METHOD FOR PRODUCING REDUCED IRON

Abstract

A method for producing reduced iron that produces reduced iron by reducing iron oxide charged in a shaft furnace, in which a heated gas mixture which contains a reducing gas and a nitrogen gas, the reducing gas containing 90 volume % or more of a hydrogen gas, is blown into the shaft furnace from a tuyere equipped at a lower portion of a reduction zone of the shaft furnace, at least part of the reducing gas is blown into a cooling zone of the reduced iron provided at a lower portion of the shaft furnace at normal temperature, and the reducing gas that has flowed up in the cooling zone is used for reduction of the iron oxide.

Claims

1. A method for producing reduced iron that produces reduced iron by reducing iron oxide charged in a shaft furnace, wherein a heated gas mixture which contains a reducing gas and a nitrogen gas, the reducing gas containing 90 volume % or more of a hydrogen gas, is blown into the shaft furnace from a tuyere equipped at a lower portion of a reduction zone of the shaft furnace, and at least part of the reducing gas is blown into a cooling zone of the reduced iron provided at a lower portion of the shaft furnace at normal temperature, and the reducing gas that has flowed up in the cooling zone is used for reduction of the iron oxide.

2. The method for producing reduced iron according to claim 1, the method comprising: separating and collecting at least unreacted hydrogen gas and nitrogen gas from a furnace top gas of the shaft furnace; and reusing the separated and collected hydrogen gas and nitrogen gas as part of the gas mixture.

3. A method for producing reduced iron that produces reduced iron by reducing iron oxide charged in a shaft furnace, wherein a heated gas mixture which contains a reducing gas and a nitrogen gas, the reducing gas containing 90 volume % or more of a hydrogen gas, is blown into the shaft furnace, and an ammonia gas is blown into a cooling zone of the reduced iron provided at a lower portion of the shaft furnace at normal temperature, and a hydrogen gas and a nitrogen gas generated by decomposition of the ammonia gas while flowing up in the cooling zone are used as part of the gas mixture.

4. The method for producing reduced iron according to claim 3, the method comprising: separating and collecting at least unreacted hydrogen gas and nitrogen gas from a furnace top gas of the shaft furnace; and reusing part of the separated and collected hydrogen gas and nitrogen gas as part of the gas mixture and using a rest as a fuel gas at a time of heating the gas mixture.

Description

BRIEF DESCRIPTION OF DRAWINGS

[0031] FIG. 1 is an explanatory view showing the process flow of a method for producing reduced iron according to a first embodiment.

[0032] FIG. 2 is a graph showing a relationship between the blow temperature of a gas mixture and a hydrogen gas intensity for each amount of a nitrogen gas added.

[0033] FIG. 3 is a graph showing a relationship between the amount of the nitrogen gas added and the hydrogen gas intensity for each blow temperature of the gas mixture.

[0034] FIG. 4 is a graph showing the relationship between the amount of the nitrogen gas added and the hydrogen gas intensity for each blow temperature of the gas mixture.

[0035] FIG. 5 is graphs showing a correlation among the intensity of a hydrogen gas that is blown into a cooling zone, the temperature and reduction degree of the reduced iron.

[0036] FIG. 6 is graphs showing the in-furnace state of a shaft furnace.

[0037] FIG. 7 is an explanatory view showing the process flow of a method for producing reduced iron according to a first modification example of the first embodiment.

[0038] FIG. 8 is an explanatory view showing the process flow of a method for producing reduced iron according to a second modification example of the first embodiment.

[0039] FIG. 9 is a graph showing a correlation among the intensity of a hydrogen gas that is blown into a cooling zone, the temperature and reduction degree of the reduced iron for each amount of a nitrogen gas added.

[0040] FIG. 10 is an explanatory view showing the process flow of a method for producing reduced iron according to a second embodiment.

[0041] FIG. 11 is a graph showing a correlation among the reaction temperature at the time of decomposing an ammonia gas into a nitrogen gas and a hydrogen gas, the volume ratio and equilibrium constant of each gas.

[0042] FIG. 12 is graphs showing a correlation among the blow temperature of a gas mixture, the intensity of the ammonia gas that is blown into a cooling zone, the intensity of the hydrogen gas that is generated by the decomposition of the ammonia gas and the intensity of a hydrogen gas that is blown into from the outside.

