United States
Environmental Protection
Agency
Industrial Environmental Reaearch
Laboratory
Reaearch Triangle Park NC 27711
EPA-400/2-7V-024
January 1979
Research and Development
Survey of Fouling, Foam,
Corrosion, and Scaling
Control in Iron and Steel
Industry Recycle Systems
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RESEARCH REPORTING SERIES
Research reports of the Office of Research and Development, U.S. Environmental
Protection Agency, have been grouped into nine series. These nine broad cate-
gories were established to facilitate further development and application of en-
vironmental technology. Elimination of traditional grouping was consciously
planned to foster technology transfer and a maximum interface in related fielr1
The nine series are:
1. Environmental Health Effects Research
2. Environmental Protection Technology
3. Ecological Research
4. Environmental Monitoring
5. Socioeconomic Environmental Studies
6. Scientific and Technical Assessment Reports (STAR)
7. Interagency Energy-Environment Research and Development
8. "Special" Reports
9. Miscellaneous Reports
This report has been assigned to the ENVIRONMENTAL PROTECTION TECH-
NOLOGY series. This series describes research performed to develop and dem-
onstrate instrumentation, equipment, and methodology to repair or prevent en-
vironmental degradation from point and non-point sources of pollution. This work
provides the new or improved technology required for the control and treatment
of pollution sources to meet environmental quality standards.
EPA REVIEW NOTICE
This report has been reviewed by the U.S. Environmental Protection Agency, and
approved for publication. Approval does not signify that the contents necessarily
reflect the views and policy of the Agency, nor does mention of trade names or
commercial products constitute endorsement or recommendation for use.
This document is available to the public through the National Technical Informa-
tion Service, Springfield, Virginia 22161.
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EPA-600/2-79-024
January 1979
Survey of Fouling, Foam,
Corrosion, and Scaling Control
in Iron and Steel Industry
Recycle Systems
by
K.S. Rajan
NT Research Institute
10 West 35th Street
Chicago, Illinois 60616
Contract No. 68-02-2617
Task No. 2-2
Program Element No. 1BB610
EPA Project Officer: John S. Ruppersberger
Industrial Environmental Research Laboratory
Office of Energy, Minerals, and Industry
Research Triangle Park, NC 27711
Prepared for
U.S. ENVIRONMENTAL PROTECTION AGENCY
Office of Research and Development
Washington, DC 20460
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ABSTRACT
The state-of-the art for fouling foaming, corrosion and scaling control
in the treatment and recycle of process waters of integrated iron and steel
mills was reviewed. Both published literature and on-site data on the
following areas collected through visits to process water treatment facili-
ties of selected iron and steel companies were examined: (1) character
of the waste waters generated in the different processes associated with iron
and steel making, (2) current treatment practices of the recireulating
systems and (3) the corrosion, scaling, fouling and foaming problems
encountered in the treatment processes and the presently adopted methods
for solving them. Possible problems that might be encountered in the
application of the present technology to waste treatment in conformity
with the 1983 effluent guidelines of the EPA were taken into consideration
and recommendations were made for appropriate Research and Development
efforts.
The waste waters of iron and steel industry are derived from non-
contact cooling and scrubbing operations. Present technology appears
adequate for the treatment and control of the non-contact cooling waters.
The treatment of the scrubber waters which are highly contaminated with
suspended and dissolved solids and a variety of pollutants consists of the
removal of the suspended solids and heat loads, decrease the dissolved
solid content by blow down and recycle. The formation of chemical scales
and deposit constitutes a major problem and this is presently being
obviated by controlling the chemical stability of the recireulating waters
through acid addition and blow down.
Research and Development efforts recommended for the process water
treatment under a high degree of recycle (1983 EPA guidelines) include
(a) effective scale inhibition and control (b) automatic monitoring of
the chemical stability of the process waters and the corrosion of the
materials of construction and (c) sensitive methods for on-line determination
and control of phosphate and phosphonate in the recireulating effluents.
-------
CONTENTS
Abstract 11
Figures IV
Tables lv
1. Introduction 1
2. Conclusions 4
3. Recommendations ' 6
Effective Scale Inhibition 6
Automatic Monitoring of the Chemical Stability 7
Corrosion and Erosion Monitoring 7
Determination of Phosphate (Phosphonate) In The
Recirculating Effluents 8
Exploration of Novel Physical Methods For
Continuous Scale Removal 8
4. Process Waters, Their Treatments And Associated Problems 9
Coke Ovens 9
Corrosion And Scaling Problems 12
Sintering Plant 12
Potential Scaling and Corrosion Conditions And
Their Control 15
Blast Furnace 15
Scaling And Corrosion Problems And Their Control 21
Steel Making Process 22
Scaling, Corrosion, Fouling and Foaming 24
Continuous Casting 24
Hot Mills 26
Probelms of Scaling, Corrosion and Fouling 30
Pickling Waste Waters 31
Cold Rolling 32
Metal Coating Operations 34
References 35
Bibliography 37
Appendices
A. Effluent Limitations Guidelines of U.S. EPA Set On
The Basis of BPCTCA And BATEA 39
B. List of Contacts 42
ill
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FIGURES
Number Page
1 Treatment of Coke-Plant Waste Waters H
2 A Modular System for Coke Plant Process Water Treatment .... 13
3 Sinter Plant Process Water Treatment And Recycle 16
4 Process Water Treatment And Recycle For Blast Furnace 20
5 Water Flow Diagram For EOF System 23
6 Spray Cooling Recirculations System 27
7 Treatment and Recirculation System for Machine Cooling Waters . 28
8 Treatment of Waste Waters of Cold Rolling Operations 33
TABLES
Number Page
1 Typical Analysis of Weak Ammonia Liquor 10
2 Variations in The Characteristics of Waste Ammonia
Liquor 14
3 Sinter Plant Scrubber Water Recirculating System 17
4 Blast Furnace Scrubber Water Recirculating System 19
5 Mold Deposits With Nature and Causes 25
6 Types of Deposits That Can Plug Spray Nozzles And
Headers 29
iv
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SECTION 1
INTRODUCTION
This report covers the results of a review made on the state-of-the-art
for fouling, foaming, corrosion and scaling control in the treatment and
recycle of process waters of integrated iron and steel mills. Our approach
in this study consisted of (i) determination of the character of the waste
waters generated in the different processes connected with the production
of iron, steel and steel products in integrated steel mills, (ii) deter-
mination of the current practices (methods) for the treatment of the waste
waters in recirculating systems, (iii) determination of the problems of
corrosion, scaling, fouling and foaming that are encountered in carrying
out the treatment of process waters and their recirculation (iv) evaluation
of the methods currently being practised by the iron and steel mills for
controling these problems (v) extrapolation of the problems under high
degree of recycle in accordance with the 1983 effluent guidelines of the
EPA, and (vi) a critical review of the problems and the currently-practised
methods of control.
The overall objectives of this study are (a) to identify the
deficiencies in the current practices, (b) to identify the potential
problem areas in extrapolating to the requirements of the 1983 guidelines
and (c) to outline the Research & Development efforts that might be under-
taken for solving the present and projected future problems.
According to published reports, there are little over 60 integrated
steel plants in this country. (An integrated steel mill is one which has
both the primary and finishing facilities situated in one location.) It
has been generally estimated by a number of reviewersl»2 that 150-180
cubic meters of water per tonne of raw steel produced are required in the
operation of an integrated steel plant. The actual consumption of water
might be as little as 5% of the above figure, if appropriate waste treatment
and recirculation practices are adopted2.
Since nearly two thirds of the total flows of water applied in an
integrated steel mill are used for the purpose of heat exchange, they
remain clean and hence do not require any major treatment. However,
the rest of the waters which come into direct contact with the off-gases
1D. Kwasnowski, International Metallurgical Reviews, 20, 137-145 (1975).
2R. Nebolsine, Iron and Steel Engineer, 44, 122-135 (1967).
-------
and the products require elaborate treatments prior to being recirculated.
In examining the character of the waste waters and their treatment methods,
it is important to consider separately the effluent waters (and their
associated pollutants) from the different steel making and finishing
operations within an integrated steel mill. For this reason, the waste
water problem is examined individually for the following major operations
(or subcategories):
(1) Coke-ovens
(2) Sinter plant
(3) Blast Furnace
(4) Steel making processes
(a) Basic oxygen furnace
(b) Electric furnace
(c) Open hearth furnace
(5) Hot mills
(6) Pickling
(7) Cold rolling
(8) Metal coating
Section 301 of the Federal Water Pollution Control Act Amendments of
1972 (PL-92-500) requires the achievement of effluent standards of critical
parameters for existing industrial point sources (waste waters discharged
to navigable waterways) by July 1, 1977 through the application of the
"Best practicable control technology currently available" (BPCTCA) and
more stringent effluent standards by July 1, 1983 by means of the "Best
Available Technology Economically Achievable" (BATEA). The proposed lim-
itation guidelines for the iron and steel industry set by the U.S. Environ-
mental Protection Agency-* on the basis of BPCTCA and BATEA are summarized
in Appendix 1.
The work undertaken toward the realization of the projected objectives
may briefly be outlined as follows:
(i) survey of published literature on the subject;
(ii) obtain related documents of EPA;
(iii) visit with steel mills and discuss with the environmental control
personnel their methods of waste water treatments;
(iv) visit with the chemical companies that are involved in the
treatment of the process waste waters of steel mills and discuss
their current methods of treatment, and problems, if any I
3
"Development Document for Effluent limitation guidelines and New Source
Performance Standards for the Steel Making Segment of the Iron and Steel
Manufacturing Point Source Category"; EPA-440/l-74-024-a.
