Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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This invention relates generally to the ield of
iron and steelmaking and more particularly to a process and
apparatus for the production of intermediate hot metal for
use in steelmaking. It involves more specifically gaseous
direct reduction of ore and melting of the prereduced ore.
Iron exists in nature generally in ~he form o~
an oxide. ~ommon forms of the oxide are hematite (Fe2d3)
and magnctite (Fe304). In order to produce steel, the iron
oxides must be reduced to substantially the metallic form.
Conventionally, this may be accomplished by reducing the
oxides with carbon~ carbon monoxide or hydrogen. Such re-
actions are usually accomplished in a blast furnace and the
resulting product is a hot metal containing about 4% of car-
bon and various impurities such as sul-fur, phosphorous~
lS manganese and silicon which have been picked up from the
ore and coke during the smelting process.
The hot metal may thereafter be refined to steel
in a steelmaking furnace. Some of the impurities, such as
carbon, silicon and manganese may be removed by oxidation
while other impurities such as sulfur and phosphorous are
normally removed by slag-metal reactlons. The process of
making steel by smelting iron ore to produce steel may be
termed an "indirect" process of steelmaking. In contrast,
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processes have been proposed for many years by which the
ore may be reduced directly ~o iron without the use of a
subsequent refining s~ep--tha so-called "direc~" reduction
process. The theory of the direct reduction process is that
upon heating of the ore in a reducing atmosphere, the oxides
will be reduced to iron and further he~ting of the reduced
iron will produce molten iron. One practical difficulty
with the direct reduction process is that the molten iron
tends to absorb and retain impurities, particularly sulfur
and phosphorous, from the ore and other raw materials used
and thus the resulting product may be unsatisfactory. For
this reason, most direct reduction processes have been
limited to the produc~ion of "prereduced" or "metallized"
pellets or briquettes intended to be melt~d and refined in
a subsequent steelmaking process.
Due to the difficulties inherent in the direct
reduction process for steelmaking, the major steelmaking
processes used during the last century have been based
upon the reduction of ore to form hot metal in a blast fur-
nace. In some cases, the hot metal has been formed by melt-
ing steel srrap and pig iron in a cupola.
Beginning in the late 1850's, the pneumatic pro-
cess represented by the bottom blown Bessemer converter was
used as a refining furn~ce. The origlnal Bessemer converter
employed a silica lining and was limited to an acld process.
Later the basic Bessemer or Thomas process was developed
which utilized a basic linlng and permitted the use of basic
slags capable of removing sulfur and phosphorous from the
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hot metal. Although the Bessem0r process typically produced
heats of steel up to 25 to 35 tons iD size in 12 to 15 minutes,
the use o air as the oxidizing ~gent resulted in an undesir-
able pick-up of nitrogen which limited the utility of the
steel produced thereby.
While the Bessemer process was the principal
steelmaking process used durlng the late 1800's, the Siemens-
Martin or open hearth process, developed in the late 1870's
soon became ascendant and remained dominant until about the
1950's. The open hearth process was capable of refining a
charge of hot metal and steel scrap or, if desired, the open
hearth could mel~ and refine a charge of cold pig iron and
scrap. Beg~nning in the late 1940's, oxygen lances were
added to the open hearth to speed up the refining process.
The use of oxygen allowed the time required to produce a
heat of steel to be reduced from a period of 10 to 12 hours
to a period of 4 to 5 hours. The dominance of the open
hearth process was due largely to its flexibility in handl-
ing various types of charges and the ability to produce high
quality steel in heats as large as several hundred tons in
size.
Shortly after the open hearth furnace began to be
used commerclally for ~teelmaking; the electric arc and the
electric induction ~urnaces were developed. The electric
furnaces, like the open hearth, were capable of wsing molten
hot metal or cold pig iron or scrap charges and, in addition,
could operate wLth a controlled atmosphere. Thus the elec-
tric furnaces were particularly suited to the refining of
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specialty steels whose premium prices could support the
generally higher operating cost of the electric furnace.
