ELECTROSTATIC SEPARATIONS OF SOLIDS FOSTER FRAAS .4ND OLIVER C. RALSTON U. S. Bureau of Mines Experiment Station, College Park, Md.
-___
S
EPARATION of mixtures of granular solids by use of Review of Principles electrostatic forces was begun in laboratories and was first recorded in the patent literature about fifty years FORCES IN THE ELECTRIC FIELD. Electrostatic separation ago. Commercial application followed about ten years later of mixtures of solids is effected by forces exerted in an electric and eventually grew to a considerable tonnage of material field. The forces on a particle in an electric field may be due treated by this method. Then other separative processes, either to an initial electric charge on the particle or merely to such as froth flotation, started to encroach on the field, and a difference between the dielectric constants of the particle and the surrounding medium. These forces are always dithe use of electrostatic processes waned. Since 1932 electrorected in such a manner that the resulting motion will cause static processes have occupied more attention and have been used more and more, especially in the mineral and food indusa maximum decrease in the total energy of the system. tries. These facts are illustrated in Figure 1, which represents an The older machines and processes were restricted t o granuelectric field between electrodes in a medium of dielectric lar solids and could not handle dusts, which was a serious constant, E. In a the variation in field intensity is perpenlimitation. Although the present commercial machines are dicular to the direction of the field; in b the variation in instill subject to this limitation, the writers have been able t o tensity is parallel to the field. Charged particles travel to adapt, on a laboratory scale, one electrostatic separation electrodes of opposite sign. Forces due to an electric charge method to dusts, all particles of which are smaller than 0.04 are always parallel to the field, whereas forces due t o a differmm. in diameter. The commercial possibilities are not yet ence in dielectric constant are always parallel to the direction evaluated, also considerable research is still needed. of maximum variation of field intensity. I n uniform elecE l e c t r o s t a t i c methods tric fields the forces due to found special application to difference in dielectric conmixtures of solids of the same stant are consequently zero. specific gravity and similar Dipoles, such as particles shape which could not be charged by the pyroelectric Basic principles and their application in electroreadily separated by other effect, and electrical conducstatic separation are discussed. Electrostatic means. In the mineral intors will be acted on similarly separators are classified according to the electric dustries such mixtures as t o substances of high dielecproperty of the solid utilized-i. e., conductance, sphalerite-pyrite, sphaleritetric constant. The behavior contact potential, dielectric constant, dielectric fluorite, and sphalerite-barite of conductors is due to the hysteresis, and pyroelectric polarization. Only were early separated electrofact that the electric field is the conductance method has been applied to any statically, but later flotation shorted through their conducimportant extent. The progress of electrostatic separations displaced the tive surfaces. separation has been retarded by several factors, Forces due t o differences in former method. Artificial particularly the lack of selectivity. Selectivity abrasives, formed in the elecdielectric constant, unlike can be increased by surface cleaning, by preforces due to initial electric tric furnace, are made from conditioning silicate minerals with hydrogen charges, remain unidirecimpure materials. The imfluoride gas and basic minerals with organic acids tional when the polarity of purities crystallize in separate and by the use of various surface active agents and the electrodes alternates even grains separable by electrooils. Although n o w seldom used, electrostatic a t very high frequencies. static methods and are still separation has diverse applications in the ore Electric fields produce in dibeing cleaned by such procdressing, chemical, and food industries. Through electrics strains which are not esses. Seeds can be separated mechanical improvements, including better insuinstantaneously eliminated and cleaned and are being lation, and a better understanding of principles on removal of the applied treated commercially by elecand limitations, this method is coming back into field. This hysteresis effect trostatic methods. increased use. By utilization of conditioned air, varies with different subNo good general discussion both this and the contact potential method of sepastances and becomes more of the principles of electroration (now under active development by the Bureau apparent, the higher the frestatic separation is available, of Mines) promise a wider field of usefulness. quency of polarity change. since most authors have been Forces are therefore exerted interested in but one specific in high-frequency fields due to type of machine. The purhvsteresis lae similarlv to pose of this paper, therefore, is to consider the principal available processes and their forces due to differences in dielectric constants(9). fundamentals, with but limited material about mechanism. A fluid medium between two charged electrodes a p parently does not remain unaffected by an electric field. Processes now in use, both commercially and in the laboraIn a fluid such as oil or air, which completely 6 l l s the tory, are included. Present applications are briefly mentioned. Sixth Chemical Englnesring Symposium held under the auspices of the Division of Industrial and Engineering Chemistry of t h e American Chemical Society a t t h e University of Michigan, Ann Arbor, Mich., pages 599-667.
