EPA-600/2-76-133
May 1976
Environmental Protection Technology Series
EVALUATION OF MAGNETICS FOR
FINE PARTICLE CONTROL
Industrial Environmental Research Laboratory
Office of Research and Development
U.S. Environmental Protection Agency
Research Triangle Park, North Carolina 27711
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RESEARCH REPORTING SERIES
Research reports of the Office of Research and Development, U.S. Environmental
Protection Agency, have been grouped into five series. These five broad
categories were established to facilitate further development and application of
environmental technology. Elimination of traditional grouping was consciously
planned to foster technology transfer and a maximum interface in related fields.
The five series are:
1. Environmental Health Effects Research
2. Environmental Protection Technology
3. Ecological Research
4. Environmental Monitoring
5. Socioeconomic Environmental Studies
This report has been assigned to the ENVIRONMENTAL PROTECTION
TECHNOLOGY series. This series describes research performed to develop and
demonstrate instrumentation, equipment, and methodology to repair or prevent
environmental degradation from point and non-point sources of pollution. This
work provides the new or improved technology required for the control and
treatment of pollution sources to meet environmental quality standards.
EPA REVIEW NOTICE
This report has been reviewed by the U.S. Environmental
Protection Agency, and approved for publication. Approval
does not signify that the contents necessarily reflect the
views and policy of the Agency, nor does mention of trade
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recommendation for use.
This document is available to the public through the National Technical Informa-
tion Service. Springfield, Virginia 22161.
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EPA-600/2-76-133
May 1976
EVALUATION
OF MAGNETICS
FOR FINE PARTICLE CONTROL
by
K. P. Ananth and L.J. Shannon
Midwest Research Institute
425 Volker Boulevard
Kansas City, Missouri 64110
Contract No. 68-02-1324, Task 26
ROAPNo. 21ADL-029
Program Element No. 1AB012
EPA Task Officer: Dennis C. Drehmel
Industrial Environmental Research Laboratory
Office of Energy, Minerals , and Industry
Research Triangle Park, NC 27711
Prepared for
U.S. ENVIRONMENTAL PROTECTION AGENCY
Office of Research and Development
Washington, DC 20460
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CONTENTS
Page
List of Figures iv
List of Tables V
Acknowledgments VI
Summary 1
Sections
I Introduction 3
II Particle Agglomeration in Magnetic Fields. .... 4
Magnetic Fields and Charged Particles 4
Magnetic Fields and Uncharged Particles 11
III Magnetic Separation 16
Operation of a HGMS 16
Principle of a HGMS 18
Laboratory Study of HGMS 24
Comparison of Magnetic Force With Conventional
Mechanisms 24
IV Conclusions 26
References 27
iii
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FIGURES
No. Page
1 The Magnetic Force F Acting on a Charged Particle. ... 5
2 Arrangement to Generate a Strongly Nonhomogeneous
Magnetic Field H, Which Can Be Used to Separate
Ferromagnetic Particles 15
3 Schematic Representation of "-im Type High Gradient
Magnetic Separator 17
4 Cross Section of Spherical Particle, Radius r_ Attached
to Ferromagnetic Wire, Radius a, Magnetized by Uniform
Magnetic Field HQ 19
5 Log-Log Plot of Magnetic (Fm) and Competing Gravitational
(Fg) and Drag Forces (F^) Versus Particle Size rp for
Magnetized Wire Whose Size is Matched to That of Particle
to be Trapped. Computed for CuO Particle Attracted by
Ferromagnetic Wire of Radius 3r_ Magnetized by 10 KOe
Field Strength and Acted Upon by Slurry With Velocity
of 5 cm/sec 21
6 Log-Log Plot of Magnetization Versus Particle Radii. ... 23
IV
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TABLES
No. Page
1 Terminal Drift Velocity of Particles in a Magnetic Field . . 7
2 Maximum Particle Charge as a Function of Particle Size ... 10
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ACKNOWLEDGMENTS
The work presented in this report was performed by Midwest Research
Institute for the Industrial Environmental Research Laboratory-RTF of the
Environmental Protection Agency as Task Order No. 26 of Contract No. 68-
02-1324. The work was performed by Dr. K. P. Ananth, Senior Environmental
Engineer, and Dr. L. J. Shannon, Assistant Director, Physical Sciences
Division. The authors appreciate the helpful comments provided by
Dr. Dennis Drehmel, the EPA Project Officer, in conducting this study.
VI
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SUMMARY
Increasing concern for better control of fine participates has di-
rected efforts toward development of new systems that are inherently more
effective than conventional control devices or seeking systems that can
improve the effectiveness of existing conventional devices in capturing
fine particulates. This evaluation -.-as undertaken to determine the effect
of using magnetic forces to: (a) agglomerate fine particulates and assist
their capture in conventional control devices} and (b) directly capture
fine particulates in magnetic devices.
