US2990268A - Pelletized iron ore concentrate composition and process for making the same - Google Patents

Description

June 27, 1961 F. D. DE VANEY 2,990,268

PELLETIZED IRON ORE CONCENTRATE COMPOSITION AND PROCESS FOR MAKING THE SAME Filed Sept. 4, 1958 2 Sheets-Sheet 2 United States Patent v 2,990,268 PELLETIZED IRON ORE CONCENTRATE COMPO- SITION AND PROCESS FOR MAKING THE SAME FredD. De Vaney, Duluth, Minn., assignor to P-M Associates, Cleveland, Ohio, a partnership Filed Sept. 4,1958, Ser. No. 758,999

2 Claims. (Cl. 75-5) smelting of iron ore additions of potentially basically reacting flux materials have to be made to the burden. In current practice, iron ore is fed to the blast furnace as relatively unsized material, sometimes supplemented by sinter or pellets, and the flux (usually limestone or dolomite) is almost invariably added in the form of coarse material, all plus 2.0-2.5 inches in size. Because of the gross size of the limestone and of the ore, little fluxing obtains until the charge has descended to a considerable depth in the blast furnace.

In current blast furnace practice it is customary to add the flux component to the charge in the form of uncalcined limestone (or, dolomite), and hence itis necessary to calcine the latter, in the course of its descent through the blast furnace, in order to make the same efiective as a reactive flux. Limestone calcines at about 1500" F. and dolomite begins to calcine at about 1300 F. In the case of either of these, calcination of the in.- side of the lump proceeds only as heat can be supplied to the immediate location of the reaction; accordingly, when working with lump stone complete calcination is not realized until after long-continued heating, to a top temperature well in excess of 1500 F. This means that uncalcined stone can get well down into the boshin some instances.

Calcination (of the stone) uses up a substantial amount of the total heat input of the furnace-approximately 700,000-800,000 B.t.u. per ton of iron, or from about 10 to about 15% of the total heat load-and develops undesirably large amounts of carbon dioxide in the furnace gases.

These, and other, disadvantages inherent. in conventional blast furnace practice have long been appreciated, and suggestions have been advanced for obviating them. Thus, it is known to modify the furnace charge by substituting a so-called self-fluxing sinter for a part of the coarse, loose, ore and limestone (or dolomite). According to that proposal, the coarsely crushed ore is mixed either with limestone or dolomite which has been crushed to minus one-fourth inch or somewhat finer or with rice stonei.e., dolomite which has been crushed to minus ice formation at lower temperatures, but such slag tends to carry iron oxidewell down in the furnace thereby increasing the hearth heat load and the amount of direct reduction occurring there. The sinter product,'moreover, has a less than desirable bulk density, viz., a bulk density of about 90 pounds per cubic foot.

According to the present invention, an iron ore concentratee. g., a concentrate from magnetic taconite or flotation concentrates from low-grade hematite ores or a gravity concentrate of non-magnetic oxidic compounds of iron from, e.g., siderite ores or the like-the particles of which are essentially all finer than minus 100 mesh in size and which include a large proportion of very fine material of the order of minus 325 mesh, in which concentrate the silica content is in excess of that which is slaggable by the basic components of the concentrate, is homogeneously mixed (a) with finely divided dolomite (or, limestone), the particles of which are all finer than 28 mesh and preferably much finer, in an amount sufiicient to combine with the excess silica of the concentrate .to yield a slag suitable for the production of pig iron in the blast furnace, together (b) with whatever additives (e.g., bentonite; soda ash; powdered coal) are to beincorporated in the resulting product and with an amount of moisture suificient to make the homogeneous admixture plastic; the resulting moist mixture is formed into pellets (i.e., small balls) in any suitable ballingapparatus (e.g., a balling drum or balling cone or equivalent balling means); and the so-formed pellets areiindurated (heat treated)-in any suitable indurating apparatus such as an up-draft or down-draft sintering machine or any known variation thereof, or any known combination of horizontal grate machine-kiln, but preferably in a vertical shaft-type indurating furnaceat an elevated temperature well above the calcining temperature of the flux component content of the pellets and in the range 20002400 F. to insure their complete induration and the complete calcination of the dolomite stone.

