Jet mill with recirculating product flow

Abstract

A jet mill has a milling chamber, a product injection nozzle configured to inject particles to be ground into the milling chamber, a primary exhaust port configured to exhaust ground particles from inside the milling chamber, a grinding nozzle configured to inject milling gas into the milling chamber, and a recirculation outlet port may be configured to collect partially ground particles from inside the milling chamber. A conduit operatively couples the recirculation outlet port to the grinding nozzle such that partially ground particles from inside the milling chamber can be recirculated through the recirculation outlet port and the conduit to the grinding nozzle and from the grinding nozzle back into the grinding chamber in a stream of mixed particles and milling gas.

Description

JET MILL WITH RECIRCULATING PRODUCT FLOWFIELD OF THE INVENTION

[0001] The present invention relates generally to a jet mill and a method of grinding particles in a jet mill.BACKGROUND

[0002] In the process of manufacturing pigment grade titanium dioxide (TiO2) particles, it is common to mill the Ti02 particles after their initial formation via either the chlorine process or the sulphate process to attain a smaller size suitable for use as a pigment. FIGS. 1 and 2 illustrate a conventional steam jet mill (also called a "micronizer") used to mill TiO2 particles to a desired size and size distribution (also called "micronizing"). This conventional spiral jet mill includes a generally planar, circular grinding chamber that is surrounded by a manifold with a number of grinding steam injectors that inject grinding steam into the grinding chamber. Typically, each of the grinding steam injectors is oriented at the same angle to radial, that is, tangentially to the centerline, to promote relatively uniform flow either clockwise or counterclockwise around the grinding chamber. A lid covers the top end of the grinding chamber and includes a single product injector nozzle, which is used to inject a stream of mixed input steam and unground granular product into the grinding chamber. The product injector nozzle is typically oriented tangentially to the centerline and at an acute angle relative to the horizontal so as to inject the stream of product and input steam along the same clockwise or counterclockwise direction as the grinding steam. As the granular particles whirl around the outer periphery of the grinding chamber, impacts with the grinding steam entering from the grinding steam injectors eventually comminute the granular particles. The smaller, milled particles eventually migrate to the center of the grinding chamber, where an exhaust outlet allows the milled particles to be ejected from the grinding chamber with the exhaust steam. An example of a conventional spiral jet mill may be seen in U.S. Patent No. 7,150,421.

[0003] Although the conventional steam jet mill is fully capable of milling (grinding) the particles to a desired size and size distribution, the conventional steam jet mill uses very large quantities of energy through the input of steam and has a relatively low efficiency due to the high throughput of steam required to grind a given amount of TiO2 particles to a desired size. Therefore, it would be desirable to have a milling system that can reduce the amount of steam or other milling gas used to grind a given quantity of particles to a desired size and / or size distribution in order to both reduceinput costs of manufacturing and reduce the environmental impact (e.g., "carbon footprint") by using less energy.SUMMARY OF THE INVENTION

[0004] In order to address one or more of the limitations discussed above, a new jet mill and a new method of grinding particles is provided by the present invention.

[0005] In one non-limiting aspect of the invention, a jet mill is provided. In some aspects of the invention, the jet mill may include a milling chamber, a product injection nozzle configured to inject particles to be ground into the milling chamber, a primary exhaust port configured to exhaust ground particles from inside the milling chamber, a grinding nozzle configured to inject milling gas into the milling chamber, and a recirculation outlet port may be configured to collect partially ground particles from inside the milling chamber. A conduit may operatively couple the recirculation outlet port to the grinding nozzle such that partially ground particles from inside the milling chamber can be recirculated through the recirculation outlet port and the conduit to the grinding nozzle and from the grinding nozzle back into the grinding chamber in a stream of mixed particles and milling gas.

[0006] In another non-limiting aspect of the invention, a method of grinding particles in a jet mill is provided. The jet mill may include a milling chamber, a product injection nozzle, and a grinding nozzle. In some aspects of the method, a stream of particles may be injected into the milling chamber through the product injection nozzle. Milling gas may be injected into the milling chamber through the grinding nozzle. A portion of the milling gas mixed with partially milled particles may be recirculated to the grinding nozzle. The recirculated milling gas and partially milled particles may be mixed with the milling gas, and the mixed recirculated milling gas and partially milled particles and milling gas may be injected into the milling chamber through the grinding nozzle.

