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ALKALINE CLEANING SUBCATEGORY
SECTION X
EFFLUENT QUALITY ATTAINABLE THROUGH THE
APPLICATION OF THE BEST AVAILABLE
TECHNOLOGY ECONOMICALLY ACHIEVABLE
Introduction
As noted earlier, the toxic metals contained in alkaline cleaning
process wastewaters are found at average levels of less than 0.15
mg/1. The discharge of these metals can only be reduced through waste
volume reduction techniques including recycle and counter current
rinse systems. Accordingly, the Agency considered two BAT model
treatment systems, both of which incorporate 90% recycle. The
blowdown from the recycle system would be further treated by
filtration in BAT Alternative 1 and by vapor compression distillation
in BAT Alternative 2. However, because the Agency could not find any
direct recycle of alkaline cleaning wastewaters or counter-current
rinse systems, and no significant quantities of toxic pollutants are
present in these wastewaters, the Agency did not promulgate BAT
limitations based upon these syterns.
BAT Alternatives
The BAT alternative treatment systems evaluated include recycle
systems to reduce the BPT model flows of 250 gal/ton and 350 gal/ton
for batch and continuous operations to 25 gal/ton and 35 gal/ton,
respectively. Vapor compression distillation systems to achieve zero
discharge by evaporating, condensing, and reusing the effluent from
the recycle systems described above constitute the second alternative.
Figure VIII-1 illustrates the two BAT treatment systems evaluated.
The effluent volumes and quality that could be achieved by these
systems are as follows:
Flow - (gal/ton)
Effluent
Concentration (1 )
mq/1
BAT Alternative
Batch
25
Continuous Lead
35
0.1
Zinc
0.1
(1) Long term average; batch or continuous
399
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Selection of BAT
The Agency has determined that alkaline cleaning wastewaters do not
contain significant quantities of toxic pollutants after compliance
with applicable BPT limitations. Accordingly, since the BPT level of
treatment provides adequate control, the Agency has not promulgated
more stringent BAT limitations.
400
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ALKALINE CLEANING SUBCATEGORY
SECTION XI
BEST CONVENTIONAL POLLUTANT CONTROL TECHNOLOGY
Introduction
The 1977 Amendments added Section 301(b)(2)(E) to the Act establishing
"best conventional pollutant control technology" (BCT) for discharges
of conventional pollutants from existing industrial point sources.
Conventional pollutants are those defined in Section 304(a)(4)
[biochemical oxygen demanding pollutants (BOD5), total suspended
solids (TSS), fecal coliform, and pH] and any additional pollutants
defined by the Administrator as "conventional" (oil and grease, 44 FR
44501, July 30, 1979).
BCT is not an additional limitation but replaces BAT for the control
of conventional pollutants. In addition to other factors specified in
section 304(b)(4)(B), the Act requires that BCT limitations be
assessed in light of a two part "cost-reasonableness test. American
Paper Institute v. EPA, 660 F.2d 954 (4th Cir. 1981). The first test
comparesthecost for private industry to reduce its conventional
pollutants with the costs to publicly owned treatment works for
similar levels of reduction in their discharge of these pollutants.
The second test examines the cost-effectiveness of additional
industrial treatment beyond BPT. EPA must find that limitations are
"reasonable" under both tests before establishing them as BCT. In no
case may BCT be less stringent than BPT.
EPA published its methodology for carrying out the BCT analysis on
August 29. 1979 (44 FR 50732). In the case mentioned above, the Court
of Appeals ordered EPA to correct data errors underlying EPA s
calculation of the first test, and to apply the second test. (EPA had
argued that a second cost test was not required.
Because of the remand in American Paper Institute v^. EPA (No. 79-115),
the Agency did not promulgate BCT limitations except for those
operations for which the BAT limitations are no more stringent than
the respective BPT limitations. Alkaline cleaning is one of the
has
the respective
subcategories where BAT was promulgated equal to BPT. The Agency
concluded that BCT limitations more stringent than BPT are not
appropriate. No additional cost for compliance with BCT is
anticipated'in this subcategory.
