IMMERSION GRINDING APPARATUS

Abstract

 An immersion grinding apparatus, comprising:
- a basket mill (22) comprising a rotor (44) located inside the basket (36)
- at least one rotating stirrer (50) located outside the basket (36), and
- a control unit (60) adapted to set the rotational speeds of the rotor (44) and of said at least one stirrer (50) to different values (n_R′, n_A′, n_Rʺ, n_Aʺ, n_R‴, n_A‴) lower than nominal speed values (n_R, n_A) when the temperatures detected by a temperature sensor (56) exceed different temperature thresholds (Tc', Ts, Tc") lower than a maximum process temperature (T_max).

Description

Field of the Invention

[0001] The present invention relates to an immersion grinding apparatus, which is adapted to perform grinding of powder or granular materials in order to reduce the granulometry of the product and which may be used for the preparation of paints, inks, colouring agents, cosmetic products, etc.

[0002] More specifically, the invention relates to an immersion grinding apparatus comprising a basket mill, adapted to be immersed into a liquid mixture contained in a process container.

Description of the related art

[0003] In order to grind powder or granular products in an immersion grinding apparatus, the product is placed into a process container having an open upper edge, and a basket mill is immersed into the product contained in the process container. Typically, the basket mill includes a basket having an external perforated wall and containing a mass of balls which are set into motion by a pin rotor, which is rotatably actuated by an electric motor. The mass of moving balls within the basket crushes the powder or granular material. The liquid product containing the granular material to be ground enters the basket through an upper opening and is expelled from the basket thanks to the centrifugal action of the balls, which keep on moving thanks to the pin rotor. One or more stirrers are normally arranged outside the basket mill, in order to agitate the liquid within the grinding container and in order to favour the passage of the liquid within the basket mill.

[0004] In such grinding apparatuses, a part of the energy employed to grind and agitate the product is converted into thermal energy, which increases the product temperature. In order to avoid overheating the product, such grinding apparatuses are generally provided with a cooling system, which includes a cooling chamber arranged in the upper part of the basket mill and a cooling interspace arranged in the side wall of the process container. A cooling liquid (usually water) is circulated in the cooling chambers, in order to remove the heat produced by the grinding assembly.

[0005] In such apparatuses, in operation, it is necessary to prevent the product temperature from exceeding a maximum process temperature, beyond which the risk exists of a degradation of the physical properties of the product or of components thereof (resins, pigments, etc.).

[0006] The known apparatuses include a temperature sensor, which arrests operation of the grinding apparatus when the detected temperature exceeds a set threshold value. When the apparatus is stopped because the product temperature exceeds the maximum process temperature, it is necessary to wait for the machine to cool down, before restarting the grinding process. This may require very long times, because the mass of the product contained in the process container exhibits a high thermal inertia. Interrupting the process because the maximum process temperature has been exceeded may also bring about problems concerning the grinding quality, e.g. because some components of the mixture may deposit on the container bottom during the interruption.

Object and Summary of the Invention

[0007] The present invention aims at providing a grinding apparatus which overcomes the problems of the prior art.

[0008] According to the present invention, said object is achieved by a grinding apparatus having the features set forth in Claim 1.

[0009] The grinding apparatus according to the present invention enables setting the rotational speed of the basket mill rotor and of the stirrer to different values, which are lower than the nominal speed values, when the product temperature detected by a temperature sensor exceeds various temperature thresholds which are lower than the maximum process temperature.

[0010] This control system helps stabilizing the product temperature during the grinding process to various equilibrium points, which are lower than the maximum process temperature of the product being mixed. Therefore, the risk of interrupting the grinding process due to a product overheating is substantially reduced.

[0011] Thus, the apparatus according to the present invention enables reducing the process times in comparison with the known solutions, and leads to an improvement of the product quality in terms of granulometry.

Brief Description of the Drawings

[0012] Further features and advantages of the present invention will become apparent in the detailed description that follows, which is given by way of nonlimiting example only, with reference to the annexed drawings, wherein:

• Figure 1 is a perspective view of a grinding apparatus according to the present invention,
• Figure 2 is a side view in partial section showing the grinding apparatus of Figure 1 associated with a process container,
• Figure 3 is a diagram of the control system of the grinding apparatus according to the present invention, and
• Figures 4, 5, 6 and 7 are diagrams illustrating the temperature varying as a function of the process time in different operating conditions.

Detailed description

[0013] With reference to Figure 1, reference number 10 denotes an immersion grinding apparatus according to the present invention.

[0014] Apparatus 10 includes a base 12 resting on the ground, wherefrom a telescopic vertical column 14 extends. An upper support structure 16 is connected to the upper end of column 14. The telescopic column 14 is controlled by a hydraulic unit 18, associated to an electric motor 20. The telescopic column 14 controls the movement of the upper support structure 16 between a lowered operating position and a raised inoperative position.

[0015] The grinding apparatus 10 includes a basket mill 22, adapted to be immersed into a liquid contained in a process container 24 (Figure 2). The basket mill 22 is connected to the upper support structure 16 by a plurality of vertical supports 26. Apparatus 10 includes a closure plate 28, arranged between the upper support structure 16 and the basket mill 22, and adapted to be applied onto an open upper edge 30 of process container 24 (Figure 2).

