DIELECTROPHORETIC BEHAVIOR OF CLAY

MINERALS

I. DIELECTROPHORETIC SEPARATION

OF CLAY MIXTURES*

by R. B. M c E u E N

Pure Oil Research Center, Crystal Lake, Illinois

A B S T R A C T

This paper describes a method and apparatus for separating clay particles according to their ability to store electrical energy. Separation is accomplished by opposing an electrical dielectrophoretic force by a mechanical centrifugal force. The apparatus used to create these forces consists of an axially rotating, fluid-filled cylinder in which a non-uniform electric field is maintained by means of radially disposed electrode vanes.

The behavior in this separator of illite, prochlorite, montmorillonite, halloysite, and kaolinite is reported. Observed differences in the dielectrophoretic force acting on these clays indicate that they can be separated one from another by this method.

The basic equations which govern the motion of particles in this separator are dis- cussed. From these equations and known electrical properties of clays it is concluded tha t the large dielectrophoretic force which acts on a clay particle must have its primary origin in an interaction between the non-uniform electric field and induced ionic space- charge which presumably is created in the particle's interlayer regions and in the loosely associated external surface layer.

I N T R O D U C T I O N

THE common physical properties of the clay minerals are very similar from group to group. Because of this, tlle clay groups are difficult to separate on the basis of particle size, density, magnetic susceptibility, shape, etc. A less common physical property of clay minerals which can be used as a basis of separation is their varying ability to store electrical energy.

That clay minerals are anomalous in their energy storage capabilities is evident from published data on their electrical properties. As early as 1020 Bairsto observed that the energy storage capability of slate, as measured by its dielectric constant at 920 cycles per second, was approximately six

*Published by permission of The Pure Oil Company.

549

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550 TWELFTH NATIONAL CONFERENCE ON CLAYS AND CLAY MINERALS

times that of marble. His data indicated that this difference should be still greater at lower frequency. In 1941 Mueller studied the Kerr effect of a sol of bentonite having a clay concentration of 2 per cent by weight and an average particle size of 20 millimicrons. The observed Kerr constant of the sol exceeded that of the most active organic liquid by a factor of 300,000.

Pohl (1958, and with collaborators, 1960) showed that movement of particles in a fluid can be brought about by intense non-uniform electric fields, if the particles are capable of storing a large enough percentage of the electric field's energy. The force producing this movement is called a dielectrophoretic force. By opposing this force with gravity, Pohl actually was able to separate mixtures of minerals in which the ability to store electrical energy differed for the mixture components.

M E T H O D OF S E P A R A T I O N

Description of Separator

The dielectrophoretic separator consists of eight radially arranged electrode vanes inside a hollow lucite cylinder filled with liquid dielectric. I t is shown assembled and dissembled in Plate 1. The non-uniform electric field is produced by grounding alternate electrodes and connecting the remaining four, which are electrically in parallel, to a source of alternating voltage. The voltage source used for these experiments was a large trans- former normally used to supply the accelerating voltage for an X-ray diffractometer. This transformer produces voltage of the order of 10 kV and current of several milliamperes at a frequency of 60 cps.

The bottom of tile separator consists of three concentrically arranged collection chambers. Rotation of tile cylindrical separator about its axis produces the centrifugal force.

The mixture to be separated is introduced into the spinning separator from the top and enters the convergent potential field at its midpoint. The dielectrophoretic force produced by the action of the field on the particles is directed towards the axis of the separator.

The centrifugal force at the point of entry of the sample determines the minimum dielectrophoretic force necessary to produce movement of a particle towards the axis of the separator. Particles experiencing insuffi- cient dielectrophoretic force move towards the periphery of the cylinder and on settling are collected in the outermost of tke concentric collection chambers.

The Dielectrophoretic Force

Mineral particles suspended in a fluid can store electrical energy in several ways. At low frequency it is possible to store energy by aligning

DIELECTROPHORETIC BEHAVIOR OF CLAY MINERALS 551

the particles so that the permanent centers of net charge within the particle produce a maximum electric field in opposition to the applied field. All other methods of electrical energy storage involve the creation of net charge centers by the action of the applied electric field. For macroscopic particles these "induction" methods are the only only ones which are of importance. This arises from the ability of tile induced charge centers to change their position without the need of reorienting the particle as a whole. Macroscopic particles in a viscous environment cannot change their orientation by 180 degrees at a rate of 60 times a second.

