U.S. Environmental Protection Agency Industrial Environmental Research EPA~600/7~78~01 3
Office of Research and Development Laboratory . An^Q
Research Triangle Park, North Carolina 27711 rQoTU3T}f 1978
CHARACTERIZATION
AND GENERATION
OF METAL AEROSOLS
Interagency
Energy-Environment
Research and Development
Program Report
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RESEARCH REPORTING SERIES
Research reports of the Office of Research and Development, U.S. Environmental
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RESEARCH AND DEVELOPMENT series. Reports in this series result from the
effort funded under the 17-agency Federal Energy/Environment Research and
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health and welfare from adverse effects of pollutants associated with energy sys-
tems. The goal of the Program is to assure the rapid development of domestic
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essary environmental data and control technology. Investigations include analy-
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EPA-600/7-78-013
February 1978
CHARACTERIZATION
AND GENERATION
OF METAL AEROSOLS
by
Neil Zimmerman, Dennis C. Drehmel, and James H. Abbott
Environmental Protection Agency
Office of Research and Development
Industrial Environmental Research Laboratory
Research Triangle Park, North Carolina 27711
Program Element No. EHE623
Prepared for
U.S. ENVIRONMENTAL PROTECTION AGENCY
Office of Research and Development
Washington, D.C. 20460
-------
Abstract
The techniques of metal aerosol generation are reviewed in this
report to establish the state-of-the-art of the technology and to guide
future researchers. Exposure to metal or metallic compound submicron
aerosols is widespread in both the industrial and general environments.
Research in areas of health effects, sampling instrumentation, and air
pollution control technology requires a reliable source of test aerosol.
Many mechanisms for the generation of metal aerosols are presented and
their applications, advantages, and disadvantages discussed. Generation
methods can be on a continuous or a batch basis, with high or low concentrations
and generation rates, and with monodisperse or polydisperse size distributions.
The method chosen and the resulting aerosol depend on the requirements
of the specific research being conducted. Metal aerosols generated by
exploding wires, electric arcs, heating to evaporation, combustion, and
dispersion are presented, with particular attention paid to particle
size characteristics.
n
-------
Table of Contents
Section Page
Abstract ii
List of Tables iv
Conversions v
Terms and Abbreviations v
Acknowledgements vi
1 Introduction 1
1.1 Metal Aerosols from the Iron and Steel Industry 2
1.2 Metal Aerosols from the Nonferrous Metals Industry 4
13. Metal Aerosols Characterized from Occupational Studies 6
1.4 Characteristics of Generated Metal Aerosols 7
2 Exploding-Wire Generators 9
3 Electric Arcs 13
3.1 Low Intensity Arcs 15
3.2 High Intensity Arcs 18
4 Heating to Evaporation 21
4.1 Metal Oxides 23
4.2 Metal Hal ides 24
4.3 Metallic Aerosols in Argon 25
5 Combustion 25
6 Dispersion 28
6.1 Using Liquid Droplets—General 28
6.2 Using Liquid Droplets—Nebulizers 30
7 Summary 31
8 References 35
-------
Tables
Number Page
1 Summary of Particle Size Data for the 3
Iron and Steel Industry
2 Summary of Particle Size Data for the 5
Nonferrous Metals Industry
3 Summary of Particle Size Data from 14
Exploding-Wire Generators
4 Summary of Particle Size Data for 29
Generators Using Combustion
iv
-------
Conversions
1 1
1 Ib
1 in.
1 ton
1 m /min
1 9
1 gr/ft3
1 ft3/min
= ICf4 ym
= 454.5 g
= 2.54 cm
= 907.2 kg
= 35.515 ft3/min
= 15.43 gr
= 2.29 g/m3
= 28.32 1/min
Terms and Abbreviations
cL = aerodynamic diameter
a
d . = mass median diameter
PSD = particle size distribution
EM = electron microscope
K = coagulation coefficient
SEM = scanning electron microscope
TEM = transmission electron microscope
o
n = concentration, particles/cm
a = geometric standard deviation
3
CMD = count median diameter
MAD = median aerodynamic diameter
-------
Acknowledgments
Discussions with Professor Benjamin Linsky of West Virginia University
and Dr. Parker Reist of the University of North Carolina at Chapel Hill
were very helpful and informative.
VI
-------
1 INTRODUCTION
An aerosol is basica'Py a suspension of solid particles and liquid
drops in a gaseous medium. They are throughout our environment, existing
naturally and as a by-product of man's activities. Particles with
diameters less than 10 mm are generally classified as aerosols. This
range of particle size is important in terms of ambient air quality,
health hazards, and industrial pollution control. In particular, fine
submicron particles, those less than 1 ym, are of most concern with
respect to visible light scattering, atmospheric dispersion, deep penetration
an'! retention in lungs, and decrease in conventional removal mechanism
efficiencies.
Aerosols can be composed of a multitude of substances depending on
t.ia materials involved in their generation. This report will be limited
to a discussion of metal aerosols. The ferrous and nonferrous metallurgical
refining, processing, and manufacturing industries make up one of the
largest as yet uncontrolled sources of anthropogenic air pollution,
second only to power plants. Because of such large amounts of metallic
compound emissions, it is important to delineate the specific characteristics
of these aerosols. Continued study of the physical, chemical, and
biological properties of these aerosols will lead to a better understanding
of their health effects as well as capabilities of controlling and
reducing their emission.
A brief review of some metal particulate emission data will help to
put the significance of the problem into perspective. Various sources
quote wide ranges of emissions for various metallurgical facilities.
Estimates of uncontrolled particulate emissions include: 8-51 Ib/ton
steel produced1"4, 20-60 Ib/ton Cu4, 50-90 Ib/ton Zn4, 12-75 Ib/ton Pb4,
4
and 81-98 Ib/ton Al . Comparisons with controlled emissions indicate
that 99.5% control is possible for Cu, Fe, Pb, and Zn, giving emissions
4
of approximately 0.1 - 0.5 Ib/ton . Controlled Al emissions are somewhat
higher, in the range of 1.6 - 19.6 Ib/ton . These five metals comprise
more than 95% of total U. S. metal consumption according to a 1976 EPA
report . Small amounts of Mn, Ti02, Ni, Zr, Sn, Sb, and Mo are
1
-------
also used. Even if emissions of many of the five major metallurgical
industries are eventually controlled, pollution problems could still
exist because of the effect of the particle size distribution on control
efficiency.
Although there are references in the literature to metal aerosol
particle size distributions, they vary widely, due partly to the nature
of the aerosol. Fumes are a type of aerosol which result from oxidation
reactions, sublimation, or condensation, mechanisms associated with the
application of heat. Most metallurgical processes involve heating and
melting at some stage, generating large amounts of fumes. The mechanisms
that generate these fumes generally operate at the molecular level,
resulting in an aerosol of largely submicron particles. These aerosols
are usually polydisperse, having a wide range of diameters.
1.1 Metal Aerosols from the Iron and Steel Industry
2
In a review of the ferrous foundry industry, Bates and Scheel
found that the particle size distributions varied from 50% weight < 1 ym
to 5% weight <2 ym. The composition also varied considerably, with the
iron content shifting from as much as 50% to only 5% depending on the
impurities present and on the stage of the process. Other components
besides iron oxides (Fe^O^ and Fe-jO^) in foundry emissions are Si, Ca,
Al, Mg, Zn, and Mn. Electric arc emissions, however, are mainly composed
of Fe and Zn oxides and volatile matter. Bates indicates that other
sources such as Holland et al. and Coulter have found electric arc
emissions to be 90-95% < 0.5 ym. Another study of electric arc emissions3
states that particle size measurements by Erickson indicate 70% weight
<5 ym, while electron photomicrographs by Allan et al. found 95% by
number of the particles < 0.5 ym in diameter.
