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electrostatic gold separator

electrostatic separation | electrostatic separator | mining equipment

electrostatic separation | electrostatic separator | mining equipment

JXSC Electrostatic Separation equipment is used for recycling of various minerals, waste metals and non-metal materials with conductivity difference such as selected white tungsten, tinstone, zirconite, andalusite, rutile, and gold placer the High tension Separator is to electrify wire electrodes with very small diameter with high-voltage direct current to produce corona electric field in the space between corona electrode and grounding electrode. The entire space close to the grounding electrode carries a negative charge (positive grounding) at the time and the mineral aggregates have the same chance to obtain electrics while passing through the corona electric field. However, due to different conductivity of minerals, they receive different electricity in the electric field and carry away different electric charges so that ore sands fall along different tracks in the electric field. In this way, good separation conditions are created for minerals with good, medium, and no conductivity. Types of electrostatic separation machines are roller electrostatic separator and ARC type electrostatic separator.

The ARC Type Electrostatic Separator machine has the structure of two rows and four stories in each row. Each floor has an earthing curved slide board (stainless steel). On the top, it has an arc-shaped high-voltage electrostatic plate (aluminum), which is stationary (but adjustable). When connected to high voltage static electricity, the ore minerals slip into high-voltage field area via earthing arc plate, then conductive minerals charged by induction and attracted to the electrode. Due to gravity, it is discharged from the front which is different from the non-conductive ore. Meanwhile, the other ion-conductive mineral has also been affected by the electric field, but will not be attracted into the lower re-sorting. The process will be conducted four times until the minerals are qualified.

The Roller Electrostatic Separator machine is used for recycling of various minerals, waste metals and non-metal materials with conductivity difference such as selected white tungsten, tinstone, zirconite, andalusite, rutile, and gold placer, the High tension Separator is to electrify wire electrodes with very small diameter with high-voltage direct current to produce corona electric field in the space between corona electrode and grounding electrode. The entire space close to the grounding electrode carries a negative charge (positive grounding) at the time and the mineral aggregates have the same chance to obtain electrics while passing through the corona electric field. However, due to different conductivity of minerals, they receive different electricity in the electric field and carry away different electric charges so that ore sands fall along different tracks in the electric field. In this way, good separation conditions are created for minerals with good, medium, and no conductivity.

Mining Equipment Manufacturers, Our Main Products: Gold Trommel, Gold Wash Plant, Dense Media Separation System, CIP, CIL, Ball Mill, Trommel Scrubber, Shaker Table, Jig Concentrator, Spiral Separator, Slurry Pump, Trommel Screen.

electrostatic separator - china henan suyuan lanning

electrostatic separator - china henan suyuan lanning

Electrostatic separation,use the differences of electrical of different material under the influence of high voltage electric field to achieve the purpose of separating.When the material particles enter into corona areas with the rotation roller, material under the electricity,centrifugal force and gravity function,add the different electrical properties of different material,different stress state make material of the falling trajectory is different,then separate out metal and nonmetallic mixtures.

Other Sorting: Metal ore sorting,fly ash decarbonization sorting,slag ash metal extraction, plastic and sand sorting,tea leaf and tea rod sorting,flaxseed kernel and shell separation, hair and other material separation.

china gold ore mining separator, gold ore mining separator manufacturers, suppliers, price

china gold ore mining separator, gold ore mining separator manufacturers, suppliers, price

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electrostatic separator

electrostatic separator

Electrostatic separation depends on a number of electrical and mechanical properties such as conductance, contact potential, dielectric constant, end particle shape. In routine testing it is desirable that the application of each of these properties be investigated. The conductance property is best utilized in a separator having an ionic field, whereas the contact potential requires a separator with a pure static field. Dielectric constant and particle shape also influence the response in these two fields. There is no separator on the market that can be adjusted for all these requirements. The separator described in this paper is satisfactory for this purpose and is provided with a dielectric electrode, which permits formation of high-intensity static fields.

Separation of minerals utilizing differences in their electrical properties may be carried out using an electrostatic separator. It will separate electrical conductors (minerals such as cassiterite and rutile) from those that are electrical non-conductors (minerals such as zircon and siderite). The principle of separation is that as the particles to be treated are passed through an intense electric field each particle acquires a charge. The conductors pass on their charge when emerging from the field while the non-conductors retain theirs momentarily.Separation is effected by passing the particles on to a rotating metal cylinder (the roll); without any external forces all the particles will follow a ballistics trajectory when leaving the roll surface.

In the presence of an electric field, the rotor becomes negatively charged, while the particles are positively charged. The non-conducting particles retain these charges and are attracted (pinned) to the roll while the conducting particles lose their charges (to the roll) and follow the normal projectory as shown in figure 9b. Separation is affected by (adjustable) splitter plates.

In practice electrostatic separators will always be tested in several stages, as one pass separations producing clean products are rare. Inherent physical properties of mineral particles (specific gravity, grain size and composites, surface coating) and their interrelationships all affect the ease of separation. In addition the number of operating variables (temperature, voltage, rotor speed and electrode configuration) make a number of trials necessary to determine optimum conditions for separation.

The essential parts of the separator are the carrier roll A, functioning as grounded electrode, the dielectric electrode, B, and the corona electrode, F shown in figure 1. A field of ions is generated by corona electrode F. This field of ions charges both the particles on the carrier roll A and the surface of the dielectric electrode B. As B rotates continuously, the region between the carrier roll and the dielectric electrode is an electric field that is as near static conditions and free from ions as it is possible to obtain. The region between the carrier roll and the corona electrode is an ionic field that may be changed from high intensity to zero intensity by rotating F about B to a position farthest from A.

The dielectric electrode has the advantage over the metallic or conductive type of electrode in that there are no spark discharges that interrupt the separation. It permits the establishment of high electric-field intensities and does not cause back deflection of conductive particles that may strike it.

Electrode B is a 3-inch-diameter pyrex-glass tube supported by bakelite end disks, V. By coating the midportion of the inside of the tube with an electrically grounded layer of Acheson aquadag graphite, the glass functions as the dielectric of a condenser. An alternative form of the dielectric electrode consists of a 3-inch -diameter methyl methacrylate plastic tube (0.13 inch wall thickness) placed as a close fit over a precision metal tube.

Roll A is a grounded, 6-inch diameter, precision metal tube supported by steel end disks. Shaft support K, detailed in figure 2, has a porous bronze bushing, a, leveling screws, d, and machined faces, b, for parts L and S. Hole f in part L is a sliding fit for the shank of part 0, a shaft support of special design (Boston Gear Works SAP 102).

A cap screw in clearance hole e fastens L rigid to K except for adjustment. This adjustment permits a variation in the angle between the horizontal and the line through the electrode and she.ft centers. The electrode-roll gap distance is adjusted by sliding the shank of part 0 in hole f of part L. Fixed positions are maintained with a set screw.