600
MAY, 1940
SEPARATION OPERATIONS
space between two horizontal plate electrodes, convective cellular vortices are produced (11). This phenomenon resembles that produced when a temperature gradient exists between the two plates.
(a 1 FIGCRE1. FORCES E~XERTED
IX AN
ELECTRIC FIELD
In addition to a force exerted on a charged particle by an externally applied electric field, a force may be exerted due to the field of the particle itself. Figure 2 illustrates the approach of a charged particle to an initially uncharged surface. When the dielectric constant of the surface is higher than the dielectric constant of the medium surrounding the two, an attraction results. This is the common result with all solid surfaces if air is the medium. When the dielectric constant of the medium is higher than that of the surface, as in b, a repulsion results. Similar behavior is obtained if dipoles, such as those formed pyroelect,rically, are substituted for singly charged particles. METHODS OF PRODUCING CHARGES. Various methods may be used to produce electric charges. These methods are summarized in Figure 3. I n a, two particles acquire charges by induction. With a nonconductor the positive and negative charges are equal, and tht: total charge is accordingly zero. In contrast, a conductive particle resting on the lower positive electrode will have its negative charge neutralized, and the total charge is positive. This method of charging will be designated as charging by conductance. Certain solids have their conductivity influenced by light (15), notably diamond by visible light and sphalerite by ultraviolet light. C,>&
60 1
When two dissimilar substances are intimately contacted and separated, they acquire electrical charges by a phenomenon commonly known as a frictional electrical effect. Pohl (15) showed that these charges are due to an initial contact potential difference between the two substances. A sufficiently close approach of the two substances results in a passage of electrical charge to balance the potential difference due to the contact potential. At sucrocJirig greater distances of separation nc electricP1 Aarge cti flow, but the potential difference increases with the increasec, distance of separation, this fact being governed by the well-known principle of the increase of potential with the increased separation of the plates of a charged condenser. In c, Figure 3, I CONDUCTANCE I CONTACT POTENTIAL the intimate contacting of two nonconductors to give fixed patches of elec/IONIZED GAS I trical charges on their surfaces is illustrated. With conductors as in d, these charges I+/ I are not fixed, and the final charge {V;is therefore (" smaller. Charging of a nonconductive particle ( b) A B 1 by contacting PYROELECTRIC PHOTOELECTRIC MISCELLANEOUS, with a grounded conductive plate =conductor -=nonconductor ____
k-~-a+
ud
-& y J
-
>&A
Applications ATTRACTION (0)
REPULSION
(b)
FIGURE2. FORCES EXERTED ON A PARTICLE DUETO ITSOWNFIELD Particles in gases containing ions predominately of one sign
will acquire electrical charges. If the particles are freely suspended in the gas, both conductors and nonconductors will receive charges as illustrated in 6 , Figure 3. Those resting on one of the electrodes will receive charges only when they are nonconductors. Ions may be produced in gases either by a pointed or small-surfaced electrode where the potential gradient is sufficiently high to disrupt the gas, or by other ionemitting objects.
Electrostatic separation of solids is but one of many electrokinetic phenomena. The magnitude of the forces is small and the solid materials to be separated in most cases must be crushed under several millimeters diameter except for lowdensity materials such as seeds. A broad classification of electrostatic processes can be made as follows: 1. Processes depending on differences in the conductance of
solids.
2. Processes depending on differences in the contact potential of solids. either with one another or with the surfaces of the seDa-
rator used. 3. Processes depending on pyroelectric polarization of cry&-
Qals.
INDUSTRIAL AND ENGINEERING CHEMISTRY
602
Processes depending on the relative dielectric constant of solids and the media in which they are suspended. 5. Miscellaneous. 4.
CONDUCTANCE SEPARATOR.Most patented methods depend on either the conductance method of charging or conductance in conjunction with ionized gases. This is probably due to the fact that the phenomenon upon which this method is based k *"'-3iLo most commonly encountered by experimenters in thF4 .eld, a result 3f the experimental simplicity and the magnitude of the electrostatic movements. The simplicity is obvious, as Figure 3-a shows. The magnitude of the electrostatic movements due to easy production of large charges screens out observation of other less prominent phenomena. Conductance separators are generalized in Figure 4. A is the initial material, B the less conductive, and C the more conductive; a and 6 are represented as having rolls beneath hoppers. In some cases these rolls are replaced by inclined chutes. The attracting electrode, indicated as being charged negative and of cylindrical form, is sometimes of other shape and polarity but is always the opposite of the roll polarity. Instead of separating the charged and uncharged particles by a dividing edge as shown, the former may be collected selectively by adhesion to a surface. Simple charging by conductance is illustrated in a, Figure 4, whereas in 6 additional charging by ions from a pin-point electrode is shown. This causes the nonconductors to adhere more firmly to the rolls.
