IMPROVED FLUID ENERGY MILL

Abstract: An improved fluid energy mill which is provided by an insert (1) having a leading edge and a trailing edge with an azimuthal angle of between 10 DEG and 300 DEG is disclosed.

Description: TITLE IMPROVED FLUID ENERGY MILL BACKGROUND OF THE INVENTION The present invention relates to fluid energy mills, in particular, to an improved fluid energy mill which is provided with a fluid dynamic control insert that maintains or improves quality of a product at lower energy consumption and at lower cost of operation. Fluid energy mills of a vortex type are well known and widely employed in certain industries because of their efficiency and economy in comminution of particulate solids. A number of early designs are described in considerable detail in U. S. Patent 2,032,827. They generally comprise a disc-shaped zone wherein an inward circular or spiral flow of the gaseous fluid causes attrition of the particles at the periphery and provides a size separation in an intermediate zone. The mill combines the function of grinding and classification within a single chamber. Since the fluid is fed into the periphery and discharged at the axis of a vortex there is a tendency for particles to be swept toward the central outlet in a spiral path. The force due to drag of the fluid acting on the suspended particle is opposed by the centrifugal force. This balance of forces can be so adjusted that coarse particles tend to return to or be held at the periphery for more attrition while smaller particles are swept to the center for collection in a cyclone and/or filters. In these mills the energy for comminution is supplied in a gaseous fluid medium injected tangentially into the vortex chamber to create and maintain the vortex. Prior art attempts to prevent premature escape of larger particles or avoid energy loss have been described in the literature. For example, U.S. Patent 3,425,638 describes a fluid energy mill having a cylindrical baffle being closed at one end and having a plurality of passageways on the cylinder surface. U.S. Patent 4,219,164 describes a fluid energy mill with upwardly flowing vortex having a circular annulus. Although various modifications have been proposed, none has proven to be wholly satisfactory and further improvements are desirable. Particularly, in the white pigment industry, there is a need to reduce the amount of oversized material passing prematurely into a resulting product. It may be necessary to increase the intensity of grinding with consequent greater costs in terms of fluid use, energy consumption and reduced capacity per mill and adverse effects on product properties. Thus, further enhancement in grinding efficiency is needed. Concomitantly, there is a need to achieve long life of inner wear liners typically used within these mills. The present invention meets these needs. SUMMARY OF THE INVENTION In accordance with this invention there is provided in a fluid energy mill of a vortex type for comminuting pulverulent materials having in combination, a disc-shaped chamber defined by a pair of opposing circular-shaped axial walls and a peripheral wall, a multiplicity of inlets extending through the peripheral wall and aligned for directing gaseous fluid into the chamber, means for charging pulverulent material to an outer portion of the chamber and discharge means for withdrawing pulverulent material and gaseous fluid along the axis of the chamber, the improvement comprising: (a) an insert, the insert having an azimuthal angle of a leading edge and a trailing edge between about 10° and about 300°, wherein the leading edge of the insert is positioned upstream, downstream or near the means for charging pulverulent material; and (b) a means for mounting the insert in the chamber, wherein the insert is operativery attached to the chamber . The fluid energy mill of this invention is characterized by the following advantages which cumulatively render it preferable to those currently available: 1. lowers total energy consumption in excess of at least 10% while maintaining or improving resulting product specifications; 2. increases production rate; 3. improved classification in the grinding zone thus narrowing the particle size distribution of the pulverulent material; 4. increases the life of mill liners; and 5. increases the uniformity of control over the operation of the mill, which provides much greater uniformity of the product. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a horizontal cross section view of a fluid energy mill embodying this invention. FIG. 2 is a horizontal cross section view setting forth an alternative embodiment of this invention. PETAfLEQ PESCMPTIQN QF THE INVENTION Most fluid energy mills are variations on a basic configuration of a disc¬ shaped chamber enclosed by two generally parallel circular plates defining axial walls and an annular rim defining a peripheral wall, the axial length or height of the chamber being substantially less than the diameter. Around the circumference of the mill are located a number of uniformly spaced jets for injecting the gaseous fluid which furnishes the energy for comminution, along with one or more injectors for feeding the pulverulent material to be comminuted. Jets are oriented such that the gaseous fluid and pulverulent material are injected tangentially to the circumference of a circle smaller than the chamber circumference. A conduit coaxial to and in direct communication with the disc shaped chamber is provided for discharge of the comminuted solids to a cyclone and/or filter for collection. The fluid energy mill of this invention can be any fluid energy mill as known in the art of the vortex type, having either top or bottom exit, and having an insert such as a vane configuration positioned within the grinding chamber as described hereinbelow. A particularly preferred base mill with no insert is described in U.S. Patent 3,726,484, the teachings of which are incorporated herein by