Ring P supports the corona emitting electrode F and its insulators Y. Adjustments with respect to the corona charging of the particles are made by rotating ring P on a steel cylinder, which is drive-fitted on the bearing housing of shaft support 0. Positions of ring P are fixed by a set screw. Either a 0.003- to 0.01-inch-diameter wire stretched parallel to the shaft center line or a row of needle points nay be used for the corona electrode. Corona wires generate fewer convective air currents then corona-needles.

A 40 kilovolt D. C. supply is satisfactory for most purposes. At higher voltages, up to 100 kilovolts, the electric supply feeder is brought in from above instead of from below, as shown in the illustration. A 60-watt, 10-megohm, 100-kilovolt resistor is connected in series with the corona electrode.

The carrier rolland dielectric electrode are driven by means of 3/8-inch V belts and cast iron variable-pitch sheaves (Browning Manufacturing Co. IVP 30). The special form of these sheaves is essential for belt tightening. J consists of two sheaves fastened on a bronze sleeve, which idles on the shaft of roll A. One sheave receives the cross belt from the main drive shaft. The other receives the belt from N, which then rotates in a direction opposite to that of roll A, and at twice the speed. An ideal speed for A in frictional electrical and conductance separations with convective charging is 150 r,p.m.

Each tray, H, hopper, W, and vibrating spout, E, is heated with 250-watt heaters such as T. A and B have, respectively, 250- and 15-watt internal heaters connected through slip rings, E. When B is constructed of methyl methacrylate plastic, a heater for this electrode is not needed. This method of heating is in contrast to continuous commercial operation where the hot feed heats the separator.

Roll A is cleaned by the wiper underneath it. However, strongly adhering corona-charged particles frequently continue past the wiper. Operation was improved by placing either a metal nonionizing electrode of polarity opposite the dielectric electrode or an ionizing electrode excited by alternating current adjacent to the wiper, so that the particles on the roll passed through the field or ions of these electrodes loosen before reaching the wiper.

part K, the wires are stretched across the roll face parallel to the roll-shaft center line so that they just touch the roll surface and are also at earth potential. The wires remain stationary as the roll rotates, and the effect is to agitate the adhering corona-charged particles. The wires are useful in improving the conductance separation of some mixtures such as rutile-zircon, but for most other conductance separations the improvement is negligible. They are not used in frictional electrical separations. Their function is to diminish entrainment of particles that would not normally adhere to the carrier electrode.

Efficient separations of the good and poor conductors in crushed ores, sands, and industrial products are obtained by adapting the principle of convective charging. In this method the corona electrode, F, is rotated toward the carrier roll, so that the particles on the carrier roll receive a corona charge. The exact position of the corona electrode is determined by the separation that results at the dividing edge, G. In addition to the corona electrode, the position of the dividing edge G, the angular setting of L, and the eloctrode-roll gap distance are varied. Combinations of poor and good conductors can be separated, which will not respond to the pure static field type of separator. An example of this is ilmenito from staurolite in certain Gulf-coast heavy sands.

Crushed ores can be carried through a special procedure of stage crushing and separation, so that the conductors are separated as they are unlocked by crushing without an unnecessary reduction in the size of the conductors. A study and application of this method will be described in a separate paper. Efficient separations are obtained with hematite, ilmenite, and chromite ores.

With convective charging, the separator will collect the blocky materials of intermaediate conductivity with the conductors, and the flat particles and slivers with the nonconductors, which is just opposite to the action of the pure- static field typo of separator.

When the corona electrode is rotated far from the carrier roll so that it is at its greatest distance from the roll, very few ions reach the particles on the roll, and the separator functions as a nonionic or pure static field type. Further decrease in stray ions is obtained by replacing tho corona wire with needles oriented to point directly at the dielectric electrode. With this adjustment, separations based on frictional electrical charging of particles may be obtained. Examples are the separation of sillimanite from quartz- and talc from tremolite.

With some minerals, sufficient frictional electrical charging results from simple pouring. Others require passage over a vibrating surface, such as that of feeder E in figure 1. The surface of feeder E is a horizontal plate whose vibration displacements are in a general vertical direction. A further description may be found in reference.

An alternative form of frictional electrifier, in which more intense vibrations may be obtained, was constructed of vertical plates. These plates, 1.6 inches high and 0.9 inch wide, are separated with spacing strips 1.6 inches high and 0.2 inch wide, placed along the long edges of the plates. With the length direction of the plates in a vertical position and the thickness of the plates and strips, 0.05 inch, the whole assembly of plates and spacing strips results in a series of cavities 0.5 inch long, 0.05 inch wide, and 1.6 inches high, with open tops raid bottoms. The plates and spacing strips are damped together by menus of two horizontal tension bars placed on each side of the assembly.

When a mineral mixture is poured through the cavities and the assembly is vibrated horizontally, which is perpendicular to the surface of the vertical plates, frictional electrification results. This electrifier, a substitute for E, is placed so that the direction of vibration and the perpendicular to the plate surfaces are parallel to the dielectric electrode shaft center line.

Biotite is separated from talc with an efficiency that is higher than with the conductance method using convective charging. With convective charging only the blocky particles of biotite are removed. This is in contrast to the pure static field separator, which removes both the blocky and thin flakes. In addition to flat particles, particles with a low conductivity and a high dielectric constant would be expected to separate more efficiently as apparent conductors in the simple static field type of separator.

Beach sands and some crushed ores receive primary wet concentration and size classification. Since the feed is thus initially wet and must be dried for electrostatic concentration, conditioning with several pounds of sulfuric acid hydrofluoric acids per ton of solid can be accomplished before drying, without much additional cost. Mineral mixtures may be conditioned in the dry state also. Dry conditioning with hydrofluoric acid vapor improves the separation of talc from tremolite. Conditioning with the same reagent in the aqueous and dry states does not always yield the same result. For example, beryl may be separated from quartz and feldspar after aqueous conditioning with hydrofluoric acid but not after dry conditioning with this reagent.

An initially wet feed is also amenable to cleaning. Attrition scrubbing is effective on wet beach sands. In one example a zircon-rutile sand that was heavily iron-stained resisted cleaning by both attrition scrubbing and chemical action with aqueous sulfur dioxide. However, by adding a small amount of metallic iron powder to the aqueous sulfur dioxide pulp, the iron-stain coating dissolved in 50 to 60 minutes. Metallic iron is a substitute for metallic zinc, cautionly used in SO bleaching of iron-stained minerals. Cleaning of particles or coating selectively with reagents promises to make electrostatic separation as versatile as flotation.

The objective of this research was separation of one highly conductive mineral from a second conductive mineral, a procedure which is impractical in commercial electrostatic separators using metal rolls. Such separators permit separation of nonconductive from conductive minerals. A new technique involved use of carrier electrode rolls specially prepared by anodizing and/or oxidizing at high temperatures to form suitable oxide surfaces. Specific oxides included aluminum, copper, and nickel. The significant result provided by the new surfaces was the extension of the separation range to higher conductivities, an extension in relative mineral conductivity from 10 -14 to 10 -11 mhos. The results are in accordance with the blocking-layer theory.

