Abstract:
A system and method for fluidized reduction of iron ore powder. Use of oxidation increases the iron ore reduction rate. Use of high-gas-velocity processing accelerates iron ore reduction speed and greatly improves the gas-treatment capabilities of a unit-cross-sectional fluidized bed. Use of parallel-connections involving reduced coal gas lessens the volume of gas passing through a single-stage fluidized bed. The invention achieves the highly-effective reduction of iron ore powder in a fluidized bed under near-atmospheric pressure.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention belongs to the fields of chemical engineering and metallurgy, and particularly relates to a system and method for fluidized iron ore powder reduction. 
       BACKGROUND OF THE INVENTION 
       [0002]    The current pig-iron smelting mainly employs the blast furnace technology, which requires uses of coke and pellet, involving a relatively long workflow. Non-blast furnace ironmaking technologies have been paid more and more attention for lessening the dependence on coke. Non-blast ironmaking technologies can be divided into two types: direct reduction and melting reduction. The product from direct reduction is sponge iron obtained from the reduction of iron ore concentrate and mainly used as raw material for electric furnace steelmaking. In melting reduction, the sponge iron is further melted so as to obtain liquid iron after separation of slag-iron. Both direct reduction and melting reduction need to undergo a gas-solid phase reduction process of the iron ore concentrate. Their difference mainly lies in the metallization ratio. Direct reduction typically requires a metallization ratio of more than 90%, whereas the metallization ratio for the gas-solid phase reduction (commonly known as pre-reduction) in melting reduction can be as low as 75% due to the presence of subsequent final reduction in a molten bath. The pre-reduction of both direct reduction and melting reduction involves basically the same processes, during which solid iron ore is reduced in a gas phase. Therefore, they are collectively referred to as iron ore reduction in the present application. There are many methods for reducing iron ore, which can be divided into different categories, such as rotary kiln, rotary furnace, shaft furnace, fluidized bed, etc., according to the types of reduction reactors. The fluidized bed reduction reactor is recognized as the most efficient iron ore reduction reactor due to its prominent advantages including direct processing of powdered ore, good heat and mass transfer, high reduction efficiency, etc. compared to other reactors. Reduction of iron ore on a fluidized bed has been investigated for several decades, and a large number of process patent applications have been filed and some technologies such as FIOR/FINMET, FINEX, HISmelt, Circofer, and Circored have undergone pilot scale tests or been industrialized. 
         [0003]    The FIOR process was first studied by the ESSO Research and Engineering Company in the 1950s. A 5 t/d laboratory-scale trial was completed in ESSO&#39;s laboratory (Baton Rouge, La., USA) in 1962, and a 300t/d factory was established in Canada (Darmouth, Nova Scotia) in 1965. In 1976, an industrial plant for producing hot briquettes with an annual output of 400,000 tons (U.S. Pat. No. 5,082,251 and U.S. Pat. No. 5,192,486) was established in Venezuela, and named as FIOR from the initials of “Fine Iron Ore Reduction”. The plant has been putting into operation continuously till today. Having been developing the FINMET technology in cooperation with Voestalpine since 1992, the FIOR Company completed the validation for the FINMET technology in 1995. A FINMET system with an annual output of 500,000 tons began to be established in January 1998, was debugged in November 1999, and was put into operation formally in May 2000. Since 2001, two FINMET systems each having an annual output of 500,000 tons have been merged into a system with an annual output of 1 million tons. H 2 +CO obtained from natural gas reforming is used as the reducing and fluidizing medium in FIOR/FINMET, in which four stages of fluidized beds are operated in series. The reducing gas with a gas pressure of 11-13 atm (gage pressure) passes through the fourth fluidized bed, the third fluidized bed, the second fluidized bed and the first fluidized bed in series. The unconsumed H 2  and CO in reduction exhaust gas are recycled after a purification process such as dust removal, decarbonization, etc. The iron ore powder is reduced on the first fluidized bed, the second fluidized bed, the third fluidized bed and the fourth fluidized bed in series (Schenk, et al.,  Particuology,  2011, 914-23). As there is no intermediary heat supplementation, the temperatures of the fluidized beds are decreased gradually from the fourth fluidized bed with a reduction temperature of about 800° C. to the first fluidized bed with a reduction temperature of only 400-500° C., at which temperature the reduction rate is so low that the first fluidized bed is mainly used for preheating. 
         [0004]    The FINEX process is a new melting reduction process based on preheating and reduction on a fluidized bed, which was developed by Pohang Iron and Steel Co. Ltd, Korean in cooperation with Voestalpine on the basis of the COREX melting reduction technology of Voestalpine. The technical development of FINEX was started in 1992; a 15 t/d laboratory scale-up experiment was completed in 1996; a 150 t/d pilot scale test was completed in 1999; a demonstration project with an annual output of 800,000 tons began to be constructed in January 2001, reaching the target output in May 2004; and an industrialized demonstration plant with an annual output of 1500,000 tons was established in May 2007. The pre-reduction part of the FINEX process (U.S. Pat. No. 5,762,681, U.S. Pat. No. 5,785,733, CN95191907.5, CN95191873.7, US20020166412, US20060119023, US20080302212, and US20080277842), which is basically the same as that of the FIOR/FINMET process, is operated using four-stage fluidized beds in series, except that the clean coal gas obtained through purification of the coal gas generated from melting reduction (in a melting reduction furnace) is used as the reducing and fluidizing medium. The reducing gas with a gas pressure of 2.3-4.0 atm (gage pressure) passes through the fourth fluidized bed, the third fluidized bed, the second fluidized bed and the first fluidized bed in series. The unconsumed H 2  and CO in reduction exhaust gas are recycled after purification processes such as dust removal, decarburization, etc. The iron ore powder is reduced while passing through the first fluidized bed, the second fluidized bed, the third fluidized bed and the fourth fluidized bed in series. Like FIOR/FINMET, no separate iron ore powder preheating unit is set up in FINEX. The first fluidized bed mainly serves for drying/preheating at a temperature of about 400° C., and the fourth fluidized bed is at a temperature of 800-900° C. The fluidized bed reduction in the FINEX process has been in operation for years with an annual output of 1500,000 tons (the actual output of reduced iron ore is expected to exceed 2700,000 tons per year). 
