EPA-340/1-79-002
CONTROL OF AIR EMISSIONS
FROM PROCESS OPERATIONS IN
THE ROCK CRUSHING INDUSTRY
by
JACA Corp.
550 Pinetown Road
Fort Washington, PA 19034
EPA Project Officer: Norman Edminsten
Region X
Enforcement Division
Contract No. 68-01 -4135
Task No. 19
Prepared for
U.S. ENVIRONMENTAL PROTECTION AGENCY
Division of Stationary Source Enforcement
Washington, DC 20460
February 1979
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ABSTRACT
Three basic methods of controlling emission from process
operations in crushed stone plants are described - dry captive
systems using fabric filters, wet suppression systems and com-
binations of these. Operational problems with these systems
associated with plant portability and product size are discussed.
Examples of good design practices and maintenance procedures for
these control options are covered. An electrostatic charged fog
technique for control of small dust particles is described and
operational problems listed. A second part analyzes the down-
wind effects of reducing emissions for worker safety.
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This report was furnished to the U.S. Environmental Protection
Agency by JACA Corp., Fort Washington, Pennsylvania, in partial fulfillment
of Contract No. 68-01-4135, Task No. 19. The contents of this report
are reproduced herein as received from the contractor. The opinions,
findings, and conclusions expressed are those of the author and not
necessarily those of the Environmental Protection Agency.
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ACKNOWLEDGEMENT
The technical discussions, data, and photographs supplied to
JACA from many sources are gratefully appreciated. We especially would
like to note the assistance of the Pennsylvania Department of Environmental
Resources, The Oregon Department of Transportation, The Oregon Water
Resources Board, Professor Stuart Hoenig of the University of Arizona,
Mike Natale of Johnson March, William Ward of Ward Engineering, James Fee
of L.B. Smith, and William Rundquist formerly of Portec Corporation, now
a technical editor for Pit § Quarry.
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SUMMARY
The basic operationally proved methods of controlling dust emissions
are: (1) suppression of the dust by the application of wet sprays preferably
with a surface active ingredient to reduce water surface tension and
(2) the use of hoods,ductwork and fabric filters in a dry capture system.
It is not unusual to find a combination suppression and capture system
with emissions from the early parts of the process (larger size stone)
controlled by suppression and later process points (smaller size stone)
controlled by captive means.
Performance of either of these systems degrades appreciably when
appropriate maintenance procedures are not followed. Operation and
maintenance practices are simple and inexpensive to follow.
Portable plants that are moved frequently encounter take-down
and set-up delays and added costs due to configuration induced difficulties
of connecting ductwork to the appropriate emission sources. These
difficulties are not insurmountable.
Water costs for wet suppression systems or combination dry (captive)
and wet suppression systems in arid regions add to production costs.
These incremental costs viewed in the industry economic context should
not preclude use of wet techniques however.
A new potential method of controlling fine particles less than
10 microns by an electrostatically charged fog is now emerging from the
experimental stage. Important operational and economic factors must be
resolved before the system will find widespread usage in stone crushing
operations.
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Efforts to control emissions from stone crushing plants benefit
both the worker who is in proximity to the equipment and the residents
of the surrounding area. Efforts of a company (except for receptor
control) to meet either the MSHA or EPA standards are mutually helpful,
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Table of Contents
SUMMARY in
TASK PURPOSE ix
INTRODUCTION 1
Industry Background 1
Sources of Air Pollution 6
SECTION I AIR POLLUTION CONTROL OF PROCESS OPERATIONS 11
Wet Suppression System 15
Baghouse System 35
Combination Wet Suppression § Baghouse System 52
Electrostatic Spray Systems 53
SECTION II EFFECTS OF MESA CONTROL STRATEGIES ON
DOWNWIND AMBIENT CONDITIONS 63
Introduction 63
Occupational Health Regulations 64
Relationship of Control for Workers to SIP Control 70
APPENDIX A: Effect of Mesa on NAAQS 73
REFERENCES 83
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List of Tables and Figures
Table No. Page
1 CRUSHED STONE PLANT SITES BY STATES 2
2 DISTRIBUTION OF ROCK CRUSHING PLANT SITES BY SIZE 5
3 MODEL WET SUPPRESSION COSTS FOR PROCESS PORTIONS OF
CRUSHED STONE PLANTS 33
4 MODEL FABRIC FILTER SYSTEM COSTS FOR PROCESS PORTIONS
OF CRUSHED STONE PLANTS 45
5 MODEL WET SUPPRESSION - FABRIC FILTER SYSTEM COSTS
FOR PROCESS PORTIONS OF CRUSHED STONE PLANTS 54
1-A EFFECT OF CHOICE OF STABILITY CLASS 81
Figure No.
1 TYPICAL HARD ROCK STONE CRUSHING PLANT OPERATING AT
300 TONS PER HOUR 9
2 TYPICAL HARD ROCK STONE CRUSHING PLANT OPERATING AT
600 TONS PER HOUR 10
3 PERMANENT CRUSHED STONE PLANT WITH COVERED SCREEN
HOUSES 12
4 DISTANT VIEW OF PLANT PRODUCING CRUSHED STONE 12
5 CLOSE UP OF PLANT SHOWN IN FIGURE 4 13
6 A STONE CRUSHING PLANT CONSISTING OF UNITIZED
PORTABLE EQUIPMENT 14
7 TYPICAL SPRAY BAR APPLICATION 16
8 TYPICAL APPLICATION POINTS FOR A WET SUPPRESSION
SYSTEM IN A CRUSHED STONE PLANT 18
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List of Figures Continued
Figure No.
9 BEFORE AND AFTER APPLICATIONS OF WET SUPPRESSION
ON A PORTABLE PLANT 20
10 BEFORE AND AFTER WET SUPPRESSION ON A SCREENING
OPERATION 21
11 BEFORE AND AFTER WET SUPPRESSION ON A PORTABLE
CRUSHER-SCREENING OPERATION 22
12 PROPORTIONER, MAIN PUMP AND SURFACTANT DRUM 26
13 CRUSHED AGRICULTURAL LIME OPERATION 37
14 GOOD PICKUP DESIGN ON A PORTABLE PLANT 37
15 GOOD HOOD DESIGN ON SCREENING OPERATIONS 38
16 HOOD WITH RUBBER SKIRT AT SECONDARY CRUSHER OUTPUT 38
17 DUCTING SUPPORTS 47
18 FLEXIBLE SECTION BETWEEN HOOD AND DUCT TAKEOFF ON
SECONDARY OF A PORTABLE PLANT 49
19 DUCTWORK CONNECTIONS TO A TERTIARY TRIPLE SCREEN UNIT
OF A PORTABLE PLANT 49
20 CRIBBING ON PRIMARY IMPELLER TYPE CRUSHER IN A
PORTABLE PLANT 50
21 EXTENSIVE CRIBBING ON TERTIARY CONE CRUSHER WITH
TRIPLE DECK SCREENS 50
22 TALL NARROW BAGHOUSE IN BACKGROUND ON A PORTABLE PLANT 51
23 TALL NARROW BAGHOUSE USED WITH A PORTABLE PLANT 51
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List of Figures Continued
Figure No. Page
24 CHARGE ON TRAP ROCK (3 RUNS) 56
25 CHARGE ON SILICA SAND (5 RUNS) 57
26 TEST SET UP CEMENT PLANT "A" 59
27 PLAN VIEW OF SURGE BUILDING CEMENT PLANT "A" 60
28 CONCENTRATION VS PARTICLE SIZE BEFORE AND AFTER
SPRAY IN SURGE BUILDING CEMENT PLANT "A" 61
1-A SKETCH OF THE GEOMETRIC RELATION BETWEEN THE
VIRTUAL SOURCE POINT, THE SOURCE AREA AND AVERAGE
POLLUTION CONCENTRATION at LI and L2 75
2-A THE PRODUCT OF a a AS A FUNCTION OF DOWNWIND DISTANCE
y z
FROM THE SOURCE 78
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TASK PURPOSE
Under Task 19 of Contract 68-01-4135 the Contractor was to describe
control systems commonly used on stone crushing plants to control process
fugitive dust. Typical annualized costs and maintenance procedures were to
be noted. Special problems in the control of portable plants in arid regions
were to be addressed.
In addition, the Contractor was to examine the possible amelioration
of downwind ambient particulate contributions from plant operations resulting
from process control designed to meet MESA (now Mine Safety and Health
Administration) requirements.
This report is divided into two sections: Section I, Air Pollution
Control of Process Operations, and Section II, Effects of MESA Control
Strategies on Downwind Ambient Conditions.
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INTRODUCTION
Industry Background
In evaluating information on control of air pollution resulting from
crushed stone operations it is desirable to have a general concept of the
industry. This includes plant distribution, size, concentration, raw
materials, customers, and market economics.
The crushed stone industry is one of the largest mining industries
in the United States with production in 1975 totaling 902 million tons,
valued at $2 billion, Crushed stone sites are found in every state except
Delaware. There were, according to the Bureau of Mines, 5,523 operating
plant sites in the United States in 1975. The plant site count by state
is shown in Table 1. The production and market for this crushed stone
is all within the United States. Export trade and foreign imports are
negligible.
The raw materials used in the crushed stone industry are derived
from all three geological classes of rock: igneous, sedimentary and
metamorphic. The specific stone used is basically a function of its
availability, quality and ability to meet the specifications and standards
for a specific product. Some stone types are not well suited to the crushed
stone market requirements. Shale, shaley limestone and slate are examples
of stone whose chemical and physical properties are undesirable. On the
other hand, the properties of limestone allow it to be used for a variety of
end products. Because of its versatility and availability, about 70% of all
stone crushed in the U.S. is limestone. Other stone types used extensively
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Table 1
CRUSHED STONE PLANT SITES BY STATES*
State Crushed Stone
Alabama 54
Alaska 116
Arizona 43
Arkansas 93
California 517
Colorado 81
Connecticut 18
Delaware
Florida 122
Georgia 74
Hawaii 34
Idaho 69
Illinois 325
Indiana 130
Iowa 337
Kansas 196
Kentucky 111
Louisiana 20
Maine 17
Maryland 31
Massachusetts 34
Michigan 49
Minnesota 102
Mississippi 8
Missouri 274
Montana 74
Nebraska 27
Nevada 96
New Hampshire 38
New Jersey 32
New Mexico 41
New York 92
North Carolina 92
North Dakota 5
Ohio 159
Oklahoma 76
Oregon 420
Pennsylvania 216
Rhode Island 4
South Carolina 34
South Dakota 21
Tennessee 135
Texas 206
Utah 26
Vermont 46
Virginia 1?T
Washington 254
West Virginia 52
Wisconsin 369
Wyoming 25
Total 5,523
*Source, Reference 1.
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for crushing are: granite, traprock, argillite, diabase, sandstone, marble,
quartzite, quartz and gneiss.
The largest market for crushed stone is the construction industry.
