Stirring mill

Description

[0001] The invention relates to a stirred mill according to the preamble of claim 1.

[0002] The invention refers to European Patent EP 1 992 412 A1, which discloses such a stirred mill. Stirred mills are used to disperse suspensions, i.e., solids in liquids. This is used, for example, in the production of adhesives, printing inks, cosmetics, or pharmaceuticals. For this purpose, the material to be ground is fed via a feed channel into a grinding chamber of the stirred mill, which is formed between the outer wall of a rotor and a container wall. Together with grinding aids, such as ceramic spheres (hereinafter also referred to as grinding media), and with the aid of tools arranged on the rotor and / or the container wall, the material is comminuted. The stirring action disperses agglomerates and breaks down crystal structures. The original particle size can be reduced from 100–500 µm to less than 3 µm.The finished product is then directed via a material discharge channel through a grinding media separation device, in particular in the form of a protective screen, to a discharge pipe. The rotor forms a kind of rotating cage around the separation device.

[0003] In such stirred mills, the grinding media should ideally be located only in the grinding chamber between the rotor and the container wall. The grinding media are accelerated and concentrated outwards by the tools attached to the rotor, which extend close to the container wall, and by the resulting centrifugal forces. If tools are arranged on the container wall, they extend towards the rotor. These tools, if any, are preferably offset from those attached to the rotor so that they can pass between the wall-mounted tools as the rotor rotates. The material being ground flows through this dense packing of grinding media in the direction of the rotor axis. Concentrating the grinding media in the grinding chamber largely prevents them from entering the discharge channel.In addition to concentrating grinding media in the annular outer grinding chamber, devices are provided in the grinding material discharge channel to prevent the grinding media from flowing through to the separation device.

[0004] The material discharge channel, which is typically located within the rotor and bounded by the rotor and the inner stator, should ideally contain no or very few grinding media. Nevertheless, the accumulation of grinding media on the separating device can repeatedly lead to increased pressure in the agitator mill, potentially reducing the product flow or even triggering automatic shutdown to prevent damage to the mill. State-of-the-art agitator mills may incorporate scrapers on the separating device to keep the grinding media away. However, this may be undesirable when dispersing thermally sensitive substances, particularly explosives or other hazardous materials, as the scrapers can generate heat within the material being ground.

[0005] DE 40 29 139 A1 and EP 0 504 836 A1 describe stirred mills with a grinding container that includes a grinding chamber which can be filled at least partially with grinding media and material to be ground.

[0006] The invention is based on the objective of preventing the accumulation of grinding media on the grinding media separation device due to drag forces and the associated clogging of the separation device.

[0007] This problem is solved according to the invention by the features of claim 1. The core of the invention consists in effectively preventing the accumulation of grinding media on the separation device by reducing the distance between the rotor and the separation device. This can be achieved by increasing the diameter of the separation device, and in particular the screen diameter. Specifically, the distance between the rotor and the separation device can be selected based on the diameter of the grinding media, instead of solely based on the size of the agitator mill. Furthermore, a maximum value for the distance, independent of the grinding media size, can be set. This measure reduces the accumulation of grinding media carried onto the separation device by drag forces. Thus, both a continuous product flow and a rapid reduction of coarse particles in the product to be dispersed are ensured.

[0008] In particular, the problem is solved by a stirred mill with the following features. The stirred mill has a grinding container and a grinding chamber bounded by a container wall, and an agitator rotatable about the central longitudinal axis with a rotor having an inner diameter d351. Tools extending towards the container wall are attached to the rotor. Furthermore, second tools extending towards the rotor can be arranged on the container wall. The stirred mill also has an inner stator located inside the rotor. A discharge channel for the ground material is formed between the rotor and an outer wall of the inner stator, through which the ground material is conveyed to the separating device and then to a discharge line. The grinding chamber is at least partially filled with grinding media with a diameter c. A separating device with a diameter d30 is arranged above the inner stator.The difference between the inner diameter of the rotor and the diameter of the separating device determines the size of the distance s between the rotor and the separating device. According to the invention, this distance s is selected depending on the diameter c of the grinding media, and s = 0.5 · (d351 - d30) ≤ 5 · c. In a further preferred embodiment, the distance s can be s ≤ 3 · c, depending on the diameter c of the grinding media. Furthermore, according to one embodiment of the invention, the distance s should not exceed 2 mm, regardless of the size of the grinding media.

[0009] Preferably, the tools attached to the rotor leave only a small gap to the container wall 9, wherein the gap has a gap width b, for which, in relation to the diameter c of the grinding media, the following applies: 4c ≤ b ≤ 6c, where the minimum gap width b is: 1.0 mm ≤ b ≤ 2.0 mm.

[0010] The inner stator is equipped with scraping tools extending towards the rotor. These scraping tools overlap each other in the direction of the central longitudinal axis and are arranged on the inner stator in such a way that, when the rotor is driven, they exert a pulse on the grinding media directed in the flow direction.

[0011] A gap is preferably formed between the scraping tools and the rotor, for whose gap width e in relation to the diameter c of the grinding media the following applies: 4c ≤ e ≤ 6c, wherein for a minimum gap width e the following applies: 1.0 mm ≤ c ≤ 2.0 mm.

[0012] Furthermore, grinding media return channels are provided in the agitator for returning the grinding media from the area of ​​the separation device to the grinding chamber.

[0013] The diameter c of the grinding media is preferably c ≤ 0.65 mm and preferably 0.02 mm ≤ c ≤ 0.3 mm.