Some ores are best concentrated by a series of stages which may involve gravity, electrostatic, and magnetic methods. Occasionally a fraction of minerals remains unseparated. It is this unseparated fraction of minerals which is of special interest. A common application of electrostatic separations divides the minerals into nonconductor and conductor. For example, conductive minerals such as ilmenite, pyrite, columbite, magnetite, cassiterite, and hematite are separated from nonconductive minerals such as quartz, zircon, monazite, and rhodolite. A separator consisting of a rotating metal cylinder and an adjacent ionizing electrode is used for these separations. The split between conductive and nonconductive minerals is regulated by adjusting the intensity of ionization. More conductive minerals are deflected into the non-conductor bin as the potential of the ionizing electrode is increased. However, a limit is finally reached where the selectivity between various conductors diminishes. This report is based on the separation of these highly conductive minerals from each other by means of a special type of carrier electrode.

Separations based on the conductivity of particles may be obtained with ionized and static electric fields. The conductivity of the particle determines the amount of charge which passes to the carrier electrode. This flow of charge from the particle to the carrier electrode is shown in figure 1. In 1A the particle receives positive ion charges from the ionizing electrode, I. The particle in positions 1 and 2 is uncharged. In position 3 a positive ion charge is received, and in position 4, part of this charge is conducted to the carrier electrode, R. In 1B both positive and negative charges are induced on the opposite sides of the particle as a result of the static field generated by electrode E. In position 4, part of the positive induced charge is conducted to the carrier electrode, R. The net particle charge in position 5 is of opposite polarity for the ionizing and static field systems, but

In the arrangement in figure 1A the dividing edge, D, is placed closer to R than in figure 1B. This is determined by the trajectory of the separated particles. The main stream tends to be attracted to R with an ionizing electrode, and to E with a static-field electrode, although in both cases there is a separation based on differences in conductivity.

The amount of mineral in fraction C versus intensity of ionization is illustrated in figure 2, where the ionization current to a fine wire electrode is plotted on a logarithmic scale, and the amount collected in fraction C is plotted on a probability scale. Other uses of the probability scale are described in a previous publication. Two sets of data are presented in

figure 2, one with a positive and the other with a negative ionizing electrode. These data show that changing the polarity of the ionizing electrode when treating minerals on metallic-carrier electrodes makes little difference.

Magnetite, galena, and the variety of conductive minerals described here are distinguished as semiconductors which may either be n or p type. The type indicates the polarity of the electric charge carriers, n for negative, and p for positive. When semiconductors are in contact with a metal surface, the conductivity across the contact interface is not the same in either direction. An established rule is that the charge carriers encounter maximum conductivity when flowing from the semiconductor to the metal. Thus with an n-type semiconductor having negative charge carriers, the conductivity would be greatest when the negative current flows from the semiconductor to the metal. With a p-type semiconductor, having positive charge carriers, the conductivity would be greatest when the equivalent of a negative current flows from the metal to the semiconductor.

An experimental verification of this fact was demonstrated by Hartmann with high resistance films of shellac about 10 -4 centimeter thick on the metal surface. For contact phases he used cuprous oxide and zinc oxide as semiconductors and copper for the metal. After addition of the shellac films, a directional conductivity or rectifying action was present at the semiconductor- metal contact. The shellac film has the effect of a barrier or blocking layer.

Barrier films are particularly applicable to electrostatic separations based on differences in conductivity. The roll-type separator, described elsewhere as most frequently used for conductivity separations, has a metal carrier electrode in the form of a rotating cylinder or roll. Soft barrier films such as those of shellac would not be practical for this application, but one of the hardest oxide films, aluminum oxide, is easily produced in film form by anodizing the aluminum metal.

To compare surfaces, only half the length of the carrier electrode was provided with the film surface, the metal surface of the remaining half length was not altered. The film of aluminum oxide on aluminum was formed by anodizing half the length of a cylinder, 5 inches long, 4 inches in outside diameter, and inch in wall thickness in 10 percent sulfuric acid for one hour; washing in dilute ammonia; and drying. The electrolysis was at a constant current ranging from 0.13 to 0.15 amperes per square inch of outer electrolyzed surface. Although the electrolytic vessel was immersed in a cooling water bath, the aluminum cylinder was approximately 50 C when removed from the electrolyte. The anodized film has a smooth satin finish.

Anodization of aluminum may be accomplished with a variety of electrolytes ranging from alkaline to acid. Dilute sulfuric acid electrolytes produce harder films than the more concentrated acid electrolytes.

Oxide films may also be formed in an oxygen atmosphere at elevated temperatures . The green form of nickel oxide, NiO, is a nonconductor and can be substituted for aluminum oxide as an insulating film. The film was formed on a nickel cylinder 4 inches in outside diameter and with a wall 0.109 inches thick by heating in air in a temperature range increasing from 900 to 1000 C in 30 minutes. After cooling, half the length of oxide covering the cylinder was removed by abrading with emery cloth. The nickel oxide film is firmly adherent, unlike most other oxide films, such as iron oxide. Some of the other metal oxides are semiconductors. Use of a nonconductive base will be described as an alternative to forming semiconductors on metal.

In contrast to the metal-semiconductor contacts previously described, a carrier electrode with a semiconductor surface provides combinations of p- and n-type semiconductor contacts. The p-n junctions have rectification proper- ties similar to metal-semiconductor junctions with barrier layers.

The semiconductor may be used in the massive state or as a layer on a sub-base. A semiconductor surface was prepared for these tests on a refractory aluminum oxide cylinder that was 5 inches long, 3- inches in outside diameter, and that had a wall 3/8 inch thick. It was of a variety sold for electric furnace construction. The surface of the cylinder was coated by painting with a water suspension of cuprous oxide, was dried, and then was heated in air to 1100 C for a short time. The coating was applied on the outside surface of the cylinder and around the ends to provide a contact with the ground potential shaft. Copper oxide reacts energetically with aluminum oxide and the time and temperature for heating must be carefully controlled. After cooling, a hard adherent surface of black cupric oxide is obtained. With insufficient heating the copper oxide is powdery, and with too long heating it disintegrates the aluminum oxide surface. The cupric oxide coating is much thicker than the nickel and aluminum oxide films on metal. Other semi- conductors may be adapted in the same way. Some of these may form a compound with a metal base.

Two alternate methods were used for comparing copper oxide with metal, either covering half the copper oxide cylinder with aluminum foil or substituting a fired on coating of ceramic silver paint in place of the oxide on half the cylinder length.

The various cylinders were mounted on a shaft with a special device constructed from parts of cast iron V-belt sheaves of adjustable pitch. Each sheave consists of two parts, one is a threaded shank that mounts on the shaft, and the other fits on a threaded shank. The conical belt surface provides a resting point for the carrier electrode cylinder. To provide a wide gap to accommodate the length of the cylinder, the shaft part of a second sheave is used as in figure 3. Although the second sheave had a smaller outside diameter, the threaded shanks of the two sheaves were the same size.

ionizing wire electrode, I, the vibrating feeder, V, the dividing edge, D, and the shield, B. Elements J and A were not used in the initial tests. They are described in the section on elimination of carrier electrode wear. In all the tests described here the cylinder rotated at constant speed of 100 rpm. The vibrating feeder consisted of two troughs, one for each half of the carrier electrode surface. The ionizing electrode, a wire 0.010 inch in diameter, was maintained at a constant angle and distance from the cylinder, but the current to the ionizing electrode was varied. The current values are for the total length of wire, which was 6 inches.