The use of magnetic fields to agglomerate either charged or uncharged
particles is of no apparent utility in industrial gas cleaning applica-
tions. In the case of charged particles, residence times clearly in excess
of those tolerable in industrial situations would be required to achieve
any appreciable agglomeration.
Dipole forces, created by external magnetic fields, can influence
the agglomeration of uncharged particles. However, significant enhance-
ment of agglomeration rates occurs only for ferromagnetic materials in
the presence of very strong magnetic fields. Since the particulate pol-
lutants emitted from most industrial sources are not ferromagnetic, the
use of dipole forces caused by magnetic fields is of no real significance.
Also, it is quite likely that, even with ferromagnetic materials, unac-
ceptable residence times would be required to effectively utilize dipole
forces.
Direct capture of fine particulates should be readily achievable,
in principle, in magnetic separators provided the particles are ferromag-
netic in nature and the magnetic force acting on them is greater than the
competing forces of gravitation and hydrodynamic drag. However, as noted
previously, many particulates of interest are only weakly ferromagnetic
or paramagnetic in nature and high magnetic field gradients are needed
to achieve particle capture. Another potential problem could involve pro-
ducing the high gradients and large magnetic forces over a surface area
large enough to trap practical numbers of particles.
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Currently, high magnetic field gradients have been generated by ap-
plying a relatively uniform background magnetic field to a ferromagnetic
structure (grids, screens, steel wool, etc.) and inducing magnetic poles
along properly oriented edges. A device using this approach is the high
gradient magnetic separator (HGMS) which has been successfully used in
mineral beneficiation and wastewater treatment. Despite the fact that
this device has not apparently been used for particulate collection in
industrial gas cleaning systems, it is attractive in principle, and war-
rants further investigation.
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SECTION I
INTRODUCTION
The search for new systems to enhance fine particulate control has
directed efforts toward evaluating the agglomeration of fine participates
by magnetic forces and toward evaluating magnetic devices for direct cap-
ture of fine particulates. If magnetic forces are effective in agglomerat-
ing fine particulates, then the agglomerates can be easily handled by
conventional control devices.
Agglomeration of particles is a complicated process. A variety of
forces causing the movement of particles toward one another may interact
to produce collisions and subsequent agglomeration. As the particles col-
lide and agglomerate, the concentration of each size changes with time,
each size interacts with every other size, and the agglomeration rates
change with size and concentration. Individual mechanisms of particle
agglomeration will be more effective under certain conditions and the
optimum utilization of one or more mechanisms will require well designed
systems.
Agglomeration of particles by magnetic forces is occasionally men-
tioned in the literature, but little effort has been devoted to either
theoretical or experimental studies of the process. Magnetic fields can
alter the motion of particles suspended in a gas stream depending upon
the magnetic permeability of the particles and magnetic fields might be
used to enhance the agglomeration of both charged and uncharged particles.
Particle agglomeration via the interaction of magnetic fields with charged
particles is reviewed in the next section. This discussion is followed
by discussions of magnetic fields and uncharged particles and magnetic
separation, primarily the high gradient magnetic separation technique
(HGMS). A brief conclusion is also presented.
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SECTION II
PARTICLE AGGLOMERATION IN MAGNETIC FIELDS
Mechanisms by which particles may agglomerate in magnetic fields
are reviewed in this section. The interactions of electrically charged
particles with no intrinsic magnetic properties are discussed first fol-
lowed by a review of the influence of permanent and induced dipole moments
on agglomeration.
MAGNETIC FIELDS AND CHARGED PARTICLES
When an electrically charged particle with no intrinsic magnetic
properties moves in a magnetic field transverse to the field lines» a
force is generated on the particle. The resulting force is termed the
Lorentz force. If a particle carrying N elementary charges "e" moves
with a velocity vpj the direction of the resulting force will be at
right angles to both the direction of the field and the direction of
motion of the particle (see Figure 1). As a result of the Lorentz force,
the particle will be diverted from its original path. The magnitude of
this force is found to be directly proportional to the velocity. If the
velocity v of the moving charged particle is not perpendicular to the
direction of the magnetic field, but makes an angle 0 with the field,
then the velocity vector vp may be resolved into two components: Vp
cos 0 in the direction of the field, and vp sin 0 perpendicular to the
field. In this general case, the force acting on the moving charged
particle is perpendicular to both the magnetic field and to vp sin 0,
and has a magnitude proportional to vp sin 0, as shown in Figure 1.
