Final Report
Contract CPA 22-69-78
FEASIBILITY STUDY OF NEW SULFUR OXIDE
CONTROL PROCESSES FOR APPLICATION
TO SMELTERS AND POWER PLANTS
Part I: The Monsanto Cat-Ox Process
for Application to Smelter Gases
Prepared for:
U.S. DEPARTMENT OF HEALTH, EDUCATION, AND WELFARE
NATIONAL AIR POLLUTION CONTROL ADMINISTRATION
DURHAM, NORTH CAROLINA
T\
b
STANFORD RESEARCH INSTITUTE
Menlo Park, California 94025 • U.S.A.
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STANFORD RESEARCH INSTITUTE
Menlo Park, California 94025 • U.S.A.
Finjl Report
Contract CPA 22-69-78
FEASIBILITY STUDY OF NEW SULFUR OXIDE
CONTROL PROCESSES FOR APPLICATION
TO SMELTERS AND POWER PLANTS
Part I: The Monsanto Cat-Ox Process
for Application to Smelter Gases
By KONRAD T SEMRAU
Prepared for
U.S. DEPARTMENT OF HEALTH, EDUCATION. AND WELFARE
NATIONAL AIR POLLUTION CONTROL ADMINISTRATION
DURHAM, NORTH CAROLINA
SRI Project PMU-7923
Approved
N K HIESTER. Director
Physical Sciences (Materials!
C J. COOK, Executive Director
Physical Sciences Division
Copy No.
8G
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CONTENTS
FOREWORD vii
I INTRODUCTION 1
II OBJECTIVES 5
III SUMMARY 7
IV PROCEDURES .- 13
A. Models 13
B. Cost Factors 13
C. Preparation of Technical Data and Cost Estimates .... 13
V PROCESS DESCRIPTION 15
VI PROCESS DATA AND COST ESTIMATES 21
A. Sulfur Oxides Recovery 21
B. Gas Cleaning 21
C. Operating Requirements for Cat-Ox Systems 22
D. Cost Estimates 22
VII GENERAL DISCUSSION 33
A. Evaluation of the Cat-Ox System 33
B. Disposal of By-Products 36
REFERENCES 39
APPENDIXES
A. SMELTER MODELS A-l
B. FACTORS AND CONDITIONS ASSUMED IN ESTIMATING CONTROL
SYSTEM COSTS B-l
C. GAS CLEANING SYSTEM C-l
iii
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ILLUSTRATIONS
Figure 1 Cat-Ox System for Smelter Gases 17
Figure 2 Capital Costs of Cat-Ox and Gas Cleaning Systems for
Smelter Gases 28
Figure 3 Annual Costs of Cat-Ox and Gas Cleaning Systems for
Smelter Gases 29
Figure 4 Model A Copper Smelters: Cat-Ox Systems on
Reverberatory Furnaces — Effect on Copper
Production Costs 30
Figure 5 Model B Copper Smelters: Cat-Ox Systems on
Reverberatory Furnaces — Effect on Copper
Production Costs 31
Figure 6 Model Zinc Smelters B, C, and D: Effect of
Cat-Ox Systems on Zinc Production Costs 32
Figure C-l Gas Cleaning System for Smelter Gases C-2
Figure C-2 Capital Cost of Gas Cleaning System for Smelter
Gases C-4
Figure C-3 Total Annual Cost of Gas Cleaning System for Smelter
Gases C-8
TABLES
Table I Summary of Estimated Costs — Cat-Ox Systems
Applied to Model Smelters 10
Table II Cat-Ox Systems for Smelter Gases — Requirements for
Materials, Utilities, and Labor 23
Table III Gas Flow Rates and Sulfuric Acid Production Rates
for Cat-Ox Systems 24
Table IV Cat-Ox Systems for Smelter Gases — Capital and
Annual Costs for Cat-Ox Systems Only 25
Table V Cat-Ox Systems for Smelter Gases — Capital and
Operating Costs for Cat-Ox and Gas Cleaning Systems . 26
iv
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CONTENTS (Concluded)
Table VI Comparison of Costs of Alternative Cat-Ox and
Contact Process Plants for Zinc Smelter Model D . . . 35
Table A-l Smelter Models — Summary of Gas Compositions and
Flow Rates and of Sulfur Emissions A-2
Table A-2 Metal Production by Model Smelters A-3
Table C-l Gas Cleaning System for Smelter Gas Liquid Flows and
Power Requirements C-5
Table C-2 Gas Cleaning System for Smelter Gases — Utilities
and Operating Labor Requirements C-6
Table C-3 Gas Cleaning System for Smelter Gases — Capital
and Total Annual Costs C-7
v
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FOREWORD
The final report for this study is presented in four separate and
independent parts;
Part I: The Monsanto Cat-Ox Process for Application to
Smelter Gases
Part II: The Wellman-Lord SO Recovery Process for Application
to Smelter Gases
Part III: The Monsanto Cat-Ox Process for Application to Power
Plant Flue Gases
Part VI: The Wellman-Lord SO Recovery Process for Application
to Power Plant Flue Gases
Information for use in this study was supplied to Stanford Research
Institute by Monsanto Company and Wellman-Lord, Inc. under terms of con-
fidentiality agreements between the U.S. Department of Health, Education,
and Welfare, Stanford Research Institute, and each of the cooperating
companies. In accordance with the agreements, Monsanto Company and
Wellman-Lord, Inc. have reviewed and released the parts of the report
dealing with their respective processes. The rights of prior review
and release are designed solely to permit the cooperating companies to
assure themselves that no proprietary or confidential data are being
revealed; they are not intended to restrict Stanford Research Institute's
rights and responsibilities tp report its conclusions so long as there
is no incidental disclosure of confidential information. Accordingly,
the release of the reports by Monsanto and Wellman-Lord does not imply
that these companies necessarily concur in all or any of the opinions,
judgments, or interpretations of fact expressed by the author, who
assumes sole responsibility for the report content.
vii
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I INTRODUCTION
Under the Systems Study for Control of Emissions — Primary Nonferrous
Smelting Industry (Contract No. PH 86-68-85), Arthur G. McKee & Company
and its subcontractor, Stanford Research Institute, carried out evaluations
of a number of sulfur oxide control processes as they might be applied to
offgases from nonferrous smelting. To permit evaluation of the technical
and economic feasibility of these control processes, a number of models of
smelters were created. Stanford Research Institute carried out the studies
necessary to determine the availability of markets for sulfur by-products
open to smelters in various areas, and the allowable production costs that
the smelters would have to attain in order to break even on the sulfur
re cov e ry ope ra t i ons.
The Division of Process Control Engineering of the National Air
Pollution Control Administration (DPCE-NAPCA) desires to extend the use-
fulness of the foregoing study by adding to it technical and economic
evaluations of new and potentially promising sulfur oxide control pro-
cesses. It also wishes to evaluate the same new processes for application
to power plants. Completion of these preliminary evaluations of the pro-
cesses will help determine their potential commercial acceptability.
DPCE-NAPCA has a specific interest in at least two control processes
being offered commercially, the Monsanto Cat-Ox process and the Wellman-
Lord SO Recovery process. However, both processes are proprietary, and,
2
as a matter of policy, DPCE-NAPCA does not wish to obtain proprietary and
confidential information on the processes. It does, nevertheless, wish to
obtain evaluations in nonconfidential terms. Broadly, DPCE-NAPCA wishes to
obtain estimates of the capital and annual costs of the control systems
for each of the assumed applications, together with appraisals of the tech-
nical constraints on each process and of the current states of develop-
ment of the processes.
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Stanford Research Institute was requested by DPCE-NAPCA to carry
out evaluations of the processes under the terms of confidentiality
agreements between the Department of Health, Education, and Welfare,
the owners of the proprietary processes, and Stanford Research Institute.
SRI, acting as a disinterested third party, was to make analyses of the
processes using information obtained from Monsanto Company and Wellman-
Lord, Inc., and to report the results to DPCE-NAPCA without compromising
any of the Monsanto or Wellman-Lord confidential data.
Because of the requirements of confidentiality, it is not permissible
to describe certain features of the Cat-Ox and Wellman-Lord processes.
The corresponding portions of the systems have had to be represented only
in terms of their general functions, and SRI's evaluation of these jgortions
has had to be presented in the jfprrn of conclusions without supporting data
or reasoning. In other instances, the parts of the systems could be de-
scribed in general, but specific details and design parameters could not
be revealed.