[0014] Further features, advantages, and details will become apparent from the following description of the invention with reference to the drawing. It shows Fig. 1 : a section of a stirred mill according to an embodiment of the invention as a vertical longitudinal section

[0015] The in Fig. 1The illustrated stirred mill has, in the usual manner, a grinding container 2 with an internal grinding chamber 8. The grinding chamber 8 is at least partially filled with grinding media 43 (not shown) with a diameter c. The stirred mill also has an internal stator 22 and a rotor 35 with a diameter d351, which is rotatable about a central longitudinal axis 19. A material discharge channel 47 is formed between an outer wall 23 of the stator 22 and the rotor 35. Tools 38 are attached to the rotor 35 and project into the grinding chamber 8. Furthermore, a second set of tools 74 are arranged on the container wall 9, offset from the tools attached to the rotor 35. Above the stator 22, a protective screen 30 is located as a separating device; this screen is rotationally symmetrical about the central longitudinal axis 19. The protective screen 30 prevents the flow of grinding media 43 into a downstream drain line 31, which is located inside the inner stator 22.A distance s is formed between the protective screen 30 and the rotor 35. The distance s can be expressed as . s = 0 , 5 ⋅ d 351 − d 30 can be described and can in particular be adjusted by selecting the diameter d30 of the protective screen 30.

[0016] When increasing the dimensions of the agitator mill, the distance s between the protective screen 30 and the rotor 35 was also increased accordingly. However, contrary to expectations, this did not lead to a proportional increase in product flow. Instead, the product flow remained below expectations.

[0017] This is related to the fact that, during operation, due to drag forces and despite a device for retaining the grinding media 43, they can be washed against the protective screen 30. This can lead to a reduction in flow rate or to complete blockage of the screen.

[0018] To effectively prevent this, the distance s between the protective screen 30 and the rotor 35 is reduced. This distance s can be selected independently of the design of the agitator mill, depending on the diameter c of the grinding media. This leads to a higher permeability of the agitator mill, as the risk of grinding media 43 being deposited against the protective screen 30 decreases, resulting in a faster reduction of coarse particles in the product to be dispersed.

[0019] According to the invention, the distance s between the protective screen 30 and the rotor 35 depends on the grinding media diameter c. s ≤ 5 ⋅ c or 0 , 5 ⋅ d 351 − d 30 ≤ 5 ⋅ c .

[0020] In another preferred embodiment, the distance s depends on the grinding media diameter c. s ≤ 3 ⋅ c .

[0021] In another preferred embodiment, the distance s is not greater than 2 mm, regardless of the size of the grinding media.

[0022] Although the previously described embodiment shows a stirred mill with a vertical central longitudinal axis 19, the described designs can also easily be used in a horizontal position or in an intermediate position.

Claims

1. Agitator mill for the treatment of flowable grinding stock, - with a milling container (2) and a milling chamber (8), which is delimited by a container wall (9), and - with a stirring mechanism having a rotor (35) that is rotatable about a central longitudinal axis (19), the said rotor having a diameter of d351, and - wherein tools (38) are attached to the rotor (35), which tools extend in the direction toward the container wall (9), and - with an internal stator (22), which is arranged inside the rotor (35), and - wherein between the rotor (35) and an outer wall (23) of the internal stator (22) a grinding stock discharge channel (47) is formed through which the grinding stock can be guided to a grinding body separating device (30) and then to a discharge line (31), and - wherein the milling chamber (8) is at least partially filled with grinding bodies having a diameter c, and - wherein above the internal stator (22) there is arranged the grinding body separating device (30) with a diameter of d30, and wherein a gap is formed that extends coaxially with respect to the central longitudinal axis (19) and defines a distance s, and wherein the distance s between the rotor (35) and the grinding body separating device (30) is given by the relationship: s = 0.5 ⋅ d 351 − d 30 ≤ 5 ⋅ c , characterized in that scraper tools that extend toward the rotor (35) are arranged on the internal stator (22), and in that the scraper tools arranged on the internal stator (22) overlap one another in the direction of the central longitudinal axis (19) and are arranged on the internal stator (22) in such manner that when the rotor (35) is driven, they exert an impulse directed in the through-flow direction on the grinding bodies.

2. Agitator mill according to claim 1, wherein the distance s between the rotor (35) and the grinding body separating device (30) is given by the relationship: s = 0.5 ⋅ d 351 − d 30 ≤ 3 ⋅ c .

3. Agitator mill according to claim 1 or 2, wherein s ≤ 2 mm.

4. Agitator mill according to claims 1 to 3, wherein the grinding body separating device (30) comprises a cylindrical protective sieve.

5. Agitator mill according to claims 1 to 4, wherein the tools (38) attached to the rotor (35) leave free only a small gap to the container wall (9), the said gap between the tools (38) and the container wall (9) having a gap width b for which, relative to the diameter c of the grinding bodies, the relationship 4c ≤ b ≤ 6c applies, the minimum gap width b being given by 1.0 mm ≤ b ≤ 2.0 mm.

6. Agitator mill according to any of claims 1 to 5, wherein between the scraper tools and the rotor (35) a gap is formed, for the gap width e of which, relative to the diameter c of the grinding bodies, the relationship 4c ≤ e ≤ 6c applies, the minimum gap width e being given by 1.0 mm ≤ c ≤ 2.0 mm.

7. Agitator mill according to any of claims 1 to 6, wherein second tools (74) are arranged on the container wall (22), which extend toward the rotor (35).

8. Agitator mill according to any of claims 1 to 7, wherein in the stirring mechanism (20) grinding body return channels are formed for returning the grinding bodies from the area of the grinding body separating device (30) back into the milling chamber (8).

9. Agitator mill according to any of claims 1 to 8, wherein the diameter c of the grinding bodies is given by: c ≤ 0.65 mm and preferably 0.02 mm ≤ c ≤ 0.3 mm.