CA1098319A - Process for the direct production of steel - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE

A process for the direct production of steel from particulate iron oxides or concentrates including two major steps in which in Step (1) the iron oxides are converted to iron carbide and in Step (2) steel is produced directly from the carbide in the basic oxygen furnace or the electric furnace.
In the production of the carbide the oxides are reduced and carburized in a single operation using a mixture of hydrogen as a reducing agent and carbon bearing substances such as propane primarily as carburizing agents. Iron carbide thus produced is introduced as all or part of the charge into a basic oxygen furnace to produce steel directly without the blast furnace step. In order to make the steel making process auto-thermal, heat is supplied either by using the hot iron carbide from Step (1) or preheating the iron carbide or by including sufficient fuel in the iron carbide to supply the required heat by combustion.

Description

10_ 8319 This application is a division of copending Canadian application Serial No. 264,471, filed October 29, 1976.

BACKGROUND OF THE INVENTION
Field of the Invention The invention lies in the field of the pyro-metallurgy of ferrous metals.
Description of the Prior Art The increasing necessity of using low grade iron ores for making steel because of the depletion of high grade ores, and economic factors, have created a demend for reduction of the costs in producing steel from iron ore. Efforts to 10"8319 reduce costs have been directed to the elimination of the use of the highly fuel-consuming blast furnace. The present invention is directed to elimination of the use of the blast furnace by converting the iron oxide to the carbide followed by producing steel directly from the carbide in the basic oxygen furnace. The conversion of iron 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 thercarbide in a basic ox~gen furnace.
U. S. Pa,ent 2,7~0,537, 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 bed 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 "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 ~ully molten steel system such as the basic oxygen 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 .o carbide. ~owever, these other direct reduction processes have the disadvantages 10~83~9 that th~ product may be pyrophoric in some cases requiring briquetting, and stickiness is not completely eliminated in some processes so that difficulties arise with the fluid bed process.
It is an object of~this invention to provide a process for making steel from iron oxide without 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 oxide to the carbide, followed by introducing the car-bide directly i~ito the basic oxygen furnace to produce steel.

SUMMARY OF T~E INVENTION

A process for the direct production of steel from particulate iron oxides or concentrates which comprises (1) converting the oxides to iron carbide in a sinsle step in a fluidized bed at low temperatures with a mixture of re-ducing and carburizing gases followed by (2) direct conversion of the car~ide 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 furnace to make the process auto-thexmal. The CO/CO2 and hydrogen to water vapor ratios of the gases in the reaction of Step ~1) are maintained at a point below which oxidation of iron carbide ~ccurs.
Off-gases from the steel making step, about 90 percent carbon monoxide, may be circulated for use as part of the re-ducing gas for the reduction and carburizing step in the fluidized bed. Material balaltce calculations show that the -. . . 10~

carbon content of the off-gas is sufficient to supply all of the carbon necessary for the redùction and carburiz~tion step.
Accordingly, when Steps 1 and 2 are performed i~ conjunction with - each other as one continuous operation, all of the carbon necessary for Step 1, subject to slight operating losses, may be provided by continuous cycling of the off-gas from Step 2 to Step 1. This eliminates the necessity for adding carbon to Step 1 with the exception of small losses occurring in normal ' operations. 'The result is that the carbon originally added to Step 1 to make iron carbide may be used over and over by re-covering it in Step 2 in the off-gas as steel is produced and reusing it in Step 1 to make more carbide. ~hen the steps are performed in conjunction with each other added Aeat is not required to make the process auto-thermal, as the product going directly from Step 1 to Step 2 is at a temperature which elimi-nates the necessity for adding heat. When Steps 1 and 2 are performed separately then the''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.
The iron carbide produced in Step 1 is added directly as the charge to the basic oxygen or electric furnace alonq with fluxing agents, alloying material and other conventional additives to produce steel directly with elimination of the conventional blast furnace step. E~eat 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/CO~ ratio in the combustion gases must be maintained below that at which iron carbide will decompose at the required preheat temperature.

10~

BR~EF DESCRIPTION OF TiIE DR~WING

The single drawing is a schematic flowsheet for the direct steel making process of the invention.

DESCRIPTION OF PREFERRED EMBODIMENTS
. . .

