US2707556A

US2707556A - Electrode edge construction

Info

Publication number: US2707556A
Application number: US271728A
Authority: US
Inventors: Eli C Gear; William A Brastad
Original assignee: General Mills Inc
Current assignee: General Mills Inc
Priority date: 1952-02-15
Filing date: 1952-02-15
Publication date: 1955-05-03
Anticipated expiration: 1972-05-03

Description

y 5 1955 a c. GEAR EI'AL 2,707,556

ELECTRODE EDGE cous'mucnon Filed Feb. 15. 1952 [III/III] l/III/I/ I III III/ I 94 FlG.3

INVENTORS E. C. GEAR WILLIAM A. BRASTAD BY 1%; W "ram/5r United States Patent ELECTRODE EDGE CONSTRUCTION Eli C. Gear and William A. Brastatl, Minneapolis, Minn.,

assignors to General Mills, Inc., a corporation of Delaware Application February 15, 1952, Serial No. 271,728

14 Claims. (Cl. 209-127) The present invention relates to electrostatic separators and more particularly to an improved electrode edge construction for such a separator.

Electrostatic separators have been proposed in which the materal to be classified is fed in a thin uniform layer along the upper surface of one electrode plate and certain selected particles are projected upwardly through openings in an upper electrode plate, which is maintained at a predetermined potential difference with respect to the lower electrode. In order to convey the material along the upper surface of the lower electrode it has been necessary to provide side plates of some sort at the lateral edges of the lower electrode to prevent undesired discharge of the material at such edges. Considerable difficulty has been encountered in the provision of satisfactory side plates.

For example, if metallic side plates are used, such plates normally tend to affect adversely the uniformity of the field at' the edges of the device. Non-uniformity of the field at such points can either result in poor separations or can cause projection of some particles of the stock over the edge and outside of the desired classifying area between the electrodes. Since the electrostatic field tendsto be concentrated at any relatively sharp edge, the bending up of the bottom electrode plate at each edge would concentrate the field at the upturned edge and leave dead spots on the surface of the lower electrode.

Where there was no provision for return of particles from these dead spots to the effective central area of the lower electrode, we found a tendency for certain types of particles to concentrate along these edges. For example, in the separation of bran from endosperm, some brannyparticles which for various reasons were unable to pass through the upper'electrode would fall back along the edges; Thus a brannier fraction would concentrate at the dead spots along these edges and would have no further opportunity to beattracted to the upper electrode.

Where non-conducting or insulating side plates have been. used, it has been considered necessary to make such plates as high as the total spacebetween the electrodes in order to prevent the escape of particles laterally from between the plates. Difficulties were encountered particularly where the upper and lower electrodes vibrated at'right angles to each other... Here there was an even greater tendency for particles to acquire alateral component of motion and be projected over the sides of the lower electrode. Higher side walls were thus requiredf With these problems of the prior art in view it is one object of the present invention to provide an improved construction for the edges of the electrode plates of an electrostatic separator of the above type.

A further object is the provision of an improved electrode edge. shape which can be used advantageously with either conducting or non-conducting materials.

Another object is the provision of an electrode edge strip which is inclined upwardly and outwardly from the lateral edges of the lower electrode plate of such a separator at an angle of or more with respect to the plate itself.

A further object is the provision of a separator in which the lower electrode extends laterally beyond the upper electrode and is inclined upwardly in the region which is outside the area of vertical projection of the upper electrode.

Another object is the provision of a separator in which the upper deck is provided with openings through which particles may be projected from the lower electrode, and in which an unperforated strip is provided on each side of the area in which the openings are located.

A further object is the provision of such an improved upper electrode in combination with a lower electrode having an edge portion inclined upwardly and intersecting the lower electrode along a line within the area of projection of the imperforate side strips of the upper electrode.

Other objects and advantages will be apparent from the following specification in which certain embodiments of the invention are described with particular reference to the accompanying drawings. In these drawings, wherein like reference characters indicate like parts,

Figure 1 is a perspective view of a complete electro static separator in which the present invention is advantageously used.

Fig. 2 is a partial sectional view on the line 2-2 of Fig. 1.

Fig. 3 is an enlarged partial sectional view, similar to Fig. 2, showing a preferred form of the present invention, and

Fig. 4 is a view similar to Fig. 2 showing still another embodiment of the invention in which the electrode edge strips are made of non-conducting material.

