Metal-making process and apparatus involving the smelting reduction of metallic oxides

Abstract: A metal-making apparatus using raw ore which contains a metallic oxide and which is in the state of particles in a wide range of size, comprising a solid-­ state prereduction furnace (F2) for preliminarily reducing raw ore in a solid state, and a smelting reduction furnace (F1) for smelting and finally reducing the prereduced ore in a molten state. In order to make possible the use of raw ore particles in a wide range of size without any such pretreatment as sizing, comminution or pelletizing, raw ore particles of larger size (7a, 7b) are prereduced in the prereduction furnace (F2) by forming a fluidized bed on its distributor (6), whereas smaller size particles (7c) are prereduced while being recirculated through the prereduction furnace. On discharge from the prereduction furnace, the prereduced ore particles of larger size are recharged by gravity into the smelting reduction furnace (F1). The prereduced ore particles of smaller size are recovered from the waste gas of the prereduction furnace, as by a cyclone separator (8), and pneumatically injected into the smelting reduction furnace (F1) as by a submerged injection nozzle (35a) or by an injection lance (51) located above the level of the bath (S, M) within the furnace (F1).

Description: This invention relates to a process and an apparatus for the production of molten metals from their ores, and more specifically to a metal-making apparatus of this general nature based upon the technology of the smelting reduction of ores containing the oxides of the desired metals. Still more specifically, the invention pertains to such a metal-making apparatus making possible the use of ores in a state of particles in a wide range of size. The smelting reduction method is one whereby metals such as iron and ferroalloys are produced by the reductive treatment of iron oxide ores or the like in a molten state. The metal-making industry has devoted research and development efforts to the enhancement of the practical utility of this method because of its potential capability of meeting the foreseeable future trend of materials and energy supply. Among the advantages of the smelting reduction method are, as far as iron making is concerned: (1) cheapness of the raw materials as compared with those required for the blast furnace method; (2) elimination of pretreatments such as sintering or pelletizing of too fine ore particles; and (3) compactness of the equipment required. Additionally, for the production of ferro-alloys, the method promises greater independence from electric energy than most other known methods. While a variety of suggestions and proposals have so far been made for the practice of the smelting reduction method, the current trend in the industry is toward the use of two furnaces in tandem, one for preliminary reduction of the ore in a solid state and the other for smelting reduction. There are many variations of this tandem-furnace method, involving different furnace types and different heat production methods. Generally, such known tandem-furnace methods are preferred by reason of the regenerative process involved; that is, the prereduction of the solid-state ore is possible by making use of the heat and reductive capability possessed by the waste gas generated from the smelting reduction furnace. The reductive gas is generated at high temperatures as a result of the reduction taking place within the smelting reduction furnace as coal or like material and oxygen gas are introduced into the metal bath therein. Upon withdrawal from the smelting reduction furnace, the reductive high-temperature gas is directed into the prereduction furnace for the preliminary reduction of the ore in the solid state. The general belief of the specialists is that the prereduction furnace should advantageously be of the so-called "fluidized bed" or fluosolids type, provided that the ore is in more or less in a state of fine particles. This type of furnace gives the properties of a quasi-fluid to the ore particles introduced therein, making possible the continuous processing of the pulverized material. As additional advantages, the complete charge of ore particles within the furnace can be maintained at a constant temperature, and the ore particles make intimate contact with the reducing gas. A typical prior art prereduction furnace of the fluidized bed type is found in Japanese Laid Open Patent Application No. 58-217615. It comprises a vessel in the form of an upstanding cylinder, with an ore supply chute and a gas exhaust conduit coupled to its top portion, and with a reducing gas supply conduit and an ore discharge chute coupled to its bottom portion. The furnace vessel has a gas distributor in the form of a grate or perforated bottom laid horizontally above the intake port of the reducing gas. Ore in the state of fine particles is charged onto the gas distributor of the furnace, whereas the high temperature reducing gas is introduced into the furnace through an inlet port positioned under the gas distributor. Stirred by the reducing gas blasted up through the gas distributor, the ore particles will become "fluid" enough to make intimate contact with the gas thereby undergoing the desired process of prereduction. The material in this condition forms a "fluidized bed", with the reducing gas "bubbling up" therethrough. After having been thus prereduced, the ore particles will be discharged from the prereduction furnace and recharged into the smelting reduction furnace for final reduction in the molten state. As so far constructed, however, the fluidized-bed type prereduction furnace has had a weakness. It imposes strict limitations upon the particle size of the ore in order to form a sufficiently "fluid" mass of the ore particles within the furnace. The reducing gas must be introduced at a rate depending upon the particles size of the ore in order to impart sufficient "fluidity" to the ore particles. Therefore, the prior art prereduction furnace does not lend itself to the processing of ore particles that differ in size over a wide range. The particle size has had to be not more than three millimeters for successful prereduction. Moreover, depending upon the particular prereduction process employed and the particular kind of ore to be processed, additional limitations have been imposed on the average particle size and on the percentage of very fine particles present. The inconveniences arising from such limitations of the prior art fluidized-bed type prereduction furnace will become apparent in the light of the fact that the iron ores available commercially as the raw materials of iron manufacture contain a considerable percentage (e.g. 30 percent) of particles exceeding three millimeters in size. Even particles of 10 millimeters or more in size are present. A conventional solution to this problem has been the sizing of ore particles by screening. Particles too coarse to be treated by the prereduction furnace of the fluidized-bed type have had to be recomminuted into the required size. Alternatively, if such coarse particles are not to be recomminuted, some other reduction means such as a shaft furnace have had to be employed. At any event, for making full use of the materials purchased, the prior art fluidized bed furnace has required some additional means such as screens and crushers, or another reducing furnace, thus incurring additional installation and running costs and additional manufacturing steps. There have also been some problems left unsolved in conjunction with the smelting reduction furnace to be connected in tandem with the solid-state prereduction furnace. The following two methods of charging ore into the furnace have been suggested: 1. The gravity charging of the whole ore particles from the top portion of the furnace. 2. The carrier-gas charging of the whole ore particles through a nozzle coupled to either the midportion or bottom portion of the furnace (Japanese Laid-Open Patent Application No. 59-113110). The first described method of gravity charging is objectionable because the finer ones of the ore particles, on being introduced into the smelting reduction furnace, tend to be blown out of the furnace by the gas generating therein. It may be contemplated to avoid such waste of the material by previously screening out the finer particles or by pelletizing the fine particles into larger ones by use of a binder. But then these measures demand the provision of screens or like sizing means, or means for pelletizing, as well as additional processing steps and additional processing time. The second recited method of carrier-gas charging, although free from the noted drawbacks of the gravity charging method, has its own shortcomings. If the pulverized material contains too coarse particles or lumps, they may clog up the charging nozzle or the conduit leading to the nozzles. It would be no satisfactory solution to make the nozzle and conduit large enough to permit the passage of such coarse lumps. For such large nozzle and conduit would require a corresponding increase in the flow rate of the carrier gas, possibly resulting in a bad effect to the reduction reaction and an undue drop in the bath temperature or in the blowing of the carrier gas through the metal bath. The usual practice, therefore, has been to pulverize the ore into sufficiently fine particles to preclude the possibility of clogging. This practice is also objectionable because of such additional means required as crushers and screens, and of the additional operation necessitated. Subject matter of the invention is a metal-making process using raw ore which contains a metallic oxide, wherein raw ore is preliminarily reduced in a solid state in a prereduction furnace (F2) by blowing high temperature reducing gas into the prereduction furnace (F2) and forming a fluidized bed therein; and prereduced ore is transferred to a smelting reduction furnace (F1) and smelted and finally reduced in a molten state therein, characterized in that raw ore being in the state of particles in a wide range of size is used; the ore particles of larger size (7a,7b) - except oversize particles (7a) which are too coarse to be fluidized - form the fluidized bed on a distributor (6) in the prereduction furnace (F2), whereas the oversize particles (7a) form a mobile layer on the distributor (6) under the fluidized bed; the ore particles of larger size (7a,7b) are prereduced by making intimate contact with the high temperature reducing gas blown up through the distributor (6); the ore particles of smaller size (7c) are carried away from the fluidized bed by waste gas and thereby prereduced; the prereduced oversize particles (7a) are discharged from a lower discharge opening (6b) at the lower end of the distributor (6) having a funnel-shaped or an inclined top surface and charged by gravity into the smelting reduction furnace (F1); the remaining ore particles of larger size (7b) are also discharged from said lower discharge opening (6b) or from an optionally provided higher discharge opening and charged by gravity into the smelting reduction furnace (F1); the prereduced ore particles of smaller size (7c) are recovered by a separator from the waste gas and either pneumatically injected into the smelting reduction furnace (F1) in total or pneumatically injected into the smelting reduction furnace (F1) in part and recirculated into the prereduction furnace (F2) in part. Subject matter of the invention is further a metal-making apparatus for carrying out such