Within the scope of the present project, it would obviously have been
impossible to inspect and evaluate independently all the company records
and design data even had the cooperating companies been requested to per-
mit this and had they acceded to the request. The author of this report,
who also conducted the study, evaluated the information provided at his
request, using his own knowledge and relevant data from the literature
and other available sources. Whenever apparent discrepancies or uncer-
tainties were noted in the information, efforts were made to secure veri-
fication or clarification from the companies. In instances where resolution
of questions was not possible, or the information required proved to be
simply unavailable, the author employed his best judgment.
Throughout the following sections of this report, information for
which other sources are not specifically cited was generally obtained from
the cooperating companies and accepted by the author either because it
could be verified from other sources or because it appeared reasonable.
In other instances, information or estimates were provided by the companies
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that could not be verified independently or judged for reasonableness;
in such cases, the companies have been specifically cited as the sources.
In still other instances, the author did not accept the information or
estimates provided and in some cases substituted his own; such cases have
also been specifically noted.
The cooperating companies, at their own option and through substantial
efforts, provided the basic capital cost estimates for the model control
systems and the information for estimation of operating and maintenance
costs. The author in this case acted as a reviewer rather than as an
estimator. The estimates were checked for reasonableness and for possible
errors or omissions. For some components and cost factors, the author
modified the estimates, or substituted others of his own where he judged
them to be more appropriate than those supplied to him. The author also
prepared cost estimates for some auxiliary systems, using separate data
sources.
By specification of the power plant and smelter models, and by review
of the results, an effort was made to ensure that the cost estimates for
both control systems were made on strictly comparable bases. Although it
is unlikely that this objective has been met fully, the deviations are
probably within the precision of the estimates themselves.
For convenience, and at the request of DPCE-NAPCA, this final report
is presented in four separate and independent parts. This part deals only
with the Monsanto Cat-Ox process as applied to control of sulfur oxides
in the offgases from copper, lead, and zinc smelters.
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II OBJECTIVES
The objectives of the part of the study covered by this report are
as follows:
1. To prepare a block flow diagram of the Cat-Ox system
showing its configuration and its relation to the smelting
processes in which the sulfur oxides are generated.
2. To present estimated mass and volume flow balances for
the Cat-Ox system.
3. To prepare preliminary engineering estimates of the capital
investment and the total annual cost (including both fixed
and variable charges) for the Cat-Ox system.
4. To apply the estimates prepared in (3) to the gas streams
of the model smelters created in the previous Systems Study
for Control of Emissions — Primary Nonferrous Smelting
Industry, and to determine the total annual cost for each
model control system. From the estimates of total annual
costs, secondary estimates are to be made of the corresponding
incremental costs of producing the nonferrous metals, both
on the gross basis (without allowance for by-product recovery
credits) and on the net basis (with allowance for by-product
recovery credits).
5. To make a qualitative appraisal of technical constraints on
the application and operation of the control system.
6. To appraise (quantitatively, to the extent permitted by
available data) the economic constraints on the application
of the control system.
7. To assess the current state of development of the Cat-Ox
system, identifying any technological deficiencies whose
elimination might enhance the applicability of the system
to smelter gases.
The accomplishment of the objectives is subject to any restrictions
that may be imposed under the terms of the confidentiality agreement
between the Government, Monsanto Company, and Stanford Research Institute,
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Ill SUMMARY
The Monsanto Cat-Ox system for sulfur oxides recovery is essentially
an adaptation of the well-known contact process for sulfuric acid manu-
facture. The gas containing both sulfur dioxide and oxygen is passed
through a fixed bed of catalyst at an appropriate temperature and most of
the sulfur dioxide is oxidized to sulfur trioxide. The gas is then passed
through an absorption tower where the sulfur trioxide is absorbed in
recirculated sulfuric acid. The Cat-Ox system has been developed prima-
rily for use on power plant flue gases, but it is also adaptable to other
dilute gas streams — such as smelter gases — that contain about 2 percent
or less of sulfur dioxide. It differs from the conventional contact pro-
cess plant in three principal respects:
1. The feed gas entering the system either must be already at a
temperature high enough for conversion of the sulfur dioxide
to the trioxide in the catalytic converter, or else auxiliary
heat must be supplied to raise the temperature. Because of
the diluteness of the sulfur dioxide, the plant is not auto-
thermal ; that is, the heat released by the oxidation of the
sulfur dioxide is insufficient to preheat the feed gas to the
reaction temperature.
2. The system operates on wet gas. The feed gas is not dried
before it enters the converter.
3. The heat in the exit gas is used to a greater or lesser degree
to concentrate the sulfuric acid formed in the final absorption
s tep. )
The version of the Cat-Ox process proposed for use on smelter gases
is similar to the "Cat-Ox reheat system" proposed for application to
existing power plants (see Part III of Final Report for this study). The
feed gas must be preheated before entering the converter. Part of the
preheating is accomplished by transfer of heat from the hot gas leaving
the converter; the rest is supplied by an indirect gas-fired heater. The
partly cooled converter exit gas enters the absorption system where the
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sulfur trioxide is absorbed. If the concentration of the acid produced
is to be about 78 percent, the absorption system will consist of a single
absorption tower as in the power plant control systems. If 93-percent
acid is to be produced, as is specified in this study, additional equip-
ment and a more complicated arrangement must be used.
In smelter applications of the Cat-Ox process, it is practical to
concentrate the acid to 93 percent because the heat required is already
available in the exit gas and there is little other possible use for it.
Although the same thing could be done in the Cat-Ox reheat system for
power plants, the cost would be relatively much greater because of the
lower concentration of sulfur oxides; use of a separate, conventional
acid concentrator should be preferable with respect to both capital and
operating costs. In the integrated Cat-Ox system for power plants, heat
taken for concentration of the acid would be lost to the power generation
cycle.
Because the gas is not dried before conversion of the sulfur dioxide
to sulfur trioxide, the amount of sulfuric acid mist formed during the
gas cooling and absorption steps is relatively much higher than that
formed in the conventional contact process. Hence, a high-efficiency
mist collector must be used to recover the mist from the tail gas. In
the Cat-Ox process, a fiber-bed mist eliminator is employed.
Contaminants (dust and fumes, vapors, and gases) in the feed gas
present the same problems in the Cat-Ox system for smelters as they do
in conventional contact process plants. It was therefore specified by
SRI as a model condition that the gas should be assumed to be cleaned
to the same degree as it would be for use in a contact process plant.
It is reasonable to expect that the residual contaminants in the clean
gas should produce no more problems in a Cat-Ox plant than they would in
a conventional contact plant.
All the basic concepts of the Cat-Ox process have been demonstrated
previously, and a process similar to the smelter-gas version of the
Cat-Ox system has been in commercial operation in Europe for a number of
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years. The Cat-Ox system is not, in principle, restricted to use on
dilute gas streams, but it offers no advantages over the contact process
for use on rich gases. The principal limitations on its application to
smelter gases appear to be economic rather than technical. The recovery
of sulfur dioxide from dilute gas streams is inherently expensive,
regardless of the process used. In addition, sulfuric acid is not a
desirable by-product in the geographical areas of the United States
where the largest number of applications of the Cat-Ox process to smel-
ters might be made. The markets for acid are limited, and they can be
more economically supplied from plants operating on richer gases.
In the model studies using the smelter models created by Arthur
G. McKee & Company, the Cat-Ox process was applied only to gas streams
containing 2 percent or less of sulfur dioxide. The results are sum-
marized in Table I, which presents the gross production cost for sulfuric
acid (100-percent basis) and the gross incremental cost of producing the
metals, before allowance of credits for sale of the 93-percent product
acid. Even with the richest of the gases, containing approximately
2 percent of sulfur dioxide, the acid production cost exceeded $ll/ton,
which would make the acid noncompetitive with that from alternative
available sources (such as rich smelter gases) in such areas as Montana-
Idaho and Arizona-New Mexico-West Texas. For gases containing less than
2 percent of sulfur dioxide, the acid production cost rose rapidly with
decrease in the sulfur dioxide concentration. Plant size was an additional
but much less important factor in the determination of the acid production
cost.
