GRINDING OF PARTICULATE MATERIAL

Abstract: Apparatus (1) for grinding a suspension of a particulate solid material. The apparatus (1) comprises a grinding chamber (2), means(7, 8) for introducing a suspension of the particulate solid material into the grinding chamber (2) and an impeller (6) which is rotated and thereby agitates the particulate grinding medium and the suspension of particulate solid material in the grinding chamber (2). The impeller (6) has at least one blade (12), the said blade (12) having a leading face (14) in the direction of rotation (R) of the impeller (6), which leading face (14) is offset relative to a plane of rotation of the impeller (6). In a preferred embodiment, an upper edge of the said leading face (14) is the leading edge and a lower edge of the said leading face is the trailing edge of each blade (12), so that the impeller (6) acts to drive the material in the grinding chamber (2) downwards towards a baseof the grinding chamber (2), thereby compresing the material in the grinding chamber (2). A baffle member (16) comprising an annular plate may be located in the grinding chamber (2) above the impeller (6) at a preset height, so that thematerial in the grinding chamber (2) is maintained in a compressed condition.

Description: Grinding o£ Particulate Material The present invention relates to grinding, and particularly to an apparatus and a method for attrition grinding of particulate mineral materials. It is well known to grind a particulate solid material suspended in a liquid medium, such as an aqueous medium. Grinding such suspensions of material is known in the art as 'wet' grinding and the typical name of a type of apparatus employed to perform such grinding is a stirred media mill or an ultrafine grinding mill. We have now devised an improved way of grinding particulate solid materials. According to a first aspect of the present invention, there is provided apparatus for grinding a suspension of a particulate solid material, which apparatus comprises: a grinding chamber for containing a particulate grinding medium; means for introducing a suspension of the particulate solid material into the grinding chamber; a rotatable impeller for agitating the particulate grinding medium and the suspension of particulate solid material in the grinding chamber, the impeller having at least one blade, wherein the or each blade of the impeller is pitched relative to the plane of rotation of the impeller; and means to drive the impeller, so as to produce a suspension of a ground particulate solid material . According to a second aspect of the present invention, there is provided apparatus for grinding a suspension of a particulate solid material, which apparatus comprises: a grinding chamber for containing a particulate grinding medium; means for introducing a suspension of the particulate solid material into the grinding chamber; a rotatable impeller for agitating the particulate grinding medium and the suspension of particulate solid material in the grinding chamber, means to drive the impeller; and a baffle member arranged over the particulate grinding medium and the suspension of particulate solid material, so as to produce a suspension of a ground particulate solid material. According to a third aspect of the present invention, there is provided a method of grinding a suspension of a particulate solid material, which method comprises : providing apparatus as defined in the first aspect of the invention; introducing into the grinding chamber the particulate grinding medium and the suspension of particulate solid material; and agitating the particulate grinding medium and the suspension of particulate solid material by rotation of the impeller, so as to produce a suspension of a ground particulate solid material. According to a fourth aspect of the present invention, there is provided a method of grinding a suspension of a particulate solid material, which method comprises : providing apparatus as defined in the second aspect of the invention; introducing into the grinding chamber the particulate grinding medium and the suspension of particulate solid material; and agitating the particulate grinding medium and the suspension of particulate solid material by rotation of the impeller, so as to produce a suspension of a ground particulate solid material. According to a fifth aspect of the present invention, there is provided apparatus for grinding a suspension of a particulate solid material, which apparatus comprises : a grinding chamber; means for introducing a suspension of the particulate solid material into the grinding chamber, an impeller having at least one blade, the said blade having a leading face in the direction of rotation of the impeller, which leading face is offset relative to a plane of rotation of the impeller. The angle of offset of the leading face relative to the plane of rotation of the impeller is preferably greater than 0° and less than 90 ° . Preferably, the impeller rotates in a substantially horizontal plane and an upper edge of the said leading face is the leading edge and a lower edge of the said leading face is the trailing edge, so that the impeller acts to drive the material in the grinding chamber downwards towards a base of the grinding chamber, thereby compressing the material in the grinding chamber. The