Contruction use accounts for about 85% of crushed stone production which
includes: dense-graded road base, concrete aggregate, bituminous aggregate,
cement, and riprap. Other major crushed stone products are: lime, agricul-
tural lime, mineral fillers, filter stone, glass, refractory stone and chemi-
cal stone.
Portable plants are often used by producers to solve a number of
production and market problems. In situations where the demand for stone
from a particular quarry does not justify the capital expenditure required
for a permanent plant, a portable unit can be used to serve a cluster of
quarries. This is often the case in the midwest and western areas where
portable plants are used to build up stockpiles of crushed products, after
which the plant is moved to the next quarry owned by the producer. It is
difficult to pin down the number of portable plants. The Bureau of Mines
reports activities at plant sites, a number of which could be served by a
single portable plant. About 28% of the reported plant sites are worked by
2
portable plant equipment, according to Bureau of Mines figures . Portable
plant sites by leading states in descending order of number of sites are:
Iowa, Wisconsin, Illinois, and Oregon.
The wide distribution of crushed stone operations is due in part
to the high costs of transportation relative to the cost of the product.
The cost of transportation of crushed stone by water is significantly less
than that for rail or truck, and where producer and user are close to water
transport, the market sphere is enlarged.
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Plant operations can range from a production capability of about
100 tons/hr. to over 1,000 tons per hour with annual production ranging
from less than 100,000 tons to over 2 million tons. Bureau of Mines data
for 1973 (see Table 2) show that about 40% of the production is from plant
sites of 900,000 tons and over annually, which account for only 10% of the
total number of plant sites. Those with an annual production of less than
100,000 tons accounted for 23% of the total number, yet only 4% of the
total production. This seemingly high concentration ratio is exaggerated
by the fact that several plant sites are serviced by one portable plant.
The crushed stone industry, nevertheless, illustrates a high concentration
ratio with plant sites producing 1 million tons or greater, exerting a pro-
nounced effect on total production.
The crushed stone industry competitive posture can be characterized
by the following:
1) Production is concentrated close to urban areas
2) Within a marketing region, yard prices are competitive
3) On public bids delivered price is competitive
4) Transportation costs are the dominant cost feature in
competition
5) Product demand is essentially inelastic
6) Competition is basically intra industry; some inter industry
competition exists where crushed stone may be substituted for
sand and gravel.
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Table 2
DISTRIBUTION OF
Plant Size
thousand tons per year
50-99
100-199
200-299
300-399
400-499
500-599
600-699
700-799
800-899
over 900
ROCK CRUSHING PLANT
No. of Plants
in size range !
989
1,075
516
387
301
215
172
129
86
430
SITES BY
fc of Total
23
25
12
9
7
5
4
3
1
4.
10
100
SIZE*
Contribution to National
Production, % of Total
4
9
7
8
8
7
6
6
4
41
100
*Source of Data, U.S. Bureau of Mines 1973 Statistics.
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The inelasticity of overall industry demand, the concentration of
quarries and plant sites near urban areas, and the high cost of product
transportation results in a localized competitive picture, although com-
petitive spheres of operation can be large in instances where the quarry
(or pit) and customer are located near a rail or barge loading facility and/
or if the quarry (or pit) produces unusual sizes or quality of material.
Sources of Air Pollution
Rock crushing activities can be broadly classified as quarrying
and beneficiation. Quarrying consists of removal of overburden, the earth
over the desired stone deposit; drilling blast holes; blasting; transport
to beneficiation facilities. Beneficiation or process facilities include
crushing, screening for size, conveying and storage. The information on
controls for rock crushing plants and the effect of MESA compliance on down-
wind ambient in this report cover the beneficiation portion of the activities
only.
The beneficiation may take place using either wet or dry techniques
at the various crushing or screening processes points. Some plants use
dry processing in all product sizes except the "fines" production, where
wet processing is often used. Where wet techniques are involved the'
air emissions are significantly reduced over those in dry process operations.
In those instances where wet process points are involved, processing of
the water effluent is needed to correct for total suspended solids (TSS)
in the wash water. Stone quarries are under a zero discharge system require-
ment which requires recirculation of the water with appropriate settling
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ponds and in some instances treatment by flocculating agents to increase
sedimentation rate.
The plants considered in this report are dry processing plants
although the raw stone may be in an initial wet condition.
Once the rock is broken into pieces which can be accommodated by the
primary crusher, either directly from a well-controlled blast or by a com-
bination of blast and drop ball, the fractured rock is trucked to the first
or primary crusher, where it is dumped onto a scalping screen and feeder
which scalps (or removes) the smaller pieces and overburden which do not
require primary crushing, therefore reducing the load on the crusher and
removing unusable material, freeing the crusher for its purpose of crushing
only the larger rocks. The scalped material is conveyed to a separate spoils
pile for disposal, while the crushed material is generally conveyed to a
surge pile (temporary storage pile) for distribution to subsequent processing
points.
Crushers utilize compression and impact to mechanically stress the
rock beyond its breaking point. Some crushers rely almost solely on impact,
whereas at the other end of the range compression is used. The more impact
involved for a given stone, the more mechanical propulsion of particles and
air turbulence tends to increase emissions.
Screens are located before each crusher. Screens agitate the
particles as they proceed across the surface so that each particle has an
opportunity to align its two smallest dimensions with the screen holes.
This process of agitation frequently developed by an eccentric action pro-
duces airborne dust by mechanical and air turbulence forces.
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Whenever a material is transferred into a bin or hopper, or from
one conveyor to the other, or into a screen deck or crusher, air is induced
and displaced. This action is largely responsible for the smaller particles
becoming airborne.
Thus the process areas that must be controlled for fugitive dust
in stone crushing beneficiation steps are:
• Crushing Operations
• Screening Operations
• Storage
• Transfer Points
The number, location, type, and size of crusher and screens vary
from plant to paint depending on many factors. These factors include type
of stone, production rate, range of products, and process space available.
Figures 1 and 2 taken from Reference 4 show typical plant operations
at 300 and 600 tons per hour. These are the model plants costed for
air pollution controls later in this section.
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Figure 1
TYPICAL HARD ROCK STONE CRUSHING PLANT OPERATING AT 300 TONS PER HOUR
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Figure 2
TYPICAL HARD ROCK STONE CRUSHING PLANT OPERATING AT 600 TONS PER HOUR
CleyOOO_
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Section I
AIR POLLUTION CONTROL OF PROCESS OPERATIONS
Dust in the process operations of crushing, conveying, and sizing
in crushed stone operations is generally controlled either by suppressing
the tendency of the dust to become airborne by applying water that has been
mixed with a suitable chemical to reduce its surface tension directly to
the material, or by capturing dust that might otherwise become airborne in
a closed dry collection system. The first will be referred to as wet sup-
pression and the latter as a baghouse system. A recently introduced system
using charged water sprays is emerging from the experimental phase but remains
to be proven in field operations for crushed stone plants. All three of
these systems will be described with emphasis on the first two since they
have been successfully used in the field while the third is not fully demon-
strated.
Crushed stone plants can be permanently placed with individual
parts of the process nearly completely covered like those shown in Figures
3, 4, or 5; or they can be highly portable, mounted on truck beds, with a
typical eight-day cycle from start of preparation for a move to completed
erection of the plant at a new site (see Figure 6).
Unique application problems with portable plants are discussed at the
end of each section on the two main control strategies covered in this
report.
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Figure 3
Permanent crushed stone plant with covered screen houses to the left.
Plant is controlled by two baghouses, one left foreground, the
second right background.
(Photo courtesy Pennsylvania Department of Environmental Resources, PennDER)
Figure 4
Distant view of plant producing crushed stone. All process units
enclosed. Plant controlled by cyclone primary and baghouse.
(Photo courtesy of PennDER)
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Figure 5
Close up of plant shown in Figure 4, emphasizing the covered, permanent
type of plant operation.
(Photo courtesy of PennDER)
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Figure 6
A Stone Crushing Plant Consisting of Unitized Portable Equipment
(Photo courtesy of Cedarapids, Iowa Manufacturing Company)
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Wet Suppression System
The use of water as a means of dust suppression has been practiced
for more than thirty years in both coal and non-coal mining operations. Water
sprays, either directed by nozzles at transfer points, crusher exits and
screening operations or applied manually by spray hose at muck piles (piles of
rock from blasting) significantly reduce emissions.
System design is not guided by equations based on the physics of the
situation, but rather on rules-of-thumb learned from many years of applica-
tion experience. A wet suppression system as the name implies prevents or
suppresses the tendency of the particles to become airborne. From the modest
amount of water added (about 1/2 of 1% of weight of stone feed), it is ob-
vious that an increase in density of a particle by means of retention of the
water is not the sole mechanism for preventing emissions. The agglomeration
of the small particles and the "sticking" of small particles to large pieces
of stone is also involved. The precise mechanism of suppression is not as
important as the fact that dust is suppressed when water is applied. As dis-
cussed later, water alone has poor wetting properties and a solution includ-
ing a surfactant or surface active ingredient is usually applied. This
substantially reduces (about 4:1) the amount of water that is required.
Application is made by spray heads mounted on a spray bar as in
Figure 7. There are hundreds of different spray head designs when the com-
binations of flow, droplet size, coverage angle, spray crossections, etc. are
considered. Droplets are generally formed by the water pressure on specially
designed channels and exit orifices in the spray heads. Very small droplet
sizes that rely on air atomization are practically never used, Different
size and types of sprays are often used on different parts of the plant - the
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Figure 7
Typical Spray Bar Application
(Photo courtesy of Johnson March)
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larger droplet types early in the production and the finer droplet size with
the smaller size materials. Choice of spray heads are affected by production
rate, stone size, and location ability. The spray bars on which the spray
heads are mounted are located with sufficient clearance from the belt, screen,
etc. to make sure that moving stone does not impact, trucks or other vehicles
are not likely to hit it (as for instance in the loading area of the primary
crusher) and so the unit can be observed and maintained with reasonable ease.
Sprays are usually located about 3 to 6 feet from the contact point - splash-
ing occurs if too close to the surface and the advantage of a fine spray will
be lost, while if spray bars are too far from the surface, crosswinds will
have more of a chance to deflect the spray away from its target. The design-
er usually examines the access problem, stone size and production rate in
determining the number, type, and location of the spray positions in a proc-
ess.
A good figure ' for the amount of solution sprayed on the stone is
1.5 gallons per ton of plant production. If chemicals that reduce the sur-
face tension of water are not used, this figure could be three or four times
greater. The 1.5 gallons of solution per ton of production is not applied
at one point, rather it is the sum of the solution distributed at various
points throughout the plant. Figure 8 shows typical application points for
such a system.
Plain water with a surface tension of 73 dynes per cm at 20 C does
not exhibit good qualities of wetting, spreading and penetrating. Use of
water as the sole spray materials therefore would require more gallons per
ton of production than a solution containing a surfactant. When surfactant
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TRUCK DUMP
INCOMING WATER LINE
Surfactant
PROPORTIONER
Figure 8
Typical Application Points for a Wet Suppression System in a Crushed
Stone Plant
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are added, the surface tension of water is reduced to 20 to 30 dynes per
cm which greatly enhances the dust suppression since the water is able to
penetrate the dust particle creating a cementing action between dust particles
so that the agglomerates thus formed are too heavy to become airborne.