Several studies have determined the particle size distributions
from iron cupolas. Two publications by Calvert6'7 give results for gray
iron and ductile iron cupolas: the mass median diameters range from
0.25 to 1.9 ym. For the Calvert data a more exact breakdown of the
results is given in Table 1 which summaries data on metal aerosols from
the iron and steel industry. In another study on a gray iron source,
-------
Table 1. SUMMARY OF PARTICLE SIZE DATA FOR THE IRON AND ?TEEL INDUSTRY
Source
Foundry
Cupola
Gray Iron
Gray Iron
Ductile Iron
Electric Arc
Ferroalloy
Ferroalloy
Author
Bates2
Calvert4
Calvert5
Cooper
Calvert5
Bates2
McCain7
Drehmel8
Percent
by Weight
< 1 ym
50
40 - 60
40 - 80
80
60 - 100
95
20
15
Mass Median
Di ameter
ym
-
0.92 - 1.15
0.54 - 1.9
0.5
0.25 - 0.84
-
4.3
1.7
Deviation
-
1.7 - 2.1
1.5 - 1.8
2.4
1.5 - 2.0
_
-
1.9
-------
Cooper8 f',und a mass median diameter of 0.5 ym. For ferroalloy furnaces,
McCain determined a mass median diameter of 4.3 ym and Drehmel10, one
of 1.9 ym. It should be noted from Table 1 that for most of the iron
and steel sources tested submicron sized particles ranged from 40 to 100
weight percent. The exception appears to be ferroalloy furnaces where
the percentage is slightly lower (15-20 weight percent). For all
sources the mass median diameter ranges from 0.25 to 4.3 ym and the
geometric deviation from 1.5 to 2.4. If it is assumed that particles of
3 ym in diameter and smaller are respirable, then it may be concluded
that metal aerosols from iron and steel operations are concentrated in a
fairly narrow size range most of which is respirable.
1.2 Metal Aerosols from the Nonferrous Metals Industry
Estimates of aluminum emissions are 40% by weight < 1 ym and 60%
by weight < 5 ym in diameter. Measurements by Hanna and Pilat at a
horizontal aluminum reduction cell show 30% by weight <1 ym, 16% by
weight < 0.2 ym, with a mass median diameter (dj) of 5.5 ym and a
geometric standard deviation (a ) of 25. They felt that coagulation and
diffusion to the exhaust duct wall was a factor in reducing the number
of particles ,< 0.5 ym and shifting the particle size distribution up.
12
Harris and Drehmel cite information by Vandegrift showing that,
on the average, at least 30% of various metal particulate emissions will
be less than 10 ym in diameter. Furthermore, Harris gives the PSD of
the emissions from various nonferrous industrial process units. For a
lead sintering machine the d^ was 0.89 ;yra. and the a was 4.4. For a
zinc sintering machine, the d was nearly the same (0.82 ym) and the
distribution more nearly monodispersed (indicated by a a of 2.1). At
the same plant the zinc roaster had very fine particulate emissions as
indicated by 90% less than 1 um and a d of 0.42 um. The size distribution
was similar with a a of 2.3. For a copper converter fewer small
particles were found and the d^ was the highest of the four units
tested (1.05 ym). The particle size distribution was also the narrowest
with a = 1.7. A summary of data on metal aerosols from the nonferrous
industry is given in Table 2. Note that all these nonferrous sources
-------
Table 2. SUMMARY OF PARTICLE SIZE DATA FOR THE
NONFERROUS METALS INDUSTRY
Source
Percent
by Weight
< 1 yni
Mass Median
Diameter
Deviation
Al Reduction
Cell
Zn Sintering
Machine
PB Sintering
Machine
Cu Converter
Zn Roaster
30
60
55
45
90
5.5
0.82
0.89
1.05
0.42
25
2.1
4.4
1.7
2.3
-------
have at laast 30% of their emissions in the submicron range; the percentage
may be as high as 90%. The d^ range from 0.42 to 5.5ym. Recalling
that the d^ for iron and steel sources ranged from 0.25 to 4.3 ym, one
may conclude that the majority of metal aerosols generated by industrial
processes are between 0.25 and 5.5ym in diameter.
1.3 Metal Aerosols Characterized from Occupational Studies
Studies have been conducted to measure the work environment of
metal workers and welders to determine their exposure potential.
1 o
Kolderup studied the fumes from a ferrosilicon open submerged-arc
furnace; he found basically a log-normal size distribution of agglomerates
ranging from 0.02 to 1 ym, consisting of spherical primary particles
ranging from 0.01 to 0.3 ym. The primarily silicon dioxide particles
appeared to form by homogeneous nucleation between Si and 02- The
agglomerates then grew by diffusion. Health effects hazards due to
nitrogen oxides (NO ) and trace elements such as Mn were reported by
14 3
McCord et al. , in addition to the 90-682 mg/m concentrations generated
with coated welding electrodes. McCord et al. indicated that, although
a large part of the sample may have been < 0.5 ym, the impinger collection
system did not collect these small particles. When silica-coated iron-
core electric arc welding electrodes are used, particles tend to be in
the size range of 0.01 to 1 ym, according to experiments by Pfefferkorn
and Desler. By developing a method to remove the iron oxide from the
sample, they observed hollow Si02 spheres, indicating that the aerosol
had an iron oxide core with a Si09 coating.
L.
In a recent study by Stettler et al. , lung biopsy specimens from
two arc-welders were compared with air samples from their work environment.
Most of the particulate material found in both samples was iron, with
other components present indicating stainless steel. Some large spherical
stainless steel particles in the range 0.5 - 3 ym were found in the lung
tissues, but most of the particles were clusters 0.1 - 0.2 ym in overall
size, made up of individual particles as small as 0.005 ym. Six times
as many irregularly shaped aluminum-containing particles on the order of
1 ym were found in Case 2 (the sicker worker) than in Case 1. Although
-------
difficulties with the filter media precluded sizing the particles from
the air samples, electron micrographs did show that clusters of particles
existed, made up of 0.05 ym primary particles. The authors point out
that there have been many conflicting reports in the literature as to
the toxicity of iron oxide and other trace materials usually found in
welding atmospheres. The observations in this study emphasize the need
for further characterization of welding fumes and animal inhalation
studies to quantify their biological impact.
Concentrations in the breathing zone of operators exposed to two
types of welding equipment, single electrode jet-arc and powder burning
arc, ranged from 20 to 35 mg/m in five steel plants in eastern Pennsylvania
With local exhaust ventilation, the exposures dropped below the Pennsylvania
Threshold Limit Value (TLV) of 10 mg/m . No definite connection between
iron oxide fumes and health effects could be drawn from this study.
A study to assess the hazards to industrial workers exposed to
metallic fumes with a median aerodynamic diameter (MAD) < 1 ym was
18
performed by exposing rats to iron oxide aerosol . Rats were exposed
to Fe90, with a MAD =0.3 ym, a = 1.8, and concentrations up to 700
3 "
mg/m . Although the rats' lung clearance half time was shorter than
man's, the Lung Dynamics Task Group model for man gave a good approximation
of the lung deposition efficiencies in the rat. The values for pulmonary
deposition are: 35% for 0.3 ym, 25% for 1 ym, and 10% for 5 ym. Results
also indicated that depositions in organs other than the lung are insignificant
for relatively insoluble particles such as iron oxide. Casarett and
19
Epstein , in another rat inhalation study, found an average deposition
rate of 60% when using a tagged iron oxide aerosol with a count median
diameter (CMD) of 0.068 ym (a = 1.62). There was a 3 to 1 ratio between
upper and lower respiratory tract deposition with 10-15% alveolar deposition.
1.4 Characteristics of Generated Metal Aerosols
A fundamental requirement for any study or project involving the
examination of certain properties of aerosols is a controllable source
of those aerosols. Laboratory generated aerosols can be used in many
-------
areas of aV pollution research; e.g., biological studies involving
inhalation and particle deposition; sampling instrumentation development,
verification, and calibration; development and evaluation of air pollution
control equipment; and evaluation of industrial hygiene ventilation and
safety equipment.
Many factors must be taken into consideration if a useful representative
aerosol is to be produced. The concentration of the aerosol in the gas
stream can vary considerably, depending on the method of generation.
The concentration to be generated depends on the needs of the research:
levels less 1 mg/m would be useful for ambient studies; values to
several hundred mg/m would apply to industrial hygiene and animal
o
inhalation studies; and concentrations of 1 g/m and above would apply
to process and stack emission analysis and control. Together with the
concentration, the gas flow rate determines the generation rate (in
units of mass per unit time), which is an indication of a generator's
capability for a certain application.
The particle size distribution (PSD) is another factor which is
radically affected by generation method. Different levels of dispersity
may apply more or less to different investigations. For example, very
controlled studies (e.g., development of a respiratory model or sampling
instrument calibration) may require a relatively monodisperse aerosol,
while control equipment testing and health effects research may be more
realistically accomplished using a polydisperse aerosol, as would be the
case in real life. Shape and density are two other factors which describe
the overall state of the aerosol.
