GB2045642A - Comminuting and classifying mill - Google Patents

Abstract

A comminuting and classifying mill of the re-entrant circulating stream jet type includes a vortex chamber (12) with nozzles (24) for producing a classifying vortex in the chamber. Material is fed in a gaseous carrier from a feed duct (42) tangentially into a feed chamber (34) and hence helically through a recess (32) in one wall of the chamber (12) into the vortex so that the material and carrier gas is added to the vortex with a rotative velocity. <IMAGE>

Description

SPECIFICATION Comminuting and classifying mill The invention relates to comminuting and classifying mills of the re-entrant circulating stream jet type commonly referred to as "Micronizers". The Micronizer is the oldest and most widely used of the re-entrant circulating stream jet grinding mills.

This type of grinding mill is described in detail in U.S. Patent Specification No.

2,032,827 of March 1 936. The basic mill includes a vortex chamber comprising an annular peripheral wall closed by two opposed lateral walls. In its preferred form, the vortex chamber is formed so that the axial length of the peripheral wall is only a small fraction of the diameter of the chamber. The peripheral wall is usually surrounded by a manifold through which high pressure gas is supplied to a plurality of nozzles positioned around the peripheral wall and angled so that the gaseous stream issuing from them propels the fluid into the chamber with both circumferential and radial components of movement to form a fluid vortex in the chamber.Comminution of the material occurs at least partly as a result of the gas jets from the nozzles forcing the heavier particles of material at the outer edge of the circulating stream to re-enter the circulating stream and cause mutual attrition between the particles. Conventionally, the material to be comminuted is fed into the chamber by jet and venturi apparatus located near the periphery of the chamber.

The introduction of feed material into the Micronizer has been a continuing problem since it was first put into commercial use.

These problems are discussed in some detail in U.S. Patent Specification No. 4,018,388.

The principal problems involve the distribution of the feed material in the proper zone and its effect upon the whirling vortex of fluid within the chamber.

Jet feeding means are mainly concerned with providing a more uniform distribution material fed into the grinding zone although they do have the advantage of giving the feed material and its carrier fluid an initial velocity.

They are principally preferred for comminuting various types of free-flowing materials because damp, viscous, or precipitate materials frequently tend to clog the feed apparatus.

The problem with screw-feeding material into the chamber arises from the fact that the circulating gases within the chamber have little or no momentum. The introduction of feed material in a static condition results in a localized shock to the conformity of flow of the circulating gases as they try to accelerate the feed material from the static condition to the velocity of the circulating load in the mill.

The apparatus described in U.S. Patent Specification No. 4,018,388 has been a commercially successful solution to the problem of improving the distribution of the feed material as it is introduced into the re-entrant circulating stream jet grinding mill. As described in that specification, a conical recess is provided in the side wall opposite to the outlet and the feed material is axially introduced into the recess so that it disperses from the apex of the cone radially axially toward the classification zone of the circulating gases. The feed material becomes entrained with the circulating gases, the vortex of which extends into the conical recess. Thus, the feed material mixes with the circulating gases and there is a uniform distribution of the feed material into the classification zone.

Although the mill of No. 4,018,388 is indeed a successful improvement in feed distribution in this type of mill, the fact remains that the fluid gas and entrained feed material is directed radially and axially in a random manner with no circulating direction. This means that the energy necessary to give it a high velocity whirling effect must be extracted from the classifying vortex. Indeed, the specification expressly points out that it uses the circular velocity of the gases in the classification zone to accelerate the feed material to a velocity in excess of the circulating load adjacent the inner periphery of the mill.

The present invention is therefore concerned with providing all of the benefits and advantages of the re-entrant circulating stream jet grinding mill described in No. 4,018,388 without extracting energy from the classifying vortex circulating within the chamber. Thus, it is one of the objects of the present invention to feed material with its carrier gas so as to provide it with a rotative velocity which will be transmitted to the classifying vortex thereby supplementing its velocity rather than extracting energy from it.

In accordance with the present invention, a comminuting and classifying mill comprises a generally circular vortex chamber with an annular peripheral wall and opposed lateral walls, the annular peripheral wall having a plurality of nozzles positioned to propel gaseous fluid, in use, into the chamber with both circumferential and radial components of movement to form a fluid vortex in the chamber; a material outlet in one of the lateral walls; a recess having a circular cross-section in the central portion of the other of the lateral walls; material feed means including a feed chamber opening into the recess, the chamber including a wall in the form of a regular surface of revolution coaxial with the recess; feed inlet means for the chamber positioned to propel, in use, gaseous carrier fluid and material into the chamber generally tangentially to the wall of the chamber whereby the material and its carrier fluid are added to the fluid vortex with a supplemental rotative velocity; means to connect the feed inlet to a source of material; and means to connect the nozzles and the feed inlet to a source of gaseous fluid under pressure.