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(v) meet with a few design engineers experienced in the design and
construction of waste-treatment facilities for the iron and steel
industry;
(vi) review the information obtained from (a) published literature
(b) steel industry environmental personnel, (c) water treatment
companies and (d) design engineers; and
(vii) on the basis of a critical examination of the information gathered
and reviewed as indicated above, delineate the current and
potential problems of corrosion, scaling, fouling and foaming
associated with the waste-water treatments in recirculating systems,
and the R&D efforts that might be appropriate.
The visits made to the different steel mills and water treatment
companies and the environmental engineers and consultants with whom we met
are listed in Appendix 2.
In the following sections of this report, the individual subcategories
(processes) of the iron and steel making operations are presented and their
waste water treatments discussed.
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SECTION 2
CONCLUSIONS
The waste waters from the different sub-categories of the iron and
steel making operations consist of non-contact cooling and scrubber waters.
The non-contact cooling waters under closed system operating conditions
do not get contaminated except for their corrosion inhibitors and descaling
compounds. No major problems of scaling, fouling and corrosion that cannot
be handled by applying the available technology have been reported in their
treatment and recycle. Systems involving open cooling towers are subject
to chemical and biological contamination. Problems of moderate corrosion,
scaling and biological fouling associated with these systems are being
controlled by the presently available technology.
The scrubber waters are highly contaminated with suspended solids,
variety of dissolved solids containing Ca, Mg, Fe, S0?~, Cl~, C0^~, and other
pollutants such as ammonia, cyanides and a few toxic heavy metals. The
current practice for their treatment and recirculation consists of (a) remov-
al of suspended solids, (b) removal of heat loads (c) decrease the dissolved
solid content by blow down from the system and (d) recycle them. Formation
of chemical scales and deposits and consequent pluggage of the spray
nozzles and supply lines appear to be a severe problem of major proportions.
A major deficiency rests with the present method of control of scale and
corrosion in the scrubber water recirculating systems. This is presently
done by controlling the chemical stability of the recirculating waters
through the addition of acids and blow down rate. The chemical stability
determination is based on calcium carbonate equilibrium and does not take
into account (a) the complexing chemical interactions of iron (and other
metals) and the anions such as cyanide, ammonia, phenol and other polyanionic
organic components and (b) physical, chemical and charge characteristics
of the finely divided suspended solids. R&D efforts should be undertaken
to develop newer and more effective methods of (1) determining the chemical
stability of the recirculating waste waters (2) automatic monitoring of the
chemical stability and (3) novel scale control methods.
Systematic data on the chemical and physical characteristics of the
process effluents before and after treatment and the nature and amounts of
inhibitors for corrosion, scaling and biological fouling are not available
in the published literature nor could they be obtained from the waste-water
treatment facilities. For an effective technical evaluation of the currently
practised waste treatment methods, it is important to know:
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(1) chemical analysis of the dissolved and suspended solids of the
influent and effluent process waters at different points along
the waste treatment,
(2) the chemical nature (structure) of the cationic and anionic
polyelectrolytes that are used for the floculations and sedimenta-
tion of the finely divided suspended solids in the clarifiers,
(3) the chemical structure of the on-line additives for the control
of scaling, corrosion and biological fouling.
Information on (2) and (3) are not available due to their proprietary nature
and systematic data on (1) are not readily available.
R&D efforts that are considered necessary for overcoming some of the
problem areas identified in this survey and for carrying out an effective
treatment and recirculation of the waste-waters, are outlined in the next
section.
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SECTION 3
RECOMMENDATIONS
On the basis of this review, it is found that a few important problem
areas do exist in the treatment and recirculation of the waste waters
from the different subcategories of the iron and steel making operations.
It is considered that R&D efforts undertaken on the following topics
might help alleviate the problems.
EFFECTIVE SCALE INHIBITION
The method currently being practised for the control of scale formation
in the recirculating waste-water streams consists of:
(1) determination of the chemical stability of the waste waters by
using Ryznar Stability Index (RSI) or Langelier's Saturation
Index (LSI),
(2) control of the chemical stability of the system by (a) acid
addition and (b) blow down
(3) on-line additions of scale inhibitors and dispersants.
In spite of this treatment, failures in the prevention of CaF2~type-,
and iron compounds-based scales accellerated by the accumulation of finely
divided suspended solids are not too uncommon in their occurrance. The
present method of control of the chemical stability of the recirculating
water system is based on Ryznar Stability Index (RSI) or the Langelier's
Saturation Index (LSI) and it does not reflect the complexing interactions
of iron and other polyvalent cations and a number of polyvalent anions
of inorganic and organic nature. The RSI or the LSI is mainly based on
calcium carbonate equilibrium in the environment of the other ions in the
system. Further, in the proposed zero discharge recirculating systems,
the concentrations of the dissolved solids will be several fold larger
than at present and therefore their reactions leading to the formation
of scales may be more complex. The scale inhibitor and dispersant systems
under these conditions should be able to interact with calcium, iron and
any other polyvalent cation in the combined presence of the different
anions of inorganic and organic nature. The R&D efforts should consist of:
an investigation of the chemical and physical characteristics of the
process effluents at several points along the recirculating system,
-------
• a critical examination of the chemical stability index and develop
modifications to include the complexing interactions discussed
above,
• physicochemical studies on the interactions of the effluent cations
with a number of selected inhibitor compounds,
• investigation of the physical and chemical characteristics of the
finely divided suspended solids and their interactions with the
candidate inhibitors, polyelectrolytes and dispersants,
• on the basis of the above, develop inhibitor (and dispersant)
systems and conditions for on-line application in an actual water-
treatment facility.
AUTOMATIC MONITORING OF THE CHEMICAL STABILITY
Since the chemical stability of the recirculating water system is
an important parameter for the control of the conditions of scaling and
corrosion in the process waters, continuous monitoring of the chemical
stability would be very useful. Hence, instrumentation for its on-line
automatic monitoring should be developed. At present, reliable automatic
monitoring devices for on-line application are not available. R&D efforts
should therefore be undertaken on the following lines:
(1) investigation of one or more physicochemical basis for an effective
measurement of this parameter
(2) design and building of the necessary exploratory instrumentation
set-up
(3) on-line testing of the operational characteristics of the
exploratory model thus developed.
CORROSION AND EROSION MONITORING
Erosion of the metallic surfaces of the materials of construction of
the treatment facility by the suspended solids of varying sizes and
hardness is an important problem. Further, corrosion of the metallic
surfaces underneath the encrustations of chemical scale is another important
problem. In order to protect the materials of construction and to prevent
any catastrophic failure, appropriate monitoring of erosion and corrosion
should be undertaken. Research and development efforts should be under-
taken toward the development of automatic corrosion and erosion probes
for on-line application. The work will consist of:
1. physicochemical studies and
2. the development of the necessary instrumentation.
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DETERMINATION OF PHOSPHATE (PHOSPHONATE) IN THE RECIRCULATING EFFLUENTS
In discussions with one of the chemical companies involved in the
treatment of process waters, it was found that for the inhibition of the
scale formation and corrosion the inhibitor compounds are added to the
recirculating process water system and controlled by a determination
of the residual phosphate or phosphonate (from the inhibitor additive)levels.
Reliable determination of the residual phosphates at the low levels and
in the combined presence of the dissolved and suspended solids in the
recirculating waters is a real problem of analytical chemical nature. The
lack of proper sensitive determination of phosphate leads to unreliable
and somewhat unpredictable control measures in waste treatment. This
problem may be magnified several fold under conditions of zero discharge
effluent limitation guidelines. Research and development efforts should
be directed toward the development of a sensitive analytical method suitable
for field application. The development of automatic monitoring instrumenta-
tion may be of added advantage.
EXPLORATION OF NOVEL PHYSICAL METHODS FOR CONTINUOUS SCALE REMOVAL
Since chemical scale formation, deposit build up and consequent pluggage
of the return lines and scrubber nozzles of the recirculating waste water
systems constitute a major problem, it may be worthwhile exploring novel
physical methods for their inhibition and control. One such possible
approach is the disintegration of "scale and deposit nuclei" build up by
the application of magnetic field to the fluid flow system. This principle
is currently being practised by a commercial company for domestic water
system and biolers. A systematic investigation on the application of this
approach might be undertaken.
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SECTION 4
PROCESS WATERS, THEIR TREATMENTS AND ASSOCIATED PROBLEMS
COKE OVENS
Coke which is nearly pure carbon is used in the blast furnace (i) as a
reducing agent for converting iron ore into iron and (ii) as a fuel. In
the United States, the conversion of coal to metallurgical coke is done
largely by the by-product recovery process. In this process, bituminous
coal is subjected to destructive distillation in the absence of air at
950-1100°C. The volatile matters evolved in this process consist of
gases and vapors. The gases include H2, CH^, ^2^6' C02» co» ethylene,
butylene, acetylene, H2S, NHo, 02 and N^. The vaporized liquids in the
gaseous mixture consist of ammonia liquor, tar and light oil. The gases,
tar and water vapor are taken through the primary coolers. The tar is
then separated from the water, which is called the waste ammonia liquor.
The direct contact cooling and cleaning of the gases and chemical by-products
gives rise to the other major waste waters of the coke-oven plant. Coke
dishcarged from the ovens is quenched by water sprays for which generally
the source water is directly used. The quenching water is recirculated.
