CA1101677A - Process and apparatus for the production of intermediate hot metal - Google Patents
Process and apparatus for the production of intermediate hot metalInfo
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- CA1101677A CA1101677A CA308,120A CA308120A CA1101677A CA 1101677 A CA1101677 A CA 1101677A CA 308120 A CA308120 A CA 308120A CA 1101677 A CA1101677 A CA 1101677A
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- Refinement Of Pig-Iron, Manufacture Of Cast Iron, And Steel Manufacture Other Than In Revolving Furnaces (AREA)
Abstract
ABSTRACT OF THE DISCLOSURE
A process and apparatus for the production of intermediate hot metal suitable for further refining into steel is disclosed.
The basic process includes the steps of heating a charge of ore in a reducing furnace having a reducing atmosphere therein comprising a mixture of reconditioned and recycled top gas from the reducing furnace and off-gas rich in hydrogen and carbon monoxide produced by a cupola melting unit to reduce the ore to iron, partially carburizing the reduced iron in the reducing furnace with carbon-containing off-gas produced by the cupola melting unit, and melting the reduced and carburized iron together with scrap, slag forming additives and fluxes in a cupola melting unit having a reducing atmosphere therein produced by the combustion of a rich fuel/oxidant mixture to form a molten slag and molten iron suitable for the further refining to produce steel. The process contemplates the further refining of the molten iron from the cupola in an electric steelmaking furnace or an oxygen steelmaking converter. In an alternative form of the process, the reduced and carburized iron is cooled within or outside the reducing furnace to form a prereduced metal pellets suitable for use as a part of the burden in a melting unit.
The apparatus comprises a refractory lined cupola melting unit portion equipped with burners capable of burning a rich fuel/oxidant mixture to produce a reducing atmosphere within the melting unit and off-gas rich in gaseous reductants, a direct reducing unitportioncommunica-ting with the upper end of the melting unit to receive the melting unit off-gas recirculating and conditioning means to recycle at least a portion of the top gas from the upper region of the reducing portion to the lower region of the reducing unit portion, means for introducing ore into the direct reduction unit portion of the apparatus, and means for introducing additives and fluxes into the melting unit portion of the apparatus.
A process and apparatus for the production of intermediate hot metal suitable for further refining into steel is disclosed.
The basic process includes the steps of heating a charge of ore in a reducing furnace having a reducing atmosphere therein comprising a mixture of reconditioned and recycled top gas from the reducing furnace and off-gas rich in hydrogen and carbon monoxide produced by a cupola melting unit to reduce the ore to iron, partially carburizing the reduced iron in the reducing furnace with carbon-containing off-gas produced by the cupola melting unit, and melting the reduced and carburized iron together with scrap, slag forming additives and fluxes in a cupola melting unit having a reducing atmosphere therein produced by the combustion of a rich fuel/oxidant mixture to form a molten slag and molten iron suitable for the further refining to produce steel. The process contemplates the further refining of the molten iron from the cupola in an electric steelmaking furnace or an oxygen steelmaking converter. In an alternative form of the process, the reduced and carburized iron is cooled within or outside the reducing furnace to form a prereduced metal pellets suitable for use as a part of the burden in a melting unit.
The apparatus comprises a refractory lined cupola melting unit portion equipped with burners capable of burning a rich fuel/oxidant mixture to produce a reducing atmosphere within the melting unit and off-gas rich in gaseous reductants, a direct reducing unitportioncommunica-ting with the upper end of the melting unit to receive the melting unit off-gas recirculating and conditioning means to recycle at least a portion of the top gas from the upper region of the reducing portion to the lower region of the reducing unit portion, means for introducing ore into the direct reduction unit portion of the apparatus, and means for introducing additives and fluxes into the melting unit portion of the apparatus.
Description
6'~
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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: ,, . .
'7~
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 6i7 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 i7~
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 :~
' . . ' :
-. . .
!
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 ' ' ' ' '~ :
,~
~ 6~7~
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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'77 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, ~, -:- :
~ 7~
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 ' . .
- 9 - ~
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 ' ~.
'77 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 ,.
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 6t7 composition of the cupola off-gas is as follows:
GAS SCF/Ton of steel 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:
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:
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 6'7~
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 It is thus apparent that while reductlon of the ore by car-
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, ~.
':
: ,, . .
'7~
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 6i7 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 i7~
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 :~
' . . ' :
-. . .
!
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 ' ' ' ' '~ :
,~
~ 6~7~
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, .
:
.
'77 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, ~, -:- :
~ 7~
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 ' . .
- 9 - ~
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 ' ~.
'77 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 :. . .
'~ ,, .~ .
.
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 ,.
