CA1095256A - Process for upgrading iron ore pellets - Google Patents

Abstract

10,983 PROCESS FOR UPGRADING IRON ORE PELLETS

ABSTRACT OF THE DISCLOSURE

In a process for hardening oxidizable green iron ore pellets in a grate-kiln furnace, said process comprising passing the grate with a bed of pellets thereon along a horizontal path through the furnace to dry and preheat the pellets and then introducing the pellets into the kiln to undergo additional heating and induration, all by contact with hot gases, said grate passing through a preheating zone in which the average pellet temperature in the bed 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 improving 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; and (c) passing the periphery of the bed of pellets on the grate through the hoods areas in such a manner that the residence time of such pellets in the hooded areas is at least about 5 seconds.

S P E C I F I C A T I O N

Description

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~ Field of the Invent_on This invention relates to a process for upgrading iron : ore pellets in a grate-kiln furnace as the pellets undergo ~ `~
hardening. ;

:` Descri~on of the Prior Art The pelletizing of iron ore concentrates for : use as charge material in blast furnaces has beeen ~- gaining in importance in the steel industryl This is the result of an attempt to meet the increased demand :~
for iron and steel with lower quality ores and ores extracted from beneficiation plants, all o which are .~ :
j , -. usually in the form of finely 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 an~ gas-solid contact in the blast furnace in order to increase the "~ ~:
rate of reduction. A secondary consideration is to ~;
- reduce the a~ount o ines blown out of the blast urnace into the gas recovery system.
:- 20 Characteristics of industrially acceptable :~ pellets are those that are strong enough to withstand ~.
degradation during stockpiling, handling, and transportation and have the capabili~y to withstand the high temperature and deyra~a-~ion forces within the blast ;`~
furnace without slumping or decrepitating.
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Typical pelletizing processes comprise forming 3/8 inch to 1 inch diameter balls of iron ore concentrate of reasonable mois~ure content in a rota~ing drum or on a rota~in~ disc and then firing the "green"

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10,g83 :
balls or pellets in a furnace to a sufficiently high temperature to harden the pellets to a strength suitable for use in blast furnaces. The green pellets oE
interest here are those which contain an oxidizable material~ usually magnetite ~Fe~4~ Other oxidizable materials are iron and ~olid fuel such as coke, coal, or charcoal~ which is some~imes added to the balling mix in a finely divided ~ta~e in order to provide additional heat to the pellets during the 10 hardening operation. The iron ore concentra~e with which we are particularly concerned here contains at ~ least about 30 percent magnetite some iron or other - iron compounds such as hematite; and a small amount of impurities such as silica, alumina and magnesia. One of ~hese concentrates is known as beneficiated taconite.
Binders are often added before or during the drum or :, :
~isc rotation to increase wet strength of the green pellets to acceptable levels for subsequent handling.
~; One of the basic ~ypes oE furnaces used 20 commercially for hardening green pellets is the grate-kiln furnace. In this Eurnace, typically, green pellets are first fed onto a chain grate where they are dried and preheated by a two-pass system using the gases from the rotary kiln. The prehea~ed pellets then drop : into the kiln where they undergo induration and are brouyht up to the final pelletizing temperature of about S 2400F before being discharged into a cooler. Air, which is used to recover the sensible heat from the hot ~ pellets, is then used as secondary air in tlle rotary ; 3~ kiln. The secondary air temperature is raised to about ::
.

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,~. ' 2400F to 2500F by use of a burner located at the discharge end of the rotary kiln. In the preheating area o~ the furnase, there is a zone in which the average pellet temperature is in the range of about 1100F to about 2200F, which zone is of interest ilere ~ and has not heretofore been deline3ted~
'- Strong bondi~g in the hardened pellets produced in the grate kiln furnace is bel~eved to be due to grain growth from ~he accompanying oxidation of magne~ite to ~ ~ hermatite and to recrystallization of ~ne hematite. The "~ exothermic oxidation reaction typically supplies about 300,000 Btu's (British thermal units) per long ton of pellets.
~- Hardened pellet strength is usually determined by compression and tumbler tests. Although specifi-cations for pellets vary dependirlg on their source and ~-, the purchaser, the minimum suggested compressive strength for individual pellets ranges from about 300 ~, ....
~'-, pounds for 1~4 inch pellets to about 800 ~o about 1500 pounds for 1 inch pellets. In the tuml~ler test, 25 pounds cf plus 1/4 inch pellets are turnbled for 200 revolutions at 24 ~ 1 rpm (revolutions per minute~ in a ,~
,' drum tumbler and then screened. Satisfactory commercial pellets generally contain less than about 6 percent of ~, minus ~8 mesh ~ines and more than 90 percent of plus 1/4 ~ . . .
;, in,ch 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 . .
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10,983 pelletizing plant is in the milllons of tons per yearrange, a small improvement in tumble index ~quality ) of about 2 percentage points9 for example, can represent significant additional income to the plant.
-~ It is understood by Shose skilled in the art that one of the important factors in irnproving the quali~y OL the pellets~ both in terms o compre~sive - strength and tumble ind~x, is to provide for a more .:
efficient conversion of magnetite to hematite in the 10 furnace, the goal being, of course, one where all of the pellets produced are essentially hematite, or, at least, of higher hematite content.
~ Oxidation of magnetite to hematite during the - pelletizing process is important not only because hematite is reduced more readily in the blast furnace in spite of its higher oxygen content, but also because in the pelletizing process, conversion of magnetite to hematite which is a strongly exothermic reaction~ favors grain growth and sintering of the particles of iron ore 20 concentrate to form hard, strong pellets that are abrasion resistant.
Since the reaction rate of magnetite in subs~antially pure oxygen is manyfold grea~er than that in aic, it has been suggestP~ ~h~t the combustion gases and air in the furnace be enriched with oxygen; howevert ;~
the volume oÇ gases circulating in a pelletizing plant ~`;
i3 s~ large that any significant increase in oxygen concentration requires uneconomic amounts of oxygen, i~e., the cost of oxygen needed ~o provide higher ;~
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30 numbers of pellets of essentially hemati~e or higher -~