Results for several minerals are given in figure 2. There is very little change with polarity change for either magnetite or galena on aluminum metal surfaces. This is not true for anodized surfaces, where the curves for each polarity are appreciably different. The greater conductivity for magnetite with a negative wire, and the greater conductivity for galena with a positive wire, suggests that the two are n- and p-type semiconductors respectively.

Similar results are given in figure 5 with mineral particles of zincite and cassiterite on nickel metal and nickel oxide surfaces; and in figure 6 with mineral particles of columbite and ilmenite on aluminum metal and anodized aluminum surfaces. Minerals in all the tests of this report were sized to the range of minus 35- to plus 65-mesh.

Returning again to figure 2 we note the quantity of conductor fraction at various ionizing electrode currents. If these data are applied to actual separations, the ratio of galena to magnetite in figure 2 should be equivalent to the composition of the deflectate (conductor fraction), provided the feed is composed of equal parts of galena and magnetite. Several conductor fraction values taken from figure 2 are presented in table 1 and calculated to ratio values. The ratio of galena to magnetite is much higher with the aluminum oxide surface.

Two tests were run to verify these data, one with an aluminum metal carrier electrode and 200 microamperes current and the other with an anodized aluminum carrier electrode and 20 microamperes current. The feed in both tests was a synthetic mixture of galena and magnetite in equal proportion. With the metal carrier electrode, one pass was sufficient to demonstrate a poor separation. The conductor or deflectate fraction C in table 2 contained only 60 percent galena.

The separation of galena from magnetite was much better with an anodized aluminum carrier electrode and 20 microamperes current. In table 3 the conductor fraction C contained 99 percent galena after one pass with a recovery of 56 percent, which is 10 percent higher than the recovery reported in table 2. The nonconductor portion recovered from the first pass (table 3) was repassed to illustrate that the high selectivity is retained in repasses, and the combined conductor product represented a recovery of 73 percent of the total galena in a 95 percent galena product.

Similarly, ratio values calculated from the data for ilmenite and columbite (figure 6) are listed in table 4. In this case comparative separation tests were run on an aluminum metal roll at 150 microamperes current and on an anodized aluminum roll at 10 microamperes. The results are summarized in tables 5 and 6. In both tests the nonconductor fraction was repassed twice. It is interesting to note that the results from using a metal roll in table 5 are in accordance with the microampere data of table 4, the concentrate representing an enrichment of ilmenite instead of columbite. The better separation is shown in table 6, in which 73 percent of the total columbite was recovered in a combined 81 percent conductor product, whereas in table 5 the same recovery is indicated in a nonconductor concentrate containing only 65 percent columbite.

The minerals previously described belong to the high conductivity range. For minerals of lower conductivity the increased separation efficiency is not obtained; for some combinations, such as nonconductors and intermediate conductors, the metal carrier electrode is more efficient. These facts are illustrated in figure 7, where the conductor fraction, or the amount of individual samples of pure mineral particles collected on the ionizing electrode side of the dividing edge, is plotted versus the logarithm of the relative resistivity of the particle. Both are on linear scales. The relative resistivity values are for single particles measured at 50 C, determined by the method described in a previous report.

roll, which had an adjacent single wire ionizing electrode, the metal roll was used with a combination of ionizing wire and static- field electrodes. The combination electrode is a typical adaptation of metal- roll, electrostatic separators now in commercial use.

The highest conductivity mineral in figure 7 is galena. The amount collected as a conductor fraction is 99 percent with the metal roll and 72 percent with the copper oxide roll. Although both curves have an abrupt dip in the quantity collected as a conductor fraction, the dip occurs at different mineral resistivities for the two roll surfaces. The dip for the metal roll is in the range from 10 13 to 10 14 ohms, while for the copper oxide roll it is in the range from 10 10 to 10 11 ohms. Minerals with a greater or lesser resistivity than this range may be separated from each other with high efficiency. For example columbite and samarskite with resistivities of 10 10 and 10 11 ohms respectively cannot be separated with the metal roll, but are efficiently separated with the copper oxide roll. Similarly samarskite and hornblende with resistivities of 10 11 and approximately 10 14 ohms, respectively, can be separated with the metal roll but cannot be separated with the copper oxide roll. Galena is one of the highest conductivity minerals. Included with galena are the sulfides of copper and iron and the arsenides of nickel and cobalt, such as niccolite, rammelsbergite, and smaltite.

The effect of a combination of ionizing and static-field electrodes was investigated with the arrangement illustrated in figure 8. R is the carrier electrode or roll; V, a feed device; D, a dividing edge; and I and E are electrodes. I is the single wire ionizing electrode used previously, but in this case it is partly surrounded by the metal shield, S, at ground potential. E is a staticfield electrode in the form of a metal cylinder 2 inches in diameter. The function of the metal shield, S, is to isolate electrodes I and E so that neither has an influence on the other. Without the shield, S, a change in the potential of electrode E would cause a change in the ionized discharge from electrode I.

Table 7 summarizes the results of treating individual samples of pure, unmixed minerals with anodized aluminum and copper oxide carrier electrodes, as well as double field electrodes. When the potential of electrode E is zero the conditions are equivalent to the previous single wire ionizing electrode tests. Tests 1 and 2 (galena on anodized aluminum) and tests 5 and 6 (rutile on copper oxide) were conducted under these conditions. The results were predictable. Rutile has a lower conductivity than galena, but with the higher conductivity copper oxide roll, the amount of conductor fraction is approximately the same. However, a significant change occurs when a potential is applied to electrode E. In tests 3 and 4 (anodized carrier electrode) the amount of conductor fraction decreases, and in tests 7 and 8 (copper oxide carrier electrode) the amount of conductor fraction increases.

The difference is that in tests 3 and 4 with the barrier-layer electrode, the potential between particle and electrode drops to a plateau value. This value is a minimum potential below which no current can flow across the barrier layer. The particle thus carries a residual charge; and when the static field is added, there is an increased adhesion of the particle to the carrier electrode.

With the copper oxide carrier electrode in tests 7 and 8 there was no barrier layer, and the potential between particle and electrode gradually dropped to zero. The particle acquired an opposite induced charge from the added static field, and thus had an increased deflection toward the staticfield electrode.

The separation response of particles is dependent on particle conductivity, roll- surface conductivity, ionizing-electrode current, and static-field intensity to such an extent that the relative separation efficiency of single and double electrodes cannot be predicted. Table 8 summarizes the results of treating individual samples of pure minerals with a single wire electrode at two ionizing currents and with a double electrode having two adjustments. Figure 9 shows the apparatus for the double electrode. The ionizing wire electrode in the double electrode arrangement is not shielded and it is therefore not possible to independently vary the ionizing current and the staticfield intensity. However, the potential is maintained approximately constant, and with the ionizing and staticfield electrodes connected to the same potential source, the ionizing current, although not measured, varies with respect to the adjustment of angle i.