The potential for collisions and possible agglomeration of particles
can be qualitatively assessed by determining the terminal drift velocity
of a particle in a magnetic field. The terminal drift velocity can be
obtained by equating the Lorentz force to the resistance of the gas, cal-
culated from the Stokes-Cunningham Law:
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F = VHsin
Direction of
Magnetic Field
Moving
Charged
Figure 1. The magnetic force F acting on a charged particle,
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3TT|J,gdvt
NqvpH = (1)
where N = number of charges on particle
q — elementary charge
Vp = velocity of particle in field
H = magnetic field
•••'.-
Ug = gas viscosity
d = particle diameter
vt = terminal drift velocity of particle '-
C — Cunningham factor
The left-hand term in Eq. (1) represents the Lorentz force and the right
term the gas resistance. Eq. (1) can be rearranged to yield the following
expression for the terminal drift velocity:
NqvpHG
Vt = 3nn d '
Eq. (2) indicates that the terminal drift velocity varies directly with
the number of charges on a particle, the particle velocity, and the mag-
netic field and inversely with the particle diameter. Since the drift
velocity varies inversely with particle size, differentials in the drift
velocity will exist in a polydisperse aerosol flowing through a magnetic
field. The possibility of particle collision and agglomeration exists
and the rate of either process will be dependent upon the magnitude of
the differential in drift velocities and the residence time in the mag-
netic field as well as the fluid mechanics of a specific system.
Eq. (2) can be used to estimate values of the terminal drift velocity.
Table 1 presents values of vt calculated using typical values of the gas
velocity encountered in gas cleaning operation. Gas velocities of 5 and
25 m/sec (1,000 and 5,000 fpm) were selected. Magnetic fields of 5 x 10^,
and 10^ gauss were used (a field of 10^ gauss represents an efficient
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Table 1. TERMINAL DRIFT VELOCITY OF PARTICLES IN A MAGNETIC FIELD
d
(cm) ,
1 x 10'4 v
1 x lO'4
I x ID'5
vp H
n (m/sec) (gauss)
1 - 5.1 5 x 103
100
1,000
i 104
100
1,000
1 . 105
100
1,000
1 25.5 5 x 103
100
1,000
1 104
100
1,000
1 105
100
1,000
1 5.1 5 x 103
100
1,000
1 10*
100
1,000
1 105
100
1,000
vt
(cm/sec)
2.4 x lO'7
2.4 x 10'5
2.4 x 10-4
4.8 x ID'7
4.8 x ID'5
4.8 x 10~4
4.8 x 10-6
4.8 x 10"4
4.8 x 10~3
1.2 x ID'6
1.2 x 10'4
1.2 x lO'3
2.4 x ID'6
2.4 x 10-4
2.4 x lO'3
2.4 x 10-5
2.4 x KT3
2.4 x ID'2
2.4 x 10-6
2.4 x ID'4
2.4 x 10~3
4.8 x 10-6
4.8 x 10"4
4.8 x lO'3
4.8 x lO-5
4.8 x 10~3
4.8 x ID'2
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Table 1. (CONCLUDED)
d vp H ,
(rm> n (m/scc) (gauss)
v ' — - ' •
1 x 10-5 i 25.5 5 x 103
100
Jh* \f \S
1,000
1 104
L
100
^ ^/ x/ • 1^
1,000
i 105
1
100
1,000
vt
(cm/sec)
1.2 x 10"5
n
1.2 x KT3
1.2 x 10-2
2.4 x 10-5
2.4 x ID'3
2.4 x 10-2
2.4 x 10-4
2.4 x ID'2
i
2.4 x 10" L
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permanent magnet). Examination of Table 1 shows that the terminal drift
velocity is less than 0.01 cm/sec except at high particle velocities and
magnetic fields indicating that magnetic fields alone do not offer any
advantages as a mechanism for primary removal* of particles in a gas
stream.
Since the force acting on the charged particle and the resultant
drift velocity are dependent upon particle size, the relative motion of
different-sized particles might be sufficient to bring them close enough
to each other to undergo collision and agglomeration. A detailed calcu-
lation of the path of particles in a magnetic field and determination of
whether or not collision and agglomeration would occur requires knowledge
of the magnetic field distribution, gas flow pattern, and charge distri-
bution on the particles.
A qualitative assessment of the likelihood of collision and agglom-
eration can be obtained by considering the magnitude of the charge on
particles, the relative drift velocities between particles and the inter-
action of various particle collision mechanisms. At the outset it should
be noted that if all the particles have a unipolar charge, collision is
unlikely because particles of like charge will repel each other. If it
is possible to achieve bipolar charging, then collision and agglomeration
might occur. An estimate of the maximum number of charges that can reside
on a particle can be obtained from various theoretical equations describ-
ing particle charging..!/ If only field charging is considered, Eq. (3)
can be used:
N = 4TTe0rE0 (3)
max o p o
where eQ = permittivity of free space
rp = particle radius
Eo = applied electric field
Table 2 presents a tabulation of maximum particle charge as a function
of particle size for an electric field of 2 x 106 volts/m.