Coke is transported to the balst furnace area and the chemicals volatilized
during coking process and taken to the by-product plant where ammonium
sulfate, tar, pitch and light oil are removed from the coke-oven gas.
The major waste water streams from the coke-oven process thus consist
of weak ammonia liquor from primary coolers, final cooler water used for
direct contact gas clenaing, benzol-plant waste water derived from stripping
operations in benzol recovery plant and cooling water from light oil
recovery plants, quench water resulting from the quenching of coke and other
wastewaters consisting of tar plant wastes containing phenols, desulfurizer
liquor containing thiocyanates and sulfides and pipeline drippings.
Weak ammonia liquor is the waste water for the primary cooler system
of coke-oven process amounting to 65 to 150 liters per tonne of coal .
The major contaminants in this liquor consist of ammonia, phenol, cyanide,
suspended solids and oil. The chemical characteristics of this waste water
vary considerably from plant to plant depending upon the coal mix, type
of oven, extent and type of by-products recovered and the recovery process
J. W. Schroeder and A. C. Naso, Iron and Steel Engineer, 1976, p. 60-66.
-------
employed. A typical analysis of the chemical composition of this waste
water is presented in Table I1. The varying characteristics of the weak
TABLE 1. TYPICAL ANALYSIS OF WEAK AMMONIA LIQUOR
Component Concentration mg/1
Ammonia 6900
Cyanide 40
Phenol 870
Thiocyanate 860
Carbon dioxide 440
Total sulphur 1000
Sulphate 35
Hydrogen sulphide 30
Chloride 11000
ammonia liquor from different sources are shown in Table 2.
The final cooling of the coke oven gas utilizes a direct contact water
system. Naphthalene is recovered as a by-product from this water. The
contaminants of this waste water are: cyanide, phenol, and ammonia.
A portion of this water is recirculated to the final coolers and some for
coke-quenching.
The waste waters from the plants for the recovery of benzol and light
oil are contaminated mainly with oil and small amounts of phenol and cyanide.
Coke-quenching practices differ considerably with the different plants.
A number of the plants use the raw water from rivers or lakes for quenching
and adopt recirculation system. Some other plants use weak ammonia liquor
for quenching.
Tar plant wastes, desulphurizer liquors and gas-line condensate leakages
constitute the other miscellaneous waste waters from the coke-oven plants.
Phenols, thiocyanates and sulfides are the main contaminants in these
waste waters.
The methods of treatment of the coke-oven waste waters essentially
consist of (1) removal of ammonia from waste ammonia liquor (WAL) by steam
distillation, (2) biological oxidation of the phenolic wastes and other
contaminants consisting of cyanides, cyanates and thiocyanates, and
(3) sedimentation.
The schematic of a possible multistage treatment of coke-plant waste-
waters indicated in the published literature*- is shown in Figure 1.
D. Kwasnowski, International Metallurgical Reviews, 20, 137-145 (1975).
10
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weaK
*} m noon i/3
Cll 1 11 * IV" MO
liquor (NH3,
phenol)
H\on "7/">l
\J\£i 1 f. v^l
wastes
, ^
(oil, phenol)
final cooler
> ti
wastes
(CN, phenol)
r
ammonia
still
^oi
air
^flotation
r**Ct
cyanide
stripper
HI
1
^
3
>
•I 1 1 k
• W"
i
CO2
f _,
bio -oxidation
L »* (phenol to
COp * H2O)
C- ^-
_
sludge
to
disposal
•
treated
k-
cfflucnt
FIGURE 1. TREATMENT OF COKE-PLANT WASTE-WATERS
[TAKEN FROM "WATER POLLUTION CONTROL IN AN INTEGRATED STEEL
PLANT", D, KWASANOWSKI, INTL, METALLURG, REV,, 2Q., 137'-145
-------
Recirculation is reported only for the final cooler and the coke-quenching
waste-water systems. A modular system has recently been developed^ for
the treatment of coke-plant waste waters which utilizes an activated carbon
adsorption module for the removal of the organics prior to steam distillation
of ammonia. In a parallel operation, the final cooler waters and other
waters pass through a clarification unit for the removal of suspended solids
and oil, followed by passage through an activated carbon absorption module.
A schematic of this is shown in Figure 2.
CORROSION AND SCALING PROBLEMS
Published information on the treatment of coke-oven waste waters
do not discuss any major problems of corrosion, scaling and fouling. On
the basis of our site visits and telephone contacts with some of the steel
companies, it was found that only a few of them carry out the biological
treatment in their own facility. After the recovery of the by-products
the effluents are generally discharged into minicipal sewers. Further
as indicated earlier, only limited recirculation is practiced in the
treatment and handling of the final cooler water and quench water. In the
event of establishing a recirculating system, there exist the possible
problems of chemical scaling (resulting from the lime addition in the steam
distillation of ammonia) and corrosion due to the possible pH drops in the
recirculating final cooler waters. Systems and conditions can be developed
for obviating these problems through the use of caustic soda as a replace-
ment for lime addition and the use of proper pH control.
SINTERING PLANT
In the sintering plant, iron-bearing wastes such as mill scale and
finely-divided dust from the basic oxygen furnace, open hearth furnace
and blast furnace are blended with fine iron ore and limestone to form
an agglomerate that would be suitable for being recharged into the blast
furnace. The mixture consisting of the iron-bearing components, limestone
and coke fines is placed on travelling grates of the sinter machine and is
heated to fusion temperature to produce the sinter. The sintering operation
also produces carbon dioxide, sulfur, chloride and fluoride. In addition
to these contaminants, the process gases contain dust, and volatilized
oil from the mill scale. The dust produced in the sintering process is
drawn through wind boxes beneath the traveling grates into the gas main
where large particles drop out into a series of dust hoppers. In order
to control the dust generated at the various points, de-duster systems
are also operated which utilize wet scrubbers. Sludge draining from the
scrubbers of the de-duster systems flows by gravity to the sinter plant
clarifier.
The wet scrubbers for cleaning the wind boxes and the gases of the
de-duster systems constitute the sources of process waste water in the
sinter plant. The contaminants of the waste waters consist of suspended
4
J. W. Schroeder and A. C. Naso, Iron and Steel Engineer, 1976, p. 60-66.
12
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DISiOlVED CAS
KOf AIION
DUAl CEll
CIAVITT fUI|«
FIGURE 2, A MODULAR SYSTEM FOR COKE PLANT PROCESS WATER TREATMENT
[TAKEN FROM "A NEW METHOD OF TREATING COKE PLANT WASTE WATER", J.W.SCHROEDER
AND A, CHARLES NASO, IND, STEEL ENGR,, P, 60-66 Q976)]
-------
TABLE 2. VARIATIONS IN THE CHARACTERISTICS OF WASTE AMMONIA LIQUOR
Waste Generated
(liters/ tonne)
Company Total /Wai
1
2 275
3 374
4 303
5 1943
**
6 346
7 343
8 626
9 251
10 417
11
175
192
160
163
167
150
196
125
138
Total
Ammonia
pH mg/1
8.7 5500
9.1 2800
9841
6500
8.5 5000
6.5-8.5 1500
7.5-8.5 3900
5.5 2500
3010
8.8-9.1 .1713-3417
Total
Cyanides
mg/1
100
140
10
65
50
15(1400)
10-100
4
10-200
Phenol
mg/1
3000
400
1753
1690
2500
550
200-300
200
350
770
660-840
*\
Taken from Carnegie Mellon Institute Report: "An Evaluation of EPA-
recommended technology for the treatment and control of Waste Waters
from by-product coke plants", G. M. Wong Chong, S. C. Caruso and
T. G. Patarlis.
**
The value in parenthesis is thiocyanate.
14
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solids (mainly iron oxide), oil and grease and acidity. Depending upon the
nature and composition of the feed mixture for the sintering process, the
concentrations of the contaminants and pH vary.
The currently practised waste treatment technology for the sinter
plant waste waters consists of (i) removal of suspended solids in a
clarifier by the addition of appropriate polyelectrolytes. The under flow
from the clarifier is vacuum-filtered within the sinter plant and the
filtered cake recharged to the sinter furnace. In a recirculating system
the over flow is re-used after the necessary blowdown followed by make-up
water additions. A schematic for the waste-water^ recirculating system
reported by Krikau and DeCaigny is shown in Figure 3 for the purpose of
illustration. Typical data on the flow and chemical parameters reported
earlier by the Interlake Corporation are presented in Table 3 for the
purpose of illustration only**.
POTENTIAL SCALING AND CORROSION CONDITIONS AND THEIR CONTROL
In the sinter plant, conditions of high basicity sinter often exist
when the lime and magnesia contents of the dusts in the sinter plant
scrubbers becomes high and consequently the scrubber water pH exceeds 12.
The current method for controlling scaling and corrosion in the scrubber
water treatment and recycle system is through the addition of acid and
blow down and thereby controlling the chemical stability of the water
(Ryznar Index). In this method, severe corrosion might be brought about
by increased acid additions and decreased blow down rate. With decreased
acid addition, a concommitant high increase in blow down rate is
necessary to bring Ryznar index to the range of 5-7 and thus prevent
conditions of scale formation. Under conditions of zero discharge, the
above method of corrosion and scale control may present problems. It is
felt that research and development eff'orts should be directed toward
the development of suitable additives for deactivating the chemical scale-
forming metal ions and at the same time controlling the pH and affording
corrosion protection.
