Electromagnetic Separators for Strongly Magnetic Minerals

Many classifications based on the method of treatment, on differences in construction, etc., have been suggested to include the different types of magnetic separators; separators with stationary magnets, and those whose magnets revolve; separators in which the ore is attracted directly against the magnet, and those which interpose a nonmagnetic belt or drum between the magnet and the particles attracted; separators which lift the magnetic particles from the mixture, and those which deflect the magnetic particles from a falling sheet of ore, and various others. The classification of most value is that based upon the types of material the different separators are suited to treat. For this reason the only classification attempted here is to distinguish between the separators designed to remove ferromagnetic minerals and those designed to treat such feebly magnetic minerals as raw siderite, limonite, etc. A number of separators have been designed to treat a finely divided feed only, and others for use as cobbing machines. The sizes of feed to which the several machines are suited will appear in the descriptions of the individual separators.

Descriptions have been published of a large number of separators whose principal claim to interest is an historical one; the only machines here described are those which are at present of commercial importance.

Ball-Norton Belt Separator

This machine employs the principle of a series of magnets of alternate polarity to effect a thorough turning over of the ore while in the influence of the magnetic field, thus permitting entrained particles of waste to fall from the concentrate. The ore is fed from a hopper by a feed roll upon a horizontal belt which serves to present it to the magnets from beneath. The magnetic particles are lifted from this feed belt by the magnets and held against a take-off belt running in the same direction and interposed between the ore stream and the magnet poles. The take-off belt is run at a greater speed than the feed belt in order to carry the ore past the magnets in a thinner layer. The belts are made of rubber-covered canvas, and means are provided to

alter the speed of belt-travel to suit different ores. As the magnetic particles are held against the take-off belt, and by its motion carried past the poles alternately opposite in sign, the loops of magnetic particles are broken and reformed as they pass from one pole to the next, permitting entrained particles to fall from the concentrate into a tailing compartment, into which the non- magnetic material remaining on the feed belt also falls. The magnetic concentrate is carried past a partition and is dropped from the last magnet into a separate compartment.

The series of magnets is made up of 12 poles, those of opposite sign being adjacent, all controlled, in the type machine, by one rheostat. By dividing the poles into two series by suitable connections, and employing an additional rheostat, two sections of the field of different intensity may be obtained.

The capacity of this machine is from 20 to 35 tons per hour of magnetite ore crushed to pass a ¼-in. aperture.

Monarch or Ball-Norton Double-Drum Separator

This machine embodies the same principle of magnet construction as the Ball-Norton belt separator. It consists of two revolv-

ing drums with nonmagnetic surfaces placed parallel and close together, within each of which is fixed a composite electromagnet made up of adjacent poles of opposite sign.

The ore is fed at the top of what may be termed the rougher drum, in passing around which the nonmagnetic particles are thoroughly eliminated, falling into a hopper below. The magnetic particles are held against the drum by the magnets within, and while passing the poles of opposite sign the loops of magnetic particles are broken and reformed, freeing the nonmagnetic particles, which are removed by a combination of gravity, centrifugal force, and the effect of a blast of air impinging upon the surface of the drum in a direction opposite to its rotation. At a point just below the horizontal diameter of this drum the ore passes beyond the influence of the magnets and is thrown, by centrifugal force, against the face of the adjacent cleaner drum where it is caught and held by the magnets. The cleaner drum revolves at a greater speed than the first drum encountered by the ore and is furnished with weaker magnets; particles of inferior permeability, which were held by the rougher drum, are here thrown off into a middling hopper; the concentrate is carried farther and thrown into a chute after passing beyond the influence of the last magnet pole. The rougher drum makes 40 revolutions per minute and the cleaner drum 50; the magnets in the rougher drum take 10.5 amperes and those in the cleaner drum 13 amperes.

The capacity of this separator, with drums 24 ins. in diameter by 24 ins. face, is from 15 to 20 tons per hour of magnetite ore, crushed to pass 16 or 20 mesh. The power required is from ½ to ¾ H. P. for operation, and from 1 to 1.5 E. H. P. for excitation.

Dellvik-Grondal Separator

This type of separator was designed for the treatment of fine material. It consists of a composite electromagnet of cylindrical form which revolves about a vertical axis. This cylinder, of cast iron, carries a series of six exciting coils, wound in circular grooves cut around its circumference. These coils are separated from each other 60 mm., and are so wound as to give fields of progressively increasing strength from top to bottom opposite the iron spaces between the coils, which form the separating surfaces.