Finally, beginning in the 1950's, the top blown
oxygen converter appeared. In the top blown process, gen-
erally known as the BOF process, pure oxygen is jetted from
above in~o a bath of hot metal and scrap. The BOF process
combined the speed o operation characteristic of the earlier
converter processes with the ability to produce steel o~
open hearth quality. Predictably, the BOF process has now
become the leading steelmaking process. Despite its many
advantages over earlier steelmaking processes, the BOF pro-
cess requires a hot metal charge amounting to about 70% of
the metallic charge and this, in ~urn, mandates that a blast
I furnace or other hot metal producing facility be available.
To supply a typical modern BOF installation, the blast fur-
nace must be capable of producing 7000 to 10000 tons of hot
metal per day. Such a blast urnace, with its auxiliary coke
oven facility, now costs upwards of $70,000,000 and is justi-
fiable only where large scale operations may be installed to
exploit large markets such as are available in many of the
developed countries. Moreover, the blast furnace requires
a large supply of metallurgical grade coke, the supply of
which is limited.
Particularly in the developing countries, as well
as ~n other areas where the market may be smaller, there is
a need for eficient steelmaking facilities having an annual
capacity in the range of 400J000 tons or less which do not
require a bl~st furnace. Proposals ~o satisfy this market
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h~ve been based upon the concept of using a direct reduction
process to conver~ iron ore having an iron content preferably
in the range above 60% and a gangue content below 7% into
pellets or briquettes metallized in the range of 80 to 95%
and then melting and refining the pelle~s or briquettes in
an electric furnace.
The usual gaseous reductant is a mixture of carbon
monoxide and hydrogen formed by steam reorming of natural
gas containing a large proportion of methane (CH4). The
endothermic reactlons involved in steam reforming are:
CH4 + C02 ~ 2C0 + 2H2 and
CH4 ~ H20 ~ C0 ~ 3H2
Where carbon monoxide ls the gaseous reductant, ~he net re-
action with hema~ite is:
Fe203 + 3C0 ~ 2Fe ~ 3C02
This is an exothermic action. Where ~he gaseous reductant
is hydrogen, the net reaction is endothermic and is shown
by the following formula:
Fe203 + 3H2 ~ 2Fe + 3H20
The reactions set forth represen~ the theoretlcal minimum
amount of reductant required to reduce the iron oxide. In
the direct gaseous reduction of ores containing hematite
(Fe203) and magnetite (Fe304), ~he higher oxides are pro-
gressively reduced ~o yield iron (Fe), carbon dioxide (COz)
and water. In addition to the reducing action referred to
above, the iron becomes carburized, generally to the range
of 1 to 1 1/2%. The carburizing reaction is as follows:
3Fe ~ 2C0 ~ Fe3C + C2
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While theoretically a 95% reduction sh~uld be attainable
within a period of about an hour, existing plants require
a period of three to six hours for the reduction process.
Over the years a large number of direct reduction
processes have been proposed. At the present time the major
gaseous direct reduction processes are the Midrex process de-
veloped by Midland Ross Corporation and the HyL process de-
veloped by the Mexican Company Hojalata y Lamina. Somewhat
similar gaseous direct reduction processes have been develop-
ed by Armco Steel Corporation and August Thyssen-Hutte A.G.
In the Midrex process a mixture of iron ore and
pellets recycled from the process is delivered to the top
of a shaft furnace where it is heated to a ~emperature o~
760C by a reducing gas containing carbon monoxide and hydro-
gen delivered to the central portion of the furnace at a
temperature of about 1000C. The reducing gas may be steam
reformed natural gas supplemented by a portion of the top
gas recycled from the furnace. The reduced ore, known as
sponge iron, is cooled in the lower portion of the reducing
furnace by circulating a cool gas through the furnace. The
Midrex process produces pellets about 1/2" in size metallized
to about ~5% and containing between .7 and 2 percent carbon.
The pellets leave the furnace at a temperature of about 40C
and are usually passivated to inhibit reoxidation during
transport or storage. For the Midrex process, it has been
estimated that about 12000 cubic feet o natural gas is re-
quired per ton of sponge iron. This translates to about 3 GK
calories per ton or 12 million Btu per ton. In addition,
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energy equivalent to about ~MM Btu per ton of iron is re-
quired for fans, blowers and pumps. If it be assumed that
the Midrex pellets are ~o be melted and refined in an effi~
cient electrical ~urnace, ~he energy required for melting and
refining is about 6L0 Kwhlton. Bearing in mind the efficiency
in transforming fossil fuels into electrical energy, it is
generally accepted that 1 KWH is equal to 10,500 Btu; thus
the energy for melting and refining the Midrex pellets is
about 6.4 million Btu/ton. The total energy required to
produce a ton o~ steel by the use of Midrex pellets is thus
on the order of 19.4 million Btu.