401
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ALKALINE CLEANING SUBCATEGORY
SECTION XII
EFFLUENT QUALITY ATTAINABLE THROUGH THE
APPLICATION OF NEW SOURCE PERFORMANCE STANDARDS
Introduction
NSPS are to represent the degree of effluent reduction achievable
through the application of the best available demonstrated control
technology (BDT), processes, operating methods, or other alternatives,
including, where practicable, a standard permitting no discharge of
pollutants. At this time, however, zero discharge is not a feasible
treatment alternative for the alkaline cleaning subcategory. As
discussed in Section VII, except for evaporative systems, there are no
technologies which could be applied to all operations in this
subcategory to attain zero discharge of process wastewater pollutants.
Evaporative technologies are energy intensive and not demonstrated in
this subcategory, or in this industry.
Identification of NSPS Alternatives
The Agency has selected two NSPS alternative treatment systems based
on the best flow (gal/ton) and the best treatment components
demonstrated in the alkaline cleaning subcategory.
A. NSPS Alternative 1
This treatment alternative is similar to the BPT model treatment
system and is shown in Figure VII1-2. The treatment components
include equalization with oil skimming, neutralization with acid,
and flocculation with polymer. Clarification provides solids
removal, followed by vacuum filtration for dewatering the sludge
collected in the clarifier.
B. NSPS Alternative 2
This treatment alternative includes the treatment alternatives
comprising NSPS-1 with the addition of filtration. This
alternative is also shown in Figure VII1-2.
The NSPS corresponding to these two alternatives are shown in Table
XII-1 Respective capital and annual costs for these alternatives
appear in Tables VII1-7 and VII1-8 for batch and continuous
operations, respectively.
Rationale for the Selection of. NSPS
The NSPS treatment alternatives include those components which achieve
the most significant removal of toxic and conventional pollutants.
403
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The Agency considered various other NSPS alternative treatment systems
including those which achieve zero discharge. However, these systems
were generally too costly. The rationale for the NSPS alternative
treatment systems and the flow and effluent concentrations follows.
Alternative Treatment Systems
Both NSPS treatment alternatives include standard chemical addition
and sedimentation. In addition, NSPS-2 includes filtration equipment.
All of these treatment components are well demonstrated in this and
other steel industry subcategories. Equalization is used to reduce
fluctuations in flow and pollutant concentrations, so that subsequent
treatment components will operate more effectively. Oil skimming is
provided to reduce any floating oils that may be present in the
wastewaters. Acid is added in a reaction tank to neutralize the pH of
the incoming wastewater to within the required range of 6.0 to 9.0.
The neutralization step is followed by polymer addition; polymer is
added to aid solids and metals removal. The polymer addition is
carried out in a mixing tank to provide proper contact between the
solids and the polymer.
After chemical addition, the wastewaters undergo preliminary
sedimentation prior to filtration. A clarifier is used in the
alternatives, since this unit will reduce suspended solids to a level
which will not interfere with the filtration equipment. Following
sedimentation the wastewaters are filtered to remove additional
particulate matter and oils. Filtration was chosen as a final step,
because it is demonstrated in the steel industry and because it is
effective at reducing the levels of solids, oils, and metals. The
cost estimates for the filtration system were based upon a multi-media
pressure filter. This type of filter is most often used in the steel
industry. However, other types of filtration systems can be used to
treat alkaline cleaning wastewaters.
Flows
Batch and Continuous Operations
A model discharge flow of 50 gal/ton for both batch and continuous
operations is the basis for the NSPS. This flow is demonstrated at
several batch and continuous operations. Seven batch operations
(approximately 26% of the batch operations submitting flow data) and
nine continuous operations (approximately 11% of the continuous
operations submitting flow data) demonstrate the model flow of 50
gal/ton. Table XII-2 presents a list of these plants and the
respective flow rates on a gallons per ton basis.
Pollutants
The Agency selected total suspended solids, oil and grease, and pH as
the pollutants to be limited at NSPS. Oil and grease was included to
provide control of the oils removed from the product in the alkaline
cleaning process. Also, oil and grease is limited in numerous steel
404
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finishing operations. Therefore, the addition of oil and grease to
the list of limited pollutants will facilitate the development of
combined standards for treatment systems. Finally, pH is limited to
ensure that the wastewaters are properly neutralized.
Effluent Concentrations
The alternative NSPS for the above treatment systems are presented in
Table XII-1. Refer to Sections IX and X for information concerning
clarification and filtration effluent concentration levels.