* i.e., a magnetic precipitator would not be feasible.
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Table 2. MAXIMUM PARTICLE CHARGE AS A
FUNCTION OF PARTICLE SIZE
(E = 2 x 106 volts/m)
Particle size
d Maximum number
(micrometer) of charges,
100 3.5 x 106
10 3.5 x 104
1 350
0.1 3.5
Inspection of Table 1 indicates that for the charge levels shown
in Table 2, relative terminal drift velocities for fine particles of the
order of 10~" to 10~4 cm/sec result for reasonable values of Vp and H.
Impaction mechanisms are precluded at such low relative velocities, but
interception and diffusion processes might result in collision and ag-
glomeration. Single particle target efficiencies of 0.2 to 0.5 for the
combined effects of interception and diffusion appear likely and some
collision and agglomeration may occur if the particles have sufficient
residence time in the magnetic field to permit the particles to traverse
the average distance between particles (i.e., the interparticle spacing
distance).
The minimum residence time required for the particles to traverse
the average distance between particles will depend upon the grain loading
of the aerosol. For aerosols with a grain loading ranging between 2.29
and 22.^9 g/m^ (i.e., 1 and 10 grains/scf), interparticle spacing dis-
tances would be of the order of 10"^ cm. If the relative drift velocities
are in the range of 10"4 to 10~6 cm/sec, residence times of 10 to 1,000
sec would be required for particles to traverse a distance of 10" cm.
A residence time of 10 sec might be feasible in an industrial applica-
tion, but 1,000 sec is clearly not. A residence time of 10 sec would re-
quire an effective magnetic field ranging from 45.72 to 228.6 m (i.e.,
150 to 750 ft) in length (depending on gas velocity). Thus, the physical
size of equipment required to treat gas streams of 2,831.7 m3/min (i.e.,
100,000 cfm) even if cylindrical, spherical, or U-shaped systems were
designed, appears to be prohibitive.
10
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Even if agglomeration could be achieved with acceptable equipment
configurations, the use of magnetic fields to agglomerate charged parti-
cles appears to have dubious value. A particle charging system of some
sophistication would be required* as a component of the system as well
as a particle collection device downstream of the magnetic agglomeration
chamber. Since the particles will probably have residual charge, an elec-
trostatic precipitator would seem to be a logical choice for the collec-
tion device. If such is the case, it is difficult to justify placing a
magnetic agglomeration system between what would basically be an electro-
static precipitator system.
The preceding qualitative analysis results in the conclusion that
the use of magnetic fields to agglomerate charged particles is not attrac-
tive. Electrostatic agglomeration and subsequent collection in an ESP or
direct collection in an ESP are more viable approaches.
MAGNETIC FIELDS AND UNCHARGED PARTICLES
Dipole forces, resulting from permanent dipole moments of aerosol
particles or dipole moments generated in a magnetic field, may be suffi-
cient to alter particle motion and cause collision and agglomeration.
Very little attention has been directed to the utilization of dipole
forces resulting from magnetic properties and fields for the agglomera-
tion of particles. The limited work on the subject is reviewed in the
next section.
Agglomeration of Particles with Permanent Dipole Moments
Aggregates of particles may form in the absence of an external mag-
netic field if the particles have permanent dipole moments. On approach-
ing one another the dipoles orient themselves so that opposite dipoles
attract. Theoretical studies of the coagulation of magnetic spherical
dipoles have been conducted by Zebel.— Zebel employed a statistical
mechanics approach in order to devise equations for the rate of coagula-
tion of weak and strong dipoles. For weak dipoles, Zebel derived the fol-
lowing expression for the coagulation rate.
f = 1 , v (4)
1 - 0.0952/
Bipolar charging would be necessary to avoid particle repulsion which
would occur between unipolar charged particles.
11
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where f = coagulation rate with dipoles
coagulation rate in absence of dipoles
r = r + r = sum of radii of particles 1 and 2
p — dipole moment of particle 1
p = dipole moment of particle 2
k = Boltzmann constant
T = absolute temperature
The term r« represents the distance of two dipoles, with moments = pj
and PT , at which their potential energy is equal to the thermal energy,
kT . Thus, r^f/ri2 -^ 1 means that at the contact distance of the parti-
cles their potential energy is large compared to the thermal energy; this
is a stro;- interaction. Alternatively, rM/r!2 <<: ^ represents a very
weak interaction. If r^ « rio » Eq. (4) simplifies to:
rr
M
For the case of strong dipole interaction, the coagulation rate is
given by Eq. (6):
M
f = ~
12
Eq. (6) is generally applicable when r^ > 1.4
Evidence of the experimental verification of either Eqs. (4), (5),
or (6) was not found in the literature. References 3 and 4 discuss the
coagulation of particles which are permanent magnets. Iron and nickel
smokes, having particles which are permanent magnets, were generated by
heating the vapors of the carbonyls in the absence of oxygen. Below the
Curie point (360° C for nickel and 770 C for iron) the smokes coagulate
12
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to thread-like aggregates but above it rounded aggregates or very short
chains are formed. Thread aggregates orient themselves in a magnetic field
and turn through 180 degrees when the field is reversed, thus showing that
the orientation is not dje to induction but due to permanent magnetism of
the particles. How the magnetism originates is not known. Smokes of iron
oxide crystals have similar properties but particles of amorphous iron ox-
ide are not ferromagnetic and do not give linear aggregates without an ex-
ternal field.
