EPA-340/1 -85-007
Inspection and Operating and Maintenance
Guidelines for Secondary Lead Smelter
Air Pollution Control
by
PEDCo Environmental, Inc.
11499 Chester Road
Cincinnati, Ohio 45246
Contract No. 68-03-2924
Work Directive 6
EPA Project Officer: John Burckle
Industrial Pollution Control Division
Industrial Environmental Research Laboratory
Office of Research and Development
U.S. Environmental Protection Agency
Cincinnati, Ohio 45268
January 1984
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DISCLAIMER
This report has been prepared for the Industrial Environmental Research
Laboratory, U.S. Environmental Protection Agency. Publication does not
signify that the contents necessarily reflect the views and policies of the
U.S. Environmental Protection Agency, nor does mention of trade names or
commercial products constitute endorsement or recommendation for use.
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FOREWORD
When energy and material resources are extracted, processed, converted,
and used, the related pollutional impacts on our environment and even on
our health often require that new and increasingly more efficient pollution
control methods be used. The Industrial Environmental Research Laboratory-
Cincinnati (lERL-Ci) assists in developing and demonstrating new and improved
methodologies that will meet these needs both efficiently and economically.
The prevention of emissions from secondary lead smelters depends upon
the procedures implemented to achieve initial compliance and remain in a
state of continuing compliance with applicable emission limitations. The
ability to remain in continuing compliance depends largely on operation and
maintenance practices. This report was developed as a tool to aid state
and local enforcement field inspectors and entry-level engineers in the
inspection of secondary lead smelters with an emphasis upon techniques to
achieve improvements in the status of continuing compliance through operations
and maintenance procedures.
David G. Stephan
Director
Industrial Environmental Research Laboratory
Cincinnati
m
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ABSTRACT
The prevention of emissions from secondary lead smelters depends upon the
procedures implemented to achieve initial compliance and remain in a state of
continuing compliance 'with applicable emission limitations. The ability to
remain in continuing compliance depends largely on operation and maintenance
practices. This manual was developed as an inspection manual incorporating
operation and maintenance information for secondary lead smelting. It presents
an overview of secondary lead operations, describes typical emission problems
associated with the material preparation, smelting, refining and oxidation
processes, and reviews the potential causes of the problems and possible
corrective measures. It also describes the types of air pollution control
equipment used in secondary lead smelting operations and typical operation and
maintenance problems experienced with this equipment.
This manual is heavily oriented towards an inspection approach emphasiz-
ing techniques to achieve improvements in the status of continuing compliance
through operations and maintenance procedures. Because it has been written
for use both as an educational and reference tool by state and local enforce-
ment field inspectors and entry-level engineers whose'familiarity with second-
ary lead operations may be. limited, it can be useful both as a training manual
and as a guidebook during field inspections.
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CONTENTS
Figures
Tables
1. Introduction
2. Description of Processes and Operations
2.1 Production process overview
2.2 Process operations
References for Section 2
3. Air Emissions Generation and Control
3.1 Process emissions
3.2 Emission controls
References for Section 3
4. Operation and Maintenance
4.1 Fugitive capture and ventilation systems
4.2 Fabric filters
4.3 Scrubbers
4.4 Afterburners
4.5 Auxiliary equipment
4.6 Instrumentation and recordkeeping
References for Section 4
5. Inspection of Secondary Lead Smelters
5.1 Inspection of control equipment and ventilation system
5.2 Safety considerations
Page
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ix
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113
Glossary
116
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FIGURES
Number
1 Secondary Lead Process Flow Diagram
2 Front-End Loader Transferring Batteries
3 Battery Decasing
4 Sloping Hearth Sweating Furnace
5 Flash Agglomeration Furnace
6 Agglomerated Dust Plug
7 Open Top Blast Furnace
8 Blast Furnace Charging
9 Blast Furnace Tuyeres and Slag Tap
10 Slag Tapping Crucible
11 Lead Tapping
12 Schematic Diagram of a Reverberatory Furnace
13 Kettle Furnace Operations
14 Pigging Operation
15 Sources of Air Emissions from Secondary Lead Plants
16 Unagglomerated Baghouse Dust
17 Tuyere Cap
18 Electron Micrograph of Lead Well Dust
19 Hairpin Cooler
20 Vacuum-Type Sweeper
21 Indoor Storage Building
22 Battery Breaking Station
23 Blast Furnace Charging Hood
24 Slag Tapping from the Blast Furnace
25 Hooding for "Hog" Casting
26 Top Charged Reverberatory Furnace with Improved Fugitive
Emissions Capture
Page
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7
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/
15
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24
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VI
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FIGURES (continued)
Number page
27 Refining Kettle Fugitive Emissions Capture 40
28 Pigging Machine with Canopy Hood 41
29 Slag Tap Hoods 44
30 Slag Piles and Tapping Vessels 45
31 Flue Dust Collection 47
32 Typical Simple Baghouse with Mechanical Shaking 53
33 Reverse Air Flexing to Clean Dust Collector Bags by Repressuring 55
34 Typical Reverse-Pulse Baghouse During Cleaning 56
35 Typical Temperature Excursion, °F 62
36 Methods of Bag Attachment in Shaker and Reverse-Air Fabric
Filters 64
37 Proper Bag Orientation for Prevention of Bag-to-Bag Contact
During Shaking 65
38 Proper Method of Installing Bag in Tube Sheet with Snap Rings 66
39 Top-Load Pulse-Jet Fabric Filter 67
40 Correct and Incorrect Installation of Bags 68
41 Abrasive Damage Caused by Accumulation of Dust on the Tube Sheet 69
42 Dust Agglomeration in Fiber Interspaces 71
43 Precoating Material for Protection of Bags from Blinding 72
44 Field Method of Determining the Effectiveness of Bag Cleaning 74
45 Impaired Cleaning in a Reverse-Air Fabric Filter 74
46 Bridging Near Baghouse Shell Caused by Cooling a Poorly
Insulated Fabric Filter 75
47 Faulty Gasket on Baghouse Access Door 76
48 Pinhole Leak Near Walkway Impaction Pattern 77
49 Indication of Cuff Bleeding 77
50 Opacity Profile of a Pulse-Jet Fabric Filter with a Pinhole
in One Bag Row 79
51 Accumulated Dust Deposits on a Pulse-Jet System Blow Pipe 80
52 Sample Daily Log 88
53 Sample Weekly Log 89
54 Venturi Scrubber System Controlling a Kettle Furnace 90
vn
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FIGURES (continued)
Number Pa9
55 A Typical Afterburner Control Device 94
56 Fan Characteristics 97
57 Sample Checklist for Raw Materials and Products 105
58 Internal Shaker Mechanism 109
59 Cake Release from Bag 110
60 Steps for External and Internal Inspection of Fabric Filters 111
61 Inspector in Work Clothes, Hardhat, and Self-Contained
Breathing Unit 114
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TABLES
Number
1
2
3
4
5
6
Sources and Emission Factors for Point and Fugitive Emissions
from Secondary Lead Smelters
Lead Distribution in Reverberatory and Blast Furnace Smelting
Operating Parameters for a 70-Mg/Day, Blast Furnace
Operating Parameters for a 37-Mg/Day Blast Furnace
Fabric Filter Operation and Maintenance Guide
Ventilation System Operation and Maintenance Guide
25
28
43
43
82
98
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SECTION 1
INTRODUCTION
The success of an air pollution abatement program ultimately depends upon
the competence of the field inspectors and the adequacy of their inspections.
The ability to identify, describe, and evaluate air pollution emissions and
those factors responsible for their occurrence is a fundamental requirement of
the inspection process. Both the availability of sound inspection procedures
and adherence to the procedures are of vital importance to the inspection
process. A control agency having adequate enforcement powers but inadequate
inspection procedures would be unlikely to make significant progress towards
attainment of air quality goals. Also, such an agency may even see erosion of
its enforcement powers as a result of adverse court decisions arising from
improperly executed inspections.
The most important aspect of maintenance of air quality is the attainment
of "continuing compliance". A "continuing compliance inspection" is an in-
spection of sources which have previously proved initial compliance with the
regulations in that they have installed the necessary air pollution control
equipment (commonly referred to as control equipment, control systems, or
simply, controls) and/or modified their process(s) to be able to meet required
emission limits on a continuing, long term basis. Most agencies perform a
continuing compliance inspection once or twice a year depending upon their
resources or any complaints received.
This inspection guide designed by the Environmental Protection Agency
(EPA) has been written and organized for use by state and local enforcement
field inspectors and entry-level engineers whose familiarity with secondary
lead operations may be limited. The guide can be useful both as a training
manual in secondary lead smelting operations and as a guidebook during field
inspections.
This guide presents an overview of secondary lead operations and de-
scribes typical emission problems from the material preparations, smelting,
refining and oxidation processes. It explains causes of the problems and
possible corrective measures. It also describes types of control equipment
used in secondary lead smelting operations and typical problems with control
equipment.
The toxicity of lead together with its relatively high vapor pressure at
operating furnace temperatures make it a serious environmental/occupational
health problem that is difficult to control economically. The EPA has prom-
ulgated a National Ambient Air Quality Standard (NAAQS) for lead of 1.5
yg/nP. The Occupational Safety and Health Administration (OSHA) has
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promulgated a workplace standard of 50 yg/irr per 8 hour exposure and a blood
level standard of 40 yg/100 ml of whole blood. Operation and maintenance
(O&M) techniques will become important tools in maintaining continued compli-
ance with standards.
The prevention of lead emissions from secondary smelters will be shown to
depend largely on O&M practices. Fabric filters are usually used to remove
particulate matter from lead smelter process and ventilation gas streams.
Because properly operated fabric filters are very efficient, little lead
particulate matter is emitted in stack gases. Fugitive emissions, however,
are a major problem. Control systems are only partially effective in captur-
ing fugitive emissions. Also, handling of the lead particulate matter after
it has been collected is a potential cause of fugitive emissions. Lead dust
escapes from the materials handling and smelting processes into the workplace
and is continually reentrained and dispersed throughout the smelter. Suffi-
cient reentrainment may occur to cause the NAAQS of 1.5 yg/m3 to be exceeded.
Continued compliance can be achieved only by applying the appropriate combina-
tion of engineering and administrative controls, and by adopting operating, .
maintenance, and housekeeping practices to make those controls work effective-
ly.
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SECTION 2
DESCRIPTION OF PROCESSES AND OPERATIONS
The secondary lead industry is relatively complex in that a number of
processing steps are combined in various sequences to produce seven principal
products. Figure 1 is a generalized flow diagram for secondary lead smelting.
There are three basic process steps at a secondary lead plant: pretreatment,
smelting, and refining. One or more furnace types (blast, reverberatory, and
kettle) are used in each process; the furnace combinations selected depend
upon the type of scrap processed and the desired product mix. These operations
and the associated equipment are discussed in this section.
2.1 PRODUCTION PROCESS OVERVIEW
As shown in Figure 1, a secondary lead plant processes lead bearing ma-
terials such as automotive batteries and battery plates, battery manufacturing
scrap, rerun blast furnace slag, reverberatory furnace slag, dross, flue dust,
and scrap lead metal (e.g., pipe and flashing) to produce lead products of
varying hardness, lead oxides, and lead alloys.
2.1.1. Pretreatment
The lead-bearing raw materials are commonly stored outdoors in large
piles and are transported around the smelter in front-end loaders and/or
trucks (Figure 2). These materials usually must be processed in some way
before they can be fed to the smelting furnaces. For example, batteries are
decased or crushed; some feed materials are subjected to sweating to recover
lead and lead alloys with low melting points; battery plates and scrap may be
premelted before charging to the smelting furnace; and collected flue dust and
dross may be agglomerated before recycling.
Preparation of whole batteries for decasing varies. Incoming batteries
are segregated by battery case material (e.g., plastic or rubber). The bat-
teries are usually cut by a saw or a shear in an enclosure for removal of bat-
tery tops and posts, but some are crushed on the ground by a track bulldozer.
The acid is drained from the batteries, and the lead plates, posts, and inter-
cell connectors are removed from battery cases, collected, and stored in a
pile for charging to the process furnaces.
In the sweating operation lead and lead alloys with low melting points
(e.g., solder, babbitt) are selectively melted and separated from pieces of
scrap iron, copper, or aluminum which remain behind intact. Radiators, cables,
bearing housings, and various items of soldered or lead-covered scrap are
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Figure 2. Front-end loader transferring batteries.
typical feed materials. Sweating is performed in direct gas or oil fired
reverberatory or rotary furnaces. Reverberatory furnaces are typically used
to process high lead content scrap and rotaries usually process low lead
content scrap.
One smelter melts battery plates and battery manufacturing scrap before
the scrap is smelted in a blast furnace. The melted scrap is tapped from the
furnace and allowed to cool into slag-like chunks. These chunks are reported
to be an ideal feed material for the blast furnace and allow rapid smelting.
Fabric filter dusts from reverberatory, rotary, and blast furnaces and
from kettles are remelted and agglomerated at many secondary lead smelters.
The agglomerated product allows better lead recovery by the smelting furnaces
and decreases fugitive emissions.
Other pretreatment operations may include crushing large pieces of scrap
with a jaw crusher to reduce the scrap to a suitable size and zinc leaching to
dissolve zinc from collected reverberatory furnace flue dust to reduce the
zinc concentration in the blast furnace feed. After the pretreatment opera-
tions, the lead-bearing materials are processed by the smelting furnaces.
2.1.2 Smelting
Smelting produces a purified lead by melting and separating lead from
metal and nonmetallic contaminants and by reducing the oxides to elemental
lead. This is accomplished by exposing the furnace charge materials to a re-
ducing atmosphere at a temperature of about 950°C (1750°F) so that the oxides
can be reduced and the sulfur and waste oxide components (silicon dioxide,
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iron oxide, and others) can be removed as slag. Smelting is carried out in
blast furnaces, reverberatory furnaces, and rotary furnaces.
Hard or antimonial leads containing about 10 percent antimony are com-
monly produced in the secondary lead industry by blast furnaces (sometimes
called cupolas or shaft furnaces). Pretreated scrap metal, coke, rerun slag,
recycled dross, flue dust, scrap iron, and fluxes (e.g., limestone) are used
as charge materials to the furnace. The process heat needed to reduce the
lead components of the charge to metallic lead is provided by the reaction of
the charged coke with blast air that is blown into the furnace.
Semi soft lead products that contain 3 to 4 percent antimony are commonly
produced in reverberatory furnaces, but can also be produced by rotary kiln
furnaces (rarely used in the United States). The lead is produced using
metallic battery parts, oxides, drosses, and other residues. This charge is
heated directly using either natural gas, oil, or coal.
2.1.3 Refining
Refining and processing the crude lead from the smelting furnaces can
consist of softening, alloying, and oxidation depending on the final products
desired. These operations can be performed in reverberatory furnaces (dis-
cussed earlier) or kettles, but kettles are most commonly used.
Intermediate smelting products, especially from blast furnaces, may con-
tain antimony or copper; either element makes the lead hard. Kettles are used
for softening processes to remove these contaminants and thus produce a soft
lead product. The process steps consist of charging the preheated kettle;
melting the charge; agitating the flux into the molten charge; skimming the
drosses; and pouring or pumping the molten metal. In some cases, molten lead
is charged directly from the smelting furnace into the refining kettle.
Often, separate kettles are used to remove various impurities.
Refining furnaces remove copper and antimony to produce soft lead and
remove arsenic and nickel to produce hard lead. Sulfur can be added to reduce
copper content while aluminum chloride, sodium nitrate, sodium hydroxide, and
air can be used to reduce antimony content. Aluminum chloride also removes
nickel.
Alloying furnaces are used to melt and mix ingots of lead and alloy mate-
rial. Antimony, tin, arsenic, copper, and nickel are the most common alloying
materials.
Oxidizing furnaces are either kettle or reverberatory furnaces which oxi-
dize lead and entrain the product lead oxides in the combustion air stream.
The product is subsequently recovered in baghouses.
Kettles are also used to remelt final products for casting.
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2.2 PROCESS OPERATIONS
This section presents a more detailed description of the operation of a
secondary lead plant.
2.2.1 Pretreatment
Battery Handling--
Preparation of whole batteries for decasing varies. Incoming batteries
are segregated by battery case material (e.g., plastic or rubber). The bat-
teries are usually cut by a saw or a metal shear in an enclosure for removal
of battery tops and posts (Figure 3), but batteries are also crushed in the
Figure 3. Battery decasing.
open by a track bulldozer. Then the acid is drained from the batteries, and
the lead plates, posts, and intercell connectors are removed from battery
cases and stored in a pile for charging to the process furnaces. The drained
acid passes to a sump, and the stored plates are sprayed with water to remove
any retained acid. This acid has a pH of about 0.6 (Mezey 1979). The acid in
the sump is neutralized with lime before it is discharged to a holding pond or
sewer system.
Empty battery cases and battery tops can be washed and shredded for land-
fill or resale. Some smelters use battery cases in the blast furnace as a
portion of the fuel. The charge is premixed to maintain a uniform material
mixture in the furnace and control furnace temperatures. Because vulcanized
rubber cases can be a source of sulfur resulting in sulfur dioxide emissions,
they are normally removed and discarded to minimize generation of sulfur
dioxide emissions in the smelting operation. From European experience, smelt-
ers which feed PVC plastic battery cases to blast furnaces on a continuous
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basis and recycle collected dust must eventually remove the lead chloride
which builds up in the furnace by leaching (Burton et al. 1980).
Sweating—
The feed materials for a secondary lead smelter typically contain various
items of soldered or lead-covered scrap derived from radiators, cables, and
bearing housings. In addition to the lead, the scrap contains higher melting
metals such as iron, copper, and aluminum components which are undesirable in
the smelting and refining stages. These undesirable components are separated
from the lead components through a process termed sweating. Sweating is a
physical separation of metals based upon melting points. In the secondary
lead process the scrap is heated in a direct-gas or oil-fired furnace of the
reverberatory or rotary type.
Reverberatory furnaces are typically used to process high content lead
scrap such as battery plates, and rotary furnaces are usually used to process
low content lead scrap (lead-sheathed cable and wires) type metal drosses.
The furnace is operated at temperatures 340° to 540°C (650° to 1000°F) above
the melting point of the lead alloys and below those of the undesirable metals,
A sloping hearth type of reverberatory furnace (illustrated in Figure 4)
is the most common furnace used for sweating. The scrap pieces are pushed or
TO STACK
BURNERS
LAUNDER
HEFRACTORY
OPENING IN CHAMBER
mil TO ALLOW HETAL
FLOW INTO HOLDING
CHAMBER
Figure 4. Sloping hearth sweating furnace.
dumped onto the furnace hearth either by hand or with a mechanical loading
device. After the metals melt and flow down the hearth, the furnace operator
rakes the pieces of unmelted scrap off the hearth and out of the furnace.
These scrap pieces may be sold to foundries or to secondary copper and alumi-
num smelters for recovery of the remaining metals. Hoes and rakes are used to
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remove the scrap through access doors. When a rotary kiln sweating furnace is
used, the scrap falls into a bin at the end of the furnace. The lead or lead
alloys are collected in a well or a holding kettle furnace. The metal can
then be cast or pumped to other furnaces for further processing.
Agglomeration—
Several secondary lead plants remelt and agglomerate fabric filter dusts
from operating furnaces to allow better lead recovery and decrease fugitive
emissions (Section 3). Where agglomeration is not practiced, flue dusts may
be recirculated directly to either the smelting blast furnace or reverberatory
furnace or stored for a later lead recovery. A large amount of this dust is
entrained in the furnace flue gas system and must again be collected by the
control system. If the flue dust is stored before recycling, the dust parti-
cles can become windblown and add to plant fugitive emissions. The agglomer-
ation process (Figure 5) melts the flue dust and fuses the particles together
IAGHOUSE
DOST HOPPER
BURNER
NOLTEN DUST
-COOLING/TRANSPORTATION
CUBICLE
Figure 5. Flash agglomeration furnace (Coleman and Vandervort 1980).
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to form a large solid piece of material in the shape of the receiving vessel.
Tipping the solidified contents of the vessel on the ground is usually suffi-
cient to break the material into lumps small enough for recharging to the
smelting furnace and large enough to prevent entrainment in the furnace flue
gas system. Since less flue dust is generated and because the volume of
recycle material is reduced by approximately 80 percent, additional lead
bearing material can be charged to the furnace which increases the smelting
rate. Blast furnaces that smelt battery scrap generate dusts that are amen-
able to agglomeration. Some dusts, however, that contain zinc or copper
cannot be processed in such a furnace either because their melting points are
too high or because they raise the melting point of the mixture above the
operating temperature. The agglomerated material is a suitable blast furnace
feed. It can also be used as a detinning agent in kettle refining. Agglomer-
Figure 6. Agglomerated dust plug.
ation, however, may not be applicable if the dust is not recycled to the blast
furnace, if the chlorine content of the dust is too low, if the smelting
furnace afterburner is not operating properly, or if the furnace top tempera-
tures are high enough to vaporize a significant quantity of metallic lead.
Recently published reports (Schwitzgebel 1981, Mackey and Bergsoe 1977,
Coleman and Vandervort 1980, Coleman and Vandervort 1979) differ about the ap-
plicability of dust agglomeration to flue dusts from secondary lead smelters.
Available data indicate that smelters differ widely in the type of scrap they
use and in their operating practices. Research is required before a recommen-
dation can be made about the use of flue dust agglomeration in a reverberatory
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furnace by any specific smelter. Flue dust reverberatory furnaces are in use
in U.S., and are similar to agglomeration furnaces. Items to consider include:
Dust composition, melting point, and generation rates
Fluctuation in dust production rates
Afterburner performance
Recycle practices
Changes in furnace feed materials
Furnace temperature control
Furnace operating temperatures depend on the dust composition. Tempera-
tures ranging from 399° to 898°C (750° to 1650°F) are the most likely oper-
ating points for agglomerating dusts generated from battery scrap smelting.
2.2.2 Smelting
Blast furnaces, reverberatory furnaces, and rotary furnaces can be used
for smelting. Reverberatory smelting furnaces are used to produce a semi soft
lead product that typically contains 3 to 4 percent antimony. Blast furnaces
produce hard or antimonial lead containing about 10 percent antimony. Rotary
kiln furnaces are occasionally used for smelting in the United States but can
produce lead products similar to those produced in reverberatory furnace.
Blast Furnaces—
Blast furnaces (sometimes called cupolas or shaft furnaces) are commonly
used for smelting in the secondary lead industry. The furnaces are refrac-
tory-lined, water-cooled, steel vessels, usually open at the top. Pretreated
• EXHAUST HOOD
AND SYSTEM
RING COLLAR
OR THIMBLE
DUCT TO
AFTERBURNER
WATER-COOLED
SHELL
REFRACTORY
WATER-COOLED
LEAD MOLD
ERES • ^c
'/.•./.. :•.::.. 7..
T
—
=3
, -X
LEAD / N SLAG
WELL POT
CHARGING
CAP OR
SKIP
Figure 7. Open top blast furnace.
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Skip hoist
Front-end loader
Figure 8. Blast furnace charging.
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scrap metal, coke, rerun slag, recycled dross, flue dust, scrap iron, and
fluxes (e.g., limestone) are charged into the furnace either in alternate
layers or as a mixed feed. Charging devices that may be used, include skip
hoists, conveyor belts, and front-end loaders (Figure 8).
The first step in blast furnace smelting is charging. A typical blast
furnace charge contains 55 to 87 percent battery scrap; 7 to 20 percent dross-
es, oxides, leaching residue, and reverberatory slag; 5 to 8 percent coke; 2
to 6 percent rerun slag; 1 to 6 percent scrap iron; 1 to 3 percent flue dust;
and 1 to 5 percent limestone. Lead drosses contain antimony, copper, caustic,
and other residues from the refining processes. Reverberatory slag may con-
tain lead, silica, tin, arsenic, copper, and antimony. Rerun slag is a highly
silicated slag tapped from previous blast furnace runs. Not all blast furnace
slag is necessarily recycled. Iron and limestone form an oxidation-retardant
slag that floats on top of the melt to prevent oxidation of the reduced lead
in the smelting furnace.
Blast air (sometimes oxygen-enriched) is introduced through tuyeres just
above the slag level in the furnace. The air reacts with coke in the charge
to produce heat, carbon monoxide, and carbon dioxide. The hot gas rises
through the charge material in the furnace, preheats it, and provides the
necessary reducing atmosphere for smelting. Smelting occurs at the tuyere
level, where carbon and carbon monoxide reacts with lead sulfates, oxides, and
carbonates to form lead, sulfur dioxide, and carbon dioxide. Temperature at
the tuyere level is approximately 1010°C (1850°F).