"Wetting Agents" is a broad category which covers such items as emul-
sifiers, solubilizers, detergents, foams, penetrants, thickeners, etc. Dust
control compounds, on the other hand, are carefully formulated blends in
which one or more special surface active agents ("surfactants") have been
incorporated. The molecules of these compounds are composed of two groups
exhibiting differing solubility characteristics. One part, usually a long
chain hydrophilic or water loving group is usually a sulfate, sulfonate,
hydroxide, ethylene oxide, etc. The other group is a long chain hydrophobic
or water hating group. When properly proportioned, these compounds effect-
ively reduce the water surface tension.
Wet suppressent systems with surfactant solutions appear effective
in dust control although there is no quantitative data to specify the degree
of control attained. Figures 9 through 11 indicate the effectiveness of well
designed wet suppression systems. Such systems, however, cannot be univer-
sally used for dust control. A spokesman for one of the largest companies
that makes and installs wet suppression systems stated that in their experi-
ence about 75% to 85% of crushed stone operations could use wet suppression
systems. Some stone type and product size operations could not use the systems.
In these cases, dry collection equipment is used or a combination wet suppres-
sion and dry collection.
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Figure 9
Before and After Applications of Wet Suppression on a Portable Plant
(Photo courtesy of Johnson March)
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Figure 10
Before and After Wet Suppression on a Screening Operation
(Photo courtesy Johnson March)
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Figure 11
Before and After Wet Suppression on a Portable Crusher-Screening Operation
(Photo courtesy of Johnson March)
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In the initial process stages of production, i.e. primary crushing
and screening and secondary crushing and screening, problems associated with
wet suppression are minimal. However, when used to control dust at tertiary
crushing and screening where small product sizes and screen openings are en-
countered, problems may arise. The wetted dust tends to blind the openings
in fine screens. In agricultural limestone where very fine material is re-
quired in a dry state, the addition of water is not practical. Many states
also have what is known as a wash out test specification on certain grades
of stone. This test is designed to measure and thereby control the amount
of -200 mesh (74 microns) particles in the product. A given amount of
product is washed in a prescribed fashion and the amount of particles
passing a 200 mesh screen weighed. If the weight of these particles exceeds
a certain amount of the product weight (usually 1% to 2%), the lot from
which the sample was taken is rejected. If the stone is to be used for cer-
tain types of concrete products, both the fines and any surfactant remains
may have to be washed out of the stone before it is acceptable. This requires
an additional plant operation with the need for more water, on the order of
several hundred gallons per ton of product processed. The water residue
would then have to be treated for solids removal before discharge to surface
waters. If discharge is to municipal facilities, a charge based quantity
discharge and a surcharge based on solids content may be levied. If the
plant discharges into a municipal system that was constructed or upgraded
with a government grant under the Federal Water Pollution Control Act, the
plant would be assessed both a User Charge and an Industrial Cost Recovery
Charge.
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A potential problem is occasionally noted relative to a tendency
of the surfactant material to cause air voids in concrete and bituminous
mixes. No data could be found to identify this as a practical problem
on a national basis. Several producers in the Pennsylvania-New York
border refrain from using surfactants in their wet suppression systems,
but no national pattern or test evidence was uncovered.
System Configuration
The method of suppression described above requires a series of spray
bars and nozzles, a supply of surfactant which is proportioned and mixed with
the water, a set of connecting pipes, pumps to force the water to the spray
heads, and appropriate filters. A method of automatically turning on the
system only when material is being produced is required, as is winterizing
equipment when the system is to be employed in beldw-freezing weather.
The system is shown below in schematic with only two spray bars
to demonstrate the equipment involved.
Incoming Water
Drum of Surfactant
Proportioner
Spray Bar #1
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Spray Bar #2
A
Spray
Controller
'M.
Control
Actuator (s)
Spray
Controller
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The function of the proportioner shown in Figure 12 is to filter
the input water if necessary, add surfactant from a drum at the rate of
one part to 1000 parts water or more, and develop a pump head of about
150 psi. This pressure is typically used at the truck dump at the front
end of the crusher and reduced to about 60 psi at the other spray bars by
means of pressure regulators.
The control actuators automatically detect presence of stone on the
conveyor and transmit the signal to the spray controllers. The actuators
operate on a variety of principles including mechanical displacement of
the conveyor belt, weight, electrical interlock with the conveyor drive
motors, by measuring the current load to drive motors, and for fine or
light weight materials that cause little conveyor deflection or motor
drag, by a device mounted on top of the conveyor that is deflected by
material on the conveyor.
A spray jet controller is mounted before each spray jet header and
consists of a filter and a method of governing the flow of the mixed
solution supplied by the proportioner.
Maintenance
According to a report prepared for the U.S. Bureau of Mines by MSA
Research Corporation in April, 1974 , adequate maintenance of dust control
systems in non-coal mining and ore processing operations seems to be the
exception, rather than the rule. The report contained results of visits
to 50 mines and 51 mills (processing plants) of which seven were crushed
stone.
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Figure 12
Proportioner, Main Pump and Surfactant Drum
(Photo courtesy of Johnson March)
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Where wet suppression systems were employed spray bars were found
where the nozzles were either completely clogged or clogged to the point
where they were ineffective. In most cases, this problem was due to
inadequately maintained water line filters. The report was generally
critical of the state of maintenance of control equipment which was reported
in a poor state of repair in relation to general plant.
As with any dust control device, a wet suppression system must be
adequately maintained to keep its efficiency at peak levels. The principal
source of trouble is in foreign material blocking the liquid flow. If the
system operates on city water the filters can go for months without serious
effects whereas if the water is from wells, rivers, or ponds more maintenance
will be involved. Although the system details vary from manufacturer to
manufacturer there are generally three filters, one at the proportioner, one
at each spray controller, and one in each spray. The main filter in the
proportioner demands the most attention because it is first to "see" the
conditions of the supply water. One manufacturer suggests the following
maintenance procedures. After some experience is gained with the system the
maintenance schedule can be adjusted. For instance if the strainer baskets
remain clean after several weekly checks this procedure could be extended.
On the other hand, the strainer basket in the proportioner might need more
frequent attention if the input water has a high amount of solids.
-27-
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DAILY MAINTENANCE
1. At time of start-up of Proportioner
a. Check pump discharge pressure.
b. Check operation of level control valve.
c. Check operation of surfactant pump.
d. Check level of surfactant compact in drum.
e. Check operation of Inlet Water Filter,
2. Visually check spray pattern and direction of all spray jets.
Clean and adjust as necessary.
WEEKLY MAINTENANCE
1. Clean strainer basket in each Flow Controller.
2. Clean strainer basket in the Proportioner.
3. Check operation of all Automatic Spray Controls.
GENERAL
1. Lubricate all equipment requiring lubrication, including
wheels on Automatic Spray Controls, when other plant equipment
is lubricated.
2. Before first freezing weather, check all heating equipment.
(Winterizing is accomplished by wrapping electrical heating
tape around pipes much the same as in winterizing residential.)
-28-
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More detailed information is usually contained in the System and
Equipment Operation and Maintenance Instructions generally furnished by
the vendors with the equipment.
Portable Plant Applications
Spray systems are readily installed in portable plants, and by the
use of flexible connections from the proportioner to the spray headers,
can accomodate changes in plant layout. In cold weather there is a problem
in winterizing such installations by using heating tape because of the
flexible nature of the hose and its heat transfer characteristics compared
to metal piping.
In areas where arid conditions are encountered, there are special
problems on water needs for the system which must be considered in setting
up the plant. As noted previously, if a surfactant is used, the total water
needs for a plant would be about 1.5 gallons per ton of product. If a
surfactant is not used, water requirements would be 4 to 5 gallons of water
per ton of production.
There are several regions of the country which have arid conditions
and where portable plants are extensively used. Eastern Oregon, for example,
is one such region of low precipitation and with high reliance on portable
plants.
Stone crushing operations in this region might obtain water from three
sources; ground water (wells or old quarries that have intercepted the water
table), surface water (rivers, streams, lakes, ponds, etc.), or tank truck.
Water wells may be constructed when a plant is to remain in one location for
-29-
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a period of time or if the plant returns periodically to the location. This,
of course, means that sufficient ground water supplies must be available to
meet the plant's needs, and because ground water occurs sporadically in
Oregon, well drilling is relatively uncommon. The cost of drilling a well
is about $20 to $25 per foot plus $15 per foot for casing (steel). Water
when available via wells occurs within 100 to 300 feet of the surface which
would cost between $3400 (drilled to 100' and cased to 75') and $10,000
(drilled to 300' and cased to 225'.).
Surface water supplies can be used when they are available, A plant
could purchase water from the owner of a farm pond. In other instances, a
stream may be temporarily dammed to serve as a source of water for the
crushing operation. The cost of water is about $10 per 1000 gallons or
about $0.01/gallon. This is the prevailing cost of water in both Eastern
and Western Oregon according to Oregon sources.
When neither ground water nor surface water supplies are available
to a stone crushing operation, water can be supplied by tank truck. These
tank trucks can be purchased or rented by the crushing plant. Crushed stone
plants with their own water trucks, are frequently of the "home-made" variety.
That is, a tank will be mounted on the bed of an old truck and used for
hauling water. The cost is less than half of a new truck. The cost of
purchasing a new tank truck can be from about $22,000 for a 4,000 gallon
truck to $43,000 for a 10,000 gallon truck (either diesel or gasoline engine
powered). The operating cost of one of these trucks is about $0.02/ton of
product (this includes labor, rent, fuel and servicing).
Another way of approaching the cost is to use rental figures set
forth in a state's method of adjusting highway construction contract
figures for unanticipated additional work. In Oregon, for example,
-30-
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g
according to Oregon highway personnel when the state requires supplemental
work done and desires to use a contractor's equipment, there is a set pay-
ment schedule on what is known as a "force account". This schedule shows
that on a weekly basis the contractor would be paid an amount of $399.65 +
15% for overhead and profit for the use of a diesel powered gravity type
sprinkling truck (the closest to a water truck on the schedule). Assuming
profit at 1/2 of the 15% figure, the weekly cost would be $430 or $10.75
per hour prorated over a 40-hour week. This figure includes all expenses,
including fuel and oil, but excludes labor. Labor at $12.35 per hour
including markup is, according to contacts with Oregon highway personnel,
a realistic labor rate to drive the water haul truck to and from a pick-up
point. Since only about 2 hours per day would be spent in driving the
truck, the hourly prorated amount would be 2/8 of $12.35 or $3.09. The
total hourly cost would be $10.75 + $3.09, or $13.84. For a 300 ton
per hour plant (one that could be supplied by one 4,000 gallon truck on
one trip per day) the cost per ton capacity would be $13.84 T 300 tons
or 4.61$ per ton. To this must be added the cost of the water.