Selection of NSPS
The Agency selected NSPS Alternative 1 as the basis for NSPS. The
Agency has promulgated NSPS for alkaline cleaning operations based
upon the best demonstrated flows noted above, and7 in order to
facilitate co-treatment of new source alkaline cleaning wastewaters
with' wastewaters from other new source steel finishing operations,
effluent quality for total suspended solids and Oil and grease
consistent with those used to develop NSPS for other subcategories.
These standards are achievable by the model treatment technology.
(See discussion in Section IX and Appendix A of Volume I). Tn®s®
concentrations are the same as those used to develop the BPT
limitations for alkaline cleaning operations. The promulgated NSPS
are presented in Table XII-3. This table also lists plants-that
demonstrate the NSPS. The NSPS model treatment system is shown in
Figure XII-1.
405
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TABLE XII-1
NEW SOURCE PERFORMANCE STANDARDS
ALKALINE CLEANING SUBCATEGORY
NSPS-1
Discharge Flow
(Gal/Ton)
Total Suspended
Solids
Oil & Grease
pH, Units
Ave.
Max.
Ave.
Max.
Batch & Continuous Operations
Concentrat ion
Basis (mg/1)
30
70
10
30
Effluent
Standards
(kg/kkg of Product)
50
0.00626
0.0146
0.00209
0.00626
Within the range of 6.0 to 9.0
NSPS-2
Discharge Flow
(Gal/Ton)
Total Suspended
Solids
Oil & Grease
pH, Units
Ave.
Max.
Ave.
Max.
15
40
10
50
0.00313
0.00834
0.00209
Within the range of 6.0 to 9.0
406 '
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TABLE XII-2
OPERATIONS DEMONSTRATING THE NSPS
DISCHARGE FLOW RATE
ALKALINE CLEANING SUBCATEGORY
Model NSPS Flow: 50 GPT
Batch
Continuous
Plant Code
0060N-01
0060N-02
0240B-02
0240B-01
0240C
0728
Discharge Flow (GPT)
42
42
28
24
7
2
Plant Code
0112A-14
0112A-12
0112A-11
0112A-15
0584F-03
0584F-04
0584F-02
0584F-01
Discharge Flow (GPT)
15
13
12
12
6
3
2
1
NOTE: The flow data for confidential operations are not listed.
407
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TABLE XII-3
JUSTIFICATION OF NSPS
ALKALINE CLEANING SUBCATEGORY
30-Day Average NSPS (kg/kkg of Product)
All Operations
Operations Achieving
the NSPS
152 0176-01
156 01121-04
TSS
0.00626
0.00048
<0.00028
Oil & Grease
0.00209
**
0.0011
PH
6.0 - 9.0
7.2 - 7.9
7.3 - 7.7
**: Standard is not supported.
408
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ALKALINE CLEANING SUBCATEGORY
SECTION XIII
PRETREATMENT STANDARDS FOR
DISCHARGES TO PUBLICLY OWNED TREATMENT WORKS
Introduction
The Agency has not promulgated pretreatment standards for alkaline
cleaning operations. Instead, the General Pretreatment Regulations,
40 CFR Part 403, will apply. The general pretreatment and categorical
pretreatment standards applying to alkaline cleaning operations are
discussed below.
General Pretreatment Standards
For detailed information on Pretreatment Standards, refer to 46 FR
9404 et seq, "General Pretreatment Regulations for Existing and New
Sources of Pollution," (January 28, 1981). See also 47 FR 4518
(February 1, 1982). In particular, 40 CFR Part 403 describes national
standards (prohibited discharges and categorical standards), revision
of categorical standards, and POTW pretreatment programs.
In considering pretreatment standards for alkaline cleaning
operations, the Agency gave primary consideration to the objectives
and requirements of the General Pretreatment Regulations.
Rationale
As discussed throughout this report, toxic pollutants are present in
untreated alkaline cleaning wastewaters at levels below or near
treatability levels of course, the conventional pollutants will
receive comparable treatment in the POTW. Hence, the Agency has not
promulgated pretreatment standards for new or existing alkaline
cleaning operations.
