US2789895A - Method of agglomerating fine iron ores - Google Patents

Description

April 23, 1957 P. D. NORA, JR 2,789,895

METHOD OF AGGLOMERATING FINE IRON ORES Filed May 19, 1954 3 Sheets-Sheet l INVENTOR. PETER D. NORA,JR

d /KM ATTORNEY April 23, 1957 P. D. NORA, JR 2,789,895

METHOD OF AGGLOMERATING FINE IRON ORES Filed May 19, 1954 s Sheets-Sheet 2 INVENTOR. PETER D. NORA, JR

ATTRNEY April 23, 1957 Filed May 19, 1954 ACCUMULATIVE PER CENT 5 Sheets-Sheet 3 IOO CONVENTIONAL HARD SINTER 90 /"7 PELLET SINTER 60 CONVENTIONAL SOFT SINTER V4 No.4 No.lO No.20 No, 40 No.60 PAN U. S. SCREEN SIZE INVENTOR.

ATTORNEY United States Patent 2,789,895 7 METHOD OF AGGLOMERATING FINE inoN ORES Peter D. Nora, In, Negaunee, Mich assignor to Jone's & Laughlin Steel Corporation, PittsbnrghyPas, a cerporation of Pennsylvania Application May 19, 1954, Serial N o. 430;793

6 Claims. (Cl. 75-''-'5)' proposed and tried for processing such finely divided ores,

fine dust, and the like into discrete particles or masses of larger size. These include 'sintering, briquetting noddlizin'gypielleti'zing, and other agglomerating processesef which only sintering is widely employed. The'sin'ter'ing' process consists of mixing ore'properly 'rnoi'stene'd with' a small amount of fuel such as coke breeze, coal 'firi'e's, and

the like; spreading themixture in a uniformlayer'over a grate, igniting it, generally at the top, and ca'using corn- 35 bustion of the fuel to proceed through the bed by draw mg air through the charge. The heat developed partially fuses or agglomerates the charge into a sint'ercak'e. which breaks up into lumps of substantial size.

The conventional sintering process makes it ,pds'siblek-to a'gglomerate into a satisfactory blastfurnace feed ores mu'ch finer'than those which can be successfully sinelted directly'iin the blast furnace. However, very finely Eivided ores, "such as result from the fine grinding necessary to liberate so-called martite and 'ta'conite ores from ganguefparticlesare not well adap'ted'to agglomeration by conventional sinte'ring. This 'is because "the 'sintering proces's'requires that air be drawn 'throughthe unsintered charge and very fine ores tend to form a cl-oselyp'acked,

bed having low air permeability.

It is an object, therefore, of my invention\toprovide-a process by which such finely divided iron ores can be sinte'red. It is another object of my invention to provide a si'nter having a novel structure. Further objeets will plana'tion of my invention.

As I have mentioned, my invention is particularly aplicable to finely divided iron ore concentrates. Ascreen analysis or such an ore concentrate to whichmy invention 60 is applicable is given in Table I.

Table I.''Scr'eenanalysis Tyler screen size:

200 mesh Table 1 indicates that more than'threeequar tersof concentrate was minus 400 mesh size. The

Table II;

Percent weight-retained.

in pal.. chemical constituents of this concentrate are-"set "out "in Table II.Chemical analysis Percent F6 \-.-V-. v i 0 H 27.25 11 9 60.69' .s'io 5.44 A1203 3.53

it is .known to agglomerate ores and ore concentrates such asthat described above into more or lessspherical particles called pellets. This may be done by introducing the ore and a measured amount of water into a drum rotating about an axis inclined slightly to the horizontal. As the material passes through this drum it balls up or forms more or less spherical pellets, the size of whichcan be controlled within a considerable range without too much difliculty. Pellets of suitable size, say one .inch or more -in diameter, can be charged directly into a blast" furnace'if they can be made strong enough to withstand the weight of the charge in the furnace shaft. The wet pellets'coming from the drum are quite plastic and .in this coarser ore particles.

