CA1095255A - Process for upgrading iron ore pellets - Google Patents
Process for upgrading iron ore pelletsInfo
- Publication number
- CA1095255A CA1095255A CA284,069A CA284069A CA1095255A CA 1095255 A CA1095255 A CA 1095255A CA 284069 A CA284069 A CA 284069A CA 1095255 A CA1095255 A CA 1095255A
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- CA
- Canada
- Prior art keywords
- pellets
- grate
- periphery
- furnace
- zone
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Expired
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Classifications
-
- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22B—PRODUCTION AND REFINING OF METALS; PRETREATMENT OF RAW MATERIALS
- C22B1/00—Preliminary treatment of ores or scrap
- C22B1/14—Agglomerating; Briquetting; Binding; Granulating
- C22B1/24—Binding; Briquetting ; Granulating
- C22B1/2413—Binding; Briquetting ; Granulating enduration of pellets
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- Engineering & Computer Science (AREA)
- Chemical & Material Sciences (AREA)
- Manufacturing & Machinery (AREA)
- Geochemistry & Mineralogy (AREA)
- Geology (AREA)
- General Life Sciences & Earth Sciences (AREA)
- Life Sciences & Earth Sciences (AREA)
- Environmental & Geological Engineering (AREA)
- Materials Engineering (AREA)
- Mechanical Engineering (AREA)
- Metallurgy (AREA)
- Organic Chemistry (AREA)
- Manufacture And Refinement Of Metals (AREA)
- Manufacture Of Iron (AREA)
Abstract
PROCESS FOR UPGRADING IRON ORE PELLETS
Abstract of the Disclosure In a process for hardening oxidizable green iron ore pellets in a horizontal grate or a circular grate furnace, said process comprising passing the grate with a bed of pellets thereon along a horizontal path through the furnace wherein said pellets are heated by contact with hot gases, the furnace having a zone in, or downstream of, the area where the peak temperature of the furnace is attained in which zone the average pellets temperature is in the range of about 1100°F to about 2200°F and in which zone the flow of gases is in a downward direction towards the pellets on the grate the improvement comprising:
(a) covering the periphery of at least part of the zone with at least two hoods to provide a hooded area on each side of the top of the grate under which the periphery of the grate passes;
(b) passing at least one oxygen stream within each hooded area in such a manner that the stream flows in a downward direction towards and through the periphery of the bed of pellets on the grate passing through the hooded areas;
(c) passing the periphery of the bed of pellets on the grate through the hooded areas in such a manner that the residence time of such pellets in the hooded areas is at least about 5 seconds.
SPECIFICATION
Abstract of the Disclosure In a process for hardening oxidizable green iron ore pellets in a horizontal grate or a circular grate furnace, said process comprising passing the grate with a bed of pellets thereon along a horizontal path through the furnace wherein said pellets are heated by contact with hot gases, the furnace having a zone in, or downstream of, the area where the peak temperature of the furnace is attained in which zone the average pellets temperature is in the range of about 1100°F to about 2200°F and in which zone the flow of gases is in a downward direction towards the pellets on the grate the improvement comprising:
(a) covering the periphery of at least part of the zone with at least two hoods to provide a hooded area on each side of the top of the grate under which the periphery of the grate passes;
(b) passing at least one oxygen stream within each hooded area in such a manner that the stream flows in a downward direction towards and through the periphery of the bed of pellets on the grate passing through the hooded areas;
(c) passing the periphery of the bed of pellets on the grate through the hooded areas in such a manner that the residence time of such pellets in the hooded areas is at least about 5 seconds.
SPECIFICATION
Description
Field of the Invention This invention relates to a process for upgrading iron ore pellets in a horizontal or circular grate furnace as the pellets undergo hardening.
Description of the Prior_Art The pelletizing of iron ore concentrates for use as cllar~c material in blast furnaces has been gaining in importance in the steel industry. This is the result of an attempt to meet *he increased demand for iron and steel with lo~er quality ores and ores extracted from beneficiation plants, all of which are usually in the fol~ of fincly ; divided particles, too finely divided for direct processing in a blast furnace.
The primary purpose of pelletizing in this industry is to improve burden permeability and gas-solid contact in the blast furnace in order to increase the rate of reduction. A secondary consideration is to ; reduce ~he amount of fines blown out of the blast furnace into the gas recovery system.
Characteristics of industrially acceptable pellets are those that are strong enough to wi*hstand degradation during stockpiling, handling~
and transportation and have the capability to withstand the high temper-ature and degradation forces within the blast furnace wi~out slumpillg or decrepitating.
