METHOD FOR DISPERSING AND MILLING PARTICLES IN A FLUID

Abstract

Described herein is a method and a system for dispersing particles in a fluid. The method includes stirring a fluid including particles in a stirring container using stirring means. During stirring the fluid is recirculated by continuously retrieving an amount of fluid from a retrieving position. Further, the particles are milled, where during each pass through the milling means a size of particles in the retrieved fluid is reduced. The retrieved fluid having passed through the milling means is continuously reintroduced into the stirring container at a reintroduction position. The reintroduced fluid is mixed with the fluid in the stirring container in a mixing region defined by the stirring means, and the retrieving position is determined such that the retrieved fluid includes particles with an average particle size differing from the average particle size of particles in the mixing region.

Claims

1. A method for dispersing and milling particles in a fluid, wherein the method comprises the steps of: introducing the fluid comprising the particles into a stirring container, wherein the particles comprise a starting average particle size, and stirring the fluid in the stirring container using stirring means positioned at a predetermined stirring position within the stirring container, and during stirring: recirculating the fluid comprising the particles by continuously retrieving an amount of fluid comprising particles from a predetermined retrieving position in the stirring container by using retrieving means, milling the particles in the continuously retrieved fluid using milling means, wherein during each pass through the milling means a size of particles in the retrieved fluid is reduced if the particles have a size above a predetermined particle size, and continuously reintroducing the retrieved fluid having passed through the milling means to the stirring container at a predetermined reintroduction position using reintroduction means, wherein the reintroduced fluid comprising the particles with a reduced size is mixed with the fluid in the stirring container in a mixing region defined by the stirring means, and wherein the retrieving position is determined such that the retrieved fluid comprises particles with an average particle size differing from the average particle size of particles in the fluid in the mixing region.

2. The method according to claim 1, wherein the mixing region is determined as a region in an inner volume of the stirring container comprising the highest change rate of an average particle size of particles in the fluid compared with particles in the fluid in other regions of the inner volume of the stirring container.

3. The method according to claim 1, wherein the method comprises determining the retrieving position by determining a retrieving region as a region within an inner volume of the stirring container, wherein the retrieving region is determined such that it comprises a highest average particle size within the stirring container compared to other regions of the stirring container, and wherein the retrieving position is provided within the determined retrieving region.

4. The method according to claim 3, wherein the mixing region and the retrieving region do not overlap.

5. The method according to claim 1, wherein the formation of the mixing region is determined based on the stirring position of the stirring means.

6. The method according to claim 1, wherein the reintroduction position is provided above the stirring position.

7. The method according to claim 6, wherein the reintroduction position is provided in an upper half of the stirring container.

8. The method according to claim 7, wherein the stirring position is provided within an upper half of the stirring container and the retrieving position is provided in a lower half of the stirring container.

9. The method according to claim 7, wherein the stirring position is provided within a lower half of the stirring container and the retrieving position is provided in an upper half of the stirring container.

10. The method according to claim 1, wherein the fluid comprises a binder and the particles comprise a colour pigment.

11. The method according to claim 1, wherein the stirring means comprise an impeller, wherein the step of stirring the fluid content of the stirring container comprises rotating the impeller.

12. The method according to claim 1, wherein the method is a control method and wherein the method comprises controlling a recirculation rate based on the average particle size in the retrieved fluid.

13. The method according to claim 12, wherein the method comprises decreasing the recirculation rate if the average particle size in the retrieved amount of fluid is lower than a predetermined first threshold and/or if a difference between the average particle size in the retrieved amount of fluid and the average particle size in the mixing region is lower than a predetermined second threshold.

14. A system for recirculating and milling particles in a fluid, the system comprising: a stirring container adapted to hold the fluid comprising the particles, and stirring means for stirring the fluid in the stirring container, wherein the stirring means are positioned at a predetermined stirring position within the stirring container, recirculating means comprising: retrieving means adapted to continuously retrieve an amount of fluid comprising particles from a predetermined retrieving position in the stirring container, milling means adapted to mill the particles in the continuously retrieved fluid, wherein during each pass through the milling means a size of the particles in the retrieved fluid is reduced if the particles have a size above a predetermined particle size, and reintroduction means adapted to continuously reintroduce the retrieved fluid having passed through the milling means to the stirring container at a predetermined reintroduction position, wherein the stirring means define a mixing region in the stirring container in which the reintroduced fluid comprising the particles with a reduced size is mixed with the fluid in the stirring container, and wherein the retrieving position is provided such that the retrieved fluid comprises particles with an average particle size differing from the average particle size of particles in the fluid in the mixing region.