411
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HOT COATING SUBCATEGORY
SECTION I
PREFACE
The USEPA has promulgated effluent limitations and standards for the
iron and steel industry pursuant to Sections 301, 304, 306, 307 and
501 of the Clean -Water Act. The regulation contains effluent
limitations guidelines for best practicable control technology
currently available (BPT), best conventional pollutant control
technology (BCT), and best available technology economically
achievable (BAT) as well as pretreatment standards for new and
existing sources (PSNS and PSES) and new source performance standards
(NSPS).
This part of the Development Document highlights the technical aspects
of EPA's study of the Hot Coating Subcategory of the Iron and Steel
Industry. Volume I of the Development Document addresses general
issues pertaining to the industry while other volumes contain specific
subcategory reports.
413
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HOT COATING SUBCATEGORY
SECTION II
CONCLUSIONS
Based upon this current study, a review of previous studies/ and
comments received on the regulation proposed on January 7, 1981 (46 FR
1858), the Agency has reached the following conclusions.
1. The Agency has established separate limitations for rinse water
discharges and discharges from fume scrubbers for hot coating
operations. The original subdivision of this subcategory is
being retained for rinsewater discharges. A separate subdivision
has been established for fume scrubber discharges.
2. The limitations for hot coating operations contained in the 1976
regulation were applicable to galvanizing and terne-coating
operations only. This regulation contains limitations for
galvanizing, terne, and hot coating operations applying other
metals. Coating metals identified as part of the other metals
subdivision include aluminum, lead, and tin, along with
combinations of these metals or combinations with zinc.
3. The Agency concluded that the model -wastewater flow rates used to
develop the previously promulgated BPT limitations for the strip,
sheet and miscellaneous products subdivision are appropriate
irrespective of the type of coating applied. The model
wastewater flow rates for operations coating nails, fasteners and
wire products were increased to reflect the larger data base
available to the Agency during this study. :
4. The concentration basis for the effluent limitations has been
revised for all pollutants to reflect additional effluent data
acquired as part of this study. Except for hexavalent chromium,
which remains unchanged, the concentration bases are more
stringent than those used to develop limitations contained in the
1976 regulation.
5. An allowance for fume scrubber wastewaters has again been
provided. The Agency has concluded that the applied flow rates
for fume scrubbers are not related to product type, production
rate, or air flow through the scrubber. Therefore, daily mass
limitations (kg/day) have been promulgated for fume scrubbers.
These limitations are to be added to the limitations for the
rinsewaters, where fume scrubbers are installed.
6. The Agency has promulgated _BPT effluent limitations for total
suspended solids, oil and grease, pH, lead, and zinc for all hot
coating subdivisions. Limitations for hexavalent chromium have
also been promulgated for those galvanizing lines that include
415
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chromate dip and rinse steps. The Agency believes that these
limitations will control the discharges of other toxic pollutants
found in hot coating wastewaters.
The Agency promulgated BAT limitations for toxic pollutants
(lead/ zinc and hexavalent chromium) that are the same as the BPT
limitations for the subdivisions covering rinsewater discharges.
The Agency found that conventional rinsewater flow reduction
methods may not be appropriate for all coating operations, and
that technologies evaluated for toxic metals removal beyond that
provided by the model BPT treatment systems either provide only
marginal incremental removal or cannot be readily retrofitted at
all existing operations. For the fume scrubber subdivision, the
promulgated BAT limitations are 15 percent of the corresponding
BPT limitations. These limitations are based upon an 85 percent
reduction in fume scrubber wastewater discharge achieved through
recycle.
The Agency has promulgated BCT limitations for conventional
pollutants (TSS and oil and grease) which are the same as the BPT
limitations for these pollutants.
A summary of the effluent loadings remaining after implementation
of BPT, BCT, BAT and PSES follows:
Direct Discharge Loadings (Tons/Yr)
Raw Waste BPT/BCT BAT
Flow, MGD
TSS
Oil and Grease
Toxic Metals
22.9
2,658
1,060
1,829
22.8
588
109
12.2
18.3
471
87.0
9.8
Indirect Discharge Loadings (Tons/yr)
Raw Waste PSES
Flow, MGD
TSS
Oil and Grease
Toxic Metals
7.5
612
217
269
5.6
142
26.3
3.0
10. Based upon facilities in place as of July 1, 1981, the Agency
estimates the following costs to the industry will result from
compliance with the BPT and BAT limitations and PSES for the hot
coating subcategory. The Agency has determined that the effluent
reduction benefits associated with compliance with the effluent
limitations and standards justify the costs presented below:
416
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BPT
BAT
PSES
Total
33.7
0.87
5.05
Costs (Millions of July 1, 1978 Dollars)
Investment Costs Annual Costs
In-Place
29.1
0.36
2.68
Required
4.60
0.51
2.37
The Agency has also determined that the
associated with compliance with new
justify those costs.
