0033] Preferably, the pins extend from one inner wall of the grinding chamber to the opposite wall without any gaps. More preferably, the pins are arranged perpendicular to the flow direction of the jet, for example, by arranging the pins with their axes perpendicular to the axis of the grinding chamber. Most preferably, the pins are arranged vertically such that they extend from the bottom to the top of the grinding chamber.

[0034] The number of pins, the diameter of the pins and the distance between the pins can be chosen to meet the requirements of the grinding task. Generally, smaller pins force the jet and the particles contained therein to enter a smaller radius around the pins. Thus, smaller pins will result in finer particles. The pin diameter also influences the lifetime of the pins. Generally, the larger the pin diameter, the longer the lifetime. In a preferred embodiment, the diameter of each pin is 2-8 mm, more preferably 3-5 mm. With this choice, safe process conditions, fine products and long pin lifetime can be achieved.

[0035] The distance between two adjacent pins in a plane is preferably chosen in the range of approximately the same order as the diameter of the pins. This ensures that in the pin section, the particles are guided from a first plane of the pins to a second plane of the pins and that clogging of the flow channel is avoided. More preferably, the ratio between the distance between two adjacent pins and the diameter of the respective pins is 0.8-1.5. In this context, "diameter" refers to the lateral diameter, i.e. the lateral extension of the pins in the plane direction.

[0036] The distance between two planes is preferably defined by the envelope of the pins in the respective planes. The envelope of a plane is a straight or curved line that is tangent to the outermost surface of the pins in that plane. The distance between two planes is then defined as the shortest line connecting the envelopes of the adjacent planes. Preferably, the ratio between the distance between two adjacent planes and the diameter of the pins in the respective planes is 0.8-1.5. In this context, "diameter" refers to the axial diameter, i.e. the extension of the pins in the direction of the longitudinal axis of the grinding chamber.

[0037] In a preferred embodiment, the pins of two adjacent planes are arranged in such a way that in the lateral direction, i.e. perpendicular to the longitudinal axis, the pins of the subsequent plane are arranged in the free channel between the two pins of the previous plane, in particular in the middle of the free channel. More preferably, the ratio between the distance between two adjacent pins in a plane and the lateral diameter of the respective pins is 0.8-1.5 and the ratio between the distance between two adjacent planes in terms of their envelopes and the axial diameter of the pins in the respective planes is 0.8-1.5. This configuration of the pins forces the jet and the particles contained therein to bend in a wave-like motion around the pins, which increases the probability of the particles colliding with the pins of the subsequent plane.

[0038] In another preferred embodiment, the pins of the three adjacent planes are arranged such that the pins of the third plane are laterally offset with respect to the pins of the second plane and the pins of the first plane. In the lateral direction, the pins of the second plane are arranged in a free passage between the two pins of the first plane, in particular in the middle of the free passage. More preferably, the ratio between the distance between two adjacent pins in the first or second plane and the lateral diameter of the respective pin is 0.8-1.5, and the ratio between the distance between the first and second plane in terms of their envelope and the axial diameter of the pins in the respective plane is 0.8-1.5. The pins in the third plane are arranged such that the pin-to-pin distance between the pins in the second plane and the pins in the third plane has an axial component and a lateral component. More preferably, the ratio between the lateral component of the pin-to-pin distance in the third plane and the lateral diameter of the respective pin is 0.8-1.5, and the ratio between the axial component of the pin-to-pin distance and the lateral component of the pin-to-pin distance is 0-2. This configuration of the pins forces the jet and the particles contained therein to bend in a wavelike motion around the pins, increasing the probability of the particles colliding with the pins in the subsequent plane. Furthermore, this configuration provides flexibility to adjust the pressure drop in the grinding chamber over a wide range.

[0039] In a preferred embodiment, the surface of the pin facing the inlet is convex. In this context, "surface" is to be understood as the area of the pin that is hit by the jet entering through the inlet. The term "convex" is to be understood in its mathematical definition: a surface of a pin is convex if a straight line between any two points of the surface extends completely within the pin. Examples of pins having a convex surface are pins having a circular, elliptical or wing-shaped cross-section. The form of the pin can be chosen according to the process and production needs. For example, pins having a circular or elliptical cross-section are generally easy to manufacture, while the manufacture of wing-shaped cross-sections can be more complex. In terms of process performance, wing-shaped cross-sections can be a better choice if there are problems with the pressure drop over the mill.

[0040] In a preferred embodiment, the pins are detachably attached inside the grinding chamber. In one embodiment, the length of the pin is greater than the portion of the pin inside the grinding chamber, and the pin is introduced into the grinding chamber through an opening that is sealed with respect to the environment. Depending on the wear of the pin inside the grinding chamber, the pin can be pushed further into the grinding chamber so that a new portion of the pin replaces the worn portion. Depending on the material of the pin and the substance to be ground, this embodiment is advantageous in view of the high wear that the pin can be subjected to, as it allows for long-time operation without the need to shut down the operation of the mill.