[0043] FIG. 13 is an explanatory view showing the process flow of a method for producing reduced iron according to a modification example of the second embodiment.

[0044] FIG. 14 is a graph showing the trial calculation result of the heat amount intensity (MJ/t-Fe) at the time of producing one ton of reduced iron using a reducing gas (900° C.) for each H.sub.2/CO volume ratio of the reducing gas.

[0045] FIG. 15 is a graph showing the trial calculation result of the reducing gas intensity (Nm.sup.3/t-Fe) at the time of producing one ton of reduced iron using a reducing gas (900° C.) for each H.sub.2/CO volume ratio of the reducing gas.

[0046] FIG. 16 is a graph showing the utilization ratio (%) of the reducing gas for each H.sub.2/CO volume ratio of the reducing gas.

[0047] FIG. 17 is a graph showing a relationship between the reducing gas intensity (Nm.sup.3/t-Fe) and the blow temperature (° C.) of the reducing gas for each H.sub.2/Co volume ratio of the reducing gas.

[0048] FIG. 18 is an explanatory view showing the process flow of a conventional shaft furnace operation.

[0049] FIG. 19 is a graph showing a relationship between the amount of a nitrogen gas added and the hydrogen gas intensity for each blow temperature of a gas mixture.

EMBODIMENTS FOR IMPLEMENTING THE INVENTION

[0050] Hereinafter, preferable embodiments of the present invention will be described in detail with reference to the accompanying drawings. Numerical limiting ranges expressed using “to” include numerical values before and after “to” as the lower limit value and the upper limit value. Numerical values expressed with “more than” or “less than” are not included in the numerical ranges.

1. First Embodiment

[0051] First, the process flow of a method for producing reduced iron (shaft furnace operation) according to a first embodiment will be described based on FIG. 1. In the first embodiment, schematically, a heated gas mixture 30 which contains a reducing gas 31 containing 90 volume % or more of a hydrogen gas and a nitrogen gas 32 is blown into a shaft furnace 10. Furthermore, in the first embodiment, at least part of the reducing gas 31 is blown into a cooling zone of the reduced iron provided at a lower portion of the shaft furnace 10 at normal temperature.

[0052] In more detail, the upper side (approximately the upper half) of the shaft furnace 10 forms a reduction zone 10a, and the lower side (approximately the lower half) forms a cooling zone 10b. The reduction zone 10a and the cooling zone 10b may not be divided as in this example, and, for example, the reduction zone 10a may be set to be longer. At a lower portion (the boundary between the reduction zone 10a and the cooling zone 10b) of the reduction zone 10a, a tuyere 10c for blowing the gas mixture 30 into the shaft furnace 10 is equipped. In addition, while not shown, not only a discharge gate of reduced iron but also a tuyere for blowing the reducing gas into the cooling zone are equipped at the lower end of the shaft furnace 10.

[0053] The method for producing reduced iron according to the first embodiment includes a step of heating the gas mixture 30 containing the reducing gas 31 and the nitrogen gas 32, a step of blowing the heated gas mixture 30 into the shaft furnace 10 and a step of blowing into at least part of the reducing gas 31 into the cooling zone 10b of reduced iron provided at the lower portion of the shaft furnace 10. Steps other than these may be the same as those of the conventional shaft furnace operation.

[0054] For example, the reducing gas 31 and the nitrogen gas 32, which are supplied from the outside, are introduced into a heating furnace 50, and the reducing gas 31 and the nitrogen gas 32 are heated together in the heating furnace 50. This makes the reducing gas 31 and the nitrogen gas 32 mixed in the heating furnace 50 to become the gas mixture 30, and the gas mixture 30 is heated up to a predetermined temperature.

