US3476243A - Pneumatic conduit type electrostatic separator - Google Patents

Description

Nov. 4, 1969 G. FERRARA ETAL 3,476,243

PNEUMATIC CONDUIT TYPE ELECTROSTATIC SEPARATOR Filed Oct. 25, 1966 2 Sheets-Sheet l BLOWER Ilill/1lll/111101111111111;l/lll/lll PNEUMATIC CONDUIT TYPE ELECTROSTATIC sEPARAToR Nov. 4, 1969 G. FERRARA ETAL 2 Sheets-Sheet E Filed Oct.

United States Patent U.S. Cl. 209-129 10 Claims ABSTRACT OF THE DSCLGSURE An electrostatic spearator for granular materials including an annular duct, a section of which has a wall including two substantially parallel planar stationary electrodes adapted to be connected to means for charging the electrodes for establishing an electric field therebetween, the electrodes ending with mutually diverging confronting terminal portions. The duct is apertured immediately after each of the terminal portions and a conduit through which a gaseous fluid is directed into the duct in a substantially tangential direction in the vicinity of the apertured portion of the duct and away from such apertured portion is provided together with means for feeding granular material into the duct.

This invention relates to a dry electrostatic separator, that is, a device adapted electrostatically to separate in dry conditions granules of mineralogically different species from a raw ground ore or by a mixture comprising at least two mineralogically different species, with the separation being carried out ,with the exploitation of electrical properties, more particularly dielectric constant and conductivity, which are different for the mineralogical species to be separated.

The prior art has disclosed several different electrostatic separators, the most interesting of which are those which resort to the free fall of material, of the type ernploying a drum and of the centrifugal type, also called cyclones.

In the free-fall type of separators (hereinafter called separators for the sake of brevity), the granules of the raw ore or the mixture as hereinbefore defined are caused freely to fall between two vertical electrodes which create an electrostatic field. Prior to being dropped between the vertical electrodes, the granules are charged with electricity by either a triboelectric or a pyroelectric effect, the granules of the one mineralogical species taking up a positive charge, and the granules of another species a negative charge, or also by causing the granules to pass in the vicinity of a ionizing electrode which emits electrons and ions due to a corona effect discharge.

In the drum-type separators, the granules of raw ore are fed on a drum rotating around its own axis in which two or more electrodes (or another drum operating as an electrode) are provided in facing relationship with the first mentioned drum with the latter and the electrodes being connected to a source of electric power which generates an electrostatic field therebetween.

In the free-fall and drum-type separators, in the neighborhood of the baseplate or beneath one of the staticfield generating electrodes, granules of positively charged mineralogical species are collected, whereas negatively charged granules are collected near the other electrode. In a few improved types of the separators, intermediate of the two static field generating electrodes and beneath them, a device is located which collects those granules which have not been collected or whose path has not been ice' sufficiently deflected by the static field. These granules which did not undergo separation and can be of different natures, are collected and mechanically conveyed on top of the separator and dropped therethrough again, thus starting a new cycle.

In electrostatic centrifugal separators, the granules of the raw ore or the mixture are kept dispersed in a gaseous stream which is fed at high speed into a centrifugal separator, where the stream takes a whirling movement. Before entering the centrifugal separator, or just as they reach the centrifugal separator, the granules are electrostatically charged in very much the same way as indicated for the free-fall separators.

Centrifugal electrostatic separators are equipped with two electrodes, one being placed at the peripheral wall of the separator and the other placed substantially near the central line thereof, so as to form a static field tending selectively to defiect the electrically charged granules. It is apparent that, for those granules which have been charged so as to be collected by the electrode placed at the separators peripheral wall, the electrostatic field force is combined with the centrifugal force possessed by the granules as they are being energetically rotated in the separator. The gaseous fiuid stream coming out of the separator is often sent to another centrifugal separator serially arranged with respect to the former, or is recycled in the separator per se.

Prior to analyzing the drawbacks affecting the electrostatic separators of the types briefly referred to above, it is fitting to outline a few phenomena accompanying electrostatic separation.