- As will be understood, in the course of being heated one-half inch and then screened to remove all particles finer than one-eighth inch in diameter-the mixture sintered on a conventional (e.g., Dwight-Lloyd) sintering machine, and the sinter cake then broken up into coarse chunks for charging to the blast furnace.

Such so-called self-fluxing sinter has some advantages over charging loose burden. However, the ingredients of the product are quite coarse, are non-uniformly distributed, and are slow to react. The pre-slagged character of conventional sinter probably permits early slag from room temperature (say, 60 F. to 2000" F.) and above in" the induration procedure, the flux component of the pellets undergoes full calcination with release of the CO contents of the carbonates (CaCO and/or MgCO Not only is this true, and not only are the pellets indurated, but also the heat-treatment efiects slag forming reactions between the basic and acidic components of the pellets. The addition of flux, in proper amount, therefore tends to increase the mechanical strength and ruggedness of the indurated pellets by reason of the in situ development of some slag in the pel-- lets.

It is to be noted, in the above connection, that in the heat-treated pellets the eventual flux is by no means fully developed and that for full development-of flux the conditions obtaining in the blast furnace (or, open hearth). are necessary; the carbonates have been fully calcined and a sufiicient amount of flux has been created to sheathe the solid particles bonding them together. This means that there has been some, but not a major amount, of fluxing of the iron ore particles by reason of the induration treatments.

The foregoing procedure provides a product in which the iron oxides and the fluxing components are very intimately and uniformly admixed. This very close proximity of the particles (of iron oxide and fluxing components) renders their fusion easy and rapid higher up in the blast furnace, drastically reduces the time required for the reduction of the iron oxides to metallic iron. Since all of the C0: of the carbonates had been driven 01f during the induration treatment, the lower part of the shaft and upper part of the bosh are freed of a heavy heatloadwhereby more rapid heating of the stock column results-the estimated reduction in the thermal load is approximately 1,200,000 B.t.u. per ton of iron and about 75 pounds of coke are saved. The absence of the previously driven off CO proves to be distinctly beneficial, in that the atmosphere in the area between the 1400 and 2400 F. isotherms of the furnace is more highly reducing than it otherwise would be; reduction of the iron oxides is initiated more quickly; a larger through-put is possible; and much less flue dust is produced as compared with a furnace burdened with merchant ore and standard lump flux components. The high bulk density of the product, viz., about 155 pounds per cubic foot, greatly increases the capacity of the blast furnace by the added amount of iron in the volume being smelted.

The product of the invention is a microscopically very porous indurated pellet characterized by the presence throughout of a slag bond which binds the ore particles and flux particles together. The greater part of the iron oxide particles thereof remain as discrete particles-sur- 4 to make the flux component addition large enough to slag all of the excess silica (or, excess bases) of the concentrate.

The invention will be described in greater particularity, with reference to the following specific examples, illustrative of the application of the principles of the present invention to a concentrate from magnetic taconite in which the iron mineral was mostly magnetite, and with reference to the accompanying drawing in which:

FIGURE 1 is a chart showing, in graph form, temperature gradients obtaining at various levels of a shaft furnace in which the process of the invention is being carried out; and

FIGURE 2 is a photomicrographic showing of the in terior of a pellet produced in accordance with the present invention.

