METHOD AND APPARATUS FOR PRODUCING MOLTEN METAL FROM POWDER STATE ORE

Abstract: A method and apparatus for producing molten metal from powder state ore through smelting process to be performed in a shaft furnace, utilized the reducing material having grain size greater than that n-types of the as blow velocity corresponding grain size to charge rom the top of a shaft furnace for forming fluidized bed at the upper section of the furnace and a reducing material filled section below the fluidized bed. The method and apparatus for smelting the powder state ore also takes the reducing material having smaller grain size to be blown into the furnace through tuyeres.

Description: The present invention relates generally to a method and apparatus for producing a molten metal from powder state ore. More specifically, the invention relates to a process for smelting powder state ore by utilizing a shaft furnace. In the recent years, the ratio of powder state ore as a material for producing metal increases. Especially, according to advance of ore dressing techniques, the ratio of the powder state ore is expected to further increase. In this viewpoint, there has been proposed a technique for smelting the powder state ore in a shaft furnace filled with carbon containing reducing material, such as coal or coke. EP-A-0 114 040 discloses a method and an apparatus for smelting sponge iron particles and pre-reduced iron ore particles of a diameter over 3mm by charging the particles into a smelter which comprises a fluidized bed of coke placed above a second stable bed of coke. Tuyeres for blowing in an oxygen-containing gas and additional reductional carbon are provided for in the lower part of each coke bed. The additional reductive carbon may be in the shape of a gas, a liquid or finely divided solid parts. A smelting process utilizing a shaft furnace has been developed and proposed. In the known process, solid state carbon containing reducing material is charged through the top of the furnace. To the furnace filled with the carbon containing reducing material, oxygen containing gas is blown through tuyeres in order to form a fluidized bed at the upper section of the furnace. Below the fluidized bed, a reduction material filled section is formed. The powder state ore is also blown into the furnace to perform smelting operation. By blowing the oxygen containing gas, substantially high temperature race ways are formed around the tuyeres. The powder state ore blown into the furnace through the tuyeres is instantly molten in the race ways. Molten ore flows down through the reducing material filled section or otherwise is fluidized with the reducing material in the fluidized bed to raise the temperature. During fluidization, reduction of the ore progresses to refine. According to progress of reduction, the density of the molten ore gradually increases. At the same time, the reduced ore repeats solidifying and melting to gradually increase the grain size. The ore with increased grain size moves down through the reducing material filled section. During downward travel, reduction of the ore is completed. At the same time, the temperature of the ore is raised to the tapping temperature. On the other hand, during the aforementioned reduction process, the ore absorbs metalloid, such as Si, Mn. Furthermore, during the reduction process, separation of the metal component and slag component occurs so that molten metal and slag are separately collected in the bottom of the furnace. Such a smelting technique is effective for efficiently producing molten metal from the powder state ore. However, the prior proposed technique has a drawback in that the reducing material to be used has to have a grain size large enough so as not to be blown away by the gas flow. The grain size of the reducing material may be variable depending upon the gas flow velocity in the furnace, which as flow velocity varies depending upon the temperature in the furnace, pressure, gas flow amount and so forth. The grain size of the reducing material at the border between blown away and not blown away in relation to the gas flow velocity will be hereafter referred to as ''gas flow velocity corresponding grain size''. In the practical operation, in consideration of fluctuation of the gas flow velocity, the grain size of the reducing material to be charged in the shaft furnace is selected to be n-times greater than the gas flow velocity corresponding grain size. In such cases, the reducing material having smaller grain size than that n-times of the gas flow velocity corresponding grain size will never be used. On the other hand, even when the reducing material which has smaller grain size than the gas flow velocity corresponding grain size, such small grain size reducing material may be easily blown away with the exhausting gas. This apparently increases the cost for producing the molten metal. On the other hand, temperature and composition of the molten metal are variable depending upon the temperature in the reducing material filled section of the furnace. Therefore, in order to stably produce high quality molten metal, it is essential to control the temperature of the reducing material filled section. The Japanese Patent First (unexamined) Publication (Tokkai) Showa 