BLAST FURNACE
The production of iron from iron ores and other iron-bearing materials
is carried out in the blast furnace. The iron-bearing materials, coke and
limestone (and dolomite) are charged into the top of the blast furnace and
hot air is blown into the bottom. Reducing gases from the coke (and from
the liquid or gaseous fuels injected with the air blast at the tuyers)
react with the oxygen of the iron ores and form porous iron. The fluxes
(limestone and dolomite) react with the impurities in the charge (burden)
Fred. G. Krikau and Roger R. DeCaigny, San Francisco Regional Technical
Meeting of the Amer. Iron and Steel Inst., 1970.
R. E. Touzalin, "Pollution Control of blast furnace plant gas scrubbers
through recirculation", Interlake Steel Corporation's report prepared for
U.S. EPA, Project 12010.
15
-------
FILTER CAKE
TO SINTER MACHINE
— I
VACUUM
FILTERS
i
\
f
VACUUM
FILTERS
I
UNDERFLOW SLURRY
COOLING
TOWER •
RECYCLE
PU.MPHOUSE
4 PUMPS'"
WATER
TO
WATER
TO
A
AIR
COOLING] AJLB i
IQW^g. igCRUSBf_R |
LJ l±
CLARIFIER
-i.
H
f'l
J 11
/
"*> y
OVER FLOW
TO
COOLIING
TOWER
PUMPS
/I i
r
f
1 1
or
lit
(Z
ft
<
_l
0
/~-
II.H. 1 J
UOIDER FLOW rO
.VACUUM PILTE_RS
UNDER FLOW TO
VACUUM FILTERS
FIGURE 3. SINTER PLANT PROCESS WATER TREATMENT AND RECYCLE
(TAKEN FROM F.G.KRIKAU AND R.R.DE CAIGNY, SAN FRANCISCO REGIONAL TECHNICAL
MEETING OF THE AMER, IRON AND STEEL INST,, 1970)
-------
TABLE 3. SINTER PLANT SCRUBBER WATER RECIRCULATING SYSTEM
Balanced Condition Parameters
Location
Make-up Water
Clarifier Influent
Clarifier Effluent
Clarifier Underflow
Blow-Down
Evaporation Loss
Flow
liters/
min
1363
2612
2385
190
1136
38
Suspended
Solids
kg /tonne
of Iron
Produced
0.043
19.16
0.47
18.7
0.22
Ammonia
Nitrogen
kg /tonne
of Iron
Produced
0.002
0.053
0.036
0.002
0.018
Cyanide
kg /tonne
of Iron
Produced
O.OOOOi
0.043
0.011
0.032
0.006
Phenol
kg /tonne
of Iron
Produced
0.000009
0.0014
0.000025
0.0013
0.000011
Iron
kg/ tonne
of Iron
Produced
0.0013
7.2
0.042
7.15
0.02
PH
8.1
10.5
11.0
11.2
11.0
r%
Taken from "Pollution Control of blast furnace plant gas scrubbers through recirculation",
R. E. Touzalin, Interlake Steel Corp. Rep. prepared for EPA Project 12010.
-------
to form the slag. Molten iron collects in the hearth of the furnace and
molten slag forms a pool on the top of the molten iron. Periodically, the
furnace is tapped and iron and slag are withdrawn. An important by-product
of the blast furnace process is its exhaust gas which is used as a fuel
for preheating the hot blast in the stoves and as supplemental fuel for
boilers.
The blast furnace gas is wet-scrubbed (i) to remove particulate matter
from the gas (ii) to reduce its moisture content and thus increase its BTU
value. Besides the suspended solids, the significant pollutants in the
blast furnace gas scrubber water are cyanide, phenols, ammonia, temperature
and pH. The particulate matter escaping into the blast furnace gas (and
subsequently removed by wet-scrubbing) consists of (i) oxides of iron,
CaC03, CaS, MgC03, Si02, CaO, MgO, KCN and carbon. The scrubbing water
attains alkaline pH1s through its turbulant interaction with the alkaline
minerals and chemicals in the fumes and dust in the top gases of the
furnace. Further because of its absorption of the thermal energy from
the hot gases, the gas cleaning water gets thermal pollution. Data on
the water characteristics of the blast furnace sampler water recirculating
system reported earlier in the literature^ are presented in Table 4 for
the purpose of illustration only.
A schematic of the blast furnace gas cleaning water recycle system
taken from Interlake's report? is shown in Figure 4 for the purpose of
illustration. Generally, the gas-cleaning system operates under positive
pressure from the blast furnace. After 50 to 75% of the particulate
matter in the blast furnace gas in removed by the dust catchers, about 99%
of the residual particulates are collected by the venturi scrubber followed
by a mist eliminator or separator. Finally, the gas cooler tower where
the flow is subjected to water sprays to cool the gas and condense any
moisture in excess of saturation. In the recirculating system the venturi
pumps send the gas-cleaning water to venturi scrubbers where the particulates
are picked up. The gas cooler water picks up the heat from the blast
furnace gas. Effluents from the venturi and gas cooler are then combined
and they flow to the clarifier (thickener). Here the suspended solids
are removed by gravity and by using chemicals and polyelectrolytes. The
resulting sludge is either removed as such or are subjected to vacuum
filtration. The process water over-flow from the thickener flows to the
hot well from where it is sent to the cooling tower. Finally the process
water from the cooling tower goes to the cold well. To control the
dissolved solids in the system, part of the water is blown down either to
a terminal treatment plant or to the source rivers, lakes or municipal sewers.
Make-up water from either of the raw water sources (rivers or lakes)
R. E. Touzalin, "Pollution Control of blast furnace plant gas scrubbers
through recirculation", Interlake Steel Corporation's report prepared
for U.S. EPA, Project 12010.
Fred G. Krikau and Roger R. DeCaigny, San Francisco Regional Technical
Meeting of the Amer. Iron and Steel Inst., 1970.
18
-------
TABLE 4. BLAST FURNACE SCRUBBER WATER RECIRCUIATING SYSTEM
v£>
Balanced Condition Parameters
Location
Make-up Water
Clarifier Influent
Clarifier Effluent
Clarifier Underflow
Blow-Down
Evaporation Loss
Flow
liters/
min
2460
17034
16277
492
1703
265
Suspended
Solids
kg /tonne
of Iron
Produced
0.08
8.65
0.35
8.3
0.035
Ammonia
Nitrogen
kg /tonne
of Iron
Produced
0.0036
0.53
0.51
0.016
0.053
Cyanide
kg /tonne
of Iron
Produced
0.000015
0.0625
0.037
0.026
0.0038
Phenol
kg/ tonne
of Iron
Produced
0.000015
0.00085
0.00002
0.00065
0.00002
Iron
kg/ tonne
of Iron
Produced
0.0023
3.95
0.08
3.85
0.009
PH
8.1
8.2
8.3
8.5
8.3
r\
Data taken from "Pollution Control of blast furnace plant gas scrubbers through recirculation",
R. E. Touzalin, Interlake Steel Corp. Rep. Prepared For EPA Project 12010.
-------
"A"
BLASJ FURNACE
GAS
\
"B"
BLAST FURNACE
GAS
CD
m
3
cc
o
CO
COOLER
\
er
LJ
m
CD
O
SLURRY
'SLURRY
s
w
AJ-1
RECYCLE
PUMPHOUSE
6._PJJMPS
ATER TO i WATER TO
BLAST FCE.I COOLING
SCRLBBER \ TOWER
& COOLERS!
,-•
JO'
CLARIF1ER
(TAKE;i FRO,",
MOi-.1 AND S
FIGURE '
.KRIKAU AL
HIST,, .'
UKPER FLOW TO
30*" Ol.AR|F!ER 3INTER PLANT
PROCESS '.-/ATER TREATMENT AND RECYCLE FOR BLAST FURNACE
R.R.DE CAIGNY, SAN FRANCISCO REGIONAL KEETING OF THE AMER.
-------
is then added to the cold well to compensate for the blow down and other
process losses. Finally, water from the cold well is recirculated to the
venturi scrubber and the gas cooler. The process water recycle system can
be one of two types, viz., combined venturi and gas cooling system or
segregated cascading system.
The above discussion does not include the waste waters that are used
for cooling various parts of the blast furnace such as the tuyers, bosh,
and hearth stoves. These waters do not pick up any chemical pollutants
or particulates, although their thermal load is increased. Depending upon
the overall water problems of a given iron and steel plant the systems for
handling the cooling waters would be using a once-through operation and
recirculating, cooling and appropriate water treatment, with or without
provision for blow down into the gas-cleaning and gas-cooling systems.
The treatment of process waters from gas-cleaning and gas-cooling
operations on a recirculating basis consists of (a) collecting the effluent
waters (b) precipitating the suspended solids in a clarifier, (c) passage
of the clarifier overflow through cooling towers to reduce the heat load,
(d) controlling the chemical stability of the process water stream by
blow-down and acid addition in order to prevent scaling and corrosion, and
(e) recirculation of the treated water to gas-cleaning and gas-cooling
operations. Besides, .polyelectrolytes, dispersants, anti-scaling, anti-
corrosion and anti-fouling chemicals are added to the recirculating water
systems at the appropriate stages. The waste-treatment methods of the
recirculating systems do not include removal of dissolved solids such as
sodium, chloride, calcium, magnesium, iron, carbonate, sulfate and the
contaminants such as ammonia, phenol, cyanide and control of pH. The
prediction and control of the chemical stability of this system of
dissolved solids is a major problem of the recycle operation. This problem
is currently being managed through blow-down and make-up.
