Note: Descriptions are shown in the official language in which they were submitted.
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~ BACKGROUND OF THE INVENTION
:~
Field of the Inven-tion
The invention lies in the i.eld o~ the pyrome-tallurgy
of ferrous metals.
Description of the Prior Art
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The increasing necessi-ty of using low grade iron
ores Eor making steel because of the depletion of high grade
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ores, and economic factors, have created a demand for reduction
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~ of the cos-ts in producing steel from iron ore. Efforts to
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reduc~ C05ts have been dlrected to the elimination of the
use of the highly Euel-consumin~ blast Eurnace. The present
invention is direc-ted to elimination of the use of the hlast
Eurnace by converting the i.ron oxide to the carbide followed
by producing steel direct1y from th.e càrbide in the basic
: oxygen furnace. The conversion of irorl oxides to carbides
for various purposes-has received some attention in the past
but there has been no known activity toward producing steel
directly from the,carbide in a basic oxygen furnace.
U. S. Patent 2,730,537, the closest prior art known,
discloses a process for converting iron oxides to carbides
- in a fluidized bed process in which carbon monoxide is used
as the chief reducing and carburizing gas. The patent teaches
that the reducing gas cannot contain hydrogen in an amount
more than 50 percent by volume of the carbon monoxide content.
It also refers to the prior art disclosing the use of hydrogen
.~ in a fluidized b~d as a reducing gas for iron oxides having
a low iron content. The reference alludes to use of the
iron carbide product for making "metallic iron" and in an
. 20 "iron production furnace" operating below the melting point
; of iron or steel; however, there is no teaching of use of the
product for introduction into a fully molten steel system
such as the basic ox.ygen furance or electric furnace. Other
!, somewhat remote prior art discloses processes for converting
:~ metallic iron to iron carbide rather than conversion of iron
oxide to the carbide.
Still other prior art discloses fluid bed processes
for the direct reduction of iron oxides to metallic iron
which in turn could be further converted to carbide. However,
these other direct reduction processes have th~ disadvantages
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that the product may be pyroE)hor:ic in sorne cases requiring
bri~luettillq, and sticki~ess is not comple~tely eliminated in
some processes so that difficul-ti~s arise wi-th the Eluid bed
process.
It is an object of this invention to provide a
; process for making steel from iron oxide withou-t the use of
a blast furnace.
It is another object of this invention to provide a
successful process for making steel from iron oxide by first con-
verting the oxidé to the carbide, followed by introducing the car-
bide directly into the basic oxygen furnace to produce steel.
SUMMP~RY OF THE INVENTION
A process for the direct production of steel Erom
particulate iron ox~des or concentra-tes which comprises (1)
converting the oxides to iron carbide in a single step in
- a fluidized bed at low temperatures with a mixtùre of re-
- ducing and carburizing gases followed by (2) direct conversion
of the carbide to steel in a basic oxygen or electric furnace.
The reducing and carburizing temperature of Step
~1) cannot exceed about 1300F with a preferred temperature
range being about 900-1200F. The carburizing of the reduced
iron to carbide may be conducted so that enough carbon is left
in the iron carbide product to supply sufficient heat upon
combustion in the basic oxygen Eurnace to ma]ce the process
auto-thermal. The CO/CO2 and hydrogen to water vapor ratios
of the gases in the reaction of Step (1) are maintained at
' 'J a point below which oxidation of iron carbide ~ccurs.
; Off-gases from the steel making ste~, about 90 percent
~!
carbon monoxide, may be circulated for use as part of the re-
- 30 ducing gas for the reduction and carburizing step in the
fluidized bed. Material balance calculations show that the
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carbon content ~ the off-~as .is sufficient to supply all of the
: carbon necessary for the reduction and carburization step.
~ccoxdingly, when Steps l and 2 are performed in conjunction wi.th
each other as one continuous opera-tion, all of the carbon
necessary for Step l, subject to slight opexaiing losses, may
be provided by continuous cycling of the off-gas from Step 2
to Step l. This eliminates the necessity for adding carbon
to Step l with the exception of small losses occurring in normal
operations. The result is that the carbon originally added to
Step l to make iron carbide may be used o~er and over by re-
.:~ covering it in Step 2 in .the off-gas as steel is produced and
~ reusing it in Step l to make more carbide. ~1hen the steps are
: performed ln conjunction with each other added heat is not
required to make the process auto-thermal, às the product going
J directly from Step 1 to Step 2 is at a tempe.rature which elimi-
nates the necessity for adding heat. When Steps l and 2 are
performed separately then theJ hot off-gas from Step 2 may be
used for preheating iron carbide or heat added by other.means
as necessary to make the steel making process auto-thermal.