Data of table 8 illustrate that one electrode system cannot be stated as being better than the other. For example, with individual samples of pure wolframite and acmite the calculated ratio of conductor fractions is greater with the single wire electrode than with the double electrode. However, with wolframite and hubnerite the reverse is true.

The greatest disadvantage in using film surfaces is wear. With semiconductor surfaces the layers may be deep enough that wear is negligible. It is also possible to select exceptionally hard materials for film surfaces. Most of the wear is caused by the abrading action of brushing adhering mineral particles off the carrier electrode. A soft bristle brush was used in the tests to minimize wear. If the particles are fed to the carrier electrode with a vibrating feeder arranged so that the drop distance to the electrode surface is small, electrode wear other than that caused by a brush should be negligible.

Adhering particles can be removed to some extent by a second or secondary ionizing electrode excited with either alternating current or reversed-polarity direct current. All particles are usually not removed by this method, but those that remain adhere only weakly and may be removed by a gentle mechanical force. Figure 4 illustrates an arrangement of secondary electrode, J, and air jet, A. Actually A is a cross section of a tube that is inch in outside diameter, is oriented parallel to the roll shaft, and is spaced to 1 inch from the roll surface. The holes for the air jet are 0.040 inches in diameter and spaced at inch intervals. An air pressure of 3 psi was adequate for residual particle removal. Electrode J is similar to I, consisting of a 0.0 10- inch- diameter wire. The ionizing current to J is opposite in polarity to the current to I and smaller. A current of approximately 3 microamperes per inch length of wire is satisfactory. When the current to I is held constant, and the current to J is varied, there is variation in the amount of conductor fraction. However, the current to J, compared to the current to I, has only about one-tenth the effect on the amount of conductor fraction.

For successful electrostatic separation, the behavior of the component minerals of the tested ore under various conditions should be known. In the course of investigation, it was found that these are difficult to measure accurately by the presently available methods. The method used by Johnson is handicapped by the fact that particles in the extreme front and rear end of the falling stream are difficult to control or observe accurately. Furthermore, this method is not applicable when the tested sample consists of more than one mineral. The deflection method, as reported by Fraas, is inadequate to show the differential amounts of mineral particles deposited at the various horizontal positions under the grounded roll of the separator. This weakness is further noted when the tested sample contains more than one mineral.

Experimental: A new apparatus termed, distribution analyzer, was used to replace both the dividing gauge and the two attached collecting chutes of a Johnson separator. This distribution analyzer consists of sixty or more 1.0-cm wide cells as shown in part in fig. 1; it has been fully described in a previous article.

Except where otherwise stated, the testing procedure used in this paper is as follows: (a) calculate the weight of feed for a 1-min run at a constant feed rate of one layer of closely packed particles falling on the grounded roll; (b) weigh and dry the feed at 100C for 12 hr; (c) place the

top of the distribution analyzer level 4 cm below the bottom of the grounded roll, so that the cells are longitudinally parallel to the grounded roll and the zero edge of a suitable cell is directly under the front vertical tangent to the grounded roll (see fig. 1); (d) with the charged roll at a negative potential of 15.5 kv, the dry hot sample is introduced into the feed chute at a constant feed rate; and (e) finally the deposited mineral particles are removed from each cell and weighed and analyzed when the tested sample contains more than one mineral.

Optimum Condition of Separation: There are two methods to ascertain the optimum condition of electrostatic separation of minerals. One is to test the ore sample directly under various conditions and is termed ore-method. This can be exemplified by the tested data of a synthetic mixture of 82 pet anthracite coal, 14 pct quartz and 4 pct pyrite by weight.

It should be noted that the surface condition of the component minerals used for the mineral-method requires special attention. For example, it was found in the course of investigation that the conductivity of lower conducting minerals was somewhat increased by contamination with higher conducting minerals, such as coal, graphite, and galena.

In comparison of the ore-method with the mineral-method, the former is more direct and accurate but requires analysis of the mineral particles deposited in each cell of the analyzer. The latter is indirect and less accurate but requires no analytical

The behavior of mineral particles in an electrostatic field is a surface phenomenon and is governed chiefly by the surface atoms and ions instead of the internal composition of mineral particles. For example, the electrostatic behavior of quartz can bealtered by changing its surface condition with chemicals and moisture.

Potential and Polarity: In regard to the effect of the magnitude of applied voltage on the behavior of mineral particles, shows that to a certain limit calcite particles are repelled farther with the increase of applied voltage. When applied voltage is increased the field intensity is also increased and consequently the electric induction of mineral particle.

Virtually all commercial use of electrostatic separation has employed separators depending on differences of conductance of the broken, solid mixtures treated by them. The two main types of conductance separators have been: (1) Those in which the separating field is as near a static field between two charged electrodes as it is possible to maintain in the face of the usual leakage that takes place between electrodes with some 5 to 50 kv tension between them and spacing of to 3 in.; (2) those in which a convective discharge of corona or spray type, but not a spark discharge, takes place between the carrier electrode and a charged electrode facing it, which is of very restricted area, such as a row of needle points or a small diameter wire parallel to the face of the carrier electrode.

The ions emitted by the corona electrode, or wire F, charge both the particles on the carrier electrode, or roll A, and the surface of the dielectric electrode B. Whereas the space between roll A and wire F contains ions, the space between electrode B and roll A is a pure static field free of ions.

In operation the ionic field from the corona electrode is balanced with the static field from the dielectric electrode. The ionic field charges the particle so that it adheres to the roll. The static field intensifies the discharge of the corona charge by interfacial conduction and permits the release of the particle. The ionic and static fields therefore have opposing effects. The values of both fields are adjusted so that the poor conductor will adhere and the good conductor will be released.

In the field F, the corona-charged particle has a greater adhesion to the roll than out of the field. When the particle is in the field but off the roll, the attraction toward the roll is less than when the particle is on the roll. This is an explanation for the violent ejection of the conductors from the roll. A great force of adhesion, if present momentarily with the conductive particles, is a factor contributing a favorable interfacial surface conductance.

If for a given particle size the separator is adjusted so that for the nonconductive particle fe = fc then for all larger particles, fe < fc, and for all smaller particles, fe > fc. The correct procedure is to select an adjustment so that fe > fc, for the maximum nonconductor particle size.

A frequency distribution of charging rates may result from either variations on each particle or variations between particles. The former has been demonstrated with straight-line probability plots. With the latter, some particles may have, by chance, a low conductance surface exposed. Regrinding would yield a new distribution.

Ilmenite, high-iron chromite, and magnetite have conductivities which correspond to specular hematite and yield equivalent results. Low-iron chromites have lower conductivities, and when associated with diopside and serpentine having apparent conductivities higher than that of quartz, size classification of the fine fraction is required. Commercial operation would require air or an electrostatic method for classification at sizes finer than 20-mesh. Electrostatic sizing will be discussed in a later paper.

