MXPA00001918A - Grinding mill - Google Patents

Abstract

A grinding mill has a rotating container (40) into which particulate material is fed. The container is rotated above critical speed to form a layer which is retained under high pressure against the container inner surface. Shearing discs (58) mounted inside the container induce shearing of the layer to promote particle fracture by shearing and abrasion in the pressurised layer. Fine ground material travels axially to the container discharge end (64). In one form of the invention, the container is rotated at sufficient speed to form a series of solidified zones (70) alternated with stirred zones (72) next to non-rotating shearing discs (58). These solidified zones act as solid discs rotating with the container.

Description

WINDMILL ANTECEDENTS OF L? INVENTION: The invention relates to a rotary mill for reducing the size of particles such as ceramics, minerals and pharmaceutical products. The above rotating mills include a cylindrical drum that rotates about a generally horizontal axis. The rotating drum is fed with particulate material such as a slurry or powder, the rotation of the drum is half to three quarters of the "critical speed" (that is, the minimum speed at which the material on the inner surface of the drum moves directly around in contact with the mill). This causes the drumming action when the feed and any grinding media moves partly up the inner wall of the drum, then falls away to collide or grind against other particles in the feed. The reduction in particle size is tmainly achieved by abrasion and impact. In conventional rotary mills, milling energy requirements increase gradually with increasing fineness of milling. For applications where fine grinding is required, the use of agitated mills, in which a body of particulate material is stirred to produce the cut of the particles and numerous low energy impacts, can be used to alleviate this problem to some degree. . However, the present application of stirred mills is restricted by the limits of the imposed reduction ratio with the upper feed size limits and the inefficiencies of energy transfer to ultra fine sizes. These constraints, together with yield limitations and media / product separation difficulties due to the effects of viscosity at ultra-fine sizes, restrict the practical and economical scope to apply that technology.

BRIEF DESCRIPTION OF THE INVENTION The present invention has as its main purpose to provide an alternative mill construction. The invention, in one form, provides a mill for particulate material, which includes a rotating vessel having an internal surface, feeding means for feeding the particulate material to the vessel, means for rotating the vessel at a sufficiently high speed, so that the particulate material forms a retained layer against the internal surface through its rotation and means that induce the cut in contact with the layer, to induce the cut in the layer. In non-vertical mills, the minimum rotational speed at which the particulate material rotates around in contact with the containers is known as the "critical velocity". The term is used herein with reference to both vertical and non-vertical mills to refer to the minimum rotational speed at which the particulate material forms a retained layer against the inner surface of the container through its rotation. The invention also provides a method of grinding in which the particulate material is fed to a container that rotates above the critical speed, to form a layer retained against the container through its rotation and which induces the cut in the layer by means that induce the cut in contact with the layer. Preferably, the cutting inducing means are mounted within and rotate relative to the container. In a first embodiment, the means that induce the cutting rotate in the direction of rotation of the container, but at a different speed. In a second embodiment, the means that induce the cut rotate in the opposite direction in relation to the container. Alternatively, the cutting inducing means may be non-rotational, based on relative rotation with the container to induce cutting of the material layer. Preferably also, the mill rotates at least three times, preferably at least ten times, the critical speed.BRIEF DESCRIPTION OF THE DRAWINGS Now preferred embodiments will be described with reference to the accompanying drawings, in which: Figure 1 is a section elevation, schematic, of a first embodiment; Figure 2 is a sectional elevation, schematic, of a second embodiment; and Figure 3 is an elongated cut-away elevation of the grinding chamber of the mill of Figure 2 during operation, showing the creation of alternating stirring and dead zones within the chamber.

DESCRIPTION OF THE PREFERRED MODALITIES The mill shown in Figure 1 has a cylindrical outer drum 10 mounted on bearings 12 to rotate about its central axis 14, driven by pulley means to drive the drum 16 attached to its outer surface. The external surface of the drum also contains cooling fins 18, which pass through a passage of cooling water 20 below the drum. A fluid particulate material feed, for example a suspension or powder, is introduced at one end of the drum from a feed hopper 21 via the feed inlet 22 and is exhaled outwardly to form a layer 23 against the inner surface of the drum. The drum is rotated sufficiently above the critical speed so that the entire mill load, and any milling media, moves directly around in contact with the drum instead of the subcritical drumming operation of the mills of the mill. previous technique. The drum is preferably rotated at least three times the critical speed, more preferably at least ten times, so that the layer of the mill load is at high pressure, compressed by the high centrifugal force. The magnitude of the compressive forces applied can be varied by varying the rotation speed of the external drum. The loading layer is mobilized by means of a disc or projections in the form of fingers or claws 24 of the member that induces the cut that rotates in the opposite direction 26 inside the drum, mounted on a central axis 28 supported on bearings 30. This axis rotates by means of a pulley that drives the shaft 32. A passage of cooling water 27 extends through the shaft 28.