Total
5.07
0.12
0.74
In-Place Required
4.31
0.05
0.39
0.76
0.07
0.35
effluent reduction benefits
source standards (NSPS, PSNS)
11. The Agency has promulgated NSPS that are 25 percent of the BPT
and BAT limitations for the subdivisions covering rinsewater
discharge. These standards are based upon the same model
treatment system, except that rinsewater discharges are reduced
by use of cascade rinsing. NSPS for the fume scrubber
subdivision are the same as the corresponding BAT limitations.
12. The Agency has promulgated pretreatment standards covering new
and existing sources (PSNS and PSES) that discharge wastewaters
to POTWs. The PSES are the same as the BAT limitations, while
the PSNS are the same as the NSPS. The standards are based upon
the same model treatment systems.
13. With regard to the remand issues, the Agency found
to hot coating operations that:
with respect
c.
Age does not significantly affect either the cost or the
ability to retrofit pollution control equipment to existing
production facilities. The Agency did, however, find that
it may not be feasible to retrofit cascade rinse systems at
all existing hot coating lines, because of configuration and
space limitations.
Its estimates of the cost of installing the model wastewater
treatment systems are sufficient to cover site-specific
conditions. The Agency compared its model based cost
estimates with actual costs reported by the industry. The
comparison showed that the Agency's cost estimates exceeded
the reported costs by 49 percent. The costs provided by the
industry included site specific and retrofit costs. Hence,
the Agency concludes that its model-based cost estimates are
sufficient to cover site-specific and retrofit costs. For
more detail on cost comparisons refer to Section VIII.
The impact of these limitations and standards upon water
consumption is insignificant. The recycle components of the
model treatment systems do not elevate the temperature of
the water to the point where evaporation becomes
significant.
417
-------
14. The Agency received comments from a small segment of the industry
suggesting that limitations should be based upon the basis of a
load per surface area coated rather than on load per production
weight basis. The Agency found that the available surface area
data was insufficient, since such records are not usually kept by
the industry and the Agency does not have an adequate data base
to develop limitations and standards on the basis of surface
area. Moreover, the Agency believes that its method of
establishing limitations and standards on the basis of quantity
of product (kg/kkg) is appropriate.
15. Table II-l presents the treatment model flow and effluent quality
data used to develop the BPT and BCT effluent limitations for the
hot coating subcategory, and Table I1-2 presents these
limitations. Table I1-3 presents the treatment model flow and
effluent quality data used to develop the BAT effluent
limitations and the NSPS, PSES, and PSNS for the hot coating
subcategory. Table II-4 presents these limitations and
standards.
418
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HOT COATING SUBCATEGORY
SECTION III
INTRODUCTION
Background
The Hot Coating Subcategory has been modified to include galvanizing,
terne coating, and hot coating with other metals. The prior
regulation (41 FR 12990) limited only galvanizing and terne coating
operations.
The originally promulgated limitations were primarily based upon data
obtained through field sampling at six hot coating facilities. This
study includes field sampling at two of the same plants and five
additional hot coating operations. In addition, an overall review of
flow and wastewater treatment components used at the hot coating
plants surveyed by basic data collection portfolios (DCPs) was
completed. Summaries of the responses to these DCPs are shown as
Table III-l for galvanizing operations, Table III-2 for terne coating
operations, and Tab,le 111-3 for hot coating operations which apply
aluminum, cadmium, lead, tin or combinations of these metals with
zinc. These tables identify products, coatings, ages, sizes,
operating modes, applied and discharged wastewater flows, control and
treatment technologies, and ultimate discharge mode for each hot
coating production line for which data have been received.
Ninty-eight percent of the responses contained sufficiently detailed
data for use in these summaries. The remaining lines were either
inactive at the time of the request, or were .being phased out.