reference. The improved fluid energy mill has an insert having a wide range of functional shapes, including plate or any curved shape such as an airfoil. The insert can optionally have slats. The insert does not need to be smooth and continuous. The insert can be a series of pins defining a curve or a series of fiat or curved shapes such as airfoils. In a preferred embodiment, the insert has an airfoil shape but it will be appreciated that the insert is functional over an extremely wide range of shapes, lengths of grinding chamber blocked, positions within the grinding chamber and operating conditions. Materials of construction of the insert can vary, and are typically hard and wear resistant. Examples include but are not limited to stainless steel, hardfaced stainless steel, 440 stainless steel, white cast iron, or ceramics comprising metal compounds of oxides, borides, carbides, nitrides and mixtures thereof. The insert is preferably constructed of a ceramic or a mixture of ceramics such as silicon carbide, silicon nitride, aluminum oxide or the like. The insert has an azimuthal angle or span ranging from about 10° and 300°, preferably between about 60° and 180° and most preferably between about 90° and 140°. The "azimuthal angle" is defined herein as the angle between a leading edge and a trailing edge of the insert within the mill, i.e., an arc of a horizon measured between a fixed point and a vertical circle passing through the center. "Leading edge" is used herein to refer to rotational flow of fluid in relation to the insert, i.e., the portion of the insert meeting the incoming fluid stream. "Trailing edge" is used herein to refer to the portion of the insert receding the incoming fluid stream. The insert is located such that the leading edge is upstream, downstream, near or at the means for charging pulverulent material, i.e., feed inlet or feed tube. A preferred distance of the feed inlet can be within about 10° of the leading edge. Preferably, the leading edge is upstream of the feed inlet, that is, the leading edge precedes the feed inlet. The feed inlet is used to introduce a pulverulent material into the mill. The feed inlet can provide introduction of feed material into the top, side, or bottom of the mill. It is preferred to have the feed inlet introduce material by a side feed. One or more feed inlets are contemplated. The insert has an angle of attack that can be positive, zero, or negative. "Angle of attack" is defined herein as an arctan of the distance of the trailing edge of the insert from a peripheral wall minus the distance of the leading edge of the insert from the peripheral wall, divided by a chord length. For determining the angle of attack, peripheral wall refers to the outer peripheral wall of the disc shaped chamber, i.e., grinding chamber. The chord length is the distance between the leading edge and the trailing edge. Surprisingly, when the angle of attack is positive there is a dramatic improvement in feed vacuum, that is, a higher feed vacuum allows more pulverulent material to be introduced into the mill. The preferred angle of attack is positive and may range from 0° to 45°, and preferably 0° to 25°, and more preferably 0° to 15 β . The radial distance of the insert from the grinding wall is not especially critical. However, this distance is preferably 10-60% of the radial distance, and more preferably, 30-40% of the radial distance at the leading or trailing edge of the insert. The insert can be placed within the mill such that it is angled or perpendicular relative to the top or bottom of the mill. Preferably, the insert is perpendicular to the bottom of the mill. The insert may be secured in place at some fixed point within or outside the chamber, for example, the insert can be fixed by attachment to an outer housing or to the inner lining. The insert can be mounted in any fashion within the mill such that the insert is physically held within the grinding chamber. The insert can be rigidly fixed in place or can be positioned such that it is capable of movement, such as oscillation about the angle of attack, while the mill is in operation. Preferably, the insert is rigidly fixed in place. The means for mounting the insert is not especially critical and will depend upon materials of construction and operating parameters of the mill. For example, an adhesive, compression between the top and bottom axial walls, or struts can be used to mount the insert to a center pin, or struts can be used to mount the insert to the top or bottom of the mill or mill housing. . The struts may or may not be movable. Alternatively, the insert can also be directly bonded to the liner of the mill by means such as bonding or as casting the insert as part of the liner or mounting the insert to the liner. Still other possible means for mounting the insert within the mill can be through a radial arm that may be movable, e.g., via cylinder or screw, to allow rotation of the insert around the grinding chamber for adjustment of operating conditions. A radial arm mount for the insert can also provide means to pivot the insert, providing the capability of varying the angle of attack. Other means for mounting the insert within the mill will be apparent to one skilled in the art using the preceding description and utilizing the present invention to its fullest extent. In operation of a fluid energy mill of this invention, any carrier gas can be used as the fluid, such as nitrogen, compressed air, helium, steam, CO2, steam under pressure, superheated steam, if desired. Other vapors or gases may be selected for use primarily on the basis of compatibility with the material being processed and provided the materials involved are not degraded by contact with the carrier gas. Pulverulent material, i.e., feed material to be ground and classified can be any solid material, inorganic or organic. Inorganic materials can be, for example, metal oxides, such as titanium dioxide, ceramics, and minerals. Organic