SCALING AND CORROSION PROBLEMS AND THEIR CONTROL
In blast furnace gas cleaning systems on recycle, the following major
failures due to scaling have been observed: (a) plugging of spray nozzles,
(b) deposits in venturi throats and (c) closing of the supply pipes by
precipitated materials. These materials have been identified as being
mainly calcium carbonate. The tendency of the system for calcium carbonate
scaling is currently being determined through the use of the Ryzner Stability
Index (RSI) or the Langelier Saturation Index (LSI). In the LSI system,
a zero index denotes that the system is in equilibrium with calcium carbonate.
A positive index indicates scaling tendencies and a negative index indicates
corrosive trends. When Ryzner stability index (RSI) is higher than 7.5
to 8.0 a corrosion condition is indicated. RSI values lower than 6 indicate
scaling tendency. Values of 6 to 7 will indicate non-scaling and
non-corrosive conditions. In view of the contribution of calcium and
alkalinity from the limestone in the flux, and the carry over of calcium
chloride, the potential for calcium carbonate exists whenever the solubility
limits are exceeded in the blast furnace gas cleaning recycle system.
The addition of sulfuric acid to control alkalinity may potentiate calcium
sulfate scaling. The control of scaling and corrosion in the blast furnace
21
-------
waste water recycle system is currently being managed by means of blow down
•and addition of acid and thereby bringing the Ryzner Stability Index
(or the Langelier Saturation Index) to the desired range.
R&D efforts are warranted for the development of low-cost non-polluting
scale-inhibitors that would inactivate Ca, Mg and Fe through sequestration.
By this means the quantity of acid added may be lessened and thus prevent
corrosion conditions.
STEEL MAKING PROCESS
The three major methods currently in use in the USA for the production
of steel are: the basic oxygen furnace, the open hearth furnace and
the electric arc furnace. In each of these methods, the raw materials
consist of hot metal (iron), scrap steel, limestone, burnt lime (CaO),
fluorspar (CaF2) and dolomite (MgC03 and CaCC>3). Slag, smoke, fume and
waste gases are the waste products. In all the three methods pure oxygen
and/or air is used to refine raw iron through the oxidation of the impuri-
ties such as silicon, carbon, phosphorus and magnanese. Since all the
three steel making processes commonly have two types of waste waters,
viz., non-contact cooling waters and the gas-scrubbing waters, we are
presenting a detailed discussion of the EOF operation only.
The basic oxygen furnace is a pear-shaped, refractory-lined open
mouth construction. The furnace is supported on trunnions mounted in
bearings and is rotated for tapping of steel ladles and dumping of slag.
The charge consists of a mixture of pig iron (70%), scrap metal ('30%) and
fluxes. Oxygen is injected into the furnace through copper-tipped steel
lances to bring about the oxidation of impurities. The waste products of
this process are heat, airborne fluxes, slag, carbon monoxide and dioxide
gases and oxides of iron emitted as submicron dust. The waste water
discharge sources of a EOF plant consists of non-contact cooling water
and the gas cleaning process water. The non-contact cooling water is
from the cooling of the hoods and the lances. It is not contaminated
except for the heat load. This can be recirculated after being cooled
and treated with inhibitors for corrosion and bacterial fouling.
Alternately, it can be utilized in the gas-cleaning system. The gas cleaning
process waters are derived from the quencher, the venturi scrubber and the
wet precipitator units.
The water treatment of the EOF gas-cleaning recirculating waters
consists of collecting the effluent water in a thickener, removal of the
suspended solids by gravity and through the addition of polyelectrolyte and
the recycle of the thickener overflow water back to the gas-cleaning
units. Inhibitor compounds are added to the recirculating waters for
controlling scaling, corrosion and bacterial growth. By means of a small
blow down, the dissolved solids content of the recirculating water system
is controlled.
The EOF waste water system of the recirculating type is illustrated by
the flow diagram (Figure 5) provided by one of the steel mills visited
by us. The system consists of two process water loops and two indirect
22
-------
K)
P. A.
VENTURI
QUENCHER
FEED TANK
SERVICE
MAKEUP
VEMTURI
HOLDJN&
TANK
CD
BLOWDOWN
SLOWDOWN
-4-
^TREATMENT R.T.
UP
SYSTEM MO- 3
COOLIMG
2 CEXLS
QUEMCHER
POHPS
QUENCHER
(2)
QUENCHER
SEAL
TAWK
(2)
HEAD TANK
(1)
SYSTEM NO. 2
CLASSI-
FIES
1
(3)
BUILDING .
FUMES IW
SVSTEH HO. 1
I
THICKENER C2)
BLOW
W. A.
VEWTUR1
SCRUBBER
TO
R3R WSPOSSl.
SLUDGE
[3UHJCE
PUMPS**)
VENTU8I
WECVCLE
VENTURI OR
SERVICE WATER
MAKE UP
SERVICE
IBLOW&QWN
COLD MILL
PUMPS (3}
COOtJWfe
(21
LAWCE WATER
CHAK6ERS
FURMAC*
HOOOS
SYSTEM WO. 4
1
LAXiCEBU)
PHJTBSS
BOOSTER
PUMPS
(2)
HOLDING
TANK
ZEALITE
SOFTNER
FIGURE 5, WATER FLOW DIAGRAM FOR BOF SYSTEM
(OBTAINED THROUGH THE COURTESY OF INLAND STEEL COMPANY'S WASTE WATER TREATMENT FACILITY)
-------
cooling water loops. System number 1 is for cooling and scrubbing the gases
evolved from the steel making furnaces. System number 2 is a scrubbing
loop for cleaning building fumes. System number 3 and number 4 are for
non-contact cooling of the furnace hoods and lances.
SCALING, CORROSION, FOULING AND FOAMING
The non-contact cooling waters of the recirculating type are treated
chemically for corrosion control. The corrosion inhibitors are of a
proprietary nature. However, it is gathered that non-chromate type of
inhibitors are in use. One such class of compounds are the aminophosphonates,
which serve the dual role of scale and corrosion inhibition. In the case
of the use of cooling towers, different biocides are being used for elimina-
ting biological growth. For scale prevention, nearly all the water treatment
companies appear to use aminophosphonates.
As a result of our discussions with the water treatment companies,
it has been found that in a number of operations adopting the recirculating
systems, severe problems of chemical deposit (scale) formation and pluggage
at the return lines, venturi throats and the recycle lines from the venturi
to the quencher are not too uncommon. The deposits have the compositions,
CaC03 - 50%, Fe-oxides - 35%, CaF2 - 5%. This problem has been occurring
in spite of their present treatment practices. The build up of total
dissolved solids, pH variations and finely divided suspended solids con-
tribute to this problem. The current method of control of this problem
is through the control of the chemical stability of the system in
combination with the addition of scale inhibitors and dispersants.
However, since the stability index method currently being used (as a
measure of the chemical stability of the waters) is based upon largely
calcium, and total alkalinity, and does not take into account the chemical
interactions of iron and the surface charge neutralization of the finely
divided suspended solids, this method therefore may not be adequate and
effective. Under conditions of Zero discharge, the problem of the
dissolved solids in the recirculating waters would be magnified considerably.
Hence R&D efforts directed toward solving this problem are necessary.
CONTINUOUS CASTING
Billets, blooms, slabs and other shapes are cast by flowing hot steel
from teeming ladles into the continuous casting molds. The casting molds
are water-cooled. On being discharged from the mold, the cast product
is cooled in a spray chamber. The product is then cut to desired length
in subsequent sections of the continuous casting mill. The water systems
serving the continuous casting process consist of mold cooling, spraying
and machine cooling water systems. For the mold and machine cooling,
closed recycle water systems are used, and the spray water system is an
open recycle system. Mold cooling uses high purity water (zeolite-softened)
because of the requirement of the high heat transfer rates through the mold
wall. The problems associated with the mold cooling water systems include
the formation of calcium carbonate scale, migratory corrosion products,
suspended solids, organics and biological fouling. In Table 5 are illustra-
ted the various deposits encountered in the mold cooling system and their
24
-------
nature and causes.
_ TABLE 5. MOLD DEPOSITS WITH NATURE AND CAUSES*
CASE I CASE II CASE III
Silica as Si02, %
Iron as Fe203, %
Loss on Ignition, %
Phosphate as P^^, %
Calcium as CaO, %
Magnesium as MgO, %
Carbonate as COo, %
4
1
7
7
38
2
31
1
85
9
-
-
-
-
4
8
29
28
16
7
-
Zinc as ZnO, %
Chromate as
Case I - Calcium Carbonate Scale Poor Softener Operation (Closed System)
Case II - Migratory Corrosion Products (Closed System)
Case III - Severe Hydraulic Fluid Leaks (Open Recirculating Tower System)
*
Taken from D. J. Juvan, "Design of Critical System For Continuous Casters,"
paper presented at the 1975 AIME-Natl. Open Hearth And Basic Oxygen Steel
Conference, Toronto, Canada, April 1975.
Current water treatment practices comprise (1) the softening of the
influent water by means of ion exchange (Zeolite softener operation) to
bring the water hardness to less than 10 ppm (to prevent calcium carbonate
scaling), (2) effective skimming of any oil and grease that infiltrate
the recirculating system (3) installing filters and strainers in the system
prior to the mold, (4) the use of corrosion inhibitors, such as the chromates,
nitrites, phosphates and zinc-bearing compounds, and (5) the use of pro-
prietary biocides for the control of biological growths.
Under the conditions of the 1983 guidelines, it would appear that
the currently practised methods of water treatment might be largely
sufficient except that non-chromate type inhibitors should be substituted.