ZO The iron carbide produced in Step l-is added directly
as the charge to the basic oxygen or electric furnace alon~ with
' fluxing agents, alloying material and other conventional additives
: to produce steel directly with elimination of the conventlonal
blast furnace step. Heat is supplied to the charge in various
ways to make the process.auto-thermal. These ways may include -
direct heating,.addition of fuel such as carbon, or producing
.~ sufficient free or combined carbon in the carbide as it is pro-
duced, or others. Sensible heat from the off-gas may be used
and the off-gas may be partially burned to provide heat to the
charge. If the latter is done the CO/C02 ratio in the combustion
. gases must be maintained below that at which iron carbide will
decompose at the required preheat temperature.
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~RIErl DESCRIPrt'ION OF 'l'HE DRAWING
The single drawinc3 is a schematic flowsheet for the
direct s-teel makiny process of the invention.
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DESCRIPTION OF PREFERRED EMBODIMENrS
~ he invention will be described in detail in con~unc-
tion with the accompanying drawing.
The basic oxygen and electric Purnace processes re-
ferred to herein for making steel are well known in -the prior
-`i art. The basic oxygen process vr basic oxygen furnace process
differs chiefly from Bessemer converters and open hearth fur-
naces in that the reactan-t used to oxidize the carbon and certain
impurities (sulfur, phosphorus, etc.) in the charge is not air,
but oxygen. The oxygen is introduced by blowing it with a lance
onto or below the surface of the molten iron.
; . .
The iron carbide produced by the pro~ess described
herein is a mixture of carbides having the molecular formulas
Fe2C and Fe3C with the Fe3C content being predominant.
The fluidized bed reactor referred to herein is of
the conventional type in which finely divided ~eed material on
a grate or other per~orate support is ~luidized by upwardly
flowing gases which may include or entirely comprise the reactant
gases. Auxiliary equipment includes heating and temperature
control and monitoring equipment, heat exchangers, scrubbers,
cyclones, gas cycling equipment, and other conventional equip-
ment. Some of this auxiliary equipment is shown schematically
in the flowsheet.
In this specification and the claims the reduction
and carburization step is referred to as Step 1 and the steel
making step as Step 2. The term "hydrogen bearing gas" includes
hydrogen gas alone and the term "carbon containing material"
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includ~s carbon ~lone.
Step I oE the overall process is the conversion o
the oxides in the iron ore concen-trate to lron carbide in the
fluidized bed unit shown in the flowsheet. The conversion
process must be carefully con-trolled to provide a product suit-
able for use in the basic oxyyen or electric furnace~ The iron
carhide is des~rable for use in these processes because it is
non-pyrophoric and resistant to weatheriny which permits trans~
~ port for long distances and storage for reasonable periods.
- 10 The oxides are reduced to iron and the iron converted
to the carbide in a continuous process in the fluid bed reactor
in which the reducing and carburizing gases are added together.
In order to prevent any sticking caused by the transient presence
of metallic iron thel temperature is maintained below about 1300F
at all times and preferably in the range o~ about 900-1200~F.
Hydrog~en is preferably used as the reducing gas although
carbon monoxide or hydrocarbon gases or mixtures of hydrogen with
CO and hydrocarbon gases may be used. The flowsheet shows the
use o~ hydrogen and carbon monoxide with water being given off.
Hydrogen is preferred as the reducing gas because thè oxidation
product of hydrogen, which is water, may be easily removed from
the furnace off-gas thus permitting recycling o~ the balance o~
the gas without the need for extensive complicated and expensive
chemical systems for removing the oxidation produats o~ carbon
whLch are carbon monoxide and carbon dioxide when carbon contain-
ing reducing gases are used.
The preferred carburi~ing gas which is mixed with the
reducing gas is propane, although carbon monoxide or other hydro-
carbon gases, or solid carbon, may be used with the lower alkyl
hydrocarbon gases being preferred. A wide range of carbonaceous
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materi~ls mcly be usecl so lorlc3 as they sunply carbon to form
iron carbide.