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L0,983 hematite content exceeds the additional income generated by the higher quality pellets~ Further, it is recognized that a large percentage of the additional oxygen is wasted, in any case, because it flows over pellets, which would be converted to essentially hematite or at least a sufficient hemati~e conten~ in a , .
,~ conventional operation.

~ An object of this invention, therefore, is to :~
. ~ ..
~ provide an improvement over conventional pelletizing ~ .
processes whereby the hematite content o~ the hardened pellets is increased and the overall quality of pellets thereby improved.
~:~ Other objects and advantages will become appaeent hereinater.
According to the present invention, such an : improvement has been discovered in a process for hardening oxidizable green iron ore pellets in a grate-kiln furnace, said process comprisin~ passing the ~ grate with a bed of pellets thereon along a horizontal path through the furnace to dry and preheat the pellets and then intr~ducin~ the pellets into the kiln to undergo additional heating and indurationO all by contact with hot ~ases, sai~ grate passing through a : preheating zone in which the average pellet temperature in the bed is in ~he range of abou~ 110~F to about 2200F and in which zone ~he flow of gases is in a downward direction towards the bed of pellets on the ~ -grate. ~:

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'., ` ~a~z~ 10,983 : The improvement comprises:
(a) covering the periphery of at least one part of the zone with at least two hoods to provide a hooded area on each side of ~he top of the grate under which the periphery of the grate passes ~ b) passing a~ 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 10 th~ough the hooded areas; and 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 pellet~ in the hooded areas is at least about 5 seconds.
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Description of the Preferred Embodiment The preparation of the green pellets has been ~ ~-referred to above and is conventional. This invention ~; is directed to that part of the pelleti~ing process whereby green pellets are hardened to the extent ~ "
20 require~ ~or use in the blast furnace. As also noted, the apparatus, i.e., the grate-kiln furnace, for carrying out the hardening aspect, the initial 1 composition of the green pellets, the basic steps in the hardening process, and the combustion gases and air (referred to as gases) used in the process are ~; conventional and are utiLi~ed here together with subject ; ;' ~ '~' :~
improvements.
The improve~ent here involv2s directing a plurity of oxygeln streams at the pellets passing throuyh .
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,983 the periphery o a particular temperature zone under a .. ~ .
set of defined conditions. ~s noted, the zone is present in conventional grate-kiln furnance oper~tions~
but until now has not been identified other than as part -~of a section of the furnace where preheating takes placeO
~ he selec~ed zone is that wh~re the average pellet temperatLIre is in the range o about 1100F to about 2200F and preferably about 1300~ to about 2000F, Further~ the zone must be one where the gases ,i .
10 ~n the furnace are flowing downward towards the pellets on the grate.
The oxyyen stream can be a mixture of gases containing a major proportion or more than 50 percent by volume oxygen. It is preEerably a mixture of gases containing at least about 90 or 95 percent by volume oxygen, however. ~he usual oxygen distributed commercially is considered to consist essentially of .~ .
:~' oxygen and it is expected that this o~ygen woul~ be the ; most easily obtained.
It is found that, by directing o~ygen at the pellets in the periphery of the selected temperature zone 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 thermal bonding which additionally raises the overall quality of the pellets.
The periphery of the zone is that area on both sides of the top of the grate running from the inside surface of each retaining wall horizon~ally towards the 30 center o~ the grate~ The retaining wall is a part of , , .
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lO,9B3 and attached to the grate and is there for the purpose of keeping the pellets from falliny off the moving ~`
grate. ~ typical retaining wall is about 8 inches high. It forms a right or oblique angle with the horizontal surface of the grate, ~he grate, retaining wall, and pellets all move together through the furnace. The distance from the inside surface of the retaininy wall towards the center of the grate to be included in the peripheral area is selected by analysis 10 (in a conventially run process) o 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.eO, measured horizontally across the yratel varies Erom grate to ~rate, the distance included in the periphery is about 3 inches to about 24 inches as measured horizontally ~rom the top of the inside surface of the retaining wall towards the center of the grate along a line perpendicular to the line of ~ravel or direction of 7~ the grate. The periphery of the zone may also be referred to as gra~e or the periphery o~ the pellet bed.
The flow o~ the oxygen stream i5 in a downward direction towarcls the bed of pellets on the horizontal grate. ~here the retaining wall is oblique, the direction of the flow ~s the same even though the pe~lets against the retaining wall are no~ on the horizontal. The stream first strikes the top of the pellet bed and then passes down through the bed and through the grat~, ~he oxygen declining in a~ount as it 3a reacts with the oxidizable materials in the pellet.

