FLUIDIZED BED TYPE REDUCTION APPARATUS FOR IRON ORES AND METHOD FOR REDUCING IRON ORES USING THE APPARATUS

Abstract: A three-stage, fluidized-bed-type reduction apparatus and a method for using it for reducing fine iron ores of wide size ranges, improving the gas utilization and reduction degree, reducing the residence time of iron ores, and increasing the prereduction rate of reduced iron. The apparatus includes serially arranged a drying/preheating furnace with a first cyclone connected to it, a primary prereduction furnace with a second cyclone connected to it, a secondary high-gas-velocity reduction furnace for finally reducing only a coarse ore portion of the prereduced iron ores at a bubbling fluidized state while carrying over the medium/fine ore portion of the iron ores, a secondary low-gas-velocity reduction furnace for finally reducing the medium/fine ores forming a bubbling fluidized bed thereof, an inner cyclone installed in the secondary low-gas-velocity reduction furnace, and a third cyclone for capturing dusty ores not captured byt the inner cyclone.

Description: FLUIDIZED BED TYPE REDUCTION APPARATUS FOR IRON ORES AND METHOD ' FOR REDUCING IRON ORES USING THE APPARATUS BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to a fluidized-bed-type reduction apparatus for reducing fine iron ores in the procedure of producing pig iron or ingot iron and a method for reducing fine iron ores using such an apparatus, and more particularly to a fluidized-bed-type reduction apparatus capable of efficiently fine reducing iron ores of wide size ranges in a stably fluidized state and a method for reducing fine iron ores using the apparatus. Description of the Prior Art Generally, conventional methods for producing pig iron from reduced iron ores include a method using blast furnaces and a method using shaft furnaces. In the latter method, iron ores reduced in a shaft furnace are melted in an electric furnace. In the case of the method for producing pig iron using blast furnaces, a large amount of coke is used as a heat source and reducing agent. In accordance with this method, iron ores are charged in the form of sintered ores in order to improve the gas-permeability and reduction. To this end, conventional methods using blast furnaces need a coke oven for producing coking coal and equipment for producing sintered ores. For this reason, the method using blast furnaces is a method requiring a huge investment and high energy consumption. Since high quality coking coal is maldistributed in the world and the amount of its reserves are diminishing, the shortage thereof becomes severe as the demand for steel increases. On the other hand, the method of reducing iron ores using shaft furnaces requires a pre-treating step for pelletizing iron ores. Since this method also uses natural gas as a heat source and reducing agent, it has a drawback that it can be commercially implemented only in areas where an easy supply of natural gas is ensured. Recently, a smelting reduction method capable of producing ingot iron from iron ores using non-coking coal in place of coke has been remarkable as a new iron production method. Such a smelting reduction method typically employs a system wherein iron ores pre-reduced in a separate furnace are completely reduced in a melting furnace to produce hot metal. In the reduction furnace, iron ores are reduced in a solid phase before they are melted. In other words, iron ores charged in the reduction furnace are reduced while being in contact with hot reducing gas generated in the melting furnace. The reduction process used in this method is classified into two types, a moving bed type and a fluidized bed type, depending on the condition how iron ores are in contact with the reducing gas. It has been known that one of the most promising methods for the reduction of fine ore ores of wide size distribution is the fluidized-bed-type process wherein the ores are reduced in a fluidized state by a reducing gas supplied through a distributor which is installed in the lower part of the reaction furnace. An example of the fluidized-bed-type reduction furnace is disclosed in Japanese Utility Model Laid-open Publication No. Sho. 58-217615, and its construction is illustrated in FIG. 1. As shown in FIG. 1, this furnace includes a cylindrical furnace body 1 in which fine iron ores are reduced. The cylindrical furnace body 1 is provided with an ore inlet 2, a reducing gas inlet 3, an reduced ore outlet 4, and an exhaust gas outlet 5. In this fluidized-bed-type furnace, fine iron ores are first charged into the cylindrical furnace body 1 through the ore inlet 2. When a reducing gas is supplied in the furnace through a gas distributor 6 at an appropriate flow rate, the iron ores form a fluidized bed above the distributor so that they can be mixed and agitated with the reducing gas, and in this state the iron ores can be reduced by the reducing gas. The reducing gas supplied in the reduction furnace forms bubbles in a layer of iron ores as if a fluid is boiled and then rises through the particle layer, thereby forming a bubbling fluidized bed. The reduced iron ores are discharged from the furnace through the ore outlet 4 and then fed to a melter gasifier. For this fluidized-bed-type reduction apparatus, it is necessary to minimize the flow rate of the reducing gas while forming an effective fluidized bed so as not only to reduce the elutriation of iron ores, but also to increase the efficiency of the reducing gas. To this end, the grain size of iron ores should be strictly limited to a certain range. In other words, the flow rate of the reducing gas required to form an effective fluidized bed is strongly dependent upon the grain size of iron ores to be reduced. As the result, it is impossible to treat iron ores of wide grain size ranges. For this reason, the grain size of iron ores to be charged in the above-mentioned fluidized bed type furnace is limited typically to 1 mm or below. However, at least 50% of iron ores supplied as a raw material in iron making industry is in the size of larger than 1 mm. In such a fluidized-bed-type reduction furnace, therefore, iron ores are screened in terms of their grain size before using, and those of a larger grain size are charged either into the reduction furnace after being pulverized to a required grain size or directly into a shaft furnaces having the above-mentioned drawback. As the result, there is an increase in installation cost due to an increase in the number of processing steps. The inventors proposed the present invention which can solve the above-mentioned problems encountered in the conventional methods based on the results of their research and experiments. SUMMARY OF THE INVENTION Therefore, an object of the invention is to provide a three-stage, fluidized-bed-type reduction apparatus which includes a single-type drying/preheating furnace for drying and preheating iron ores, a single-type primary reduction furnace for pre-reducing the fine iron ores, and twin-type final reduction furnace for finally reducing iron ores, thereby being capable of efficiently reducing iron ores of wide size ranges in a stably fluidized state, maximizing the gas use rate in its fluidized-bed-type reduction furnaces, reducing the residence time of iron ores, and increasing the production rate of reduced iron, and to provide an optimal method for reducing the fine iron ores using this apparatus. Another object of the invention is to provide a fluidized-bed-type reduction apparatus which includes a drying/preheating furnace for drying and preheating iron ores and twin type reduction furnaces for primarily and finally reducing the iron ores, thereby being capable of efficiently reducing fine iron ores of wide size ranges in a stably fluidized state, achieving a uniform reduction irrespective of the grain size of iron ores, and improving the reduction rate, gas use rate and gas consumption rate, and to provide an optimal method for reducing the fine iron ores using this apparatus. In accordance with one aspect, the present invention provides a fluidized bed type reduction apparatus for reducing fine iron ores of wide size distribution, comprising: a drying/preheating furnace for drying and preheating iron ores supplied from a hopper; a first cyclone for capturing dusty iron ores contained in exhaust gas from the drying/preheating furnace; a primary reduction furnace for pre-reducing the dried/preheated iron ores and dust ores captured by the first cyclone in a bubbling fluidized state; a second cyclone for capturing dusty iron ores contained in exhaust gas from the primary reduction furnace; a secondary high-gas-velocity reduction furnace for finally reducing only a coarse ore portion of the pre- reduced iron ores in a bubbling fluidized state while elutriating a medium/fine ore portion of the iron ores; a secondary low-gas-velocity reduction furnace for finally reducing the medium/fine ores elutriated from the secondary high-gas-velocity reduction furnace while forming a bubbling fluidized bed thereof; and a third cyclone for capturing dusty ores contained in exhaust gas from the secondary low-gas-velocity ore reduction furnace. In accordance with another aspect, the present invention provides a fluidized-bed-type reduction apparatus for reducing fine iron ores of wide size distribution, comprising: a drying/preheating furnace for drying and preheating iron ores supplied from a hopper; a first cyclone for capturing dusty iron ores contained in exhaust gas from the drying/preheating furnace; a primary high-gas- velocity reduction furnace for primarily reducing (pre- reducing) only a coarse ore portion of the preheated iron ores discharged from the drying/preheating furnace at a bubbling fluidized state while elutriating a medium/fine ore portion of the preheated iron ores; a primary low-gas- velocity reduction furnace for pre-reducing the medium/fine iron ores elutriated from the primary high-gas-velocity reduction furnace while forming a bubbling fluidized bed thereof; a second cyclone for capturing dusty iron ores contained in exhaust gas from the primary low-gas-velocity reduction furnace; a secondary high-gas-velocity reduction furnace for finally reducing the coarse iron ores pre- reduced in the primary high-gas-velocity reduction furnace while forming a bubbling fluidized bed thereof; a secondary low-gas-velocity reduction furnace for finally reducing the medium/fine iron ores pre-reduced in the primary low-gas- velocity reduction furnace while forming a bubbling fluidized bed thereof; a third cyclone for capturing dusty iron ores contained in exhaust gas from the secondary low- gas-velocity reduction furnace, the third cyclone re¬ circulating the captured dusty iron ores to either a lower portion of the secondary low-gas-velocity reduction furnace or a lower portion of a elter gasifier; and a fourth cyclone for capturing dusty ores contained in exhaust gas from the secondary high-gas-velocity reduction furnace. In accordance with another aspect, the present invention provides a method for reducing fine iron ores of wide size distribution, comprising the steps of: drying and preheating iron ores supplied from a hopper in a drying/ preheating furnace; pre-reducing the dried/preheated iron ores in a primary reduction furnace; finally reducing a coarse ore portion of the pre-reduced iron ores in a secondary high-gas-velocity reduction furnace while forming a bubbling fluidized bed thereof; and finally reducing a medium/fine ore portion of the pre-reduced iron ores in a secondary low-gas-velocity reduction furnace while forming a bubbling fluidized bed thereof. In accordance with another aspect, the present invention provides a method for reducing fine iron ores of wide size distribution, comprising the steps of: drying and preheating iron ores supplied from a hopper in a drying/preheating furnace; pre-reducing a coarse ore portion of the dried/ preheated iron ores in a primary high-gas-velocity - reduction furnace while forming a bubbling fluidized bed thereof; pre-reducing a medium/fine ore portion of 'the dried/preheated iron ores in a primary low-gaε-velocity reduction furnace; finally reducing the pre-reduced coarse iron ores in a secondary high-gas- velocity reduction furnace while forming a bubbling fluidized bed thereof; and finally reducing the pre-reduced medium/fine iron ores in a secondary low-gas-velocity reduction furnace while forming a bubbling fluidized bed thereof. BRIEF DESCRIPTION OF THE DRAWINGS Other objects and aspects of the invention will become apparent from the following description of embodiments in conjunction with the accompanying drawings in which: FIG. 1 is a schematic diagram illustrating a conventional fluidized-bed-type reduction furnace for reducing iron ores; and FIG. 2 is a schematic diagram illustrating a fluid- bed-type reduction apparatus for reducing fine iron ores of wide size distribution in accordance with an embodiment of the present invention; FIG. 3 is a schematic diagram illustrating a fluid- bed-type reduction apparatus for reducing fine iron ores of wide size distribution in accordance with another embodiment of the present invention; FIG. 4 is a schematic diagram illustrating a fluid- bed-type reduction apparatus for reducing fine iron ores of wide size distribution in accordance with another embodiment of the present invention; FIG. 5 is a graph illustrating the degradation of iron ore with reduction time in a fluidized bed; FIG. 6 is a graph illustrating variations in terminal velocity with the pressure of the reducing gas, respectively represented for the pre-reduction (pre- reduction) and final reduction at different grain sizes; and FIG. 7 is a graph illustrating variations in minimum fluidizing velocity with the pressure of the reducing gas, respectively represented for the pre-reduction (pre¬ reduction) and final reduction at different grain sizes. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS In FIG. 2, a fluidized-bed-type reduction apparatus for reducing fine iron ores of wide size distribution in accordance with the present invention is illustrated. As shown in FIG. 2, the fluidized-bed-type reduction apparatus includes a drying/preheating furnace 10 for drying and preheating iron ores supplied from a hopper 90, a primary reduction furnace 20 for pre-reducing the dried/preheated iron ores from the drying/preheating furnace 10, a secondary high-gas-velocity reduction furnace 30 for finally reducing a coarse ore portion of the iron ores pre-reduced in the primary reduction furnace by forming a bubbling fluidized bed thereof while elutriating a medium/fine ore portion of the iron ores into a secondary low-gas-velocity reduction furnace, and a secondary low- gas-velocity reduction furnace 40 for finally reducing the medium/fine iron ores supplied from the secondary high-gas- velocity reduction furnace 30 while forming a bubbling fluidized bed thereof. The drying/preheating furnace 10 has a enlarged-upper- reduced-lower cylindrical body in which the diameter of its lower portion is smaller than that of its upper portion. That is, the body of the drying/preheating furnace 10 includes an enlarged upper-cylindrical section 101, an intermediate tapered section 102 of which diameter increases smoothly from its lower end to its upper end, and a reduced lower-cylindrical section 103. The drying/ preheating furnace 10 is provided at the bottom portion with a first gas inlet 11 for receiving exhaust gas from the primary reduction furnace 20. A first gas distributor 12 is installed at the lower section 103 of the drying/preheating furnace 10 to evenly distribute the exhaust gas supplied through the first gas inlet 11 in the drying/preheating furnace 10. The drying/preheating furnace 10 is also provided at the side wall of its lower section 103 with a first ore inlet 15 for receiving iron ores from the hopper 90, a first coarse ore outlet 13 for discharging a coarse ore portion of the dried/preheated iron ores from the drying/preheating furnace 10 and a first medium/fine ore outlet 14 for discharging a medium/fine ore portion of the dried/preheated iron ores from the drying/preheating furnace 10. A first exhaust gas outlet 16 is provided at the side wall of the upper section 101 to discharge exhaust gas from the drying/preheating furnace 10. The first ore inlet 15 is connected to the hopper 90 via an ore supply line 91 so that iron ores can be fed therethrough. The first exhaust gas outlet 16 is connected to a first cyclone 50 via a first exhaust gas line 53. Both the first coarse and medium/fine ore outlets 13 and 14 are connected to the primary reduction furnace 20 via a first duct line 17 so that iron ores can be fed therethrough. The first cyclone 50 has a first dusty ore discharge line 51 and a first cleaned gas discharge line 52. The first dusty ore discharge line 51 is connected at one end thereof to the first duct line 17 so that iron ores are fed therethrough. Preferably, a first purging gas line P which stems from a second cleaned gas discharge line 62 of the primary reduction furnace 20, is connected to a bended portion of the first duct line 17 so as to supply a small amount of purging gas to the first duct line 17, thereby preventing the duct line 17 from being plugged by iron ores being fed in the line 17. ' For the same purpose, another purging gas line (not shown) connected to the second cleaned gas discharge line 62 may be connected to a bended portion of the first dusty ore discharge line 51. Similarly to the drying/preheating furnace 10, the primary reduction furnace 20 has a enlarged-upper-reduced- lower cylindrical furnace body in which the diameter of its lower portion is smaller than that of its upper portion. That is, the body of the primary reduction furnace 20 includes an enlarged upper-cylindrical section 201, an intermediate tapered section 202 of which diameter increases smoothly from its lower end to its upper end, and a reduced lower-cylindrical section 203. The primary reduction furnace 20 is provided at the bottom portion thereof with a second gas inlet 21 for receiving exhaust gas from the secondary low-gaε-velocity reduction furnace 40. A second gas distributor 22 is installed at the lower section 203 of the primary reduction furnace 20 to evenly distribute the exhaust gas supplied through the second gas inlet 21 in the primary reduction furnace 20. The primary reduction furnace 20 is also provided at the side wall of its lower section 203 with a second ore inlet 25 for receiving iron ores from the drying/preheating furnace 10, a second coarse ore outlet 23 for discharging a coarse ore portion of the pre-reduced iron ores from the primary reduction furnace 20 and a second medium/fine ore outlet 24 for discharging a medium/fine ore portion of the pre-reduced iron ores from the primary reduction furnace 20. A second exhaust gas outlet 26 is provided at the side wall of the upper section 201 to discharge exhaust gas from the primary reduction furnace 20. The second ore inlet 25 is connected to the drying/ preheating furnace 10 via the first circulation line 17 so as to receive the dried/preheated iron ores from the drying/preheating furnace 10. The second exhaust gas outlet 26 is connected to a second cyclone 60 via a second exhaust gas line 63 so that exhaust gas is fed therethrough. Both the second coarse and medium/fine ore outlets 23 and 24 are connected to the secondary high-gas- velocity reduction furnace 30 via a second duct line 27 so that iron ores are fed therethrough. To the bottom portion of the second cyclone 60, a second dusty ore discharge line 61 is connected at its one end. The second clean gas discharge line 62 which branch out the first purging gas line P 1 is connected at one end thereof to the top portion of the second cyclone 60. The second dusty ore discharge line 61 is connected at the other end thereof to the second duct line 27. Preferably, a second purging gas line P 2 , which stems from a first reducing gas supply line 31a of the secondary high-gas-velocity reduction furnace 30, is connected to a bended portion of the second dusty ore discharge line 61 so as to supply a small amount of purging gas to the line 61, thereby preventing the line 61 from being plugged by iron ores being fed in the line 61. For the same purpose, another purging gas line (not shown) connected to the first reducing gas supply line 31a may be connected to a bended portion of the second duct line 27. The second cleaned gas discharge line 62 is connected to the first gas inlet 11 of the drying/preheating furnace 10. On the other hand, the second duct line 27 is connected at one end thereof to a third ore inlet 35 of the secondary high-gas-velocity reduction furnace 30 in order to supply the pre-reduced iron ores from the primary reduction furnace 20. Meanwhile, the secondary high-gas-velocity reduction furnace 30 has a cylindrical body having a uniform diameter throughout its length. The secondary high-gas-velocity reduction furnace 30 is provided at the bottom portion thereof with a third gas inlet 31 for receiving a reducing gas from a reducing gas source (not shown). A third gas distributor 32 is installed at the lower portion of the secondary high-gas-velocity reduction furnace 30 to distribute the reducing gas supplied through the third gas inlet 31 in the secondary high-gas-velocity reduction furnace 30. At one side of the lower portion of the secondary high-gas-velocity reduction furnace 30, a third ore inlet 35 is provided to receive the pre-reduced iron ores from the primary reduction furnace 20 along with dusty iron ores captured by the first cyclone 60. The third gas inlet 31 is connected to the first reducing gas supply line 31a which is connected to the reducing gas source (not shown) . The secondary high-gas-velocity reduction furnace 30 also has a third coarse ore outlet 33 for outwardly discharging a coarse ore portion of the finally reduced iron ores in the secondary high-gas-velocity reduction furnace 30, and a third medium/fine ore outlet 34 for discharging(elutriating) a medium/fine ore portion of the finally reduced iron ores from the secondary high-gas- velocity reduction furnace 30 to the secondary low-gaε- velocity reduction furnace 40 along with exhaust gas from the secondary high-gas-velocity reduction furnace 30. To the third coarse ore outlet 33, a third coarse ore discharge line 33a is connected. On the other hand, the third medium/fine ore outlet 34 is connected to a secondary low-gas-velocity reduction furnace 40 via a third medium/fine ore discharge line 34a. The secondary low-gas-velocity reduction furnace 40 has a enlarged-upper-reduced-lower cylindrical body in which the diameter of its lower portion is smaller than that of its upper portion. That is, the body of the secondary medium/fine ore reduction furnace 40 includes an enlarged upper-cylindrical section 401, an intermediate tapered section 402 of which diameter increases smoothly from its lower end to its upper end, and a reduced lower- cylindrical section 403. The secondary low-gas-velocity reduction furnace 40 is provided at the bottom portion thereof with a fourth gas inlet 41 for receiving a reducing gas from a reducing gas source (not shown) . A fourth gas distributor 42 is stalled at the lower section 403 of the secondary low-gas-velocity reduction furnace 40 to homogeneously distribute the reducing gas supplied through the fourth gas inlet 41 in the secondary low-gas-velocity reduction furnace 40. The fourth gas inlet 41 is connected to a second reducing gas supply line 41a communicating with the reducing gas source (not shown) so that a reducing gas can be fed therethrough. The second reducing gas supply line 41a may be merged with the first reducing gas supply line 31a. The secondary low-gaε-velocity reduction furnace 40 is also provided at the side wall of its lower section 403 with a third medium/fine ore outlet 44 for outwardly diεcharging medium/fine iron ores finally reduced in the secondary low-gaε-velocity reduction furnace 40. Alεo, the εecondary low-gaε-velocity reduction furnace 40 is provided at the side wall of its upper section 401 with a third exhaust gas outlet 46 for discharging exhaust gas from the εecondary low-gas-velocity reduction furnace 40. Preferably, an inner cyclone 80 is installed in the secondary low-gas-velocity reduction furnace 40. The secondary low-gaε-velocity reduction furnace 40 reduces the medium/fine iron ores received therein at a bubbling fluidized state by the reducing gas merged from the εecondary high-gas-velocity reduction furnace 30 and secondary low-gas-velocity reduction furnace 40. At this time, an ultra-fine, namely, dusty portion of the iron ores is elutriated by exhaust gas in the εecondary low-gas- velocity reduction furnace 40. The inner cyclone 80 receives the dusty- iron ores entrained in the exhaust gas from the secondary low-gaε-velocity reduction furnace 40 and separates them from the exhauεt gaε, and then re¬ circulates them to the lower portion of the εecondary low- gaε-velocity reduction furnace 40 while diεcharging the exhauεt gaε to a third cyclone 70 via a third exhaust gaε line 73. The fourth medium/fine ore outlet 44 is connected to a reduced iron storing veεεel (not εhown) via a third duct line 44a. The third exhaust gas line 73 is connected to the third cyclone 70 through the third exhaust gaε outlet 46. The third cyclone 70 is connected at its bottom portion to one end of a third dusty ore discharge line 71 and at its top portion to one end of a third cleaned gas discharge line 72. The other end of the third dusty ore discharge line 71 is connected to the third duct line 44a. The third dusty ore discharge line 71 intersects with the third medium/fine ore discharge line 34a. Preferably, a bi-directional discharge valve 71a is installed at the intersection between the lines 34a and 71 so that the dusty iron oreε captured in the third cyclone 70 can be re¬ circulated to the lower portion of the εecondary low-gas- velocity reduction furnace 40 or supplied to a subsequent stage. If the εecondary low-gaε-velocity reduction furnace 40 does not include the inner cyclone 80, the third cyclone 70 is directly connected to the secondary low-gas-velocity reduction furnace 40 via the third exhaust gas line 73. In order to increase the upper limit of grain size of the coarse ore portion of the iron ores to be reduced in the fluidized-bed-type reduction apparatus of the present invention, the drying/preheating furnace 10, primary reduction furnace 20 and εecondary coarεe ore reduction furnace 30 may have a more reduced dimension at their lower sections. In this connection, FIG. 3 illustrates a fluidized-bed-type reduction apparatus constructed to accommodate an increase in the grain size of the coarεe ore portion of iron ores to be