US3420656A - Process for forming hard oxide pellets and product thereof - Google Patents

Description

Jan. 7, 1969 v, ME ET AL PROCESS FOR FORMING HARD OXIDE PELLET S AND PRODUCT THEREOF Sheet Filed Sept. 2, 1966 Pelletizer lNVENTORS Valentine Mekler Morgan C. Sze

Ward J. Bloomer William V. Bauer BY 773mm, 2W

Fig. l.

ATTORNEYS Jan. 7, 1969 PROCESS FOR FORMING HARD OXIDE PELLETS AND PRODUCT THEREOF Filed Sept. 2, 1966 Sheet 3 of 3 9 re 8 7 ea c e 400 Hea ter m Pelletizer 400 Pellet 750 Heater Ggs sml Muffle 5 Gasolme J Coker I000 Distillate A J Muffle 6 o a clnar Calciner I600 A Coker Gas 7 r Heavy Oil |soo J Reducer i 2ooo L 2 2z ,J Air 7 Water Steam Flue Gas 2000 Boiler ||5 Air Heater INVENTORS Valentine lgekler Morgan 0. ze Flg. 4. War d .1. Bloomer Wilham V. Bauer BY 77Za/z/n &

ATTORNEYS United States Patent 3,420,656 PROCESS FOR FORMING HARD OXIDE PELLETS AND PRODUCT THEREOF Valentine Mekler, New York, and Morgan C. Sze, Garden City, N.Y., Ward J. Bloomer, Westfield, N.J., and William V. Bauer, New York, N.Y., assignors to The Lummus Company, New York, N.Y., a corporation of Delaware Filed Sept. 2, 1966, Ser. No. 577,055 .U-S. Cl. 75.5

Int. Cl. C21b 1/10 20 Claims ABSTRACT OF THE DISCLOSURE This invention relates generally to a process for agglomerating, hardening and partially reducing an oxide ore prior to its introduction into a metallurgical furnace. More specifically, the invention relates to the treatment of iron oxide ores, such as taconite fines, by agglomeration at 400500 F. using a heavy oil residue as binder, coking at 700l000 F., and hardening and reducing at 1500-2200 F., preferably below about 2100 F. The invention also relates to the pellets themselves, which are reduced about 1540%, with some metallic iron and with little or no free carbon. Partial reduction per se is not the main objective of the invention, but it does provide the crushing strength necessary for blast furnace usage, and of course the reduction raises blast furnace efliciency. I

As is well known in the art, pig iron is produced from iron ore, coke and limestone introduced into a blast furnace. At the point of entry of the air into the blast furnace, high temperatures are attained as a result of the exothermic heat of combustion of coke with air. The carbon monoxide formed by the incomplete combustion of coke and unburned coke reduce the iron oxide to iron. Impurities present in the ore and coke form a slag with the limestone. Molten pig iron and slag are periodically removed from the hearth of the blast furnace.

Since the blast furnace is a shaft furnace, it is necessary that the ore, coke and limestone be aggregates of fairly large particle size. The amount of fines in the charge materials should be minimized since fines can be blown out of the furnace as dust and can also plug up the shaft, thereby resulting in a high pressure drop. A number of factors are of importance in regard to the materials charged to the furnace. For example, since metallurgical coke is relatively expensive, it is obvious that the amount required per ton of product should be minimized. Furthermore, lower coke requirements reduce the amount of ash necessary to be slagged out, andthereby reduce the amount of limestone required to .be introduced into the blast furnace, consequently resulting in higher furnace capacity and lower production cost per ton of product.

Recently, taconite ore-a low-grade iron oxide ore has been increasingly mined as a result of decreasing supply of high grade ores. Benefication of taconite usually involves grinding the ore to form very small ice particles of fines, followed by magnetic separation of iron values (Fe O The fines are agglomerated into pellets prior to charging the ore into a blast furnace. Agglomeration of the ore generally consists of pelletizing or briquetting the fines with the assistance of a binding agent, and then heating the resulting material to a high sintering temperature in the order of 2200 2400 F., to increase the strength of the pellets. In commercial pelletizing plants, bentonite clay and some solid carbonaceous fuel are added to the concentrate. The mixture is then passed through a balling drum (where the wet bentonite-concentrate mixture is made into balls of, for example, A to size); a vibrating feeder and a sintering machine wherein, at a temperature of approximately 2350 F., hardened pellets are produced. The pellets are thereafter cooled, screened and stored for shipment.

Briquetting differs from pelletizing only in that the binder-ore mixture is formed into shape by pressing rather than by balling, as for example in briquetting rolls, or by extrusion.