process, comprising: a prereduction furnace (F2); means for forming a fluidized bed of ore particles in the prereduction furnace (F2); a smelting reduction furnace (F1); means for transferring prereduced ore particles from the prereduction furnace (F2) to the smelting reduction furnace (F1), characterized by: a gas distributor (6) in the prereduction furnace (F2), the distributor (6) having a funnel-shaped or an inclined top surface, and a lower discharge opening (6b) at the lower end of the distributor (6) for the discharge of ore particles of larger size (7a,7b); a discharge chute (4) communicating the discharge opening (6b) with the interior of the smelting reduction furnace (F1); an exhaust conduit (5) of the prereduction furnace (F2); a separator (8) coupled to the exhaust conduit (5), the separator (8) in operation recovering ore particles of smaller size (7c) from the waste gas flowing in the exhaust conduit (5); a communication conduit between the separator and the interior of the smelting reduction furnace and injection means for pneumatically injecting at least part of the recovered ore particles of smaller size into the smelting reduction furnace; whereby the apparatus is adapted for the use of raw ore being in the state of particles in a wide range of size. A process and an apparatus comprising the features indicated in the two proceeding paragraphs before the word "characterized" are known from the document "Stahl und Eisen" 107 (1987) Nr. 22, page 1020, Fig. 15 and corresponding text. Exclusively prereduced fine ore is transferred from the prereduction furnace to the smelting reduction furnace. The document "Stahl und Eisen" 107 (1987) Nr. 22 shows on page 1020, Fig. 16 and corresponding text an apparatus for producing metal from lump ore or coarse ore particles the ore being first prereduced in a prereduction furnace and then charged by gravity into a smelting reduction furnace. The document "Patent Abstracts of Japan" Vol. 11 No. 24 (C-399) (2471), January 23, 1987 (JP-A-61-199009) shows a smelting reduction furnace, into which prereduced granular ore is charged. A first finer proportion of the ore is blown into the molten iron through tuyeres, and a second coarser proportion of the ore is dropped from the top of the furnace toward the molten iron. The present invention solves all the above discussed problems of the prior art and provides an improved metal-making apparatus of the type comprising a tandem arrangement of a solid-state prereduction furnace and a smelting reduction furnace for the production of iron or other metal by the reduction of the oxide of the metal. Particularly, the invention seeks to make possible the use, as raw materials, of pulverized ores of widely varying particle sizes without the need for screening, recomminuting, or pelletizing them. Further the invention seeks to accomplish a thoroughly streamlined processing of such raw materials from their solid-state prereduction to smelting reduction. With the process and the apparatus according to the invention raw ore is used which contains a metallic oxide and which is in the state of particles with a wade range in size. The metal-making apparatus comprises a solid-state prereduction furnace for preliminarily reducing the raw ore in a solid state. The prereduction furnace has a gas distributor on which is deposited a charge of the raw ore particles in a wide range in size. High temperature reducing gas is blown up through the gas distributor and the charge of raw ore particles thereon. Consequently, the ore particles of larger size form a fluidized bed on the gas distributor and are prereduced by making intimate contact with the high temperature reducing gas, whereas the ore particles of smaller size are carried away from the fluidized bed by the waste gas and thereby prereduced. Discharged from the prereduction furnace, the prereduced ore particles of larger size are recharged by gravity into a smelting reduction furnace. The prereduced ore particles of smaller size, on the other hand, are recovered from the waste gas of the prereduction furnace and recharged into the smelting reduction furnace by pneumatic injection. The prereduced ore particles of both sizes are smelted in the smelting reduction furnace and finally reduced in the molten state to provide the desired metal. Possibly, the raw ore particles charged into the prereduction furnace may include those too coarse to form a fluidized bed together with other medium-size particles. Such oversize ore particles will form, instead, what may be called a "mobile layer" over the gas distributor and under the fluidized bed of the medium-size particles. The gas distributor has a funnel-shaped or otherwise inclined top surface, so that the oversize ore particles will be prereduced while slowly traveling over the inclined surface of the grate toward a discharge opening formed therein. The prereduced oversize particles will be discharged from the prereduction furnace together with the medium-size particles, for joint gravity charging into the smelting reduction furnace. The raw ore particles too fine to form a fluidized bed, on the other hand, will be carried away from over the fluidized bed by the upflow of the high temperature reducing gas. Carried by the waste gas, such undersize particles will flow into a cyclone separator or like gas-solid separator, which is disposed outside the prereduction furnace, thereby to be recovered from the waste gas. The recovered undersize ore particles may be recharged into the prereduction furnace. While being so recirculated, the undersize ore particles will also be prereduced, so that part of the recirculating particles may be withdrawn from the recirculation path for pneumatic injection into the smelting reduction furnace. Thus, although the solid-state