         [0005]    The Circofer process and Circored process are iron ore reduction processes on coal-based and gas-based fluidized beds, respectively, developed by Lurgi, Germany (Lurgi has sold its metallurgical business to Outokumpu, Finland). In the Circored process (U.S. Pat. No. 5,603,748, and Schenk et al., Particuology, 2011, 914-23), hydrogen is used as the reducing medium, and two-stage fluidized beds in series are employed for reduction. First, the iron ore powder is dried and preheated to 850-900° C. in a preheating unit (a combined transferring fluidized bed-cyclone-venturi preheater), and then enters into a circulating fluidized bed at a temperature of 850-900° C. for pre-reduction (first-stage reduction). The pre-reduced iron ore powder discharged from the circulating fluidized bed enters into a bubbling fluidized bed for final reduction (second-stage reduction). The bubbling fluidized bed is a horizontal transverse multi-sectional (multi-stage) fluidized bed with an operational temperature of 630-650° C. The operational pressure of both fluidized beds is 4 atm (gage pressure). The reduced iron ore powder discharged from the bubbling fluidized bed is heated by a rapid heater to a temperature above 680° C. as required for briquetting, and enters into a hot briquetting section for being briquetted. The reduction exhaust gas is recycled after treatments such as exchanging heat with circulating gas for sensible heat recycling, purification, compression, etc. Lurgi began to study the technology of iron ore reduction on circulating fluidized beds in the 1970s, primarily focused on the development of the technology with coal as the reducing medium from 1973 to 1990, and later shifted to research on the reduction technology on fluidized beds with hydrogen as the medium in 1993. In 1996, the company began to establish a demonstration project with an annual output of 500,000 tons in its plant in Trinidad, the construction of which was completed in March 1999. In May 1999, the first batch of hot briquetted iron was obtained. From 1999 to 2001, the system was debugged, optimized and restructured. In August 2001, a targeted goal of 63.6 t/h of HBI was successfully achieved. From August to November in 2001, about 130,000 tons of HBI was produced continuously. However, the system has been shut down for market reasons since November 2001. The Circofer process (US20070256519, CN100587080C, and CN100540698C) is a coal-based fluidized reduction process of iron ore developed by the Lurgi/Outokumpu company (Orth, et al.,  Minerals Engineering,  2007, 854-861). In this process, the iron ore powder exchanges heat with the exhaust gas discharged from the first fluidized bed in a combined cyclone-venturi heater. After being preheated, the iron ore powder enters into the front chamber of the first fluidized bed; meanwhile, coal powder is added and oxygen is introduced into the front chamber. The iron ore powder is preheated by the heat generated from partial combustion of the coal powder, during which reductive gas is generated at the same time. The preheated iron ore powder and the generated gas enter into the main bed of the first fluidized bed from the top of the front chamber of the first fluidized bed. The clean coal gas obtained through purification of the exhaust enters into the bottom of the first fluidized bed as the fluidizing and reducing medium. After being reduced, the iron ore powder is discharged from the lower part of the first fluidized bed into the second fluidized bed for subsequent reduction. The sponge iron obtained from the reduction process is discharged from the second fluidized bed, passes through a hot magnetic separator to remove semi coke particles contained therein, and then goes into a smelting and separating furnace for slag-iron separation. The hot exhaust gas discharged from the top of the first fluidized bed passes through the cyclone dust remover for separation, and then enters into a combined cyclone-venturi preheater to heat up the iron ore powder while cooling down the exhaust gas. After passing through a waste heat boiler for heat recycling, the gas undergoes further dust removal through a bag-type dust remover and a venture-type dust remover and CO 2  removal through a CO 2  remover to yield clean coal gas for recycling as the fluidizing and reducing medium. Lurgi established a pilot scale test platform of a circulating fluidized bed with a diameter of 700 mm and an output of 5 t/d for developing the Cirfofer technology. Until 2003, over ten rounds of tests have been conducted with a total running time of more than 70 days. However, from reports in existing documents, the Circofer technology only passed the above pilot scale test, without further reported pilot scale test or industrialized application. 
         [0006]    In the HISmelt process, a transport bed, in combination with four-stage cyclone preheaters, preheats the iron ore powder before entering into a smelting and separating furnace (http://www.hismelt.com; Schenk et al.,  Particuology,  2011, 914-23). As it essentially does not belong to iron ore reduction technologies, and is not in close association with the present application, this technology is not detailed herein. 
         [0007]    Except for the above fluidized-bed reduction processes which either have undergone pilot scale tests or further have been industrialized, many patents relating to the processes for reducing iron ore on fluidized beds have been filed home and aboard. In these patents, 2- to 4-stage fluidized beds are generally used for reduction, which is similar to the FIOR/FINMET and FINMET processes. Some of the processes are even essentially the same as the above two processes, whereas others differ in different combinations of stages of fluidized beds, powder preheating mode, gas preheating mode, gas operation mode, etc., thus forming a number of granted patents. It also indicates from one aspect that there is still a huge innovation space for the iron ore fluidized reduction process from the different combinations of the main aspects of stages of fluidized beds, iron powder preheating, reducing gas preheating and gas operation mode. The iron ore fluidized reduction processes home and abroad are analyzed from the above respects as follows. 
         [0008]    1) Stages of fluidized beds: four-stage fluidized beds are used in the mainstream FIOR/FINMET and FINEX processes. Patents that employ similar four-stage fluidized beds for reduction also include: US20120328465 (2012), CN101892339 (2012), CN101397597 (2010), CN101519707 (2010), CN100560739 (2009), US20080277842 (2008), AU2001265669 (2001), etc., wherein their iron ore powder reduction parts are essentially the same as those of FIOR/FINMET and FINEX. The patents such as CN103221555 (2013), CN102127611 (2012), U.S. Pat. No. 6,960,238 (2005), U.S. Pat. No. 6,736,876 (2004), US20020166412 (2002), and U.S. Pat. No. 5,785,733 (1998) employ three-stage fluidized beds for reduction. Except for the Lurgi&#39;s process, CN201563469 (2010), CN101333575 (2010), and CN101906501 (2010) relate to two-stage fluidized beds for reduction. 
         [0009]    2) Powder preheating: In the Lurgi&#39;s process, a combined circulating fluidized bed-cyclone preheater-venturi unit is used. In CN101906501 (2010), five-stage cyclone preheaters are used to preheat iron ore powder. In CN101333575 (2010), a slope furnace is used to preheat iron ore powder. In other patents without specific iron ore powder preheating unit, the last stage fluidized bed functions to preheat the iron ore powder, which is, in fact, essentially similar to those of the FIOR/FINMET and FINEX processes. 
         [0010]    3) Gas preheating: a gas preheating unit is set up in the Lurgi&#39;s fluidized-bed reduction process, whereas in many patents including the patents relating to the FIOR/FINMET process (U.S. Pat. No. 5,082,251, and U.S. Pat. No. 5,192,486) and the FINEX process (U.S. Pat. No. 5,762,681, and U.S. Pat. No. 5,785,733), no preheating unit is involved or contained. However, if the gas is not preheated, the temperature of the fluidized bed cannot be maintained at 800° C. or above, which is required for the reaction. In some processes including those in patents such as CN10151970 (2010), CN101906501 (2010), AU2001265669 (2001), and U.S. Pat. No. 6,736,876 (2004), the hot gas discharged from a melting gasifier is introduced directly into the final stage fluidized bed so that the sensible heat of the hot gas from the melting gasifier can be directly utilized. However, the reducing capability of the hot gas would be weakened if the gases generated during the melting and separating process, such as CO 2 , H 2 O, etc., are not removed. Actually, a gas preheating unit is set up in the actual flow of FIOR/FINMET (Schenk et al.,  Particuology,  2011, 914-23), i.e., the reducing gas is preheated by a preheater before passing through the fourth stage fluidized bed, the third stage fluidized bed, the second stage fluidized bed and the first stage fluidized bed sequentially. As the reaction process absorbs heat in general, when no preheater is set up thereamong, the temperature of the fluidized bed falls gradually, reaching as low as 400-500° C. in the last stage fluidized bed (the first fluidized bed), resulting in poor reducing capabilities. To resolve this problem, U.S. Pat. No. 6,960,238 (2005) suggests that oxygen/air is introduced into hot gas before the gas enters into each fluidized bed so that the reducing gas would be directly heated up through its partially combustion. Though the temperature of the gas is elevated by direct partial combustion, the CO 2  and H 2 O generated from combustion would greatly lower the reduction potential and weaken the reducing capability of the gas, which is quite adverse to the reduction process. 