b FIGURE 4. CONDUCTANCE SEPARATORS In Figure 4-c material is moved mechanically from A to B on a flat surface, such as a grounded vibrating table or a moving belt. A superimposed electrode causes the more conductive material to be levitated and affected less by the mechanical movement. With proper tilting this more conductive material is removed a t C. The applied potential in Figure 4 c may be constant in magnitude and polarity, constant in polarity but intermittently reduced to zero, or alternating in polarity. Interruption and alternation of potential cause material t o be repeatedly levitated and redeposited on the table, and results in a number of successive separations during one passage over the table. The alternations and interruptions may also be synchronized with the table vibrations. The effect of these repetitions is t o give higher selectivity and higher efficiency of separation. The selectivity of the separators just described depends not only on conductivity differences of the constituents but also on differences in the density, particle size, and particle shape. CONTACTPOTENTIAL SEPARATOR.The literature describes processes and experimental results (3, 8, 13) in which the con-
VOL. 32, NO. 5
tact potential was utilized in electrostatic separation, although this fact was often not recognized. A common result is a dual source of electrical charge due to an accompanying charge by conductance (8). In work on separation a t the Bureau of Mines (16) specific use of the contact potential has been made. Two types of apparatus used in these investigations are shown in Figure 5 . In a, particles from hopper A are fed onto a horizontal plate vibrating with both a horizontal and vertical component as illustrated. These vibrations
(0 )
FIGURE5 .
( b) C021T.4CT POTENTI.4L SEPARATORS
cause the particles to move toward baffle plate F , where they fall vertically into the electric field between the enclosed electrodes and are separated according to the polarity of their charges. It has been found that vibrating surfaces, such as plate E , produce more enhanced electrical charging than mere passage down inclined chutes or vertical zigzag passageways. The more vigorous the vibration, the more enhanced is the charging. Plate E is constructed of electrically conductive material whose contact potential lies between the contact potentials of the constituents to be separated; i. e., the plate is positive toward one constituent and negative toward the other. On passage of powdered material, a nonconducting vibrating plate will acquire areas of high electrical charge which not only result in erratic charging of the powdered material but also in causing its adhesion. The accumulating charge must be dissipated in some manner. This may be accomplished and still permit suitable electrification of the powdered material, by superimposing, a short distance above the plate, an electrically grounded hot wire grid. Such a grid must be adjusted to a proper temperature and will emit either positive or negative ions according to the potential gradient developed by the fixed plate charges. Figure 5-6 illustrates a contact potential separator suitable for material with a particle diameter smaller than 0.04 mm. Particles from hopper A are suspended in dry gas and forced through a sinuous-path passageway where they become electrically charged. On emergence between two open-structure electrodes, such as grids or screens, they are separated according to the polarity of their charge. Separated fractions leave a t B and C, and unseparated a t D. Another separator which probably operates by frictional electrical charges is that of Overstrom (14) for the separation of biotite mica from sands (Figure 6 ) . A mixture of mica and quartz from hopper A is electrified by passage over a set of staggered glass plates. Because of its shape the flat mica, after becoming frictionally charged, has greater adhesion powers and tends to cling to the bottom edge of the plates. Thus the mica is collected a t C and the quartz a t B. PYROELECTRIC SEPARATOR.Overstrom (14) also made use of the pyroelectric effect for the separation of quartz from feldspar. In Figure 7 heated quartz and feldspar from a steam-jacketed hopper, A , is fed onto a roll directly below. A drop in temperature after deposition on the roll causes pyro-
MAY, 1940
S E P A R A T 1ON O P E R A T I ON S
electric polarization to appear on the quartz, with r e s u l t i n g adhesion to the roll. The adhering quartz is collected a t B and D , and the feldspar a t C. D I E L E CTnICCONSTANT SEP-4R m o n . Very few
electrostatic s e p a r a t o r s described in the patent literature are dependent on differences in dielectric constant. Hatfield (6) has done considerable work in this direction. The principle of his separator is illustrated in Figure 8. A mineral mixture from hopper A is allowed to fall C o u i t e a y . Sutton, Steelr & Steele, Inc. into an electrically nonconELECTRO-FLOT" SEPARATOR WITH ducting liquid Tn-o SEPARATING ZOKES having a dielectric constant between the dielectric constants of the constituents to be separated. An alternating potential, V , applied to the pointed surfaces causes the particles of high dielectric constant to adhere to the points while the others are repelled and pass freely to collecting chamber C below. On removal of potential Ti, the adhering constituent is dislodged and collected a t B. Hatfield's method is not being used commercially but is an excellent laboratory means of separating ores. ~IISCELLANEOUS S E P m A T o n s . There are patents in which the use of dielectric hysteresis is claimed. Krasny-Ergen (9) described the utilization of this property for the separation of colloids. The previous descriptions have covered in general all of the United States patents. Additional special types and various general types with special features have not been described because of apparent lack of importance.