[0007] In some optional configurations, the grinding nozzle may include a mixing chamber, a first entry port configured to direct a stream of milling gas into the mixing chamber, a second entry port configured to direct a stream of particles into the mixing chamber, and an injector port configured to direct the stream of mixed particles and milling gas from the mixing chamber into the milling chamber. The conduit may operatively couple the recirculation outlet port to the second entry port on the grinding nozzle.

[0008] In some optional configurations, the grinding nozzle may include a venturi configured to receive the stream of mixed particles and milling gas. For example, in some arrangements, theventuri may be operatively disposed between the mixing chamber and the injector port so that the stream of mixed milling gas and particles from the mixing chamber are directed through the venturi before being injected back into the mixing chamber.

[0009] In some optional configurations, the recirculation outlet may be disposed along a radially peripheral wall of the milling chamber.

[0010] In some optional configurations, the grinding nozzle may be oriented to inject the stream of mixed particles and milling gas into the mixing chamber tangentially along the vortex.

[0011] In some optional configurations, the recirculation port may be oriented substantially radially to the central axis of the vortex.

[0012] In some optional configurations, at least two or more of the grinding nozzles may be distributed around a radial periphery of the milling chamber.

[0013] In some optional configurations, at least two or more of the recirculation outlets may be distributed around the radial periphery of the milling chamber.

[0014] In some optional configurations, the product injection nozzle is disposed in a lid covering the milling chamber.

[0015] In some optional configurations, two or more of the product injection nozzles may be oriented to inject particles into the milling chamber radially inwardly from the radial periphery of the milling chamber.

[0016] In some optional configurations, a milling gas injector port may be disposed on the radial periphery of the milling chamber and configured to inject milling gas into the milling chamber. The grinding nozzle may be coupled to the milling gas injector port.

[0017] In some optional configurations, the mixed recirculated milling gas and partially ground particles and milling gas may be accelerated through a venturi to further mill the partially ground particles.

[0018] In some optional configurations, the milling chamber may have a peripheral wall radially outward from a centerline of the vortex. The product injection nozzle may inject the stream of particles into the milling chamber at a location between the peripheral wall and the centerline of the vortex.

[0019] In some optional configurations, the grinding nozzle may inject the milling gas and partially ground particles along the peripheral wall.

[0020] In some optional configurations, a portion of the milling gas mixed with partially groundparticles may be recirculated to two or more of the grinding nozzles.

[0021] In some configurations and methods, the spiral jet mill and / or method of the present invention may provide faster and / or more efficient milling of small particles, such as TiO2 pigment particles, providing up to 50% or more energy savings relative to conventional jet mills. Other advantages, uses, and / or characteristics will become apparent upon review of the following detailed description and the drawings.BRIEF DESCRIPTION OF THE DRAWINGS

[0022] FIG. 1 is a side cross-sectional view of a conventional steam jet mill for grinding titanium dioxide particles;

[0023] FIG. 2 is a top cross-sectional view of the conventional steam jet mill of FIG. 1;

[0024] FIG. 3 Is a schematic top view of a jet mill according to a non-limiting example embodiment of the present invention;

[0025] FIG. 4 is an axial cross-sectional view of one of the grinding nozzles of the jet mill of FIG. 3; and

[0026] FIG. 5 is a top cross-sectional view of the jet mill of FIG. 3.DETAILED DESCRIPTION

[0027] The following description is meant to describe what is shown in the drawings and / or to various contemplated embodiments shown and / or related to what is shown in the drawings. The embodiment(s) shown in the drawings are non-limiting examples and do not themselves define the invention. Any of the features shown and / or described in relation to one embodiment may be combined with any one or more features shown and / or described relative to another embodiment, and any feature shown in one of the embodiments may be omitted in other embodiments. Any dimensions shown in the drawings are exemplary only and are not intended to limit the scope of the invention. Use of the term "a" and "an" is used as an open-ended inclusive expression that is not to be limited to only one of such feature but may include more than one of such feature unless otherwise stated.