materials can be, for example, pharmaceuticals or coal. The present invention provides an improved fluid energy mill having an insert positioned inside of the mill such that it partially blocks a mean free path of a grinding fluid and ground particles as they attempt to exit the grinding portion of the mill grinding chamber. The insert redefines the fluid (grind fluid plus feed material particles) flow direction, and the absolute pressure regions established within the fluid energy mill. It is believed the insert is not only a physical barrier to undesirable pathways of partially ground particles, it is also a fluid dynamic device that directly alters the velocity, mean free path, and absolute pressure of the grinding fluid in localized regions of the fluid energy mill, resulting in previously unknown control of the operating parameters of a fluid energy mill. Referring now to the drawings, like reference numerals and reference characters have the same significance. FIGURE 1 is a schematic horizontal cross section view of a fluid energy mill of this invention. Insert (1) is a curved shape showing a positive angle of attack, having distance (A) of the trailing edge from the grinding wall greater than distance (B) of the leading edge from the grinding wall. Mill inner wear liner (2) provides the grinding wall. Inlet opening (3) provides for introduction of pulverulent material through the top of the mill cover. Ring jet openings (4) in mill inner wear liner (2) provide for introduction of fluid into the mill. A multiplicity of ring jet openings (4) is preferred. Inserts (5) and (6) show alternative embodiment locations for the insert, at zero angle of attack [distance (A) is equal to distance (B)] and at negative angle of attack [distance (A) is less than distance (B)], respectively. Direction of internal fluid flow (7) is also shown. FIGURE 2 is a schematic horizontal cross section view of a fluid energy mill of this invention. Figure 2 differs from Figure 1 with respect to inlet opening (3). Inlet opening (3) in Figure 2 provides for introduction of feed material through a side opening in the mill inner wear liner (2). To give a clearer understanding of the invention, the following Example is construed as illustrative and not limitative of the underlying principles of the invention in any way whatsoever. EXΔMELE An airfoil shaped insert constructed of stainless steel having an azimuthal angle of 120°, a positive angle of attack of 5° v. as mounted with a radial arm pinned in the center of a fluid energy mill of the vortex type creating an "L" cross section. The insert was pinned such that it was held rigidly in place. This apparatus was tested in a commercial plant and five T.O2 pigments were tested. Also, five Tiθ2 pigments were tested without the presence of the insert (Control). Products were compared. A practical method of evaluating the mill action was used, i.e., measurement of gloss and particle size for coatings grades (Table 1), and screen and particle size for plastics grades (Table 2). Steam to pigment rates and feed rates were also measured. Inasr £ojιttpi Gloss 68 - 76 67 - 76 % >0.6 6 - 16 5 -16 S/P ratio 20- 45% less steam Feed rate 4-26% more rate IΔB E Insert Control Screen 11 11 %>0.6 8 8 S/P ratio ' 30% less steam Feed rate 9% more rate Use of the insert in the fluid energy mill reduced the quantity of steam required to grind the pigments and provided improved or comparable quality of the resulting product. For certain pigments, the feed rates were enhanced when the insert was used without detrimental effects on product quality. A further result of these tests was that the liner was still functional at a lifetime of about two to four times longer than normal life expectancy of such a liner. As used herein, Gloss is determined by formulating a pigment sample into a test paint, which is prepared by using a sandmilled dispersion of Tiθ2 in an alkyd-melamine baking system or in the case of waterbome systems by drawdowns of high speed dispersed emulsion paints, sprayed on an aluminum panel and compared with panels of known gloss values. % >0.6 is the fraction of particles greater than 0.6 microns in size. Particle size distribution of the pigment products was measured by sedimentation analysis, with a SedigraphΦ (Micromeritics Instrument Corp., Norcross, GA) after dispersion in suspension by fixed level sonication. S/P ratio is the improvement in steam to pigment ratio when the insert was present in the fluid energy mill relative to the steam to pigment ratio when there was no insert present in the mill. Improvement in S P ratio reduces the energy costs related to operating the mill and also can provide higher feed rate of pigment. Feed rate is the increase in feed rate of pigment when the insert is present in the mill relative to the feed rate without the insert present. Increase in feed rate allows operation of the mill at higher throughput of pigment, therefore, higher production rates. Screen is a test of dispersion. A 50 wt% concentrate of T-O2Λ0W- density polyethylene was prepared in a BanburyΦ-type mixer (available from Farrel Corp., Ansonia, CT), chopped into small granules, aad extruded on Killion Extruder through a 325 mesh screen. The undispersed Tiθ2 grit particles retained on the screen were measured on a Texas Nuclear single element analyzer. The higher the number, the poorer the dispersion of the T.O2 in the plastic. Having thus described and exemplified the invention with a certain degree of particularity, it should be appreciated that the following Claims are not to be limited but are to be afforded a scope commensurate with the wording of each element of the Claims and equivalents thereof. From the foregoing description, one skilled in the art can easily ascertain the essential characteristics of this invention, and without departing from the spirit and scope thereof, can make various changes and modifications of the invention to adapt it to various usages and conditions.