[0016] The basket mill 22 includes an upper cap 32, which is connected to the lower ends of vertical supports 26. Cap 32 may be hollow and may host a cooling chamber 34 wherein, in operation, a cooling liquid circulates.

[0017] The basket mill 22 includes a basket 36 which is fixed to the top cap 32 and extends downwards from top cap 32. The basket 36 has a closed bottom wall and a perforated side wall 40. Basket 36 contains a mass of balls 42, having a diameter larger than the diameter of the perforations of side wall 40.

[0018] Referring to Figure 2, basket mill 22 moreover comprises a rotor 44 extending inside basket 36 and being at least partially immersed in the mass of balls 42. The rotor 44 may be a pin rotor, and it is fixed to the lower end of a drive shaft 46 extending through an opening 48 of top cap 32. The drive shaft 46 is rotatable around its own longitudinal vertical axis, and it is rotatably driven by a first electric motor 48 carried by the upper support structure 16.

[0019] The grinding apparatus 10 comprises at least one rotating stirrer 50, located outside basket 36. In the illustrated example, apparatus 10 includes two rotating stirrers 50, carried by respective vertical rotating shafts 52 which extend downwards from the upper support structure 16. The shafts 52 of the rotating stirrers 50 are rotatably driven by a second electric motor 54, which is carried by the upper support structure 16.

[0020] Basket mill 32 includes a temperature sensor 56 (Figure 1) carried by one of the vertical supports 26, and arranged in such a way as to be immersed in the liquid during operation.

[0021] Figure 3 schematically shows the temperature control circuit of the grinding apparatus 10. The temperature control circuit includes an electric panel 58, which comprises a control unit 60 (PLC) and two inverters 62, 64 which respectively drive the electric motor 48, driving the rotor of basket mill 22, and the electric motor 54, driving the rotating stirrers 50. A user interface 66 enables setting the characteristic parameters of the grinding process.

[0022] The operation of apparatus 10 is as follows.

[0023] The process container 24, which contains a mixture of liquids and powder or granular materials, is placed at a workstation below the basket mill 22. In order to position the grinding container 24 in the workstation, the telescopic column 14 is extended, in such a way as to move the basket mill 22 above container 24. As shown in Figure 2, the process container 24 may be provided, on the side wall thereof, with a cooling chamber 68 which, when process container 24 is located in the workstation, is connected to a liquid feeding apparatus via a connector 70.

[0024] When the process container 24 which contains the product to be ground is correctly positioned in the workstation, the upper support structure 16 of apparatus 10 is lowered, so that the basket mill 22 is immersed into the fluid product contained in the process container 24. The closure plate 28 is positioned on the upper edge 30 of container 24, in order to close the process volume. Then, the rotor 44 of basket mill 22 and the stirrers 50 are rotatably driven by the respective motors 48, 54. The rotor 44 sets the mass of balls 42 into motion inside basket 40. The moving balls 42 perform a grinding and crushing action on the powder or granular material, and mix the solid material with the liquid base. The liquid product contained in basket 36 is expelled through the external perforated wall 40 of basket 36 thanks to the centrifugal action of balls 42. Within basket 36 a vacuum is generated which sucks the liquid product back into basket 36 through the opening 47 surrounding the drive shaft 46 of rotor 44.

[0025] At the same time, the stirrers 50 produce a whirling movement of the product within grinding container 24, which favours the product turnover within basket 36. In operation, the cooling chamber 34 located within the cap 32 of basket mill 22 is connected to a cooling liquid feeding circuit via hydraulic connectors (Figure 2) arranged on supports 26.

[0026] The energy needed to grind and agitate the product is partially converted into thermal energy, with a consequent increase of the product temperature. A part of the thermal energy fed into the product during the process is removed by the cooling circuits. However, the cooling system associated to the immersion mill 22 and to the grinding container 24 is generally incapable of removing all the thermal energy fed into the product during grinding. Therefore, the product temperature progressively increases during the process.

[0027] The temperature control system controls the rotational speed of the rotor 44 of basket mill 22 and of the stirrers 50 as a function of the temperature detected by sensor 56.

[0028] The temperature control system controls the following parameters:

• T_max - maximum process temperature: it is the maximum temperature value of the product, beyond which the risk exists of a degradation of the physical properties of the product or of components thereof (resins, pigments etc.),
• T_s - threshold temperature: it is the temperature value of the desired thermal equilibrium point during the grinding process,
• R_c - control range: it is the temperature range, centred on threshold temperature T_s, within which the control process takes place in order to stabilize the process temperature around threshold temperature T_s.

[0029] Referring to Figures 4 to 7, once the threshold temperature T_s and the control range R_c have been established, a lower control temperature T_c' and an upper control temperature T_c" are found as follows:

[0030] For the operation of the temperature control system it is necessary to input into control unit 60, via user interface 66, the following operating parameters:

• n_R : nominal rotational speed of the basket mill rotor,
• n_A : nominal speed of the stirrers,
• T_max: maximum process temperature,
• T_s : threshold temperature,
• R_c : control temperature range,
• c_R : speed reduction coefficient of the mill rotor;
• c_A : speed reduction coefficient of the stirrers.