may show "a higher'permeability .to air than a bed of simiappear in the course of the following description and ex- 1 condition will'support very little weight without deforming or disintegrating. The physical strength of such pel-- lets'is considerably increased by drying and .may be fur-- 5 ther increased by heating them 'to elevated temperatures. Such'drying' o'r'h'eatingrequires apparatus especially cons'tr'ucted for such purpose and is a -rathertedious process, however, since rapid heating turns the moisture in the pellets to steam, causing them to disintegrate. l have found that pellets formed as described above can be si'nt'ered in a conventional sintering machine such as'a Dwight-Lloyd or a Greenawalt machine, and that the product so formed has a novel structure. .I have found that pellet disintegration can be .prevented and ,thatmoistrue-containing pellets can be successfully sintered the properties of the moist pellet are controlledand the bed of pellets on the rate of the sintering machine is covered with alayer of'unagglomerated iron ore in amanner to be described.

It is known to add ,a certain proportionof me as pol-- let's, granules or briquettes, to a sinte'r mix in the same manner as return sinter fines arev added With the object of improving the permeability to air of the bed of sinter mix. Usually the ore belowsome selected screen size is agglomerated and added in this way to the of A bed of sinte'r mix .so formed lar'lo're c'onv'entionallyv prepared. Thisincrease in bed permeability disappears, however, as soon as .sintering begins. My invention diifers markedly .fromlsuch known ,pr'a'ctices in that the great bulk of the sinteringmachine chai ge is inthe form of pellets, and these arenot mixed with unagglomerated ore.

The fir'st step in my process is the mixing with the finely divided iron ore of a .small proportion .of carbonaceous ,fuel. Thisfuel may be of any type conventionally used in sintering and should be added in amount less than that sullicient to sinter the ore in the conventional manner. .In

conventional sintering of iron ores, fuel equal .to about 6% to 8% of the charge weight isadded to the ore. I ,have'found that fuel in amounts between 2% and 4% ofthe weight of the oreiis 'sufiicient for my pellets, and thatthisiamoun't, although too low to ignite by itself, will provide enough heat to sustain sintering if sintering is commenced by the ignition .of an upperlay'er of ore richer .infue'l. If "fuel much beyond 4% is added, the pellets willifu's'eto such an extent'that the .pas'sage'o'f airthrough the"fhatgeishindered. It is desirable thatthe fuel be ground to are'aso'nable fineness so that it'will mix unifor 1y with'thefinel'y divid'ed ironore. Minuse'S mesh I coal-1 satisfactory.

The rriirtture'ofore and fuel isth en fonned in'to'pellts in" Patented Apr. 23, 1957 was highly permeable to air.

a horizontally rotating drum in the manner known to the art. Suificient water must be introduced into the drum, usually in the form of a spray, to enable the ore to ball satisfactorily. The amount of water necessary ranges from about 10% to about 20% of the weight of the ore. Most ores contain an appreciable amount of water, and those beneficiated by wet processes may contain enough Water to form pellets without any further moistening. I prefer to usepellets averaging about one-quarter inch in diameter and not larger than about three-eighths inch, as pellets much larger than this require an extended time for thorough sintering. The pellets should not be so small that they pack firmly on the grate and reduce the bed permeability to air. I find that pellets which are held on a 6 mesh Tyler screen are about the smallest which can be satisfactorily sintered. The openings in such a screen are .131 inch square.

The moisture-containing pellets direct from the pelletizmg drum may then be spread directly upon the grate of a conventional sintering machine. As these wet pellets do not have great mechanical strength, the depth of the bed must not be so great that the Weight of the upper layers deforms the pellets at the bottom. If this occurs, the pellets at the bottom spread out and form a layer of low air permeability, thus slowing down the sintering operation. For Wet pellets about one-quarter inch size of the concentrate of Table II the pellet bed depth should not exceed about inches, to permit the application of an upper layer as described below. It will be understood, however, that a bed of pellets of this depth weighs as much as a considerably deeper bed of coarse ore because of the compacting of the fine ore in the pelletizing operation.

