EPA-650/2-75-063
July 1975
Environmental Protection Technology Series
OF .1.LOY FURNACE
o
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EPA-650/2-75-063
A STUDY
OF FERROALLOY FURNACE
PRODUCT FLEXIBILITY
by
C. E. Mob ley and A. O. Hoffman
Battelle Columbus Laboratories
505 King Avenue
Columbus, Ohio 43201
Contract No. 68-02-1323 (Task 18)
ROAP No. 21AUY-011
Program Element No. 1AB015
EPA Project Officer: R.D.Rovang
Control Systems Laboratory
National Environmental Research Center
Research Triangle Park, North Carolina 27711
Prepared for
U.S. ENVIRONMENTAL PROTECTION AGENCY
OFFICE OF RESEARCH AND DEVELOPMENT
WASHINGTON, D. C, 20460
July 1975
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EPA REVIEW NOTICE
This report has been reviewed by the National Environmental Research
Center - Research Triangle Park, Office of Research and Development,
EPA, and approved for publication. Approval does not signify that the
contents necessarily reflect the views and policies of the Environmental
Protection Agency, nor does mention of trade names or commercial
products constitute endorsement or recommendation for use.
RESEARCH REPORTING SERIES
Research reports of the Office of Research and Development, U.S. Environ-
mental Protection Agency, have been grouped into series. These broad
categories were established to facilitate further development and applica-
tion of environmental technology. Elimination of traditional grouping was
consciously planned to foster technology transfer and maximum interface
in related fields. These 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
9. MISCELLANEOUS
This report has been assigned to the ENVIRONMENTAL PROTECTION
TECHNOLOGY series. This series describes research performed to
develop and demonstrate instrumentation, equipment and methodology
to repair or prevent environmental 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.
This document is available to the public for sale through the National
Technical Information Service, Springfield, Virginia 2Z161.
Publication No. EPA-650/2-75-063
11
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TABLE OF CONTENTS
Page
OVERVIEW AND EXECUTIVE SUMMARY v
BACKGROUND AND INTRODUCTION 1
SUBTASK STATEMENTS AND RESPONSES 6
Subtask 1: Describe, in General, Ferroalloy Furnace
Designs and How These Designs Vary According to the
Product Being Produced 6
Subtask 2: Define The Parameters Which Must Be
Considered When Switching Products in Both Open
and Sealed Ferroalloy Furnaces 12
Subtask 3: Identify and Compare General Procedures for
Changing From One Product to Another in Open and Sealed
Ferroalloy Furnaces 26
Subtask 4: Identify Deviations from Optimum Conditions
and Other Penalties (Including Economics) Which Are In-
volved in Switching Products in Open and Sealed Furnaces. . 29
Subtask 5: Do Case Studies of the Product Families
Which are Most Frequently Interchanged Based on Typical
Open and Sealed Furnaces 33
Subtask 6: Draw Conclusions, Where Possible, on the
Limitations to Flexibility Which Would Be Encountered
in the Domestic Ferroalloys Industry if all Future
Capacity Expansion Were to Consist of Sealed Furnaces ... 35
Subtask 7: Make Recommendations for Additional
Research, Development, and Possible Demonstration
Programs Which Could be Undertaken by EPA to Aid in
the Solution of the Product Flexibility Problem
Identified in Subtask 6 40
Battelle's Recommendations Relative to Open and
Sealed Ferroalloy Furnaces 41
Battelle's Suggestions for Possible Research
Projects 43
Possible Research Projects 44
SUMMARY AND CONCLUSIONS 47
References 51
APPENDIX: MITSUBISHI RESEARCH INSTITUTE REPORT ON
SEALED FERROALLOY FURNACES OPERATIONS IN JAPAN .... A-0.1
iii
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LIST OF TABLES
Page
Table 1. Furnace Design and Operation Data for
Selected Ferroalloy 18
LIST OF FIGURES
Figure 1. Schematic Illustration of Electric-Arc Ferroalloy
Furnace
Figure 2. K-Factor Versus Electrode Powder Density for
Selected Ferroalloys 13
Figure 3. Curves Relating Furnace Loads, Voltages, "K"
Factors, and Electrode Currents 15
Figure 4. Furnace Load Versus Electrode Diameter 24
iv
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OVERVIEW AND EXECUTIVE SUMMARY
This program was undertaken to provide the U. S. Environmental
Protection Agency (EPA) with insight into (1) the extent to which the U. S.
ferroalloy industry uses furnace-product flexibility, (2) the effect of
ferroalloy-furnace design (specifically a comparison of open versus totally
sealed furnaces) on product flexibility, and (3) research and development
areas which could enhance product flexibility in sealed furnaces and/or
make sealed ferroalloy furnaces more practical in the U. S. industry.
With one exception, all ferroalloy furnaces are of the "open"
type in the United States. Elsewhere, particularly in Japan, many ferro-
alloy furnaces are of the "sealed" type. An EPA study has indicated that
the sealed ferroalloy furnaces offer the potential to be operated with
lower pollution of the atmosphere than the other types of furnaces. In
light of this advantage, the question has been raised as to why sealed
ferroalloy furnaces are not presently utilized to a greater degree in the
United States. Reservations about the "flexibility" of sealed furnaces
relative to open furnaces have been one of the primary deterrents cited
concerning the lack of adoption of sealed furnaces in the United States.
Flexibility of a ferroalloy furnace is the term selected to
describe the ability to change the furnace from the production of one type
or family of ferroalloys to another family requiring different smelting
conditions. Furnaces with high flexibility can be used to manufacture a
wide variety of ferroalloys, usually with only short changeover periods
and/or low loss of production occurring during product switches. Furnaces
with low flexibility are generally designed to produce ferroalloys under
conditions that are optimum for a specific type or family. Low flexibility
furnaces do not readily lend themselves to product conversions, as product
changes in these furnaces may incur significant penalties in technology,
changeover time, and costs.
Based upon an extensive literature review and numerous interviews
with representatives of ferroalloy companies, it is concluded that the
U. S. ferroalloy industry has used and is likely to continue utilizing
high furnace-product flexibility to remain competitive in a changing
domestic and international marketplace.
In comparing the product flexibility of open and sealed furnaces,
it is concluded that sealed ferroalloy furnaces are less flexible than
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comparable, open-hooded furnaces, because (1) it has not yet been demon-
strated feasible to smelt all the common ferroalloys in sealed furnaces,
and (2) a sealed-furnace operation generally requires greater charge
preparation and attention to operating details than a comparable, open-
hooded furnace on the same product. U. S. ferroalloy producers are
reluctant to install sealed furnaces until either (1) a long-term market
demand is assured for that particular sealed-furnace product and/or (2)
it has been demonstrated feasible to smelt essentially all the ferro-
alloys, specifically the high-silicon-content ferroalloys, under sealed-
furnace conditions.
Battelle investigators recommend that EPA (1) conduct a cross-
media assessment of the overall (i.e., air, water, solid waste, etc.) en-
vironmental characteristics of open-hooded and sealed ferroalloy furnaces
to define future policy statements, and (2) assist in evaluating the design
and performance of baghouses used with ferroalloy furnaces with the ob-
jective of securing methods for lowering the annual total emissions from
existing and future ferroalloy furnaces.
The investigators also formulated two suggestions for additional
programs aimed at increased utilization of sealed ferroalloy furnaces and/or
lower furnace emissions from U. S. operations. These suggested programs
involve (1) determination of whether the substitution of iron-ore pellets
(as used in Japan) for ferrous scrap (as used in the U. S.) decreases
the need for stoking and improves the ease of operation in the smelting
of 50 and 75 percent ferrosilicon alloys, and (2) research directed at
insuring that sealed furnaces are safe operations through the development
of improved furnace-monitoring techniques and devices.
vi
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A STUDY OF FERROALLOY FURNACE PRODUCT FLEXIBILITY
BACKGROUND AND INTRODUCTION
There are three basic types of electric-arc ferroalloy furnaces
with respect to the design and operation of the furnace offgas system.
These three types are the open, the semiclosed, and the totally closed
furnace. The totally closed furnace is herein referred to as the "sealed"
furnace. In the operation of the open ferroalloy furnaces, the carbon
monoxide by-product released during smelting is burned above the mix and
the products of combustion (and the large volume of diluting/cooling ex-
cess air) are then cleaned to remove particulate matter. In the semi-
closed and sealed furnace operations, a cover is placed over the fur-
naces to collect the carbon monoxide-rich offgas, without combustion.
This is done to clean a minimum amount of gas per ton of product and/or
to collect the carbon monoxide for its fuel value. While both the semi-
closed and sealed furnaces are equipped with covers to exclude air from
the furnace interior, they differ in the design of the seals between the
furnace cover and the movable-electrode columns. In the semiclosed fur-
nace, the mix being charged is used to achieve a quasi-seal between the
furnace cover and the electrodes. More positive seals are utilized
with the sealed furnaces. The sealing performance of mix seals on semi-
closed furnaces is not considered satisfactory by furnace operators or
the EPA^l' and it is-, therefore, anticipated that new U. S. ferroalloy
furnaces will be either of the open or sealed design.
Based upon U. S. Environmental Protection Agency (EPA) studies
of the emissions associated with ferroalloy furnaces, it has been re-
ported that sealed furnaces equipped with appropriate air-pollution^cpntrpl
devices release significantly less particulate emissions than open-hooded fur-
(1 2}
naces with their air-pollution-control devices^ ' '. EPA has emphasized
the advantages (from air-pollution and energy standpoints) of sealed
furnaces compared with open-hooded furnaces and has considered setting air stan-
dards which would encourage the use of sealed furnaces'2' •*). u. S.
ferroalloy producers have expressed reluctance to install sealed fur-
naces, indicating that the use of sealed furnaces would lower the producers'
product flexibility (i.e., their ability to change from one ferroalloy
type to another in one furnace).
(1) References are listed on page 51
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To give a greater insight into the issue of furnace-product
flexibility, EPA, under a basic ordering agreement, requested that Battelle's
Columbus Laboratories (BCL) conduct a study of ferroalloy furnace-product
flexibility. The specific objectives of the BCL study were
(1) To compare the flexibility of open and totally
closed (sealed) ferroalloy furnaces as it re-
lates and applies to the U. S. ferroalloy in-
dustry, so that the significance of product
flexibility can be ascertained
(2) To identify existing technology and applied
engineering practices which can be used to
achieve additional flexibility in sealed fur-
naces without further research and development
efforts
(3) To develop, where warranted, recommendations for an
additional program which could be undertaken by
EPA to (a) make sealed furnaces practical in the
domestic industry if they are not at the present
time and/or (b) develop or demonstrate techniques
or systems to allow both ferroalloy product flexi-
bility and lower furnace emissions from U. S.
ferroalloy furnaces.
To achieve the above objectives, seven subtasks were
formulated by EPA for which responses were to be prepared by BCL.
The subtask statements, as presented in the Work Statement,
were:
(1) "Describe, in general, ferroalloy furnace designs
and how these designs vary according to the product
being produced.
(2) Define the parameters vhich must be considered when
switching products in both open and totally enclosed
ferroalloy furnaces.
(3) Identify and compare general procedures for changing
from one product family to another in open and totally
enclosed ferroalloy furnaces (and semienclosed fur-
naces, if these appear relevant).
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(4) Identify deviations from optimum conditions and other
penalties (including economics) which are involved in
switching products in open and totally enclosed
furnaces.
(5) Do case studies of the product families which are most
frequently interchanged based on typical open and
totally enclosed (sealed) ferroalloy furnaces.
(6) Draw conclusions, where possible, on the limitations
to flexibility which would be encountered in the do-
mestic ferroalloy industry if all future capacity ex-
pansion were to consist of totally enclosed furnaces.
(7) Make recommendations for additional research, develop-
ment, and possible demonstration programs which could
be undertaken by EPA to aid in the solution of the
product flexibility problem identified in Subtask 6. "
The Battelle-prepared responses for each subtask inquiry are given sequentially
in the following text. Following the last subtask response (No. 7), a summary
and conclusion section is presented*
As part of this study, Mitsubishi Research Institute, Inc. (MRI),
on a subcontract basis conducted a study of sealed ferroalloy furnace
operjitions in Japan. A series of questions concerning Japanese sealed
ferroalloy furn.ice operations was formulated by BCL personnel and pre-
sented to MRI for answering. The complete MRI report (with the question-
answer format) is presented in the Apnendix. Because most of the sealed
furnaces in the world are in Japan, the MRI report was used to give BCL
personnel additional insight on the flexibility of sealed furnaces.
Prior to addressing the Subtask questions and responses, it is
appropriate to define what is meant in this report by "ferroalloys" and
to clarify the intended meaning of furnace flexibility. Listings of ferro-
alloys and their corresponding chemical analyses are available in the
literature^1 ' ^'. For the purposes of this study, primary atten-
tion is directed at those ferroalloys produced in the greatest quantities,
that is the manganese, si]icon, and chromium families of alloys, including
standard ferromannanese (Std FcMn) , silicomanganeue (SiMn) , ferrosilicon
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(50 FeSi and 75 FeSi) , silicon metal (Si), ferrochronrium (FcCr) , and
ferrochrome-silicon (FeCrSi)*. Other ferroalloys, such as the rare-earth
silicides and calcium silicide, etc., are introduced and included as
specific entries in the presentation of the flexibility issue.
Flexibility is defined as the "capability of responding
or conforming to changing or new situations"^. Furnace flexibility
herein refers to the capability for changing or adjusting particular
ferroalloy furnaces to produce different ferroalloy products. It is
important to note that a time-frame is not inherently defined as part
of the "flexibility" statement; no qualifying statement as to the
length of time required to respond to the changing situation is included
in the flexibility definition.
Based upon our review of the literature and on interviews with
ferroalloy-industry representatives, It appears advantageous to consider
at least two types of flexibility, that is, short-term and long-term,
when discussing ferroalloy furnace flexibility. Short-term flexibility
as presented here, refers to the capability of carrying out product
changes in a given furnace within or over a period of several months. Long-
term flexibility relates to the capability and/or practice of effect-
ing a product change in a furnace on an annual or longer-term basis.
These time-frame generalizations to the flexibility statement are not
absolute, but are introduced as relative guidelines to simplify the dis-
cussion of ferroalloy-furnace flexibility.
The U. S. ferroalloy industry utilizes both short-term and lonjt-
term furnace flexibility. For example, one ferroalloy plant visited
during this study produces a variety of speciality ferroalloy products
and typically produces about ten different alloys in a given
furnace per year. While this example of short-term furnace flexi-
bility may represent the most product changes in a given furnace, other
* The cited group of ferroalloys accounts for about 75 percent of the
aporoximately 2 million tons of ferroalloys produced in the U. S.
ferroalloy furnaces annually.
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ferroalloy plants also utilize relatively small open furnaces to produce a
variety of specialty products on a similar short-term flexibility basis.
As indicated in several communications between EPA and the
U. S. ferroalloy industry, practically all (if not all) of the U. S.
ferroalloy companies have used and expect to continue utilizing long-
term furnace fJexibility to remain economically competitive in a
changing product-demand marketplace. Examples of U. S. ferro-
nlloy company's long-term furnace flexibility practice are presented
in the "Statement on Proposed Air Quality Standards for High Carbon
Ferromangane.se" prepared by the Ferroalloys Association^'.
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SUB TASK STATEMENTS AND RljSPONSKS
Subtask 1; Describe, in General. Ferroalloy Furnace Do si RUS atid
How These Designs Vary According to the Product being Produced
Design details for ferroalloy furnaces are presented in several
f 6— Q ^
texts on the theory and practices of electric-arc smelting ~ .
While there are numerous minor design variations, most 3-electrode, 3-
phase electric-arc furnaces are quite similar in their basic design. Thu
basic smelting furnace consists of a cylindrical furnace shell, the interior
of which is lined with insulating bricks and a carbon hearth. Electrical
energy is continuously supplied to the reaction area through three
vertically suspended prebaked carbon or self-baking (i.e., Soderberg)
electrodes arranged in a delta formation. The mix material is batch fed to
the top of the descending bed of charge material and the reaction products
(i.e., ferroalloys and slags) are periodically or, in some cases, con-
tinuously tapped from the side of the furnace. A schematic illustration
of a submerged-arc ferroalloy furnace is shown in Figure 1.