Figure 9. Blast furnace tuyeres and slag tap.
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Furnace slag floats on top of the lead in the furnace and is tapped in-
termittently from a hole that is drilled through the furnace refractory that
is located at a higher level than the lead tap hole and usually 180 degrees
from the lead tap hole. The hole is then sealed with a fire-clay plug after
the slag is tapped. The furnace slag level is monitored by removing the tu-
yere covers, peering through glass inserts in the tuyere covers, checking the
tuyere air pressure, timing previous slag taps, or a combination of these
techniques. At no time should the slag level be allowed to rise above the
tuyere level. Quite often, rods must be inserted through the tuyeres to clean
slag accretions that form when slag splashes the tuyeres. This is commonly
referred to as "punching the tuyeres."
Slag is usually tapped into crucibles for cooling (Figure 10). If enough
iron is present in the furnace charge, a liquid called matte, which consists
Figure 10. Slag tapping crucible.
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of lead, iron, and sulfur, forms at the bottom of the slag crucible. Matte
differs from slag and lead bullion. Most of the sulfur (95 to 98 percent) in
the feed material accumulates in the matte or slag/matte mixture. The matte
can be broken from the slag when the slag crucible is cooled and dumped.
Thus, the slag, with its higher lead content, can be charged to the blast fur-
nace without recycling most of the sulfur. Not all the slag, however, is
necessarily recycled. .
Molten lead settles to a lead well at the bottom of the furnace. Normal-
ly, it is continuously tapped from the lead well through a hole in the furnace
wall. The temperature of the molten lead is 871° to 898°C (1600° to 1650°F).
Next, the lead is fed by gravity via a runner into a water-cooled mold or a
Figure 11. Lead tapping.
holding kettle where the temperature is ,371° to 538°C (700° to 1000°F). The
crude lead is then cooled and cast into 227-kg (500-lb) sows or into hogs,
which range from 680 to 1360 kg (1500 to 3000 Ib).
Additional limestone can be added to the furnace to reduce the lead con-
tent of the ferrosilicate blast furnace slag. This addition, however, raises
the melting point of the slag mixture, which contains silicon oxide (8102).
ferrous oxide (FeO), and calcium oxide (CaO). Adding extra limestone also
increases slag viscosity, so that the slag is sticky and difficult to remove
from the furnace. Oxygen lances can be used to heat such slag and make it
flow. Extreme care must be taken, however, to prevent overheating of the
refractory, causing hot spots and ultimately, a sudden furnace rupture. This
is particularly critical where water cooling is used to prevent the possibil-
ity of water contacting hot metal, resulting in a "metal explosion" from the
rapid steam generation.
15
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Blast furnace capacities range from 15 to 40 Mg (16 to 44 tons) of lead/
day per square meter of furnace cross section. The operating capacity is
usually limited by the amount of gas that can be drawn through the furnace
without overloading the gas cleaning equipment. Each smelting furnace opera-
tor can vary a number of parameters to increase production, but as the upper
limit of production is approached, the likelihood of upsets that create envi-
ronmental problems increase (see Section 3).
Temperature control is accomplished by adjusting the coke feed rate,
blast air rate, or oxygen content of the blast air. Some smelters also have
blast air preheat temperature controls. A relatively high ratio of coke (8 to
9 percent) is used in the charge to maximize production, and a relatively
large operating temperature range can be achieved within the furnace by blast
air control. Rapid smelting can thus be achieved by raising temperatures
throughout the furnace with high blast air rates, preheat temperatures, and
oxygen ratios.
The blast furnace operator may lose control of the furnace temperature.
If the gases ignite, the fire cannot be extinguished simply by shutting off
the blast air. In such cases, additional charge material without coke must
usually be added to reduce the heat and extinguish the fire. Placing a steel
cover on top can also help put out the fire.
Reverberatory Furnaces—
Reverberatory furnaces are typically rectangular and have an arched roof.
The melt is directly exposed to the furnace burner flame. Figure 12 shows a
COMBUSTION AIR
— FUEL GAS OR OIL
EXHAUST GASES
AIR LANCE
FLUX
LEAD
PRODUCT
Figure 12. Schematic diagram of a reverberatory furnace.
16
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schematic diagram of a reverberatory furnace. As discussed earlier, rever-
beratory furnaces are also used for sweating, melting, dust agglomeration,
softening, and refining. Typically, a specific reverberatory furnace, is used
for only one of these functions. The processing steps vary, depending on the
furnace function.
Battery plates, lead oxides, drosses from refining kettles, fluxes, and
collected flue dusts are feed materials for reverberatory furnaces. Typical
feed mechanisms are ram feeders and top charging doors with conveyors. Both
feed mechanisms require manual attention. The high temperatures within re-
verberatory furnaces immediately vaporize any water that is introduced with
the scrap. The consequent rapid increase in gas flow from the furnace can
cause temporary overloading of the exhaust ventilation and process gas han-
dling systems. Predrying of the raw material can prevent this problem.
Furnace temperatures vary with the operation and the type of fuel fired.
Oil flames produce cooler, more radiant flames than natural gas. Although
furnace exit gas temperatures may be as high as 1204°C (2200°F) in smelting
applications, slag temperatures in the furnace rarely exceed 1010°C (1850°F),
and metal temperature rarely exceed 955°C (1750°F).
The slag floats on top of the melted lead and may be tapped from one or
more locations around the furnace. Access or inspection doors are usually
provided at or just above the desired slag level. When the desired slag level
is reached, the tap hole is opened and the slag drained from the furnace. A
refractory launder (chute) is usually provided to allow the slag to flow into
a receiving ladle. The receiving ladle should be allowed to cool under an
exhaust hood until a crust forms on the slag. Ventilation should be provided
for the slag hole, launder, and ladle to allow access during slag tapping.
In some cases, reverberatory slags are too viscous to flow freely from the
furnace. In such cases, the tapping operation requires the furnace operator
to rake out the slag through a slag door having a launder attached to it. If
slag tapping can be done at only one location in the furnace, it may be
necessary to push the slag from several points around the furnace toward the
slag tap door.
Crude lead may be tapped (as described in the discussion of blast fur-
naces) or pumped from the reverberatory furnace. This lead has less impuri-
ties than from the blast furnaces and is normally a soft lead. Often the lead
is tapped into a holding kettle that is held at 427° to 538°C (800° to 1000°F).
The crude lead is either cast into sows or hogs for later processing or can be
pumped to refining and/or alloying kettles or to a casting operation for final
processing.
A number of secondary lead smelters use reverberatory furnaces to produce
soft or semisoft lead directly from battery plates and lead scrap. The fur-
nace can be adjusted to oxidize all of the antimony in the scrap, but only
part of the lead. A highly oxidized, antimonial slag is produced as a result.
This slag is typically processed in either a rotary or a blast furnace to re-
cover the metal values. The soft or semisoft lead can be sent to the refining
or alloying kettle furnaces.
17
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The recent increase in demand for soft lead for the manufacture of cal-
cium-lead-tin (Ca-Pb-Sn) batteries has increased the need for reverberatory
smelting and softening capacity. The decision to use a blast furnace/rever-
beratory furnace combination or only a reverberatory furnace is usually based
on site-specific lead markets. Smelters who receive Ca-Pb-Sn batteries and
battery manufacturing scrap and who supply manufacturers of Ca-Pb-Sn batteries
may elect not to make antimonial lead.
Rotary Kiln Furnaces--
Rotary kiln furnaces are not used as widely as blast furnaces and rever-
beratory furnaces for smelting at secondary lead plants in the United States.
One operating kiln is a 177 foot long, 10 foot diameter cement kiln that has
been converted to lead smelting specifications (Egan 1980). The furnace is
inclined slightly to the horozontal. Brick is used to form a dam around the
perimeter of the discharge and to collect the metal.
Feed materials (lead bearing materials, fluxes, and coke) can be fed by a
conveyor system into the higher end of the kiln. The feed is exposed to
furnace temperatures of up to 1300°C (2400°F) generated mainly by the combus-
tion of coke in the charge. A burner is located at the discharge end of the
kiln to initiate the coke combustion as well as for slag temperature control.
At the discharge end, the slag is floating on top of the hot metal and is
continuously removed from the furnace by flowing over the brick dam. Lead is
tapped periodically by stopping the kiln, breaking open the tap hole located
behind the dam, and then rotating the kiln until the tap hole, points down
(Egan 1980). The lead produced by rotary kiln smelting is identical to that
produced in reverberatory furnaces as discussed above.
2.2.3 Refining (softening, alloying, and oxidation)
Refining and processing the crude lead from the smelting furnaces can
consist of softening, alloying, and oxidation depending on the final products.
Since intermediate smelting products, especially from blast furnaces, may con-
tain antimony or copper (either element makes the lead hard), these contami-
nants must be removed to produce a soft lead product. Molten or cast lead
bullion, fluxes (including sodium hydroxide, sodium nitrate, aluminum chlo-
ride, aluminum, sawdust, sulfur, and calcium chloride), and air may be inputs
to this process. Sodium nitrate (NaN03), sodium hydroxide (NaOH), and air are
used to reduce the antimony content of the charge. The addition of sodium
nitrate and sodium hydroxide to the melt produces a slag/dross material that
contains NaSbOs. Aluminum reacts preferentially with copper, antimony, and
nickel to form drosses that can be skimmed off the melt surface; sulfur is
used to dross copper. Calcium chloride is used for detinning. Alloying
processes involve adjustment of the metals content of smelting furnace lead to
produce a desired lead alloy. Common alloying agents are antimony, copper,
silver, and tin. Lead is oxidized to produce battery lead oxide (PbO contain-
ing approximatley 20 percent lead metal), and lead oxide pigments such as
lead monoxide (PbO) and red lead
Kettle furnaces are generally used to refine and process lead because
they allow better control of impurities and further alloying. Most kettles
are heated by oil- or gas-fired burners located in brick-lined pits below the
kettles as shown in Figure 13.
18
-------
lURNER
SUPPORTS EXHAUST DUCT
KIXER FOR FOR Pb OXIDES
MIXER PRODUCT I ON^_TO_FMR1C
FILTER
BURNER
a. Kettle, pit, and exhaust system.
b. Melting operation for refining
and alloying.
c. Kettle just before pouring.
Figure 13. Kettle furnace operations.
19
-------
Kettles are used for softening processes to remove antimony and/or copper
and thus produce a soft lead product. Kettle sizes generally range from 1 to
135 Mg (1 to 150 tons). The process steps are charging the preheated kettle,
melting the charge, agitating the flux into the molten charge, skimming the
drosses, and pouring or pumping the molten metal. In some cases, molten lead
is charged directly from the smelting furnace into the refining kettle. Often,
separate kettles are used to remove various impurities. Fluxing temperatures
generally range between 375° and 485°C (700° and 900°F). Sometimes copper is
precipitated by lowering the temperature to 325°C (620°F). Aluminum chloride
or sawdust may be mixed into the dross to decrease the chemical bond between
lead droplets and dross. The dross is skimmed or tapped from the furnace
before the softened lead is tapped. The dross is usually reprocessed in a
blast furnace to recover metal values.
The process steps for alloying are similar to those used for softening
except that alloying agents are added to the melted lead and mixed into the
charge before pouring and casting into pigs (Figure 14).
Oxidizing furnaces are either kettle or reverberatory furnaces which
oxidize lead and entrain the product lead oxides in the combustion air stream.
The product is subsequently recovered in baghouses at high efficiency. Bat-
tery lead oxide (PbO containing approximately 20 percent lead metal) is pro-
duced by kettle oxidation. The process steps are: charging the kettle by
gravity with molten lead, agitating the molten lead with paddles, inducing a
draft of air over the surface of the melt through a duct leading to a bag-
house, and collecting the lead and lead oxide fumes in a baghouse. Lead oxide
pigments, lead monoxide (PbO), and red lead (Pb304) are produced by rever-
beratory oxidation. The process steps are charging the preheated reverbera-
tory furnace with molten lead, agitating and simultaneously oxidizing the
molten lead, and removing the lead oxide form the furnace and cooling rapidly.
Either lead monoxide (PbO) or red lead (Pb304) is produced by controlling the
degree of oxidation.
2.2.4 Casting (Pigging)
The softened lead and lead alloy products are cast into pigs (ingots)
with a pigging machine (Figure 14). The pigging machine is similar to a con-
veyor with molds forming the conveyor belt. Molds receive the liquid hot
metal at one end of the machine, and by the time the mold is moved to the
other end, the lead ingot (pig) is solidified. The pig is then dumped from
the mold. The mold is then recycled to receive more hot metal.
20
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Figure 14. Pigging operation
21
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REFERENCES FOR SECTION 2
1.
2.
3.
4.
5.
6.
7.
Burton, D. J., et al. 1980. Control Technology Assessment - the Second
Nonferrous Smelting Industry. National Institute for Occupational Safety
and Health Technical Report No. DHHS 80-143. Radian Corporation, Austin,
Texas.
Coleman, R. T., and R. Vandervort. 1979. Demonstration of the Bergsoe
Agglomeration Furnace and Best Management Practices at a Secondary Lead
Smelter. Report DCN 79-201-010-02. Radian Corporation, Austin, Texas.
Coleman, R. T., and R. Vandervort. 1980. Evaluation of Paul Bergsoe and
Son Secondary Lead Smelter. EPA-600/2-80-022.
Egan, R. C., et al. 1980. Rotary Kiln Smelting of Secondary Lead.
Lead-Zinc-Tin '80, Proceedings of a World Symposium on Metallurgy and
Environmental Control, Sponsored by AIME - Las Vegas, Nevada National
Lead Industries, Inc. Hightstown, New Jersey.
Mackey, T. S., and S. Bergsoe.
Journal of Metals, 29(1):12.
1977. Flash Agglomeration of Flue Dust.
Mezey, E. J. 1979. Characterization of Priority Pollutants From a
Secondary Lead and Battery Manufacturing Facility. EPA-600/2-79-039.
Schwitzgebel, K. 1981. Flue Dust Agglomeration in the Secondary Lead
Industry. Journal of Metals, 33(1):38-41.
22
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SECTION 3
AIR EMISSIONS GENERATION AND CONTROL
Furnace technology in the secondary lead industry has remained essentially
unchanged for more than 50 years. Many furnace combinations are used, depend-
ing on the scrap supply and the desired product. Each furnace produces point
and fugitive emissions. The lead content of particulate
furnace varies with the furnace feed material, furnace
oxidation potential of the furnace gas. Figure 15 indicates
of environmental emissions from secondary lead processes.
source emissions
matter from each
temperature, and
the major sources
Table 1 lists sources of fugitive emissions from secondary lead smelters.
Fugitive emission rates vary with each operation. When the furnaces are open
(e.g., during charging) fugitive emission rates are highest. At other times,
fugitive emission rates can be essentially nil. Smelting furnaces account for
a significant portion of both process and fugitive emissions, but vehicular
traffic, flue dust handling, and cleanup operations can create additional
fugitive emissions problems that are independent of furnace operating condi-
tions. In some older smelters, the accumulation of dust settled on the ground
over the years has created a continual reentrainment problem arising from
windblown dust that results in ambient lead levels exceeding 1.5 yg/rn^ off
the smelter property, even after the facility has been shut down. The Section
3 References (Page 49) lists important sources for data on emissions from
secondary lead smelting operations.
Major changes in secondary lead smelters have been the addition of emis-
sion control systems and control methods for process and ventilation gases.
Acceptable control practices can be divided into process controls, process
emission controls, and fugitive emission controls. Control practices are
further classified as engineering controls and procedural controls. Engineer-
ing controls include the following:
Isolation of process or employee
Enclosure of process or employee
Ventilation (either local exhaust and capture or dilution)
Substitution of materials, processes, equipment, or operating practices
Physical modification of contaminants (e.g., wetting the material)
Reduction or elimination of emission producing energy to the source
(°.n.3 not overheating molten metal or not sweeping dry, dusty materials)
23
-------
c
s-
•i—
to
<*-
O
O)
O
S-
13
O
OO
a)
S-
a>
24
-------
TABLE 1. SOURCES AND EMISSION FACTORS FOR POINT AND FUGITIVE
EMISSIONS FROM SECONDARY LEAD SMELTERS3»&
Source
Raw material handling and transfer
(scrap lead, scrap iron, coke,
limestone, etc. )
Lead and iron scrap burning
Battery decasing
Crushing or shredding
Rotary or reverberatory furnace
sweating
Charging
Tapping
Scrap removal
Reverberatory furnace smelting
Charging
Slag tapping
Lead tapping/casting
Blast furnace smelting
Charging
Slag tapping
Lead tapping/casting
Holding pot
Reverberatory furnace softening
Charging
Tapping (dressing, skimming,
lead removal)
Kettle softening/refining
Charging
Tapping (dressing, skimming,
lead removal )
Kettle alloying/refining
Charging
Tapping, dressing, skimming
Kettle oxidation
Charging
Reverberatory furnace oxidation
Charging
Casting (pigging)6
Flue dust handling and transfer
Vehicular traffic
Traffic on paved roads
Traffic on unpaved roads
Hooding, ductwork, control
device, or furnace leaks
Point source emissions, Ib/ton
Particulate
NA
NA
NA
NA
32-70
147
193
NA
NA
0.8
<40
NA
NA
NA
NA
NA
Lead
NA
NA
NA
NA
7-16
34
44
NA
NA
0.2
NA
NA
NA
NA
NA
NA
Basisc
E
B
B
B
I
Fugitive emissions, Ib/ton
Particulate
NA
NA
NA
NA
1.6-3.5
2.8-15.7
NA
NA
0.04
NA
NA
0.88
NA
NA
NA
Lead
NA
NA
NA
NA
0.4-1.8
0.6-3.6
NA
NA
0.01
NA
NA
0.2
NA
NA
NA
Basisc
E
E
/
E
E
NA - data not available.
' Source: Reference 9.
b All emission factors are based on the quantity of material charged to the furnace (except particulate
kettle oxidation).
c The basis of the emission factor refers to the method from which.the emission factor was obtained.
B - Emission factor based on source test data and is rated (EPA 1980) as above average.
E - Engineering estimate supportable by visual observation and emission tests for similar sources.
These emission factors are rated (EPA 1980) poor. '
Factors based on amount of lead oxide produced.
e Factors based on amount of lead cast.
25
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Use of applicable control devices (e.g., central vacuum systems or
electrostatic foggers)
Procedural controls include the following:
Operating practice instructions
Employee work practices, job descriptions, and incentives
Housekeeping practices and policies
Maintenance practices and policies
This section describes both point and fugitive process emission sources
and their associated control alternatives.
3.1 PROCESS EMISSIONS
Air emissions are generated at all secondary lead plant operations. Raw
material handling and preparation, smelting, and refining processes are sources
of point as well as fugitive emissions. In addition to lead, antimony, nickel,
tin, and zinc, arsenic is also a pollutant of concern. Arsenic is found in
varying amounts in lead scrap. It is contained in primary lead unless removed
in refining, and it is often added during lead alloying as a hardening agent.
Antimonial lead (hard lead) is used primarily in the posts and grids of lead-
acid storage batteries and for lead cable sheathing (PEDCo 1982). The arsenic
in such alloys ranges from 0.15 percent for antimonial lead to no more than
0.5 percent for arsenical lead (Burgess 1976).
3.1.1 Raw Material Handling and Pretreatment
Raw Material Handling and Transport--
Lead bearing feed materials are commonly stored outdoors in large piles
(Figure 16) and are transported around the smelter in front-end loaders and/or
trucks. If these materials are allowed to dry, wind erosion becomes a major
source of fugitive emissions. Indoor storage is possible and would signifi-
cantly reduce the fugitive emissions but the high cost of large structures has
discouraged many facilities from using this technique. There are no estimates
available for fugitive emissions from raw material handling and transport
operations.
Battery Handling-
Battery handling and breaking consists of the following operations: seg-
regation by the type of battery case material; battery sawing or crushing;
acid draining; and lead scrap removal. Emissions consist mainly of sulfuric
acid mist and dusts containing dirt, battery case material and lead compounds.
There are no available emission factor estimates from battery handling opera-
tions.
26
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Figure 16. Unagglomerated baghouse dust.
Sweating—
Sweating furnaces operate with gas exit temperatures of 538° to 816°C
(1000° to 1500°F) from both reverberatory and rotary sweating furnaces. At-
mospheric emissions consist of fume, dust, soot, participates, and combustion
products, including sulfur dioxide (S02). The S02 emissions are from the
combustion of sulfur compounds from both the scrap and the fuel. Particulate
emissions from the stack range from 16 to 35 kg/Mg (32-70 Ib/ton) of material
feed with dust loadings of 3.2 to 10.3 g/m3 (1.4-4.5 gr/ft3)(EPA 1980). Lead
emissions are estimated to be 4 to 8 kg/Mg (7-16 Ib/ton) (EPA 1980). Fugitive
emissions are generated during charging and removing the scrap from the
furnace. Estimated fugitive emissions from sweating furnaces are 0.8 to 1.8
kg/Mg (1.6-3.5 Ib/ton) for particulates and 0.2 to 0.4 kg/Mg (0.4-1.8 Ib/ton)
for lead (EPA 1980).
Reverberatory furnaces for sweating require the removal of solid scrap
parts after the lead or solder has been melted away. Hoes or rakes are used
to remove the scrap through access doors. This operation can create fugitive
emissions and drag dust from the furnace hearth. ,
Dust Agglomeration— ,
Dust agglomeration is not performed at all plants. It is frequently used
to minimize fugitive emissions when recycling fabric filter dusts from rever-
beratory, blast, and rotary furnaces. There are no readily available emission
estimates for dust agglomeration furnaces.
27
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3.1.2 Smelting
Blast Furnace—
The process gas temperature at the exit from American blast furnaces
normally range from 121° to 425°C (250° to 800°F). This temperature strongly
influences the quantity and composition of material in the process gases from
individual furnaces (Schwitzgebel 1981). Blast furnace emissions include car-
bon monoxide (CO), sulfur dioxide (S02), sulfuric acid (h^SCty), and particulate
matter that contains lead, antimony, chlorine, sulfur, and organic materials.
Table 2 shows lead distribution in blast and reverberatory furnace dust.
TABLE 2. LEAD DISTRIBUTION IN REVERBERATORY
AND BLAST FURNACE SMELTING (Burton 1980)
Reverberatory furnace
smelting
Blast furnace
smelting
Lead in
raw material
charge
47
70
Lead in slag
46
8
Lead in
fumes and
dusts
7
12
A Canadian study of fabric filter and flue dusts from 10 sources shows that
the lead content in the sources ranged from 15.8 to 68.7 percent with an
average of 51.7 percent (McDonnell and Hi!born 1978). Another source indi-
cated that the lead blast furnace flue dust is 80 to 90 percent metal and that
70 to 80 percent of the metal is lead (i.e., that the flue dust is 56 to 72
percent lead).* Point source emissions from blast furnace operation are esti-
mated to average 97 kg/Mg (193 Ib/ton) of material charged for particulate, 22
kg/Mg (44 Ib/ton) for lead, and 27 kg/Mg (53 Ib/ton) for S02 (EPA 1980).
Fugitive emissions from the blast furnace result from the following
operations:
Furnace charging
Tuyere punching
Slag tapping
Crude lead tapping
Process upsets
Emission estimates are not available for each of these operations, but
total fugitive emissions from smelting operations are estimated to be 1.4 to
7.9 kg/Mg (2.8-15.7 Ib/ton) of charge material for particulate and 0.3 to 1.8
kg/Mg (0.6-3.6 Ib/ton) for lead (EPA 1980).
Front-end loaders are potential fugitive emissions sources during the
charging operation, as are the skip hoist, conveyors, and furnace charging
area itself. Skip hoists and conveyors in this application are likely to
collect dust and battery residue that may be subsequently dispersed into the
air when the machine is impacted or vibrated vigorously.
* Personal communication with F. Ledbetter, U.S. EPA, Region IV, December
18, 1979.
28
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Routine tending of a blast furnace involves examination of each tuyere
either by removing the tuyere cover or by viewing it through a transparent
window in the cover (Figure 17). When the tuyere cover is removed for
TRANSPARENT
WINDOW
TUYERE CAP ROTATES
FOR PUNCHING
•NOTCH TO PERMIT
INSERTION OF
PUNCHING BAR
Figure 17. Tuyere cap.
inspection of the slag level or for punching, some emissions occur. The
transparent window in the tuyere cover minimizes the necessity to remove the
cover and hence reduces emissions.