DCP responses were solicited from about five-sixths of the domestic
hot coating line operators which represents 97 percent of the nation's
hot coating capacity. The Agency's data collection effort focused on
acquiring data from the ten largest steel companies, from selected
other companies known to have wastewater treatment systems in place,
and, from a representatve group of the smaller operators. This
approach has provided data on lines as small as 525 pounds per turn
and as large as 940 tons per turn. The largest steel corporation in
the country provided data for 28 hot coating lines varying in size
from a 1.8 ton per turn wire coating line to a 321 ton per turn
continuous strip and sheet galvanizing operation. The Agency is
confident that the DCP responses are representative of all hot coating
operations, including those plants not solicited for data. Following
a review of the DCP responses, detailed data collection portfolios
(D-DCPs) requesting information on existing wastewater treatment
practices, and cost and effluent data were forwarded to nine
operations, including one operation which was previously sampled.
Overall, field sampling covered 14% of the plants with annual
capacities totaling about 17% of the estimated domestic hot coating
capacity. Detailed pollutant concentration and load data as well as
423
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cost data were sought from plants accounting for an additional 17% of
the national production capacity, and basic data were requested for
plants comprising 97% of national capacity. Table II1-4 summarizes
the data base for the entire hot coating subcategory.
The Agency obtained both in-process and end-of-pipe samples during the
field sampling visits. Data for raw wastewater and effluent
characteristics, water use and cost information supplied for
individual plants from historical records were also obtained during
such visits. NPDES permit application data were of limited value for
the purposes of this study since most of these data are for outfalls
serving more than one operation. However, NPDES self monitoring data
for selected plants with well designed and operated treatment
facilities were evaluated to characterize the performance of the model
wastewater treatment systems.
The alternative treatment systems and effluent limitations were
derived from available data for the actual performance of existing
plants. Other plants were reviewed for demonstrated technologies
which, together with field sampling data, provide the basis for
various BAT, BCT, NSPS, PSES, and PSNS treatment systems.
Descriptions of Hot Coating Operations
Hot coating processes in the steel industry involve the immersion of
clean steel into baths of molten metal for the purpose of depositing a
thin layer of the metal onto the steel surface. These coatings
provide desired qualities, such as resistance to corrosion, safety
from contamination, or a decorative bright appearance. Finished
products retain the strength of steel while gaining the improved
surface quality of the coated metal for a fraction of the cost of
products made entirely of that metal alone.
All methods for applying protective coatings to steel products require
careful attention to proper surface preparation - the primary and most
important step in the coating process. Without proper surface
preparation, good adhesion is impossible. Surface preparation methods
vary depending upon the type of coating applied and upon the shape of
the surface being coated, but all methods aim at cleanliness and
uniformity of the surface. The most common methods used are acid
pickling to remove scale or rust, alkaline or solvent cleaning to
remove oils and greases, and physical desurfacing with abrasives to
eliminate surface imperfections.
The two major classes of .metallic coating operations in the industry
are hot coating and cold coating. Zinc, terne, and aluminum coatings
are most often applied from molten metal baths, while tin and chromium
are usually applied electrolytically from plating solutions. Cold
Coating operations are being addressed separately as part of the Metal
Finishing Industry.
424
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Hot Coating
Hot-dipped coating using baths of molten metal is practiced in the
steel industry as a batch-dip operation for sheet, plate, pipe or
other pre-formed products, or on a continuous basis for coiled wire or
strip and sheet. Operating processes vary, depending on the coating
being applied. Refer to Figures III-l, III-2 and III-3 for typical
process flow diagrams for galvanizing (zinc) coating operations, and
to Figures III-4 and III-5, for terne coating and aluminizing process
flow diagrams. While aluminum is shown as an example of other metal
coating, similar processes are used for the cadmium, lead, hot-dipped
tin, and mixtures of various metals. Other coating lines may also be
batch dip or continuous operations.
Galvanizing
The batch-dip operation normally follows hot rolling, batch annealing,
cold rolling, and pre-forming or sizing operations. Rolling
lubricants are removed by alkaline cleaning, and final surface
preparation is usually provided by mild acid pickling in stationary
tubs with slight agitation. Following pickling, residual acid and
iron salts are removed by an alkaline dip, water rinsing, or prolonged
immersion in boiling water. The latter practice has the added
advantage of minimizing hydrogen embrittlement- Clean base metal
forms are then conveyed, manually or by moving belt,-through the flux
box section of the coating pot, and immersed in the molten metal.