[0041] In many applications, the pins will be subject to wear and thus some or all of the pins will need to be replaced from time to time. In a preferred embodiment, at least some of the pins comprise a sensor capable of detecting a measure of wear of the respective pin. In a first variant of this embodiment, the sensor is an acceleration sensor capable of detecting wear due to the speed at which particles collide with the pin. An example of such a sensor is a piezoelectric sensor. In a second variant of this embodiment, the sensor comprises means to excite the pin with a vibration scan signal and means to capture the frequency response of the pin to calculate friction or wear. In a third variant of this embodiment, the sensor is capable of measuring the electrical conductivity of the pin. In one example, the respective pin is made of a non-conductive material, such as a ceramic material, and contains a wire, such as a metal wire, inside the pin. Once the pin is worn to the wire, the wire breaks and the sensor detects the sudden change in electrical conductivity. The jet mill of this embodiment can contain a single type of sensor or a mix of sensor types. Providing sensors in at least some of the pins in the mill enables condition monitoring and predictive maintenance of the mill, thereby saving operational costs.

[0042] In a preferred embodiment, the pin segments each have 2-5 pin planes.

[0043] It is further preferred that the length of the acceleration segment is larger than the longitudinal distance between the planes in the pin segment. An advantage of this embodiment is that the particles can reach a higher speed and thus have a higher impact energy in the acceleration segment.

[0044] The distance between two adjacent pin segments and thus the length of the acceleration segment between these pin segments is preferably chosen between 20-200 mm. This range has proven to provide a large operating window to achieve high particle speeds in a mill with a reasonable length.

[0045] The number of pin segments is preferably chosen between 1-10, more preferably between 2-5. This range has proven to be a good compromise between particle speed, impact per particle and pressure drop along the grinding chamber. The larger the impact force needed, the fewer pin segments should be chosen.

[0046] In a preferred embodiment, an acceleration chamber with an inlet and an outlet is connected to the grinding chamber, the outlet of the acceleration chamber being the inlet of the grinding chamber. Providing an acceleration chamber at the inlet of the mill enables an increase of the impact energy of the particles colliding with the surface of the pins in the first pin segment. It is further preferred that the acceleration chamber has a conical shape.

[0047] For this embodiment with an acceleration chamber at the inlet, it is further preferred that the inlet of the acceleration chamber has a smaller cross-sectional area than the outlet thereof. In one variant, the ratio of the inlet width of the grinding chamber to the inlet width of the acceleration chamber is preferably between 1-7. The ratio of the length of the acceleration chamber to the inlet width of the grinding chamber is preferably between 2-10.

[0048] In another variant, the cross-section of the inlet and outlet of the acceleration chamber differ in size and / or shape. It is particularly preferred that the acceleration chamber inlet has a circular shape, while the acceleration chamber outlet has a rectangular shape. This variant is particularly suitable for connecting a duct to the acceleration chamber inlet, for example a duct through which pressurized air is fed to the mill. The injector of the particles to be ground can be connected to this duct or directly to the acceleration chamber.

[0049] In a preferred embodiment, the pins are made of a material selected from the group consisting of wear-resistant steel or wear-resistant ceramic, in particular a material selected from the group consisting of wear-resistant steel, corundum, silicon carbide, tungsten carbide.

[0050] In a preferred embodiment, the height of the grinding chamber is 3-10 mm, in particular 5-6 mm. This range has proven to be a good compromise between grinding capacity and separation efficiency in a subsequent classifier.

[0051] The jet mill of the present invention can be used as a standalone device or in combination with other devices or components, for example in combination with a classifier. In a preferred embodiment, the outlet of the grinding chamber is coupled to the inlet of a classifier capable of separating fine particles from coarse particles. It is further preferred that said classifier is capable of simultaneously separating multiple product fractions having at least one fine particle fraction and at least one coarse particle fraction. The coupling of the outlet of the grinding chamber to the inlet of the classifier can be direct or indirect, for example through a pipe or hose.

[0052] It is particularly preferred that the classifier is based on the Coanda effect. This facilitates the simultaneous production of very fine particles and multiple product fractions. Another advantage of this mill and classifier combination is the absence of moving parts in direct contact with the gas loaded with solids.

[0053] Some embodiments of the jet mill of the present invention are particularly advantageous when a classifier, in particular a Coanda classifier, is coupled to the outlet of the mill.

[0054] In a preferred embodiment, the last section of the mill serves to finally accelerate the particles to a similar speed as the grinding gas. The advantage is that particles with a similar speed as the grinding gas can achieve a better classification result in a Coanda classifier.

[0055] In a preferred embodiment, the cross-section of the last section of the mill is reduced in order to increase the gas speed before the gas enters the Coanda classifier. The advantage is that the higher the gas and particle speed, the finer the cut size in the Coanda classifier.

[0056] In another preferred embodiment, the cross-section of the last section of the mill is widened into a de Laval nozzle type shape. This also increases the gas speed towards the end of the mill before entering the classifier.

[0057] The measures to accelerate the gas into the classifier are particularly advantageous for very fine separations in the range of microns to sub-microns.

[0058] In a preferred embodiment of the coupling system, the outlet of the coarse fraction is recirculated to the inlet of the grinding chamber or to the inlet of the acceleration chamber.

[0059] In another preferred embodiment of the coupling system, fresh particles to be ground are fed from the outlet of the coarse fraction to the inlet of the grinding chamber or to the inlet of the acceleration chamber.