reduced in accordance with another embodiment of the present invention. In FIG. 3, elements respectively corresponding to those in FIG. 2 are denoted by the same reference numerals. As εhown in FIG. 3, the fluidized-bed-type reduction apparatus, which is denoted by the reference numeral 200, includes a drying/preheating furnace IOA having a double- tapered cylindrical structure, a primary reduction furnace 20A having a double-tapered cylindrical structure, and a εecondary high-gaε-velocity reduction furnace 30A having a single-tapered cylindrical structure. That is, the drying/preheating furnace IOA includes an upper section 101A, a first tapered section 102A, an intermediate section 104A, a second tapered section 105A, and a lower section 103A. Similar to the drying/preheating furnace IOA, the primary reduction furnace 20A includes an upper section 201A, a first tapered section 202A, an intermediate section 204A, a second tapered section 205A, and a lower section 203A. On the other hand, the εecondary coarse ore reduction furnace 30A includes an upper section 30A, a tapered section 302A and a lower εection 303A. In accordance with either embodiment of the present invention, the fluidized-bed-type reduction apparatus may include a elter gasifier 99 for melting finally-reduced coarse and medium/fine ores to produce ingot iron. In this instance, exhaust gaε generated from the melter gaεifier 99 may be used as a reducing gaε for the εecondary low-gas- velocity reduction furnace 40 and the secondary high-gas- velocity reduction furnace 30A as can be seen in FIG. 3. The reason why the drying/preheating furnaces 10 and 10A, primary reduction furnaces 20 and 20A, and the secondary high-gaε-velocity reduction furnace 30 have the tapered cylindrical εtructure is not only to actively fluidize iron ores including medium/fine ore portions at each lower furnace section, thereby improving the utilization degree of the reducing gaε and the gaε consumption rate, but also to decrease the gas velocity of the reducing gaε in each upper furnace section, thereby suppreεεing the iron oreε from being elutriated. The iron ores to be treated in the drying/preheating furnaces 10 and IOA, and primary reduction furnaces 20 and 20A include a coarse ore portion as well aε medium/fine ore portionε. On the other hand, in the εecondary high-gas- velocity reduction furnaces 30 and 30A, only the coarεe iron oreε εhould be reduced whereaε in the secondary low- gas-velocity reduction furnace 40 only the medium/fine iron ores should be reduced. For this reason, the gas velocities used in these furnaces are different each other. For the drying/preheating furnaces 10 and 10A, primary reduction furnaces 20 and 20A, and secondary low-gas- velocity reduction furnace 40, it is preferred that the gas velocity of the reducing gas in each associated lower furnace section should be kept equal to 1.5 to 3.0 times the minimum fluidizing velocity of the iron oreε. The gas velocity of the reducing gas in each upper furnace section, however, should be kept lower than the terminal velocity of the iron ores being elutriated. For the secondary high- gas-velocity reduction furnaces 30 and 30A which are in a cylindrical shape involving a uniform gas velocity throughout its length, it is preferred that the gas velocity of the reducing gas be kept equal to 1.5 to 3.0 times the minimum fluidizing velocity in order to achieve a bubbling fluidization of coarse ores. In order to provide an optimum gas velocity, the drying/preheating furnace 10 and primary reduction furnace 20 preferably have, at each asεociated lower εection thereof, an inner diameter equal to 1.2 to 1.8 times the inner diameter of the secondary high-gas-velocity reduction furnace 30. If the inner diameter of each lower section of the furnaces 10 and 20 is smaller than 1.2 times the inner diameter of the furnace 30, the gaε velocity of the reducing gaε in the furnaceε 10 and 20 becomeε too high and it reεultε in an exceεεively faεt fluidization of fine iron oreε. In thiε caεe, the reaction between the iron oreε and the reducing gaε hardly takes place. On the other hand, if the inner diameter of each lower section of the furnaces 10 and 20 is larger than 1.8 times the inner diameter of the furnace 30, the gaε velocity of the reducing gaε in the furnaces 10 and 20 becomes too low and it results in a defluidizing phenomenon. It is also preferred that the drying/preheating furnace 10 and primary reduction furnace 20 have, at each asεociated lower section thereof, a height equal to 7.0 to 12.0 times the inner diameter of the same lower εection. It is also preferred that the secondary high-gas-velocity reduction furnace 30 have a height equal to 20 to 30 times the inner diameter thereof. Preferably, the drying/preheating furnace 10 and primary reduction furnace 20 also have, at each associated upper section thereof, an inner diameter equal to 3.5 to 5.0 times the inner diameter of the secondary high-gaε- velocity reduction furnace 30. If the inner diameter of each upper εection of the furnaces 10 and 20 is smaller than 3.5 times the inner diameter of the furnace 30, the gas velocity of the reducing gas in the furnaces 10 and 20 becomes too high and it results in an excessively fast fluidization of fine iron ores. On the other hand if the inner diameter of each upper sectionε of the furnaceε 10 and 20 is larger than 5.0 times the inner diameter of the furnace 30, the gaε velocity of the reducing gaε in the furnaces 10 and 20 becomeε too low and causes a defluidization phenomenon. The drying/preheating furnace 10 and primary reduction furnace 20 preferably have, at each aεεociated upper section thereof, a height equal to 2.0 to 4.0 times the inner diameter of the same upper εection. Meanwhile, the εecondary low-gas-velocity reduction furnace 40 preferably has, at the lower section thereof, an inner diameter equal to 2.0 to 3.0 times the inner diameter of the εecondary high-gaε-velocity reduction furnace 30. If the inner diamer of lower section of the furnace 40 is smaller than 2.0 times the inner diameter of the furnace 30, the gaε velocity of the reducing gaε in the furnace 40 becomeε too high and it cauεe an exceεεively faεt fluidization of fine iron oreε. If the inner diameter of lower εection of the furnace 40 iε larger than 3.0 timeε the inner diameter of the furnace 30, the gas velocity of the reducing gas in the furnace 40 becomeε too low and it causeε a defluidization phenomenon. Preferably, the secondary low-gas-velocity reduction furnace 40 also has, at the upper section thereof, an inner diameter equal to 2.8 to 4.0 times the inner diameter of the secondary high-gas-velocity reduction furnace 30. If the inner diameter of upper εection of the furnace 40 iε smaller than 2.8 times the inner diameter of the furnace 30, the gas velocity of the reducing gaε in the furnace 40 becomes too high and it causeε an excessively fast fluidization of fine iron oreε. On the other hand, if the inner diameter of the upper εection of the furnace 40 is larger than 4.0 times the inner diameter of the furnace 30, the gas velocity of the reducing gas in the furnace 40 becomes too low and it causes a defluidization phenomenon. It is also preferred that the secondary low-gas- velocity reduction furnace 40 have, at the lower section thereof, a height equal to 7.0 to 12.0 times the inner diameter of the same lower section and, at the upper section thereof, a height equal to 2.0 to 4.0 times the inner diameter of -the same upper section. On the other hand, in the case of the fluidized-bed- type reduction apparatus εhown in FIG. 3, the lower εection 303A of the secondary high-gaε-velocity reduction furnace 30A, which coverε from the diεtributor 32 to a height correεponding to 5 to 15% of the entire height of the furnace 30A, iε modified in reduced inner diameter equal to 0.5 to 0.9 timeε the inner diameter of the upper εection 301A. The lower εection 103A of the drying/preheating furnace IOA, which coverε from the diεtributor 12 to a height correεponding to 5 to 15% of the entire height of the furnace IOA, iε modified in reduced inner diameter equal to 0.5 to 0.9 times the inner diameter of the intermediate section 104A. The lower εection 203A of the primary reduction furnace 20A, which coverε from the diεtributor 22 to a height corresponding to 5 to 15% of the entire height of the furnace 20A, iε also modified in reduced inner diameter equal to 0.5 to 0.9 times the inner diameter of the intermediate section 204A. With such εtructureε, in the case of the drying/preheating furnace 10A and primary reduction furnace 20A, the gas velocity in each lower furnace section becomes higher than that in each asεociated intermediate furnace εection by 1.3 to 4.0 timeε. In the caεe of the εecondary high-gas-velocity reduction furnace 30A, the gas velocity in the lower furnace section becomeε higher than that in the upper furnace εection by 1.3 to 4.0 times with the above- mentioned modification. Accordingly, it becomes posεible to make the fluidization of coarse iron ores diεtributed in the lower εection of each furnace more active. As a result, sticking among the coarse iron oreε due to their defluidization at a high temperature can be prevented. Furthermore, the maximum grain εize of fluidizable iron oreε can be increaεed, thereby widening the usable range of raw materials. Preferably, the εecondary high-gaε-velocity reduction furnace 30A has, at the lower section thereof, a height equal to 2.0 to ' 3.0 times the inner diameter of the same lower εection and, at the upper εection thereof, a height equal to 15 to 20 timeε the inner diameter of the same upper section. Based on theories and experimentε, the inventors found out that the degradation of iron ores is almost completed at the early stage of the fluidizing reduction carried out at a high temperature and also that the gas consumption rate can be improved by carrying out the fluidizing reduction separately for two different grain size portions of iron oreε, namely, coarεe and medium/fine grain size portions. The inventors also discovered that the gaε velocities required for the fluidization of fine iron ores are the same between the pre-reduction and the final reduction because the density of iron ores decreases as the reduction is carried out, while the density of the reducing gas increaseε. Taking theεe facts into consideration, the present invention proposeε another fluidized bed type reduction apparatuε which iε illustrated in FIG. 4. As shown in FIG. 4, this apparatus, which iε denoted by the reference numeral 300, includes a drying/preheating furnace 10B for drying and preheating iron ores supplied from a hopper 90B while forming a bubbling fluidized bed thereof. Downstream of the drying/preheating furnace 10B, a primary high-gas-velocity reduction furnace 20C is arranged to elutriate a medium/fine ore portion of the dried/preheated iron ores by the exhaust gas in the furnace 20C, thereby separating them from a coarse ore portion of the iron ores. The primary high-gas-velocity reduction furnace 20C primarily reduces only the coarεe ore portion of the dried/preheated iron oreε remained in the furnace 20C while forming a bubbling fluidized bed thereof. The apparatuε alεo includeε a primary low-gaε-velocity reduction furnace 20B for primarily reducing the elutriated medium/fine iron ores from the furnace 20C while forming a bubbling fluidized bed thereof, a secondary high-gaε- velocity reduction furnace 30B for finally reducing the primarily reduced coarεe iron oreε while forming a bubbling fluidized bed thereof, and a secondary low-gas-velocity reduction furnace 40B for finally reducing the primarily reduced medium/fine iron oreε while forming a bubbling fluidized bed thereof. The drying/preheating furnace 10B is provided at the bottom portion thereof with a firεt gaε inlet 11B for receiving exhauεt gas from a second cyclone 60B. A first gas distributor 12B is installed at the lower section of the drying/preheating furnace 10B. The drying/preheating furnace 10B is also provided at one side thereof with a first ore inlet 15B for receiving iron oreε from the hopper 9OB, and at the other side thereof with a first coarse ore outlet 13B and a first medium/fine ore outlet 14B. These ore outlets 13B and 14B are connected to the primary high- gas-velocity reduction furnace 20C via a first duct line 17B so that iron oreε can be fed therethrough. A firεt exhauεt gas outlet 16B is provided at the side of the upper section 101 opposite to the firεt ore inlet 15B to discharge exhaust gas from the drying/preheating furnace 10B. This first exhaust gas outlet 16B iε connected to a firεt cyclone 50B via a first exhaust gas line 53B. The first cyclone 50B iε adapted to capture duεty iron ores contained in the exhaust gas from the drying/preheating furnace 10B. The first cyclone 50B iε connected at itε bottom portion to a firεt duεty ore discharge line 5IB and at its top portion to a first cleaned exhauεt gaε line 52B. The firεt duεty ore diεcharge line 5IB iε connected at one end thereof to the firεt duct line 17B εo that iron ores can be fed therethrough. The primary low-gas-velocity reduction furnace 20B iε provided at the bottom portion thereof with a εecond gaε inlet 2IB for receiving exhauεt gaε from a third cyclone 70B. A εecond gas diεtributor 22B iε inεtalled at the lower εection of the primary low-gaε-velocity reduction furnace 20B. The