Several disadvantages characterize the present practice of pelletizing. High sintering temperatures are required to provide hardness and high strength to the pellets consequently using substantial quantities of fuel. Such pellets are not large aggregates and therefore high blast rates in the furnace cannot be used. This is especially so, since the size of the pellets is much smaller than the size of the coke and limestone used. The optimum size range of the pellets should be near the size range of the limestone and coke. Additionally, the use of an ash-containing carbonaceous fuel to supply the heat for agglomeration increases the amount of slag which has to be produced. Since the finished pellets are not reduced at all, they cannot effect any saving in metallurgical coke requirements. Highly reduced pellets are produced by various processes, all of which use solid carbonaceous fuels for reduction and for heat. The pellets have varying amounts of residual carbon. The ash content is inversely related to the qualityand costof the solid fuel.

Numerous procedures have been proposed to overcome disadvantages of the character indicated above. For example, it has been proposed to use a tar oil or a petroleum oil with an iron oxide concentrate, rather than a solid carbonaceous fuel, but strong, reduced pellets were not produced. It has also been proposed to use an asphalt with an oxide ore; however, even when temperatures of the order of 1500-l600 F. are used in forming a carbon-impregnated oxide ore aggregate, little physical strength is developed. A further proposal of prior workers was to initially mix the ore with tar at the mine site to render it non-dusting. At the steel mill, it Was mixed with powdered coal and this mixture was briquetted and carbonized. Of course the coal addition brings back the ash problem noted above. Accordingly, while a number of such proposals have been made, there has been little or no success in providing oxide ore pellets having sufi'icient hardness to withstand the rigor of mechanical handling and thermal shock before and after being introduced to an agglomerating furnace. Nor has there been success in reducing the fuel requirements in preparing such agglomerates, or the ash content of the pellets.

It is an object of the present invention, therefor, to provide a process for forming oxide ore in pellet form of desired strength and hardness, of suitable size for efiicient use in a metallurgical furnace, and of low ash content.

It is another object of the invention to form such pellets at relatively low temperatures and at an economical level of fuel consumption.

A further object is to provide partially reduced iron oxide ore in pellet form of said desired characteristics, whereby blast furnace coke requirements are reduced.

Another object of the invention is to provide iron oxide ore in pellet form containing low residual free carbon.

Still another object of the invention is to provide a process for forming oxide ore pellets utilizing inexpensive carbonaceous materials.

Yet another object of the invention is to provide a hard, partially reduced iron oxide pellet having the abovenoted characteristics.

Various other objects and advantages of the invention will become clear from the following description of several embodiments thereof, and the novel features will be particularly pointed out in connection with the appended claims. The description will refer to the accompanying drawings, which are illustrative only and should not be interpreted in a limiting sense, and in which:

FIGURE 1 is a greatly simplified, pictorial flow sheet depicting the essential elements of the process as carried out in illustrative apparatus;

FIGURE 2 is a greatly simplified, schematic flow sheet depicting the solids flow in a second embodiment of the invention;

FIGURE 3 is a great-1y simplified, schematic flow sheet depicting the solids flow and liquid recovery in an embodiment of the invention designed to product 3000 t.p.d. of product pellets; and

FIGURE 4 is a simplified, schematic flow sheet depicting gas and liquid flows for the embodiment illustrated in FIGURE 3.

While the description made herein of the invention is primarily directed to the treatment of iron oxide ores, it is to be understood that other oxides ores can be treated.

The foregoing objects are realized by a process comprising the following steps:

(a) Mixing the fiine ore with a fluid carbonaceous material and agglomerating the mixture into pellets, all

at a temperature of about 400-500 F. These two steps are preferably carried out continuously in a suitable pelletizing machine so that the resulting pellets show a uniform cross-section of the ore and binder.

(b) Heating the green pellets to a temperature in the range of 7001000 F. to coke the heavy hydrocarbon binder, driving off volatile components and resulting in a pellet having coke evenly dispersed therein. Heating at this stage should be indirect to avoid combustion of the volatiles or dilution thereof with the heating gases.

Further heating the pellet to a temperature in the range of 1500 2200 F., during which heating the pellet is hardened and partially reduced.

Optionally, limestone or the like may be added during mixing, particularly if slag-formers are present in the ore, or if either the ore or the hydrocarbon contain sulfur or sulfur compounds.

The last heating should of course be carried out in a non-oxidizing atmosphere. Subsequent cooling of the pellets should also be conducted in the absence of air or oxidizing gases, until the pellets reach a temperature where reoxidation is no problem (below about 600 F.).