For the copper reverberatory furnaces, the gross incremental cost of
producing copper (without by-product credit) ranged from 0.4 to 0.8 cent/lb,
depending upon sulfur dioxide concentration and plant size. For the model
zinc smelters, the gross incremental cost of producing zinc, resulting
from control of sulfur oxide emissions in dilute gases, ranged from 0.9 to
2.6 cents/lb. These incremental costs would be reduced relatively little
by any by-product credits likely to be realizable. However, in the case
of copper, the gross incremental cost is a small fraction of the current
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Table I
SUMMARY OF ESTIMATED COSTS
CAT-OX SYSTEMS APPLIED TO MODEL SMELTERS
Model
Smelter
Copper
Model A
Model B
Zinc
Model B
Model C
Model D
Gas
Stream
Reverberatory
Furnace
Small
Medium
Large
Reverberatory
Furnace
Small
Medium
Large
Roaster
Sinter
Plant
Roaster
Sinter-
Roaster
soa
Concentration
1.89
0.91
0.9
0.5
0.8
2.0
Gross Acid
Production
Cost
($/ton)1
15.54
12.62
11.37
38.28
30.29
27.00
26.82
69.18
25.89
11.75
Gross
Incremental
Cost of
Metal
Production
0.84
0.67
0.61
0.59
0.47
0.42
1.75
0.86
1.97
0.92
Includes both fixed and variable charges. See Appendix B
for bases.
Before allowance for by-product acid credit.
10
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price of copper, or even of the increase in the price of copper that
has taken place in the period 1969-1970. On the other hand, the incre-
mental costs for production of zinc are larger not only absolutely but
also in relation to zinc prices.
11
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IV PROCEDURES
A. Models
The models of the hypothetical smelters, which are presented in
Appendix A, are the same as those formulated and used in the previous
9
study of smelter emission control by Arthur G. McKee & Company. Some
supplemental conditions, necessary to the specific analyses of the
Cat-Ox and Wellman-Lord processes, were specified by SRI and are also
given in Appendix A.
B. Cost Factors
The factors used in making the cost estimates for the control
9
systems were also taken from the McKee report, and are presented in
Appendix B. Some supplemental factors needed specifically for the
present study were specified by SRI.
The estimates of the prices that might be obtained for sulfur by-
9
products were also taken from the McKee report.
C. Preparation of Technical Data and Cost Estimates
The Monsanto Company prepared the technical designs for the model
control systems, based on the model conditions specified by SRI, and
estimated the capital investments and the utility and maintenance
requirements. In making the estimates, Monsanto used as its basis a
relatively detailed cost estimate that it had made for a proposed Cat-
Ox plant for an actual smelter. They estimated the capital costs for
the ten hypothetical installations individually by applying appropriate
ratios to the costs of the component parts of the base-case plant. The
author reviewed the estimates, but since the design data and specific
component configurations were in most cases not revealed by Monsanto,
it was generally impractical to make a critical, independent analysis
13
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of the capital cost estimates. The cost breakdowns for individual major
components of some of the systems were supplied by Monsanto, however, and
were surveyed only for general reasonableness and consistency with the
model specifications.
Because the original cost estimates made by Monsanto for the base-
case plant were made on the basis of assumptions somewhat different from
those used in the present study, the capital and operating cost estimates
for the ten hypothetical installations had to be modified. The modifi-
cations of the estimates were made by the author after obtaining additional
information from, and reviewing proposed changes with, Monsanto Company.
The cost estimates for the gas cleaning system used ahead of the
Cat-Ox system were prepared by the author (see Appendix C).
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PROCESS DESCRIPTION
The Monsanto Cat-Ox process is essentially a variation on the con-
tact process for sulfuric acid manufacture, which is extensively treated
4
in the literature. The process was developed primarily for removal of
sulfur dioxide from the flue gases of coal- or oil-fired power plants '
' ' in which the sulfur dioxide concentration usually falls within
the range from 0.1 to 0.4 percent by volume, but it can be used on richer
gases. It differs from the conventional contact process in three prin-
cipal respects:
1. Because of the diluteness of the sulfur dioxide, the plant is
not autothermal; that is, the heat released by the oxidation
of the sulfur dioxide is insufficient to preheat the feed gas
to the reaction temperature (generally above 800 F). Hence,
if the gas is not already near that temperature, auxiliary
heat must be employed.
2. The system operates on wet gas. The feed gas is not dried
before it enters the converter (catalytic reactor). Hence,
the amount of sulfuric acid mist formed is relatively much
greater than that formed in the conventional contact process,
and a collector must be employed to recover the mist from the
tail gas.
3. The heat in the exit gas from the converter is used to a greater
or lesser degree to concentrate the sulfuric acid formed in the
final absorption step. The concentration of the acid produced
depends upon the temperature to which the flue gas stream is
reduced during contact with the acid in the absorber.
All of the component basic concepts of the Cat-Ox system have been
demonstrated previously under some circumstances. Catalytic oxidation
of sulfur dioxide at low concentrations has been demonstrated previously
12
in the laboratory and in pilot plant studies that were preliminary
parts of the development of the Cat-Ox system. A similar process (SNPA-
Topsoe) has been in large-scale commercial operation at Lacq, France for
several years on the incinerated tail gases from a Glaus sulfur plant;
15
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there the concentration of sulfur dioxide is of the order of 1.0 percent,
and a two-stage absorption process is used to produce 94-percent acid.
Some relatively conventional contact plants for manufacture of
4
sulfuric acid from hydrogen sulfide have been built for use on wet gases.
The hot gas from the combustion of hydrogen sulfide (partly cooled in a
waste heat boiler and diluted with air to give a sulfur dioxide concen-
tration of about 7.5 percent) enters the converter without being dried.
The hot exit gas from the converter is not cooled, as is customary in
contact plants, but goes directly to an absorber. Because of the rela-
tively high water content of the gas stream, it is not feasible to produce
acid stronger than 93 to 94 percent. Also, relatively large quantities
of sulfuric acid mist are formed that must be removed from the tail gas
with some-type of mist collector. The principal advantage sought in this
type of plant is a reduction in capital cost.
This report treats a variation of the Cat-Ox process proposed for
application to smelter gases (Fig. 1), which is essentially similar to
the Cat-Ox reheat system intended for application to existing power
plants. The system treated is a conceptual design; no pilot or commer-
cial plant has been constructed and tested. However, as noted above,
components of the system have, in one way or another, been tested else-
where.
The Cat-Ox system shown in Fig. 1 is designed for use on gases con-
taining 2 percent or less of sulfur dioxide. In principle, the Cat-Ox
process is applicable to rich as well as lean gases. As applied to rich
gases, it would be very similar to the wet-gas plants designed for use
on the gases from combustion of hydrogen sulfide, which are described
4
above and in the literature. However, preliminary discussions between
Monsanto and SRI representatives gave indications that, for use on rich
smelter gases, the Cat-Ox process would have no substantial advantages
over the conventional contact process and might suffer some additional
penalties. Hence, no estimates were made for Cat-Ox plants to handle
gases containing more than 2 percent of sulfur dioxide.
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Tart Gas
Clean
Smelter
Product Acid
(93%)
Cooling
Water
Air
TA-7923-1
FIGURE 1 CAT-OX SYSTEM FOR SMELTER GASES
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When used on smelter gases, the Cat-Ox process is subject to the
same potential problems from contaminated feed gas as is the conventional
contact process — catalyst-bed plugging, catalyst poisoning, and corrosion.
However, it can reasonably be assumed that if the feed gas is purified to
the degree acceptable for use in contact plants, the residual contaminants
should produce no more problems in a Cat-Ox plant than in a standard con-
tact plant. Monsanto has prepared conceptual designs for smelter gas
Cat-Ox plants based on the assumption that the gases contained more inert
solids than are normally acceptable in contact plants, but that no excessive
quantities of catalyst poisons or corrosive materials (such as lead and
zinc oxides, fluorides, chloride, and arsenic) were present. For the present
study, SRI specified that the smelter gas entering the Cat-Ox system should
be assumed to: (1) have been purified in a gas cleaning system typical of
those used with contact sulfuric acid plants, (2) contain residual conta-
minants in concentrations at least as low as are specified by Donovan and
Stuber for acceptability in contact plants, and (3) be at 110 F and satu-
rated with water vapor. The capital and operating costs of the gas puri-
fication system are charged against sulfur oxide recovery, but were estimated
separately by SRI from literature data (see Appendix C and Section VI) and
are presented as separate items.
The cleani cold gas from the gas cleaning system enters a blower
(Fig. 1) that forces the gas through the Cat-Ox system. From the blower
the gas goes first to the heat exchanger where it is partly heated by
the hot exit gas from the converter, then to the natural-gas-fired pre-
heater where it is raised to a suitable temperature (in the range of 800
to 900 F) for catalytic oxidation of the sulfur dioxide in the converter.