         [0011]    4) Gas operation mode: in all existing processes, gas is operated in series and the reaction pressures vary significantly. The operating gage pressure is 11-13 atm for FIOR/FINMET; 2-4 atm. for FINEX; and 4 atm. for Circored. The operating gage pressure in CN100560739 is 4-10 atm. In some patents, such as CN101519707 (2010) and CN102127611 (2012), an operating pressure of 1-10 atm is provided. However, it is not operationally practical due to the wide range, because ten times of difference between the highest and the lowest operating pressures means the same ten times of difference between the highest and the lowest linear speeds of the fluidized beds, and generally it is hard for fluidized beds to have such operational flexibility. In many other patents, such as CN101333575 (2010), 101563469 (2010), CN103221555 (2013), and CN101892339 (2012), etc., the operating pressure is not described. 
         [0012]    Although compared to a shaft furnace, the fluidized bed has many advantages including good contacting between the gas phase and solid phase, high heat and mass transfer efficiency, etc., the energy consumption for producing hot briquetted iron by the FIOR/FINMET process is about 15.0 GJ per ton, which is much higher than 10.5 GJ of the MIDREX process based on the shaft furnace, suggesting that the processes based on the fluidized bed still have much room for improvement in reducing energy consumption. Existing processes for reducing iron ore powder on fluidized beds still have the following two respects of problems. 
         [0013]    High operational pressure: in both FIOR/FINMET and FINEX processes, high operating pressure is used (for example, the operating pressure in FIOR/FINMET is 12-14 atm). Since the gas compression process is energy-intensive, the gas compression consumption would be lowered greatly if the operating pressure can decrease to near atmosphere pressure (the operating pressure in MIDREX is 1-1.5 atm), and in turn the efficiency of fluidized-bed reduction of iron ore would be improved. One of the main reasons of using high-pressure operation in existing processes is that the diameter of the fluidized bed reactor can be reduced thereby. Since reducing iron ore to metallic iron by gas demands highly in thermodynamics, the iron trioxide in iron ore can be reduced to metallic iron only when the gas is excessive in a large amount. Therefore, a large amount of gas is required in the reduction process. An operation under the atmospheric pressure usually requires an oversized diameter of the fluidized bed. For example, in a 1-million-ton system using the FINMET process with an operating pressure of 12-14 atm, the diameter of the fluidized bed is still 5 m. It can be calculated that if the operation is conducted under the atmospheric pressure, the diameter of the fluidized bed will reach 17.7 m. 
         [0014]    (2) low reduction efficiency: Although most processes use four-stage fluidized beds for reduction, no intermediary heat supplementation for the gas passing from the fourth stage to the first stage fluidized beds in series results in gradual reduction of the reaction temperature from the fourth stage to the first stage, with the temperature in the first stage fluidized bed of only 400-500° C. Since iron ore exhibits slow reduction kinetics under a temperature below 700° C., the last two stages of the conventional four-stage fluidized beds play a minor role in reduction, resulting in a low overall reduction efficiency. 
         [0015]    In summary, lowering operating pressure of the fluidized beds and increasing the overall reduction efficiency of the multi-stage fluidized beds through process and technical innovation are the key for reducing energy consumption during iron ore reduction on fluidized beds and improving economic efficiency of the reduction process. 
       SUMMARY OF THE INVENTION 
       [0016]    The present invention aims to provide a system and method for fluidized iron ore powder reduction, which can achieve highly effective reduction of iron ore powder in a fluidized bed under a near-normal pressure. 
         [0017]    In order to achieve the above object, the present invention is implemented as follows. 
         [0018]    The present invention provides a system for fluidized iron ore powder reduction, comprising: an iron ore powder bin  1 , a screw feeder  2 , an oxidizing fluidized bed  3 , a primary feeder  4 , a primary circulating fluidized bed  5 , a secondary feeder  6 , a secondary circulating fluidized bed  7 , a tertiary feeder  8 , a tertiary circulating fluidized bed  9 , a discharger  10 , a product bin  11 , a burning chamber  12 , a primary coal gas preheater  13 , a secondary coal gas preheater  14 , a tertiary coal gas preheater  15 , a cyclone dust collector  16 , a bag dust collector  17 , an induced draft fan  18 , and a chimney  19 . 
         [0019]    The oxidizing fluidized bed  3  comprises an oxidizing fluidized bed body  31 , a first cyclone separator  32 , a second cyclone separator  33 , and a feeder  34 . 
         [0020]    The primary circulating fluidized bed  5  comprises a primary riser  51 , a third cyclone separator  52 , a fourth cyclone separator  53  and a primary circulating dipleg  54 . 
         [0021]    The secondary circulating fluidized bed  7  comprises a secondary riser  71 , a fifth cyclone separator  72 , a sixth cyclone separator  73  and a secondary circulating dipleg  74 . 
         [0022]    The tertiary circulating fluidized bed  9  comprises a tertiary riser  91 , a seventh cyclone separator  92 , an eighth cyclone separator  93  and a tertiary circulating dipleg  94 . 
         [0023]    A feed outlet at the bottom of the iron ore powder bin  1  is provided with a pipeline which is connected to a feed inlet of the screw feeder  2 , and the iron ore powder is added to the screw feeder  2  through the pipeline. A feed outlet of the screw feeder  2  is connected to a gas outlet of the first cyclone separator  32  and an inlet of the second cyclone separator  33  through a pipeline. 
         [0024]    The oxidizing fluidized bed body  31  is provided with a gas outlet and a feed outlet at the upper portion, a gas inlet at the bottom, and a feed inlet at the lower portion. The gas outlet of the oxidizing fluidized bed body  31  is connected to an inlet of the first cyclone separator  32 , the feed inlet of the oxidizing fluidized bed body  31  is connected to a feed outlet of the feeder  34 , the gas inlet of the oxidizing fluidized bed body  31  is connected to a gas outlet of the burning chamber  12  through a pipeline, and the feed outlet of the oxidizing fluidized bed body  31  is connected to a feed inlet of the primary feeder  4  through a pipeline. The inlet of the first cyclone separator  32  is also connected to a feed outlet of the second cyclone separator  33 , a feed outlet of the cyclone dust collector  16 , and a feed outlet of the bag dust collector  17 , respectively, through a pipeline, a feed outlet of the first cyclone separator  32  is connected to a feed inlet of the feeder  34 , and a gas inlet and feed outlet of the feeder  34  are connected to an air main pipe and the feed inlet of the oxidizing fluidized bed body  31 , respectively, through a pipeline. An inlet of the second cyclone separator  33  is connected to the feed outlet of the screw feeder  2  and the gas outlet of the first cyclone separator through a pipeline, a gas outlet of the second cyclone separator is connected to an inlet of the cyclone dust collector  16 , and the feed outlet of the second cyclone separator  33  is connected to the inlet of the first cyclone separator  32 . 