The upper portion of the furnace-charge materials may be
thought of as a particle bed, with the hot carbon monoxide offgas as-
cending through and heating the descending mix components. The basic
reaction which occurs within the furnace reaction zone Is the high-
temperature reduction of an oxide with carbon to yield the ferroalloy plus
carbon monoxide. Large quantities of carbon monoxide are formed as part
of the smelting reaction for all the ferroalloy smelting operations. The
thermochemistry of the various ferroalloy smelting operations is des-
cribed by Elyutin^9).
While generally quite similar in design, there are several furnace-
design features which are associated with specific ferroalloys. For example,
while the circular (or near circular) furnace shell with three electrodes in
a triangular configuration is the predominate furnace of the industry, some
companies prefer the so-called "packet" furnace with a rectangular crucible
>v
with in-line electrodes for the production of silicon metal . Silicon
metal is also produced in the 3-electrode, circular furnace common to
other alloys. Because the comparison of packet versus circular or in-line versus
Various in-line electrode furnaces have been proposed for smelting
particular ferroalloys(10).
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3 (in Delta Formation)
ELECTRODES "
CHARGE
MATERIAL
REFRACTORY
LINING
SHELL
CRUCIBLE
S~T- TAP HOLE
FIGURE 1. SCHEMATIC ILLUSTRATION OF
ELECTRIC-ARC FERROALLOY FURNACE
(Third electrode is not shown.)
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8
delta electrode configuration is not a major issue in the comparison of
open versus sealed furnaces, no further consideration will
be given to the in-line electrodes or packet-type furnaces.
The taphole configuration is another furnace-design feature
which depends somewhat on the particular type of ferroalloy produced.
In smelting some ferroalloys, a slag is produced as a process by-product
(e.g., in the smelting of Std. FeMn and FeCr), while little or no slag
&
occurs in smelting other ferroalloys (e.g., silicon and ferrosilicon) .
For those cases wherein a slag by-product occurs, the tapholes may be
situated in such a manner that the ferroalloy and the slag components
can be tapped from separate sites to allow separation of the two portions,
Such a tapping arrangement is not required for "dry" operations. Like
the packet versus cylindrical furnaces, the issue of taphole configura-
tion has only little to do with the issue of product flexibility wifch
open and sealed ferroalloy furnaces.
A furnace-design feature which does have direct bearing on
the flexibility issue is the need for and incorporation of auxiliary
means of controlling charge descent and flow into the reaction zones.
One means of affecting the descent of charge material into the reaction
zones is by stoking (i.e., rabbling the charge). Another is by rotating
the furnace shell. With many ferroalloys (e.g., FeMn, SiMn, FeCr, 50%
FeSi), the use of sized and controlled-property mix components is
adequate to insure that the charge materials will descend uniformly
under reasonably controlled smelting conditions. However, this in not
the case with high-silicon ferroalloys. As the silicon content
of the alloy increases, the extent to which silicon monoxide (S10) vapor
is generated also increases. This SiO vapor condenses throughout the
descending portion of the charge and can form an impervious crust or
"bridge" in the mix. This crusting in turn causes a buildup of the hot
by-product gases beneath the crust. When sufficient internal pressure
* These smelting processes in which no slag by-product occurs are fre-
quently referred to as "dry" processes.
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has developed, the gases either flow along the electrode
sides (by-passing the crust) or erupt through the charge. Eruption
can throw hot mix for distances of tens of feet*. The Jetting of
gases along the electrodes represents a significant loss
nl" energy Us contrasted to uniform gas flow up through the charge) and a
significant increase in both silicon loss and dust generation. The
Jons nf hot SIO vapor along the electrodes results in the release of
larBe quantities of super-fine fume; i.e., this SiO is not con-
densed on the charge and is not returned to the reaction zone. Stoking
is the primary measure used to break up the crusts once they are formed,
or as a preventive practice to keep thorn from forming initially.
Rotating the furnace shell is another means utilized to pro-
vide more uniform control of the mix descent and to minimize crust
formation and "blowing"^" .
While stoking can be and has been incorporated in sealed-furnace
operations, stukLnp, Ls more.- easily practiced with open furnaces where a
furnace operator lias access to and a complete view of the mix surface.
Likewise, while a rotating shell can be designed with a totally closed
cover, the rotation of the shell with an open top is simpler and more
amenable to engineering design and operation. As previously stated,
the need for stoking and/or shell rotation is related to the silicon con-
tent and/or the content of other chemical species which are volatile
at high temperatures but which condense out in the descending charge, forming
viscous and impervious crusts. The tendency for bridging and crusting
and, in turn, the need to stoke the charge, coupled with the extremely
high temperatures of the offgas, are the reasons why such ferroalloys
as siJLeon and FeSi with silicon content greater than 75 percent have not
to date been produced in scaled furnaces. This is also true for the
rare-earth silicides. Those alloys for which the uniform flow of charge
can be controlled by sizing and preparation of the mix components (with-
out the neyd for extensive stoking and/or shell rotation) arc amenable
* The erupt.Lon of the gas from the mix Is appropriately
cnlled a "blow".
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10
to either open or sealed furnace production. Thus, standard FeMn, SiMn,
FeCr, and 50 percent FeSi can he produced with only minor stoking if
the charge materials are properly sized and prepared prior to charging.
These ferroalloys are presently made in both open and sealed furnaces.
Another particular furnace-design feature associated with
sealed furnaces producing ferromanganese is (he use of drying and pre-
reducing procedures to prepare the manganese mix prior to smelting. It
should be noted in this instance that the emphasis is shifted to the
complete mix drying and prereduction system rather than only a
smelting-furnace operation. Drying and prereduction of the manganese-
bearing mix lowers the probability of a furnace blow resulting from
mix cave-ins and rapid reduction of the large quantities of the high-
manganese oxide contained in the untreated ore. While drying and pre-
reduction are not an absolute requirement for smelting ferrotnanganese In
sealed furnaces, utilization of this practice significantly increases
the operator's confidence that the sealed-furnace operation will be smooth.
In turn, a smooth operation is less prone to dangerous furnace explosions and
requires less stoking. The use of drying and prereduction of manganese charge
materials could be (and sometimes is) used as part of an open furrotnanganese
furnace operation, and should contribute the same improvement in operating
conditions on the open furnaces as with sealed furnaces. However, this more complex
mix preparation is less needed on the open furnaces, because furnace blows are
generally less harmful on open furnaces and the open-furnace operation can
counteract to a greater extent the occurence of blows by stoking and rabbling the mLx
In summary, there are notable minor variations in furnace design
throughout the industry. Here the word "design" is used In a general
sense. Specific parameters of "design" that ncrtain to particular ferro-
alloys are discussed in the following suhtasks.
In the general sense of the word design, the seeking requirement
is a major factor in assessing the ease with which an ouc-n furnace opera-
tion can be converted to a sealed operation. The other general furnace
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11
desi-gn variations have little direct effect on the issue of open versus
sealed furnaces and furnace flexibility.
Furnace size is an element of significance with regard to the
flexibility issue. This issue will be addressed in Subtasks 2 and 4.
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12
Subtask 2; Define The Parameters Which Must Be Considered When
Switching Products in Both Open and Sealed Ferroalloy Furnaces
The parameters which affect the successful smelting of the
various ferroalloys are described in texts on electric-arc furnace
( ft — f^
operations . As presented by Robiette(6), the boundaries
within which a given elertrlc-arc furnace can be utilized are establish-
ed by
(1) the maximum electrodp current (and/or current density),
(2) the range of transformer secondary voltage taps,
(3) the transformer capacity,
(A) the acceptable natural power factor,
and (5) the maximum operating resistance.
The operating resistance is, to a .large extent, determined by
the specific resistivity of the mix materials. The interrelations be-
tween the charge resistance and the other electrical and geometrical
parameters of the furnace (i.e., electrode-to-hcarth potential, electrode
currentiand diameter) were empirically established and set forth as K-
factor versus Electrode Power Density curves by Andreas'*-^ and Kt'lly l^'.
The K-factor, or peripheral resistance of the charge, is defined as:
K = (E/I) irD, [1]
where E = electrode to hearth voltage (volts)
I = electrode current (amperes)
and D = electrode diameter (cm).
*
Curves for K-factor versus electrode power density for selected ferro-
JUJU
alloys are shown in Figure 2
As Indicated in Figure 2, a characteristic K-Cactor - electrode
power density relation is associated with the smelting of a given ferro-
alloy . By combining the K-factor - electrode power density curves
with the electrical formulations for the furnace circuit (i.e., the
electrode critical current as a function of the electrode diameter); the
* The electrode power density is the electrical power carried in each
electrode (kw), divided by the electrode cross-sectional area (cm2).
** In metric units.
*** Persson(13) indicates that the elcctrode-to hearth potential divided
by the square root of the electrode diameter (i.e., E/^D~)is another
equivalent unique characterizing parameter for each ferroalloy.
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13
1.2
1.0 „
§ 0.8
o
g
H
0.6 „
0.4
0.2 -
Hi. C. Ferrochrome
„..'_.
oa :
O.|2 i 043 0.4 .0,5
I j ! • i | i
IELECTRODE POWER DENSITY,
'
0,6
0.7
_,
FIGURE 2. K- FACTOR VERSUS ELECTRODE POWER
I ; DENSITY FOR SELECTED FERROALLOYS
! I
•—1
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14
furnace power as a function of einctrode current, tun voltage, and power
factor), a set of technically acceptable electric smeLting conditions
can be established for a given ferroalloy. Curves of the type shown Ln
Figure 3, relating furnace load, voltages, K-factors, and electrode
currents may be used as furnace"operating charts^^^ lor smelting a
given alloy. Knowledge and control of the mix conductivity and its K-
factor, and the maximum allowable electrode current allows one to estab-
lish the maximum furnace load for given available tap voltages . For ex-
ample, referring to Figure 3, if the electrode current must not exceed
60,000 amperes and the smelting of the alloy is recommended over a range
of 0.50 to 0.55 ohm-cm, then the maximum furnace load will range from
about 14.2 to 15.9 MW, with voltages between 161 and 175.
To obtain maximum transformer utilization at a given voltage,
the furnace is generally operated with the greatest ;i.l.lowable current
that will also satisfy the other conditions of charge resistance and
power factor. It is desirable to have the current-conduction paths re-
stricted to the "reaction" zones between the tips of the electrodes nnd
the hearth. To achieve this design condition, the voltage is
adjusted or prescribed such that there is little or no current flow
through the charge between the electrodes. If a maximum voltage Is ex-
ceeded in smelting a given ferroalloy, thf current flow between the
electrodes increases and becomes more difficult to regulate. Creatfjr
electrode motion is required when there is excessive current flow between
the electrodes than when the predominate current flow is between the
electrode tips and the hearth. Under extremely high-voltage-input con-
ditions, the electrode penetration is shallow (i.e., the electrodes arc
high in the mix) and there may be large open areas or zones under each
electrode.
In the 3-electrode furnaces there are three interconnected reaction
zones near the hearth level . These reaction zones are essentially flxt'd
relative to the furnace structure, but have a contitm.-i] flow-through of
materials. The material in the reaction zones is continuously under-
going reduction and melting. While the concept of "reaction" zones ha.-.
* There is more art and empirical know-how behind this statement than
science. Science, however, is making inroads in this
-------
15
w
g
(Xl
Electrode Current
65,000 amperes
60,000 amperes
200V
'K" FACTOR, Ohm-Cm
FIGURE 3.
CURVES RELATING FURNACE LOADS. VOLTAGES, "K"
FACTORS, AND ELECTRODE CURRENTS(14)
-------
16
been successfully applied to furnace design and operation, the current-
flow patterns and the precise size and shape of the zones have not been
experimentally determined.
The current-carrying capacity of particular diameters of electrodes is
an intrinsic furnace-design limitation. As the electrode-current density
(and, in turn, the electrode-power density) increases, the surface tem-
perature of the electrode increases. The temperature of the electrode
portion immersed within the charge is relatively unimportant , because the heat
developed in that section is dissipated directly to the charge. However,
the exposed electrode surface (i.e., the portion above the charge) is
critical, because it oxidizes and/or deteriorates rapidly as its surface tempera-
ature rises. Thus, the limiting electrode-current density is determined
by the physical properties of the electrode material and the amount of
exposed surface area. The maximum allowable electrode current is a
principal design feature, in conjunction with the K-factor - electrode
power density relations.
Thus, the electrical-design parameters for the ferroalloy furnaces
are essentially contained within the K-factor - electrode power density
and the electrode diameter - critical current relations. However, there
are also geometrical parameters to be considered in designing and operating
the furnace. The electrode diameter is a design parameter of extreme
significance as will be discussed later in this section. Practically all
of the geometrical features of the furnace are derived from or scaled to the
electrode diameter. -The electrode diameter is inherent in the K-factor -
electrode power density relation and in considerations of the critical cur-
rent density for available electrode sizes.
Other geometrical factors of importance for smelting a particu-
lar ferroalloy are the electrode spacing (herein specified as the distance
between electrode centers) and the crucible and shell dimensions. The
preferred electrode spacing for smelting a given ferroalloy is based
primarily on experience. The ratio of electrode spacing to electrode
diameter (ES/D) varies from slightly less than 2 to somewhat less than
3 for most of the common ferroalloys. Generally, the higher the silicon
content of the alloy, the closer the electrodes are grouped. Thus,
-------
1.7
silicon metal is smelted in furnaces with electrode~spacing-to-diameter
ratios of 1.9 to 2.1, while 50 percent ferrosilicon operations utilize
ratios of 2.2 to 2.7.
Kelly( ' suggests that the crucible and shell diameters should
scale with the electrode spacing. Data taken from the literature indi-
cate that the ratio of shell diameter to electrode diameter (S/D) falls
within the range of 6 to 8 for the common ferroalloys.
Furnace depth is one of the more difficult design parameters to
reduce to a formal relation with the other design parameters. It has
(12)
been suggested that the ratio of electrode penetration (i.e., the dis-
tance the electrode is submerged in the mix) to the distance between the
electrode tips and the hearth could provide a design parameter for pre-
dicting desirable furnace depths. Typically, this ratio varies from
3.0 to 0.3 for the standard ferroalloys.
Furnace designers have developed today's ferroalloy furnaces for
particular alloys by collecting design and performance data from previous
furnaces and attempting to improve upon earlier designs. Ferroalloy-
furnace designers point out that this is an ongoing process, and that the
"optimum" furnace remains to be defined for many ferroalloys.
To more fully develop the interrelation of the design variables
of ferroalloy furnaces for given products, ferroalloy-furnace design and
operating data were collected and tabulated from the open literature
and from field-trip data. These data are presented in Table 1. The data
entries include furnace rating, furnace load, electrode diameter, ratio of
electrode spacing to electrode diameter, ratio of shell diameter to electrode
diameter, crucible depth, electrode penetration, K-factor, electrode-power
density, Persson's parameter^13), tap voltage, electrode-current density,
electrical energy consumed (kwhr) per tonne of product, the percent operating
time, the recovery percentage for the element, and the furnace type (i.e.,
open or sealed), and reference numbers identifying the source of the data.
Entries are grouped according to the ferroalloy product.
The ranges of K-factors and electrode-power densities, as well as
Persson's electrode-to-nearth potential factor presented in the table are in
keeping with the values depicted in Figure 2. Other Table 1 design and operating
-------
18
TABLE 1. FURNACE DESIGN AND OPERATION DATA FOR SELECTED FERROALLOYS
Alloy
Silicon
15% FeSi
50% FeSl
50.6* SI
Standard FeMn
74.fff.Mn
78% Mn
14. 8% MB
75.9J.Mn
78-82% Mn
S.Mn
High Carbon FeCr
FeCrSi
41% Ct. 41% Si
37% Ci. 47% Si
36% Ct. 40% Si
36% Ci. 42. 3% Si
Furnace
Ruling,
MVA
7
12
12
29
36
9
-
8.5
15
21.3
-
16. 5
21
-
15
43
60
60
-
-
16
40
30
36
72
-
-
-
45
72
11.25
43
13.5
13.5
25
40
Furnace
Load.