[0028] A jet mill for grinding particles, such as titanium dioxide (TiO2) pigment particles, according to some non-limiting aspects of the invention may be configured to recirculate at least some partially ground pigment particles from a milling chamber inside the jet mill through one or moregrinding nozzles and back into the milling chamber. By recirculating partially ground particles through the grinding nozzles, the jet mill may in some configurations reduce the amount of energy needed to grind the particles to a desired size and / or reduce the amount of time required to grind the particles to the desired size. For example, in configurations where the jet mill uses steam to grind the particles ("micronizing"), it is believed that recirculating partially ground particles through the grinding nozzles could reduce the amount of steam needed to grind the particles to the desire size by about 25% to 50%, which would provide a significant energy savings and reduce the environmental impact of using the jet mill. In some arrangements, the steam energy input savings could be more than 50% or less than 25%, depending on the specific configuration and operating parameters of a given jet mill.

[0029] Turning now to the drawings, FIGS. 3 and 5 illustrate a jet mill 10 according to a non-limiting example implementing the invention, and FIG. 4 illustrates a grinding nozzle 18 according to a nonlimiting example that is incorporated into the jet mill 10. The jet mill 10 has a milling chamber 12 formed inside a main body. In this example, the main body is generally cylindrical in shape with a generally cylindrical sidewall and a top side and a bottom side that, together, enclose the milling chamber 12. However, the main body may have other shapes. The milling chamber 12 forms a hollow chamber inside the main body that is configured to allow a vortex of gases and / or particles to swirl around an axis while particles and steam and / or other gases are injected at high velocity into the milling chamber. In this example, the milling chamber 12 is generally cylindrical in shape corresponding generally to the shape of the main body and has a cylindrical peripheral side wall 34 that is radially spaced from and surrounds a central vertical axis of the milling chamber, a bottom wall, and a lid 36 that covers a top opening into the milling chamber. However, the milling chamber 12 could have other non-circular shapes, such as oval or polyhedron.

[0030] At least one product injection nozzle 14 is provided to inject particulate product, such as TiO2 pigment particles, into the milling chamber 12 during a milling process. In this example, the jet mill 10 includes two product injection nozzles 14. Each product injection nozzle 14 is preferably configured to inject the particles to be milled along a vortex formed in the milling chamber 12. In this example, each nozzle 14 extends through the lid 36 at an oblique downward angle and is aligned offset from the centerline of the milling chamber 12 and aligned generally tangentially to a radius from the centerline so that the flow of injected particles is coincident with the counterclockwise (as seen from the top view) direction and flow of the vortex formed inside the milling chamber. In other arrangements, the product injection nozzles 14 could be arranged differently and / or there could be only one or more than two product injection nozzles. The product injection nozzles 14 are operatively couple to one or more supplies of the particles that are to be milled in the jet mill 10.The supply of particles may be located remote from the jet mill and may be operatively coupled to the injection nozzles 14, for example, by a conduit attached to exterior flanges of the injection nozzles.

[0031] At least one primary exhaust port 16 is configured to exhaust fully ground particles out of the milling chamber 12 when they have been milled down to a desired size. In this example, the exhaust port 16 is disposed in the lid 36 and extends upwardly near a central axis of the vortex, preferably aligned with the central axis of the milling chamber 12. In this configuration, as the particles are ground to smaller sizes, they will slowly migrate radially inwardly in the vortex due to their smaller mass and eventually be ejected out through the exhaust port 16 along with the gases and / or milling steam. However, other configurations for the exhaust port, such as extending downwardly from the bottom wall of the milling chamber 12 or some other configuration could be used.