[0031] In operation, when the temperature detected by sensor 56 is lower than the lower control temperature T_c', the rotor of the basket mill and the stirrers are operated at the respective nominal speeds n_R and n_A.

[0032] During the grinding process, the product temperature increases progressively, as a function of time t, as shown in the diagram of Figure 4.

[0033] When the product temperature exceeds the lower control temperature T_c', the control unit 60 reduces the rotational speed of rotor 44 and of stirrers 50 to the values n_R' and n_A', given by the average values between the nominal speed n_A and the nominal speed reduced by the reduction coefficient, according to the following formulae:

[0034] This speed reduction helps stabilize the temperature, as shown in the diagram of Figure 4.

[0035] If, after reducing the rotational speed to the values n_R' and n_A', the product temperature exceeds the threshold temperature T_s, the control unit 60 causes a second speed reduction, by setting the rotational speed of the rotors and of the stirrers to values n_R" e n_A", given by the average value between the former speed and the former speed reduced by the reduction coefficient, as shown in the following formulae:

[0036] The new temperature equilibrium point is shown in the diagram of Figure 5.

[0037] At this point, if the product temperature keeps on increasing, until it exceeds the upper control temperature T_c", the control unit further reduces the rotational speed of rotor 44 and of stirrers 50 to values n_R"' e n_A"', given by the average value between the former speed and the former speed reduced by the reduction coefficient, as shown in the following formulae:

[0038] The product temperature tends to get stabilized, as shown in Figure 6.

[0039] Should the product temperature exceed the value of the maximum process temperature T_max, the control unit 60 reduces the rotational speed of rotor 44 and of stirrers 50 to zero, as shown in Figure 7.

[0040] Therefore, the control system helps preventing the process temperature from exceeding the maximum temperature by progressively reducing temperature to successively decreasing values. The temperature gradient is directly connected to the percentage of speed reduction.

[0041] The control system according to the present invention greatly reduces the need to stop the grinding process and to wait for the temperature to fall below the maximum process temperature. Thus, the grinding apparatus according to the present invention enables achieving shorter process times and a better grinding quality, because it helps avoiding interruptions in the grinding process.

[0042] Of course, without prejudice to the principle of the invention, the implementation details and the embodiments may widely vary from what has been described and shown herein, without departing from the scope of the invention as set forth by the Claims that follow.

Claims

1. An immersion grinding apparatus, comprising:

- an upper support structure (16), movable vertically between a lowered working position and a raised inoperative position,

- a basket mill (22) connected to the upper support structure (16) by vertical supports (26) and comprising: a top cap (32) connected to the upper support structure (16) by means of said vertical supports (26), a basket (36) attached to the top cap (32) and extending downward from the top cap (32), a mass of balls (42) contained in the basket (36), a rotor (44) located inside the basket (36) and connected to the lower end of a vertical drive shaft (46) which extends through an opening (47) of the top cap (32),

- at least one rotating stirrer (50) located outside the basket (36),

- a first electric motor (48) which rotates the rotor (44) of the basket mill (22),

- a second electric motor (54) which rotates said at least one stirrer (50),

- a temperature sensor (56) arranged to detect the temperature of the product during grinding, and

- a control unit (60) which controls the first and second electric motor (48, 54) as a function of signals provided by said temperature sensor (56) and interrupts the operation of the apparatus when the temperature detected by said sensor temperature (56) exceeds a maximum process temperature (T_max),

characterized in that the control unit (60) is configured to set the rotation speeds of the rotor (44) and of said at least one stirrer (50) to different values (n_R', n_A', n_R", n_A", n_R"', n_A'") lower than nominal speed values (n_R, n_A) when the temperatures detected by said temperature sensor (56) exceed different temperature thresholds (Tc', Ts, Tc") lower than maximum process temperature (T_max).

2. The apparatus according to claim 1, wherein said control unit (60) is configured to store a threshold temperature (Ts), a lower control temperature (Tc') lower than the threshold temperature (Ts) and an upper control temperature (Tc") higher than the threshold temperature and lower than the maximum process temperature (T_max), and is configured to set a first reduced speed (n_R', n_A') of the rotor and said at least one agitator when the temperature detected by said temperature sensor (56) is comprised between the lower control temperature (Tc') and the threshold temperature (Ts), a second reduced speed (n_R", n_A") when the temperature detected by said temperature sensor (56) is comprised between said threshold temperature (Ts) and said upper control temperature (Tc") and a third reduced speed (n_R"', n_A"') when the temperature detected by said temperature sensor (56) is between the upper control temperature (Tc") and the maximum process temperature (T_max).

3. The apparatus according to claim 2, wherein the control unit (60) is configured to calculate each of said reduced speeds as an arithmetic average between the previous speeds and the previous speeds reduced by a reduction coefficient (C_R, C_A).

Drawing