SCREW MILL

Abstract: A screw mill (1), for example, used for producing fine powder grains of calcium carbonate used as bulking agent for paper comprises a continuous screw (70) and discontinuous vanes (72) which enable efficient grinding both of slurry and dry materials without causing clogging of the mill. The clearance between the hub (44) and the rotor (52) can be finely adjusted by a screw type adjustor (54). This makes it possible to easily adjust the grain size and to produce ground materials having a spheriodicity of 0.3 to 0.03 which are best suited for the bulking agent for paper.

Description: This invention relates to a screw mill, for example, to a screw mill used for producing calcium carbonate powder used as a bulking agent for paper comprising a hollow cylindrical member having a first inlet port which is arranged at the rear end of the cylindrical member, material to be milled being transferred from the rear end to the front end of the cylindrical member during operation of the screw mill, a shaft which is formed with a continuous screw and extends forwardly from the first inlet port, a forwardly diverging rotor which is secured to the shaft and is formed on its outer periphery with grooves, an annular member which is secured to the hollow cylindrical member around the rotor and has a forwardly diverging inner peripheral surface which is formed with grooves facing the grooves of the rotor, means for rotating the shaft about its longitudinal axis, and means enabling relative movement between the annular member and the rotor along the longitudinal axis of the shaft. Such a type of construction is disclosed in FR-A-1197065 (Colombo) and is intended to be used for crushing olives and fruit to a paste. This mill has a single inlet port and the shaft has a constant diameter along its length. The grooves of the annular member and rotor extend parallel to the longitudinal axis of the shaft and the angles subtended by the frustro conical surfaces of the annular member and the rotor are different from each other. Also the second said means enables the rotor to be moved along the shaft towards and away from the annular member. GB-A-11852/A.D. 1904 (Prenzel) discloses a grinding mill designed for use in connection with roller flour mills. The mill comprises a hollow member which is tapered on the inside in opposite directions from a position between its ends. The hollow member has an inlet port which is arranged at the rear end of the hollow member. A shaft extends through the hollow member and carries a grinding roller, which is shaped to correspond with the hollow member. The grinding roller is formed with a continuous screw which extends from its rear end adjacent to the inlet port up to the largest diameter position of the grinding roller. From the largest diameter position of the grinding roller to the front end, the grinding roller and the hollow member have spiral grooves. A forwardly converging rotor is secured to the shaft at the front end of the grinding roller and has grooves in its outer periphery. An annular member is secured to the hollow member around the rotor, the inner peripheral surface of the annular member corresponding to the outer periphery of the rotor and being formed with grooves facing the grooves of the rotor. Means is provided for enabling relative movement between the annular member and the rotor along the longitudinal axis of the shaft. Several other known systems for grinding particles of small diameters are, for example, as follows: (1) an impacting and pressing type; this type is carried out, for example, by using a jaw crusher or an impact crusher; (2) a pressing and shearing type; this type is carried out, for example, by using a roller mill; (3) an autogenous grinding type (for example, striking, pressing, rolling, and high speed rolling and impacting types); this type is carried out by using an aerofall mill, a jet mill, a rolling cylinder mill, an attrition mill or a super-micron mill; (4) a type using a grinding medium, this type is carried out by using a ball mill, a rod mill, a vibration ball mill or an attrition mill; (5) a type of combination of the types of (1) - (4). However, none of these types can easily control the grain size and also cannot manufacture the powder grains each having spheroidicity less than 0.3 which is suitable for a bulking agent for paper. The spheroidicity A of each powder grain is defined as follows: where "m" is the observed mass of one grain; the observed mass "m" is found by detecting the number of grains in a predetermined weight of a sample, by means of a particle counter using the electrical resistance method (Caulter's principle), and then dividing the weight by the number of grains and, "m'" is the mass of one grain which is assumed to be spherical and is calculated from a volumetric mean diameter ( d ') of a cube formed by surfaces each being tangential to the grain. This is calculated as follows: where, "d'" is found by an image analysis system. Thus, the spheroidicity A of a sphere is 1, and the spheroidicity A of a coin-shaped grain having a thickness of 1/10 diamter is 0.15. The present invention is characterised in that: the hollow cylindrical member has a second inlet port which is arranged at the front end of the cylindrical member, the shaft has a diameter which gradually increases towards the front of the cylindrical member, the continuous screw extends to a position just before the second inlet port, the shaft is formed with multiple stages of vanes which are arranged at the front of the continuous screw, the continuous screw and the vanes are in contact with the inner cylindrical surface of the hollow cylindrical member, the rotor is secured to the front end of the shaft and its grooves are inclined relative to the longitudinal axis of the shaft, the annular member is secured to the front end of the hollow cylindrical member and its inner peripheral surface corresponds to the outer periphery of the rotor, and the second said means enables the annular member to be moved against the rotor. In the accompanying drawings; Figure 1 is a cross-sectional side elevation view of a screw mill; Figure 2 is a perspective view of the annular member of the screw mill of Figure 1; Figure 3 is a perspective view showing an exploded condition of a clearance adjusting apparatus used in the screw mill of Figure 1; and, Figure 4 is a cross-sectional view taken along the line IV-IV of Figure 1. A screw mill 1 has a base 2 on which a first bearing unit 6 and a second bearing unit 8 for rotatably supporting a shaft 4 are mounted at right and left hand ends, respectively. Also mounted on the base 2 is a hydraulic piston and cylinder unit 10 for driving a hollow cylindrical member 12 axially along the base 2. The hollow cylindrical member 12 is slidably supported on the base 2 by means of supporting units 14 and 16 and slide plates 18 and 20. The sliding motion of the cylindrical member 12 is guided, for example, by a dove-tail guiding means 60 (Figure 4). The cylindrical member 12 is provided with a first inlet port 24 for slurry materials and a second inlet port 26 for dry materials at rear and front ends of the cylindrical member 12, respectively. The material to be ground flows from the inlet ports 24 and 26 towards the front of the screw mill 1 (that is, towards the left-hand of Figure 1). When grinding slurry, it is fed into the screw mill 1 through the first inlet port 24. On the other hand, dry material is fed through the second inlet port 26. Two inlet ports 30 and 32 for a dispersing agent are formed on the top of the cylindrical member 12 between the first and second inlet ports 24 and 26 and another inlet port 34 for a dispersing agent is formed in front of the second inlet port 26. A drain 36 for washing water is also formed on the bottom of the cylindrical member 12. A forwardly diverging taper portion 40 is formed on the inner surface of the front end of the cylindrical member 12. An annular member 44 is secured to the front end of the hollow cylindrical member 12. The member 44 has a forwardly diverging inner pheripheral surface and is formed with grooves 42 (Figure 2) on the inner pheripheral surface thereof. An outlet ring 62 for receiving the ground powder is mounted on the front end of the annular member 44, and a chute 64 for receiving and guiding the ground powder to one side of the screw mill 1 is mounted below the outlet ring 62. The rear end of the front supporting unit 16 is connected to the front end of the plunger 50 of the hydraulic piston and cylinder unit 10. A clearance adjusting device 54 is connected to the supporting unit 16 so as to finely adjust the clearance between the annular member 44 and a rotor 52 which will be hereinafter explained. As shown in Figure 3, the clearance adjusting device 54 is composed of a pair of tapered members 56 and 58 and a screw means 66 for moving the tapered member 56 along a line perpendicular to the axis of the cylindrical member 12. The shaft 4 is rotatably supported within the hollow cylindrical member 12 and has a diameter gradually increasing toward the front of the screw mill 1. The shaft 4 is provided with a continuous helical screw 70 extending from the first inlet port 24 to a position just before the second inlet port 26 and also is provided with discontinuous vanes 72 in multiple stages (two stages are shown in Figure 1) arranged in a region in front of the continuous screw 70. The height of the screw 70 and the vanes 72 are so determined that the crests thereof contact the inner surface of the cylindrical member 12. The vanes 72 of the front stage gradually increase their height so that the crests thereof contact the inner surface of the tapered portion 40 of the cylindrical member 12. The rotor 52 is secured to the front end of the shaft 4 and has grooves 80 formed on the periphery thereof. The direction of the