The ore, in suspension in water, is fed from a launder against the topmost magnetic ring. This launder, which is curved to cover about 90 degrees of the magnetic cylinder, is supplemented by four other similar launders below it, which serve to catch and return against the drum any material thrown off by its revolution.

The magnetic particles stick to the rings between the coils, those not held by the first ring being caught and held by one of the lower rings, each of which has a field of greater strength than the ring next above it. Nonmagnetic particles are washed from the concentrate by a stream of water which plays against the cylinder. By the revolution of the cylinder the magnetic particles adhering to it are carried opposite a wooden cylinder, carrying secondary magnets, which is mounted parallel to the magnetic cylinder, and which revolves in the opposite direction. This wooden cylinder is studded with a number of iron pegs so placed

as to come opposite the magnetic rings of the separating cylinder. These pegs, distant 5 mm. from the magnetic rings, concentrate the lines of force from these rings upon their points, giving rise to local fields of greater intensity than the primaries, and so cause the magnetic particles to leap across the gap and attach themselves to the pegs. By the revolution of the wooden cylinder these pegs are carried beyond the influence of the primaries, lose their secondarily induced magnetism, and drop their burden of magnetic particles, which removal is aided by a stream of water.

The capacity of this machine is from 30 to 45 metric tons per 24 hours of magnetite ore, crushed to pass a 1-mm. aperture. The magnets require 6 amperes at 31 volts. The separating cylinder makes 25 R.P.M. and the take-off cylinder 225 R.P.M.

Grondal Type II Separator

This separator consists of two iron disks fastened, 60 mm. apart, to a vertical standard, the space between the disks being occupied by the exciting coils. The disks and coils are stationary. This circular magnet is covered with a brass ring, around

the periphery of which a series of iron strips are mounted; and which are magnetized from the disks as long as they are adjacent to them. The distance between the disks and the brass ring is so varied that the iron strips are magnetized during one half of the revolution only. The ore is slimed and fed, in suspension in water, against the brass ring through launders similar to those employed in the Dellvik-Grondal separator. The magnetic particles stick to the iron strips during half the revolution, are thoroughly washed with a jet of water, and, on passing beyond the influence of the magnetic disks, are washed off the strips by a jet of water. The iron strips are coated at the top with a layer of lead and antimony. This layer is thickest at the top of the strip, gradually shading off until at the bottom of each strip the ore comes into direct contact with the iron; this is done to give a field of steadily increasing strength on each strip in the direction of passage of the ore.

Grondal Type III Separator

This separator consists of a fixed electromagnet with hatchet-shaped pole pieces enclosed in brass drums which revolve at 80 revolutions per minute. The surfaces of the drums are fitted with strips of iron which form secondary poles, and against which the magnetic particles are attracted. The ore is introduced into a tank beneath the revolving drum, which is suspended just above the level of the water; the sharp edges of the pole pieces give rise to a concentration of the lines of force which serves to lift the particles of pure magnetite out of the water and against the drum, where they stick to the secondary magnets and are carried by the revolution of the drum out of the field and discharged into a launder. The particles forming the middling product are not lifted from the water, but are sufficiently attracted to separate them from the waste and are discharged through an overflow at the side of the tank. The nonmagnetic particles fall to the bottom of the tank and are discharged through pipes. Generally two drums are combined in a twin machine which requires 2 H.P. for operation, and 3.5 amperes at 110 volts for excitation of the magnets. The capacity of this machine is 50 tons in 24 hours.

Grondal Type IV Separator

This type of separator was designed to deliver magnetite concentrate as dry as possible from a wet separation. It consists of a brass disk revolving at 1450 R.P.M. beneath an electromagnet whose pole pieces taper to an edge at their lower extremities. The

slimed ore is delivered by a launder into a tank beneath the brass disk, and the magnetic particles are drawn up against the disk, from which they are thrown off by centrifugal force in a nearly dry state. About 1 H.P. is required for operation, and 3.5 amperes at 110 volts for excitation of the magnet.

Grondal Type V Separator

This machine consists of a brass drum which revolves on a horizontal axis and encloses a series of magnets of alternate polarity of the Ball-Norton type. The difference between the working of this machine and that of the Ball-Norton consists in

feeding the finely crushed ore in the former case, in a stream of water into a tank beneath the separating drum, from which it is raised by the magnets against the drum. This machine requires 1 H.P. for operation and 4 to 5 amperes at 110 volts for excitation. It is said to have treated 100 tons of crude ore in 24 hours.