[0055] As described above, the reducing gas 31 contains 90 volume % or more (volume % with respect to the total volume of the reducing gas 31) of a hydrogen gas. That is, the hydrogen gas concentration of the reducing gas 31 becomes 90 volume % or more. From the viewpoint of iron and steel manufacturing with CO.sub.2 zero emission, the hydrogen gas concentration of the reducing gas 31 is preferably as high as possible in a range of 90 volume % or more and preferably 100 volume % (that is, the reducing gas 31 is only composed of a hydrogen gas). Additionally, as a method for heating the gas mixture 30, an electric heater is preferably used, and, in a case where the gas mixture is heated by combustion heating, a combustion gas mainly containing hydrogen is preferable.

[0056] In a case where the hydrogen gas concentration of the reducing gas 31 becomes 90 volume % or more and less than 100 volume %, the reducing gas 31 may contain a reducing gas other than a hydrogen gas. As such a reducing gas, for example, not only a CO gas but also a hydrocarbon gas and the like are included. The hydrocarbon gas generates a CO gas in the shaft furnace. In the following description, unless particularly otherwise described, description will be made on the assumption that the hydrogen gas concentration of the reducing gas 31 is 100 volume %, that is, the reducing gas 31 is only composed of a hydrogen gas.

[0057] The nitrogen gas 32 is an inert gas that is not directly involved in any reduction reactions in the shaft furnace and simply functions as a carrier that carries sensible heat into the shaft furnace 10. Therefore, according to the first embodiment, there is no need to apply a heating load only to the hydrogen gas, which makes it possible to carry out shaft furnace operation at an appropriate blow temperature (predetermined temperature).

[0058] The amount of the nitrogen gas 32 added to the reducing gas 31 will be described below in detail, but the effects of the present embodiment (the reduction of the hydrogen gas intensity and the dropping of the blow temperature of the hydrogen gas) can be obtained only by slightly adding the nitrogen gas 32 to the reducing gas 31. On the other hand, when the nitrogen gas 32 is excessively added, the deceleration of the reduction reaction rate of iron oxide due to a decrease in the hydrogen concentration in the gas mixture 30 surpasses the effect of compensating the reduction reaction heat by heat supply from the nitrogen gas 32. In this case, the effects of the present embodiment are saturated. From such a viewpoint, the amount of the nitrogen gas 32 added is preferably 90 volume % or less of the reducing gas 31.

[0059] The gas mixture 30 is preferably composed only of the above-described reducing gas 31 and nitrogen gas 32 but may contain a gas other than the reducing gas 31 and the nitrogen gas 32 to an extent that the effects of the present embodiment are not affected.

[0060] In the heating furnace 50, the gas mixture 30 is heated up to a predetermined temperature (the temperature of the gas mixture 30 at the time of being blown into the shaft furnace, that is, the blow temperature). The predetermined temperature may be adjusted as appropriate depending on the status or the like of shaft furnace operation, and, as described below, the predetermined temperature can be dropped to be lower than that in a case where the nitrogen gas 32 is not added. This is because the nitrogen gas 32 functions as a carrier of sensible heat. The predetermined temperature is preferably 900° C. or lower. The lower limit value of the predetermined temperature is not particularly limited as long as shaft furnace operation by the first embodiment is possible and may be, for example, approximately 750° C.

[0061] The gas mixture 30 is heated up to the predetermined temperature and then blown into the shaft furnace 10. An iron oxide raw material 20 is charged from the top of the shaft furnace 10. The kind of the iron oxide raw material 20 does not particularly matter and may be the same as in the conventional shaft furnace operation. An example of the iron oxide raw material 20 is iron oxide pellets. The iron oxide raw material 20 charged in the shaft furnace 10 descends in the reduction zone 10a. The arrow X in the drawing indicates the moving direction of the iron oxide raw material 20 or reduced iron 21 in the shaft furnace 10.

[0062] The gas mixture 30 blown into the shaft furnace 10 flows up in the reduction zone 10a of the shaft furnace 10. The reducing gas 31 in the gas mixture 30 reduces the iron oxide raw material 20 that is descending in the reduction zone 10a, whereby the reduced iron 21 is produced. A reduction reaction by the hydrogen gas is an endothermic reaction, but the reduction reaction heat is covered not only by sensible heat from the reducing gas 31 but also by sensible heat from the nitrogen gas 32. The reduction degree of the reduced iron 21 becomes approximately 100% when the reduced iron 21 reaches the tuyere level (the same height as the tuyere 10c), and the temperature of the reduced iron 21 is raised to the level of the blow temperature. A furnace top gas 40 is exhausted from the furnace top of the shaft furnace 10. The furnace top gas 40 contains not only an unreacted hydrogen gas but also water vapor and the nitrogen gas 32.