On top of the pellet bed so formed I then form a layer of a conventional sinter mix of iron ore and fuel. The ore may be the same finely divided ore from which the pellets are formed or it may be a coarser ore. Return fines may also be included in this layer, if desired. The fuel content of this layer should be sufficient to ignite but need not be more. I find that fuel amounting to about 8% by weight of the layer is sufiicient. The size of the fuel incorporated in this layer is not critical. I find that minus one-quarterinch coke is a satisfactory fuel. The weight of this upper layer should not be less than about 5% of the weight of the pellet bed andmay be more, if desired, up to the weight at which bed penneability to air is adversely aifected or the bottommost pellets are deformed. I have had good success with upper layers amounting to one-third the weight of the pellet bed when the upper layer In general, however, the great bulk of the charge should be in the form of pellets. The fuel in the upper layer is then ignited and the charge sintered in the conventional manner.

I findthat a charge prepared as above described can be sintered considerably more rapidly than the same weight of the same type of ore of a particle size range well adapted for conventional sintering. In one installation with which I am familiar a charge prepared in accordance with my invention as above described was sintered in seventeen minutes, whereas a conventional charge of the same weight required thirty-three minutes for sintering. I believe this increased speed of sintering is attributable to the relatively great permeability to air of the pellet bed of my invention. The wet pellets could not have been sintered at all, however, without the upper layer, which appears to transfer heat to the pellets below in the form of combustion gases in such manner that the pellets dry without spalling or disintegrating.

My process is facilitated if the grates of the sintering machine before being charged with my pellets are first covered with a thin layer of coarse ore, return fines, or dried or semi-hardened pellet fragments. For convenience I de nominate such material coarse ore-like particles. The presence of a hearth layer of coarse ore-like particles appreciably increases the permeability to air of the'charge,

arsasae apparently by permitting excess moisture at the bottom of the charge to drain away from the bottommost pellets. If hot return fines from the sintering operation are used in this hearth layer, the bottommost pellets charged on to it are partially dried and hardened, thus increasing the depth of bed they can support.

A significant advantage of my process is the saving in fuel it makes possible. As I have mentioned, conventional sintering requires fuel equal to about 6% to 8% of the charge weight. My process requires this proportion of fuel for the upper layer only, but the pellets forming the remainder of the charge and accounting for perhaps three-quarters or more of its weight require only 2% to 4% of their weight as fuel.

My invention provides sinter of novel structure and properties. These are illustrated in the attached figures to which reference is now made.

Figure 1 illustrates two sections of sinter cake, the sections being the full height of the cake which resulted from the sintering of a 4 /2 inch pellet bed plus a 1% inch upper layer.

Figure 2 shows three random pieces of sinter from a broken sinter cake.

Figure 3 is a photomicrograph at a magnification of 300 diameter of a section through sinter of my invention.

Figure 4 is a similar photomicrograph at the same magnification of conventional hard sinter.

Figure 5 is a graph illustrating the comparative resistance to crushing of the sinter of my invention and conventional hard and soft sinter.

It will be observed that the sinter illustrated in Figures 1 and 2 is formed of a large number of individual pellets fused sufficiently to join together but not so much that their identities have disappeared.

The microstructure of my sinter is shown in Figure 3. In this figure the individual ore grains 1 are clearly visible. Most of these grains 2-2 are in direct contact with one another and are bonded by particle-to-particle fusion. The darker colored constituent 3 visible in may grains is magnetite; the lighter colored constituent 4 is hematite formed by oxidation of magnetite. The gray constituent 5 is slag. The black areas 66 are pits or voids in the specimen.

The microstructure of conventional hard magnetite sinter is shown in Figure 4. Here none of the grains 1 is in direct contact with another, but all these grains, which are magnetite, are bonded by the slag matrix 5. The black areas 6, again, are pits.

Figures 3 and 4 demonstrate that my sinter contains substantially less slag than conventional hard sinter and that nearly all the grains in my sinter are directly bonded together without the intervention of any slag. Because of this desirable structure, my sinter is as easily reduced in the blast furnace as the best conventional soft sinter.