prereduction furnace is basically of the fluidized-bed type, the raw ore particles charged therein may contain those both too large and too small to form a fluidized bed. All such particles of a wide range of size can be prereduced in the solid state without the inconveniences encountered heretofore. According to a further feature of the invention, the oversize and medium-size particles and the undersize particles are separately discharged from the prereduction furnace and separately charged into the smelting reduction furnace. The oversize and medium-size particles are charged by gravity into the smelting reduction furnace, there being little hazard of such particles being blasted away by the gas generated within and rising from the smelting reduction furnace. The undersize particles are pneumatically charged into the smelting reduction furnace, as by an injection nozzle submerged in the metal or slag bath within the furnace or by an injection lance above the level of the bath. Advantageously, such undersize particles can be smoothly conveyed through a conduit system of relatively small diameter and by a carrier gas supplied at a relatively low rate. The above and other features and advantages of this invention and the manner of realizing them will become more apparent, and the invention itself will best be understood, from a study of the following description and appended claims, with reference to the attached drawings showing some preferable embodiments of the invention. FIG. 1 is a diagrammatic illustration of the metal-making apparatus constructed in accordance with the novel concepts of the invention, the apparatus being herein shown adapted for the production of iron from iron ore; FIG. 2 is an enlarged vertical section of the prereduction furnace used in the apparatus of FIG. 1; and FIG. 3 is a partial diagrammatic illustration of another preferred embodiment of the invention, featuring alternative means for charging prereduced ore particles of smaller size into the smelting reduction furnace. The apparatus of the invention is shown in FIG. 1 as adapted for the production of iron by reductive treatments of fines and larger size iron ore. Broadly, the exemplified apparatus comprises a tandem configuration of a smelting reduction furnace F1 (hereinafter referred to as the smelting reduction furnace) and a solid-state prereduction furnace F2 (hereinafter referred to as the prereduction furnace). The smelting reduction furnace F1 reduces the prereduced iron ore particles in the molten state, with the consequent production of high temperature reductive gas. The prereduction furnace F2 utilizes this gas for preliminarily reducing the raw iron ore particles in a solid state. The smelting reduction furnace F1 is of the molten iron bath type capable of holding the molten iron M and slag S in the molten state within a refractory-lined vessel. Reduction takes place within the smelting reduction furnace F1 as the prereduced ore particles, coal, lime, and oxygen are fed into the furnace, as will be detailed subsequently. As illustrated on an enlarged scale in FIG. 2, the prereduction furnace F2 comprises a vessel, complete with a refractory lining, which is in the form of an upstanding cylinder. An ore supply conduit or chute 2 is coupled to the midportion of the prereduction furnace F2 for charging raw iron ore particles of a wide range of size therein through an inlet port 2 a . A reducing gas supply conduit 3 is coupled to the bottom end portion of the furnace for introducing reducing gas at a high temperature through its inlet port 3 b . A reference back to FIG. 1 will reveal that this reducing gas is supplied from the smelting reduction furnace F1. A prereduced ore discharge chute 4 extends downwardly from the furnace for the delivery of the prereduced ore particles of relatively large size toward the smelting reduction furnace F1. Further a waste gas exhaust conduit 5 is coupled to the top end portion of the furnace vessel for the withdrawal of the waste reductive gas from within the furnace through an exhaust port 5 a . Within the prereduction furnace F2 an openwork bottom or grate type gas distributor 6 (hereinafter referenced to as the distributor) is disposed in a position above the reducing gas inlet port 3 b and below the raw ore inlet port 2 a . The distributor 6 has a multiplicity of perforations or like openings 6 a to permit the reducing gas to flow up therethrough and hence to create diffused gas streams needed for the prereduction of the charged ore particles. The distributor 6, or at least its top surface, is funnel-shaped, with the top surface having an angle α with respect to the plane of the horizon. This angle α is greater than the angle of repose of the ore particles to be charged into the furnace. Positioned centrally of the distributor 6, an ore discharge port 6 b communicates with the discharge chute 4 for the discharge of the prereduced coarse and medium-size ore particles. A solid-gas separator such as, typically, a cyclone separator 8 is coupled to the waste gas exhaust conduit 5 for collecting and recovering the fine ore particles contained in the waste gas from the prereduction furnace F2. Freed of such ore particles, the waste gas is directed into an exhaust duct 11. The recovered fine ore particles, on the other hand, fall into a chute 10 a depending from the cyclone separator 8 and communicating with both a recirculation chute 10 b and a discharge chute 10 c . The recirculating ore flow rate and discharging ore flow rate are controlled by the control valve 9 and 25, respectively. The recirculation chute 10b is open to the interior of the prereduction furnace F2 at a point that is approximately at the same level as the inlet port 2 a of the raw ore particles. Therefore, recovered from the