It should be noted that very few particles possess permanent magnetic
properties (only iron, nickel, and cobalt and some alloys are ferromag-
netic). Because only a few particles have permanent magnetic properties,
limited opportunities will exist to exploit particle agglomeration by this
technique. Agglomeration by this mechanism will occur naturally whenever
it is possible and efforts to enhance the process appear to have little
merit.
Dipole Moments Generated in a Magnetic Field
Dipole moments can be generated when uncharged particles are placed
in a magnetic field. If a homogeneous magnetic field of strength H is
used, it will polarize spherical aerosol particles with radius rp to
dipoles having moments:
•* f\
(7)
where u = magnetic permeability of particle.
By making simplifying assumptions regarding particle motion and assum-
ing that V1 is equal to unity (i.e., ferromagnetic material), Fuchs^-'
u + 2
has shown that a distinct increase occurs in the factor f (see Eq. (4))
when rM/rJ -> 20.
Under the above conditions (i.e., strong attraction), the ratio of
the rate of coagulation with and without dipole forces is:
23
(8)
From the preceding it is readily apparent that acceleration of agglomera-
tion of fine particles by induced magnetic dipoles will only occur in
strong fields aid for ferromagnetic materials.
13
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If a nonhomogeneous magnetic field (Figure 2) is used, a force will
act on the particles which might result in deflection of particles with
resultant collision and agglomeration. For this case, Zebel presents the
following equation for the
K = a/2 grad H (9)
n. _ i ^
where a =
u- = magnetic permeability of particle
r = particle radius
P
Detailed calculations of the path of particles and determination of
whether or not they will collide and agglomerate require a knowledge of
the magnetic field distribution, particle size distribution, particle
grain loadings, magnetic permeabilities of particles, and the gas flow
pattern. As was noted previously, only few materials possess high mag-
netic permeabilities and it is likely that only ferromagnetic materials
would show any significant agglomeration.
14
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Figure 2. Arrangement to generate a strongly
nonhomogeneous magnetic field H, which can
be used to separate ferromagnetic particles
(N and S designate the north and south pole,
respectively).
15
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SECTION III
MAGNETIC SEPARATION
Magnetic separation is an old technique whose use has been restricted
in the past to the separation of strongly magnetic materials like iron and
magnetite. A good example of this type of separator is the conventional
drum-type magnetic separator.—' Currently, magnetic separation of weakly
magnetic materials (weak paramagnetic materials) is made possible with
the aid of high magnetic field gradients. One such device is the "high
gradient magnetic separator" (HGMS). Since particulates emitted from most
industrial sources are not ferromagnetic, this discussion will be limited
to the more widely applicable HGMS.
The literature contains several articles on HGMS and its operating
principle and potential application areas. The application areas
discussed include mineral beneficiation (of taconite iron ores), coal
desulfurization, and wastewater treatment. Unfortunately, none of the
papers discuss the potential use of HGMS in controlling particulates in
industrial gas streams. However, the principle and operation of the de-
vice are independent of the application area. A brief discussion of the
HGMS concept is presented next.
OPERATION OF A HGMS
7,9/
Figure 3 is a schematic of a Kolm type HGMS."""^ A magnet designed
to produce a strong variable field in the canister volume is used. This
volume is packed with a matrix of filamentary, ferromagnetic material.
These filaments produce large surface areas of high magnetic gradients
along their edges and they resist compression and clumping in the applied
magnetic field. The filaments are chosen to match the size of the parti-
cles to be removed in order to optimize the magnetic forces. The feed ,
in a fluid (usually water) slurry, is passed down through the canister.""
The fluid and nonmagnetic particles pass easily through the relatively
open structure of the matrix. The trapped magnetic particles are easily
washed out when the applied field is reduced to zero.-^
16
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Magnet
Field
Stainless
Steel Wool
Matrix
Filtered^.
Liquid Out
Figure 3« Schematic representation of Kolm type high gradient
magnetic separator.£jZ'
17
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PRINCIPLE OF A HGMS
The principle of operation of a HGMS is reviewed in detail in Ref-
erence 7. A brief summary of the review highlighting those aspects which
are critical to particulate collection in industrial gas streams is pre-
sented in the following paragraphs.