primary low-gaε-velocity reduction furnace 2OB iε alεo provided at one side of its lower section with a second medium/fine ore outlet 24B and at the other side of the lower section with a second medium/fine ore inlet 25B. The primary low-gas-velocity reduction furnace 20B alεo haε a εecond exhauεt gaε outlet 26B at the εide of itε upper εection oppoεite to the εecond medium/fine ore outlet 24B in order to diεcharge exhauεt gaε from the primary low-gas-velocity reduction furnace 20B. The second cyclone 60B serveε to capture duεty iron oreε contained in the exhauεt gaε from the primary low-gaε- velocity reduction furnace 20B. The εecond cyclone 60B iε connected at itε bottom portion to a εecond duεty ore discharge line 6IB and at its top portion to a second cleaned exhauεt gaε line 62B. The second exhauεt gas outlet 26B is connected to the second cyclone 60B via a second exhauεt gas line 63B so that gaε can be fed therethrough. The primary high-gas-velocity reduction furnace 20C is provided at the bottom portion thereof with a third gas inlet 21C for receiving exhauεt gas from a fourth cyclone 70C. A third gas distributor 22C iε inεtalled at the lower εection of the primary high-gaε-velocity reduction furnace 20C. The primary high-gas-velocity reduction furnace 20C iε alεo provided at one side of itε lower εection with a εecond coarεe ore outlet 23C and at one εide of the upper section with a third medium/fine ore outlet 24C. This outlet 24C can alεo be an exhaust gas outlet from the furnace 20C. The primary high-gaε-velocity reduction furnace 20C alεo has a third ore inlet 25C at the other side of itε lower section. The third medium/fine ore outlet 24C iε connected to the εecond ore inlet 25B of the primary low-gaε-velocity reduction furnace 2OB via a firεt medium/fine ore discharge line 24aC so that oreε are fed therethrough. The second duεty ore discharge line 61B intersects with the first medium/fine ore discharge line 24aC. Preferably, a bi¬ directional discharge valve 64B iε inεtalled at the intersection between lines 24aC and 6IB so that the dusty iron ores captured in the second cyclone 60B can be re- circulated to the primary low-gaε-velocity reduction furnace 20B or directly re-circulated to the lower εection of the εecondary low-gaε-velocity reduction furnace 40B. The εecondary high-gaε-velocity reduction furnace 30B iε provided at the bottom portion thereof with a fourth gaε inlet 3IB for receiving exhauεt gaε from a melter gaεifier 99 via a fifth exhauεt gaε line 99b. A fourth gaε diεtributor 32B iε inεtalled at the lower εection of the εecondary high-gaε-velocity reduction furnace 30B. The secondary high-gas-velocity reduction furnace 30B is also provided at one side of itε lower εection with a fourth ore inlet 35B and at the other εide of the lower εection with a third coarεe ore outlet 33B. The secondary high-gas- velocity reduction furnace 30B also has a third exhaust gaε outlet 36B at the side of its upper εection oppoεite to the fourth ore inlet 35B in order to diεcharge exhauεt gas from the εecondary high-gaε-velocity reduction furnace 30B. The fourth cyclone 70C εerveε to capture dusty iron ores contained in the exhaust gas from the secondary high- gas-velocity reduction furnace 30B. The fourth cyclone 70C is connected at its bottom portion to a fourth dusty ore discharge line 71C and at its top portion to a fourth cleaned exhaust gaε line 72C being .connected at one end thereof with the third gaε inlet 21C. The third exhauεt gaε outlet 36B iε connected to the fourth cyclone 70C via a fourth exhauεt gas discharge line 73C so that gas iε fed upεtream therethrough. The εecondary high-gas-velocity reduction furnace 30B is connected to the primary high-gaε-velocity reduction furnace 20C via a firεt coarεe ore diεcharge line 35aB and to the melter gasifier 99 via a second coarse ore discharge line 33aB so that ores can be fed downstream therethrough. The secondary low-gaε-velocity reduction furnace 40B is provided at the bottom portion thereof with a fifth gas inlet 4IB for receiving exhaust gas from the melter gasifier 99 via a sixth exhaust gas discharge line 99C. A fifth gaε diεtributor 42B is installed at the lower section of the secondary low-gaε-velocity reduction furnace 40B. The εecondary low-gas-velocity reduction furnace 40B is also provided at one εide of its lower section with a fifth ore inlet 43B and a third medium/fine ore outlet 44B. At the other side of the lower section, a sixth ore inlet 45B is provided. The secondary low-gaε-velocity reduction furnace 40B alεo haε at one εide of itε upper εection a fourth exhauεt gaε outlet 46B. Preferably, an inner cyclone 80B iε installed in the secondary low-gas-velocity reduction furnace 40B. The inner cyclone 80B separateε duεty oreε, which are elutriated by the merged reducing gas in the secondary low- gas-velocity reduction furnace 40B, from exhaust gaε generated from the secondary low-gas-velocity reduction furnace 40B. The inner cyclone 80B re-circulates the εeparated duεty iron oreε to the lower εection of the εecondary low-gaε-velocity reduction furnace 40B while diεcharging the cleaned exhauεt gas to a third cyclone 7OB via a third exhaust gas line 73B. The sixth ore inlet 45B is connected to the second medium/fine ore outlet 24B via a second medium/fine ore diεcharge line 45aB. The εecond dusty ore discharge line 61B is connected at one end thereof to the εecond medium/fine ore diεcharge line 45aB. The third medium/fine ore outlet 44B iε connected to the lower portion of the melter gaεifier 99 via a third medium/fine ore 'diεcharge line 43aB. The fifth ore inlet 43B iε connected to the fourth cyclone 70C via the fourth duεty ore diεcharge line 71C. The third medium/fine ore discharge line 43aB is connected to the fourth dusty ore discharge line 71C via a second duct line 47B. The third cyclone 7OB iε connected at itε bottom portion to a third duεty ore diεcharge line 7IB and at itε top portion to a third cleaned exhauεt gas line 72B being connected at one end thereof with the εecond circulation line 47B. When the εecondary medium/fine ore reduction furnace 40B doeε not include the inner cyclone 80B, the third cyclone 7OB iε directly connected to the εecondary low-gaε-velocity reduction furnace 40B via the third exhauεt gaε diεcharge line 73B. The third dusty ore discharge line 7IB interεectε with the fourth duεty ore diεcharge line 71C. Preferably, a bi-directional discharge valve 74B is inεtalled at the intersection between lines 7IB and 71C. At the connection between the fourth dusty ore diεcharge line 71c and εecond circulation line 47B, another bi-directional diεcharge valve 74C iε preferably provided. Preferably, purging gas lines P are connected to each bended portion of the firεt circulation line 17B, firεt coarεe ore diεcharge line 35aB and εecond medium/fine ore diεcharge line 45aB εo aε to supply a small amount of purging gas to those lineε, thereby preventing the lineε from being plugged by iron oreε being fed through the lineε. The drying/preheating furnace 10B, primary low-gaε- velocity reduction furnace 2OB and εecondary low-gas- velocity reduction furnace 40B all have a enlarged-upper- reduced-lower cylindrical furnace body in which the diameter of their lower portions iε εmaller than thoεe of their upper portions. That is, each body of theεe furnaceε includes an enlarged upper cylindrical εection, an intermediate tapered εection increaεing in diameter from itε lower end to itε upper end, and a reduced lower cylindrical εection. On the other hand, the primary and εecondary high-gaε-velocity reduction furnaceε 20C and 30B have a cylindrical furnace body having a uniform diameter throughout itε length. It iε preferred that the drying/preheating furnace 10B, primary low-gas-velocity reduction furnace 20B and secondary low-gas-velocity reduction furnace 40B have, at each asεociated lower section thereof, a height equal to 7.0 to 12.0 times the inner diameter of the same lower section. It is alεo preferred that the primary and εecondary high-gas-velocity reduction furnaces 20C and 30B have a height equal to 20 to 30 times the inner diameter thereof. In FIG. 4, the reference numeral 99a denotes a molten iron outlet. Now, a method for producing reduced iron or molten pig iron using the fluidized bed type reduction apparatus having the arrangement shown in FIG. 2 in accordance with the preεent invention will be described. Aε εhown in FIG. 2, fine iron ores contained in the hopper 90 are supplied to the drying/preheating furnace 10 through the ore supply line 91 and first ore inlet 15. The drying/preheating furnace 10 is also supplied with exhaust gas from the primary reduction furnace 20 through the second cyclone 60, the second cleaned exhaust gas line 62 and first gas inlet 11. Thiε exhauεt gaε is uniformly distributed in the drying/preheating furnace 10 by means of the first gas distributor 12. By the uniformly distributed gaε, the iron oreε are dried and preheated at a bubbling fluidized εtate. The dried/preheated iron oreε are then fed to the primary reduction furnace 20 via the coarεe ore outlet 13, medium/fine ore outlet 14, firεt duct line 17 and εecond ore inlet 25. The exhaust gaε iε discharged outward from the drying/preheating furnace 10, in which iron ores are dried and preheated by the exhauεt gaε before being exhauεted via the firεt exhauεt gas outlet 16, first exhaust gaε line 53, firεt cyclone 50 and firεt cleaned gaε diεcharge line 52 in order. Dusty iron oreε contained in the exhauεt gas are captured in the first cyclone 50 and then recirculate to the primary reduction furnace 20 via the first dusty ore discharge line 51 and second ore inlet 25. The dried/preheated iron ores supplied in the primary reduction furnace 20 are then pre-reduced at a bubbling fluidized state by the exhaust gas which is fed to the primary reduction furnace 20 via the third cyclone 70, third cleaned exhaust gas line 72, second gas inlet 21 and second gas distributor 22. The pre-reduced iron ores are discharged through either the coarεe ore outlet 23 or medium/fine ore outlet 24, and then fed to the εecondary high-gaε-velocity reduction furnace 30 via the εecond duct line 27 and third ore inlet 35. In the primary reduction furnace 20, exhauεt gaε is also generated while iron ores are primarily reduced. This exhaust gas iε diεcharged from the primary reduction furnace 20 via the εecond exhauεt gaε outlet 26, εecond exhauεt gaε line 63, εecond cyclone 60 and εecond cleaned exhauεt gaε line 22 in order and then introduced in the drying/preheating furnace 10. Duεty iron oreε contained in the exhauεt gaε are captured in the second cyclone 60 and then recirculated to the secondary high-gas-velocity reduction furnace 30 via the second dusty ore discharge line 61, second circulation line 27 and third ore inlet 35. The εecondary high-gaε-velocity reduction furnace 30 finally reduceε a coarεe ore portion of the iron oreε received therein at a bubbling fluidized εtate by a reducing gaε εupplied in the furnace 30 via the firεt reducing gas supply line 31a, third gas inlet 31 and third gas diεtributor 32. At this time, a medium/fine ore portion of the iron oreε iε elutriated by exhauεt gaε generated in the εecondary high-gaε-velocity reduction furnace 30. The gaε velocity of the reducing gaε εupplied in the secondary coarεe ore reduction furnace 30 should always be higher than the terminal velocity of medium/fine ore. The medium/fine iron ores elutriated by the exhaust gaε are then carried over to the εecondary low-gaε-velocity reduction furnace 40 via the third medium/fine ore outlet 34, third medium/fine ore diεcharge line 34a and fourth ore inlet 45. On the other hand, the finally reduced coarse iron ores are fed to a reduced iron storing veεεel or to the melter gaεifier 99 (aε in the case of FIG. 3) via the εecond coarεe ore outlet 33 and third coarεe ore discharge line 33a. Meanwhile, the medium/fine iron ores supplied in the secondary low-gaε-velocity reduction furnace 40 are finally reduced at a bubbling fluidized εtate by a reducing gaε εupplied in the furnace 40 via the εecond reducing gaε supply line 41a, fourth gas inlet 41 and fourth gaε diεtributor 42. The finally reduced medium/fine iron oreε are fed to the reduced iron εtoring veεεel or to the melter gaεifier 99 (aε in the caεe of FIG. 3) via the εecond medium/fine ore outlet 44 and third medium/fine ore diεcharge line 44a. The flow rate of the reducing gaε εupplied in the εecondary medium/fine ore reduction furnace 40 εhould alwayε be higher than the mean minimum fluidizing velocity of medium/fine ore. Exhauεt gaε generated during finally reducing medium/fine iron oreε in the secondary low-gas-velocity reduction furnace 40 iε introduced in the primary reduction furnace 20 after being diεcharged through the inner cyclone 80, third exhauεt gaε outlet 73, third cyclone 70, and third clean gaε diεcharge line 72 in order. Duεty iron ores contained in the exhauεt gaε are separated from the exhauεt gaε in the inner cyclone 80 and then recirculated to the third duct line 44a via the third duεty ore discharge line 71. When the reduced iron ores are charged into the melter gasifier 99, they are melted, thereby producing molten pig iron. On the other hand, the reduced iron stored in the reduced iron storing vessel will be εubεequently tranεported to a next εtage for variouε purpoεe. On the other hand, it iε preferred that the gaε