MW
7
9
10. 5
17
20
1
8.5
-
11
12.5
15
-
16.1
17
45
8.8
10.9
12.8
17.1
27.7
42
45-50
7.5
S.5
13
25
27-30
29
30
10
25-27
26
30
33
3.9
35
10
10
23.5-25
24
Electrode
Diameter,
cm
89
89
89
127
140
89
89
95
105
101.6
114.3
115
120
127
180
89
101.5
114
127
140
175
140
105.4
101.6
142
no
190
190
190
89
170
150
165
190
89
165
89
89
145
150
ES/DO)
1.77-1.94
2,42
2.23
2.0
2.0
1.91
2.23
2.26
2.52
2.225
2.57
2.18
-
2.40
2.16
2.23
2.35
2.57
2.40
-
-
2.13
2.55
2.2
-
2.47
2.32
2.31
2.5
2.74
2.35
2.5
2 31
2.S
2.43
1.92
2.57
2.57
2.1
2.365
S/D<2>
7.4
8.9
8.9
6.1
-
7.4
1.54
-
-7.7
7.35
6.53
-
-
7.54
6.55
7.54
7.50
6.53
7.54
7. SO
8.14
8.0
6.65
8.4
7.07
7.94
6.58
6.58
8.0
8.9
9.8
.
6.46
8.0
>1.88
6.25
31.9
>7.9
6.93
7.67
Crucible
Depth.
m
1.8
2.13
2.05
-
—
-1.8
1.88
1.98
2.4
-2.2
2.21
2.35
-
2.92
-
1.88
2.41
2.21
2.92
-
-
2.49
2.18
2.23
3.35
4.95
5.1
4.0
6.3
-
4.7
-
-3
6.3
2.36
3.3
2.33
2 33
3.1
3.9
Electrode
Penetration.
in
-
-1.37
-2.59
2.3-2.4
2.3-2.4
1.07
-1.2'
-
-
1.52
-1.5*
1.2
-
2.2'
-
1.2
1.7
1.5
2.2
-
-
2.3
1.4
-
-
-
0.9-1.5
-
1.5-2.5
-
0.9-1.5
-
-1.7
1. 5-2. 5
1.3-1.5
-
1.37
1.31
-1.73
™
.,K.(3)
ohm-cm
0.58-0.31
-
-
-
-
-
0.51-0.41
0.48
0.41-0.51
-
-
0.58
0.56-0.10
0.55
0.49
0.64-0.83
-
0.86-0.61
0.31-0.46
0.57-0.73
0.64-0.83
0.51-0.13
0.60
0.11
0.43-0.58
0.33-0.20
0.38-C.25
0.26
High SI
0.76-0.40
0 43-0.30
Electrode
Power
Density.
KW/cm2
0.375
0.48
0.56
0.45
0.43
0.31-0.465
0.315
0.46
-
-
0.51
0.50
-
0.41
0.45
0.59
0.51-0.635
0.47
0.45
0.42
0.45
0.60
0.58
1.03
0.17-0.33
0.29
0.35
0.27
0.37
0.335
0.34
0.35
0.31-0.54
0.535
0.38
0.49
0.47
0.39
0. 50-0. 83
0.48
O.MS
0.62-0.83
0.54
0.54
0.49
0.45
(1) ES/D = Electrode Spacing (centc:-to-cemer)/Elecuodc Diameter
(2) S/D - Shell Diamcier/Llertrode Diameter.
(3) "K" = K Factor = Peripheral Resistance.
•Estimated Values.
-------
19
TABLE! 1. (Continued)
B/VBM).
valct/Vcm
8.6
8. 625
8.4-8.8
10.0
9.9
10. 9
5.6
Top
Voltage,
volis
120-130
ISO
155-1CO
220
230
120
140
140
-
17U
100
200
182
210
-
135
no
190
195
-233
-298
300-320
115-120
120-140
-
204
-
Electrode
Current
Density,
amp/crn^
6.S
6.98
7.78
5.5
5.2
5.0-5.5
6.78
7.17
5.0
-
-
5.5
5.8
5.3
4.65
-5.5
1.56
5.72
4.77
4.99
5.6
4.6
7. 0-1. 75
4.1
-
3.7
4.4
-
Energy
Consumption
per Tonne.
KWIIr/i
15,400
13, 780
19,400
12,400
•
-
9,680
-
9,570
-
8,100
9,450
9,194
-
9,213
9.018
8,080-8,820
5,500
5.710
5.512
5.446
5.203
5.600-G.OOO
5. 600-6. 000
- 4, 500
2,600
3.000-3.200
-
2.680
1,960"
2.650
3. 1(4. 4 Max) 2.645
7.38
B.I
6.62
285
125- ISO
-
300
177
-
-168
-168
160- 180
—
4.7
-8.1
<5.0
4.1
5.8
4.2
6.9
6.9
-A 4
™
2,200
4,840
-
4. 850
4,410-4,740
4. 850
-4,000
4.620
3.932
- 4. 400
8.140
7 .358
8.846
7. 500-8. 600
7,050
Ope wing Element
Time, Recovery,
percent pcnem
71.351
89.9 58. 4 Si
92 77-81 SI
-
-
-
85.1 SI
-
-
8451
86. 6 Si
-
86. 2 SI
85. 1 SI
-
93.2 Si
93 SI
96. 3 Si
93. 5 SI
.
-
- 95 97- 99 Si
78-BOMn
95 73 Mn
-
93. 5 71 Mn
98+
O3
94 85 Mn
98*
92.9 85.3Cr
-
93-95 88-90 Ci
90-96 82-91 Cr
93 84-92 Cr
90.6Cr
Electrode
Type'5'
P
P
P
-
-
P
P
P
S
s
P
s
s
s
s
s
s
s
s
s
s
s
s
P
s
P
P
5
Furnace
Type<6>
O
0
O
0
O
O
O
0
s
0-R
O
0
O
0
0
O
0
0
O
s
s
Seml-C
s
s
s
s
s
s
5
C-R
Reference
Number
1.2.3
4
6
S
6
Field Data
1.8. 3. 7
8
9
10
11
8
9
10
12
9
13
1.2.3
9
9
9
9
14
14
15
1.2.3
16
17
IB
19
Field Data
20
Field Data
1.2.3
21
Field Data
Field Data
22
Field Data
1.2.3
23
24
1.2.3
25
25
Field Data
26
(4) k/V5 = I'urssnn Elcclruh: in llc.inh I'niciiiul Factor
(S) EluuroJe Ty|« Code
P = rith,il«:
S = Socluiburi;.
(6) FuniiHL Type CouV
(1 -Open
S = Scaled (CMally tUvidl
Scmi-C = Sciiii-CnviTL'd
R = Itotaiuig.
"Prereduced Charge Material).
-------
20
References - Table 1
(1) Kelly, W. M., "Design and Construction of the Submerged Arc Furnace",
Metal Progress, Vol. 73, May, 1958.
(2) Persson, J. H., "The Significance of Electroclc-to-Hearth Voltage
in Electric Smelting Furnaces", ATME Electric Furnace Proceedings
1970, Vol. 28, pp. 168-169.
(3) Engineering and Cost Study of the Ferroalloy Industry, U. S. En-
vironmental Protection Agency, Office of Air and Wastu Management,
EPA 450/2-74-008, May, 1974.
(4) Fairchild, W. T., Journal of Metals, Vol. 22, 1970, pp. 55 to 58.
(5) Ascik, A. L., "Manufacture of Silicon Metal in a Three-Phase Sta-
tionary Submerged Arc Furnace", ATME Electric Furnace Proceedings,
1964, Vol. 21, pp. 330 to 336.
(6) Wolfe, J. and Pait, R. , Interchangeable Sheds on a 25,000 KVA
Electric-Smelting Furnace, AIME Electric Furnace Proceedings, .1964
Vol. 22, pp. 153 to 156.
(7) Elyuten, V. P., et al, Production of Ferroalloys Electrometallurgy,
Second Edition, Translated from Russian, Published by NatJon.il
Technical Information Service, U. S. Department of Commerce,
TT-61-11429.
(8) Wise, W. H., "Manufacture of 75 Percent Ferrosilicon i.n Large
Submierged-Arc Furnaces", AIME Electric Furnace Proceedings,
(9) Dann, T. E., and Wise, W. H., "Comparative Operating Characteristics
of Large Versus Small Ferroalloy Units", AIME Electric Furnace Pro-
ceedings , Vol. 22, 1964, pp. 150-152
(10) Zherdev, I. T., et al, Critical Rate of Rotation for Ferrosilicon
Baths, Stal (English) 5, 1968, pp, 406 to 408.
(11) Horibe, K. , Totally-Closed Electric Furnace for 75 Percent Ferro-
silicon Production.
(12) Semenco, V. E., "Melting 75 and 65 Percent Ferrosilicon in Air-
Enclosed Furnace", Stal (in English) 1969, pp. 563 to 565.
(13) Schneider, W. R., and Eyrich, J. F., "Operation of a 75 MVA, 75
Percent Ferrosilicon Furnace with Heat Recovery Equipment", AIHE
Electric Furnace Proceedings, Preprint, 1974, Meeting, Pittsburgh,
Pennsylvania
(14) Tada, Y., et al, Development of 50% Si Fe-Si Smelting by the Ore
Process with a Large Closed-type ElccLrIc Furnace.
-------
21
References - Table 1 (continued)
(15) Lopuszynski, T. W., et al, "Design and Operation of a 45 megawatt,
50 Percent Ferrosilicon Furnace", AIMS Electric Furnace Proceedings.
Vol. 30, 1972, pp. 89 to 93.
(16) Harmon, C. N., "Production of High Carbon Ferromanganese with Dis-
card Slag Practice", AIMS Electric Furnace Proceedings, Vol. 28, 1970,
pp. 112 to 116.
(17) McClure, R. N., "Manufacture of Standard Ferromanganese in Electric
Furnaces", AIME Electric Furnace Proceedings. Vol. 19, 1961, pp. 307
to 314.
(18) Hopper, R. T., "The Production of Ferromanganese", AIME Electric
Furnace Proceedings, Vol. 25, 1967, pp. 141 to 145.
(19) Chisaki, T., and Takeuchi, K., "Electric Smelting of High Carbon
Ferromanganese with Preheated-Prereduced Materials at Kashima Works",
AIME Electric Furnace Proceedings, Preprint, 1974 Meeting, Pittsburgh,
Pennsylvania.
(20) Rossemyr, L., "Manganese Alloy Production in a Large Submerged Arc
Furnace", AIME Electric Furnace Proceedings. Vol. 28, 1970, pp. 121
r.o 123.
(21) Oxaai, J., "Manufacture of Silicomanganesc", AIME Electric Furnace
Proceedings. Vol. 19, 1961, pp. 315 to 344.
(22) Gamroth, R. R., "Operation of 30,000-kw Submerged Air Furnace on
Silicomanganese", AIME Electric Furnace Proceedings^ Vol. 27, 1969,
pp. 164 to 166.
(23) Leeper, R. A., and Dyrdek, T. J., "Smelting of High-Carbon Ferro-
chromium in a Three -Phase Electric Furnace", AIME Electric Furnace
Proceedings, Vol. 23, 1965, pp. 110 to 114.
(24) Scott, J. W., "Design of a 75-kw High Carbon Ferrochrome Furnace
Equipped with an Electrostatic Precipitator", AIME Electric Furnace
Proceedings. Vol. 29, 1971, pp. 80 to 82.
(25) Madronic, I., "Use of Chromium Ore Briquetts in the Manufacture of
Ferrochrome Silicon", AIME Electric Furnace Proceedings, Vol. 28,
1970, pp. 174 to 178.
(26) Nagasawa, S., "Ferrochromium Silicon Refining in Closed-Type Furnace",
AIME Electric Furnace Proceedings, Vol. 31, 1973, pp. 125 to 130.
-------
22
data of particular interest include (1) ranges of electrode spacing used
with each alloy, (2) use of prebaked versus Soderberg electrodes, and (3)
the electrode diameter versus furnace load for the various alloys.
As indicated in Table 1, the electrode spacing (center to
center) typically is 2.2 to 2.6 times the electrode diameter for all
the ferroalloys considered, with the exception of silicon metal. Silicon-
metal operations utilize electrode spacings of about 2.0 times the
fr
electrode diameter . It appears that with the exception of silicon, each
and all of the ferroalloys considered can be successfully produced with
electrode spacings in the above-cited range (i.e., 2.2 to 2.6 times
electrode diameter). While individual companies and particular furnace
operators have defined the optimum or preferred electrode spacing for
smelting particular alloys, it does not appear that the electrode spacing
in itself is a restricting parameter to furnace flexibility. That is,
in general, the electrode spacing need not be changed in carrying out a
product change. However, some furnace operators do adjust electrode
spacing (on some open furnaces) when making a product change.
Prebaked electrodes, which are capable of carrying a greater
current density than the Soderberg (or self-baking) type of electrode
are used predominately in the smelting of the high-silicon-content alloys
(e.g., silicon metal and 751 FcSi). However, Soderberg electrodes are
available in larger diameters than the prebaked types and are, therefore,
used predominately in today's larger ferroalloy furnaces. As presented
in Table 1, Soderberg electrodes are widely used in smelting of standard
FeMn and silicomanganese. While a given type of electrode may be pre-
ferred for smelting any of the given ferroalloys, furnace flexibility is
affected primarily through considerations of the power and current-
carrying capacity associated with the particular electrode used.
* The one silicon furnace entry in Table 1 wherein an ES/o ratio of 2.43
was used was an inefficient operation and was corrected, in part, by
reducing the ES/D ratio to 2.23.
-------
23
As indicated in Table 1, the electrode diameters (and, in turn,
all the parameters which scale with electrode diameter, such as electrode
spacing and shell diameter, etc.) increase with the furnace load. The
furnace load - electrode diameter data for the silicon alloys (i.e.,
silicon metal, 75% and 50% ferrosilicon), silicomanganese, and ferro-
manganese are plotted in Figure 4. As shown in Figure 4, most of the
furnace load - electrode diameter data for the given ferroalloys indi-
cate that the furnace load is proportional to the square of the electrode
diameter (i.e., W a D2) . Such a load - electrode, diameter dependence
is anticipated if the maximum electrode current-carrying capacity (i.e.,
current density) is proportional to the reciprocal of the
square root of the electrode diameter.
The specific point of interest to be derived from Figure 4 is
that the furnace load - electrode diameter relations not only differ for
the ferrosilicon alloys and for ferromanganese furnaces, but that the extent
of difference increases with increasing furnace load and/or electrode
diameter (i.e., with furnace size).
Thus, as indicated by Figure 4, .\ furnace with 90 cm-diameter
electrodes (specifically Soderberg electrodes) would probably operate
at a load of about 10 megawatts for smelting 50 and 75% ferrosilicon
and about 6 megawatts for smelting ferromanganese. Such a difference
in furnace load (i.e., 6 to 10 megawatts) is not particularly difficult
to obtain or achieve with a variable-tap transformer with an acceptable
power rating and operating power factor. However, a furnace which
utilized 180-cm-diameter electrodes would probably be designed to operate
at a load of 37 or more megawatts for smelting the ferrosilicon alloys
and 25 megawatts for the ferromanganese operation. While a transformer
and associated electrical system with a variable power output over the
above range (i.e., 25 to 37 megawatts) is certainly possible, the cost
of such a system is quite high. In general, the voltage range of the
transformer and the total range of allowable power input, are determined
by capital-cost considerations. The transformer costs increr.se rapidly
with both the number of voltage taps and the size (i.e., power
-------
24
50
P
a
T
o
i—i
IT)
i-i
<£>
I _ Si Alloys-
i ! W = 11.4 D"1
30
/:
*•/
T
,
/
n)
o
a)
o
20 L
10
a
J
rh-r
-j—
Fekn
W =»_7!. 7!
I ' !
—f- — r—i—r--~
; .! : I I ' :
i ..-i !—i—r
jg Silicon!
a-
"t'T
rir r~r
..!...!
IGURE 4. __ FURNACE^ LOAD, VERSUS! ELEQTRpI)| DIAMEtER! ; ! ! j.
([Arrows indicate overlapping data po|ints.j) ; 1
.___,. . ._„. ._ .. ._i_^.. _.._
60
80
100
120 140 160
Electrode Diameter (cm)
180
200
-------
25
rating) of the transformer. In addition, terms of the ferroalloy com-
pany's contract with the electric power company regarding firm, inter-
rupted and dump power also influence the design versatility of any in-
stalled transformer system.