Despite the fact that there is a considerable difference in precipi-
tation between eastern and western Oregon, adequate water supplies are
available if trucking is done and the prevailing rate of about $10 per
1000 gallons of water holds in both regions . The water cost, assuming
1.5 gallons per ton of product would add 1.5$ to the trucking cost for a
total of 6.11$ (4.61$ + 1.5$) per ton of stone crushed for a 300 ton per
hour plant. For a 600 ton per hour plant the cost is 4.45$ (see Table 3)
per ton added.
-31-
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Therefore, the trucking cost alone more than doubles per ton
control costs by wet suppression of a plant required to haul water over
a plant which is not required to haul water. This cost of control, however,
viewed in relation to product cost and market posture of the industry is
not unreasonable. While operations using wet suppressent methods in
arid regions will experience higher operating costs, the inelastic nature
of the market and the strong effect transportation exerts on price indicates
that such costs can be passed through with negligible change in product
demand.
A portable 300 ton per hour plant using a wet suppression system
would experience control costs of 9.84£ (see Table 3) per ton if water
had to be hauled. The price of crushed stone FOB quarry is on the order
of $2.50 per ton. It varies according to location of the quarry and size
stone produced. The average price of 1975 stone received by one state
depot in Pennsylvania in response to competitive bids was $2.41 for 2A
q
stone, and 2.73
-------
Table 3
MODEL WET SUPPRESSION COSTS
FOR PROCESS PORTIONS OF CRUSHED STONE PLANTS
Capital Costs for Installed System
Small Plant Large Plant
500 Tons/Hr. 600 Tons/Hr.
Wet Suppression Equipment $58,643 $69,108
Annualized Capital Costs $ 7,330 $ 8,639
(12.50% of Capital)
2
Operating § Maintenance
Electricity § .04/KWH $ 115 $ 288
Maintenance Operation £ Supplies $14,048 $22,398
Total Annualized O&M $14,163 $22,686
Total Annualized Cost $21,493 $31,325
(Capital + O&M)
Cost/Ton (excluding water hauling 3.73^ 2.72$
£ water costs in arid
regions)
Added Cost/Ton for water $ hauling
in arid regions 6. Hi 4.45
-------
At that time the average price of a cubic yard of concrete in place
was $125. The average price of the stone at the quarry was $1.73 per ton;
FOB delivered it cost approximately $3.00. This was less than 1.4% and
2.5%, respectively, of the price of the concrete. Thus a 10% increase in
the price of the stone at the quarry would have a negligible effect on the
per yard cost of concrete in place, increasing the price by $.17. The
effect of a 10% increase in the price of stone would have about a $7 increase
in the price of a $30,000 home; a $500,000 building would be increased by
$180, a $400,000 per mile highway by $640 per mile and a $360,000 school
building by $140.
Transportation costs make the industry one of local competition
with the notable exceptions of some stone sources with access to cheap
water transportation. Stone transportation costs range roughly from 12
to 19
-------
Baghouse Systems
Dust in process operations of crushed stone plants is often
controlled by dry collection techniques using appropriate methods to capture
the dust and transport it to one or more baghouses. Whether a central
baghouse is used or two separate baghouses depends principally on the
layout of the plant and is decided on the basis of installation and operational
economics. Baghouses must be used in those operations where stone type and/
or small product size preclude the use of wet suppression techniques.
Principal System Components
A baghouse system in crushed stone process operations can be examined
in the light of the four functions which make up the total system design:
• Pick up design
• Ducting design
• Filtering design
• Filter catch handling
Pick Up Design:
This is a key factor in a dry collection system. If the dust that
would otherwise become fugitive is not properly "picked up", the system will
fail in its job. A pick up system consists of a hood or enclosure and an
indraft sufficiently high to entrain the dust that may otherwise become
fugitive. The indraft velocity in feet per minute multiplied by the area
of the hood opening in square feet yields the volume flow in cfm required
to adequately vent a particular dust emission point to the connecting duct
-35-
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and thence to the baghouse. There is general agreement on indraft design.
A minimum draft flow of 200 fpm at pick up points is frequently used.
Some general rules applied at various operational points are:
1. Volume flow rate at conveyor transfer points of 350 cfm per
foot of belt width for belt speeds of less than 200 feet per
minute and 500 cfm per foot of belt for speeds over 200 feet
per minute.
2. Volume flow rates of 100 cfm per square foot of casing cross
section for bucket elevators.
3. Volume flow rates of 50 cfm per square foot of screen area
on vibrating screens either single or multiple deck.
Not only must the necessary indraft flow be provided, but the
emission point should be covered as completely as possible to avoid the
effects of wind interference which would otherwise blow the particle away.
For example if a 200 feet per minute (fpm) indraft velocity is used, a
cross wind of 10 miles per hour would exert a cross draft of 880 feet per
minute on the particle. Its resulting trajectory rather than being up and
into' the hood and to the transfer duct would then be at 87° to the intended
direction, and would likely not be captured. Good enclosure and hood
design are indicated in Figures 13, 14 and 15.
It is common practice to use abrasion resistant rubber liners around
hoods venting crushers or transfer points (see Figure 16). These liners are
often constructed with slits in the lower edges so that they can be slightly
larger than the openings, but not impede material flow on conveyor access
points and crusher openings. It is good practice to cover conveyor belts to
prevent dust being carried away by the wind. Screen houses are frequently
enclosed and vented from the top.
-36-
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Figure 13
Crushed Agricultural Line Operation Showing Good Enclosure Design at Transfer Points
(Photo courtesy of PennDER)
Figure 14
Good pickup design on a portable plant
(JACA Photo)
-37-
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Figure 15
Good Hood Design on Screening Operations of an Agricultural Crushed Stone Plant
(Photo courtesy PennDER)
Figure 16
Hood With Rubber Skirt at Secondary
Crusher Output
(JACA Photo)
-38-
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Ducting:
Air velocities in the ducts leading from the hoods to the baghouse are
typically 3500 to 4500 fpm. This velocity is needed to keep the particles
entrained in the air stream rather than settling out at bends, turns and
connecting points in the ductwork system. Tight bends should be avoided
both because particles may be knocked out there and because of abrasion of the
metal walls. In addition bends and abrupt connecting of ductwork introduces
frictional air losses which must be compensated for by larger fans and motors.
Vibration is also a problem at crusher and screen pick ups which is frequent-
ly solved by using a flexible section at the inlet to minimize damage.
Fabric Filters:
The actual filtering operation in crushed stone operations is a reason-
ably simple one. Problems such as high gas temperatures and corrosive atmos-
pheres that constitute design problems in many baghouse installations are not
present. Nomex and fiberglass bags need not be used, insulation is not
generally required, and temperature adjustment and temperature safety devices
are not called for. Sizing, as in any baghouse, remains a critical design
factor. The ratio of the amount of air by volume to the lateral area of the
bags, the so called air-to-cloth ratio, can vary from 2 or 3 to 1 for mechan-
ically shaken bags to 6 or 7 to one for pulse air type units. In the mechan-
ically shaken bag method the baghouse is larger because of the lower air to
cloth ratio and the fact that one compnrimiMii must !><• tiiKon of) ••( ro.'un fur
shaking, while in the positive air blowdown the full system is essentially
always on stream. While mechanical shake baghouses are larger for a given
gas handling capacity, they require less in the way of bag cleaning equip-
ment. Air blowdown requires sets of solenoids and air headers on each row
of bags, air Venturis for each bag, and an air compressor to supply the nec-
essary blow down air.
-39-
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The fan is usually located on the side of the baghouse opposite the
baghouse inlet - the "clean side" of the system. The air is pulled through
the baghouse giving rise to the term negative pressure baghouse. Thus, the
fan blades do not experience the wear they might otherwise if the fan was
located on the dirty side of the system. The negative pressure aspects, how-
ever, means that the baghouse should be well sealed, and leaks in ducting
promptly repaired.
Dust Disposal:
What is done with the captured fines varies from plant to plant
depending on the product being manufactured and "outside11 markets for the
fines in that particular area.
Dust hoppers usually have an air lock valve emptying the bin either
into a truck or via a screw conveyor or a pneumatic conveyor to some other
direct use or storage for future use. For instance, the baghouse controlling
the primary crusher of report 75-STN-3 periodically discharged the catch
during the cleaning cycle through an air lock onto the covered conveyor lead-
ing to the secondary crusher. Its discharge of about 10 tons daily is fed
directly back into the process. The second baghouse which collects material
from secondary and tertiary crushing and screening operations discharges
directly into an agricultural stone bin and is used as final product.
13
The baghouse reported in 75-STN-78 discharges a total of about four
tons per day into a truck. The material was then used as choke material in
road based stone.
Whether the material is reintroduced into the product, sold separately
as fines or discarded in a suitable dump site depends on the markets open
to a particular plant.
-40-
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Maintenance
Failure of a conveyor, a crusher, or screening operations would
constitute immediate plant problems, production might stop, and if the nature
of screen failure involved holes in the screen out of size rejects would
mount. Therefore these portions of the plant receive preventative mainten-
ance to reduce potential downtime. If failure occurred prompt attention
would be taken to restore production and assure quality control. The dry
pollution collection system is not integral to the plant - the plant can
produce quality stone even if the pollution control equipment is not function-
ing properly. Therefore there is a temptation to devote more maintenance
attention to the productive processes rather than the control process.
This was noted by Reference 7, a study on control efficiency in non coal
mines, both surface and deep, that stated, "It is obvious that production
maintenance has priority over dust control maintenance."
Maintenance starts with enclosures and enclosure skirts. As mentioned
earlier in this section proper enclosures plus an adequate indraft is essen-
tial for capture of what might otherwise be fugitive dust. Enclosures can be
quickly examined - once a week should be sufficient - to detect frayed skirts
or loose or ill fitting enclosures.
Ducting should have suitable clean out holes which should be examined
for dust build-up first on a weekly basis and then adjusted to more or less
frequent cleaning depending on the amount of build-up. Ducting should also
be inspected weekly for holes and separations. If the enclosures and skirts
have a reasonable fit and the ductwork is clear and without holes the remain-
ing points to inspect are the baghouse and operating mechanisms and the air
moving apparatus.
-41-
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The amount of air moving through the system is important because
the amount of air for a given ductwork and hood design determines indraft
and duct conveying velocities. If there is reduced airflow the system
may be working ineffectively. Airflow and systems operation can be checked
very easily by the aid of two inexpensive instruments, costing less than
$100 each - a simple pressure gauge (either u-type water gauge, or magne-
helic) and a hand tachometer.
The pressure gauge is used to measure system pressure drop and
pressure drop across the baghouse. The tachometer is used to measure the
fan speed.
As air moves along ductwork, especially at restrictions, joints,
bends etc. it encounters a resistance to flow reflected in a static pressure
drop There is also a drop in moving through the baghouse, and around any
dampers that might be in the system. System resistance can increase if
dampers are incorrectly set, bags in the baghouse blind, and ducts become
obstructed. If the system pressure drop increases the capacity of the system
to handle the air flow has decreased and it will not have sufficient
indraft.