Further, open cooling towers should preferably be eliminated from the
recirculating water system so that atmospheric contamination, corrosion and
fouling problems associated with them could be obviated.
The spray water system gets contaminated most among the three systems.
The contaminants include iron oxide scale, oil, grease and other foreign
objects. Because of the use of recirculating cooling tower systems,
airborne contaminants, corrosion and biological fouling constitute the
25
-------
additional contaminants. As a result of these contaminants, the major
problem encountered in the spray water system is fouling and plugging of
spray nozzles. The spray cooling recirculating system^ is schematically
illustrated in Figure 6.
Various types of deposits which have been found to plug up the spray
nozzles and headers" are illustrated in Table 6.
The current practice for water treatment in the spray water recircula-
ting system consist of (a) settling of the heavy solids and the iron
oxide scales, (b) skimming of the oil and grease (c) sedimentation of the
finely divided solids through the additions of polyelectrolytes or (d) high-
rate sand filters, (e) the addition of corrosion inhibitors and microbiocides
and (f) installation of on-line filters. These methods should be adequate
for the treatment of recirculating systems under the zero discharge
conditions required by the 1983 EPA guidelines. However, the use of
non-chromate type of corrosion inhibitors and non-polluting dispersants
for the avoidance of the "nozzle-plugging" problem should be instituted.
The machine cooling water system services several pieces of
equipment. It also cools the mold water through a heat exchanger. In the
recirculating system for the machine cooling waters, open cooling towers
are often installed in the circuit** as illustrated in Figure 7. The
contaminants picked up by this water system are similar to those of the
spray water system. High quality water is required for this system in
view of its heat exchanger use. The current treatment methods include
sedimentation of any suspended solids, installation of filters in the circuit
and the addition of corrosion inhibitors, deposit control agents and
microbiocides. Excepting for the use of non-chromate inhibitors, the
present methods should be effective and compatible under the zero discharge
condition of the 1983 guidelines.
HOT MILLS
In all integrated steel mills, steel slabs are put through a hot-
strip rolling process. In this process the slab, sheet, bloom billet or
bar is being oxidized, cooled and washed with a high pressure water spray.
The layer of iron oxide (mill scale) formed on the entire surface of the
steel, as it is being rolled, is broken away from the steel. It then
falls through the roll tables into the flume or sewer through which high
velocity water is flowing. Large amounts of lubrication greases and oils
from the rolling machinery as well as other mill debris find their way
into the flume. The principal contaminants in the effluents from the hot
mills are mill scale, oil and grease. The quantity of scale produced is
estimated at about 3% of the output of the mill. The volume of water
used in the hot forming operation, the nature and quantities of the mill
scale in the waste waters varies widely with different steel mills.
D. J. Juvan, "Design of Critical System for Continuous Casters", paper
presented at the 1975 AIME-National Open Hearth And Basic Oxygen Steel
Conference, Toronto, Canada, April 1975.
26
-------
f
MAKE-UP
\
\SPRAY/
\TOWER/
SPRAY WATER
FILTERS
^_n— 1
y STAINLESS STEEL
~ / SPRAY HEADERS
/ & PIPING
\r
FLUME
SEDIMENTATION
BASIN
SCALE
PIT
STORAGE
TANK
MACHINE WATER
MOLD STORAGE
TANK OVERFLOW
OIL
SKIMMER
OVERFLOW
WEIR
FIGURE 6, SPRAY-COOLING RECIRCULATING SYSTEM
Society of AIME)
27
-------
to
oo
__ /MOLD HEAT
^EXCHANGER
r FURNACE
COOLING
FIGURE 7, TREATMENT AND RECIRCULATION SYSTEM FOR MACHINE COOLING WATERS
[TAKEN FROM,, "DESIGN OF CRITICAL SYSTEM FOR CONTINUOUS CASTERS", BY D,J, JUVAN,
1975 AIME-NATL, OPEN HEARTH AND BASIC OXYGEN STEEL CONFERENCE, TORONTO, CANADA,]
(Iron and Steel Society of AIME)
-------
TABLE 6. TYPES OF DEPOSITS THAT CAN PLUG SPRAY NOZZLES AND HEADERS*
Constituent, %
Silica, Si02
Aluminum, A120»
Iron, Fe
Loss on Ignition
Phosphate, P2°s
Magnesium, MgO
Zinc, ZnO
Sulfate, SO™
Calcium, CaO
Carbonate, C09
CASE I
1
-
79
4
1
1
2
-
7
5
CAl II
3
1
68
19
-
-
4
3
1
1
CASE III
8
5
13
65
2
1
4
-
2
_
Case I - Corrosion of System Piping and Headers
Case II - Unsatisfactory Oil and Scale Removal
Case III - Biological Fouling (30-40% Bacterial Slime)
Taken from "Design of Critical Systems For Continuous Casters",
by D. J. Juvan, 1975 AIME Natl. Open Hearth and Basic Oxygen Steel
conference, Toronto, Canada.
29
-------
11000 - 38000 liters of water per tonne of steel rolled might be applied
in this operation.9-12
The hot mill operation generate two types of waste waters, viz.,
waters from the furnace cooling and process waters from the hot rolling
operations. The furnace cooling waters do not get contaminated except
for heat load. Depending upon the water problems of the individual steel
mills, the furnace cooling water can be cooled, treated for corrosion
inhibition and recirculated; or else blown down into the recirculating
process waters of the hot rolling operations.
The treatment of the process waste waters from the hot mill operations
briefly consists of (1) collecting the waste waters in a scale pit,
(2) removing the scales by gravity settling, (3) removal of the oil by
means of an oil skimmer, (4) removal of the finely divided scales, suspended
solids and oil by using a deep-bed sand filter (5) passage of the treated
water through cooling tower to reduce the thermal load and (6) recirculate
the water to the hot-strip mill after a small blow down in order to control
the dissolved solids content of the recirculating water.
The chemical additives to the recirculating water system consist
of (1) polyelectrolyte feed to flocculate the finely divided scales and
other suspended particulates (2) acid and other descaling compounds to
control the chemical stability of the water (Ryzner Stability Index) and
(3) chemicals for controlling bacterial growth and foaming.
PROBLEMS OF SCALING, CORROSION AND FOULING
On the basis of a review of the published data and discussions with
the environmental control personnel of the steel mills that we visited,
it appears that no serious corrosion problems are encountered under the
recirculating conditions of operation at present. The formation of deposits
and chemical scales is a problem which is being controlled through the
addition of scale inhibitors and dispersing compounds recommended by the
water treatment companies. Under zero discharge conditions of the 1983 EPA
effluent guidelines, it is expected that the problem of chemical scale
formation might be more severe due to potentially increased dissolved
solids and finely divided suspended solids content of the recirculating
waters and the lack of automatic monitoring and control of the chemical
stability of the waters. R&D efforts directed toward the development of
an effective method for chemical stability of aqueous systems should be
undertaken.
9
J. P. Gravenstreter and R. J. Sanday, Iron and Steel Engineer, 46, 85
(1969).
C. Browman, Blast Furnace Steel Plant, 59, 19 (1971).
K. S. Patton, Iron and Steel Engr., 48, 98 (1971).
Tl. Nebolsine, Iron and Steel Engr., 48, 85 (1971)\
30
-------
PICKLING WAC; ; WATERS
Since a large percent of the steel products produced in an integrated
steel mill undergo acid pickling for removing the scale, the problem of
handling the spent pickle liquor and the rinse water, their treatment for
re-use or safe disposal is of great importance. Either sulfuric or
hydrochloric acid is used in the acid baths. The waste waters from the
pickling operation consist of spent pickle liquor, acid rinse water and fume
scrubber water. From published data, it is known that the spent pickle
liquor contains 6-12% iron, 1-11% free acid, traces of heavy metals, oil
and suspended solids-'-. Several methods are currently being practiced by
the steel industry for handling this waste, i.e., (1) regenerating the
acid (HCl or I^SCfy) and recycling it to pickling operations, (2) deep well
injection, (3) use as a source of iron and iron compounds (4) use for
removal of phosphate from municipal sewage plants and (5) precipitating
the iron through neutralization followed by removal of the precipitate
and discharging the clarified effluent into rivers and streams. The
feasibility of a closed loop system for the treatment of the waste pickle
liquor was recently reported.13 The closed-loop system consists of (a) a low
temperature crystallization of FeSC>4"7H20 from sulfuric acid waste liquor,
(b) ion exchange adsorption of the ferrous iron from the ferrous sulfate,
(c) oxidation of the ferrous to the ferric iron, (d) stripping the adsorbed
iron from the ion exchange by HN03, (e) hydrolysis of the Ferric nitrate
to Fe2C>3 and HN03 and (f) recovery and recycle of the HN03 back to the ion
exchange unit. The sulfuric acid recovered from the crystallizer is recycled
back to the pickling operation.
The rinse water wastes generated by acid pickling are neutralized
with lime, allowed to settle and the effluent discharged into rivers and
streams. The gummy sludge produced is disposed at remote land areas or
into deep sea. All these methods of handling the pickle rinse water cause
some degree of environmental problem. Nippon Steel Corporation of Japanl^
recently has developed a recirculating ion exchange method for the treatment
of acid-rinse water generated in the pickling process.
Fume scrubber water system absorbs the corrosive mist and vapors given
off from the surface of the acid-pickling baths. Water from this system
is combined with spent acid-rinse wastes and treated for acid removal and
recycling.