         [0025]    A gas outlet of the cyclone dust collector  16  is connected to a gas inlet of the bag dust collector  17  through a pipeline, and the feed outlet of the cyclone dust collector  16  is connected to the inlet of the first cyclone separator  32  through a pipeline. The gas inlet of the bag dust collector  17  is connected to an inlet of the induced draft fan  18 , the feed outlet of the bag dust collector  17  is connected to the inlet of the first cyclone separator  32  through a pipeline, and a gas outlet of the induced draft fan  18  is connected to the chimney  19 . 
         [0026]    The primary feeder  4  is provided with a feed inlet, a gas inlet and a feed outlet. The feed inlet of the primary feeder  4  is connected to the feed outlet of the oxidizing fluidized bed body  31  through a pipeline, the gas inlet of the primary feeder  4  is connected to the coal gas main pipe, and the feed outlet of the primary feeder  4  is connected to a feed inlet of the primary riser  51  through a pipeline. 
         [0027]    The primary riser  51  is provided with a gas outlet at the top, a gas inlet at the bottom, and a feed inlet and a circulating return port at the lower portion. The gas outlet of the primary riser  51  is connected to an inlet of the third cyclone separator  52 , the feed inlet of the primary riser  51  is connected to the feed outlet of the primary feeder  4 , and the gas inlet of the primary riser  51  is connected to a coal gas outlet of the primary coal gas preheater  13  through a pipeline. A gas outlet of the third cyclone separator  52  is connected to an inlet of the fourth cyclone separator  53 , and a discharge port of the third cyclone separator  52  is connected to the primary circulating dipleg  54 . The primary circulating dipleg  54  is provide with a return port and a discharge port at the lower portion, and the return port of the primary circulating dipleg  54  is connected to the circulating return port of the primary riser  51  through a pipeline, the discharge port of the primary circulating dipleg  54  is connected to a lower feed inlet of the secondary riser  71  through a pipeline, and a gas inlet of the primary circulating dipleg  54  is connected to the coal gas main pipe. The gas outlet of the fourth cyclone separator  53  is connected to an inlet of a reduction exhaust gas treatment system, and a discharge port of the fourth cyclone separator  53  is connected to a feed inlet of the secondary feeder  6  through a pipeline. 
         [0028]    The secondary feeder  6  is provided with a feed inlet, a gas inlet and a feed outlet. The feed inlet of the secondary feeder  6  is connected to the discharge port of the fourth cyclone separator  53 , the gas inlet of the secondary feeder  6  is connected to the coal gas main pipe, and the feed outlet of the secondary feeder  6  is connected to the an upper feed inlet of the secondary riser  71  through a pipeline. 
         [0029]    The secondary riser  71  provided with a gas outlet at the top, an upper feed inlet at the upper portion, a gas inlet at the bottom, and the lower feed inlet and a circulating return port at the lower portion. The gas outlet of the secondary riser  71  is connected to an inlet of the fifth cyclone separator  72 , the upper feed inlet of the secondary riser  71  is connected to the feed outlet of the secondary feeder  6 , the lower feed inlet of the secondary riser  71  is connected to the discharge port of the primary circulating dipleg  54  through a pipeline, and the gas inlet of the secondary riser  71  is connected to a coal gas outlet of the secondary coal gas preheater  14  through a pipeline. A gas outlet of the fifth cyclone separator  72  is connected to an inlet of the sixth cyclone separator  73 , and a discharge port of the fifth cyclone separator  72  is connected to the secondary circulating dipleg  74 . The secondary circulating dipleg  74  is provided with a return port and a discharge port at the lower portion. The return port of the secondary circulating dipleg  74  is connected to the circulating return port of the secondary riser  71  through a pipeline, the discharge port of the secondary circulating dipleg  74  is connected to a lower feed inlet of the tertiary riser  91  through a pipeline, and the gas inlet of the secondary circulating dipleg  74  is connected to the coal gas main pipe through a pipeline. A gas outlet of the sixth cyclone separator  73  is connected to the inlet of the reduction exhaust gas treatment system, and a discharge port of the sixth cyclone separator  73  is connected to a feed inlet of the tertiary feeder  8  through a pipeline. 
         [0030]    The tertiary feeder  8  is provided with a feed inlet, a gas inlet and a feed outlet. The feed inlet of the tertiary feeder  8  is connected to a discharge port of the sixth cyclone separator  73 , the gas inlet of the tertiary feeder  8  is connected to the coal gas main pipe, and the feed outlet of the tertiary feeder  8  is connected to the an upper feed inlet of the tertiary riser  91  through a pipeline. 
         [0031]    The tertiary riser  91  is provided with a gas outlet at the top, an upper feed inlet at the upper portion, a gas inlet at the bottom, and a lower feed inlet and a circulating return port at the lower portion. The gas outlet of the tertiary riser  91  is connected to an inlet of the seventh cyclone separator  92 , the upper feed inlet of the tertiary riser  91  is connected to the feed outlet of tertiary feeder  8 , the lower feed inlet of the tertiary riser  91  is connected to the discharge port of the secondary circulating dipleg  74  through a pipeline, and the gas inlet of the tertiary riser  91  is connected to a coal gas outlet of the tertiary coal gas preheater  15  through a pipeline. A gas outlet of the seventh cyclone separator  92  is connected to an inlet of the eighth cyclone separator  93 , and a discharge port of the seventh cyclone separator  92  is connected to the tertiary circulating dipleg  94 . The tertiary circulating dipleg  94  is provided with a return port and a discharge port at the lower portion. The return port of the tertiary circulating dipleg  94  is connected to the circulating return port of the tertiary riser  91  through a pipeline, the discharge port of the tertiary circulating dipleg  94  is connected to the product bin  11  through a pipeline, and a gas inlet of the tertiary circulating dipleg  94  is connected to the coal gas main pipe through a pipeline. A gas outlet of the eighth cyclone separator  93  is connected to the inlet of the reduction exhaust gas treatment system, and a discharge port of the eighth cyclone separator  93  is connected to a feed inlet of the discharger  10  through a pipeline. 
         [0032]    The discharger  10  is provided with a feed inlet, a gas inlet and a feed outlet. The feed inlet of the discharger  10  is connected to the discharge port of the eighth cyclone separator  93 , the gas inlet of the discharger  10  is connected to the coal gas main pipe, and the feed outlet of the discharger  10  is connected to the product bin  11  through a pipeline. 
         [0033]    The burning chamber  12  heats the air by burning the coal gas. It is provided with a nozzle that is connected to the coal gas main pipe and the air main pipe, and a gas outlet that is connected to the gas inlet at the bottom of the oxidizing fluidized bed body  31 . The gas inlet of the burning chamber  12  is connected to the air main pipe. 
         [0034]    The coal gas preheater is divided into the primary coal gas preheater  13 , the secondary coal gas preheater  14  and the tertiary coal gas preheater  15 , which preheat the fluidized coal gas required for the primary circulating fluidized bed  5 , the secondary circulating fluidized bed  7  and the tertiary circulating fluidized bed  9 , respectively. Any one of the three primary coal gas preheaters is provided with a nozzle, a flue gas outlet, a coal gas inlet and a coal gas outlet. The nozzle is used for burning the coal gas to generate a high temperature flue gas to provide the heat required for preheating the reducing coal gas, and the nozzle is connected to the air main pipe and the coal gas main pipe through a pipeline. The flue gas outlet is connected to a flue gas main pipe, and the discharged flue gas enters into a flue gas treatment system via the flue gas main pipe for heat recovery. The coal gas inlet is connected to the coal gas main pipe, and the coal gas outlet of the primary coal gas preheater is connected to the gas inlet at the bottom of the primary riser, the coal gas outlet of the secondary coal gas preheater is connected to the gas inlet at the bottom of the secondary riser, and the coal gas outlet of the tertiary coal gas preheater is connected to the gas inlet at the bottom of the tertiary riser. 