The system is designed to give an exit concentration of sulfur dioxide
not exceeding 500 parts per million with inlet concentrations of up to 2.0
percent. The converter is therefore designed for higher conversions than
are the converters for the power plant units. With the richer smelter gas
streams, the heat released in the oxidation of the sulfur dioxide is suf-
ficient to raise the temperature of the gas to levels that may require
control. The design features used to provide for the desired conversion
18
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and for gas temperature control are based on established principles and
techniques but are not shown in Fig. 1 or described here, because Monsanto
does not wish to disclose the particular approaches used.
After being partly cooled in the heat exchanger, the converter exit
gas enters the absorption section of the plant, where product acid is
recovered. The acid recovered may be either relatively weak (78 percent,
for example) or strong (93 percent). The absorption system for production
of the weak acids is essentially identical to that used in the power plant
Cat-Ox system. ' Producing the strong acid requires more equipment
and a more complicated system. This latter system is not shown in Fig. 1
or described here because Monsanto does not wish to make a public dis-
closure of the specific method used. The system is, however, a feasible
one that has been anticipated in the prior art.
After leaving the absorption system, the gas passes through a Brink
or Cat-Ox mist eliminator, where the residual sulfuric acid mist and
entrained acid droplets are removed.
In the gas-to-gas heat exchanger of the Cat-Ox system, cold-end
corrosion must be avoided as it must be also in the similar exchangers
of the power plant control systems. In the smelter gas plant, corrosion
is prevented by the use of cocurrent instead of counter-current flow of
the cold and hot gases, but at the expense of reduced heat recovery from
the hot gas stream.
In the smelter gas Cat-Ox system, as in conventional contact acid
plants, the economic balance generally favors incurring relatively high
power costs in order to reduce capital investment. Consequently, the
converter is designed to use relatively deep catalyst beds and high gas
velocities while minimizing unit size. The pressure drop across the
clean converter is 27 inches of water, and that through the entire system
(when clean) is approximately 85 to 90 inches. Allowance is made for an
additional 60 inches of water pressure drop due to dirt buildup.
With the feed gas cleaned to the specified degree, it is anticipated
that it will not be necessary to remove and clean the catalyst, or clean
19
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the mist eliminator, more than once per year. This maintenance work can
be accomplished during the annual shutdown. Monsanto estimates the loss
of catalyst during a single cleaning operation at 5 percent, on the basis
of experience with conventional contact acid plants.
The production of 93-percent sulfuric acid was specified by SRI.
Weaker acid could be produced, as noted above, in a simpler and less
expensive absorption system. However, this modification would apparently
reduce the capital investment by no more than 5 to 6 percent and also
would not reduce the operating cost by any substantial fraction. The
weak acid would be salable only under special circumstances. Previous
9
market studies have shown that markets for even strong acid are limited
in precisely those areas where there are the greatest potential applica-
tions of the Cat-Ox process to smelter gases (see Section VII).
Dry, hot cleaning of the smelter gas fed to the Cat-Ox unit would
not necessarily result in major savings even if it were undertaken on
the hazardous assumption that no volatile or gaseous contaminants (such
as fluorides, chlorides, and arsenic) are present. Current, conventional
hot electrostatic precipitator installations could probably be expected
to allow passage of at least five to ten times the amount of dust that
would penetrate the combination wet cleaning system (see Appendix C).
The amount of catalyst cleaning and catalyst loss would be increased
proportionately, as would be the maintenance required on the mist elimi-
nator. The necessity for auxiliary heat would remain, since all of the
smelter gas streams are (at least once they have been cooled to permit
dry cleaning) still at temperatures below those required for operation
o
of the converter, which are generally above 800 F. Many dilute smelter
o 9
gas streams are at temperatures in the range of 200 to 500 F.
20
-------
VI PROCESS DATA AND COST ESTIMATES
A. Sulfur Oxides Recovery
In the smelter models (Appendix A) the distribution of the emitted
sulfur oxides between sulfur dioxide and sulfur trioxide is in most
cases not given. Where the concentration of sulfur trioxide in the gas
is not specified, the concentration of sulfur dioxide is calculated on
the assumption that all of the sulfur is in the form of sulfur dioxide
(see Table A-l). In fact, a variable quantity of sulfur trioxide (equi-
valent to a few percent of the total sulfur emission) will be present in
all cases, and will be removed in the gas cleaning system that precedes
the sulfur dioxide recovery system. However, for the sake of simplifica-
tion, it was assumed in this study that all of the sulfur was in the form
of sulfur dioxide and was treated in the sulfur dioxide control system.
The inputs and outputs of sulfur from the recovery systems were calculated
on this basis.
9
In the McKee study, different collection efficiencies for sulfur
dioxide were assumed for the various control systems modeled, as appeared
appropriate for the systems considered. In the present study, a uniform
standard of performance was set for both of the recovery systems (Cat-Ox
and Wellman-Lord) that were treated (see Appendix A). Both systems appeared
to be capable of attaining or exceeding the efficiencies specified, at least
within the ranges of input sulfur dioxide concentrations for which their
application was appropriate. The calculated collections of sulfur dioxide
and yields of by-products for the model plants were based on attainment
of the specified efficiencies.
B. Gas Cleaning
The costs (capital and operating) of preliminary gas cleaning (see
Appendix C) were taken to be the same for both the Cat-Ox and Wellraan-Lord
21
-------
systems. However, the costs for the gas cleaning and sulfur dioxide
recovery systems are presented separately in this report, and are
combined only in final totals.
The capital cost for the gas cleaning system does not include that
for the fan, which is assigned to the sulfur dioxide recovery system.
However, the operating cost for the gas cleaning system does include the
cost of electrical "power consumed by the fan that moves the gas through
the gas cleaning system.
The maximum allowable concentrations of impurities in the cleaned
gas (Appendix A and Reference 3) are apparently higher, in at least a few
instances, than the levels that are favored by some other authorities
or that are attained in some installations. For sulfuric acid mist,
3
Donovan and Stuber give an upper limit of 0.022 grain/std cu ft, which
2
is assumed in this study, whereas Carter favors a maximum of 0.0012
Q
grain/cu ft. Heinrich and Anderson report attainment of residual acid
mist and arsenic concentrations as low as 0.0000022 grain/cu ft with a
wet electrostatic precipitator.
C. Operating Requirements for Cat-Ox Systems
The factors used to calculate the materials, utilities, and labor
required for the various model control systems are summarized in Table II.
They are based on information obtained from Monsanto Company. The cost
of catalyst makeup is based on cleaning the catalyst once per year, with
an attendant loss of 5 percent.
D. Cost Estimates
The gas flow rates and the annual sulfuric acid production rates for
the model Cat-Ox systems are presented in Table III. The capital and(
total annual costs for the model Cat-Ox systems, alone, are presented1 in
Table IV. In Table V: the summarized capital and annual costs of both
the gas cleaning and Cat-Ox systems are presented separately and then
added to give the total costs for the complete sulfur dioxide recovery
systems. Table V also shows the contributions of both parts of the systems
22
-------
Table II
CAT-OX SYSTEMS FOR SMELTER GASES
REQUIREMENTS FOR MATERIALS, UTILITIES, AND LABOR
Catalyst Makeup
Annual cost = $0.105 per SCFM of gas rate
Makeup Cooling Water
5.66 gpm per 1000 SCFM of gas rate
Electrical Power
440 kwh per day per 1000 kwh SCFM of gas rate
Natural Gas
Labor
1162 CFH per 1000 SCFM of gas rate
1
3 men per shift - 72 man-hours per day
Supervision
2 man-hours per shift = 6 man-hours per day
Estimates of labor and supervision based on experience with
conventional contact acid plants.