velocity of gaε in each lower εection of the drying/preheating furnaceε 10 and IOA, primary reduction furnaceε 20 and 20A, and εecondary low-gaε-velocity reduction furnace 40 iε kept at a value equal to 1.5 to 3.0 timeε the minimum fluidizing velocity of the iron oreε reεiding in each aεεociated fluidized bed. For these furnaces, it is also preferred that the gas velocity of gaε in each upper section be kept lower than the terminal velocity of the iron ores being elutriated. For the secondary high-gas-velocity reduction furnaceε 30 and 30A, the gaε velocity of gas preferably is kept at a value equal to 1.5 to 3.0 times the minimum fluidizing velocity of coarse iron ore. It is also preferred that the pressure and temperature of gas εupplied to each of the εecondary low-gas-velocity reduction furnace 40 and secondary high- gaε-velocity reduction furnaceε 30 and 30A range from 2 to 4 atm. and 800 to 900°C, reεpectively. For the drying/preheating furnaceε 10 and 10A, primary reduction furnaceε 20 and 20A, and εecondary low-gas-velocity reduction furnace 40, the presεure drop occurring in each furnace preferably rangeε from 0.3 to 0.6 atm. and the temperature drop occurring in each furnace preferably ranges from 30 to 80°C. Preferably, the residence time of iron oreε in each -furnace iε 20 to 40 minuteε. Another method for producing reduced iron or molten pig iron uεing the fluidized bed type reduction apparatuε having the arrangement εhown in FIG. 4 in accordance with the preεent invention will now be deεcribed. Aε εhown in FIG. 4, fine iron oreε contained in the hopper 9OB are supplied to the drying/preheating furnace 10B and then dried and preheated in the drying/preheating furnace 10B by exhaust gas supplied from the second cyclone 60B. The dried/preheated iron ores are then fed to the lower portion of the primary high-gas-velocity reduction furnace 20C which receives exhaust gaε from the fourth cyclone 70C aε a reducing gaε. The gaε velocity of the reducing gaε the primary high-gaε-velocity reduction furnace 20C iε controlled to be higher than the terminal velocity of medium/fine ore, thereby elutriating a medium/fine ore portion of the iron oreε by the exhaust gas. Aε a result, this medium/fine ore portion is separated from the coarse ore portion of the iron ores. The coarse ores remained in the primary coarse ore reduction furnace 20C are primarily reduced while forming a bubbling fluidized bed. The coarse oreε are then fed to the εecondary high-gas-velocity reduction furnace 3OB through the second coarse ore outlet 23C. On the other hand, the medium/fine ores elutriated by the exhaust gas in the primary high-gas-velocity reduction furnace 20C are transported to the lower section of the primary medium/fine ore reduction furnace 20B via the first medium/fine ore discharge line 24aC. In the primary low-gas-velocity reduction furnace 2OB, the medium/fine ores are primarily reduced at a bubbling fluidized state by exhaust gaε εupplied from the third cyclone 70B in a value of higher than the mean minimum fluidizing velocity of medium/fine ore. After being reduced, the medium/fine oreε are fed to the εecondary low-gas-velocity reduction furnace 40B. In the primary low-gas-velocity reduction furnace 20B, duεty oreε are elutriated by the reducing gaε supplied in the primary low-gas-velocity reduction furnace 2OB via either the firεt medium/fine ore diεcharge line 24aC or second gas ore inlet 2IB. These duεty oreε are then fed along with exhauεt gaε from in the primary low-gaε-velocity reduction furnace 20B to the εecond cyclone 60B, in which the dusty ores are captured. By the bi-directional discharge valve 64B, the captured dusty ores are re¬ circulated to the primary low-gaε-velocity reduction furnace 20B or εecondary low-gas-velocity reduction furnace 40B so that they will be primarily or finally reduced. The reduced medium/fine and duεty oreε are diεcharged through the εecond medium/fine ore outlet 24B and εecond dusty ore discharge line 6IB, reεpectively. The coarεe oreε supplied in the εecondary high-gaε-velocity reduction furnace 30B are finally reduced while forming a bubbling fluidized bed in the furnace 30B by exhauεt gas εupplied from the melter gasifier 99. After being reduced, the coarse ores are charged into the melter gasifier 99 via the third coarse ore outlet 33B and second coarεe ore discharge line 33aB. The melter gasifier 99 melts the received ores, thereby producing pig iron. The gaε velocity of the reducing gaε εupplied in the secondary high-gas-velocity reduction furnace 3OB is equal to that in the primary high-gas-velocity reduction furnace 20C. Dusty iron ores elutriated from the secondary high- gaε-velocity reduction furnace 3OB are captured in the fourth cyclone 70C and then circulated to the secondary low-gaε-velocity reduction furnace 40B or directly charged into the melter gasifier 99 by the use of the bi¬ directional discharge valve 74C. The medium/fine oreε εupplied in the εecondary low-gaε-velocity reduction furnace 40B are finally reduced at a bubbling fluidized bed in the furnace 40B by exhauεt gas supplied from the melter gasifier 99. The gas velocity of the reducing gas supplied in the εecondary low-gaε-velocity reduction furnace 40B iε equal to that in the primary low-gaε-velocity ore reduction furnace 2OB. Duεty iron oreε elutriated in the εecondary low-gaε-velocity reduction furnace 40B are captured by the inner cyclone 80B and third cyclone 70B and then recirculated to the εecondary low-gas-velocity reduction furnace 40B so that they can be finally reduced. The dusty iron ores captured in the third cyclone 70B may also be directly charged to the melter gaεifier 99 by the use of the bi-directional discharge valve 74B. The finally reduced medium/fine and dusty ores are discharged through the third medium/fine ore outlet 44B and third dusty ore discharge line 7IB, respectively. The oreε are then charged into the melter gaεifier 99 via the third medium/fine ore outlet 43aB. The melter gaεifier 99 eltε the received ores, thereby producing pig iron. In accordance with this embodiment, two twin type fluidized furnaces (the primary low-gas-velocity reduction furnace 20B, primary high-gas-velocity reduction furnace 20C, secondary low-gas-velocity reduction furnace 4OB, and secondary high-gas-velocity reduction furnace 30B) are employed in order to reduce the gaε consumption rate as much aε posεible. At the pre-reduction εtep, iron ores are separated into a coarse ore portion and a medium/fine portion so that theεe portionε can be individually pre- reduced at a fluidized state. At the final reduction step, the ore portions are also individually subjected to a final reduction. The apparatus of this embodiment utilizes the advantage of the conventional twin type fluidized beds 2OB and 20C in which the upper portion is connected to the lower portion via a duct line 24aC. Accordingly, iron oreε can be appropriately εeparated into a coarεe ore portion and a medium/fine ore portion from the pre-reduction εtage. Since the degradation of iron oreε iε almoεt completed at the preheating and pre-reduction εtep, the εeparated coarεe and medium/fine oreε are pre-reduced and finally reduced independently. In the final reduction εtage, however, a twin fluidized bedε but without a connection between the low-gaε-velocity and high-gaε-velocity reduction furnaceε iε employed. With εuch a εtructure, the present invention solveε the problem involved in the conventional twin type εtructure cauεed by the mixing of gaεeε. Since the primary and εecondary high-gaε-velocity reduction furnaceε 20C and 3OB, or the primary and secondary low-gas-velocity reduction furnaceε 20B and 40B are the εame in εhape and di enεion, they can be connected in a gaε εtream. In thiε caεe, the fluidization operations of these two reactors can be εimultaneouεly controlled in a gaε velocity. Accordingly, it iε possible to εimplify the entire operation of the apparatuε. In accordance with the preεent invention, the εingle type drying/preheating furnace 10B, twin type primary low-gaε-velocity reduction furnace 20B and twin type εecondary low-gaε-velocity reduction furnace 40B have a enlarged-upper-reduced-lower cylindrical body in which the diameter of their lower portionε iε εmaller than thoεe of their upper portionε εo aε not only to obtain an active bubbling fluidization of iron oreε in each reduced lower section thereof, thereby increasing the gaε utilization degree, but alεo to decreaεe the gaε velocity of gaε in its enlarged upper εection thereof, thereby εuppreεεing an elutriation of duεty ores while reducing the gas consumption rate. For theεe furnaceε, the gaε velocity of the reducing gaε in each lower εection εhould be equal to 1.5 to 3.0 timeε the minimum fluidizing velocity of the iron oreε to be charged in a fluidized bed of each associated furnace. The gas velocity of the reducing gaε in each upper section, however, should be lower than the terminal velocity of the iron ores being elutriated. For the primary and secondary high-gas-velocity reduction furnaces 20C and -30B each having a uniform cylindrical structure involving a uniform gas velocity throughout its length, the gaε velocity of the reducing gaε εhould be equal to 1.5 to 3.0 timeε the minimum fluidizing velocity of coarse iron ore. It is also preferred that the preεεure and temperature of gas supplied to each of the secondary high-gas-velocity reduction furnace 30B and εecondary low-gaε-velocity reduction furnace 40B range from 2 to 4 atm. and 800 to 900°C, reεpectively. For the drying/preheating furnace 10B, primary and εecondary low-gaε-velocity reduction furnaceε 20B and 40B, and primary and εecondary high-gaε- velocity reduction furnaceε 20C and 30B, the preεεure drop occurring in each furnace preferably rangeε from 0.3 to 0.6 atm. and the temperature drop occurring in each furnace preferably rangeε from 30 to 80°C. Preferably, the residence time of iron ores in each furnace is 20 to 40 minutes. The present invention will be understood more readily with reference to the following exampleε; however theεe exampleε are only intended to illuεtrate the invention and are not to be conεtrued to limit the εcope of the present invention. Example 1 A fluidized-bed-type reduction apparatuε having the conεtruction aε εhown in FIG. 2 waε prepared. Thiε fluidized-bed-type reduction apparatuε had the following dimenεion: 1) Inner Diameter and Height of Each Fluidized-Bed- Type Furnace - Drying/Preheating Furnace Inner Diameter of Lower Section: 0.3 m; Height of Lower Section: 4.0 m; Inner Diameter of Upper Section: 0.90 m; Height of Upper Section: 2.50 m; - Primary Reduction Furnace Inner 'Diameter of Lower Section: 0.3 m; Height of Lower Section: 4.0 m; Inner Diameter of Upper Section: 0.90 m; Height of Upper Section: 2.50 m; - Secondary High-Gas-Velocity Reduction Furnace Inner Diameter: 0.2 m; Height: 5.20 m; - Secondary Low-Gas-Velocity Reduction Furnace Inner Diameter of Lower Section: 0.50 m; Height of Lower Section: 2.70 m; Inner Diameter of Upper Section: 0.90 m; Height of Upper Section: 2.30 m. Using the fluidized-bed-type reduction apparatus as mentioned above, the production of reduced iron and molten pig iron was carried out under the following conditionε: 2) Charge and Diεcharge of Iron Oreε - Compoεition of Fine Iron Oreε T.Fe: 62.36%, Si0 2 : 5.65%, Al 2 0 3 : 2.91%, S: 0.007%, and P: 0.065%; - Particle Size Diεtribution < 0.25 mm = 22%, 0.25 mm - 1.0 mm = 28%, and 1.0 mm - 5.0 mm = 50%; - Feed Rate: 6 Kg/ in - Diεcharge Rate from Third Coarεe Ore Outlet: 3.0 Kg/min - Diεcharge Rate from Third Medium/Fine Ore Outlet: 1.2 Kg/min - Diεcharge Rate from Third Duεty Ore Outlet: 3.0 Kg/min 3) Reducing Gas - Composition : CO: 65%, H 2 : 25%, and C0 2 + H 2 0: 10%; - Temperature : about 850°C; and - Preεεure : 2.3 Kgf/cm 2 4) Superficial Gaε Velocity in Each Furnace Drying/Preheating Furnace - Superficial Gaε Velocity in Lower Section: 2.22 m/ε; - Superficial Gaε Velocity in Upper Section: 0.25 m/ε; Primary Reduction Furnace - Superficial Gaε Velocity in Lower Section: 2.22 m/ε; - Superficial Gaε Velocity in Upper Section 0.25 m/ε; Secondary High-Gaε-Velocity Reduction Furnace: 3.0 m/ε; Secondary Low-Gaε-Velocity Reduction Furnace - Superficial Gaε Velocity in Lower Section: 0.32 m/ε; - Superficial Gaε Velocity in Upper Section: 0.25 m/ε; Under the above conditionε, it waε poεεible to maintain the iron oreε at a bubbling fluidized εtate in each of the drying/preheating furnace 10, primary reduction furnace 20, εecondary high-gas-velocity reduction furnace 30 and secondary low-gas-velocity reduction furnace 40. The mean gas utilization degree was about 29%, and the gas consumption rate was about 1,290 Nm 3 /t-ore. Mean reduction degree of iron oreε reεpectively diεcharged through the third coarεe ore outlet 33 of the εecondary high-gaε-velocity reduction furnace 30A, the third medium/fine ore outlet 43 of the εecondary low-gaε- velocity reduction furnace and the third duεty ore diεcharge line 71 were 87%, 89% and 84%. After 60 minuteε elapεed from the beginning of the reduction, reduced iron waε diεcharged. The reduce iron discharged from the third coarse ore outlet 33 of the εecondary high-gaε-velocity reduction furnace 30 haε a grain εize diεtribution conεiεting of < 0.2 mm: 8%, 0.2 mm - 0.5 mm: 11%, 0.5 mm - 1.0 mm: 14%, 1.0 mm - 3.0 mm: 57% and 3.0 mm - 5.0 mm: 10%. On the other hand, the reduce iron diεcharged