When the granules of raw ore or the mixture are charged under the action of ionizing electrodes, all the granules which are being charged take up a negative charge. Prior to coming between the static-field generating electrodes, the negatively charged granules are caused to contact a positively charged, or merely grounded, surface, whereat the conductive species granules shuffle off their negative charge, taking up by induction and conduction a positive charge, whereas the nonconductive species granules, or the granules whose conductivity is smaller than that considered above, do not lose their negative charge or lose it only in part. For the electrostatic separation to be efficient, it is essential that all the ionized granules may electrically contact the positively charged surface, and that means that a single or sufi'iciently thin layer of granules should cover at every instant of time the surface. This requirement cannot be fulfilled by the free-fall separators (where the ionized granules, prior to entering the static field, freely fall onto a positively charged or grounded sloping plate) or by the drum separators (where the granules fall onto a positively charged or grounded drum) when very fine grained ores are treated, such as below 20G-250 mesh. Such a requirement could be theoretically fulfilled only by assigning to the separators a throughput capacity, i.e. an efficiency, so low as to render the electrostatic processing unpractical. The problem can be lsolved by replacing the static or substantially static contact of the ionized granules with the positively charged or grounded surface, or by a strongly dynamic contact such as could be obtained by hurling the granules against the surface and forcing them to slide or roll thereon under the urge of inertial or centrifugal forces. To achieve this result, the granules should not be allowed to fall freely under the action of their gravity pull, since it has been ascertained that they should be entrained by an air or gas stream impinging the grounded or positively charged surface, as it occurs in the electrostatic separators of the centrifugal type.

Another phenomenon which deserves attention is the granule cakeup, which is due both to residual moisture and electrostatic forces, and the phenomenon is particularly conspicuous in very finely ground granules. Recalling that a caked particle, which comprises granules of different mineralogical species can have an electrically neutral behavior, in which case it passes through the static field without swerving, or it can take an overall positive or negative charge, in which case it is deviated towards the negative or the positive electrode thus entraining those granules which have been caked but should have been deviated towards the opposite electrode, it can be realized how diflicult, if not impossible, it is to obtain a selective separation of very fine granules from a raw ore or a mixture. For this shortcoming to be obviated in a full satisfactory manner, it has been ascertained that the granules should be dispersed and conveyed by a gaseous stream, similarly to what is experienced in the centrifugal separators.

Another phenomenon Which is essential and of vital significance for an efficient separation of different mineralogical species, lies in that the granules which are caused to pass between the static-field generating electrodes are to follow predetermined paths in order to be deviated by the static field through a predetermined extent and within precise limits, as necessary for effecting an efficient separation.

In both the drumand free-fall separators the granules over a certain size follow, under the action of the gravity pull and the resistance opposed by the air through which they are moved, well determined paths upon which the electrostatic field can act in a way which can be determined a priori in a precise and measurable way. If the grains, contrarywise, are very fine, the resistance of air, which forces them to follow random, not well defined paths, has a considerable bearing on their falling motion. The randomly nature of these paths is further infiuenced and intensified by the action of the air whirls near the rotating drum and of the static breeze caused by the ionizing electrodes if the granules are charged by ionization. To the ends of an efficient separation, the action of the electric field cannot produce any favorable effect on the random paths, since in many cases the granules are driven thereby out of the electric field towards the surrounding environment. It is clearly apparent that the serious drawback discussed just now is greatly intensified in the centrifugal separators, where the air which conveys the granules into the electrostatic field area takes: strongly whirling movements. The result is that the centrifugal separators are in most of the cases wholly incapable of effecting the separation of different mineralogical species of an ore which has been very finely ground, at least in part, a requirement which is otherwise essential for the treatment of many ores. The conventional centrifugal separators are affected by a further drawback consisting in that they do not carry out, as an airstream passes through the separator, a sharp separation of the positively charged granules from the negatively charged ones and from those which have not been collected by the two static-field generating electrodes. As a matter of fact, air coming out of the separator to be recycled or sent to a subsequent separator, is discharged near the bottom of one of the electrodes and entrains a portion of those granules which had been collected bythe electrode, bringing moreover into contact with the granules collected by the electrode and gathered at the base thereof, granules of different species or cakes which contaminate the species collected thereat.