The terms hard and soft as applied to sinter refer primarily to its physical strength, particularly its resistance to crushing. Hard sinter is made by extending the time of sintering, or increasing the fuel content of the sinter mix, or both, so that more complete fusion of the mix' is obtained and a mechanically strong sinter is produced. Such sinter does not easily crush under the load of the burden in a blast furnace, and a substantial hardness of this type is desired by blast furnace operators.v It is recognized, however, that sinter hardness is conventionally attained at the expense of reducibility. Soft sinter is less completely fused than hard sinter and is mechanically weaker, but is more easily reduced in the blast furnace.

Figure 5 illustrates graphically the results of crush tests performed on conventional hard sinter, conventional soft sinter, and the sinter of my invention. These tests were performed in the manner described in the paperv Laboratory studies on iron ore sintering and testing, by F. M. Hamilton and H. F. Ameen, published in the T ransactions of the American Institute of Mining and Metal lurgical Engineers, Volume 187, December 1950, Mining Engineering, pages 1275-1282. The sinter to be tested is placed in a hollow shell in which is inserted a plunger or anvil. A weight of 150 pounds is dropped on this anvil from a height of 53 /2 inches. The material is then removed and screen analyzed. The particle size distribution so attained is plotted on the graph of Figure 5. The graph for the pellet sinter of my invention is seen to fall intermediate those for conventional hard and soft sinter, showing that my sinter, though not as hard as conventional hard sinter, is much harder than soft sinter. As has been mentioned, however, my sinter is as easily reduced in the blast furnace as conventional soft sinter.

I claim:

1. The method of agglomerating finely divided iron ore comprising forming a moist mixture of said ore and finely divided carbonaceous fuel in amount insufficient to sinter said ore, forming said mixture into pellets of an average diameter not exceeding about three-eighths of an inch, spreading said moisture-containing pellets in a bed of a depth less than that depth which causes crushing of the moist bottom pellets, spreading on said pellet bed an upper layer of iron ore mixed with carbonaceous fuel in amount suflicient to ignite and sinter said upper layer,

igniting said upper layer and sintering said pellet bed from the top down.

2. The method of claim 1 in which the weight of the upper layer is not less than about 5% of the weight of the pellet bed.

3. The method of claim 1 in which the fuel content of the pellets is between about 2% to about 4% of their weight.

4. The method of claim 1 larger than six mesh.

5. The method of claim 1 in which the fuel content of the upper layer does not exceed about 8% of its weight.

6. The method of claim 1 in which the moisture-containing pellets are spread on a. hearth layer of coarse orelike particles.

in which the pellets are References Cited in the file of this patent UNITED STATES PATENTS 2,582,386 Komarek et al Jan. 15, 1952 2,608,481 Royster Aug. 26, 1952 2,631,941 Cole Mar. 17, 1953 2,672,412 Burrow Mar. 16, 1954

Claims (1)

1. THE METHOD OF AGGLOMERATING FINEKY DIVIDED IRON ORE COMPRISING FORMING A MOIST MIXTURE OF SAID ORE AND FINELY DIVIDED CARBONACEOUS FUEL IN AMOUT INSUFFICIENT TO SINTER SAID ORE, FORMING SAID MIXTURE INTO PELLETS OF AN AVERAGE DIAMETER NOT EXCEEDING ABOUT THREE-EIGHTS OF AN INCH, SPREADING SAID MOISTURE-CONTAINING PELLETS IN A BED OF A DEPTH LESS THAN THE DEPTH WHICH CAUSES CRUSHES OF THE MOIST BOTTOM PELLETS, SPREADING ON SAID PELLET BED AN UPPER LAYER OF IRON ORE MIXED WITH CARBONACEOUS FUEL IN AMOUNT SUFFICIENT TO IGNITE AND SINTER SAID UPPER LAYER, IGNITING SAID UPPER LAYER AND SINTERING SAID PELLET BED FROM THE TOP DOWN.

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