Net charge centers can be induced in the bulk of a particle. These induced charge centers are expressed in terms of the dielectric constant, ep, of the particle. Net charge centers can also be induced by charge migration along the surface of the particle. These induced charge centers are expressed in terms of surface conductivity, A, along the particle's surface. Charges also accumulate at the interface between two media of differing conductivity, g.

When the charge centers are induced by a non-uniform electric field a force couple is created which tends to move the particle. For the separator described here, this dielectrophoretic force can be approximated by:

F

(2ee + ep) ~ + gp +

where e, is the dielectric constant of the liquid dielectic into which the particle is subsequently introduced; AV is the rms value of the alternating voltage applied; air is the ratio of the particle's radius to its distance from the axis of the separator; ep, gp and h are, respectively, the dielectric constant, bulk conductivity, and surface conductivity of the particle; and

is the angular frequency of the applied voltage. A derivation of the full equation is available from the author, but need not be reproduced in these proceedings.

Inspection of this equation shows that the dielectrophoretic force increases as the square of the applied voltage and as the cube of particle radius. This indicates that it is desirable to use high voltages and macro- scopic particles. The only instance in which this force is not directed toward the axis of the separator is when the dielectric constant of the particle is less than that of the suspending fluid and the frequency dependent con- ductivity term is small in comparison with the dielectric constant depen- dent terms. This conclusion has been verified by observing that air bubbles move away from the region of maximum field strength for the electrode configuration and liquid dielectric used in this separator. The dependence of the dielectrophoretic force on surface polarization is of importance and is the subject of the second article of this series.

$52 TWELFTH NATIONAL CONFERENCE ON CLAYS AND CLAY MINERALS

E X P E R I M E N T A L R E S U L T S

Description of Clay Groups Studied

The dielectrophoretic behavior of clay minerals of the kaolinite (7A), talc-mica-montmorillonite (10A), and chlorite (14A) groups has been observed in the separator. A detailed discussion of the composition and structure of these layer lattices or clay groups is given by Warshaw and Roy (1959). Five A.P.I. Standard clays have been studied:

Kaolinite, Bath, South Carolina Haloysite, Eureka, Utah Montmorillonite, Upton, Wyoming Illite, Morris, Illinois Prochlorite, Chester, Vermont

Kaolinite (7A) Group

Montmorillonite (10A) Group

Chlorite (14A) Group

Didectrophoretic Behavior of Clays Studied

Several observations were made during this experimental work which show that certain clay minerals can be separated from other clay minerals. These observations may also help to explain the effects of the field on polar particles and to predict the possibilities for separation of other materials. They may be summarized as follows:

(1) At sufficiently low rates of rotation, some fractions of all clays migrate in the direction of increasing field strength. At points of maximum field strength some of these particles coalesce to form filaments. The larger filaments form complete bridges between adjacent electrodes.

(2) At sufficiently high rates of rotation, certain minerals collect only at the periphery of the separator (i.e. points of minimum field strength).

(3) A wide variation in the bridge-forming ability of different minerals was observed.

(4) Not all particles of a particular clay which migrate in the direction of increasing field strength participate in the tormation of bridges.

(5) Morphology and physical strength of the bridges are different for different clay minerals.

(6) Bridges do not carry excessive current (e.g. massive bridges of illite and/or montmoriUonite do not short-circuit the electrodes).

The observations listed above demonstrate clearly that separation of minerals by the combined action of centrifugal and dielectrophoretic forces is possible. Interpretation of these observations is complicated by the

PLATE 1.--Dielectrophoretic separator.

PLATE 2A.--Chlorite accumulation at periphery of separator.

/

PLATE 2B.--I l l i te bridges at points of maximum field strength.

PLATE 3A.--Halloysi te bridges at points of maximum field strength.

PLATE 3B.--Montmoril lonite bridges a t points of maximum field strength.

DIELECTROPHORETIC BEHAVIOR OF CLAY MINERALS 553

fact that grain-size has not been controlled, but the wide variation in behavior of the various minerals studied cannot be explained by grain-size difference alone. Plate 2A shows how essentially all of a pure chlorite sample tends to accumulate at the periphery of the separator. Rotation is 200 rpm and the voltage setting is 10 kV.

Plate 2B shows the accumulation of illite as bridges at the center of the separator under the same operating conditions.