Another serious shortcoming of the centrifugal separators stands in that they suffer from heavy pressure drops, so that it is necessary to mount blowers in the path of the recycled air, with the blowers being rapidly worn out in contact with the granulated ore.

In order that these shortcomings may be eliminated, an object of the present invention is to provide an electrostatic separator wherein the granulated ore to be treated is conveyed by an air or gas stream which causes the granules, the finest ones included, to travel over Well defined paths at least in a section of the separator, in which static-field generating electrodes are positioned.

Another object is to provide a separator wherein a pneumatic conveyance of the granules may take place according to a cycle in which those granules which have not been collected by either static-field generating electrode are continually separated and the non-separated granules are pneumatically recycled in a continuous closed cycle without contacting the granules collected at the bases of the two electrodes, and in which the drive is transferred to the closed cycle airstream by injecting into the cycle a continuous and adjusted air or gas stream rather than by direct mechanical means.

Still another object is to provide a separator in which the granulated ore to lbe treated is conveyed by a gas stream which, if the electric charges are imparted to the granules by emission of an ionizing electrode, causes the ionized granules to come into a vigorous dynamic contact with a positively charged or grounded surface, with the settling of the granules by gravity somewhere in the separator and the granule caking up being simultaneously prevented thereby.

Yet another object is to provide an electrostatic separator permitting an efficient separation of granules of different mineralogical species, having a size less than 20() mesh and even less than 400 mesh.

Such an electrostatic separator comprises an annular duct, a section of which has two static-field generating electrodes, the electrodes being substantially parallel to one another and one end of each electrode diverging from the confronting end of the other electrode, an opening formed through the wall of the annular duct in correspondence with the diverging end of each of the electrodes, a perforation formed through the wall of the annular duct in the neighbourhood of the openings, a tubular member being inserted with one into the perforation to intersect the annular duct substantially in tangential relationship With respect to the axis of the duct in the intersection area, by means for feeding a gaseous uid under pressure into the tubular member and means for feeding the granules to be separated, the position of the tubular member relative to the annular duct being such that the gaseous fluid fed into the tubular member penetrates the annular duct with a trend of motion away from the diverging ends of the two electrodes. In order that a clearer understanding of the structure of the inventive electrostatic separator may be attained, and its features and the advantages obtainable with the use thereof may be fully conspicuous, a few exemplary, non-limiting embodiments will now be described with reference to the accompanying drawings, wherein:

FIGURE l is a diagrammatical cross-sectional view taken along a vertical plane, of a separator in which the electric charge of the granules takes place by triboelectricity.

FIGURES 2 and 3 are also diagrammatically illustrative of vertical sectional views of two different embodiments of separators wherein the electric charge is imparted to granules by ionization, and

FIGURES 4 and 5 are diagrammatical illustrations of two embodiments of the shapes of the static-field generating electrodes of a separator, as compared with the shapes of the electrodes shown in FIGS. 1 to 3.

The separator shown in FIG. l comprises an annular duct 1, substantially rectangular in cross-section, to which two electrodes 2 and 3 are integrally connected, one of which is connected to the negative terminal and the other one to the positive terminal, respectively, of an electric generator, the connections having been omitted for clarity since they are well known. Let us assume, for example, that the electrode 2 is connected to the negative terminal and the electrode 3 to the positive terminal. The electrodes 2 and 3 are electrically insulated from the remaining part of the body of the annular duct, which duct could be made of a metallic conductive material. Electrode 3 could also be grounded as well as the remaining part of the annular ducts body. ln this case, obviously the electrode 3 need not be insulated with respect to the remaining part of the duct 1, of which it can be an integral part. As viewed in FIG. 1, the electrodes 2 and 3 are substantially planar and parallel to each other along a major fraction of their length and have either end, 4 or 5, respectively, diverging with respect to the other one. In correspondence with, and beneath each end 4 and 5 of the electrodes, an opening is formed through the wall of the annular duct 1, with the width of the opening being adjustable by means of a movable vane hingedly connected to the body of the conduit 1. The width of the opening formed in the vicinity of the electrode 2 is adjustable by a sector 6 which is rotatable about a hinge 7, and the width of the opening formed in the neighborhood of the electrode 3 can be adjusted by means of a sector 8 rotatable about a hinge 9.