By proper balancing of the ra-tios of the hydrogen
and carbon bearing materials, it is possib:le to restrict the
hydrogen to a reducing function and the carbon to a carburizing
function. This can readily be done by maintaining quantities
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of hydrogen bearing gases which are in excess of the quantities
of the carbon bearing gases.
~ ecause of the equilibrium conditions involved
in hydrogen-carbon-oxygen gas systems, the required hydrogen-
carbon ratios will automatically require that methane be present
in the gas system. The quantity o-f methane present will be
~ a function of carbon to hydrogen ratios, as well as temperature
":
and pressure conditions.
Representative tests and results from an extensive
test program using the reduction and carburi~ation procedure
described above in a fluid bed reactor are presented in the
following Table 1.
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The carbon content in -the -Einal produc-t varies as the
percen-t iron oxide in -the feed materials varies. Lower gr~de
ores with lower iron contents will au-toma-tically yield products
with lower carbon contents.
The volume of hydroyen in the hydrogen-carbon monoxide
reducing and carburizing mixture in the flu,idized unik should exceed
the volume OL carbon monoxide, the preferred amount of hydrogen be-
ing over about 60 percent by volume of the carbon monoxide present.
The results show production by Step 1 oE the process
of clean iron carbide which is highly suitable for use in the
basic oxygen or electric Eurnace. X-ray diffra~tion analysis
showed the carbon to be present as iro,n carbide with no free
carbon or metallic iron. The product was found to be non-
pyrophoric. Simulated weathering tests showed that the product
was stable in oxidizing atmospheres containing water vapor up
to a temperature of 250C.
The results also show that Step 1 of the process is
highly successful in producing iron carbide directly from iron
oxides when operated within temperature ranges of about 1020F -
1170F using hydrogen to water vapor ratios between S to 1 and
8 to 1 and CO/CO2 ratios between about 1 to 1 ~nd 5 to 1. As
stated herein, Step 1 can be successfully operated within a
temperature range of about 900-1300F, a hydrogen to water vap~r
ratio of about 2.5 to 1 to about 8 to 1 and a CO/CO2 ratio of
' about 1 to 1, up to about 4 to 1. Under these conditions,
methane will be present in quantities ranging from 1 to 70 percent
~, ' by volume of the gas system containing the prescribed amounts,' of hydrogen, water vapor, CO, and CO2. It was Eound that Step 1
would not operate outside these ran~es to successfully produce
~, 30 iron carbide.
Step 2 of the overall proce'ss is the conversion of the
, iron carbide to steel in the basic oxygen furnace. Because of the
' nature of the basic oxygen furnace process, special conditions
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apply -to the processing of iron carbide to steel by this pro-
cess as compared to other steel making processes in furnaces.
If Steps 1 and 2 are close-coupled so that -the iron
carbide comes ou-t oE the ~luid bed unit at an elevated temper-
ature of about 1100-1300F and at that temperature is added
directly to the basic oxygen furnace, then the heat calculations
show that no added heat is required and the process is continuous
and auto-thermal.
The modification shown in the flowsheet wherein the
of~~gases are being sent directly to the fluidized bed unit is
used when Steps 1 and 2 are close-coupled in time. In this mod-
ification of the process substantially all of the carbon used in
the fluidized bed unit to convert the oxides to iron carbide is
recovered as CO in the furnace and recycled to the fluidized bed
unit to be reused in again ma]cing iron carbide.
If for~purposes of transport or storage the Step 1
product becomes or is cooled before Step 2, then heat must be
readded either in the form of reheating the product or adding
extra fuel to Step 2.
.
: 20 :Heat balance calculations show that at~ ambient temper-
: ature iron carbide does not contain sufficient fuel value so that
the reaction taking place in the basic oxygen ~urnace is auto-
thermal without adding heat to the charge.
.; The additional heat required to make the reaction self-
;' sustaining may be supplied in a number of ways. The off-gas from
the basi.c oxygen furnace produced by the process~ing of iron carbide
contains about 90 peraent carbon monoxide in addition to substan-
tial sensible heat. The sensible heat may be used through heat
: exchangers or otherwise to heat the incoming iron carbide~ By
burning part of the off-gas, sufficient heat is achieved for
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augmenting the sensible heat to effect the required preheat-
ing of the incomlng iron carbide char~e to make the process
auto-thenn~l. Under some conditions the sensible heat alone
is suEficient. The heat for the preheating can be obtained
entirely from combustion of the off-gas. The pre~erred pre-
heat temperature range i~ from about 1300F to about 2000F.
l'ests conducted with iron carbide in a gaseous medium
simulating that of the combustlon products from partial com-
bustion of the off-gas showed that the iron carbide is not only
stable under these conditions but actually increased in carbon
content from 5~9 to 7.1 percent due to the formation of the
~ Fe2C carbide from the normally predominant Fe3C. To achieve
: this result the CO/CO2 ratio in the combustion gas must be
between 1 to 1 and 2l to 1 when attaining preheat temperatures
of 900-1300F.