Since the air handling system is open to atmosphere at both ends,
the system pressure drop is numerically equal to the static pressure
across the fan. It is much easier to measure the static pressure drop across
the fan than to measure all the individual pressure drops that make up the
system pressure drop. A 3/8' pipe metal static tube can be inserted and
welded into the outlet and inlet side of the fan positioned opposite to the
direction of flow to measure only the static pressure. The inlet side is
first connected via a suitable flexible tube to the low pressure end of the
gauge, and a reading taken. The flexible tube is then attached to the out-
-42-
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let side of the fan and the high pressure end, and a second reading is taken.
The difference is the pressure drop in the system. If these readings are
taken when the system is known to be in good operative condition, a diff-
erence of - 20% or more at a later date from the initial condiction at
satisfactory operation indicates some system problem. Lower pressures
point to leaks between the dirty and clean side of the baghouse and/or
large holes in the ductwork, or reduced fan speed. Higher pressures indicate
such problems as obstructions in the ducts, blinding of filter bags in the
baghouse and dents in the ductwork.
Test points for static pressure should also be provided across the
dirty and clean sides of the baghouse, A pressure gauge can be permanently
placed there, and is frequently included with the equipment. This narrows
the diagnosis to the baghouse. If pressure is lower than normal this can mean
that a hole or tear has developed in one or more of the bags, a clamp has
become loose at the plenum spearators, a separation has occurred in the seal
or weld of the plenum separator. If the pressure is higher than normal
it can mean that the bags are blinding either because of damp material or
incomplete cleaning.
The tachometer can be used to measure the fan RPM by pressing the
tachometer tip firmly against the center of the fan shaft while the plant
is in operation. Since most fans encountered in stone crushing operations
are positive displacement devices the amount of air handled is directly pro-
portional to the speed of the fan. A decrease in fan speed which might be
due to operational wear or stretch of the belts used in the drive will re-
sult in a decrease in air handling capacity of the system including the
critical indraft portion.
-43-
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Bag cleaning mechanisms should be checked about once a week. Mechani-
cal shakers should be checked for lubrication and wear. Positive pressure
air cleaning devices should be checked for solenoid operation, and com-
pressor performance about once per week. Manufacturers of compressors and
bags provide maintenance instruction with their equipment. These are usually
broken down into daily (mostly visual), weekly, and monthly maintenance pro-
cedures. Special sections are frequently included on preparations for lengthy
shutdown periods (viz. over winter months in northern states) and turn on.
Control Costs
Control costs for a model plant fabric filter system installed is set
forth in Table 4. The plant installation estimated was shown in Figures 1
and 2.
Portable Plant Applications
Portable plant operations with fabric filter systems are faced with
certain problems if frequent plant movement is required.
Five portable stone crushing plant manufacturers were contacted
regarding baghouse systems for portable plants. None offered baghouses
as a standard part of their portable rock processing plant line. Three
of the manufacturers who make portable asphalt plants offer such units in-
1 2
tegral to their portable plant line. We were also told by two companies '
which serve only the after market in control systems and who offer both wet
suppression and baghouse systems that they have never applied baghouses to
-44-
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Table 4
MODEL FABRIC FILTER SYSTEM COSTS
FOR PROCESS PORTIONS OF CRUSHED STONE PLANTS
Capital Costs for Installed System
Small Plant Large Plant
500 Tons/Hr. 600 Tons/Hr.
Fabric Filter System
Equipment Costs Installed $125,922 $200,802
Annualized Capital Costs $15,740 $25,100
(12.5% of Capital)
2
Operating § Maintenance
Electricity @ .04/KWH $ 8,083 $15,110
Operations § Supplies $19,589 $50,153
Total Annual 0§M $27,672
Total Annualized Cost $43,412
Cost/Ton 7.54
-------
portable plants although both had supplied wet suppression systems to many
portable plants. The difficulty of applying a baghouse system to portable
plants does not stem from making the baghouse proper portable since that is
frequently done for asphalt concrete plants whose configuration is constant
and where only one (in the case of drum mix plants) or at least no more than
three or four duct connections (in hot mix asphalt batch plants) to the
plant are involved. In portable stone processing, difficulty arises from
the multiplicity of ductwork connections and supports, different plant lay-
outs at new sites, and resulting changes in dust loads. This can be
demonstrated by examining one plant in Pennsylvania, and one located in
Maryland.
Both of these plants consist of three portable units. The plant
located in Pennsylvania produced about 2000 tons per day comprised of five
different product sizes. The operation consists of a primary impeller
crusher, secondary crusher and double screen deck and tertiary cone crusher
and triple screens. There are seven pick-up points, four in the secondary
section and three on the tertiary.
The Maryland unit produces about 1600 tons per day comprised of two
products. It includes a primary jaw, a single shaker screen, and a
secondary cone crusher with double-deck screens. There are eight pick-up
points, one on the primary, two on the shaker screen and five on the double-
deck and secondary crusher.
The ducting from both operations is quite extensive and is supported
by outriggers welded to various pieces of operating equipment (see Figure 17)
or ground mounted A frames. The pick up hoods are also attached to operating
equipment and often have a flexible section to help dissipate equipment
vibration. (See Figure 18). Some connections are welded directly to pieces
-46-
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Figure 17
Outrigger Support on Far Left of Duct and Ground Mounted Support on
Right. Center Input Duct Shows Poor Design with Three Short Radius
Bends, and a Perpendicular Duct Intersection. Wear is Evidenced by
White Sections That are Taped.
(JACA Photo)
-47-
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of equipment as shown in Figure 19.
The baghouses at both locations were tall units having 132 bags each
arranged in 11 rows of 12 bags. (See Figures 22 § 23). In shipment they
were designed to be laid on their side on a flat bed. The tall narrow
configuration simplifies dust load out and reduces the number of air headers
and air solenoid valves used in the air pulse cleaning system. In the
Pennsylvania plant the baghouse was mounted with tie brackets on a concrete
base while the Maryland plant was placed on rail-road ties and secured
with four guy wires from the top of the baghouse to a power pole and parts
of the stone operating equipment.
Difficulty is encountered in breaking down this equipment and rein-
stalling it at another location. The A frames must be dismantled and the
ducts must be cut into pieces that can be stored for highway travel. The
outriggers and ducts must be cut free. It is estimated that this dismantling
and reinstallation plus the same for the baghouse essentially doubles the
time normally required to move the plant when configuration at the new site
is identical to the old. The most difficult problem in such reinstallation
is lining up the ductwork with the holes cut in the operating units and
the various hoods. In the Pennsylvania plant, duct connections are made
to two operating units which are on long flat beds which are positioned
by truck, the dollys removed, and the units cribbed as shown in Figures 20
and 21. The Maryland system requires interconnections to three units. It
is difficult to maneuver the equipment beds to the position where seven
or eight ductwork connections can match up. Field cutting and fitting then
must be undertaken to perfect the fit.
-48-
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Figure 18
Flexible Section Between Hood and
Duct Takeoff on Secondary of a
Portable Plant
(JACA Photo)
Figure 19
Ductwork Connections to a Tertiary Triple Screen Unit of a Portable Plant.
Holes Were Cut in Unit and Duct Welded in. Duct Radius and Intersection
Points Show Good Design
(JACA Photo)
-49-
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-50-
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Figure 22
Tall Narrow Baghouse in Background
on a Portable Plant
Baghouse attached to frame mounted
on concrete slab. Caged ladder and
handrails in accordance with OSHA.
Bin empties into truck. Note un-
usual design of exit stack which
exits from clean plenum of the bag-
house (top of filter house) and
then runs horizontally (view ob-
scured by foreground equipment)
(JACA Photo)
Figure 25
Tall Narrow Baghouse Used With a
Portable Plant
Baghouse has been placed on ground
and steadied by guy wires. Baghouse
empties from screw at the bottom of
the house going to the left. Filter
catch is merely piled there and pro-
tected by a tarpaulin. Damp condi-
tion of this site required use of
system less than 10% of the time.
In dry areas this arrangement would
not be adequate.
(JACA Photo)
-51-
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This situation is greatly aggravated if the stone processing operation
changes configuration. Local terrain limitations and or product requirements
might dictate that the process units comprising the operation be grouped in
other than a straight line. This is not infrequent and might involve a
semicircle, T, or L configuration. In this case the ductwork would require
redesign and considerable installation modifications.
Another difficulty, not as dramatic as the reconfiguration problem
lies in the necessity to keep the ductwork compact for shipment but pro-
viding low air movement friction loss and wear. A rule of thumb in duct
design is to have radius turns equal to at least two diameters, and to
keep intersections of ducts at 30 or less. These practices reduce wear
on the ductwork and fans and motors and conserve energy but make the system
more difficult to transport.
While astute design can affect a reasonable engineering compromise
between system life and energy needs on the one hand and compactness on the
other, the problem of ducting for multiple connections in a reconfigured
deployment of process equipment is of more serious proportions. It would
appear to be a principal reason for the fact that such systems are not
generally offered as a part of the overall original process equipment.
Combination Wet Suppression and Baghouse Systems
When water availability is not a problem and where fine size products
that really agglomerate are being made (crushing and screening operations on
-1/4" particles) a combination system is often employed. Wet suppression is
generally used at the initial portions of the process, i.e., the primary
crushing and screening operations, reclaim feeders and conveyor transfer
-52-
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points. Emission points following these process activities such as
secondary and tertiary crusher and screen and recirculating conveyors would
be controlled by a baghouse collector. Combination systems have higher
annualized costs than full wet suppression systems, but less than fully dry
systems when water is readily available. When water must be trucked in the
added costs of the hauling must be included in the analysis. Since water is
applied to only a portion of the system, the water use will be about 40%
that of the total wet suppression system or about 2
-------
Table 5
MODEL WET SUPPRESSION - FABRIC FILTER SYSTEM COSTS
FOR PROCESS PORTIONS OF CRUSHED STONE PLANTS1
Capital Costs for Installed System
Small Plant
300 Tons/Hr.
Fabric Filter and Wet
Suppression Equipment Installed
$107,852
Large Plant
600 Tons/Hr.
$132,840
Annualized Capital Cost $13,482
(12.5% of Capital)
2
Operating § Maintenance
Electricity § .04/KWH $ 2,909
Maintenance, Operation £ Supplies $ 18,396
Total Annual 0$M $21,305
Total Annualized Cost $34,787
(Capital + 0§M)
6.04
-------
As is not infrequently the situation in scientific work, the develop-
ment of this technique to control dust was ancillary to the main thrust
of a study conducted at the University of Arizona under the aegis of the
National Aeronautics and Space Administration, Beginning in 1973 a number
of studies were performed to determine if electrostatic charging was a factor
in the levitation of dust on Mars. Since no Mars dust samples were avail-
able, tests of necessity were run on a variety of industrial and naturally
occurring particulate materials. Results of this work are described in a
14
report dated August, 1977. The report contains a large amount of experimen-
tal data on dust charges and dust reduction results after applying various
amounts of charged spray. In broad terms particles below 8 microns tend
to be charged negatively while larger particles tend to have positive charges
or to be uncharged. However, there is considerable variation among dusts
and the same dust with various impurities. The physics underlying the
charge phenomena and the settling mechanism is not fully developed but a
government funded study is currently under way to provide a better under-
standing of the phenomena
The variety of charge type and strength is illustrated by two bar
charts taken from the report . The first, Figure 24, shows that trap
rock, a hard ingenous rock often found in crushed stone quarries, exhibits
negative charge for all discrete particle size ranges tested up to 11
microns. Silica sand (also frequently found in stone quarries) in contrast
exhibits positive as well as negative charges at some particle sizes (See
Figure 25).