In the vicinity of the sectors 6 and 8, on the lower wall of the annular conduit 1, a perforation is formed in which an end of a tubular member 10 is inserted, and the width of the perforation is adjustable by a movable sector 11 mounted on a pin 12. The control mechanisms enabling the rotation of the sectors 6, 8 and 11 to be effected about the respective pivots and independently of each other, have not been shown for clarity in FIG. 1, since they are obvious and their illustration is not vital for an understanding of the operability of the separator. Above the movable sector 11, a hopper 13 lled with ground raw ore is affixed to the annular duct 1 and the bottom thereof communicates with the inside of the annular duct. It should be added that in the tubular member 10 means are provided for feeding a gaseous fluid under pressure, with such means being, for example, a blower (not shown) which feeds the tubular member 10 with compressed air. Let us now assume that compressed air, coming from a blower, is fed via the tubular member 10 to the annular duct. The fed air, whose rate of flow and speed can be varied by the action of the sector 11, transfers its drive to the air which is already in the duct 1, causing the air to circulate in a loop within the annular duct. A portion of air equal to the one fed-in through the tubular member 10 is dumped out of the annular duct through the opening whose width is adjustable by the sectors 6 and 8. It should also be noticed that the flow of air fedin in the conduit 1 via the tubular member 10 causes a negative pressure to be produced at the bottom of the hopper 13 (upstream of the air intake port) where the raw ore is fed.

During the operation of the separator shown in FIG. 1, the granules of different mineralogical species of the raw ore (or mixture) fed via the hopper 13, and the recycled granules, on account of the different magnitudes of their dielectric constants, are electrically charged with opposite signs by the triboelectricity produced by the impact of the granules with one another, the friction between the granules and the Walls of the annular duct and the friction between the granules and the gaseous stream. The eiciency of the impact between the granules is enhanced by the whirling motion of the airflow in the duct 1, by a high percentage of granules within the airstream, by the high speed of the airstream circulated within the duct, by the intake speed of air from tubular member 10 into the duct and by the curvature of the duct. The efficiency of the friction between the granules and the inner walls of the annular duct is elevated, not only by the speed and turbulence of the air flowing through the duct, but also by the nature of the material which forms the inner walls, the latter being made, with advantage, of a material having a dielectric constant intermediate between those of the mineralogical species to be separated.

The effect of the friction between the granules and the gaseous streams is influenced by the whirling motion of the gaseous stream in the annular duct and the speed at which the gas is fed into the duct via the tubular member 10. Obviously, the electric charging in the separator of granules of different mineralogical species can take place by pyroelectricity or resorting to particular gaseous fluids which, as is known to those skilled in the art, encourage the electric charging of the granules and, more particularly, the dierentation of the charges for granules of different mineralogical species.