15. A method of using recirculation means according to claim 14, the method comprising retrieving means, milling means and reintroduction means with a stirring container comprising stirring means such that a number of passes through the milling means required to reach a predetermined average particle size throughout a fluid within the stirring container is decreased.

16. The method according to claim 6, wherein the reintroduction position is provided at or near a surface of the fluid in the stirring container.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0034] In the Following Drawings:

[0035] FIG. 1 shows schematically and exemplarily an embodiment of a system for dispersing and milling particles in a fluid by recirculation,

[0036] FIG. 2 shows a flowchart exemplarily illustrating an embodiment of a method for dispersing and milling particles in a fluid by recirculation,

[0037] FIG. 3 shows schematically and exemplarily an illustration of processes taking place during an execution of an embodiment of the method for milling particles in a fluid by recirculation, and

[0038] FIG. 4, comprising FIG. 4A to 4E, illustrates schematically and exemplarily an internal fluid flow for an embodiment of the system for milling particles in a fluid by recirculation.

DETAILED DESCRIPTION OF EMBODIMENTS

[0039] FIG. 1 shows schematically and exemplarily an embodiment of a system for dispersing and milling particles in a fluid by recirculation. In this embodiment, the system 100 comprises a stirring container 110 into which a fluid 160 comprising particles 164 can be introduced. Stirring means 120 are provided at a stirring position 121 within the stirring container 110. In this exemplary embodiment, the stirring position 121 is provided within a lower half of the stirring container 110. In this embodiment, the stirring means 120 comprise an impeller, wherein the impeller is rotated and causes turbulent fluid flows in the vicinity of the impeller. In this embodiment, the impeller is a Cowsle disk. In a preferred embodiment the impeller is rotated with a rotation speed of 2 to 3 times the rotational speed used for dispersing the particles before the recirculating and milling is started. Preferably, the speed of the impeller is regulated such that a vortex is generated in the stirring container 110. The turbulent fluid flows in the vicinity of the impeller define a mixing region 161 in which the fluid 160 and fluid that is reintroduced into the stirring container 120 can be mixed. The system 100 further comprises recirculating means comprising retrieving means 130, milling means 140 and reintroduction means 150.

[0040] In this embodiment, the retrieving means 130 refer to a pipe that is connected with a pump 170 for pumping fluid that is retrieved from the stirring container 110 by the retrieving means 130 through the retrieving means 130 to the milling means 140 and further through the reintroduction means 150 back into the stirring container 110. The retrieving means 130 preferably comprise an opening that is configured to use the rotational energy of the fluid flowing into the pipe to facilitate the operation of the pump 170. For instance, the opening of the pipe can ba adapted such that the retrieving means 130 opens in the direction of the fluid flow of the surrounding fluid, for instance, the end of the pipe can be cut in a diagonal manner to form an elliptical opening in the direction of the fluid flow.

[0041] The retrieving means 130 are positioned such that the fluid 160, together with particles 164 within the fluid 160, is retrieved from the stirring container 110 at a stirring position 131. The retrieving position 131 is chosen such that it lies within a region 162 of the stirring container 110 that has in this particular embodiment a cylindrical ring shape. The retrieving region 162 is defined as a region comprising the highest average particle size with respect to other regions within the container 110. The retrieving region 162 has been found for this particular construction of the stirring container 110, for instance, by experiments and/or numerical simulations. For instance, during a numerical simulation the simulated stirring container 110 might be divided into a plurality of equally sized small volumes, wherein for each of the volumes the average particle size over time during the recirculation process can be calculated. The retrieving region 162 is then defined as a region in which the volumes show the highest average particle size of all volumes over the time of the recirculation process. The retrieving means 130 can then be constructed such that the retrieving position 131 is found anywhere within the retrieving region 162.