[0060] In a preferred embodiment, the feed of the mill and the recirculated coarse fraction of the Coanda classifier are fed through the same injector system. For this purpose, the fresh feed material of the mill can be fed with a device that decouples the ambient air pressure from the potential vacuum in the injector system, for example by using a gas-tight rotary unit valve or a screw conveyor. The coarse fraction from the Coanda classifier is preferably separated from the classification gas flow, for example by a cyclone and a rotary unit valve. The transport back into the injector system can be realized, for example, by pneumatic transport of the suction air of the pneumatic injectors or by a solid transport system such as a conveyor belt or a screw conveyor.

[0061] In a preferred embodiment of the method for grinding and classifying solid particles, in which the outlet of the grinding chamber is recirculated to the inlet of a classifier that is capable of separating fine particles from coarse particles, the method comprises the steps of (a) injecting particles into a jet, (b) feeding the jet comprising the injected particles into a jet mill of the invention, and (c) separating at least one fine particle fraction from the feed material in the classifier.

[0062] All preferred embodiments of the jet mill and the coupling system are also preferred embodiments of the method of the invention for grinding and classifying solid particles in a jet mill or a coupling system.

[0063] For all embodiments, the jet is preferably a high-speed flow of gas or dry steam. For particle sizes of the ground particles of about 1 micron or less than 1 micron, dry steam is particularly preferred.

[0064] The jet mill of the invention can advantageously be used for grinding a variety of types of particles, for example magnetic materials, battery materials, active ingredients such as ibuprofen, citric acid or magnesium carbonate, pigments (for example for paints), metal organic frameworks, carbonyl iron powder.

[0065] The invention will be explained in more detail below with reference to the drawings. The drawings are to be interpreted as a principle illustration. They do not constitute any limitation of the invention, for example with regard to specific dimensions or design variants. In the drawings:

[0066] Figure 1 A longitudinal top view of a first embodiment of the jet mill of the invention is shown.

[0067] Figure 2 shows a longitudinal top view of the second embodiment of the jet mill of the present invention.

[0068] Figure 3 shows a top view of a segment of a grinding chamber with 4 pin planes.

[0069] Figure 4 shows a top view of a segment of a grinding chamber with 3 pin planes.

[0070] Figure 5 shows a schematic view of the first embodiment of the jet mill coupled to a Coanda classifier.

[0071] Figure 6 shows a schematic view of the second embodiment of the jet mill coupled to a Coanda classifier.

[0072] Figure 7 shows a longitudinal top view of the jet mill of the comparative example.

[0073] Figure 8 shows a longitudinal top view of the second embodiment of the jet mill of the present invention.

[0074] Figure 9 shows a comparison of the particle size distribution obtained by grinding particles in the jet mill of Figure 7 and Figure 8

[0075] List of used reference signs:

[0076] 1 grinding chamber

[0077] 2 longitudinal axis

[0078] 3 grinding chamber inlet

[0079] 4 grinding chamber outlet

[0080] 5 pin

[0081] 6 acceleration chamber

[0082] 7 acceleration chamber inlet

[0083] 11 gas source

[0084] 12 additional gas

[0085] 13 additional gas

[0086] 14 fine particle outlet

[0087] 15 medium particle outlet

[0088] 16 coarse particle 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 pin plane

[0095] Figure 1 A longitudinal section of a jet mill as first embodiment of the invention is shown. The jet mill comprises a grinding chamber 1 having a longitudinal axis 2, an inlet 3 at one end of the axis and an outlet 4 at the opposite end of the axis. Within the grinding chamber 1 there are 15 pins 5 arranged in 3 pin sections, each pin section having 5 pins 5. Between the pin sections there is an acceleration section, each acceleration section being free of pins. Counting from the inlet 3 to the outlet 4, the first pin section comprises 3 pin planes. In the first plane, 2 pins 5 are arranged symmetrically with respect to the longitudinal axis 2. The second plane comprises 1 pin 5 arranged in the center of the grinding chamber on the longitudinal axis 2. The third plane comprises 2 pins 5 attached to the left and right wall of the grinding chamber 1, respectively. The second pin section comprises a plane of 2 pins 5. The arrangement of the pins in the first plane of the second pin section is identical to the arrangement in the first plane of the first pin section. The second plane of the second pin section comprises 3 pins 5. One pin is arranged in the center of the grinding chamber on the longitudinal axis 2. The other 2 pins are attached to the left and right wall of the grinding chamber, respectively. The arrangement of the pins in the third pin section is identical to the arrangement of the second pin section. All planes are distant from each other in the longitudinal direction. The pins of one plane are laterally offset with respect to the pins of the adjacent plane, because the center of the pin axis in one plane and the center of the pin axis in the subsequent plane lie on different lines parallel to the longitudinal axis of the mixing chamber.

[0096] An acceleration chamber 6 having an inlet 7 and an outlet is connected to the grinding chamber 1, the outlet of the acceleration chamber being the inlet 3 of the grinding chamber. The cross section of the grinding chamber 1 and the acceleration chamber 6 is rectangular, the cross sectional area of the inlet 7 of the acceleration chamber being smaller than its outlet.