The Armco process is broadly similar to the Midrex
process although the reducing reaction is conducted at a tem-
perature of about 900 C. The Purofer process of August
Thyssen-Huette is also similar but is per~ormed at a temper-
ature of about 1000C and the product normally is briquet~ed.
An analysis of the Armco process indicates that about 12500
cubic feet of natural gas is required per ton o~ sponge iron
as compared with 12000 cubic feet of natural gas per ton for
the Midrex process. This difference is the result of the
different engineering details of the two processes. Assuming
that the same electric furnace was used to process the product
of the Armco product as was used for the Midrex product, the
total energy requirement to produce a ton o~ steel would be
about 19.9 mlllLon Btu.
In contrast to the Midrex and Armco processes which
may be described as progressive-feed vertical shaft processes
which produce a moving bed, the HyL process i8 a batch-feed,
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fixed-bed process. In the HyL process, a batch of ore is
placed in a shaft-type reactor vessel and is successively
~reated with an initial reducing gas, a final reducing gas,
and a cooling gas. By providing four reaction vessels
S operated sequentlally, a substantially continuous operation
may be attained. The estimated Btu requirements to produce
a ton of metallic iron at room temperature are about 20
million Btu if lump ore is reduced and about 17 million Btu
i~ oxide pellets are used. Again, additional energy in the
amount of about 6.4 million Btu is required to complete the
refining and produce steel.
The thPrmodynamic energy requirement for melting
a ton of iron at room temperature is about 900,000 Btu. Thus
the overall thermal efficiency of the electric urnace melt-
ing operation is only of the order of 16-20%. It is ~or this
reason that it has been generally believed that the conven-
tional blast furnace-oxygen steelmaking combina~ion represents
a more efficient process than any of ~he presently extant
direct reduction-electric furnace processes,
With the foregoing in mind and with a view to re-
ducing the total energy requirements for refining a charge
to steel we provide in accordance with the invention a pro-
cess for ~he production of intermediate hot metal for use
in steelmaking) comprising introducing a charge of ore con-
taining oxides of iron and gangue into a reducing furnace,
and heating the charge in a reducing furnace by means of a
reducing atmosphere to reduce the oxides o~ iron substanti~
ally to iron, characterized in that a charge of said ore in
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the form of lumps, briquettes, pellets or other agglomer
ates is introduced into said reducing furrlace ~or reduction
therein in a reducing atmospheré comprising a mixture of
top gas recycled from said reducing furnace and off-gas
containing hydrogen and carbon monoxide produced in a melt-
ing unit, a por~ion of the iron thus reduced being carburiz-
ed within the reducing furnace to iron carbide by reacting
the iron with a portion of the carbon monoxide contained
in said top gas and with said off-gas from said melting unit,
introducing said reduced and carburized iron and said gangue
together with slag ~orming additives and ~luxes into the
upper region of a melting unit, and melting said reduced and
carburized iron, said gangue and said slag orming additives
and fluxes under a reducing atmosphere produced by the com-
bustion of a rich fuel/oxidant mixture to produce a molten
slag and molten metal comprising essentially iron and carbon~
By interfacing our gaseous direct reduction process
with the melting unit, the off-gas of the melting unit may
be used to provide a portion or all of the gaseous reductants
required for the reduction of ore which in turn is fed direct-
ly into the melting unit along with iron-containing metals
such as cast iron or steel scrap to produce a hot metal hav-
ing a carbon content on the order of 1 to 2%. If desired,
the melting unit may be operated to produce an excess quan-
tity of reducing gases which may be used either to produce
a surplus of prereduced metal or as a source of energy for
other purposes. The intermediate material produced in the
melting unit may be reined to a desired low carbon steel
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in an electric furnace or oxyge~ con~er~er as hereinafter
more ully disclosed, By virtue of the present invention
a charge of prereduced metal and ~s~ iron scrap, wherein
the prereduced metal may comprise between 30 and 100% (pre-
ferably 40 to 60%) of the charge, may be refined ~o steel
with a reduction of up to about 3~/O in the total energy
requirement, including the energy required to reduce the
ore.