Figure 1 shows X-ray patterns which indicate that partial separation of a micronized mixture of montmorillonite and kaolinite can be accom- plished by rotating the separator at 76 rpm while applying 8 kV to the

jI, Ik,

3.4 (A) 8.5(A) I'r,O(A) MONTMORILLONITE PEAKS 3.6(A) 7.2 (A) KAOUNtTE pZAKS

FIGURE 1.--X-ray analyses showing partial separation of montmorillomte from kaolinite.

electrodes. Patterns 1, 2, and 3 of Fig. 1 show the increases in montmoril- lonite from an area of minimum field strength (pattern 1), to an area of maximum field strength (pattern 3). This degree of separation was accom- plished with only one pass through the separator. A cleaner separation could be effected by recycling.

554 TWELFTH NATIONAL CONFERENCE ON CLAYS AND CLAY MINERALS

D I S C U S S I O N

Bridge Morphology and Strength

Some of the anomalies which have been observed during these experi- ments cannot be easily explained. It was not the object of this separator, for example, to form bridges of the more polar particles. This additional effect does, however, provide informat;on on the polarization mechanism. The differing morphology of the bridges formed by different clay groups is thought to be related to differing areas of charge displacement on the surfaces of the clay particles. Clays of the kaolinite group would be expected to have greater charge displacement at the broken ends of the layers where unbound oxygens occur, whereas clays of the montmorillonite group would have greater displacement on the surfaces of the layers. The halloysite bridges are made up of thin, hair-like fibres which may be constructed by connecting the particles at the broken end of the layers while the mont- morillonite bridges are broad bands which could be constructed by layer surface connection (see Plate 3). This manner of interconnection of the particles could also explain the fact that for bridges of similar size, mont- morillonite bridges are stronger than those of haUoysite.

Validity Test of Derived Equations

Calculation of the centrifugal force against which particles have been observed to move determines the lower limit of the dielectrophoretic force acting on the particles. It has been observed that 10 micron montmoril- lonite particles having a mks density contrast of 1.8 • 103 kg/m ~ are able to move from the point of sample introduction (r = 0.03 m) towards the axis of the separator when the rotation rate is 100 rpm and. 10 kV is applied to the electrodes. This observation indicates that under these conditions the dielectrophoretic force exceeds 2.5 • 10 -11 newtons. If equation (1) properly describes the behavior of montmorillonite particles in the separator it must provide a calculated force in excess of this value for the above conditions.

In order to evaluate equation (1), however, it is necessary to have representative values of the required electrical parameters. Unfortunately very little is known about the low frequency electrical properties of clays. H. Van Olphen and M. H. Waxman (1958) calculate from theoretical con- siderations that the average surface conductance for internal and external surfaces of sodium bentonite should be of the order of 3 • 10 -9 mhos. This value is approximately one hundred times smaller than the average observed surface conductivity of Pyrex glass in contact with 0.01 N KC1 (see Overbeek, 1952). Von Hipple (1954) gives values for the dielectric constant and dielectric-loss tangent for Canadian mica. From these measurements the bulk conductivity, at 100 cycles per second, normal

D I E L E C T R O P H O R E T I C B E H A V I O R O F C L A Y M I N E R A L S 5 5 5

to the mica sheets can be determined as 6 • l0 - n mhos/meter. The dielectric constant in a direction normal to the mica sheets determined from these measurements is approximately 7. The conductivity, gf, of the No. 1 white oil used for these experiments should be of the order of 5 • l0 -15 mhos/meter. The dielectric constant, el, of this oil has been determined to be approximately 2.

TABLE 1.--EXPECTED VALUES OF ]~LECTRICAL PARAMETERS

gp = 6 • 10 - 1 1 m h o s / m e t e r 3 • 1 0 - g m h o s

ep ~ 7 • e o f a r a d s / m e t e r g] = 5 X 10 - 1 ~ m h o s / m e t e r e/ = 2 • e o f a r a d s / m e t e r e e = e / f o r l o w p a r t i c l e c o n c e n t r a t i o n ge = gf f o r l o w p a r t i c l e c o n c e n t r a t i o n

1 e o = ~ • 10 - 9 f a r a d s / m e t e r ( d i e l e c t r i c c o n s t a n t o f f r ee s p a c e )

Table 1 lists the electrical parameters which will be used to describe the electrical behavior of a polarizable particle in the separator. Substitution of these values into equation (1) gives