At any rate, the expedients enumerated above for imparting, or encouraging the takeup of, electric charges of opposite signs to the granules should be such as not to disturb the flow of air between the electrodes 2 and 3 where the granules are to travel over paths which should be as regular and uniform as practicable. The airborne granules, as they come between the electrodes 2 and 3, undergo the effect of the static field existing therebetween. The positively charged granules are collected by the electrode 2, and the negatively charged granules by the electrode 3. The separation of the positive granules from the negative ones is particularly pronounced near the ends 4 and 5 of the electrodes, inasmuch as near the end 4 there are virtually only granules belonging to the species which had taken up positive charges, whereas near the end 5 there are virtually only granules of the negatively charged species. The granules which move in the neighborhood of the electrode ends 4 and 5 are dumped outside of the duct 1 through the openings adjusted by the sectors 6 and 8, and it is clearly obvious that, by decreasing or increasing the widths of the openings by means of the sectors 6 and 8, it is possible to collect, at the base of the electrodes 2 and 3, granules having a higher, or a lower concentration, respectively, of the mineralogical species which had taken up a charge such as to be collected by the electrode concerned. The granules which are not dumped through the openings formed at the bases of the electrodes, that is, the granules which did not undergo any electrostatic separation, are conveyed by the gaseous stream between the sectors 6 and 8 and then recycled. It should be noticed, at the outset, that an efficient separation can be obtained also for extremely nely ground granules in that all the granules conveyed by the gaseous stream go, between the electrodes, along substantially regular paths.

When the ore being treated in the separator contains a considerable amount of very finely sized granules for which the separation is obviously harder to perform, the amount of granules which have not been separated during progress of a cycle and consequently to be recycled, is greater: if so, a continuous and automatic recycling of the granules which had not been separated during an operative cycle of the separator described herein, is of vital importance. The trajectories of the granules in the inter-electrode space can be modified in a way which can be defined a priori or determined by trial and error, in addition to its being modified by varying the gradient of potential between the electrodes and also by varying the speed of air (or gas) in the closed loop within the annular duct. It is necessary, in any case, that the speed of air in the duct is always such as to ensure the pneumatical conveyance of the granules in any point of the duct. To obtain high speeds of the airflow within the annular duct, high rates of flow of air need not be supplied through the tubular member 10 since, by increasing the pressure of air in the tubular member 10 and decreasing, with the sector 11, the width of the opening at which the tubular member intersects the annular duct, it is possible to administer to the air flowing in a closed loop an energy which is adequate to impart the desired speed to the airflow. This is of paramount importance, inasmuch as it permits to optimize the ratio of the rate of flow of air circulating between the electrodes, to the rate of flow of air coming out of the separator together with the separated granules. By operating with sufiiciently high values of the ratio, the phenomena of entrainment of particles towards the outlet ports can be coniined within preselected limits, the width of the outlet ports being adjusted by the sectors 6 and `8. The occurrence of the phenomenon has a considerable bearing when separating very finely sized granules, whose paths are influenced by the trend of the airstream in the interelectrode space to an extent which is more significant than that experienced with coarser granules. A value of the ratio which has been found satisfactory in many instances lies in the range from to 6, and the corresponding speed of air in the annular duct is in the range from 1 to 3 meters a second. It is to be seen that the duct 1 can take shapes other than those shown, with the outline of the conduit still being an annular one. The hopper 13, instead of being arranged and inserted in the annular duct 1, can optionally be positioned and inserted in the tubular member 10.

With reference to FIGURE 2, it is seen that this gure, and also FIGURE 3, shows a separator akin to that shown in FIGURE 1 and which comprises structural cornponent parts which are equal, or entirely equivalent, to those described in connection with FIGURE 1, and corresponding parts bear the same reference numerals for simplicity and consciseness, to dispense with their description.

The separator of FIGURE 2 has an upper curved portion in correspondence with which two electrodes are mounted within the duct 1. More exactly, the outer peripheral wall of the duct 1 mounts an electrode 14 which is connected to the positive terminal of a high voltage power generator (or, also and preferably, grounded) whereas an ionizing electrode `15, lconnected to the minus terminal of the high voltage generator, is mounted on the inner peripheral wall of the duct. The ionizing electrode can consist of a metal plate having curled-up edges, and carries a bundle of wires, such as tungsten wires, spaced apart from the plate. Alternatively, the ionizing electrode can consist of a thin-gauged metal gauze or blades arranged perpendicularly to the surface of the electrode 14 or also by pointed electrodes so as to determine a convective discharge of ions and electrons towards the electrode 14. The electric charging, by ionic effect, of the granules of ore treated in the separator can take place concurrently with, or entirely independently on, the charging of the same granules due to triboelectric effects as hereinbefore defined.