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

[0098] Figure 2A longitudinal section of a jet mill is shown as a second embodiment of the invention. The jet mill comprises a grinding chamber 1 having a longitudinal axis 2, an inlet 3 at one end of the axis and an outlet 4 at the opposite end of the axis. Within the grinding chamber 1, there are 24 pins 5 arranged in two pin sections, where in each pin section 12 pins 5 are arranged in 4 planes. Between the pin sections there is an acceleration section without pins. Counting from the inlet 3 to the outlet 4, the first plane of the first pin section comprises 3 pins. One pin is attached to the right wall of the grinding chamber and the other 2 pins are arranged with equal lateral distance between the pins. The lateral distance of the leftmost pin to the left wall is the same as the lateral distance between the pins in the plane. The pins in the second plane of the first pin section are arranged in a similar way as the pins in the first plane, but are laterally offset from the pins of the first plane. The leftmost pin is attached to the left wall of the grinding chamber and the other 2 pins are arranged with equal lateral distance between the pins. The lateral distance of the rightmost pin to the right wall is the same as the lateral distance between the pins in the plane. The pins of the third plane are arranged in a similar way as the pins in the first plane and the pins of the fourth plane are arranged in a similar way as the pins in the second plane. All pins in the first pin section have the same cylindrical shape. Their cross section is circular, so the surface of the pins facing the inlet 3 is convex.

[0099] The diameter of the pins of the second pin section is larger than the diameter of the pins of the first pin section. Counting from the inlet 3 to the outlet 4, the first plane of the second pin section comprises 3 pins. One pin is attached to the left wall of the grinding chamber and the other 2 pins are arranged with equal lateral distance between the pins. The cross section of the leftmost pin is semi-circular and the cross section of the other 2 pins is circular. The lateral distance of the rightmost pin to the right wall is the same as the lateral extension of the leftmost pin attached to the wall. The pins in the second plane of the second pin section are arranged in a similar way as the pins in the first plane, but are laterally offset from the pins of the first plane. The rightmost pin is attached to the right wall of the grinding chamber and the other 2 pins are arranged with equal lateral distance between the pins. The cross section of the rightmost pin is semi-circular and the cross section of the other 2 pins is circular. The lateral distance of the leftmost pin to the left wall is the same as the lateral extension of the rightmost pin attached to the wall. The pins of the third plane are arranged in a similar way as the pins in the first plane and the pins of the fourth plane are arranged in a similar way as the pins in the second plane. The cross section of all pins in the second pin section is circular or semi-circular, so the surface of the pins facing the inlet 3 is convex.

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

[0101] An acceleration chamber 6 having an inlet 7 and an outlet is connected to the grinding chamber 1, the outlet of the acceleration chamber is the inlet 3 of the grinding chamber. The cross section of the grinding chamber 1 and the acceleration chamber 6 is rectangular, the cross sectional area of the inlet 7 of the acceleration chamber is smaller than the outlet of the acceleration chamber.

[0102] Figure 3A longitudinal sectional view of a pin segment of a grinding chamber, as another exemplary embodiment of the present invention, is shown. The pin segment includes four pin planes. Counting from left to right in the jet direction, the first plane P1 and the third plane P3 each include three pins. One pin is attached to the right wall of the grinding chamber, one pin is attached to the left wall of the grinding chamber, and one pin is arranged at the transverse center of the grinding chamber on the longitudinal axis. Figure 3 (The axis is not shown in the diagram). The second plane P2 and the fourth plane P4 each contain two pins, each arranged symmetrically with respect to the longitudinal axis between the axis and the channel wall. All pins in this segment have the same cylindrical shape with a circular cross-section.

[0103] In the transverse direction, i.e., perpendicular to the longitudinal axis, the pins of the subsequent plane are arranged in the middle of the free passage between the two pins of the previous plane. The ratio of the transverse distance L1 between two adjacent pins in the plane to the diameter of the corresponding pin is preferably 0.8-1.5. Figure 3 In the example shown, the ratio is 1.25.

[0104] The axial distance A1 between two adjacent planes is defined by the envelope of pins in the corresponding planes. Figure 3 In the example shown, the envelope of the plane is the line tangent to the outermost surface of the pin in that plane, as shown by the dashed line in plane P3. Preferably, the ratio of the axial distance A1 between two adjacent planes to the diameter of the pin in the corresponding plane is 0.8-1.5. Figure 3 In the example shown, the ratio is 1.15.

[0105] Figure 4 A longitudinal sectional view of a pin segment of a grinding chamber, as another exemplary embodiment of the present invention, is shown. The pin segment includes three pin planes. Counting from left to right in the jet direction, the first plane P1 includes two pins, which are arranged on the left and right sides of the longitudinal axis of the grinding chamber. Figure 4 (This axis is not shown in the diagram). The second plane P2 includes one pin arranged at the center of the grinding chamber on the longitudinal axis. The third plane P3 includes two pins attached to the left and right walls of the grinding chamber. All pins in this section have the same cylindrical shape with a circular cross-section.

[0106] The pin in the third plane P3 is laterally offset from the pins in the second plane P2 and the first plane P1. Laterally, the pin in plane P2 is positioned in the middle of the free passage between the two pins in plane P1.