configuration of the impeller provided in the apparatus of the invention causes enhanced grinding of the suspension of particulate material introduced into the apparatus. The impeller may comprise an annular body having a central opening which may be disposed on a substantially vertically orientated rotatable shaft . An upper end of the shaft may be connected to a drive means to rotatably drive the shaft and the impeller during operation of the apparatus. It is preferred that the annular body of the impeller may comprise a plurality of blades, each of the blades projecting radially from the annular body and preferably being equidistantly spaced from the other around an exterior face of the annular body, and each of the blades being pitched relative to the plane of rotation of the impeller. In this specification, the 'pitch' of a blade is defined as the angle between the chord of the blade (or the leading face of the blade in the direction of rotation of the impeller) and the plane of rotation of the blades of the impeller. The impeller may have an annular body and a plurality of blades which may be unitary, or, alternatively, the plurality of blades may be fixedly secured to the annular body. It is preferred that the pitched blades have a pitch angle in the range of from about 30° to about 60°. Optimally, the pitch angle is about 45° which during use of the apparatus of the invention advantageously produces a ground particulate material product having the desired particle size properties. It is also preferred that the plane of rotation of the impeller may be substantially perpendicular to a substantially vertically orientated rotatable impeller shaft. Preferably, each blade of the plurality of blades of the impeller has a substantially equivalent pitch angle; in other words, the chord of each blade may have a substantially equivalent gradient. The preferred orientation of the pitched blades provides enhanced grinding of the suspension of particulate mineral material , to produce a ground mineral product having the required particle size with a reduce energy input. Preferably, a front (leading) face and/or a rear (trailing) face of each blade may be substantially flat (planar) . In an embodiment of the apparatus of the invention, the impeller may comprise an arrangement of one or more tiers. Preferably, there are two, three or four tiers to further improve the performance of the method of the invention. A two tier impeller provided in such apparatus may be called a double-pitched impeller. Preferably, the two tiered impeller may be arranged to rotate in unison and the upper tier of the impeller may be upwardly spaced from the lower tier so as to maximise the grinding effect of the impeller to comminute the suspension of particulate material introduced into the grinding chamber. Preferably, the impeller blades of the upper tier may be pitched at substantially an equivalent pitch angle to the impeller blades of the lower tier. Known prior art configurations of 'wet' grinding apparatus do not utilise a pitched impeller as defined by the present invention and hence such known prior art configurations do not provide the advantages as described herein. Generally, the effectiveness of a grinding process may be attributed to the fineness of product produced and the yield of product of the desired particle size. A comminuted product mainly consisting of particles having a particle size less than 2μm is generally recognised in the art as being an aptly ground product. The ground particulate product produced by the method of the invention advantageously consists of a greater yield of particles having a finer particle size than a ground product produced by known prior art grinding apparatus. Preferably, at least 90% by weight of the ground particulate product comprises particles having a particle size less than 5μm; and preferably at least 70% by weight of the ground product comprises particles having a particle size less than 2μm. Furthermore, use of the apparatus of the invention provided with a pitched impeller is more cost-effective and energy efficient than known prior art grinding apparatus . The apparatus of the invention uses less energy during operation and achieves an improved level of grinding compared to prior art configurations. The amount of energy used by apparatus in accordance with the invention when grinding materials such as kaolin may be in the range of from about 20kWh/t to 150kWh/t; optimally the energy to be applied is about 55kWh/t to 150kWh/t. The amount of energy to be applied when grinding materials such as calcium carbonate is preferably up to 300kWh/t. In tests conducted by the applicant, the use of an impeller having blades with a 45° pitch rather than pin type (cylindrical) blades in a conventional wet grinding test chamber decreased by approximately half the energy input to achieve a ground product in which 80% of the particles were smaller than 2μm. The use of a pitched impeller as described hereinbefore in the apparatus of the invention causes improved mixing of the charge resulting in improved grinding of the suspension of material introduced into the apparatus. In this specification, the 'charge' is defined as the mixture of the particulate grinding medium and the suspension of particulate solid material contained in the grinding chamber