from the third medium/fine ore outlet 43 of the εecondary low-gas-velocity reduction furnace 40 has a grain size distribution consiεting of < 0.05 mm: 1.0%, 0.05 mm - 0.1 mm: 17%, 0.1 mm - 0.2 mm: 72%, and 0.2 mm - 0.5 mm: 10%. The amount and grain εize of the reduced iron diεcharged from each outlet could be controlled by the gaε velocity of the reducing gaε being εupplied. A variation in reduction degree could alεo be found when the residence time of iron oreε in each furnace waε changed. Example 2 A fluidized-bed-type reduction apparatuε having the conεtruction aε shown in FIG. 3 was prepared. This fluidized-bed-type reduction apparatuε had the following dimenεion: 1) Inner Diameter and Height of Each Fluidized-Bed- Type Furnace - Drying/Preheating Furnace Inner Diameter of Lower Section near Diεtributor: 0.22 m; Height of Lower Section near Diεtributor: 0.70 m; Inner Diameter of Intermediate Section: 0.30 m; Height of Intermediate Section: 3.50 m; Inner Diameter of Upper Section: 0.90 ; Height of Upper Section: 2.50 m; - Primary Reduction Furnace Inner Diameter of Lower Section near Diεtributor: 0.22 m; Height of Lower Section near Diεtributor: 0.70 m; Inner Diameter of Intermediate Section: 0.30 m; Height of Intermediate Section: 3.50 m; Inner Diameter of Upper Section: 0.90 m; Height of Upper Section: 2.50 m; - Secondary High-Gas-Velocity Reduction Furnace Inner Diameter of Lower Section near Diεtributor: 0.26 ; Height of Lower Section near Diεtributor: 0.50 ; Inner Diameter of Upper Section: 0.30 m; Height of Upper Section: 4.70 m; - Secondary Low-Gaε-Velocity Reduction Furnace Inner Diameter of Lower Section: 0.50 m; Height of Lower Section: 2.70 m; Inner Diameter of Upper Section: 0.90 m; Height of Upper Section: 2.30 m. Uεing the fluidized-bed-type reduction apparatuε aε mentioned above, the production of reduced iron and molten pig iron waε carried out under the following conditions: 2) Charge and Discharge of Iron Ores - Composition of Fine Iron Ores T.Fe: 62.36%, Si0 2 : 5.65%, Al 2 0 3 : 2.91%, S: 0.007%, and P: 0.065%; - Particle Size Diεtribution < 0.25 mm = 20%, 0.25 mm - 1.0 mm = 23%, 1.0 mm - 5.0 mm = 45%, and 5.0 mm - 8.0 mm = 12%; - Feed Rate: 6 Kg/min - Diεcharge Rate from Third Coarεe Ore Outlet: 3.0 Kg/min - Diεcharge Rate from Third Medium/Fine Ore Outlet: 1.2 Kg/min - Diεcharge Rate from Third Dusty Ore Outlet: 0.06 Kg/min 3) Reducing Gas - Composition : CO: 65%, H 2 : 25%, and C0 2 + H 2 0: 10%; - Temperature : about 850°C; and - Pressure : 2.3 Kgf/cm 2 4) Superficial Gas Velocity in Each Furnace Drying/Preheating Furnace - Superficial Gas Velocity in Lower Section near Distributor: 4.12 m/ε; - Superficial Gaε Velocity in Intermediate Section: 2.22 m/ε; - Superficial Gaε Velocity in Upper Section: 0.25 m/ε; Primary Reduction Furnace - Superficial Gaε Velocity in Lower Section near Diεtributor: 4.12 m/ε; - Superficial Gaε Velocity in Intermediate Section: 2.22 m/ε; - Superficial Gaε Velocity in Upper Section: 0.25 m/ε; Secondary High-Gaε-Velocity Reduction Furnace - Superficial Gas Velocity in Lower Section near Distributor: 4.0 m/s; - Superficial Gaε Velocity in Upper Section: 3.0 m/ε; Secondary Low-Gaε-Velocity Reduction Furnace - Superficial Gaε Velocity in Lower Section: 0.32 m/ε; - Superficial Gas Velocity in Upper Section: 0.25 m/s; A fluidizing reduction waε carried out for iron oreε of wide εize diεtribution, namely, iron oreε having a grain size of leεε than 8 mm while obtaining an active bubbling fluidization of coarεe iron oreε (otherwiεe it would be mainly diεtributed- in the lower εection of each furnace) under the above-mentioned conditionε. In thiε example, a mean reduction degree of 89% waε obtained at the εame gaε conεumption rate and gaε utilization degree aε thoεe in Example 1 related to the fluidizing reduction for iron oreε having a grain εize of leεε than 5 mm. Example 3 0.4 Kg of iron oreε having a grain εizeε ranging from 1 mm to 5 mm were reduced by a reducing gaε in a laboratory-scale fluidized-bed-type reduction furnace at a temperature of about 850°C. The εuperficial gaε velocity of the reducing gaε waε about 4 m/ε. The reducing gaε had a compoεition of CO: 65%, C0 2 : 0.5%, H 2 : 25% and Ar: 5%. The degradation of the iron oreε waε checked. The result is shown in FIG. 5. Reεpective terminal velocities at 1, 2, 3 and 4 atm. were calculated for the case of fluidizing iron oreε at 850°C uεing a reducing gaε which had a compoεition of CO: 46%, C0 2 : 15%, H 2 : 20%, H 2 0: 8% and N 2 : 11% in the primary reduction and a compoεition of CO: 59%, C0 2 : 3%, H 2 : 25%, H 2 0: 2% and N 2 : 11% in the final reduction. The reεultε are εhown in FIG. 6. Respective minimum fluidizing velocities at 1, 2, 3 and 4 atm. to obtain individual fluidizations for different grain sizes were calculated by the Wen and Yu formula for the case of fluidizing iron ores at 850°C using a reducing gas which had a composition of CO: 46%, C0 2 : 15%, H 2 : 20%, H 2 0: 8% and N 2 : 11% in the primary reduction and a composition of CO: 59%, C0 2 : 3%, H 2 : 25%, H 2 0: 2% and N 2 : 11% in the final reduction. The reεults are shown in FIG. 7. Referring to FIG. 5, it can be found that the degradation of iron oreε iε almoεt completed at the early εtage (in about 15 minuteε) of the hot fluidizing reduction. Referring to FIG. 6, it can be found that the terminal velocity in the primary reaction iε nearly equal to that in the final reaction. FIG. 7 εhowε that the minimum fluidizing velocity required in the primary reaction iε nearly equal to that required in the final reaction. The apparatuε having the construction of FIG. 4 is just the fluidized-bed-type reduction furnace made on the baεiε of such facts in accordance with the present invention. Example 4 The fluidized-bed-type reduction furnace used in this example had the construction εhown in FIG. 4, which includeε the drying/preheating furnace 10B for drying and preheating iron oreε, the primary low-gaε-velocity reduction furnace 20B for pre-reducing a medium/fine ore portion of the dried/ preheated iron oreε, and the εecondary low-gaε-velocity reduction furnace 40B for finally reducing the pre-reduced medium/fine oreε. Theεe furnaceε 10B, 20B and 40B have a enlarged-upper-reduced- lower cylindrical body in which the diameter of their lower portionε iε εmaller than thoεe of their upper portionε εo aε not only to obtain an active bubbling fluidization of iron ores in each reduced lower εection thereof, thereby increaεing the gaε utilization degree, but alεo to decreaεe the gaε velocity in their enlarged upper εection thereof, thereby εuppreεεing an elutriation of duεty oreε. The conεtruction alεo includes the primary high-gas-velocity reduction furnace 20C for pre-reducing only the coarse ore portion of the dried/preheated iron oreε while elutriating the medium/fine oreε, and the secondary high-gas-velocity reduction furnace 30B for finally reducing the pre-reduced coarse iron ores. These furnaces 20C and 30B have a cylindrical furnace body of a uniform diameter throughout its length. When a reducing gaε of about 850°C is supplied to both the secondary high-gas-velocity and low-gas-velocity reduction furnaces 30B and 40B respectively through their reducing gas inlets 3IB and 4IB and their gas distributorε 32B and 42B while charging iron oreε of wide size diεtribution to the drying/preheating furnace 10B. The iron oreε are dried and preheated while forming a bubbling fluidized bed in the furnace 10B. The dried/preheated iron ores are then fed to the primary high-gas-velocity reduction furnace 20C. In the primary high-gas-velocity reduction furnace 20C, coarεe iron ores are pre-reduced in a bubbling fluidized bed while elutriating the medium/fine portion of the dried/pre-heated. The pre-reduced coarse iron oreε are fed to the εecondary high-gaε-velocity reduction furnace 30B in which they are, in turn, finally reduced. After being finally reduced, the coarεe iron oreε are diεcharged through the third coarεe ore outlet 33B. On the other hand, the medium/fine iron ores carried over from the primary high-gaε-velocity reduction furnace 20C are pre-reduced in the primary low-gas-velocity reduction furnace 20B while forming a bubbling fluidized bed. After being pre-reduced, the medium/fine oreε are fed to the εecondary low-gaε-velocity reduction furnace 40B in which they are, in turn, finally reduced. The medium/fine iron ores are then diεcharged through the third medium/fine ore outlet 44B. The fluidized bed type reduction apparatuε used in this example had the following dimensionε: 1) Inner Diameter and Height of Each Fluidized-Bed- Type Furnace - Drying/Preheating Furnace Inner Diameter of Lower Section: 0.3 m; Height of Lower Section: 4.0 ; Inner Diameter of Upper Section: 0.9 m; Height of Upper Section: 2.5 ; - Primary High-Gas-Velocity Reduction Furnace Inner Diameter 0.2 m; Height 5.2 ; - Primary Low-Gas-Velocity Reduction Furnace Inner Diameter of Lower Section: 0.5 ; Height of Lower Section: 2.7 m; Inner Diameter of Upper Section: 0.9 ; Height of Upper Section: 2.3 m; - Secondary High-Gas-Velocity Reduction Furnace Inner Diameter: 0.2 ; Height: 5.2 ; - Secondary Low-Gas-Velocity Reduction Furnace Inner Diameter of Lower Section: 0.5 m; Height of Lower Section: 2.7 m; Inner Diameter of Upper Section: 0.9 ; Height of Upper Section: 2.3 m. Using the fluidized-bed-type reduction apparatus as mentioned above, the production of reduced iron and molten pig iron waε carried out under the following conditions: 2) Charge and Discharge of Iron Ores - Composition of Fine Iron Oreε T.Fe: 62.36%, Si0 2 : 5.65%, Al 2 0 3 : 2.91%, S: 0.007%, and P: 0.065%; - Particle Size Distribution < 0.25 mm = 22%, 0.25 mm - 1.0 mm = 28%, and 1.0 mm - 5.0 mm = 50%; - Feed Rate: 5 Kg/min - Discharge Rate from Third Coarse Ore Outlet: 1.7 Kg/min - Diεcharge Rate from Third Medium/Fine Ore Outlet: 2.2 Kg/min 3) Reducing Gaε - Compoεition : CO: 65%, H 2 : 25%, and C0 2 + H 2 0: 10%; - Temperature : about 850°C; and - Preεεure : 2.3 Kgf/cm 2 4) Superficial Gaε Velocity in Each Furnace Drying/Preheating Furnace - Superficial Gas Velocity in Lower Section: 2.22 m/ε; - Superficial Gas Velocity in Upper Section: 0.25 m/s; Primary High-Gas-Velocity Reduction Furnace: 3.0 m/s; Primary High-Gas-Velocity Reduction Furnace - Superficial Gas Velocity in Lower Section: 0.32 m/ε; - Superficial Gas Velocity in Upper Section: 0.25 m/s; Secondary High-Gas-Velocity Reduction Furnace: 3.0 m/s; Secondary Low-Gaε-Velocity Reduction Furnace - Superficial Gas Velocity in Lower Section: 0.32 m/ε; - Superficial Gas Velocity in Upper Section: 0.25 m/s; Under the above conditions, it waε possible to maintain the iron ores at a bubbling fluidized state in each of the drying/preheating furnace 10B, primary coarse ore reduction furnace 20B, primary medium/fine ore reduction furnace 20B, secondary high-gas-velocity reduction furnace 30B and secondary low-gas-velocity reduction furnace 40B. At the primary reduction step, the iron ores were εeparated into a coarεe ore portion and a medium/fine ore portion. These ore portions were individually subjected to the pre-reduction and final reduction. In thiε example, the mean gaε utilization degree was about 30%, and the gas consumption rate waε about 1,270 Nm 3 /t-ore. After 60 minuteε elapεed from the beginning of the reduction, reduced iron of a mean reduction degree of about 90% could be diεcharged through the third coarεe and medium/fine ore outletε 33B and 44B. Aε apparent from the above deεcription, in accordance with the preεent invention, iron oreε are εeparated in termε of the grain εize after being dried and preheated in the drying/pre-heating furnace 10B. Coarεe iron ores are pre-reduced in the primary high-gas-velocity reduction furnace 20B and then finally reduced in the secondary high- gaε-velocity reduction furnace 30B. On the other hand, medium/fine iron oreε are pre-reduced in the primary low- gaε-velocity reduction furnace 20B and then finally reduced in the εecondary low-gaε-velocity reduction furnace 40B. Accordingly, it iε poεεible to produce reduced iron of a relatively uniform reduction degree irrespective of the grain size of iron ores while obtaining a excellent segregation between coarse and medium/fine oreε. It iε alεo poεεible to εupply reduced iron with an appropriate εize depending on the supply plant and supply position to the melter-gaεifier. The amount and grain εize of the reduced iron discharged through either the third coarse ore outlet 33B or third medium/fine ore outlet 44B can be adjuεted by the gaε velocity of the reducing gaε. The reduction degree of iron ores can be alεo controlled by the residence time of the iron oreε in each furnace. Since all the three-εtage, fluidized-bed-type reduction operationε are carried out uεing the bubbling fluidization in accordance with the preεent invention, the gaε utilization degree and gaε conεumption rate can be improved. The fluidized-bed-type reduction apparatuε alεo haε a conεtruction capable of preventing a plugging of lines and a non-fluidization even after the operation is performed for a long period of time. Although the preferred embodiments of the invention have been disclosed for illustrative purposes, those skilled in the art will appreciate that various modificationε, additionε and εubstitutionε are poεεible, without departing from the εcope and baεic idea of the invention aε diεcloεed in the accompanying claimε.