waste gas by the cyclone separator 8, the fine ore particles are at least in part recharged into the prereduction furnace F2 for recirculation. The discharge chute 10 c communicates with the smelting reduction furnace F1, as will be later described in more detail with referring to FIG. 1. The fine ore particles recovered from the waste gas may therefore be withdrawn from their recirculation path and fed toward the smelting reduction furnace F1 by the discharging ore flow rate control valve 25 in communication with the discharge chute 10 c . Preferably, the recirculating ore flow rate control valve 9 should be of the type capable of preventing reverse gas flow from prereduction furnace F2 to cyclone separator 8 therethrough. The interior of the prereduction furnace F2 has a reduced diameter portion 1 b located above the distributor 6. This reduced diameter portion serves to permit the reducing gas to flow therethrough at a sufficient rate to "fluidize" the ore particles of widely varying sizes on the distributor 6. A greater diameter portion 1 a , overlying the reduced diameter portion 1 b , is effective to retard the gas speed and hence to set a limit on the size of the finer particles that are carried away by the waste gas from the prereduction furnace F2. Such being the construction of the prereduction furnace F2, the iron ore particles in a wide range of size may be charged onto the distributor 6 through the ore inlet port 2 a . The high temperature reducing gas may also be directed from the smelting reduction furnace F1 into the prereduction furnace F2 through the supply conduit 3. Flowing upwardly through the perforations in the distributor 6, the gas will reduce the overlying charge of ore particles. After having been thus prereduced, the large- and medium-size particles will be discharged though the discharge chute 4 whereas the fine particles will be carried away by the waste gas into the exhaust conduit 5. More specifically, during the progress of ore reduction within the prereduction furnace F2, the mediumsize particles will form a "fluidized bed" 7 b at the reduced diameter portion 1 b over the distributor 6 and/or the greater diameter size portion 1 a , with the diffuse streams of reducing gas bubbling up therethrough at high speed. However, those ore particles which are too coarse to be fluidized will form, instead, a mobile layer 7 a , slowly traveling over the sloping top surface of the distributor 6 toward the central discharge port 6 b . Such largesize particles will be prereduced while so traveling over the distributor. It is assumed that the raw ore particles charged into the prereduction furnace F2 include those too small to stay in the fluidized bed over the distributor 6. Such smaller size particles will be blown up by the reducing gas in the larger diameter portion 1 a and thence into the exhaust conduit 5. Then, separated from the waste gas by the cyclone separator 8, the fine particles will be carried back into the prereduction furnace F2 via the recirculation chute 10 b , thereby forming a recirculating flow of such particles designated 7 c . The fine particles will also be reduced while being so recirculated, and will be discharged through the discharge chute 10 c as a discharging ore flow rate control valve is placed in the chute 10 c . Notwithstanding the showing of FIG. 2 it is not essential that both coarse and medium-size particles be discharged through the common discharge chute 4. Alternatively, the medium-size particles may be discharged through an additional chute or conduit in direct communication with the reduced diameter portion 1 b of the prereduction furnace F2. Also, the interior of the prereduction furnace F2 may taper toward the distributor 6, instead of being formed into the portions 1 a and 1 b of distinctly different diameters. As a further possible modification the prereduction furnace F2 may be provided with a inclined plane grate set at an angle to the plane of the horizon, in substitution for the funnel-shaped grate 6. Advantageously, the invention permits the independent control of the residence time of coarse and medium-size ore particles within the prereduction furnace F2, and of the residence time of fine ore particles within the recirculation path. The rate of discharge of coarse and medium-size particles is determined by a discharge control valve 22, FIG. 1, on the discharge chute 4, and the rate of discharge of fine particles by the circulating ore flow rate control valve 9 on their recirculation path. Being dependent upon the rates of discharge, the residence times may be independently controlled by the valves 9 and 22. The following is a report of the experiment conducted by the instant applicant on the solid-state prereduction of iron ore with the furnace F2 of FIG. 2. The conditions of the experiment were: 1. Raw Material Iron ore Particle size distribution: More than 10 mm 2 %. 5 - 10 mm 18 %. 0.5 - 5 mm 31 %. Less than 0.5 mm 49 %. Charging temperature: 450 degrees C. 2. Reducing Gas Composition: Carbon monoxide (CO) 39 %. Carbon dioxide (CO₂) 21 %. Hydrogen (H₂) 14 %. Steam (H₂O) 12 %. Nitrogen (N₂) 14 %. Temperature: 1030 degrees C. 3. Dimensions of Prereduction Furnace Diameter D a of furnace portion 1 a = 280 mm. Diameter D b of furnace portion 1 b = 200 mm. Height H a of exhaust conduit 5 from bottom of furnace portion 1 a = 4000 mm. Vertical dimension H b of furnace portion 1 b = 500 mm. Grate angle α = 40 degrees. As a result of the experiment under the above specified conditions, prereduced iron ore particles were recovered from both discharge chutes 4 and 10 c , normally with a reduction rate of approximately 30 %. Of the recovered material, 49 % was from the chute 4, and 51 % from the chute 10 c . The material recovered from the chute 4 contained 