23
-------
Table III
GAS FLOW RATES AND SULFURIC ACID PRODUCTION
RATES FOR CAT-OX SYSTEMS
Model
Smelter
Copper Model A
Reve rbe ra tory
Furnaces
Small
Medium
Large
Copper Model B
Reverberatory
Furnaces
Small
Medium
Large
Zinc Model B
Roaster
Sinter Plant
Zinc Model C
Roaster
Zinc Model D
Sinter-Roaster
Gas Flow Rate
ACFM
(110°F, 1 atm, sat'd)
81 ,000
162,000
243,000
48 ,800
97,700
146,600
154,400
55,500
409,600
166,900
Sulfuric Acid
Production
tons/year
(100% basis)
81,180
162,360
243,540
23 ,500
47,190
70,620
73,590
14,030
172,260
177,870
24
-------
Table IV
CAT-OX SYSTEMS FOR SHELTER GASES
CAPITAL AND ANNUAL COSTS FOR CAT-OX SYSTEMS ONLY1
Item
Capital Investment ($)
Annual Cost ($)
A. Depreciation
B. Direct Operating Cost
1. Labor
2. Supervision
3. Payroll benefits
4. Maintenance materials
5. Factory supplies
6. Catalyst makeup
7. Electricity
8. Natural gas
9. Cooling water makeup
C. Indirect Costs
1. Controllable
2. Noncontrollable
Total Annual Cost ($)
( Cross )
Copper Hod el A
Reverberatory Furnaces
Small
2,460,000
164,000
"• —
89 , 100
9,405
24,630
73,800
12 , 300
7,340
101,500
257,320
3,760
86,150
73 , 800
903,105
Medium
3,760,000
250,670
89,100
9,405
24,630
112 , 800
18,800
14,680
203,000
514,800
7,520
105,650
112,800
1,463,855
Large
4,820,000
321,335
89 , 100
9,405
24,630
144,600
24,100
22,020
304,500
772,000
11,280
121,550
144,600
1,989,120
Copper Model B
Reverberatory Furnaces
Small
1,691,000
112,735
89 , 100
9,405
24,630
50,730
8,460
4,425
61 , 20O
155,160
1,280
74,620
50,730
643,475
Medium
2,731,000
182,070
89 , 100
9,405
24,630
81,930
13,660
8,850
122 , 400
310,320
4,540
90,220
81,930
1,019, OSS
Large
3,570,000
238,000
89,100.
9,405
24,630
107,100
17,850
13,280
183 , 700
465,600
6,800
102,800
107,100
1,365,365
Zinc Model B
Roaster
3,650,000
243,335
89 , 100
9,405
24,630
109,500
18,250
13,990
193,400
490,400
7,160
104,000
109,500
1,412,670
Sinter
Plant
1,817,000
121,135
89 , 100
9,405
24,630
54,510
9,090
5,030
69,600
176,320
2,580
76 , 150
54,510
692,060
Zinc
Model C
Roaster
8,620,000
574,670
89,100
9,405
24,630
258,600
43,100
37,110
513,200
1,301,200
19 , 000
178,550
258,600
3,307,165
Zinc
Model D
Sinter-
Roaster
3,815,000
254,335
89 , 100
9,405
24,630
114,450
19,080
15,120
209,100
530,000
7,740
106,480
114,450
1,493,990
(0
Not including gas cleaning systems.
-------
Table V
CAT-OX SYSTEMS FOR SMELTER GASES
CAPITAL AND OPERATING COSTS FOR CAT-OX AND GAS CLEANING SYSTEMS
Capital Investment ($)
Gas Cleaning System
Cat-Ox System
Total
Annual Cost ($)
Gas Cleaning System
Cat-Ox System
Total
Acid Production Cost
($/ton 100% acid)
Gas Cleaning System
Cat-Ox System
Total
Gross Cost per Unit Quantity
of Metal Produced1
(e/lb)
Gas Cleaning System
Cat-Ox System
Total
Copper Model A
Reverberatory Furnaces
Small
1,830,000
2,460,000
4,290,000
358,000
903,000
1,261,000
4.41
11.13
15.54
0.24
0.60
0.84
Medium
2,950,000
3,760,000
6,710,000
583,000
1,464,000
2,047,000
3.59
9.03
12.62
0.19
0.48
0.67
Large
3,900,000
4,820,000
8,720,000
780,000
1,989,000
2,769,000
3.20
8.17
11.37
0.17
0.44
0.61
Copper Model B
Reverberatory Furnaces
Small
1,280,000
1,691,000
2,971,000
256,000
643,000
899,000
10.89
27.39
38.28
0.17
0.42
O.S9
Medium
2 , 080 , 000
2,731,000
4,811,000
408,000
1,019,000
1,427,000
8.65
21.64
30.29
0.13
0.34
0.47
Large
2,750,000
3,570,000
6,320,000
540,000
1,365,000
1,905,000
7.65
19.35
27.00
0.12
0.30
0.42
Zinc Model B
Roaster
.
2,850,000
3,650,000
6,500,000
560,000
1,413,000
1,973,000
7.61
19.21
26.82
0.50
1.25
1.75
Sinter
Plant
1,410,000
1,817,000
3,227,000
278,000
692,000
970,000
19.82
49.36
69.18
0.25
0.61
0.86
Zinc
Model C
Roaster
5, 550, 000
8,620,000
14,170,000
1,150,000
3,307,000
4,457,000
6.68
19.21
25.89
0.51
1.46
1.97
Zinc
Model D
Sinter-
Roaster
3,000,000
3,815,000
6,815,000
595,000
1,494,000
2,089,000
3.35
8.40
11.75
0.26
0.66
0.92
CO
Before allowance for by-product credits.
-------
to the total acid production costs and to the total gross incremental
costs of producing metal.
The capital and total annual costs of the gas cleaning systems and
Cat-Ox systems and the combinations of the two are shown graphically
in Figs. 2 and 3. The data points from which the cost curves were con-
structed were taken from Table V. It can be seen that the estimated
capital cost of the Cat-Ox system is not a function of the sulfur dioxide
concentration, at least over the range of concentrations considered (0.5
to 2.0 percent). The relationship of capital cost to system gas-handling
capacity is, however, affected by the size of the installation. In the
largest plant (410,000 CFM at 110°F and 1 atm), some of the components
in the system are composed of multiple units, which increases the ratio
of cost to capacity. It appears that for plants larger than about
400,000 CFM capacity, the capital costs will be about proportional to
capacity. However, in plants of up to about 250,000 CFM capacity, the
capital cost is proportional only to about the 0.6 power of capacity.
The annual costs of the Cat-Ox system (Fig. 3) reflect the capital
costs, but are essentially independent of sulfur dioxide concentration.
Hence, the unit costs of producing sulfuric acid are decreased appreci-
ably by increase in plant size, but are radically increased by decreases
in the sulfur dioxide concentration.
In Figs. 4 through 6 the changes in the unit production costs of
the metals, resulting from the application of Cat-Ox systems to the
various model smelters, are shown graphically as functions of the net
selling price of the product acid at the smelter. The net selling price
is equal to the gross selling price less sales cost and overhead, and
will, of course, be dependent upon the particular location of the plant.
As is noted below (see Section VII), it is unlikely that at most loca-
tions it would be possible to realize sufficient return from the sale
of acid to offset a substantial part of the gross incremental cost of
producing the metal. The model smelters that have the best opportuni-
ties are the Model A copper smelters and the Model D zinc smelter, which
have the richest gases (about 2 percent of sulfur dioxide).
27
-------
100
10
o.
U
TOTAL SYSTEM
CAT-OX SYSTEM
GAS CLEANING SYSTEM
I I I
10,000 100,000
GAS FLOW RATE—cu ft/min (110°F, 1 atm)
1,000,000
TA-7923-2
FIGURE 2 CAPITAL COSTS OF CAT-OX AND GAS CLEANING SYSTEMS
FOR SMELTER GASES
28
-------
10
2
=
o
T)
I 1.0
6
D
Z
Z
0.1
10,000
i—i—i i i i iu
TOTAL SYSTEM
CAT-OX SYSTEM
GAS CLEANING SYSTEM
I I I I I I I
100,000
GAS FLOW RATE—cu ft/min (110°F, 1 atm)
FIGURE 3 ANNUAL COSTS OF CAT-OX AND GAS CLEANING SYSTEMS
FOR SMELTER GASES
1,000,000
TA-7923-3
29
-------
i
8
•8
o
8 -1
UJ
O
I
5 -2
LARGE
5 10 15 20
NET SELLING PRICE OF SULFURIC ACID AT SMELTER— dollars/ton of 100% acid
25
TA-7923-4
FIGURE 4 MODEL A COPPER SMELTERS: CAT-OX SYSTEMS ON
REVERBERATORY FURNACES—EFFECT ON COPPER
PRODUCTION COSTS
30
-------
^
I
e
o
o
QC
111
o
1
o
-2
SMALL
MEDIUM
LARGE-
I
I
10 20 30 40 50
NET SELLING PRICE OF SULFURIC ACID AT SMELTER— dollars/ton of 100% acid
TA-7923-5
FIGURE 5 MODEL B COPPER SMELTERS: CAT-OX SYSTEMS ON
REVERBERATORY FURNACES—EFFECT ON COPPER
PRODUCTION COSTS
31
-------
o
o
o
a.