Slag is tapped from holes bored through the furnace refractory between
the lead well surface and the tuyere level. One to four tap holes may be
used, depending on the size and shape of the furnace. This process is labor
intensive and can be a source of fugitive emissions from the time the holes
are tapped until they are sealed with fire-clay plugs. The slag tapping
operation is repeated every 15 to 20 minutes, depending on the rate of opera-
tion and the material being smelted.
Crude lead tapping is another source of fugitive emissions from the blast
furnace. These emissions occur as the crude lead is being continuously tapped
from a tap hole below the slag level (Figure 18).
Fugitive emissions can also occur if the production limitation of the
furnace is exceeded. Furnace capacity is usually limited by the gas handling
capacity of the control equipment attached to the furnace. If the gas han-
dling capacity is exceeded, excess emissions occur through leaks in the system;
e.g., leaks from the furnace top, flanges, inspection covers, or doors. Even
if local exhaust ventilation is provided for such fugitive emissions, fires
and explosions can occur because these fugitive emissions bypass the after-
burner and are ducted to the sanitary baghouse. At the high end of the temper-
ature range, (e.g., 425°C gas exit temperature) fires in the scrap charge are
a danger. Thus, control of the temperature profile within the furnace has an
important effect on furnace emissions.
29
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Figure 18. Electron micrograph of lead well dust. Magnification - 4750 X.
Reverberatory Furnaces--
Reverberatory furnace off-gas temperatures range from 538° to 1315°C
(1000° to 2400°F), depending on the function of the furnace, the operation
being performed, the firing rate, and the feed material. The furnace gases
are usually well oxidized. In most furnaces, excess combustion air is sup-
plied to ensure that adequate combustion occurs at the burner end of the
furnace, where smelting occurs. The furnaces are also operated at a very
slightly negative pressure, typically between -2.48 and -24.8 Pa (-0.01 and
-0.1 in. H£0). This causes a slight flow of air into the furnace that helps
to complete the combustion of hot gases and particulate matter.
Reverberatory emissions can be expected to be similar to those from blast
furnace smelting if battery scrap is the furnace feed material. Reverberatory
furnaces that are charged with crude lead instead of scrap will not emit
chlorine or sulfur compounds; those compounds were already removed in the
production of the crude lead.
Emissions from reverberatory furnaces used for smelting have been esti-
mated to be an average of 74 kg/Mg (147 Ib/ton) of material charged for par-
ticulate, 17 kg/Mg (34 Ib/ton) for lead, and 40 kg/Mg (80 Ib/ton) for S02 (EPA
1980). Off gas temperatures range from 530° to 1315°C (1000° to 2400°F).
Fugitive emissions result from the following operations:
o
o
o
o
Furnace charging
Slag tapping
Crude lead tapping
Cooler (hairpin) cleaning
(where applicable)
30
-------
No specific fugitive emission data were available for reverberatory
smelting. One estimate, however, for fugitive emissions from smelting is 1.4
to 7.9 kg/Mg (2.8-15.7 Ib/ton) of feed material for particulate, and 0.3 to
1.8 kg/Mg (0.6-3.6 Ib/ton) for lead (EPA 1980).
Furnace charging is a significant source of fugitive emissions. Ram
feeders and top charging doors with conveyors are typical feeding mechanisms
for reverberatory furnaces. Both feed mechanisms require manual attention.
Although exhaust ventilation and partial enclosure is possible, the high
temperatures within reverberatory furnaces immediately vaporize any water that
is introduced with the scrap. The consequent rapid increase in gas flow from
the furnace can cause temporary overloading of the exhaust ventilation and
process gas handling systems. Fugitive dust is emitted from access doors and
charge ports, and through the furnace refractory. Leaks through the refractory
are probably the most difficult fugitive emission problem to solve with
reverberatory furnaces.
Fugitive emissions can escape during slag tapping. Slag may be tapped
from one or more locations around the furnace. Access or inspection doors are
usually provided at or just above the desired slag level. Fugitive emissions
escape when the doors are opened. When the desired slag level is reached, the
tap hole is opened and the slag drained from the furnace.
In some cases, reverberatory slags are dusty. They are often too viscous
to flow freely from the furnace. In such cases, the tapping operation requires
the furnace operator to rake out the slag through a slag door having a launder
attached to it. If slag tapping can be done at only one location in the
furnace, it may be necessary to push the slag from several points around the
furnace toward the slag tap door. When access doors for pushing the slag are
open, the potential for fugitive emissions is great, especially if moist
charge materials are being fed to the furnace.
During crude lead tapping, fugitive emissions will escape from the launder
and receiving kettle, ladle, or casting operation during the metal taps.
Reverberatory gases may be cooled in hairpin type coolers (Figure 19) or
by dilution with process or ventilation gases prior to particulate collection.
The long continuous run of ductwork which makes up a hairpin cooler appears as
several verticle rows of hairpins. The gases are cooled in hairpin coolers
because of the heat loss from the large surface area of the ductwork. Some
particulate matter collects at the base of the hairpin coolers and must be
periodically removed. Unless this collected material is conveyed automatically
in an enclosed system, it can create a fugitive emission problem.
3.1.3 Refining (Softening, Alloying, and Oxidation)
Process gases, fumes, and dusts are emitted from kettle furnaces used for
refining processes. Combustion gases are often vented directly to the atmo-
sphere without being mixed with the process gases. Emissions from softening
and alloying are estimated to be 0.4 kg/Mg (0.8 Ib/ton) of material charged
for particulate, and 0.1 kg/Mg (0.2 Ib/ton) for lead (EPA 1980). The upper
limit for lead oxide escaping from product collection for kettle oxidaton is
20 I^/MC (A" iH/ton) of lead oxide produced for particulate (EPA 1980).
31
-------
Figure 19. Hairpin cooler.
Lead refining and the production of lead, tin, and antimonial alloys pose
potential fugitive emission problems because of the employees' continued con-
tact with the process. Charging, melting, fluxing, mixing, dressing, skimming,
and tapping operations require the operator to open kettle hood doors and to
partially disrupt local exhaust ventilation. Lead dust and fume, agitated
molten metal, high temperatures, and a number of chemical agents all contrib-
ute to the potential for fugitive emissions. Total fugitives from kettle
refining are estimated to be 0.02 kg/Mg (0.04 Ib/ton) of material charged for
particulate and 0.004 kg/Mg (0.01 Ib/ton) for lead (EPA 1980).
Little charge preparation is required for kettles. Fugitive emissions
may result from melting the residual lead .inside the lead pumps (i.e., the
lead in the pumps solidifies between uses and heating is necessary to melt the
contained lead so that the pump impeller is freed) so that the pumps can be
used. This can best be done by immersing the pump in the kettle, since the
pumps are relatively portable and thus, lead fumes are captured by the kettle
hood.
Fugitive emissions occur during fluxing, mixing, and dressing, and skim-
ming operations through openings required to complete these operations. The
proper use of the hood doors during these operations can minimize openings as
well as fugitive emissions. The higher metal temperatures, additional agita-
tion, and presence of dusty dross materials create a greater fugitive dust
potential.
Antimonial drosses are a major fugitive problem with kettle furnaces.
These dusty materials are usually skimmed manually from the lead surface with
hoes, rakes, or shovels. During the dressing operation, kettle temperatures
are typically above 343°C (650°F), kettle doors are at least partially open,
32
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and the normal flow pattern of the local exhaust ventilation is disturbed.
Some dross is invariably spilled as it is removed from the kettle.
Fugitive emissions occur during molten metal transfer when the hood is
opened to insert the pump. It is not practical to reduce the kettle tempera-
ture prior to opening the hood and inserting the pump to keep emissions low.
A side-entry, slant-in pump assembly may be present however, to eliminate the
need to open the hood during the pumping operation.
Casting--
Fugitive emissions occur during the casting operation. These have been
estimated to be 0.44 kg/Mg (0.88 Ib/ton) of metal cast for particulate and 0.1
kg/Mg (0.2 Ib/ton) for lead (EPA 1980).
3.2 EMISSION CONTROLS
Process point and fugitive emission control systems are required to
minimize pollution from secondary lead processes. These controls include
fugitive emissions capture and control by ventilation systems (i.e., hooding)
and other techniques (i.e., sprays), control equipment such as fabric filters,
wet scrubbers, and afterburners, and procedural controls such as plant opera-
tion and maintenance practices. This section will address the application of
these control systems and techniques. The design and operation and maintenance
requirements for these systems will be discussed in Section 4.
3.2.1 Fugitive Capture and Control
Materials Handling and Preparation-
Material handling operations emit fines, coke breeze, and flue dust.
These may be recovered at smelters by hand sweeping or mechanical and/or
vacuum sweepers. Sweeping can be done with mechanical or vacuum-type sweepers
Figure 20.
Vacuum-type sweeper.
33
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to avoid reentrainment of dust. If sweepers are not properly maintained, they
also may cause reentrainment of dust. Motorized mechanical or vacuum sweepers
or a central vacuum system should be used to recover fines. Normally, the
recovered material should be recharged to one of the furnaces.
Paving of the entire smelter area including the raw material storage and
preparation areas, transportation areas, emission collection areas, and tempo-
rary waste material or dust storage areas together with systematic, periodic
wetting of paved areas minimize dust reentrainment. Runoff water should be
collected and combined with acid drainage and any process rinse water. After
neutralization, this collected water should be sent to a holding pond or to a
clarifier/thickener. Periodically the lead bearing sludge can be returned to
the furnaces for additional metal recovery. The clarified and neutralized
pond water can be recycled to the process.
If the plant area is unpaved, water spraying of the roads and storage
areas should be performed on a routine schedule to minimize dust reentrain-
ment. A wetting agent should be used in the water spraying operations. All
vehicles transporting flue or waste dust or dusty raw materials should be
covered or enclosed.
Raw material piles are commonly sprinkled with water to control wind
erosion. The acidic runoff water must be collected. Although it can be
neutralized easily, discharge of the treated water and disposal of the result-
ant sludge can be difficult. The treated water can be recirculated in a
closed system, and the sludge can be recycled to the smelting furnace (Coleman
and Vandervort 1980). This technique has been used with some success at a
secondary lead smelter in England where high winds are a regular daily occur-
rence.
Indoor storage is a possible control measure, but the cost of large
structures for that purpose has discouraged many facilities from using this
Figure 21. Indoor storage building.
34
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technique. An alternative technique is to use outside storage sheds with
sprinkler systems. Live-bottom hoppers that feed to conveyor belts have also
been suggested as a feasible technique for enclosing and handling raw mate-
rials, but no such systems are in use at the present time.
Enclosure and wetting techniques seem to be the most effective means to
control fugitive emissions caused by front-end loaders. Lead smelters in
Denmark and Sweden have attempted to enclose scrap handling and transport op-
erations. The irregular receipt of materials, however, resulted in inventories
that exceeded enclosed storage capacities. Constant wetting of paved areas at
these smelters helped to reduce ambient lead levels in the yard area to a
range of 12 to 18 yg/m^ during one test period (Coleman and Vandervort 1980).
The battery processing area (breaking and crushing) is usually controlled.
Water spraying of crushed batteries to minimize acid mist fumes is maintained.
The battery breaking station is hooded to prevent fugitive emissions.
Figure 22. Battery breaking station.
Smelting—
During blast furnace operation, fugitive emissions occur during charging,
tuyere punching, slag tapping, and crude lead tapping. To ensure maximum
collection during charging, many smelters have successfully enclosed the
furnace charging area with canopy or box-like hoods to contain any dust that
is emitted. Local exhaust ventilation hooding above the tuyeres should be
provided to help capture fugitive emissions. These hoods may also function as
secondary slag tapping hoods.
35
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STEEL
COVER
FOR
FURNACE
TOP
PLATFORM
AT FURNACE
TOP
FUGITIVE EMISSIONS
TO COLLECTOR
CHAINS OR
CONVEYOR BELT
MATERIAL
DUCT TO
AFTERBURNER
CHARGE
mm
RING COLLAR
OR THIMBLE
SEALED BY CHARGE TO
PREVENT EXCESS EMISSIONS
^BLAST FURNACE
SHELL
Figure 23. Blast furnace charging hood.
The exhaust ventilation system is extremely important in minimizing fugi-
tive emissions from slag tapping. Exhaust hoods must provide worker access to
the slag hole without removing the hood. The design should ensure adequate
ventilation of the slag launder and ladle and offer easy access for personnel
to open the slag hole. Some slagging areas are completely enclosed having two
swinging doors for removal of the slag pot or for opening or sealing the slag
hole (Figures 24a and 24b).
Necessary exhaust ventilation for the lead launder and receiving vessel
for hog casting varies in design with the specific smelter tapping configura-
tion. Hooding should allow access to the tap hole, launder, and receiving
vessel while ventilation should maintain at least a 152-cm/s (300-ft/min) hood
face velocity. Hooding should be as close as possible to the source of emis-
sions. Every attempt should be made to eliminate cross-drafts near these
hoods (i.e., hanging lengths of heavy chains on the hoods). Access doors
should be considered to eliminate the need for hood removal when the tap hole
is tended (Figures 25a and 25b).
In reverberatory furnace operations, fugitive dust is emitted from access
doors and charge ports, and through the furnace refractory. Leaks through the
refractory are probably the most difficult fugitive emission problem to solve
with reverberatory furnaces. Some smelters have attempted to enclose rever-
beratory furnaces with steel plate to prevent such emissions. Enclosures,
however, cannot be made air tight because joints must be left in the steel to
36
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Figure 24. Slag tapping from the blast furnace.
37
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a. Before total enclosure
b. After total enclosure
Figure 25. Hooding for "Hog" casting.
38
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allow for furnace expansion and contraction. In addition, buckling of the
plates is common and creates additional spaces for emissions to occur. More
complete furnace enclosure and greater exhaust ventilation in the form of a
canopy hood may be required.
Furnace charging doors, access doors, slag holes, and metal taps should
be hooded and ventilated. A top charge reverberatory furnace with movable
curtains and panels for the improved capture of fugitive emissions is shown in
Figure 26. The exact configuration of the hoods and ductwork depends on the
TO FLUE CAS
CONTROL SYSTEM
TO EXHAUST SYSTEM
FOR CHARGING HOOD
TO EXHAUST SYSTEM FOR
TAPPING/POURING HOOD
MOVABLE
PANEL
MOVABLE
CURTAINS
MODIFICATION TO
TAPPING/POURING
HOOD TO HELP
CONTROL EMISSIONS
FROM SLAG REMOVAL
Figure 26.
Top charged reverberatory furnace with improved
fugitive emissions capture.
locations of the charging door, access doors, slag holes, and metal taps. The
charging door hood may not be sufficient to capture emissions that result from
a wet charge. If that is the case, enclosure of the entire furnace may be
necessary to control fugitives. Fugitive emissions also escape from the
launder and receiving kettle, ladle, or casting operation during the metal
taps. Unlike blast furnaces, reverberatory furnaces are tapped intermittently.
Exhaust ventilation should be provided at each emission point.
Refining—
Process gases, fumes, and dust are usually collected from kettle furnaces
by a hood over the kettle as shown in Figure 27. Kettles cannot be provided
with close fitting exhaust hoods because of the need to open the hoods fre-
quently.
The control objective is to ensure sufficient ventilation for worst-case
conditions. The following engineering controls are recommended:
Design of hoods to provide adequate local exhaust ventilation [i.e., to
provide face velocities greater than 152 cm/s (300 ft/min) in the hood
openings when all hood doors open]
39
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Figure 27. Refining kettle fugitive emissions capture.
Construction of hoods from durable metal plates, which allow insertion/ex-
traction of charge materials, dressing agents, drosses, pumps, mixers,
and ladles with minimum hood openings
Use of ventilation systems that deliver clean, tempered air to each
kettle work area
Complete enclosure of kettles and exhaust ventilation of the entire
enclosure.
Provision of vacuum and water hose connection and floor drains in the
kettle area to facilitate cleanup and washdown after the kettles are
charged
New hood installations should be tested before any lead is charged to the
kettles. Some design changes may be required to prevent serious fugitive
emissions. Smoke tube, vane or hot wire anemometer, and pi tot tubes flow
measurements are suggested.
40
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An exhaust enclosure for the pigging (casting) machine should be used.
The molten metal reservoir or trough of the pigging machine is of primary
importance. An enclosure can be fashioned with hinged, side access doors, but
a canopy hood may suffice.
Figure 28. Pigging machine with canopy hood.
3.2.2 Control Equipment
The control equipment most commonly used in secondary lead smelters are
fabric filters, scrubbers, and afterburners. Fabric filters are the most
common devices used to remove particulate matter from secondary lead smelter
process and ventilation gas streams. This section presents the application of
these control devices while the design and operation of the control equipment
is discussed in Section 4.
Materials Handling and Preparation--
The controls for these operations consist mainly of sprays and enclosures
and control equipment (i.e., fabric filters, scrubbers, and afterburners) are
not usually necessary. When battery breaking is hooded, a scrubber is typi-
cally used to control sulfuric acid mist and particulates.
Smelting—
The blast and reverberatory furnaces used for smelting account for about
88 percent of total lead emissions from the secondary lead industry (EPA
1980). Using the proper control equipment is crucial for minimizing emissions
from these processes.
The exhaust gases from the blast furnace are controlled in a fabric
filter after passing through an afterburner. An afterburner torch in the top
41
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of the furnace combusts the carbon monoxide and hydrocarbons in the exhaust
gas. Complete combustion ensures that fires or explosions do not occur in the
fabric filter and that condensible hydrocarbons do not form sticky masses that
can foul the fabric filter. Fugitive emissions from the blast furnace opera-
tions (i.e., tuyere punching, slag tapping, and crude lead tapping), can be
ducted to the fabric filter without first passing through an afterburner.
Reverberatory furnace gases are usually well oxidized and do not require
an afterburner. All exhausts from the enclosures and ventilation system can
be sent directly to a fabric filter or scrubber. Reverberatory furnace par-
ticulates are larger than those emitted from blast furnaces and are thus more
suitable than blast furnaces for control by scrubber.
Refining and Casting--
The hoods from kettle furnaces and casting operations are usually vented
to a fabric filter or scrubber.
3.2.3 Procedural Controls
Procedural controls are those that use operating techniques or operating
parameters to decrease emissions. These are mainly applicable to furnace
operation and will be discussed, in this section, by furnace type.
Blast Furnace--
Furnace operating parameters vary widely among smelters.
controls can be illustrated with an example case study.
Procedural
Table 3 shows a set of operating parameters for a 70-Mg/day (77-ton/day)
blast furnace (Coleman and Vandervort 1980). Similar data are reported in
Table 4 for a 37-Mg/day (41-ton/day) furnace (Coleman and Vandervort 1979).
The top temperature of the larger furnace is between 77° and 180°C (170° to
356°F); that of the smaller one is 425°C (797°F). The higher top temperature
results in greater emisisons and makes temperature control more difficult.
Additionally, the low afterburner set point 260°C (500°F) of the smaller
furnace may result in incomplete combustion. No combustion occurs at all if
the process gas temperature is high enough to cause the afterburner tempera-
ture control to shut the burner off. This represents a dangerous operating
practice where emissions might adversely affect downstream emissions.
The operating parameters of each furnace must be examined on a case-by-
case basis to determine the changes needed to improve emission control. The
data in Table 3 can be used as a basis for initial comparison of blast furnac-
es. The operating parameters used by a given plant should take into account
the associated emission control equipment. The blast furnace feed should be
kept above the ring collar to minimize emissions during charging.
A thermocouple for measuring the melt tap temperature is not essential
but can be helpful when operating the furnace to minimize emissions. The
temperature of the melt is related to furnace operation and production rate.
If this temperature is known, an operator can assess furnace operation. The
production rate is typically measured and recorded, whereas material feed
rates are not. Given the tapping temperature, an experienced operator can
42
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TABLE 3. OPERATING PARAMETERS FOR A 70-Mg/DAY BLAST FURNACE'
Oxygen flow r»te
Oxygen/air pressure
Oxygen/air temperature
Blast air flow rate
Water jacket temperature (inlet)
Water jacket temperature (outlet)
Furnace top temperature (process)
Afterburner temperature (inlet)
Afterburner temperature (outlet)
Afterburner temperature .(outlet
after fresh air dilution)
Afterburner temperature (outlet
before mix point)
Fabric filter temperature
(average inlet)
Stack temperature
Stack gas flow rate
56-114 NuiVh (33-67 $cfn)
10-12 M>» (41-46 1n. H20)
472°-495°C (882°-923°F)
3240-3740 NmVh (1900-2200 scfm)
47.5°-52.5°C (117.5°-126.5°F)
57°-64.50C (134.6°-a48.1°F)
770-180°C (170.6°-356°F)
90°-175°C (194°-347°F)
700°-800°C (1292°-1472°F)
415°-460°C (779°-860cF)
290°-330°C (554°-626°F)
104°-132"C (219°-269.6°F)
82C-90°C (179.6°-194°F)
115,000-120,000 NmVh (67,700-71,000
scfm)
Source: Col emsn »nd Vandervort 1980.
TABLE 4. OPERATING PARAMETERS FOR A 37-Mg/DAY BLAST FURNACE'
Oxygen enrichment
Blast air flow rate
Water jacket temperature (inlet)
Water jacket temperature (outlet)
Furnace top temperature
Afterburner temperature set point
Fabric filter temperature (inlet)
Fabric filter lime injection rate
Fabric filter air-to-cloth ratio
Stack temperature
Stack gas flow rate
0.5 to 2.5 percent
3400-4250 NWh (2000-2500 scfm)
71°C (160°F)
90°C (194°F)
425°C (797°F)
260°C (500°F)
104°C (219°F)
11.4 kg/h (25 Ib/h)
27.3 m3/h per m2 (1.49 ftVmin
per ft2)
70°-85CC (158°-185°F)
32,000 Nm3/h '(19,000 scfm)
8 Source: Coleman »nd Vandervort 1979.
43
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determine if the furnace feed rates and firing rates are proper and thus
optimize production and minimize emissions.
Crude lead tapping and slag tapping are major sources of fugitive emis-
sions from blast furnaces. Every attempt should be made, to eliminate cross
drafts near the exhaust ventilation for the lead launder and receiving vessel
(i.e., hanging chains on the hood). Side access doors should be considered to
eliminate the need for hood removal when the tap hole is tended.
The slag is generally cooled under the slag tap hood until a solid crust
forms. The area should be provided with vacuum and water hose connections and
floor drains to facilitate cleanup in this area and thus help minimize fugi-
tive dust from this area.
Figure 29. Slag tap hoods.
A modern rotary system -
An empty ladle is placed
upon the rotary platform
in Position 1 and rotated
to the slag tap (Position 2),
where it is filled with slag.
It is then moved under the
cooling hood (Position 3),
and finally when cooled, back
to Position 1 where it is
removed and taken to the
yard.
If the slag is stored or disposed of in an open storage area or slag
pile, fugitive emissions may occur. Covering or wetting of the pile can be
beneficial, but water disposal problems must be considered. Runoff from the
slag pile should be neutralized before it is reused or discharged (Figure 30)
Reverberatory Furnaces—
Procedural controls for reverberatory furnaces are similar to those for
blast fumacps. There are, however, a few differences.
44
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Figure 30. Slag piles and tapping vessels.
Reverberatory gases may be cooled in hairpin type coolers or by dilution
with process or ventilation gases prior to particulate collection. Some
particulate matter collects in hairpin type and must be periodically removed.
Unless this collected material is conveyed automatically in an enclosed system,
it can create a fugitive emission problem. If dilution cooling is used, the
particulate handling and treatment problem is confined to the fabric filter
area only.
Although exhaust ventilation and partial enclosure may be used to control
reverberatory emissions, the ventilation system may become overloaded when wet
charges are added. The high temperatures within reverberatory furnaces imme-
diately vaporize any water that is introduced with the scrap. The consequent
rapid gas flow can cause temporary overloading of the gas handling system. To
prevent this occurrence wet charges may be pre-dried by use of a calciner
prior to being fed to the furnace.
Reverberatory furnace slag is usually recharged to the furnaces. Storage
safeguards as used for blast furnace slag should be observed. Slag from
reverberatory furnaces should also be treated in the same manner as blast
furnace slag.
Kettle Furnaces—
Particulate emissions are released from kettle furnaces during charging,
fluxing, mixing, dressing, and skimming. The following operating procedures
can minimize emissions during these operations:
Start with kettle at 343°C (650°F), mixer raised and out of way, and all
charging doors open.