603
ductivitia were highly divergent, as in the case of zircon and ilmenite, where their densities differed as in mixtures of zircon and quartz, or when the particle shapes were dissimilar as in mixtures of mica and a blocky material. Most materials separated by other methods do not have these unique properties. Recently Johnson (7) and the Bureau of Mines independently developed the idea of preconditioning the surfaces of solid particles before electrostatic separation. The use of acidic gases, applied before or in the separator itself, has brought fruitful results. For the silicate minerals, such as mixtures of quartz and feldspar, hydrofluoric acid gas is most effective; it forms a conductive film of potassium and aluminum fluorides on the feldspar particles, whereas quartz particles are merely etched and silicon tetrafluoride vapor passes away. Heating to assure dryness after hydrofluoric acid treatment is beneficial. Numerous methods for selective filming of mineral mixtures are available to enhance the sharpness in electrostatic separations. Basic minerals, such as limestone, dolomite, magnesite, or borax, can be filmed by contacting with air containing vapors of acids such as acetic or benzoic. Treatment with surface-active reagents and oils may be applied either in wet pulps or in the semidry state, followed by drying. Many years ago separation of sphalerite from pyrite was made possible only by treatment with a solution of copper sulfate before drying, the sphalerite thereby receiving a conductive film of copper sulfide. Another important factor discovered in the Bureau of Mines work (16) was the previously mentioned application of the contact potential methods of separation. Thus, after hydrofluoric acid vapor treatment, feldspar is positive and quartz negative in contact potential toward an aluminum surface in the vibrating electrifier of Figure 5-a. Spodumene is positive whereas albite, microcline, and quartz in the same ore
C
FIGURE6 OVERSTROMMICA SEP.4RATOR
FIGURE 7 FIGURE8 PYROELECTRICD I E L E c T R I c SEPARATOR CONSTANT SEPARATOR
Progress of Electrostatic Separation Electrostatic methods have not been utilized so widely in the separation of solids as certain others, particularly flotation. Some of the factors which have contributed to this slow development are: lack of selectivity in separation, limited range of particle size capable of separation, necessity for close sizing, lack of suitable sources of high potential, lack of good insulating materials for construction, strong competition of other methods of separation, exorbitant patent royalties, and occasional unexplained failure of electrostatic separators to work, probably as a result of weather variations or some undetermined electrical influence. INCREASE OF SELECTIVITY. Up t o a few years ago there was a deplorable lack of selectivity in electrostatic separation. Solid constituents could be separated only when their con-
are negative toward an aluminum surface. Preconditioning with hydrogen fluoride vapor makes spodumene more conductive than the other minerals named. The same pretreatment also improves sepa'ration of zoisite from plagioclase feldspar. After benzoic acid vapor treatment, ulexite is positive and bentonite negative when vibrated on a plate with a cupric sulfide coating; the conductivities are also more divergent after the conditioning, ulexite being rendered less conductive. Doubtless the polar end of benzoic acid attaches itself to the basic ulexite surface, and the hydrocarbon ends are oriented toward the outer surface. The use of surface-active reagents is illustrated in the conductivity separation of halite (natural sodium chloride) and sylvite (natural potassium chloride). The dry salt mixture
604
INDUSTRIAL AND ENGINEERING CHEMISTRY
VOL. 32, NO. 5
SENSITIVITY TO MOISTURE. The authors find that the contact potential method of separation is very sensitive toward humidity changes, and that often a small concentration of moiature in the surrounding air is detrimental. Some of the variations and unexplained difficulties with conductivity separation would also probably be removed if properly conditioned air were supplied to the electrostatic separating machines or the rooms in which they are installed. Present Status of Electrostatic Separation
Courtesy, Ritter Products Corporation
FOURLARGESEPARATORS WHICHARE TOTALLY ENCLOSED AND PRACTICALLY DUSTPROOF