Further details of the invention will become ap-
parent to those skilled in the art from the following de-
tailed description of the inven~ion and the accompanying
drawings in which:
Figure 1 is a schemat~c block diagram showing the
interfacing of a gaseous direct reduction process with a
melting unit operating with a reducing atmosphere and
ollowed by a rafining furnace to produce a low carbon steel;
Figure 2 is a similar schematic block diagram show-
ing the inter~acing of a gaseous direct reduction process
with a melting unit and a refining urnace but including a
step whereby the prereduced mstal is cooled prior to admis-
sion into the melting uni~;
Figure 3 is a graph showing the effect on ~he melt-
ing rate of a melting unit according to the present inven
tion as a function of the proportion o~ prereduced metal em-
ployed in the melting unit charge;
Figure 4 is a graph showing a comparison of the
energy required to produce steel using the best commerclal
practice and the energy required ~or the present process
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as a function o~ the proportion o~ prereduced metal in the
charge;
Figure 5 is a graph showing the percen~ o~ energy
saved by the process of the present invention compared with
the best commercial practice as a function of the proportion
of prereduced metal in the charge;
Figure 6 is a diagrammatic drawing of an apparatus
capable of use in the performance of the process in which
both the scrap and the ore are introduced into, and pass
through, the reducing furnace;
Figure 7 is a diagrammatic drawing of an apparatus
wherein the scrap, flux and additives are introduced direct-
ly into the melting unit.
Referring now to Fig. 1, 10 denotes a gaseous di-
rect reduction furnace, 12 is a charge vestibule for the re-
ception of hot prereduced metal, scr~p, limestone and coke
and 14 is a melting unit specially adapted to ~erate with
a reducing atmosphere and to produce o~f-gas rich in hydro-
gen (H2) and carbon monoxide (CO). A refining and super-
heating furnace is indicated at 16. Oxygen required for
combustion in tha melting unit 14 and for refining in the
furnace 16 is supplied by an oxygen facility 18.
A portion of the top gas from the gaseous direct
reduction furnace is conditioned in the gas conditioner 20
and then recycled to the direct reductlon furnace 10. The
remainder of the top gas from the direct reduction unit passes
through an energy recovery unit 22 and is thereafter ex-
hausted to the atmosphere.
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For convenience the embodiment of the invention
shown in Figure 1 will be described wittl reference to an
exemplary operation wherein substantially equal amounts of
cast iron scrap and prersduced metal are melted and refined
to produce a short ton of low carbon steel. It will be ap-
preciated as set forth in more detail below, ~hat the pro-
cess is applicable to ~erations in which the prereduced
metal may cons~itute between about 30 and 100% o the charge
and to operations where the melting uni~ charge may also in-
clude iron-containing metals such as cast iron scrap, steel
scrap or a mixture of both. Of course the quantity and com-
position of the off gas and the temperatures at various points
in the process will be affected by the specific nature of
the charge materials. Although the process begins wlth the
reduction of ore in the gaseous direct reduction furnace 10,
the process is controlled essentially by the operatlon of
the melting unit 14 and may, therefore, conveniently be de-
scribed beginning with this unit.
The melting unit 14 is known as the Consolidated-
Wingaersheek Cupola, or C-W Cupola, and is equipped with
burners capable of combusting oxygen and natural gas or
fuel oil at about half the stoichiometric ratio to produce
a reducing atmosphere within the cupola and off-gas rich in
hydrogen and carbon monoxlde. To produce a short ton of low
carbon steel, the charge vestibule 12 of the cupola 14 is
charged with prereduced metal 24 and scrap 26. The prere
duced metal 24 which may, for example, comprise Midrex
pellets, has a carbon content o about l~0~/o and a weig~t
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of about 1079 pounds. The scrap charge 26 includes 1061
pounds of iron having a carbon content ~ about 3.5~/~, 87
pounds of limestone and 22 pounds of coke. The scrap
charge 26 is cold but the prereduced metal 24 is preferably
hot as received from the direct reduc~ion furnace 10.