As a result of capital and operating cost considerations,
large ferroalloy furnaces and, in turn, the required large transformer
systems and electrode columns are generally designed specifically for
the production of one ferroalloy or a limited group of ferroalloys re-
quiring similar smelting conditions. Based on the Figure 4 data and
economic considerations, it appears that furnace size is a major para-
meter relative to the issue of furnace flexibility.
Based upon this review of the theory and practice for ferro-
alloy furnace design and operation, the major operating parameters which
must be adjusted to effect any product change in a given furnace are
(1) the mix composition
and (2) the voltage and power to secure proper smelting
conditions.
As will be discussed in Subtask 3, many of today's ferroalloy
furnaces are equipped with on-line voltage changers which allow the
furnace operator to stepwise change the transformer tap voltage without
a major interruption of the power supplied to the furnace and charge.
The ability to change tap voltage (and, in turn, input power) as the
charge resistance changes in conjunction with the change in mix composi-
tion, allows the furnace operator to effect a continuous, gradual product
change in a given furnace.
While generally not a major prerequisite to achieving a product
change, some furnace operators also adjust electrode spacings when carry-
ing out a product change. It is the opinion of the Battelle researchers
that, in general, the electrode spacing need not be changed in carrying
out most product changes.
BCL personnel realize that the foregoing presentation of opera-
tion parameter relations are not simple or even "straight forward". The
relationships required to understand and parameterize a total furnace opera-
tion are couched in mathematical terms. A mathematical formulation was
not presented herein, but rather a simplified overview of each parameter
and the effects of parameter changes.
-------
26
Subtask 3; Identify and Compare General Procedures for Changing
From One Product Family to Another in Open and Sealed Ferroalloy Furnaces
Prior to describing the general procedures involved in carry-
ing out a change of product in a given furnace, it is advantageous to
describe what is meant by a given furnace. In this report, a given
furnace is taken to indicate that the following furnace components or
parameters are prescribed and fixed (i.e., they are not subject to
change):
(1) the installed transformer and associated
electrical system (e.g., the bus bars),
(2) the electrode diameter,
and (3) the furnace crucible and shell.
To change any of the above furnace components (or parameters)
requires major alteration to the unit. The above components are generally
only changed in a complete retrofit and/or overhaul situation , resulting
essentially in a new "piven" furnace. While the above components can be
changed (i.e., essentially a new furnace can be constructed in place of a
previous one), such major alterations are long-term events and
do not affect short-term furnace flexibility.
Now, let us turn our attention to those procedures and/or
conditions which are involved in carrying out a change of product in a
given ferroalloy furnace.
In effecting a change of product in a furnace, several basic
operating conditions must be adjusted or changed. These basic or funda-
mental changes include:
(1) the change of mix composition
and (2) adjustment of voltage and power for proper
smelting conditions.
That the mix composition must be changed to effect a change in product
is obvious. For example, in changing from a 50% ferrosilicon to a 75%
ferrosilicon operation, less scrap and more silica and
reductant (e.g. coke) are charged per unit of mix fed to the furnace.
Mix compositions for smelting the various ferroalloys arc available in
in the literature(6» 9» 16>.
* InterlakeUS) has reported on the use of interchangeable crucible
and shell units for increasing the flexibility of one of their
furnaces.
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27
As described in the Subtask 2 section on parameters which must
be considered in performing a product change, the voltage and, in turn,
the furnace load are adjusted (using the K-factor-elcctrode power density
data) in any change of product. The particular adjustment of tap voltage
and furnace load will depend on both the product change and the allow-
able range of voltage (and power) for the given transformer.
Most of today's transformer-furnace systems are equipped with
on-line voltage-changing facilities which enable the furnace operator to
incrementally raise or lower the tap voltage with a simple switch (a tap-
voltage selector). As the composition of the mix charged to the furnace
is altered to secure a product change, the charge resistance is also
altered. For a fixed voltage, the current flow through the electrodes
will change with the change in mix resistance.
The furnace operator may monitor both furnace voltage and
current and can adjust the working voltage via the on-line voltage selector
to maintain the current at a prescribed value within the allowed working
range. The maximum current allowed is determined by the current-carrying
capacity of the electrical system components (e.g., transformer,
and electrodes). In general, the furnace operator will attempt to
utilize a current slightly less than the allowed maximum, in order to
obtain a near-maximum total power input under all smelting conditions.
The above changes are required in any product change, inde-
pendent of the furnace being open or sealed.
There are two basic methods for securing a change of product;
(1) a gradual change of product by continuing to feed the new mix com-
position to the operating furnace; and (2) to shut down the furnace, dig
out and remove the old product mix from the furnace, and recharge and
start again with the new product. The decision to utilize one or the
other of these approaches to a product change depends on a number of
factors, such as (1) feasibility of a continuous (i.e., furnace-on) product
change, (2) the time required to effect the product change from Alloy No.
1 to on-grade Alloy No. 2, and (3) the marketability of the off-grade pro-
duct (s) produced during the product-change period.
The gradual and continuous change of product is the most fre-
quently utilized procedure. Ferroalloy-company representatives estimated
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28
that better than 95 percent of the U. S. industry's product changes were
performed as continuous operations, without shutdown of the furnaces.
Those product changes wherein the furnace is shut down and subsequently
restarted on a new product are only used when the product changeover
can be concurrent with a shutdown of the furnace for major repairs and
overhaul.
During a gradual product changeover period in the switching of
products with significantly different smelting conditions, the furnace
operation may not be so "smooth" nor so well controlled as during "steady-
state" operations. In the anticipation of trouble, sealed furnaces may
be opened during changeover to observe closely the furnace top and to
be in a position to stoke, if necessary. In these instances, the emission-
control equipment would have difficulty handling the increased flow of
gases and the offgases may be released without cleaning. If this "safety"
practice were followed (i.e., sealed furnaces were operated in an open mode
during product transition), the air-pollution control and energy advantages
of sealed furnaces might be lowered.
Some furnace operators also adjust electrode spacings when
carrying out a product change in a given furnace. An adjustment of
electrode spacing is relatively easy on some open furnaces. However,
with a sealed furnace, the furnace cover is designed to seal against
the electrodes at a particular design spacing. Thus, the electrode
spacing on a sealed furnace is essentially fixed and constant. While the
data in Table 1 indicate that most of the ferroalloys can be smelted
with an electrode-spacing-to-diameter ratio between 2.2 and 2.6, the
ability to adjust the electrode spacing in conjunction with a product
change gives the furnace operator additional freedom to improve the smelt-
ing operation.
The general procedures for changing from one ferroalloy product
to another in open or sealed furnaces should be quite similar (i.e.,
change in mix composition, voltage, etc.). However, because of the
accessibility of the charge in an open furnace (and, in turn, the
ability to stoke and rabble the charge), an open furnace is inherently
more flexible (i.e., more product changes can be carried out) than a
sealed furnace.
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Subtask A; Identify Deviations from Optimum Conditions and
Other Penalties (Including Economics) Which are
Involved in Switching Products in Open and Sealed Furnaces
The incentive for changing the ferroalloy produced in a given
furnace is purely one of economics. In general, a product change has
been made for one of two basic reasons—(1) the raw materials which were
used are no longer available at an acceptable price level or (2) the
market demand and/or profitability associated with a new product suf-
ficiently exceeds that of the old product to warrant a furnace change-
over. U. S. ferroalloy companies have practiced furnace flexibility
for both of the above-stated reasons and have indicated that long-term
furnace flexibility is a desirable consideration in any new furnace
designs, because of the possible reduced availability of manganese and
chromium ores to U. S. smelters. As noted in the Introduction, many
speciality ferroalloy producers use small (^10 MW) furnaces to produce
several ferroalloys per year in a given furnace. In this case, the
market demand is limited and the output of a year-long campaign on one
of these specialty ferroalloys would exceed the total marketable
quantity several times over. These companies practice short-term fur-
nace flexibility to remain profitable and competitive.
Thus, while there may be operating "penalties" associated
with a product change in a given furnace, there should be no economic
penalty, assuming that the company management has adequately and accurately
analyzed the business market. That is, even though the furnace operation
may be "rough" and require greater operating attention, the product switch
is designed to be profitable.
When a product change is made in a given furnace, be it open
or sealed, there are several factors which influence the overall profit-
ability of the change. Several of these factors to be considered are:
(1) The required down-time to achieve the product
switch and/or the loss associated with the
production of off-grade or nonsalable alloys,,
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30
(2) the required power consumption per ton product,
and (3) the change in furnace productivity (i.e., tons
of ferroalloy produced per month).
The time required to achieve a product change is, in part, a
function of the specific alloys involved, the size of the furnace, and the
furnace design (e.g., whether the furnace is open or sealed). A minimum
conversion time is associated with a gradual product change between ferro-
alloys of similar smelting conditions (i.e., 50% *• 75% ferrosilicon) in
small open furnaces (of the order of 10 megawatt load). A conversion time
of about several days to a week is anticipated for the above case and,
also, the product associated with the conversion period is often salable
as off-grade material. As the furnace size increases and smelting con-
ditions differ, the time required for conversion increases. Typically,
4 to 8 weeks may be required to convert a reasonably large (about 30 MW)
open furnace from one product to another. In some cases, this time
period is downtime during which ao ferroalloy is produced.
There is very little field experience to date to define precisely
the time required for a product change in a largo sealed furnace. However,
for products with a major change In smelting conditions, the conversion
time is expected to exceed that of a compariable open furnace because of
the additional time required to adjust and stabilize the operation prior
to the replacement of the cover seal. It is anticipated that the conver-
sion time required for a large sealed furnace could exceed that of an open
furnace by several weeks.
To date, the procedures for carrying out a product change be-
tween ferroalloys requiring significantly different smelting conditions
in a large sealed furnace have not been presented in the open literature.
Based on the MRI study (see the Appendix) and discussions with the U. S.
ferroalloy companies, the large sealed furnaces operated to date are es-
sentially single-product furnaces and are likely to remain single-product
operations for the life of the furnace. Because there are no field data as to
how a product change would be performed within a large sealed furnace,
our presentation is based upon our best estimate of this technology. We
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31
anticipate that a large sealed furnace may be operated in the open mode
(l.e, without the sealed cover) during any major product change. Thus,
whereas a product change with an open furnace would essentially utilize
the same air-pollution-control devices as during its steady-state opera-
tion, additional air-pollution-control systems and/or permission to operate
the furnace with increased emissions may be required in the changeover of
a sealed furnace. This premise is based on the belief that the successful
sealed-furnace operation requires greater control of the process than the
corresponding open-furnace operation. The product change with a sealed
furnace is anticipated to involve (1) opening the furnace (bypassing the
air-pollution-control system) and (2) stabilizing the operation on the
new product prior to resealing. Such a procedure may have a direct bear-
ing on the overall air cleanliness rating of the operation.
As stated in the Sub task 2 and 3 sections, the voltage and fur-
nace load are generally changed when carrying out a product change in a
given furnace. These changes, in turn, may causu an electrical energy
consumption penalty. For example, if the furnace and transformer system
were designed for optimum power input for smelting one ferroalloy, a shift
from that alloy to another may cause a change in power factor and in energy
usage. Thus, the operator may have to pay for more electrical energy than
would be required in a system specifically designed (and optimized) for
smelting the new product.
There is an anticipated change in furnace productivity with
any change in furnace product. As indicated in Table 1, the electrical
energy consumption per tonne of product (Kwhr/tonne) varies with the
particular ferroalloys. For example, approximately 15,000 Kwhr are
consumed to produce a tonne of silicon metal in the U. S. practice,
while a tonne of standard ferromanganese requires about 2600 Kwhr. The
anticipated productivity of a furnace is the furnace load divided by the
specific energy consumption per tonne of alloy times the operating time
factor. This is:
Q = (P/EC ) x Operating Time Factor, [2]
where Q = quantity (tonne) of ferroalloy produced per hour,
P = furnace load in kilowatts
and EC = required energy consumption in kilowatt-hour per tonne of alloy.
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32
Based on the estimated furnace output (i.e., tonnes of product
per unit time), the market demand and the profit margin (i.e., the
sales price minus the production cost), the ferroalloy company manage-
ment can formulate their policies and decisions regarding furnace
flexibility.
Throughout this discussion of the penalties (and gains) as-
sociated with product changes in a given furnace, it has been assumed
that the product change is feasible and possible within the particular
furnace. There have been cases where a product change was initiated
and the ultimate penalty of zero production occurred; that is, the fur-
nace could not be tapped under the prescribed smelting conditions.
While this condition of an untappable furnace has occurred, it is
rare in today's operations.
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33
Subtask 5; Do Case Studies of the Product Families Which are
Most Frequently Interchanged Based on Typical Open and Sealed Furnaces
The examples of product changes cited in this section are based
on open literature data and information provided by individual U. S.
ferroalloy companies. Because of the longer history of usage and the
inherent greater product flexibility associated with open furnaces, there
are significantly more examples of product changes with the open-type
furnaces than with sealed furnaces.
The most frequent and/or common changes of product within given
open furnaces are those involving similar smelting conditions. For ex-
ample, product changes of silicomanganese to standard ferromanganese or
vice versa (hereafter represented as SiHn +• FeMn) or 50% ferrosilicon to
75% FeSi to silicon metal or vice versa (i.e., 50 FeSi <- 75 FeSi «- Si)
are fairly common. Other reasonably frequent product changes in open fur-
nace include the sequence 50% ferrosilicon to ferrochrome-silicon and then
to ferrochrome (i.e. , 50 FeSi «- FeCrSi «- FeCr) . Changes of alloy grade within
a family are also common (e.g., FeCr «- blocking chrome «- charging chrome). The
product switch between ferrosilicon alloys and ferromanganese is less
frequent, but this change has been performed. As a generalization, one
can state that all the permutations of product changes have been carried
out with small open furnaces. The time required for a particular product
change has typically been 1 to 2 weeks, depending on the extent of dif-
ference in required smelting conditions for the particular ferroalloys
involved.
The versatility of a small open ferroalloy furnace is evidenced
by the operation of what may be termed the "universal" furnace, in which
essentially all the common ferroalloys have been successfully smelted.
The basic "universal" furnace consisted of three 89-cm-diameter electrodes,
pq
with a 2.44-meter center-to-center-electrode spacing (i.e., / = 2.74)
SD
and a 7.92-meter diameter shell (i.e., / « 8.9). The open furnace was
equipped with a variable-tap transformer to allow the smelting of all the
common ferroalloys in one furnace. These basic furnaces were used for
many years to smelt all the common ferroalloys. The only major parameters
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changed (besides mix composition) in performing a product charge were
voltage and furnace load. For example, ferromanganese was smelted with
a furnace load of 6 to 7 megawatts, silicomanganese with about 8 megawatts,
ferrochromium with about 10 megawatts and 50% ferrosilicon with 12 to 14 mega-
watts. Stoking was practiced with all these operations.
The examples of short-term furnace flexibility, wherein up to
ten specialty ferroalloys (including the rare-earth silicides, calcium
silicon, and the ferrosilicons) are produced per year in a single ferro-
alloy furnace are also achieved with relatively small (up to about 10-
megawatt) open furnaces.
Some longer term product flexibility is also associated with
larger (about 30-megawatt) open furnaces. u- s« ferro-
alloy companies cited numerous examples of product changes (e.g->
FeSi •*• FeCr) carried out on large open ferroalloy furnaces. These pro-
duct changes typically required several weeks for completion and were
dictated by a major shift in market demands.
To date, sealed furnaces are designed and operated essentially
as single-product furnaces. Because the majority of the world's sealed ferro-
alloy furnaces are located in Japan, a subcontracted study through
Mitsubishi Research Institute, Inc. was made of the Japanese ferroalloy
industry. The full text of the M*I study of the Japanese ferroalloy sealed
furnace operations is presented in the Appendix. The MRI study supports
the previous statement that the Japanese sealed furnaces are currently de-
signed and operated as one-product furnaces. The greatest number of
sealed ferroalloy furnaces is producing standard ferromanganese. The
ferromanganese sealed-furnace operation can and has been converted to the
production of silicomanganese and vice versa; however, no other product
changes are known of in sealed furnaces. The lack of demonstrated flexi-
bility for sealed furnaces may be due in part to (1) the relatively large
size of most of the new sealed furnaces and (2) the lack of operating ex-
perience associated with the sealed-furnace operations. In either case,
no case histories baaed on foreign or domestic field experience is avail-
able relating to product changes (other than FeMn «- SiMn) in sealed ferro-
alloy furnaces.