62-56537 discloses a method for producing molten metal from powder state ore by forming the fluidized bed of the reducing material and the reducing material filled section in the shaft furnace. However, the disclosed prior art connot control the temperature of the reducing material filled section. Therefore, it was not possible to stably perform production of the molten metal and maintain the quality of the produced molten metal at a satisfactorily high level. Therefore, it is an object of the present invention to provide a method and apparatus for producing molten metal from powder state ore in a shaft furnace, which can control the temperature of the reducing material filled section in the shaft furnace so as to control temperature and composition of the molten metal. In order to accomplish the aforementioned objects, the present invention utilizes the method and apparatus as defined in the claims. In the preferred embodiment, the border grain size is determined relative to a minimum grain size of the burden which is not blown away from the furnace with an exhaust gas. In practice, the border grain size is set at a value n-times greater than the minimum grain size. In the alternative, the border grain size of the burden is set at 3 mm diameter. The present invention will be understood more fully from the detailed description given herebelow and from the accompanying drawings of the preferred embodiments of the invention. In the drawings: Fig. 1 is a diagrammatical illustration of the first embodiment of a shaft furnace arrangement which implements the preferred embodiment of method for producing molten metal from powder state ore through a smelting process; Fig. 2 is a diagrammatical illustration of the second embodiment of a shaft furnace arrangement for implementing the preferred smelting process for producing molten metal from powder state ore; and Figs. 3 and 4 show variation of Si content and tapping temperature through the operation period in the smelting process of the second embodiment and a comparative example. Referring now to the drawings, particularly to Fig. 1, the first embodiment of a shaft furnace arrangement is particularly designed for smelting and/or reducing power state ore for obtaining molten metal. A shaft furnace 6 is employed for implementing the preferred process of smelting operation. A reducing material pre-treatment furnace 14 is also provided for performing pre-treatment of solid state carbon containing reducing material, such as coke. In the pre-treatment furnace 14, pre-heating of the reducing material may be performed. The pre-treatment for the reducing material to be performed in the pre-treatment furnace also includes classifying or sizing of the reducing material. Reducing material having grain size larger than or equal to n-times of gas flow velocity corresponding grain size, is selected to be transferred through a reducing material outlet 15 of the pre-treatment furnace 14 and a reducing material transferring passage way 15a, to be charged into the shaft furnace as a burden of the furnace. The reducing material charged in the shaft furnace 6 forms a fluidized bed 5 at the upper section thereof, and a solid state reducing material filled section 4, wherein the fluidized bed is formed above the solid state reducing material filled section. The gas flow velocity corresponding grain size of the solid state reducing material may be arithmetically derived on the basis of the temperature, pressure and as flow amount, gas flow velocity in the furnace, apparent density of the reducing material, density of gas and viscosity coefficient, utilizing Allen's formula or Newton's formula. In the shown embodiment, the grain size of the solid state reducing material to be charged to the furnace 6 is selected to be larger than or equal to twice the gas flow velocity corresponding grain size. The shaft furnace arrangement further includes an ore pre-treatment furnace 16 which performs pre-treatment for powder state ore. In the pre-treatment for powder state ore in the ore pre-treatment furnace 16, the pre-fluidization and pre-reduction of the ore is performed. The pre-treated ore is transferred through the outlet 17 and an ore passageway 17a as a constituent of the burden to be charged through the top of furnace. Part of the pre-treated ore is delivered through an ore passageway 17b to be introduced into the fluidized bed 5 in the furnace. The shaft furnace 6 is provided with two vertically offset groups of tuyeres 3 and 8. One group of tuyeres 3 is located at a lower elevation than the other 8 and directed to the solid state reducing material filled section 4. On the other hand, the other group of tuyeres 8 is directed toward the fluidized bed 5. The tuyeres 3 located at the lower elevation will be hereafter referred to as ''lower tuyeres'' and the other tuyeres 8 located at upper elevation will be hereafter referred to as ''upper tuyeres''. The lower and upper tuyeres 3 and 8 are respectively connected to an oxygen containing gas source 2 to introduce therefrom oxygen containing reduction gas through gas passageways 2a and 2b. The oxygen containing gas introduced into the furnace through the upper tuyeres 8 serves for fluidization of the reduction material to form the