With this construction, the material is not only uniformly distributed into the classification zone, but this is also accomplished by supplementing rather than extracting energy from the circulating vortex of gas and material within the mill vortex chamber.

A jet and anvil comminuting means may be positioned in such a manner that the discharge from the anvil chamber tangentially enters the feed chamber. The material comminuted by the jet and anvil grinding means or otherwise fed to the feed inlet can be feed material entering the mill and/or a portion of the load in the mill which is extracted through a duct connected adjacent the periphery of the vortex chamber and recirculated.

Undesirable contaminants may be removed from the material being comminuted by providing an annular shoulder at the junction of the feed chamber and conical recess, and venting means to remove material diverted by the shoulder.

Some examples of mills constructed in accordance with the invention are illustrated in the accompanying drawings, in which: Figure 1 is a vertical axial sectional view of one mill; Figure 2 is a section taken on the line 2-2 in Fig. 1; Figure 3 is a section taken on the line 3-3 in Fig. 1; Figure 4 is a partial sectional view of an alternate jet and anvil feeding means for the mill of Fig. 1; Figure 5 is a sectional view of another example of mill showing the mill with apparatus to recirculate the load through a jet and anvil comminuting means; Figure 6 is a section taken on the line 6-6 in Fig. 5; and, Figure 7 is a partial sectional view of another example of mill.

Fig. 1 shows a comminuting and classifying mill 10 of the re-entrant circulating stream jet type. The mill 10 includes a circular comminuting and vortex chamber 12 with an annular peripheral wall 14 and opposed lateral walls 16 and 18. The walls 14, 16 and 18 are removably held together by C-clamps 20 and 22 so that the mill 10 may be readily disassembled and cleaned.

As best shown in Fig. 2, a plurality of nozzles 24 are spaced around the whole of peripheral wall 1 4 and angled relatively to the radius of chamber 1 2 so that fluid jet streams emitted from them move with both a forward (i.e. circumferential) and radial components.

Thus, as is known in the art, the nozzles create the circulating vortex within the chamber 12.

Surrounding the annular peripheral wall 14 is an annular manifold 26 connected to a source of gaseous fluid under pressure (not shown) through the duct 28. An outlet duct 30 for comminuted material extends through the wall 1 8 and its opening is preferably coaxial with the axis of the circular chamber 12.

The axial wall 1 6 is provided with a central recess 32 which as shown is frusto-conical in shape.The larger diameter end of the recess 32 is coplanar with the lateral wall 1 6 and its smaller diameter end is remote from it as shown. The smaller diameter or apex of the recess 32 opens into a feed chamber 34 which as shown includes cylindrical wall 36 is closed by end wall 38. Although the feed chamber 34 is shown in Fig. 1 as being cylindrical, it should be understood that it can take other shapes as explained and illustrated below. But chamber 34 should have a circular cross-section or otherwise be in the form of a regular surface of revolution. Moreover, the axis of the wall 36 is preferably coaxial with the axis of the recess 32.

As best illustrated in Fig. 3, the chamber 34 is provided with an inlet 40 which extends through the wall 36 in such a manner that material fed through the duct 42 flows into the chamber 34 tangentially with the wall 36.

The feed materiak is inserted into the apparatus 10 through the funnel 44 and is entrained by the carrier fluid from the nozzle 46 which injects the material into the venturi passage 48, where it is accelerated and propelled through the duct 42 and inlet 40 into the chamber 34. The nozzle 46 is connected to a source of carrier fluid under pressure (not shown).

The diameter of the feed chamber 34 rhust be of sufficient dimension in relation to the total amount of carrier gas and material tangentially directed into it that it will enter the apex of the recess 32 with a greater rotational than upward directional component. By way of example but not limitation, it has been determined that in most applications an axial length equal to the diameter of the feed chamber 34 is satisfactory. Further, the diameter must be considered in relation to the recess 32. In this regard, satisfactory results are obtained when the diameter of feed chamber 32 is approximately one-half (1 /2) the larger diameter of recess 32. As explained below, some variation is required when the feed chamber is designed to remove undesirable contaminants from the material or when the carrier gas includes recirculating material drawn from the apparatus.

