US2398792A - Electrostatic sizing of materials - Google Patents

Description

April 23, 4 H. B. J OHNSON 2,398,792

ELECTROSTATIC SIZING- OF MATERIALS Filed Oct. 22, 1943 200 a; 100 my 100 50 J0 Mes/ Qza ucGDrS .50 22 10 Mes/Z V 1 N1 EN TOR. f/erZarZBJoZuzson Patented Apr. 2a, 1946 2,398,792

UNITED STATES PATENT OFFICE ELECTROSTATIC SIZING or MATERIALS Herbert B. Johnson, Rochester, N. Y., assignor to Bitter Products Corporation, Rochester, N. Y., a corporation oi New York Application October 22, 1943, Serial No. 507,288 6 Claims. (Cl. 209-127) This invention relates to the dry sizing, gradpurpose of reducing the load on the different ing, or classification of mixed particles of masized screens, so as to greatly increase the lift of terial according to their diiierent sizes and in the screen or bolting cloth and assure more accutheir natural state, without separation of the rate sizing of the finished product to meet very particles according to their electrostatic suscep- 5 close United States or Tyler standard specifitibility based on chemical composition or other cations. difierence in kind, and one object of the inven- Many groups of mixed particles, such as feldtion is to provide an improved and commercially spar, fiuorspar, mica, biotite, nepheline, and the useful process for eflectlng such sizing, without like, contain particles of various-shapes, such as the use of the screen cloths commonly employed n sliver, flat particles and particles with rough irfor the purpose. regular edges which, in passing over the usual In the screening of fine powders and other mas re n 010th. tend to Wedge in and quickly blind terials containing particles which accumulate a the r n pen n s and r a ly r e h surface static charge when coming into contact r ning icien y. Another object of the p eswith vibrating screen cloth of various materials, is cut invention is to provide an imp m h such as metals, silk, and the like, the static for sizing materials t in s h particles so charge so accumulated causes the particles to agas to avoid such diillcultles and increase the glomerate, clutter together, or form balls which Screening effieiencycannot pass through the screen openings. Such Another Object is o provide an improved pr difllculties not only tend to greatly reduce the m ess f l s g partic es acc rding to size. o screening capacity and greatly increase the cost, future treatment such as the concentration or but also to lower the screening eiliciency, and Separation of i d Par i s ac r t th i another object of the present invention is toprodifiel'ences in pe ific gravity, magnetic P vide an improved process for obviating these difbiiity. electrostatic susceptibility. or particular ilculties in the sizing oisuch materials. is Shape- Another object is to provide an improved proc- A further (Meet is Pmvide an impmved and egg for sizing many materials of a highly hygfl)- less expensive method for removing from many scopic nature which due to th rapid accumumaterials, requiring further treatment 01' balllation of moisture, tend to agglomerate, cluster, dung, the fine dust Particles, such as would Pass ball up and block the openings of screen cloth. 8 a hundred mesh screen, eliminating the Many chemical salts, such as common salt, 'potdust nuisance and Wing the Wanting ash salts and the like, are of a highly corrosive dmmlsin handling such materialsnature which causes corrosion and rust that not Stm further object is to Provide a more only blinds the screen cloth openings, but injects cient process for removing the finer sized parinto the material impurities in the form of rust 5 times Produced during Particle reduction, crush scales which seriously impair the appearance of 111g grinding ns, t ach step of the rethe screen products and defeat the purpose of duction, 80 that 8. minimum 8111011111", Of inpalpable producin chemically pure and marketable proddust or slimes is produced in crushing ores, rocks, ucts. Another object of the invention is to afchemical Salts, and many food Productsford an improved process for sizing such 40 To these and other ends the invention resides give materials so as t avoid t m mt in certain improvements and combinations of fen-ed t parts, all as will be hereinafter more fully de- Another object of t invention is t pruvide a scribed, the novel features being pointed out in the process for removing, t minimum breakage claims at the end of the specification. handling, the finer sizes 01' crystals of potash and 4s Inthe drawing: many other salts which frequently have a lower Fig. 1 is an elevational view. p rtly in section value and cause the coarser cyrstals to cake or and P y mm i s w a h d an cement together so as to impair the appearance apparatus for separating and classifying particles of the crystal salts and seriously interfere with of finely divided materials according to size, cmthe free-flowing qualities desired in handling and 50 bodying the present invention, and packaging operations. Fig. 2 is a similar view showing repeated appli- Another object is to provide an advantageous cation of the invention to effect a succession of method for classifying particles according to size separations in diflerent size classifications. for further accurate screening with standard It has been known heretofore that the particles mesh screen cloth or silk bolting cloth, for the as c! a mixture of finely divided materials may be separated by delivering the mixture on a rotary conveying electrode into a strong electric field produced between the conveying electrode and a second electrode spaced therefrom and having a smooth surface and substantial size and charged to a high potential relative to the conveying electrode of, say, 10,000 volts or more, so as to cause the particles of better conducting material, substantially irrespective of size, to be repelled from the conveying electrode and thereby separated from particles of material of lower conductivity, as described, for example, in my prior United States Patents Nos. 2,135,716 and 2,246,253. In another known type of method and apparatus, the electrode spaced from the conveying electrode has been of small size, such as fine wire, or provided with sharp points, and charged to a similarly high potential relative to the conveying electrode so as to produce a spray discharge and ion bombardment of the particles on the conveying electrode, with the result that the particles of materials of lower conductivity, substantially irrespective of size, are caused to cling to the conveying electrode so as to be depressed and separated from the better conducting material.