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electrostatic separation of minerals | environmental xprt

electrostatic separation of minerals | environmental xprt

Electrostatic phenomenon dates back to 600 700 B.C. when people noticed static electric effects (attraction of hair) when amber was rubbed with fur. Indeed, the word electrostatic comes from the ancient Greek word electron meaning amber. Electrostatic beneficiation of dry granular plant-based food (i.e, organic) materials have been investigated for over 140 years, with the first patent for electrostatic separation of wheat flour middlings filed as early as 1880. In the late 1890s, Thomas Edison designed a machine to separate non-magnetic iron ore from sand-like particles in the absence of water. His electrostatic separator accepted a thin film of dry particles to pass over an electrically charged drum. During the Gold Rush in the Ortiz Mountains southeast of Santa Fe in New Mexico, Edison learned that gold would stick to the drum while sand was repelled, resulting in the first patented a gold concentrator using electrostatic principles. In the early 1900s, electrostatic separation became popular in mineral processing, until the advent of froth flotation. However, in more recent times, the move towards environmentally-friendly processing techniques (i.e. reducing the use of chemicals) and with many mineral processing plants facing water supply issues, there is an increased interest in the electrostatic separation of minerals in a dry state. Principles of ElectroStatic Separation Every mineral species has electrostatic properties that can potentially allow separation in an external electrostatic field. Mineral processing categorises minerals into conductors and non-conductors (dielectric) materials. When an electrostatic field is applied they behave differently, thus enabling a separation. Conductor minerals (most sulphides) will lose the charge to an earthed surface, whilst non-conductors will retain the charge and be pinned to the earthed surface. This effect is known as image charge and is the operating principle of the ElectroStatic Separator. Technical description of the ElectroStatic Separator

Electrostatic phenomenon dates back to 600 700 B.C. when people noticed static electric effects (attraction of hair) when amber was rubbed with fur. Indeed, the word electrostatic comes from the ancient Greek word electron meaning amber.

Electrostatic beneficiation of dry granular plant-based food (i.e, organic) materials have been investigated for over 140 years, with the first patent for electrostatic separation of wheat flour middlings filed as early as 1880. In the late 1890s, Thomas Edison designed a machine to separate non-magnetic iron ore from sand-like particles in the absence of water. His electrostatic separator accepted a thin film of dry particles to pass over an electrically charged drum. During the Gold Rush in the Ortiz Mountains southeast of Santa Fe in New Mexico, Edison learned that gold would stick to the drum while sand was repelled, resulting in the first patented a gold concentrator using electrostatic principles.

In the early 1900s, electrostatic separation became popular in mineral processing, until the advent of froth flotation. However, in more recent times, the move towards environmentally-friendly processing techniques (i.e. reducing the use of chemicals) and with many mineral processing plants facing water supply issues, there is an increased interest in the electrostatic separation of minerals in a dry state.

Every mineral species has electrostatic properties that can potentially allow separation in an external electrostatic field. Mineral processing categorises minerals into conductors and non-conductors (dielectric) materials. When an electrostatic field is applied they behave differently, thus enabling a separation.

Conductor minerals (most sulphides) will lose the charge to an earthed surface, whilst non-conductors will retain the charge and be pinned to the earthed surface. This effect is known as image charge and is the operating principle of the ElectroStatic Separator.

A feed of conducting (Pyrite FeS2) and non-conducting (Silica Sand SiO2) powders is fed via a vibratory feeder onto an earthed, stainless-steel roll revolving at a pre-determined rate. A strategically positioned electrode assembly, at around 20 from the vertical, charges the feed at a high voltage (20 30 KeV). As the mineral particles leave the vibratory feeder and land on the roll, two behaviours are observed:

The mineralogy of a sample dictates the ElectroStatic Separator settings. Controlled laboratory tests at Buntings Centre of Excellence (near Birmingham in the UK) identify the key parameters needed to optimise and then meet the separation objective. The key control variables for separation are:

The list of minerals classed as conductive and non-conductive minerals is extensive. Such mineral mixes are ideally suited for separation using the ElectroStatic Separator. The following list is not comprehensive, but is provided as a guide.

For the primary separation, a Magnetic Separator (typically an Induced Magnetic Roll or Rare Earth Roll Separator) removes magnetically-susceptible para and ferro magnetic minerals (e.g. ilmenite, garnet, monazite). The secondary electrostatic separation focuses on separating the remaining minerals (e.g. zircon sand, silica and rutile).

When processing beach sands and similar minerals reserves, the electrostatic separator has the advantage of processing materials in a dry state (unlike froth flotation). In a beach sands processing plant, a 1.5m wide ElectroStatic Separator would typically process between 3-5 tonnes per hour.

Mineral processors have never faced greater challenges. There is a focus on mining lower quality reserves or reprocessing waste materials, both requiring higher degrees of processing. Environmental considerations are paramount, whether that is availability of resource (i.e water) or the effect on the local natural habitat. These considerations often dictate the processing method.

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electrostatic separation - an overview | sciencedirect topics

electrostatic separation - an overview | sciencedirect topics

Electrostatic separation is a beneficiation technique that exploits the differences in conductivity between different minerals to achieve separation (Higashiyama and Asano, 2007; Kelly and Spottiswood, 1989a,b,c).

Electrostatic separation works on the natural conductivity properties between minerals in feed. Separation is between economic ore constituents, noneconomic contaminants, and gangue. The common units are high-tension plate and screen electrostatic separator. The electrostatic plate separators work by passing a stream of particles over a charged anode. The electrostatic minerals lose electrons to the plate and are pulled away from other particles due to induced attraction to the anode. The dry stream of moving particles is preferred between 75 and 250m, with close size distribution and uniformity of shape for efficient separation. It is used for separating monazite, spinel, sillimanite, tourmaline, garnet, zircon, rutile, and ilmenite from heavy beach/stream placer sand. The electrostatic technique with local modification is extensively used in Australia, Indonesia, Malaysia, and India bordering Indian Ocean for separation of mineral sands.

There are three distinct stages in electrostatic separation processes: particle charging, separation at the grounded surface, and separation caused by the trajectory of the particles. Particle charging can occur by three possible mechanisms: contacting of dissimilar materials, ion bombardment, and induction.

Contacting, followed by separation, of dissimilar materials results in one being positively charged and the other negatively charged. Bulk movement with repeated contacts is necessary for sufficient charging.