The Grondal Slime Separator

This is a stationary electro-magnet with two beveled-edge pole pieces which are suspended above V-shaped settling tanks. The slime, in suspension in water, is introduced at one side of the tank in a shallow stream which flows beneath the pole pieces to a similar discharge at the opposite side. The current on the magnet, which is suspended close to the water level, but not dipping into the water, is regulated so as to be just too weak to lift magnetic particles out of the water. The magnetic particles form

bunches in the water beneath the pole pieces and fall to the bottom of the tank, from which they are discharged through a pipe. This apparatus is frequently employed for dewatering the pulp from ball mills, in which case a stream of clear water is introduced into the tank at the bottom; the sand falls to the bottom and is discharged through a pipe along with the bunches of magnetic slime collected beneath the magnets. By regulation of the velocity of the stream of pulp and the amount of clear water added, the size of particles carried over the waste discharge may be adjusted to suit the ore under treatment.

Wetherill Type F Separator

This machine comprises a separating armature, built up of alternate disks of magnetic and nonmagnetic material. Upon revolution between the primary magnets secondary poles are set up at

the edges of the magnetic plates, or disks, of the armature, focusing the lines of force from the primaries and causing magnetic particles to stick to the armature until carried beyond the influence of the primary poles. The waste drops off the armature into a receptacle, while the magnetic particles are held until the neutral point is reached, where the magnetism of the disks changes from plus to minus, when they fall into a receptacle. The change in magnetism is gradual, so that by means of suitable partitions, several products may be made on the same separator, the strongly magnetic being the last to fall from the armature.

The machine is built in one size only, with 30-in. poles, but the magnets are wound for various strengths of current. The capacity of the machine is large: the makers claim that 400 tons are put through these machines at Mineville, N. Y., in 24 hours.

Froeding Separator

This separator consists of a round table of brass 3 mm. thick, and 1.45 meters in diameter, which slopes from center to circumference. Beneath this separating surface, which revolves, there is a system of 12 stationary magnets, arranged radially to cover 6/7

of the surface of the table; beneath the sector, representing 1/7 of the area, there is a gap without magnetic attraction. The magnets are of alternate polarity and have their corners beveled to concentrate the lines of force at the periphery, and are spaced 50 mm. apart. Above the table is a series of movable perforated pipes, which deliver a spray of wash water on the ore under separation. The ore is delivered in a stream of water at the center of the table, and spreads out in a layer of decreasing thickness toward the periphery. The magnetic particles are held against the surface of the table and carried by its revolution to the sector where there is no magnet and here washed off. The nonmagnetic particles are washed off the table by the wash water from the pipes. The alternate polarity of the magnets causes the magnetic particles to turn over in passing from one magnet to another, the entrained waste liberated during this process being washed off by the sprays from the pipes, which are hung 40 mm. above the table. The two products are caught in separate launders at the periphery of the table. Magnetic particles are prevented from being washed off the table by the concentration of the magnetic field due to the beveling of the magnets mentioned before. The capacity of the machine is 2 metric tons per hour, at 10 R.P.M.; 150 liters of wash water are used per minute; ½ H.P. is sufficient to operate the moving parts, while the magnets require 8 amperes at 100 volts for excitation.

Ericksson Separator

The construction of this machine is best understood from the accompanying figures. The magnets A and the coils C revolve about the shaft B. The magnet wheels are divided into 21 spokes, the spokes on each side being opposite one another. Between the two halves of the magnet is an annular slot, extending completely around the circle; the walls of this slot are thin sheets of nonmagnetic metal, and this space is filled with water to the height of the axle. The ore is fed by a stream of water at E; the magnetic particles form bridges in the fields between the opposite spokes, and are carried around by the revolution of the magnets. At K a launder is introduced into the slot, receiving the bridges of magnetic particles, which are washed out of the machine through this launder .by a strong jet of water. The magnetic material is washed, and waste particles removed, by sprays of water playing on the bridges across the slot between the time it is lifted above the water level and the time of its encountering the discharge launder. The nonmagnetic particles fall to the bottom of the tank and are discharged at H. A float, J, is connected with the discharge opening, H, by a rod; when the water rises above the proper level, because of the introduction of the

feed, the discharge gate at H is opened and the surplus water, along with the waste, flows from the machine. The capacity of this separator is about 2 metric tons per hour; the magnets take 20 amperes at 110 volts. Nonmagnetic slimes which do not settle readily are drawn off from time to time through the pipe F.