Added heat to make the process auto-thermal can be
supplied wholly or in part by direc-t heating of the Fe3C charye
with an external heat source. Sufficient carbon may be added
to the iron carbide to provide the required additional heat by
combustion during the process. The amount of carbon added varies
from about 3 to S weight percent of the iron carbide charge.~ The
carbon may be addecl directl~ to the iron carbide by preheating the
iron carbide in carbon bearing gases having a CO/CO2 ratio greater
than 1 to 1.
Heat may be supplied by reaction of the basic oxygen
furnace off-gas with incoming iron carbide. The necessary carbon
content of the iron carbide to furnish the required heat upon com-
bustion can be supplied during Step 1 of the process described
above by adjusting the content of the carbonaceous material in
the reacting gas mixture of the fluidi2ed bed to provide for the
production of sufficient Fe2C in the Fe3C product. As shown in
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the flowsh~et, hot scrap metal may ~e added -to the basic oxygen
furnace charge.
Step 2 of the process may also include the addition
of pig iron to the iron carbide charye in the b~sic oxygen and
electric furnaces. A significan-t advantage of this feature is
that iron carbide can then be added for cooling in an amount
three times that of scrap iron which can be ad~ed to conventional
basic oxygen furnace processes for cooling. Iron carbide or
this purpose can be added in an amount up to about 60 percent
by weight of the iron carbide-pig iron charge. One advantage
of this is that present pig iron furnaces can be continued in
operation in conjunction with the present process.
The invention includes all of the above procedures
alone or combined for, providing the necessary heat for the iron
carbide charge to make the reaction in the basic oxygen furnace
auto-thermal.
If Step 2 is conducted in the electric furnace, any
extraneous heat required may be supplied by means o~ the elec-
trical evergy normally used in this type of furnace.
Step 1 of the process provides a conve~ient and e~fec-
tive means for concentrating low ~rade non-~agnetic ores to
separate the iron ore from the gangue. As the iron carbide pro-
duced from non-magnetic ores is magnetic ik is only necessary to
process non-maynetic ore, such as, oxidized taconites, in accor-
dance with Step 1 to convert the iron oxide therein to iron carbide
and subject the trea-ted ore to magnetic separation to separate
the magnetic iron carbide from the gangue. The iron carbide re-
covered may then be used in Step 2 of the process of the invention.
A number of advantages of:the invention are apparent
from the above description. Its principal advantage is that it
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eliminates the expensive inte:rrnediate blast furnace step in
converting iron ore to steel. ~hen the~ two steps are performed
in conjunction no added hea;t is necessary for the second step
and carbon monoxide from -the second step provides -the necessary
carbon ~or carbonization of reduced iron in the first step so
-that the carbon can b~ reused continuously in bo-th steps. Step
1 includes the production of water as a by-produck, thus simpli~
fying the recovery of by-product carbon containing gases. This
step can be performed to give a product having a high enough
ratio of Fe2C to Fe3C to provide a high enough carbon content in
the charge for the basic oxygen furnace to make the steel
making process auto-thermal.
Advantages of Step 2 are that it provides sources
of heat for making :this step auto-thermal without the use of
extra materials, i.e., sensible heat from the off-gases can be
used or the C0 in the off-yases can be burned to provide the
necessary heat, or the C0 can be reacted with the iron carbide
...:
; from Step '1 to raise the ratio of Fe2C to Fe3C in the charge
. so that sufficient carbon will be available for combustion
to supply the augmenting heat to make Step 2 auto-thermal.
When pig iron is added to the charge, large amounts of iron
;' carbide can be added for cooling. The,overall process is
practically pollution-fxee and provides 'for maximum conserva-
tion and reuse of non-product reactants. A further advantage
of the overall process is that it results in a saving in trans-
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;.' portation costs when the carbide is made near -the mine before
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:. transport to the steel making furnace as iron carbide represents
' a higher percentage of usable material -than the oxide.
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