The invention will be described in detail in conjunc-tion with the accompanying drawing.
The-basic oxygen and electric furnace processes re-ferred to herein for making steel are ~ell known in the prior art. The basic oxygen process or basic oxygen furnace process differs chiefly from Bessemer converte-rs and open hearth fur-naces in that the reactant 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 ~y the process 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 feed material on a grate or other perforate support is fluidized 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, scru~bers, cyclones, gas cycling equipment, and other conventional equip-ment. Some of this auxiliary e~uipment is shown schematically in the ~lowsheet.
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"

10~83~

.

includ~s carbon alone~ -Step 1 of the overall process is the conversion of the oxides in the iron ore conccntrate to iron carhide in the fluidized bed unit shown in the flowsheet. The conversion process must be carefully controlle~ to provide a product suit-able for use in the basic oxygen or electric furnace. The iron carbide is desirable 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.
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 the,temperature is maintained below about 1300F
at all times and preferably in the range of abo~t 900-1200F.
Hydrogen 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 yiven off.
Hydrogen is preferred as the reducing gas because the oxidation product of hydrogen, which is water, may be easily removed from the furnace off-gas thus permitting recycling of the balance of the gas without the need for extensive complicated and expensive chemical systems for removing the oxidation products of carbon which are carbon monoxide and carbon dioxide when carbon contain-ing reducing gases are used.
The preferred carburizing gas which is mixed with the reducing gas is propane, although carbon mono~ide or other hydro-carbon gases, or solid carbon, may be used with the lower alkyl hydrocarbon gases being preferred. ~ wide range of carbonaceous 1~8319 materials may be used so long as they suDply carbon to form iron carbide.
By proper balancing of the ratios of the hydrogen and carbon bearing materials, it is possible to restrict the hydrogen to a reducing func~ion and the carbon to a carburizing function. This can readily be done by maintaining quantities of hydrogen bearinq 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 of 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 carburization procedure described above in a fluid bed reactor are presented in the following Table I.

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dP ~ ~ U
N ~ 0 X
~ ~1 0 U~ ~
h ~ ~ S ~ U O U
1~1 Q~ G~ ~a ~ ~ "' ~~ ~' F~ h 1~ --I Q)~1 O ~ aJ~ G) O~ O U U ~ 3 a ~o ~o~o ~ ~o ~o ~o~o ~o O h O ~ O O O O O
U
t~ ~ o a~
O ~P
h ~ Lr~o~ er ~ ~ ~ u-P.
~ o cs~~r o ~ r~
O ~P
~.) ~ D
~ ~ ~ ao ''~ o dP
u~ C ) c~ o ~r ~ ~ u~ o ~ :S
U~
~ n ~ Z<w o q~
O O O ~1 ~D ~ UlU~
~dP
Z ~C t` ~ ~ ~ ~ ~I` CO
H r-- t` I` I` ~D ~U~1l') E~
~) ::~ ~ Co 0 ~0 ~ ~ ~ ~ o O~ ~4 _1 ~ o o .-1 ~
~; E~
Hl~l ~ ,.
~3tJ ~ U~

~ ~ ,~ ~C O O o o o o o o U~ ~1 ~'~
C~ ~ ~
O ~ ~ 1 .' U~
X ~ ~
1~~ N E~ ~ n ~ . . .

~: ~ ~ ~ ~ h ~1 0 t- ~o a) ~ ~ o o o o o o 14P:;~ ~ 3 ~1 O o o O O oo S O o o O o o Oo U~
O ~ + + + + + ++
~ O O O O O OOf~
U~ l ~ l l 3 . ql al o o a) Q~ Ql ~1 ~ V ~ ~ J~ ~Jr1 rl U
O .,1 .~.,~ .,1 .~1.,~ ~ J~ a~
~ JJ~ ~ ~ ~~ a a E~ ~
~ a) E l ~ S
to ~ I m m m m c~
~ o m o ~ o E~ ~ ~ ~ ~ <" ~ ~ ~r ~ ~r ~