Coated products are withdrawn from the bath and dried by a warm air
blast, or chemically treated with ammonium chloride, sulfur dioxide,
chromate or phosphate solutions to provide special finishes and
surface characteristics. The product may then be rinsed with water
and prepared for shipment.
Continuous hot-dip galvanizing accounts for more than 60% of total
galvanizing production. The simplest version starts with annealed and
tempered steel which receives a mild muriatic acid (HC1) pickle and
rinse, then proceeds directly through a layer of fluxing agent to the
molten zinc bath. The coated product is dried and recoiled, or cut to
size for shipment. More elaborate continuous galvanizing lines
include additional stages preceding and following the hot-dip step.
At least one strip galvanizer incorporates a sequence of pickling in
hot sulfuric acid; rinsing and scrubbing with brushes; a dip into a
hot alkaline cleaning solution; scrubbing in alkaline solutions; an
electrolytic hot alkaline cleaning step, rinsing and scrubbing with
brushes; a light pickle in hot sulfuric acid; rinsing and scrubbing
with brushes; a dip into a hot zinc sulfate flux bath; a hot dip into
molten zinc; dip and spray with chromate or phosphate solutions; a
final water rinse; drying with hot air; and recoiling.
Other producers use a so-called "furnace line" to .anneal the steel
product prior to coating with zinc. Without annealing, incoming coils
to hot coating operations are very hard following cold reduction.
Furnace line operators include annealing as follows: cold rolled coils
are given a hot alkaline cleaning, rinsing, and scrubbing; and
425
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pickling in hot acid followed by water rinses. The strip is then
placed in a controlled atmosphere heating chamber (annealing furnace)
up to 60 meters (200 ft.) in length with a series of independently
controlled heat zones to provide temperatures required for annealing,
yet sufficient cooling so that strip exits the furnace at temperatures
slightly above the molten bath temperature. A mixture of NX gas
(principally nitrogen, with controlled amounts of methane, carbon
monoxide, and carbon dioxide) and cracked ammonia is used in some
annealing furnaces to prevent oxidation and decarburization. The
strip is discharged from the exit end of the furnace below the surface
of the molten zinc bath. A sinker roll submerged near the surface of
the zinc bath is used for controlling the thickness and distribution
of the coating. Forced air blasts are used to cool the exiting strip
and to help solidify the zinc coating. Chromate or phosphate chemical
treatments may be provided at this point to retard formation of white
corrosion products on« the coating. A final rinse and drying step may
also follow. Finished coated strip is recoiled or cut to size ready
for shipment.
Another type of furnace line subjects cold rolled strip to a complex
furnace gas containing hydrogen chloride. After annealing and
cooling, a mild hydrochloric acid pickling is completed just prior to
the flux section of a conventional molten zinc pot. In place of the
usual exit rolls for controlling coating thickness, flexible wipes are
used to yield very thin, but extremely adherent zinc coatings.
Terne Metal
Terne is an inexpensive, corrosion-resistant hot-dipped coating
consisting of lead and tin in a ratio typically in the range of five
or six to one. Lead alone does not alloy with iron, but does form a
cohesive solution with tin, which in turn alloys readily with iron,
although requiring higher temperatures than for tin alone. Most of
the terne coated material is used in the automobile industry to
manufacture gasoline tanks, with lesser amounts going into the
production of automotive mufflers, oil pans, air cleaners, and
radiator parts. Other end products made of terne metal include
roofing materials, portable fire extinguishers, and burial caskets.
As in the case of hot-dipped galvanizing processes, both batch and
continuous terne coating processes are used, although the continuous
process is used to supply by far the larger portion of the market.
Both metals used in terne coating are corrosion-resistant, as is their
combination. But since both lead and tin are cathodic to iron in most
environments, corrosion is actually accelerated if any portion of the
base metal is exposed. For this reason, terne coatings are usually
thicker than other metallic coatings. For maximum corrosion
resistance, even the thickest terne coatings benefit from painting or
other protective finishing.
The batch-dip terne coating operation normally is performed on cold
reduced, batch annealed, and temper rolled coils cut into sheets.
