US2806777A - Crust-bearing iron oxide agglomerate - Google Patents

Description

Sept' 17, 1957 J. H. VEALE ETAL 2,806,777

CRUST-BEARING IRON OXIDE AGGLOMERATE Filed Oct. 22, 1954 N07' CONTACTE 0R DOL M/TE,

I N VEN T ORS.

AREA Co/v TAcrE WITH LIME Unite? arent @nice entier-BEARING moN oxmn Acetonmnarn John H. Veale, Coal City, and Howard il'. West, .oiie-t, ill., assignors to Illinois Clay Products Company, Joliet, lli., a corporation of illinois Application ctoher 22, 1954, Serial No. 464,922

6 Claims. (Cl. 754-5) This invention relates to a crust-bearing iron ore agglomerate by which is meant a pellet, nodule or briquette of an iron oxide such as hematite, magnetite or taconite, which has a hardened crust joined internally along an irregular surface to the true composition of the agglomerate. The elements of the crust are principally those of the agglomerate, but the compounds in the crust are different from those of the agglomerate, giving to the crust greater density and toughness.

The invention is to be read in connection with the disclosure of a method of making these agglomerates in copending application Serial No. 456,584, filed September 16, 1954. The product disclosed here can be made by any of the methods described in that application as well as other methods.

The object of this invention is to provide an iron oxide agglomerate of either regular or irregular shape which has a crust of substantial thickness and greater density than the main body of the agglomerate. This crust must be much tougher than the surface of an agglomerate which lacks it. It must be of a somewhat similar chemical composition and preferably one that will be of assistance in the blast furnace or open hearth. It must not contain elements which are troublesome in the blast furnace and open hearth furnace.

The need for a crust-bearing iron ore agglomerate resides in the fact that the iron oxide agglomerates must be transported over long distances, and while quite hard pellets or nodules of these ores have been formed by sintering, pelletizing and nodulizing processes, the surfaces are not suiiiciently hard to prevent small pieces from spelling ot during handling. This is particularly true of nodules and pellets which have an uneven surface with projecting points of material which can readily be knocked off. These pellets and nodules during handling usually do not break into two or three parts. Rather, they grind oft small particles from their surfaces, losing on the average ten percent of their weight in the course of a movement from the Mesabi Range to Pittsburgh. These fines are smaller than a pea and are not separated from the pellets prior to charging a furnace. They are readily blown out of the blast furnace into the dust catcher. During recent years, the pressures of the blast in a blast furnace have been steadily increased in order to shorten the time of reduction, and the blast can blow increasingly large particles into the dust catcher.

The product is described in connection with drawings, wherein:

Figures l. and 2 are a perspective and a cross section respectively, of an end of a taconite brick treated With seven percent of lime;

Figures 3 and 4 are a perspective View and a sectional View respectively of the end of a taconite brick treated with three percent lime;

Figure 5 is anenlargement of Figure 3;

Figure 6 is a full scale view of a nodule; and,

Figure 7 is a full scale View of a pellet.

Applicants will give a few examples of the product first. All of the bricks used in the first four examples are identical, that is, they were made from the same agglomerate batch. The batch consisted of 98% taconite of a 20D-mesh size thoroughly mixed with a binder such as bentonite clay in the amount of 2%, and then heated to a temperature in the neighborhood of 1500 F. to form a bond which holds the brick together. For the first four examples, these bricks were allowed to cool down before using in the examples, but this step of `cooling is immaterial. As the copending application shows, the initial binding of the materials of the agglomerate is carried to a temperature of around 2300" F. and then the crust-forming step is performed with the use of calcium carbonate (limestone), calcium oxide (lime) or calcium-magnesium carbonate (dolomite).

Example l Dry hydrated lime (Ca(OH)2) is mixed with =a small quantity of water to produce a thick slurry which is then buttered over the end of the brick 10 to a thickness such that the gross amount of the lime, if applied to all surfaces of the brick, would be 7% by Weight of the total taconite brick. The coating is then allowed to dry. The brick is then placed in a furnace and its temperature raised to a point in excess of 2300 F. When the calcium oxide-iron oxide eutectic of about 2192" F. is reached, there is a contraction of the materials on the surface of the brick to form calcium ferrites which are denser than the iron oxides of the taconite, i. e., occupy less space. The surface l2, in Figure l, was smooth when the process began. After the fusion, the surface is deeply pitted with valleys and recesses as much as one-eighth to three-sixteenth inch deep. The ridges such as 14 are exceedingly hard, much harder than the inside of the brick. The elfect of the fusion of the lime and the ironoxides appears to have been felt as much as one-eighth to three-sixteenth of an inch inwardly of the surface of the brick as shown by the numeral 16 in Figure 2. The surface has a lava-like appearance with considerable sheen indicating a glass-like condition.