A variety of hydrocarbon materials may be employed in the process of the invention. Any heavy residual oil, vacuum residual oil, coal tar pitch or the like may generally be employed; it is desirable that the material have a Conradson Carbon content of at least 15% and preferably 20%, and have a 5% boiling point of at least 650 F. The term 5% boiling point means that 95% of the residual oil boils above the specified temperature. It is further noted that a coal tar pitch would produce more coke and less distillate than the oils discussed hereinbelow and in the examples.

The pellets produced by the process will vary in composition depending on the amount of carbonaceous material added and the length of time at the Various temperatures. It is preferred, however, to produce a pellet reduced about 1540% (25-35% preferred), in terms of total combined oxygen, in which there is some metallization and little or no free carbon. Such a pellet is characterized by a very high crushing strength and a fine porous structure which is eminently suitable for use in a blast furnace or other reduction device. In the case of pellets made from taconite or other high-grade concentrates, both the coke rate and fluxing required for reduction in the blast furnace are significant-1y reduced. The amount of metallic iron will not generally exceed about 5% when the pellets are intended for blast furnace usage, but it will be understood that by adjustment of oil addition and residence time, a sponge iron product can be produced. Preferably, the reduction is carried out so that the pellet, as discharged from the reducer, contains about or more FeO, and, as noted above, little or no free carbon and some metallic iron. If cooling is very slow, it is possible that the FeO will disproportionate into Fe and Fe O at least according to some prior workers. Under near equilibrium conditions, a metallic iron content of 20% may be achieved. However, under normal (nonoxidizing) gas cooling, which is quite rapid, the FeO structure is substantially retained. For this reason the pellet structure is characterized as it is prior to cooling or after rapid cooling. Regardless of cooling rate, pellets of the preferred composition have an oxygen-to-iron (weight) ratio in the range of about 0.15 to 0.34.

In FIGURE 1 there is illustrated a very simple embodiment of the invention illustrating the essential steps thereof. Residual oil preheated so as to be flowable and ore from a bin are supplied to a rotating pelletizing drum. The oil and ore may be separately preheated to the preferred pelletizing temperature before they are charged, or the pelletizing drum may be provided with heating means. Oil is sprayed continuously on the ore, which agglomerates during its passage through the pelletizer. Any vapors evolved during this stage are combined with coker distillate for recovery of valuable constituents.

The pellets discharging from the pelletizing kiln pass through an intermediate hopper and are charged into the coking kiln, which is an indirectly fired travelling grate kiln. In this kiln, the pellets are heated to about 700- 1000 F. and vapors are driven off. The oil vapors are then passed to a recovery section from an outlet at one end. A seal hopper passes the coked pellets to the hardening and reducing kiln, which in this instance is a directfired, travelling grate kiln, where the pellets are heated to 1500-2000 F. or higher. Removal of combustion gases is via a line in the charge end which leads to appropriate solids removal and heat recovery equipment (not shown). A travelling grate kiln is preferred for this service as the 1000 F. pellets discharged from the coker will probably not be hard enough to withstand handling in a rotary kiln.

The hardened pellets pass from the reducing kiln through another seal hopper to a pellet cooler, where their sensible heat is given up with the production of high-pressure steam. This pellet cooler is provided with inlets and outlets on the tube side for steam generation, inlets and outlets for supplying an inert or non-oxidizing atmosphere to the pellets, and an outlet for discharging cool pellets 600 F.) to conveyor. The high pressure steam generated in this cooler may be conveniently utilized to preheat the oil prior to injection into the pelletizing kiln or to drive any rotating equipment.

In FIGURE 2 there is illustrated "a more detailed embodiment of the invention. As noted above, the formation of green pellets is preferably carried out in a pelletizing drum and in this embodiment the ore and oil are preheated (not shown) and the drum itself isnot heated. The fines, which are generally at least 70% minus 325 mesh, are conveyed to the pelletizing drum having been mixed with recycle dust and, if desired, limestone prior to preheating. A preheated heavy oil is supplied to the drums in thegsame manner as illustrated in FIGURE 1. As shown in FIGURE 2, recycled solids come from a product screen and a green pellet screen, but it .will be understood that solids will also be collected by cyclones orother equipment from various gas flows. Where appropriate, recycled material should be suitably crushed and ground before charging to the pelletizing drum.

The oil injection and pelletizing operation is carried out at 400 to 500 F. by preheating in order to secure a viscosity of the'residue oil suitably low to provide an effective. oil spray and to ensure that the oil intimately wets the charging ore and the accreting pellets to achieve uniform dispersion of the carbon for reduction.