A simple expression for the magnetic force acting on a particle is
(in the x component):
Fx = / (Mp - Mm) 5it • dV
J dx
In Eq. (10), M^ and M are the magnetization of the particle and medium,
respectively; V is the volume of the particle and dH/dx the magnetic
field gradient. According to Eq, (10), for a specific particle size (volume)
any increase in magnetization or the field gradient should improve the mag-
netic force. The magnetization (M) itself is a function of the susceptibil-
ity* and the magnetic field strength as shown in Eq. (11).
M = xB (11)
Substitution of Eq. (11) in Eq. (10) and integration yields;
Fx = (v - v ) . .
x P ™ dx
where b is a point within the particle having volume V (see Figure 4).
In Eq. (12), Yp, Xjn an<^ V are intrinsic properties. The only
parameters that can be varied to enhance FX are the magnetic field
strength B(b) and the field gradient dH(b)/dx. Even with a constant
field strength, the magnetic force could be improved by altering the
field gradient. For an applied field HQ less than bulk satuation field
H , along the axis, the magnetic field gradient at the particle is given;—
JU Q 2
v^n _ _ Ou o
dx ° b3
Therefore, there is a certain filament radius (a) which will maximize the
magnetic force on a given particle. Oberteuffer-i' reports that this
Magnetic susceptibility, y = u/uo - 1 where "u" is the magnetic per-
meability of the material (particle) and "uo" is the magnetic permea-
bility of vacuum.
18
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FERROMAGNETIC
WIRE
Figure 4. Cross section of spherical particle,, radius r
^
attached to ferromagnetic wire, radius a, magnetized
by uniform magnetic field H .
19
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radius should be of the same order of magnitude as the particle radius and,
to be more precise, is three times the particle radius. One apparent dis-
advantage is the fact that a high efficiency of magnetic separation can be
achieved only for certain sized particles in a polydispersed gas stream.
It is also to be noted that if the magnetic force falls off signifi-
cantly over a distance of one particle diameter, each trapping site in
the separator will have a capacity of approximately one particle. There-
fore to increase the number of sites in separators it is desirable to use
fibers or wires since the number of trapping sites is approximately the
same as the volume of the fibers.!/
Another consideration in the HGMS is the need to determine the range
of particle radius over which the magnetic force is greater than the com-
peting forces of gravity and hydrodynamic drag. The gravitational force
is given by:
Fg = ^ Tir^ (pp - pf) g (13)
where r is the particle radius, p and pr the densities of the par-
ticle and medium, respectively, and g the local accleration of gravity.
Since F is a function of r^ the gravitational force will be signi-
ficant for large particles. The hydrodynamic drag force given by:
Fd = 6T77]rpv (14)
is significant for small particles. In Eq. (14), T] is the viscosity of
the medium and v is the velocity of the particle relative to the stream.
Figure 5 shows the magnetic and competing forces versus particle size for
a gradient-matched separator where the filament size is matched with the
size of the particle to be trapped.—'
It can be observed from Figure 5 that the magnetic force (F ) exceeds
the hydrodynamic drag force (Fd) in a water slurry, only for particles of
size greater than 1 to 10 urn. This result means that for CuO particles
< 1 urn in size the hydrodynamic drag force predominates in a water slurry
system and capture by magnetic forces is ruled out. We have extended this
analysis to an air system to determine the applicability of magnetics in
gas cleaning which is of greater concern to us. The Fd lines for air at
two different velocities have been calculated and included in Figure 5.
It is interesting to note that in an air system the magnetic force (Fm)
dominates the drag force (Fd) even for particles < 0.1 urn at a velocity
of 5 cm/sec. Higher particle velocities shift the effectiveness of the
magnetic force toward larger particles. For an air velocity of 50 cm/
sec, which is close to the typical value for an industrial gas system,
the magnetic force is effective down to about 0.3 urn.
20
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X
-o
1Q
,-2
10'
,-4
10
,-6
10'
r8
10'
rlO
Fd (Water) @ Velocity of 5 cm/sec
Fd (Air) @ Velocity of 50 cm/sec
Fd (Air) @ Velocity of 5 cm/sec
J_
10
- O.lcm .
_ I I
10
0.01 0.1 1 100 1 100
PARTICLE RADIUS, rp (/Am )
Figure 5. Log-log plot of magnetic (Fm) and competing gravitational
(Fg) and drag forces (F^) versus particle size r_ for magnetized
wire whose size is matched to that of particle to be trapped. Com-
puted for CuO particle attracted by ferromagnetic wire of radius
3r magnetized by 10 KOe field strength and acted upon by slurry
with velocity of 5 cm/sec.2/
21
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If, instead of CuO particles, one uses Fe30^ which has a much higher
magnetic permeability, then the line depicting magnetic force (Fm) in
Figure 5 would move to the left. This shift is shown by the dashed line.