The ore granules coming between the electrodes 14 and 15, by conveyance with a gaseous stream, are negatively charged. Inasmuch as the electrode 14 is extended along the external periphery of the arcuate portion of the annular duct, the granules are hurled by centrifugal force against the electrode. Here the ionized granules of conductive mineralogical species shule off their negative charge (taken up when passing near the ionizing electrode) and are positively charged by induction and conduction. The ionized granules of an insulating mineralogical species or anyhow less conductive than the one considered above, do not lose (or lose only in part) their negative charge, since the separators design is such that the granules of the less conductive mineralogical species are contacted by the electrode 14 during a time shorter than the inversion time, that is, the time which is necessary for the negative charges to be dispersed. The granules coming between the electrodes 2 and 3 are thus positively charged for either mineralogical species` and negatively charged for another mineralogical species which is less conductive than the former. The separation between the static-field generating electrodes takes place in the same way and under the same conditions as described above in connection with FIG. 1.

Electrode 14 shown in FIG. 2 has `an angular width wider than the ionizing electrode 15 and extends up to the vicinity of the extreme upper edge of the electrode 3 with respect to which it has been shown insulated in the drawing. It is obvious, however, that the electrode 14 and the electrode 3 could possibly be non-insulated from one another or even could be integral with the housing of the duct 1 when the latter is made of a conductive material and is grounded for reasons of safety. The ionizing electrode 15 which in FIG. 2 has been shown with its extreme edges considerably spaced apart from the upper extreme edge of the electrode 2, could even extend up to the vicinity of the upper extreme edge of electrode 2, with respect to which it could even be non-insulated, while being still insulated with respect to the housing of the duct \1. The basic feature of the separator which charges the granules by ionic effect is, in addition to all those features which have provento be essential for the separator described in connection with FIGURE 1, that the electrode 14 is present and extended up to the outer periphery of an arcuate portion of the annular duct in correspondence with, or downstream of, an area in which an ionizing electrode has been catered for.

FIGURE 3 shows -a separator akin to that of FIG. 2, but in which an arcuate portion of the annular duct near the upper end of the electrodes 2 and 3 a single, positively charged electrode 16 is provided, (it could be grounded, instead), whereas an ionizing electrode 17 and a backing electrode l8r, connected to the high voltage generator, are placed upstream of the electrode 16 in a rectilinear portion of the duct 1.

The electric charging of the ore granules dispersed in the gaseous fluid flowing within the annular duct is very efficient between the ionizing electrodes since the uidborne granules have a movement with a gyratory component causing them to be exposed to the action of the ionizing electrodes their entire surface throughout. The dynamic contact obtained by hurling the granules, upon charging them negatively between the ionizing electrodes, against the surface of an electrode 14 (FIG. 2) or 16 (FIG. 3) grounded or connected tol a positive power source, forcing them to slide or roll over the surface under the urge of inertial and contrifugal forces, is also a kind of contact which is very efficient to the end of the reversal of the charge of the granules of the more conductive mineralogical species, an outstanding factor when extremely finely ground granules are involved.

The shape of the electrodes shown FIGS. 1 to 3 can be Varied in certain particular cases, with a curved trend being also provided. FIGURE 4 shows a constructional modificatie nof the shape of the electrodes to be imagined, for example, as applied to the separator of FIG. 2, shown only in part. As seen in FIG. 4, while one of the static field generating electrodes, precisely the electrode indicated at 19, is the same as the electrode 2 described above and has the same characteristics, while the other electrode, indicated at 20, differs from the electrode 3 of FIGS. 1 to 3 in that it has a portion 21 bent towards the inside of the duct. The effect of the portion 21 of the electrode 20 is to modify the direction of the airflow between the electrode and electrode 19, thus influencing the path of the granules so as to collect, at the base of electrode 20, granules, as compared with that which would occur by employing two electrodes `of the type shown in FIGS. l to 3, a greater concentration of granules to be collected by the electrode 20 and, concurrently, a smaller concentration of granules to be collected by the electrode 19.