Fuel 28, preferably natural gas comprising essen-
tially methane (CH4), and an oxidizer 30, preferably oxygen,
are mixed and burned in the cupola burners. Equal quantities,
in this case about 6783 SCF ~standard cubic feet), of methane
and oxygen are burned to produce heat and a reducing a~mos-
phere within the cupola according to the reaction:
CH4 ~ 2 ~ ~CO, C02, H2, and H20]
Within the cupola charge, a number of reactions occur in
addition to melting of the iron contained in the scrap and
ore. In summary from these include:
CaC03 ~ CaQ + C02
CaO ~ SiO2 ~ CaSiO3
C ~ C02 ~ 2CO ;
C ~ H20 -- -- ~ H2 +
As a result of these reactions the gangue contained in the
ore and the coke and limestone form about 142 pounds of a
molten slag which may be slagged from the cupola and the
iron from the scrap and ore form approximately a ton of
ho~ metal containing about 2.3% carbon. In addition, off-
gas comprising water (H20), carbon rnonoxide (CO), hydro-
gen (H2) and carbon dioxide (C02) leaves the cupola 14 at
a temperature o approximately 2000F. The approximate
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composition of the cupola off-gas is as follows:
GAS SCF/Ton of steel
H20 5586
CO 5586
S H2 798
C2 1178
While the cupola of-gas is rich in hydrogen and
carbon monoxide, both of which are effective as reducing
: agents, the off-gas also contains wa~er vapor and carbon
dioxide which may inhibit the r~ducing reaction. The re-
duction of ore may be regarded as a four step process by
reference to the oxi.dation state wherein the ore progresses
from hematite ~Fe203) to magnetits (Fe304) to wustite (FeO)
to iron (Fe) Where the gaseous reductants are hydrogen
and carbon monoxide, th~ com~ined reactions may be repre-
sented as follows:
CO CO2
3Fe203 t H2 ~ 2Fe304 ~
CO C~2
Fe304 ~ H2 ~~~~~~~~~3 FeO
CO ~ CO2
FeO ~ H2 - - )Fe H20
At t~mperatures below 560C, wustite (FeO) is unstable and
the reduction of Fe304 proceeds directly to Fe as follows:
CO CO2
Fe304 ~ 4 H2 ~ 3 Fe ~ 4 ~
However, although the equilibrium for the reduction oi
hematite to magnetite is such th~t either Co or H2is very
efficient at all temperatures, the equilibrium for the sub-
sequent reducing steps is less favorable and depends both
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on the temperature and the ratio of CO2 to CO and H20 to
H2. If we define Kl as ~he equillbrium eonstant for the
reduction of iron ore by CO and K2 as the equil.ibrlum con-
stant for the reduction of iron ore by hydrogen, then
C2 H2 ~
Kl = _ and K2 = - ~ and i~ can be shown that
at 1600F, Kl must be less than .48 to maintain a reducing
atmosphere but that as the temperature is reduced to 800F,
Kl may be increased to about 1.35. On the other hand, at
1600F~ K2 must be less ~han 0.55 but as the temperature
falls to 800F, ~ must be decreased to about 0.15 to maln-
tain a redueing condition.
It will be appreciated that the cupola of~-gas
may enter the gaseous reduction ~urnace at 1600F - 800F
and leave the furnace at 600-9OOaF so that the reducing re~
actions are being performed over a range of temperatures.
Moreover, as the reduction proceeds, the reactlons produce
both CO2 and H20 which tend to increase the values of K
and K2 respectively.
Based upon the composition o~ the cupola off-gas
set forth above~ Kl = .21 while K2 = .7 derived respectively
as follows:
C2 1178 and H20 5586
CO 5586 H2 7980
It is thus apparent that while reductlon of the ore by car-
2~ bon monoxide will be strongly favored, the reduction of the
ore beyond the magnetite oxidation level will be inhibited
unless K2 is lowered substantially. To this end, the cupola
off-gas is introduced into the lower region 38 of the gaseous
ydirect reduction furnace 10, passed in counter flow relatlon
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~hrough the furnace and wi~hdrawn at ~he upper end 40 o~
the furnace. As explained more fully below a portion o the
top gas 42 from the direct reduction furnace is directed
into the air conditioner 20 where the gas ls cooled and
treated to remove a portion of the water and carbon dioxide
and thereafter reheated. The rehea~ed and conditioned gas
44 is mixed wlth the gas stream 36 leaving the cupola 14 and
re-enters the direct reduction furnace 10 at 38. As a re-
sult of the removal of substantial quan~ities o water and
carbon dioxide in the conditioner 20, the values of Kl and
K2 may be maintained well below .4 and .5 respectively so
that reduction of the iron oxides by both hydrogen and car-
bon monoxide will occur within the direct reduction furnace
10.