of the apparatus, which term is commonly known to skilled people in the art. Such improved grinding is believed by the applicant to be caused by grinding of the mineral material suspension relatively near, or in the region of, an interior face of the wall of the grinding chamber. Grinding may be initiated by rotation of the pitched impeller at a suitable velocity so as to fluidise the (bed of) grinding medium and cause mixing of the grinding medium with the suspension of material to be ground. A suitable rotational velocity of the pitched impeller may be in the range of from 50rpm to 400rpm. In accordance with the second aspect of the present invention, a baffle member is arranged over the particulate grinding medium and the suspension of particulate solid material. The baffle member may be formed of an annular baffle plate having a central opening which may receive therethrough a substantially vertically orientated rotatable shaft. The baffle plate may be made of a plastics material, such as, for example a polyurethane material. Most preferably, the baffle member is of rigid construction and may be made of steel . It may be lined with an antiwear component such as a polyurethane coating or lining. Preferably, the baffle member is removably secured to an interior face of the wall of the grinding chamber, or, alternatively, to a top end of the grinding chamber. Preferably, the baffle member lies in a plane which is substantially perpendicular to a rotational axis of the impeller shaft. Preferably, the baffle member is located at a preset height over the charge, such that during operation of the apparatus the baffle member ensures that the charge is in a relatively compressed condition. Such location of the baffle member optimises the energy delivered to each unit volume of the charge (through rotation of the impeller) thereby maximising the grinding of the mineral material suspension. The baffle member also serves to limit the extent to which the charge rides up the interior face of the wall of the grinding chamber. In tests conducted by the applicant, the addition of a baffle member to a conventional wet grinding test chamber decreased by approximately 40%, the energy input to achieve a ground product in which 80% of the particles were smaller than 2μm. The provision of a baffle member in the apparatus of the first and fifth aspects of the invention also further enhances the technical advantages which may be achieved by the provision of a pitched impeller as hereinbefore described. For instance, the yield of ground particulate product having a fine particle size (of ideally less than 2μm) is further improved and the amount of energy applied to the apparatus is further reduced resulting in a particularly preferred embodiment of the invention. The particulate grinding medium which may be employed according to the method of the invention may be of a natural or a synthetic material. The grinding medium may comprise balls, beads or pellets of any hard mineral, ceramic or metallic material; such materials may include, for example, alumina, zirconia, zirconium, silicate, aluminium silicate or the mullite-rich material which is produced by calcining kaolinitic clay at a temperature in the range of from 1300°C to 1800°C. Alternatively, particles of natural sand of a suitable particle size may be used in the method of the invention. Generally, the type of and particle size of grinding medium to be selected for use in the invention may be dependent on the properties, such as, eg. the particle size of and the chemical composition of the feed suspension of inorganic material to be ground. Preferably, the particulate grinding medium comprises particles having an average diameter in the range of from 0.1mm to 6.0mm and, more preferably in the range of from 0.2mm to 4.0mm. Preferably, the grinding medium (or media) may be present in an amount of from 40% to 70% by volume of the charge; and, more preferably in an amount of about 60% by volume of the charge. The suspension of particulate solid material to be comminuted by the method of the invention may be of an inorganic material and preferably a mineral material. The mineral material may be used in its naturally occurring form or its chemically synthesised form. Such materials may include, for example calcium carbonate, a kaolin clay and any other suitable mineral material . The particulate inorganic material may preferably be present in an amount of from 10% to 85% by weight of the suspension; more preferably, the inorganic material may be present in an amount of from 20% to 80% by weight of the suspension. Most preferably, the inorganic material may be present in an amount of about 30% to 75% by weight of the suspension. The suspension of solid material to be comminuted may be of a relatively high viscosity, in which case a dispersing agent may preferably be added to the suspension prior to comminution by the method of the invention. The dispersing agent may be, for example, a water soluble condensed phosphate, a water soluble salt of a polysilicic acid or a polyelectrolyte, for example a water soluble salt of a poly (acrylic acid) or of a poly (methacrylic acid) having a number average molecular weight not greater than 20,000. The amount of the dispersing agent used would generally be in the range of from 0.1 to 2.0% by