-55-
-------
II
PARTICLE SIZE
MICROMETERS
Figure 24
CHARGE ON TRAP ROCK
3 RUNS
INDICATES SPREAD
OF THE DATA
W/////////////M/M
I 1
3 2
CHARGE ( ARBITRARY UNITS )
0
Ci
-------
PARTICLE SIZE
MICROMETERS
6 -
cn
•~j
i
0
I-
WI///tlJ/llffh ABOVE II
-1
INDICATES SPREAD
OF THE DATA
Figure 25
CHARGE ON SILICA SAND
5 RUNS
I
• » »
d
:AD ///.
'IL
\ -. .,
'• r • • ' "•
7///////////////////////////^^
\
i I I
1
1
— n_ I
1 1 t_.
12
10 8 6 4 2
CHARGE (ARBITRARY UNITS)
0
-------
The method employed uses small charged water particle fog. The
oppositely charged fog droplets enhance contact between the particulates
and the fog droplets. After contact is made, the wetted particulates agglom-
erate and fall rapidly. The device consists of a modified commercial
electrostatic paint spray gun that uses compressed air to atomize the
water droplets. The droplets can be formed uncharged, or with positive or
negative charges as desired.
Dust tunnel studies were conducted on a variety of dust samples
using various quantities of water and different charge polarities. Water
flow rates were nominally 30 ml/min per gun (0.475 gallon/hour) and air flow
100 standard cubic feet per hour. All of these studies indicated reductions
in dust concentration for particles smaller than 9 microns in the test dust
tunne1.
Field data most closely related to crushed stone operations were
gathered at a cement plant in Arizona. Samples of dust were first taken
from the belt conveyor in the quarry surge building and tested in the dust
tunnel for sign which indicated a preponderence of negatively charged dust.
The in-plant test utilized two modified REA guns made by the Ransburg
Corporation of Indiana. These two guns were mounted as shown in Figure 26,
A sampling head was then located at belt level (5', 152 cms.) and
about two feet from the edge of the belt. Floor to ceiling curtains were
arranged as shown in Figure 27 to prevent dust blowing in or out of the
test area. Results of this in-plant test are shown in Figure 28 for
particles up to nine microns. The data indicate reduction of dust level,
with the most improvement coming after 30 minutes of dust loading,
-58-
-------
Figure 26
TEST SET UP CEMENT PLANT "A"
o
FOG GUNS
/////// -1 '///'"'
/ * '////////'/
-59-
-------
Figure 27
PLAN VIEW OF SURGE BUILDING CEMENT PLANT "A"
WALL
FOG GUNS
$
-EDGE OF BELT PLATFORM
j SAMPLING
| STATION
{ 5' ( 152 cm )
| ABOVE FLOOR
1
I
^^^^ FLOOR T
1 ^ CEILING
CURTAIN
x, — • — • •
8* / ?£. £.
\ \
2' { 50.7 cm)
51 (i
0 .^^^
I
I
c m i • • - •»
52 cm }
-60-
JACA CORP. • ENVIRONMENTAL CONSULTANTS & ENGINEERS
-------
!2
10
e
to
bO
H
H-l
C/5
Figure 28
CONCENTRATION VS. PARTICLE SIZE
BEFORE AND AFTER SPRAY IN
SURGE BUILDING CEMENT PLANT "A"
WATER F'LOW RATE
60 ml / min
AIR FLOW RATE
200 S C F H
INITIAL DUST LEVEL
DATA
UNCERTAIN
DUST LEVEL AFTER 5 MINUTES
OF POSITIVE FOG
DUST LEVEL AFTER 30
'MINUTES OF POSITIVE FOG
b
2 3
PARTICLE
4567
SIZE (MICROMETERS)
-61-
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It is not possible to report on annualized costs for different
crushed stone installations as was done in the case of baghouse and wet
suppression systems because the system has not been deployed in the field.
Individual guns consisting of a water and air atomizing head and a self-
contained device to charge the particles for either polarity sell for
$2,000 each from the Ransburg Corporation. One or more device must be
located at each process source of dust, the area shielded from wind action,
and the necessary water and air connections made. The air use of 100 SCFPH
per gun is low so that even if 20 units were required (two at each of 10
emission points) plant air could readily meet the need. Water consumption
for twenty units would be 9.5 gallons per hour, a small fraction of the
450 gallons per hour of water needed for a production rate of 300 tons per
hour of crushed stone in a wet surpression system.
In summary, the electrostatic spray system has been applied experi-
mentally to the control of fine dusts (less than 10 microns) with varying
degrees of efficiency. There has not been operational application to
crushed stone operations at this time to the author's knowledge.
Because it operates most efficiently on small particles and is
essentially not helpful on particles larger than 10 microns, it cannot be
characterized as a direct practical, alternative to the baghouse or wet
suppression techniques extensively used in control of dust from crushed
stone process operations. It should be considered for further experimentation
on the smaller size dust generated by tertiary crushing which sometimes
cannot be controlled by wet suppression systems because of the desire to
keep the product dry. It might also be applied to dust before entering a
device whose efficiency is dependent on particle mass diameter such as a
cyclone or wet scrubber. Removal of the fine particles before the existing
control device tends to improve its efficiency and reduce the energy
necessary to effect separation by particle dynamics.
-62-
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Section II
EFFECTS OF MESA CONTROL STRATEGIES
ON DOWNWIND AMBIENT CONDITIONS
Introduction
The Mining and Enforcement Safety Administration (MESA)* and state
agencies have promulgated and enforced worker health standards which have
the potential for interaction with the enforcement of regulations promul-
gated pursuant to the Clean Air Act. The purpose of the work reported in
this section of the report was to briefly examine the effects of control
strategies for MESA on downwind ambient conditions.
Emission controls installed for worker protection under MESA or
state occupational health requirements were found to ameliorate downwind
ambient particulate contributions from plant operations. With certain
simplifying assumptions the salutary effect on National Ambient Air Quality
Standards can be calculated. However the effect of MESA and other occupa-
tional health regulations on current SIP regulations often cannot be
quantitatively evaluated because SIP regulations use a different measurement
criteria or are often semi-quantifiable.
It is important to note that control of process emissions for air
pollution reasons, which is favored by MSHA over controlling the receptor
(worker) by using protective respiration devices or a work enclosure,
etc., is significantly aided by use of air pollution control devices
noted in the main section of this report.
*MESA has recently been moved from the Bureau of Mines, Department of
the Interior to the Department of Labor. The changeover has been recc
and the name has been changed to the Mine Safety 6 Health Administration
-63-
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Occupational Health Regulations
Stone quarries, also referred to as surface mining operations or
simply mining in this discussion, can be regulated by more than one agency.
Occupational health and safety regulations which may have an impact on air
pollution control by the rock crushing industry exist at the Federal level.
In addition some states also have occupational health regulations that
pertain to this industry. Since there is considerable variation in state
activity we will generally use Oregon and Pennsylvania as illustrative
examples.
MESA. The federal Mining Enforcement and Safety Administration (MESA),
an agency of the U.S. Department of Interior, was created in 1973 to take
over from the Bureau of Mines the authority to develop and enforce health
and safety standards under the Federal Coal Mine Health and Safety Act
and the Federal Metallic and Nonmetallic Mine Safety Act (FM§NMSA),
30 U.S.C. §721 et seq.
The FM^NMSA, which applies essentially to all mining operations,
both deep and surface, grants MESA authority to enter and inspect all
subject mines. The Act empowers MESA to promulgate two types of health
and safety standards: voluntary and mandatory. Voluntary standards may
be set to protect life, promote health and safety, and prevent accidents.
Voluntary standards are similar to guidelines: operators may not be
penalized for violating them. MESA may also set mandatory standards to
regulate practices reasonably expected to cause death or serious physical
harm. Violations of mandatory standards may lead to imposition of the
penalties specified in the Act.
The states may be delegated inspection and enforcement authority
-64-
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under the FMSNMSA if they submit a plan that is approved by the Department
of Interior. If the state plan is approved,MESA essentially turns over
the enforcement of the standards to the state. To date six states have
been delegated inspection and enforcement authority. Oregon and Pennsylvania
are among those that have not been delegated such authority. State mine
regulatory laws that are not in conflict with FMSNMSA, or that regulate
mines more strictly than the Act, are not superseded by it, so Pennsylvania's
law and the FM&NMSA are both enforced in Pennsylvania. This is also true
in Oregon, except that an accomodation has been reached between the two
enforcement agencies so that little overlap occurs in field enforcement.
OSHA. The Occupational Safety and Health Act of 1970, 29 U.S.C. §65
et seq., created the federal Occupational Safety and Health Administration
and gave it authority to enter and inspect all business establishments in
the United States and its territories. OSHA has authority to set and
enforce standards, issue citations, assess fines, and petition federal
courts to close down establishments where employees are threatened by
imminent hazards.
OSHA currently exercises no jurisdiction over mining enterprises,
except for practices occurring in buildings outside the mine or pit (crushing,
screening, and conveying are deemed not "outside the pit"). While neither
the OSHA Act nor the FMfiNMSA preclude OSHA from setting and enforcing regula-
tions pertaining to mines, any such action would be a mere duplication of
MESA's efforts. OSHA has issued no safety or health standards for mines to
date, nor does it presently enter and inspect mines, despite its legal
authority to do both.
EPA - SIP. The federal Environmental Protection Agency is charged
with enforcing the federal Clean Air Act (CAA), 42 U.S.C. §1857 et seq.
-65-
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The CAA is based on a regulatory concept involving ambient air quality and
emissions limitations; it does not give EPA authority to set exposure levels
of air pollutants for workers. EPA, in any case, only retains primary
enforcement authority over hazardous emissions (including asbestos) and
emissions from certain new sources (not yet including any quarrying opera-
tions). Enforcement of emission limitations for non-hazardous pollutants
from existing sources can be delegated to the states after approval of
their state implementation plans. EPA can enforce state SIP's if state
enforcement is inadequate.
State Health Agencies. Some states also have state industrial health
organizations that regulate and inspect mining operations. For example,
when the Pennsylvania Department of Environmental Resources (DER) was
created in 1970, among the powers it was delegated was the power to set and
enforce health and safety regulations for mines, previously invested in
the Department of Labor and Industry. DER's authority over mines is
derived from the Pennsylvania Surface Mining Conservation and Reclamation
Act, 52 P.S. §1396.1 et seq. The inspection and enforcement is handled
by the Bureau of Occupational Health.