To generate liquid or solid aerosols the basic input requirement is
energy. In general, the more energy per unit of material, the smaller
is the particle size attained due to increased surface area. This energy
input can take two basic forms to generate either dispersion-redistribution
20
aerosols or condensation aerosols. In the former, liquid aerosols are
normally dispersed by nebulizers and sprays in various configurations,
while solid particles can be redistributed by blowers, grinders, vibrators,
or resuspension in liquids. Condensation aerosols are created by heating
a material to the vapor state, by combustion, or by chemical reaction.
-------
20-23
Many good reviews of aerosol generation are available which
discuss various techniques and mechanisms involved in this important
field. This report will examine these topics as they apply specifically
to metal aerosols. Although this sometimes involves overlap with techniques
(such as nebulization) applicable to liquids or solids, other techniques
(such as exploding wires and electric arcs) are unique to the generation
of metal aerosols.
Researchers have been interested in the characteristics of metal
aerosols since the early 1950's. Advances in generating and analyzing
aeiosols in the past 5 years have increased the desirability of using
submicron diameter metal particles. The major generation techniques
that will be reviewed are: exploding wires; electric arc, both low and
high intensity; vaporization; combustion; and liquid resuspension systems
a- -1 associated thermal treatment. Inherent in many of these methods
will be factors influencing the quality and usefulness of the aerosol.
Such factors include the freshness or staleness of the material, the
moisture content and temperature of the aerosol-gas stream system, and
the presence of combustion products and other gases.
Condensation processes will be discussed first. In general these
processes (e.g., exploding wires, electric arcs, combustion, and vaporization)
produce very high concentrations of very small particles, with large
size distributions. The high concentration promotes rapid agglomeration
by diffusion, which changes the particle size distribution.
2 EXPLODING-WIRE GENERATORS
The general principle of this method is to conduct as much stored
electrical energy through a thin piece of wire in as short a time as
possible. Both amount of energy and time of transfer are important
parameters. If the amount of energy transferred exceeds that which
would cause surface evaporation, the wire expands as a superheated vapor
until it explodes. The wire thus changes state and becomes a charged
plasma of metal vapor which cools and condenses into a submicron metal
aerosol. The plasma consists of the metal vapor and ionized gas.
Thermal coagulation and Brownian diffusion occur rapidly, creating
-------
charged chained groups of particles. A basic series of texts containing
extensive information on the exploding wire phenomenon was compiled by
Chase and Moore during the period 1958-1968. More recent literature
reviewing characteristics of the aerosols is presented in a chronological
manner.
Metal aerosols from exploded wires of 15 different materials were
studied by Karioris and Fish.25 With a current surge from a discharging
4000 joule capacitor, spherical metallic or metal oxide particles with a
CMD of 0.02 ym are formed. This median diameter was observed to decrease
with an increase in voltage. Although agglomerate sizing was not done,
coagulation occurred quite rapidly and could be observed directly in a
Tyndall beam. By diluting in the ratio of 20 to 1 and mixing for an
hour, a stable aerosol of mostly isolated spherical primary particles
7fi
could be attained. Duerksen et al. improved upon the previous exploding-
wire generator with construction of a smaller, self-contained unit.
They reported obtaining reproducible spherical particles in the range of
0.01 - 0.1 ym for use in studies of particle properties when exposed to
various humidity, radiation, and gaseous contaminant conditions. Other
27
investigators using the same equipment studied tagged S02 adsorption
rate with aerosols generated from exploded Fe, Al, and Pb wires. They
reported that chemisorption accounted for a monolayer coating of S02 on
the particles but physical adsorption continued to add up to 75 times as
thick a layer of S02-
28
A modification was developed by Tomaides and Whitby which altered
the energy triggering method. In their generator, the wire was brought
into contact with a fixed electrode, completing the electrical circuit
and discharging the energy. They also observed the relationship of
decreasing the CMD and the a as specific charge energy (energy per unit
length) was increased. A typical aerosol generated by this process had
a CMD = 0.03 ym and a = 2.0. The aggregates formed by these primary
particles, however, experienced an increase in CMD with an increase in
specific charge energy. It was theorized that the smaller particles
promoted greater coagulation, ultimately resulting in a larger but log-
normal distributed chained aggregate aerosol.
10
-------
Phalen29 Investigated the properties of an exploded silver wire
aerosol with particular emphasis on an evaluation of its suitability for
inhalation studies. He determined the coagulation constant to be 2 x
-93 93
10 cm /sec, with an initial concentration of 10 particles/cm ,
suggesting a coagulation half-time of approximately 0.5 seconds. Accordingly,
he aimed a sharp blast of air (2 liters) at the wire cloud, triggered
within a few tenths of a second after the explosion to delay the coagulation.
A large dilution factor then maintained a stable aerosol with a mean
aerodynamic diameter (MAD) of 0.3 \an. Due to the configuration of the
aggTomerated aerosols with their greatly increased surface area per
vccime, the particles are more chemically reactive at the surface, can
retain more electric charge, and can be more easily solubilized. These
factors can affect aerosol interaction with biological systems and make
them more hazardous than was previously expected.
Rather than simply characterizing the aerosol, Sherman concentrated
on the relationship between the particle size characteristics and the
actual amount of energy transferred to the wire. With his circuitry and
measuring instrumentation, he was able to determine the energy, in
joules, as a function of time, in microseconds. Transmission electron
microscope (TEM) analysis of Ag, Cd, and Zn aerosol collected on mica
coated plates (positioned 8 cm from the wire) indicated that the mean
particle size decreased as energy transfer increased. That is, the
faster the metal expansion, or explosion, the greater the supersaturation
and rate of nucleation. He observed that Cd and Zn had a narrower size
distribution than the Ag aerosol. This may be explained by the fact
that Ag, having a larger surface tension, would tend to have a slower
nucleation rate yielding a wider size distribution of nuclei. It is
interesting to note that Sherman saw no change in PSD over several
minutes—the electron photomicrographs showed that almost no agglomeration
took place. His major observations (when no chemical reactions occurred
and pure metal aerosols formed) were: particles were spherical in shape
and ranged from 0.005 to 0.1 ^m in diameter; the PSD was roughly log-
normal; and the mean particle size was inversely proportional to the
energy transferred per unit length of wire.
n
-------
Just as the energy transfer that Sherman studied is one of the most
critical parameters in determining aerosol characteristics prior to the
explosion, the phenomenon of agglomeration or coagulation, seen by most
experimenters, is one of the most critical parameters after the explosion.
31
Wegrzyn studied the effect the bipolar charge has on the stability of
the generated aerosol. Using a gold wire, exploded in an argon-filled
chamber, he observed an aerosol whose primary particle mean diameter
ranged from 0.01 to 0.1 ym with a typical effective diameter of 0.032 ym.
The percentage of charged particles was slightly less than 50%. The
charge concentration of his aerosol (approximately 100,0007cm ) was well
within the capabilities of a 2 millicurie Krypton-85 neutralizer at a
flow rate of 10 1/min. Wegrzyn then compared the concentration in
particles/cm over a 30 minute period after the explosion with and
without exposure to the neutralizer to observe the coagulation coefficient
(K) with respect to time. After the aerosol was neutralized to a standard
Boltzman distribution of charge, K was fairly stable within a range of
-10 3
21-29 x 10 cm /sec; however, without neutralization, values of K were
approximately a factor of 2 higher during the first 600 seconds, before
dropping to the lower values. He also noticed an increase in the total
mass concentration with respect to time which may indicate the increasing
vapor adsorptive capability of the coagulating aerosol. His results
reinforce previous findings that exploding wire aerosols are reproducible
(submicron, normal, or log-normal) in primary particle size distribution
and highly charged.
Radioactive tagged iron oxide aerosols from an exploding wire were
32 33
analyzed by Kops et al. ' with an electron microscope and a Stober
centrifugal aerosol spectrometer (which deposits the aerosol along a
strip depending on the aerodynamic diameter) to study fluid drag on the
chained aggregates. The electron microscope examination showed that the
primary particle size distribution was approximately log-normal. From
the centrifuge data of iron oxide aggregates with six different primary
PSD's, they consistently observed two distinct log-normal distributed
diameter ranges which depended on the concentration (n) in the chamber.