The present invention employs neither of the above types of method and apparatus, involving instead a relatively weak electric field on the conveying electrode produced by a spaced electrode of intermediate size and having a relatively low potential difference from the conveying electrode, so as to avoid differential action on particles of different conductivity but effecting a moderate depressing action on the particles of smaller size, as the particlesof larger size are moved in the opposite direction by the centrifugal force imparted by the conveying electrode, thereby advantageously coordinating and combining the opposite effects of a suitable electric field and centrifugal force to classify the particles according to size.

I have discovered, more particularly that the particles of a finely divided mixture may be separated and classified according to the sizes of the particles, irrespective of their materials and resulting electrostatic susceptibility, by a combination and coordination of centrifugal force and electrostatic force, through control and coordination of the peripheral speed of the rotary conveying electrode and of the potential difference between it and the charged electrode, in conJunction with electrode surfaces of suitable characteristics.

The centrifugal force applied by a rotary electrode tends to impel the particles of greater mass forwardly to the outside of the particle stream falling from the electrode, while the particles of less mass tend to drop more nearly vertically at the rear side of the stream, with the result of a partial separation and distribution of the particles according to their relative mass and size. Such centrifugal separation, however, has been found insufilcient and inadequate to assure a continuous and even spread of the particles according to size, and I have found that such centrifugal sizing action may be supplemented and rendered successful for the purpose by applying against it a coordinated electrostatic force of suitable character. Investigations involving many tests on metallic and non-metallic minerals, chemical salts, and other materials, have proven that the surfaces and diameters of the electrodes and their difference in potential are important factors in applying the electrostatic force against the centrifugal force, in order to obtain emcient sizing action without producing any separation of the particles according to their different kinds of materials and electrostatic susceptibility.

In the application of electrostatic forces as used in the electrostatic separation of particles according to differences in the materials of which they are composed, it has been found that the separating action is reversed as the diameter of the charged electrode is increased from .005 of an inch to 2 or 3 inches. For example, if a fine wire having a diameter up to 6 inch is used as the charged electrode, the resulting ion bombardment causes the particles to stick to the surface of the grounded electrode. As. the diameter of the charged electrode increases from to 2 inches, or more, there is an attracting action of the charged electrode and a repelling'of the more conductive particles from the grounded electrode.

It has been found through many experiments and tests that charged electrodes ranging in diameter from inch to of an inch have a more neutral repelling or attracting action and that there is a definite relationship between the charged electrode diameters ranging from 1 6 inch to inch and the potential difference employed between the charged and grounded electrodes. For example, ,in employing charged electrodes ranging in diameter from 1*; inch to inch, there is practically no electrostatic separation of the particles according to the material of which they are composed, or interference with the free sizing action of the particles, if the potential is maintained below about 7,000 volts. If the potential difference is increased above about 7,000 volts, electrostatic separation of the particles according to their materials begins to take place and increases in proportion tothe increase in the difference in potential between the electrodes, so as to defeat the desired separation according to size.