For maximum cutting, the shaft is rotated rapidly in the direction opposite to the drum 10. Alternatively, the shaft may be rotated in the same direction as the drum, but at a different speed. This last arrangement eliminates a "dead" cycle within the load layer at which the rotational force "G" is zero, and reduces the energy requirements of the mill. The particles in the loading layer are subjected to intense, interparticle and / or particle shear stresses due to the agitation of the projections 24 which rotate through the compressed loading layer. The high pressure due to the rotation of the loading layer increases the transfer of energy from the projections to the load, thereby transferring a relatively large proportion of the feed energy available directly to the particles as stress promoting the fracture. The cutting of the compressed solids layer produces both cutting and fracture by abrasion of the particles, with sufficient energy to produce localized stresses and fracture applied simultaneously to a large proportion of the total particle population within the mill. The net result is a high distribution of very fine particles, with the capacity to sustain an effective fracture by this mechanism at high speeds of expansion of the particles inside the mill.

In addition to the abrasion fracture, the particles can also fracture due to the compression force of the media and pressure per unit volume of the solid particles, due to the exaggerated "gravitational" force inside the mill. The magnitude of this compression force and the densities densified particle / particle and particle / media can be varied. It is believed that there is also some fracture due to cracking and wear of the surfaces of the particles resulting from impacts at a higher speed., but to a lesser degree than the abrasion fracture. The discharge end 33 of the mill drum 10 has an annular retaining plate 34 which extends radially inwardly from the inner surface of the drum. The larger centrifugal force acting on the heavy average particles causes the media to be retained within the mill radially outwardly of the holding plate 34 and therefore, to remain inside the mill while the fine product is displaced by the incoming feed and passes radially towards the retaining plate and towards a discharge trough 36. Figures 2 and 3 illustrate a vertical mill constructed in accordance with a second embodiment, which includes non-rotating cutting members. The rotating drum 40 of the mill is mounted on a vertical rotational shaft 42 supported on the frame 44 by bearings 46, and is rotated at high speed via the pulley that drives the drum 48. The mill is initially loaded with a mixture of media of grinding, fed from the media hopper 50 via the balloon valve 52, and a feed or suspension powder fed through the feed port 54. The load passes down through the stationary feed tube 55 to the drum . Feed impellers 56 attached to the rotating drum impart rotational movement to the load, which forms a highly compressed layer held against the inner surface of the drum. In the embodiment of Figures 2 and 3, the member that induces cutting inside the drum is stationary, consisting of one or more radial discs 58 attached to a fixed shaft 60. The disc has openings 62 in the region of the free inner surface 63 of the loaded layer to allow axial movement of the finely ground material through the mill to the discharge end. If fingers or other projections are used instead of disks 58, openings 62 are not required. After the initial loading is introduced, no additional grinding media is added but a continuous flow of feed is fed via the feed gate 54. The mill is adapted to receive feed suspensions with a high solids content, for example 50-90% solids, typically 55-75%, depending on the feed material and the required reduction size. The grinding media and the larger particles in the loading layer will tend not to move axially through the mill, due to the high compressive forces on the load. Instead of the radial migration of the particles, where the larger particles introduced into the feed suspension migrate radially outward through the load, due to the high centrifugal force and are subjected to grinding and fracturing by efficient mechanisms discussed above with reference to Figure 1. When the particle size is reduced, the smaller particles migrate radially inward until they reach the free internal surface of the charge layer, which reaches a pressure (gauge) site of zero . The fine particles reaching the free surface can then move axially through the mill, through the openings 62 in the discs, pass radially inward of the discharge ring 64 and into the discharge trough 66. A doctor blade 68 can be fixed to the stationary shaft 60 to keep the material flowing freely through the discharge ring.