3 3
For n > 5 x 10 /cm , the aerodynamic diameter (dfl) was proportional to
n1/3; for n< 5 x 103/cm3, da was proportional to n1/6. Typical mean d
values of the two regions were 0.259 and 0.925 ym. This is as expected,
12
-------
since a higher concentration would tend to promote more coagulation and
a wider subsequent diameter spread; whereas, a lower concentration would
have a lower coagulation rate and, hence, a more monodisperse aerosol.
Their results are of importance in describing the coagulation of solid
particles. It has generally been assumed that smaller solid as well as
liquid aerosols unite to form one large new particle, implying that the
Stokes diameter of a chain aggregate equals its volume equivalent
diameter. Kops et al., however, from their calculations of d . have
a
shown that the Stokes diameters of chain aggregates are less than their
volume equivalent diameters.
Exploding wire aerosol generators have been utilized and studied
extensively for the past 30 years. Many of the phenomena are fairly
well understood and, with the various refinements that have been incorporated
in their use, the aerosol is quite useful and dependable in the proper
application. The inherent batch operation nature of exploding wires,
however, limits its usefulness when larger, more continuous quantities
of aerosol are desired or required. Moreover, as shown in Table 3, the
size distribution of particles generated by exploding wire techniques is
very small (0.005 - 1 ym) and does not simulate industrial metal aerosols
which range from 0.25 to 5.5 pm.
3 ELECTRIC ARCS
One metal aerosol generation method that is not limited to small,
batch type conditions is the electric arc. The technique of striking an
arc between two electrodes has been recognized since the late 19th
century. Continued development led to the discovery of the high intensity
arc mode by Beck in Germany in 1910. Early aerosol scientists such as
34
Whytlaw-Gray were making use of electric arcs to study the properties
of metal fumes in the early 1920's. Ultramicroscopy allowed those
researchers to observe particles in the 0.01 - 0.2 um range. With the
advent of electron microscopy and other advances in particle sizing and
counting methodology, the study of electric arcs continued with emphasis
on the characterization and use of the aerosol being generated. Research
with both low and high intensity arcs has contributed to the understanding
of metal aerosols.
13
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Table 3. SUMMARY OF PARTICLE SIZE DATA FROM EXPLODING-WIRE GENERATORS
Author
Data Type
Result, ym
Karioris
Duerksen
Tomaides
2Q
Phalen"
Sherman
Wegrzyn
32
^
26
28
30
31
Count mean diameter
Range
Count mean diameter
Mass average diameter
Range
Effective diameter
Mean diameter
0.02
0.01 - 0.1
0.03
0.3
0.005 - 0.1
0.032
0.26, 0.93
14
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3.1 Low Intensity Arcs
A low intensity arc normally consists of one or two consumable
electrodes creating a gap across which an electric current can be passed
in a continuous spark. A great deal of the electrical energy is transformed
into thermal energy, which melts and partially vaporizes any material
within the arc with a melting or boiling temperature less than the arc
temperature. Typical arc-spot temperatures (actual spot on metal that
arc touches) are approximately 3000°C. The exact configuration of the
apparatus will determine the dispersion of the melted and vaporized
metal. As the material leaves the arc zone, it rapidly cools, causing
melted drops to solidify and vaporized material to condense and solidify
into an aerosol. The gas in the arc zone between the anode and the
cathode becomes ionized and exists as a plasma. Since the majority of
the voltage drop occurs in this region, most of the energy transfer is
in the form of radiant heat to the plasma rather than conductive and
convective heat to the electrode which is to be melted. This condition
of energy distribution is optimized with the high intensity arc which is
discussed later. The aerosol generated by the low intensity arc is
influenced by many input parameters such as voltage and current applied
to the arc, rate of electrode consumption, and temperature in the arc
zone.
Since the consumable electrode is the initial source of aerosolized
35
metal, its physical characteristics must be understood. Amson developed
a model for predicting the voltage along the "stick-out" (the exposed
electrode between the current pick-up point and the arc interface) in
terms of arc current, wire feed rate, and exposed length. Carslaw and
Jaeger also present a well-developed model of temperature distribution
within the electrode.
Recognizing the need for the generation capability of aerosols<1 ym
from an industrial hygiene standpoint, a group of Japanese investigators
generated Fe, Pb, and W aerosols with an electric arc. Rather than
using a continuous DC current, they discharged 9000 volts AC intermittently
(300 cycles/minute) across the electrodes. Sampling with an electrostatic
precipitator sampler and analyzing with an electron microscope indicated
15
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spherical Fe and Pb oxide particles with little agglomeration. The
count mode diameter of the iron oxide was 0.02 ym; for the tungsten, it
was 0.04 ym, with a general range of 0.015 - 0.15 ym. They concluded
that this was a viable method of generating submicron particles, but did
not attempt to explain the formation mechanisms.
OQ
Harvey et al. generated many metal aerosols with a DC arc to
investigate the structure and forms of the primary particles of the
smokes. Electron microscope analysis confirmed earlier investigations
(such as by Whytlaw-Gray et al. ) that all the particles formed charged
chain-like aggregates. Additionally, the extent of the chain formation
for Au and Ag smoke appeared to be strongly dependent on the pressure of
the gaseous atmosphere. The Au and Ag aerosols (from heated wire filaments)
were formed of pure metal; the other metals investigated formed oxides,
except that Pb and Bi formed metal particles coated with an oxide. From
electron and x-ray diffraction analysis, some typical shapes, and primary
and aggregate dimensions are: ZnO--hexagonal, 4$ (lft = 10" ) primary
particles, and 4000$ long rod-shaped aggregates; Fe203~ cubic, 8$
primary particles, and 300$ aggregates; and Al£0,—cubic, 16$ primary
particles, and 300-500$ agglomerates.
Although researchers had made many estimates of aerosol shape and
39
size, Reichelt was among the first to critically examine the operational
parameters and their influence on the aerosol particle shape and size
distribution. Experimenting with Fe, Al, and Cu electrodes in air, CO^,
argon, and ^» he could detect no lower size limit with the capabilities
of the electron microscope (size range from 100$ to 20 ym). These
particles are created as some of the metal evaporates due to the high
temperature at the spot where the electric arc contacts the electrode.
Even if voltage and current are maintained constant, the arc cannot be
considered a candidate for use as a monodisperse aerosol generator due
to the nonuniform nature of the arc, which can move about quickly,
changing its length and spot location. In terms of total aerosol production,
current has a great influence: the weight of aerosol is proportional to
the current cubed. Thinner electrodes and greater separation optimize
and increase production. Analyzing particles with a Goetz aerosol
16
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centrifuge, Reichelt found that the particles generated were not spherical
and formed chains. He also noted that the current had little effect on
the Stokes diameter (diameter of a sphere with the same density and
settling velocity as the particle in question) particle size of the
chain aggregates. Increasing the flow rate did decrease the agglomerate
size, basically by diluting and dispersing the particles, reducing the
coagulation rate. With electron microscope analysis it was found that
current had a large effect on the size of the primary particles—an
increasing current increased the size. This tends to explain the fact
that the influence of current on Stokes diameter is not so pronounced,
since larger particles will coagulate less. Other aerosol characteristics
(e.g., the number and size of polyhedra and crystals) varied with flow
rate, direction of flow (horizontal or vertical), and chemical composition.
In conclusion, Reichelt felt that the arc was not an ideal aerosol
generator, due to the rather uncontrolled and complex processes involved
and his results of varying PSD's with consecutive samples.
Beginning with a configuration similar to Reichelt1s with two
40
electrodes inserted into an exhaust duct, Linsky et al. investigated
the generation of iron and zinc oxide. The initial generator consisted
of one consumable wire electrode and one nonconsumable tungsten electrode.
Results indicated that 75% of the agglomerated iron oxide particles were
<0.1 urn; primary particles had a mean diameter of approximately 0.02 ym.
The agglomerate size data probably was altered since the measurement
technique was resuspension in liquid for Coulter Counter sizing. Linsky
et al. later examined a commercially available wire spray metal!izer,
used to coat industrial parts with a fine metal layer. The equipment
consisted of two consumable wire electrodes mounted on a gun; a compressed
air nozzle centered between them dispersed the melted and vaporized
material. This configuration was much more controllable than the earlier
one or Reichelt's equipment, and reproducible PSD's and concentrations
were obtained. It was estimated that agglomeration occurred within the
first 0.1 second after generation, forming highly charged chain aggregates
whose mean diameter was 0.41 ym; the mean diameter of the primary particles
was <0.01 ym. The maximum aerosol generation rate of the Linsky metallizer
was approximately 8 g/min which corresponded to a typical conversion
efficiency of wire metal into usable aerosol of 8-15%.