         [0035]    The present invention further provides a method of fluidized iron ore powder reduction based on the above system, comprising: allowing a powder and gas to enter into and pass through the above system simultaneously in the following way: the iron ore powder enters into the bubbling fluidized bed body  31  through the second cyclone separator  33 , the cyclone dust collector  16 , the first cyclone separator  32  and the feeder  34  from the iron ore powder bin  1  via the screw feeder  2 . After being discharged from an upper feed outlet of the bubbling fluidized bed body  31 , the iron ore powder enters into the primary riser  51  of the primary circulating fluidized bed  5  via the primary feeder  4 ; and is discharged from the outlet of the primary riser  51  into the secondary riser  71  via the third cyclone separator  52  and the primary circulating dipleg  54 . The powder collected by the fourth cyclone separator  53  also enters into the secondary riser  71  via the secondary feeder  6 ; and is discharged from the outlet of the secondary riser  71  into the tertiary riser  91  via the fifth cyclone separator  72  and the primary circulating dipleg  74 . The powder collected by the sixth cyclone separator  73  also enters into the secondary riser  91  via the tertiary feeder  8 . The reduced powder product from the tertiary circulating fluidized bed  9  is discharged into the product bin  11  via the seventh cyclone separator  92  and the primary circulating dipleg  94 , and the powder product collected by the eighth cyclone separator  93  also enters into the product bin  11  via the discharger  10 . After being preheated in the burning chamber  12 , the air enters into the bubbling fluidized bed body  31  to contact and oxidize the iron ore powder. The coal gas and air enter into the burning chamber  12  via the nozzle for preheating the air through burning. Meanwhile, the air also enters into the bubbling fluidized bed body  31  via the gas inlet at the bottom of the feeder  34 . The gas discharged from the gas outlet of the bubbling fluidized bed body  31  contacts and exchanges heat with the iron ore powder through the first cyclone separator  32  and the second cyclone separator  33 , and is sent to the chimney  19  via the induced draft fan  18  for discharging after being dedusted by the cyclone dust collector  16  and the bag dust collector  17 . The coal gas enters into the primary riser  51 , the secondary riser  71  and the tertiary riser  91 , respectively, to contact with the iron ore powder for reduction after being preheated by the primary coal gas preheater  13 , the secondary coal gas preheater  14  and the tertiary coal gas preheater  15 . In the meantime, the coal gas enters into the reduction system through the corresponding gas inlets at the bottom of the primary feeder  4 , the primary circulating dipleg  54 , the secondary feeder  6 , the secondary circulating dipleg  74 , the tertiary feeder  8 , the tertiary circulating dipleg  94  and the discharger  10 , respectively, via the pipeline. The gas discharged by the primary riser  51  is dedusted by the third cyclone separator  52  and the fourth cyclone separator  53 , and then enters into the reduction exhaust gas treatment system. The gas discharged by the secondary riser  71  enters into the reduction exhaust gas treatment system via the pipeline after being dedusted by the fifth cyclone separator  72  and the sixth cyclone separator  73 . The gas discharged by the tertiary riser  91  enters into the reduction exhaust gas treatment system via the pipeline after being dedusted by the seventh cyclone separator  92  and the eighth cyclone separator  93 . The coal gas and air enter into the primary coal gas preheater  13 , the secondary coal gas preheater  14  and the tertiary coal gas preheater  15  via the nozzle for combustion to generate heat for preheating the coal gas, and then enter into the flue gas treatment system via the pipeline. 
         [0036]    The further improvement of the present invention lies in that the oxidization of the iron ore powder is conducted in a bubbling fluidized bed at 850-950° C. 
         [0037]    The still further improvement of the present invention lies in that the reducing coal gas is preheated by the primary coal gas preheater  13 , the secondary coal gas preheater  14 , and the tertiary coal gas preheater  15  to a high temperature, which is 850-950° C. 
         [0038]    Another improvement of the present invention lies in that the oxidized iron ore powder is reduced by three-stage circulating fluidized beds, wherein the circulating fluidized bed is operated at a high gas velocity of 5-10 m/s, and the reduction is conducted at 800-900° C. 
         [0039]    The yet further improvement of the process of the present invention lies in that the reduction process is conducted at a low pressure, wherein the low pressure is an operating pressure of less than 1 atm. 
         [0040]    The present invention offers the following advantages relative to the prior art: 
         [0041]    1) The reducing gas enters into multi-stage fluidized bed reactors in a parallel mode, thereby effectively reducing the amount of reducing gas passing through an individual fluidized bed. 
         [0042]    2) The reducing coal gas is preheated separately, such that each stage of the reducing fluidized bed can be operated at 850-950° C., thereby increasing the overall reduction efficiency of the multi-stage fluidized beds. 
         [0043]    3) The operation at a high gas velocity significantly increases the gas throughput per cross-sectional area of the fluidized bed, while reducing the mass transfer resistance and increasing the reduction efficiency. 
         [0044]    4) The iron ore is oxidized first, which can increase the reduction rate of the iron ore. 
         [0045]    When the iron ore powder is reduced by the present invention, the reactor, at a pressure of less than 1 atm, can be of a size which is the same as or smaller than that of the reactor used in the conventional process under the high pressure operation, thereby overcoming the deficiency of high-power gas compression resulting from the high pressure in the traditional technology. Additionally, the present invention can increases the reduction rate of the iron ore by oxidization and decrease the diffusion resistance of the reduction process due to a high gas velocity. Moreover, the present invention has advantages such as high reducing efficiency and low energy consumption in reduction process and the like, and can improve the economy in the process of reducing the iron ore powder. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0046]    The accompanying drawings are provided to further illustrate the present invention, and constitute a part of the specification. The drawings, together with the examples of the present invention, are used to explain the present invention, and not construed as limiting the present invention. 
           [0047]      FIG. 1  is a schematic diagram illustrating the configuration of a system for fluidized iron ore powder reduction according to the present invention: 
       
    
    
       [0048]    Reference signs:  1 , iron ore powder bin;  2 , screw feeder;  3 , oxidizing fluidized bed;  31 , fluidized bed body;  32 , first cyclone separator;  33 , second cyclone separator;  34 , feeder;  4 , primary feeder;  5 , primary circulating fluidized bed;  51 , primary riser;  52 , third cyclone separator;  53 , fourth cyclone separator;  54 , primary circulating dipleg;  6 , secondary feeder;  7 , secondary circulating fluidized bed;  71 , secondary riser;  72 , fifth cyclone separator;  73 , sixth cyclone separator;  74 , secondary circulating dipleg;  8 , tertiary feeder;  9 , tertiary circulating fluidized bed;  91 , tertiary riser;  92 , seventh cyclone separator;  93 , eighth cyclone separator;  94 , tertiary circulating dipleg;  10 , discharger;  11 , product bin;  12 , burning chamber;  13 , primary coal gas preheater;  14 , secondary coal gas preheater;  15 , tertiary coal gas preheater;  16 , cyclone dust collector;  17 , bag dust collector;  18 , induced draft fan;  19 , chimney. 