-The retention time of the pellets in the balling and mixing apparatus should be sufiicient to permit the relatively slow growth of balls from an original minuscule size to an eventual /2, or 1 to 1 /2 inch size, as desired.

When discharged from the pelletizing drum, undersize maybe screened. At this stage they have little mechanical strength and must be handled carefully. The green pellets are conveyed to an indirectly-fired travelling grate kiln similar to that shown in FIGURE 1 for coking. Of course, any device capable of indirect heating and in which the pellets are quiescent can be employed. Indirect heating is necessary so that the distillate can be recovered separately without combustion inside the kiln or dilution with combustion gases. Such'indirect-heated furnaces are well known in the art. Oil distillate passing out of the kiln is sent to the recovery unit. The coking kiln is operated at a temperature which will distill essentially all volatile matter out of the pellets and thoroughly coke the remaining carbon. This is generally in the range of 750 to 1000? F., but inthis embodiment a higher discharge temperature is desirable so that the pellets will be sufiicientlyhard to be subsequently treated in a direct-fired rotating kiln. A two-section kiln may be employed in this service. The cracked vapors begin to evolve in volume at, about 750 F. and resemble in composition those secured in a-conventional delayed coking operation. The non-condensible gaseous portion is rich in methane and hydrogen and, after recovery, is used in the fuel firing systems, as noted hereinbelow. Since the reduction of Fe O to Fe O begins at about 850 F. and does not become appreciable until about 1100 is reached, reduction in the coker, even at the higher discharge temperature, is relatively minor; the major heat requirements are thus due to sensible heat dutiesand the endothermic cracking of hydrocarbons in .theresidual oil.

-"Thedistillate may be passed to a condenser where its sensible heat is recovered, and thence to a knock-out drum from which liquid and non-condensibles are withdrawn separately or, it may flow directly to a fractionating to'wer, without being cooled, and processed as vapors from delayed coking drums are normally handled, In most instances a'portion of i the recovered distillate is used f for fuel purposes, and appropriate local storage 'facilitiestnot shown) must be provided.

As noted above, thecoked pellets should have sutficient mechanical strength to withstand subsequent handling and treatment by the time, they leave the coking kiln. The e oked pellets are transferred, while hot, to the charge end of the reducer kiln, where hardening and partial reduc'tion takes place. A hopper seal wherein the pellets form a gasseal is preferred. This'kiln is a direct-fired rotating kiln'as shown, but may be an indirectly or directly"fired"tunnel'kiln' or other suitable device capable of maintaining temperatures in the desired 1500-2100 F.

range, 'particularly'if thepellets are still soft. As all of "the reductantis contained in the pellets themselves, the kiln atmosphere need not be strongly reducing, but of course it should be non-oxidizing. This is readily accomplished by using a deficiency or air in the firing mechanism. Residence time in the reducer kiln will vary,

depending on degree of reduction desired and other factors, but about /2 to 2 hours is normal. When'the reducer kiln is directly fired, by means of a burner located in the discharge end, it may be desirable to inject some additional air at one or more .points upstream therefrom so that unburned fuel will be burned and the. entire kiln maintained within the desired temperature range. This socalled side air injection is well known in the art, and is done by mounting a fan on the outside of the kiln shell and blowing air into the kiln interior through a stainless steel pipe. Alternatively a substantial portion of the combustion gases withdrawn from the charge end can be recycled to the burner end where they will mix with the combustion products and provide a more uniform temperature distribution.

After discharge from the reducer kiln, the pellets are cooled, still in a non-oxidizing atmosphere, to a temperature under 600 F. Below this temperature surface reoxidation is not a problem, and further cooling is done with air in a second cooler. Such coolers are of conventional design.

The cooled pellets are then screened, with undersize being recycled to the pelletizing drum after appropriate grinding. Oversize pellets are ready for storage or shipment.

FIGURES 3 and 4 are considered in detail in Example II set forth hereinbelow, but it is of interest to compare them with FIGURES 1 and 2. FIGURES 3 and 4 relate to a plant designed to produce 3000 t.p.d. of pellets.

On this scale, it is economical to use a fluidized bed heater to initially heat the concentrate. Further, a separate pellet heater prior to the coker and a calcining stage thereafter are desirable, although in terms of actual process ing units it may be desirable to make the pellet heating stage part of the pelletizer or coker, and locate the calcining stage at the end of the coking kiln or the charge end of the reducer kiln.