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35
Subtask 6: Draw Conclusions. Where Possible, on the
Limitations to Flexibility Which Would Be Encountered
in the Domestic Ferroalloys Industry if All Future
Capacity Expansion Were to Consist of Sealed Furnaces
Recognizing that sealed-furnace operations have not been demon-
strated feasible for smelting some ferroalloys, the installation of only
sealed furnaces in the future would limit the production of
the new furnaces (by comparison with open furnaces) to only those products
amenable to sealed-furnace production. The literature and field data
indicate that the current sealed-furnace product capability is as shown
below:
Demonstrated Not
Operational Marginal Feasible
FeMn 75 FeSi Si
SiMn FeCrSi (1 stage) CaSi
FeCr Rare-Earth Silicides
50 FeSi
FeMn, SiMn, and FeCr are currently smelted in large sealed furnaces
throughout the world. Fifty percent FeSi is smelted in relatively large
sealed furnaces in Japan. However, the large U. S. 50% FeSi furnaces are
all open.
Japanese sealed-furnace smelting of 50 percent ferrosilicon
differs from the U. S. open furnace practice in that the Japanese use
siliceous ore as their source of iron. Steel scrap is the iron-bearing
charge material used throughout the U. S. industry to produce ferrosilicon.
While a greater electrical energy should be required per tonne of ferro-
silicon product with the ore practice relative to the scrap method, the
Japanese ferroalloy representatives indicate'the ore usage is justified
in terms of allowing better mix sizing and feed control and yielding a
smoother smelting operation (Refer to Yakagi Interview, pages A5.1 and 5.2
of the Appendix). U. S. ferroalloy industry representatives have indicated
that, while the sealed furnace smelting of 50 FeSi is clearly demonstrated
with the use of ore, the sealed furnace production of ferrosilicon
with a steel scrap charge has not yet been proven feasible.
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36
The smelting of 75 FeSi and single-stage FeCrSi in large sealed
furnaces is presented herein as marginal; that is, these alloys have been
smelted in relatively small (by today's standards) sealed furnaces, but
further development is required before large sealed-furnace operations are
proven. From both the theoretical and practical views, the smelting of
silicon metal and the rare earth silicides in sealed furnaces is not yet
feasible.
As previously stated, the average size of the new installed fur-
naces has shown a steady increase over the last several decades . Thus,
the significance of furnace size relative to the ability to seal the furnace
on a given product is extremely important. In general, a large open furnace
is more difficult to operate smoothly than a small open furnace, particularly
if stoking is required. This is, in part, the reason that U. S. ferroalloy
producers do not accept the operation of a 11 MW sealed-furnace operation
with 75 FeSi as confirmation or demonstration of feasibility for smelting
75 FeSi in a large (e.g., 45 MW) furnace.
As implied in the Mitsubishi Research Institute report on the
Japanese sealed-furnace operations, a sealed ferroalloy furnace is less flexible
than an open furnace. The Japanese essentially present the sealed furnace as
a single- product furnace. However, product flexibility, particularly long-
term flexibility, is a matter of definition and degree.
It is of some interest to herein speculate on the differing need
for furnace-product flexibility in foreign countries relative to the United
States. As noted in the Introduction, most of the world's sealed ferro-
alloy furnaces are in Japan. Japan currently has 39 totally sealed ferro-
alloy furnace operations (Refer to Page A.2.1. of the Appendix). There is
only one sealed furnace currently producing a major ferroalloy product
(silicomanganese) in the United States .
* The particular U. S. scaled furnace was originally designed and operated
for the production of pig iron. Operational difficulties were encountered
in converting this sealed furnace to the production of ferrosilicon and
ferromanganese. The furnace is currently utilized to produce silico-
manganese. Because this particular furnace was not specifically designed
for the smelting of ferroalloys, it is the position of BCL personnel
that this particular sealed furnace should not be cited as a pro or
con in judging the attributes of sealed-furnace technology.
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37
In the United States, producers of ferroalloys are concerned that
the ability of sealed furnaces to switch from product to product is or may
be less than the ability of open furnaces to make such switches. In Japan,
the ferroalloy producers generally do not have the same concern about
furnace "flexibility". Thus, sealed furnaces generally face a more receptive
attitude in Japan than in the United States. One can question the reason
for this difference in attitude. The answer involves some speculation,
but basically appears to the present BCL investigators to be the result of
(1) differences in the relationships between the Government and the ferro-
alloy producers and (2) differences in the relationships between competing
producers of ferroalloys.
As indicated in a previous EPA-prepared document^ ', differences
do exist between Japan and the United States with respect to Government
involvement and interactions with industry with regard to taxation, sub-
sidy payments for promotion of industrial growth, export policies, and
other aspects. Although the specific mechanisms are obscure when viewed
by outsiders, it is a fact that Government - industry relationships in
Japan are such that the Government is in a strong position to influence
the output and operating practices of even private-sector manufacturers.
The Japanese government has at its disposal methods for applying pressure on
industrial manufacturers to a degree not practiced in the United States.
One form of this pressure appears to be as follows.
A Government planning group (frequently MITI, The
Ministry of International Trade and Industry) devises
a national plan for the production of some commodity,
say, for example, ferroalloys. The plan takes into
account existing facilities, domestic demand, export
and import potentials, costs of production, availability
of raw materials, and even social factors. One pos-
sible conclusion from such an analysis is that it
would be better overall for the national economy for
Plant 1 to specialize in Product A, Plant 2 to specialize
in Product B, etc.
Although the organization of the Government-industry relationship does not
necessarily permit "control" of the manufacturers to follow the plan, in
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38
practice the types of pressure available to the Government generally re-
sult in acceptance of the plan by the individual manufacturers.
The Japanese system, therefore, in essence evolves to a type of
allocation of tonnage and products among various manufacturers. The
tendency is for each manufacturer to be responsible for the production of
a narrower range of products, but at a higher rate of output than if a
larger number of his competitors were also making the same products. To
the degree that sealed ferroalloy furnaces have limited flexibility rela-
tive to open furnces, this Government-industry relationship makes sealed
furnaces more attractive to the Limited-product companies that tend to
evolve more extensively in Japan than in the United States.
The Government-industry relationships in Japan also has an in-
fluence on the relationships that develop between competing ferroalloy
companies in the private sector. Somewhat assured of a large market for
their specific products, and somewhat insulated from the threat of un-
expected foreign intrusion of another producer into their prime market,
individual ferroalloy companies operate in an environment that encourges
them to exchange technical and research information through industrial
associations.
Less there be misunderstanding, the Japanese system as described
here according to our understanding of the system, involves little in the
form of formal "controls". An individual manufacturer can on his own
initiative deviate from the plan and take his chances in open competition.
This has been done from time to time, but the resulting experiences have
been such that such deviations involve more risks than does compliance
with the plan. Furthermore, it should be recognized that the Japanese
system of "planning and pressure" is not limited to the ferroalloy in-
dustry, but extends also into other industries, of which the steel in-
dustry is another example.
In the United States, there is no equivalent of the Japanese
national plans, and the pressures available to the Government to influence
the tonnage or type of product made by any manufacturer usually are in-
significant. The result is that U. S. producers of ferroalloys compete
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39
vigorously with each other over a wide range of products, often changing
product mix or tonnage based on each producer's evaluation of the total
market, sources of raw materials, import and export situations, actions
of his competitors, and relative profitability of various products. In
this environment, flexibility of ferroalloy furnaces takes on great im-
portance. Large sealed furnaces, especially in their present stage of
development are not so acceptable in the U. S. system as in the Japanese
system and, in the final analysis, this difference arises from difference
in business modes between the United States and Japanese ferroalloy
companies.
The U. S. producers are concerned about the availability of
chromium and manganese ores to the U. S. for the future smelting of
ferrochrome, ferromanganese, and related products. Because of this
long-range concern, U. S. producers would prefer the installation of
large open furnaces. Thus if a forced product switch were made
(i.e., manganese and/or chrome products to silicon products), the
adjustment could be carried out more easily in existing furnaces.
In conclusion, if all new furnace installations were sealed
furnaces, flexibility would be curtailed. Short-term product
flexibility, which is essentially associated with the smelting of
specialty products in small furnaces, would be severely affected, if not
eliminated. Long-term flexibility would be possible. However, the
difficulty, as measured by the time and cost, associated with a product
change in a large sealed furnace would be greater than the corresponding
product change in a large open furnace.
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Subtask 7; Make Recommendations for Additional
Research, Development, and Possible
Demonstration Programs Which Could be Undertaken by
EPA to Aid in the Solution of the Product
Flexibility Problem Identified in Subtask 6
As has been shown, a movement cowards large, sealed ferro-
alloy furnaces is a movement towards furnace inflexibility. The develop-
ment and acceptance of sealed ferroalloy furnace operations is in its
early stages*. As the state of the art evolves, it is considered probable
that large ferroalloy companies in the United States having large resources
(capital and technology), and an assured market, will in the future install
sealed furnaces dedicated to one product. Battelle has no recommendations
for research and development that could make a sealed furnace more flexible
in terms of product shifts.
As indicated in this report, some ferroalloys cannot as yet be pro-
duced in sealed furnaces. Also, to retain some elements of product flexi-
bility, some ferroalloy producers want to install only open furnaces in the future.
On large furnaces this flexibility aspect may be more theoretical than practical,
but the flexibility aspect is more available with open furnaces than with sealed
furnaces. It is judged, therefore, that there will continue to be many open
furnaces even in the future. Battells has, therefore, included recommendations
for (1) reassessment and comparison of the relative environmental impact of open
and sealed furnaces via a cross media impact type study, and (2) approaches to be
considered for the improvement of the air-pollution control of open furnaces .
The EPA is interested in aiding a movement towards sealed ferroalloy
furnaces for new installations in the United States. As stated by Japanese
ferroalloy company representatives, the normal development of sealed furnace
operations is to improve the smelting performance in open furnaces to the
point where sealing of the operation can be technically justified. Battelle
researchers have, therefore, included in this Subtask some suggestions for
possible research projects that could improve smelting operations in general.
* Semiclosed (i.e. , mix seal) ferroalloy furnaces and totally sealed pig
iron and calcium carbide furnace operations predate the introduction of
sealed ferroalloy furnaces. Sealed ferroalloy furnaces came into use
in the mid to late 1960's.
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41
Battalia's Recommendations Relative to
Open and Sealed Ferroalloy Furnaces
(A) It is recommended that the EPA study and evaluate
the comparative environmental-impact level of both
sealed and open furnaces using the criteria of all
of the expected environmental guidelines of the
future. These criteria should include water- and
air-pollution guidelines and waste-control standards,
and energy considerations. Also included in this
evaluation should be the expected improvements in
baghouse performance (if any) and expected improve-
ments in water-treatment methods to achieve zero-
discharge standards.
It is appreciated that EPA has stated that "sealed" furnaces are
inherently superior from an air-pollution-control aspect C1). While this
superiority in one environmental area is significant, the need exists to
define that furnace system which is best from the viewpoint of total environ-
mental impact. To quote a recent paper entitled "Assessing Cross-Media
Impacts"(18),
"Certain control technologies aimed at achieving specific
limits generate new waste streams which, in turn, require
controls. Without a systematic assessment of these im-
pacts, there is a substantial risk that pollution control
strategies for controlling pollution in one medium will
exacerbate problems in another medium".
Such may be the case with sealed ferroalloy furnaces.
In sealed ferroalloy furnaces coupled to wet-scrubber systems,
there is no combustion of the organics released from the operation prior
to the delivery of these gases to the wet-scrubber system. All of the
volatiles from the coal, coke, electrodes, and wood chips then enter the
scrubbers. Under these conditions it is probable that some of these
organics report both to the cleaned carbon monoxide gas (which is later
combusted under varying conditions) and to the recycled scrubber water.
This point is recognized in the EPA waste-water-treatment guidelines for
the ferroalloy industry. In these guidelines, cyanide and phenols are
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42
expected in the waste water from wet air-pollution-control devices on
sealed ferroalloy furnaces. These pollutants are not encountered in the
waste water from open ferroalloy furnaces.
There is also concern over the buildup (and elimination) of
soluble elements and compounds (organic and inorganic) in totally recycled
scrubber water (from sealed furnaces) over time. There is a need to
evaluate ferroalloy furnaces of the sealed and open variety from a view-
point of total environment impact.
These potential water problems have already been evaluated in
part by the water-pollution-confrol sections of EPA. From one viewpoint,
a sealed ferroalloy furnace can be regarded as a coal gasifier. The over-
all environmental impact of coal gasification projects is being evaluated
and the knowledge developed on these projects can be used to evaluate the
air- and water-pollution control aspects of both sealed and closed furnaces.
Battelle researchers also recommend that the air-pollution-control
aspects of sealed ferroalloy furnaces be reviewed taking into considera-
tion the emissions from sintering and drying operations often associated
with the use of upgraded material in sealed furnaces. Sintering and dry-
ing operations are often difficult to control adequately from an air-
pollution point of view and, if required as part of the ferroalloy-furnace
operation, those sources of air pollution should be examined in any
evaluation of the total system.
(B) It is recommended that EPA evaluate the designs, op-
eration, and performance of the baghouses associated with
ferroalloy operations. This evaluation should be from
the viewpoint of aiding in decreasing the annual average
emission weight (per ton of a particular alloy) in
future baghouse installations for both old and new
furnaces. At this time, it is not technically
possible to produce silicon metal (and several other
ferroalloys) in sealed furnaces. There is, therefore,
a need to improve baghouse performance with open
furnaces to lower the overall emission rate and to
approach the particulate emission rate of sealed
furnaces.
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43
It appears that there will be a need to operate some new and
old ferroalloy furnaces in the open mode with dry dust collection.
Ferroalloy producers are, in part, dissatisfied with the efficiency and
operating-time performance of baghouses. It is recommended that EPA
sponsor research and evaluation studies to improve the performance of
baghouses on ferroalloy furnaces. This study should take into consider-
ation the special or different characteristics of the ferroalloy furnace
dust and develop new approaches to improving baghouse performance
collecting these materials. The possibility exists that the industry may
lend support (possibly as a joint sponsor) to such a program.
In the evaluation portion of this baghouse study, information
should be obtained on the factors influencing the operating-time perfor-
mance of existing baghouses. Some baghouse designs or equipment require
considerable "down-time" for maintenance and repairs. Some of the problems
arc (1) difficulty in locating and eliminating individual broken or torn
bags and (2) maintenance of the fans and blowers required to operate these
units. Baghouses are most often purchased on a competitive-bidding basis
and only some purchasers are in the technical position to specify desired
design improvements that relate to their type of particulate dust. These
design improvements either remain proprietary or they become incorporated
into units sold to other companies, provided that the price remains com-
petitive. There is thus only a slow improvement in baghouse performance,
most of which (say the buyers) indirectly results from efforts by individual
ferroalloy companies. There is a need for an overall evaluation from the
user's viewpoint and an acceleration in the improvement of baghouse per-
formance as the result of a concentrated and innovative technical input.
Battelle's Suggestions for Possible Research Projects
Because of EPA's interest in sealed furnaces, Battelle researchers
have considered research and development projects that would promote the
development (and acceptance) of sealed furnaces.
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44
Overall, it is judged that additional conferences and dis-
cussions will be required between EPA and ferroalloy-industry execu-
tives before specific research programs are formulated. A problem
to be resolved is to avoid duplication of confidential in-house research
and development projects being carried out at competing ferroalloy com-
panies. As an example, Battelle researchers until recently were expect-
ing to suggest research in the fields of (1) development of computer con-
trols on electric smelting furnaces to smooth the operations in prepara-
tion for eventual operation in sealed furnaces and (2) salvaging the
heat generated from combustion in open furnaces (using close-coupled
waste-heat boilers). Further consideration of these suggestions was
abandoned when two papers on these subjects were given at a recent meeting
(19 20^
of the AIME Electric Furnace Conference (December 11 and 12, 1974) ' "'•
It is possible that some or even .all of the following research projects
may already be underway at individual ferroalloy companies.