The Surface Mining Act gives DER exclusive authority to enter and
inspect any surface mine in the state. The Act also empowers DER to issue
and enforce such regulations as are necessary to protect mine workers and
public health and safety. This Act does not distinguish between voluntary
and mandatory standards; all standards issued by DER are mandatory and
enforceable. The state regulations and those of MESA are essentially
identical insofar as worker exposure is concerned,
Oregon's situation in respect to state involvement with occupational
health at surface mining installations is similar in theory, but not in
-66-
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practice. There is regulatory overlap between MESA and the Accident
Prevention Division (APD) of the Worker's Compensation Department
(formerly Workman's Compensation Board). The APD and MESA, however,
have an arrangement under which MESA has primary inspection responsibility.
APD only inspects if they happen to be on the site for another purpose, and
have determined that there has not been a recent inspection. Copies of
inspection reports are exchanged between APD and MESA.
Comparing Pennsylvania, the leading stone producing state, and Oregon,
approximately the thirteenth in terms of production, insofar as regulatory
bodies concerned with air pollution at stone crushing operations we see the
following:
Enforcement Agencies for Fugitive Dust
From Stone Crushing Operations
Pennsylvania Oregon
Pennsylvania Department of
Environmental Resources
(Bureau of Air Quality and
Noise Control) enforces
EPA approved SIP.
Pennsylvania Department of
Environmental Resources
(Bureau of Occupational
Health) inspects surface
mines and enforces worker
standards.
MESA inspects and enforces
worker standards in surface
mines.
Oregon Department of
Environmental Quality
enforces EPA approved
SIP.
Oregon APD of the Worker's
Compensation Department
inspects and enforces worker
standards in a variety of
industries including surface
mines.
Same.
-67-
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Thus there is an overlap in both Pennsylvania and Oregon between
state and MESA inspections and enforcement authority.
Worker Exposure Regulations. The MESA standards that apply to
worker related air contaminants are mandatory in nature. They are based
on the threshold limit values (TLV's) for nuisance dusts and dusts con-
taining quartz and asbestos recommended by the American Conference of
Governmental Industrial Hygienists (ACGIH). The following common TLV's
are therefore applicable to the rock crushing industry:
Substance TLV
Nuisance Dust 10mg/m3
Quartz (10mg/m3) T (% Respirable Quartz + 2)
Asbestos 5 fibers/ml, (fibers greater than 5y in length
Under these regulations, no employee in a rock crushing plant is to be
exposed to an averaged 8 hour working day dust concentration greater than
the TLV. For 5% Quartz content, the above TLV formula reduces to:
10 mg/m3 T (5% + 2) = 1.43 mg/m3 or 1430 yg/m3
SIP Regulations on Fugitive Dust. The SIP requirements for non
stack emitted dust (fugitive dust) in states vary to the extreme. Several
examples will illustrate this point. The reader is cautioned that the
following examples note highlights of the regulations, and are not to be
considered as exhaustive or definitive legal treatments.
-68-
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Pennsylvania permits no fugitive dust unless the operation falls in
either of eight categories (construction, agriculture, etc.) or is classified
by PennDER as an "insignificant" source. Stone crushing is not inlcuded in
the eight categories and each operation is judged on its own as to whether
or not it is an insignificant source. If significant no fugitive emissions
are permitted, while if insignificant, measurements are made downwind at the
property line using a Konimeter, a device which is intended to provide particle
count and size information. The DER regulations specify no more than 150
particles per cubic centimeter downwind air above background at the property
line. These instruments have been found to be unacceptable for this purpose
by this contractor and the state, and their use has been abandoned.
Oregon does not have a performance regulation on fugitive emission,
but requires the "installation of hoods, fans, and fabric filters to enclose
and vent the handling of dusty materials" (OAR 21-060, 20). These regulations
apply however only if a nuisance condition exists or if any other regulation
is violated. The existence of a nuisance condition is determined by such
factors as population density, duration of the activity and others (OAR 21-050
(2)). This regulatory arrangement leads to the not uncommon problem posed
by having objective criteria which cannot be quantitatively applied.
In addition, existing sources outside Special Control areas are
subject to a 40% opacity limitation (Ringelmann 2). All new sources and
existing sources outside the Special Control areas are subject to a 20%
opacity limitation (Ringelmann 1) (OAR 21-015).
-69-
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Arizona fugitive dust regulations require the use of "reasonable
precautions" to prevent particulates from becoming airborne from certain
activities. For crushing, screening, handling, or conveying of materials,
such reasonable precautions are defined to include (but are not limited to)
spray bars, wetting agents, and hoods (Arizona Rules and Regulations R9-3-302
CD)).
In Texas, in the Standard Metropolitan Statistical Areas (SMSAs) in
which federal ambient air quality standards for particulates are exceeded, fine
material may not be handled, transported, or stored unless stockpiles and
other surfaces from which airborn dust may arise are coated with water,
chemicals, or other suitable materials; hoods, fans, and filters are used
to collect dusty materials; and open vehicles are covered and materials
wetted (Texas Regulation I, Rule 104). Portable rock crushers on site less
than six months and engaged in public works projects are exempt from
visible emissions and process rate standards (Rules 103 and 105) as long as
they are located at least one mile outside the nearest town limits, occupied
facility, or recreational area; equipped with cyclones, wet scrubbers, or
water sprays (or equivalent) at transfer pointc; do not create a nuisance;
and not in Dallas or Harris counties (Regulation I, Rule 106).
In Utah, all sources emitting over 100 tons of particulates annually
(uncontrolled) must achieve 85% control. No methods are specified (Utah State
Division of Health Code of Air Conservation Regulations §2.3.1).
Relationship of Control for Workers to SIP Control
An interesting topic concerns the extent to which control of fugitive
dust emissions from stone crushing operations to satisfy MESA type requirements
-70-
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aids in the attainment of SIP standards. This is a very difficult subject.
MESA requirements are based on a dosage concept that involves a pollution field
and a receptor (the worker). The TLV's are based on an eight hour working day
exposure. The method of test for worker dust exposure is to attach on the upper
torso clothing of a worker the input section of a calibrated vacuum pump so that
it provides an inspired volume flow rate similar to a normal adult worker. The
worker shuts the device off during work breaks so that the air inspired by the
device is similar to that inhaled by a worker acting on the job over an eight
hour period. Dust is separated into respirable and non respirable fractions
by a small cyclone in the device and the respirable portion caught on a filter.
Gravimetric determination of the filter catch is made and the weight
of dust divided by the volume of air passed through the device over an eight
hour period. The result is an eight hour average exposure which is then
compared to the pertinent TLV. Anything that will reduce the worker's exposure
such as work rotation, dust free booths, personal protection (respirators) can
be used. Note that the pollution field has not been reduced, only the workers
exposure to that field has been lessened. MESA prefers engineering controls
at the source (emission controls) or at the receptor (by enclosing the employee
in a dust-free booth). Source emission controls are therefore the only ones
that could possibly be analyzed as to the degree to which MESA standards attain-
ment helps SIP attainment. Because, as was shown earlier, the SIP standards for
fugitive dust vary significantly among the states and are usually not given in
concentration terms it is extremely difficult to make such analysis.
A statement of the question in different terms helps the analysis:
To what extent does the attainment of MESA standards by emission control
-71-
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strategies on otherwise non MESA compliance operations improve the National
Air Quality STandards (NAAQS) in the vicinity of the operation? NAAQS for
particulates are:
Annual daily geometric mean: Primary 75pg/m3
Secondary 60yg/m
Maximum 24 hour average: Primary 260ug/m
3
Secondary ISOpg/m
The restated question may be addressed more effectively than the
former question, and if certain simplifying assumptions are made, a mathe-
matical expression relating such variables as distance to the property line,
wind speed, etc. can be formulated. Appendix A of this report includes an
example of how such an analysis can be made, given with underlying
assumptions.
The plant with the particular geometry described in Appendix A has
reduced its contribution to the 24 hour average ambient at the property
line by nearly 53% (from 286.1ygm-3 to 135. lygnr3) by meeting MESA
standards for the working environment.* Differing backgrounds, geometries,
work shifts, and before-control worker ambient levels would yield differing
results in accordance with equations 1, 2 and 3 of the Appendix, and the
correction from 8 hour TLV to a 24 hour ambient. Thus the enforcement of
applicable emission regulations and worker exposure provisions are
considered mutually supportive and complementary to the other program
objectives.
*For a given r and stability class. See the Appendix for the ranpe of
possibilities.
-72-
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APPENDIX A
Example Analysis of Effect of MESA on NMQS
-73-
-------
The question addressed is, what extent will pollution controls
intended to meet one standard (i.e. MESA standards) contribute toward
meeting another standard (i.e. National Ambient Air Quality Standards)?
The answer depends upon the extent to which the pollutant being regulated
contributes to the pollutant governed by the second standard. In this
discussion we will assume that the second pollutant is identical to the
first. Also, we will assume inert pollutants (e.g. 5% quartz stone dust) with
f-r
no fall out, an ambient particulate background concentration of 100 yg/m^, and
atmospheric conditions such that the dilution downstream from the source is
governed by the Gaussian plume relationship.
It is reasonable to use the Gaussian model in this situation since we
are not concerned with particle fallout, rather only those particles that do
not fall out within the plant property line. In terms of MESA Standards we
are concerned with respirable size particles, i.e. 0.1 ym to 10 ym diameter,
and their control. These particles remain airborne for sufficient time periods
to consider their dispersion according to the Gaussian plume model.
As a starting point, we define the following pollution concentrations
in micrograms per cubic meter:
XR - pollutant regulated by the first standard;
X? - pollutant governed by the second standard;
X , , x?h - concentrations of XR and x2 before controls are instituted;
XD Xo - concentrations of XD and X9 after controls are instituted.
Ra ^.a K /
These relations follow by definition also:
AXR " XRb " XRa ;
AX2 = X2b - X2a .
-74-
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These represent the controlled pollutant concentrations.
Let us now examine how AXD and AX~ are related as a function of
K £
distance d downwind of the pollution source area. To do so we can envision
a Gaussian plume dispersion model extending downwind of the plant site area
A as sketched in Figure 1-A. The pollution concentration at ground level
averaged over about one hour is sketched at LI and L? showing the pollution
spread out and decreased in concentration. In a later example, L_ will be
taken to be the boundary of the plant property at which NAAQS must be met
and LI is the critical point with regard to MESA standards.
The ground level concentration is a Maximum at the plume centerline
and decreases from L, to L_ according to the relation
irayazy
where
X(ygm~ ) is the pollutant concentration,
Q(ygs ) is the source strength,
2
0yoz(m ) is the product of Gaussian plume dispersion parameters and,
y(ms ) is wind speed.
The above expression is for a continuous point source with ground level
release.
From the above relationship we see that the ratio of concentraion at
!.,„ to that at L, is r where;
Turner, D.B., 1969: Workbook of Atmospheric Dispersion Estimates. Public
Health Service, HEW.
-75-
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Virtual
Source
of
Pollution
Source Area
A.