97 % of coarse and medium-size particles (particle size more than 0.5 mm). The material recovered from the other chute 10 c contained 97 % of fine particles (particle size less than 0.5 mm). The foregoing results of the experiment are believed to justify the following mathematical analyses of gas and ore behaviors within the prereduction furnace. The flow rate of the reducing gas just above the distributor 6, that is, at the reduced diameter furnace portion 1 b , was 7.0 meters per second, which is enough to "fluidize" only ore particles of five millimeters or less in size. The gas flow rate at the larger diameter furnace portion 1 a was 5.0 meters per second, which is enough to enable the gas to carry ore particles of 0.5 millimeter or less away from the furnace. Consequently, out of the iron ore particles of varying sizes that had been charged into the prereduction furnace F2, those exceeding five millimeters in size formed a mobile layer 7 a on the distributor 6; those between 0.5 and five millimeters formed a "bubbling" quasi-fluid layer b at the reduced diameter furnace portion 1 b ; and those of less than 0.5 millimeter in size recirculated through the closed path. It has also been confirmed from the experiment that: (1) the particle sizes of the prereduced ore discharged from the two separate chutes 4 and 10 c depend on the rate of introduction of the reducing gas into the furnace; and (2) the rates of reduction of the two different grades of ore particles depend upon their residence times within the furnace. The advantages gained by the prereduction furnace F2 of the foregoing construction and operation may be summarized as follows: 1. Raw ore particles in a wide range of size can be directly charged into the prereduction furnace without any such pretreatments as comminution, pelletizing, or sizing. 2. Prereduced ore particles of small size and medium and large sizes can be separately recovered from the prereduction furnace, for separate delivery to the smelting reduction furnace F1. 3. The residence times of the two different classes of ore particles can be separately determined, making possible the easy and accurate control of their expected reduction rates. 4. The coarse ore particles directly overlying the distributor serve to prevent the finer particles from falling down through the distributor when the introduction of the reducing gas into the furnace is terminated. The perforations or openings of the distributor can therefore be made sufficiently large to avoid clogging. 5. The maximum size of particles to be "fluidized" and the maximum size of particles to be recirculated can be predetermined at desired values by varying the relative cross sectional areas of the top and bottom portions 1 a and 1 b of the prereduction furnace, so that a higher rate of reduction can be realized than heretofore. Referring back to FIG. 1, it will be seen that the discharge chute 4 of the prereduction furnace F2 communicates with a storage vessel 23 via the discharge control valve 22. The other discharge chute 10 c of the prereduction furnace F2 communicates with another storage vessel 26 via another discharge control valve 25. Therefore, recovered from the prereduction furnace F2, the coarse and medium-size ore particles will be temporarily stored in the storage vessel 23, and the fine ore particles in the other storage vessel 26, prior to separate delivery to the smelting reduction furnace F1. Both storage vessels 23 and 26 have built-in weighing instrument, not shown, for measuring the amounts of the incoming prereduced ore particles. Accordingly, as a charge control valve 24 is opened, the coarse and medium-size particles will be charged onto the slag S of the molten iron bath within the smelting reduction furnace F1 by falling by gravity through a supply chute 34 which is open to an inlet port 34 a above the level of the bath. Likewise, as another charge control valve 27 is opened, the fine ore particles will be carried through a supply conduit 35 by a carrier gas and blown directly into the molten iron M or the slag S within the smelting reduction furnace F1 through an injection nozzle 35 a submerged in the bath. The carrier gas, which may be either an inert gas such as nitrogen or a reductive gas such as carbon monoxide, is supplied from its source, not shown, through a conduit 29 in constant communication with the ore supply conduit 35. A heater 30 is provided downstream of the unshown gas supply for preheating the carrier gas in this particular embodiment. The first recited charge control valve 24 should be of the type capable of maintaining the pressure differential between the storage vessel 23 and the supply chute 34. For the desired smelting reduction of the prereduced iron ore, the smelting reduction furnace F1 is supplied with oxygen, coal, and lime, in addition to the prereduced ore particles. Oxygen is blasted directly into the molten iron M through a supply conduit 38. As an alternative, the oxygen gas can be introduced into the smelting reduction furnace through a lance inserted from the upper opening of the furnace. Preferably, and as shown, relatively coarse particles and relatively fine particles of coal and lime should be fed separately into the smelting reduction furnace F1, just like the two different classes of prereduced ore particles from the prereduction furnace F2. The coarse coal and lime particles may be fed by gravity through a supply chute 36 onto the iron bath. The fine coal and lime particles may be conveyed by a carrier gas, such as that used for carrying the prereduced fine ore particles, through a supply conduit 37 and blown directly into the molten iron M. As has been mentioned with reference to FIG. 2, the reductive high-temperature gas generated within the smelting reduction furnace