[0032] At least one grinding nozzle 18 is configured to inject milling gas (also called grinding gas), such as milling steam, into the milling chamber 12 to mill the particles inside the milling chamber to smaller sizes. In this example, the jet mill includes four grinding nozzles 18; however, fewer than four or more than four grinding nozzles could be used. Unlike previous jet mills, the grinding nozzle 18 is configured not only to inject the milling gas, but to also inject particles to be ground at the same time with the milling gas. In this way, the grinding nozzle 18 can grind the particles both as they flow through the grinding nozzle itself and as they impact other particles inside the milling chamber 12. Thus, injecting both milling gas (e.g., steam) and particles (e.g., TiO2 pigment particles) mixed together through the grinding nozzle 18, the jet mill provides additional milling points over conventional jet mills, which allows the particles to be milled faster than in conventional jet mills. As best seen in FIG. 4, the grinding nozzle 18 has a first entry port 22 for receiving milling gas from a pressurized source of the milling gas and a second entry port 24 for receiving a flow of particles to be ground and / or re-ground. The first and second entry ports 22 and 24 direct their respective streams of milling gas and particles into a mixing chamber 20 inside a main body of the grinding nozzle 18 where the milling gas and particles mix together. An injector port 26 is disposed downstream from the mixing chamber 20 and arranged to inject the mixed stream of milling gas and particles from the mixing chamber 20 into the milling chamber 12. The injector port 26 is coupled to the main body of the jet mill and configured to inject a stream of mixed particles and milling gas from the mixing chamber 20 into the milling chamber 12. In this example, the injector port 26 couples to an exterior flange on the outer side of the main body and an injection port extends through the main body and the peripheral wall of the milling chamber 12. However, in other arrangements, the injector port 26 may itself extend through the main body of the jet mill or be otherwise configured to inject themixed stream of milling gas and particles into the milling chamber 12. In other embodiments, the recirculated particles and the (clean) milling gas may be mixed at a location upstream of the grinding nozzle 18 rather than directly in the grinding nozzle and subsequently conducted through the grinding nozzle 18.

[0033] Preferably, at least one or more of the grinding nozzles 18 is aligned to inject the stream of mixed milling gas and particles tangential to the central axis of the milling chamber in the direction of the vortex in order to create and / or help maintain the vortex flow of gases and particles inside the milling chamber 12 during operation. In this example, each of the four grinding nozzles 18 is aligned to inject its respective stream tangentially in a counterclockwise direction (as seen from the top) to create a counterclockwise vortex inside the milling chamber, and the four grinding nozzles are angularly spaced approximately equally around the outer periphery of the main body and the peripheral wall 34 at approximately 90°. However, any one or more of the grinding nozzles 18 may be oriented differently and / or spaced differently. The action of injecting the mixed stream of milling gas and particles together through the grinding nozzle 18 provides at least some additional milling of the particles at least from the collisions between the injected particles and particles already swirling in the vortex. However, to provide even more milling action inside the grinding nozzle 18, a venturi 32 may optionally be disposed between the mixing chamber 20 and the injector port 26 so that the particles in the milling gas flowing from the mixing chamber 20 to the injector port 26 experience a pressure drop at the venturi to pull apart agglomerated particles and / or hit each other at an increased velocity to break apart agglomerated particles or otherwise provide milling action. Thus, the additional grinding / milling action obtained with the grinding nozzles can be even further improved if the grinding nozzles are designed like the product injection nozzles to have a venturi form. In this way, TiO2 pigment particles can be sucked out of the grinding chamber via a suction line via the resulting negative pressure, and then accelerated by means of the injector (grinding nozzle) back into the grinding chamber.

[0034] The grinding nozzles 18 may be coupled to the milling chamber 12 with any convenient mechanism. In this example, milling gas injector ports 38 are disposed on the outer radial surface of the peripheral wall 34 that provide through bores through the peripheral wall into the grinding area inside the milling chamber 12. Each milling gas injector port 38 includes a flange on the radially exterior side of the peripheral wall 34 to which a complementary flange on the injector port 26 of the grinding nozzle can be attached, for example with bolts and / or welds. In other examples, the injector port 26 of the grinding nozzle may project at least partially through the peripheral wall 34 to deliver the stream of grinding gas (with or without the recirculated particles) directly into the milling chamber 12. Other configurations for operatively coupling the grinding nozzles 18 to the millingchamber 12 so as to deliver the stream of grinding gas and optionally recirculated particles may be used.