fluidized bed. On the other hand, the oxygen containing gas introduced into the furnace via the lower tuyeres 3 serves for reducing the ore travelling through the solid state reducing material filled section 4. The ore passageway 17a is connected to the gas passageways 2b. Therefore, the powder state ore fed through the ore passageway 17a is introduced into the gas passageways 2b and is blown into the fluidized bed 5 in the furnace 6 via the upper tuyeres 8. The ore introduced into the fluidized bed 5 is fluidized to drop through the solid state reducing material filled section 5. During dropping through the solid state reducing material filled section 4, the ore is molten and reduced. Furthermore, during dropping, molten metal 10 and slag 11 are separated to be separately collected in the bottom of the furnace. The molten metal 10 collected in the bottom of the shaft furnace 6 is tapped via tapping notch 12. On the other hand, reducing material having a grain size smaller than n-times of the gas flow velocity corresponding grain size, is collected by a collector 20 and fed through a reducing dust passageway 21. The passageway 21 is connected to the gas passageways 2a and 2b. The ratio of the reducing material to be introduced into the gas passageways 2a and 2b may be adjusted in view of the grain distribution of the reducing material to be charged through the reducing material transferring passageway 15a so that the temperature in the solid state reducing material filled section 4 can be controlled to be adapted for the molten metal to be produced. During the smelting operation, exhaust gas rising through the solid state reducing material filled section 4 and the fluidized bed 5 is collected and circulated into the reducing material pre-treatment furnace 14 and the ore pre-treatment furnace 16. The exhaust gas introduced into the reducing material pre-treatment furnace is utilized as distillation gas for distilling the reducing material in the pre-treatment. In order to confirm the performance of the aforementioned first embodiment of the smelting process, experiments are performed. In the experimentation, the shaft furnace of 1.2m diameter furnace was used. EXAMPLE 1 In the experimentation of example 1, the gas flow velocity corresponding grain size derived was 0.5 mm. The reducing material of 20 to 1 mm grain size is charged through the top of the shaft furnace. The reducing material having grain size smaller than 1 mm was introduced into the furnace through the upper and lower tuyeres 3 and 8. Overall charge amount of the reducing material was 1040 kg/h. To this, the amount of the reducing material to be introduced into the fluidized bed through the upper tuyeres 8 was 95 kg/h (9.1% of overall reducing material amount). The amount of the reducing material to be introduced into the solid state reducing material filled section 4 was 61 kg/h (5.9% of the overall amount of the reducing material). From the condition set forth above, 11.8 tons of pig iron could be produced per day. EXAMPLE 2 In the experimentation of example 2, the gas flow velocity corresponding grain size derived was 0.5 mm. The reducing material of 20 to 1 mm grain size is charged through the top of the shaft furnace. The reducing material of a grain size smaller than 1 mm was introduced into the furnace through the upper and lower tuyeres 3 and 8. Overall charge amount of the reducing material was 997 kg/h. To this, the amount of the reducing material to be introduced into the fluidized bed through the upper tuyeres 8 was 78 kg/h (7.8% of overall reducing material amount). The amount of the reducing material to be introduced into the solid state reducing material to be introduced into the solid state reducing material filled section 4 was 111 kg/h (11.1% of the overall amount of the reducing material). From the condition set forth above, 11.2 tons of pig iron could be produced per day. As will be seen herefrom, since the grain size of the reducing material charged through the top of the furnace was smaller than that used in example 1, the small grain size reducing material to be introduced into the solid state reducing material filled section 4 was increased in comparison with that in example 1. In either examples, the small grain size reducing material introduced into the fluidized bed 5 becomes high temperature particle. Since the powder state ore is introduced into the fluidized bed 5 together with the reducing material, the molten ore tends to adhere to the surface of the small grain size reducing material. This makes reduction of the ore more efficient. In another embodiment of the smelting process of the molten metal from the powder state ore, operation is performed by charging reducing material of grain size greater than or equal to 3 mm diameter. The reducing material of the grain size smaller than 3 mm diameter is discharged through the upper and the lower tuyeres 8 and 3. In order to separate the large grain size reducing material which has a grain size greater than or equal to 3 mm diameter and the small grain size reducing material which has the grain size smaller than 3 mm diameter, classification of the reducing material may be performed in the reducing material pre-treatment furnace and associated classification