~ Typical pelletizing processes comprise forming 3/8 inch to 1 inch }1 diameter balls of iron ore concentrate of reasonable moisture content in a rotating dru~ or on a rotating disc and then iring *he "green"
b~lls or pellets in a furnace to a sufficiently high *emperature to harden *he pellets to a strength suitable for use in blast furnaces. The green ; pellets of interest here are those which contain an oxidi~able material,
Description of the Prior_Art The pelletizing of iron ore concentrates for use as cllar~c material in blast furnaces has been gaining in importance in the steel industry. This is the result of an attempt to meet *he increased demand for iron and steel with lo~er quality ores and ores extracted from beneficiation plants, all of which are usually in the fol~ of fincly ; divided particles, too finely divided for direct processing in a blast furnace.
The primary purpose of pelletizing in this industry is to improve burden permeability and gas-solid contact in the blast furnace in order to increase the rate of reduction. A secondary consideration is to ; reduce ~he amount of fines blown out of the blast furnace into the gas recovery system.
Characteristics of industrially acceptable pellets are those that are strong enough to wi*hstand degradation during stockpiling, handling~
and transportation and have the capability to withstand the high temper-ature and degradation forces within the blast furnace wi~out slumpillg or decrepitating.
~ Typical pelletizing processes comprise forming 3/8 inch to 1 inch }1 diameter balls of iron ore concentrate of reasonable moisture content in a rotating dru~ or on a rotating disc and then iring *he "green"
b~lls or pellets in a furnace to a sufficiently high *emperature to harden *he pellets to a strength suitable for use in blast furnaces. The green ; pellets of interest here are those which contain an oxidi~able material,
-2 ' .. . .
~i 10,982 usually magnetite ~Fe304). Other oxidizable materials are iron and solid fuel, such as co~e, coal, or charcoal, which is sometimcs ~ddcd to the balling mix in a finely divided state in order to provide additional heat to the pellets duri.ng the hardening operation. The iron ore c~ncentrate with which we are particularly concerned here contains at least about 30 percent magnetite; some iron or other iron compo~ds such as hematite; and a small arnount of impurities such as silica, alumina and magnesia. One of these concentrates is known as beneiciated taconite. Binders are often added before or during the drum or disc rotation to increase the wet strength of the green pellets to acceptable levels for subsequent handling.
One of the types of furnaces used commercially for hardening green pellets is the horizontal grate furnace.
The process practiced in ~his furnace is basically a modified sintering process. Typically, green pellets (sometimes with the fuel addi~ive mentioned above) are continuously fed at the upstream end of ~;
the furnace onto a moving grate to form a bed having a depth of about 14 inches. As the continuous grate moves downs~ream~ the pellets are subjected to contact wi~h combustion gases and/or air in updraft and downdraft drying, preheating, ignition (firing), induration, heat recuperation, and cooling before being discharged at the downstream end of the furnac0 as product. Various updraft-downdraft combinations particularly in the drying and cooling steps, are currently practiced.
A modification of the basic horizontal grate process includes the reuse of burned pellets as the bottom and side layers on the grate with green pellets placed above and between these layers. Recently a circular grate in the shape of a toroid has been subs~ituted for the horizontal grate thus eliminating the need for a return strand, but ~he circular grate is otherwise similar to the horizontal grate in operation. Temper-atures in the horizontal and circular grate systems reach a pea~ of ~;
~i 10,982 usually magnetite ~Fe304). Other oxidizable materials are iron and solid fuel, such as co~e, coal, or charcoal, which is sometimcs ~ddcd to the balling mix in a finely divided state in order to provide additional heat to the pellets duri.ng the hardening operation. The iron ore c~ncentrate with which we are particularly concerned here contains at least about 30 percent magnetite; some iron or other iron compo~ds such as hematite; and a small arnount of impurities such as silica, alumina and magnesia. One of these concentrates is known as beneiciated taconite. Binders are often added before or during the drum or disc rotation to increase the wet strength of the green pellets to acceptable levels for subsequent handling.
One of the types of furnaces used commercially for hardening green pellets is the horizontal grate furnace.
The process practiced in ~his furnace is basically a modified sintering process. Typically, green pellets (sometimes with the fuel addi~ive mentioned above) are continuously fed at the upstream end of ~;
the furnace onto a moving grate to form a bed having a depth of about 14 inches. As the continuous grate moves downs~ream~ the pellets are subjected to contact wi~h combustion gases and/or air in updraft and downdraft drying, preheating, ignition (firing), induration, heat recuperation, and cooling before being discharged at the downstream end of the furnac0 as product. Various updraft-downdraft combinations particularly in the drying and cooling steps, are currently practiced.