As shown in FIGURE 3, the liquid recovery section is illustrated as producing five separate fractions, as might be done in a single combination fractionating tower. A small stream may be withdrawn from the bottom of the tower and recycled directly to the pelletizer in order to recycle any ore fines contained in the coker vapors and trapped in the fractionator liquid streams. This is labeled as a slurry stream in the drawing, and is distinguished from the heavy oil stream, which is the highest-boiling wholly liquid stream that is withdrawn. The lighest fraction of gas (tower overhead), comprising noncondensibles and hydrocarbons up to propane (C is shown as being used as fuel in the reducer along with the heavy oil stream; those skilled in the art will appreciate that these streams are utilized separately, the gaseous stream being burned with a substantial deficiency of air to insure maintenance of a reducing atmosphere.

In the embodiment of FIGURE 3, separate C -400 F. gasoline and distillate oil streams are withdrawn as byproducts. As shown in Example II, the thermal economy of the process is'such that a credit can be obtained from these upgraded products.

The gas and liquid flows for the embodiment of FIG- URE 3 are shown in FIGURE 4. Briefly, incoming air is first preheated by indirect heat exchange against the exhaust gases from the ore preheater and the pellet heater. The air, at about 500 F., is further heated to about 1100" F. in a fired heater. The hot air is passed primarily to the reducer burner, with minor sidesstreams being taken off for combustion in the muflies of the calciner and coker.

Coker gas and heavy oil from the liquid recovery still are ditional air being added at each stage. Efiluent from the coker mufile is at about 1410 F., and is used for heating (without further combustion) in the ore and recycle heaters and in the pellet heater, either in series or in parallel. The combined etiluent, now at about 600 F. and still containing considerable carbon monoxide, which is combustible, is first used to preheat incoming air, and then, after further cooling to about 100 F. (where water vapor is condensed) it is used to cool finished pellets. The gas is heated to about 1700 F. during the latter operation by the pellets, and is then used as fuel in the air preheater, which produces steam as a byproduct. For the conditions set forth in Example II, stream production is 150,000 lb./hr.

Understanding of the invention will be facilitated by referring to the following specific examples. Example I presents detailed data on a small scale operation and Example II provides thermal and material balances for a plant producing three thousand tons per day of hardened, partially reduced pellets. Example HI discusses the pellet composition and FIGURE 5.

EXAMPLE I Eight-hundred pounds of commercially purchased taconite concentrate had the composition shown in Table I and sieve analysis shown in Table II.

Table I.Chemical composition The steps of the process were carried out in a batch kiln externally wound with heating cable and equipped to provide an inert (N atmosphere. Some pre-dried concentrate was charged and the kiln was preheated to 450 -F. Nitrogen-atomized hot heavy-oil residue was sprayed in while the kiln was turning at 8 r.p.m., and additional concentrate was continuously added. The residual oil was a topped crude with an IBP of 700 F., and 14 API gravity and Saybolt Universal viscosity of 320 and 145 seconds at 210 F. and 250 F, respectively. Oil addition was controlled so that, in the finished (coked) pellet, there was 0.092 pound of coke per pound of Fe.

After the pellets attained a satisfactory size (100% plus /1 inch), rotation was stopped and the temperature was raised to and held at, successively, 750, 800 and 865 F. levels. The oil distilled off largely at the 750 F. level.

The pellets were transferred, while hot, to a horizontal mufile furnace and heated successively to 1500 F. and 2000 F., again in a nitrogen atmosphere. Considerable weight loss resulted from calcination and oxide reduction.

After 3 hours at 2,000 F., the furnace was turned off 8 and the pellets were'allowed to cool, the nitrogen atmosphere being retained to prevent reoxidation. Y An assortment of the pellets was subjected to crushing strength tests and chemical analysis. All pellets tested of /2 to /8" size h-ad crushing (i.e. compressive) strength in excess of 300 pounds, as compared to 200-250 pounds exhibited by clay-pelletized taconites. Chemical analysis is set forth in Table III.

Table III.Pellet composition Reduction was about 31%, and in this instance the pellet was 80.34% FeO and 9.35% Fe O EXAMPLE II Commercial taconite ore of the composition shown in Table I of Example I is preheated to 400 F. and fed continuously into a rotary pelletizer, together with recycle fines. A petroleum residuum fraction having the properties shown in Table IV is preheated to 400 F. and sprayed continuously into the pelletizer.