In addition to the reducing reactions noted above~
the reduced iron is carburized in the reducing furnace to
about 1.0% carbon according to the reaction:
3 Fe ~ 2 C0 ~ Fe3C + C2
The temperature relationships within the reducing furnace
10 must be regulated closely in order to maintain the rate
of the reducing reaction at a maximum but limiting the tem-
peratures, particularly in the lower regions of the ~urn~ce,
so as to prevent sintering or agglomeration of the reducad
ore. This may be accomplished in part, through the opera-
tion of the gas conditioner 20 whlch can be controlled to
maintain the desired temperature of the gas entering the
reduction furnace 10 at point 38. Moreover, the proportion
of the top gas 42 which is recycled can be selected so that
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the reducing gas is recycled a plurality o-f times through
the reducing furnace 10. The precise e~ent of the re-
circulation will, of course, depend upon the nature and
composition of the raw ore 46 and the cupola off-gas 36 and
the operation of the gas conditioner 20.
In the present example, 1516 pounds o~ ore com-
prising 1422 pounds of hematite and 94 pounds of gangue
are charged into the reducing furnace 10 to yield 1079
pounds of prereduced metal 24. The portion of the top gas
48 from the reducing furnace 10 which is not recycled, may
be burned with air and additlonal fuel, if necessary, to
produce the heat required to reheat the top gas which passes
through the conditioner 20 in the energy recovery unit 22.
Any excess of energy available from the reducing furnace
top gas 48 may be used, for example, to generate steam.
As noted above the metal 34 leaving the cupola
14 has a carbon content of about 2.3% and may have a ~em-
perature in excess of 2500F. Further refining in the
steelmaking furnace 16 is required to produce steel having
a carbon content in the range of 0.1%. The furnace 16 is
preferably an electric furnace or an oxygen converter.
With the components shown in Fig. 1, about 72Z, SCF of oxy-
gen 50 is required stoichiometrically to oxidize the carbon
in the metal 34 from an initial level o 2.3% to a f-Lnal
level of 0.1%. The reac~lon of oxygen and carbon is exo-
thermic and will raise the temperature of the final steel
52 to the desired t~pping temperature of about 2960F.
An energy balance for the process exemplified in
Fig. 1 reveals that about 7.19 x 106 Btu is provided in the
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form of fuel (natural gas); 1.2 x 106 Btu is provided
electrically to produce the oxygen used for combustion and
refining and about 0.66 x 106 ~tu is provided as electrical
power for gas conditioning ~nd the operation of fans and
blowers. Thus the total energy per ton of steel according
to the present process is about 9.05 x 106 Btu.
Figure 3 is a char~ showing the relationship be-
tween cupola melting rate and percent of prereduced metal
in the charge. Lines 54 and 56 represent a range of test
data in a cupola having burners operated to produce a reduc-
ing atmosphere within the cupola and off-gas rich in hydro-
gen and carbon monoxide. In general, these data indicate
that as the percentage of prereduced metal in the cupola
charge is increased, the melting rate is decreased. This
data is replotted respectively at lines 58 and 60 on a .scale
showing the percentage of the melting rate in a cupola
operated with no prereduced metal in the charge. Line 62
is taken from Fig. 1 of the article "The Us8 of Sponge Iron
in Foundriesl' appearing at page 53 of the September, 1976
issue of Modern Casting and shows results similar to those
obtained by applicant with respect to the effect of pre-
reduced metal on cupola melting rates.