A modification of the basic horizontal grate process includes the reuse of burned pellets as the bottom and side layers on the grate with green pellets placed above and between these layers. Recently a circular grate in the shape of a toroid has been subs~ituted for the horizontal grate thus eliminating the need for a return strand, but ~he circular grate is otherwise similar to the horizontal grate in operation. Temper-atures in the horizontal and circular grate systems reach a pea~ of ~;
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about 2400F in ~he ignition and induration sections going do~n to about 200F to 400F at discharge.
In ~he ignition, induratio~ or cooling sections, there is a zone in whLch the average pellet temperature is in the range of ahout 1100F to ^
about 2200F., which is of interest here and which here~ofore has not been delineated.
Strong bonding in the hardened pellets produced in the grate furnace is believed to be due to grain growth rom the accompanying oxidation of magnetite to hematite and ~o recrystallization of the hematite. The exothermic oxidation reac~ion typically supplies about 300,000 Btu's (British thermal units) per long ton of pellets.
Hardened pellet streng~h is usually determined by comp~ession and tumbler tests. Although specifications for pellets vary dcpcndin~
on their source and the purchaser, the minimum suggested compressive strength for individual pellets ranges from about 300 pounds for 1/~ inch pellets to about 800 ~o about 1500 pounds for 1 inch pellets. In the tumbler test, 25 pounds of plus 1/4 inch pellets are tumbled for 200 revoluticns at Z4 1 1 rpm (revolutions per minute) in a drum tumbler and ~hen screened. Sa~isfactory commercial pellets gcncrally contai less than about 6 percent of minus 28 mesh fines and more than about 90percent of plL~ 1/4 inch pellets after the tumble test. In some cases, the tumble index has been modified to measure only the plus 1/4 inch pellets present before and remaining after the tumble test and the price paid per long ton of pellets shipped is adjusted accordingly.
Since production at a pelletizing plant is in the millions of tons per year range, a small improvement in tumble index ~quality) of about 2 percentage points, for example, can represent si~nificant additional income ~D ~he plant.
, . . .
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'`'` ' ~ 10,9~ , ; It is understood by those skilled in the art that one of the - important factors in improving the quality of the pellets, both in terms of co~pressive strength and tumble index, is to provide for a more efficien~ conversion of magnetite to hematite in the furnace, the ~ v goal being, of course, one where all of the pellets produced are essentially hematite, or, at least, of higher hematite conteTlt.
Oxidation of magnetite to hematite during the pe~letizing process is important not only because hematite is reduced more readily in the blast furnace in spite of its higher oxygen con~ent, but also because in the pelletizing process, conversion of magne~ite to hematite,which is a strongly exothermic reaction, favors grain growth and sintering of the particles of iron ore concentrate to form hard, strong pellets that are abrasion resistant.
Since the reaction rate of magnetite in substantially pure oxygen is manyfold grea~er than that in air, it has been sugges~ed ~hat the combustion gases and air in the furnace be enriched wi*h oxygen; however, the volume of gases circulating in a pelletizing plant is so large that any significant increase in oxygen concentration requires uneconomic amounts of oxygen, i.e., the cost of oxygen needed ~o provide higher numbers of pelle~s of essentially hematite or higher hemati~.c con~ent exceeds the additional income generated by the higher quality pellèts.
Further, it is recognized that a large percentage of the additional oxygen is was~ed, in any case, because it 10ws over pellets, which would be converted to essentially hematite or at least a sufficient hematite content in a conventional operation.
.
-~ Summary of the Invention . I
An object of this invention, ~herefore, is to provide an improvement over conventional pelletizing processes whereby the hem~tite content of ;- the hardened pellets is increased, and the overall ~uality of pellets is thereby i~proved.
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Other objec~s and advan~ages will become apparent hereinafter.
According to the present invention, such an improvement has been discovered in a process for hardening oxidizable green iron ore pellets in a horizontal grate or a circular grate furnace, said process comprising passing the grate with a bed of pellets thereon along a horizon~al path ; through ~he furnace ~herein said pellets ~re hc~tcd b~- ~ont:lct ~irll h~t gases, the furnace haYing a ~one in, or downstream of, the area where the peak temperature of the furnace is at~ainedJ in which zone the average pellet temperature is in the range of about 1100~F to about 2200F and in which zone the ~low of gases is in a downward direction towards the bed of pellets on the grate.