The applicant has found that, at the high rotational speeds at which these mills are operated, preferably at least 100 times the gravity, for example, up to 20 times the gravity, the areas in the load away from the cutting discs 58 pack solids and rotate at the same time with the rotating drum. This can be advantageously used by separating the cutting discs a distance sufficient to create "dead" load zones of solids between successive discs and adjacent to the end faces of the rotating drum. Those dead zones 70, shown by. the darker shading in Figure 3, effectively act as solid discs that extend into the inner wall of the drum, parallel to and rotating at high speed relative to the discs. This creates an extremely high cutting speed in the agitated load regions 72 (shown in the form of lighter shading in Figure 3) adjacent to the disk, while protecting the end surfaces of the drum from excessive wear. The minimum separation of the disc required to create this phenomenon of agitated zone / dead zone will vary depending on the rotational speed and the material used, but in cases of an extremely high G force and a high solids content, it can be as small as 50 mm Compared with the embodiment of Figure 1, the embodiment of Figures 2 and 3 have the advantage of requiring less energy, since it is not necessary to actuate the member that induces the cut. The energy requirement of the mill can be further reduced by reducing the length of the milling chamber and using only a single cutting disk. The environment of high "gravity" within the mill according to the invention, extends the practical and economic limits of crushing with mill with conventional agitation with respect to the larger size of the feed,. the reduction relationships, energy efficiency and performance. Although particular embodiments of this invention have been described, it will be apparent to those skilled in the art that the present invention may be carried out in other specific ways without departing from the essential features thereof. The present modalities and examples should therefore be considered, in all respects, as illustrative and not restrictive, the scope of the invention is indicated by the appended claims rather than by the foregoing description, and it is intended that all changes that fall within of the meaning and scope of the equivalence of the claims are covered in it.

Claims (14)

1. A mill for particulate material, characterized in that it includes a rotating container having an internal surface, feeding means for feeding the particulate material to the container, means for rotating the container at a sufficiently high speed, so that the particulate material forms a layer retained against the internal surface through its rotation, and means that induce the cut in contact with the layer to induce the cut in the layer.

2. The mill in accordance with the claim 1, characterized in that the cutting means are mounted inside and rotate relative to the container.

3. The mill in accordance with the claim 2, characterized in that the cutting means rotate in a direction of rotation of the container. The mill according to claim 2, characterized in that the cutting means rotates in the opposite direction with respect to the container. The mill according to claim 1, characterized in that the container rotates at least three times the minimum speed at which the particulate material forms a retained layer against the internal surface of the container through its rotation. The mill according to claim 1, characterized in that the container rotates at least 10 times the minimum speed at which the particulate material forms a retained layer against the internal surface of the container through its rotation. The mill according to claim 1, characterized in that the container rotates at a speed sufficient to induce a force of at least 100 times the gravity on the layer of particulate material. 8.. The mill according to claim 1, characterized in that the container rotates at a speed sufficient to cause one or more zones to solidify substantially in the layer of particulate material. 9. The mill in accordance with the claim 8, characterized in that the cutting means create one or more agitated zones in the layer of particulate material, the agitated zones are located between the cutting means and the solidified zones. 10. The mill in accordance with the claim 9, characterized in that it includes a plurality of cutting means axially spaced along the container, to create alternating solidified and agitated zones. The mill according to claim 10, characterized in that the cutting means include radial cutting discs that extend towards the layer of particulate material. The mill according to claim 11, characterized in that the discs do not rotate. 13. The mill in accordance with the claim 1, characterized in that the cutting means include one or more radial members which extend towards the layer of particulate material. 1

4. The mill according to claim 13, characterized in that the cutting means do not rotate. 1

5. The mill according to claim 13, characterized in that the cutting means includes one or more radial discs mounted inside the container. 1

6. The mill according to claim 15, characterized in that the disks include openings therethrough, which allow the axial passage of the ground material along the container. The mill according to claim 15, characterized in that the cutting means includes a plurality of radial discs. 18. A method for grinding particulate material, characterized in that it includes feeding the particulate material to a container, which has an internal surface, to rotate the container at a sufficiently high speed, so that the particulate material forms a retained layer against the surface internal through its rotation, and contact the layer with means that induce the cut to induce the cut in the layer.