Curve 64 is based upon the melting rate data of
curve 54 for a cupola operated at half the stoichiomatic
ratio of oxygen and fuel so as to produce a reducing atmos-
phere and off-gas rich in C0 and H2. Curve 64 demonstrates
that a sufficient quantity of off-gas may be generated to
effect the reduction of ore under any desired charging con-
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dition. Curve 64 shows that about 75% o~ the off-gas
generated by the cupola is required to reduce ~ suflcient
amount of ore to constitute 5~/O of the cupola charge. As
set forth above, the remainder of the of~-gas may then be
burned to provide the energy for conditioning the top gas
from the reducing furnace. Where the cupola is operated at
a lower melting rate as shown by curve 66, a smaller propor-
tion of the off-gas is required for reduction of the ore
and a surplus of energy in the form of reducing furnace top
gas becomes available.
Figure 4 is a graph showing a comparison on an
energy basis of a typical process according to the present
invention and the best known commercial steelmaking process
involving the direct reduction o~ ore followed by melting
and refining in an electric arc furnace for v~rious percent~
ages of prereduced metal in the charge. Line 68 shows the
energy required to produce steel by direct reduction and an
electric arc furnace using between 0 and 100% prereduced
metal as the charge. It will be noted that a~ 0% prereduced
metal in the electric furnace charge the energy requirement
is about 5 x 106 Btu/ton while with 100% prereduced metal
in the charge the energy requirement is about 19.4 x 106 Btu/ton.
Line 70 represents the process according to ~he present in-
vention wherein the off-gas rom the cupola melting unit
provides the reductants required for the reduction o the
ore. The data from Fig. 4 have been replotted in Fig. 5 to
show the typical percent saving in energy possible with the
process of the present invention compared with the best
commercial process of dlrect reduction followed by melting
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and refining in an electric arc furnace. From Fig. 5 it
will be appreciated that the process of the present inven-
tion will result in energy savings of about 3~/O for a charge
including about 50% prereduced metal.
5 . . Figure 7 shows in diagrammatic form an apparatus
in which the invention according to the process set forth
in Fig. 1 may be performed. The melting unit por~ion of
the apparatus is indicated generally at 72 while the charge
vestibule is shown at 74 and the direct reduction furnace
at 76. The melting unit 72 and charge vestibule 74 are con-
tained in a generally cylindrical steel shell 78 which is
lined with an appropriate refractory material 80. Additional
refractory material 82, preferably in ~he form of shaped
bricks, is placed in~eriorly of the refractory material 80
so as to define a hearth 84, a heating and melting region
86 and a charge receiving region 88. Communicating with the
hearth region 84 are a plurality of combustion chambers 90
adapted to receive burners 92. The burners 92 are effective
to burn a rich fuel/oxidant mixture so as to produce com-
bustion products rich in hydrogen and carbon monoxide.
spout 94 communicates with the hearth 84 slightly above the
bottom thereof to direct the molten metal from the melting
unit into an oxygen converter or electric urnace (not shown)
for further reining to produce steel. One end of a refrac-
tory lined additive passage 96 communicates with the charge
vestibule 74 while the other end communLcates with an addi-
tive hopper 98 through a gas sealing valve 100. The out-
board end of the additive hopper 98 is also fitted with a
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gas tight closure 102. Additives comp~ising iron or steel
scrap, coke,limestone and fluxes may be placed in the hopper
98 and in~roduced into the charge vestibule 74 as required.
The direct reduction furnace 76 comprises a gen-
erally cylindrical s~eel shell portion 104 having a refrac-
tory lining 106 which communlcates wi~h the charge vestibule
74 through a refractory lined converging sec~ion 108 and an
orifice 110. At least one orlfice 113 is formed in the upper
region of the direct reduction furnace 76 for the egress of
gas. A~ least one orifice 115 is provided in the charge
vestibule 74 through which reconditioned gas from the direct
reduction furnace 76 may be recirculated into the charge-
receiving region 88 of the charge vestibule 74 and thence
through the orifice 110 and the interior 112 of the furnace
76. Of course, fresh gaseous reductants may also be mixed
into the reconditioned gases if desired.
The top of the direct reduction furnace 76 is
closed by a charge hopper 114 provided with appropriate
gas sealing means (not shown).