[0035] At least one recirculation outlet port 28 is configured to collect milling gas and partially ground particles from inside the milling chamber 12 for recirculation to the at least one grinding nozzle 18. In this example, the jet mill 10 has four recirculation outlet ports 28, corresponding the number of grinding nozzles 18. However, other configurations are possible for recirculating milling gas and partially ground particles from the milling chamber 12 back to one or more of the grinding nozzles 18. Each recirculation outlet port 28 provides an opening through the peripheral wall 34 and side wall of the main body of the jet mill. In this example, each recirculation outlet port 28 is aligned radially with the central axis of the milling chamber 12 so as to collect milling gas and particles traveling along the peripheral wall of the milling chamber. In other arrangements, any one or more of the recirculation outlet ports may be aligned non-radially to the central axis. For example, any one or more of the recirculation outlet ports 28 may be aligned tangentially, for example in the opposite clockwise or counterclockwise direction as the grinding nozzles 18.

[0036] A conduit 30 operatively couples each recirculation outlet port 28 with the second entry port 24 of a respective one of the grinding nozzles 18 so as to direct and recirculate mixed milling gas and partially ground particles from the recirculation outlet port to the second entry port. In this example, each conduit 30 is a hose or pipe with opposite ends coupled directly to the respective recirculation outlet port 28 second entry port 24. However, in other arrangements for collecting and / or conveying the mixed milling gas and partially ground particles from the recirculation outlet ports 28 to the grinding nozzles 18, such as with a manifold or other type of conveyance conduit.

[0037] In one nonlimiting example method of using a jet mill, such as the jet mill 10 described above, according to some aspects of the invention, a stream of particles to be milled to smaller particle sizes (unmilled particles) is injected into the milling chamber through one or more product injection nozzles 14. Typically, the particles are unground / unmilled particles, but it is possible that the particles may have been milled previously and simply require additional milling / grinding. For ease of reference, the particles being injected from the product injection nozzles 14 may be referred as unmilled particles even if they have been previously milled at some other time / place, with the understanding that the term "unmilled" as used here refers to the fact that they have not yet been milled in this particular jet mill at the time of this process. The product injection nozzles 14 may be aligned so as to inject the particles in the direction of a vortex formed inside the milling chamber 12. The product injection nozzles 14 may be aligned to inject the milling gas in the direction of the vortex so as to cause and / or help form or maintain the vortex swirling around inside the milling chamber. Milling gas is also injected into the milling chamber through one or more grinding nozzles18. The milling gas may be injected simultaneously with the product from the product injection nozzles 14 so that the injected particles and gases swirl around the milling chamber 12 in a vortex. As the particles swirl around the vortex in the milling chamber 12, the particles are milled to smaller sizes. Lighter and smaller fully milled particles tend to migrate centrally and up and out the exhaust port 15 with exhausted milling gas, whereas larger, heavier particles that are only partially milled are more likely to travel near the radially outer perimeter of the milling chamber 12. As the partially milled particles travel around the outer perimeter of the milling chamber, at least some of the partially milled particles enter into the recirculation outlet ports 28, by suction from the grinding nozzles 18 and / or by centrifugal force and / or positive pressure inside the milling chamber and are directed via the respective conduit 30 to the second entry port 24 of the respective grinding nozzle 18. The recirculated partially ground particles then mix with the stream of grinding gas entering the first entry port 22 in the mixing chamber 20 and are re-injected into the milling chamber 12 through the injector port 26 to collide with other particles already swirling around inside the milling chamber, thereby providing additional grinding collisions. If the grinding nozzle includes the venturi 32, the particles are further ground by passing through the venturi. Eventually, as the partially ground particles are ground to smaller and smaller sizes until they are fully ground particles, they will also migrate radially inwardly and out the exhaust port 16. In this manner, the particles being ground in the jet mill 10 are subject to more milling action per unit of input milling energy due to the recirculation created by the jet mill 10 than conventional jet mills that do not include a recirculation system.

[0038] The foregoing detailed description and the embodiment(s) shown in the drawings are meant purely as examples and are not intended as limitations of the invention. As previously mentioned, the invention may include any one or more the various features described and / or shown herein in any workable combination, including fewer and / or more than all the features shown and / or described relative a particular embodiment and / or different combinations of features than shown and / or described relative a particular embodiment. As such, it is the following claims that are intended to define the invention and not the foregoing description of the example embodiment(s).