Example 2 To a portion 18 of the top of a clean taconite brick is buttered a thick lime slurry to .a thickness such that if the brick were buttered all over, the total amount of lime would be approximately 3% `of the total Weight of the brick. The slurry is permitted to dry, it being an object of the applicants to avoid water playing any part in the reaction. The brick is then heated as before to a temperature in excess of 2300 F. and when the lime iron oxide eutectic is reached, there is formed a lime ferrite crust as illustrated in Figures 3 and 4. All of the Figures 1 4 are to full scale. Where the 3% by weight of lime is used, the contraction into valleys is much less pronounced. Rather, the entire surface seems to contract more evenly toward the center of the agglomerate. As

shown in Figure 4, the surface recedes not to exceed 3/32 of' an inch, `and the degree of penetration 19 does not appear to be much more than 1/32 of an inch.

Example 3 A taconite brick has a layer of dehydrated lime laid across it in the same quantity as that applied as a slurry in Example 2. The result is identical.

Example 4 A taconite brick is heated to a temperature in excess of 2300 Rand with a spray gun lime of a mesh size of 20G-300 mesh is sprayed onto the surface of the hot brick in an amountv comparable to that applied by the other methods in Example 2 or 3. Upon cooling, the appearance of the brick was not perceptibly different from that shownk in Figures 3 and 4, thus indicatingl ascesi/7"? that the fact that the lime is impinged upon the surface instead of just laid upon the surface, or lightly contacted therewith, matters little. ln all cases, there is a contraction of the surface as the calcium Vferrites. are formed.

The next three examples are all of the commercial product, namely, the three forms called nodules, pellets and cinders. All three of these agglomerates are composed of about 70%V iron oxide in the form of taconite. Applicants examples utilize in each'instance the commercial Output of ore beneciating plants presently in operation in the Mesabi.

Example A taconite nodule is formed in a rotary kiln, which has a refractory lining as described in the copending application. To the pulverized taconiteY ore, lime is added. A typical charge is the one used for the nodules which the applicants subsequently experimented with. This particular charge contained 2.44% by weight of lime. There was approximately 7.33% by weight of free silica (SiOzlpresent. This charge is deposited in the feed end of the kiln and as it advances along the long rotating tunnel, its temperature is raised to about 2400 F. The tumbling and the rise in the temperature cause the formation of calcium silicates by a solid state reaction which occurs at slightly above 1500 F. This reaction is not to be confused with a fusion, for the eutectic of lime and silicon is comparatively high, namely, 27007 F. In order for this solid state reaction to occur, the silicon must be in the free silica`form and this happens to be true of most of the silicon in taconite. The amount of lime present is insuicient-to combine with all of the Silica in the charge, and once the silicate is formed, it Will not decompose to form a calcium ferrite with iron when the temperature of the eutectic of calcium oxide and iron oxide is attained. 0n leaving the kiln, the nodule is cooled and it is believed that there is no calcium ferrite present, at least in an appreciable amount. And this is true 'even though the temperature in the Vnodulizing process is carried above the eutecticV of calcium oxide and iron oxide. The nodule has a grayish appearance and when broken appears to be constant in its chemical composition throughout. is shown to full size in Figure 6.

Applicants dipped several of these taconite nodules in a slurry of water and lime until upon drying, the weight of the lime to the gross Weight of the Vnodule varied from 1 to 4%. The additional lime was put on the nodule by means of dipping and drying until the coating was thicker for some nodules than for others. These nodules and two or three which had not been dipped in the lime slurry, were placed in a furnace and the temperature raised to a point above the eutectic of iron oxide and calcium oxide, at about 2400 F. The temperature was held at this point for about an hour, and then the nodules were withdrawn from the furnace and slowly cooled.

The applicants then broke the nodule with a hammer for the purpose of comparing their hardness. The nodules which had not been placed in the furnace, that is, were in the same condition as when they left the beneflciating process at the Mesabi, broke most easily. The nodules which hadV been reheated and slowly cooled were more dilicult to break, and this indicated that the reheating and slow cooling relieved the nodule of strains and hence made it more didcult to break. The color of the reheated nodules was much the same as those from the Range. The nodules which were lime coated were hardest to break. An examination of the section of a nodule showed a cross section similar to those of FiguresV A typical nodule over that of the brick. In the case of the nodule having about 4% by weight of lime on its surface, an examination showed that apparently the lime had by capillary action entered to the heart of the nodule and formed what is believed to be calcium ferrite bonds throughout. Bearing in mind that Example 1 shows a lime proportion of 7% on the brick, it is evident that the nodule is much more porous than the pressed brick used in Example l. Applicants have not shown cross sections in the drawings of the nodules because line drawings of nodule or pellet sections lack instructive definition.