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Usually, the introduction or injection of the oxygen stream into the hooded area is made in the desired downward direction, but it c~n be introduced into the hooded area i~ any direction, e.g~, in a horizontal direc~ion from the sides of the hood, and dispersed within the hood, which will serve to direct ~he flow in the downward direction.
One or more hoods are provided to cover the periphery on each side of the grate. The hoods are made 10 of conventional material~ which will withstand furnace temperatures. Refractories are usually used. The oxygen stream is introduced under the hood in such a manner that the downward flow referred to above is accomplished whethec directly or indirectly. The introduction under the hood can be through an open pipe, a capped and perForated pipe, or through a series of jets which are located to foLlow the path of the pellets. The hood serves to obstruct the flow of furnace gases over that area of the periphery which it 20 serves and minimizes dilution of the oxyyen streams within the hooded area.
It will be understood by those skilled in the art that the term "hood" or "hooded area" contemplate the utilization of enclosures, canopies, closed end tunnels, tents, compaLtments ~r any shielding device which permits ~he oxygen s~ream to contact the pellets without being diluted to any grea~ extent while permitting the unreacted o~ygen to join the main stream of furnace gases. The wid~h of the hood, i.e9 ~ that 30 part measured from the top of ~he inside surface of the :: .;,~, , , , ,- , ;

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10,983 retaining wall horizontally towards the center of the grate i5 sufficient to cover the periphery of the zone as des ribed above. The length of the hood, i.e.~ tha~
dimension measured along a line running parallel to the movement of the gra~e is sufficien~ to provide ~he required residence time for the incsmpletely oxldized pellets in the hooded area, the residence time being at least about 5 seconds and preferably at least about lO
secondsr It will be understood ~hat the entire length 10 of the ~one within the periphery does not have to be subjected to oxygen treat~ent 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 ~ime except the bounds oE practicality, i.e~, when complete oxidation has been achieved, although an upper limit oE about 30 seconds is preferred.
Typically, the grate moves at abouk 250 inches per minute and the Elow of oxygen is kept constant.
Therefore, the len~th oE the hood is adjusted to provide 20 the required residence time based on the speed of the grate~ eOg., in order to provide a residence time of 15 seconds and assuming the grate is moving at a rate of 200 inches per minute, the internal length of the hood ~`
may be readily calculated as follows: -internal length of hood ~ 20i0nln hes x 6~-aee~ x 15 seco - 5~ inches ~ :

The most desirable Location for the hood is near the hot end of the grate section before the pellets are tumbled into the rotary kiln. An additional ~ 10~983 advantage of subject process is that oxidationy which usually occurs in the cooler of a conventionally operated grate-kiln, is esential1y avoided thus maintaining proper oper~tion and efficiency of the cooler.
The amount of oxygen supplied to the periphery of the zone is usually suficient to convert essentially all of the magnetite in the periphery of khe zone to .: :
hematite as determined on a theoretical basis. The same 1~ analysis as mentioned above for the determination of flow rate can, of course, be used to determine this amount, It is preferred that about 0.3 mole to about 2 moles of oxygen be used for each mole of magnetite passing through the periphery of the defined zone. The higher the quality oE the pellet product desired, the higher the amoun~ of oxygen ~hich may be used, however~
In any case, the quality will be upgradedO

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Claims (7)

10,983 I claim:

1. In a process for hardening oxidizable green iron ore pellets in a grate-kiln furnace, said process comprising passing the grate with a bed of pellets thereon along a horizontal path through the furnace to dry and preheat the pellets and then introducing the pellets into the kiln to undergo additional heating and induration, all by contact with hot gases, said grate passing through a preheating zone in which the average pellet temperature in the bed 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; and (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 10,983 pellets in the zone have an average temperature 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.