17
-------
Zimm^man41achieved similar results using identical equipment, with
an approximate conversion efficiency of 4% and a highly charged, agglomerated
aerosol. Approximately 80% of this aerosol had an aerodynamic diameter <
1 ym, based on a cascade impactor. More aerosol characterization work
needs to be done on this system to determine if a specified set of input
parameters (e.g., voltage, wire feed rate, compressed air pressure,
exhaust air flow rate, and wire composition) can generate a reproducible,
predictable aerosol.
3.2 High Intensity Arcs
A high intensity arc is similar in configuration to the low intensity
arc. The difference lies in the current density and efficient use of
the electrical energy. The high intensity arc is established at a
sudden transition point when the current density exceeds a critical
value for an ordinary arc. In the arc crater, the temperature can
increase by more than a factor of 2 to 7000-8000°C. As previously
mentioned, in the low intensity arc the largest voltage drop occurs in
the plasma between the electrodes, while the remaining drop is approximately
10% across each electrode. As the arc current increases, the current
density remains approximately constant as the arc crater (assuming
electrode diameters larger than wires) increases on the surface of the
anode until it fills the entire anode face area. Additional energy is
dissipated by radiation and conduction of heat until the increased
energy can't be removed fast enough. Then the anode temperature begins
to rise until the boiling point is reached. Beyond this point large
amounts of vapor are generated by the anode material. A superheated
region develops near the anode in this tail flame which can reach values
to twice the boiling point of the anode material (e.g., 4827°C for
carbon; 2750°C for iron), dissociating it into elemental molecules which
then recondense as they travel out of the hot zone.
The high intensity arc was proposed by Sheer and Korman42 as a
means of energy transfer in the processing of materials, such as refining
of metal ores. The nonconducting portion of the electrodes can be as
high as 80-85%, with the balance being carbon. They obtained silica
18
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fumes with an average particle size of 0.03 - 0.04 ym with an 80% silica/
20% carbon electrode. Harris et al.43 later proposed the use of the arc
for processing rhodonite for extraction of manganese from siliceous
ores. No size data was given in their study.
Holmgren et al. applied the high intensity arc principle specifically
to submicron particle formation for industrial applications. By consuming
the solid anode, a highly ionized plasma anode tail flame exists, so
that there are many available condensation nuclei upon which submicron
particles can form. The rapid temperature drop does not allow much
particle growth leaving primary particles in the 10-1000$ size range.
Increasing the dilution-cooling quench rate decreases the size. Increased
quenching also decreases the initial coagulation rate by dispersion of
the particles and thus limits the PSD. Agglomeration, however, cannot
be avoided, and chain aggregates will form. According to the authors,
the increased chemical reactivity, sintering rate, and surface energy of
submicron particles make this a promising process. For the same reasons
it also could be very useful in aerosol generation applications.
Although the anode temperature of a high intensity arc can reach
8000°C, the area around the cathode can achieve temperatures in the
AR dfi d
-------
Researchers in Italy (Boffa and Pfender47) have made a similar
application of the high intensity arc principle with the added variation
that the quench gas transpires through the anode as the anode is ablated,
or consumed. This transpiration not only provides dilution and cooling
air but appears to limit coagulation as well. Their results show that
they can control mean particle size between 458 and 1 ym with o <
g
1.2. A a this low is generally considered to be a measure of practical
monodispersity. The major controlling parameters are the arc current
and the cooling gas flow rate. The mean particle size increases with
increasing temperature, due to increased current, because the anode
ablation rate increases causing a greater density of particles in the
area near the anode. This increase in number of particles influences
the coagulation and growth rate. The mean particle size decreases,
however, with an increase in flow rate due to enhanced cooling and
dispersing.
48
Boffa et al. later altered their generation system to make it
easier to use. The high intensity arc is maintained between two nonconsumable
electrodes with argon gas feeding into the cathode area. The plasma jet
and the argon transpire through a porous consumable matrix. The plasma
ablates the matrix material while the gas quenches and dilutes the
vapor. Similar PSD results were obtained with this method. No generation
rate information was given for either variation of this method.
49
Another group of Italians, Tarroni et al. , developed an apparatus
very similar in principle to the work of Korman et al. Tarroni's
group utilized a Collison atomizer (with dialyzed Fe^O., suspended in
argon) and a dust feeder (with iron particles) to feed the material into
the plasma arc near the cathode. CMD's of 0.005 and 0.004 ym, respectively,
each with o = 1.3, were obtained.
50
A review by Fannick and Corn indicates the extent of industrial
hygiene hazards of plasma torches and arcs. Concentrations of various
fumes generated at plasma torch facilities were presented. Many exposures
of Fe203 exceed the Threshhold Limit Value (TLV) of 10 mg/m3, ranging
from 11.9 to 35.1 mg/m . From another source, they reported total fumes
o
(consisting of 10% chromium) ranging from 24.5 to 31.3 mg/m without
20
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exhaust ventilation and 0.4-5.7 mg/m with ventilation. Although this
data is presented from an industrial hygiene standpoint, it indicates
the generation capability of high intensity plasma arcs in industry.
4 HEATING TO EVAPORATION
Rather than imparting electric energy to the metallic material as
quickly as possible in a violent action, such as the electric arc and
exploding wire, other researchers have studied the effect of applying a
steady source of heat: dirdctly, as with a lower level of electric
current; or indirectly, as with a furnace chamber. This also results in
the vaporization of a steady stream of metal vapor which condenses into
a submicron aerosol. Some controlling parameters include temperature of
the material, composition and dimensions of the material, and dilution-
carrier gas flow rate. In addition to the development of practical test
aerosol generators, the purpose of many of the following studies was to
further investigate the underlying principles involved in the evaporation/
condensation mechanisms.
51
Turkevich stated his purpose as not only examining the nature of
air contaminant fumes, but also, from an acadmeic standpoint, determining
how unordered gaseous atoms condense to ordered ultrafine particles. In
the region in which molecules aggregate in sufficient numbers to promote
condensation, theoretical and experimental evidence seems to indicate
that a limiting lower particle size exists, based on the fact that
surface tension (varying directly with the square of the radius) tends
to disrupt the particle while lattice energy (varying directly with the
cube of the radius) tends to hold the particle together. Thus, the
point of formation of a particle is complex, involving many parameters.
For his purpose, Turkevich felt that the particles produced by an exploding
wire were too irregularly shaped. He attributed this to the nature of
the particle formation, which he considered to be basically disintegration.
A process which agglomerates or condenses is considered more uniform for
the study purposes. Particles from two aggregation processes, DC electric
arc and electrically heated wire, were examined. The smoke from the
electric arc suggested that the shape of the submicron particle was
often characteristic of the chemical composition of the parent material,
21
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especially as the particle size decreased. Turkevich attempted to study
the stages in the evaporation/condensation process by passing current
through a tungsten wire enclosed in a vacuum chamber bell jar, and
collecting particles or promoting condensation on collection surfaces
(collodion grids, later used directly in electron microscope analysis)
positioned at different distances from the wire. With a high vacuum
(10 mm Hg) in the chamber, the vapor appeared to migrate to each surface
before condensing on it, giving a similar uniform deposit on each,
regardless of distance. As the pressure in the chamber increased,
however, he was able to detect three distinct areas where different
processes predominated: evaporation, nucleation, and coagulation.
Evaluations of particles were based on the average size and the PSD.
From his observations he concluded that the final diameter of the particle
is inversely proportional to the rate of nucleation, a phenomenon observed
by others.
ro_CT
Spumy and various co-workers " investigated the generation of
metals and metal compounds by sublimation and vaporization. Spurny
prefers condensation over spraying as a means of aerosol generation,
stating that the former results in a more monodisperse, controllable,
CO
and reproducible aerosol . Two methods which Spurny uses in most of
his research are: 1) heating metals in a furnace (usually preferred for
low melting point metals); and 2) passing current through a wire. The
major difference between these methods is the time necessary to achieve
a stable aerosol. Furnaces can take hours, while heated wires achieve
steady-state conditions in a matter of minutes. In general, condensation
methods can produce aerosols within a size range from 0.001 to 1
Dilution to a concentration
coagulation of the aerosol.