The carrier fluid and entrained material are constrained by wall 36 to whirl around the inside of feed chamber 34. The material and carrier fluid also move axially with a high velocity helical action toward and into the recess 32 where it transfers its rotary energy to the classifying vortex of the mill itself. In this manner, the carrier gas and entrained material add energy to the circulating vortex in the classifying zone. As pointed out in No.

4,018,388, the rotary velocity in the classifying zone exceeds the velocity of the circulating load adjacent to the inner periphery of the mill. Moreover, the axial introduction of the material into the apex of the recess, centrally located in relation to the periphery of the apparatus, permits the material to be dispersed radially and axially resulting in a more uniform distribution of the feed material into the classification zone.

For a number of materials, it is advantageous to remove a portion of the circulating load and recirculate that material in the same manner as feed material. Figs. 5 and 6 show how this advantage may be accomplished. The structural elements of Figs. 5 and 6 which are the same as the structural elements of the embodiment shown in Figs. 1 to 3 are indicated by primed numbers but their function is not again described so as to avoid unnecessary repetition.

The mill 60 is a comminuting and classifying apparatus of the re-entrant circulating stream jet type. It differs from the mill 10 in that it is provided with a means to remove a portion of the circulating load from the mill and recirculate that portion through the feed chamber 34' together with the feed material.

As shown, a recirculation duct 62 opens into the chamber 12' adjacent to the peripheral wall 14'. Duct 62 conducts load material and gaseous fluid to a point adjacent the exit end of nozzle 64 which is connected to a source of gaseous fluid under pressure (not shown). In a manner known in the art, the high velocity gas exiting from nozzle 64 entrains the load material from recirculating duct 62 and it is accelerated in the venturi 66. The entrained material is therefore driven at high velocity into the impact chamber 68.

Impact chamber 68 is provided with an anvil 70 whose surface is a hardened material such as tungsten carbide. This provides an effective jet and anvil grinding means for additional comminution of the recirculated material.

The feed material is inserted through funnel 72 and is entrained by the gaseous fluid under pressure from nozzle 74 which injects the material into the venturi 76. Venturi 76 accelerates the material and propels it against the anvil 70 where it undergoes primary com- minution.

The comminuted feed material and comminuted recirculated material rebound from the anvil 70 and flow together with the gaseous fluid through feed duct 48' into feed chamber 34'. As described above, in respect to feed mill 10, the material enters feed chamber 34' at a high velocity and generally tangential to the wall 36' so that it acquires a rotative velocity and commences whirling helically toward and into the recess 32'.

It has been determined that for operation of a mill where a portion of the load is recirculated, the diameter of the feed chamber 34' should be slightly less than the largest diameter of the frusto-conical recess 32'. This dimension is satisfactory for a majority of the comminution processes, particularly where two-fifths (2/5) of the total gas volume. used in the operation of the mill is supplied to the feed chamber 34'. In general, this gas comprises the carrier fluid for the original feed material plus the gas drawn through recirculating duct 62, plus gas issuing from nozzle 64.

It should also be understood that primary jet and anvil comminution of the feed material as illustrated in Fig. 5 and 6 is not required for use within a mill that recirculates a portion of the load. The fed material can be fed directly into the feed chamber as in Figs. 1 to 3.

In addition, it may under some circumstances be advantageous to provide primary comminution of the feed material only. This is illustrated in Fig. 4 where in the mill is the reentrant circulating stream jet type and may or may not, as desired, recirculate a portion of the load without jet and anvil comminution.

In Fig. 4, the elements which are the same as those of the embodiment of Figs. 1 to 3 are shown with double primed numbers but to avoid unnecessary repetition, they are not otherwise described.

As shown in Fig. 4, the feed material is inserted through funnel 80 and is entrained by gaseous fluid under pressure from nozzle 82. Nozzle 82 injects the material into an venturi 84 where it is accelerated and propelled at a high velocity against an anvil 86.

Anvil 86 is mounted within an impact chamber 88 and is provided with a hardened surface, such, as by way of example, tungsten carbide. The comminuted feed material flows from impact chamber 88 through feed duct 42' into feed chamber 34'. It is injected at a high velocity into feed chamber 34" tangentially to the wall and flows into the recess 32" in the manner heretofore described.