device. In order to implement the second embodiment of the smelting method, an experiment is performed utilizing the shaft-type reduction furnace which has 1.2 m of internal diameter, 5 m of height and about 10 tons of production capacity of pig iron per day, which, in turn, has a production capacity for about 5 tons of ferrochromium per day. The large grain size reducing material was charged through the top of the furnace. The small grain size reducing material was then blown into the fluidized bed 5 and the solid state reducing material filled section 4 via the upper and lower tuyeres 8 and 3. The distribution rate of the small grain size reducing material was adjusted depending upon the temperature of the molten metal to be produced. By utilizing the aforementioned facility, the smelting operation was performed for producing pig iron and ferrochromium from iron ore from Australia and chromite from South Africa, the compositions of which are shown in the appended table 1. In order to compare with the inventive examples 3 and 4, comparative experiments were also performed, the results of which are shown as comparative examples 1 and 2 in the appended table 2. In the comparative experiments, the overall amount of reducing material was charged through the top of the furnace regardless of the grain size. On the other hand, in the inventive examples 3 and 4, the distribution rate of the small grain size reducing material was adjusted so that the rate may be changed in a range of 0% to 100% depending upon the tapping temperature. The results of the experimentation is shown in the appended table 2. As will be seen from the table 2, it should be appreciated that, by using large grain size reducing material as a burden to be charged through the top of the furnace for forming the fluidized bed and the solid state reducing material filled section in the furnace and blowing the small grain size reducing material through the tuyeres at controlled distribution rate, the total consumption of the reducing material could be decreased. In addition, the tapping temperature and Si concentration of the molten metal can be maintained at a narrower variation range in relation to the desired tapping temperature and desired Si concentration, in comparison with the variation range of the comparative examples. Fig. 2 shows the second embodiment of the shaft furnace arrangement according to the invention, which implements the preferred smelting process for producing molten metal from powder state ore. In the shown second embodiment, the elements of the same construction and same functions to that of the foregoing first embodiment are represented by the same reference numerals to the foregoing first embodiment. For the elements represented by the common reference numerals to the foregoing Fig. 1, a detailed description has not been included in order to avoid redundancy of the recitation. The shaft furnace arrangement of Fig. 2 is characterized by a dust collecting unit 30. The dust collecting unit 30 collects dust of reducing material which flows away from the charged reducing material layer with the exhaust gas. The dust collecting unit 30 recovers the reducing material dust and recirculates some to the passage 21. On the other hand, the dust collecting unit 30 may feed the high temperature exhaust gas to the reducing material pre-treatment furnace (not shown in Fig. 2) and the ore pre-treatment furnace (not shown in Fig. 2) for utilizing the heat of the exhaust gas in pre-treatment. In the shown embodiment, the passage way 21 is branched to have two branches 21a and 21b emanating from a distribution unit 31. The branch 21a is connected to the gas flow passageway 2a and the branch 21b is connected to the gas flow passageway 2b. A gas distribution unit 32 is disposed between the gas flow passageways 2a and 2b for adjusting distribution of the oxygen containing gas to flow therethrough. Though it is not clearly shown in Fig. 2, it may be possible to connect the dust collecting unit to the reducing material pre-treatment furnace to receive therefrom the small grain size reducing material which has a grain size smaller than n-times of the gas flow velocity corresponding grain size. In the shown construction, the amount of small grain reducing material to be distributed to the branches 21a and 21b is varied so as to control consumption of the charged large grain reducing material. Namely, if the amount of available large grain material becomes low, which will cause lowering of the molten metal temperature and consequently a reduction in the quality of the metal produced, the amount of small grain material added to the solid material filled section 4, via the branch 21a and the lower tuyeres 3, is increased. By increasing the amount of small grain material present in the solid material section 4, combustion occurs in both large and small grain materials, which reduces the amount of large grain material required to maintain the temperature of the solid-state reducing material in the solid material section 4, at a desired temperature. Further, this lengthens the period in which the large grain material may be held in the solid material section 4 which allows the temperature of the material in the solid material section 4 to rise. This, in