4
Dilution to a concentration of 10 particles/cm is necessary to prevent
Heicklen and Luria presented a review of experimental results on
kinetics of homogenous particle nucleation and growth. They acknowledge
that metal oxide data in this area is limited. The authors indicate
three means by which heterogeneous nucleation can form a homogeneous
particle: 1) condensation on an ion; 2) condensation on a neutral nucleating
particle; and 3) coprecipitation of two or more species. Homogeneous
nucleation occurs when a critical supersaturation is reached or a gas
22
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phase reaction occurs. Metal ions react with 02 to give metal oxides.
Some rate coefficient data is given for these reactions.
4.1 Metal Oxides
In studies conducted by Maiwald and Polydorova , a 0.5 mm
tungsten wire was vaporized by heating the metal to 850°C in an air
flow. A problem inherent to this approach is the continual decrease in
wire diameter which increases the current per unit length unless the
voltage is regulated. Presence of an oxide layer appears to increase
tf The oxygen content of the carrier gas could be decreased to limit
the formation of oxide but then the gas stream's usefulness for inhalation
tests would be affected unless 02 additions were made. Results from EM
analysis indicate that particles range from 0.03 to 0.5 ym and are
coagulated. An increase in temperature caused an increase in particle
size due to the increased coagulation.
A similar study was also conducted by Polydorova with platinum
wire. At high wire surface temperatures (1330-1510°C), platinum oxide
forms on the surface. Since the vapor pressure of the oxide is less
than that of the pure metal, platinum oxide aerosols are generated. The
higher vapor pressure at higher surface temperatures generates larger
particles in higher concentrations. Keeping temperature constant, an
increase in air flow dilutes the vapor and reduces the coagulation rate.
r e T
Aerosols in concentrations of 10 - 10 particles/cm with size ranging
from 0.018 to 0.048 ym were generated in low air flows (3-6 1/min).
Fumes of Fe, Zn, and Cd oxides were generated by heating small
rn
amounts of the metals until vaporization occurred. These simulated
welding and brazing fumes were used to compare the sampling efficiencies
of two industrial hygiene filter samplers. All samples had data showing
that most particles were < 1 ym. The authors cite work by Jarnuszkiewicz
indicating that the major portion of welding fume is <0.5 ym.
23
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4.2 Metal Halides
Metal aerosols, specifically Ag, have been widely used in the study
of ice and rain nuceleation. Requirements for an aerosol for this
application include being continuous, steady-state, and free of combustion
products and oversized particles. Barchet and McKenzie63 generated
silver iodide aerosols by electroplating nichrome wires with Ag, exposing
them to I vapor to form a Agl coating, and then heating the wire by
passing current through it. Particle production is controlled by sublimation
from the wire and recondensation in the quench air. A broad range of
particle sizes was obtained, with 7% <0.1 ym and a geometric mean
diameter of 0.01 ym. They estimated that approximately 50% of the small
particles were lost by coagulation and diffusion by the time the sample
reached the condensation nuclei counter.
Another Agl aerosol generation approach was taken by Poc and
64
Rolleau to attempt to eliminate all impurities from possibly influencing
the ice nucleating mechanism. A two-step procedure was used: first,
passing pure dry N2 over a crucible of Ag heated to 950°C to generate a
pure Ag aerosol; and second, exposing this aerosol to a steady flow of I
vapor. Electron microscopy indicated an Ag aerosol with a concentration
/•
-------
particles formed in the first furnace. Spumy also introduced various
materials to the double furnace as fine powders in a gas stream of dry
air or helium. An Fe2CL aerosol with a size range from 0.002 to 0.02
urn was produced with a vaporizing temperature of 450-800°C, well below
the melting point (1535°C). The range of generation rate capability of
this method is not given but Spurny stated that these aerosols could be
useful in experimental aerosol physics and chemistry studies, filtration
studies, and biological research. As a variation of the heated wire
method, Spurny et al. * heated a hollow tube of platinum by passing an
electric current through it; dry air flowing through the center of the
tube carried away the evaporated metal. They again observed that the
particle size increased with increasing temperature, but decreased with
increasing gas flow rates.
4.3 Metallic Aerosols in Argon
65
A heat-pulse cloud chamber was used by Buckle and Pointon to
study the condensation of metallic aerosols in argon. A highly supersaturated
vapor is produced by passing a current through a coil to flash-heat the
metal sample above background temperature. The vapor rapidly cools and
condenses to form a suspension of droplets or solid particles. Zn and
Pb aerosols were produced in this way, but difficulties were encountered
with Ca and Cd. The authors feel that the formation of micron-size
metallic aerosols is attributed to nucleation and growth under conditions
of large temperature and concentration gradients near the supersaturator.
In continued studies of Cd aerosol formation, Buckle and Pointon ' observed
that the particles were shaped like prisms and spheroids with a variety
of surface features; e.g., dished areas and concentric rings and layers.
This roughened platelike surface effect was thought to be caused by room
temperature condensation of additional vapor onto the aerosol particles.
They noted that the PSD was constant and log-normal with a geometric
mean of approximately 2 ym, regardless of particle configuration.
5 COMBUSTION
Melting and vaporizing metallic materials can be achieved by means
other than heating; e.g., combustion, feeding the material into a flame,
25
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or combining it with combustible materials and igniting the mixture.
Methods of generating metal aerosols by combustion are more varied than
the previously discussed methods since there are many ways of introducing
the metal material into the flame. These methods would still be classified
as condensation, however, since the original phase, whether liquid or
solids, if vaporized in the flame, then followed by condensation.
The problems of removing submicron-size iron fumes from open-hearth
CQ
furnaces were studied by Billings et al. in the mid-1950's using
combustion to generate Fe^O., for laboratory tests. Actually, two methods
were used: 1) flake iron powder was injected into the air side of an
air/oxygen/acetylene flame where it was oxidized exothermally; and 2)
iron carbonyl was burned in an oxygen/acetylene flame. The second
method provided a somewhat more controlled generation. Concentrations
o
from these procedures ranged from 0.04 to 0.50 gr/ft . In field tests,
3
they found furnace emission concentrations of 0.02-0.34 gr/ft . A
particle size analysis of the furnace emissions indicated a mass median
diameter of 0.65 - 0.82 ym, while the CMD was 0.047 - 0.057 ym. Thus
even in earlier research with metal aerosols, the necessity for simulation
of actual emissions was recognized.
Almost 20 years later another group of researchers, Brain et
al. , modified the same equipment to continue the study of Fe203
aerosol generation from iron pentacarbonyl. They bubbled Np through the
Fe(CO)r and decomposed the vapor in a 500°C furnace cylinder. A concentration
3
of 340 mg/m was achieved, composed of an agglomerated polydisperse
aerosol with a CMD of 0.4 ym (a = 2) and a mass median diameter of
1.6 ym. The primary spherical particles which make up the log-normal
distributed agglomerates range in size from 0.01 - 0.03 ym. The authors
estimated that the agglomeration occurs rapidly (with a half-time of 0.1
second) in the furnace. Keeping the furnace temperature above 500°C
greatly reduces the toxic fumes and CO, making the aerosol suitable for
animal studies.
A generator to produce uranium fume, U.OP, was devised by Glauberman
70
and Breslin in the mid-19501s. A bed of uranium chips supported on a
slowly rotating wire mesh turntable was burned in an argon/oxygen gas
26
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stream. The authors demonstrated that the median particle size could be
pre-selected in a range of 0.05 - 0.2 ym (a of 2.5-3.1) by establishing
the appropriate argon to oxygen ratio and total gas flow rate. Their
particular chamber design allowed sustained generation for 6 minutes.
Formenti et al. , working mainly with Ti02, prepared metal oxide
aerosols by carrying the vapor of metal chlorides into an oxygen/hydrogen
flame. They observed both spherical and polyhedral (crystal) nonporous
particles in the size range of 0.01 - 0.2 vm. The major parameters
influencing size and shape include gas flow rate, chloride concentration,
72
flame temperature, and residence time in the flame. Juillet et al.
continued the research with many other metal chloride solutions. Both
groups noted that the aerosol exhibited photocatalytic properties in the
presence of ultraviolet light, properties not found in large or porous
particles.
