WET STIRRED BALL MILL

Abstract: A vertical wet stirred ball mill (21) is provided according to this invention, wherein a slurry raw material fed from the top of the mill body is stirred together with a grinding medium in said mill body under pressure and the material is gradually exhausted from the bottom of said mill body as it is milled. A spring-loaded conical separation valve (22) is fitted at an exhaust port of said body. The separation valve (22) retracts when the pressure within the mill body is increased due to plugging, thereby the annular clearance around the separation valve is enlarged to increase the amount of exhaust, leading to elimination of the plugging.

Description: The present invention relates to a wet stirred ball mill which effects grinding of a slurry raw material by stirring it together with grinding media such as steel ball, ceramic ball, glass bead, etc. into a fine particle in the mill body, particularly to a wet stirred ball mill, wherein a slurry raw material is fed into the mill body of the closed type mill and is continuously exhausted as it is milled with the grinding medium under pressure. This type of wet stirred ball mills are well known and available in many variations. The most popular one is of a type wherein a slurry raw material is fed from the bottom of the mill body, milled with a grinding medium by stirring under pressure, from which the medium is then separated, to exhaust the material by allowing it to overflow together with the waste media from the top of the mill body. Now referring to the feeding mode, while there is Attritor mill Model S, manufactured by Union Process Inc. of Akron, Ohio, in which a slurry raw material is fed from the top of the mill body and exhausted from the bottom thereof, this type of mill actually is that of top-open and resort to batchwise system in which stirring is conducted under application of no pressure and not a pressurizing continuous mill. In either type of the mills described above, the resulting slurry product to be exhausted from the mill must be separated from the medium. For such a purpose, a screen is often provided at the exhaust port through which the slurry product is allowed to be exhausted. However, plugging is liable to occur particularly in the pressurizing mill. A method of separating the medium other than using a screen is also known, in which a rotor fit in the exhaust port is adopted to be driven for rotation to allow the slurry product to be exhausted through the annular clearance formed around the rotor and to separate, on the clearance, the grinding medium having a larger size than the aperture of the clearance. In such a constitution, while plugging in the clearance may not easily be caused, the peripheral edge of the exhaust port and the rotor may easily be subject to abrasion. Thus, even the medium which is still usable may also be exhausted easily. Moreover, an inclusion of the medium in the clearance may be caused during the rotation of the rotor, and if it should happen and if the medium is of ceramic, said medium will be broken by said inclusion. Also, there has been proposed a device in which the exhaust port is adopted to be vibrated for preventing plugging thereof. The one as disclosed in Japanese Provisional Patent Publication No. 216747/1986 may be given as an example of such type of mill, in which a conical (cone-shaped) valve is fitted at the exhaust port with sufficient clearance and designed to be vibrated to prevent plugging of the exhaust port around said valve. However, in this device the grinding medium is exhausted together with the slurry product continuously, and the medium is separated outside the mill body. Accordingly, the valve does not have a function to separate the grinding medium. It is possible to impart a function of separating the grinding medium to the valve by narrowing the clearance of the exhaust port. In such an instance, however, it is not possible to eliminate the plugging by enlarging the clearance only when plugging occurred, since the valve vibrates irrespective of the presence or absence of plugging around the valve, to cause increase and decrease of the clearance repeatedly. Moreover, in such a mill, the gain and loss of the clearance caused by the vibration is constant, thereby plugging may not be eliminated in some cases depending. on the degree of plugging. Although it is possible to overcome the problem of plugging by making the gain and loss of clearance larger,a wider clearance will be provided, when it is not desired, to increase the amount of exhaust. Accordingly, it becomes difficult to keep the liquid level constant, and also the dwell time that the slurry raw material dwells in the mill body will be reduced, making it impossible to obtain a product with a desired particle size through one pass. Moreover, the medium which is still effective will be easily exhausted. GB-A-2030471 describes a wet ball mill including a separator with a movable valve which moves in response to the pressure of the mill. The valve is movable periodically by a given amount. That is to say the gap size is changed by a fixed amount by a vibrating mechanism irrespective of the pressure in the mill. Thus if a large particle becomes caught in the gap clogging can occur. DE-A-2026733 describes a wet ball mill having a screen with a shutter which can move up and down in response to the pressure of the mill. The shutter