       DETAILED DESCRIPTION OF THE INVENTION 
       [0049]    In order to make the purpose, technical solutions and advantages of the present invention more apparent, a clear and complete description about the technical solutions of the examples in the present invention will be provided below with reference to the accompanying drawings of the examples of the present invention. Obviously, the examples described below are part of the examples of the present invention, rather than all of the examples. Other examples obtained by those of the ordinary skill in the art based on the examples of the present invention without creative effort also fall into the scope of protection of the present invention. 
       EXAMPLE 1 
       [0050]    As shown in  FIG. 1 , a system for fluidized iron ore powder reduction comprises: an iron ore powder bin  1 , a screw feeder  2 , an oxidizing fluidized bed  3 , a primary feeder  4 , a primary circulating fluidized bed  5 , a secondary feeder  6 , a secondary circulating fluidized bed  7 , a tertiary feeder  8 , a tertiary circulating fluidized bed  9 , a discharger  10 , a product bin  11 , a burning chamber  12 , a primary coal gas preheater  13 , a secondary coal gas preheater  14 , and a tertiary coal gas preheater  15 , a cyclone dust collector  16 , a bag dust collector  17 , an induced draft fan  18 , and a chimney  19 . 
         [0051]    The oxidizing fluidized bed  3  comprises an oxidizing fluidized bed body  31 , a first cyclone separator  32 , a second cyclone separator  33 , and a feeder  34 . 
         [0052]    The primary circulating fluidized bed  5  comprises a primary riser  51 , a third cyclone separator  52 , a fourth cyclone separator  53  and a primary circulating dipleg  54 . 
         [0053]    The secondary circulating fluidized bed  7  comprises a secondary riser  71 , a fifth cyclone separator  72 , a sixth cyclone separator  73  and a secondary circulating dipleg  74 . 
         [0054]    The tertiary circulating fluidized bed  9  comprises a tertiary riser  91 , a seventh cyclone separator  92 , an eighth cyclone separator  93  and a tertiary circulating dipleg  94 . 
         [0055]    A feed outlet at the bottom of the iron ore powder bin  1  is provided with a pipeline, which is connected to a feed inlet of the screw feeder  2 , and the iron ore powder is added to the screw feeder  2  through the pipeline. A feed outlet of the screw feeder  2  is connected to a gas outlet of the first cyclone separator  32  and an inlet of the second cyclone separator  33  through a pipeline. 
         [0056]    The oxidizing fluidized bed body  31  is provided with a gas outlet and a feed outlet at the upper portion, a gas inlet at the bottom, and a feed inlet at the lower portion. The gas outlet of the oxidizing fluidized bed body  31  is connected to an inlet of the first cyclone separator  32 , the feed inlet of the oxidizing fluidized bed body  31  is connected to a feed outlet of the feeder  34 , the gas inlet of the oxidizing fluidized bed body  31  is connected to a gas outlet of the burning chamber  12  through a pipeline, and the feed outlet of the oxidizing fluidized bed body  31  is connected to a feed inlet of the primary feeder  4  through a pipeline. The inlet of the first cyclone separator  32  is also connected to a feed outlet of the second cyclone separator  33 , a feed outlet of the cyclone dust collector  16 , and a feed outlet of the bag dust collector  17 , respectively, through a pipeline, a feed outlet of the first cyclone separator  32  is connected to a feed inlet of the feeder  34 , and a gas inlet and feed outlet of the feeder  34  are connected to an air main pipe and the feed inlet of the oxidizing fluidized bed body  31 , respectively, through a pipeline. An inlet of the second cyclone separator  33  is connected to the feed outlet of the screw feeder  2  and the gas outlet of the first cyclone separator  32  through a pipeline, a gas outlet of the second cyclone separator  33  is connected to an inlet of the cyclone dust collector  16 , and the feed outlet of the second cyclone separator  33  is connected to the inlet of the first cyclone separator  32 . 
         [0057]    A gas outlet of the cyclone dust collector  16  is connected to a gas inlet of the bag dust collector  17  through a pipeline, and the feed outlet of the cyclone dust collector  16  is connected to the inlet of the first cyclone separator  32  through a pipeline. The gas inlet of the bag dust collector  17  is connected to an inlet of the induced draft fan  18 , the feed outlet of the bag dust collector is connected to the inlet of the first cyclone separator  32  through a pipeline, and a gas outlet of the induced draft fan  18  is connected to the chimney  19 . 
         [0058]    The primary feeder  4  is provided with a feed inlet, a gas inlet and a feed outlet. The feed inlet of the primary feeder  4  is connected to the feed outlet of the oxidizing fluidized bed body  31  through a pipeline. The gas inlet of the primary feeder  4  is connected to the coal gas main pipe. The feed outlet of the primary feeder  4  is connected to a feed inlet of the primary riser  51  through a pipeline. 
         [0059]    The primary riser  51  is provided with a gas outlet at the top, a gas inlet at the bottom, and a feed inlet and a circulating return port at the lower portion. The gas outlet of the primary riser  51  is connected to an inlet of the third cyclone separator  52 , the feed inlet of the primary riser  51  is connected to the feed outlet of the primary feeder  4 , and the gas inlet of the primary riser  51  is connected to a coal gas outlet of the primary coal gas preheater  13  through a pipeline. A gas outlet of the third cyclone separator  52  is connected to an inlet of the fourth cyclone separator  53 , and a discharge port of the third cyclone separator  52  is connected to the primary circulating dipleg  54 . The primary circulating dipleg  54  is provide with a return port and a discharge port at the lower portion. The return port of the primary circulating dipleg  54  is connected to the circulating return port of the primary riser  51  through a pipeline, the discharge port of the primary circulating dipleg  54  is connected to a lower feed inlet of the secondary riser  71  through a pipeline, and a gas inlet of the primary circulating dipleg  54  is connected to the coal gas main pipe. The gas outlet of the fourth cyclone separator  53  is connected to an inlet of a reduction exhaust gas treatment system, and a discharge port of the fourth cyclone separator  53  is connected to a feed inlet of the secondary feeder  6  through a pipeline. 
         [0060]    The secondary feeder  6  is provided with a feed inlet, a gas inlet and a feed outlet. The feed inlet of the secondary feeder  6  is connected to the discharge port of the fourth cyclone separator  53 , the gas inlet of the secondary feeder  6  is connected to the coal gas main pipe, and the feed outlet of the secondary feeder  6  is connected to the an upper feed inlet of the secondary riser  71  through a pipeline. 