By rubbing a tile on the surface of the wholly untreated taconite nodule, a slight reddish hue appears. In the case of the reheated nodule, this reddish hue can still be obtained. However, the nodule that has been lime treated does not develop any appreciable amount of reddish color on the surface by ling, Since the reddish hue indicates the Vpresence of iron `oxide in the FezOs form, it is apparent that the color test shows that either the iron oxide has remained in the magnetite form (FesOi), or calcium ferrite has the same color as magnetite.Y Applicants believe the latter is true.

Example 6 Taconite pellets such as that shown in Figure 7 are formed by the process disclosed in the copending application. These pellets are burned at a temperature of about 2400 F. `and usually are of a size slightlyy smaller than that shown in Figure 7. The pellets are agglomerated by means of bentonite or some other clay. Ready for shipment, they have a slightly reddish hue. Rubbing them with a sharp surface such as a file brings out the hue more clearly. The pellets surface is much smoother than the surface of the nodule, Figures 6 and 7 correctly indicating this difference in smoothness. However, the nodule is considerably harder than the pellet. Moreover, when the pellet is broken with a hammer, a substantial amount of nes from the interior of the pellet is almost always produced; whereas, when the nodule is broken with a hammer, it is much more likely to break into three or four large pieces with only a small amount o lines.

Applicants treated several of the pellets in thesame way that they treated the nodules in Example 5, and then placed the lime coated pellets in the furnace with some uncoated pellets. The firing was carried out in the same way, the temperature being brought up to 2400 F. where it remained for perhaps an hour.

Upon removal and cooling, the color of both the lime coated and the uncoated pellets had become darker but the uncoated pellets still displayed a slightly reddish hue which could be brought out by abrasive action on the surface.k The lime coated pellet was considerably harder, or a fairly uniform gray, the layers as shown in Figures 2 and 4 not being pronounced. There was considerable sheen to the material.

Example 7 Taconite sinters were treated with lime in the same way as the nodules and pellets in Examples 5 and 6. A sinter is not shown in the drawings because it has no typical shape. In this process at the Range,'the pulverized taconite is mixed with fine coke and burned, the sinter forming at a temperature below 2400 F. and inasmuch as the constituents are moving along a tray, generally in a horizontal path, the size and the shape of the sinters is not uniform. The product is composed of particles ranging from the size of a pellet to agglomerates larger than nodules and the shapes ,are very irregular. However, the sinters have approximately the same percentage of magnetite and show the same general characteristics of thepellets and the nodules under the influence of the lime coating.

From the foregoing examples, it is evident that the amount of lime or dolomite that is to be used depends upon the degree of hardness that one wishes to attain. The degree of hardness again depends upon the amount of ferrite formed, firstly on the surface, and secondly beneath the surface. The porosity of the agglomerate determines how much lime can be combined with the iron oxide. As indicated by the examples of the bricks, Where the agglomerate has been pressed so that there are few pores, even a heavy coating of lime does not penetrate the surface of the bricks very deeply. In the case of the nodules, however, an hour in the furnace is suicient to permit the lime to thoroughly penetrate the nodule.

Applicants performed several experiments with bricks, nodules and pellets, using dolomitic limestone instead of straight lime. Here the calcium oxide and magnesium oxide combined with the iron to form a calcium-magnesium ferrite. Magnesium oxide alone does not produce the crust at the temperatures here used.

The foregoing examples all relate to agglomerating taconite, that is, the rock bearing iron oxide in the magnetite form (FesO-i). However, it is unlikely that the calcium ferrite or the calcium-magnesium ferrite form by utilizing the FesOt iron oxide. Rather, it is always formed with the hematite form (FezOs). The use of limestone, or lime, or dolomitic limestone would, therefore, seem useful in encrusting hematite ines which are increasingly the output of the Mesabi Range and which will probably be the exclusive output of Venezuela. These nes have all of the disadvantages in the blast furnace and open hearth heretofore ascribed to the pulverized taconite. Experiment shows that applicants step of encrusting agglomerates functions well for hematite lines.