moves to enlarge the screen if the pressure within the mill is increased due to clogging. This mill also can become clogged by large particles. An object of the present invention is to prevent plugging in an exhaust port which is one of the technical subjects to be improved in a wet stirred ball mill in which a grinding medium is separated at the exhaust port. According to the present invention there is provided a wet stirred ball mill comprising a closed type mill body provided with a stirring device in which a slurry raw material to be fed continuously is stirred in admixture with a grinding medium under pressure and after said medium is separated, said material is exhausted, characterised in that a pressure control valve device is provided therein comprising a generally conical separation valve retractably fitted to an exhaust port and an operation means to allow advancing and retracting of said separation valve by means of the pressure within the mill body, whereby the separation valve retracts corresponding to said pressure to provide a wider clearance, when plugging occurs in the clearance and thereby the pressure within the mill body is increased. By use of this mill, the grains of the medium (the medium usually has a size of 5 to 100 times as large as the clearance) having a size larger than the aperture of the clearance is separated on the clearance (see Fig. 8). When plugging becomes serious, the the pressure within the mill body will be increased with the decrease in the amount of exhaust, with which the separation valve retracts to enlarge the clearance. Clearance will be enlarged until the plugging is eliminated, and rapidly reduced with the sudden decrease of pressure after elimination of the plugging. Each time when plugging occurs, the above operations are repeated successively (see Fig. 9). It should be noted that the aperture of the clearance when plugging occurs is not constant, and it may be great or small depending on the occasion. Also the interval between the plugging occurrence may not necessarily be constant. As the stirring device for stirring the slurry raw material together with the grinding medium in the mill body, one having a disc fixed on the rotary shaft or having pins protruding radially therefrom is used, to conduct predominantly stirring in the direction orthogonal with the rotary shaft. The operation means to be employed in the above mill may be exemplified by a device comprising a spring device for supporting the separation valve such as helical spring, air spring, etc., a device for detecting the pressure within the mill body, and an operation mechanism which allows the separation valve to advance or retract corresponding to the pressure detected by said device for detecting pressure within the mill body. The separation valve may preferably be disposed at a portion closer to the peripheral wall of the mill body apart from the rotary shaft of the stirring device. If said valve is provided in alignment with the rotary shaft, stirring effect to be obtained may be smaller and the grinding medium is liable to be retained at the exhaust port. On the contrary, greater stirring effect can be obtained by providing said valve at said peripheral portion to enhance smooth flow of the raw material, thereby to allow for good passage of the liquid. In such a constitution, a plural separation valves can also be provided. The separation valve may also be vibrated usually or when plugging should occur, as well as it is advanced or retracted corresponding to the pressure. When the pressure within the mill body is increased to enlarge the clearance, smaller grains of the medium which have considerably been worn and which may not be exhausted through the normal aperture of clearance will be allowed to be exhausted therethrough. However, the medium is caught in the clearance to cause plugging during the advancing stroke of the valve with the reduction of pressure after elimination of the plugging, which will interfere the advancing movement thereof. In such an instance, when the valve is vibrated, the medium caught in the clearance will be knocked out into or outside of the mill body. In the above mill, inclusion of the medium may not easily be induced, since the separation valve is in a static state unless pressure change occurs. Also if such an inclusion should occur due to the enlargement of the clearance, the medium is retained as included therein; and, if it is made of ceramic, it may not be damaged. However, the grain size of the medium is several tens to several tens of thousands times as large as an ordinary product, and if this medium is included, the clearance will be enlarged to increase the amount of the slurry to be exhausted. As a result, not only maintenance of the liquid level will be difficult, but also the dwell time that the slurry raw material dwells in the mill will be reduced, to induce a problem that a product having a desired particle size may not be obtained through one pass. This problem can be solved by forming either or both of the separation valve and the valve seat to which the separation valve is fitted is (are) made of a soft material. When medium inclusion should occur, part of the medium will be embedded in either or both of the valve and the valve seat, thereby a clearance narrower than the diameter of the medium will be provided (see Fig. 7). The valve seat and the valve can be formed entirely or