F1 is delivered to the prereduction furnace F2 for prereducing the raw ore particles therein. Toward this end the smelting reduction furnace F1 has its open top covered by a hood 39 which is in constant communication with a duct 40. The reducing gas supply conduit 3 communicates with the duct 40 via a solid-gas separator such as a cyclone separator 41. Thus, freed from solids such as dust and iron particles by the cyclone separator 41, the reducing gas will be delivered at a high temperature from smelting reduction furnace F1 to the prereduction furnace F2. The advantageous effects obtained in connection with the smelting reduction furnace F1 may be enumerated as follows: 1. The prereduced ore particles of large and medium size can be chuted by gravity into the smelting reduction furnace F1 with little or no likelihood of being blasted away from within the furnace by the gas generated by the molten iron M. 2. The prereduced ore particles of smaller size are injected directly into the molten metal M by being pneumatically conveyed through the supply conduit 35 and injection nozzle 35 a . If these ore particles are less than one millimeter in size, for example, then the supply conduit 35 and injection nozzle 35 a can be as small in inside diameter as from 10 to 13 millimeters. The carrier gas can be supplied at a relatively small rate for conveying such fine particles through such small diameter conduit and nozzle without the fear of clogging. Additionally, the wear of the supply conduit and injection nozzle is reduced to a minimum, and there is practically no possibility of the carrier gas blowing through the molten iron M. 3. The prereduced ore particles at an elevated temperature (approximately 800 degrees C. within the prereduction furnace F2) are fed directly from prereduction furnace to smelting reduction furnace F1 without any such intermediate treatment as sizing, comminution or pelletizing. If the storage vessels 23 and 26, etc., are suitably insulated against heat, the sensible heat of the prereduced ore can be fully utilized for its molten-state reduction in the smelting reduction furnace F1. 4. Since the carrier gas can be supplied at a relatively low rate for conveying the prereduced ore particles of smaller size into the smelting reduction furnace F1, as has been pointed out, the smelting reduction furnace would suffer little temperature drop even if, contrary to the showing of FIG. 1, the carrier gas were not preheated. 5. When the delivery of the prereduced fine ore particles from the storage vessel 26 is suspended, the supply of the carrier gas into the conduit 35 must be continued in order to prevent the inflow of the molten iron M into the injection nozzle 35 a . However, by preheating the carrier gas to a temperature range of, say, 700 - 800 degrees C. by the heater 30, the temperature difference between the prereduced ore and the gas can be reduced to such an extent as to preclude the danger of the thermal stress cracking of the supply conduit 35, even if the conduit is lined with a ceramic to resist wear. 6. Being not exposed to oxidizing gas, the prereduced ore will not be reoxidized during transfer from prereduction furnace F2 to smelting reduction furnace F1. The two separate grades of prereduced ore particles may be charged into the smelting reduction furnace F1 at the rates determined by the weighing instrument built into the storage vessels 23 and 26 and by the charge control valves 24 and 27. FIG. 3 shows an alternative embodiment of the invention, featuring a different method of charging the prereduced ore particles of smaller size into the smelting reduction furnace F1. The storage vessel 26 for the finer ore particles communicates with an injection lance 51 via the charge control valve 27, herein shown as a rotary feeder, and a supply conduit 46. The prereduced fine ore particles are fed through the supply conduit 46 and injection lance 51 by a carrier gas fed through a conduit 45. The injection lance 51 extends downwardly into the smelting reduction furnace F1 through its open top and terminates short of the level of the iron bath. The fine ore particles are to be charged into the smelting reduction furnace F1 from the tip of the injection lance 51 held out of contact with the iron bath. The FIG. 3 embodiment is akin in the other constructional details to that of FIGS. 1 and 2. A pronounced advantage of the FIG. 3 apparatus is that neither the molten iron M nor slag S is to flow into the injection lance 51 when the introduction of the prereduced fine ore particles is suspended. Unlike the preceding embodiment, therefore, the carrier gas need not be blasted through the injection lance during the nonintroduction of the fine ore particles, so that the bath in the smelting reduction furnace will suffer no temperature drop even if the carrier gas is not preheated. The supply conduit 46 and injection lance 51 are immune against thermal stress cracking as they are not cooled by the carrier gas during the nonintroduction of the fine ore particles. The use of the injection lance 51 offers an additional advantage of being also used for such purposes, in addition to the injection of the prereduced fine ore particles, as the temperature measurement and sampling of the molten iron M, the charging of coal and lime in finely divided form, and the blasting of oxygen into the smelting reduction furnace F1. Despite the foregoing detailed disclosure, it is not desired that the invention be limited by the exact showings of the drawings or the description thereof. A variety of modifications or alternations of the illustrated embodiments may be resorted to in order to conform to the requirements of each specific metal-making operation or to design preferences, without departure from the scope of the invention.