Battelie's suggested research programs relate mainly to im-
proving the technology of smelting operations for 75% ferrosilicon. This
particular alloy is selected as a target for production improvement be-
cause (1) it is expected that there may be a future emphasis on silicon
alloys in the United States, (2) this is probably the next major ferro-
alloy to be "tamed" by advancing aealed-furnace technology, and (3)
improvements in the production technology of this alloy can be trans-
ferred to other alloys (including Si metal).
Possible Research Prelects
(A) Determine whether the substitution of iron-ore
pellets for ferrous scrap in the production of
50 and 75% ferrosilicon alloys decreases the need
for stoking these operations and results in a smoother
operation.
U. S. producers of ferrosilicon alloys use ferrous scrap in the
charge to supply the ferrous component of the alloy. Japanese manufacturers,
on the other hand, use iron ore or siliceous iron ores to provide the
ferrous component of the alloy. U. S. producers prefer the use of scrap
because, in their evaluation, it is a more economical source of iron. A
Japanese producer (see Yahagi Iron Company information in the Appendix)
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45
claims that (1") the use of ore permits a "low refining temperature (1220 C)
to be used that makes it convenient to totally seal 50% FeSi furnaces";
(2) the use of ore permits proper charge sizing, minimizes segregation,
and promotes a smooth operation, and (3) the use of ore prevents the con-
tact of quartzite and improves the movement of the charge because it avoids
"the softening and sticking to each other" of the quartzite pieces.
The subject of iron ore usage versus scrap usage in ferro-
silicon production is controversial in the United States with most (if
not all) of the producers having negative opinions on the use of iron
ores in this application. However, Battelle researchers suggest that this sub-
ject be studied in a demonstration program. It is suggested that this
topic be studied in some small experimental electric-smelting furnace
even though there are some negative sentiments in the industry relevant
to the validity of the conclusions that can be drawn from small-scale
tests.
As part of the above experimental program it is also suggested
that lumps of coal char incorporating silica fines be used as part of
the charge to the test furnaces. The intimate mixture of the major raw
materials to a ferrosilicon operation nay improve the operation of these
furnaces. The Russians are experimenting with the approach^ '. It is
suggested that this coal-char-plus-silica-fines agglomerate could be pro-
duced in the United States with traveling-grate coker units'
Overall, all ferroalloy companies are interested in upgrading
their reducing agents. Further study and/or information from the industry
is required before it is possible to judge whether improved reductants
could significantly improve the smoothness of ferrosilicon (and other)
operations.
(B) At one time, the EPA was considering a requirement
that the U. S. ferroalloy industry construct only
sealed furnaces for those ferroalloy products that
can be produced in sealed furnaces, as a requisite
part of the best-demonstrated air-pollution-control
technology' . If this requirement may develop in
the future, it is suggested that EPA sponsor re-
search directed at insuring that sealed furnaces are
safe operations.
* One such unit is the Peabody Coal Company's traveling-grate coker
in Tennessee.
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46
For a ferroalloy company which has learned to produce 50% and
75% ferrosilicon quietly and smoothly in open furnaces (probably with
computer controls), without a need for excessive stoking, it is not
difficult to foresee a future safe operation in sealed furnaces. However,
if all future ferrosilicon operations are to be conducted in sealed fur-
naces, it is suggested that EPA should be interested in aiding the
development of controls that will improve the safety of operation of
all sealed ferrosilicon operations.
Japanese producers of ferroalloys in sealed furnaces maintain
that operations in sealed furnaces are safe provided that the procedures are
developed on open furnaces and provided that there are sufficient controls on the
follow-on sealed furnaces (see Appendix: MRI Study on Sealed Ferro-
alloy Furnace Operations in Japan).
A serious problem is the development of pressure in the high-
temperature cavity(s) deep in the charge resulting from crusting or
sealing of the top mix. At the present time there is no direct signal
or indication of the contained pressure inside ferrosilicon or
silicon-metal operations. Excessive pressure can result either in ex-
cessive "blowing" (with attendent overheating of the furnace roof and
high loss of silica fume) or in heaving of mix out of the crucible
and, in extreme cases, furnace explosions. All present control methods
are indirect or secondary in nature and there is no direct method of
knowing or anticipating when a furnace will erupt. It is to be expected
that some U. S. furnace operators will, in the interest of lowering costs,
be using low-grade local charpe materials. In these instances, it would
be particularly important that improved controls be developed for sealed-
furnace operations.
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47
No attempt has been made on this project to develop concepts
for the improved control of silicon smelting operations. On first evalua-
tion, it would appear that the internal pressure could be monitored by the
use of hollow electrodes and pressure measuring equipment. However, one
company which has pioneered the use of hollow electrodes for feeding fines
to electric furnaces is not enthusiastic about this approach to improve
controls. Here again, if EPA is interested in improved furnace controls,
it is suggested that conferences be held with representatives of the U. S.
ferroalloy companies. It is considered probable by the Battalia personnel
that research on this subject may be in progress at some companies.
SUMMARY AND CONCLUSIONS
The major topic of this study is ferroalloy furnace flexibility.
Flexibility is a relative term which has been discussed, in this report,
in terms of (1) open versus sealed furnaces, (2) small versus large fur-
naces, and (3) by ferroalloy type. The subject is both complex and con-
troversial and it can be misleading to summarize via generalizations.
However, six general conclusions are presented here as a summary concern-
ing the flexibility issue.
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48
(1) The U. S. ferroalloy industry has used and will
probably continue to use furnace flexibility
(on both a short-term and long-term basis) to
remain competitive in a changing product-demand
market in the United States.
(2) Sealed furnaces are less flexible (i.e., it would
be more difficult, time consuming, and costly to
effect a product change) than comparable,
open-hooded furnaces.
(3) It has not yet been demonstrated feasible to smelt
all the common ferroalloys in sealed furnaces. Sealed
furnaces have been successfully used (mainly in
Japan) to produce ferromanganese, si lie manganese,
ferrochrome, ferrochromesilicon and the ferro-
silicon alloys with silicon contents of
75% or less. The high-silicon-content (75 Si)
ferrosilicons, silicon metal, and the rare-earth
silicides have not been commercially prepared under
sealed-furnace conditions.
A sealed-furnace operation requires greater charge
preparation and attention to operating details than
a comparable open-hooded furnace on the same
product. However, a sealed furnace often exhibits
a greater operating time percentage and a good ef-
ficiency rating (e.g., kwhr/tonne, element recovery
percentage, etc.) as a result of the additional
attention given to the operating practice (see
Table 1).
(4) In general, the newer and, in turn, larger ferroalloy
furnaces are currently used to produce one product
(or, as with FeMn and SiMn, to make very similar
products). However, U. S. companies desire the ability
to change products (even in the newer furnaces) if
market conditions require or warrant such a change.
(5) A large (open or sealed) furnace is less flexible
than a small furnace. The time and coat associated
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49
with a product change in a large furnace is signifi-
cantly greater than for the corresponding change in
a small furnace.
(5) Combining Conclusions (2) and (5) above, a graphic
presentation of the degree of flexibility associated
with ferroalloy furnaces is as follows:
Small Opcm Small Sealed Large Open Large Sealed
Furnace Furnace Furnace Furnace
Max
Flexil
*.-•
Lmum
bilitv
-* A... .
• - - >
Min
Flexi
imum
hility
(6) A meaningful comparison between open and sealed
ferroalloy furnaces In carrying out a product change
is extremely difficult to formulate, as there are
multiple differences in furnace designs and
operating procedures from plant to plant. Based up-
on the furnace design and operating data presented
in Table 1, it anpears that a Riven ferroalloy product
can be successfully made with n range of operating
conditions (such as electrode spacing to electrode
diameter ratio, electrode penetration, etc.).
While the ranges of operating conditions for various products
overlap, the ability to switch products is presently dependent
on the operator's ability to stoke and observe the electrode
penetration and the charge-level condition. Thus, it may be
that to change products in a sealed furnace, the furnace would
have to be opened, the product change carried out, and then
the furnace returned to operation in the sealed condition. As
sealed ferroalloy furnace operations are a relatively recent
development, the data base for carrying out a product change
(e.g., from standard ferromanganese (FeMn) to silicomanganese
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50
(SiMn) or vice versa), Is not simply a matter of changing
the charge materials. A product change requiring a major
adjustment of smelting conditions may require that the
furnace be opened to make the product changeover.
These conclusions are essentially in accord with the statements
concerning ferroalloy furnace flexibility presented in a recent EPA report
on Standards of Performance for Electric Arc Furnace for the Production of
Ferroalloys(2).
Relative to the second specific objective of this study, BCL
personnel could not identify any existing technology and applied engineer-
ing practices which could be applied simply to existing furnace facilities
to allow the achievement of additional furnace flexibility in sealed fur-
naces without further long-term research and development efforts.
However, BCL personnel recommend that EPA (1) study and evaluate
the overall (i.e., air, water, solid waste, etc.) environmental character-
istics of open-hooded and sealed ferroalloy furnaces to establish future
policy statements, and (2) evaluate the design and operation and performance
of baghouse associated with ferroalloy furnaces with the objective of de-
fining methods of decreasing the annual total emissions from existing and
future furnaces.
The BCL-formulated suggestions for additional programs which
could be undertaken by EPA (possibly jointly with the U. S. ferroalloy
industry) to enhance the utilization of sealed furnaces and/or lower
furnace emission from U. S. furnaces include:
(A) Determine whether the substitution of iron-ore pellets
(as used in Japan) for ferrous scrap (as used in the
U. S.) decreases the need for stoking and improves
the ease of operation in the smelting of 50 and 75 per-
cent ferrosilicon alloys.
and (B) Sponsor research directed at insuring that sealed
furnaces are safe operations through the develop-
ment of improved furnace-monitoring techniques and
devices.
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51
References
(1) Engineering and Cost Study of the Ferroalloy Industry, James 0.
Dealy and Arthur M. Killin, U. S. Environmental Protection Agency,
EPA - 450/2-74-008, May, 1974.
(2) Background Information for Standards of Performances: Electric
Submerged Arc Furnaces for Production of Ferroalloys, Vol. 1,
U. S. Environmental Protection Agency, EPA-450/2-74-018A.
(3) Feik-ral Register, Vol. 39, Nn. 204, Monday, October 21, 1974.
(4) Webster's Seventh New Collegiate Dictionary, G & C Merriam Company,
Publishers, 1965.
(5) Statement on Proposed Air Quality Standards for High Carbon Ferro-
manganese, Silcomanganese and Calcium Carbide, The Ferroalloy
Association, Washington, T). C., 1973.
(6) A.G.E. Robiette, Electric Smelting Processes, John Wiley & Sons,
Hew York, 1973.
(7) C. L. Mantell, Electrochemical Engineering, Fourth Edition, MaGraw-
Hill Book Company, Hew York, 1960.
(8) V. Paschkis and J. Persson, Industrial Electric Furnaces and Ap-
pliances, Second Edition, Interscience Publishers, Inc., New York,
1960.
(9) V. P. Elyutin, et al, Production of Ferroalloys Electrometallurgy,
Second Edition, Published for the National Science Foundation, Wash-
ington, D. C. and the Department of the Interior, by the Israel
Program for Scientific Translations, 1957.
(10) J. A. Persson, "Six-Electrode-In-Llne Smelting Furnaces", Electric
Furnace Proceedings, AIME, Vol. 31, 1973.
(11) F. Andreas, "Design and Control of Ferroalloy Furnaces", AIEE
Transactions, Vol. 69, 1950.
(12) W. M. Kelly, "Design and Construction of the Submerged Arc Furnaces",
Metal Progress, Vol 73, 1958.
(13) J. A. Persson, "The Significance of Electrode-to-Hearth Voltage in
Electric Smelting Furnaces", Electric Furnace Conference Proceedings,
AIME, Vol. 28, 1970.
0/0 Z. B. Wowk, "Characteristics of a Submerped-Arc Furnace", Electric
Furnace Conference Proceedings, AIME, Vol. 22, 1964.
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52
(15) J. C. Wolfe and R. R. Pait, "Interchangeable Shells on a 29,000 KVA
Electric Smelting Furnace", Electric Furnace Conference Proceedings,
AIME, Vol. 22, 1964.
(16) Development Document for Proposed Effluent Limitations Guidelines
and New Source Performance Standards for the Smelting and Slag Pro-
cessing Segment of the Ferroalloy Manufacturing Point Source Cate-
gory, U. S. Environmental Protection Agency, Effluent Guidelines
Division, 1973.
(17) C. H. Schanche, N. Bugge, and 0. Braaten, "Today's Trend Toward
Larger Electric Smelting Furnaces - Some Features in Design and
Operation", Electric Furnace Conference Proceedings, AIME, Vol. 24,
1966.
(18) H. Reiquam, N. Dee, and P. Choi, "Assessing Cross-Media Impacts",
Environmental Science and Technology. Vol. 9, No. 2, pp. 118-120,
February, 1975.
(19) W. R. Schneider and J. F. Eyrich, "Operation of a 75 MVA, 75% Ferro-
silicon Furnace with Heat Recovery Equipment", Electric Furnace
Conference Proceedings, AIME, Vol. 32, 1975.
(20) W. L. Wilbern, "Computer Control of Submerged Arc Ferroalloy Fur-
nace Operations", Electric Furnace Conference Proceedings, AIME,
Vol. 32, 1974.
(21) S. I. Khetrick, et. al, "Production of 45% Ferrosilicon Using a
Briquetted Charge", Stal. December, 1966.
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A-0.1
APPENDIX
MITSUBISHI RESEARCH INSTITUTE REPORT ON
SEALED FERROALLOY FURNACE OPERATIONS IN JAPAN
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A-0.2
TABLE OF CONTENTS
Page
A-l. MRI REPORT COVER LETTER A-l.l
A-2. MRI INTERVIEW WITH JAPANESE FERROALLOY ASSOCIATION A-2.1-A-2.7
A-3. MRI INTERVIEW WITH JOETSU DENRO KOZYO COMPANY A-3.1-A-3.3
A-4. COMMENTS AND OBSERVATIONS BY MITSUBISHI RESEARCH
INSTITUTE, INC., BASED ON JOETSU STUDY A-4. l-A-4.2
A-5. MRI INTERVIEW WITH YAHAGI IRON COMPANY A-5.l-A-5.4
A-6. MRI INTERVIEW WITH TANABE KAKOKI COMPANY A-6.1-A-6. 3
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A-l.l
MRI REPORT COVER LETTER
MITSUBISHI RESEARCH INSTITUTE, INC.
TELEPHONE 03/214-1331
TELEX 222-2287
CABLE MRISYSTEMS
HI8IYA PARK BUILDING
1. YURAKUCHO I-CHOME
CHIYODA-KU TOKYO
December 27, 1974
Dr. C. E. Moblcy
Primary Operations Section
Battelle Columbus Laboratories
SOS King Avenue
Columbus, Ohio 43201
U.S.A.
Dear Dr. Mobley:
I am sending under the separate cover the report on
sealed ferro-alloy furnace operations in Japan requested
by your purchase order No. J-827S.
Our conclusions for this research are as follows.
1. It is not technically feasible at present for
every company to convert the production of 75%
FeSi to totally sealed furnace. Judging from
experiences in Japan, introduction of sealed
furnaces should be in paralled with the operation
of open furnaces, through which technical develop-
ment be achieved and experiences be accumulated.
2. 50t FcSi can be safely produced with sealed
furnaces without any pollution problems.
Especially, the process using ore as raw material
has been technically established.
3. Those interviewees in Japan arc ready to sell
their know-hows to foreign companies.
4. A scaled furnace builder in Japan has actual experi-
ences in supplying furnaces to foreign companies over-
seas except for the U.S. The company considers that
the export directly to the U.S. ferro-alloy producers
is rather difficult, since they seem to have their
own accumulation of technology. Therefore, the company
wishes to find an appropriate furnace builder as the
partner in the U.S. to make its know-hows available
in the U.S.
Very truly yours,
YS:sm
/Research Director
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A-2.1
A-2. MRI INTERVIEW WITH JAPANESE FERROALLOY ASSOCIATION
Q. 1. How many totally sealed ferroalloy furnaces are there in operation
in Japan? What percentage of the total Japanese furnace capacity?