Charging by ion bombardment occurs as the air between the particle and electrode conducts by a corona discharge. If the particle is a nonconductor, it does not lose charge while in contact with the grounded rotor and is held to the surface by its own image force Fi. This force represents the attraction between the charged nonconducting particle and the grounded surface, the latter being equivalent to a similar charge of opposite sign in a mirror image position behind the surface. The force is given by

Induction charging of a particle occurs on a grounded surface in the presence of an electric field. Both the conducting and nonconducting particles initially develop opposite charges on opposing faces, but because the conducting particle loses one of these charges to the grounded rotor, it develops an equipotential surface and experiences an electrical force Fe away from the rotor given by

The separation that occurs at a grounded surface then results from the differing forces on the particles. With high tension separators, the important forces are the image force Fi and the centrifugal force. Equating these forces yields the pinning factor

Because all electrostatic separators give incomplete separations, multiple stages are used. The probability of collection of any component at each stage tends to remain constant, so that the process can be modeled by a probability relationship such as Eq. (24). The probability p will have some relationship to the parameters described earlier.

As a first step in the refining process, water, inorganic salts, suspended solids, and water-soluble trace metal contaminants are removed by desalting using chemical or electrostatic separation. This process is usually considered a part of the crude distillation unit. The desalted crude is continuously drawn from the top of the settling tanks and sent to the crude fractionation unit. Distillation of crude oil into straight-run cuts occurs in atmospheric and vacuum towers. The main fractions obtained have specific boiling-point ranges and can be classified in order of decreasing volatility into gases, light distillates, middle distillates, gas oils, and residue. The composition of the products is directly related to the characteristics of the crude processed. Desalted crude is processed in a vertical distillation column at pressures slightly above atmospheric and at temperatures ranging from 345 to 370C (heating above these temperatures may cause undesirable thermal cracking). In order to further distill the residue from atmospheric distillation at higher temperatures, reduced pressure is required to prevent thermal cracking. Vacuum distillation resembles atmospheric distillation except that larger diameter columns are used to maintain comparable vapor velocities at the reduced pressures.

Sieving is carried out in the physical recycling process to classify the different sized particles based on the various sizes of sieve apertures to the desired particulate size for separation. Sieving is not only been utilized to prepare a uniformly sized feed but also to upgrade metal contents (Kaya, 2016). The screening is essential as the particle size and shape of metals are different from that of plastics and ceramics. Rotating screen is used mainly for metal recovery in WEEE recycling process.

Based on the variation in shape, density, and electric conductivity of metallic and nonmetallic materials in WEEE electrostatic separation are considered as a promising way to recover metals from pulverized WPCBs. Recycling industry basically used shape separation by tilted plate and sieves. Copper recovery is promising by an inclined conveyor with a vibrating plate from electric cable waste, printed circuit board scrap, and waste television and personal computers in Japan (Cui and Forssberg, 2003).

Magnetic, electrostatic, and density separation are mechanical separation techniques that have been widely used in urban mining of WEEE. Low-intensity drum separators are the standard method of magnetic separation for the recovery of ferromagnetic metals from nonferrous metals and other nonmagnetic wastes (Hsu et al., 2019). Magnetic separation is in general performed first, followed by shredding or grinding to fine particle size, and after that electrostatic separation is applied. High-intensity separators are used for possible separation of copper alloys from the waste matrix (Veit et al., 2005). Through an intense magnetic field, copper alloys with relatively high mass susceptibility (Al multicompound bronze), copper alloys with medium mass susceptibility (Mn multicompound bronze, special brass) and copper alloys with low mass susceptibility and/or diamagnetic material behavior (Sn and Sn multicompound bronze, Pb and Pb multicompound bronze, and brass with low Fe content) can be separated (Cui and Forssberg, 2003).

Electrostatic separation is considered as advantageous compared to the other physical techniques as it is smooth operation, less hazardous, and requires less energy (Lu and Xu, 2016). Electrostatic separation is based upon electrical conductivity and separates the nonconductive materials from the conductive ones. Although the electrostatic separators were initially recovered nonferrous metals from automobile scrap or municipal solid waste, now widely used for WEEE utilized explicitly for the recovery of copper or aluminum from chopped electric wires and cables and recovery of copper and precious metals from printed circuit board scrap (Lu and Xu, 2016). It has been observed that the multistage process is needed to separate conductors from nonconductors (Hsu et al., 2019). Both corona discharging and eddy current-based electrostatic separation have received significant attention in the separation of ferrous and nonferrous metals and the separation of plastics from the plastic and metal mixture. Particle size has become a limiting factor, along with the sticking effect of larger particles in terms of corona separation, whereas eddy current-based electrostatic separation depends on the flow of the particles (Cui and Forssberg, 2003).

Gravity separation is considered as the best physical separation option for nonmetals from the metals by different specific gravities. Density separation is dependent on the density and the size of the components. Viscous liquids such as tetrabromoethane can serve as the separation medium where the metals can be separated from the plastics or ceramics. Conventional gravity separators that are used in E-waste recycling are water or airflow tables, dense media separation, and sifting. Density separation techniques that have extensively been used in the mineral processing industry are now applied into E-waste recycling as WEEE consists of many plastics, with a density less than 2.0g/cm3; light metal, primarily Al and glass, with a density of 2.7g/cm3; and heavy metals, predominantly Cu and ferromagnetic, with a density more than 7g/cm3 (Kaya, 2016). The enriched fractions are treated by chemical techniques: pyrometallurgical and hydrometallurgical processes after mechanical/physical treatments in order to extract precious metals.

The idea of UCC is that the ash-free coal (AFC) can be used more efficiently to provide electricity either via direct firing into the gas turbine followed by a steam cycle or via catalytic gasification followed by fuel cell. Chemical cleaning of coal to obtain more orless AFC can be broadly classified into two: physical and chemical. Fig.2.9 shows the physical and chemical coal cleaning pathways and the characteristics of the cleaned coal [31]. In physical cleaning, there is no involvement of chemicals and thecoal structure does not change. Physical cleaning involves the removal of minerals by methods such as gravity separation, froth flotation, electrostatic separation, magnetic separation, oil agglomeration, and air dense medium fluidization [32]. In the air dense mediumfluidized bed process, the lighter coal particles containing fewer minerals floats on the surface of the bed and the particles with the higher mineral content sink.

Even though physical coal cleaning is easy and low cost, it does have the problem that only impurities in the larger sized particles can be cleaned (e.g., gravity separation requires particle size >0.5mm). Physical cleaning cannot remove finely distributed minerals and chemically bounded species within a coal matrix.

Chemical cleaning methods are better than physical methods because chemical processes can remove finely distributed minerals and organically bounded chemical species. The chemical cleaning methods can be divided into two categories: use of chemicals that dissolve minerals, usually called as UCC, and solvents for dissolving the coal-like matter, usually called AFC. In UCC, there is no change in the coal structure of the product, whereas in AFC, the coal structure is changed. In UCC, the ash content ranges between 0.1% and 5%, whereas in AFC, ash content is less than 0.1%. The details of the production process, yield advantages, and disadvantages of these processes have been presented in our recent review [31]. Extraction of organic matters using NMP (N-methyl-2-pyrrolidone) to produce AFC also reduces sulfur; in particular, inorganic sulfur is completely removed [33].