Sketch of
Pollution
Concentration
at Edge of Site
Plume
Transport
Directior
Sketch of
Pollution
Concentration
Further Downwind
Figure 1-A: Sketch of the Geometric Relation Between
the Virtual Source Point 0', the Source Area As
and Average Pollution Concentration at L^ and L,2
-76-
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« 2f£2l . hH^ (Equation 1)
X(Ljj [ayaz](L2) M
The ratio of Gaussian parameter products can be estimated for various values
of L, and L~ from the following table derived from the graph of Figure 2-A for
various stability classes. This table presents the values of the Gaussian
parameter products a a for various stability categories at the correspond-
y z
ing downwind distance.
2
Gaussian Parameter Products, 00 (m ) for Various Stability Classes
Downwind Distance,
Class B
Class C
Class D
Class E
And Downwind
L(km) .1
120 4
96 1
40
25
Distance
.5
,500
,800
660
360
L(km)
1
16,000
6,400
2,100
1,103
5
400,000
115,000
26,500
12,000
10
off graph
400,000
74,000
32,000
Applying the distance reduction to AX- which would be at L7 and
AXR at L, shows that
o X2bCL2^ = rXRb^Ll^ (Equation 2)
° X2a(V = rXRa(V (Equation 3)
These relations show how pollution concentrations due to control measures
initiated at point LI relative to pertinent regulations translate to pollu-
tant concentration at point L2 relative to another standard. Thus under the
simplifying assumptions made, any reduction in source strength from the
various fugitive sources will have the effect of a corresponding decrease in
-77-
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0.1
1 10
Distance Downwind, km
100
Figure 2-A:The Product of ayaz as a Function of Downwind Distance From
the Source. (A Represents Unstable Conditions and F very stable.)
Source: Turner, D.B., Workbook of Atmospheric Dispersion Estimates, U.S. EPA.
-78-
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their contributions to the downwind ambient. How this would work in the case
of a stone crushing operation can be seen in the following example of a stone
crushing operation processing 5% quartz stone with a plant area ambient con-
_3
centration of dust at 7150 ygm . The TLV criteria for 5% quartz dust es-
tablished by MESA is 1430ygm as noted earlier in this section of the report.
To meet this standard an 80% reduction of quartz bearing dust is accomplished
at the plant site by employing emission controls. For this operating site
with L,=lkm, what is the impact of these operating site controls at the plant
boundary 4 km beyond the operating area, i.e. L =5 km?
Notice that the particulate concentration measured in the plant area
-3
includes the background concentration of lOOygm . To determine only the
plant's contribution to the ambient concentration at the plant boundary, the
background concentration should be subtracted from the plant area concentra-
tions before applying equations2 and 3. Thus the plant area concentration
attributed to plant operations prior to control, xRh is 7050 ygm" and after
control, xRa, is-1330 yg/m .
The following calculations exemplify the use of equations 2 and 3
in determining the effect of controls initiated within the plant area on
the ambient concentration at the property line. Calculations are based on
atmospheric stability class D.
From Figure 2-A and equation 1 we find that:
r= °y°z (Ll) = 2100m2 = 0.0792
°y°z (L ) 26500m2
During an eight hour work day
X2b (L2) = r (xRb) = 0.0792 (7050) = 558.4 yg/m3
X7a (L9) = r (XD ) = 0.0792 (1330) = 105.3 yg/m3
^ d £ Kcl
3
AX9 (L9) - 453.1 yg/m
-79-
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The contribution of the plant to the ambient particulate concentration
_5
at the plant boundary before and after control is thus 558.4 ygm and 105.3
ygnf , respectively. There is approximately an 80% reduction in the plant's
contribution to the ambient. Since we assumed a background concentration
of 100 ygm , the actual ambient concentration of particulates 5 km downwind
_•? _3
would be 658.4 ygm before control and 205.3 ygm after controls were initi-
ated at the plant. These values must then be converted to 24 hour average
concentrations to be compared to the NAAQS for particulates of 260 pgm
_3
During non-working hours the ambient concentration is 100 ygm . There-
fore, the 24 hour average concentration is
24 hr ,
X2b (L2) - 1/3 (658.4) + 2/3 (100) - 286.1 y g/in
and
24 hr .
X2a (L2) = 1/3 (205.3) + 2/3 (100) = 135.1 yg/m
Thus the plant with this particular geometry (the assumed LI and L^)
for atmospheric stability class D has reduced its contribution to the 24 hour
average ambient at the property line by nearly 53% by meeting MESA standards
for the working environment. In doing so, the ambient concentration was re-
duced below the NAAQS for particulates of 260 ygm" . Differing backgrounds,
atmospheric stability classes, geometries, shifts, and before control worker
ambient levels would yield differing results in accordance with equations 1, 2,
and 3 and the correction from 8 hour TLV to a 24 hour ambient.
Table 1-A shows the effect of the choice of stability class on the 24
hour average ambient concentration. The values were developed in the same
-80-
-------
manner as the preceding calculations, only the value of r was changed in
accordance with the Gaussian parameter products from Figure 2-A.
Table 1-A
B
. 0425
199.9
118.8
40.57
C
.0557
230.9
124.7
45.99
D
.0792
286.1
135.1
52.78
E
.0917
315.8
140.7
55.45
Stability Class
_24 hr
*2b (V
_24 hr
X2a (L2)
% reduction of ambient
24 hr. avg. concentra-
tion
This table shows that the 24 hours average concentration at the prop-
erty line decreases going from Class E to B as the atmosphere becomes more
unstable. Obviously, this is because the diffusion capability of the atmos-
phere increases in proportion to the atmospheric turbulence. Granted, the
background concentration would also vary with atmospheric stability, but
this effect is beyond the scope of our analysis. Additionally, the percent
reduction of the ambient 24 hour average concentration decreases as the
atmosphere becomes more unstable. This is because the contribution to the
ambient from the plant during the 8 hour working day decreased with the ratio
of the Gaussian parameter products while the background concentration was
assumed constant for the 24 hour period. Thus the effect of the controls
initiated at the plant is reduced in terms of the 24 hour average concentra-
tion.
Notice that for stability classes B and C the 24 hour average concen-
tration at the property line is already within the limit of the NAAQS before
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controls were initiated at the plant. Thus, pollutant control is not necessary
to meet these standards if the property line is 5km from the source. Stabil-
ity Class D, neutral stability, is more or less the worst case situation in
terms of downwind centerline concentration during an 8 hour work day. By
definition stability class E does not occur during the day. This shows that
by controlling particulate concentrations at the plant by 80% in order to
meet MESA standards a corresponding 53% reduction in the ambient concentra-
tion at the property line will result which also meets the NAAQS. As pre-
viously stated, different background concentrations, plant geometries, shifts
and before control worker ambient levels will yield different numerical re-
sults.
Controls installed for air pollution control reasons also have the
salutary effect of reducing worker ambient levels. Thus MESA (now MSHA)
and EPA regulations have mutually desirable effects.
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REFERENCES
1. Bureau of Mines, Department of the Interior, Minerals Yearbook, 1975.
2. Private communication with the U.S. Bureau of Mines, Non Metallic
Minerals Division (based on 1972 figures).
3. JACA Corp., Differential Impact of Pollution Control Requirements on
Small vs. Large Businesses in Grain and Stone Industries, for Small
Business Administration under Contract No. SBA-1757-PRA-74, May 1975.
4. Evans, Robert J., Methods and Costs of Dust Control in Stone Crushing
Operations, U.S. Bureau of Mines, Circular 8669, 1974.
5. Discussion with William Ward of Ward Engineering, Swarthmore, PA re
Aquadyne System.
6. Discussion with Mike Natale of Johnson March Co., Philadelphia, PA re
Chem-Jet System.
7. MSA Research Corp., Survey of Past and Present Methods Used to Control
Respirable Dust in Noncoal Mines and Ore Processing Mills, for U.S.
Bureau of Mines, April 30, 1974.NTIS No. PB 240662.
8. Johnson March Company recommendations for Chem-Jet System maintenance.
9. Communications with M.D. Glenn, Oregon Department of Transportation,
Highway Division, Construction Section - Materials and George Sanford -
Cost Analysis.
10. Telephone conversation with William McCall - Oregon Water Resources
Board.
11. Engineering Science, Inc., Air Pollution Emission Test for EPA under
Contract No. 68-02-1406, Task 7, Report No. 75-STN-3.
12. Discussions with James Fee of L.B. Smith Company, Camp Hill, PA.
13. Clayton Environmental Consultants, Inc., Air Pollution Emission Test,
for EPA under Contract No. 68-02-1406, Task 6, EPA Report No. 75-STN-7.
14. Hoenig, Stuart, The Use of Electrostatically Charged Fog for Control
of Dust From Open Sources, EPA Grant No. R 805228010, August 1977.
15. Visit with Dr. Stuart Hoenig of the University of Arizona, Tuscon, AZ.
16. Hoenig, Stuart, Use of Electrostatically Charged Fog for Control of
Fugitive Emissions. iiPA-600/7-77-131 . Nnypmh^-r 1077 ' —
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BIBLIOGRAPHIC DATA
SHEET
1. Report No.
13.
iient's .icce jsioa No.
4. Title and Subtitle
Control of Air Emissions From Process Operations in
the Rock Crushing Industry
5. Report Date
February 1978
6.
7. Author(s)
JACA Corp.
8. Performing Organization Kept.
No.
9. Performing Organization Name and Address
JACA Corporation
550 Pinetown Road
Fort Washington, PA 19034
10. Provct/Task/tt'oik Unit No.
11. Contract/Grant No.
68-01-4135-Task-19
12. Sponsoring Organization Name and Address
Division of Stationary Source Enforcement
U.S. Environmental Protection Agency
Washington, DC 20460
13. Type of Report & Period
Covered
Applied Research
14.
15. Supplementary Notes
16. Abstracts
Three basic methods of controlling emissions from process operations
in crushed stone plants are described - dry captive systems using
fabric filters, wet suppression systems and combinations of these.
Operational problems with these systems associated with plant por-
tability and product size are discussed. Examples of good design
practices and maintenance procedures for these control options are
covered. An electrostatic charged fog technique for control of small
dust particles is described and operational problems listed. A second
part analyzes the downwind effects of reducing emissions for worker
safety.
17. Key Words and Docuraer: Analysis. 17a. Descriptors
Rock Crushing Industry
Crushed Stone Plants
Controlling Emissions from Process Operations
Dry Captive Systems Using Fabric Filters/crushed stone plants
Wet Suppression Systems/Crushed stone plants
Operations problems-controlling emissions-crushed stone plants
I7b. Ideiitifiers/Open-Ended Terms
~\
I7c. COSATI Field/Group
18. Availability Statement
19. Security Class (This
Report) .... ,
unclassified
20. Security Class (This
unclassifie
21. No. of Pages
22. Hiice
FORM NTis-35 iHEv. 10-73) ENDORSED BY ANSI AND UNESCO.
THIS FORM MAY BE REPRODUCED
USCOMM-DC B2B5-f»7<
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