The shift in the line caused by particles of high magnetic permeability
would thus enable the capture of particles well into the submicron region.
For a given range of magnetization, the range of particles separable
in a HGMS and in an "ideal" separator have also been reported for a water
slurry system.^' The "ideal" separator has a wire (filament) radius ap-
proximating the particle size (i.e., it is gradient-matched) and is as-
sumed to operate at a magnetic field intensity of 10 KOe* and a water
slurry velocity of 5 cm/sec. The lower and upper limits of particles that
can be captured in such a device have been calculated by equating the com-
peting forces, i.e., magnetic force (Fm) with the drag force (F
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103
102
10
c
at
o
c
s
8
o>
10
-1
10
-2
0.01 0.10 1.0
Particle Radius,fp (fj.m )
Figure 6. Log-log plot of magnetization versus particle radii
10
23
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* Gas stream flow rates/velocities.
* Gas stream viscosity and density.
* Selection of appropriate wire diameter.
* Selection and generation of required field intensities.
LABORATORY STUDY OF HGMS
Unfortunately, there is no direct experience available in the use of
such a system for particulate control in industrial gas streams. Appar-
ently, the only study that is being conducted to determine the use of a
HGMS in scrubbing fine particles in gas streams is a laboratory study at
Research Triangle Institute (RTI) under EPA sponsorship. The gas stream
under test at RTI is dust from the basic oxygen furnace in prefiltered
room air. Efficiencies of the order of 90% for particulates > 0.3 um have
been observed with an optical particle counter in the preliminary experi-
ments — and no other data are available.
COMPARISON OF MAGNETIC FORCE WITH CONVENTIONAL MECHANISMS
It is interesting to compare particulate capture due to conventional
mechanisms such as impaction, interception, and diffusion with that due to
magnetic forces. Let us take a case where there is no magnetic force acting
on the particle and the wires act as targets for interception, inertial im-
paction, and diffusion of the particles at a gas velocity of 5 cm/sec. Let
us also assume that the wire diameter is three times the particle diameter
in order to conform to a gradient -matched magnetic separator.
The interception efficiency is given by:—'
T( = 1 + R - - 1 - (15)
(1 + R)
where R = ratio of particle diameter to that of the intercepting body
(in our case, R = 0.33). The inertial impaction parameter is given by:—'
2
. C pp v Dp
- ~ (16)
18ugDb104
where C = empirical correction factor for resistance of gas to move-
ment of small particles
= 1 (assumed for 2 um particle)
24
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p = particle density in g/cc = 2.8 g/cc (equivalent to iron
foundry particles)
v = velocity of particle in cm/ sec
= velocity of gas, = 5 cm/ sec
D = particle s i . ;-c in urn
Ug = viscosity of gas in poises =-- 1.8 x 10 poises (for air)
D, = diameter of wire in um
= 6 um
The diffusional efficiency is;-!-t'
1], = H_ (1 + 0.55 Re1/2 Sc1/3) (17)
Pe n
n v
where Pe = peclet number = Re Sc - —I —
£
I) vp
Re1 = Reynolds number = — ^ -
I' g
Sc - Schmidt number = li&
p^
In Eq . (17), I) is the particle diameter (2 urn) , v is the particle ve-
locity (5 cm/sec), P the density of the particle (2.8 g/cc), ug - vis-
cosity of air and S the diffusivity of t he particle (assumed to he 2.7 x
For the' conditions indicated above the intercept' ion efficiency is cal-
culated lo be r)87. and the impact ion parameter is 0.','9. At the latter value
the impact ion efficiency is x.ero. — -' The diffusional efficiency is • 17,.—'
If, under the above conditions, magnetic forces can help capture all of the
2-\nn particles, I hen systems ntili/.ing mai'.netic forces could be rated as
being superior to those de-vices which only make use of the conventional
mechanisms. Inspection of Figure 5 shows that at a gas (air) velocity of
5 cm/sec the magnetic loree is effective up to about 0.05 |im. One obvious
route- to enirincr the performance- ol a magnetic separator is to use magnetic
seeding e> I the- gas stream to he- scrubbed with high penm-ab i 1 i t v p.irticu-
late-s. However, on an industrial basis this approach does not appear via-
ble-.
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SECTION IV
CONCLUSIONS
The use of magnetic fields to agglomerate either charged or uncharged
particles is of no apparent utility in industrial gas. cleaning applica-
tions. In the case of charged particles, residence times clearly in excess
of those tolerable in industrial situations would be required to achieve
any appreciable agglomeration. Only ferromagnetic materials could be ag-
glomerated by dipole forces and the paucity of ferromagnetic materials
in most industrial emissions makes that approach nearly useless. Also,
it is quite likely that unacceptable residence times would be required
to effectively utilize dipole forces.