The improvement comprises:
(a) covering the periphery of at least part of the zone ~ith at least two hoods to pro~ide a hooded area on each side of ~he top of the grate under which the periphery of the grate passes;
(b) passing at least one oxygen stream within each hooded area ~:, ; in such a manner that the stream flows in a downward direction towards and through the perlphery of the bed of pellets on the grate passing through the hooded areas;
~c~ passing the periphery of the bed o pellets on the grate ..
through the hooded areas in such a manner that the residence time of such pellets in the hooded areas is at leas~ about 5 seconds.
Descri~tion of the Preferred Embodiment ! The preparation of the green pellets has been referred to above .,ji : ~
- and is conventional. This invention is directed to that part of the pelletizing process whereby green pellets are hardened to the extent re-i~ quired for use in the blast furnace. As al50 noted, the apparatus, i.e.
~.
the horizontal or circular grate furnace, for carrying out the hardening aspect, the composition of ~he green pellets, the basic steps in the hardening process, and ~he combustion gases and air (referred to as gases~
. . , ~i -6-. :~
- ~ ;
- . 10,952 used in the process are conventional and are utilized here t~gether with subject improvement.
The improvement here involves directing a plurality of o~ygen strea at the pellets passing through the periphery of a particular temperature zone under a set of defined conditions. As notcd, ~he ~one is prescn~
in conventional longitudinal grate and circular grate furllace operations, but until now has not been identified other than as part of those sections of the ~urnace where oxidation, ignition, induration, and cooling ta~e place.
Th~ selected zone is that where the average pellet temperature is in the range of about 1100F to about 2200F and preferably about 1300F to about 2000F, and is in, or downstream of, the area where the peak temperatu~e of the furnace is attain~d. Further, the ~one must be sne where the gases in the furnace are flowing downward towar~s the pellets on the grate.
' ' i .' -;
The oxygen stream can be a mix~ure of gases con~aining a major proportion or more than 50 percen~ by volume oxygen. It is preferably a mixture of gases containing at least about 90 or 95 percer.t by volume oxygen, however. The usual oxygen distributed commercia]ly is considered to consist essentially of oxygen and it is expected ~hat this oxyge would be the most easily obtained.
It is found that, by directing oxygen at the pellets in the periphery of the selected temperature ~one under the defined conditions, maximum oxidation can be achieved with minimum oxygen consumption and the temperature of the pellets is raised thereby to provide more efficient ~hermal bonding which additionally raises the overall quality of the - . . .
pellets.
~........................................................................ .
~7-., ., .
10~982 The periphery of ~he zone is tha~ area on both sides of the top o the grate running from the inside surface of each retaining wall horizontally towards the center of the grate. The retaining wall is a par~ of and attached to the grate and is there for the purp~sc of keeping the pellets from falling off the moving grate. A typical retaining wall is about 16 inches high. It forms a right or oblique angle with the horizontal surface of the grate. The grate~ re~aining wall, and pellets all move together through the furnace.' The distance from the inside surface of the retaining wall towards the center of the grate to be included in the peripheral area is selected b~ analysis (in a conventionally run process) of samples of the pellets passing through the zone to determine the location of the bulk of the incompletely oxidized pellets. Although the width of the grates, i.e., measured horizontally across the ~ratc, varies from grate to grate, the distance included in the periphery is about 3 inches to about 24 inches as measured horizontally from the top of the inside surface of the retaining wall towards the center of the grate along a line perpendicular to the line of travel or direction of ,,; .
the grate. The periphery of the zone may also be referred to as the - periphery of tha gra~e or the periphery of the pellet bed.
The flow of the oxygen stream is in a downward direction towards the bed of pellets on the hori70ntal gratc. ~'here thc retainin~ wall is oblique, the direction of the flow is the same even though the pellets against the retaining wall are not on ~he horizontal. The stream first strikes the top of the pellet bed and then passes down through the bed and through the grate, the oxygen declining in amount as it reacts wi~h the oxidizable materials in the pellet.
Usually, the introduction or injection of the oxygen stream into the hooded area is made in the desired downward direction, but it can be introduced into the hooded area in any direction, e.g. in a horizontal . : .
direc~ion from the sides of ~he hood, and dispersed ~ithin the hood, which will serve to direct the flow in the down~ard direction.
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.
~' 10,9~
-One or m~re hoods are provided to co~er the periphery on eachside of the grate. The hoods are made of conventional materials which will withstand furnace temperatures. Refractories are usually used.
The oxygen stream i5 introduced'under the hood in sueh a manner'that downward flow referred to above is accomplished whether directly or indirectly. The introduction under the hood can be through an open pipe, a capped and perforated pipe, or through a series cf jets which re located to follow the path of the pellets. The hood serves to obstruct the flow of furnace gases over tha~ area of the periphery which it serves and ~inimizes dilution of the oxygen streams within the hooded area.