Charge large metal scrap or sows (castings from smelting furnace) to the
rear center of the pot.
45
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Position mixer over mixer door in raised position and close mixer door.
Add bulk of flux material and complete charging to the front of the pot.
Close charge door and increase pot heating rate.
Lower mixer into molten pot and mix.
Add more fluxes or additives through a chute or feed pipe (with doors
closed).
Stop mixer, open dressing hood doors, and skim off dross material.
Because large doors need not be opened for mixer insertion and fluxing while
the pot is hot, major improvements in hood performance can be realized by
keeping the doors closed.
Kettles cannot be provided with close fitting exhaust hoods because of
the need to open the hoods. Major input materials include lead ingots, lead
sows, and molten lead. Ingots and sows are typically loaded into kettles by
chain hoists, mobile hoists, or other front-end loading devices. Kettle
temperatures should be kept below 343°C (650°F) until charging is complete to
minimize fumes during loading. If lead is pumped into the kettle, hood doors
should be kept closed while the molten lead is being charged into the kettle.
Drums, containing collected dross, that do not seal properly should be
removed from service. Dross drums should not be reopened until the dross is
to be recycled to the smelting furnace. No open' drums of dross should be
allowed outside an enclosed or exhaust-ventilated area. Provisions may be
present to ventilate the dross containers as they are loaded and thus minimize
fugitive emissions. The ventilation system, if present, may be an extension
of the kettle system or a separate system.
Flue and Waste Dust Handling—
Many of the precautions and operating procedures to minimize fugitive
dusts from flue and waste dust handling activities were discussed earlier
under raw material handling. This section will discuss the operational con-
trols available from the time the dust is collected in control equipment until
it is transferred to storage or directly recycled back to the furnace.
The dust collected in dropout chambers, spark arresters, and fabric
filter hoppers should ideally be recycled by enclosed conveyors directly to an
agglomeration or slag furnace. If this facility does not exist and the dust
Is not recycled directly back to a furnace, provisions for temporary dust
storage by means of drums or hoppers should be available. Drums or hoppers
should be available in the event that maintenance or malfunction of the
furnace or screw conveyor precludes immediate recycle (Figures 31a and 31b).
When flue dust is melted in an agglomerating or slag furnace, it is
collected in a pot or thimble, and allowed to solidify. The solidified flue
dust when dumped can be broken into lumps suitable for recharging to the blast
or reverberatory furnace. Schwitzgebel (1981) reports that the agglomerating
46
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a. Improper collection without dust drum or hopper.
b. Proper collection in dust hopper.
Figure 31. Flue dust- collection.
47
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furnace reduces the flue dust volume by 80 percent. When subjected to abrasion
or severe weathering, the agglomerated dust can become powdery and be reen-
trained. During long-term storage, fugitive emissions can be minimized by
covering, drumming, or wetting the material. Wetting requires water runoff
collection and discharge to a sump for neutralization.
Flue dust conveyors should be enclosed, and conveyor transition points,
discharge points, and storage points should be equipped with emission con-
trols, such as capture hoods ducted to a collection unit. Collection hoppers,
ladles, and thimbles should be covered. Where the flue dust is dumped from a
container, ladle, or thimble for storage, an emission collection system should
be used at the point of discharge arid storage. Waste dust transported to a
pond or storage area should be wetted prior to transport for fugitive emission
control. A truck carrying flue or waste dust for off-site disposal should be
enclosed, or the material should be encapsulated or pelletized prior to move-
ment. The outside of the truck should also be washed down prior to leaving
the plant.
Scrubber discharges should be neutralized and directed to a pond system.
Solids can be prepared as a sludge or pug mill product and recharged to the
furnaces. The clarified water can be reused for scrubbing. All ponds should
be lined or made impervious to leaching.
If the collected material is discarded, the disposal area must be paved
or lined to prevent leaching. Runoff from the disposal area must be collected
and neutralized. If dust is stored, it must be covered or wetted to prevent
reentrainment. If pond disposal is used, the pond must be lined, and its pH
must be controlled. Landfill materials (e.g., battery cases) must be washed
before disposal; all others (e.g., sludges and dust) should be properly
neutralized or fixed and the soil should be prepared to avoid leaching.
Bags removed from the baghouse can be charged to a furnace for disposal
and recovery of entrained metal. If stored, they should be placed in closed
drums or containers.
3.2.4 Miscellaneous Controls
Job Descriptions—
Each smelter worker must be acquainted with the hazards of working with
lead, the danger of personal exposure, and the need to control process and
fugitive emissions. Each worker should be made familiar with pollution con-
trols for each process operation and should be instructed in the proper use
and maintenance of pollution control equipment. Additionally, each work area
should be cleaned during each shift.
Personnel Monitoring—
The Occupational Safety and Health Administration requires that tests be
conducted at regular intervals to determine lead levels of employee's blood;
and employees with high lead levels must be moved to clean jobs until those
lead levels improve. Thus, management has a strong incentive to make the
workers use the control devices and protective equipment that are provided.
Some plants use a bonus system to give workers an additional incentive to
follow nrocpdures that keep their blood levels acceptable.
48
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Traffic Controls--
Control of fugitive emissions caused by vehicular traffic is very im-
portant. Provisions must be made to keep traffic areas continuously wetted.
Personal vehicles should not be allowed in work areas. Truck and rail-
road car washing should be used to prevent tracking of lead beyond the smelter
boundaries. Provisions for collection, treatment, and recycle of the wash
water should also be required.
Inventory Management—
Because raw materials may be received at irregular intervals, sufficient
storage area should be provided so that the scrap receiving and storage does
not interfere with normal smelter operations. Finely divided materials are
also stored and can create severe fugitive emission problems unless properly
stored and handled. Provisions should be made for storage of lead bearing
materials so as to prevent wind erosion of the scrap piles. Wetting, enclo-
sure, exhaust ventilation, or some combination of control measures should be
considered for raw material storage piles. Good housekeeping should be fol-
lowed and control measures should be well maintained and strictly followed.
49
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REFERENCES FOR SECTION 3
1. Burgess, R. P., Jr., and D. H. Sargent. 1976. Technical and Microeco-
nomic Analysis of Arsenic and Its Compounds. U.S. Environmental Protec-
tion Agency, Washington, D.C. EPA-560/6-76-016.
2. Burton, D. J., et al. 1980. Control Technology Assessment. - the Secondary
Nonferrous Smelting Industry. National Institute for Occupational Safety
and Health Technical Report No. DHHS 80-143. Radian Corporation, Austin,
Texas.
3. Coleman, R. T., and R. Vandervort. 1979. Demonstration of the Bergsoe
Agglomeration Furnace and Best Management Practices at a Secondary Lead
Smelter. Report DCN 79-201-010-02. Radian Corporation, Austin, Texas.
4. Coleman, R. T., and R. Vandervort. 1980. Evaluation of Paul Bergsoe &
Son Secondary Lead Smelter. EPA-600/2-80-022.
5. McDonnel, D. B., and C. Hilborn. 1978. Alkali Fusion of Glass Fiber
Filters; Analysis of Secondary Lead Emissions Particulate. Journal of
the Air Pollution Control Association, 28(9):933-934.
6. Mezey, E. J. 1979. Characterization of Priority Pollutants From a
Secondary Lead and Battery Manufacturing Facility. EPA 600/2-79-039.
7. Page, C. 1976. Special Report - Source Characterization and Demon-
stration Opportunities for the Secondary Lead Smelters. Prepared for
the U.S. Environmental Protection Agency under Contract No. 68-02-1319
(Task 49).
8. Schwitzgebel, K. 1981. Flue Dust Agglomeration in the Secondary
Lead Industry, Journal of Metals, 33(1):38-41.
9. U.S. Environmental Protection Agency. 1980. Secondary Lead Processing.
Compilation of Air Pollution Emission Factors. 2nd ed. AP-42.
50
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SECTION 4
OPERATION AND MAINTENANCE
Compliance with air pollution control requirements over the long term
depends on both appropriate design and construction as well as operation and
maintenance procedures for emission control devices. Proper instrumentation
and recordkeeping procedures are also critical to the success of a plant's
operation and maintenance program which in turn results in fewer emissions
from a secondary lead plant. This section discusses both the design consider-
ations for emission control and ancillary equipment and operation and mainte-
nance procedures required to provide reliable equipment operation and pollutant
collection. Fugitive capture systems, fabric filters, and scrubbers, and an-
cillary equipment such as afterburners, fans, pumps, and ducting are addressed.
4.1 FUGITIVE CAPTURE AND VENTILATION SYSTEMS
Ventilation systems serve two important functions: removal of pollutants
from the workplace to protect workers from overexposure, and removal of the
pollutants from the vented gas stream to provide environmental protection.
Such systems are composed of two major components - the capture hood and an
air pollution control system, usually a fabric filter called a sanitary bag-
house.
The primary objective of the hooding/ventilation system is to catch all
lead emitted by a given process and minimize lead concentrations in the work-
place environment. This aspect is critical because any material that escapes
capture cannot pass through the control equipment to be collected. Such
material will thus escape into the atmosphere or settle nearby to create a
potential reentrainment problem. In a lead smelter this problem is particu-
larly severe because lead is a highly toxic material. To be effective, it
must have a physical configuration that does not interfere with the operation
and a sufficient flow of directed air to capture emissions under all operating
conditions. Hood design principals are well known for most applications, and
only a few general principals are discussed here. For a given situation, the
size of the hood is influenced by the proximity of the hood to the source,
i.e., close placement requires a much smaller hood than distant placement.
As would be expected, a greater gas handling capacity is needed for a large
(distant) hood in order to achieve the velocities needed to move the polluted
air into the hood. Because larger hoods require the handling of larger gas
volumes, they increase the cost of the pollution control equipment and oper-
ating costs. Hood size and placement are also influenced by the need for
equipment access. Hoods may interfere with operators requiring access by
overhead cranes or other materials handling equipment. Access can be re-
stricted by close fitting hoods or larger, distant hoods which reduce overhead
51
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access. Movable hoods are used to solve access problems. These must be de-
signed to minimize warpage, heavy surface wear, and other factors to maintain
easy movement so they will remain functional and be used conscientiously by
the operators.
Operators and management tend to neglect operations that are auxiliary to
the smelting process, such as the ventilation systems. Management must con-
scientiously pursue an aggressive program to ensure that ventilation systems
are functional and safe work procedures are followed if the workplace and the
environment are to be protected from fugitive emission pollution sources.
A properly designed hood is of such a shape and size that it encloses
the process to the maximum extent practical and creates sufficiently high air
velocities at exposed points to cause the emissions from a specific process to
enter the exhaust system. Capture velocity is the air velocity necessary in
front of a hood to overcome opposing air currents and to capture the contami-
nated air by forcing it to flow into the exhaust hood. Capture velocities for
various conditions are given below (ACGIH 1980).
Condition
Capture velocity,
cm/s (ft/min)
Contaminant released at low velocity into
quiet air
Contaminant released at low velocity into
air at low velocity
Contaminant released into rapidly moving air
Contaminant released at high velocity into
rapidly moving air
25-50 (50-100)
50-100 (100-200)
100-254 (200-500)
254-1016 (500-2000)
Capture velocities on the order of 102 to 153 cm/s (200 to 300 ft/min) should
be adequate for most lead smelting operations that require ventilation control.
4.2 FABRIC FILTERS
In fabric filtration, an assembly of tubular shaped fabric bags is housed
in a steel fabricated structure called a baghouse. Because of this relation-
ship, the terms "fabric filter" and "baghouse" are generally considered
synonymous and used interchangeably. The dust laden gas is passed through the
bags to filter out the particulate; as the particulate collects on the bags,
the pressure drop across the baghouse increases. The bags are periodically
cleaned to prevent excessive pressure drop buildup. The cleaning cycle is
controlled by a timing mechanism. Fabric filters are usually characterized
according to the cleaning method used. There are three basic types of fabric
filter used in the secondary lead smelters. These are the shaker, reverse
flow, and pulse (or reverse) jet types.
52
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At secondary lead smelters, fabric filters are generally the control
devices of choice for most operations. Shaker and reverse air fabric filters
are used to control process emissions from reverberatory and blast furnaces.
Pulse jet fabric filters are used to control emissions in fugitive capture and
ventilation exhausts, smelting furnaces, and refining furnaces. They are also
normally used as the product recovery equipment in the production of lead
oxides.
4.2.1 Fabric Filter Descriptions
Shaker—
In shaker-type units, the filter bags are hung from a structural frame-
work. The structure is supported so that it will oscillate freely when driven
by an electric motor. At set intervals, a damper is used to isolate a bag-
house compartment so that no gas flows through it. The bags are then shaken
for a preset period of time. The collected dust is dislodged from the bags
(i.e., cleaning) and falls into a hopper from which it is subsequently removed.
A conventional shaker-type fabric filter is shown in Figure 32. As shown,
particulate-laden gas enters below the tube sheet and passes from the inside
bag surface to the outside surface. Particles are captured on a cake of dust
that gradually builds up as filtration continues. This dust, cake is removed
SHAKER
MOTOR
-HANGERS
CLEAN AIR
SIDE
OUTLET
PIPE
FILTER
BAGS
BAFFLE
PLATE
TUBE
SHEET
HOPPER
Figure 32. Typical simple baghouse with mechanical shaking.
53
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periodically by the mechanical shaking of the filter fabric. Shaking is
accomplished by the rapid horizontal motion of the filter bag induced by a
mechanical shaker bar attached at the top of the bag. The shaking creates a
standing wave in the bag and causes flexing of the fabric. The flexing causes
the dust cake to crack, and portions are released from the fabric surface. A
portion of the dust cake remains on the bag surface and in the interstices of
the fabric. The cleaning intensity is controlled by bag tension and by the
amplitude, frequency, and duration of shaking. The residual dust cake gives a
minimum resistance to gas flow, causing a static pressure drop that is higher
than that of a new clean fabric. Woven fabric is used in shaker type collec-
tors. Because of the low cleaning intensity achievable in this type of clean-
ing design, the gas flow is stopped before cleaning to eliminate particle
reentrainment and allow dust cake release. The cleaning may be done by bag,
row, section, or compartment.
Normally, shaker-type fabric filters are limited to low superficial
velocities of less than 3 ft/min. This means that the total gas flow rate (at
operating temperature and pressure) divided by the total cloth area available
should not exceed the stated "velocity". This parameter is usually referred
to as the air-to-cloth (A/C) ratio and is expressed in ft3/ft2-min. High
values may lead to excessive particulate penetration or blinding, which result
in reduced fabric life. Typical A/C ratios for shaker-type fabric filters
range from 1.0 to 2.5 ft3/ft2-min.
Mechanical shaker-type units differ with regard to the shaker assembly
design, bag length and arrangement, and the type of fabric. This design is
applicable to both very small and large control systems.
Reverse-Flow or Reverse Air—
Reverse-flow baghouses are equipped with a secondary fan that forces air
through the bags in an isolated compartment in the direction opposite to that
of filtration. The reverse flow can be supplied by cleaned exhaust gases or
by a secondary high-pressure fan supplying ambient air. This action collapses
the bag and breaks the dust layer. When the filter bags are reinflated by
being brought back on line, the broken dust layer is dislodged from the bag
and falls into the hopper. If the main process fan is located downstream of
the baghouse, the reduced pressure in the structure may eliminate the need for
an auxiliary fan. Sometimes shaking and reverse-flow cleaning mechanisms are
combined in the same baghouse unit.
The dust cake can be collected either on the inside or outside of the
bag. In most reverse air fabric filters, an entire compartment is temporarily
isolated for cleaning, as described above. Numerous other approaches are used
commercially.
In filters with internal cake collection, cleaning is accomplished during
off-line operation with individual compartments isolated. The filter bag may
require anti-collapse rings to prevent complete collapse of the bag and dust
bridging. Cake release may be increased by rapid reinflation of the bag,
creating a snap in the surface followed by a short period of reverse air flow.
54
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V
V
n
^*>
3
EXHAUST
REPRESSURING
VALVE
CLEANED
VENT GAS
TO FILTER
INLET
SIDE VIEW
FILTERING
SIDE VIEW
COLLAPSING
T7 INLET
VALVE
SIDE VIEW
CLEANING
Figure 33. Reverse air flexing to clean dust collector bags by repressuring
(with collection inside the bag).
Fabrics in reverse-air collectors may be woven or felt. The felts are
normally restricted to external surface collection by means of high-pressure
reverse air.
Reverse air filters are usually limited to A/C ratios of from 1.0 to 2.5
ft3/ft2-min, but the ratio may be higher depending on application. Although
suitable for a wide range of applications, these designs are not usually
competitive for use with very small gas flows, i.e., less than 5000 actual
cubic feet per minute (acfm).
Pulse Jet-
Pulse jet, fabric filters (also called reverse-pulse) use a short pulse
of compressed air directed from the top to the bottom of each bag to clean the
bags. This burst, usually less than one-tenth of a second, draws in other air
as it passes through a nozzle or venturi. The resulting combined air mass
expands the bag and loosens and removes the collected dust. In pulse jet
units, the filtering is always done on the exterior bag surface.
The bags, supported by inner retainers (called cages), are suspended from
an upper cell plate. Compressed air is supplied through a manifold-solenoid
assembly (Figure 34) into the blow pipes shown in an end view. Venturis
mounted in the bag entry area are intended to improve the shock effect. The
baffle plate shown at the gas inlet is intended to prevent particles from
abrading the bag.
55
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SOLENOID
VALVES
TO
EXHAUSTER
DUST
LADEN
AIR
COMPRESSED AIR
BLOW PIPE
INDUCED FLOW
CLEANED
EXHAUST
FILTER CLOTH
BAG RETAINER
ASPERATED
AIR
/BLOW
.*: PIPE
o
^4P
PULSE
- AIR
JET
- LOOSENED
DUST CAKE
NORMAL
OPERATION
PULSE JET
CLEANING
FILTER
CAKE '
ANT I
COLLAPSE
RING
MATERIAL
DISCHARGE
Figure 34. Typical reverse-pulse baghouse during cleaning.
By using pulse jet cleaning, the dust deposit is removed with only a
brief interruption of the filtering flow. The fabric suffers a minimum of
flexural wear and the filter installation is smaller because the fabric is in
use practically all the time. Most pulse equipment utilizes felt rather than
woven cloth. With felt, the filtration velocity can be 3 to 4 times that used
in shake or reverse flow equipment, so the size of the pulse jet unit is
smaller.
The components of a pulse cleaning filter include an air compessor, a
storage or surge tank, piping, solenoids and nozzles, and some models use
Venturis and fabric support gridwork as well. Few, if any, ducting dampers
and their associated controls are needed. Because no moving parts are re-
quired, the pulse method has an advantage in terms of maintenance. Many units
also have top access to the bags so that the housing does not have to be
entered to replace the bags.
Just as in shake cleaning, in which all the cleaning energy must be
applied at one end of the bag, so in pulse cleaning the effectiveness of
56
.
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cleaning decreases with length of bag. For this reason short bags are used In
pulse equipment, rarely if ever over three meters (ten feet) in length. Since
there is little fabric motion, the bags or tubes can be packed slightly
closer without interchaffing.
Like other types of equipment, pulsed cleaning has its disadvantages. It
is limited in temperature to around 450°F at present, because felt materials
are not available for higher temperature. High pulse pressure can damage the
fabric by over-stretching it. Felts tend to plug in depth rather than blind.
They may have to be cleaned rather than discarded because of their greater
value. Felt, compressed air, power, and the compressor are all relatively
expensive so that economic balances are different for pulse equipment. The
electric power used to run the compressor can equal that for the primary fan.
As a result, equipment sizes and shapes are different. Pulsed equipment may
be best for some applications and simply not economical for others.
4.2.2 Design Considerations
Prior to design of any fabric filter, the following parameters must be
determined: maximum gas volume, maximum gas temperature, temperature profile
of cyclic operations, dust loading, particle size, and gas composition. These
parameters dictate what type of fabric may be used and whether special corro-
sion-resistant material or insulation is needed. The buyer has considerable
freedom in selecting the type of fabric filter (i.e., reverse-air, shaker, or
pulse-jet cleaning) and the degree of accessibility and compartmentation of
the fabric filter. Previous experience, however, has shown that some appli-
cations are more successful than others because of both initial design and
maintenance considerations.
Generally, air-to-cloth (A/C) ratios are dictated by a combination of the
application and cleaning mechanism used. Both shaker and reverse-air fabric
filters have A/C ratios of 1.0 to 2.5; and ratios of 1.5 to 2.25 are most
prevalent. The shaker fabric filter can have slightly higher A/C ratios
because of the greater amount of energy available for bag cleaning when the
bags are properly tensioned. Pulse-jet baghouses used to recover lead oxide
or to control fugitive emissions from the furnace tapping areas and refining
kettles typically have A/C ratios-of 4 to 6.5. Pressure drops of 746 to 995
Pa (3 to 4 in. H20) are typical for these applications. Air to cloth ratios
of fabric filters for material handling and transfer points typically range
from 6.5 to 10 [with a pressure drop of 995 to 1493 Pa (4 to 6 in. h^O)]. An
increase in A/C ratio decreases capital cost because the number of bags and
size of the fabric filter decreases. As the A/C ratio increases, however, the
pressure drop across the unit and the energy required to clean the fabric
increase.
During the selection and design of a fabric filter, it is simple to in-
corporate design factors that improve accessibility and thus enhance the
inspection and maintenance capabilities of plant personnel. Such factors
generally increase initial costs, but can reduce maintenance time and costs.
Easier, faster maintenance also reduces fugitive emissions from these opera-
tions. Large, easily opened doors enhance accessibility. They allow easy
57
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entry by personnel wearing self-contained breathing apparatus and carrying
tools. Larger walkways may also be required to minimize damage to the bags
along the walkway caused by maintenance personnel during bag installation. In
shaker and reverse-air fabric filters, access to the shaker mechanism and bag
tensioning hangers enhances inspection and maintenance capabilities. In
pulse-jet fabric filters, bag bleeding and pinhole leaks can easily be seen on
the clean side of the fabric filter. Access for inspection (typically top
access) must be provided. Both top access and top loading bags are recommended
to reduce maintenance time for a pulse-jet fabric filter.
Selecting the appropriate fabric for the bags is very critical. The fab-
ric types used in secondary lead smelter applications include acrylic/wool
blends, wool, OrlonR, cotton, polyester, fiberglass, and polyester/fiberglass
blends. Fibers used in air pollution control applications must be strongly
resistant to chemical attack (acid in secondary lead applications) and moder-
ately resistant to abrasion. The fibers have a maximum service temperature
which is related to their chemical composition and strength-temperature rela-
tionship. Synthetic fibers lose strength at elevated temperatures and become
plastic. Natural fibers (wool, cotton) char at high temperatures and become
bri ttle.
Fabrics may be of woven or felted construction. Woven fabrics are
typically used in reverse air on shaker applications where air-to-cloth ratios
are low. Felted fabrics are typically used in pulse jet or reverse air (ex-
ternal filtering) where air-to-cloth ratios and cleaning energies are higher.
The porosity of the fabric is influenced by the weave structure, tightness,
and yarn weight. In the United States, the final weight of the fabric is
expressed in ounces/square yard. Typical weights are 8 to 16 oz/yd^, and the
porosity (expressed as clean fabric permeability).is typically between 13 and
55 acfm/ft2.
Fabrics may have surface finishes or constructions that reduce dust
penetration (napped or felted surface on woven fabrics) or improve cake re-
lease (by singeing). Fabric finish is extremely important for inside bag
filtering where nodules develop, restricting filtering (increase pressure
drop) and interfering with cake release. Singeing the interior surface to
remove fibers not tightly bound in the weave reduces the formation of nodules.
Another important design consideration is "bag reach" (i.e., the number
of bags in any row from the access walkway to the end of the row). A typical
reach is five or six bags. Such a reach requires the removal of several bags
to replace a bag far from the point of access and increases the chance of
damage to surrounding bags. A reach of three or four bags represents a bal-
ance between increased baghouse shell size, and reduced maintenance cost.
Some pulse-jet fabric filters impose another maintenance hardship by
providing only "dirty side" access without a walkway. In some units nearly
all the bags must be removed to reach the farthest bag from the access hatch-
way. In this situation, changing all the bags is simpler and less costly than
searching for and replacing worn or defective bags. Dirty side access to
58
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pulse-jets fabric filters is important, however, because it can reveal air
inleakage at the hatch, cake release problems, and moisture problems in the
cleaning system.