Direct magnetic separation on the other hand appears to be of poten-
tial value for fine particulate control in industrial gas streams. However,
it has never been used for this application and only a laboratory scale
study is currently under way to test the HGMS for particulate control in
specific gas streams (i.e., streams containing high permeability particles)
A detailed review of the results of the present -study including aspects
such as fractional efficiency versus magnetic permeability of the particle,
particulate entrainment, wire (filament) diameter, and pressure drop would
be critical prior to further investigation. Also, the economic aspects of
the device, particularly the energy required to generate large magnetic
gradients in field size units, should be evaluated in the absence of super-
conducting magnets. Thus, it can only be said that the HGMS is attractive,
in principle, for particulate control in gas streams and warrants further
investigation to assess its actual utility.
26
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REFERENCES
10 Robinson, M., Electrostatic Precipitation, published in Air Pollution
Control, Part I, Wiley-Interscience (1971).
2. Zebel, G., Staub. 19(11), 381 (1959).
3. Berscher, D., and A. Winkel, A. Phys. Chem., H6A, 1 (1936).
4. Berscher, D., and A. Winkel, Naturwiss., 2J5, 420 (1937).
5. Fuchs, N. A., The Mechanics of Aerosols, Pergamon Press, New York
(1964).
6. Zebel, G., Staub (English Iraslation), 28(7), 1 (1968).
7. Oberteuffer, J. A., IEEE Transactions on Magnetics, MAG;40(2), 223,
June 1974.
8. Kolm, H., J. Oberteuffer, and D. Kelland, Scientific American, p.
47, November 1975.
9. Oberteuffer, J. A., IEEE Transactions on Magnetics. MAG-9_(3), 303,
September 1973.
10. Kelland, D. R., IEEE Transactions on Magnetics, MAG-9(3), 3079
September 1973.
11. Trindade, S. C., and H. H. Kolm, IEEE Transactions on Magnetics,
MAG-9(3), 310, September 1973.
12. De Latour, C., IEEE Transactions on Magnetics, MAg-9^3), 314, September
1973.
13. Private communication with Mr. Charles Gooding, Research Triangle
Institute, North Carolina, February 9, 1976.
14. Strauss, W., Industrial Gas Cleaning. Pergamon Press, New York (1966).
27
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1. REPORT NO.
TECHNICAL REPORT DATA
(Please read Inunctions on the reverse before completing)
EPA-600/2-76-133
I. TITLE AND SUBTITLE
2.
3. RECIPIENT'S ACCESSION-NO.
Evaluation of Magnetics for Fine Particle Control
5. REPORT DATE
May 1976
6. PERFORMING ORGANIZATION CODE
7. AUTHOR(S)
8. PERFORMING ORGANIZATION REPORT NO.
K. P. Ananth and L. J. Shannon
9. PERFORMING ORGANIZATION NAME AND ADDRESS
Midwest Research Institute
425 Volker Boulevard
Kansas City, Missouri 64110
1O. PROGRAM ELEMENT NO.
1AB012; ROAP 21ADL-029
11. CONTRACT/GRANT NO.
68-02-1324, Task 26
12. SPONSORING AGENCY NAME AND ADDRESS
EPA, Office of Research and Development
Industrial Environmental Research Laboratory
Research Triangle Park, NC 27711
13. TYPE OF REPORT AND PERIOD COVERED
Task Final; 2/75-2/76
14. SPONSORING AGENCY CODE
EPA-ORD
15. SUPPLEMENTARY NOTES Task officer for this report is D. C. Drehmel, Mail Drop 61,
Ext 2925.
16. ABSTRACT
The report gives results of an evaluation of the effectiveness of magnetic
agglomeration/separation techniques in enhancing fine particulate capture. Whereas
residence times and magnetic fields required to achieve agglomeration via
magnetic forces appear to be unrealistic, magnetic separation--especially high
gradient magnetic separation--appears to be attractive in principle.
7.
KEY WORDS AND DOCUMENT ANALYSIS
DESCRIPTORS
b.lDENTIFIERS/OPEN ENDED TERMS
c. COSATI Field/Group
Air Pollution
Magnetic Properties
Agglomeration
Separation
Dust
Air Pollution Control
Stationary Sources
Fine Particulate
13B
20C
11G
3. DISTRIBUTION STATEMENT
Unlimited
19. SECURITY CLASS {ThisReport)
Unclassified
21. NO. OF PAGES
34
20. SECURITY CLASS (Thispage)
Unclassified
22. PRICE
EPA Form 2220-1 (9-73)
28
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