It will be understood by those skilled in ~he art that the ~er~s "hood" or "hooded area" contemplate the utili~ation of enclosures, canopies, closed end tulmels, tents, compartments, or any shielding device which permits the oxy~en strean~ to contact thc pellets without being diluted to any great extent while permitting the unreacted oxygen to join the main stream of furnace gases. The width of the hood~ i.e., that part measured from the top of the inside surface of the retaining wall horizontall~ ~o~ards the center of the grate, is 1~ 20 sufficient ~o cover the periphery sf ~he zone as described above. The length of the hood, i.e., that dimension measured along a linc runnillg parallel to the movement of the grate is sufficient to provide the required residence time for the incompletely oxidized pellets in the hooded area, the residence time being at least about 5 seconds and ,,, preferably at least about 10 seconds. It will be understood that ~he entire length of the zone within the periphery does not have to be sub-jected to oxygen treatment but only a sufficient length to insure that the residence time condition for the peripheral pellets is met. There is no upper limit for residence ti~e except the bounds of practicality, i.e., when complete oxidation has been acl-ieved, although an up~cr limit of about 30 seconds is preferred.
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10,982 `
:
Typically, the grate moves at about 50 to about 250 inches per minute and the flow of oxygen is kept cons~ant. Therefore, the length of the hood is adjusted to provide the required residence time based on the speed of ~he grate, e.g.,in order to provide a contact time of 15 seconds and assuming ~he grate is moving at a ra~e of 230 inches per minute, ~he internal length of the hood may be readily calculated as follows:
. . .
internal length of hood = 200 lnches mi x 15 seconds minute 60 seconds - 50 inches The amoun~ of oxygen supplied to the periphery of the ~one is " usually sufficient to convert essentially all of the magnetite in the periphery of the zone to hematite as determined on a theoretical basis. The same analysis as mentioned above for the determination of ~, .
the periph~ry can, of course, be used to determine this amount. It is preferred that about 0.30 moles to about 2 moles of oxygen be used for each mole of magnetite passing through the periphery of the .. ..
`r'' de~ined zone. The higher the quali~y of the pellet product desired, ~he higher the amount o oxygen which may be used9 ho\~ever. In any ~ case, the quality will be upgraded.
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about 2400F in ~he ignition and induration sections going do~n to about 200F to 400F at discharge.
In ~he ignition, induratio~ or cooling sections, there is a zone in whLch the average pellet temperature is in the range of ahout 1100F to ^
about 2200F., which is of interest here and which here~ofore has not been delineated.
Strong bonding in the hardened pellets produced in the grate furnace is believed to be due to grain growth rom the accompanying oxidation of magnetite to hematite and ~o recrystallization of the hematite. The exothermic oxidation reac~ion typically supplies about 300,000 Btu's (British thermal units) per long ton of pellets.
Hardened pellet streng~h is usually determined by comp~ession and tumbler tests. Although specifications for pellets vary dcpcndin~
on their source and the purchaser, the minimum suggested compressive strength for individual pellets ranges from about 300 pounds for 1/~ inch pellets to about 800 ~o about 1500 pounds for 1 inch pellets. In the tumbler test, 25 pounds of plus 1/4 inch pellets are tumbled for 200 revoluticns at Z4 1 1 rpm (revolutions per minute) in a drum tumbler and ~hen screened. Sa~isfactory commercial pellets gcncrally contai less than about 6 percent of minus 28 mesh fines and more than about 90percent of plL~ 1/4 inch pellets after the tumble test. In some cases, the tumble index has been modified to measure only the plus 1/4 inch pellets present before and remaining after the tumble test and the price paid per long ton of pellets shipped is adjusted accordingly.
Since production at a pelletizing plant is in the millions of tons per year range, a small improvement in tumble index ~quality) of about 2 percentage points, for example, can represent si~nificant additional income ~D ~he plant.
, . . .
.;~' "
; ~
.,, . . ~. . . . ::.. .
'`'` ' ~ 10,9~ , ; It is understood by those skilled in the art that one of the - important factors in improving the quality of the pellets, both in terms of co~pressive strength and tumble index, is to provide for a more efficien~ conversion of magnetite to hematite in the furnace, the ~ v goal being, of course, one where all of the pellets produced are essentially hematite, or, at least, of higher hematite conteTlt.
Oxidation of magnetite to hematite during the pe~letizing process is important not only because hematite is reduced more readily in the blast furnace in spite of its higher oxygen con~ent, but also because in the pelletizing process, conversion of magne~ite to hematite,which is a strongly exothermic reaction, favors grain growth and sintering of the particles of iron ore concentrate to form hard, strong pellets that are abrasion resistant.