curve of the grooves 80 is the same as that of the vanes 72. The front end of the rotor 52 is secured to a shaft 84 rotatably mounted on the second bearing unit 8 and outlet groove member 82 is mounted around the shaft 84. The operation of the screw mill will now be described. Prior to the grinding operation, the clearance between the annular member 44 and the rotor 52 is adjusted by the screw means 66 and the hydraulic piston and cylinder unit 10 is operated to urge the annular member 44 against the rotor 52 at a predetermined pressure. Then the shaft 4 is rotated by a power source (not shown) by means of a chain (not shown) and a sprocket wheel 90. If the material to be ground is liquid or slurry, it is fed into the first inlet port 24. On the other hand, if the material to be ground is dry particles, it is fed into the second inlet port 26. The dispersing agents are fed into the ports 30, 32 and 34, if desired. The slurry material fed into the first inlet port 24 is pressed and increases in density and reduces in volume while it is passed forward through the space between the cylindrical member 12 and the shaft 4, which gradually increases in diameter toward the front. Then the material is forced into the clearance between the annular member 44 and the rotor 52 by the multi-stage vanes 72 and ground therebetween and finally leaves the screw mill 1 through the chute 64. On the other hand, dry material fed into the second inlet port 26 is forced into the clearance between the annular member 44 and the rotor 52 by the multi-stage vanes 72 and ground into fine powder therebetween and finally leaves the screw mill 1 through the chute 64. The combination of the continuous screw and discontinuous vanes makes it possible to efficiently grind both slurry and dry materials without causing clogging of the mill, Also the clearance between the annular member and the rotor can be finely adjusted by the screw type adjuster. This makes it possible to easily adjust the grain size and to produce ground material having the spheroidicity of 0.3 to 0.03 which is best suited for the bulking agent for paper.

Claims: A screw mill comprising: a hollow cylindrical member (12) having a first inlet port (24) which is arranged at the rear end of the cylindrical member (12), material to be milled being transferred from the rear end to the front end of the cylindrical member (12) during operation of the screw mill, a shaft (4) which is formed with a continuous screw (70) and extends forwardly from the first inlet port (24), a forwardly diverging rotor (52) which is secured to the shaft (4) and is formed on its outer periphery with grooves (80), an annular member (44) which is secured to the hollow cylindrical member (12) around the rotor (52) and has a forwardly diverging inner peripheral surface which is formed with grooves (42) facing the grooves (80) of the rotor (52), means for rotating the shaft member (4) about its longitudinal axis, and means (10, 50) enabling relative movement between the annular member (44) and the rotor (52) along the longitudinal axis of the shaft (4), characterised in that: the hollow cylindrical member (12) has a second inlet port (26) which is arranged at the front end of the cylindrical member (12), the shaft (4) has a diameter which gradually increases towards the front of the cylindrical member (12), the continuous screw (70) extends to a position just before the second inlet port (26), the shaft (4) is formed with multiple stages of vanes (72) which are arranged at the front of the continuous screw (70), the continuous screw (70) and the vanes (72) are in contact with the inner cylindrical surface of the hollow cylindrical member (12), the rotor (52) is secured to the front end of the shaft (4) and its grooves (80) are inclined relative to the longitudinal axis of the shaft (4), the annular member (44) is secured to the front end of the hollow cylindrical member (12) and its inner peripheral surface corresponds to the outer periphery of the rotor (52), and the second said means (10, 50) enables the annular member (44) to be moved against the rotor (52). A screw mill as claimed in Claim 1, wherein the vanes (72) are discontinuously arranged in two stages. A screw mill as claimed in Claim 1 or 2, wherein second said means comprises a hydraulic piston and cylinder unit (10) arranged to act on the cylindrical member (12). A screw mill as claimed in any preceding claim, and further including a clearance adjusting apparatus (54) for adjusting the clearance between the annular member (44) and the rotor (52). A screw mill according to Claim 4, wherein the clearance adjusting apparatus comprises a pair of tapered members (56, 58) and a screw means (66) for moving the tapered members along a line perpendicular to the longitudinal axis of the cylindrical member (12).

Inventors: AKAZAWA, TOYO
Applicants: JMC CORPORATION
Application year: 1987
Main classification: B02C19
IPC codes: B02C19