Claims: WHAT IS CLAIMED IS: 1. A fluidized-bed-type reduction apparatus for reducing fine iron oreε of wide εize distribution, comprising: a drying/preheating furnace for drying and preheating fine iron oreε εupplied from a hopper, the drying/preheating furnace including a firεt gaε inlet provided at a bottom portion thereof, a firεt gas distributor installed at a lower portion thereof, a firεt ore inlet provided at a εide wall of the lower portion, a first coarse ore outlet provided at the εide wall of the lower portion, a firεt medium/fine ore outlet provided at the εide wall of the lower portion, and a firεt exhauεt gaε outlet provided at a εide wall of an upper portion thereof; a primary reduction furnace for pre-reducing the dried/preheated iron oreε εupplied from the drying/preheating furnace, the primary reduction furnace including a εecond gaε inlet provided at a bottom portion thereof, a εecond gaε diεtributor inεtalled at a lower portion thereof, a second ore inlet provided at a εide wall of the lower portion, a εecond coarεe ore outlet provided at the side wall of the lower portion, a second medium/fine ore outlet provided at the εide wall of the lower portion, and a εecond exhaust gaε outlet provided at a εide wall of an upper portion thereof; a εecondary high-gaε-velocity reduction furnace for finally reducing the pre-reduced iron oreε εupplied from the primary reduction furnace, the εecondary high-gas- velocity reduction furnace including a third gaε inlet provided at a bottom portion thereof, a third gaε diεtributor inεtalled at a lower portion thereof, a third ore inlet provided at a εide wall of the lower portion, a third coarεe ore outlet provided at the εide wall of the lower portion, and a third medium/fine ore outlet, which iε alεo an outlet of exhauεt gaε from the εecondary high-gaε- velocity reduction furnace, provided at a εide wall of an upper portion thereof; a εecondary low-gaε-velocity reduction furnace for finally reducing iron oreε elutriated from the εecondary high-gaε-velocity reduction furnace, the εecondary low-gaε- velocity reduction furnace including a fourth gaε inlet provided at a bottom portion thereof, a fourth gaε distributor installed in a lower portion thereof, a fourth ore inlet, which is also an inlet of gaε, provided at a εide wall of the lower portion, a fourth medium/fine ore outlet provided at the εide wall of the lower portion, and a third exhauεt gaε outlet provided at a εide wall of an upper portion thereof; a firεt cyclone for capturing duεty iron oreε contained in an exhauεt gaε from the drying/preheating furnace and recycling the captured duεty iron oreε to the primary reduction furnace while outwardly diεcharging the exhauεt gaε, the firεt cyclone being connected to the firεt exhauεt gaε outlet of the drying/preheating furnace via a first exhaust gaε line, being connected at a top portion thereof to a firεt cleaned exhauεt gas line, and being connected at a bottom portion thereof to a firεt duεty ore discharge line; a second cyclone for capturing duεty iron oreε contained in an exhauεt gas from the primary reduction furnace and recycling the captured duεty iron oreε to the secondary high-gaε-velocity reduction furnace while εupplying the cleaned exhauεt gaε to the drying/preheating furnace, the εecond cyclone being connected to the εecond exhauεt gaε outlet of the primary reduction furnace via a εecond exhauεt gaε line, being connected at a top portion thereof to the firεt gas inlet of the drying/preheating furnace via a second cleaned exhaust gaε line, and being connected at a bottom portion thereof to a εecond duεty ore diεcharge line; a third cyclone for capturing duεty iron oreε contained in an exhauεt gaε from the εecondary low-gaε- velocity reduction furnace and recirculating the cleaned exhauεt gaε to the primary reduction furnace, the third cyclone being connected to the εecondary low-gaε-velocity reduction furnace via a • third exhauεt gaε line, being connected at a top portion thereof to the εecond gas inlet of the primary reduction furnace via a third cleaned exhaust gas line, and being connected at a bottom portion thereof to a third dusty ore discharge line; a first duct line for connecting the firεt medium/fine and coarεe ore outlets of the drying/preheating furnace to the second ore inlet of the primary reduction furnace εo that the iron ores can be fed therethrough, and to the firεt duct line the first dusty ore discharge line is connected; a second duct line for connecting the second medium/fine and coarεe ore outlets of the primary reduction furnace to the third ore inlet of the secondary high-gas- velocity reduction furnace εo that the iron oreε can be fed therethrough, and to the εecond duct line the second dusty ore discharge line is connected; a third medium/fine ore discharge line for connecting the third medium/fine ore outlet to the fourth ore inlet of the secondary low-gaε-velocity reduction furnace εo that the iron ores can be fed therethrough; and a third duct line connected to the fourth medium/fine ore outlet of the secondary low-gas-velocity reduction furnace and to the third dusty ore outlet. 2. The fluidized-bed-type reduction apparatuε in accordance with claim 1, wherein each lower portion of the drying/preheating furnace and the first reduction furnace has an inner diameter eσual to 1.2 to 1.8 times the inner diameter of the secondary high-gas-velocity reduction furnace and each upper portion of them haε an inner diameter equal to about 3.5 to 5.0 timeε the inner diameter of the εecondary high-gas-velocity reduction furnace, and the lower portion of the secondary low-gas-velocity reduction furnace haε an inner diameter equal to about 2.0 to 3.0 timeε the inner diameter of the εecondary high-gaε- velocity reduction furnace and the upper portion of the f rnace haε an inner diameter equal to about 2.8 to 4.0 timeε the inner diameter of the εecondary high-gaε-velocity reduction furnace. 3. The fluidized-bed-type reduction apparatuε in accordance with claim 1 or 2, wherein each lower portion of the drying/preheating furnace, the primary reduction furnace and the εecondary low-gaε-velocity reduction furnace haε a height equal to 7.0 to 12.0 times its inner diameter and each upper portion of them has a height equal to 2.0 to 4.0 timeε itε inner diameter, and the εecondary high-gaε-velocity reduction furnace has a height equal to 20 to 30 times its inner diameter thereof. 4. The fluidized-bed-type reduction apparatuε in accordance with claim 1 or 2, wherein each of the drying/ preheating furnace and the primary reduction furnace compriεeε a cylindrical body including an enlarged upper εection, a firεt tapered εection increasing in diameter from itε lower end to itε upper end, an intermediate εection having a diameter εmaller than the upper εection, a εecond tapered εection increaεing in diameter from itε lower end to itε upper end, and a reduced lower section having a diameter smaller than the intermediate εection, and the secondary high-gas-velocity reduction furnace compriseε a cylindrical body including an enlarged upper εection, a tapered εection increaεing in diameter from itε lower end to its upper end, and a reduced lower section. 5. The fluidized-bed-type reduction apparatuε in accordance with claim 3, wherein each of the drying/preheating furnace and the primary reduction furnace compriεeε a cylindrical body including an enlarged upper section, a firεt tapered εection increaεing in diameter from itε lower end to itε upper end, an intermediate εection having a diameter smaller than the upper section, a second tapered section increasing in diameter from itε lower end to itε upper end, and a reduced lower εection having a diameter εmaller than the intermediate εection, and the εecondary high-gaε-velocity reduction furnace compriseε a cylindrical body including an enlarged upper section, a tapered section increasing in diameter from its lower end to itε upper end, and a reduced lower εection. 6. The fluidized-bed-type reduction apparatus in accordance with claim 1 or 2, wherein the third coarεe ore outlet and the third duct line are connected to a melter gaεifier εo that iron ores can be discharged into the melter gaεifier, and the third gaε inlet of the εecondary high-gaε-velocity reduction furnace and the fourth gaε inlet of the εecondary low-gaε-velocity reduction furnace are connected to the melter gaεifier εo that exhauεt gaε from the melter gaεifier can be supplied to the secondary high-gas-velocity and low-gas-velocity reduction furnaces. 7. The fluidized-bed-type reduction apparatus in accordance with claim 4, wherein the third coarse ore outlet and the third duct line are connected to a melter gaεifier εuch that iron oreε are diεcharged into the melter gasifier, and the third gas inlet of the secondary high- gas-velocity reduction furnace and the fourth gas inlet of the εecondary low-gaε-velocity reduction furnace are connected to the melter gaεifier εo that exhauεt gaε from the melter gaεifier iε εupplied to the εecondary high-gaε- velocity and low-gaε-velocity reduction furnaceε. 8. The fluidized-bed-type reduction apparatus in accordance with claim 5, wherein the third coarse ore outlet and the third duct line are connected to a melter gaεifier so that iron ores can be diεcharged into the melter gaεifier, and the third gaε inlet of the εecondary high-gaε-velocity reduction furnace and the fourth gaε inlet of the εecondary low-gaε-velocity reduction furnace are connected to the melter gasifier so that exhauεt gaε from the melting furnace can be εupplied to the secondary high-gas-velocity and low-gaε-velocity reduction furnaceε. 9. The fluidized-bed-type reduction apparatuε in accordance with claim 1 or 2, further compriεing an inner cyclone inεtalled in the upper portion of the εecondary low-gaε-velocity reduction furnace and connected to the third cyclone via the third exhauεt gaε line extending through the third exhauεt gaε outlet. 10. The fluidized-bed-type reduction apparatus in accordance with claim 3, further compriεing an inner cyclone inεtalled in the upper portion of the εecondary low-gaε-velocity reduction furnace and connected to the third cyclone via the third exhauεt gas line extending through the third exhaust gas outlet. 11. The fluidized-bed-type reduction apparatus in accordance with claim 4, further, comprising an inner cyclone installed in the upper portion of the secondary low-gas-velocity reduction furnace and connected to the third cyclone via the third exhaust gas line extending through the third exhauεt gaε outlet. 12. The fluidized-bed-type reduction apparatus in accordance with claim 5, further compriεing an inner cyclone inεtalled in the upper portion of the εecondary low-gaε-velocity reduction furnace and connected to the third cyclone via the third exhauεt gaε line extending through the third exhauεt gaε outlet. 13. The fluidized-bed-type reduction apparatus in accordance with claim 6, further comprising an inner cyclone installed in the upper portion of the εecondary low-gaε-velocity reduction furnace and connected to the third cyclone via the third exhauεt gaε line extending through the third exhauεt gaε outlet. 14. The fluidized-bed-type reduction apparatuε in accordance with claim 7 or 8, further compriεing an inner cyclone installed in the upper portion of the secondary low-gas-velocity reduction furnace and connected to the third cyclone via the third exhaust gaε line extending through the third exhauεt gaε outlet. 15. The fluidized-bed-type reduction apparatuε in accordance with claim 1 or 2, further compriεing firεt and εecond purging gaε lines connected to bended portions of the first and second duct lines for supplying a εmall amount of purging gaε to each correεponding line. 16. The fluidized-bed-type reduction apparatus in accordance with claim 3, further comprising firεt and εecond purging gaε lineε connected to bended portions of the firεt and εecond duct lineε for εupplying a small amount of purging gas to each correεponding line. 17. The fluidized-bed-type reduction apparatuε in accordance with claim 4, further comprising firεt and εecond purging gaε lineε connected to bended portionε of the first and second duct lineε for supplying a small amount of purging gas to each corresponding line. 18. The fluidized-bed-type reduction apparatus in accordance with claim 5, further comprising first and second purging gaε lineε connected to bended portions of the firεt and second duct lines for supplying a εmall amount of purging gas to each corresponding line. 19. The fluidized-bed-type reduction apparatus in accordance with claim 6, further compriεing firεt and εecond purging gaε lines connected to bended portionε of the first and εecond duct lineε for supplying a small amount of purging gaε to each correεponding line. 20. The fluidized-bed-type reduction apparatuε in accordance with claim 7 or 8, further compriεing firεt and εecond purging gaε lineε connected to bended portionε of the first and second circulation lines for εupplying a εmall amount of purging gas to each correεponding line. 21. The fluidized-bed-type reduction apparatuε in accordance with claim 9, further compriεing firεt and εecond purging gaε lineε connected to bended portionε of the firεt and εecond circulation lineε for supplying a small amount of purging gaε to each correεponding line. 22. The fluidized-bed-type reduction apparatus in accordance with any one of claimε 10 to 13, further comprising firεt and εecond purging gaε lineε connected to bended portionε of the firεt and εecond circulation lineε for supplying a small amount of purging gas to each corresponding line. 23. The fluidized-bed-type reduction apparatus in accordance with claim 14, further compriεing firεt and εecond purging gaε lineε connected to bended portionε of the first and second circulation lines for supplying a small amount of purging gaε to each corresponding line. 24. A fluidized-bed-type reduction apparatuε for reducing iron ores of wide size distribution, compriεing: a drying/preheating furnace for drying and preheating iron oreε supplied from a hopper by an exhauεt gaε from a second cyclone while forming a bubbling fluidized bed of the iron ores; a firεt cyclone for capturing duεty iron oreε contained in an exhauεt gas from the drying/preheating furnace; a primary high-gas-velocity reduction furnace for pre- reducing only a coarse ore portion of the dried/preheated iron ores diεcharged from the drying/preheating furnace at a bubbling fluidized εtate by an exhauεt gaε from a fourth cyclone while forming a bubbling fluidized bed of the coarεe iron oreε, the primary high-gaε-velocity reducing furnace alεo elutriating a medium/fine ore portion of the dried/preheated iron oreε by the exhauεt gaε from the fourth cyclone; a primary low-gaε-velocity reduction furnace for pre- reducing the medium/fine iron ores carried over to a lower portion thereof via the primary high-gas-velocity reduction furnace and a first medium/fine ore discharge line by an exhauεt gaε from a third cyclone while forming a bubbling fluidized bed of the medium/fine iron oreε; the second cyclone for capturing dusty iron oreε contained in exhauεt gaε from the primary low-gaε-velocity reduction furnace, the εecond cyclone re-circulating the captured duεty iron oreε to the εecond medium/fine ore diεcharge line which iε connected to the ore inlet of the primary low-gaε-velocity reduction furnace or directly re¬ circulating to a lower portion of the εecondary low-gaε- velocity reduction furnace; a εecondary high-gaε-velocity reduction furnace for finally reducing the pre-reduced coarεe iron oreε diεcharged from the primary high-gaε-velocity reduction furnace by an exhauεt gaε from a melter gasifier while forming a bubbling fluidized bed of the pre-reduced coarse iron ores; a secondary low-gas-velocity reduction furnace for finally reducing the pre-reduced medium/fine iron ores discharged from the primary low-gas-velocity reduction furnace by the exhaust gas from the melter gaεifier while forming a bubbling fluidized bed of the pre-reduced medium/fine iron oreε; the third cyclone for capturing dusty iron oreε contained in an exhauεt gaε from the secondary medium/fine ore reduction furnace, the third cyclone re-circulating the captured duεty iron oreε to the lower portion of the εecondary low-gas-velocity reduction furnace or directly diεcharging the captured duεty iron oreε to a lower portion of the melter gasifier; and the fourth cyclone for capturing duεty oreε contained in an exhauεt gas diεcharged from the εecondary high-gaε- velocity reduction furnace, the fourth cyclone re¬ circulating the captured duεty iron ores to the lower portion of the secondary low-gaε-velocity reduction furnace or directly diεcharging the captured duεty iron oreε to a lower portion of the melter gaεifier. 