It will be appreciated that appropriate quantities
of ore may be introduced intb the direct reduction furnace
76 to react with the gaseous reductants and produce pre-
reduced metal which may then be admitted to the charge
vestibule region 88 together with the desired quantity of
scrap, fluxes and additives to form the charge for the
melting unit 72. In the hearth portion of the melting uni~
temperatures in the range of 3000 to 4000F are produced to
melt the charge and form a pool of hot metal 116 suitable
'
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for final refining in a steelmaking vessel. As shown in
Fig. 7, the hearth portion 84 of the melting unit is of
smaller diameter than the heating and meltingregion 86 of
the melting unit so as to provide a circumferential shoulder
118 to support the burden in the melting uni~. The effect
is to produce an arched combustion chamber and avoid the
risk of solid material falling into the molten pool 116
and possibly quenching and solidifying the pool. The molten
pool 116 is an impor~ant aspect of the melting unit hearth
design in that it protects the refractory bottom of the
melting unit and simultaneously absorbs heat from the gaseous
combustion products. Moreover, retention of ~ quantity of
molten metal in the pool provides an opportunity for the
fluxes and other additive agents ~o react with the slag and
promote the desired slag-metal reactions.
Preferably the combustion chambers 90 terminate
at their inward ends in reduced sections 120 which increase
the velocity of the produc~s of combustion to form a gaseous
Jet capable of penetrating into the hearth region 84 and
creatlng a highly turbulent region in which hea~ transfer
to the burden and to the molten pool 116 is enhanced.
Figure 2 shows in the form of a block diagram a
modification o~ the process shown in Fig. 1. The principal
difference lies in the step 11 of cooling the prereduced
metal within, or immediately upon its exit rom, the direct
reducing furnace 10 to ambient temperature. By the use o
this technique, the operating rate of the furnace 10 is not
directly tied to the operation of the melting unit 14 and
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it is therefore pGssible to utilize more nearly the full
reducing capacity of the melter off-gas to increase the
production o~ prareduced metal. Of course, by cooling the
prereduced metal portion of the melting unit charge the
sensible heat of the prereduced metal is lost and must be
made up in the melting unit through the combustion of an
additional amount o fuel. This will necessarily increase
the total energy requirement of the process and therefore
i decrease the eficiency somewhat. Except as noted above,
the process shown in Fig. 2 is the same as that shown in
Fig. 1 as indicated by the use of the same re~erence char-
acters. The excess prereduced metal may be used in other
steelmaking operations or sold as an item of commerce.
Figure 6 illustrates a modified form of apparatus
~or use in the practice of the process of the present inven-
tion. The apparatus differs from that shown in Fig. 7 in
that the charge vestibule 74 and the associated additive
passage and additive hopper have been eliminated. The
~ elements of the apparatu~ of Fig. 7 are indicated by the
same reference numerals and perform a similar function in
both embodiments of the apparatus. In the embodiment of
Fig. 6~ the required quantities of scrap, fluxes and other
! additives together with the ore are charged into the dLrect
reduction furnace 76 through the charge hopper 114. The
effect of this modification is that the scrap, fluxes and
other additives will be preheated in the direct reduction
furnace 76 instead of in the upper regions of the melting
unit 72. Thus, ~he temperature of the melting unit off-gas
~ 6~7
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will be somewhat highér than that pro~lced in the embodl
men~ o~ Fig. 7. As noted above it is necessary to limit
the maximum temperature of the melting unit o~f-gas to
prevent sintering o~ agglomeration of the charge within
the reducing furnace 76.
In Figs. 1 and 2 it is indicated that the lron ore
is introduced into the reducing furnace 10 while the scrap,
i.e., the iron-containing metals, and other additives in-
cluding cokeg limestone and 1uxes are introduced into the
charge-receiving portion of the melting unit 14. It will
be appreciated that, as indicated in Fig. 6, all of the
charge materials may be introduced into and passed through
the reducing furnace 10, if desired. In this event, a por-
tion of the heating load of the melting unit 14 will be
transferred to the reducing furnace 10. As a result, the
temperature of the off-gas leaving the melting unit 14 will
be somewhat higher.
The terms and expressions which have been employed
are used as terms of description and not of limitation and
there is no intention, in the use of such terms and expres-
sions, of excluding any equivalents of the features shown
and described or portions thereof, but it is recognized
that various modifications are possible within the scope
of the invention claimed.