In general, the objects of the present invention are accomplished by providing an electrode edge construction in which the lateral edges are inclined upwardly and make an' obtuse angle with respect to the main central area of the electrode, such obtuse angle being in the range from 135 to (x) where x is the angle of repose of the material under the particular conditions of agitation and field strength encountered in a given operation. Preferably the angle formed by the upwardly inclined edge strip and the central area of the lower electrode is just slightly less than (180-x) The inclination of the strip is thus sufficient to urge the material back toward the central area of the electrode where it canbe acted upon by the uniform field and projected toward the openings in the upper electrode. At the same time, this obtuse angle is great enough so that distortion of the field uniformity at the edges will be minimized. Preferably this upwardly inclined electrode edge area extends outwardly beyond the area of the upper electrode and in any event should be outside the area of projection of the central perforated or open area of the upper electrode.

In Fig. 1 of the drawings, an electrostatic separator of the type described above is designated generally at 20. The device includes

frame members

22 and 24 on which a lower electrode plate 26 is mounted. An upper electrode 28 is supported by frame members 30 and 32 a predetermined distance above the lower electrode and substantially parallel thereto. A supply hopper 34 at one end of the device is provided with an adjustable feed gate 36 for control of the rate of feed of material across the lower electrode 26. The assembly just described, including the

frame members

22 and 24, is mounted for rapid reciprocation in a direction having both perpendicular and horizontal components, by means of inclined supporting springs such as 38 extending above a suitable base 40. An electromagnetic vibrator unit, indicated generally at 42, may be utilized to provide the desired 3 reciprocation. This vibrator unit is energized by suitable sqnu s ions with a newer pa k 44- The lower and

upper electrode plates

26 and 28 are connected by

wires

46 and 48, respectively, to a power un by w ic he P a e re maintaine a he sired potential difference with respect to each other. T e PQW?! u Q may nc ude a o t met 52. and a adj s n kno 4 or con o t e relative d ficr ncc of Pot tia he u p r e ec ro ,8 is provid d w th ope ings or perforations 56 through which selected particles may be projected from the lower electrode 26, Vertical edges 58 on the sides of the upper electrode 28 serve to retain h P oceed ma a o he up er u ac at e upp isa th so a it m b co eyed alon he ele tr and discharged through a spout 60 into a suitable container 62. A similar spout 64 on the end of the lower electrode plate 26 discharges material from that plate iaga'aae hacontainer 66 According to the present invention a new and improved construction is provided for the edges. of the lower electrode 26. As illustrated in Fig. 2, these edges include upwardly and outwardly inclined surfaces 68 which intersect the top surface of the lower electrode 26 along a longitudinally extending line 67. As further shown in Fig. 2, the openings or perforations 56 in the upper electrode 28 dofnot extend throughout the entire width of the electrode but cover only the central area thereof. Thus an unperforated strip 57 is provided at each edge of the perforated central area of the upper electrode. As shown in Fig. 2, the line of intersection 67 between the inclined electrode edge portion 68 in the fiat central electrode portion 26 lies just outside the vertical area of projection of the perforated central area of the upper electrode, or, in other words, within the vertical area of projectionof the unperforated strip 57 at the edge of the upper electrode. Thus there are no perforations 56 above any portion of the inclined edge surface at 68.

According to the present invention, as pointed out above, the inclined edge strip 68 forms an obtuse angle with the central area of the lower electrode 26, this angle being designated at A in the figure. It is this angle A which should lie within the range from 135 to (l80x), where x is the angle of repose of the particular material under the conditions of agitation and electric field strength. Obviously the angle of repose may vary, even for the same material, if the conditions of operation are changed; The greater the degree of vibration and/or the greater the electrostatic field strength,

the smaller this angle of repose is likely to be for a given material.

By providing an inclined edge strip within these angular limits, and preferably with an angle as close as possible to or just slightly less than (180x), the electrode edge offers the least possible interference with the uniformity of field which is consistent with return of the material toward the central elfective area of the electrodes. Furthermore, the inclination of the lower electrode in this area results in a similar inclination of the electrostatic field. This inclined field area tends to move particles inwardly from the outer edge toward the center and thus cooperates with the effect of gravity in urging the particles inwardly along the incline.

The uppermost portion at the outer edge of the strip 68 according to this invention does not need to project all the way up to the level of the upper electrode 28. Actually, the vertical height of outer wall from the lower electrode 26 to the upper edge 69 is preferably less than half the total vertical distance bctweenthe upper and lower electrodes 26 and 28.- Thus a substantial saving in material is made possible by the inclined edge construction discussed above.