partly at strategic portions, for example, of Teflon or urethane rubber. In order to eliminate the inclusion of medium, it is also effective to form the opening edge of the valve seat into a shape of knife edge. The smaller is the contact area of said edge with the medium, the less will be the possibility of catching the medium in the clearance. If the valve seat having a knife edge shaped opening edge has resilience, the inclusion of medium will be eliminated more effectively. It is because provided that the inclusion of medium should happen, the edge springs up and down to knock off the media included in the clearance as shown in Fig. 6. While the raw material may be fed from the bottom of the mill body and exhausted from the top thereof as practiced in many prior art mills, preferably it is fed from the top and exhausted from the bottom. In this regard, the present inventors have made various experiments with vertical pressurizing continuous mill in which a slurry raw material is fed from the top of the mill body and examined the state of medium distribution within the mill body. They found, as a result, that among the medium, grains of relatively large size gather in the upper part of the mill body and the grain size gradually becomes smaller toward the bottom as shown in Fig. 5. They came to know from these findings that the raw material moves downward as it is milled and effective grinding can be achieved since it is successively milled with the grains with smaller sizes toward the bottom; that stable operation of the mill can be obtained by virtue of small variation in grain size distribution in the mill body and thereby of small variation in grinding property of the resulting product, since the grains of the medium move toward the lower part of the mill body as they are worn gradually to be exhausted as the waste medium continuously, while new medium is supplied from the top; that bulk density of the medium increases and also does the pressure between the grains of the medium by means of the weight of the medium itself, the dynamic energy of the slurry raw material moving downward and pressure, whereby the phenomenon of so-called short pass, wherein the raw material fed to the mill is immediately exhausted without being stirred sufficiently therein, can be eliminated by virtue of the improved bluk density of the medium. Accordingly, far more effective grinding can be achieved. The following Table shows the conditions and the results of comparative experiments conducted with respect to the cases wherein the slurry raw material was fed from the bottom of the mill body and the cases wherein said material was fed from the top of the mill body. The raw material used in each run in the above Table was calcium carbonate (CaCo₃), in which the raw material No. 1 had an initial maximum particle size of 35 µ and an initial mean particle size of 2.3 µ; the raw material No. 2 had an initial maximum particle size of 100 µ and an initial mean particle size of 13 µ; and the grinding medium had a grain size of 1 mm. In each run, the revolution was set such that the power of the mill may be 1.5 KW. As can be seen from the Table, it was necessary to set the revolution in the top feeding mode at a lower level than in the bottom feeding mode. This demonstrates that in the top feeding mode, bulk density of the medium becomes higher and that short pass of the slurry through the mill can be reduced. As a result, a sharp particle size distribution line can be obtained for the resulting product. Namely, it can be deduced that particles come to have rather uniform particle size and variations in the particle sizes can be reduced. Therefore, another object of the present invention is to enhance smooth flow of the raw material, thereby to allow for good passage of the liquid by disposing a separation valve at a portion closer to the peripheral wall of the mill body apart from the rotary shaft of the stirring device. Another object of the present invention is to prevent inclusion of medium in the clearance which leads to plugging by vibrating the separation valve usually or as necessary as well as said valve is advanced or retracted corresponding to the pressure within the mill body, even when smaller grains of the medium which may not be exhausted through the normal size of clearance should be included therein. Still another object of the present invention is to form either or both of the separation valve and the valve seat to which said valve is fitted of a soft material to reduce enlargement of the clearance even when smaller grains of the medium should be included therein. Another object of the present invention is to form the opening edge of the valve seat into a shape of knife edge to reduce the possibility of catching of the medium in the clearance; and further to impart resilience to the edge of the valve seat to knock out the grains of the medium when they are included. Another object of the present invention is to eliminate short pass of slurry raw material to achieve effective grinding thereof by feeding it from the top of a vertical mill and exhaust it from the bottom. Other objects and advantages of the present invention will become apparent from the following description on the embodiments given with reference to the drawings. In the drawings, Fig. 1 shows a flow sheet; Fig. 2 shows a cross-sectional view of the exhaust port in the flow sheet as illustrated