[0107] The ratio of the lateral distance L1 between two pins in plane P1 to the diameter of the corresponding pin is preferably 0.8-1.5. Figure 4 In the example shown, the scale is 0.95. The axial distance A1 between plane P1 and plane P2 is defined by the envelope of the pins in the respective planes. Figure 4In the example shown, the envelope of a plane is a line that is tangent to the outermost surface of the pin in that plane, as indicated by the dashed line in plane P1. Preferably, the ratio of the axial distance A1 between the envelopes of the first plane P1 and the second plane P2 to the axial diameter of the pin in the respective plane is 0.8-1.5. In Figure 4 In the example shown, 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 an adjacent pin in the third plane P2 has an axial component A2 and a lateral component L2. Preferably, the ratio of the lateral component L2 of the pin-pin distance in the third plane P3 to the lateral diameter of the respective pin is 0.8-1.5. In Figure 4 In the example shown, this ratio is 1.25. Further preferably, the ratio of the axial component A2 of the pin-pin distance to the lateral component L2 of the pin-pin distance is 0-2. In Figure 4 In the example shown, this ratio is 1.

[0109] Figure 5 A schematic diagram of a first embodiment of a jet mill coupled to a Coanda classifier is shown. The particles to be milled are fed into the gaseous medium and into the inlet of the jet mill in the gas source 11. The outlet of the jet mill is directly coupled to the inlet of the Coanda classifier. Coanda classifiers are known in the art (e.g. Heinrich Schubert (Editor): 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, namely a fine particle fraction, a medium particle fraction and a coarse particle fraction. Additional gas flows 12 and 13 without particle load can be used to influence the separation into the three fractions. The fine particle fraction is withdrawn from the classifier through the fine particle outlet 14. The medium particle fraction is withdrawn from the classifier through the medium particle outlet 15. The coarse particle fraction is withdrawn from the classifier through the coarse particle outlet 16 and recirculated to the gas source 11, thus to the inlet of the milling chamber of the jet mill. Fresh particles to be milled can be fed directly into the gas source 11 and / or into the recirculation flow from the coarse particle outlet 16 back to the gas source 11. This option is shown in Figure 5 stream 17.

[0111] Figure 6 A schematic diagram of a second embodiment of a jet mill coupled to a Coanda classifier is shown. Figure 5 the first embodiment shown and Figure 6The difference between the second embodiment shown is that the coarse particles removed from the classifier are fed into a further separation unit 18 before being recirculated to the jet mill inlet. The separation unit 18 can comprise any suitable separation device, in particular a filter, a cyclone or a combination of filter and cyclone. The coarse solid fraction separated in the separation unit is preferably pneumatically isolated from the gas stream, for example by means of a rotating unit valve or similar device such as a screw conveyor. By these means, decoupling between the negative pressure of the injector of the gas source 11 and the negative pressure in the Coanda classifier can be achieved. This facilitates separate adjustment of the feed rate of the mill and the separation conditions in the Coanda classifier. The cleaned offgas 19 is withdrawn from the separation unit 18. Example

[0112] A jet mill according to the present application was compared to a state-of-the-art opposed jet mill and a spiral jet mill. In each of the three mills, limestone powder ("Jurapele 150-300" by Omya Gmbh, Cologne, Germany) was ground. The particle size parameters of the powder were as follows:

[0113] D10 [pm] 4.6 D50 [pm] 119.0 D90 [pm] 225.0

[0114] In each case, the grinding pressure of the respective mill was set as high as possible, since this leads to the highest product fineness. Then all mills were loaded until the particle size distribution of the ground product became significantly coarser or the mill reached a critical operating state.

[0115] Comparative Example 1

[0116] As comparative example 1, an opposed jet mill (type AFG 100 by Hosokawa Alpine AG, Augsburg, Germany) was used. The mill was operated with 3 nozzles each having a diameter of 1.9 mm to provide a grinding gas at a pressure of 7 bar. The material was fed into the grinding chamber with a screw feeder at a feed rate of 4 kg / h. The diameter of the turn 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.

[0117] Comparative Example 2

[0118] As comparative example 2, a spiral jet mill with a grinding chamber diameter of 170 mm and a grinding chamber height of 15 mm was used. The spiral jet mill was equipped with 10 cylindrical grinding gas nozzles each having a diameter of 1.5 mm, which were evenly distributed over the circumference of the grinding chamber. The grinding gas pressure was 3.6 bar. The diameter of the injection nozzles was 2.5 mm and the diameter of the booster nozzles was 8 mm. The injection nozzle pressure was 3.8 bar. The vortex finder of the mill had a circular shape with a diameter of 40 mm.

[0119] Example 1 of the Invention

[0120] A jet mill similar to the one shown in Figure 1 embodiment of the application. The jet mill comprises a grinding chamber having 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 grinding chamber, there are 15 pins arranged in 3 pin segments, each pin segment having 5 pins. Each pin segment comprises 2 pin planes, said planes being perpendicular to the longitudinal axis. Counting from the inlet to the outlet, in the first plane, 2 pins are arranged symmetrically with respect to the longitudinal axis. The second plane comprises 3 pins. One pin is arranged in the center of the grinding chamber on the longitudinal axis. The other two pins are attached to the left and right wall of the grinding chamber, respectively. All planes are distanced from each other. The longitudinal distance between the first plane in each pin segment and the respective second plane is 10 mm. The pins of one plane are laterally offset with respect to the pins of the adjacent plane, since the pin axis center in one plane and the pin axis center in the subsequent plane are located on different lines parallel to the longitudinal axis of the mixing chamber. The pins are made of silicon carbide. All pins have the same cylindrical shape. Their cross section is a circle with a diameter of 4 mm, so the pin surface facing the inlet is convex. Thus, the longitudinal distance between the first plane in each pin segment and the respective second plane is 6 mm in terms of its envelope. Between the pin segments there is an acceleration segment, each acceleration segment having no pins. The length of the two acceleration segments is 36 mm each.