TABLE NO. 1

Screen analysis of raw materials rounded by sheaths of slag acting as hinder: the iron Conch" Dolomite, Furnace oxide particles are not sigmficantly fused. The product RawMaterial-S1ze Percent Percent Feed, Peris characterized, further, in that all of its iron content is in the highest state of oxidation, regardless of its initial oxidation state. That is to say, where the concentrate $23 fig was derived from a magnetic taconite and contained mag- I08 I10 :18 netite as, the sole or chief iron mineral, the iron content a; of the indurated pellets therefrom is all Fe O and a sub- I15 1125 I02 stantial amount (-20%) of the latter is in a magnetic '23 g; {3% form. 111 67 191 73 12100 In the foregoing it has been assumed that in the usual 83'87 6M5 8M5 case the iron ore concentrate will contain an excess of Total. 100.00 100.00 100.00 silica over that content which would be slaggable by the TABLE NO. 2

Chemical analysis of raw materials Material Iron Phos. Silica Mang. Alum. Lime Mag. Sul. 110g Coneentrate-. 04.91 .006 7.98 .26 .1s .31 .19 Dolomite .90 .011 1. a0 .03 .60 29.46 20. 25 .000 40.00 Furnace Feed... 5s 2s 006 7.08 .23 24 4. 25 2. 91 .001 6.33

bases contents of the concentrate, and that, therefore, in the carrying out of the present invention the added flux component will be dolomite or limestone. However, the principles of the invention are equally applicable in a situation where the bases contents of the iron ore concentrate are in excess of the silica content: in such case, the added flux component" is finely divided silica or mineral rich in silica and the amount added is suflicient to slag the excess bases of the ore.

While in the foregoing it has been stated to be preferable to addto the iron ore concentrate-sufiicient base-yielding flux component to combine with all of the excess silica of the concentrate, it is Wholly within the intent and scope of the invention to add a somewhat lesser stoichiometric amount of the base-yielding flux component to the end that a substantial amount of flux is formed in the pellets but the pellets are not fully selffluxing. It is noted, in this connection, that the blast furnace operator conventionally uses 13 pounds CaO plus MgO for every 10 pounds of acidic components (disre- 'garding oxides of iron) of the ore. In a situation where the excess acidic components of an iron ore concentrate would require-for forming a fully self-fluxing p'ellet the addition of about 505 lbs. per long ton dry weight of the concentrate, of dolomite, it is within the concept of the present invention to reduce the dolomite content of the admixture to about 350 lbs./l.t., whereby to attain a substantial approach to true self-fluxing. However, since the fullest advantages of the invention are associated with a fully self-fluxing product, it is preferred In Examples Nos. 1 and 2, the pellet composition was as follows:

In Example No. 3 the pellet composition difiered from the above only in that no powdered coal was added.

To insure uniform mixing of the dolomite (and other additives) with the magnetite concentrate, and to .provide careful control of the moisture content of the balling drum feed, weighed portions of each were fed to -a mix-. ing drum, pulped with water, and then filtered, a drum filter, to provide a homogeneous filter cake containing about 10% by weight of moisture. The filter cake was fed to the balling drum described in US. Patent No. 2,831,210, in which latter the mixture balled readily producing plus 4; inch raw pellets having good average physical properties as shown in Table No. 3 above.

The pellets were fed to the top of a column ofsimilar US Patent No. 2,816,016 and therein were subjected to iilduration treatment.

After a preliminary trial run, :a series of threeiindura? tion experiments were conducted. In the preliminary chamber temperature had to be maintained substantially lower than 2300 F.

Furnace operation aura -Test No.5

[2000 F. temperature (without coal) 1 In experiment No. 1 the combustion chamber tempera- $33 3 rate 59 1 5 x ture was maintained at 2100f F., and operating condi- Oil consumpfim; I 5 39 L/Lt. fired pellets." tion were held constant for a 17-hour run. Air blow. "ff 7'?" I In experiments Nos. 2 and 3, the combustion chamber Mair; shaft 184 cfm temperature was reduced to 2000 F. (as noted above, in Combustion 96 d m: experiment No. 3, the pellets contained no, powdered V coal). Total 280 c.f.m.