1.3 • 10 -12 newtons

for a particle of radius 10 microns at the point of sample introduction (i.e. r = 0.03 m) when the applied voltage is 10 kV. The derived value for the dielectrophoretic force is thus too low by a factor of 19. This force deficiency can, however, be made up by assigning to the effective dielectric constant of the suspending fluid a value of around 40. I t could then be argued that this would be reasonable when the particle is surrounded by other polarizable particles. This is not, however, the only way in which the behavior of montmorillonite differs from that predicted by theory. When equation (1) is used with excepted values of the electrical parameters listed in Table 1, it predicts a dielectric relaxation time of the order of 10 -s seconds. Data to be presented in the second paper of this series indicate that the relaxation time for trioctahedral montmorillonite, however, is several orders of magnitude larger than this. The conclusion which must be drawn is that the linear electrical parameters, determined for clays at low field strength, cannot account for the forces observed to act on clay particles subjected to intense non-uniform electric fields. I t is not likely that the inclusion of particle anisotropy would modify the theory enough to remove the above mentioned short-comings (see Sillars, 1937 and O'Konski, 1960).

The major limitation of the theory as presented is that it disregards all space-charge effects. Mobile charges are assumed to accumulate only at the boundaries between the various phases. This major limitation is, however, not easily overcome, for when space-charge effects become dominant the law of superposition of potential is no longer applicable and

556 TWELFTH NATIONAL CONFERENCE ON CLAYS AND CLAY MINERALS

solut ions canno t be ob ta ined in close form (Macdonald, 1955). The im- po r t an t conclusion to be drawn from this s t u dy is, then, t ha t since the l inear electrical parameters are no t capable of expla in ing the observed force on the particle, this force mus t have its p r imary origin in an interac- t ion between the non-un i fo rm electric field and induced ionic space-charge which p resumab ly is created in the part icle 's in te r layer regions and in the loosely associated external surface layer.

A C K N O W L E D G M E N T

The writer wishes to express b.is t h a n k s to Mr. D. B. Speights who was a co-invest igator on the ini t ia l phases of this work and is responsible for the clay group descript ion included in this paper.

R E F E R E N C E S

Bairsto, G. E. (1920) On the variation with frequency of the conductivity and dielectric constant of dielectrics for high-frequency oscillations: Proc. Roy. Soc., v.96, pp.363- 382.

Kao, K. C. (1961) Some eleetromechanical effects on dielectrics: British Journal of Applied Physics, v.12, pp.629-632.

Macdonald, J. R. (1955) Note on theories of time-varying space-charge polarization: Journal of Chemical Physics, v.23, pp.2308-2309.

Mueller, Hans (1941) Electro-optical field mapping: Journal of the optical Society of America, v.31, pp.286--291.

O'Konski, C. T. (1960) Electric properties of macromolecules. V. Theory of ionic polarization in polyelectrolytes: Journal of Physical Chemistry, v.64, pp.605-619.

Overbeck, J. Th. G. (1962) Eleetrokinetie phenomena: Chapter 5 "Colloid Science" Kruyt, H. R., editor, New York, Elsevier Publishing Company, 236 pp.

Pohl, H. A. (1958) Some effects of non-uniform fields on dielectrics: Journal of Applied Physics, v.29, No. 8, pp. 1182-1188.

Pohl, H. A., (1960) Non-uniform field effects in poorly conducting media: Journal of the Electrochemical Society, v.107, pp.386-390.

Pohl, H. A., and Schwar, J. P. (1960) Particle separations by non-uniform electric, fields in liquid dielectrics, batch methods: Journal of the Electrochemical Society, v. 107, pp.383-385.

Pohl, H. A., and Plymale, C. E. (1960) Continuous separations of suspensions by non- uniform electric fields in liquid dielectrics: Journal of the Electrochemical Society, v . 107, pp.390-396.

Sillars, B. A. (1937) The properties of a dielectric containing semi-conducting particles of various shapes: J. Inst. Eler Engrs. (London), v.80, pp.378-394.

Van Olphen, H., and Waxman, M. H. (1958) Surface conductance of sodium bentonite in water: Clays and Clay Minerals, Nat. Acad. of Sci.--Natl. Research Council, publ. 566, pp. 61-80.

Von Hipple, A. R., editor (1954) Dielectric material and applications. New York, John Wiley, 313 pp.

Warshaw, C. M., and Roy, R. (1959) The classification and a scheme for identification of layer silicates: Contribution No. 59-40 (preliminary), College of Mineral Industries, The Penn. State Univ., Univ. Park, Pa., 72 pp.