Claims: A metal-making process using raw ore which contains a metallic oxide, wherein raw ore is preliminarily reduced in a solid state in a prereduction furnace (F2) by blowing high temperature reducing gas into the prereduction furnace (F2) and forming a fluidized bed therein; and prereduced ore is transferred to a smelting reduction furnace (F1) and smelted and finally reduced in a molten state therein, characterized in that raw ore being in the state of particles in a wide range of size is used; the ore particles of larger size (7a,7b) - except oversize particles (7a) which are too coarse to be fluidized - form the fluidized bed on a distributor (6) in the prereduction furnace (F2), whereas the oversize particles (7a) form a mobile layer on the distributor (6) under the fluidized bed; the ore particles of larger size (7a,7b) are prereduced by making intimate contact with the high temperature reducing gas blown up through the distributor (6); the ore particles of smaller size (7c) are carried away from the fluidized bed by waste gas and thereby prereduced; the prereduced oversize particles (7a) are discharged from a lower discharge opening (6b) at the lower end of the distributor (6) having a funnel-shaped or an inclined top surface and charged by gravity into the smelting reduction furnace (F1); the remaining ore particles of larger size (7b) are also discharged from said lower discharge opening (6b) or from an optionally provided higher discharge opening and charged by gravity into the smelting reduction furnace (F1); the prereduced ore particles of smaller size (7c) are recovered by a separator from the waste gas and either pneumatically injected into the smelting reduction furnace (F1) in total or pneumatically injected into the smelting reduction furnace (F1) in part and recirculated into the prereduction furnace (F2) in part. A process as claimed in claim 1, characterized in that prereduced ore particles of smaller size (7c) are pneumatically injected below the level of molten metal (M) or slag (S) in the smelting reduction furnace (F1). A process as claimed in claim 1, characterized in that the prereduced ore particles of smaller size (7c) are pneumatically injected by an injection lance (51) terminating shortly above the level of molten metal (M) and slag (S) in the smelting reduction furnace (F1). A metal-making apparatus for carrying out the process as claimed in any one of claims 1 to 3, comprising: a prereduction furnace (F2); means for forming a fluidized bed of ore particles in the prereduction furnace (F2); a smelting reduction furnace (F1); means for transferring prereduced ore particles from the prereduction furnace (F2) to the smelting reduction furnace (F1), characterized by: a gas distributor (6) in the prereduction furnace (F2), the distributor (6) having a funnel-shaped or an inclined top surface, and a lower discharge opening (6b) at the lower end of the distributor (6) for the discharge of ore particles of larger size (7a, 7b); a discharge chute (4) communicating the discharge opening (6b) with the interior of the smelting reduction furnace (F1); an exhaust conduit (5) of the prereduction furnace (F2); a separator (8) coupled to the exhaust conduit (5), the separator (8) in operation recovering ore particles of smaller size (7c) from the waste gas flowing in the exhaust conduit (5); a communication conduit (10c, 35) between the separator (8) and the interior of the smelting reduction furnace (F1) and injection means (29, 35, 35a; 45, 46, 51) for pneumatically injecting at least part of the recovered ore particles of smaller size (7c) into the smelting reduction furnace (F1); whereby the apparatus is adapted for the use of raw ore being in the state of particles in a wide range of size. An apparatus as claimed in claim 4, characterized by a recirculation conduit (10b) for recirculating part of the recovered ore particles of smaller size (7c) back into the prereduction furnace (F2). An apparatus as claimed in claim 4 and 5, characterized in that the angle (α) of the distributor (6) top surface with respect to the plane of the horizon is greater than the angle of repose of the ore particles loaded thereon. An apparatus as claimed in any one of claims 4 to 6, characterized in that the prereduction furnace (F2) is substantially in the shape of an upstanding cylinder and has a first portion (1b) of reduced inside diameter immediately over the distributor (6), and a second portion (1a) of greater inside diameter over the first portion (1b). An apparatus as claimed in any one of claims 4 to 7, characterized by an injection nozzle (35a) for the ore particles of smaller size (7c), the injection nozzle (35a) being submerged in the molten metal (M) and slag (S) bath in the smelting reduction furnace (F1). An apparatus as claimed in any one of claims 4 to 7, characterized by an injection lance (51) for the ore particles of smaller size (7c) extending into the smelting reduction furnace (F1) and terminating shortly above the level of the molten metal (M) or slag (S) bath therein. An apparatus as claimed in any one of claims 4 to 9, characterized by a heater (39) for preheating a carrier gas used for carrying and injecting the ore particles of smaller size into the smelting reduction furnace (F1).

Inventors: KISHIMOTO MITSUHARU, MAEDA TAKUYA, MURAKAMI KEIKICHI, TAKEMURA YOSHIHIKO, YAJIMA KENICHI, KISHIMOTO, MITSUHARU, MAEDA, TAKUYA, MURAKAMI, KEIKICHI, TAKEMURA, YOSHIHIKO, YAJIMA, KENICHI
Applicants: KAWASAKI HEAVY IND LTD, KAWASAKI JUKOGYO KABUSHIKI KAISHA
Application year: 1988
Main classification: C21B13
IPC codes: C21B13