         [0061]    The secondary riser  71  provided with a gas outlet at the top, an upper feed inlet at the upper portion, a gas inlet at the bottom, and the lower feed inlet and a circulating return port at the lower portion. The gas outlet of the secondary riser  71  is connected to an inlet of the fifth cyclone separator  72 , the upper feed inlet of the secondary riser  71  is connected to the feed outlet of the secondary feeder  6 , the lower feed inlet of the secondary riser  71  is connected to the discharge port of the primary circulating dipleg  54  through a pipeline, and the gas inlet of the secondary riser  71  is connected to a coal gas outlet of the secondary coal gas preheater  14  through a pipeline. A gas outlet of the fifth cyclone separator  72  is connected to an inlet of the sixth cyclone separator  73 , and a discharge port of the fifth cyclone separator  72  is connected to the secondary circulating dipleg  74 . The secondary circulating dipleg  74  is provided with a return port and a discharge port at the lower portion. The return port of the secondary circulating dipleg  74  is connected to the circulating return port of the secondary riser  71  through a pipeline, the discharge port of the secondary circulating dipleg  74  is connected to a lower feed inlet of the tertiary riser  91  through a pipeline, and the gas inlet of the secondary circulating dipleg  74  is connected to the coal gas main pipe through a pipeline. A gas outlet of the sixth cyclone separator  73  is connected to the inlet of the reduction exhaust gas treatment system, and a discharge port of the sixth cyclone separator  73  is connected to a feed inlet of the tertiary feeder  8  through a pipeline. 
         [0062]    The tertiary feeder  8  is provided with a feed inlet, a gas inlet and a feed outlet. The feed inlet of the tertiary feeder  8  is connected to a discharge port of the sixth cyclone separator  73 , the gas inlet of the tertiary feeder  8  is connected to the coal gas main pipe, and the feed outlet of the tertiary feeder  8  is connected to the an upper feed inlet of the tertiary riser  91  through a pipeline. 
         [0063]    The tertiary riser  91  is provided with a gas outlet at the top, an upper feed inlet at the upper portion, a gas inlet at the bottom, and a lower feed inlet and a circulating return port at the lower portion. The gas outlet of the tertiary riser  91  is connected to an inlet of the seventh cyclone separator  92 , the upper feed inlet of the tertiary riser  91  is connected to the feed outlet of tertiary feeder  8 , the lower feed inlet of the tertiary riser  91  is connected to the discharge port of the secondary circulating dipleg  74  through a pipeline, and the gas inlet of the tertiary riser  91  is connected to a coal gas outlet of the tertiary coal gas preheater  15  through a pipeline. A gas outlet of the seventh cyclone separator  92  is connected to an inlet of the eighth cyclone separator  93 , and a discharge port of the seventh cyclone separator  92  is connected to the tertiary circulating dipleg  94 . The tertiary circulating dipleg  94  is provided with a return port and a discharge port at the lower portion, and the return port of the tertiary circulating dipleg  94  is connected to the circulating return port of the tertiary riser  91  through a pipeline, the discharge port of the tertiary circulating dipleg  94  is connected to the product bin  11  through a pipeline, and a gas inlet of the tertiary circulating dipleg  94  is connected to the coal gas main pipe through a pipeline. A gas outlet of the eighth cyclone separator  93  is connected to the inlet of the reduction exhaust gas treatment system, and a discharge port of the eighth cyclone separator  93  is connected to a feed inlet of the discharger  10  through a pipeline. 
         [0064]    The discharger  10  is provided with a feed inlet, a gas inlet and a feed outlet. The feed inlet of the discharger  10  is connected to the discharge port of the eighth cyclone separator  93 , the gas inlet of the discharger  10  is connected to the coal gas main pipe, and the feed outlet of the discharger  10  is connected to the product bin  11  through a pipeline. 
         [0065]    The burning chamber  12  heats the air by burning the coal gas. It is provided with a nozzle that is connected to the coal gas main pipe and the air main pipe, and a gas outlet that is connected to the gas inlet at the bottom of the oxidizing fluidized bed body  31  through a pipeline. The gas inlet of the burning chamber  12  is connected to the air main pipe. 
         [0066]    The coal gas preheater is divided into the primary coal gas preheater  13 , the secondary coal gas preheater  14  and the tertiary coal gas preheater  15 , which preheat the fluidized coal gas required for the primary circulating fluidized bed  5 , the secondary circulating fluidized bed  7  and the tertiary circulating fluidized bed  9 , respectively. Any one of the three coal gas preheaters is provided with a nozzle, a flue gas outlet, a coal gas inlet and a coal gas outlet. The nozzle is used for burning the coal gas to generate a high temperature flue gas to provide the heat for preheating the reducing coal gas, and the nozzle is connected to the air main pipe and the coal gas main pipe through a pipeline. The flue gas outlet is connected to a flue gas main pipe, and the discharged flue gas enters into a flue gas treatment system via the flue gas main pipe for heat recovery. The coal gas inlet is connected to the coal gas main pipe, the coal gas outlet of the primary coal gas preheater is connected to the gas inlet at the bottom of the primary riser, the coal gas outlet of the secondary coal gas preheater is connected to the gas inlet at the bottom of the secondary riser, and the coal gas outlet of the tertiary coal gas preheater is connected to the gas inlet at the bottom of the tertiary riser. 
         [0067]    A bubbling fluidized bed is used as the oxidizing fluidized bed in the present example. 
       EXAMPLE 2 
       [0068]    A method for reducing iron ore powder by the above fluidized reduction system comprises the following steps: the iron ore powder is sent to the inlet of the second cyclone separator  33  from the iron ore powder bin  1  via the screw feeder  2 . The iron ore powder separated and collected by the second cyclone separator  33  and the iron ore powder collected by the cyclone dust collector  16  and the bag dust collector  17  respectively enter into the inlet of the first cyclone separator  32  through a pipeline, and then enter into the first cyclone separator  32  for gas-solid separation after being mixed with the exhaust gas discharged by the oxidizing the fluidized bed body  31 . The iron ore powder is discharged from the feed outlet at the bottom of the first cyclone separator  32  into the oxidizing fluidized bed body  31  via the feeder  34 . The iron ore powder is subjected to an oxidization reaction in the oxidizing fluidized bed body  31 , and then discharged from the feed outlet at the upper portion of the oxidizing fluidized bed body  31  into the primary riser  51  via the primary feeder  4 . After being reduced in the primary circulating fluidized bed, it is discharged from the primary circulating dipleg  54  into the secondary riser  71  of the secondary circulating fluidized bed  7  via the lower feed inlet. Meanwhile, the iron ore powder collected by the fourth cyclone separator  53  enters into the secondary riser  71  from the upper feed inlet via the secondary feeder  6 . The iron ore powder is reacted in the secondary circulating fluidized bed  7  and then discharged from the secondary circulating dipleg  74  into the tertiary riser  91  of the tertiary circulating fluidized bed  9  via the lower feed inlet. Meanwhile, the iron ore powder collected by the sixth cyclone separator  73  enters into the tertiary riser  91  from the upper feed inlet via the secondary feeder  8 . The iron ore powder is reduced in the tertiary circulating fluidized bed  9  and then discharged from the tertiary circulating dipleg  94  into the product bin  11 . Meanwhile, the iron ore powder collected by the eighth cyclone separator  93  enters into the product bin  11  via the discharger  10 . 