Is it possible to obtain the total number (of sealed furnaces), the
alloy type per furnace, and the rated and actual average power in-
put and ton of product per year per furnace and age of furnace?
A. 1-1 At the present time, 39 units of totally sealed ferroalloy furnaces
are in operation in Japan.
A. 1-2 We refrain from revealing the production capacity. However, the
following table shows the production during the period from April
1 to June 30:
(Ton) Insiders* Outsiders*
Fe-Mn
Si-Mn
Fe-Si
Fe-Cr
Others
163,000
150,000
63,000
150,000
29,000
555,000
2,900
5,400
33,900
2,300
66,700
111,500
Total 666,500 Ton
The above figures show the highest record in the past history.
A. 1-3 Power rating of furnaces in June, 1973
Total furnaces Sealed furnaces
Unit Rating (KVA) Unit Rating (KVA)
He Fe-Mn
Si-Mn
Fe-Si
He Fe-Cr
Si-Cr
M & LC
Fe-Mn
11
36-'
43
32
8
16
248,200
479,700
692,500
336,000
101,000
55,200
9
15
3
4
1
7
239,400
304,000
120,000
91,000
40,000
-
A. 1-4 We are unable to give answers to other questions. It is necessary
for MRI to conduct an additional market research to obtain proper
answers.
Q. 2. Of the sealed furnaces, what is the listing of gas cleaning
equipment? (bag house, scrubbers, scrubbers plus wet
electrostatic)
A. 2. We can not reply to this question. An additional market research
is required. However, the characteristics in Japan are;
1. Baghouses are used for open tyce furnaces.
2. Scrubbers or scrubber + Wet electrostatic facilities
are used for sealed furnaces.
* We believe that these terms are used to designate members and nonmembers
of the Japanese Ferroalloy Association, respectively*
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A-2.2
Q. 3. As of 1073 it was understood that there was one sealed furnace
(Joethu Electric Furnace Industry Company) in operation in
Japan making 75 percent FeSi.. This we understand is a small
furnace. Is this furnace st::ll in operation? Are there other
sealed furnaces making 75 percent FeSi now? Are there larger
furnaces in operation or going to be put into operation on
this difficult alloy?
A. 3-1 Yes, the furnace is in operation at the present time.
A. 3-2 Only 75%-Fe-Si is produced.
A. 3-3 No larger furnaces are scheduled to be put in operation.
Q. 4. Is the small furnace (making 75 percent FeSi) kept on this
product or is it necessary to switch to 50 percent FeSi (or
other products) for periods to clean out the silicon carbide
that may have formed in the furnace?
A. A . Only 75%-Fe-Si is produced.
Q. 5. In all ferrosilicon and silicon operations there is an input
quartzite to reductant ratio with a variable loss of silicon
monoxide. Are there any attempts being made to measure the
rate of loss of silicon (as & compound) in order to adjust
the input quartzite to reductant ratio? Is it possible to
control a ferrosilicon or silicon furnace accurately without
knowing the silicon losses hour by hour? (This question is
asked because in the U. S. high-silicon alloys represent a
problem in manufacture because of bridging, high-temperature
blows, and serious control problems. On the other hand, EPA
descriptions of high-silicon alloy production in sealed
furnaces in Japan indicate that there are no particular
problems, difficulties, or system "upsets".)
A. 5. There are no particular problems, difficulties or system
"upsets", associated with quartzite addition. As you pointed
out, the input is adjusted by the weight and no other ways are
used.
Q. 6- The U. S. EPA visitors to Japan were impressed by the dust-
collection efficiency of various scrubber and wet electro-
static precipitator arrangements on sealed furnaces in Japan.
The use of a wet electrostatic precipitator following two
scrubbers in series represents a new approach to collection
equipment to Battelle researchers. In this regard we have the
following questions:
(a) In the series arrangement of: scrubber-scrubber-wet
electrostatic precipitator, what percent of dust col-
lection (of the total) occurs at the wet. electrostatic
precipitator?
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A-2.3
(b) Does this arrangement control and collect super-
fine silica (Si02) fume?
(c) Is this arrangement more efficient than a well-
maintained bag house on a sealed furnace? Why?
(d) On ferrosilicon operations is there any official
data on dust release to the atmosphere by (1) type
of collection system, (2) type of alloy, (3) weight
per MW-hour?
(e) Are there any bag house installations in Japan
connected to ferrosilicon operations both on
sealed and open furnaces. If the answer is yes,
what is the relative performance (in terms of
emissions to the atmosphere in grams per M7-hr)?
A. 6. Since the manufacturers concerned have directly submitted the data
to EPA, this Association knows nothing about the contents of data.
Q. 7. Do the safety records (or injury data) of the Japanese Ferroalloy
Association indicate that sealed furnaces are as safe as open
furnaces?
A. 7. No accidents have been recorded. (Refer Yahagi's counter
opinion.)
Q. 8. Are there any sealed ferrosilicon furnaces connected to suction
bag houses? If yes, what is the reported emissions rate to the
atmosphere?
A. 8. There is one (1) unit at Showa Denko. ^ is used. No data
available.
Q. 9. Is there any information on the emissions of organic volatile
compounds (by weight and by compounds) from sealed furnaces?
It is appreciated that all the gas from sealed furnaces is com-
pletely burned either as fuel or in flaring, but what amount
of the volatile organics end up as input to the water-control
system? U. S. ferroalloy companies are concerned about using
scrubbers on sealed furnaces because they occasionally use a
rather high percentage of coal and/or wood chips in the charge
both for economic and operational considerations. The probable
condensation of coal and wood volatiles would appear to have
many problems in common with coal coking and tar processing.
Does the Japanese Ferroalloy Association have any information
on the amount of organics that may appear in the scrubber water
on sealed furnaces? Does the industry have a position or pre-
ference for wet versus dry collection techniques? To what ex-
tent are water effluents considered a pollution problem and
how is the industry dealing with the water effluents?
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A-2. A
A. 9. No data available.
Q. 10. It is appreciated that of the many advances made in ferroalloy
operations in Japan, two improvements concern themselves with
(a) well distributed unsegregated feed to each electrode, and
(b) monitoring the charge consumed at each electrode. Given
that the operator has information of when bridging starts at
one electrode in a sealed furnace, what action does he take to
return the consumption of charge material to steady flow conditions?
A. 10. Since the quality of electrode is excellent and the facilities used
for charging materials into the furnace are superior, no troubles
have been experienced.
Q. 11.' In general, what actions are taken (and what actions are possible)
in a sealed furnace to improve any "rough" operation that may
develop? It is appreciated that drying the mix, sizing the mix,
and preventing segregation on charging improves the operation of
any ferroalloy furnace. However, in the judgment of many, no ferro-
alloy furnace is actually in close hour-by-hour control. An er-
roneous mix change, for example, can result in furnace difficulties
2 to 4 hours later. In instances when this does happen, what cor-
rective actions arc possible?
A. 11. No particular mix-preparation is made for the materials. The
operations used for the closed furnaces are equal to those used
for the open furnaces. (Refer Yahagi's counter opinion.)
Q. 12. In sealed furnaces, are electrodes ever raised to determine the
length submerged in the charge:? If this is not possible, what
method aside from estimated eJectrode consumption is used to es-
timate electrode length?
A. 12. We depend on experiment.
Q. 13. In selecting quartzite for use in the production of silicon-bearing
alloys is there any qualification other than chemical analysis? Is
the resistance to thermal shattering determined? What is the source
of quartzite in Japan and are there any local quartzites that are
not used because of the problems of bridging that may be related
to this particular quartzite?
A. 13-1 Chemical analysis is used for qualification.
A. 13-2 Since the specifications of supplyers are trusted, no special test-
ing is carried out and there has been no troubles.
A. 13-3 In Japan, quartzites is produced in Fukushima, Mikawa, Fukui, etc.
As a matter of fact, quartzites is abundantly produced elsewhere
in Japan. In order to save the transportation cost, the locally
produced quartzites is selected by each suitable ferro alloy manu-
facturer who is located at different area.
-------
A-2.5
A. 13-4 There has been no problems of bridging related to any particular
quartzitcs. (Refer Yahagi's answer.)
Q. 14. It is judged, by Battelle-Columbus, that the operation of a sealed
furnace requires more knowledge, expertise, and understanding than
the operation of an "open" furnace making the same alloy. Is this
correct? If this is indeed correct, it is difficult to convert an
operator used to working an "open" furnace to operating a closed
furnace, making the same alloy? Is special training required to
operate a totally sealed furnace over and above that given for
open furnace operation? In the United States, many operators are
very reluctant to "give up" their control of the operation based
on the visual observation of the top of the charge. Is this
also true in Japan?
A. 14-1 Yes.
A. 14-2 It is easy to convert an operator used to working an "open" fur-
nace to operating a close furnace, making the same alloy.
A. 14-3 Special training is required to operate closed furnaces.
A. 14-4 Most of operations and actions are controlled by instruments
which are observed in the control room, but the furnace operating
conditions are determined through the inspection shutter by visual
observation, if necessary.
Q. 15. Has any method been developed to measure or obtain direct warning
of either (a) deep bridging down in the mix and/or (b) pressure
build-up in the scaled furnace. Again it is appreciated that a
sealed furnace may require extensive mxi preparation (like a
modern Japanese blast furnace) and mix preparation improves opera*-
tions in general. However, in a ferroalloy furnace there is no
indirect indication of internal pressure, or has this been develop-
ed? If not, would this be a worthwhile topic of research?
A. 15-1 The pressure within the cover of sealed furnace is measured.
A. 15-2 Since the relative position of the electrode and the material
charge chute is designed excellently, there has been no trouble.
A. 15-3 The method of removing furnace gases and dust is very excellent—
this is another reason which contributes to eliminating troubles.
Q. 16. Mix preparation in blast furnace operation had the effect of in-
creasing productivity. It is appreciated that ferroalloy furnaces
do not work on the same principle (blowing air into carbon to
produce heat and reducing agent) but are there any indications
that mix preparation improves the productivity of ferroalloy
furnaces? We would be interested in the productivity figures on
two furnaces (open and sealed, ordinary mix and prepared mix) hav-
ing about the same transformer rating. Is information of this
type available Ln Japan?
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A-2.6
A. 16. We do not think productivity can be improved, but the working
conditions can be improved and pollution can be reduced.
Q. 17. Is the collection and effective utilization of the
carbon monoxide off-gas from scaled furnaces the
economic justification for sealing ferroalloy furnaces
and preparing the mix for these furnaces? If this is
correct, would it be possible to elaborate on the
gains?
[In this instance, the Battelle researchers appreciate
the gains made by the Japanese in installing the OG
system on Japanese LD vessels.]
A. 17. With most of sealed furnaces, gases from the furnaces are
collected and used for boilers. Some plants sell such gases
to outsiders. Nippon Kokan plans to use this off-gas from
sealed furnace for pre-reduction of raw materials or chemical
industry.
Q. 18. If absolutely necessary, at what cost and at what time
loss can a sealed fcrromanganese furnace be converted
to a furnace for producing 75 percent ferrosilicon?
Here the Battelle investigators appreciate that this
is not an efficient procedure, but for the moment if
this had to be done, how flexible are sealed furnaces?
Please elaborate. (In this question we are assuming
that the off-grade product made during the switchover
can be sold somewhere at a discount). Our interest is
mainly in the expense involved in adjusting electrode
diameter and electrode working diameter with a scaled
furnace. )
A. 18-1 Fe-Mn furnaces cannot be converted to Fe-Si furnaces.
This kind of conversion is not practiced in Japan, be-
cause each furnace is highly specialized. Even if con-
verted, the depth of furnace is different (technically
speaking). A large amount of slag is formed in case
of Mn operation.
A. 18-2 For the data of electrode, refer to MRI note on Electrode Diameter
Calculation.
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A-2.7
MRI NOTE ON ELECTRODE DIAMETER CALCULATIONS
Ferro Alloy
Std. Ferromanganese
ii
it
ii
Silicomanganese
ii
it
it
75 Percent
Ferrosilicon
n
n
ii
n
Furnace
Designation
A
B
C
D
E
F
G
H
I
J
K
L
M
N
Furnace
Load,
Kilowatts
25.0
14.1
15.0
26.0
25.0
15.0
18.4
30.0
16.0
12.6
14.0
24.0
30.0
26.0
Electrode Diameter, cm
Calculations
Kelley Aono
200 160
150 130
151 132
207 163
195 161
150 137
166 145
210 173
137 125
124 120
127 120
168 145
190 160
173 150
Actual
170
143
145
170
170
135
147
170
125
110
105
135
140
140
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A-3.1
A-3. MRI INTERVIEW WITH JOETSU DENRO KOZYO COMPANY
Q. 1. The EPA representatives have reported that this company is operating a
12 MW furnace to produce 75 percent FeSi, without special mix preparation.
It is also reported that this company has determined that the silica fume
collected at this furnace is an order of magnitude larger in size than
similar fume from open furnaces. Our general questions are as follows:
(a) Is this small furnace still producing 75% FeSi?
(b) To establish operating reliability, will this company
report for one year:
(1) The tons of various alloys made in this sealed furnace?
(2) Average MW-hrs per ton of each product.
(3) Number of operating hours per one year (power on time).
(4) Average MW hour input per ton of product.
(5) Range of MW input (high and low).
(6) Average number of repair or maintenance hours per week.
It is appreciated that Joetsu may be varying the power input to this
sealed furnace depending on power availability. If this is the case,
we would appreciate knowing the power availability schedule.
A. 1. (a) Yes, as of now, this furnace produces 75% FeSi.
(b)-l Only 75% FeSi was produced.
(b)-2 8100 kWh/tapped metal ton.
(b)-3 We refrain from answering.
(b)-4 Refer to (b)-2.
(b)-5 6000 - 12,000 kw.
(b)-6 The furnace has not been repaired since 1971. It is, however,
scheduled to repair it in 1975.
Note: The charging chutes and contact chutes are maintained
at all times to prevent abrasion caused by raw material.
(b)-7 Electric power availability.
10,000 - 11,000 kw load is the target value. However,
it drops down to 6000 kw in the dry seasons in winter
and summer.
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A-3.2
Q. 2. Several countries have reported difficulties with attempting to
produce 75 percent FeSi in sealed furnaces. Besides the expected
dust build-up problems, the major operating problem was the crusting
of the mix. In some instances special cokes were used in an attempt
to correct this problem. What action is taken at this company to
overcome or prevent the typical bridging problems with 75 percent FeSi
production?
A. 2-1 No special cokes are used.
A. 2-2 The problem has been solved by improving the facilities concerned.
That means that the sealed furnace is provided with the fixed and
the mobile poking devices and, therefore, there is no trouble of
"dust build-up".
Q. 3. Does this company plan a larger furnace? If they do not plan to
utilize a large furnace due to current economic or market reasons,
do they consider a larger 75 FeSi furnace feasible?
A. 3. Yes, this company plans to build a sealed furnace of 26,000 - 30,000 kva,
but as of now, this plan is pending due to the increasing power rate
and the problems in the FeSi market. Technically speaking, however,
there is no problem to build a large scale sealed furnace.
Q. 4. Japanese technical descriptions of this 75% FeSi operation have
not indicated any special technological factors that would appear
to have solved the production problems with this alloy whether
using an open or a closed furnace. Are there special considerations?
(A satisfactory answer would be that there is proprietary technical
information other than engineering modifications.)
A. 4. There is a difference between the closed furnace and the open furnace
in regard to the engineering of design itself. Especially, with the
experience to operate open furnaces, a highly efficient operation of
closed furnaces is possible. However, in order to learn the operating
procedure in a short period of time, a "know-how" should be used for
the operation.
Q. 5. Of particular interest (to the world) is the reported growth in
size of the fume from this furnace making collection a relatively
simple matter. It would be interesting to know if this fume-
growth factor has been confirmed particularly in the fume collected
routinely at the dust collector? It is possible to visualize
growth in particle size of deposits in and on the furnace structure.
Growth in transit or flight to the collector could, however, represent
new technical information that would rapidly encourage the use of
sealed furnaces on 75% FeSi with Japanese know-how.