Forsgren Separator

This separator comprises five independent separating zone; which may be employed, if desired, on different ores and with different strengths of field. This machine consists of two concentric brass rings mounted with soft-iron secondary poles attached to a spider which, by revolution about a vertical axis, causes the rings to pass between the poles of five fixed electro-magnets spaced 72 degrees apart. The ore is fed in the annular space between the brass rings at points opposite the primary magnets; the magnetic particles in the ore attach themselves to the secondary magnets, while the nonmagnetic particles fall past them into a tailing chute. As the rotation of the brass rings carries the secondarily induced magnets past the fixed primaries they lose their magnetism and the attracted particles fall, first the feebly magnetic particles, which drop into a middling chute, and finally the strongly magnetic particles which drop into a concentrate chute.

From ½ to 3 H.P. is required for operation, and from 3 to 3.5 amperes for the excitation of each primary magnet. The capacity

of this machine varies with the size of the material treated: operating on magnetite ore crushed to 1.2 mm. it handles 1 2/3 metric tons per separating zone per hour; arranged for cobbing, it handles 2.5 metric tons per separating zone per hour for sizes up to If ins. The brass rings rotate at a speed of from 5 to 10 R.P.M.

Edison Separator

This machine consists of a series of bar magnets in front of which the ore is allowed to fall in a thin sheet. The magnetic particles are attracted sufficiently to alter their trajectory but

not enough to draw them against the magnets. The falling sheet thus divided is caught in separate chutes or hoppers. The current on the magnets and the distance from the falling ore sheet to the face of the magnet are capable of adjustment to suit different ores.

A single magnet may be employed to effect the separation, or a number of units in series. In the mill at Edison, N. J., two systems were employed, the first to produce a clean tailing product and a second for the cleaning of the concentrate from the first magnets.

The preliminary magnets are arranged as shown in Fig. 13. The ore is fed past each end of the magnets, the magnetic product passing from the machine from each magnet, while the nonmagnetic particles are successively re-treated. This arrangement produces a clean tailing with very little loss in magnetic material. The magnets are 12 ins. long, 4 ins. thick and have a separating face 4 ft. 6 ins. wide. The cores are of cast iron (as the magnets are never saturated) and are wound with No. 4 copper wire. The three magnets are wired in series, and each has a different winding, the upper with the fewest and the lower with the greatest number of turns, giving separating fields of constantly increasing strength in the direction of travel of the ore. The magnets are excited by 15 amperes at 80 volts. The capacity of the series is 16 tons per hour of ore crushed to pass 0.06 in. A second series of magnets is used to re-treat the magnetic product from the above-described machine after drying and recrushing. The arrangement is the same, but the magnets are 8 ins. long, and are wound with No. 6 wire: the capacity is 2.25 tons per hour on material crushed to pass 0.02 in.; tailings from the last magnet are waste. These magnets take 10 amperes at 120 volts.

The cleaning magnets are arranged in a series of five units, and treat the concentrate from the preliminary magnets after the removal of dust. With this machine the object is the production of a clean magnetic product, and the magnets are arranged as shown above to repeatedly re-treat the magnetite, the tailing being discharged after passing each unit. The magnets are 4 ins. long, 2 ins. thick and have a separating face 4 ft. 6 ins. wide. They all have the same winding of No. 6 wire, are connected in series and take 17 amperes at 100 volts. The tailing from the upper magnet in this series is run to waste, while the tailing from the four lower magnets is regarded as middling and sent back for re- treatment. The capacity of this machine is about 0.9 ton per hour.

Edison Belt Separator

This machine consists of a belt 7 ft. wide which travels over two pulleys revolving about horizontal axes in the same vertical plane. Behind the side of the belt which travels upward are placed several electro-magnets staggered across the belt, adjacent magnets being of opposite polarity. The ore is fed against the belt opposite the lowest magnet, the magnetic material adheres to the belt and is carried upward and across it as a result of the arrangement of the poles of the magnets; the nonmagnetic particles fall from the belt. The material fed is in a fine state of division and forms tufts on the surface of the belt which turn over and over in their passage across and up the belt, liberating any particles of entrained waste. The upper magnet extends

further toward the edge of the belt than the lower magnets, and the magnetic particles are dropped from it into a series of small buckets riveted to the edge of the belt, and so discharged from the machine. This separator is designed for the removal of non-magnetic particles from a finely divided feed.