Table IV.--Petroleum residuum fraction Gravity API 7.4 Conradson carbon No "percent... 19.6

The resulting green pellets are fed continuously into a travelling grate kiln, wherein the pellet temperature is raised from 400 F. to 750 F. by direct contact with hot gases. In a subsequent section of this kiln the pellets are further heated to 1000 F. This high temperature portion of the kiln is partitioned from the low-temperature section so as to minimize gas fiow or interchange between between the two sections. In the high-temperature zone, coking of the pellet binder material takes place. The cracked vapors are passed to a fractionating tower from which the following streams are removed: Colrer Gas (C s and lighter), Gasoline (C(s to 400 BR), Light Distillate Oil (400 to 750 F. BR), and Heavy Distillate Oil (750 to 1100 F. B.R.). Alternatively, of course, the cracked vapors may be just cooled to condense and recover a wide boiling range distillate oil and a fuel gas. Any ore fines entrained by the coker gases are scrubbed in the bottom of the still and are removed as slurry with some of the still bottoms. This slurry is recycled to the pelletizer.

The coked pellets are fed to a second travelling .grat'e kiln, known as the calciner, wherein the pellet temperature is increased from 1000 F. to 1600 F. The material is heated indirectly by radiation from an overhead muffie. The coke binder in the pellets is devolatilized, generating a so-called coker gas which is subsequently used as fuel. Some reduction of the iron oxides in the pellets takes place, generating CO and CO v The 1600 F. calcined pellets arequite strong andv can withstand subsequent treatment in a rotary kiln without excessive decrepitation. The rotary kiln is called the reducer. Here the pellets are heated by direct radiation from products of combustion to 2000, The coke contained in the feed pellets is virtually completely reacted, resulting in the reduction of the iron oxides to lower oxides and a small amount of iron. This reduction achievesinduration, so that the cooled product pelletsexhibit a big crushing strength, usually exceeding 300 lbs.

The reduced and hardened pellets are introduced into the top of a shaft cooler. Here, cool F.) inert gas, free of air or oxidizing gases, is passed counter-current to the descending pellets, cooling the latter toapproximately F.

The cooled pellets are screened and the sized product is Many variations of this processing scheme are possible: conveyed to storage. The undersize material, about 2% For example, the reducer could be a travelling grate kiln. of the total cooler output, is ground and recycledto the The temperature levels can be varied considerably,- for ore heater. The material flows, to produce 250,000 lb./hr. example, the calciner pellet discharge temperature could or 3,000 t.p.d. of product pellets, are shown in Table V. 5 be set at 1700 F. instead of l600.F. at =1700 F., sig- The fuel economy ofthis process is demonstrated in nificant reduction will occur. Also,,instead of combining the last column of Table V, which indicates residuum feed the, pellet heater and coker functions in one travelling of 1.22 bbl. and liquid by-product (gasoline and distilgrate kiln, other combinations are quite possible. 7 late oil) recovery of 0.73 bbl. per short ton of product The gas flow pattern can be modified to meet specific pellets. The actual fuel cost is further reduced by the dif- 10 plan requirements. All of the 1410 F. gas from t e ke ferential in the per-barrel value of the upgraded liquid muffie chamber can be passed through the pellet preheat by-product as compared to the distressed residuum feed- Zone-leaving at about 990 F. (instead of 730 F..); all stock. ormost of this gas can be passed through the ore and The fuel economy results from the gas and liquid flow recyc heater- In some cases, it ay be advantageous to system, shown in FIG. 4. The coker gas and calciner gas spill some of the 1410 g tly to theboiler-air are burned with a deficiency of air (preheated t-o preheater burner to improve temperature control and re- 1'100 F.). This is augmented with combustion of the dllee Cooling Water requirements heavy oil to release the energy required in the rotary kiln The following advantageous teatllres of the general reducer. The efliuent gas from the reducer is rich in CO, Processing scheme should he noted: Although Wide latidue to the CO content of the products of combustion (the tude ill Processing equipment is Permissible, and desirable gaseous f l b i b d ith a d fi i f i due to variations in ore, residuum, etc., the use of lowaugmented by the CO generated by the ore reduction. An attrition devices (such as travelling grate kilns) P to a amount of the ffl t gas may be ,recycled the kiln pellet temperature level of 1400- to 1800 F. insures low inlet to lower the inlet gas temperature and achieve a Pellet degradation ahd'ioW ore y requirements. Use fairly even temperature distribution in the kiln. This reof indirect heating n t 7 F- to about 1500-170Q F. circulation is, however, not always required. range insures fairly complete recovery of gaseous fuels The net reducer efiiuent gas, at about 1865 F., is inand liquid y-p Also, use o reducing atmosphere troduced into the heating chamber of the travelling grate and relatively uniform temperature in the redheel' Permit l i A li l preheated i 1100 R) is introduced relatively rapid reduction to proceed to the desired level. into this gas to burn some of the contained CO as required deslred action is further aided y the gradual and b h h b l continuous release of CO (and equilibrium concentration The calciner eflluent gas, at 1550 F., passes to the of 2) from the surface of the Pellet bed, acting as a heating chamberof the coker More preheated (11006 R) gas blanket to prevent excessive contact of the bed with air is introduced, burning a little more of the CO to mainthe combustion gases above the tain heat balance. The gas issues from the mufiled heath s of a relatively reducing gas as the Cooling ing chamber at about 14100 About of this medium in the pellet cooler, combined with the relatively 14 0 gas is then passed directly through the pellets rapid heat transfer due to direct contact exchange, inon the travelling grate. Four passes are used to achieve sures agalnst Pellet However, good heat counter current heat exchange This portion (60%) of covery may be accomplished with suitable heat recovery the gas stream leaves the travelling grate kiln (pellet 40 equlpment as shown In heater-calciner) at about 730 F.