Pulse-jet fabric filters commonly encounter cake release and bag blinding
problems because of dirty compressed air for the cleaning pulse. An in-line
air dryer or trap to remove water and oil from the supply line is recommended.
The surge tank placement and the location of the pulse-air offtakes also
affect the amount of water and oil blown into the bags. Placing the pulse-air
offtakes above the surge tank allows water and oil to collect in the tank
rather than pass into the baghouse and the filter media. A purge valve lo-
cated at the bottom of the tank allows periodic bleeding of the collected
water and oil.
Control of gas temperature requires attention in the design of fabric
filters, particularly units serving "hot" sources. Cooling loops and dilution
air usually keep gas temperature below the maximum that the bag fabric can
handle. The thermal swing or temperature cycle encountered during normal
operation is particularly important. Although many blast furnaces are equipped
with afterburners without temperature controls, the gas temperature tends to
decrease after each charge. The use of smaller charges at more frequent
intervals will minimize gas temperature fluctuations. Duct systems are gener-
ally designed with dilution air inlets and cooling loops to decrease the gas
temperature. If the gas is constantly cycling above and below the acid dew-
point, condensation and corrosion may occur. In this case, particulate
matter becomes sticky and difficult to remove from the bags. Dust cake build-
up causes a bag to stretch because of increased weight. Finally, the bag can
tear loose from its hanger and could damage other bags. The stretching of the
fabric tends to decrease cleaning efficiency, increase "bag bridging" (buildup
of collected dust inside the bag), and cause excessive wear at the bag cuff.
In shaker fabric filters, increasing shaker intensity to reduce dust cake
buildup increases wear of both the fabric and the shaker mechanism. Feedback
temperature controls on the afterburner and dilution air damper can maintain
the temperature between set limits. The most successful facilities use Teflon-
coated fiberglass bags in a temperature range of 138° to 163°C (280° to 325°F).
Lower operating temperatures (to a minimum of about 95°C (200°F)) are possible
with Dacron bags and with acrylic and wool bags. The lower temperature limit
is determined by the acid dewpoint (condensation temperature). One method to
control the temperature of the gas coming, from the afterburner is to permit
outside air to be drawn into the system (i.e., the system operates under
negative draft) before the baghouse with the air leakage rate controlled by a
damper and a temperature sensitive controller located at the baghouse inlet.
Insulating the fabric filter helps maintain the temperature of the gas
while it is being cleaned. Temperatures above the acid dewpoint minimize cor-
rosion of hanger components, doors, and walls. Internal shell components can
be lined with appropriate corrosion-resistant materials if necessary. Insula-
tion should be applied to the baghouse shell, hoppers, and doors. Structural
steel may be placed on the internal portion of the shell. The insulation may
then be applied evenly across the shell to reduce "cold spots," which promote
corrosion of the shell. In addition to reducing corrosion and improving cake
release, insulation can reduce material handling problems in the hoppers.
59
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Correct hopper slope angle, uniform insulation, sequential operating
vibrators, and continuous dust removal can minimize hopper bridging. Access
hatches (insulated) and anvils for vibrators or for manual rapping should be
provided for dust removal in the event of hopper bridging. Continuous opera-
tion of the dust removal system may be assured by electrically interlocking
rotary airlocks and screw conveyors to the exhaust fan.
After removal from the fabric filter, dust must be properly handled and
disposed of. One option is to feed the dust to an agglomeration furnace,
where it is melted for recycle to the smelting furnace. When dust is fed
directly to the smelting furnace, much of the dust will be simply reentrained
in the furnace off-gas and returned directly to the fabric filter.
In summary, selection and design of a fabric filter depend upon the spe-
cific process variables, the desired accessibility, and the available cleaning
energy. If the unit is well designed and all other factors are held constant,
costs resulting from increased accessibility and lower A/C ratios are generally
offset by lower energy and maintenance costs.
4.2.3 Operation and Maintenance Considerations
Theoretically, fabric filters can achieve mass collection efficiencies in
excess of 99.5 percent when particles are as small as 0.1 ym. In practice,
many process conditions and installation problems can reduce both the collec-
tion effeciency and the time available for service. Fabric filters require
extensive preventive maintenance and inspection to reduce periods of excess
emissions.
This section discusses malfunctions of fabric filters used to control
process and fugitive emissions from blast and reverberatory furnaces. It also
suggests means of avoiding or correcting malfunctions. Such means include
proper instruction and training in system maintenance, establishment of a
preventive maintenance program, and use of instruments and records to diagnose
and correct system deficiencies. The goals of the program are to ensure
continuous compliance with standards, to extend system life, and to reduce
operation and maintenance costs.
The discussion assumes that all normal mechanical, electrical, and lubri-
cation procedures involved in plant operation have been conducted in accord-
ance with good maintenance practices'and as defined by equipment manufacturers'
operating instructions or O&M manuals.
Factors Affecting Fabric Life--
The fabric types used in fabric filters in the secondary lead industry are
acrylic, acrylic/wool, wool, Orion, cotton, polyester, fiberglass, and poly-
ester/fiberglass. Failure of the filter to maintain high efficiency is usu-
ally caused by fabric failure resulting from thermal degradation, chemical
attack, or mechanical injury including abrasion. The reasons for failures
may not be readily apparent and may result from subtle changes in the equip-
ment caused by wear and deterioration. These factors are interrelated.
60
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Dust is removed from the gas stream by passing the gas through a porous
fabric upon which the dust deposits and builds a dust cake layer. The effi-
ciency of dust collection depends on the integrity of the fabric structure
supporting the dust cake. Any deterioration of the fabric structure that
allows localized failure increases the penetration of dust through the system.
Each fabric has a continuous and maximum operating temperature suited to
its properties. The temperature specified for continuous use is based on the
rate of thermal degradation (polymer chain cleavage) that occurs at a given
temperature. This temperature also influences the rate of abrasion and effect
of chemicals on the fabric. For example, deterioration of the finish on
fiberglass bags increases as the gas temperature increases. As the finish
decomposes, the effective life of the fiberglass is reduced because of abra-
sion and/or chemical effects.
Temperature excursions—The most common cause of polymer chain breakage
is exposure to high temperatures. Exposure to temperatures at or near the
recommended continuous levels results in random chain breakage with reduced
tensile strength over the life of the fabric. Typical life of acrylic bags at
104°C (220°F) is between 6 and 18 months in service on a blast furnace. The
life may be greatly reduced if the fabric filter is simultaneously exposed to
acids and moisture.
Exposure to temperatures above the recommended continuous operating
temperatures for a few minutes may not result in immediate failure, but will
reduce the overall life of the fabric. The effects of repeated temperature
excursions on tensile strength are cumulative.
The exposure of the fabric to temperatures above the maximum exposure
temperature can result in immediate failure because of the complete loss of
strength and permanent elongation (melting). Poor operation of the furnace
and/or afterburner can result in burning the fabric by sparks and carbon
buildup and oil on the bag surface.
Prevention of excessive fabric failure and periodic excess emissions
requires installation of monitors to determine the temperature cycles of the
system. It is an unfortunate misconception that short temperature excursions
do not cause permanent damage. It is necessary to inspect temperature charts
to determine the potential for short-term failure caused by the excursions.
Also a high-temperature alarm with an automatic method for bag protection
(e.g., quenching, dilution, or bypass) should be provided. Figure 35 shows a
typical temperature chart illustrating temperature excursions up to 150°C
(300°F).
Chemical attack—Polyester is generally rated as resistant to alkali
attack, but at temperatures above 93°C (200°F) in the presence of moisture,
the polymer degrades rapidly. Cotton and Nomex are particularly susceptible
to sulfuric acid attack below the acid dewpoint. The tensile strength of the
fiber is reduced as the polymer chains are broken.
61
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Figure 35. Typical temperature excursion, °F.
In general, the fiber begins to lose tensile strength immediately upon
exposure to the gas stream. The life of the fiber depends on proper fiber
choice for application to acid gases such as SO?, hydrogen chloride (HC1), and
hydrogen fluoride (HF). The fabric filter should be operated at the lowest
temperature consistent with avoiding moisture or acid condensation.
At sources that frequently shut down, bags can quickly be destroyed
because of temperature excursions through the acid and moisture dewpoints.
Shutdown should be accomplished by exhausting flue gases from the filter with
dilution air (ambient) before cooling gases below the dewpoint. The purging
removes the S02 and water vapor before condensation can occur on bag surfaces.
Mechanical injury/abrasion—Mechanical injury is perhaps the most diffi-
cult type of failure to prevent as its principal causes are equipment deteri-
oration and operator error. It may occur abruptly as from an inadvertent
screwdriver puncture, or it may result over a longer period of time from ac-
cellerated wear caused by abrasion or excessive mechanical stressing.
The failure of the fabric may occur over a long period of time because of
the abrasive action of dust particles on individual fibers in the structure.
The failure may result from general abrasion over a large area or specific
attacks in concentrated areas.
62
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General abrasion of the fabric is a common occurrence and is accepted as
the normal failure mode of the bag over its life. This type of failure cannot
be prevented, but the rate of abrasion can be reduced by proper installation
of bags to avoid bag-to-bag contact and by reduction of the amount of dust
being handled. A cyclone may be installed as a precleaner to remove larger
particles and reduce inlet loading.
Local intensive abrasion, which results in premature bag failure, is
undesirable and can be prevented. High abrasion rates are commonly associated
with improper bag installation or design flaws in the collector. Each case of
abrasion failure must be addressed separately to determine if corrective
action may be taken to reduce the frequency of failure.
Bag Installation—
The improper installation of bags can result in premature failure of the
bags and increased emissions. These failures can result in significant costs
and also reduce production if downtime is required to change bags. This
subsection is included to supplement manufacturers' instructions for installa-
tion of bags. The items covered are those that have been demonstrated by
field experience to result in high bag failure rates.
Shaker and reverse-air systems—Bags in shaker and reverse-air systems
should be installed from the outer walls toward the center of the compartment.
Bags should be hung according to manufacturers' recommendations by loop/hanger,
eye-bolt/J-hook, or tongue/hanger assemblies. Each bag should also be in-
spected before hanging to ensure that it has no holes, is the proper size, and
has a proper seam. Normally, the bags should be hung by row, the cuffs should
be placed over thimbles, and the ring clamps should be attached. The fit of
the bag over the thimble should be checked, and loose fitting bags should be
discarded. Small bags that fail to meet specifications should not be forced
over the thimbles.
In shaker and reverse-air fabric filters, the bag can be attached to the
tube sheet by a thimble and clamp ring design or by a snap ring design.
Figure 36 shows the two methods of attachment. Dust enters the baghouse
filter at the hopper in a horizontal direction and must turn vertically to
enter the tube sheet thimbles. Heavy particles with higher inertia do not
follow the flow and therefore do not enter the opening parallel to the thimble
walls. The particles impact on the walls of the thimble and, if the thimble
is short, on the fabric above the thimble. The action of the particles strik-
ing at an angle to the fiber surface increases abrasion. Roughly 90 percent
of bag failures occur near the thimble. The use of double-layered fabric
(cuffs) or longer thimbles reduces the failure rate.
In the snap ring system no thimble is used, and in some cases a cuff is
not used. This exposes the bag to rapid abrasion a few inches above the snap
ring. Add-on tube sheet thimbles may be used to reduce the effect.
Baffle plates or diffusers may be used to deposit large particles in the
hopper before they contact the bags. The orientation of the plates is criti-
cal, however, because deflection of incoming gas into the hopper can resuspend
collected dust and increase effective dust loading through the tube sheet.
63
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THIMBLE AND CLAMP RING DESIGN
CLAMP
POOR
BAG
SHORT
THIHBLE
,INCREASED
* ABRASION
SHORT CUFF
TUBE SHEET
GAS FLOW
I
BETTER
BAG
LONG
THIMBLE
GAS FLOW
LONG CUFF AND
REDUCED ABRASION
TUBE SHEET
POOR
BAG
CUFF
WITH
SNAP
RING
J
POOR
SNAP RING DESIGN
SHORT CUFF
NO THIMBLE
INCREASED
S ABRASION
BETTER
BAG
CUFF WITH
SNAP RING
LONG CUFF AND
,REDUCED ABRASION
•TUBE SHEET
AND
THIMBLE
Figure 36. Methods of bag attachment in shaker and
reverse-air fabric filters.
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The resuspension is reduced if the hoppers are operated with continuous dust
removal; thus, dust remains below the gas inlet.
After a bag is clamped, the tension should be adjusted to the manufac-
turer's specifications by using a spring tensioning device or tightening the
bag to a known length. In no case should the bag be allowed to hang freely
and fold over the thimble. Also, tension must be uniform in all bags to
provide uniform cleaning efficiency.
Each row should be installed in a similar manner. Installation -from the
outer walls toward the center of the compartment is required because of the
deflection of the tube sheet as tension is applied. After installation, the
tension of bags should be rechecked and adjusted as necessary. The use of
proper tension reduces bag failures at the cuff, lessens wear on thimbles, and
improves cleaning efficiency.
After bags are in service the fabric may stretch and cause slackness.
The expansion is not uniform, and the seam may not stretch as rapidly as the
woven portion of the bag. The bag can consequently take on the appearance of
a "banana," and bags may come in contact with each other, resulting in abrasive
damage. Such contact can be reduced by orienting the bags on each side of the
walkway with seams facing to the outside of the compartment. Figure 37 shows
the proper bag orientation for prevention of bag-to-bag contact during shak-
ing.
SIDE VIEW
SEAM
ORIENTED TO
OUTSIDE WALL
•SEAM
ORIENTED TO
OUTSIDE WALL
I
WALKWAY
Figure 37. Proper bag orientation for prevention of
bag-to-bag contact during shaking.
65
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In the nonthimble design, improper installation of the snap ring can
result in dust penetration between the tube sheet and the bag cuff. If there
is a question concerning the seating of a ring, the bag should be removed and
reinstalled. If an adequate fit cannot be achieved, the bag should be dis-
carded.
The proper installation of the bag requires collapse of the ring inward
with the fingers and insertion of the cuff into the tube sheet opening. The
circular portion of the ring should be placed in the seat, and the ring should
be released. Fingers should be placed inside the ring allowing the bag to
collapse into the tube sheet opening, and the ring should be pressed into
place. The bag should be tensioned as necessary (Figure 38).
SNAP RING
WOUND WITH
FIBER
TUBE SHEET
Figure 38. Proper method of installing bag in tube sheet with snap rings.
Pulse-jet systems—A pulse-jet fabric filter can be top load or bottom
access. A top load system (Figure 39) has the advantage of ease of bag re-
moval without interference with adjacent bags. Normal installation in a top
load system requires placement of a bag over a cage and placement of the cage
and bag through a tube sheet. The venturi is then placed in the cage opening
and secured by pressed fit or held down by clamps, depending on the manfactur-
er's design. Care must be taken that the bag has been correctly positioned
over the cage, that the seam is not twisted, and that the bag fits tightly
over the bottom of the cage. In systems using a receiving groove in the
venturi for the cage, the cage tongue must be secured in the venturi groove
before the retaining clamp is tightened. Failure to make a tight seal can
66
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result in bag/cage drops during pulsing and/or misalignment of the cage. Dur-
ing periods of complete system rebagging, it is advisable to use a pneumatic
screw driver with adjustable torque to tighten clamps. This provides uniform
installation, whereas hand tightening can vary because of fatigue of mainte-
nance personnel. Uniform clamp tension and alignment of cages have been shown
to reduce cage drops and excess emissions.
Figure 39. Top-load pulse-jet filter (Ecolaire Environmental Company).
Bent or misaligned cages should not be used. Cages should be inspected
for corrosion, broken wires, or sharp edges that may penetrate the bag surface.
Gaskets used between the venturi and tube sheet should be checked to
ensure that they are elastic (not hard or deteriorated) and that the surfaces
are sealed. Chemical attack can destroy gaskets or caulking. Such destruc-
tion allows dust penetration around the bag seal.
When a bag farther than the second row from the walkway must be replaced,
the intervening bags should be temporarily removed to allow safe installation
of the replacement bag. Otherwise, the intervening bags can be stretched and
damaged, the proper installation and tensioning of the replacement bag can be
difficult. Figure 40 illustrates correct and incorrect installation of bags.
To avoid snagging and puncturing bags, maintenance personnel should not carry
tools while in the compartment.
67
-------
oo
QO
DAMAGE
CORRECT INCORRECT
Figure 40. Correct and incorrect installation of bags.
When dust is collected outside the fabric (e.g., by pulse-jet fabric
filters), abrasion may occur in the lower portion of the bag because of the
direct impact of particles on the bag surface. Baffles are required to dis-
tribute the incoming particles evenly and provide a uniform velocity over the
bags. If a high failure rate is occurring in the lower bag area near the
inlet, this mechanism should be investigated as the possible cause.
It is common practice not to remove dust that accumulates on the clean
side of the tube sheet. The presence of dust is not a significant problem as
long as penetration is not occurring. Heavy dust accumulation, however,
results in rapid abrasive failure of serviceable bags. When the dust that has
been emitted from previous bag failures settles on the tube sheet and collects
around a bag, the weight collapses the bag and forms an orifice (Figure 41).
The reduction in area increases gas velocity and therefore abrasive damage to
the bag in the area of the restriction. The increased tension of the bag also
results in abrasion of the bag where it contacts the top edge of the thimble.
Prompt removal of accumulated dust from the tube sheet after a bag failure can
reduce damage to other bags.
68
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BAG
DAMAGE
OCCURS HERE
Figure 41. Abrasive damage caused by accumulation of dust on the tube sheet.
If penetration occurs, deposited material should be removed from the tube
sheet before installation or replacement of bags. This aids in location of
failed bags and prevents damage,to the remaining bags.
Bag Cleaning—
The resistance to gas flow through the filter results from two mechanisms,
fabric resistance and dust cake resistance. The fabric, when first installed,
has a resistance defined in terms of the permeability. The resistance in-
creases with gas velocity through the fabric and is a function of fabric
construction and weight. The static pressure drop across the fabric increases
as the dust cake increases on the fabric surface. At some preset point, the
cake must be removed to reduce the resistance.
The effectiveness of removal is related to cleaning energy expended and
dust cake properties. In general, sufficient residual dust penetrates the
fabric structure to provide an additional static pressure drop after cleaning
above the original clean fabric level. This incremental resistance caused by
the residual dust cake remaining on the fabric is normal and attempts should
not be made to obtain pressure drops achieved when the bags were new. At-
tempts to maintain static pressure drop at new bag levels by high cleaning
energy cause rapid bag failure and also decrease the bag's efficiency to
collect dust.
69
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Cleaning Cycle—In normal operation, the cleaning cycle is controlled by
timers and the bags are periodically cleaned on a time scheduled basis. The
cycle is timed so that the system pressure drop remains between the upper
limit and the lower level desired after cleaning. The duration and intensity
of cleaning is controlled to achieve the lower pressure drop level (i.e., the
after cleaning pressure drop) in as short a time as possible without undue
wear of the bags. These are determined by experience. After installation of
a complete set of new bags, the cleaning cycle time period should be reset
after the bags have been in service one or two days. This allows the operator
to set the timer to achieve the desired pressure drop range over the cleaning
cycle (from the completion of one cleaning cycle to the beginning of the
next). The operator or inspector (when reviewing the operating logs) should
look for any increase (above the previous operating levels) in the lower
(after the cleaning cycle) and upper (before the next cycle begins) pressure
drops across the bags. An increase in these static pressure drops indicates a
change in fabric/cake resistance. This change can result from changes in
amount of cake buildup retained, furnace charge composition, afterburner
temperature, oil deposits from plastics in the charge, or moisture from in-
leakage.
A measurement of static pressure drop should be made on a periodic basis
to determine relative changes in dust cake resistance. A gradual increase in
resistance can indicate oil deposits, fine particulate blinding of fabric, or
moisture inleakage. The increase may be tolerated if it is not severe or if
it does not decrease ventilation performance because of decreased volume of
gas exhausted.
If an increase in pressure drop occurs, attempts should be made to diag-
nose the cause (oil, moisture, carbon), and corrective action should be taken.
An increase in cleaning energy beyond manufacturers' recommendations should
not be made, because it shortens bag life.
Because the stack pressure drop is a linear function of the filtering
velocity, and thus the air-to-cloth ratio, the installation of additional bags
(i.e., an increase in baghouse capacity) can be used to reduce an unacceptably
high pressure drop where other remedies fail.
Cake release—The ability to remove collected particulates (cake) from
the fabric surface determines the cleaning frequency required for the filter
system. Factors that affect the energy necessary to remove the cake include
cake composition, porosity, and the effectiveness of the energy transfer to
the cake/fiber interface.
The most common reason for poor cake release in the secondary lead indus-
try is agglomeration in the bag due to oil, water, or carbonaceous particulates.
Oil vaporized from the charge materials or from malfunctioning afterburner
systems coats the bags and causes agglomeration of dust particles and coating
of fibers. The cake does not break effectively when flexed, and additional
energy is required to remove it from the surface. In severe cases the dust
accumulated in fiber interspaces agglomerates and results in an increased
cleaned bag resistance. As the blinding increases, the system static pressure
drop increases sharply and cannot be reduced to the usual post-cleaning levels
during the normal cleaning cycle. Because of uneven cake removal, gas veloc-
ity increases in local areas and reduces bag life through abrasion.
70
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Figure 42. Dust agglomeration in fiber interspaces.
The presence of moisture either from operation below the dewpoint or from
inleakage through the shell presents a similar problem. The cake release is
impaired, and increased energy is required to remove the cake. As the cake
adsorbs moisture, the specific particle volume increases. The swelling of the
dust deposited in the fiber interspaces reduces porosity and causes fiber
damage.
Specifically, wool fiber is hydroscopic and may adsorb two times its
weight in water. Under these conditions, the fiber increases in volume by 150
percent. The increased volume reduces the open space between fibers and
increases gas flow resistance.
Oil on bags may be detected by placing samples of the dust cake in water,
agitating the mix, and then allowing the mix to settle. Oils will separate
and appear as a sheen on the liquid surface. A small amount of oil or tar
particles may be tolerated if the dust cake remains loose and removable. If
oil interferes with cake release, a precoating material such as limestone may
be introduced into the fabric filter inlet to adsorb the oils and protect the
bags (Figure 43). The use of such systems must be carefully considered,
however, because the coat treats the symptom and does not eliminate the cause
of concern--the oil. Attempts should be made to eliminate the source of oil
or vnH'ir-n ?mO,m-t-c tr> acceptable levels.
71
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CLOTH
PRECOATING
MATERIAL
DIRT
SMOKE
TAR
Figure 43. Precoating material for protection of bags from blinding.
(collection inside bag)
If limestone precoats are used, it must be noted that bag failures which
are not corrected result in increased emissions because of the higher grain
loading entering the collector. Also, any inleakage or cooling that allows
the gas stream to pass through the dewpoint promotes hardening of the dust
cake and fabric failure. Continued operation below the dewpoint muds the
bags; when the mud dries, a hard cake is produced. The cake cannot be removed
without damage to the bags.
Proper design and operation of a furnace afterburner can eliminate the
problems caused by carbonaceous particulate and oils. Dilution air or coolers
can help insure condensation of metal fumes (particularly chlorides) prior to
their entering the bag filter.
When the process is shut down, it is advisable to continue to operate the
baghouse for one complete cycle (including cleaning). This operation purges
it with clean air to avoid condensation and ensure that bag contaminants are
removed.
Cleaning intensity—The removal of the dust cake requires the breaking of
the cake structure. The mechanism and energy (intensity) required to accom-
plish this is a function of previous items discussed.
72
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Too little energy does not break the cake, and too much energy increases
bag failure because of fiber abrasion. The proper intensity is defined as the
minimum amount necessary to remove the proper amount of cake. Each system is
unique, and identical systems at a site may not have the same cake release
properties because of source variability and/or gas stream characteristics.
Therefore, the required cleaning intensity must be matched to the system.
Because the intensity of cleaning is typically related to shaker ampli-
tude and frequency in shaker collectors, it is assumed that cleaning is uni-
form in all compartments and that all bags in each compartment are equally
clean. In practice this is not the case. The amplitude and force transmitted
through each bag are functions of the distance from the drive mechanism in the
shaker assembly, wear and clearance of linkages, bag tension, and dust cake
properties between bag locations.
The natural segregation of particles by mass because of inertia! forces
in the collector inlet can result in a cake with larger particles, higher
porosity, and better cake release properties opposite the inlet. The segre-
gation is greatly reduced if a baffle plate is used.