If the feed material is damp when using jet and anvil, it is desirable to use a heated gas, such as superheated steam. In the operation of a mill, such as the mill 60' shown in Figs.

5 and 6, there will be more recirculated material than original feed material. Moreover, the amount of fluid energy required to impact the recirculated material against the anvil 70 is necessarily more than that required to inject the original feed material. As a result, there is a considerable drying effect resulting from the merging of the dry fluid entrained recirculating material with the damp feed material.

Some damp feed materials tend to build up on the anvil and then drop off in lump form thereby clogging the feed duct 42'. When these conditions occur, it is desirable to elimi nate the anvil 70 and have the two gaseous streams impact each other as is known in the art and was first described in U.S. Patent Specification No. 697,505 of 1902.

It should also be noted that steam operated Micronizers and other re-entrant circulating stream jet type mills have been used to remove flint particles from china clay by peripheral venting. Although this method of removing undesirable flint has been commercially accepted, the apparatus discharges more clay with the flint than is considered to be optimum.

The circulating load concentrated at the periphery of the chamber 12' is the material having the highest specific gravity, the least grindability and the coarsest fractions being comminuted. Because of the axial restriction at the periphery of re-entrant circulating stream jet mills, such as Micronizers, the turbulent circulating stream of material has considerable radial extent. This is necessary so that sufficient material will circulate in front of the jets issuing from the nozzles for them to pick up the material adjacent the peripheral wall and drive it, in re-entrant manner, through the circulating steam for impact grinding.Because of this radial depth of material, prior art peripheral venting means have been unable to make a very selective cut unless the feeding of material is stopped periodically to allow the product to be slectively removed leaving the contaminants in the mill as described in U.S. Patent Specification No.

2,390,678. Still further, this process has proved rather difficult to automate on large, high temperature steam mills resulting in a non-uniform product. Stated otherwise, a uniform rate of material feeding is a known condition for obtaining uniform product size in re-entrant circulating steam jet mills.

As previously indicated, one of the objects is to provide new means for removing undesirable contaminants from the material undergoing comminution. The apparatus shown in Fig. 7 is specifically directed to removing undesirable contaminants from the material undergoing comminution without removing an unacceptable quantity of material which ultimately would become a desirable product when further comminuted.

The mill illustrated in Fig. 7 is a comminuting and classifying apparatus of the re-entrant circulating stream jet type and is generally indicated by the numeral 90. To avoid unnecessary redundant description, elements of the mill 90 which are the same as elements of the mill 10 are indicated by triple primed numbers.

As in the case of the mill 60 illustrated in Figs. 5 and 6, the mill 90 is provided with means for recirculating a portion of the load.

This recirculating means includes the recirculating duct 92 which conducts load material from chamber 12"' to the impact chamber 94 where it undergoes jet and anvil comminution as described in relation to the mill 60.

In the same manner, material is fed from a funnel into the impact chamber 94. Combined recirculating material and feed material flow through feed duct so 42"' into a feed chamber 1 00. It enters feed chamber 100 tangentialiy to a wall 102 and thus is constrained to flow in the form of a helix through the chamber 100 into recess 32"' as heretofore described.

In the mill 90, the entrance to the recirculating duct 92 is spaced radially inward from the peripheral wall. By way of comparison, the entrance to the duct 62 of the mill 60 is adjacent the peripheral wall 14'. This inward spacing of the entrance to the duct 92 permits a finer average comminution in the mill proper and reduces the percentage of desirable material that is vented from the mill together with the undesirable material. For the same reason, it may be preferable initially to feed material directly into the chamber 12"' as is conventionally done in re-entrant circulating stream jet mills. For this purpose, the feed material is inserted through a funnel 106 and is entrained by gaseous carrier fluid under pressure from a nozzle 108 which injects the material into a venturi 100 and through the lateral wall 18"' into the chamber 12"'.

Under certain conditions, such as for example the feeding of damp materials, it is desirable to introduce the feed material tangentially into the apex of the recess 32"'. For this purpose, the apparatus 90 is provided with a funnel 112, nozzle 114 and venturi 116 which accelerates the feed material into an impact chamber 11 8 against an anvil 1 20 in the manner described in detail in the embodiment of Fig. 4. The feed material is comminuted in impact chamber 11 8 and flows through duct 1 22 into the recess 32"' adjacent the apex thereof.