My improved method is best described in connection with a suitable apparatus for carrying it out, shown in Fig. 1 of the drawing, in which 35 is a feed hopper for the finely divided mixture of particles to be sized. At 36 is an adjustable fee'd gate, of any known or suitable variety, for controlling the rate of delivery of the particles to the surface of a rotary cylindrical electrode 31 of substantial size having a diameter of several inches. The surface of electrode 31 is preferably knurled, fluted, corrugated or otherwise roughened, with suitable reference to the size of particles to be treated and the type of materials to be handled. Electrode 31 is rotated at a suitable peripheral speed for picking up, rotating and discharging the particles at the same speed into an electrical field produced between electrode 31, which is grounded as at 38, and an electrode 39 connected as at 40 with a suitable potential source such as a rectifier unit of the usual or any suitable variety, energized by a transformer connected through a reversible switch with a service power line, as conveniently-indicated in connection with Fig. 2. The difference in voltage between electrodes 31 and 39 is of lower range'than that commonly employed in electrostatic separation, ranging to not more than about 7,000 volts.

Electrode 39 preferably has a smooth cylindrical surface and a diameter of not less than 1 g of an inch and not more than about of an inch, and may be either rotating or stationary. It is located, as shown, in suitably spaced relation with the point in which the particles drop from the conveying electrode 31, preferably with the centers of the two electrodes in a common horizontal plane.

In carrying out the present process by the described apparatus shown in Fig. 1, the rotary electrode 3'! is given a peripheral speed adapted to deliver the particles in a definite stream with the particles thereof subjected to centrifugal forces proportional to their respective masses and tending to project them somewhat forwardly from the point of delivery from the electrode. The electrostatic force imposed by electrode 39 operates in opposition to such centrifugal force, in inverse ratio to the sizes of the particles, so that they are depressed backwardly and fanned out, as shown, with the finer particles falling farthest to the rear, as at 42. A series of dividers 43 are preferably provided and adjusted to separately collect the particles in each particular size range, as shown. Any known or suitable means may be provided, such as the variable resistance indicated at 64, for varying the voltage applied to electrode 39.

The peripheral speed of the conveying electrode 37 and the particular voltage of the charged electrode 39 are selectively adjusted and coordinated with each other for operation at best efiiciency to suit variations in the particle size and particular kind of material to be handled. The roughened surface of the conveying electrode operates effectively to discharge the particles at a corresponding surface speed, without any substantial tendency to set up spray discharge or, ion bombardment, because of the relatively low voltage of the charged electrode 39. The charged electrode is of such impressed voltage and size asto produce a limited depression of the particles in inverse ratio to their size and in opposition to the applied centrifugal force, so that the particlesare separated'and fanned out according to size without any tendency to separation according to the kind of material and its electrostatic susceptibility.

I have found, for example, that with a roughened, grounded, conveying electrode of several inches in diameter, rotating at a peripheral speed between, say, 100 to 450 feet per minute, and a charged electrode a; of an inch in diameter at a potential of 3500 volts, four size classifications are easily produced when treating phosphate rock, phosphate flotation concentrates, copper stamp mill tailings, cement rock, potash salts, feldspar, fiuorspar, abrasives and other metallic and nonmetallic ores and rocks, which have been preliminarily crushed to pass a six mesh screen and surface dried. With the voltage and other details of the exemplary embodiment here referred to, the charged electrode 39 is preferably located with its center spaced about an inch from the material on the conveying electrode, but different materials and different preliminary particle sizes may require some variation in the surface roughening and peripheral speed of the conveying electrode, the spacing of the electrodes, the potential of the charged electrode and perhaps its diameter, but such variations are readily made in setting up the apparatus and calibrating it to the particular operating conditions, for optimum efficiency, as will be readily apparent to those skilled in the art. It may also prove desirable, in connection with some materials, to reverse the polarity of the charged electrode, as described in my prior Patent No. 2,197,864.

The invention may be applied to repeated treatments of a given material, using a plurality of pairs of electrodes and dividers, as shown in Fig. 2, where the feed hopper is shown at 45,

with an adjustable feed gate 44 for feeding a uniform stream of material to the rotary grounded conveying electrode 41. Opposite electrode 41 is a charged electrode 48, these electrodes having the same construction, arrangement and operation as described in connection with Fig. 1. comprising suitable means for varying the surface speed of the conveying electrode and the voltage of the charged electrode. The finest grade of particles separated at these electrodes, passing a two hundred mesh screen, for example, is separated by a suitable divider 49, while the remaining rades are received by suitable collecting surfaces indicated at 50, and fed to a second pair of electrodes BI and 52, similarly constructed and operated. The finest grade of particles separated by this pair of electrodes is separately collected by a divider 53, while the remaining grades are received by suitable collecting surfaces indicated generally at 54 and fed to a final pair of electrodes 55 and 56. Electrodes 55 and 56 are similarly constructed and operated and produce a final separation into two particle sizes, indicated at 51 and 58, which are separately collected by suitable means comprising a divider 59, as shown.