Since the reaction rate of magnetite in substantially pure oxygen is manyfold grea~er than that in air, it has been sugges~ed ~hat the combustion gases and air in the furnace be enriched wi*h oxygen; however, the volume of gases circulating in a pelletizing plant is so large that any significant increase in oxygen concentration requires uneconomic amounts of oxygen, i.e., the cost of oxygen needed ~o provide higher numbers of pelle~s of essentially hematite or higher hemati~.c con~ent exceeds the additional income generated by the higher quality pellèts.
Further, it is recognized that a large percentage of the additional oxygen is was~ed, in any case, because it 10ws over pellets, which would be converted to essentially hematite or at least a sufficient hematite content in a conventional operation.
.
-~ Summary of the Invention . I
An object of this invention, ~herefore, is to provide an improvement over conventional pelletizing processes whereby the hem~tite content of ;- the hardened pellets is increased, and the overall ~uality of pellets is thereby i~proved.
,~, .
. ~ , .... .
. . ~ : - . - - ~ . - .
-( ~ 10,9~2 .
~5~
Other objec~s and advan~ages will become apparent hereinafter.
According to the present invention, such an improvement has been discovered in a process for hardening oxidizable green iron ore pellets in a horizontal grate or a circular grate furnace, said process comprising passing the grate with a bed of pellets thereon along a horizon~al path ; through ~he furnace ~herein said pellets ~re hc~tcd b~- ~ont:lct ~irll h~t gases, the furnace haYing a ~one in, or downstream of, the area where the peak temperature of the furnace is at~ainedJ in which zone the average pellet temperature is in the range of about 1100~F to about 2200F and in which zone the ~low of gases is in a downward direction towards the bed of pellets on the grate.
The improvement comprises:
(a) covering the periphery of at least part of the zone ~ith at least two hoods to pro~ide a hooded area on each side of ~he top of the grate under which the periphery of the grate passes;
(b) passing at least one oxygen stream within each hooded area ~:, ; in such a manner that the stream flows in a downward direction towards and through the perlphery of the bed of pellets on the grate passing through the hooded areas;
~c~ passing the periphery of the bed o pellets on the grate ..
through the hooded areas in such a manner that the residence time of such pellets in the hooded areas is at leas~ about 5 seconds.
Descri~tion of the Preferred Embodiment ! The preparation of the green pellets has been referred to above .,ji : ~
- and is conventional. This invention is directed to that part of the pelletizing process whereby green pellets are hardened to the extent re-i~ quired for use in the blast furnace. As al50 noted, the apparatus, i.e.
~.
the horizontal or circular grate furnace, for carrying out the hardening aspect, the composition of ~he green pellets, the basic steps in the hardening process, and ~he combustion gases and air (referred to as gases~
. . , ~i -6-. :~
- ~ ;
- . 10,952 used in the process are conventional and are utilized here t~gether with subject improvement.
The improvement here involves directing a plurality of o~ygen strea at the pellets passing through the periphery of a particular temperature zone under a set of defined conditions. As notcd, ~he ~one is prescn~
in conventional longitudinal grate and circular grate furllace operations, but until now has not been identified other than as part of those sections of the ~urnace where oxidation, ignition, induration, and cooling ta~e place.
Th~ selected zone is that where the average pellet temperature is in the range of about 1100F to about 2200F and preferably about 1300F to about 2000F, and is in, or downstream of, the area where the peak temperatu~e of the furnace is attain~d. Further, the ~one must be sne where the gases in the furnace are flowing downward towar~s the pellets on the grate.
' ' i .' -;
The oxygen stream can be a mix~ure of gases con~aining a major proportion or more than 50 percen~ by volume oxygen. It is preferably a mixture of gases containing at least about 90 or 95 percer.t by volume oxygen, however. The usual oxygen distributed commercia]ly is considered to consist essentially of oxygen and it is expected ~hat this oxyge would be the most easily obtained.
It is found that, by directing oxygen at the pellets in the periphery of the selected temperature ~one under the defined conditions, maximum oxidation can be achieved with minimum oxygen consumption and the temperature of the pellets is raised thereby to provide more efficient ~hermal bonding which additionally raises the overall quality of the - . . .
pellets.
~........................................................................ .
~7-., ., .