,TABLE NO. 4 Temperatures: I Furnace operating dataTest N0. 1 gg z 'g s e i 2000 [2100 F. temperature or heating gas] H 740 F. Feed rate .59 1.t./h1'. 20 G 1960 F. Discharge rate .54 l.t./hr. Bottom stove- Oil consumption 5.93 gal./l.t. fired pellets. 1 E' 1370 F. Air blow: p D 890 F. Main shaft 196 c.f.m. J Pellet discharge 500 F. Combustion chamber 96 c.f.m. Pressures:

Top 0.30" mercury. 292 Middle 0.30" mercury. Temperames; Bottom 0.45" mercury.

chamber Static 0.50" mercury. H 1 0 The temperature data set out in Tables 4; 5 and 6 G 5 above were secured at the following thermocouple posi- Bottom stve tions within the apparatus:

E 0 F. Combustion chamberin inlet end of insulated pas- D 50 sageway between combustion chamber and furnace shaft; Pellet discharge 50 R Hperiphery of charge colnmn'substantially midway Pressures: between top and bottom of upper stove; 4 a Top Q25" mercury. Gperiphery of charge column substantially midway Middle 030" mercury between level H and level of introduction of heating Bottom 135" mercury, 40 gas from combustion chamber; I Static 55" mercury E-periphery of charge column adjacent to butbe- TABLE NO 5 neath level of introduction of heating gas from com bustion chamber; v I T. Furnace operating data-Test N0. 2 D-periphery of charge column substantially id- [2000. F temperature (with coal way between top and-bottom of bottom stove.

Feedrate .59 l.t./hr. TA LE NO. 7 Discharge Tate Ph sical r0 erties 0 red ellets on consumption 5.44 gal./l.t. fired pellets. y p p f fi I Airblowi P d t 2100F F e I Main shaft 210 c.f.m. to no With Coal viiiii ooei %i' i2i 1o1 ;ti Combustion chamber 96 c.f.m.

Total 314 c.f.m. Test No #1 #2 #3 Temperatures:

Combustion chamber 2000 P. Size: Percentwt. Percentwt. Pei-centuri- Top stove 55 i?" g; ig'gg .I-I 1860 F. p10 G 21000 2:78 5131 21in Bottom stove- 1.10 I 2.05 4.48

100.00 100.00 100.00 Pellet 558315; ifiii iiiifilii lfiit: iii: 32 33 3% Pressures; Specific Gravity 3. 94 3. 74 3. 63

Top 0.25 mercury.

TABLE NO. 8

Chemical analysis of fired pellets Assay Percent Test No. Product Iron Ferrous Ferric Phos. Sil. Mang. Lime Mag- Iron Iron V nesia 2,100 1. With 0010..-.--" 58.16 .55 57.51 .000 7. e5 .23 4Ia0 2.5'2' 2,000F.Wlth C0al-.-- 58.16 .54 57.52 .000 7.08 .23 4.37. 2.51 2,000" F. Without O0al.... 58.14 .66 57.4s .005 7.66 .23 4.28 2. 55'

In tests Nos; 1, 2 and 3, the readings of the traveling thermocouple, at the core of the furnace, were as shown in FIGURE 1 of the accompanying drawing, which shows in graph form the temperature gradients obtaining in these tests. It is noted in this connection that the instruments'used were not capable of registering temperatures above 2500 =F..

In spite of the high furnace core temperatures, no difiiculty with fused pellets was eneountered in tests 1-3 inelusive. a I I MINERALOGICAL STUDY or FIRED PELLE'I'S Macroscopically, the pellets when broken open appeared to be a completely fused slag mass. However, polished cross sections through the pellets, when studied under the microscope, presented a very different picture. The individual iron ore particles could be readily distinguished. In the case of the pellets fired at lower temperature, a combination of voids and glassy slag separated hematite particles on which the cusps had "been blunted, and some of the dolomite residue could be distinguished. In the samples indurated at 2100 F., with added coal, many of the smaller grains were seen to have migrated together through what must have been a fairly fluid slag, and apparently through the action of surface tensionto have been drawn together into spherical shapes, looking like clustersof grapes in a slag matrix. Slag appeared to be the principal factor governing bonding in all cases.