[0121] An acceleration chamber having an inlet and an outlet is connected to the grinding chamber, the outlet of the acceleration chamber being the inlet of the grinding chamber. The length of the acceleration chamber is 50 mm. The length of the grinding chamber is 165 mm. The cross section of the grinding chamber and the acceleration chamber is rectangular. The width of the grinding chamber is 20 mm, its height is 5 mm. The width of the acceleration chamber inlet is 9 mm.

[0122] Based on the Coanda effect, the outlet of the grinding chamber is connected to the inlet of the classifier. The overall setup is as shown in Figure 6 .

[0123] The feed material is fed into the suction pipe of the injector using a screw conveyor. In the injector, the solid feed material is dispersed in a jet of milling gas. The dispersed material is accelerated in the acceleration chamber so that the particles reach a speed similar to the speed of the milling gas. Subsequently, the particles collide with the pins of the first pin section and are mechanically shattered by the impact. In addition, the particle-particle contact between the particles reflected by the pins and the particles dispersed in the milling gas results in high-energy impacts, which lead to particle breakage. After the first pin section, the particles are re-accelerated until they collide with the first pin of the second pin section. After the third pin section, the particles are again accelerated by the remaining milling gas pressure so that they can enter the Coanda classifier with a speed similar to the milling gas. The main part of the milling gas entering the Coanda classifier is forced to a curved motion along the curved shape of the Coanda inlet. The fine particles with a high specific surface follow the curved motion of the gas flow. The particles with a lower specific surface, e.g. medium-sized or coarse particles, can only partially follow the curved motion of the gas and deviate less from their initial straight motion. Thus, a parabolic distribution of fine particles to coarse particles can be achieved within the housing of the Coanda classifier. By adjusting the flow divider in the path of the different-sized particles, they can be separated into a fine particle fraction and a coarse particle fraction. In order to optimize the flight path of the particles in the Coanda classifier and thus the separation performance, additional gas is sucked into the Coanda classifier. The separated fine particles are sucked into a filter to remove the solid phase from the gas phase. The coarse particle fraction collected at the bottom of the cyclone is brought out of the cyclone by a screw conveyor and then returned into the suction pipe of the injector. Thus, the coarse particles are mixed with fresh material and fed back into the mill.

[0124] The parameters and results of the grinding experiments are shown in the following table:

[0125] Comparative Example 1 Comparative Example 2 Example of the Invention Feed rate [kg / h] 4 2 10 Grinding pressure [bar] 7 3.6 7.8 Volume flow [Nm 3 / h]]]> 62 86.6 36 Specific loading [g / m 3 ]]]> 65 23 278 Specific energy [kWh / kg] 1.12 1.86 0.28 D10 [pm] 0.4 0.6 0.3 D50 [pm] 3.0 3.7 3.0 D90 [pm] 5.7 9.7 6.5

[0126] As can be seen from the above table, the product fineness as a result of the grinding of the powder in the jet mill according to the invention is very similar to the product fineness obtained using a opposed jet mill. The product of the grinding method in the screw jet mill is coarser.

[0127] Due to its design, the jet mill according to the invention can be operated at significantly higher milling gas (jet) to particle loadings. Due to the high solid loadings and the relatively low volume flow of the milling gas, the specific energy consumption of the grinding method according to the invention is much lower than in prior art methods. In the above example, the specific energy consumption is 4 times lower than in an opposed jet mill and 6.6 times lower than in a screw jet mill.

[0128] Comparative Example 3

[0129] In another set of experiments, the influence of the intermediate acceleration section was investigated. The material to be ground was the same limestone powder as in the previous examples ("Jurapele 150-300" by Omya Gmbh, Cologne, Germany). In each case, the grinding pressure of the respective mill was set to 8 bar (absolute), and the feed rate of the limestone particles was set to 18 kg / h.

[0130] As comparative example 3, a jet mill of the type shown in Fig. 1 was used. Figure 7 The jet mill of the example shown in Fig. 1. Figure 7 A longitudinal top view showing the principle of a jet mill is shown. The jet mill comprises a grinding chamber 1 having 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 within the grinding chamber 1, there are 24 pins 5 arranged in 16 pin planes, which planes are perpendicular to the longitudinal axis 2. Counting from the inlet 3 to the outlet 4, in the first plane, 2 pins are arranged symmetrically with respect to the longitudinal axis 2. The second plane comprises 1 pin, which is arranged in the center of the grinding chamber on the longitudinal axis 2. The pattern of the first and second plane is repeated 7 times. Thus, the arrangement of the pins in the third, fifth, seventh, ninth, eleventh, thirteenth and fifteenth plane is identical to the arrangement of the pins in the first plane, while the arrangement of the pins in the second, fourth, sixth, eighth, tenth, twelfth, fourteenth and sixteenth plane is identical to the arrangement of the pins in the fourth plane.