Z
u -1
CD
O
-2
10 20 30 40 50
NET SELLING PRICE OF SULFURIC ACID AT SMELTER— dollars/ton of 100% acid
TA-7923-6
FIGURE 6 MODEL ZINC SMELTERS B, C, AND D: EFFECTS OF
CAT-OX SYSTEMS ON ZINC PRODUCTION COSTS
32
-------
VII GENERAL DISCUSSION
A. Evaluation of the Cat-Ox System
There are no reasons to doubt the technical feasibility of the
—b-asig_Cat-Ox process. As is noted above, the basic process (essentially,
the contact process) is fully established. The version of the Cat-Ox
system proposed for application to smelters need not be seriously limited
by the types of problems encountered in power plant applications (see
Part III of Final Report for this study).
There are three principal technical problems (actual or potential)
in the Cat-Ox process; these are listed below in their order of impor-
tance:
1. Dust collection
2. Acid mist formation and collection
3. Corrosion.
Of these three, corrosion is dealt with most readily. It presents
its greatest threat in heat exchangers, where it can be avoided, but
only at the expense of reduced heat recovery and of increased capital
and operating costs. Dust collection is potentially by far the greatest
problem, one which is apparently not yet fully resolved in the Cat-Ox
systems designed for power plant applications. However, in the case of
smelter gas Cat-Ox plants, where the feed gas is to be cleaned by well
established methods, dust should not present any problems that cannot be
anticipated fully on the basis of experience with standard contact plants.
It should be possible to build a Cat-Ox plant for such service with
reasonable confidence by using experience gained in the design of con-
ventional acid plants.
Acid mist collection presents at least a potential problem primarily
because of its interrelationship with the dust collection problem. Col-
lection of the acid mist itself is not likely to present any serious
33
-------
problems, since fibrous mist eliminators of the Brink type have been
amply demonstrated to give both high efficiency and reliability in mist
collection. On the other hand, if there are solid particles in the gas
stream, they will tend to plug the fiber bed. However, with gas of the
cleanliness specified in this study, the rate of solids buildup in the
mist eliminator will probably be sufficiently low that no unreasonable
amount of maintenance will be required.
^—
The Cat-Ox system is appropriate only for use on relatively dilute
gases (those containing about 2 percent or less of sulfur dioxide).
For operation on richer gases, it has no obvious advantages over the
standard contact process. Without detailed comparative analysis, it is
not possible to determine at what (low) sulfur dioxide, concentration
level in smelter gas the Cat-Ox process would begin to show a net economic
advantage over the conventional contact process in which the feed gas is
dehumidified and then dried before it enters the converter. The conven-
tional process should avoid the potential corrosion problems associated
with direct conversion of the wet gas. Auxiliary heat will be required
4
at sulfur dioxide concentrations below a minimum of about 3.5 percent,
but more of the heat used can be recovered by exchange because the
danger of reaching the acid dew point temperature will be eliminated,
and less heat will be consumed in concentrating the acid produced. On
the other hand, additional energy must be consumed in refrigeration in
order to dehumidify the feed gas and maintain the water balance necessary
to produce concentrated acid. At sulfur dioxide concentrations under
2 percent, it will become progressively less practical to employ the
refrigeration necessary to permit production of acid as strong as 93
percent.
In Table VI the summarized capital and operating costs of alternative
Cat-Ox and contact process plants are presented for Zinc Smelter Model D,1
where the concentration of sulfur dioxide in the gas is 2.0 percent.
The capital and operating costs for the contact process plant include
allowances for use of auxiliary heat and of refrigeration of the cooling
water, and were drawn from Reference 9. The indicated costs show a
34
-------
Table VI
COMPARISON OF COSTS OF ALTERNATIVE CAT-OX
AND CONTACT PROCESS PLANTS FOR ZINC SMELTER MODEL D
Control System
Capital Cost ($)
Annual Cost ($)
Production Cost of
93 -percent Acid
($/ton 100% acid)
I
Cat-Ox
Plant
6,815,000
2,089,000
11.75
Contact Process
Plant2'
9,800,000
2,700,000
15.20
Includes gas cleaning system.
2
Includes auxiliary gas heater and refrigeration system for cooling
water.
3
Data from Reference 9.
substantial saving for the Cat-Ox system, although — in view of the
probable precision of the estimates — the actual cost differences might
not necessarily be as large as estimated. Presumably, the comparison
should increasingly favor the Cat-Ox system as the sulfur dioxide con-
centration in the gas is reduced below 2.0 percent.
The most severe apparentjlimitations on the application of the Cat-
Ox process to smelter gases are economic rather than technical. The
costs of recovering sulfur by-products from dilute gases are inherently
high, and this affects not only the Cat-Ox process but all alternative
recovery processes. A specific limitation on the Cat-Ox process is
that its product is sulfurie acid. As is discussed below, the geographi-
cal areas of the United States in which there is the greatest potential
application of the Cat-Ox process are those in which there are only
limited markets for the acid.
35
-------
B. Disposal of By-Products
The sizes and availabilities of markets for sulfur by-products
g
producible at smelters have been presented in the McKee report, in
which estimated by-product prices were based on an assumed Gulf Coast
sulfur price of $30/long ton, f.o.b. At the time of the previous
9
study it was estimated that the price of $30 was a likely average
for the period up to 1975. However, sulfur supplies have since shifted
from shortages to surpluses in a period of only about a year, and sul-
fur prices have become chaotic. The development of sulfur surpluses
has followed a continuing period of low activity in the market for fer-
tilizers, which provides the largest outlet for sulfur.
In the next five years the withdrawal of marginal producers and a
possible revival of the fertilizer market may cause sulfur prices to
rise from their present lows. However, the figure of $30/long ton now
appears to be an optimistic one. It perhaps represents the upper end
of the probable range of Gulf Coast prices to be encountered in the
period to 1975. It cannot even be assumed that the Gulf Coast sulfur
price will continue to maintain its previous status as the base line
for world sulfur prices. Nevertheless, if the above limitations are
recognized, the assumption of the $30/long ton price is probably as
good as any that can be made at this time. The estimates of sulfur
by-product prices in smelter areas that are presented in Reference 9
can therefore be used as a first approximation if it is understood that
they probably represent the most favorable situation that can be anti-
cipated.
Most of the U.S. smelter units to which the Cat-Ox process might
be applied are located in two regions, one comprised of Idaho and Montana,
and the other of Arizona, New Mexico, and western Texas. In neither of
these regions are there sufficient markets — current or foreseeable —
to absorb all of the sulfuric acid potentially producible from smelter
9
gases. The amount of sulfuric acid needed can be produced more econo-
mically from available rich gases than it can be from dilute gases by
36
-------
the Cat-Ox process or any other process for treating such gases. Local
conditions at some specific smelters may make the Cat-Ox process a desirable
one for treating the weak gases, even though probably not economic in the
conventional sense. Net selling prices for acid at the smelters may fre-
quently be $4/ton or less even for the quantities of acid that can be
sold at all.
37
-------
REFERENCES
1. Brink, J. A., Jr., W. F. Burggrabe, and L. E. Greenwell, Mist
Eliminators for Sulfuric Acid Plants, Chera. Eng. Progr. 64_ (11).
82-86 (Nov. 1968)
2. Carter, B. M. , Sulfuric Acid, Kirk-Othmer Encyclopedia of Chemical
Technology, 1st Ed., 13^ 458-501 (1954)
3. Donovan, J. R., and P. J. Stuber, Sulfuric Acid Production from
Ore Roaster Gases, J. Metals 19_ (11), 45-50 (Nov. 1967)
4. Duecker, W. W., and J. K. West (Eds.), "The Manufacture of Sulfuric
Acid," Reinhold Publishing Co., New York (1959)
5. Guyot, G., SNPA Process for the Treatment of Residual Gases with
Low Sulfur Dioxide Concentration, Chira. Ind., Genie Chim. 101 (1),
31-34 (Jan. 1969)
6. Guyot, G., Production of Concentrated Sulfuric Acid from Sulfur-
Containing Gases with High Water Vapor Content, Chim. Ind., Genie
Cliim. 101 (6), 813-816 (Mar. 1969)
7. Guyot, G., and J. P. Zwilling, SNPA's Process for H SO Production
Developed with Eye on Air Pollution, Oil Gas J. 64 T47J, 198-200
(Nov. 21, 1966)
8. Heinrich, R. F., and J. R. Anderson, Electro-Precipitation, in
Cremer and Davies, "Chemical Engineering Practice," Vol. 3, pp. 484-534,
Butterworths, London (1956)
9. McKee & Company, Arthur G., Systems Study for Control of Emissions —
Primary Nonferrous Smelting Industry, Final Report to National Air
Pollution Control Administration, June 1969; Contract No. PH 86-68-85.