10~83~9 The carbon content in the final product varies as the percent iron oxide in the feed materials varies. Lower grade ores with lower iron contents will automatically yield products with lower carbon contents.
The volume of hydroyen in the hydrogen-carbon monoxide reducing and carburizing mixture in the fluidized unit should exceed the volume of 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 S~ep I of the process of clean iron carbide which is highly suitable for use in the basic oxygen or electric furnace. X-ray diffraction analysis showed the carbon to be present as iron 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 5 to 1 and 8 to 1 and CO/CO2 ratios between about 1 to 1 and 5 to 1. As stated herein, Step 1 can be successfully operated within a temperature range of about 900-1300F, a hydrogen to water vapor 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 C02. It was round that Step 1 would not operate outside these ran~es to successfully produce iron carbide.
Step 2 of the overall process is the conversion of the iron carbide to steel in the basic o~ygen furnace. Because of the nature of the basic oxygen furnace process, special conditions _g_ ~8319 apply to the processing of iron carbide to steel by this pro-cess as compared to other steel making processes in furn~ces.
If Steps 1 and 2 are close-coupled so that the iron carbide comes out of the fluid bed unit at an elevated temper~
ature of about 1100-1300F and at that temperature is added directly to the basic oxygen furnace r then the heat calculations show that no added heat lS required and the process is continuous and auto-thermal.
The modjification 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 making iron carbide.
I' for~purposes of transport or storag~ the Step 1 product becomes or is cooled before Step 2, then heat must be readded either in the form of reheating the produc-t or adding extra fuel to Step 2.
Heat balance calculations show that at ambient temp~r-ature iron carbide does not contain sufficient fuel value so that the reaction taking place in the basic oxygen furnace is auto-thermal without adding heat to the charse.
The additional heat required to make the reaction self-sustaining may be supplied in a num~er of ways. The off-gas from the basic oxygen furnace produced by the processing of iron carbid~
contains about 90 percent 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 -10- ' 10~83~9 augmenting the sensible heat to effect the required preheat-ing of the incoming iron carbide charge to make the process auto-the~nal. Under some conditions the sensible heat alone is sufficient. The heat for the preheating can be obtained entirely from combustion of the off-gas. The preerred pre-heat temperature range is from about 1300F to about 2000F.
Tests conducted with iron carbide in a ~aseous medium simulating that of the combustion 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 2Ito 1 when attaining preheat temperatures of 900-1300F.
Added heat to make the process auto-thermal can be supplied wholly or in part by direct heating of the Fe3C charge with an external heat source. Sufficient carbon may be added ~o the iron carbide to provide the required additional heat by combustion during the process. The amount of carbon added varies from about 3 to 5 weight percent of the iron carbide charge. The carbon may be added directly 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 fluiclized bed to provide for the production of sufficient Fe2C in the Fe3C product. As shown in 10~8319 the flowsheet, hot scrap metal may be adde~ to the basic oxygen ~urnace charge.
Step 2 of the process may also include the addition of pig iron to the iron carbide charge in the basic oxygen and electric furnaces. A significant 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 added to conventiona]
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 con~unction with the present process.
The invention includes all of the above procedures alone or combined fo~r providing the necessary heat for the iron carbide charge to makè 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 of the elec-trical evergy normally used in this type of furnace.
Step 1 of the process provides a conve~ient and effec-tive means for concentrating low ~rade non-ma~netic ores to separate the iron ore from the gangue-. As the iron carbide pro-duced from non-magnetic ores is magnetic it is only necessary to process non-magnetic ore, such as, oxidized taconites, in accor-dance with Step 1 to convert the iron oxide therein to iron carbide and subject the treated ore to magnetic separation to separate the ma~netic 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 invelltion are apparent from the above description. Its principal advanta~e is that it 1~98319 eliminates the expensive interme~iate blast furnace step in converting iron ore to steel. When the two steps are performed in conjunction no added heat is necessary for the second step and carbon monoxide from the second step provides the necessary carbon for carbonization of reduced iron in the first step so that the carbon can be reused continuously in both steps. Step 1 includes the production of water as a by-product, thus sim~
fying the recovery of by-product carbon containing gases. This step can be performed to give a product having a high enouyh ratio of Fe2C to Fe3C to provide a high enough carbon content in the charge for the basic oxygen furnace to make the steel ma~ing 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 CO in the off-gases can be burned to provide the necessar~ heat, or the CO 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-free 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-portation costs when the carbide is made near the mine before transport to the steel making furnace as iron carbide represents a higher percentage of usable material than the oxide.

Claims (6)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A process for conversion of iron oxides to iron carbide which comprises reducing the oxides and converting the iron to iron carbide in one step in a fluidized bed at a temperature not exceeding 1300°F with a mixture of a hydrogen bearing gas and a carbon containing material which provides carbon for the iron carbide, the mixture containing hydrogen in an amount over 60 percent by volume of the carbon monoxide present.

2. The process of claim 1 in which the ratio of hydrogen to formed water in the reaction medium of the fluidized bed is maintained from about 2.5 to 1 to about 8 to 1 and the ratio of CO/CO2 is maintained from about 1 to 1 to about 4 to 1, the prescribed CO/CO2 - hydrogen/H20 ratios being essentially in equilibrium with methane.

3. The process of claim 1 in which the carbon con-taining material is solid carbon.

4. The process of claim 1 in which the carbon con-taining material is a lower alkyl hydrocarbon gas.

5. The process of claim 4 in which the gas is propane.

6. The process of claim 1 in which the temperature of the mixture is between about 1100°F and about 1300°F.