in Fig. 1; Fig. 3 is another embodiment of the exhaust port of the mill showing in cross-section; Fig. 4 shows another embodiment of the exhaust port schematically which is disposed offset from the center of the mill; Fig. 5 is a graph showing distribution of the medium in the mill body; Fig. 6 is an operation view of the resilient edge of the valve seat; Fig. 7 is an enlarged partial cross-sectional view of a state where a grain of the medium is included in the clearance of the mill whose separation valve has been formed of a soft material; Fig. 8 shows a state when the medium is separated on the clearance for exhausting the slurry material; and Fig. 9 is a diagram showing the relationship between the pressure within the mill body, clearance aperture and the amount of the raw material to be exhausted. DETAILED DESCRIPTION OF THE INVENTION The vertical mill body 21 comprises a cylindrical pressurized vessel having a constricted lower portion and a discharge port 22 provided at the bottom; said mill body 21 being designed to be cooled with cold water to be passed through the jacket 26 provided over the external surface of the mill body 21. A stirring device 29, comprising a vertical shaft 25 sealed by a shaft seal 24 and driven by a motor 23 for rotation and a plural pins protruding radially and with a regular interval along the longitudinal direction of the vertical shaft 25, is provided at the the shaft center. A slurry raw material, to be fed from the feedstock tank 31 to the top of the mill body 21 by means of a pump 35, is stirred by a stirring device 29 under pressure in admixture with the grinding medium having been placed in the mill body 21, and gradually moves downward as it is milled to a fine particle to be exhausted together with the waste medium through the layer of the medium on the exhaust port 22. The resulting slurry product exhausted is passed through a screen 33 via a product tank 31, from which the waste medium is then removed, and forwarded to a storage tank not shown by a pump 35. The exhaust port is formed in a valve seat 37, which comprises a circular opening 38 having a tapered opening edge such that the opening may be larger toward the lower surface of the valve seat 37 as shown in Fig. 2. A separation valve 39 having a conical top is fitted to the circular opening 38 to form an annular clearance 41 with valve seat 37 therebetween. The separation valve 39 is fixed on the upper end of the shaft 43, and the shaft 43 is slidably inserted to the shaft center of the standard 45. Also, the separation valve 39 is fitted with the standard 45 on the upper end. The standard 45 has a cylindrical lower portion, and a plunger 49 fixed to the shaft 43 is fitted to the cavity 47. A helical spring 51 is disposed in the space defined between the plunger 49 and the bottom of the cavity 47. The shaft 43 and the separation valve 39 connected to said shaft 43 are pushed up by the function of the helical spring 51 and pushed down by the pressure within the mill body against the function of the helical spring 51. Thus, the clearance 41 formed between the valve seat 37 and the valve 39 varies. The numerals 53, 54 and 55 each show an air vent, and the air vents 54 and 55 formed in the upper and lower portions of the plunger 49 are adopted to be capable of introducing compressed air such that the separation valve 39 may be retractable as the advancing or retracting movement of the plunger 49 when compressed air is introduced into one of these air vents. OPERATION The mill device of the present invention has a constitution as described heretofore, in which the clearance between the valve and the valve seat is kept at a predetermined aperture when the pressure within the mill is maintained at a constant level and also when the mill is operated under normal condition, to allow separation of the medium from the slurry product to be left on said clearance and exhaust of said product in fixed amounts from the clearance 41 through the discharge port 57 together with the waste medium to be forwarded to the product tank 31. However, when plugging begins to occur in the clearance, the separation valve 39 descends resisting against the function of the helical spring 51 until it is equilibrium with the resistance of said spring 51 with the increase of the pressure of the slurry within the mill. Accordingly, the clearance will be enlarged and thus the amount of slurry to be exhausted will increase suddenly to eliminate the plugging. When the plugging is eliminated and the pressure within the mill body is reduced, the separation valve 39 is ascended. Thus, although the amount of exhaust may temporarily increase, it will return to a normal condition after elimination of the plugging. Fig. 3 shows another embodiment of the device for operating the separation valve, wherein a rack 65 is formed in the lower part of a shaft 63 slidably fitted to the bearing 61 and engaged with a pinion 69 driven by a servo motor 67 for rotation. The servo motor 67 is controlled by a pressure detection mechanism for detecting the pressure within the mill body and adopted to be rotatable normally and reversely with a required number of revolution corresponding to the pressure change within the mill body. When the servo motor 67 is rotated with a required number of revolution normally or reversely according to the pressure change within the mill body, a separation valve 71 is adopted