FIGURE 5 is illustrative of another modification of the shape of the static-field generating electrodes, supposedly applied to the separator of FIGURE 2l. In FIGURE 5, while an electrode 22 retains the shape of the electrode 3 of FIGS. 1 to 3, the other static-field generating electrode, indicated at 23, has a louvered configuration. The electrode 23, in practice, can be formed by a plurality of slats 24 hinged about horizontal axes, fixed and perpendicular to the sheet of drawing, so as to vary their slant. The louvered configuration of the electrode is necessary for separating mineral species having a short inversion time (in practice, a host of metallic ores such as galena, pyrite, iron and manganese ores and others), that is of ores whose granules, when in contact with an oppositely charged electrode invert their own charges very rapidly and completely so that, as they enter the static field, their path is immediately deflected so as to point towards the oppositely 4charged electrode (electrode 23) under a wide angle of incidence.

Should this occur with an electrode such as shown FIGS. 1 to 3, the granules could rebound as they impinge onto the electrode, after having shuffled off their charge, due to the above mentioned low inversion time, and so there Would be a chance that the granules may be conveyed towards the opening formed at the base of the The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. An electrostatic separator for granular material comprising wall means defining an annular duct, a section of said duct having its wall including two substantially parallel planar stationary electrodes adapted to be connected to means for charging the electrodes to establish an electric field therebetween, said electrodes ending with mutually diverging confronting terminal portions, said duct opposite electrode together with the granules collected 10 having an aperture defined by walls spaced closer together thereby. The louvered shape of the electrode 23 is such than the diverging terminal end portions of said electhat, even if the granules impinge thereover and rebound, trodes, a conduit through which a gaseous fluid is directed they pass through the inter-louver gap and are collected into said duct in a substantially tangential direction in the at the bottom of a hood 25 which encircles either, or both, 15 vicinity of said aperture and away from said terminal porstatic-eld generating electrodes. If so, the hood 25 should tions, and means for feeding granular material into said be fitted with devices adapted to set apart granules of `difduct, with the fluid moving the granular material through ferent mineralogical species as separated by the separator. the duct whereby positively charged granules are col- The combination of the different kinds of static-field lected by one electrode and negatively charged granules generating electrodes as shown FIGS. 1 to 5, enables the 20 by the other electrode and the granules not undergoing separator, in practice, to be modified as to its behaviour electrostatic separation entering said aperture for rein any desired sense, consistently with the result intended cycling. to be achieved by the separation. 2. The separator as claimed in claim 1 in which said To cast a light on the results obtainable with a separaaperture is provided with movable sectors rotatably tor such as described above, the results of one of the 25 mounted about pivots on the wall means of the annular numerous tests performed by the applicants with an induct. ventive separator will be set forth. 3. The separator as claimed in claim 1 in which said In the test in point, a hematitic ore, coming from an conduit is provided with a movable sector rotatable about African sill and having a gangue predominantly formed a pivot provided on said wall means. by a silica in the form of quartz with a minimum amount 4. The separator as claimed in claim 1 in which said of silicate, has been considered. The ore has been ground annular duct has a substantially vertical orientation. to a grit size of less than 70 mesh and has been treated in 5. The separator as claimed in claim 1 in which said the separator. To study the behavior of the separator electrodes are planar in thatl portion of their length at towards the several mesh ratings which compose the tried which they are substantially parallel. ore, a grit size classification has been instituted and a 6. The separator as claimed in claim 1 in which atleast chemical analysis of the granules separated by the Sepone of said electrodes is provided with a portion bent arator was made. The attached Table tabulates the results towards the other electrode, obtained and the recovered iron values, both with respect 7, The separator as Claimed in claim 1 in which at least to the total for each mesh rating and to the fed-in one of said electrodes is provided withaplurality of transmaterial. verse slots.