Oils and greases are removed by alkaline or solvent (mineral spirits)
426
-------
cleaning, and final surface preparation requires a hydrochloric acid
dip just prior to coating. Excess acid is squeezed from the sheets by
rubber rolls. The sheets are then conveyed through a flux box
containing a hot solution of zinc chloride in hydrochloric acid, or a
molten zinc chloride salt bath to remove residual iron oxides and to
provide dry steel surface. The sheets are then passed downward
through a molten terne metal bath maintained at 325°C to 360°C <6170F
to 680°F), where the coating is applied, then upward through an oil
bath floating atop the terne pot. This oil tends to maintain the high
temperature long enough for oil rolls to control deposition and
coating thickness evenly over the sheet surfaces. Although most
batch-dipped terne coatings use a single unit as described above, the
wider range of coating weights sometimes requires a pass through a
second molten metal bath of the same type, but including another oil
bath instead of the zinc chloride flux box prior to the application of
the second coat.
The steel strip fed to a continuous terne coating operation receives
the same preliminary treatment as the steel processed on the batch-dip
line except that it remains in the coil form, and the cleaning
procedure prior to pickling is most often done electrolytically. The
normal sequence is oil and grease removal in an electrolytic alkaline
unit; rinsing and scrubbing with brushes; pickling; terne coating|
and, oiling by a process similar to batch dipping. After cooling,
residual oils are removed in a "branner", which consists of tandem
sets of cleaning rolls made of thousands of tightly compressed flannel
discs. Middlings from grain milling, called bran, are fed to the
first set of rolls to absorb moisture and excess oil, while tne
remaining rolls distribute a light oil film evenly over the entire
coated surface. The final product is then recoiled, or cut to size
for shipment as terne coated flats. Additional detail for a terne
line is illustrated in Figure III-4.
Aluminum
Another metallic coating applied using the hot-dip technique is
aluminum. Products made of aluminum coated steel include bright and
matte finished sheets and strip used as building materials in marine,
industrial, or other environments where a high degree of resistance to
corrosion is required. Aluminum coated wire is used for chain-linK
and field fencing, barbed wire, telephone wire, and screening.
The batch coating process uses either a conventional molten metal
bath, as in zinc or terne coating, or a special cementation process
called calorizing. Thoroughly cleaned, degreased, and dried steel
products are packed in a rotating drum, along with a mixture of
aluminum powder, aluminum oxide, and ammonium-chloride. As the drum
rotates inside a furnace at 940°C-955°C (1,724<>F-1,751<>F) a reducing
gas is passed into the drum, and the mixture is tumbled for 4-5 hours.
A cohesive solution of aluminum in iron, richest in aluminum near the
surface, forms the coating. This type of coating is especially
effective in protecting steel from oxidation at high temperatures,
427
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hence it is used in pyrometer and superheater tubes, and in a variety
of oil refinery applications.
The continuous aluminum coating process starts with cold rolled steel
strip or steel wire. The strip lines are usually furnace lines, with
an annealing step just prior to the hot-dip in molten aluminum. The
sequence is much the same as zinc coating on a furnace line. The cold
rolled steel coils are cleaned in a hot alkaline solution, rinsed, and
given a mild pickling in hot acid, followed by a final rinse. An
annealing furnace softens the otherwise hard carbon steel, and the
coating is applied immediately following the furnace. The strip
exiting the aluminum bath is cooled, oiled if required, and recoiled
or cut to size for shipment. There is usually no chemical treatment
or final rinse following the aluminizing dip.
In making aluminum-coated wire products by the hot-dipped process,
clean, cold-drawn carbon-steel wire is passed through the molten
aluminum bath at 660°C-680°C (1,220°F, 256°F). This temperature is
high enough to soften the carbon-steel wire sufficiently that
annealing is not required, but the tensile strength of the wire is
reduced, rendering it unsuited for certain applications. This problem
is readily corrected by cold-drawing the coated wire, which not only
raises the tensile strength, but also provides a very bright final
finish to the coating.
Additional
III-5.
detail for an aluminizing line is illustrated in Figure
Other Hot-Dipped Metal Coatings
Other hot coating operations involve combinations of zinc and
aluminum, zinc and cadmium, or zinc, tin and cadmium. There are also
some wire coating operations which use molten tin, or cadmium alone as
the coating agent. However, the latter processes comprise a minor
fraction of hot-dipped coating operations. Most tin plating
production at steel plants is electrolytic, as is all chromium plating
and a limited amount of zinc coating.
428
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