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 6t7 composition of the cupola off-gas is as follows:
GAS SCF/Ton of steel 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:
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:
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 6'7~
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 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 ~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:
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 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 7~
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-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 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 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 ' 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 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 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.
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 7~
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-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 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 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 ' 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 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 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.
Claims (12)
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:
1. A process for the production of intermediate hot metal for use in steelmaking, comprising introducing a charge of ore containing 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 of iron substantially to iron, characterized in that a charge of said ore in the form of lumps, briquettes, pellets or other agglomerates is introduced into said reducing furnace for reduction therein in a reducing atmosphere comprising a mixture of top gas re-cycled from said reducing furnace and off-gas containing hydrogen and carbon monoxide produced in a melting unit, a portion of the iron thus reduced being carburized with-in 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 forming additives and fluxes into the upper region of a melting unit, and melting said reduced and carburized iron, said gangue and said slag forming additives and fluxes under a reducing atmosphere produced by the combustion of a rich fuel/oxidant mixture to produce a molten slag and molten metal comprising es-sentially iron and carbon.
2. The process of claim 1, wherein the reduced and carburized iron introduced into the melting unit com-prises between 30% and 70% of the metallic charge of the melting unit and the balance of the metallic charge omprises iron-containing metals.
3. The process of claim 1, wherein the molten metal comprising essentially iron and carbon is further re-fined in an electric furnace or in an oxygen converter to produce steel.
4. The process of claim 1, wherein the reduced and carburized iron is cooled before introduction into the melting unit.
5. The process of claim 4, wherein the reduced and carburized iron comprises between 40% and 60% of the metallic charge of the melting unit and the balance of the charge comprises iron-containing metals.
6. The process of claim 1, 2 or 3, characterized in that the charge introduced into the reducing furnace includes slag forming additives and fluxes in addition to said ore.
7. The process of claim 1, characterized in that the reducing atmosphere in the reducing furnace comprises a mixture of top gas containing hydrogen, water, carbon monoxide and carbon dioxide, recycled from said reduc-ing furnace, and off-gas containing hydrogen, carbon monoxide, water and carbon dioxide produced in said melting unit, a portion of said iron being carburized to iron carbide within said reducing furnace by reacting the iron values with a portion of the carbon monoxide contained in said top gas and said off-gas from said melting unit, a portion of said top gas from said re-ducing furnace being cooled and conditioned to remove a portion of the water and carbon dioxide therein and then reheated before recycling together with said off-gas from said melting unit into said reducing furnace.
8. The process of claim 7, characterized in that a portion of the heat required to reheat said condition-ed top gas is produced by the combustion of another por-tion of said top gas with an oxidant.
9. An apparatus for the production of intermediate hot metal for use in steelmaking, comprising a refrac-tory lined reducing furnace having openings at the top and bottom thereof, charging means communicating with said top opening of said reducing furnace for introduc-ing therein at least iron ore for reduction thereof, a refractory lined vertical melting unit communicating at its upper end with said bottom opening of said reducing furnace and having a hearth region near the lower end thereof and a melting region effectively communicating between said hearth region and said bottom opening of said reducing furnace, first duct means communicating at one end with the upper region of said reducing furnace and at the other end with gas conditioning means, second duct means communicating at one end with said gas condi-tioning means and at the other end with said reduced iron ore before entering the melting unit, a plurality of combustion chambers communicating with said hearth region of said melting unit, each combustion chamber having positioned therein a burner, and a spout com-municating with said hearth region of said melting unit near the bottom thereof.
10. The apparatus of claim 9, wherein said melting region has a larger diameter than the diameter of said hearth region so as to define a shoulder therebetween.
11. The apparatus of claim 9, wherein said combus-tion chambers communicate with said hearth region through a reduced diameter portion whereby the velocity of the combustion products entering said hearth region is in-creased.
12. The apparatus of claim 9, 10 or 11, wherein the melting unit has a charge-receiving region near the upper end thereof, that the melting region communicates between said charge-receiving region and said hearth region, and that the second duct means communicates at said other end thereof with said charge-receiving region of said melting unit, charging means being provided communicating with said charge-receiving region of said melting point.
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA308,120A CA1101677A (en) | 1978-07-25 | 1978-07-25 | Process and apparatus for the production of intermediate hot metal |
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA308,120A CA1101677A (en) | 1978-07-25 | 1978-07-25 | Process and apparatus for the production of intermediate hot metal |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1101677A true CA1101677A (en) | 1981-05-26 |
Family
ID=4111984
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA308,120A Expired CA1101677A (en) | 1978-07-25 | 1978-07-25 | Process and apparatus for the production of intermediate hot metal |
Country Status (1)
| Country | Link |
|---|---|
| CA (1) | CA1101677A (en) |
-
1978
- 1978-07-25 CA CA308,120A patent/CA1101677A/en not_active Expired
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