Neither the published literature nor discussions with the environmental
control personnel of the different steel mills which we visited have
brought out any problems of scaling or fouling. However, in view of the
strongly acidic nature of the waste waters, several corrosion problems are
D. Kwasnowski, International Metallurgical Reviews, JZO, 137-145 (1975).
13
J. C. Peterson, EPA Report No. EPA-600/2-77-127, (1977); NTIS-PB-270-090.
14
"The Kimitsu Ion Exchange System for pickle rinse water treatment", by
Nippon Steel Corporation, 1975.
31
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expected in the supply lines and in the process equipment. This problem
can be controlled by appropriate surface treatments of the materials of
construction and through the addition of suitable corrosion inhibitors.
In conforming to the 1983 EPA effluent guidelines, it is felt that
the existing technology can adequately handle the controlling of the
potential problems of corrosion and fouling.
COLD ROLLING
Since the cold rolling operations consist of removing the rust preventive
oil coating from the steel strips and processing them in the presence of lu-
bricants consisting of oil and water, the waste waters generated in this
process contain both floating and emulsified oils, detergents, cleaning
chemicals, mill scale and other suspended solids. The current waste
treatment practices consist of (1) skimming the floating oil, (2) filtration
through a multilayer sand and gravel filter to remove suspended solids
and emulsified oil and (3) further removal of oil in a thickener. •*• •>
Alternately, the waste waters are subjected to aeration and the floating
oils are skimmed. The emulsified oils left in the waters are treated with
acid together with ferric or aluminum sulfate (or chloride) to split the
emulsion into oily and aqueous layers. After removing the oil, the pH of
the system is raised to precipitate the iron (or aluminum), allowed to settle
and filtered. The treated effluent is either sent to terminal treatment
facility or discharged into the plant outfalls. The solid residue is
disposed at distant land sites or into deep ocean. A schematic of the
treatment of the cold-rolling waste waters taken from the published
literature-*- is shown in Figure 8.
On the basis of the published data, it would appear that chemical
scaling and corrosion might not be problems of critical concern in the
currently practised methods. Biological fouling and foaming are encountered
and the currently practised technology is adequate for their control.
However, under conditions of zero discharge in accordance with the 1983
effluent guidelines, the recirculating system would show considerable
build-up of dissolved solids and finely divided solids (chemically pre-
cipitated material) and oil and grease which in turn could be expected to
result in deposit formation and pluggage. Further, finely divided solids
from bacterial growth could present a problem in a recirculating system
under zero discharge conditions. In the event of recycling the treated
effluents, control of the chemical stability of the effluent waters and
addition of dispersants and scale inhibitors should be considered.
15
v C. R. Seymons and J. Water, J. Water Pollution Control Federation,
43_, 2280 (1971).
1D. Kwasnowski, International Metallurgical Reviews, 20, 137-145 (1975).
32
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CO
oily
waste -
waters
(oil,
solids)
skimmed
oil
mill
sump
solids-liquid
separator
sludge i >
treated
effluent
backwash
water
deep-
bed
filters
backwash
sludge
FIGURE 8, TREATMENT OF WASTE WATERS OF COLD-ROLLING OPERATIONS
[TAKEN FROM "WATER POLLUTION CONTROL IN AN INTEGRATED STEEL PLANT"
BY D, KWASNOWSKI, INTL, METALURGY, REV,, 2CL 137-145 (1975)]
-------
METAL COATING OPERATIONS
The metal coating operations include hot galvanizing, lead-tin alloy
coating and, electrolytic plating of tin and chromium. The waste streams
from these operations contain zinc, tin, fluorides, chromates, cyanides,
acids and alkalis. Currently practised waste treatment methods consist of
precipitation of the metals as their hydroxides, precipitation of the
fluorides as the calcium fluoride and destruction of the cyanides by an
alkaline chlorination method or alternately by biological oxidation methods.
Ion exchange methods are also applied for the recovery of chromium from
the waste waters^>2. Recently an innovative rinse-and-recovery system for
metal finishing processes has been developed which consists of a two stage
solvent spray rinse followed by a single stage aqueous immersion rinse
with no plating solution exits to the sewer*". Although severe corrosion
and scaling problems may not be encountered, the toxicity of the treated
effluents is of great concern. Chromium, lead, zinc and cyanide are among
the toxic components. Methods for their automatic (on-line) determination
and control should be instituted into the recirculating system.
D. Kwasnowski, International Metallurgical Reviews, 20, 137-145 (1975),
2R. Nebolsine, Iron and Steel Engineer, 44, 122-135 (1967).
W. C. Trnka and C. J. Novotny, EPA Report No. 600/2-77-099 (1977).
34
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REFERENCES
1. Kwasnowski, D. International Metallurgical Reviews, 20, 137-145 (1975).
2. Nebolsine, R. Iron and Steel Engineer, 44, 122-135 (1967).
3. "Development Document for Effluent limitation guidelines and New
Source Performance Standards for the Steel Making Segment of the Iron
and Steel Manufacturing Point Source Category"; EPA-440/l-74-024-a.
4. Schroeder, J. W., and A. C. Naso, Iron and Steel Engineer, 1976,
p. 60-66.
5. Wong-Chong, G. M., S. C. Caruso and T. G. Patarlis. Carnegie-Mellon
Institute Report entitled, "An Evaluation of EPA recommended technology
for the treatment and control of waste waters from by-product coke-
plants - Alternate 2.
6. Touzalin, R. E. "Pollution Control of blast furnace plant gas scrubbers
through recirculation", Interlake Steel Corporation's report prepared
for U.S. EPA, Project 12010.
7. Rrikau, Fred G., and Roger R. DeCaigny. San Francisco Regional
Technical Meeting of the Amer. Iron and Steel Inst., 1970.
8. Juvan, D. J. "Design of Critical System for Continuous Casters",
paper presented at the 1975 AIME-National Open Hearth And Basic
Oxygen Steel Conference, Toronto, Canada, April 1975.
9. Gravenstreter, J. P., and R. J. Sanday. Iron and Steel Engineer,
46_, 85 (1969).
10. Browman, C. Blast Furnace Steel Plant, 59, 19 (1971).
11. Patton, R. S. Iron and Steel Engr,, 48., 98 (1971).
12. Nebolsine, R. Iron and Steel Engr., 48. 85 (1971).
13. Peterson, J. C. EPA Report No. EPA-600/2-77-127, (1977); NTIS-PB-
270-090.
14. "The Kimitsu Ion Exchange System for pickle rinse water treatment",
by Nippon Steel Corporation, 1975.
35
-------
15. Seymons, C. R., and J. Water. J. Water Pollution Control Federation,
43, 2280 (1971).
16. Trnka, W. C., and C. J. Novotny. EPA Report No. 600/2-77-099 (1977),
36
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BIBLIOGRAPHY
1. Wong-Cheng, G. M., and S. C. Caruso. Carnegie-Mellon Report entitled,
"An evaluation of the treatment and control technology recommended
by EPA for the blast furnace (iron) waste water", Aug. 1976.
s
2. Steiner, B. A., and R. J. Thompson. Jl. Air Pollution Control
Association, 27 (11) 1069-75 (1977).
3. Martin, J. R. Steel Times Annual Review, 1975, p. 678-682.
4. Myatt, R. T., R. J. Aston and K. S. Johnson. Iron and Steel
International, 4£, 421-424 (1973).
5. Anon. "BOF facility and combination mill in full operation at
Bethlehem", Iron and Steel Engineer, Aug. 1969, p. 88-94.
6. Plumer, F. J. Iron and Steel Engr., 46, 124-132 (1969).
7. Wallace, DeYarman. Iron and Steel Engr., 47, 83-87 (1970).
8. Bell, J. P, Industrial Wastes, 1976, p. 20-22.
9. Harris, E. R., and F. R. Beiser. Jl. Air Poll. Control Assoc., 1965,
46-49.
10. Hodsden, J. B., and W. L. Vankley. Iron and Steel Engr., 51,
49-53 (1974).
11. Corey, B. M., and A. C. Elliot. Iron and Steel Engr., 51, 81-87 (1974).
12. Smith, D. W. Jl. Water Poll. Control Federation, 48_ (6) 1287-93 (1976).
13. Temmel, F. M. Am. Iron and Steel Inst., Regional Meeting at
San Francisco, 1971, p. 343-362.
14. Theegarten, H. F., and R. K. Von Hartman. Iron and Steel Engr.,
50, 67-74 (1973).
15. Anon. Iron and Steel, Spl. Issue 1972, p. 150-159.
16. Cook, Y. W. Iron and Steel International, 1974, p. 393-402.
37
-------
17. Nebolsine, R., I. Pouschine. Iron and Steel Engr., 49, 89-91 (1972).
18. "Integrated Steel Plant Pollution Study For Zero Water And Minimum
Air Discharge", Hydrotechnic Corporation Report to EPA, Contract
No. 68022626.
19. "Development Document For Interim Final Effluent Limitations guidelines
And Proposed Hew Source Performance Standards For the FORMING,
FINISHING AND SPECIALITY STEEL Segments of the IRON and STEEL MAKING
INDUSTRY", Volume II; EPA Report No. 440/1-76/048-b Group 1,
Phase II (March 1976).
20. Nelson, R. E., and D. L. Bunn, Amer. Iron and Steel Inst. Regional
Tech. Meeting, San Francisco, 1973.