10. Monsanto Company, St. Louis, Missouri, Air Pollution Control for
Electric Utilities, Bulletin (Undated — issued 1970)
11. Monsanto Enviro-Chem Systems, Inc., St. Louis, Missouri, Cat-Ox
System for Existing Power Generating Stations, Bulletin (Undated —
issued 1970)
12. Napier, D. H., and M. H. Stone, Catalytic Oxidation of Sulfur
Dioxide at Low Concentrations, J. Appl. Chem. 8_ (12), 781-786
(Dec. 1958)
39
-------
13. Stites, J. G., Jr., W. R. Horlacher, Jr., J. L. Bachofer, Jr., and
J. S. Bartman, Removing SO from Flue Gas, Chem. Eng. Progr. 65
(10), 74-79 (Oct. 1969)
14. Tigges, A. J. , Recovery of Values from Sulfur-Dioxide-Containing
Flue Gases, Brit. Pat. No. 1,074,937 (July 5, 1967)
15. Zawadzki, E. A., Removal of Sulfur Dioxide from Flue Gases: The
BCR Catalytic Gas Phase Oxidation Process, Trans. Soc. Mining
Engrs. AIME 232, 241-246 (Sept. 1965)
40
-------
Appendix A
SMELTER MODELS
The models of hypothetical copper, lead, and zinc smelters were
created by Arthur G. McKee & Company, and are described in full in the
A2
McKee report. The essential features of the models directly pertinent
to the present study are summarized here. Some additional model condi-
tions, specific to the present study, were set by SRI, as indicated
below.
In Table A-l are presented the data on gas stream compositions, gas
flow rates, and sulfur emissions for each of the model smelters. The
data on metal production at the model smelters are given in Table A-2.
The remainder of the model conditions are summarized below. Conditions
specified by SRI are noted by an asterisk.
A. Plant Operating Time
330 days/year
24 hours/day
B. Gas Cleaning
1. Gases are assumed to have been cleaned in a hot electro-
static precipitator to a residual dust and fume content of
0.1 grain/std cu ft. The cost of hot, dry gas cleaning is
not charged against sulfur oxides recovery..
*2. The gas leaving the hot precipitator is assumed to be cleaned
in a system typical of those used for cleaning feed gases
to a contact sulfuric acid plant (see Appendix C). The
costs (capital and operating) of this secondary cleaning
system are charged against sulfur oxides recovery. The
cleaned gas is assumed to be at 110 F and essentially 1 atm
pressure, and to be saturated with water vapor. It is
further assumed that the residual concentrations of impuri-
ties are at least as low as those Donovan and Stuber* have
specified to be acceptable in contact sulfuric acid plants:
A-l
-------
Table A-l
SMELTER MODELS
SUMMARY OF GAS COMPOSITIONS AND FLOW RATES AND OF SULFUR EMISSIONS
Plant
Copper
Zinc
Lead
Model
A
B
A
B
C
D
A
Process Unit
Reverberatory
Furnace
Converters
Roaster
Re ver ber at ory
Furnace
Converters
Roaster
Sinter plant
Roaster
Sinter plant
Roaster
Sinter -Roaster
Sinter plant
Gas
Temp.
550
645
550
550
637
600
500
600
300
400
300
400
Gas Composition
S°2
1.89
3.82
8.00
0.91
3.78
7.1
0.048
0.9
0.5
0.8
2.0
5.0
S°3
0.1
0.0016
0.29
°2
6.48
5.71
16.1
10.9
18.0
18.0
18.0
16.0
16.0
12.0
H2°
11.7
34.8
0.05
3.2
Gas Flow Rate
1,000 SCFM
(32°F, 1 atm)
Small
69.9
64.8 1
36.0
42.15
58. 043
19.31
29.8
133.2
47.9
—
—
10.67
Medium
139.8
129.6 2
72.0
84.3
116.1 4
38.62
59.6
—
353.4
144.0
21.34
Large
209.7
194.4 2
108
126.5
174.1 4
57.93
89.4
--
—
—
42.68
Sulfur Equivalent
(short tons /day)
Small
84.6
158.9
183.8
24.6
141.6
89.6
0.9
77.1
15.4
—
—
34.3
Medium
169.2
317.8
367.6
49.2
283.2
179.2
1.8
—
181.8
185.2
68.6
Large
253.8
476.7
551.4
73.8
424.8
268.8
2.7
—
—
—
137.2
T
to
1 Size recovery plant for 137% of average flow.
2 Size recovery plant for 130% of average flow.
3 Size recovery plant for 130% of average flow if gases combined with roaster gases.
4 Size recovery plant for 123% of average flow if gases combined with roaster gases.
-------
Table A-2
METAL PRODUCTION BY MODEL SMELTERS
Metal Production
Metal
Copper
Size
B
Zinc
B
C
D
Lead
Small
Medium
Large
Small
Medium
Large
Small
Medium
Large
Small
Medium
Medium
Small
Med ium
Large
tons/day
230
460
690
230
460
690
171.4
342.8
514.2
171.4
342.8
342.8
142.9
285.8
571.6
tons/year
75,900
152,000
228,000
75,900
152,000
228,000
56,600
113,000
170,000
56,600
113,000
113,000
47,200
94,300
189,000
A-3
-------
Concentration,
grains/std cu ft,
Contaminant dry basis
Chlorides (as Cl) 0.0005
Fluorides (as F) 0.0001
Arsenic (as As 0 ) 0.0005
Lead (as Pb) 0.0005
Mercury (as Hg) 0.0001
Selenium (as Se) 0.022
H SO mist (100%) 0.022
Total Solids (dust) 0.0005
*C. Sulfur Dioxide Collection Efficiency
The performance of the primary sulfur dioxide collection system is
to be as follows, defined in terms of collection efficiency or of sulfur
dioxide concentration in the exit gas:
SO Concentration Efficiency (%) or
in Feed Gas Concentration in
(%) Exit Gas (ppm)
Under 1.0 500 ppm
1.0 to 2.0 95%
Over 2.0 98%
In cases where sulfur dioxide is concentrated for conversion to
another product in a secondary process, the efficiency of the conversion
step is to be taken as a reasonable one for the process assumed.
*D. Cooling Water
The capital and operating costs of the sulfur dioxide control system
are to include the costs of a water cooling tower. Cooling water costs
will be for makeup only.
References
Al. Donovan, J. R., and P. J. Stuber, Sulfuric Acid Production from
Ore Roaster Gases, J. Metals 1£ (11), 45^50 (Nov. 1967)
A2. McKee & Company, Arthur, G., Systems Study for Control of Emissions
— Primary Nonferrous Smelting Industry, Final Report to National
Air Pollution Control Administration, June 1969, Contract No.
PH 86-68-85.
A-4
-------
Appendix B
FACTORS AND CONDITIONS ASSUMED IN
ESTIMATING CONTROL SYSTEM COSTS
The following conditions and cost factors were taken from the
Bl
report by Arthur G. McKee & Company, except that the items marke
with an asterisk are additional conditions specified by SRI.
A. Capital Costs
1. Capital costs do not include:
(a) Working capital
(b) Contingencies
(c) Cost of land
(d) Inventory
(e) Interest on investment
(f) Offsite utilities
(g) Steam generators
(h) Plant access
(i) Cost of dry, hot gas cleaning
2. The cost of the wet cleaning system for the gas is charged to
sulfur oxides control, but is estimated as a separate item
(see Appendix C).
BI Depreciation
15-year, straight-line.
!