In contrast with certain prior art arrangements in which the field may be concentrated at sharp retaining edges of the lower electrode and thus provide dead spots in the field within the main central area of the electrodes, the device described above in Fig. 2 provides for a gradual change in uniformity of the field from the line of intersection 67 toward the outer cdge of the inclined surface 68. The inclination of the surface at an angle in the specified range will insure the movement of material particles from the edge back toward the central area of the electrode, both because of the corresponding inclination the field and because of the effect of gravity. The absence of perforations above this inclined electrode edge 68 will prevent separations in an area Where the field strength differs slightly from that throughout the main area of the device. In a sense, then, the particular electrode edge construction provides an electrical barrier at the sides of the device which combines with the gravity effect to prevent the lateral escape of particles and insure greater precision of separation, without the necessity of extending a physical barrier over a major portion of the vertical distance between the electrodes.

The advantages of the present invention are further illustrated in the preferred embodiment shown in Fig. 3 in which the lower electrode 78 extends outwardly beyond the vertical area of projection of the upper electrode 90. In this case, the lower electrode is provided with an outer edge portion 80 which inclines upwardly and outwardly in the manner described above. This inclined electrodecdge portion 80 is, in this case, of conducting material and has a vertical outer leg82 spacing the outermost portion of the strip the desired distance above the outer edge 84 of the lower electrode 78. The line of intersection 86 between the inclined electrode edge strip 80 and the main portion 78 of the lower electrode lies almost directly beneath or just slightly inside the outer edge 88 0f the upper electrode 90.

Just as in the previous cases, upper electrode 90 is provided with openings or perforations 92 throughout a central area, while an. imperforate strip 94 is provided at each edge just, outside the. perforated central: area. The line of intersection 86 accordingly falls just slightly within or at the outer edge of the area of vertical projection of this imperforate area 94.

Here again the angle A between the inclined electrode edge surface 80 and the main electrode surface 78 lies within the limits set forth above. Among other things, the preferred device of Fig. 3, with the inclined; lower electrode edge 80 projecting outwardly beyond the area of vertical projection of the upper electrode, offers the least possible interference with the uniformity of the field within the effective separating area, of the electrodes. and permits the use of the narrowest possible imperforate strip 94 at each edge of the upper electrode. Similarly, this arrangement provides maximum inclination of: the electrostatic field at these edges and thus increases'the-etfective forces tending to. resist lateral. escape of the particles.

In Fig; 4* still another embodiment of the invention is illustrated; In this case the upper. electrode ZSis-substantially. the same as that illustrated in Fig. 2 and is-provided with a central area having openings or perforations 56 and imperforateside strips 57. between the central area andthe electrode edges. 58.

The lower electrode 96, in this case, isprovided' with edge strips 98-of electrically non-conducting or insulating material. Each edge strip 98 includes an inner or centrally extending flat portion 100 having an upper flat surface IGZadapted to fit directly beneath and in contact with the lower surface of the electrode plate 96. Each; strip 98 alsohas anupwardly. and outwardly'inclinedupper surface 104 extending from the outer-edge ofthe electrode 96 and forming an obtuse angle with the plane of the-surface102-withinthe limits previously discussed.

Between the flat surface 102" of the inner portion of the strip 98 and'theinclinedouter edge portion 104 of the strip, a vertical shoulder 106 is provided. This'vertical'shoulderhasa height exactly equal'to the-thickness of the electrode plate 96 so that the insulating strip 98 effectively fits around the outer edge and lower surface of the electrode plate. Thus a line of intersection 108 is provided between the inclined edge portion 104 and the upper surface of the electrode 96, this line of intersection coinciding, in this particular case, with the outer edge of the electrode itself.

In this embodiment the width of the metallic or electrically conducting electrode portion 96 is substantially the same as the width of the upper electrode 28 so that the line of intersection 108 lies directly beneath or just within the outer edge 58 of the upper electrode. Thus the inclined insulated edge portion 104 projects outwardly be yond the area of vertical projection of the upper electrode. Because of the insulating characteristics of this inclined portion there will be substantially no elfect on the field outside the plates and the forces tending to restore material toward the central area of the lower electrode 96 will be largely those due to gravity and the vibration of the lower plate.

Each electrode edge strip 98 is illustrated with a substantially vertical outer wall 110 and a substantially horizontal bottom wall 112 to complete the unit. The central or inwardly extending fiat portion 100 of each strip is further provided with means for attachment of the strip to the electrode 96. In this case the attachment means consists of a series of bolts 114 provided with heads 116 fitting in recesses in the upper surface of the electrode plate 96 so that the tops of the bolt heads 116 are flush with the upper surface of the electrode. Retaining nuts 118 complete the assembly.

According to the foregoing description, a number of embodiments of the present invention have been described which accomplish the objects set forth at the beginning of this specification and which are not subject to the disadvantages of prior devices.