10~982 The periphery of ~he zone is tha~ area on both sides of the top o the grate running from the inside surface of each retaining wall horizontally towards the center of the grate. The retaining wall is a par~ of and attached to the grate and is there for the purp~sc of keeping the pellets from falling off the moving grate. A typical retaining wall is about 16 inches high. It forms a right or oblique angle with the horizontal surface of the grate. The grate~ re~aining wall, and pellets all move together through the furnace.' The distance from the inside surface of the retaining wall towards the center of the grate to be included in the peripheral area is selected b~ analysis (in a conventionally run process) of samples of the pellets passing through the zone to determine the location of the bulk of the incompletely oxidized pellets. Although the width of the grates, i.e., measured horizontally across the ~ratc, varies from grate to grate, the distance included in the periphery is about 3 inches to about 24 inches as measured horizontally from the top of the inside surface of the retaining wall towards the center of the grate along a line perpendicular to the line of travel or direction of ,,; .
the grate. The periphery of the zone may also be referred to as the - periphery of tha gra~e or the periphery of the pellet bed.
The flow of the oxygen stream is in a downward direction towards the bed of pellets on the hori70ntal gratc. ~'here thc retainin~ wall is oblique, the direction of the flow is the same even though the pellets against the retaining wall are not on ~he horizontal. The stream first strikes the top of the pellet bed and then passes down through the bed and through the grate, the oxygen declining in amount as it reacts wi~h the oxidizable materials in the pellet.
Usually, the introduction or injection of the oxygen stream into the hooded area is made in the desired downward direction, but it can be introduced into the hooded area in any direction, e.g. in a horizontal . : .
direc~ion from the sides of ~he hood, and dispersed ~ithin the hood, which will serve to direct the flow in the down~ard direction.
:
:,~
.
~' 10,9~
-One or m~re hoods are provided to co~er the periphery on eachside of the grate. The hoods are made of conventional materials which will withstand furnace temperatures. Refractories are usually used.
The oxygen stream i5 introduced'under the hood in sueh a manner'that downward flow referred to above is accomplished whether directly or indirectly. The introduction under the hood can be through an open pipe, a capped and perforated pipe, or through a series cf jets which re located to follow the path of the pellets. The hood serves to obstruct the flow of furnace gases over tha~ area of the periphery which it serves and ~inimizes dilution of the oxygen streams within the hooded area.
It will be understood by those skilled in ~he art that the ~er~s "hood" or "hooded area" contemplate the utili~ation of enclosures, canopies, closed end tulmels, tents, compartments, or any shielding device which permits the oxy~en strean~ to contact thc pellets without being diluted to any great extent while permitting the unreacted oxygen to join the main stream of furnace gases. The width of the hood~ i.e., that part measured from the top of the inside surface of the retaining wall horizontall~ ~o~ards the center of the grate, is 1~ 20 sufficient ~o cover the periphery sf ~he zone as described above. The length of the hood, i.e., that dimension measured along a linc runnillg parallel to the movement of the grate is sufficient to provide the required residence time for the incompletely oxidized pellets in the hooded area, the residence time being at least about 5 seconds and ,,, preferably at least about 10 seconds. It will be understood that ~he entire length of the zone within the periphery does not have to be sub-jected to oxygen treatment but only a sufficient length to insure that the residence time condition for the peripheral pellets is met. There is no upper limit for residence ti~e except the bounds of practicality, i.e., when complete oxidation has been acl-ieved, although an up~cr limit of about 30 seconds is preferred.
_ g ~
10,982 `
:
Typically, the grate moves at about 50 to about 250 inches per minute and the flow of oxygen is kept cons~ant. Therefore, the length of the hood is adjusted to provide the required residence time based on the speed of ~he grate, e.g.,in order to provide a contact time of 15 seconds and assuming ~he grate is moving at a ra~e of 230 inches per minute, ~he internal length of the hood may be readily calculated as follows:
. . .
internal length of hood = 200 lnches mi x 15 seconds minute 60 seconds - 50 inches The amoun~ of oxygen supplied to the periphery of the ~one is " usually sufficient to convert essentially all of the magnetite in the periphery of the zone to hematite as determined on a theoretical basis. The same analysis as mentioned above for the determination of ~, .
the periph~ry can, of course, be used to determine this amount. It is preferred that about 0.30 moles to about 2 moles of oxygen be used for each mole of magnetite passing through the periphery of the .. ..
`r'' de~ined zone. The higher the quali~y of the pellet product desired, ~he higher the amount o oxygen which may be used9 ho\~ever. In any ~ case, the quality will be upgraded.
i.
.
,", .
, - , . .:
"' :
,.~
".;, , ,. ~ :
:: ' ''~ 10 ~.