to ascend to change the clearance 73 between said valve 71 and a valve seat 72. Fig. 4 shows an embodiment wherein a separation valve 81 is spaced from the shaft center of a stirring device 82 and disposed at a portion closer to the peripheral wall of a mill body 83. Since a larger stirring action by the stirring device 82 can be obtained along the peripheral portion rather than in the shaft center position, the separation valve is disposed at a portion closer to the peripheral wall of the mill body to enhance smooth flow of the raw material, thereby to allow for good passage of the liquid. In the above embodiments, while the separation valve 39, 71 or 81 is descended by the pressure increase of the slurry in the mill body as described above, thereby the clearance is enlarged to increase the amount of exhaust temporarily, relatively larger grains of the medium which would not normally be exhausted through said clearance may be allowed to be exhausted together with the slurry product with the increase of the clearance 41 or 73. Relatively larger grains of the grinding medium, which are going to slip out of the clearance having been enlarged when the valve is ascending with the pressure reduction due to the temporary increase of the exhaust amount, will be included inbetween the valve seat and the separation valve to cause plugging of the clearance. Thus, the separation valve may sometimes fail to return to the normal state. Such inclusion of medium may be moderated to some extent by forming the opening edge of the valve seat into a shape of knife edge to provide a condition in which the medium is not easily be caught in the clearance, i.e. to make the medium to be escaped therefrom easily. However, in addition to the above, for example, a vibrator can be connected to the shaft 43 or 63 to vibrate said valve usually or when the medium inclusion occurs; or, as shown in Fig. 6, resilience can be imparted to the opening edge of the valve seat 85 to eliminate the medium inclusion. More specifically, the medium included can thus be knocked out into or outside of the mill body. The inclusion of medium in clearance further causes the following problems in addition to the problem of difficulty in resetting the separation valve. To describe in detail, since the size of the grinding medium are in many cases several tens to several tens of thousands larger than that of the slurry product, the clearance will be enlarged if the medium is included therein to increase the amount of slurry to be exhausted. Therefore, not only maintenance of the liquid level will be difficult, but also dwell time that the slurry raw material dwells within the mill body will be shorter, resulting in the failure of obtaining a product having a desired particle size through one pass. Fig. 7 shows an embodiment wherein the separation valve 91 is formed of a urethane rubber in order to cope with the above problem; in which if the medium 95 is included in the clearance 93, it is designed that the medium may be partly embedded in the separation valve 91 for providing a clearance h narrower than the grain size m of the grinding medium 95.

Claims: A wet stirred ball mill comprising a closed type mill body (21) provided with a stirring device (29) in which a slurry raw material to be fed continuously is stirred in admixture with a grinding medium under pressure and after said medium is separated, said material is exhausted, characterized in that a pressure control valve device is provided therein comprising a generally conical separation valve (39, 71, 81, 86, 91) retractably fitted to an exhaust port (22) and an operation means (51) to allow advancing and retracting of said separation valve by means of the pressure within the mill body, whereby the separation valve (39, 71, 81, 86, 92) retracts corresponding to said pressure to provide a wider clearance, when plugging occurs in the clearance and thereby the pressure within the mill body is increased. A mill according to Claim 1, wherein the operation means comprises a spring device (51) for supporting the separation valve. A mill according to Claim 1, wherein the operation means comprises a pressure detection device for detecting the pressure within the mill body and an operation mechanism (67) for advancing or retracting the separation valve corresponding to the pressure detected by said detection device. A mill according to Claim 1, wherein the separation valve (81) is spaced from the shaft center of the stirring device (82) and disposed at a portion closer to the peripheral wall of the mill body. A mill according to Claim 1, wherein the separation valve (39, 71, 81, 86, 91) is vibrated usually or as required. A mill according to Claim 1, wherein either or both of the valve seat (37, 72, 85) in which an exhaust port is formed and the separation valve (39, 71, 81, 86, 91) fitted to said exhaust port is (are) made of a soft material. A mill according to Claim 1, wherein the opening edge of the exhaust port has a shape of knife edge. A mill according to Claim 7, wherein the opening edge of the exhaust port has resilience. A mill according to Claim 1, wherein the raw material is fed from the top of the mill body (21) and exhausted from the bottom thereof.

Inventors: INKYO, MITSUGI, KITADA, TASUTAKE, TAHARA, TAKASHI
Applicants: KOTOBUKI ENGINEERING & MANUFACTURING CO LTD
Application year: 1987
Main classification: B02C17
IPC codes: B02C17