The proper cleaning of the bag requires the flexing of the surface to
dislodge the cake. If bag tension is low, the bag may be flexed adequately at
the top, but the standing wave dampens as it is transmitted downward. The in-
stallation of each bag must be checked to ensure proper tension. Manufactur-
ers' literature should be consulted to determine the correct tension method.
The fabric may elongate because of the weight of dust collected between clean-
ing cycles or bag tongues .may slip in hangers. Thus, tension may change with
time of service.
Assuming a reasonable dust cake depth before cleaning, the weight of dust
before cleaning may be between 27 and 55 kg/bag (60 and 120 Ib/bag). This can
increase dramatically if oil or water reduces cake release.
A method used to evaluate the cleaning effectiveness and dust distribu-
tion across the tube sheet is to grasp the bag near the bottom just after a
cleaning cycle has occurred and vigorously shake the fabric while the bag is
tightly closed. The dust cake above is released and falls into the closed
bag. The volume of dust above the restriction indicates the amount of sep-
arable dust cake remaining after cleaning (Figure 44). Several bags can be
tested in this manner to assure that all bags are properly cleaned. When
doing this test, proper precautions should be taken to avoid inhaling the dust
emitted. The variation in cleaning should be noted with regard to possible
causes. As mentioned, overtension can damage bags because of abrasion.
In reverse-air collectors, the cake is released by collapsing the bag
with reversal of gas flow. The bag is flexed, and the cake removed from the
surface by the cleaning gas. In systems with short bags [i.e., bags less than
2.5 m (8 ft) long], bags may be allowed to collapse almost completely. The
bag must be reinflated in a snap action, and a dwell time must be allowed for
the dislodged cake to flow from the bag before gas filtration commences. In
73
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THICKNESS OF WALL
IS EXAGGERATED
-BAG
THIMBLE NOT -SHOWN
Figure 44. Field method of determining the effectiveness of bag cleaning.
this case low bag tension results in complete closure of the bag near the
thimble, reduction of reverse gas flow through the bag, and consequently
reduction of cleaning efficiency (Figure 45).
SIDE VIEW
FLOW OF REVERSE AIR
TOP VIEW
AREAS WHERE
CLEANING
IS PREVENTED
BECAUSE OF
BAG CLOSING
Figure 45. Impaired cleaning in a reverse-air fabric filter.
74
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Tube sheet bridging—In shaker or reverse-air fabric filters, dust cake
is collected on the interior surface of the bags. The removal of the collected
dust cake requires the free fall of the dust through the thimble and into the
dust hopper. When systems are uninsulated or when the gas temperature is near
the dewpoint, cake accumulates on the underside of the tube sheet and on the
inside of the thimbles. Heat is transferred from the tube sheet to the unin-
sulated baghouse shell. The colder metal reduces dust temperature and causes
agglomeration and deposition on the surface.
As caking increases, the ability of the dust to discharge through the
thimble is reduced. Eventually, complete bridging of the bag results. In
severe cases, the accumulation may extend several feet into the bag. The
bridging occurs most commonly near the baghouse shell (outside rows) or near
doors where air inleakage from deteriorated gaskets occurs (Figures 46, 47).
The bridge may normally be dislodged by flexing the bag with the hand near the
top of the thimble. The dust above the bridge, because it is exposed to the
gas stream, is free flowing and discharges after the cooler cake is broken
near the tube sheet.
Figure 46. Bridging near baghouse shell caused by cooling a
poorly insulated fabric filter.
75
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Figure 47. Faulty gasket on baghouse access door.
The breaking of the cake only returns the bag to service for a short
period; bridging soon recurs. Continued operation in this condition decreases
net cloth area and increases pressure drop. The higher A/C ratio increases
bag abrasion and decreases bag life. The solution to the problem is to reduce
heat loss through the tube sheet/shell by installing insulation or increasing
the system temperature (e.g., increasing afterburner temperature).
Hopper bridging—"Bridging" is a term applied to the blocking of dust
discharge through an opening by the agglomeration of the dust. Bridging
commonly occurs a short distance above the apex of fabric filter hoppers and
results in partial or complete closure of the discharge.
Common causes of the agglomeration are moisture, oils, and temperature
drop. In fabric filters that operate below or near the dewpoint, the added
drop in temperature in the hopper as a result of radiative cooling initiates
agglomeration of the dust. Moisture enhances agglomeration of the dust, and
cake gradually builds up. The area available for dust discharge is reduced
and complete bridging eventually occurs. Agglomeration can be initiated by a
drop in dust temperature resulting from air inleakage through flanges, gas-
kets, doors, or weld failures in the hopper.
Continuous or repeated occurrences of hopper bridging indicate a chronic
temperature or moisture control problem in the ventilation and control equip-
ment system. Careful inspection of hoppers should be made to determine gas
inleakage points, and repairs should be made. If an adequate temperature
cannot be maintained, installation of insulation or an increase in afterburner
temperature may be required. Bridging is not a common problem in tight
systems that are insulated and that operate at proper temperatures.
76
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Detection of Dust Penetration--
To maintain continuous compliance with emission limits, the operator must
identify and correct deficiencies in the system in an effective and timely
manner.
Depending on the particle size of the dust being collected, the filter
exhaust may be visible. Ideally, the operator should determine the visible
emissions (opacity) during a period when the mass emission level is known
(e.g., during a stack test) and use this value as a baseline. An'increase in
visible emissions should be used as an indicator of increased penetration. An
increase in mass emissions may occur without a noticeable increase in opacity
if the increase is caused by large particles (those greater than 5 ym in
diameter). Because of the uncertainty of the relationship between mass and
visible emissions, frequent internal inspections of the "clean side" of the
filter tube sheet are needed to determine if bag failure or dust penetration
is occurring.
The following subsections are provided to give guidance to maintenance
personnel in detecting the point of dust penetration. The methods are de-
signed to minimize the time required to determine the point of penetration and
therefore reduce excess emission and/or equipment downtime.
Shaker and reverse-air systems—As noted, excessive dust accumulation on
the clean side of the tube sheet can damage bags. It is therefore necessary
to remove the accumulated dust after a bag break.. This removal allows the
operator to evaluate the filter at a future date for evidence of penetration.
When a pinhole occurs in the fabric surface, an orifice is formed through
which gas passes at high velocity. Particles that pass through the opening
immediately begin to lose velocity, and the heavier particles settle to the
tube sheet. The pattern of accumulated dust indicates the general area of the
penetration. If the pinhole is near a walkway, the high velocity gas stream
(jet) normally produces an impaction pattern (Figure 48). If the pinhole is
farther back in the bags, the area between bags quickly fills with dust be-
cause of the impaction of particles on adjacent bags. The depth of the deposits
normally indicates the general area in which dust penetration is occurring.
The operator should not enter the compartment without evaluating initial
deposits on the tube sheet. It is almost impossible to use dust patterns for
detection after an operator walks through them.
Cuff bleeding is usually indicated by a general deposit around the bag
with a cone-like depression in the dust at the bag. The gases passing by the
seal ring form small orifices and deposit dust around the bags. When the
filter is isolated for inspection, the dust falls back into the orifice leav-
ing a cone-like depression (Figure 49). These penetration points are easily
detected if the tube sheet is cleaned frequently to remove accumulated dust.
Roughly 90 percent of bag failures occur near the bag thimble, primarily
because of improper installation or tension. If the cause of failure is not
immediately obvious, the inspector should begin at the thimble and then move
up the bag.
77
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AREA BLOWN CLEAN BY AIR STREAM FROM PINHOLE LEAK
Figure 48. Pinhole leak near walkway impaction pattern.
BAG
CUFF WITH
SNAP RING
LONG CUFF AND
/REDUCED ABRASION
DEPRESSION
INDICATING
CUFF BLEEDING
GAS FLOW
TUBE SHEET
AND
THIMBLE
Figure 49-. Indication of cuff bleeding.
78
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Pulse-jet_systems--Causes of dust penetration are not as easily detected
in pulse-jet filters as in shaker or reverse-air systems. Because gas exits
the cage opening (tube sheet) at high velocity, the particles do not deposit
in the area of the penetration, but are generally spread over the whole clean
side plenum.
Two mechanisms allow the identification of pinholes in these systems.
Because the cleaning mechanism is semicontinuous and the gas stream is not
interrupted for cleaning, small pinholes in the fabric are covered by the
collected cake during filtering periods except shortly after pulse cleaning.
The removal of the cake by the pulse allows penetration for a short period (2
to 3 seconds) before the cake is reestablished. The penetration results in an
increase in stack opacity (puff), which coincides with the cleaning of the row
containing the defective bag. Figure 50 is an opacity profile of a pulse-jet
fabric filter with a pinhole in one bag row.
o
0-
o
30
25
20
15
10
5
0
— __
—
—
— t— CM m «f
O O O O
GZ o: ct 2
" 1
PINHOLE IN FABRIC ~
—
if) ID r~- __
§ g g. ' '
o; c£. o:
1 I
10 20 30 40 50
TIME, seconds
60
70
Figure 50. Opacity profile of a pulse-jet fabric filter with
a pinhole in one bag row.
In systems in which the clean side of the filter is accessible, the
penetrating dust accumulates on the underside of the compressed air blow tube
at the stagnation point on the pipe (Figure 51). A routine check by sliding
the hand along the blow tubes indicates the deposit. If these deposits are
cleaned periodically, the method may be used to indicate the presence of a
pinhole at a later date.
79
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BLOW PIPE
ACCUMULATED DUST
COMPRESSED
AIR
BAG
Figure 51. Accumulated dust deposits on a pulse-jet system blow pipe.
Use of fluorescent dye—Several firms sell bag leak detection systems
that use fluorescent dye and ultraviolet (UV) light. The dye is injected into
the dirty gas stream and penetrates into the clean side of the collector
through pinholes. After isolating the compartment, the penetration points may
be located by detecting the fluorescent dye under UV light. The use of the
tracer is not of benefit if the system is in gross failure because the dye
penetrates the entire clean side plenum and does not indicate an individual
source of penetration. The method is very useful and time saving after major
failure mechanisms have been eliminated.
General System Operation-
Continued operation of the collector system requires that all of the
associated systems that interface with the collector function as designed.
This subsection discusses several of the major failure mechanisms in these
systems.
Dampers—Dampers are used to direct gas flows or isolate compartments for
cleaning or repair. If these dampers do not function to seal the compartment
in shaker fabric filters or to change the direction of gas flow in reverse-air
fabric filters, proper cleaning of the bags cannot be accomplished. Malfunc-
tion increases pressure drop, but in multiple-compartment systems does not
necessarily shut the system down. Because all dampers leak under adverse
conditions, dampers and seats should be inspected to minimize leakage.
Compressed gas system—The air used to activate dampers .and pulse-clean
bags must be clean and dry. An in-line gas dryer (such as a dessicant, re-
frigerant, or filter) should be used to remove oil and water from the gas
stream prior to introduction to the filter. As a safety precaution, a reserve
tank with blowdown should be used at the filter to collect oil and water. If
80
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not collected, oil and water blind bags and can freeze in diaphragms during
cold weather. The dryer should be serviced according to the manufacturer's
recommendations. An internal inspection of the filter bags should be con-
ducted periodically to check for oil and water.
Pulse diaphragms—Pulse diaphragms are used to open the valve seat in
pulse-jet cleaning systems and provide a sharp finite surge of compressed gas
through the blow tube to the venturi. The diaphragm in the closed position is
held against the seat by compressed air and a spring. The compressed air is
discharged through a solenoid valve and creates a pressure differential, which
pulls the diaphragm from the seat. This momentarily allows passage of gas
under the seat. Closure of the solenoid valve reestablishes the seal.
The solenoid commonly fails because of water freezing in the gas stream
or because of electrical failure. In either case, the cleaning pulse can not
be initiated. If the solenoid does not seat, a constant release of compressed
gas can be heard. The cleaning system can also fail because of diaphragm
rupture or improper diaphragm seating. Constant bleeding of compressed gas
into the blow tube can be heard. Both sounds will be a constant hissing.
A reduction in cleaning efficiency can occur if the diaphragm returns to
the seat sluggishly. This can be caused by water, oil, or grit fouling the
return spring, and can be heard as a sharp pulse that trails off.
In evaluating the pulse cleaning system, personnel should inspect the
reserve air tank for water and listen for malfunctions of each pulse system
through one cleaning cycle.
Preventive Maintenance--
The proper timing of maintenance is important to maintain compliance with
emission standards and reduce maintenance cost. Crisis maintenance cannot
provide the level of continuous compliance required. When properly carried
out, a preventive maintenance plan reduces maintenance time and makes effi-
cient, effective use of the limited number of personnel employed at the plant.
Many problems with fabric filter systems result from long-term causes
that gradually accumulate. A gasket leak around a door does not immediately
result in system failure; but if allowed to continue, it can cause bag blind-
ing, increased pressure drop, dust removal problems, and corrosion of the bag-
house shell. Over many months, the shell can deteriorate, and gas penetration
increases at an accelerated rate. Normally the symptoms (such as bridging or
bag blinding) are treated, but the cause (the gasket leak) is not noted (Fig-
ure 47). If a crisis maintenance approach is used, the system can deteriorate
almost beyond repair before it receives proper attention and it may be diffi-
cult to pinpoint the actual cause. Thus, such an approach can require re-
placement of major portions of the system. A preventive maintenance program,
however, can be used to avoid such a situation.
In a preventive maintenance program, maintenance personnel should be
advised of all factors that can cause component failures. Table 5 presents a
guide for fabric filter operation and maintenance. Critical areas must also
be regularly inspected, and accurate logs must be kept.
81
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TABLE 5. FABRIC FILTER OPERATION AND MAINTENANCE GUIDE
NOTE: When information applies to a specific type of fabric filter the
following code is used:
RF - Reverse Flow
PJ - Pulse Jet
S - Shaker
Symptom
Dirty discharge at
stack
High baghouse pres-
sure drop
Cause
Bags leaking
Bag clamps not sealing
Failure of seals in
joints at clean/dirty
air connection
Insufficient filter
cake
Bags too porous
Baghouse undersized
Bag cleaning mechanism
not adjusted properly
(continued)
82
Remedy
Replace bags
Tie off bags and replace at a
later date
Isolate leaking compartment
if allowable without upset-
ting system
Check and tighten clamps
Smooth out cloth under clamp
and reel amp
Caulk and tighten clamps
Smooth out cloth under clamp
and reel amp
Allow more dust to build up
on bags by cleaning less
frequently
Use a precoating of dust on
bags (S, RF)
Send bag in for permeability
test and review with manu-
facturer
Consult manufacturer
Install double bags
Add more compartments or
modules
Increase cleaning frequency
Clean for longer duration
Clean more vigorously (must
check with manufacturer
before implementing)
-------
TABLE 5 (continued)
Symptom
Cause
Remedy
Compressed air pres-
sure too low (PJ)
Repressuring pressure
too low (RF)
Shaking not vigorous
Isolation damper
valves not closing
(S, RF)
Isolation damper
valves not opening
(S, RF)
Bag tension too loose
(S)
Pulsing valves failed
(PJ)
Air volume greater
than design
Cleaning time failure
Not capable of re-
moving dust from bags
Increase pressure
Decrease duration and/or
frequency
Check dryer and clean if
necessary
Check for obstruction in
piping
Speed up repressuring fan
Check for leaks
Check damper valve seals
Increase shaker speed (check
with manufacturer)
Check linkage
Check seals
Check air supply of pneumatic
operators
Check linkage
Check air supply on pneumatic
operators
Tighten bags
Check diaphragm valves
Check solenoid valves
Damper system to design point
Install fan amperage controls
Check to see if timer is
indexing to all contacts
Check output on all terminals
Send sample of dust to manu-
facturer
Send bag to lab for analysis
for blinding
Dryclean or replace bags
Reduce air flow
(continued)
83
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TABLE 5 (continued)
Symptom
Cause
Remedy
High bag failure:
wearing out
High bag failure:
burning
Excessive reentrain-
ment of dust
Incorrect pressure
reading
Baffle plate worn out
Too much dust
Cleaning cycle too
frequent
Inlet air not properly
baffled from bags
Shaking too violent
(S)
Repressuring pressure
too high (RF)
Pulsing pressure too
high (PJ)
Cages have barbs (PJ)
Stratification of hot
and cold gases
Sparks entering bag-
house
Thermocouple failed
Failure of cooling
device
Continuously empty hopper
Clean rows of bags randomly,
instead of sequentially
(PJ)
Clean out pressure taps
Check hoses for leaks
Check for proper fluid in
manometer
Check diaphragm in gauge
Replace baffle plate
Install primary collector
Slow down cleaning
Consult manufacturer
Slow down shaking mechanism
(consult manufacturer)
Reduce pressure
Reduce pressure
Remove and smooth out barbs
Force turbulence in duct with
baffles
Install spark arrester
Replace and determine cause
of failure
Review design and work with
manufacturer
(continued)
84
-------
TABLE 5 (continued)
Symptom
Cause
Remedy
High bag failure:
decomposition
Moisture in baghouse
High compressed air
consumption (PJ)
Reduced compressed
air pressure (PJ)
(continued)
Bag material improper
for chemical composi-
tion of gas or dust
Operating below acid
dew point
System not purged
after shutdown
Wall temperature below
dew point
Cold spots at struc-
tural members
Compressed air intro-
ducing water (PJ)
Repressuring air
causing condensation
(RF)
Cleaning cycle too
frequent
Pulse too long
Pressure too high
Diaphragm valve
failure
Compressed air con-
sumption too high
Restrictions in piping
Analyze gas and dust and
check with manufacturer
Treat with neutralizer before
baghouse
Increase gas temperature
Bypass and startup
Keep fan running for 5 to 10
minutes to purge exhaust
gases from baghouse after
process is shut down
Raise gas temperature
Insulate unit
Lower dew point by keeping
moisture out of system
Fully insulate structural
members
Check automatic drains
Install aftercooler
Install dryer
Preheat repressuring air
Use process gas as source of
repressuring air
Reducing cleaning cycle if
possible
Reduce duration (after ini-
tial shock all other com-
pressed air is wasted)
Reduce supply pressure if
possible
Check diaphragms and springs
Check solenoid valve
See above
Check piping
85
-------
TABLE 5 (continued)
Symptom
Cause
Remedy
Reduced compressed
air consumption (PJ)
High fan motor amper-
age/low air volume
High screw conveyor
wear
Material bridging in
hopper
Frequent screw con-
veyor/air lock
failure
Dryer plugged
Supply line too small
Compressor worn
Pulsing valves not
working
Timer failed
High baghouse pressure
Screw conveyor under-
sized
Conveyor speed too
high
Moisture in baghouse
Dust being stored in
hopper
Hopper slope insuffi-
cient
Conveyor opening too
small
Equipment undersized
Screw conveyor mis-
aligned
Overloading components
Replace desiccant or bypass
dryer if allowed
Consult design
Replace rings
Check diaphragms
Check springs
Check solenoid valves
Check terminal outputs
See above
Measure hourly collection of
dust and consult manufacturer
Slow down speed
See above
Add hopper heaters
Remove dust continuously
Rework or replace hoppers
Use a wide-flared trough
Consult manufacturer
Align conveyor
Check sizing to see that each
component is capable of han-
dling a 100% delivery from
the previous component
(continued)
86
-------
TABLE 5 (continued)
Symptom
Cause
Remedy
High pneumatic con-
veyor wear
Pneumatic conveyor
pipes plugging
Pneumatic blower too
fast
Piping undersized
Elbow radius too short
Overloading pneumatic
conveyor
Moisture in dust
Slow down blower
Review design and slow blower
or increase pipe size
Replace with long radius
el bows
Review design
See above
Figures 52 and 53 are sample daily and weekly logs. These logs can give
the maintenance department a continuous record of system performance.
87
-------
Location:.
Tine:
Visible emissions:
Pressure drop:
Temperature (inlet):
Fan current:
Dust discharge functioning:
Hopper bridged:
Screw conveyor functioning:
Compressed air pressure:
Compartment shaking:
Diaphragms pulsing:
Reverse-air fan functioning:
%
in. H20
°F
amperes
yes
yes
yes
psig
yes
yes
yes _
no
no
no
_ no
no
no
Figure 52. Sample daily log.
88
-------
Location:
Reported by:
Visible emissions:
Pressure drop:
Temperature (chart):
Temperature above set point:
Solenoids not functioning:
Compressed air dryer functioning:
Water in reserve tank blowdown:
Reverse air damper closing:
Air inleakage: Doors
Hopper
Airlock
Flanges
Shaker motor functioning:
Bags shaking: Comp. 1
Comp. 2
Comp. 3
Comp. 4
Cleaning efficiency: Comp. 1
Comp. 2
Comp. 3
Comp. 4
Bag tension: Comp. 1
Comp. 2
Comp. 3
Comp. 4
Clean side deposits: Comp. 1
Comp. 2
Comp. 3
Comp. 4
Pinholes: Comp. 1
Comp. 2
Comp. 3
Comp. 4
Corrosion:
Moisture:
Bag bridging: Comp. 1
Comp. 2
Comp. 3
Comp. 4
Time:
Date:
Low %
Low in. H2
Low , °F
High %
0 High in. H20
High °F
occurrences min/week
Bag row
Bag row
Bag row
Bag row
yes .
yes
yes
yes
yes
yes
yes
yes
yes
yes
yes
yes
good
good
good
good
good
good
good
good
yes
yes
yes
yes
yes
yes
yes
yes
yes
yes
yes
yes
yes
yes
no
no
no
no
no
no
no
no
no
no
no
no
bad
bad
bad
bad
bad
bad
bad
bad
no in. accumulation
no in. accumulation
no in. accumulation
no in. accumulation
no Bag row Bag No.
no Bag row Bag No.
no Bag row Bag No.
no Bag row Bag No.
no
no
no Bag row Bag No.
no Bag row Bag No.
no Bag row Bag No.
no Bag row Bag No.
Figure 53. Sample weekly Tog.
89
-------
4.3 SCRUBBERS
Wet scrubbers are used less often than fabric filters because they require
more energy to attain the desired level of emission control and they require
water treatment facilities. Wet scrubbers are not used to control emissions
from lead oxide production or material handling and transfer because fabric
filters are much more cost-effective for product recovery. When used, scrub-
bers are most frequently associated with control of emissions from refining
kettles.
FLUE GAS
GAS
ABSORPTION
TOWER
HOOD
FUEL"
1
WATER
KETTLE
FURNACE
SURGE TANK
PUMPS
CAUSTIC TANK
Figure 54. Venturi scrubber system controlling a kettle furnace.
Gas streams to a venturi scrubber from kettles typically contain fine
lead and lead oxide fumes and particulate matter generated by the addition of
sulfur, phosphorus, sodium nitrate, sawdust, and other fluxes in the refining
process. The gas temperature entering the scrubber in this application is
typically 49° to 60°C (120° to 140°F) because a large quantity of ambient air
passes through the hooding. Thus, a presaturator stage is not needed to
decrease the gas temperature. The water is typically introduced to the ven-
turi throat either by spray nozzles or by a flooded weir system. In either
case, the acceleration of the gas stream in the converging section and throat
of the venturi provides the shear force to transform the incoming water to
small droplets.
The primary collection mechanism for a venturi scrubber is particle
impaction on the water droplets. The efficiency of this mechanism depends on
the size of the particulates. Particulates with diameters from 0.1 to 0.5 ym
require a high energy expenditure to obtain the high pressure drop needed for
collection by impaction. Some of these particles may be collected by a diffu-
sion mechanism in the divergent section of the venturi, but collection effi-
ciency by this mechanism is typically not very high.
90
-------
After collection of the particle on the water droplet, the droplet must
be separated from the gas stream. This is typically accomplished by a cy-
clonic separator that may be either a separate component or an integral por-
tion of the stack base.
4.3.1 Design
Important factors in scrubber design are good water distribution in the
scrubber throat and either use of nonplugging nozzles or provisions allowing
individual removal of nozzles from the scrubber .exterior for inspection and
replacement while the scrubber is on line. A hatch to provide access for
periodic inspection of the scrubber throat should be .included. A method of
determining water flow rate, venturi pressure drop, and fan power utilization
is useful in determining venturi performance. Typically, venturi scrubbers
are constructed of 316L stainless steel to resist corrosion.
Most of the energy required by a venturi scrubber is used to accelerate
the water droplets to the velocity of the gas stream. At the throat of the
venturi, water droplets have essentially no velocity but gas streams typically
have velocities of 7,500 to 15,000 cm/s (14,700 to 29,500 ft/rain) for optimum
collection efficiency. This velocity gradient allows particulates to impact
upon the larger water droplets. Adding more water provides more water droplet
"targets" and increases particulate collection. Increasing the amount of
water accelerated also increases the scrubber pressure drop and energy use.