The remainder (40%) of the 1410 F. gas leaving the EXAMPLE HI coker mufiled zone is passed through the ore and under- A pellet picked at random from those produced in size recycle heatenThis is a shallow-bed fluidized bed Example I was sectioned, mounted and photomicrounit. The powder is heated to 400 F., and the gas issues 5 graphed so as to display the microstructure resulting from at about 410 F. the process of the invention.

TABLE V.MATERIAL FLOWS, 3,000 T.P.D. PELLEI PRODUCTION Ore Recycle Coker and Li .b ds. Fl

Feed Resid. undersize Green Coked Product calciner Fuel i bbillt oii (dry) material pellets pellets pellets cracked Flow Grav. of prod. gases rate API pellets Iron oxides 246. 8

- Total 270. 2 Ore/Pellet AnaL:

Fe, Wt. percent- OlFe, Wt.'rati0 Coke/Fe, Wt. ratio N o'rE.-All Flows in 1,000s of lbs/hr.

1 The cool gases (730 and 410 F.) are heat exchanged A porous, essentially two-phase structure is present, the

fagainst cool air, thenthey are cooled with water to 100 F. lesser phase being gangue materials and the majority of The cooled. gases, still reducing 'dueto residual CO conthe structure being iron oxide. It is believed that the tent, are passed counter-currently through the descending etching'technique employed selectively removed metallic bed of'pellets-in the pellet cooler. The-gases are thereby iron from the surface. Macroscopic examinationrevealed heated to about 1685" F.' that the cross-section of the pellet was entirely'uniform. The reheated gases are burned with additional air With the method used it was not possible to distinguish (cold) to consume the residual C0, and are used to prebetween various iron oxide compositions. I heat-air to the desired level (1100 F;) and to generate It is to be understod that many other modifications can plant steam. be made to the process illustrated in the drawings. For

I 11 example, the concentrate and oil can be milled at the 400-500 F. temperature to form an even mixture, and then briquetted or extruded, while hot, in conventional equipment. The green briquettes are then treated in the same manner as the green pellets. Further, sufiicient carbonaceous material'can be added to bring about substantially complete metallization, with the production of sponge iron pellets suitable for melting and finishing, for example, in an electric furnace.

The preferred pellet composition, discussed hereinabove, is decidedly advantageous, however, for use in a blast furnace. For example, at a 30% reduction level the coke rate in a blast furnace can be 200 lb./ton of hot metal lower than for unreduced pellets. This drop in the coke rate allows the production rate of the furnace to be increased by about 20% at the same wind rate. The exact degree of reduction desired in any instance will depend on economic consideration.

Various other changes in the details, steps, materials and arrangements of parts, which have been herein described and illustrated in order to explain the nature of the invention, may be made by those skilled in the art, within the principle and scope of the invention as defined in the appended claims.

The present application is related to copending US. application Ser. No. 316,359, filed Oct. 15, 1963, now abandoned, and assigned to the same assignee.

What is claimed is:

1. A hardened, thermally treated, partially reduced iron-oxide bearing pellet which, prior to cooling or after rapid cooling is characterized by a finely porous structure and consists essentially of:

at least about 70% FeO;

essentially no free carbon;

O-5% metallic iron;

balance higher iron oxides and impurities;

said pellet being chemically reduced about 15-40%.

2. The pellet as claimed in claim 1, wherein the compressive strength is at least 300 pounds, and size is about /2 to 1 /2 inches in diameter.