[0131] All planes are remote from each other. The longitudinal distance between two planes is 10 mm. The pins of one plane are laterally offset from the pins of the adjacent plane, since the center of the pin axis in one plane and the center of the axis in the subsequent plane are located on different lines parallel to the longitudinal axis of the mixing chamber. The pins are made of silicon carbide. All pins have the same cylindrical shape. Their cross section is a circle with a diameter of 4 mm.

[0132] An acceleration chamber 6 having an inlet 7 and an outlet is connected to the grinding chamber 1, the outlet of the acceleration chamber 6 being the inlet 3 of the grinding chamber 1. The length of the acceleration chamber is 50 mm. The length of the grinding chamber is 165 mm. The cross section of the grinding chamber and the acceleration chamber is rectangular. The width of the grinding chamber is 20 mm, its height is 5 mm. The width of the acceleration chamber inlet is 9 mm.

[0133] The feed material is supplied to the suction pipe of the injector using a screw conveyor. In the injector, the solid feed material is dispersed in the jet of grinding gas. The dispersed material is accelerated in the acceleration chamber 6 so that the particles reach a speed similar to the speed of the grinding gas. Subsequently, the particles collide with the pins 5 of the first pin section and are mechanically impact ground. In addition, particle-particle contacts between the particles reflected by the pins and the particles dispersed in the grinding gas result in high-energy impacts, which lead to particle breakage. The limestone particles are collected at the outlet 4 of the jet mill and their particle size is determined.

[0134] Example 2 of the Invention

[0135] The jet mill of the embodiment shown is used as another embodiment of the invention. Figure 8 A longitudinal top view showing the principle of the jet mill. Figure 8 A longitudinal top view showing the principle of the jet mill.

[0136] The jet mill comprises a grinding chamber 1 having 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 within the grinding chamber 1, 20 pins 5 are arranged, which have 4 pin sections, each pin section having 5 pins. Each pin section comprises 2 pin planes, which are perpendicular to the longitudinal axis 2. Counting from the inlet 3 to the outlet 4, in the first plane, 2 pins 5 are arranged symmetrically with respect to the longitudinal axis. The second plane comprises 3 pins 5. One pin is arranged in the center of the grinding chamber on the longitudinal axis. The other two pins are attached to the left and right wall of the grinding chamber, respectively. The arrangement of the pins in the second, third and fourth pin section is identical to the arrangement of the pins in the first pin section.

[0137] All planes are distanced from each other. The longitudinal distance between the first plane and the respective second plane in each pin section is 10 mm. The pins of one plane are laterally offset with respect to the pins of the adjacent plane, since the pin axis center in one plane and the pin axis center in the subsequent plane are located on different lines parallel to the longitudinal axis of the mixing chamber. The pins are made of silicon carbide. All pins have the same cylindrical shape. Their cross section is a circle with a diameter of 4 mm, so the pin surface facing the inlet is convex. Thus, the longitudinal distance between the first plane and the respective second plane in each pin section in terms of its envelope is 6 mm. Between the pin sections there is an acceleration section, each acceleration section being free of pins. The length of the three acceleration sections is 36 mm, respectively.

[0138] An acceleration chamber 6 having an inlet 7 and an outlet is connected to the grinding chamber 1, the outlet of the acceleration chamber 6 being the inlet 3 of the grinding chamber 1. The length of the acceleration chamber is 50 mm. The length of the grinding chamber is 165 mm. The cross section of the grinding chamber and the acceleration chamber is rectangular. The width of the grinding chamber is 20 mm, its height is 5 mm. The width of the acceleration chamber inlet is 9 mm.

[0139] The feed material is supplied to the suction pipe of the injector using a screw conveyor. In the injector, the solid feed material is dispersed in a jet of milling gas. The dispersed material is accelerated in the acceleration chamber 6 so that the particles reach a speed similar to the speed of the milling gas. Subsequently, the particles collide with the pins 5 of the first pin section and are mechanically impact pulverized. In addition, the particle-particle contacts between the particles reflected by the pins and the particles dispersed in the milling gas result in high-energy impacts, which lead to particle breakage. After the first pin section, the particles are accelerated again until they collide with the first pins 5 of the second pin section. After the second pin section, the particles are accelerated again until they collide with the first pins 5 of the third pin section. After the third pin section, the particles are accelerated again until they collide with the first pins 5 of the fourth pin section. The limestone particles are collected at the outlet 4 of the jet mill and their particle size is determined.

[0140] Figure 9 A comparison of the particle size distributions obtained by milling particles in the jet mill of comparative example 3 Figure 7 and inventive example 2 Figure 8 is shown. On the abscissa, the particle size in micrometers (pm) is given. The ordinate shows the mass fraction in percent. The dotted line represents the feed material, which is characterized by about 80% of the particles being larger than 50 pm and about 60% of the particles being larger than 100 pm.

[0141] The dashed line shows the particle size distribution of a particle sample obtained at the outlet of the jet mill of comparative example 3. About 54% of the particles in this sample are smaller than 50 pm and about 30% of the particles are still larger than 100 pm.

[0142] The solid line shows the particle size distribution of a particle sample obtained at the outlet of the jet mill of inventive example 2. About 62% of the particles in this sample are smaller than 50 pm and about 14% of the particles are larger than 100 pm.