weight, based on the weight of the dry particulate solid material. The suspension may be introduced into the grinding chamber of the apparatus at a temperature in the range of from 4°C to 100 °C. An embodiment of the invention will now be described by way of example only and with reference to. the accompanying drawings, in which: Figure 1 is a sectional view in schematic form of an apparatus embodying the invention which is arranged to grind a suspension of a particulate solid material by a method embodying the invention. Figure 2 is a perspective view from above of a pitched impeller for use in an apparatus embodying the invention as illustrated, for instance, in Figure 1. Referring to Figure 1, there- is shown a grinding apparatus generally designated by reference numeral 1 comprising a grinding chamber 2 containing a mixture 3 of a particulate grinding medium and a suspension of a particulate solid material. A rotatable vertically orientated shaft 4 is suspended in the grinding chamber 3 , an upper end of the shaft 4 being supported above the grinding chamber. A motor 5 is connected to the shaft by means of a suitable gear arrangement . A double pitched impeller 6 having two tiers 6a, 6b is secured to the shaft 4. A conduit 7 having an inlet opening 8 is provided at the base of the grinding chamber 2 to supply a feed suspension of particulate solid material to the chamber 2 to be comminuted. The inlet opening 8 is centrally located at the base of the grinding chamber 2 under a free end of the vertically orientated shaft 4. An annular baffle plate 16, located at a preset height above the impeller 6, is fixed to a side wall 18 of the grinding chamber 2. The drive shaft 4 extends through an opening 20 in the baffle plate 16. Referring to Figure 2, there is shown a tier (6a, 6b) of the pitched impeller 6 for use in the apparatus of Figure 1. The pitched impeller 6 comprises an annular body 11 having a central opening for disposing the impeller 6 onto the shaft 4, and, six blades 12 radially projecting from the annular body 11. Each blade 12 is pitched at an equivalent angle of about 45° relative to the plane of rotation of the impeller 6, such that a leading face 14 of each blade in the direction of rotation R of the impeller faces in a direction with a downward component . As the feed suspension is supplied to the grinding chamber 2, the shaft 4 and the pitched impeller 6 rotate to fluidise the particulate grinding medium and cause mixing of the grinding medium with the suspension of particulate solid material, resulting in grinding of the particulate solid suspension. The action of the pitched blades 6 and baffle plate 16 combine to compress the particulate solid suspension and improve the grinding efficiency. The comminuted solid suspension comprising particles of a desired particle size exits the grinding chamber 2 through a mesh 9 and is carried along a conduit 10 for further refining if necessary. The invention will now be described with reference to the following example. Example A kaolin suspension containing 30% by weight solids was introduced through inlet conduit 7 into a grinding apparatus of the type illustrated in Figure 1. Silica sand present in an amount of 60% by volume was used as the grinding medium in the apparatus . The magnitude of energy applied to the apparatus was 75kWh/t. The comminuted suspension obtained from the apparatus through conduit 10 was found to contain 92% by weight of particles having a particle size less than 2μm. The above experiment was also carried out using conventional prior art apparatus and a ground kaolin suspension containing 88% by weight of particles having a particle size less than 2μm was obtained. These results show that an improved comminuted product suspension having a finer particle size can be produced by use of the apparatus and method of the invention.

Claims: CLAIMS 1. Apparatus for grinding a suspension of a particulate solid material, which apparatus comprises : a grinding chamber for containing a particulate grinding medium; means for introducing a suspension of the particulate solid material into the grinding chamber; a rotatable impeller for agitating the particulate grinding medium and the suspension of particulate solid material in the grinding chamber, the impeller having at least one blade, wherein the or each blade of the impeller is pitched relative to the plane of rotation of the impeller; and means to drive the impeller, so as to produce a suspension of a ground particulate solid material. 2. Apparatus as claimed in claim 1, in which the said plane of rotation of the impeller is substantially horizontal. 3. Apparatus as claimed in claim 1 or 2 , in which the impeller comprises a hub, the or each blade of the impeller projecting from the hub substantially at right angles to the rotational axis of the impeller. 4. Apparatus as claimed in claim 3 , in which the hub comprises an annular body having a central opening which receives a rotatable drive shaft. 5. Apparatus as claimed in claim 4, in which the drive shaft is fixed to the impeller at one end and is connected to drive means at its other end. 6. Apparatus as claimed in any one of claims 3 to 5 , in which the blades are integrally formed with the hub of the impeller. 