As shown in Fig. 7, the feed chamber 100 differs from the previously described feed chambers in that the wall 102 is frustroconical with the smaller diameter or apex thereof joining the apex of the recess 32"'. However, the apex of the conical wall 102 is slightly larger than the apex of the recess 32"' so that the wall of recess 32"' overhangs the interior of feed chamber 100 and forms a annular shoulder 1 30 which functions to divert undesirable particles.

The undesirable material is vented from the space between the top of wall 102 and annular shoulder 1 30 through one or more curved slots 1 32 formed in the top of wall 102 and connected to appropriate exhaust means (not shown).

The operation of the mill 90 is somewhat similar to the operation of the mill 60. Material discharged through recirculating duct 92 is conveyed into impact chamber 94 for jet and anvil comminution. From impact chamber 94, the material is carried through duct 42"' into feed chamber 1 00.

It has been found that the conical shape of feed chamber 100 and the shoulder 130 is particularly useful for the removal of small quantities of undesirable particles such as flint from china clay or pyrites (which contain undesirable sulphur) or ash producing shale from coal. Still further, it is preferable that the height of feed chamber 100 be approximately twice its largest diameter. For some material, an even greater ratio of height to diameter may be desirable.

Since the recirculating material enters feed chamber 100 at or adjacent to the bottom and tangentially to the wall 102 thereof, the material will spread over the considerable volume of the chamber 100 as it rises axially in a rotational manner. As a result, there is very little radial depth of material adjacent the wall 102 such as when venting adjacent the periphery of the mill as shown in Fig. 5.

As the gaseous fluid and entrained material converge axially due to the conical shape of the feed chamber 100, a sorting such as when venting adjacent the periphery of the mill as shown in Fig. 7 occurs.

The sorting effect is obtained because the higher specific gravity and larger particles concentrate as a thin layer against the wall 102 while the finer fractions spread radially inward toward the axis of the mill and enter the apex of recess 32"'.

It follows that the radial dimension of the shoulder 1 30 must be limited to prevent venting too much desirable material together with the contaminants. By properly dimensioning the radial extent of shoulder 130, a very selective removal of contaminants is possible. In general, the larger the mill, the greater the radial extent of the should 1 30.

Thus, large mills designed for several ton per hour of material throughput and which have a lesser quantity of contaminants in the feed material. By way of example but not limitation, a 24 inch mill having a throughput of 1 ton per hour of feed material having 8% contaminants was found to operate in a satisfactory manner with an annular shoulder of three-eighths (3/8) of an inch.

Claims (9)

1. A comminuting and classifying mill, comprising a generally circular vortex chamber with an annular peripheral wall and opposed lateral walls, the annular peripheral wall having a plurality of nozzles positioned to propel, in use, gaseous fluid into the chamber with both circumferential and radial components of movement to form a fluid vortex in the chamber; a material outlet in one of the lateral walls; a recess having a circular crosssection in the central portion of the other of the lateral walls; material feed means including a feed chamber opening into the recess, the chamber including a wall in the form of a regular surface of revolution coaxial with the recess; feed inlet means for the chamber positioned to propel, in use, gaseous carrier fluid and material into the chamber generally tangential to the wall of the chamber whereby the material and its carrier fluid are added to the fluid vortex with a supplemental rotative velocity; means to connect the feed inlet to a source of material; and means to connect the nozzles and the feed inlet to a source of gaseous fluid under pressure.

2. A mill according to claim 1, wherein the means to connect the feed inlet to a source of material includes means to connect the inlet to receive material being fed to the mill.

3. A mill according to claim 1, wherein the means to connect the feed inlet to a source of material includes means to connect the inlet to receive material recirculated from the vortex chamber.

4. A mill according to any one of the preceding claims, wherein the means to connect the feed inlet to a source of material includes jet and anvil comminuting means.

5. A mill according to any one of the preceding claims, wherein the recess is frustoconical in configuration with the smaller diameter end thereof remote from the vortex chamber.

6. A mill according to any one of the preceding claims, wherein the chamber is cylindrical.

7. A mill according to any one of claims 1 to 6, wherein the chamber is frusto-conical with the smaller diameter end thereof adjacent to the recess.

8. A mill according to any one of the preceding claims, wherein an annular material diverting shoulder is positioned adjacent to the junction of the chamber and recess for selectively diverting particles of material to a vent opening, and venting means for venting the particles of material from the mill.

9. A mill according to claim 1, substantially as described with reference to any one of the examples illustrated in the accompanying drawings.