25. A fluidized-bed-type reduction apparatuε for reducing fine iron oreε of wide εize diεtribution, compriεing: a drying/preheating furnace for drying and preheating iron oreε εupplied from a hopper, the drying/preheating furnace including a first gaε inlet provided at a bottom portion thereof, a firεt gaε diεtributor inεtalled at a lower portion thereof, a first ore inlet provided at a side wall of the lower portion, a first coarse ore outlet provided at the side wall of the lower portion, a first medium/fine ore outlet provided at the side wall of the lower portion, and a firεt exhauεt gaε outlet provided at a εide wall of an upper portion thereof; a primary high-gaε-velocity reduction furnace for pre- reducing a coarεe ore portion of the dried/preheated iron oreε εupplied from the drying/preheating furnace at a bubbling fluidized εtate while elutriating a medium/fine ore portion of the dried/preheated iron oreε, the primary high-gaε-velocity reduction furnace including a third gas inlet provided at a bottom portion thereof, a third gaε diεtributor inεtalled at a lower portion thereof, a third ore inlet provided at a εide wall of the lower portion, a third coarεe ore outlet provided at the side wall of the lower portion, a second medium/fine ore outlet, which is also an outlet of exhaust gas from the primary high-gas- velocity reduction furnace provided at the εide wall of an upper portion thereof, and a third ore inlet provided at the εide wall of the lower portion; a primary reduction furnace for pre-reducing the dried/preheated iron oreε εupplied from the drying/preheating furnace, the primary reduction furnace including a εecond gaε inlet provided at a bottom portion thereof, a εecond gaε diεtributor inεtalled at a lower portion thereof, a εecond ore inlet provided at a εide wall of the lower portion, a εecond coarεe ore outlet provided at the εide wall of the lower portion, a second medium/fine ore outlet provided at the side wall of the lower portion, and a second exhaust gaε outlet provided at a εide wall of an upper portion thereof; a primary low-gaε-velocity reduction furnace for pre- reducing iron oreε carried over from the primary high-gaε- velocity reduction furnace while forming a bubbling fluidized bed thereof, the primary low-gaε-velocity reduction furnace including a εecond gaε inlet provided at a bottom portion thereof, a εecond gaε diεtributor inεtalled at a lower portion thereof, a εecond medium/fine ore outlet provided at the εide wall of the lower portion, and a εecond exhauεt gaε outlet provided at a εide wall of an upper portion thereof; a εecondary high-gaε-velocity reduction furnace for finally reducing the pre-reduced coarεe iron oreε εupplied from the primary high-gaε-velocity reduction furnace, the εecondary high-gaε-velocity reduction furnace including a fourth gaε inlet provided at a bottom portion thereof, a fourth gaε diεtributor inεtalled in a lower portion thereof, a fourth ore inlet provided at a εide wall of the lower portion, a third coarεe ore outlet provided at the εide wall of the lower portion, and a third exhauεt gaε outlet provided at a εide wall of an upper portion thereof; a εecondary low-gaε-velocity reduction furnace for finally reducing the medium/fine iron oreε εupplied from the primary low-gaε-velocity reduction furnace, the εecondary low-gaε-velocity reduction furnace including a fifth gaε inlet provided at a bottom portion thereof, a fifth gaε diεtributor inεtalled in a lower portion thereof, a fifth ore inlet provided at a εide wall of the lower portion, a εixth ore inlet provided at a εide of the lower portion, a third medium/fine ore outlet provided at the εide wall of the lower portion, and a fourth exhauεt gaε outlet provided at a εide wall of an upper portion thereof; a firεt cyclone for capturing dusty iron ores contained in an exhaust gaε diεcharged from the drying/preheating furnace and recirculating the collected duεty iron oreε to the primary high-gaε-velocity reduction furnace while outwardly diεcharging the cleaned exhauεt gaε, the firεt cyclone being connected to the firεt exhauεt gaε outlet of the drying/ preheating furnace via a firεt exhauεt gaε diεcharge line, being connected at a top portion thereof to a firεt cleaned exhauεt gaε line, and being connected at a bottom portion thereof to a firεt duεty ore discharge line; a second cyclone for capturing dusty iron oreε contained in an exhauεt gaε diεcharged from the primary low-gaε-velocity reduction furnace and recycling the captured duεty iron oreε to the primary low-gaε-velocity reduction furnace or the εecondary low-gaε-velocity reduction furnace while εupplying the cleaned exhauεt gaε to the drying/preheating furnace, the εecond cyclone being connected to the second exhaust gaε outlet of the primary low-gaε-velocity reduction furnace via a second exhauεt gaε line, being connected at a top portion thereof to the firεt gaε inlet of the drying/preheating furnace via a εecond cleaned exhauεt gaε line, and being connected at a bottom portion thereof to a εecond duεty ore discharge line; a third cyclone for capturing dusty iron ores contained in an exhaust gas discharged from the secondary low-gaε-velocity reduction furnace and recycling the captured duεty iron oreε to the εecondary low-gaε-velocity reduction furnace or directly charging to a melter gaεifier while supplying the cleaned exhaust gas to the primary low- gas-velocity reduction furnace, the third cyclone being connected to the secondary low-gas-velocity reduction furnace via a third exhaust gas line, being connected at a top portion thereof to the second gas inlet of the primary low-gas-velocity reduction furnace via a third cleaned exhaust gas line, and being connected at a bottom portion thereof to a third duεty ore diεcharge line; a fourth cyclone for capturing duεty iron oreε contained in an exhauεt gaε discharged from the secondary high-gas-velocity reduction furnace and supplying the captured dusty iron ores to the secondary low-gaε-velocity reduction furnace or a melter gaεifier while εupplying the cleaned exhauεt gaε to the primary high-gaε-velocity reduction furnace, the fourth cyclone being connected to the secondary high-gas-velocity reduction furnace via a fourth exhaust gaε line, being connected at a top portion thereof to the third gas inlet of the primary high-gas- velocity reduction furnace via a fourth cleaned exhauεt gaε line, and being connected at a bottom portion thereof to a fourth dusty ore diεcharge line; a firεt duct line for connecting the firεt medium/fine and coarse ore outlets of the drying/preheating furnace to the second ore inlet of the primary high-gas-velocity reduction furnace, and to the first duct line the first dusty ore discharge line is connected; a first coarse ore discharge line for connecting the second coarεe ore outlet of the primary high-gas-velocity reduction furnace to the fourth ore inlet of the secondary high-gas-velocity reduction furnace; a first medium/fine ore discharge line for connecting the third medium/fine ore outlet of the primary high-gaε- velocity reduction furnace to the εecond ore inlet of the primary low-gaε-velocity reduction furnace; a εecond coarεe ore diεcharge line for connecting the third coarse ore outlet of the secondary high-gaε-velocity reduction furnace to the melter gaεifier; a second medium/fine ore discharge line for connecting the second medium/fine ore outlet of the primary low-gas- velocity reduction furnace to the sixth ore inlet, and to the second medium/fine ore discharge line being connected to the second dusty ore discharge line in connected; a third medium/fine ore diεcharge line for connecting the fourth medium/fine ore outlet of the εecondary medium/fine ore reduction furnace to the melter gasifier, a εecond duct line for connecting the fourth duεty ore diεcharge line to the third medium/fine ore diεcharge line, the εecond duct line being connected to the third duεty ore discharge line; the third and fourth dusty ore discharge lineε being interεect with each other; a bi-directional diεcharge valve provided at the interεection between the third and fourth duεty ore diεcharge lineε; and another bi-directional diεcharge valve provided at the connection between the εecond duct line and fourth duεty ore diεcharge line. 26. The fluidized-bed-type reduction apparatuε in accordance with claim 25, further compriεing purging gas lines connected to bended portions of the firεt circulation line, firεt coarεe ore diεcharge line and εecond medium/fine ore diεcharge lineε for εupplying a εmall amount of purging gaε to the correεponding lineε, reεpectively. 27. The fluidized-bed-type reduction apparatuε in accordance with any one of claims 24 to 26, wherein the primary high-gaε-velocity reduction furnace haε the εame εhape and εize aε the εecondary high-gaε-velocity reduction furnace, and the primary low-gaε-velocity reduction furnace haε the εame εhape and εize aε the εecondary low-gas- velocity reduction furnace. 28. The fluidized-bed-type reduction apparatuε in accordance with any one of claimε 24 to 26, wherein each lower portion of the drying/preheating furnace, the primary low-gaε-velocity reduction furnace and the εecondary low- gaε-velocity reduction furnace each has a height equal to 7 to 12 timeε the inner diameter of itε lower portion and each upper portion of them has a height equal to 2 to 4 timeε itε inner diameter, and the primary and εecondary high-gaε-velocity reduction furnaceε each has a height equal to 20 to 30 timeε itε inner diameter thereof. 29. The fluidized-bed-type reduction apparatuε in accordance with claim 27, wherein each lower portion of the drying/preheating furnace, the primary low-gaε-velocity reduction furnace and the εecondary low-gaε-velocity reduction furnace haε a height equal to 7 to 12 timeε itε inner diameter and each upper portion of them haε a height equal to 2 to 4 times its inner diameter, and each of the primary and secondary high-gas-velocity reduction furnaceε haε a height equal to 20 to 30 timeε itε inner diameter thereof. 30. The fluidized-bed-type reduction apparatuε in accordance with any one of claimε 24 to 26, further compriεing an inner cyclone inεtalled in the upper portion of the εecondary low-gaε-velocity reduction furnace and connected to the third cyclone via the third exhauεt gaε line. 31. The fluidized-bed-type reduction apparatuε in accordance with claimε 27, further comprising an inner cyclone installed in the upper portion of the secondary low-gaε-velocity reduction furnace and connected to the third cyclone via the third exhauεt gaε line. 32. The fluidized-bed-type reduction apparatuε in accordance with claims 28, further, compriεing an inner cyclone installed in the upper portion of the εecondary low-gaε-velocity reduction furnace and connected to the third cyclone via the third exhauεt gaε line. 33. The fluidized-bed-type reduction apparatuε in accordance with claimε 29, further compriεing an inner cyclone installed in the upper portion of the secondary low-gaε-velocity reduction furnace and connected to the third cyclone via the third exhauεt gaε line. 34. A method for reducing iron ores using a fluidized- bed-type reduction apparatus, comprising the stepε of: drying and preheating iron oreε εupplied from a hopper In a drying/preheating furnace; pre-reducing the dried/preheated iron oreε in a primary reduction furnace; finally reducing a coarse ore portion of the pre- reduced iron oreε in a εecondary high-gaε-velocity reduction furnace while forming a bubbling fluidized bed thereof; and finally reducing a medium/fine ore portion of the pre- reduced iron ores in a secondary low-gas-velocity reduction furnace while forming a bubbling fluidized bed thereof. 35. The method in accordance with claim 34, wherein the gas velocity in each lower portion of the drying/preheating furnace, the primary reduction furnace and the secondary low-gas-velocity reduction furnace iε equal to 1.5 to 3.0 timeε the minimum fluidizing velocity of the iron ores to be charged in a fluidized bed of each asεociated furnace, the gaε velocity of gaε in each upper portion of the furnaceε, however, iε lower than the terminal velocity of the iron oreε being elutriated. The gaε velocity in the εecondary high-gaε-velocity reduction furnace iε equal to 1.5 to 3.0 timeε the minimum fluidizing velocity of the coarεe iron oreε. 36. The method in accordance with claim 34 or 35, wherein the preεεure and temperature of gaε εupplied to either the εecondary low-gaε-velocity reduction furnace or the secondary high-gaε-velocity reduction furnace are 2 to 4 atm. and 800 to 900°C, respectively, the presεure and temperature drops occurring in each of the drying/ preheating furnace, the secondary high-gas-velocity reduction furnace and the secondary low-gas-velocity reduction furnace are 0.3 to 0.6 atm. and 30 to 80°C, respectively, and the residence time of the iron ores in each furnace is 20 to 40 minuteε. 37. A method for reducing iron oreε uεing a fluidized- bed-type reduction apparatuε, compriεing the εtepε of: drying and preheating iron oreε εupplied from a hopper in a drying/preheating furnace; pre-reducing a coarεe ore portion of the dried/ preheated iron oreε in a primary high-gaε-velocity reduction furnace while forming a bubbling fluidized bed thereof; pre-reducing a medium/fine ore portion of the dried/preheated iron oreε in a primary low-gaε-velocity reduction furnace; finally reducing the pre-reduced coarεe iron ores in a εecondary high-gaε-velocity reduction furnace while forming a bubbling fluidized bed thereof; and finally reducing the pre-reduced medium/fine iron oreε in a εecondary low-gaε-velocity reduction furnace while forming a bubbling fluidized bed thereof. 38. The method in accordance with claim 37, wherein the gas velocity in each lower portion of the drying/ preheating furnace, the primary low-gaε-velocity reduction furnace and the εecondary low-gaε-velocity reduction furnace iε equal to 1.5 to 3.0 timeε the minimum fluidizing velocity of the iron oreε to be charged in a fluidized bed of each associated furnace, the gaε velocity of gaε in each upper portion of the furnaceε, however, iε lower than the terminal velocity of the iron oreε being elutriated, and the gaε velocity in either the primary high-gas-velocity reduction furnace or the secondary high-gaε-velocity reduction furnace iε equal to 1.5 to 3.0 timeε the minimum fluidizing velocity of the coarεe iron oreε. 39. The method in accordance with claim 37 or 38, wherein the preεεure and temperature of gaε supplied to either the secondary high-gas-velocity reduction furnace or the εecondary low-gaε-velocity reduction furnace are 2 to 4 atm. and 800 to 900°C, reεpectively, the preεεure and temperature dropε occurring in each of the drying/ preheating furnace, the primary low-gaε-velocity reduction furnace, the primary high-gaε-velocity reduction furnace, the εecondary high-gaε-velocity reduction furnace and the εecondary low-gaε-velocity reduction furnace are 0.3 to 0.6 atm. and 30 to 80°C, reεpectively, and the reεidence time of the iron oreε in each furnace iε 20 to 40 minuteε.

Inventors: CHUNG UOO CHANG, JUNG BONG JIN, KEPPLINGER WERNER L, KIM HANG GOO, KIM YONG HA, LEE IL OCK, SCHENK JOHANNES L, WALLNER FELIX, CHUNG, UOO, CHANG;, JUNG, BONG, JIN;, KEPPLINGER, WERNER, L., KIM, HANG, GOO;, KIM, YONG, HA;, LEE, IL, OCK;, SCHENK, JOHANNES, L., WALLNER, FELIX
Applicants: CHUNG UOO CHANG, JUNG BONG JIN, KEPPLINGER WERNER L, KIM HANG GOO, KIM YONG HA, LEE IL OCK, PO HANG IRON & STEEL, RES INST IND SCIENCE & TECH, SCHENK JOHANNES L, VOEST ALPINE IND ANLAGEN, WALLNER FELIX, CHUNG, UOO, CHANG;, JUNG, BONG, JIN;, KEPPLINGER, WERNER, L., KIM, HANG, GOO;, KIM, YONG, HA;, LEE, IL, OCK;, POHANG IRON & STEEL CO., LTD., RESEARCH INSTITUTE OF INDUSTRIAL SCIENCE & TECHNOL, SCHENK, JOHANNES, L., VOEST-ALPINE INDUSTRIEANLAGENBAU GMBH, WALLNER, FELIX
Application year: 1995
Main classification: C21B13
IPC codes: C21B11, C21B13