[0143] The milling process carried out in the jet mill of the present invention with intermediate acceleration sections results in smaller particles and a more uniform particle size distribution, which can also be derived from the curves in Figure 9 , where the slope of the solid line in the range of 20-100 pm is much steeper than the slope of the dotted line in the same range.

[0144] Further experiments with triboluminescent material show that in the mill of Figure 7 the breakage of the particles mainly occurs in the first two planes of the mill. In the mill of the present invention shown in Figure 8 , a continuous strong luminescence can be observed on all pins of the mill, which clearly indicates that the milling process is more intensive due to the acceleration sections.

Claims

1. A jet mill comprising a grinding chamber (1) having a longitudinal axis (2), an inlet (3) at one end of the axis and an outlet (4) at the opposite end of the axis, the grinding chamber (1) comprising a plurality of pins (5) arranged in the free-flow cross section of the grinding chamber (1), wherein the pins (5) are arranged in at least two planes perpendicular to the longitudinal axis (2), said planes being longitudinally distanced from each other and the pins (5) of one plane being laterally offset with respect to the pins (5) of a subsequent plane, characterized in that, The grinding chamber (1) is divided into alternating pin sections and acceleration sections, the pin sections each having at least two pin (5) planes, and the acceleration sections being pin-free.

2. The jet mill of claim 1, wherein, The surface of the pins (5) facing the inlet (3) of the grinding chamber is convex.

3. The jet mill of claim 1, wherein, The pins (5) are removably attached within the grinding chamber (1).

4. The jet mill of claim 2, wherein, The pins (5) are removably attached within the grinding chamber (1).

5. The jet mill of claim 1, wherein, At least some of the pins (5) comprise a sensor capable of detecting a measure of wear of the respective pin.

6. The jet mill of claim 2, wherein, At least some of the pins (5) comprise a sensor capable of detecting a measure of wear of the respective pin.

7. The jet mill of claim 3, wherein, At least some of the pins (5) comprise a sensor capable of detecting a measure of wear of the respective pin.

8. The jet mill of claim 4, wherein, At least some of the pins (5) comprise a sensor capable of detecting a measure of wear of the respective pin.

9. The jet mill of any one of claims 1-8, wherein, The pin sections each have 2-5 pin (5) planes.

10. The jet mill of any one of claims 1-8, wherein, The length of the acceleration sections is greater than the longitudinal distance between the planes in the pin sections.

11. The jet mill of claim 9, wherein, The length of the acceleration sections is greater than the longitudinal distance between the planes in the pin sections.

12. The jet mill of any one of claims 1-8, wherein, An acceleration chamber (6) having an inlet (7) and an outlet is connected to the grinding chamber (1), the outlet of the acceleration chamber (6) being the inlet (3) of the grinding chamber.

13. The jet mill of claim 11, wherein, An acceleration chamber (6) having an inlet (7) and an outlet is connected to the grinding chamber (1), the outlet of the acceleration chamber (6) being the inlet (3) of the grinding chamber.

14. The jet mill of claim 12, wherein, The inlet (7) of the acceleration chamber (6) has a smaller cross-sectional area than its outlet.

15. The jet mill of claim 13, wherein, The inlet (7) of the acceleration chamber (6) has a smaller cross-sectional area than its outlet.

16. The jet mill of any one of claims 1-8, wherein, The height of the grinding chamber (1) is 3-10 mm.

17. The jet mill of claim 15, wherein, The height of the grinding chamber (1) is 3-10 mm.

18. The jet mill of claim 16, wherein, The height of the grinding chamber (1) is 5-6 mm.

19. The jet mill of any one of claims 1-8, wherein, The pins (5) are made of a material selected from wear-resistant steel or wear-resistant ceramic.

20. The jet mill of claim 17, wherein, The pins (5) are made of a material selected from wear-resistant steel or wear-resistant ceramic.

21. The jet mill of claim 19, wherein, The pins (5) are made of a material selected from wear-resistant steel, corundum, silicon carbide, tungsten carbide.

22. The jet mill of any one of claims 1-8, wherein, The outlet (4) of the grinding chamber is coupled to an inlet of a classifier capable of separating fine particles from coarse particles.

23. The jet mill of claim 20, wherein, The outlet (4) of the grinding chamber is coupled to an inlet of a classifier capable of separating fine particles from coarse particles.

24. The jet mill of claim 22, wherein, The outlet (4) of the grinding chamber is coupled to an inlet of a classifier based on the Coanda effect.

25. The jet mill of claim 22, wherein, The outlet of the coarse fraction is recirculated to the inlet (3) of the grinding chamber or to the inlet (7) of the acceleration chamber.

26. The jet mill of claim 23, wherein, The outlet of the coarse fraction is recirculated to the inlet (3) of the grinding chamber or to the inlet (7) of the acceleration chamber.

27. A method of milling solid particles, comprising the steps of: (a) injecting particles into a jet, and (b) feeding the jet comprising the injected particles into a jet mill according to any one of claims 1-26.

28. The method according to claim 27, wherein the outlet of the grinding chamber is coupled to an inlet of a classifier capable of separating fine particles from coarse particles, and at least one fine particle fraction is separated from the feed material in the classifier.

29. The method according to claim 28, wherein the outlet of the grinding chamber is coupled to an inlet of a classifier based on the Coanda effect, and at least one fine particle fraction is separated from the feed material in the classifier.

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