         [0069]    The coal gas, via the coal gas main pipe, is divided into three parts which enter into the primary coal gas preheater  13 , the secondary coal gas preheater  14  and the tertiary coal gas preheater  15 , respectively. After being preheated by the coal gas preheaters  13 ,  14  and  15 , the coal gas is discharged from the coal gas outlet of the primary coal gas preheater  13 , the secondary coal gas preheater  14  and the tertiary coal gas preheater  15  into the primary riser  51 , the secondary riser  71  and the tertiary riser  91 , respectively, via the gas inlet at the bottom. After a reduction reaction with the iron ore powder in the primary circulating fluidized bed  5 , secondary circulating fluidized bed  7  and the tertiary circulating fluidized bed  9 , it is discharged from the primary riser  51 , the secondary riser  71  and the tertiary riser  91  and subjected to gas-solid separation through the third cyclone separator  52 , the fifth cyclone separator  72  and the seventh cyclone separator  92 . Subsequently, it is subjected to further gas-solid separation through the fourth cyclone separator  53 , the sixth cyclone separator  73  and the eighth cyclone separator  93  and then enters into the reduction exhaust gas treatment system. The heat required for preheating the coal gas in the coal gas preheaters  13 ,  14  and  15  is obtained through combustion of the coal gas by the nozzle. The coal gas and air from the coal gas main pipe and the air main pipe are sent to the nozzles of the primary coal gas preheater  13 , the secondary coal gas preheater  14  and the tertiary coal gas preheater  15 , respectively, through the pipeline, and burned in the primary coal gas preheater  13 , the secondary coal gas preheater  14  and the tertiary coal gas preheater  15  after being lighted by the nozzle. Then, they are discharged through the flue gas outlets of the primary coal gas preheater  13 , the secondary coal gas preheater  14  and the tertiary coal gas preheater  15  into the flue gas treatment system. The air enters into the burning chamber  12  via the air main pipe. Meanwhile, the coal gas and the air enter into the nozzle of the burning chamber from the coal gas main pipe and the air main pipe, and after being lighted by the nozzle, are burned in the burning chamber  12  to provide heat for the heating air. The air is heated in the burning chamber and then enters into the oxidizing fluidized bed body  31  from the gas inlet at the bottom. After an oxidization reaction with the iron ore powder in the oxidizing fluidized bed body  31 , it is dedusted by the first cyclone separator  32  and the second cyclone separator  33 , and then enters into the cyclone dust collector  16  and the bag dust collector  16  for further dedusting. Finally, it is discharged through the chimney  19  via the induced draft fan  18 . 
       EXAMPLE 3 
       [0070]    A Philippine marine placer (vanadium titano-magnetite) comprised of 63.5% Fe 3 O 4 , 2.5% Fe 2 O 3 , 22.0% FeTiO 3 , 1.5% CaO, 3.2% MgO, 3.9% Al 2 O 3  and 3.4% SiO 2  (percentages by mass) was processed according to the present invention. The iron ore powder had a particle size of 0.1-0.3 mm. After being oxidized in the oxidizing fluidized bed according to the present invention, the oxidization ratio of the Fe(II) in the vanadium titano-magnetite reached 80.5% at 850° C. for 30 min; 90.8% at 850° C. for 60 min; 85.3% at 900° C. for 20 min; and 93.6% at 950° C. for 30 min. The oxidized vanadium titano-magnetite was reduced in the three-stage circulating fluidized beds according to the above process of the present invention. When each stage of the circulating fluidized bed was operated under the conditions of a reduction temperature of 800° C., a reduction time of 30 minutes and an operating gas velocity of 5 m/s with a corresponding maximum operating pressure of 0.36 atm, the metallization ratio of the iron ore powder from the outlet of the third stage circulating fluidized bed reached 83.7%. When each stage of the circulating fluidized bed was operated under the conditions of a reduction temperature of 850° C., a reduction time of 30 minutes and an operating gas velocity of 10 m/s with a corresponding maximum operating pressure of 0.58 atm, the metallization ratio of the iron ore powder from the outlet of the third stage circulating fluidized bed reached 87.2%. When each stage of the circulating fluidized bed was operated under the conditions of a reduction temperature of 850° C., a reduction time of 60 minutes and an operating gas velocity of 9 m/s with a corresponding maximum operating pressure of 0.53 atm, the metallization ratio of the iron ore powder from the outlet of the third stage circulating fluidized bed reached 94.1%. When each stage of the circulating fluidized bed was operated under the conditions of a reduction temperature of 900° C., a reduction time of 30 minutes and an operating gas velocity of 9 m/s with a corresponding maximum operating pressure of 0.56 atm, the metallization ratio of the iron ore powder from the outlet of the third stage circulating fluidized bed reached 91.4%. In contrast, when only preheating was conducted without oxidization before the reduction, the following results were obtained for the same vanadium titano-magnetite reduced through three-stage circulating fluidized beds. When each stage was operated under the conditions of a reduction temperature of 850° C., a reduction time of 30 minutes and an operating gas velocity of 10 m/s with a corresponding maximum operating pressure of 0.59 atm, the metallization ratio of the iron ore powder from the outlet of the third stage circulating fluidized bed were only 62.9%. When each stage was operated under the conditions of a reduction temperature of 900° C., a reduction time of 30 minutes and an operating gas velocity of 9 m/s with a corresponding maximum operating pressure of 0.56 atm, the metallization ratio of the iron ore powder from the outlet of the third stage circulating fluidized bed only reached 70.2%. 
       EXAMPLE 4 
       [0071]    A common iron ore comprised of 82.5% Fe 3 O 4 , 3.7% CaO, 1.2% MgO, 3.8% Al 2 O 3  and 8.8% SiO 2  (percentages by mass) was processed according to the present invention. The iron ore powder had a particle size of 0.15-1.0 mm. After being oxidized in the oxidizing fluidized bed according to the above process of the present invention, the oxidization ratio of the Fe(II) in the iron ore powder reached 87.3% at 850° C. for 30 min; 95.8% at 850° C. for 60 min; 90.3% at 900° C. for 20 min; and 94.9% at 950° C. for 30 min. The oxidized iron ore powder was reduced in three stage circulating fluidized beds according to the above process of the present invention. When each stage of the circulating fluidized bed was operated under the conditions of a reduction temperature of 800° C., a reduction time of 30 minutes and an operating gas velocity of 5 m/s with a corresponding maximum operating pressure of 0.35 atm, the metallization ratio of the iron ore powder from the outlet of the third stage circulating fluidized bed reached 80.7%. When each stage of the circulating fluidized bed was operated under the conditions of a reduction temperature of 850° C., a reduction time of 30 minutes and an operating gas velocity of 10 m/s with a corresponding maximum operating pressure of 0.58 atm, the metallization ratio of the iron ore powder from the outlet of the third stage circulating fluidized bed reached 86.7%. When each stage of the circulating fluidized bed was operated under the conditions of a reduction temperature of 900° C., a reduction time of 30 minutes and an operating gas velocity of 9 m/s with a corresponding maximum operating pressure of 0.55 atm, the metallization ratio of the iron ore powder from the outlet of the third stage circulating fluidized bed reached 94.8%. 
         [0072]    Finally, it should be noted that the above examples are merely provided for illustrating the technical solutions of the present invention, rather than limiting the present invention. Although the present invention has been described in detail with reference to the above examples, those of ordinary skill in the art will appreciate that they can still modify the technical solutions described in each example above, or make equivalent replacements for some technical features therein. However, such modifications or replacements will not render the essence of corresponding technical solutions departing from the spirit and scope of the technical solutions in each example of the present invention.