[0042] After the fluid comprising the particles is retrieved by the retrieving means 130, the fluid flows through the retrieving means 130 to the milling means 140. The milling means can comprise any kind of mill that allows to reduce the size of the particles 164 within the retrieved fluid 160.

[0043] The retrieved fluid 160 comprising the particles 165 with reduced size is then reintroduced into the stirring container 110 by the reintroduction means 150 at a reintroduction position 151. In this embodiment, the reintroduction position 151 is provided near a surface 163 of the fluid 160 within the stirring container 110. In an alternative embodiment the reintroduction position 151 can be provided above the surface of the fluid 160 such that the reintroduced fluid is provided against an inner wall of the stirring container 110 and flows down to the surface of the fluid 160 along the inner wall of the stirring container 110.

[0044] FIG. 1 further shows an exemplary flow of the reintroduced fluid represented by the milled and thus smaller particles 165. After the reintroduction of the fluid comprising the milled particles 165 the fluid circulates at the top of the surface of the fluid in the stirring container in a spiral form, wherein the fluid in the respective spiral is then sucked down in the middle of the spiral into a vortex formed, in this embodiment, along and around a shaft holding the stirring means 120 until the fluid with the smaller particles 165 reaches the stirring means 120. Such a fluid flow is also called a helicoidal stream. In the region of the stirring means 120 the fluid is then subject to an expulsion in a radial area to form substantially a disk volume, wherein this disk volume is then growing with time like a “stack of plates”. This general fluid flow in a stirring container can be subject to certain modifications based on the exact structure and design of the stirring means. For instance, if the stirring means 121 comprises blades linked to the shaft by simple rods, the helicoidal stream takes the form of a cylinder with a diameter greater than the shaft and never touching the shaft. If the blades are welded directly to the shaft the helicoidal stream directly touches the shaft like a coat. However, the general principle of the fluid flow of the reintroduced fluid as described above remains independent of the specific embodiment of the stirring means 120.

[0045] Further, FIG. 1 shows a modification of the above described embodiment. In this modification indicated by the dotted lines an additional retrieving means 132 is provided comprising a valve 133. In this embodiment a small amount of fluid, for instance, 20 to 30 percent of the amount of fluid retrieved by the retrieving means 130 is also retrieved by the additional retrieving means 132 at the bottom of the stirring container 110. The valve 133 is provided to regulate the amount of fluid retrieved by the additional retrieving means 132. Preferably the valve is controlled based on the current pumping performance of the pump 170. For instance, if the viscosity of the fluid is such that the pumping performance is not optimal, the valve can be opened and the additional retrieved fluid following the force of gravity can support the pumping of the fluid. Moreover, it is preferred that if cavitation occurs in the pump 170, for instance, if cavitation noise is detected, the valve 133 is used to provide more retrieved fluid from the additional retrieving means 132 to avoid damage to the pump. This embodiment is especially advantageous in applications comprising high viscosity fluids.

[0046] FIG. 2 shows another embodiment of the system for milling particles in a fluid by recirculation schematically and exemplarily, wherein components that are similar to the above described embodiment of the system shown in FIG. 1 comprise the same reference signs. In the system 200 the stirring means 220 are provided at the stirring position 221, wherein the stirring position 221 in this embodiment is provided in the upper half of the stirring container 110. Accordingly, also the mixing region 261 can be found in this embodiment in the upper half of the stirring container 110. It has been determined that the retrieving region 262 in this embodiment can be found in the lower half of the stirring container 110. Thus, in this embodiment retrieving means 230 are positioned such that the retrieving position 231 is provided within the retrieving region 262. In particular, in this embodiment the retrieving position 231 is provided at the bottom of the stirring container 110.

[0047] With reference to FIGS. 3 and 4, in the following a method for dispersing and milling particles by recirculation in a fluid according to the invention will be schematically and exemplarily explained. In a first step 310 of the method 300 a fluid 160 comprising particles 164 with a starting average particle size is introduced into a stirring container 110 as, for instance, shown in FIG. 1. In a next step 320 the fluid 160 within the stirring container is stirred using stirring means 120 as, for instance, also shown in FIG. 1. During the stirring the fluid 160 is recirculated in step 330.