38
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APPENDIX A
EFFLUENT LIMITATIONS GUIDELINES OF U.S. EPA SET ON THE BASIS OF BPCTCA AND BATEA
BPCTCA
kg/kkg product
BATEA
kg/kkg product
u>
MS
Subcategory
Coke-oven
(By-product recovery
process)
Beehive-oven Process
Sinter Plant
Blast Furnace
(iron)
Pollutant Parameter For
Cyanide
Phenol
Ammonia
Oil and Grease
Suspended Solids
Sulfide
No discharge of process
Suspended Solids
Oil and Grease
Sulfide
Fluoride
PH
Suspended Solids
Cyanide
Phenol
Ammonia
Sulfide
Fluoride
pH
Maximum
Any One
0.0657
0.0045
0.2736
0.0327
0.1095
Daily
Average over Maximum
Day 30 days
0.0219
0.0015
0.0912
0.0109
0.0365
waste water pollutants to
0.0312
0.0063
0.0780
0.0234
0.0063
0.1953
0.0104
0.0021
6.0 to 9.
0.0260
0.0078
0.0021
0.0651
6.0 to 9.
For Any One Day
0.0003
0.0006
0.0126
0.0126
0.0312
0.0003
navigable waters
0.0156
0.0063
0.00018
0.0126
0
0.0390
0.0004
0.0008
0.0156
0.0005
0.0312
0
Daily
Average over
30 days
0.0001
0.0042
0.0042
0.0042
0.0104
0.0001
0.0052
0.0021
0.00006
0.0042
0.0130
0.00013
0.00026
0.0052
0.00016
0.0104
-------
•p-
o
APPENDIX A (cont.)
EFFLUENT LIMITATIONS GUIDELINES OF U.S. EPA SET ON THE BASIS OF BPCTCA AND BATEA
Subcategory
Blast Furnace
(Ferromanganese)
s
Basic oxygen
Furnace
(semi-wet air
pollution methods)
Pollutant Parameter
Suspended Solids
Cyanide
Phenol
Ammonia
Sulfide
Manganese
PH
Suspended Solids
Fluoride
PH
BTCTCA
kg/kkg product
Daily
Maximum Average over
For Any One Day 30 days
0.3129 0.1043
0.4689 0.1563
0.0624 0.0208
1.5636 0.5212
6.0 to 9.0
No discharge of process
pollutants to navigable
BATEA
kg/kkg
Maximum
product
Daily
Average over
For Any One Day 30 days
0.0780
0.0008
0.0016
0.0312
0.0009
0.0156
waste water
waters.
0.0206
0.00026
0.00052
0.0003
0.0052
0.0052
Basic oxygen
Furnace
(Wet Air Pollution
control methods)
Open Hearth
Suspended Solids
Fluoride
PH
Suspended Solids
Fluoride
Nitrate (N0_)
Zinc
PH
Electric Arc Furnace Suspended Solids
(Semi-wet air Zinc
pollution control) Fluoride
PH
0.0312
0.0104
6.0 to 9.0
0.0156 0.0052
0.0126 0.0042
6.0 to 9.0
0.0312
0.104
0.0156
0.0126
0.0282
0.0030
0.0052
0.0042
0.0094
0.0010
6.0 to 9.0
6.0 to 9.0
No discharge of process waste water
pollutants to navigable waters
-------
APPENDIX A (coiit.)
EFFLUENT LIMITATIONS GUIDELINES OF U.S. EPA SET ON THE BASIS OF BPCTCA AND BATEA
Subcategory
Electric Arc Furnace
(Wet Air Pollution
Control methods)
Vacuum Degssing
Continuous Casting
^
Pollutant Parameter
Suspended Solids
Fluoride
Zinc
PH
Suspended Solids
Zinc
Manganese
Lead
Nitrate (as N0_)
PH
Suspended Solids
Oil and Grease
PH
BTCTCA
kg/kkg product
Daily
Maximum Average over
For Any One Day 30 days
0.0312 0.0104
6.0 to 9.0
0.0156 0.0052
6.0 to 9.0
0.0780 0.0260
0.0234 0.0078
6.0 to 9.0
BATEA
kg/kkg product
Maximum
For Any One Day
0.0156
0.0126
0.0030
6.0 to
0.0078
0.0015
0.0015
0.00015
0.0141
6.0 to 9
0.0156
0.0156
6.0 to 9
Daily
Average over
30 days
0.0052
0.0042
0.0010
9.0
0.0026
0.0005
0.0005
0.00005
0.0047
.0
0.0052
0.0052
.0
Taken from "Development of Effluent Limitation guidelines and New Source Performance Standards For The
Steel Making Segment of the Iron and Steel Manufacturing Point Source Category", EPA-440/1-74-024-1)
-------
APPENDIX B
In connection with the development of information of the current
waste-treatment practices of the iron and steel industry, a number of
integrated steel mills and water treatment companies were visited. Also a
number of environmental engineers and specialists were consulted in this area.
Following is a list of the contacts made in connection with this work.
The steel companies and the personnel visited are as follows:
(i) Inland Steel Co.
East Chicago Indiana
Mr. John Brough
Director of Air and Water Control
(ii) Interlake Steel Corp.
Chicago, Illinois >
Mr. Fred Krikau
Director, Environmental Control
(iii) U.S. Steel Gary Works
Gary, Indiana
Mr. J. H. Dickerson
Supdt. of Environmental Control
(iv) Armco Steel Corp.
Middletown, Ohio
Mr. Donald R. Perander
Environmental Engineering
(v) Kaiser Steel Corp.
Fontana, California
Mr. R. E. Garner
Asst. to Works Manager
Chemical Companies and Personnel Visited:
(i) Nalco Chemical Co.
Oakbrook, Illinois
Mr. Alex J. Bajusz
Industry Manager for Steel and Primary Metals
42
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(ii) Betz Laboratories
Lansing, Illinois
Mr. Richard Stone
Area Manager for Steel Industry
(iii) Calgon Corp.
Pittsburgh, Pa.
Mr. George Peabody
Market Manager, Water Managment
Mr. L. J. Persinski
Manager, Industrial Water Treatment Research
Other Groups;
(i) Chester Engineers,
Environmental Engineers & Planners
Coraopolis, Pa.
Mr. Walter Zabban
Industrail Waste Consultant
(ii) Hydrotechnic Corp.
New York, NY
Mr. H. J. Kohllman
Vice President,
Engineering Manager
(iii) Prof. George St. Pierre,
Department of Metallurgy
Ohio State University
Columbus, Ohio
(iv) Prof. J. Patterson
Environmental Engineering
Illinois Institute of Technology
Chicago, Illinois
(v) American Iron and Steel Institute
Task Force Committee headed by
Mr. William Benzer at Pittsburgh, Pa.
43
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TECHNICAL REPORT DATA
(Please read Instructions on the reverse before completing)
. REPORT NO.
EPA-600/2-79-024
2.
3. RECIPIENT'S ACCESSION-NO.
». TITLE AND SUBTITLE
Survey of Fouling, Foam, Corrosion, and Scaling
Control in Iron and Steel Industry Recycle Systems
6. REPORT DATE
January 1979
6. PERFORMING ORGANIZATION CODE
7. AUTHOR(S)
K.S. Rajan
8. PERFORMING ORGANIZATION REPORT NO.
9. PERFORMING ORGANIZATION NAME AND ADDRESS
TJT Research Institute
10 West 35th Street
Chicago, Illinois 60616
10. PROGRAM ELEMENT NO.
1BB610
11. CONTRACT/GRANT NO.
8-02-2617, Task 2-2
12. SPONSORING AGENCY NAME AND ADDRESS
EPA, Office of Research and Development
Industrial Environmental Research Laboratory
Research Triangle Park, NC 27711
13. TYPE OF REPORT AND PERIOD COVERED
Task Final; 8/77 - 1/78
14. SPONSORING AGENCY CODE
EPA/600/13
is. SUPPLEMENTARY NOTES
62, 919/541-2733.
project officer is John S. Ruppersberger, Mail Drop
is. ABSTRACT The report gives results of & review of the state-of-the-art for fouling,
foaming, corrosion, and scaling control in the treatment and recycle of process
waters of integrated iron and steel mills. Areas examined were: (1) the character
of the wastewaters generated in the different processes associated with iron and
steel making, (2) current treatment practices of the recirculating systems, and (3)
corrosion, scaling, fouling,"and foaming problems encountered in the treatment
processes and current methods for solving them. Much of the iron and steel industry
wastewater is derived from noncontact cooling and scrubbing operations. Present
technology appears adequate for treatment and control of the noncontact cooling
waters. Treatment of the scrubber waters, which are highly contaminated with sus-
pended and dissolved solids and a variety of pollutants, consists of removing the
suspended solids and heat loads, decreasing the dissolved solid content by blowdown,
and recycling the process water. Chemical scales and deposits constitute a major
problem, and are minimized by controlling the chemical stability of the recircuiating
waters through acid addition and blowdown. Research and development recommended
for process water treatment under a high degree of recycle include effective scale
inhibition and control, and automatic process water chemical stability monitoring.
17.
KEY WORDS AND DOCUMENT ANALYSIS
DESCRIPTORS
b.IDENTIFIERS/OPEN ENDED TERMS
c. COSATI Field/Group
Pollution
Scaling
Iron and Steel Industry
Waste Water
Circulation
Foaming
Corrosion
Cooling Water
Pollution Control
Stationary Sources
Recycling
Scrubbing Water
13B
11F
13H,07A
11C
13A
18. DISTRIBUTION STATEMENT
Unlimited
19. SECURITY CLASS (ThisReport)
Unclassified
21. NO. OF PAGES
48
20. SECURITY CLASS (Thispage)
Unclassified
22. PRICE
EPA Form 2220-1 (9-73)
44
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