Ball-Norton Single-Drum Cobbing Separator

This machine is used for cobbing ores which are not necessarily dry; the ore fed is coarse (1½ ins.) and the separator puts through a large tonnage with the idea of making a clean concentrate of the pure magnetite pieces, while the tailing is re-treated on other separators after crushing. The separator consists of a drum with nonmagnetic surface which revolves about a composite magnet in the form of a sector of a circle. The attraction is exerted by 16 electro-magnets attached to a spider and mounted

on the shaft of the drum. The magnets are stationary and cover a little more than 180 degrees of the circumference of the drum. They are of alternate polarity, which causes the ore to turn over as it is carried past each of the 16 poles by the revolution of the drum. This turning over permits the nonmagnetic particles to drop off the drum into the tailing hopper. The ore is fed near the top of the drum, and the strongly magnetic pieces are carried past the tailing hopper and thrown off by centrifugal force as they pass beyond the influence of the last magnet, falling into a concentrate chute. The amperage is regulated so as to pick out the pure pieces of mineral only, allowing composite pieces of ore and waste to go into the tailing to be separated after crushing.

Wenstrom Separator

This machine consists of a drum made up of alternately magnetic and nonmagnetic bars, which revolves about a horizontal axis and encloses a stationary magnet. The stationary magnet is cylindrical in form and is placed eccentrically within the revolving drum; it carries four circular projections, or ridges, between which are wound the exciting coils, so connected that adjacent projections have opposite polarity. The surface of the drum is made up of soft iron bars with nonmagnetic spaces between them usually filled with strips of wood. The bars have projections from the inner surface of the drum which engage the projections from the magnet, making them practically prolongations of the poles of

the magnet. The projections on alternate bars engage alternately the north and south poles of the stationary magnet, giving adjacent bars opposite polarity. The projections from the magnet are cut away on one side of a vertical diameter of the drum. The ore is fed at the top of the drum and is carried forward by its revolution; the magnetic pieces are held by the magnetic bars until the vertical diameter is passed, when they fall into a hopper upon the bars becoming demagnetized. The waste falls into a hopper in front on the drum. This machine is designed to treat lump ores which need not necessarily be dry. It is made in two sizes: the larger size is capable of separating 4-in. lumps, is 27 ins. in diameter and 24 ins. across the face, takes 15 amperes at 110 volts and has a capacity of from 5 to 7 tons per hour. A smaller size has a capacity of 3 tons per hour on ore 1.5 ins. maximum size.

New Wenstrom Cobbing Separator

This is a modification of the machine above described. The distance between the ribs making up the surface of the drum of this separator is varied to suit the size of the ore to be treated. For the finer sizes, from 1/8 to 1 1/8 ins., the drum is covered with a sheath of German silver. For treating coarse ores the drum is made in diameters from 2 ft. 10 ins. to 3 ft. 4 ins.; the length

of the drum face is 2 ft. Recently some of these machines have been built with twice this width and divided into two sections, one side for coarse and the other for fine material. The drums make from 16 to 20 revolutions per minute; the electro-magnet requires from 15 to 20 amperes at 110 volts for excitation. The capacity of this separator varies from 5 to 10 tons of crude ore per hour.

Grondal Cobbing Separator

resembles the Wenstrom machine, the drum being made up of ribs alternately iron and brass. The former are ½ in. wide and the latter 3/16 in. wide. The drum is operated at a speed of 30 revolutions per minute.

Dings Separator

This separator consists of an inclined shaking conveyor which serves to carry the material to be separated beneath two wheels, each studded with secondarily induced magnets and revolving about vertical axes. The ore is fed from a hopper at the head of the inclined conveyor, and is transported by the shaking movement through four zones of separation, due to the magnet wheels. The first magnet encountered by the ore carries the less current and separates the strongly magnetic particles only; the second magnet carries a greater current and separates a middling product; the nonmagnetic tailing passes off the end of the shaking conveyor.

The conveyor is a tray made up of a sheet of 3/16 in. steel covered with asbestos and mounted upon hangers. A shaking movement is imparted to the conveyor by an eccentric, the movement being upward at the feed end and also in the direction of the travel of the ore. The usual speed is 440 strokes per minute. While passing over this conveyor the ore is kept constantly in agitation, thus lessening the chance of entrainment. The conveyor is 18 ins. wide and 7 ft. long, and may be raised or lowered by means of hand wheels on the hangers, thereby altering its distance from the magnets. By raising one end only, a different and gradually increasing distance from the plate to the magnet wheels may be obtained at each of the four zones of separation. This separator is also built with a conveyor belt in the place of the shaking conveyor.