turn, raises the temperature of the molten metal passing through the reducing material filled section and can thus stably maintain the composition and quality of the molten metal and the resulting product. On the other hand, when an excessive volume of large grain size reducing material is within the reducing material filled section, the temperature of the molten metal tends to be excessively high. In this case, the small grain size reducing material to be introduced into the reducing material filled section 4 is reduced. By this, consumption of the large grain size reducing material is increased to lower the temperature in the reducing material filled section and thereby lower the temperature of the molten metal. In order to check the performance of the shown second embodiment of the shaft furnace and the preferred smelting process, an experiment was performed by utilising a shaft type reduction furnace 6 which has a capacity for producing about 10 tons of pig iron per day and about 5 tons of ferrochromium. In the experimentation, iron ore from Australia and chromite from South Africa, the compositions of which are shown in the appended table 1, are used for producing pig iron and ferrochromium. The amount of the small grain size reducing material to be distributed to the fluidized bed 5 and the reducing material filled section 4 were adjusted depending upon the molten metal temperature and Si concentration. The appended table 3 shows composition of the dust used in smelting operation for the aforementioned ores. In order to compare with the results obtained from the preferred process, comparative experiments were performed. The results of the experimentations of the preferred process and the comparative examples are shown in the appended table 4. In comparative examples 3 and 5, smelting operations are performed without blowing in the small grain size reducing material through the tuyeres. In comparative example 4, the distribution rate of the small grain size reducing material is fixed at 1 : 1 to blow into the fluidized base 5 and the reducing material filled section 4. In example 5, distribution of the amounts of the small grain size reducing material to be blown through the upper tuyeres 8 and the lower tuyeres 3 are adjusted in view of the tapping temperature and Si concentration. In this case, the adjustment range of the distribution of the amount of the small grain size reducing material was 20 to 80% in both the upper and lower tuyeres. On the other hand, in the examples 6 and 7, the distribution of the amounts of the small grain size reducing material to be blown through the upper and lower tuyeres are adjusted in a range of 0 to 100% in view of the tapping temperature. From the experimentation set forth above, it was observed that actual tapping temperature and Si concentration in comparative examples 3 and 4 did not match the desired values and fluctuated within a wide range. On the other hand, in examples 5 and 6, the tapping temperature matches the desired value and fluctuated within a small range close to the desired value. In addition, in examples 7 and 6, the Si concentrations were maintained at a substantially narrow range across the desired value. In the case of comparative example 5, both the tapping temperature and Si concentration fluctuated within a substantial range across the desired value. To the contrary, in example 7 both the tapping temperature and Si concentration could be maintained within a substantial narrow fluctuation range across the desired value. The results of the experimentation are shown in the appended table 4. As can be seen from table 4, it will be appreciated that by adjusting the small grain size reducing material distribution at the upper and lower tuyeres, the tapping temperature and Si concentration can be stably maintained at approximately the desired values. Therefore, consumption of the reducing material can be economized. Figs. 3 and 4 show variation of tapping temperature and Si concentration in comparative example 5 and example 6. In example 6, the distribution of the amount of the small grain size of the reducing material was derived according to the following formulas: when a > b + 50 α = 0 when b + 50 > a > b - 50 α = 0.5 - 0.01 x (a - b) when a < b - 50 α = 1.0 where a: tapping temperature; b: desired tapping temperature α: distribution rate of the small grain size reducing material for the lower tuyere relative to the total amount of the small grain size reducing material to be discharged into the furnace. As can be seen from Figs. 3 and 4, it will be appreciated that by adjusting the small grain size reducing material distribution at the upper and lower tuyeres the tapping temperature and Si concentration can be maintained approximately at the desired values. As will be appreciated herefrom, in the smelting process according to the present invention, substantially constant and high quality molten metal can be produced with high efficiency of the reducing material, such as coal or coke by discharging controlled distribution of the small grain size reducing material which tends to be blown away if charged from the top of the furnace as a burden. Therefore, the present invention fulfills all of the objects and advantages sought therefor.