It has been demonstrated in practical operation of the invention that coordinating adjustments of the centrifugal and electrostatic forces employed, in connection with suitable location of the divider, produce definite sizes of materials which, tested by the usual screen test, indicate at 88-92% sizing efficiency, according to standard screen operating measurements.

The apparatus employed to carry out the present process may be totally enclosed, so that the impalpable dust can be removed pneumatically and collected in the usual way, thereby preventing any accumulation of dust to the point of explosive hazard. The different conveyor speeds and the difierent polarities and potentials of the electrodes required may be readily calibrated for various materials and particle sizes for adaptin the machine to different uses, the controlling factors being well within the range of commercial application.

I claim:

1. The process of separating particles according to their different sizes from a finely divided mixture thereof comprising the steps of applying to said mixture a centrifugal force adapted to deliver the same in a definite stream with the particles thereof subjected to said force in proportion to their respective masses, subjecting said particles in space to an electrostatic field between cylindrical electrodes having a potential difference of about thirty-five hundred volts acting to depress particles of smaller size without separation of the particles according toconductivity, the smaller of said electrodes having a diameter of not less than approximately one-sixteenth of an inch and not more than approximately threefourths of an inch, to separate the smaller from the larger particles in said stream in opposition to said centrifugal force, and separately collecting particles of difi'erent sizes.

, 2. The process of separating particles accordin to their different sizes from afinely divided mixture thereof comprising the steps ofapplying to said mixture a centrifugal force'adapted to deliver the same in a definite stream with the particles thereof subjected to said force in proportion to their respective masses. subjecting said particles in space to an electrostatic field between cylindrical electrodeshaving a potential the smaller of said electrodes having a diameter of not'less than approximately one-sixteenth of an inch and not more than approximately threefourths of an inch, to separate the smaller from the larger particles in said stream in opposition to said centrifugal force, and separately collect- 'ing the particles of different sizes.

3. The process of separating particles accord-v ing to their different sizes from a finely divided mixture thereof comprising the steps of feeding said mixture on a cylindrical electrode rotating at a speed adapted to deliver the same in a definite stream with the particles thereof subjected to centrifugal forces in proportion to their 'respec-, tive masses, delivering said stream into an electrostatic field formed between said electrode and a smaller cylindrical electrode having a potential difference therefrom of not more than about 7,000 volts, said smaller electrode having a diameter of not less than about one-sixteenth of an inch and not more than about three-fourths of an inch, to separate the smaller from the larger particles in said stream in opposition to said centrifugalforce, and separately collecting the particles of different sizes.

4. The process of separating particles accord- I ing to their different sizes from a finely divided mixture thereof comprising the steps of. conveying said mixture on a cylindrical electrode having a roughened surface and rotated at a speed adapted to deliver the same in a definite inch, and separately collecting theparticlesof different sizes.

7 diflerence of not more than substantially 7,000 volts,

5. The process of separating particles according to their different sizes from a finely divided mixture thereof comprising the stepsof feeding said mixture on a cylindrical electrode rotating at a speed adapted to'deliver the same in a definite stream with the particles thereof subjected to centrifugal forces in proportion to their respective masses, delivering said stream into an electrostatic field formed between said electrode and a smaller cylindrical electrode having a potential difference therefrom of not more than about seven thousand volts acting to depress particles of smaller size without separation of the particles according to conductivity, said smaller electrode having a diameter of not less than about onesixteenth of an inch and not more than about three-fourths of an inch and said electrodes having their centers substantially in the same horizontal plane, to separate the smaller from the larger particles insaid' stream in opposition to said centrifugal. force and separately collecting the particles of different sizes.

6. The process of separating particles according totheir different sizes from a finely divided mixture thereof comprising the steps of conveying said mixture on a cyilndrical electrode having a roughened surface and rotated at a speed adapted to deliver the same in a definite stream with the particles thereofsubjected to centrifugal forces proportional to their respective masses, delivering said stream into an electrostatic field formed between said electrode and a smaller cylindrical electrode having its center located substantially inthe same horizontal plane with the center of said conveying electrodeand located about one inch from the surface of said conveying electrode, maintaining a potential difference between said electrodes of not more than approximately 7000 volts, said smaller electrode having a smooth surface and a diameter of not less than approximately one-sixteenth of an inch and not more than approximately three-fourths of an inch, and separately collecting the particles of different sizes.

HERBERT B. JOHNSON.

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