7. Apparatus as claimed in any one of the preceding claims in which the pitch of the or each blade relative to the plane of rotation of the impeller is between 30 and 60°. 8. Apparatus as claimed in claim 7, in which the pitch of the or each blade of the impeller relative to the plane of rotation of the impeller is 45°. 9. Apparatus as claimed in any one of the preceding claims, in which the impeller has a plurality of blades and each of the blades is pitched at substantially the same angle relative to the plane of rotation of the impeller. 10. Apparatus as claimed in any one of the preceding claims, in which the impeller comprises a plurality of blades which are arranged in two or more tiers. 11. Apparatus as claimed in claim 10, in which the tiers are located one above the other in a substantially vertically aligned stack. 12. Apparatus as claimed in claim 11, in which each tier of blades is mounted on a respective hub, the hubs being mounted on a common drive shaft. 13. Apparatus as claimed in any one of claims 10 to 12, in which the blades of respective tiers of the impeller are pitched at the same angle relative to the plane of rotation of the impeller. 14. Apparatus as claimed in any one of the preceding claims, in which a baffle member is located in the grinding chamber above the impeller. 15. Apparatus as claimed in claim 14, in which the baffle member comprises a plate which is disposed in a plane which is substantially parallel to the plane of rotation of the impeller. 16. Apparatus as claimed in claim 14 or 15, in which the baffle member has a central opening which receives a drive shaft of the impeller. 17. Apparatus as claimed in any one of claims 14 to 16, in which the baffle member is of rigid construction and is provided with an anti-wear component . 18. Apparatus as claimed in claim 17, in which the anti-wear component comprises a polyurethane coating lining or element. 19. Apparatus as claimed in any one of claims 14 to 18, in which the baffle member is removably secured to a wall of the grinding chamber. 20. Apparatus as claimed in any one of claims 14 to 19, in which the baffle plate is located at a preset height over the suspension of particulate solid material in the grinding chamber. 21. Apparatus as claimed in any one of the preceding claims, in which the grinding medium comprises particles having an average diameter in the range of from 0.1mm to 6mm. 22. Apparatus as claimed in claim 21, in which the grinding medium has an average diameter in the range of from 0.2mm to 4mm. 23. Apparatus as claimed in claim 21 or 22, in which the grinding medium is present in an amount of from 40% to 70% by volume of the material in the grinding chamber. 24. Apparatus for grinding a suspension of a particulate solid material, which apparatus comprises : a grinding chamber for containing a particulate grinding medium; means for introducing a suspension of the particulate solid material into the grinding chamber; a rotatable impeller for agitating the particulate grinding medium and the suspension of particulate solid material in the grinding chamber, means to drive the impeller; and a baffle member arranged over the particulate grinding medium and the suspension of particulate solid material, so as to produce a suspension of a ground particulate solid material. 25. Apparatus as claimed in claim 24, in which the baffle member comprises a plate which is disposed in a plane which is substantially parallel to a plane of rotation of the impeller. 26. Apparatus as claimed in claim 24 or 25, in which the baffle member has a central opening which receives a drive shaft of the impeller. 27. Apparatus as claimed in any one of claims 24 to 26, in which the baffle member is of rigid construction and is provided with an anti-wear component . 28. Apparatus as claimed in claim 27, in which the anti-wear component comprises a polyurethane coating, lining or element. 29. Apparatus as claimed in any one of claims 24 to 28, in which the baffle member is removably secured to a wall of the grinding chamber. 30. Apparatus as claimed in any one of claims 24 to 29, in which the baffle plate is located at a preset height over the suspension of particulate solid material in the grinding chamber. 31. A method of grinding a suspension of a particulate solid material, which method comprises: providing apparatus as defined in the first aspect of the invention; introducing into the grinding chamber the particulate grinding medium and the suspension of particulate solid material; and agitating the particulate grinding medium and the suspension of particulate solid material by rotation of the impeller, so as to produce a suspension of a ground particulate solid material. 32. A method of grinding a suspension of a particulate solid material, which method comprises: providing apparatus as defined in the second aspect of the invention; introducing into the grinding chamber the particulate grinding medium and the suspension of particulate solid material; and agitating the particulate grinding medium and the suspension of particulate solid material by rotation of the impeller, so as to produce a suspension of a ground particulate solid material.

Inventors: PEARCE DAVID ANTHONY, PEARCE, DAVID, ANTHONY
Applicants: IMERYS MINERALS LTD, PEARCE DAVID ANTHONY, IMERYS MINERALS LIMITED, PEARCE, DAVID, ANTHONY
Application year: 2001
Main classification: B02C17
IPC codes: B02C17