Claims: A method for producing molten metal from powder state ore comprising the steps of: providing a furnace chamber filled with a carbon containing reducing material as a burden, to form a fluidized bed and solid burden layer below said fluidized bed, said burden having a grain size greater than a predetermined border grain size which is determined in relation to gas flow velocity in the furnace; supplying a mixture of an oxygen containing gas, powder state ore and reducing material dust which has a grain size smaller than said border grain size to a first tuyere directed to said fluidized bed; supplying a mixture of said oxygen containing gas and said reducing material dust through a second tuyere directed to said solid burden layer; and distributing said reducing material dust for said first and second tuyeres at respectively controlled distribution rates thereby controlling the burden temperature. A method as set forth in claim 1, wherein in the step of distribution of said reducing material dust at a controlled distribution rate, said distribution rate of said reducing material dust for said first and second tuyere is determined depending upon a given tapping temperature of the molten metal. A method as set forth in claim 1 or 2 wherein in said step of distributing reducing material dust at the controlled distribution rate, said distribution rate is determined depending upon a given desired Si concentration of the molten metal to be produced. A method as set forth in claim 2, wherein said distribution rate for said second tuyere is increased when the molten metal temperature is lower than said desired tapping temperature and decreased when the molten metal temperature is higher than said desired tapping temperature. A method as set forth in any one of claims 1 to 4, wherein said border grain size is determined relative to a minimum grain size of said burden which is not blown away from the furnace with an exhaust gas. A method as set forth in claim 5, wherein said border grain size is set at a value n-times greater than said minimum grain size. A method as set forth in claim 6, wherein said border grain size of said burden is set at 3mm diameter. A method as set forth in any one of claims 1 to 7, which further comprise a step of collecting the reducing material dust contained in an exhaust gas of the furnace for recirculating the collected dust through said first and second tuyeres. A furnace for producing molten metal from powder state ore, in particular for carrying out the method of claim 1 comprising: a furnace chamber filled with a carbon containing reducing material as a burden, to form a fluidized bed and solid burden layer below said fluidized bed, said burden having a grain size greater than a predetermined border grain size which is determined in relation to gas flow velocity in the furance; a first tuyere directed to said fluidized bed; a second tuyere directed to said solid burden layer; first means associated with said first tuyere for supplying the latter with a mixture of an oxygen containing gas, powder state ore and reducing material dust which has a grain size smaller than said border grain size; second means associated with said second tuyere for supplying the latter with a mixture of said oxygen containing gas and said reducing material dust; and reducing material dust control means for distributing said reducing material dust for said first and second means at respectively controlled distribution rates. A furnace as set forth in claim 9, wherein said reducing material dust control means determines said distribution rate of said reducing material dust for said first and second tuyere depending upon a given tapping temperature of the molten metal. A furnace as set forth in claims 9 or 10, wherein said reducing material dust control means determines said distribution rates of said reducing material dust for said first and second tuyeres depending upon a given desired Si concentration of the molten metal to be produced. A furnace as set forth in claim 10, wherein said reducing material dust control means increases the distribution rates of said reducing material dust for said second tuyere when the molten metal temperature is lower than said desired tapping temperature and decreases the distribution rate of said reducing material dust for said second tuyere when the molten metal temperature is higher than said desired tapping temperature. A furnace as set forth in any one of claims 9 to 12, wherein said reducing material dust control means is designed for collecting the reducing material dust contained in an exhaust gas of the furnace for recirculating the collected dust through said first and second tuyeres. A furnace as set forth in claim 13, wherein said reducing material dust control means determines said distribution rate of said reducing material dust for said first and second tuyere depending upon a given tapping temperature of the molten metal.

Inventors: HAMADA, TAKAO, ITAYA, HIROSHI, KATAYAMA, HIDESHI, MOMOKAWA, HIDEYUKI, TAKEUCHI, SHINOBU, USHIJIMA, TAKASHI
Applicants: KAWASAKI STEEL CORPORATION
Application year: 1987
Main classification: C21B13
IPC codes: C21B13, C22B5