Example 8 To hematite fines of the Mesabi area containing substantial amounts of silicates was added 1-3% clay with water, and mixed and pressed into brick. After drying, the surface was coated with enough lime to form about a 3% by weight lime coating. The brick was then heated to the eutectic of lime and iron oxide, and thereafter cooled. 'Ihe eutectic of hematite (Fe2O3) and lime (CaO) is the same as the eutectic of magnetite (FesOt) and lime. The fusion of the surface of the hematite brick, therefore, occurs at the same temperature. However, it probably occurs more rapidly, hence the glassy surface appears quicker and the ability of the lime to further penetrate into the brick is reduced.

Applicants have found that bricks, pellets or nodules made of hematite and treated with lime in accordance with the first ve examples will all come out with a calcium ferrite crust comparable to that obtained in those examples. The general principle is that up to about 7% or 8% of the gross weight of the pellet, the addition of lime will increase the fusion and shrinkage on the surface, making the pellet more dense because of the greater density of the crust. Applicants iind that the use of more than 8% lime is unnecessary. The most desirable amount is not entirely clear. This depends upon a great many factors, particularly the surface area of the nodule, and the roughness of the surface contour may enormously increase the surface area.

In the foregoing examples, the amount of calcium oxide has always been stated in terms of weight of the agglomerate. This was the only practical measuring value which the applicants had for irregularly shaped objects. The surface area is what is important, and inasmuch as for a given weight, the surface weight increases as the mesh size gets smaller, the amount of lime added will have to be increased accordingly. It can be said at this point that ordinarily the amount of lime or limestone put in a blast furnace with iron oxide is about equal in weight to the iron oxides. It follows that the amount of lime added to the pellets or briquettes is not excessive.

It should be noted that in the original blast furnace practice, the charge consisted of one-third coke, one-third ore, and one-third limestone. In this charge, the amount of iron oxide, i. e., the iron oxide of hematite ore, was only about 40% by Weight of the ore. This means that the ratio of limestone to impurities in the ore was about three to two. When applicants added 7% lime by weight to taconite ore, which is 70% iron oxide, they were providing a lime ratio for fluxing the impurities of the ore of about one to three, or one to four, which means that the amount of limestone that must -be added in the blast furnace is greatly reduced.

ln place of lime, applicants may use limestone which with heat converts to lime. Importantly, as disclosed in the copending application, dolomite in a similar mesh size may be substituted for lime in all instances. Where substituted, the crust is composed of calcium-magnesium ferrites.

Having thus described their invention, what applicants claim is:

1. An iron ore agglomerate element comprising a core consisting essentially of iron oxide and coated with a eutectic crust consisting essentially of a substance selected from the group consisting of calcium ferrite, magnesium ferrite and mixtures thereof.

2. An iron ore agglomerate element comprising a core consisting essentially of iron oxide and coated with a eutectic crust consisting essentially of calcium ferrite.

3. An iron ore agglomerate element comprising a core consisting essentially of iron oxide and coated with a eutectic crust consisting essentially of calcium and magnesium ferrites.

4. An iron ore agglomerate element comprising a core consisting essentially of iron oxide and coated with a eutectic crust consisting essentially of a substance selected from the group consisting of calcium ferrite, magnesium ferrite and mixtures thereof, said ferrite crust being present in an amount ybetween 2% and 7% of the total weight of the agglomerate.

5. A taconite agglomerate consisting essentially of a core consisting essentially of magnetite and silicate coated with a eutectic crust consisting essentially of a substance selected from the group consisting of calcium ferrite, magnesium ferrite and mixtures thereof.

6. A taconite nodule consisting essentially of magnetite particles bonded together with calcium silicate to form an agglomerate, the surface of said agglomerate being covered with a fused eutectic layer of a composition consisting essentially of a substance selected from the group consisting of calcium ferrite, magnesium ferrite and mixtures thereof, said fused layer being present in an amount between 2% and 7% of the total weight of the nodule.

References Cited in the file of this patent UNITED STATES PATENTS OTHER REFERENCES A. S. M. Review of Current Metal Literature, January 1949, page 23.

Claims (1)

1. AN IRON ORE AGGLOMERATE ELEMENT COMPRISING A CORE CONSISTING ESSENTIALLY OF IRON OXIDE COATED WITH A EUTECTIC CRUST CONSISTING ESSENTIALLY OF A SUBSTANCE SELECTED FROM THE GROUP CONSISTING OF CALCUM FERRITE, MAGNESIUM FERRITE AND MIXTURE THEREOF.

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