Certain of the features shown but not claimed herein are claimed in our copending application, Serial No. 108,476, filed August 4, 1949, entitled Method and Apparatus for Electrostatic Separation.

Since minor variations and changes in the exact details of construction will be apparent to persons skilled in this field, it is intended that this invention shall cover all such changes and modifications as fall within the spirit and scope of the attached claims.

Now, therefore, we claim:

1. An electrostatic separator having spaced upper and lower electrodes, means for feeding a thin uniform stream of material along the lower electrode and beneath the upper electrode, means for maintaining a difference of electrical potential between the electrodes and thereby providing an electrostatic field of predetermined strength by which selected particles of material are attracted from the lower electrode to the upper electrode, and means on the lateral edges of the lower electrode providing a longitudinally extending upwardly and outwardly gently sloping retaining edge, said retaining edge having a total vertical height less than half the vertical distance between the upper and lower electrodes.

2. An electrostatic separator according to claim 1 in which the lowest and innermost portion of the retaining edge means meets the lower electrode at an angle greater than 135.

3. An electrostatic separator having spaced upper and lower electrodes, means for feeding a thin uniform stream of material along the lower electrode and beneath the upper electrode, means for maintaining a difference of electrical potential between the electrodes, the upper electrode having perforations through which selected particles of material are projected from the lower electrode, and a material-retaining lateral edge on the lower electrode, said edge being inclined upwardly and outwardly from a line on the lower electrode just outside the vertically projected area of the upper electrodeperforations.

4. An electrostatic separator according to claim 3 in which the total vertical height of the inclined electrode edge is less than half the total vertical distance between the upper and lower electrodes.

5. An,electrostatic separator according to claim 3 in which the line along which the inclined retaining edge intersects the lower electrode lies substantially vertically beneath the outer edge of the upper electrode.

6. An electrostatic separator having spaced upper and lower electrodes, means for feeding a thin uniform stream of material along the lower electrode and beneath the upper electrode, means for maintaining a difference of electrical potential between the electrodes, the upper electrode having a longitudinally extending central area provided with openings through which selected particles of material are projected from the lower electrode, said upper electrode also having an outer unperforated region extending longitudinally of the electrode outwardly of said area, and a longitudinally extending material-retaining lateral edge on the lower electrode, said retaining edge being inclined upwardly and outwardly from a longitudinal line on the lower electrode located outside the vertical projection of said central area.

7. An electrostatic separator according to claim 6 in which the line along which the inclined retaining edge intersects the lower electrode is located within the vertical projection of said unperforated region.

8. An electrostatic separator according to claim 6 in which the retaining edge on the lower electrode is upwardly inclined at an angle of at least 135 and lessthan (180x) with respect to the plane of the lower electrode, where x is the angle of repose of the material under the particular operating conditions of the lower electrode.

9. An electrostatic separator having spaced upper and lower electrodes, means for feeding a thin uniform stream of material along the lower electrode and beneath the upper electrode, means for maintaining a difference of electrical potential between the electrodes, the upper electrode having a plurality of openings through which selected particles of material are projected from the lower electrode, and the lower electrode having a longitudinallyextending material retaining edge inclined upwardly and outwardly at an angle of from 135 to (l-x) with respect to the lower electrode, where x is the angle of repose of the material under the particular operating conditions of the lower electrode.

10. An electrostatic separator according to claim 9 in which the retaining edge is inclined at an angle just slightly less than (180x) with respect to the lower electrode.

11. An electrostatic separator according to claim 9 in which the total vertical height of the inclined retaining edge is less than half the total vertical distance between the upper and lower electrodes.

12. An electrostatic separator according to claim 9 in which said material-retaining edge comprises a strip of insulating material having an inner horizontal portion extending inwardly beneath and in contact with the bottom of the lower electrode.

13. A material-retaining edge strip for one of the electrode plates of an electrostatic separator, said strip having an inner fiat top portion fitting beneath and in contact with the bottom of said electrode, an outwardly and upwardly inclined outer top surface extending at an angle greater than and less than 180 with respect to said inner flat top portion, and a vertical shoulder between said inclined outer surface and said flat inner top portion, the height of said shoulder being equal to the thickness of the electrode plate.

14. An electrode edge according to claim 13 in which the inner fiat portion has means for attachment of the strip to said electrode.

(References on following page) 7 8 References Cited in the file of this patent FOREIGN PATENTS UNITED STATES PATENTS 112,021 Australia Dec. 12, 1940 714,649 Sutton N 25 :1902 7055007 rmany wMar. 13, 9

1,729,589 Mordey Sept. 24, 1929 5 2,328,577 Oglesby Sept. 7, 1943