Claims: CLAIMS What is Claimed is: 1. In a fluid energy mill of a vortex type for comminuting pulverulent materials having in combination, a disc-shaped chamber defined by a pair of opposing circular-shaped axial walls and a peripheral wall, a multiplicity of inlets extending through the peripheral wall and aligned for directing gaseous fluid into the chamber, means for charging pulverulent material to an outer portion of the chamber and discharge means for withdrawing pulverulent material and gaseous fluid along the axis of the chamber, the improvement comprising: (a) an insert, the insert having an azimuthal angle of a leading edge and a trailing edge between about 10° and about 300°, wherein the leading edge of the insert is positioned upstream, downstream or near the means for charging pulverulent material ; and (b) a means for mounting the insert in the chamber, wherein the insert is operatively attached to the chamber . 2. The mill of claim 1, wherein the azimuthal angle of the insert is between about 60° and about 180°. 3. The mill of claim 2, wherein the Bzimυthal angle of the insert is between about 90° and about 140°. 4. The mill of claim 1 or claim 2 or claim 3, wherein the insert has an angle of attack selected from the group consisting of positive, zero and negative. 5. The mill of claim 4, wherein the insert has an airfoil shape and a material of construction selected from the group consisting of stainless steel, hardfaced stainless steel, 440 stainless steel, cast iron and ceramic. 6. The mill of claim 5, where the material of construction of the insert is a ceramic selected from metal compounds of borides. carbides, nitrides and mixtures thereof. 7. The mill of claim 5, wherein the insert is rigidly fixed in the center of the disc shaped chamber. 8. In a fluid energy mill of a vortex type for comminuting pulverulent materials having in combination, a disc-shaped chamber defined by a pair of opposing circular-shaped axial walls and a peripheral wall, a multiplicity of inlets extending through the peripheral wall and aligned for directing gaseous fluid into the chamber, means for charging pulverulent material to an outer portion of the chamber and discharge means for withdrawing pulverulent material and gaseous fluid along the axis of the chamber, the improvement comprising: a stainless steel insert mounted with a radial arm pinned in the center of a chamber and having an azimuthal angle of a leading edge and a trailing edge of the insert of about 90° and 140° and a positive angle of attack of about 0° and 45°, wherein the leading edge is positioned upstream to the means for charging pulveruent material through a side opening in a mill liner. 9. The fluid energy mill of claim 1 or claim 8 wherein the pulverulent material is TIO2. 10. In a vortex type fluid energy mill disc-shaped chamber for comminuting pulverulent materials with steam, the improvement comprising a fluid dynamic energy control insert positioned in the disc-shaped chamber, wherein the insert has an azimuthal angle of a leading edge and a trailing edge between about 10° and about 300° and the insert alters a mean free path of the steam and pulverulent material.

Inventors: CAPELLE WILLIAM EDWARD JR, CONNOLLY JOHN DONALD JR, DE LA VEAUX STEPHAN CLAUDE, DIAZ ANA ESTELA, LANCI JOHN PHILLIP SR, SCHURR GEORGE ALAN, CAPELLE, WILLIAM, EDWARD, JR., CONNOLLY, JOHN, DONALD, JR., DE LA VEAUX, STEPHAN, CLAUDE, DIAZ, ANA, ESTELA, LANCI, JOHN, PHILLIP, SR., SCHURR, GEORGE, ALAN
Applicants: DU PONT, E.I. DU PONT DE NEMOURS AND COMPANY
Application year: 1997
Main classification: B02C19
IPC codes: B02C19