C. Direct Operating Costs (except utilities)
1. Labor $3.75/man-hour
2. Supervision 4.75/man-hour
3. Payroll benefits 25% of labor + supervision
4. Maintenance materials 3% of capital cost of plant
5. Factory supplies 0.5% of capital cost of plant
B-l
-------
D. Indirect Costs
1. Controllable indirect costs (maintenance labor, laboratory,
overhead, and supervision)
50% of labor + supervision + maintenance materials
2. Noncontrollable indirect costs (local taxes and insurance)
3% of capital cost of plant
E. Utilities
1. Electrical power 1^/kwh
2. Steam 80
-------
Appendix C
GAS CLEANING SYSTEM
The flowsheet for the gas cleaning system (Fig. C-l) was adapted
from that portrayed for contact sulfuric acid plants in the Arthur G.
C2
McKee & Company final report. The inlet gas conditions were also
assumed to be the same as those specified in the McKee report, except
that a single, average value was taken for the SO concentration (0.2%
o
by volume). Under the conditions assumed, the water vapor condensed
from the incoming gas is sufficient to balance the outflow of weak acid
waste, so that no makeup scrubbing water is required. The cooling water
used in the heat exchanger to cool the recirculated scrubbing water is
assumed to be itself cooled in a water cooling tower. The water cooling
tower is not shown in the flowsheet but is assumed to be part of the sys-
tem. It is treated as a separate unit for convenience in cost accounting,
even though the same water cooling tower would probably in most cases
handle the cooling water for both the gas cleaning and the sulfur dioxide
recovery systems.
The capital cost for the gas cleaning system (not including water
Cl
cooling tower) was taken from the paper by J. M. Connor. Data points
were taken from cost curve No. 2 of Connor's Fig. 5, corresponding to
indirect cooling with a 30 F approach between the cold gas and cooling
water temperatures. The system capacity was converted from the original
basis (tons per day of 100% H SO with gas containing 8% SO2) to the
basis of volumetric gas flow at system exit conditions (110 F, 1 atm, sat'd),
Connor's estimates were presumably made for the type of gas cleaning
system illustrated in his paper, a scrubber-cooler followed by an indirect
tubular type of gas cooler, with an electrostatic mist precipitator for
final cleanup. Although the scrubbing and cooling equipment may therefore
be different from that assumed in the present case, Connor's cost estimates
(which were admittedly approximate) were taken to be sufficiently precise
C-l
-------
Partially Cleaned
Gas from Hot
Electrostatic
Precipitator
DB 600°F
WB 150°F
Clean Gas
O
I
DO
Sludge
and
acid waste
ELECTROSTATIC
MIST
PRECIPITATOR
GAS SCRUBBER
HUMIDIFIER
Makeup
Water
TA-7923-7
FIGURE C-1 GAS CLEANING SYSTEM FOR SMELTER GASES
-------
for use in this analysis.
Connor's curves did not extend beyond a capacity equivalent to
about 100,000 CFM of gas, but the plotted curve (Fig. C-2) was extrap-
olated arbitrarily to 1,000,000 CFM. The actual upper limit of validity
probably corresponds to no more than 300 to 400,000 CFM. Above this
size range, the use of multiple units of equipment may make the cost
more nearly proportional to the first power of gas flow rate. However,
none of the individual gas flows in the smelter models exceeds about
400,000 CFM, above which the curve in Fig. C-2 is shown as a broken line.
i
To obtain the curve of capital costs (Fig. C-2) for use in the pres-
ent study, the capital cost of a water cooling tower was added to the
cost of the remainder of the system as adapted from Connor's data.
The gas and water flows through the system were determined by making
heat and material balances based on the conditions shown in Fig. C-l.
Power requirements were estimated from literature data in a few instances,
and calculated from guesstimates of gas pressure drop and pump heads in
the rest. Utilities, labor requirements, and cost factors employed are
summarized in Tables C-l and C-2. The standard cost factors used were the
same as those employed elsewhere in the study (Appendix B).
The total annual costs were calculated for five gas flow capacities
ranging from 10,000 to 1,000,000 CFM (see Table C-3), using the data of
Fig. C-2 and Tables C-l and C-2, and were used to prepare the curve of
annual cost, Fig. C-3. The curve probably is not valid for capacities
greater than 300 to 400,000 CFM, and is shown as a broken line above the
latter level. '
References
Cl. Connor, J. M., Economics of Sulfuric Acid Manufacture, Chem. Eng.
Progr. 64 (11), 59-65 (Nov. 1968)
C2. McKee & Company, Arthur G., Systems Study for Control of Emissions —
Primary Nonferrous Smelting Industry, Final Report to National Air
Pollution Control Administration, June 1969, Contract No. PH 86-68-85.
C-3
-------
10
1.0
CL
<
o
0.1
10,000
I I I I I I I 1 I
1 I I
I J I
100,000
GAS FLOW RATE—cu ft/min (110°F, 1 atm, sat'd)
1,000,000
TA-7923-8
FIGURE C-2 CAPITAL COST OF GAS CLEANING SYSTEM FOR SMELTER GASES
C-4
-------
Table C-l
GAS CLEANING SYSTEM FOR SMELTER GAS
LIQUID FLOWS AND POWER REQUIREMENTS
Item
Gas Flow through System
Ap = 14 in. w.c.
Scrubber-Humidifier
Circulated water
Gas Cooling Tower
Circulated water
Cooling Water Circulation
Waste Acid
(21% H2S04)
Water Cooling Tower
Circulated water
Makeup water
Subtotal
Electrostatic Precipitator
Total
Basis
Gas Flow = 1000 CFM at 110°F, 1 atm.sat'd
Liquid Flow
7.86 gpm
35.7 gpm
41.7 gpm
0.212 gpm
41.7 gpm
4.17 gpm
Power
3.50 hp
0.43 hp
1.50 hp
1.40 hp
0.06 hp
1.91 hp
8.80 hp
6.57 kw
1.50 kw
8.07 kw _
C-5
-------
Table C-2
GAS CLEANING SYSTEM FOR SMELTER GASES
UTILITIES AND OPERATING LABOR REQUIREMENTS
Item
Electrical Power
Gas flow only
Total
Makeup Cooling
Water
Waste Acid
Treatment
Operating Labor
Supervision
Payroll Benefits
Unit Cost
$0.01 A»h
$0.02/1000 gal
$0.50/1000 gal
$3.75/hr
$4.75/hr
25% of Labor +
Supervision
Utilities Basis:
Gas Flow = 1000 CFM @
110°F, 1 atm, sat'd
Quantity
2.62 few
8.07 kw
4.17 gpm
0.212 gpm
12 hrs/day
2 hrs/day
Annual Cost
($)
207.50
639.14
39.63
50.37
MjSSO1
3.1351
4,4961
Plant operating time = 330 days = 7920 hrs/yr.
Labor costs are total for plant regardless of size.
C-6
-------
Table C-3
GAS CLEANING SYSTEM FOR SMELTER GASES
CAPITAL AND TOTAL ANNUAL COSTS
Item
Capital Investment
Annual Cost
A . Depr ec i at ion
B. Direct Operating Cost
1 . Labor
2. Supervision
3. Payroll benefits
4. Maintenance Materials
5. Factory supplies
6. Electricity
7. Makeup water
8. Waste acid treatment
C. Indirect Costs
1. Controllable
2. Noncontrollable
Total Annual Cost
Cost
Basis
$
$/CFM
$/yr
$/yr
$/yr per
CFM
Gas Flow Rate-CFM at 110eF, 1 atm, saturated
10,000
436,000
43.60
29,100
14,850
3,135
4,496
13,080
2,180
6,390
396
504
15,530
13,080
102,741
10.27
30,000
930,000
31.00
62,000
14,850
3,135
4,496
27,900
4,650
19,170
1,190
1,510
22,940
27,900
189,741
6.33
100,000
2,104,000
21.04
140,200
14,850
3,135
4,496
63,120
10,520
63,910
3,960
5,040
40,550
63,120
412,901
4.13
300,000 -
4,480,000
14.93
298,670
14,850
3,135
4,496
134,400
22,400
191,740
11,890
15,110
76,190
134,400
907,281
3.02
1,000,000
10,156,000
10.16
677,100
14,850
3,135
4,496
304,680
50,780
639,140
39,630
50,370
161,330
304,680
2,250,191
2.25
o
-------
10
o
•a
1.0
z
i
I
I I I I I I I
0.1
10.000 100.000 1,000,000
GAS FLOW RATE—cu ft/min (110°F, 1 atm, safd)
TA-7923-9
FIGURE C-3 TOTAL ANNUAL COST OF GAS CLEANING SYSTEM FOR SMELTER GASES
C-8
------- |