FIG. 2 is a photomicrographic showing, at 350 diameters, of interior of a pellet, with coal added, fired at a combustion chamber temperature of 2000 F. In connection with the data set forth in Table No. 7, supra, it should be noted that the specific gravity or apparent density of the self-fiuxing pellets (i.e., from 3.94 to 3.63, depending upon temperature of firing and upon amount of coal used in the pellets) is substantially higher than is the apparent density-3.50 to 3.35, depending upon amount of coal usedof pellets formed from the same ore concentrate and treated in the same manner but without the llux addition. This appreciable gain in density is due to the shrinking efiect that takes place when suchpellets are fired. This is undoubtedly due to the fact that there is a great deal more of the slag phase present under the flux-added condition which permits greater mobility of the particle and permits such shrinking. This added specific gravity is of great benefit in the blast furnace as it contributes to the amount of iron present in the charge, and it has a separate advantage form'aterial used in the open hearth since its added specific gravity permits such pellets to sink through the slag layer much easier.

That the particle size of the added flux component has a significant bearing on the quality of the indurated -self-fluxing" pellets containing it is demonstrated by the findings of the following specific example.

The starting material was an iron ore (Hilton) concentrate, analyzing approximately 1.88% SiO 0.004% P 0.50% CaO-l-MgO The screen analysis of the concentrate was substantially all minus 28 mesh with about 80 weight percent finer than 350 mesh. Because of its low content of acidic 1.6 lbs. soda ash and 12.0 lbs. bentonite,

the particle size of the limestone being varied as'follows:

Test #l allminus 14 mesh,

8 Test #2--a1l minus 28 mesh, Test #3all minus 65 mesh, and Test #4--all minus 100 mesh.

In each instance the resulting homogeneous admixture was formed into small balls or pellets and the resulting pellets were indurated-in the manner described above at a top temperature of about 235 0 F. In each instance the dry compression strength of the unfired pellets and the compression strength of the indurated (fired) pellets were determined by standard procedure.

Pellets formed of the same mixture but containing no added limestone (or other base-yielding fluxing component) were found to have a dry compression strength (unflred) of 35 pounds, and a fired compression strength of about 1800 pounds.

The strength data of Tests 1-4, incl., were as follows:

Dry (Unfired) Fired Pounds There was a marked increase in quality as the maximum particle size of the added limestone was decreased. An examination of the fired pellets showed that those in which the coarser sizes of limestone had been used contained voids due to calcination of the relatively large pieces of limestone, whereas at sizes finer than 65 mesh no voids could be detected and the internal structure of the pellets was quite homogeneous. Accordingly, the addition of basic flux component as coarse as, or coarser than, 28 mesh, maximum, is harmful to the physical properties of the unfired pellets and also of the fired pellets notwithstanding the probability that the self-fluxing properties of the pellets containing the coarser limestone had not been seriously impaired. In sum, the flux component added to the fine magnetite concentrate, before pelletizing, should be as fine as minus 28 mesh or finer, because such fine material permits thorough mixing of the constituents and yields a more nearly homogeneous product.

As is stated hereinbefore, the thermal treatment leading to the product of this invention is not confined to the use of a shaft-type furnace, since said treatment can be effected equally well using known indurating apparatus of the traveling grate type which may employ either the updraft or the downdraft principle, provided only that the above-described conditions are observed.

I claim:

I. In the process of pelletizing iron ore concentrates, the particles of which are essentially all finer than minus 100 mesh and include a large proportion of material finer than 325 mesh, in which concentrate the iron oxides are associated with gan-gue material including acidic components in excess of the combined proportion of its basic components, according to which pelletizing process a moist concentrate is homogeneously admixed with finely divided solid carbonaceous fuel and bentonite and the mixture is formed into pellets and the pellets are indurated at an elevated temperature in excess of 1832 F., the improvement which consists in homogeneously admixing with the moist concentrate, prior to balling, a

finely divided alkaline earth metal carbonate rock the particles of which are all minus 28 mesh, the amount of such added carbonate rock being such as to yield, upon calcination thereof, a slag-forming basic reagent in an amount suificient to slag all of said excess acidic components, and heat-treating the resulting discrete pellets by contactinga mass of the discrete pellets with a stream of free oxygen-containing heating gas introduced into the pellets mass at a temperature of from about 2000 to 9 about 2100 F., the heat-treatment being continued for a period suflicient to efiect (a) the complete calcination of the carbonate rock and (b) substantially complete combination of the calcined material with all of the acidic components of the pellets thereby substantially completely pre-slagging the gangue constituents of the pellets to the production of mechanically strong and rugged self-fluxing iron oxide concentrate pellets.