[0048] The recirculating and the effects of the recirculating will be explained in more detail also with reference to FIG. 4. FIG. 4 shows schematically and exemplarily the effects of the method for dispersing particles in a fluid. In FIG. 4A, the state of the fluid comprising the particles at the beginning of the recirculation process is shown. In this state the particles comprise mainly the same size, i.e. a first particle size. In step 331 an amount of fluid is continuously retrieved from the stirring container from a retrieving position that is shown in FIG. 4A in accordance with the embodiment of the system shown in FIG. 1, i.e. the fluid is retrieved at the left side of the container in the upper half of the container. Since in this state the fluid within the container only comprises particles of the first particle size, the retrieved fluid also only comprises particles of the first particle size. In step 332 the retrieved fluid is provided to milling means that reduce the size of the particles within the fluid. Accordingly, after milling, the retrieved fluid comprises particles with a reduced particle size, i.e. comprising a second particle size. To facilitate the understanding in the schematic process shown in FIG. 4, the second particle size also refers to a predetermined final particle size that should be provided in the end product of the process, wherein for explanatory reasons the particles of the second particle size will pass the mill unchanged. The retrieved fluid comprising the particles with reduced particle size, here with the second particle size, is in step 333 reintroduced into the stirring container near the surface of the fluid within the fluid container, as can be seen in FIG. 4B. It is also shown in FIG. 4B that due to vortexing the fluid comprising the particles with the second particle size flow along the surface of the fluid in the stirring container and then along a vortex around a rod holding the impeller downwards to the impeller without substantially mixing with the fluid within the container comprising particles of the first particle size. Only in a region in which the impeller induces turbulent flow the fluid comprising the particles of the second particle size and the fluid within the container still comprising particles of the first particle size are mixed. Accordingly, as can also be seen in FIG. 4B, in this region an average particle size is reduced very fast due to the continuous provision of particles of the second particle size into this region. Contrary to this, the average particle size in the retrieving region still substantially corresponds to the first particle size and changes only very slowly due to slow transportation processes from the mixing region into the upper regions of the stirring container. Such a slow transportation process can be schematically seen in FIGS. 4C and 4D. Moreover, it is shown in FIGS. 4C and 4D that due to the continuous provision of small particles to the mixing region and the retrieving of the larger particles from the retrieving region the overall average particle size is decreased, wherein the average particle size decreases faster in the mixing region than in the retrieving region until the average particle size in the mixing region mainly corresponds to the second particle size. FIG. 4E shows a state at which a reintroduction process is stopped, since the average particle size of the particles within the retrieved fluid is below a threshold lying only slightly above the second particle size, such that the final product fulfils a predetermined quality criterion.

[0049] Although in the above embodiments a specific configuration showing a specific flow pattern was described, in other embodiments the flow pattern can be different. For instance, also other flow patterns of the fluid and, accordingly, other specific configurations of the stirring container and the stirring means that allow to determine a region with higher average particle sizes can be contemplated. In general, the invention refers to retrieving fluid for milling from a region of a stirring container comprising on average more particles that are still to big, i.e. retrieve fluid with particles having a higher average particle size then other regions of the stirring container, wherein according to this principle the efficiency of the milling process can be increased and a milling product of high quality can be reached in a shorter time.

[0050] Other variations to the disclosed embodiments can be understood and effected by those skilled in the art and practising the planned invention from the study of the drawings, the disclosure and the appendant claims.

[0051] In the claims, the word “comprising” does not exclude other elements or steps, and the indefinite article “a” or “an” does not exclude a plurality.

[0052] Any reference signs in the claims should not be construed as limiting the scope.

[0053] The invention refers to a method and a system for dispersing and milling particles in a fluid. The method comprises stirring a fluid comprising particles in a stirring container using stirring means. During stirring the fluid is recirculated by continuously retrieving an amount of fluid from a retrieving position. Further, the particles are milled, wherein during each pass through the milling means a size of particles in the retrieved fluid is reduced. Moreover, the retrieved fluid having passed through the milling means is continuously reintroduced into the stirring container at a reintroduction position. The reintroduced fluid is mixed with the fluid in the stirring container in a mixing region defined by the stirring means, and the retrieving position is determined such that the retrieved fluid comprises particles with an average particle size differing from the average particle size of particles in the mixing region.