Typical pressure drops of wet scrubbers applied to refining kettles range from
5722 to 9952 Pa (23 to 40 in. H?0) at liquid-to-gas (L/G) ratios of 0.4 to 0.6
Iiter/m3 (8 to 15 gal/1000 acfm).
Weir sumps are normally used to allow particulates to settle out of the
scrubber water. Holdup times of 2 to 3 hours are typical. BTeedoff lines are
usually not provided, and losses resulting from evaporation and droplet carry-
over are typically made up at the sump by maintaining a certain water level
with a float and lever connected to a water valve. The sump must be period-
ically drained and emptied of sludge, which is usually landfilled.
Scrubbers can also be used for S02 collection. These are typically spray
towers or packed-bed absorbers designed specifically to collect SO? with
caustic scrubbing solutions. This application is currently limited in use and
will not be discussed here.
4.3.2 Operation and Maintenance
Wet scrubbers can provide continuous, reliable service when they are
operated properly and regular maintenance is performed. Poor operation and
maintenance leads to component failure, which can result in poor scrubber
performance and/or damage to the system. Most scrubber failures result from
abrasion, corrosion, solids buildup, and wear of rotating parts. Common
failure modes for individual components are discussed below.
91
-------
Nozzle Plugging--
Nozzle plugging is one of the most common malfunctions in scrubbers.
Plugged nozzles reduce the L/G ratio or cause maldistribution of the liquid.
Nozzle plugging results from improper nozzle selection, excessive solids in
scrubbing liquors, poor pump operation, or poor sump design. Remedies for
nozzle plugging include replacement with nozzles of a different type, frequent
cleaning of the nozzles, and a reduction of liquor solids content by increas-
ing liquor blowdown and makeup water rates. Nozzle plugging can be detected
by observing the liquid spray pattern the nozzles produce. If the nozzles are
not accessible while the pumps are operating, they should be checked during
scrubber shutdowns for evidence Of caking over the nozzle openings. A reduc-
tion in water flow rate during scrubber operation is an additional symptom of
nozzle plugging.
Solids Buildup--
Solids buildup is another problem common to wet scrubbers and one that is
often difficult to control. The two types of solids buildup are sedimentation
and chemical scaling. Sedimentation occurs when a layer of particles becomes
attached to a surface or settles in areas of low turbulence. Sedimentation
can lead to plugging of pipes and ducts or to buildup on internal parts.
Chemical scaling results from a chemical reaction of two or more species to
form a precipitate on the surfaces of scrubber components.
Solids buildup may occur in piping, sumps, instrumentation lines, or
ductwork, and may lead to reduced scrubber efficiency and major equipment
failure. Most scrubbers using open pipes cannot reliably tolerate liquor
slurries of over 15 percent solids by weight. It is usually best to maintain
solids content at less than 6 to 8 percent (Schifftner, 1979). Techniques to
control scaling include increasing the L/G ratio, controlling pH, providing
greater residence time in the holding tank, and adding other chemical agents
such as dispersants. Solids buildup can be detected by inspection of accessi-
ble components and by inspection of the inner surfaces of piping, tubing, and
ductwork at removable fittings and hatches.
Corrosion—
Corrosion problems arise frequently in wet scrubbers, especially when the
gases being cleaned contain acid-forming compounds or "soluble electrolytic
compounds such as the oxides of sulfur encountered in secondary lead smelting.
The combustion of fossil fuels, especially coal, coke, and residual fuel oil,
yields oxides of sulfur, which can produce sulfuric acids in scrubbing liquors.
Recirculation of scrubbing liquors greatly increases the concentrations of any
corrosive agents they contain. Chlorides contained in the scrubbing liquor
can cause chloride stress corrosion of stainless steel.
Prevention of corrosion is best handled through proper choice of materi-
als of construction and through pH control. When a pH control system is to be
the principle defense against corrosion, it will require regular maintenance
at frequent intervals, especially at the pH electrodes. Another common operat-
ing problem occurs when scrubber liquor blowdown rates are reduced to limit
92
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the emission of pollutants into surface waters. Reducing or eliminating
blowdown can so greatly increase the acid and electrolyte concentrations in
the liquor that otherwise acceptable materials of construction becom ineffec-
tive against corrosion.
Abrasion —
Abrasion can occur where gases or scrubbing liquors containing high
concentrations of abrasive particulate are in the turbulent mode or are sub-
jected to a sudden change in flow direction. Typical wear areas in scrubbing
systems include venturi throats, walls of centrifugal mist collectors near the
inlet duct, and elbows in the ductwork (Schifftner, 1979). Solutions to
abrasion wear include the use of precleaning devices and the use of large-
radius turns in ductwork.
Pump wear is also a common problem in scrubber systems. Pump housings,
impellers, and seals are subject to abrasion and corrosion by scrubber slur-
ries. Rubber linings and special -alloy pump materials are often used to
reduce abrasion and corrosion of the housings or impellers. Installation of a
water flush in the seals can help reduce wear of the seals (Czuchra, 1979).
Preventive Maintenance —
Preventive maintenance is an important tool in assuring the continuous
operation of scrubber systems. Preventive maintenance programs for scrubbers
should include periodic inspection of equipment, replacement of worn parts,
periodic cleaning of components prone to plugging, maintenance of an adequate
spare parts inventory, and, recording of all maintenance performed on scrubber
equipment.
All worn parts and malfunctioning equipment should be serviced as they
are discovered to prevent deterioration of system performance and to prevent
damage to equipment. This requires an adequate inventory of spare parts for
the system. Parts that must be maintained in stock include nozzles, bearings,
pump seals, liners for pumps with replaceable liners, pump impellers, wear
plates for fan wheels with wear plates, pH probes, and valve parts (Fontana
and Greene, 1967). Records should be made of all maintenance performed and
all parts replaced. This information is useful in planning subsequent pre-
ventive maintenance schedules and in determining the type and number of re-
placement parts needed.
4.4 AFTERBURNERS
The primary function of an afterburner on a blast furnace is to oxidize
carbon monoxide (CO) and hydrogen sulfide (HeS) from the blast furnace to form
carbon dioxide (CO?) and S02- Afterburners may also be designed to oxidize
organic particulates (residual hydrocarbons) that result from charging "plugs"
(i.e., battery terminals) or pieces of battery casing. It is advisable to
avoid the very tacky particulates caused by incomplete combustion of the
plastic battery cases by reducing the quantity of plastic in the feed.
93
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4.4.1 Design
The actual design of afterburners depends on temperature, contact time,
and mixing of the gas streams. The temperature needed for complete combustion
must be above the auto-ignition temperature of the most difficult compound.
The contact time is usually approximately 1 second. Temperatures needed at
these low contact times are approximately 980°C (1800°F). Mixing must be
insured by adding the necessary amounts of other air or oxygen for the combus-
tion to take place.
Some facilities use afterburners as gas reheater because quantities of
air inleakage or dilution air lower gas temperature enough to cause problems
in the control equipment. In these cases, oxidation of CO, h^S, and organic
participates is more difficult, and larger quantities of fuel and more reac-
tion time must be provided.
Afterfurners (Figure 55) may be cylindrical or rectangular in cross sec-
tion and are usually constructed of refractory materials inside a steel shell.
The afterburner consists of a mixing section to provide for contact between
the contaminated gases and the burner flame, and a combustion section where
the hydrocarbons are destroyed by incineration.
FLAME SENSOR-
BURNER -\
REFRACTORY•
INSULATION-
TURBULENT EXPANSION ZONE-
BLAST FURNACE
EXHAUST INLET
COMPRESSION ZONE
COOLING AIR
INDUCTION SYSTEM
(ADJUSTABLE)
EXHAUST
OUTLET
Figure 55. A typical afterburner control device (EPA 1973)
94
-------
4.4.2 Operation and Maintenance
Proper operation of an afterburner system requires control of contaminant
quantity and characteristics and requires regular maintenance of the burners.
As with other particulate control devices, complete instrumentation and an
effective preventive maintenance program are necessary.
Burners—
Burners are high-maintenance items because of the high temperatures and
small orifices (Ross, 1977). When the contaminated gas stream is used as the
combustion air, fouling of the orifices and/or deposits in the air delivery
lines can occur. Impurities within the oil can lead to similar problems. A
second problem is improper sizing of the burner(s). This can lead to low gas
temperatures resulting in incomplete oxidation of particulate matter. Burners
with poorly adjusted air-fuel ratios can generate soot, which fouls downstream
heat exchange surfaces.
Minimizing of burner problems is facilitated by providing a means of vi-
sually checking the flame for proper luminosity, length, and stability. Also,
an adequate inventory of spare parts should be kept. If fouling continues, a
precleaner may be economical. Finally, such afterburner instrumentation as
the flame sensor and the temperature controller should be checked regularly.
Effluent Characteristics—
Variability of effluent quantity and heat content should be minimized by
controlling the process operation. Excess concentrations of combustible gases
and vapors can lead to high temperature excursions, which damage the after-
burner shell. High gas flow rates lead to poor particle oxidation resulting
from decreased residence time and decreased reaction temperature.
Contaminants containing sulfur or chlorine compounds may be oxidized to
highly corrosive species such as hydrochloric acid vapors and sulfuric acid
vapors. These could result in chemical attack of the afterburner shell under
certain circumstances. Special materials of construction are required when
these contaminants are present.
4.5 AUXILIARY EQUIPMENT
4.5.1 Fans
Either axial or centrifugal fans may be used in typical smelter ventila-
tion applications. Axial fans are often selected for high-volume, low-pressure-
drop applications such as building and local ventilation. Centrifugal fans
are usually selected for high-pressure-drop applications such as process
exhaust gas cleaning.
Centrifugal fans are divided into three subclassifications: forward
curved, straight (or radial), and backward curved. Forward-curved centrifugal
fans are compact, have low tip speeds, and thus are quiet. They are usually
used against moderate static pressure (e.g., in air conditioning systems) and
95
-------
not recommended for the movement of dust or fumes, which can adhere to the
short curved blades and cause imbalance. Straight (or radial) fans have
intermediate tip speeds, are noisier than forward-curved fans, and are used in
dirty environments. Backward-curved fans have high tip speeds and high effi-
ciencies. Because material can adhere to the blades, these fans are seldom
used to move extremely dirty gas streams.
Fan characteristics are reported in graphs or tables relating volume flow
rate to horsepower and static pressure. Figure 56 displays generalized charac-
teristics for the fans discussed (American Conference of Governmental Indus-
trial Hygienists 1980).
As the air flow through a given system increases, so does the static
pressure. As the flow rate increases through a fan, the static pressure
increases, reaches a maximum, and decreases. For a given fan and duct system,
the two characteristic curves cross at a point where the system will operate.
Any selected fan and duct system will equilibrate at a flow rate and fan
static pressure. The flow rate can be increased by reducing the system re-
sistance or by increasing fan performance. When changes are made, new per-
formance criteria can be estimated by the following fan laws:
Air flow varies directly with fan speed.
Total pressure and static pressure vary with the square of the fan
speed.
Fan horsepower varies with the cube of the fan speed.
Fan wear is a common problem. Forced-draft fans often suffer abrasion
because of exposure to particulate-laden gases. Wear problems in forced-draft
fans can be addressed by the use of spacial wear-resistant alloys, by reduc-
tion of fan rotation speeds (by installing a larger fan), or by moving of the
fan to an induced-draft location on the clean air side of the gas cleaning
system. Induced-draft fans can undergo corrosion or solids buildup on the
blades if mist is carried over from the liquid entrainment separator. Induced-
draft fan problems can be addressed by use of corrosion-resistant materials or
by improving liquid entrainment separation.
Table 6 summarizes causes and remedies for common ventilation system
symptoms.
96
-------
FORWARD-CURVED BLADES
ee O
B. 3:
VOLUME
BACKWARD-CURVED BLADES
UJ 3
tt O
=> 0.
UJ CC
tt O
CL X
STATIC
PRESSURE
VOLUME
STRAIGHT (OR RADIAL) BLADES
VANE-AXIAL FAN
uj or
ec o
a. oc.
^ CD
STATIC
PRESSURE
VOLUME
BRAKE HORSEPOWER
Figure 56. Fan characteristics (American Conference of Governmental
Industrial Hygienists 1980).
97
-------
TABLE 6. VENTILATION SYSTEM OPERATION AND MAINTENANCE GUIDE
Symptom
Cause
Remedy
Dust escaping at
source
Low fan motor amper-
age/low air volume
Fan motor overloading
Air volume too high
Low air volume
Ducts leaking
Improper duct flow
balancing
Improper hood design
Fan and motor sheaves
reversed
Ducts plugged with
dust
Fan damper closed
System static pressure
too high
Fan not operating per
design
Belts slipping
Air volume too high
Motor not sized for
cold start
Ducts leaking
Insufficient static
pressure
98
See below
Patch leaks so air does not
bypass source
Adjust blast gates in branch
ducts
Close open areas around dust
source
Check for cross drafts that
overcome suction
Check for dust being thrown
away from hood by belt, etc.
Check drawings and reverse
sheaves
Clean out ducts and check
duct velocities
Open damper and lock in
position
Measure static on both sides
and compare with design
pressure
Duct velocity too high
Duct design not proper
Check fan inlet configuration
• and be sure even airflow
exists
Check tension and adjust
See below
Damper fan at startup
Reduce fan speed
Provide heat faster
Replace motor
Patch leaks
Close damper valve
Slow down fan
-------
TABLE 6 (continued)
Symptom
Excessive fan wear
Excessive fan
vibration
Cause
Improper fan
Fan speed too high
Buildup of dust on
blades
Wrong fan wheel for
application
Sheaves not balanced
Bearings worn
Remedy
Check with fan manufacturer
to see if fan is correct for
application
Check with manufacturer
Clean off and check to see if
fan is handling too much
dust (see above)
Do not allow any water in fan
(check drain, look for con-
densation, etc.)
Check with manufacturer
Have sheaves dynamically
balanced
Replace bearings
4.6 INSTRUMENTATION AND RECORDKEEPING
The use of instrumentation and recordkeeping procedures can provide a
cost-effective method of improving and optimizing both process operation and
control equipment performance. Well designed equipment is necessary to obtain
maximum benefit from the instrumentation and recordkeeping. Performance
problems caused by poorly designed equipment are likely to continue until such
equipment is upgraded or replaced. Parameters measured by the instruments and
data assimilated through recordkeeping should provide operators with informa-
tion concerning both instantaneous and long-term performance characteristics
of the process and the emission control equipment. The specific objectives
are:
Instantaneous evaluation of process operation
Increased data for troubleshooting both process and emission control
equipment
Optimization of process variables and control equipment performance
Enhancement of preventive maintenance scheduling
Operator alertness to conditions that may damage the equipment
Reduction of malfunctions
99
-------
A comprehensive package of instruments and recordkeeping should be de-
veloped to aid in achieving the plant operating objectives. Although a compre-
hensive package of instruments and recordkeeping requires capital investment
and manpower, costs should be offset by increased production and control
efficiency and by reduced energy and maintenance cost for the process and
emission control systems.
The primary instruments used at a secondary lead facility are pressure
and temperature indicators. These instruments provide a static pressure pro-
file and temperature profile for each ventilation system. Profiles start from
the process and end at the exhaust point. They can be used to determine a
baseline case, which represents satisfactory operation as well as the design
basis for the installed air pollution control equipment. Any major deviation
from baseline parameters indicates either a process change or a malfunction
requiring maintenance. With baseline values established, plant personnel may
take steps to optimize O&M practices.
4.6.1 Instrumentation
Fabric Filters Applied for Furnace and Process Emissions—
Instruments for furnaces and process emission fabric filters have been
limited at most secondary lead facilities. The following minimum instruments
are recommended: a temperature indicator (thermocouple) and a static pressure
indicator (e.g., a magnehelic gauge) at the outlet of the furnace, static
pressure and temperature indicators at the inlet and outlet of the process
emission fabric filter, and an indicator of the fan motor current. These
instruments allow personnel to determine the gas flow rate and the amount of
gas cooling between the furnace afterburner and the outlet of the process
emission fabric filter. Automatic temperature recorders should be used to
provide operating records and may be used for automatic control of afterburner
firing rate and air dilution. In addition, alarms can be connected to alert
operators to temperature excursions. Temperature indicators at the inlet and
outlet of the process emission fabric filter should be used to detect air
inleakage, in conjunction with static pressure indicators to measure pressure
drop across the fabric filter. The temperature and static pressure at the
fabric filter outlet are also used to establish fan operating speed. Because
fan speed is usually constant, only a periodic check is necessary. The static
pressure, temperature, fan speed, and fan motor horsepower are used to estab-
lish the gas flow rate through the fabric filter. Additional sampling points
(static pressure taps) should be installed to allow further analysis of the
ventilation system. These sampling points should include the inlet to the
blast furnace afterburner, the inlet to the cooling loops, and each leg of the
cooling loops. Although permanent instruments are not needed at these points,
provisions for troubleshooting with portable instruments should be included.
A static pressure indicator at the duct from the hood may be needed to
assess the system performance. When combined with a visual check, use of such
an indicator can confirm proper hood design and fugitive emission capture.
Continuous measurement of hood face velocity is usually impractical although
this should be calculated after hood installation.
100
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At facilities that use pulse-jet fabric filters for fugitive emission
control, a pressure indicator should be installed to determine the pulse
header pressure. Further, an alarm can be connected to such an indicator to
signal the operator when pulse pressure drops below a preset value [e.g., 483
kPa (70 lb/in.2)].
Static pressure indicators must withstand high temperature and dust load-
ings. Because the most common problem with pressure indicators is plugging of
the taps, provisions for cleaning the taps must be included. The use of port-
able instruments and the installation of pressure taps in the ductwork allow
easy periodic checks of ductwork parameters. Pressure measurements alone
indicate the permeability of the cloth, how heavy the dust deposit is before
cleaning, how complete the cleaning is, and whether the fabric is starting to
plug or blind.
Fabric Filters Applied for Fugitive Emissions--
Fugitive emission fabric filters require a minimal number of instruments.
Inlet and outlet static pressures and temperatures should be measured. These
data can be used to determine pressure drop across the fabric filter, fan
temperature, and fan static pressure. Also, the fan motor current should be
measured to determine gas flow rate at the fixed fan speed.
Wet Scrubbers—
When used, wet scrubbers are typically installed to control emissions
from refining kettles. In this application, a single temperature indicator is
necessary to determine fan operating temperature for calculation of gas flow
rate. A simple dial thermometer at the inlet or outlet of the fan is suffi-
cient. Periodic calibration checks are needed to ensure that vibration does
not affect the accuracy of the thermometer.
The fan current, venturi pressure drop, and water flow rate should be
monitored. As previously mentioned, fan speed is considered a constant until
changed by plant personnel and need not be monitored on a continuous basis.
Pressure drop can be monitored by the same methods as those used with a fabric
filter. Liquid flow rate to the scrubber can be monitored by various flow-
meters, including rotameters, orifice meters, or ultrasonic/doppler shift
meters. Measuring the pressure from the pump to the scrubber header does not
eliminate the need to ensure that scrubber liquid reaches the scrubber in the
prescribed quantities.
When materials of construction must be protected or when the scrubber is
used to remove S02, the pH should be monitored. Continuous pH monitoring
requires frequent inspection and cleaning of the probe, particularly when the
monitor output is interfaced with a device to control addition of caustic
solution. Many facilities use batch neutralization or add caustic according
to a prescribed schedule because pH meters have typically not provided ade-
quate service. One promising method of determining pH is sidestream monitor-
ing, in which a sample of the sump liquor is withdrawn for pH analysis.
Sidestream monitoring may be either continuous or periodic.
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Afterburners—
An afterburner requires an outlet temperature indicator, a fuel flow rate
indicator, and a fuel use integrator. In oil-fired systems, air atomization
pressure may also be measured to determine proper burner operation. The tem-
perature indicator (i.e., thermocouple) may be connected to a feedback device
that controls the fuel firing rate and maintains the design outlet temperature.
An inlet thermocouple may be added to the system and used with the outlet
thermocouple in a feedforward/feedback device that controls the afterburner
firing rate.
Continuous monitoring of oxygen and carbon dioxide is not practiced at
secondary lead smelters. Grab samples and an Orsat analyzer, however, can be
used to optimize the performance of the blast furnace afterburner.
4.6.2 Recordkeeping Procedures
Recordkeeping procedures must be tailored to the manpower constraints and
associated costs of each facility. Some minimal records should be kept and
periodically analyzed to determine correlations between operating practices
and cycles. The plant operator, however, should not be burdened with obtain-
ing information of little value for process optimization or good preventive
maintenance.
Daily records should include charts from all temperature instruments,
data on the fabric filter pressure drop before and after cleaning (if appli-
cable) and an estimate of the average or typical volume of gas handled. The
A/C ratio should be calculated for fabric filters, and any changes should be
noted. Also, daily records should indicate production rates and all upsets or
malfunctions. Internal inspection results and bag failures should be recorded.
Records should describe the general condition of each compartment and the
specific location and nature of all bag failures. Periodic analysis of fabric
filter records can indicate trends (e.g., specific types of bag failures in a
specific location) and suggest the need for modification. In addition, peri-
odic review of records can suggest optimization strategies for both production
and preventive maintenance and thus can reduce unexpected downtime and lost
production.
Similar daily records should be maintained for scrubbers and afterburners.
Daily fuel consumption and fuel use rate cycles should be recorded for after-
burners. Scrubber liquor flow rate, fan current, and pressure drop should be
recorded. Again, both long-term and short-term trends are important for
optimizing production and preventive maintenance.
Initially, static pressure and temperature profiles should be checked
weekly or biweekly to determine system characteristics. As experience.in-
creases, checks can be less frequent. In addition, instruments should be
checked at least weekly or biweekly, if not daily.
102
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REFERENCES FOR SECTION 4
1. American Conference of Governmental Industrial Hygienists. 1980. Indus-
trial Ventilation: A Manual of Recommended Practice, 1980. Lansing,
Michigan.
2. Czuchra, P. A. 1979. Operation and Maintenance of a Particulate Scrubber
System's Ancillary Components. Presented at the U.S. EPA Environmental
Research Information Center Seminar on Operation and Maintenance of Air
Pollution Equipment for Particulate Control, Atlanta, Georgia.
3. Fontana, M. G., and N. D. Greene. 1967. Corrosion Engineering. McGraw-
Hill. New York, New York.
4. Pangborn Bulletin. 1978. Unit Type CN-2 Collector. Pangborn A. Kennecott
Co., Hagerstown, Maryland.
5. Ross, R. 1977. Incinerators, An Operation and Maintenance of Air Pollu-
tion Control Equipment. Technomic Publishing.
6. Schifftner, K. C. 1979. Venturi Scrubber Operation and Maintenance.
Presented at the U.S. EPA Environmental Research Information Center
Seminar on Operation and Maintenance of Air Pollution Equipment for
Particulate Control, Atlanta, Georgia.
7. U.S. Environmental Protection Agency. 1973. Air Pollution Engineering
Manual, Second Edition. AP-40. Research Triangle Park, North Carolina.
103
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SECTION 5
INSPECTION OF SECONDARY LEAD SMELTERS
Inspection of a secondary lead smelter requires that data be recorded on
site for later use in evaluating smelter emission control and compliance
practices. The following items are useful to ensure that the inspection is
complete and to maximize the pertinent information that can be obtained while
the inspector is on site:
0 Smelter plot plan • ,
0 Engineering drawings or sketches of equipment and specifications
0 Process flowsheet and equipment checklist
0 Raw material/product checklist
0 Individual process worksheets
0 Emission control equipment (systems) acceptance or performance test
results
0 Maintenance records
Most of these items can be obtained during the file review at the appropriate
EPA Regional and local offices.
Prior to the inspection, the worksheets, process flows, and maintenance
records should be reviewed with the plant's representative at the plant.
Organizing these items prior to the actual inspection helps ensure that all
necessary data are obtained.
Plot Plan
The plot plan should show entrances, major buildings, paved and unpaved
areas, and major equipment items to scale. Other appropriate details should
be included to provide orientation.
Equipment Drawings
Photographs or sketches of major equipment items are useful for reference
or comparison when a smelter control system is evaluated. Traffic flow, major
process equipment, and control equipment should be shown to allow for easy
reference at a later date.