The primary magnets are fixed, and consist of two steel cores, which carry the windings and connect the pole pieces. These pole pieces are made in the form of circular arcs to correspond with the secondary magnets revolving below. The secondary magnets are made of laminated steel and are disposed around the periphery of a bronze carrying wheel 30 ins. in diameter; they project as cylindrical knobs about 1 in. below the carrier, and their upper ends are U-shaped to engage closely, but not to touch, the pole pieces of the primary magnets. The magnetic circuit is completed through the steel plate beneath the asbestos covering of the conveyor. As the individual secondarily-induced magnets are carried by the revolution of the carrying wheel beyond the fields of the primaries, they lose their magnetism and allow the attracted particles to drop off. These magnets reverse their polarity before entering the field of the opposite pole of the primary, causing a thorough discharge of their burden of magnetic particles. Troughs are provided to carry away the magnetic particles dropped, and may be so arranged as to deliver four distinct products, if it is desired.

In operation, a variety of adjustments may be made, to suit different ores, by altering the amperage on the primary magnets, by changing the distance from the conveyor to the secondary magnets, and by altering the inclination of the conveyor. The capacity of the machine may be taken at 1 ton per hour of properly roasted blende-pyrite concentrate. About one mechanical horse power is required for operation, and from ½ to 2 electrical horse power for excitation.

Humboldt-Wetherill Tandem Separator Type VII

In this a broad conveyor feed belt transports the ore to be separated beneath highly magnetized rollers. These rollers, which revolve in the same direction as the travel of the belt beneath them, pick up the magnetic particles from the ore stream and deposit them on cross belts which remove them to one side. At the end of each cross belt is another magnet which acts upon the magnetic particles as they are thrown off the cross belt, diverting them into suitable receptacles, according to their permeabilities, giving a double separation of the magnetic particles. These separators may be operated at high speed and are said to have a large capacity on strongly magnetic ore or artificial magnetite.

Cleveland-Knowles Separator

This machine comprises a conveyor belt which serves to transport the material to be separated beneath two cylindrical electromagnets which revolve about vertical axes at a height of approximately 1 in. above the belt. The first magnet encountered by the ore, usually called the rougher magnet, is the weaker of the two and attracts the more strongly magnetic particles of the ore only; the second, or cleaner, magnet carries a higher amperage on a greater number of turns, and removes such magnetic particles as were not attracted by the first magnet, making a middling product; the nonmagnetic particles pass off the end of the belt. This machine is made in two sizes, with 12-in. and 21-in. belts respectively; a description of the 21-in. belt machine will serve for both.

The belt of seamless rubber on a heavy canvas base is carried on two 18-in. pulleys and driven from a line shaft through the pulley at the feed end; provision for taking up stretch in the belt is made by capstan bolts working against the sliding bearing of the pulley at the discharge end. The belt is kept level beneath the magnets by three liner pulleys which are capable of adjustment to permit the regulation of the distance between the magnets and the surface of the ore stream.

The magnets are cylinders 26.5 ins. in diameter, the rougher of cast iron and the cleaner of cast steel and are set to overhang the belt at one side. An annular space ½ in. in width is turned out of the bottom of the magnets 1 3/8 ins. from the periphery and is filled with spelter; the magnetic circuit is from the outside shell across the spelter gap to the inner core of the magnet about which the coils are wound. The magnetic particles are attracted and form a bridge across the spelter ring, and, by the revolution of the magnets, are carried to one side where they are scraped off by a brass scraper.

The normal speed of the conveyor belt when treating artificial magnetite is 100 feet per minute, and the speed of the magnets is 40 R.P.M. At this speed the operator is capable of treating 1 ton per hour of properly roasted blende-pyrite concentrate of average grade and crushed to pass 4 mesh. The capacity of the 12-in. machine is about one half that amount. The amperage employed varies with the ore and the quality of the roast from ½ to 2 amperes on the rougher magnet and from 3.5 to 10 amperes on the cleaner magnet.

Stern-Type Wet Separator

This separator is built to separate wet concentrates and finely divided material. It is said not to require a preliminary classification of the feed, and to work well on very finely divided ore. This machine consists of a number of electro-magnets mounted on a spider which revolves in a tank partly filled with water. The ends of the revolving magnets are connected by the shaft with the walls of the tank, which form the opposite poles; the separation is accomplished in this space, between the ends of the moving magnets and the cylindrical wall of the tank. The ore is fed into the machine at one side, the moving magnets pick up the magnetic particles and carry them above the water level, where they are washed off into a launder by a strong jet of water: the non- magnetic particles are drawn off through the bottom of the tank. The movement of the magnets through the water stirs up the ore thoroughly and permits a thorough separation. The machine operates on a 0.5 H.P. and requires 10 amperes for excitation of the magnets.