Claims

CLAIMS1. A jet mill (10) comprising: a milling chamber (12); a product injection nozzle (14) configured to inject particles to be ground into the milling chamber; a primary exhaust port (16) configured to exhaust ground particles from inside the milling chamber; a grinding nozzle (18) configured to inject milling gas into the milling chamber; a recirculation outlet port (28) configured to collect partially ground particles from inside the milling chamber; and a conduit (30) operatively coupling the recirculation outlet port to the grinding nozzle, wherein partially ground particles from inside the milling chamber can be recirculated through the recirculation outlet port and the conduit to the grinding nozzle and from the grinding nozzle back into the grinding chamber in a stream of mixed particles and milling gas.

2. The jet mill of claim 1, wherein the grinding nozzle (18) comprises: a mixing chamber (20); a first entry port (22) configured to direct a stream of milling gas into the mixing chamber; a second entry port (24) configured to direct a stream of particles into the mixing chamber; and an injector port (26) configured to direct the stream of mixed particles and milling gas from the mixing chamber into the milling chamber, wherein the conduit (30) operatively couples the recirculation outlet port to the second entry port on the grinding nozzle.

3. The jet mill of claim 2 any one of the previous claims, wherein the grinding nozzle comprises a venturi (32) configured to receive the stream of mixed particles and milling gas.

4. The jet mill of claim 1 or any one of the previous claims, wherein the recirculation outlet(28) is disposed along a radially peripheral wall (34) of the milling chamber.

5. The jet mill of claim 1 or any one of the previous claims, wherein the grinding nozzle (18) is oriented to inject the stream of mixed particles and milling gas into the mixing chamber (12) tangentially along a vortex.

6. The jet mill of claim 1 or any one of the previous claims, wherein the recirculation port (28) is oriented substantially radially to the central axis of the vortex.

7. The jet mill of claim 1 or any one of the previous claims, comprising a plurality of the grinding nozzles (18) distributed around a radial periphery of the milling chamber (12).

8. The jet mill of claim 1 or any one of the previous claims, comprising a plurality of the recirculation outlets (28) distributed around a radial periphery of the milling chamber (12).

9. The jet mill of claim 1 or any one of the previous claims, wherein the product injection nozzle (14) is disposed in a lid (36) covering the milling chamber (12).

10. The jet mill of claim 1 or any one of the previous claims, comprising a plurality of the product injection nozzles (14) oriented to inject particles into the milling chamber (12) radially inwardly from a radial periphery of the milling chamber.

11. The jet mill of claim 1 or any one of the previous claims, comprising a milling gas injector port (38) disposed on a radial periphery of the milling chamber and configured to inject milling gas into the milling chamber (12), wherein the grinding nozzle (18) is coupled to the milling gas injector port.

12. A method of grinding particles in a jet mill (10) comprising a milling chamber (12), a product injection nozzle (14), and a grinding nozzle (18), the method comprising:injecting a stream of particles into the milling chamber (12) through the product injection nozzle (14); injecting milling gas into the milling chamber (12) through the grinding nozzle (18); recirculating a portion of the milling gas mixed with partially milled particles from inside the milling chamber (12) to the grinding nozzle (18); mixing the recirculated milling gas and partially milled particles with the milling gas; and injecting the mixed recirculated milling gas and partially milled particles and milling gas into the milling chamber (12) through the grinding nozzle (18).

13. The method of claim 12, wherein the step of injecting the mixed recirculated milling gas and partially ground particles and milling gas comprises accelerating the mixed recirculated milling gas and partially ground particles and milling gas through a venturi (32) to further mill the partially ground particles.

14. The method of any one of claims 12-13, wherein the milling chamber (12) has a peripheral wall (34) radially outward from a centerline of the vortex, and the product injection nozzle (14) injects the stream of particles into the milling chamber at a location between the peripheral wall and the centerline of the vortex.

15. The method of any one of claims 12-14, wherein the milling chamber (12) has a peripheral wall (34) radially outward from a centerline of the vortex, and the grinding nozzle (18) injects the milling gas and partially ground particles along the peripheral wall, and optionally, wherein the jet mill comprises a plurality of the grinding nozzles (18), wherein the step of recirculating comprises recirculating a portion of the milling gas mixed with partially ground particles to at least some, and preferably each of the plurality of grinding nozzles (18).

Images