CA1084273A - Process for the direct production of steel - Google Patents

Abstract

PROCESS FOR THE DIRECT
PRODUCTION OF STEEL

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

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~ BACKGROUND OF THE INVENTION
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Field of the Inven-tion The invention lies in the i.eld o~ the pyrome-tallurgy of ferrous metals.

Description of the Prior Art ::, . ~
The increasing necessi-ty of using low grade iron ores Eor making steel because of the depletion of high grade .. ..
ores, and economic factors, have created a demand for reduction .~
~ of the cos-ts in producing steel from iron ore. Efforts to , I ~ / ~ ~ ~, : .

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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'73 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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2~3 ~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.
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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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~8~273 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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~ ~84~73 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 :, :
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 : ~ _g_ .
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1~84;~73 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.
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: 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 ., ~ ` .

~8'~ 3 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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~89L;~'~3 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-.: ~
;.' 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.

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Claims (31)

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

1. A process for the direct production of steel from iron oxides which comprises:
(a) converting the iron in the oxides to iron carbide; and (b) converting the iron in the iron carbide directly to steel in the basic oxygen furnace or in the electric furnace.

2. A process for the direct production of steel from iron oxides which comprises:
(a) converting the iron in the oxides to iron carbide; and (b) converting the iron in the iron carbide directly to steel in the basic oxygen furnace.

3. The process of claim 2 in which in step (b) the conversion is accomplished by oxidizing the carbon in the iron carbide to carbon monoxide with the heat released thereby providing heat for operation of the basic oxygen furnace.

4. The process of claim 1 in which the iron carbide is processed to steel in the electric furnace.

5. A process for the direct production of steel from iron oxides which comprises:
(a) 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 the carbon for the iron carbide, the hydrogen being present in an amount exceeding 60% by volume of the CO present; and (b) adding the iron carbide to a basic oxygen furnace and processing it to steel by the basic oxygen furnace process without the addition of external heat.

6. The process of claim 5 in which steps (a) and (b) are performed in conjunction and the iron carbide of step (a) is added directly to the basic oxygen furnace without substantial heat loss so that the process is auto-thermal.

7. The process of claim 6 in which the iron carbide is at a temperature between about 1100 F to 1300°F
when it leaves step (a).

8. The process of claim 6 in which off-gas is cycled from step (b) to step (a).

9. The process of claim 8 in which the off-gas provides substantially all of the carbon for step (a).

10. The process of claim 5 in which in step (a) 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 CO/CO2 ratio is maintained from about 1 to 1 to about 4 to 1, the prescribed CO/CO2 - hydrogen/H2O
ratios being essentially in equilibrium with methane.

11. The process of claim 5 in which the volume of hydrogen exceeds the volume of CO in said fluidized bed.

12. The process of claim 5 in which the carbon con-taining material is solid carbon.

13. The process of claim 5 in which the carbon con-taining material is lower alkyl hydrocarbon gas.

14. The process of claim 13 in which the gas is propane.

15. The process of claim 10 in which the temperature of the reaction gas mixture is between about 1100°F and 1300°F

16. The process of claim 5 in which the iron carbide of step (a) has lost heat before step (b) is performed and sufficient heat in a preheat step is added to it for step (b) to render the reactions occurring in the conversion of the iron carbide to steel in the basic oxygen furnace auto-thermal.

17. The process of claim 16 in which the iron carbide charge is preheated to a temperature between about 1300°F
and about 2000°F.

18. The process of claim 16 in which at least part of the heat for the preheat step is derived from off-gases from step (b).

19. The process of claim 16 in which at least some of the heat for the preheat step is derived from sensible heat in the off-gases.

20. The process of claim 16 in which at least some of the heat for the preheat step is derived from combustion of CO in the off-gases.

21. The process of claim 20 in which the CO/CO2 ratio in the gas combustion products is maintained from about 1:1 to about 2:1 when attaining preheat temperatures of 900-1300°F.

22. The process of claim 16 in which the heat for the preheat step is provided by reacting at least part of the off-gas to produce Fe2C in the iron carbide from step (a).

23. The process of claim 22 in which the composition of the reactive medium of step (a) is adjusted to provide the required amount of Fe2C in the iron carbide charge.

24. The process of claim 5 in which the iron carbide of step (a) has lost heat before step (b) is performed and sufficient fuel is added to it to provide additional heat upon combustion in the process to render the reactions occurring in the conversion of the iron carbide to steel in the basic oxygen furnace auto-thermal.

25. The process of claim 24 in which the fuel is carbon.

26. The process of claim 25 in which the carbon is added in an amount from about 3 to 5 weight percent of the iron carbide charge.

27. The process of claim 25 in which the carbon is added directly to the iron carbide charge.

28. The process of claim 5 in which molten pig iron is added to the iron carbide charge in step (b) and iron carbide is used to control the temperature of the melt.

29. The process of claim 28 in which the temperature controlling iron carbide is added in an amount up to about 60 weight percent of the iron carbide-pig iron charge.

30. A process for the direct production of steel from iron oxides which comprises:
(a) reducing the oxides and converting the iron to iron carbide in one step in a fluidized bed at a tem-perature 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 hydrogen being present in an amount exceeding 60% by volume of the CO present; and (b) adding the iron carbide to an electric furnace and processing it to steel.

31. The process as defined in claim 30 in which steps (a) and (b) are performed in conjunction and the iron carbide of step (a) is added directly to the electric furnace without substantial heat loss.