Primosigh Wet Separator

In general principle this machine resembles the Primosigh separator for dry ores described in the following chapter. The material to be separated, in a fine state of division, is fed in suspension in a stream of water into the grooves at the top of the magnet

cylinder, which is suspended above a spitzkasten so as to be immersed in water during a part of its revolution. The nonmagnetic particles drop away from the pole pieces as soon as they reach the water, while the magnetic particles are carried above the surface of the water and removed by a series of secondarily induced magnet points as in the dry separator. This machine is adapted to the treatment of fine material. Upon a feed ranging from 0.25 mm. down to dust the capacity for a machine with four separating grooves is 0.4 metric ton per hour. Twelve amperes at 80 volts are required for excitation, and ¼ H.P. for revolution.

Leuschner Table Separator

This separator is designed to treat slime. It consists of a round table with flat surface which rotates above a series of fixed electro-magnets. The magnetic particles are held against the surface of the table by the magnets beneath, while the nonmagnetic

slime is washed off by jets of water. Beneath one sector of the table there is no magnet and here the magnetic particles are washed off into a separate launder.

Humboldt Single-Roller Separator for Wet Separation

This is similar to the above-described separator for the treatment of dry ores. The drum revolves partly in water, and the material to be separated is fed against it, near the lower vertical diameter. The nonmagnetic particles sink to the bottom while the magnetic particles are carried farther by the revolution of the drum and washed off by a stream of water. A stream of wash water is directed against the magnetic particles while held against the drum, to remove nonmagnetic dust and entrained particles. The drum is protected by a water-tight mantle of sheet copper. The capacity of these machines varies, with the kind of ore and the size treated, from 500 to 4000 pounds per hour.

Herbele Wet Separator

In this separator a series of electro-magnets is enclosed in a water-tight casing. An endless belt travels around pulleys at top and bottom of the case containing the magnets, the belt mov-

ing downward close to the casting on the side of the ore feed. This apparatus is set vertically in a tank filled with water to a point above the top of the magnets. The ore, best below 30 mesh, as the machine is intended to treat fine material, is fed at the top of the belt in a stream of water; the nonmagnetic particles fall and are carried straight down by the flow of water, while the magnetic particles, held against the belt by the magnets, are carried around the lower pulley and dropped into a separate hopper. The construction is best understood from the above figure where A is the point at which the ore is fed, in suspension in water; B, the belt which conveys the magnetic particles past the magnets; c-c’, the pulleys about which the belt runs; E, the concentrate hopper; F, the concentrate discharge; G, the tailing hopper; H, the tailing discharge. The actual separation of the non-magnetic particles from the magnetic takes place at the end of the shield shown close to and opposite the lowest magnet. The feed and discharge of both concentrate and tailing are continuous. The belt is 2 ft. 6 ins. wide. The capacity of the machine reaches 35 tons per 24 hours.

Odling Separator

In this machine a conveyor belt serves to carry the ore beneath an electro-magnet whose poles extend across, and just above, the conveyor belt. A cross belt running beneath the poles carries the magnetic particles attracted against it to one side, where they are discharged into a chute. The nonmagnetic particles are discharged off the end of the conveyor belt.

Humboldt Single-Roller Separator: For Dry Separation

This machine consists of a drum whose face is made up of alternately magnetic and nonmagnetic bars, revolving about fixed internal electro-magnets. The primary magnets are placed to cover a part of the lower diameter of the drum; the secondary magnets, carried on the face of the drum, become magnetized by induction while passing the primaries, and pick up the magnetic particles from the stream of ore which is fed beneath the drum. The magnetic particles drop off the drum as the secondary mag- nets become demagnetized on passing out of the field of the primaries. The whole machine is covered with a dust-tight hood. The capacity varies, with the kind of ore and the size to which it is reduced, from 700 to 3000 pounds per hour.

Ferraris Cross-Belt Separator

This machine comprises a series of six inverted horseshoe magnets placed in line, with a single take-off belt running immediately beneath the poles of all the magnets, and six feed belts running

below the take-off belt and at right angles to it, each feed belt supplying a magnet. The poles of the horseshoe magnets are bent in toward each other, giving a concentrated field at right angles to the feed belts. The magnets are fitted with an iron projection extending a few inches beyond the ends of the poles in the direction of travel of the take-off belt, permitting the magnetic particles to be carried to one side and dropped past the feed belts into separate hoppers. Each magnet is fed with a different size of ore

except two magnets, which both treat ore passing through a 1 mm. screen, as this size preponderates. Mounted on the separator frame are shaking screens, which deliver sized products into separate hoppers, which in turn deliver on to the feed belts. The feed belts are 12 ins. wide and travel 1.5 ft. per second. The height between these belts and the magnets is capable of adjustment through the small guide rollers shown just below the magnets. The distance through which the magnetic particles are lifted varies from 30 to 40 mm. Each magnet requires 2 amperes at 50 volts. The capacity of the apparatus is slightly over 1 metric ton per hour.