2. As a new product, a charge material for the blast furnace and for the open hearth, consisting essentially of discrete indurated pellets, each pellet consisting essentially of finely divided iron ore particles in a matrix of slag containing the nonferrous constituents of the iron ore in slagged form, the iron content of the pellets being in the highest state of oxidation and a substantial proportion '10 of said iron content being in magnetic form, there being present in said slag matrix a content of basic slag-forming component amounting to from about 100% to about 130% of the stoichiometric equivalent of the content of acidic slag-forming component present therein, said pellets being mechanically strong, rugged and self-fluxing.

References Cited in the file of this patent UNITED STATES PATENTS 1,847,596 Cavers et a1. Mar. 1, 1932 2,696,432 Davis Dec. 7, 1954 2,805,141 Apuli Sept. 3, 1957 FOREIGN PATENTS 642,339 Great Britain Aug. 30, 1950

Claims (1)

1. IN THE PROCESS OF PELLETIZING IRON CORE CONCENTRATES, THE PARTICLES OF WHICH ARE ESSENTIALLY ALL FINER THAN MINUS 100 MESH AND INCLUDE A LARGE PROPORTION OF MATERIAL FINER THAN 325 MESH, IN WHICH CONCENTRATE THE IRON OXIDES ARE ASSOCIATED WITH GANGUE MATERIAL INCLUDING ACIDIC COMPONENTS IN EXCESS OF THE COMBINED PROPORTION OF ITS BASIC COMPONENTS, ACCORDING TO WHICH PELLETIZING PROCESS A MOIST CONCENTRATE IS HOMOGENEOUSLY ADMIXED WITH FINELY DIVIDED SOLID CARBONACEOUS FUEL AND BENTONITE AND THE MIXTURE IS FORMED INTO PELLETS AND THE PELLETS ARE INDURATED AT AN ELEVATED TEMPERATURE IN EXCESS OF 1832*F., THE IMPROVEMENT WHICH CONSISTS IN HOMOGENEOUSLY ADMIXING WITH THE MOIST CONCENTRATE, PRIOR TO BALLING, A FINELY DIVIDED ALKALINE EARTH METAL CARBONATE ROCK THE PARTICLES OF WHICH ARE ALL MINUS 28 MESH, THE AMOUNT OF SUCH ADDED CARBONATE ROCK BEING SUCH AS TO YIELD, UPON CALCINATION THEREOF, A SLAG-FORMING BASIC REAGENT IN AN AMOUNT SUFFICIENT TO SLAG ALL OF SAID EXCESS ACIDIC COMPONENTS, AND HEAT-TREATING THE RESULTING DISCRETE PELLETS BY CONTACTING A MASS OF THE DISCRETE PELLETS WITH A STREAM OF FREE OXYGEN-CONTAINING HEATING GAS INTRODUCED INTO THE PELLETS MASS AT A TEMPERATURE OF FROM ABOUT 2000* TO ABOUT 2100*F., THE HEAT-TREATMENT BEING CONTINUED FOR A PERIOD SUFFICIENT TO EFFECT (A) THE COMPLETE CALCINATION OF THE CARBONATE ROCK AND (B) SUBSTANTIALLY COMPLETE COMBINATION OF THE CALCINED MATERIAL WITH ALL OF THE ACIDIC COMPONENTS OF THE PELLETS THEREBY SUBSTANTIALLY COMPLETELY PRE-SLAGGING THE GANGUE CONSTITUENTS OF THE PELLETS TO THE PRODUCTION OF MECHINICALLY STRONG AND RUGGED SELF-FLUXING IRON OXIDE CONCENTRATE PELLETS.

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