The 'fable reports under the entry A the data relating 8. The separator as claimed in claim 7 in which said O tlle Ininefalogieal species colleeed at the base 0f the slots are confined by a plurality of movable slats mounted negative static-field generating electrode; the entry B re- 0n xed pivots,

Ports the data related to the mineralogieal species Whiel'l 9. The separator as claimed in claim 1 in which said has been separated at the base of the positive 0r the annular duct in the vicinity of the ends of said electrodes grounded eleCrOde, and the entry C indicates the data opposite said terminal portions is of curved configuration, relative to the total of the mesh rating COI1Cerned the inner peripheral face of said curved portion being pro- AS Seen in 'Ille table, the separator is considerably ef' vided with an ionizing electrode adapted to be connected ficient even in the finest mesh ratings, since, for grit size to the negative terminal of a high voltage power genclasses between 325 and 400 mesh and Under 400 mesh erator, and the inner face of the Outer peripheral surface Values Of the minel'alogioal sPeCieS Collected at the base 50 of the curved portion being provided with an electrode of the negative electrode have been attained Whioli are connected to a voltage higher than said negative voltage. more than 68% Fe in both Cases, along With recoveries 10. The separator as claimed in claim 1 in which said of Fe With respect t0 the total 0f the mesli Tamigi of annular duct in the vicinity of the ends of said electrodes Inoi'e than 97% and more than 84% reSPeclvely' remote from said terminal portions being of curved con- It cal? be Sald by Way of summatlonthat hlgh'efclncy 55 figuration, the inner face of the outer periphery of the Separatlons gener'fmy as emclent as 90% and Sometlme; curved portion being provided with an electrode adapted over cari be Obtamed that a rmuneatwogeatmlnt Od to be connected to the positive terminal of a power genores havmg a Very ne mesh 512e. un er m-es an erator, and the inner face of said duct in the portion even under 400 mesh can be carried out and highly Sethereof intermediate said conduit and curved portion belective separation can be effected and that it is possible f l d f l t to separate, at the base of the static field generating elecmg Provided Wlth tYVO ac1 ng e ectr0 es one o Sald as nodes, granules exhibiting a high concentration of the named'electrodes being an ionizing electrode and the other mineralogical species collected by each of the electrodes a backing electrode Wllll One 0f Sald last named eleetrOdeS involved. being adapted to be connected to the negative terminal of TABLE Distribution Fe on mesh Distribution Fe on Feeding, Weights, percent Fe Contents, percent Rating, percent percent A B C A B C A B C A B C Mesh Ratings:

------------------ aa ari as aa as iii 669 a; :ai as 12.62 3.37 15.99 68.58 5.13 55. 21 98.04 1.96 100.00 15.19 0. 30 15.49 4.87 1.65 6.52 68.36 7.37 52.92 96.47 3.53 100.00 5.85 0.21 6.06 11.35 4.13 15. 4s 67.91 4.69 51.04 97.55 2.45 100. 00 13.52 0. 3 4 13. 86 4.41 1.15 5.56 68.13 6.62 55.41 97.53 2.47 100. 00 5.27 0.13 5. 40 9.90 3.86 13. 76 68.47 32.15 58. 26 84.53 15.47 100. 00 11.89 2.18 14.07

1 1 a high voltage generator and the other with the positive 3,308,944 terminal of the power generator. 3,341,008

References Cited UNITED STATES PATENTS 489,249 701,417 6/1902 swart 209-128 999894 2,706,044 4/1955 Cook 209-127 722889 2,738,067 3/1956 Cook 209-127 3,111,398 11/1963 Jones 209-127 X 12 3/1967 Chamberlain 209-129 X 9/1967 Mayer 209-127 X FOREIGN PATENTS 9/ 19 18 France. 9/ 1951 France. 7/ 1942 Germany.

FRANK W. LUTTER, Primary Examiner

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