can be agitated with pieces of alundum containing an abJorbed solution of octyl amine hydrochloride’ and then with pieces of alundum containing absorbed oil. The crystals of sylvite are selectively filmed with oil in this manner and become much less conductive than the halite. In many cases the selectivity has been increased after the particles have been cleaned by abrasion and dedusting (16). Certain solids have increased selectivity by substituting contact potential methods for conductivity methods. Examples we removal of tremolite from dolomite, dolomite from talc, snd sphalerite from fluorite. In the specific examples of solids cited here the various properties such as conductance and contact potential are not always characteristic of the mineralogical identity, since in Jome cases these properties are dependent on the presence of amall amounts of impurities. RANGEOF PARTICLE SIZE. Electrostatic separation has been limited in application by the impossibility of separating materials of finer sizes. The separator illustrated in Figure 5b, which may partly solve this problem, can separate materials having highly divergent contact potentials such as sphalerite and fluorite. It is possible that expensive close sizing can be eliminated to a certain extent in separators such as those in Figures 4-b, .%a, and 5-b. IMPROVEMENTS IN EQUIPMENT. I n the past it was necessary to use mechanical rectifiers and electrostatic generators as sources of high potentials. These devices were not entirely satisfactory. Of late, economical electron-tube rectifiers have been made commercially available. Further advancement in this field is to be expected. Paralleling this is a similar advancement in the availability of new inexpensive electrical insulating materials. : This reagent
was suggeated b y J. E. Norman of t h e Bureau of Mines
Electrostatic methods are being used extensively in the food industries where dry separations are desired and where it is desirable to operate without contamination with reagents. Notable examples are the separation of insect and animal contamination from seeds such as mustard, and the separation of hulls and other components from seeds and beans. Pest seeds can be separated from various economic seeds. Bartlett ( 2 ) claims to have separated water-cress seed from rice, and leaf and stem material from raisins. He also says that shelled nut meats can be separated from shell material. Sutton (17) states that sorrel can be separated from alsike seed, and curled dock from clover. Quartz and feldspar are now separated by the conductance method after the preliminary hydrofluoric acid treatment. These same two mineral constituents can also be separated by the pyroelectric method. At Monterey, Calif., the Overstrom mica separation has been in operation for ten years a t the plant of Delmonte Properties Company. Zircon is being purified by removal of ilmenite and rutile. The former is an easy mineral t o remove, and many public demonstrations include this separation. Removal of mica from graphite was practiced for some years after the World War until domestic graphite was almost abandoned through loss of markets. Abrasive grains, such as silicon carbide, can be recovered from grindings, quartz from fluorite or from phosphate rock, and clay particles from kernite. Ferrosilicon is being separated from electrically fused alumina abrasive. Installation costs of electrostatic separators are said by Sutton and Jarman (17) to be about the same as for electromagnetic separators. In small installations the cost is as high as $3000 per ton per hour, with sharply reduced costs for larger plants.
Literature Cited Banerji, S. K., Indian J. Phgdcs, 12,409-37(1938). Bartlett, H.W., U. 8. Patent 2,174,681(Oct. 3, 1939). Blake, L. I.,and Morscher, L. N ; ,Ibid., 924,032(June 8 , 1909). Glockler. G.,and Lind, 5. C., Electrochemistry of Gases and Other Dielectrics”, p. 379, New York, John Wiley & Sone. 1939. Guest. P., U. 9. Bur. Mines, Bull. 368 (1933). Hatfield, H. S., Bull. Inst. Mining Met., No. 233 (1924); U.S. Patent 1,498,911(June 24,1924). Johnson, F. R., U. S. Patent 2,090,418(Aug. 17,1937). Johnson, H. B., T r a m . Am. Inst. Mining Met. Engrs., 134,40923 (1939). Krasny-Ergen, W., Hochfrequenztech u.Electroakustik, 48,126-33 (1936). Lenard, P.,Wied. Ann., 46,584-636 (1892). Luntz, M.. C o m p t . rend., 208, 1886-8 (1939). Medi, E., Atti accad. Lincei, Classe sei. fis.,mat. nut., 26, 159-65 (1937). O’Brien, B., U.S. Patent 2,168,681(Aug. 8, 1939). Overstrom, G. ,+.,Ibid., 1,679,73940(Aug. 7,1928). Pohl, R. W., Physical Principles of Electricity and Magnetism’’, London, Blackie and Son, 1930. Ralston, 0. C., U. S. Bur. Mines, Rept. Investigatione 3427 (1938)and 3473 (1939); Circ. 6974 (1937). Sutton, H. M., and Jarman, G. W., meeting -4m.Inst. Mining Met. Engrs., Feb., 1940. P n s L I s z m D by permiasion of the Director, subject t o oopyright.)
U. S. Bureau
of Mines. (Not