Claims (7)
- I claim:
In a process for hardening oxidizable green iron ore pellets in a horizontal grate or a circular grate furnace, said process comprising passing the grate with a bed of pellets thereon along a horizontal path through the furnace wherein said pellets are heated by contact with hot gases, the furnace having a zone in, or downstream of, the area where the peak temperature of the furnace is attained, in which zone the average pellet temperature is in the range of about 1100°F to about 2200°F and in which zone the flow of gases is in a downward direction towards the bed of pellets on the grate the improvement comprising:
(a) covering the periphery of at least part of the zone with at least two hoods to provide a hooded area on each side of the top of the grate under which the periphery of the grate passes;
(b) passing at least one oxygen stream within each hooded area in such a manner that the stream flows in a downward direction towards and through the periphery of the bed of pellets on the grate passing through the hooded areas;
(c) passing the periphery of the bed of pellets on the grate through the hooded areas in such a manner that the residence time of such pellets in the hooded areas is at least about 5 seconds. - 2. The process defined in claim 1 wherein the average pellet temperature in the zone is in the range of about 1300°F to about 2000°F.
- 3. The process defined in claim 2 wherein the residence time is at least about 10 seconds.
- 4. The process defined in claim 3 wherein the amount of oxygen used is in excess of that theoretically required to convert any magnetite in the periphery to hematite.
- 5. The process defined in claim 1 wherein the streams consist essentially of oxygen.
- 6. The process defined in claim 3 wherein the streams consist essentially of oxygen.
- 7. The process defined in claim 4 wherein the streams consist essentially of oxygen.
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| US71213776A | 1976-08-06 | 1976-08-06 | |
| US712,137 | 1976-08-06 |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1095255A true CA1095255A (en) | 1981-02-10 |
Family
ID=24860897
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA284,069A Expired CA1095255A (en) | 1976-08-06 | 1977-08-04 | Process for upgrading iron ore pellets |
Country Status (13)
| Country | Link |
|---|---|
| JP (1) | JPS5319120A (en) |
| AU (1) | AU505875B2 (en) |
| BE (1) | BE857556A (en) |
| BR (1) | BR7705179A (en) |
| CA (1) | CA1095255A (en) |
| DE (1) | DE2735370A1 (en) |
| ES (1) | ES461385A1 (en) |
| FI (1) | FI772371A (en) |
| FR (1) | FR2360677A1 (en) |
| NL (1) | NL7708715A (en) |
| NO (1) | NO772759L (en) |
| SE (1) | SE7708933L (en) |
| ZA (1) | ZA774460B (en) |
Families Citing this family (2)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| AU558199B2 (en) * | 1982-09-16 | 1987-01-22 | Ishihara Sangyo Kaisha Ltd. | Production of magnetic powder |
| US4774012A (en) * | 1986-01-30 | 1988-09-27 | Ishihara Sangyo Kaisha, Ltd. | Cobalt-containing ferromagnetic iron oxide powder and process for producing the same |
-
1977
- 1977-07-22 ZA ZA00774460A patent/ZA774460B/en unknown
- 1977-08-04 CA CA284,069A patent/CA1095255A/en not_active Expired
- 1977-08-05 JP JP9347277A patent/JPS5319120A/en active Pending
- 1977-08-05 ES ES461385A patent/ES461385A1/en not_active Expired
- 1977-08-05 FR FR7724259A patent/FR2360677A1/en not_active Withdrawn
- 1977-08-05 BR BR7705179A patent/BR7705179A/en unknown
- 1977-08-05 NL NL7708715A patent/NL7708715A/en not_active Application Discontinuation
- 1977-08-05 AU AU27665/77A patent/AU505875B2/en not_active Expired
- 1977-08-05 FI FI772371A patent/FI772371A/fi not_active Application Discontinuation
- 1977-08-05 DE DE19772735370 patent/DE2735370A1/en active Pending
- 1977-08-05 NO NO772759A patent/NO772759L/en unknown
- 1977-08-05 SE SE7708933A patent/SE7708933L/en not_active Application Discontinuation
- 1977-08-05 BE BE179969A patent/BE857556A/en unknown
Also Published As
| Publication number | Publication date |
|---|---|
| ES461385A1 (en) | 1978-05-16 |
| NL7708715A (en) | 1978-02-08 |
| AU2766577A (en) | 1979-02-08 |
| JPS5319120A (en) | 1978-02-22 |
| BR7705179A (en) | 1978-05-30 |
| FI772371A (en) | 1978-02-07 |
| ZA774460B (en) | 1978-06-28 |
| FR2360677A1 (en) | 1978-03-03 |
| DE2735370A1 (en) | 1978-02-09 |
| NO772759L (en) | 1978-02-07 |
| SE7708933L (en) | 1978-02-07 |
| BE857556A (en) | 1978-02-06 |
| AU505875B2 (en) | 1979-12-06 |
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