Process Flowsheet and Equipment Checklist
An inspector should have a clear idea of the process flow and major
equipment items in the smelter. A copy of Figure 57 could be used as a
worksheet in any secondary lead smelter once corrected for that plant. The
equipment associated with each process should be listed.
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RAW MATERIALS
Whole batteries
Battery plates
Solders
Misc. lead scrap*
Dross - old^
- ne*r
Flue dust - old
- new
Tetraethyl lead
residue
Rerun slag
Coke
Iron or steel
Limestone (CaC03)
Silica (Si02)
Other: a.
b.
c.
FEED RATE
* Electronics, crude pigs or sows,
clean scrap, alloy metal, flash-
ing, pipe, sheet, came lead, etc.
Material from outside sources.
* In-house recycle naterial.
PRODUCTS
Finished Products
Soft lead
Semisoft lead
Hard (antimonial) lead
Lead alloys
Battery oxide
Other: a.
b.
c.
d.
Intermediates/Residue
Crude or Herd lead
Slags a.
b.
Matte
Dross a.
b.
c.
Flue dust a.
PRODUCTION RATE
b.
Other a.
b.
Figure 57. Sample checklist for raw materials and products.
105
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Raw Material/Product Checklist
For each process, the sample checklist shown in Figure 56 may be useful
for recording the types and quantities of raw materials and products.
Individual Process Worksheets
Individual worksheets may be prepared to collect pertinent process in-
formation based upon current operating logs, maintenance records, and design
specifications. The worksheet varies with each furnace being inspected, but
should include operating temperatures, operating cycle frequency, descriptions
of equipment and area conditions, and furnace operating procedures.
5.1 INSPECTION OF CONTROL EQUIPMENT AND VENTILATION SYSTEM
The purpose of inspecting control equipment is to evaluate system per-
formance with respect to regulatory requirements and operation and maintenance
procedures. Inspections can vary in detail, depending upon the objective.
For example, cursory inspections can be performed by only quick, external
.examination of the control equipment and recording of several temperature and
static pressure readings. Thorough inspection requires more detailed analysis
of the control system and perhaps an internal inspection of the equipment.
Inspection requirements should be tailored to the characteristics of each
facility, and the time available for each inspection.
An inspector, upon arrival at the plant to be inspected, should determine
existing plant self monitoring procedures by questioning plant management.
Any records which show control system operating parameters and production
levels should be quickly reviewed to estimate plant "baseline values". "Base-
line values" are useful for inspection of control equipment and evaluation of
operating parameters. A baseline value is the value of a given parameter
(e.g., fan static pressure and temperature or fabric filter pressure drop)
when all the equipment in a system is operated in a manner that provides
acceptable performance. In general, a single value should not be established
for each parameter to indicate acceptable performance. Rather, a range of
values is established, although this range can be relatively narrow. Actual
operating values, obtained during the inspection, can be compared with base-
line values to evaluate the system. If actual operating values differ signif-
icantly from baseline values, operation of the process or control equipment
has changed enough to warrant further questioning of plant personnel to deter-
mine probable reasons for these deviations.
5.1.1 General Indications of Control Equipment Performance
Some parameters can indicate the performance of the control equipment.
These include opacity and fan parameters.
Opacity—
Opacity observations are very useful for determining control equipment
performance at secondary lead facilities. Particles generated by these fa-
cilities are typically 5 um or less in diameter and scatter Tight if released
106
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to the atmosphere. Observation of the opacity and comparison of this value
with the baseline value provides the initial indicator of any process or
control equipment performance variation. It is important to observe not only
the magnitude of any change, but also the characteristics of any opacity
readings to determine correspondence with process operations. For example,
opacity puffs that correspond with cleaning cycles may indicate pinhole leaks
in fabric filters, or opacity puffs may correspond with charging operations.
The opacity of emissions from fabric filter outlets is typically 0 to 5
percent. Opacity increases only when process changes occur (e.g., when coke
or plastics in the charge material create hydrocarbons that condense after
passing through the fabic filter) or when control equipment fails. Thus,
opacity changes can indicate the need for control equipment maintenance.
Observing the opacity of emissions from wet scrubbers can be complicated
by the presence of a steam plume. Because the operating temperature of most
scrubbers is relatively low, however, the steam plume is typically short. In
general, no opacity is observed at the steam dissipation point except during
refining operations (i.e., during addition of sulfur, phosphorus, and sawdust
to refining kettles). Thus, opacity observations can be useful during these
operations and can indicate changes in scrubber operating parameters.
Fan Parameters—
Fan parameters are generally the parameters that plant and regulatory
personnel use the least in evaluating control equipment performance. The
procedure for evaluating fan performance is straightforward and relatively
quick. Each fan has a unique static pressure/motor current/gas volume curve
at any given speed. Because fan speed is usually fixed (and must be manually
and intentionally changed by plant personnel), it need not be measured each
time plant personnel check the system. Parameters to be measured are the gas
temperature at the fan, the inlet and outlet static pressure, and the fan
motor current. Because most fan curves are plotted for 21°C (70°F) at stan-
dard conditions, static pressure and motor current measurements must be cor-
rected to standard conditions. Once corrected to standard conditions, the gas
volume through the fan may be read directly from the curve. This gas volume
is useful in determining scrubber throat velocities, A/C ratios, and air
inleakage rates if pitot tube measurements are also available from points
upstream of the fan.
Baseline values may simplify the process. If all parameters remain
unchanged, then the fan is probably moving the same quantity of gas as it
moved when the baseline values were obtained. Any appreciable change of
parameters indicates a change in the gas flow, and a new volume should be
determined from the curve. If temperature and fan speed remain unchanged, a
decrease in fan static pressure indicates an increase in gas volume handled
and an increase in fan motor current. The opposite is true for an increase in
fan static pressure. If temperature or fan speed changes, fan laws should be
applied to determine the new gas volume.
Several effects may be observed in fan parameters during certain opera-
tions at a facility. For example, a gradual increase in pressure drop across
a shaker or reverse-air fabric filter between cleaning cycles may be observed
107
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in the change of fan parameters. This increase will reduce gas flow, reduce
suction at the ventilation,pickup point, and permit fugitive emissions. Thus,
this increase in pressure drop indicates the need to reduce the time between
cleaning cycles.
Fan vibration should be observed. Slight vibration is normal, but moder-
ate to heavy vibration may indicate significant particle penetration, buildup
on fan blades, or wear of fan blades (if the fan is downstream of the control
device). If severe fan vibration is discovered, the appropriate personnel
should be notified, and the process should be immediately shut down. Severe
fan vibration indicates imminent fan failure. If a blade breaks away from the
fan wheel, the resulting imbalance can easily break the wheel off the shaft
and destroy the housing. "Fan explosions," as they are called, are generally
preventable through routine inspection. The inspector should inspect the fan
(while the fan is off and locked out of service). A light particulate coating
is normal but a heavy coating indicates particulate buildup which could cause
excessive wear of the blades.
5.1.2 Guidelines for Control Equipment Inspection
Fabric Filters—
The most frequently used indicator for fabric filter operation is the
pressure drop across the filter medium. This parameter, however, only indi-
cates significant changes in fabric filter operation caused by bag breakage or
blinding. Small changes in fabric filter pressure drop may not be important
on a short-term basis. A gradual long-term increase in pressure drop may
indicate the need to change or clean bags. Thus, fabric filter pressure drop
should be measured and recorded.
The cleaning system operation should be confirmed, and the timing se-
quence should be recorded. Pulse-jet fabric filters typically operate in a
continuous cleaning mode with an automatic timer to trigger the solenoids.
Shaker and reverse-air fabric filters may be operated by timers or by pres-
sure-drop sensors to maintain a preset pressure drop. The triggering of the
pulse-jet solenoids is easily heard, and a qualitative judgment concerning
cleaning effectiveness may be made from the sound of the pulse. A quick,
sharp pulse will indicate that the pulse-jet solenoid is probably properly
operating while a sluggish sound will indicate that something is wrong. Re-
verse-air fabric filters should be checked for proper damper activation.
Shaker mechanism operations are more difficult to evaluate because most moving
parts are inside the baghouse shell; however, external components (if any) of
the shaker mechanism should be checked (Figure 58).
The waste handling system (including hoppers, airlocks, and conveyors)
should be checked for proper operation. Rotary airlocks and screw conveyors
should be operated at all times to prevent hopper bridging. Hoppers should
not be used for storage of captured particulates. Although hoppers may be
insulated, the hopper discharge should be warm to the touch. A cold hopper
discharge on a "hot source" may indicate hopper plugging. In addition, damage
to hoppers and hopper insulation should be noted and corrected because of the
danger of hopper bridging and corrosion.
108
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Figure 58. Internal shaker mechanism.
If a fabric filter is on the negative-pressure side of a fan, fabric
filter hatches should be checked for inleakage. This can generally be de-
tected by the "whistling" of air rushing in past the door and gasket. Al-
though minor inleakage may cause no problems, steps should be taken to avoid
cake release, moisture, and corrosion problems normally associated with air
inleakage.
The previous steps provide some information, but the only reliable method
of evaluating fabric filter performance is internal inspection. Safety is a
prime concern in performing an internal inspection. The baghouse shell is a
confined area containing asphyxiants (CO and C02), little oxygen (less than
19.5% oxygen), and toxic substances (lead and H2$). Generally, special equip-
ment is required to enter these baghouses. Section 5.2 provides more informa-
tion on safety considerations.
Upon entering the fabric filter from the clean side, an inspector should
look for evidence of air inleakage past the door gasket. Any dust deposits on
the floor should be noted. Their presence may indicate bag leaks, and deposit
patterns may aid in the location of these leaks. The location of any bags out
of position (dropped) should be noted. If applicable, the shaker mechanism
should be operated to determine proper and effective shaking action. Cake
release from the bags should be checked. It may be desirable to operate the
fabric filter compartment through a cleaning cycle and to enter the compart-
ment just after cleaning. This helps an inspector assess the cleaning sys-
tem's effectiveness. Pulse-jet fabric filters typically require entry into
both the clean side and dirty side of the fabric filter to complete the
assessment (Figure 59).
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Figure 59. Cake release from bag.
The actual internal inspection takes little time. The majority of the
time required is used in preparing equipment and in isolating and opening
compartments. The location and nature of all bag failures should be recorded
for maintenance schedule optimization. The use of a fluorescent dye and a
portable UV light may be helpful in identifying problems. Pinholes and bag
seal leaks are easily spotted. Figure 60 indicates the steps for internal and
external inspections of fabric filters.
Wet Scrubbers—
Inspection of a wet scrubber relies heavily on baseline values because
internal inspection of the equipment is not likely. An inspector must attempt
to quantify any change in efficiency based upon a change in the water flow
rate, pressure drop, gas volume (throat velocity), or gas temperature. The
only other parameter that can significantly affect scrubber performance is a
shift in the particle size distribution. Such a shift is difficult to mea-
sure, but generally decreases collection efficiency, when the shift is to
smaller sizes, when all other parameters are held constant.
When examining a scrubber, an inspector should determine whether all
pumps are operating and valves are open. Water flow indicators are usually
provided to show water flow in addition to pressure gauges. Although water
pressure at the scrubber nozzles is important, pumps can produce a line pres-
sure with little or no water flow even when nozzles are completely or par-
tially plugged.
110
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EXTERNAL INSPECTION
INTERNAL INSPECTION
CHECK PRESSURE DROP ACROSS
EACH COMPARTMENT.
i
CHECK CONDITION OF LINES
AND PRESSURE GAUGES.
1
CHECK CLEANING SYSTEM
EQUIPMENT:
PULSE-JET MECHANISM
SOLENOIDS
REVERSE-AIR BLOWERS
SHAKERS
1
CHECK SOLIDS REMOVAL
EQUIPMENT:
SCREW CONVEYOR
PNEUMATIC SYSTEM
HEATERS
VIBRATORS
INVESTIGATE ANY OTHER
INDICATIONS OF NONOPTIMAL
PERFORMANCE.
CHECK BAGS:
BAG TEARS
BAG DETERIORATION
DROPPED BAGS
OILY BAGS
WET BAGS
IMPROPER BAG TENSION
DEPOSITS ON FLOOR
I
CHECK CLEAN AIR CHAMBER
FOR POSSIBLE LEAKS.
I
CHECK HOPPERS FOR INCOM-
PLETE SOLIDS REMOVAL AND
FOR CORROSION.
Figure 60. Steps for external and internal inspection of fabric filters.
Ill
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The inlet and outlet gas temperatures should be measured. There should
be a slight change in gas temperature even though the inlet gas temperature is
relatively low. Scrubber liquor conditions (i.e., pH and temperature) should
be measured. A sample of the scrubber liquor should be periodically obtained
to determine solids content. In addition, all makeup and bleedoff line water
flows should be obtained and recorded.
The scrubber pressure drop is usually the primary indicator of a change
in scrubber operation. A decrease in pressure drop may indicate a decrease in
gas volume or water flow rate, whereas an increase may indicate the opposite.
When pressure drop information is combined with data on gas volume and water
flow rate to calculate the L/6 ratio and throat velocity, the results indicate
any shift of performance.
The main problem with scrubbers in the secondary lead industry has been
high suspended solids content. These scrubbers are typically designed with a
continuous recirculation system, a settling pond, and makeup water for evapo-
rative losses only. Over a period of time, solids become more concentrated in
the scrubber liquor, cause nozzles to erode or plug, and increase pump wear.
Nozzle erosion generally results in inefficient injection of the water and may
result in a maldistribution of liquor to the scrubber throat. Nozzle pluggage
is more likely than nozzle erosion. Although liquor flow may remain at normal
levels, pressure drop across the scrubber decreases, and outlet gas tempera-
tures may rise. Plugged nozzles must be found by examining each nozzle. An
excessive amount of suspended solids may also cause pump impeller wear, which
reduces the liquor flow rate to the scrubber. This is usually characterized
by a decrease in pressure drop, as well as a lower flow rate to the scrubber.
Because of the low gas stream temperature, other problems with liquor
solids content and evaporation are not typically encountered. Corrosion is
generally limited because of construction practices (e.g., the use of 316
stainless steel). Little day-to-day variation in scrubber operation is ex-
pected.
J
5.1.3 Guidelines for Ventilation System Inspection
For inspection purposes, the ventilation system is defined as the duct-
work leading from the emission points to the control devices. Static pressure
taps are recommended throughout the length of ductwork to provide data on air
inleakage and ductwork plugging.
The ventilation system inspection should include routine measurement of
the,temperature arid static pressure at the furnace or kettle hood outlet, at
the afterburner outlet (if applicable), at the inlet and outlet of the cooling
loops, and at some point downstream of dilution air dampers. These parameters
vary with production rate^ and ambient temperature and operating logs should
be examined to determine long-term baseline values, which take into account
normal process variations.
Two major problems with ventilation systems are duct plugging and ex-
cessive air inleakage, both of which cause changes in the static pressure and
112
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temperature profiles of the ductwork system. An inspector should carefully
examine the ventilation system to identify the problem. Duct blockage is
characterized by a large increase in static pressure between the blockage and
fan. Blockage results in reduction of hood face velocity and causes failure
of the hooding to capture fugitive emissions. It has been typically observed
in ductwork bends and cooling loops. Part of this problem may be attributable
to insufficient duct velocity (improper duct sizing), allowing sedimentation
of dust particles. Most of this problem, however, is caused by excessive
cooling of the gas stream, which tends to change the particulate matter into
sticky particles that deposit in the duct.
Air inleakage may be a major contributing factor to duct blockage by
lowering the gas temperature. Although air inleakage may not occur at a
single point, it is characterized by lower static pressures and lower tem-
peratures downstream of the inleakage points. As is the case with duct
blockage, excessive inleakage may cause fugitive emissions because of a de-
crease in collection efficiency at the emission point.
In addition to the measurement of static pressure and temperature at the
emission point, the face velocity and positioning of all fugitive hooding
should be checked. Improper positioning of hooding or hood damage may result
in the reduction of capture efficiencies. Hood face velocities of 76 to 406
cm/s (150 to 800 ft/min) are typical for this application. In addition to
these face velocities, a negative pressure of at least 24 to 49 Pa (0.1 to 0.2
in. HoO) should be maintained at fugitive and process emission points to ac-
commodate any surges in gas volume and emissions. Lower duct static pressures
indicate an undersized or underperforming fan.
5.2 SAFETY CONSIDERATIONS
Special procedures and safety equipment are required to perform the in-
spection and maintenance procedures discussed in this document. Personnel and
inspectors must be protected from potentially hazardous and toxic substances
and alerted to potential hazards.
Although the facility provides safety equipment and training required by
regulations of the Occupational Safety and Health Administration (OSHA) or of
state and local agencies, the inspector or individual worker is personally
responsible for following prescribed safety procedures and precautions to
ensure personal safety. This safety is a combination of knowledge of poten-
tial hazards and "common sense" to avoid unnecessary risks. Items of typical
safety equipment for a worker in the secondary lead industry include a hardhat,
safety glasses or goggles, steel-toed workshoes, disposable work clothes, and
a filter mask approved by the National Institute for Occupational Safety and
Health (NIOSH) or the Mine Safety and Health Administration. Because OSHA
regulations require that a worker change from street clothes to work clothes
when entering a secondary lead facility and from work clothes to street
clothes when leaving the facility, exposure to lead is minimized for both the
worker and other members of the worker's household (Figure 61).
113
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Figure 61. Inspector in work clothes, hardhat,
and self-contained breathing unit.
Typical safety equipment items are adequate only for an external inspec-
tion of control equipment. An internal inspection requires additional equip-
ment, including devices for supplying uncontaminated oxygen to an inspector
and for monitoring gas conditions to determine flammable gas mixtures. Most
secondary lead facilities use fabric filters, therefore this discussion
focuses on safe entry into a fabric filter. An inspector must be protected
from high temperatures, from potentially lethal doses of H2S and CO, and from
high concentrations of lead.
Internal inspection of an operating fabric filter requires the isolation
of each compartment from the incoming gas stream prior to entry by personnel.
Gas temperature should be measured. The maximum allowable temperature for
safe entry is generally considered to be 49°C (120°F). If necessary, purge
air should be used to cool the fabric filter compartment and to reduce CO and
concentrations.
Prior to entry by an inspector, the oxygen and H?S concentrations in each
compartment should be measured. Combustible levels of CO are not common be-
cause of the large quantity of dilution air added at the charging area, but it
is advisable to check for CO also. If the oxygen content is below 19.5
114
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percent, either air-line respirators or self-contained breathing units must be
used. In addition, H2S levels above 1 to 3 parts per million represent a
potential hazard to personnel. Although H2S has a characteristic "rotten egg"
odor at low concentrations, personnel may, due to olfactory fatigue, unknow-
ingly wander into an area where lethal concentrations of H2S have built up.
An instrument or indicator tube should be used to test for the presence of F^S
although these units may yield inaccurate readings due to chemical inter-
ferences.
In no instance should a confined entry be performed alone. At least two
persons should be involved during confined entry (one for actual entry and one
for rescue). The person designated for rescue must be equipped for entry at
all times while the other person is inside the confined area. This means that
all safety equipment should be donned and operational. All equipment entry
and lockout procedures should be observed. The inspection of each compartment
should be rapid and efficient. The inspector should spend only the minimum
amount of time in each compartment required for the internal inspection.
Under no circumstances should the inspector exceed the allowable time dictated
by his breathing unit or other safety practice.
Air-line respirators or self-contained breathing units should be used
when the facility is operational and when a compartment has been isolated for
inspection or on-line maintenance (because isolation dampers tend to leak).
Entry during shutdown periods (after the fabric filter has been purged and is
cool) may not require respirators or breathing units, although proper tests
must still be performed prior to entry. Conditions at each facility dictate
the steps required for safe entry into control equipment. The choice of air-
line respirators or self-contained breathing units depends on economics and
convenience. Although an inspector remains attached to an external air supply
hose, air-line respirators tend to be less bulky and less restrictive in tight
fabric filter passages than self-contained units. On the other hand, self-
contained units do not restrict movement with an external hose. Because both
air-line respirators and self-contained units put additional strain on the
cardiovascular and pulmonary systems, inspectors should have a physical ex-
amination before using such devices. Under no circumstances should such
devices be used for more than 4 hours and 40 minutes.
The foregoing is an extremely basic discussion of safety requirements and
should not under any circumstances be considered complete. This section is
intended only to alert the inspector to the problem and to indicate the need
to acquire detailed information from his own Safety Officer and to consult
with the safety and other personnel of the plant visited regarding proper
safety practice for each installation.
115
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GLOSSARY
Babbitt Metal - Either of two alloys used for lining bearings such as (1) a
lead-base alloy containing 1 to 10 percent tin and 10 to 15 percent
antimony, with or without some arsenic; or (2) a tin-base alloy, espe-
cially one containing 2 to 8 percent copper and 5 to 15 percent antimony.
Dross - The scum that forms on the surface of molten metals largely because of
oxidation, but sometimes because of the rising of impurities to the
surface.
"Hard" Lead - Lead alloy in which the high degree of malleability characteris-
tic of pure lead ("soft" lead) is taken away by the presence or the
addition of alloying agents or impurities, of which antimony is the most
common.
Hog - A crude lead casting, from the smelting process that typically weighs
from 680 to 1360 kg (1500 to 3000 Ib).
Launder - A trough channel or gutter for the conveyance of molten lead from
the lead well in the furnace into a water-cooled mold or a holding
kettle.
Pig or Ingot - A casting of metal (as lead or iron) convenient for storage,
transportation, or melting.
Rerun Slag - A highly silicated slag tapped from previous blast furnace runs
that is recycled as a blast furnace feed material.
Sow - A crude lead casting from the smelting process that typically weighs
about 227 kg (500 Ib).
Sweating - A physical separation of metals based upon melting points, used to
recover lead from scrap such as radiators, cables, and bearing housings.
Tuyere - A tube or opening, passing through the refractory lining of a blast
furnace, through which air is blown as part of the smelting process.
116
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TECHNICAL REPORT DATA
(Please rcaJ Imirucltt'iis on the reverse before cvmplelingi
1. REPORT NO.
2.
3. RECIPIENT'S ACCESSION NO.
. TITLE AND SUBTITLE
Inspection and Operating and Maintenance Guidelines
for Secondary Lead Smelters
5. REPORT DATE
6. PERFORMING ORGANIZATION CODE
7. AUTHOR(S)
B. PERFORMING ORGANIZATION REPORT NO.
9. PERFORMING ORGANIZATION NAME AND ADDRESS
PEDCo Environmental, Inc.
11499 Chester Road
Cincinnati, Ohio 45246
10. PROGRAM ELEMENT NO.
1AB604
11. CONTRACT/GRANT NO
68-03-2924
12. SPONSORING AGENCY NAME AND ADDRESS
Industrial Environmental Research Laboratory
Office of Research and Development
U.S. Environmental Protection Agency
Cincinnati, Ohio 45268
13. TYPE OF REPORT AND PERIOD COVERED
Final
14. SPONSORING AGENCY CODE
EPA/600/12
15. SUPPLEMENTARY NOTES
16. ABSTRACTThe prevention of emissions from secondary lead smelters depends upon the
procedures implemented to achieve initial compliance and remain in a state of
continuing compliance with applicable emission limitations. The ability to remain in
continuing compliance depends largely on operation and maintenance practices. This
manual was developed as an inspection manual incorporating operation and maintenance
information for secondary lead smelting. It presents an overview of secondary lead
operations, describes typical emission problems associated with the material prepara-
tion, smelting, refining and oxidation processes and reviews the potential causes of
the problems and possible corrective measures. It also describes the types of air
pollution control equipment used in secondary lead smelting operations and typical
O&M problems experienced with this equipment.
The manual is heavily oriented towards an inspection approach emphasizing
techniques to achieve improvements in the status of continuing compliance through
operations and maintenance procedures. It has been written for use both as an
educational and reference tool by state and local enforcement field inspectors and
entry-level engineers whose familiarity with secondary lead operations may be limited,
and, as such, can be useful both as a training manual and as a guidebook during field
inspections.
17.
KEY WORDS AND DOCUMENT ANALYSIS
DESCRIPTORS
b. IDENTIFIERS/OPEN ENDEDTERM5
c COSATI Iicld'Group
18. DISTRIBUTION STATEMENT
Release to Public
19, SECURITY CLASS fTlns Report/
Unclassified
21. NO.- OF PAGES
125
20. SECURITY CLASS (This page/
Unclassified
22. PRICE
EPA Form 2220-1 (R«». 4-77) PREVIOUS EDITION is OBSOLETE
117
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