Sorting processes are most often required in order to provide type sorted fractions for recompounding processes. Dry sorting processes apply mechanical (sieves) or electrostatic forces or they sort by means of spectroscopic detection systems. Sieves or air classifiers are applied to reduce the amount of fine particles in waste plastic fractions, since the fines contain small metal particles from grinding processes and dust collected on plastic surfaces in the use or reclamation phase. These foreign materials may worsen the mechanical properties of recycled plastics. However, sieving or air classification have no impact on the FR system.

Electrostatic separators are applied to separate mixtures of two or three different polymers by means of electrostatic forces [24]. If one of the three plastics in the input stream contains (B)FR, this process may separate FR equipped from other plastics. However, such defined input streams are most likely found in postindustrial waste and are generally not present in real postconsumer e-waste. Therefore, electrostatic separation of postconsumer plastics in e-waste is only applied to fractions obtained by preceding processes (e.g. sink and float fractions).

Manual spectroscopic detection devices have been applied in recycling operations of dismantling plants, whereas these techniques are applied to comparably large plastic parts (unground casings, etc.). NIR and midinfrared MIR can be applied to sort plastic parts according to their polymer types unless their color is too dark to produce analyzable spectra. ABS and ABS equipped with TBBPA can be differentiated by NIR, whereas ABS and ABS equipped with other BFR cannot be distinguished by this technique. Therefore, NIR is not a powerful tool to sort FR and FR-free polymers.

In contrast, bromine or phosphorous can be detected by sliding spark spectroscopy (SSS), which can be performed by low cost manual devices. Thus, the combination of SSS and NIR provides information suitable for an effective sorting of FR-free polymers. As both instrument scan plastics within seconds, their application in dismantling plants appears to be a reasonable and low cost recycling approach for light plastics.

However, from an economic point of view, automated sensor-based sorting isconsidered to be more effective than manual approaches. These techniques are applied to coarsely ground plastic waste and subject particles with 550mm to the sensor system e.g. on a band conveyer. An NIR camera or an alternative sensor system analyzes the particles coming across and controls a blowing device that may then blow a particle out of a waste stream, if the sensor results indicate thepresence (or absence) of a given polymer. These sorting techniques are rather effective; however, they work best with highly concentrated target polymers (>80%). As long as a complex mixture is to be segregated, the sorting will have to be done in several consecutive sorting steps in order to produce reasonably pure polymer fractions. This is necessary since the mechanical sorting by blowing off particles always leads to a small amount of falsely sorted particles. This is exemplified in Table 5, which provides experimental data on the automated NIR separation of ground gray monitor casings. Whereas the separation of ABS and PS has not been completely optimized in this example, PC/ABS can well be segregated from ABS and PS. However, there is a small percentage of 6% ABS and PS particles, which have been sorted falsely into the PC/ABS fraction.

An innovative approach applies laser spectroscopy to sort plastics automatically by a sensor technique that produces analyzable spectra even for black polymers [43]. Thus, NIR and laser spectroscopy provide two powerful sorting tools with respect to polymer type. XRT is also used as detector in automated sorting systems and can differentiate polymers of different specific atomic densities. As the optical density is significantly increased by BFR, e.g. it has been shown that XRT may separate BFR-free polymers from BFR polymers. As for the manual approach, a combination of two automated sensor-based technologies appears to be capable of sorting pure and BFR-free polymer fractions from WEEE.

Electrostatic separation is a beneficiation technique that exploits the differences in conductivity between different minerals to achieve separation (Higashiyama and Asano, 2007; Kelly and Spottiswood, 1989a,b,c). Electrostatic separation techniques are typically only used when alternative processing techniques will not suffice, as the comminution steps in mineral processing flowsheets are generally wet processes and the energy requirements to drive off all moisture prior to electrostatic separation can be significant (Kelly and Spottiswood, 1989a). In the context of rare earth mineral processing, the typical use of electrostatic separation is in the separation of monazite and xenotime from gangue minerals with similar specific gravity and magnetic properties (Ferron et al., 1991; Zhang and Edwards, 2012). A specific example of this is when xenotime, which is more strongly paramagnetic than monazite, is concentrated with ilmenite after magnetic separation of heavy mineral sands (Gupta and Krishnamurthy, 1992). In this case the only means by which xenotime may be removed from the ilmenite is via electrostatic separation, as ilmenite is conductive but xenotime is not (Gupta and Krishnamurthy, 1992).

Electrostatic separation is a valuable technique for heavy mineral sand beneficiation, and the successful application of this process to separate ultrafine (<37m) coal particles may present an opportunity to treat the fines produced in many currently operating mineral processing circuits that account for significant rare earth losses (Higashiyama and Asano, 2007). Unfortunately, all electrostatic separation techniques (drum-type, belt-type, plate-type etc.) suffer from the requirement that the feed material must be completely dry (Higashiyama and Asano, 2007). Aside from the heavy mineral sand deposits, almost all other discovered rare earth deposits (aside from the ion-adsorbed clays in southern China) require some form of comminution prior to separation and these grinding operations are heavily reliant on slurried feed material. The energy costs associated with completely drying a ground ore prior to an electrostatic separation step are likely to be far too cost-prohibitive for such a process to be applied on an industrial scale.

Electrostatic separation can be applied in ilmenite upgrading because of the conductivity difference between ilmenite and its co-existing minerals such as titanaugite and zircon. High-tension (electrical) and electrostatic plate separators are commonly employed for this purpose (Australia, 2019). The successful industrial application is reported in Australia for the selective separation of ilmenite, rutile, zircon and monazite by means of electrostatic separation, together with magnetite separation in view of their different magnetic and electrical properties at various elevated temperatures (Australia, 2019; Farjana et al., 2018; Jones, 2009). As a combined processing technology, rare earth drum magnets are first used to remove ilmenite from mined heavy mineral concentrate feed, from which most of the ilmenite can be recovered. Then non-magnetic minerals are processed by electrostatic separation to obtain conductor minerals (rutile and leucoxene) and non-conductors (consisting of zircon, kyanite, quartz, monazite and staurolite). Subsequently, the non-conductors are treated by gravity separation to remove the lower specific gravity material (quartz, kyanite, garnet, and staurolite) from the higher specific gravity zircon. After gravity separation, electrostatic separation is used again to remove residual conductors from the zircon and induced roll magnets are used to separate traces of monazite and staurolite from zircon.

As far as the authors knowledge, no report on the current application of electrostatic separation in ilmenite industry can be found in China. Electrostatic separation has been used to prepare high purity ilmenite and titanaugite in labs, and also used for theoretical studies on separating ilmenite from its associated gangue minerals such as quartz and zircon. Ziemski and Holtham (2005) used electrostatic separation to separate titanium minerals (ilmenite and zircon) using a new theory of particle discharge in high tension roll (HTR) separation, and the influence of particle bed and size was considered. Triboelectric separation of ilmenite from quartz is also successful in lab experiments, provided optimum airflow rate, feed rate and voltage (Yang et al., 2018). Overall, there are some limitations impeding the popularization and industrial use of electrostatic separation in ilmenite dressing, such as high requirement for feedstock quality in terms of moisture content (completely dry) and uniform granularity, safe operation for workers (dust problem) and limited handling capacity (<5 t/h).

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