Ferraris Drum Separator

This machine comprises a shaking conveyor which feeds the material to be separated from a hopper upon a conveyor belt, which in turn presents it to a magnetic drum, fitted with a belt serving to remove the particles attracted. The magnetic drum is

composed of a series of composite pole pieces which dovetail into one another in a manner best understood from an inspection of the accompanying illustrations. The poles are insulated by a filling of zinc, the whole forming a smooth surface. The exciting coils are placed within the drum, connections with the dynamo being made through disks which dip into cups containing mercury. This machine, fitted with a belt 16 ins. wide, treats about 500 kgm. of calcined limonite-calamine ore per hour. The magnets require 1.5 amperes at 110 volts for excitation.

Vial Separator

The deviation of magnetic particles from a falling sheet of finely divided ore is the principle upon which this separator operates. The attraction is exerted by six horseshoe magnets separated by bronze rings. These magnets, which are arranged horizontally, are enclosed in a brass cylinder which revolves at from 8 to 10 R.P.M. The ore is fed in a thin sheet at a distance of from 5 to 25 mm. from the brass cylinder. The magnetic particles are drawn toward the magnets but are prevented from adhering to them by the brass cylinder; the magnetic and nonmagnetic products are divided by an adjustable diaphragm and fall into separate hoppers. The capacity of the machine is 500 kgm. per hour. The entire apparatus is enclosed in a sheet-iron housing to prevent air currents, which would interfere with the separation. The machine treats material passing a screen with 1.5 mm. holes

Heberle Dry Separator

This machine consists of a brass drum revolving about a series of fixed electro-magnets. The ore is fed against the drum at a horizontal diameter. The nonmagnetic particles fall past

the drum into a hopper, while the magnetic particles are held against the surface of the drum by the magnets and are carried, by its revolution over the top of the drum to fall into a separate hopper. The drum makes 36 R.P.M. 6 to 7 amperes at 65 volts are required for excitation, and 1/8 H.P. for revolution.

Humboldt Ring Separator

This consists of an annular magnet suspended in a horizontal plane within a circular casing. The ore is guided to the magnet by a conical shield, and, passing between the magnet and the casing, the magnetic particles are drawn inward, while the nonmagnetic particles fall past the magnet unaffected. The two products are gathered in two concentric inverted cones, the inner receiving the magnetic portion and delivering it from the separator by means of a spout through the lower, or outer, cone. The separator contains no moving parts. In lieu of an air gap between poles the separation is effected in a zone of dispersion caused by

a narrowing of the enclosing casing, which induces a magnetic resistance. The operation of the separator is best understood by inspection of the figure given above. The magnetic ring has a diameter of 40 cm., or a separating periphery of about 1.25 meters. The separator is said to have a capacity of 1 metric ton per hour.

Knowles Magnetic Separator

This separator consists of a stationary primary magnet between the poles of which a belt, which is studded with small secondary magnets, is caused to travel. The construction is made clear in the accompanying illustration. The ore is fed from a

hopper upon a reciprocating feed plate, which in turn delivers it upon a reciprocating conveyor plate; this conveyor plate brings the ore close to the belt carrying the secondary magnets; the plate and belt gradually approach each other, causing the ore particles to move in a magnetic field of constantly increasing strength. The magnetic particles are picked up by the secondary magnets and held until carried past the primary magnet, when they are gradually dropped off in inverse order to their magnetic permeabilities by the gradually decreasing strength of the secondary magnets. The nonmagnetic material falls from the end of the conveyor plate into a separate hopper. The upper pole of the primary magnet is beveled, coming to an edge at its lower end, thus giving a concentrated field at this point: the lower pole is rounded, and being movable, an adjustment of the concentration of the magnetic field is obtainable. The secondary magnets are soft-steel rivets, with serrated washers on the lower side of the belt, there are about 200 of these rivets per square foot of belt,

copper plated to prevent rusting. The speed of belt travel is 250 ft. per minute. The machine is designed for sizes from 6 to 36-in. belt width, having capacities from 7 to 46 tons per 24 hours.