Larger quantities of material can be generated by spraying a dilute
metal or metal salt solution in an atomizer burner. Dharmarajan and
73
West used this method to generate metal oxide aerosols in the size
range of 0.1 - 10 ym. Additional work is needed to characterize the
particle size, to determine the controllability of the particle size
distribution, and to eliminate the soot that accompanies the metal
oxides.
A similar approach was taken by Carroz et al. to demonstrate
continuous high mass flow rate generation of fresh, fine, inorganic
particulate aerosols, to simulate emissions from coal power plants,
electric arc and basic oxygen furnaces, and Zn smelters. The fume
generation apparatus consists of a solvent burner with a spray nozzle,
pressurized tank, and ductwork. Inorganic metallic oxides are generated
from the burning flammable solutions of metal nitrates and other salts.
The hot gases and excess air reduce agglomeration. They have obtained
3 9
particle loadings as high as 16.8 g/m and concentrations of 10
particles/cm . Analyses with an electric mobility aerosol analyzer,
optical counter, condensation nuclei counter, thermal precipitator, and
electron microscope have indicated that the aerosol consists of hollow
spheres (approximately 20 ym) and chain aggregates with mass median
27
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diameters of approximately 1 ym. Methods were developed to vary S02
concentration and particle resistivity to better simulate power plant
emissions over a wide range of operating conditions. The method is very
flexible and can be used to generate aerosols of many different oxides
and chlorides. Data from all methods is summarized in Table 4.
6 DISPERSION
Various approaches to aerosol generation can be classified as
dispersion processes. The basic difference between dispersion and
condensation categories is that the former involves mainly physical
processes; the other is related more to chemical processes and phase
changes. Dispersion encompasses the formation of two types of aerosols,
solid particles and liquid droplets.
Dispersion of solid particles, by grinding or resuspension of
pulverized material, results in a highly polydisperse aerosol, with
doubtful reproducibility and control of size, shape, and concentration.
Brain et al. cite mid-19501s references of the use of a Wright Dust
Feeder and a commercial blender to resuspend iron oxide and iron powder.
Problems stem from the fact that the closely packed, sometimes charged
particles tend to remain agglomerated and do not represent actual emissions.
6.1 Liquid Droplets--General
Generation of liquid droplets can consist of four basic types: 1)
nebulization; 2) spraying; 3) rotation (spinning disks); and 4) vibration.75
These methods can be used with many kinds of solutions including those
with a dilute concentration of solids suspended in the liquid. The
droplets also can be treated after formation such as by heating to
evaporate liquid, leaving a solid residue, or by thermal decomposition
to alter the chemical composition of the material. By combining the
many alternatives of dispersion generation, numerous aerosol generation
techniques have been devised. Examples of some of these typical applications
which generate metal aerosols are presented in this review.
28
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Table 4. SUMMARY OF PARTICLE SIZE DATA FOR GENERATORS
USING COMBUSTION
Author
Data Type
Result, ym
Billings68
Brain69
70
Glauberman
Formenti
Dharmarajan
Carroz
74
Mass median diameter
Count "
Mass median diameter
Count "
Median particle size
Range
Range
Mass median diameter
0.65 - 0.82
0.047 - 0.057
1.6
0.4
0.05 - 0.2
0.01 - 0.2
0.1 - 10
1.3
29
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75-77
A number of researchers have tested spinning disk aerosol
generators using an iron oxide solution. A fairly monodisperse aerosol
is attainable but only for diameters larger than 1 ym. Albert et al.75
were able to generate iron oxide particles in the range of 1-8 ym.
Smaller, less monodisperse aerosols can be made by utilizing the satellite
drops rather than the larger drops (this wastes 90% of the mass). The
disadvantages are that the particles need heat to stabilize their structure
and that the particles cannot be used directly for inhalation studies.
Because the air stream contains solvent vapors, the aerosol must be
collected and then redispersed, making this an awkward research tool.
Spertell and Lippman76 also had difficulties, producing only 200
o
particles/cm and being unable to maintain a stable test aerosol for
long intervals. Even using an aerosol concentrator, they could not
generate a usable aerosol less than 3 ym.
Caplan et al., like Albert et al., were able to generate a fairly
monodisperse aerosol, but only in the size range of 2-6 ym.
6.2 Using Liquid Droplets—Nebulizers
A Lauterbach nebulizer driven by compressed air was used by
19
Casarett and Epstein to aerosolize a suspension of FeO.,. ps^ data
determined with electron microscope analysis indicated a CMD = 0.068 ym
and o = 1.62. The authors concluded that Fe20~ was an adequate material
for use in inhalation studies although continued work is needed to fully
characterize the aerosol, especially in the area of solubility.
78
Waite and Ramsden describe an ultrasonic nebulizer which generates
radioactive, stable, insoluble particles of Fe^O.,. A solution prepared
from ferric chloride and ferric nitrate results in hollow spheres of
Fe203 when the solution is nebulized and dried. These hollow spheres
have a CMD of 0.03 - 0.5 ym when the initial solution droplets are 3 ym.
The authors note that an aerodynamic median diameter (AMD) rather than
CMD should be used to describe the diameter of the iron oxide aerosol
since the spheres are hollow rather than solid. To make this conversion,
the density of the aerosol particles is needed; it will be less than the
30
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bulk density of iron oxide or of a solid iron oxide particle. Their
observations show a significant drop in density between 0.55 and 0.3 ym.
This process is not only interesting, but also useful in situations
where the amount of material needed is limited.
79
The work of Raabe et al. appears more promising but still only
allows a small amount of aerosol to be generated. Solutions of metals
in the form of oxalates, citrates, and tartrates are nebulized into
droplets and then thermally decomposed in a furnace at 1150°C. A spherical
oxide aerosol results with no organic solvent contamination. A zirconium
oxide aerosol was generated and found to have a size of approximately
3 ym. The authors state that the more monodisperse the droplet distribution
system is, the more monodisperse will be the final particles.
80
This project was continued by Kotrappa and Wilkinson, using the
Lovelace nebulizer, which the authors classify as a high specific output
nebulizer. They generated droplets from a colloidal iron hydroxide
solution which was nebulized and dried to form iron oxide particles in
the range of 0.3 to 3.5 ym, with a particle material density of 2.56
g/cm (density was independent of particle size).
Aqueous solutions of metallic carbonates, acetates, and oxalates
were generated with Lovelace and Collison nebulizers by Horstman et
81 82
al. and Friedman and Horstman. Cd, Ni, and Mn solutions were used.
The droplets were dried in a furnace with temperature of 600-1000°C to
decompose them to an aerosol with a mass median diameter of 0.15 -0.6 ym
and a of 1.8-2.6.
As a research tool the approach of generating liquid drops and
treating them thermally appears useful, but larger, more continuous
aerosol production has not yet been shown to be feasible.
7 SUMMARY
Exposure to metal or metallic compound submicron aerosols is
widespread, both to workers in an industrial setting and to the general
population. In fact, the metals industries are second only to power
31
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plants in total nationwide man-made emissions to the atmosphere. Research
in areas of effects and control of metal aerosols requires sources of
test aerosols. Submicron aerosols are needed for inhalation experiments
(deposition and clearance of particles in the lungs), air pollution
control equipment testing, filter testing, and sampling instrumentation
development and calibration. To be useful, however, the aerosol must be
reproducible, with a predictable characteristic particle size and range
of diameter. Moreover, to simulate industrial emissions of metal aerosols,
the appropriate range of diameters is from 0.25 to 5.5 ym.
There are many possible methods for the generation of laboratory
test aerosols. These methods can be on a continuous or a batch basis,
with high or low concentrations and generation rates, and with monodisperse
or polydisperse size distributions. The method to be chosen and the
resulting aerosol depend on the requirements of the specific research
being conducted.
Metal aerosol generation can be thought of as being in two major
divisions, condensation and dispersion. Condensation includes the
formation of new particles from a vapor state which has been created by
evaporation, sublimation, or combustion of some parent material. Dispersion
includes any method involving a redistribution of the parent material
into a smaller particle size. This could include grinding for solids or
some droplet formation mechanism for liquids and solids in a liquid
suspension. Treatment of the drops after formation by application of
heat for evaporation or thermal decomposition is sometimes utilized to
alter the droplet characteristics into a more useful aerosol. Condensation
aerosols are usually <1 ym, mainly existing as agglomerate chains (0.4-
0.8 ym typical sizes) composed of charged primary spherical particles
(0.001-0.01 ym). Dispersion aerosols are usually > 1 ym, but are more
controllable and generally more monodisperse than those formed by condensation.