3. A thermally treated, partially reduced iron-oxide bearing pellet which, prior to cooling or after rapid cooling, is characterized by a finely porous structure and a crushing strength of at least 300 pounds, a size of about h to 1 /2 inches in diameter, and consists essentially of:

at least 70% FeO;

-5 metallic iron;

0-5 CaO;

essentially no free carbon;

balance higher iron oxides and impurities;

said pellet being chemically reduced about 25-35%.

4. A hardened pellet consisting of:

0-20% metallic iron;

0-5 CaO;

essentially no free carbon;

balance iron oxides and impurities;

said pellet being characterized by an oxygen-to-iron weight ratio in the range of about 0.15 to 0.34, and having a finely porous structure and a crushing strength of at least 300 pounds.

5. The process for agglomerating and beneficiating an oxide ore comprising:

forming pellets by agglomerating said ore with a heavy hydrocarbon oil having a 5% boiling point of at least 650 F., said agglomeration being carried out at a lower but still elevated temperature where said oil is fluid;

heating the pellets thus for-med to a temperature between said boiling point and the temperature where significant reduction of said ore begins to occur for a sufiicient time to coke said oil and bake said pellets;

and

further heating said pellets to reducing temperature,

whereby said pellets are hardened and residual carbon therein effects reduction.

12 6. The process as claimed in claim 5, wherein said pelletizing is carried out at a temperature in the range of 400 to 650 F. 7'. The process as claimed in claim 5, wherein said coking is carried out at a temperature in the range of 700 to 1000 F.

8. The process as claimed in claim 5, wherein said reduction and hardening are carried out at a temperature in the range of 1500 to 2200 F. in a non-oxidizing atmosphere.

9. The process as claimed in claim 7, wherein said coking is carried out in an indirectly fired heater wherein said pellets are in a quiescent state, and additionally comprising recovering distilled volatiles driven off during said coking.

10. The process as claimed in claim 8, wherein said reduction and hardening are carried out in a direct-fired rotary kiln and said non-oxidizing atmosphere is provided by burning fuel with a deficiency of air.

11. The process as claimed in claim 5, wherein the quantity of said oil and said reduction are'controlled to provide a finished pellet reduced about 15-40%, having 0-5 metallic iron, and essentially no free carbon.

12. The process for producing hardened, prereduced iron oxide pellets reduced about 15-40% comprising:

forming green pellets by mixing iron ore with a heavy hydrocarbon oil, said oil having a 5% boiling point in the range of about 650 to 1100 F., at a temperature in the range of about 400 to 500 F.;

coking said green pellets by heating to a temperature in the range of about 700 to 1000 F. for a sufficient time to coke the heavy hydrocarbons and permit the cracked volatile constituents of said oil to be driven off;

hardening and reducing said pellets by heating to a temperature in the range of about 1500 to 2200 F. in a non-oxidizing atmosphere; and

cooling the hardened and reduced pellets in a nonoxidizing atmosphere.

13. The process as claimed in claim 12, wherein said recovered volatiles are utilized to provide endothermic reaction heat.

14. The process as claimed in claim 12, wherein said ore is a taconite concentrate.

15. The process as claimed in claim 12, wherein said oil addition and said reduction are controlled to produce a pellet containing 0-5% metallic iron and essentially no free carbon.

16. The process as claimed in claim 13, wherein:

volatiles recovered from said coking are fractionated and separate fuel gas and fuel oil fractions are recovered;

said fractions are bur-ned'with a deficiency of preheated air in said hardening and reducing step;

combustion gases resulting fiom said hardening and reducing step are used to supply heat to said coking step, and to preheat said air and said iron ore; and the cooled combustion gases are used to cool said pellets.

17. The process as claimed in claim 16, wherein combustion gases used in said cooling step are burned with air, and resulting combustion gases are utilized to further preheat said air.

18. The process as claimed in claim 16, wherein said coked pellets are calcined at a'temperature of 1000 to 1600" F. prior to reduction, heat therefor being supplied by combustion gases from said hardening and reducing step. Y t

19. The process as claimed in claim 18, wherein additional preheated air is burned with said combustion gases in said coking and calcining steps.

20. The process as claimed in claim 12, wherein said ore and said oil are separately preheated to 400 to 500 F. prior to formation of said pellets.

(References on following page) References Cited UNITED STATES PATENTS Culberson et a1. 754

Lesher 754 Peras 75-26 Ban 753 3,341,322 9/1967 Bailey 75--26 3,351,459 11/1967 M1115 75-4 L. SEWAYNE RUTLEDGE, Primary Examiner. ERNEST L. WEISE, Assistant Examiner.

US. Cl. X.R. 754, 5

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