A. 5. The characteristics of sealed furnace is such that the size and
shape of dust are fundamentally different. The fact that the
size of dust is large makes the closed furnace fundametally
different from the open furnace in regard to the know-how of
operation, structure and mechanism of furnace, etc.
-------
A-3.3
A. 5-1 For the shape and size of dust, refer to Joetsu Report.
Note: This report was made by checking the dry dust (before
soaked in water).
A. 5-2 Can be seen with naked eyes.
Q. 6. Most high-silicon operations build up a deposit of silicon
carbide in the bottom of the furnace after some period
forcing switching to 50% silicon operation for a period.
Others keep operating by rotating the furnace. What is
the practice of Joetsu Electric?
A. 6. The furnace has not been repaired. The 50% FeSi will not
be produced.
Q. 7. What is the electrode spacing to electrode diameter ratio
for the sealed 75 FeSi furnace?
A. 7. Electrode ratio is 1.52 which is used formally for producing
carbide.
-------
A-4.1
A-4. COMMENTS AND OBSERVATIONS BY MITSUBISHI
RESEARCH INSTITUTE. INC.. BASED ON JOETSU STUDY
(1) U.S.S.R. is using closed furnaces to produce 75% FeSi. Please refer
to the book; "Steel in USSR", November, 1972, page 881. Closed furnace
melting 75% FeSi in USSR.
(2) The know-how possessed by Joetsu is "How to dispose of the dust peculiar
to the closed furnace operation".
(a) How to prevent the dust from being accumulated in the furnace
cover, i.e., how effectively ventilate the gas in the furnace
cover.
(b) To ensure a stabilized operation by detecting a signal from the
furnace (related to the proper operation).
(3) The closed furnace of Joetsu is manufactured by Tanabe Kakoki Co.
(refer visiting report of Tanabe Kakoki Co.). Tanabe and Joetsu are
under a special agreement for their closed furnaces. Whenever
Tanabe sells a furnace to other company, Joetsu receives a "fee".
from Tanabe. Tanabe has already supplied a FeMn furnace to UCC in
USA and, as of now, received about 25 inquiries from companies of
different countries in the world.*
(4) A dry type baghouse is used for the dust collector of closed furnace
of Showa Denko (Toyama Works).
(5) With closed furnaces, gases are not released into the atmosphere and,
therefore, the environment is kept clean, but special care is required
in operation because incompletely combusted gases and CO gas are
handled.
(6) In case of Joetsu, The speed of scrubber is so slow that it should
rather be called "washer".
(7) Since the electric resistance of Japanese cokes is low at a high
temperature, coal is used partially. By doing so, tar from coal is
a problem for the water system.
(8) Joetsu makes electrodes at their own factory. No bridging is caused
by the paste of electrode by controlling the quality and temperature
of electrodes as well as the quality of paste.
(9) The raw material is charged to the surrounding area of one (1) electrode
through four (4) charging chutes. Also, there is one chute common at
the center. Furthermore, since there is a fixed and mobile poking
device, no abnormal condition occurs in the furnace.
This is somewhat incorrect as the sealed ferromanganese furnace supplied to
Union Carbide Corporation by Tanabe is part of a Canadian ferroalloy plant
operated by Union Carbide, Canada.
-------
A-4.2
(10) We understand that, in the USA, the electrode consumption is
measured by means of the load cell, but we are doubtful of the
effects of this method. In Japan, we rely on the operator's
experience and, so far, have no troubles.
(11) Joetsu buys quartzites from Fukui and Ashikaga.
(12) When the melting point of quartzities is low, banking occurs.
However, there has been no operational accident or trouble by
using the purchased quartzites.
(13) In case of Joetsu, the exhaust gas is used for boilers.
1500 m^/FeSi ton is produced.
(14) Some companies in Japan are usi.ng wood chips. Rate of wood
chips consumed; 350 kg/ton.
-------
A-5.1
A-5. MRI INTERVIEW WITH YAHAGI IRON COMPANY
The production results on 50% FeSi on a large sealed furnace are
impressive. Possible questions for this company are as follows.
Q. 1. Is this company considering making 75% FeSi in their furnace?
A. 1. Yes. The company is considering making 75% FeSi in the closed furnace.
Reasons: Since the FeSi of this company is produced by the ore
method in this company, the amount of gases (02, H2> generated
from the product is less than those generated by scrap process.
Since the FeSi of this company has excellent quality, it has been
enjoying a good reputation.
Furthermore, in case of the closed furnace operation, a low
refining temperature (1,220 C) is used in the ore method of
this company and, therefore, it is very convenient to totally
seal the furnace.
Q. 2. If not, what are the problems to be overcome?
A. 2.
Q. 3. It is understood that this operation (50% FeSi) is carried
out using siliceous iron ore which keeps the fraction of
quartzite charged from touching and fusing. Could this company
produce 50% FeSi routinely using only quartzite and coke?
A. 3. This company uses special coal.
Reasons: If special coal is not used, proper reducing reaction
can not be ensured.
Q. 4. Are there any indications of large sized silica fume in the off-
takes of this furnace?
A. 4. Only very fine dry Si02 is obtained.
Reasons: Differences from other types of closed furnace in
obtaining Si02 are (a) refining method, (b) raw materials,
and (c) temperature.
The offtakes are not clogged with the Si02-
Q. 5. Would this company consider an operation using quartzite • coke,
coal, and iron scrap? We would be particularly interested in
any information on the effect on the smoothness of operation
using iron or steel scrap.
-------
A-5.2
A. 5-1 Iron scrap has never been used.
A. 5-2 Even if the price of the iron scrap becomes low, we are most
certain that the low temperature refining of FeSi would
continue to be more attractive and beneficial.
A. 5-3 In order to ensure the smoothness of operation, the "preparation
of material mix" is very important. In the operation using
the scrap, the "mix preparation" is made for the sizes of raw
materials such as scale, chips, etc, and the size of the quartzite,
beforehand, but a segregation is unavoidable when the materials are
charged, thus the smooth operation being impaired. This is the
reason why this company uses "Ore". In case of "Ore", sizing can
be done properly against the size of quartzite and other charging
materials.
Q. 6. It is probable that hanging is encountered in the smelting zones
at times. What action is taken when this occurs. What danger
exists if a hanging condition is not corrected?
A. 6-1 If the experience, controlling, etc are not enough and "hanging"
is encountered, the operation of closed furnace can not be performed,
Therefore, a company (operators, workers, etc) must obtain enough
experience and acquire the know-hows by operating open furnaces and
learn how to prevent hanging before proceeding into operating
closed furnaces.
MRI Note: A large amount of Si02 fume is especially produced in
a high temperature operation for which scrap, quartzite and coke
are used as raw material.
Rate of fume produced is; 300 g/M3 gas x 1500 M3/ton
FeSi = 450 Kg/ton FeSi.
A. 6-2 Around the year of 1970, there was an accident caused by hanging
of closed furnace (not in Yahagi) . In this accident, a furnace
cover was blown up and two operators in the control room were
killed by the explosion
Q. 7. Is the silica fume collected at the collection equipment
monitored to serve as an adjustment on the silica to carbon
ratio of the mix?
A. 7. The operation cannot be performed only by changing the carbon
ratio by monitoring the fume generating rate. The following
matters should be taken into consideration.
(a) Temperature of gas (in offtakes and furnace cover) .
(b) Tapping temperature.
-------
A-5.3
A. 7. (Continued)
(c) Analyzed values of CO, C02, and H2 in the gas.
^d) Contents of Al and Ca in the metal.
Also, the problems should be solved from the standpoint of
facilities concerned, because the electric unbalance should
be prevented by checking the balance of consumption of raw
material with reference to each electrode.
(Patent No. US 3,499,970)
Q. 8. If this company uses coal as part of the charge material,
what effect is noted in the water treatment system, if any?
A. 8. Cyanogen and heavy metals are contained in water and, even when
the water is passed through a thickener and a pre-filter, it
cannot be reconverted to pure (fresh) water.
At the present time there is no problem, because a perfectly
closed cycle is used for the water system. There is, however,
a minor problem which stems from excessive bubbles produced in
the water system due to the effects of tar from coal.
Q. 9. If possible, please ask this company for performance figures
(particularly for the sealed 45 and 60 kva furnaces) over an
entire year as follows:
(a) Number of operating hours per year
(b) Tonnage of alloys (by type produced)
(c) Average MW hours per ton of product
(d) Average MW during operating period
(e) Range in MW (high and low)
(f) Electrical energy supply conditions?
A. 9. Please refer to the Yahagi Report.
The closed furnace should not be evaluated on the basis of
the operating hours, because there is a restriction in the electric
power supply.
Reasons:
(1) The restriction in electric power supply at the
peak time of day.
(2) Restriction in the power consumption in winter and summer.
(3) Night factor (38% of total consumption should be alloted
during 8 hours at night); 12 hours/month is always used
for periodical maintenance of closed furnaces.
-------
A-5.4
Q. 10. What is electrode spacing to electrode diameter ratio for the
two sealed 50 FeSi furnaces?
A. 10. This is the specific "know-how", and the answer to this
question cannot be obtained .
Q. 11. Is this company, or the equipment supplier, ready to export
the know-how and furnace design for producing 50% FeSi under
other operating conditions? What are the restrictions? What
are the requirements?
A. 11. Yes, this company is ready to export the know-how, patent,
and furnace design. However, the decision shall be made
according to the possibility of territorial conflict with
the would be licensee in marketing the product. That is, the
decision shall be made on t.he case-by-case basis.
Q. 12. Is the quartzite given any qualification other than chemical
analysis? Is spalling resistance determined?
A. 12-1 The softening point of quartzite within the range of 1,600 C -
1700 C is measured.
Any quartzite which is softened quickly can not be used for
Si refining.
A. 12-2 Thermal shocks are also tested.
Other comments:
(1) It is said that, in general, furnaces for FeSi and for FeMn
cannot be substituted for each other.
(2) The (electric) power density of refining zone for FeMn is
35W/cm2, and 85W/cm2 for FeSi.
We believe that if the furnace capacity and structure are
appropriately designed, a mutually substitutable furnace
can be constructed, but not economical.
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A-6.1
A-6. MRI INTERVIEW WITH TANABE KAKOKI COMPANY
General Consideration design
for Closed Furnace
When a closed furnace is used, a technical bottle neck is how to
handle CO gas produced from the carbon contained in the raw material. On the
other hand, since the air flow-in rate is low, the absolute amount of disposed
gas is small and the amount of produced gas is only a few times more than
that of the theoretically estimated gases. Because of these reasons, the
dust collecting system is much smaller when compared with that of the open
electric furnaces. However, since the concentration of CO gas is 50 - 70 percent,
it is unavoidable to use a wet type dust collector as long as the technical level
remains in the present condition. The reasons why the wet type is used are:
When a dry or an electric method is used, air may flow-in, air lock may occur
when dust is taken out, the rotating units may be overheated, mechanical shock
may occur, gas explosion may be caused by an arc, etc. Now, the matters
concerning the plan, operation and maintenance of dust collecting system shall
be discussed with reference to the above matters. First, "dust" is produced
as follows: the raw material is sent from the mixing tank to the plural
number of raw material throwing tanks installed on the top of furnaces. Then,
a fixed quantity of raw material is alternately thrown into the furnace through
the air locked gate provided on the lower part of the above tanks. Generally
speaking, th« raw material in the furnace consists of (from the top to the
bottom) the layer drying, preheated layer, the slag and molten metal on the
bottom.
The exhaust gas rises in the reverse direction of the above described
sequence of layers and the heat of heat-gns is exchanged with that of the
layers of raw materials and the lower raw material in the raw material throwing-
tanks. Then, it filters the coarse grains of dust in the raw material layer
and sends the gas and fine grains to the dust collecting system provided
outside. In this case, if the pretreatment of raw material is insufficient,
the water content in the raw material moves into the gas, condenses on the
inner walls of pipes and may cause a clogging in piping. In order to prevent
this trouble, the temperature of inner wall of pipe and dew point' of gas
should be calculated with reference to the gas temperature, gas calories,
ambient temperature, etc and necessary measures such as heat-insulation, heating,
etc of external walls of piping should be taken.
Besides the above matters, there is another effective method. Let
water flow through the piping to protect the inner wall of pipe with a film
of water, preventing the dust from sticking.
In order to ensure reliability and effects of operation of the wet
type dust collecting system, it is necessary to combine different types of
dust collectors of different functions, thus making it possible to cope with
troubles of every possible cause. For example, a combination of Taizen washer,
Venturi scrubber, mist catcher, etc or a combination of a low pressure Venturi
and a high pressure Venturi-cyclime scrubber.
-------
A-6.2
When combining a low pressure Venturi, Taizen washer, filling tower,
etc, it is necessary to take into consideration the characteristics of each
element referring to the limitation of the amount of weter used by the factory,
the capacity of waste water treatment, etc. When designing a system and if
water and dust should be removed from each dust collector, the diameter of
grain, physical properties of dust at each dust collecting stage, etc should
be predicted properly and the system should be provided with such a function
which can fully cope with the sinking speed, condensing property, PH values
of drained water, etc. Generally, in case of the wet type dust collector,
the ferroalloy dust is often hardened while sinking and cannot be discharged
to the outside and, when water is used for recycling, the substances dissolved
in the liquid are educed by the chaige in the density and humidity, thus
often resulting in clogging the syscem. From this point of view, the amount
of water to be added should be decided by taking into consideration the
economy of factory water as well as the safety in operation. Another important
matter is that each dust collector should be provided with a bypass route so that,
when repairing or replacing the equipment, it is not necessary to stop the
entire system. It is quite natural that the density of discharged "dust"
should be kept lower than the value prescribed by laws concerned. In case of
a closed furnace, CO gas is combusted in the flare-stack before it is
released into the atmosphere. It is possible that the aforementioned heavy
metals, other solid substances, dissolved substances, harmful substances
such as cyanogen may be mixed in the drained water and, therefore, this is
another problem that should be taken into consideration when designing the
drained water disposing facilities.
Cyanogen was detected by a public research organization a few
years ago. Cyanogen gas is produced as the carbonic substance reacts by
heat in the closed furnace. This phenomenon is not observed in open furnaces.
When designing a wet type dust collecting system, it is inevitable to take
necessary measures that can remove smoothly the drops of liquid from the inside
of equipment. This is because the driving source is a gaseous body of small
density which is the characteristics of this system.
Characteristics of Dust and System for
Ferro-Cr and Ferro-SiCr Open Furnace
In case of Cr, the temperature is at an intermediate level of FeSi
and FeSi and FeSiMn but the dust is comparatively coarse and large -- for
example, with the centrifugal circulation type multi-cyclone with Z = > 150,
the exhaust gas density was 0.1 - 0.1 G/Nm3. In case of an open electric
furnace, damage on electrodes, calcination of paste, condensation point
of carbon, tar volatilization, etc are taken into consideration and in case
of the actual example, the load of electrode calcination (approximately
1/3 of power consumption required foir melting) and a back-filter used for
filtering only the amount of occasionally produced disposed gas should be
provided. In case of Cr, there are many large coarse particles at the
preduster and when Z = > 150, it is just as described above, but with silicon
steel plate multicyclone, the conical section was damaged in approximately
6 months.
-------
A-6.3
As a general theory, Z = 30 of the common idea is to use a
gravity settling type preduster and provide a back-filter on the rear stage.
For the dust discharging system of the first stage dust collection, it is
better to use a double damper type air lock method rather than using a rotary
type. However, if the effects of preduster is insufficient, the runners
of blower will wear excessively, thus unabling to perform operation and,
therefore, it is necessary to keep spare runners at all times. Depending
upon the temperature at the inlet of preduster, it may be better to use a
multicyclone and apply a rubber lining is force-fitted into the back-filter.
When FeSiCr Is compared with FeCr, FeSiCr has finer particles. In case of
a wet type, the filtering thickness is approximately < 2m/m by Oliver Test,
thus making it difficult to apply it for industrial use. Water can be
separated from dust very quickly but is condensed and becomes very hard.
It was, therefore, difficult to dispose of it mechanically.
From the above point of view, the dry method is the best way to
dispose of FeCr and FeSiCr.
-------
A-6.5
TECHNICAL REPORT DATA
(Pit osc read liiilnictiviH on the rci t r
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