The exploding wire generator has been studied and used for many
years. With carefully regulated operation, a useful reproducible metal
aerosol can be produced, but only in small amounts (the mass of the
32
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wire is the limiting factor) and on a batch basis. A high coagulation
rate and polydisperse aerosol are due to the local high concentration in
the plasma following an explosion. Immediate dilution partially alleviates
this problem.
The low intensity electric arc generator includes easily accessible
components, but offers a low metal to aerosol conversion efficiency. The
process, consisting of vaporizing and melting the metal electrodes, is
not a highly controllable system due to the arc flame instability. The
product is normally a polydisperse agglomerated aerosol whose reproducibility
is not yet totally proven. It does, however, come closest to simulating
industrial emissions; from that standpoint and because of its ease of
operation, it is a worthwhile system. Generation rates as high as 8
g/minute have been reported. The high intensity arc offers a much
higher conversion efficiency and vaporization rate and operates with a
more controllable stable flame. Its aerosols are still somewhat polydisperse
due to coagulation. Electrode transpiration dilution helps reduce the
tendency to coagulate. Some difficulty has been noted in feeding the
powderized material to the electric arc. It has been suggested that it
may be most useful to utilize this process in the basic configuration of
an arc struck across an electrode and a metal-filled crucible. Generation
rates as high as 40 g/minute have been reported with the high intensity
arc.
Heating metallic material to evaporation by passing current through
the material or by direct heating in a furnace provides very small
quantities of aerosol. Most experiments using this process are theoretical,
investigating basic principles of evaporation and condensation. Since
low concentrations are produced, coagulation is not as great a problem.
Some difficulties occur when heating wires, since the constantly decreasing
diameter creates a non-steady state current.
Combustion processes such as spray solvent burning have high generation
rate capabilities. The exhaust gas stream containing the aerosol may be
hot and contain products of combustion such as CO, NO , and H~0 which
A £
may be used to simulate exhaust gases found in the industrial source
33
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under investigation. Combustion processes provide particles in the 0.01
to 10 ym range which simulate closely the range found in industrial
sources.
Direct production of solid particles from grinding or redispersion
methods is generally not too successful from the standpoint of uniform
size and shape. Liquid droplet formation methods such as spray nebulization,
spinning disks, and ultrasonic nebulization have been very successful
producing monodisperse aerosols. Generally, however, these aerosols are >
1 ym. Researchers have not yet developed this into a continuous, high
concentration method.
In summary, the ability to generate a known reproducible source of
aerosols, specifically metallic, for laboratory and research purposes is
very important. A variety of significant work has been conducted in
this field. Of the techniques studied only electric arcs and combustion
have demonstrated the capability for continuous generation of large
quantitities of metal aerosols which simulate the particle size of
industrial sources.
34
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8 REFERENCES
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36
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21. Mercer, T. T., Aerosol Technology in Hazard Evaluation,
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32. Kops, J., VandeVate, J. F., Jermans, L., and Dibbets, G., "The
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unpublished data, 1977.
38
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42. Sheer, C., and Korman, S., "The High-Intensity Arc in Process
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Hygiene Aspects and a Survey of Current United States Practices for
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51. Turkevich, J.f "UHrafine Particles in the Gas Phase," Fundamental
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Nucleation and Growth," International J. Chemical Kinetics. (Symp. 1,
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Translation), 25_(12):13-15, 1965.
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Oxide Aerosol," Staub (English Translation), 25(12):16-18, 1965.
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61. Polydorova, M., "The Manufacture and Properties of a Platinum
Oxide Aerosol," Staub (English Translation), 27_(8)-5-8, 1967.
62. Tomb, T. F., Treaftis, ri, M., and Beckert, A. F., "Quantitating
the Efficiency of the Coal Mine Personnel Respirable Dust Sampler for
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63. Barchet, W. R., and McKenzie, R. S., "Silver Iodide Aerosol
Production by a Plated Hot-Wire Generate""," -I^Jtogligd..Meteorology,
1^:1151-1155, 1975.
64. Poc, M., and Roulleau, M., "The Ice Nucleating Properties of
an Ultrafine Aerosol of Pure Silver Iodide," J. Applied Meteorology,
14:1146-1150, 1975.
65. Buckle, E. R., and Poincon, K. C., "Condensation and Evaporation
of Metallic Aerosols," Faraday Symposia of the Chemical Society, 3:78-84,
1973.
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Cadmium Aerosols," Atmospheric Environment, 3:1335-1337, 1974.
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Aerosols," J. Materials Science, 10:365-378, 1975.
68. Billings, C. E., Small, V<. 3., and Silver-man, I., "Pilot-Plant
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^(3):159-165, 1955.
69. Brain, J. D., Valberg, P. A., Sorokin, S. P., and Hinds, W. C.,
"An Iron Oxide Aerosol Suitable for Animal Exposures," Environmental
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70. Glauberman, H., and Breslin, A. J., "The Generation of a Solid
Sub-micron Test Aerosol," AIHA Quarterly, ]_7:401-404, Dec., 1956.
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71. Formenti, M., Juillet, F., Meriaudeau, P., Teichner, S. J.,
and Vergnon, P., "Preparation in a Hydrogen-Oxygen Flame of Ultrafine
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73. Dharmarajan, V., and West, P. W., "A Precise Method for the
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J. R., "Fabrication of Monodisperse Lucite and Iron Oxide Particles with
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42
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43
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TECHNICAL REPORT DATA
(Please read Inductions on the reverse before completing)
. REPORT NO.
EPA-600/7-78-013
2.
3. RECIPIENT'S ACCESSION NO.
. TITLE AND SUBTITLE
Characterization and Generation of Metal Aerosols
5. REPORT DATE
February 1978
6. PERFORMING ORGANIZATION CODE
.AUTHOR(S)Neil Zimmermari) Dennis C. Drehmel, and
James H. Abbott
8. PERFORMING ORGANIZATION REPORT NO.
9. PERFORMING ORGANIZATION NAME AND ADDRESS
U.S. Environmental Protection Agency
Industrial Environmental Research Laboratory
Research Triangle Park, North Carolina 27711
10. PROGRAM ELEMENT NO.
EHE623
11. CONTRACT/GRANT NO.
NA (Inhouse)
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
Final: 6/76-6/77
14. SPONSORING AGENCY CODE
EPA/600/13
15. SUPPLEMENTARY NOTES
61, 919/541-2925.
project off icer/auttior is D.C. Drehmel, Mail Drop
16 ABSTRACT The report reviews techniques of metal aerosol generation for the purpose
of establishing the state-of-the-art of the technology and guiding future researchers.
Exposure to metal or metallic compound submicron aerosols is widespread in both
industrial and general environments. Research in areas of health effects, sampling
instrumentation, and air pollution control technology requires a reliable source of
test aerosol. The report presents many mechanisms for generating metal aerosols,
and discusses their applications, advantages, and disadvantages. Generation methods
can be on a continuous or batch basis, with high or low concentrations and generation
rates, and with monodisperse or polydisperse size distributions. The method chosen
and the resulting aerosol depend on the requirements of the specific research being
conducted. Metal aerosols generated by exploding wires, electric arcs, heating to
evaporation, combustion, and dispersion are presented, with particular attention paid
to particle size characteristics.
17.
KEY WORDS AND DOCUMENT ANALYSIS
DESCRIPTORS
b.IDENTIFIERS/OPEN ENDED TERMS
c. COSATI Field/Group
Air Pollution
Aerosols
Metallurgy
Aerosol Generators
Properties
Testing
Exploding Wires
Electric Arcs
Evaporation
Combustion
Dispersions
Size Determination
Air Pollution Control
Stationary Sources
Particulates
Metal Aerosols
Characterization
13B
07D
11F
15B
14B
19A,14B
20C
21B
18. DISTRIBUTION STATEMENT
Unlimited
19. SECURITY CLASS (This Report)
Unclassified
21. NO. OF PAGES
50
20. SECURITY CLASS (Thispage)
Unclassified
22. PRICE
EPA Form 2220-1 (9-73)
44
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