[0063] After that, the reduced iron 21 moves into the cooling zone 10b. At least part of the reducing gas 31 that is supplied from the outside is blown into (the lower portion of) the cooling zone 10b at normal temperature. That is, in the first embodiment, part of the reducing gas 31 that is supplied from the outside (the reducing gas 31 that is supplied along a supply line (a) in FIG. 1) is blown into the cooling zone 10b at normal temperature, and the rest (the reducing gas 31 that is supplied along a supply line (b) in FIG. 1) is heated together with the nitrogen gas 32 and blown into the reduction zone 10a of the shaft furnace 10 from the tuyere 10c. The reducing gas 31 blown into the cooling zone 10b cools the reduced iron 21 while flowing up in the cooling zone 10b. Accordingly, the reducing gas 31 is heated by the sensible heat of the reduced iron 21. In addition, the reducing gas 31 that has flowed up and has been heated in the cooling zone is used for the reduction of the iron oxide raw material 20. The sensible heat of the reduced iron 21 is also sensible heat supplied to the gas mixture 30 by the heating furnace 50, and thus the reducing gas 31 blown into the cooling zone 10b collects part of sensible heat supplied to the gas mixture 30 by the heating furnace 50. Therefore, the heating load of the reducing gas is reduced, and the thermal efficiency improves.

[0064] Here, “normal temperature” is not particularly limited as long as normal temperature is within a temperature range that is recognized as normal temperature in a technical field to which the present invention belongs and may be in a range of, for example, approximately 25±10° C. Furthermore, the intensity (blowing amount) (Nm.sup.3/t-Fe) of the reducing gas 31 that is blown into the cooling zone 10b is preferably set such that, for example, the reduction degree of the reduced iron 21 that is exhausted from the shaft furnace 10 becomes 100% (or a value close to 100%, for example, 95%) and the temperature of the reduced iron 21 is cooled to approximately normal temperature (for example, normal temperature to approximately normal temperature+30° C.). For example, in a case where the blow temperature becomes 900° C. and the amount of the nitrogen gas 32 added becomes 330 Nm.sup.3/t-Fe, a specific range of the blowing amount becomes approximately 450 to 550 Nm.sup.3/t-Fe (refer to FIG. 4 and FIG. 5).

[0065] Next, among the effects of the first embodiment, an effect of the gas mixture 30 will be described. The present inventors simulated the shaft furnace operation according to the first embodiment using the above-described mathematical model. In addition, shaft furnace operation in which the nitrogen gas 32 was not added was also simulated in order for comparison. The results are shown in FIG. 2 and FIG. 3. The calculation conditions are the same as those in Table 1. In addition, the hydrogen gas concentration of the reducing gas 31 was set to 100 volume %. In addition, the reduction degree of the reduced iron 21 was made to become 100% at the tuyere level. In addition, in the present simulation, the reducing gas 31 was fully injected into the heating furnace 50.

[0066] FIG. 2 shows the relationship between the blow temperature (° C.) of the gas mixture 30 and the hydrogen gas intensity (Nm.sup.3/t-Fe) for each amount of the nitrogen gas 32 added. The graph L1 indicates the above-described relationship when the nitrogen gas is not added, the graph L2 indicates the above-described relationship when 250 Nm.sup.3/t-Fe of the nitrogen gas is added to the reducing gas 31 and the graph L3 indicates the above-described relationship when 500 Nm.sup.3/t-Fe of the nitrogen gas is added to the reducing gas 31. Therefore, the graphs L2 and L3 correspond to the shaft furnace operation according to the first embodiment. Since the hydrogen gas concentration of the reducing gas 31 is 100 volume %, the hydrogen gas intensity can be read as the reducing gas intensity. According to the graphs L2 and L3 in FIG. 2, when the blow temperature becomes 900° C., the reducing gas intensity becomes approximately 1500 to 1700 Nm.sup.3/t-Fe. FIG. 15 shows that, in the conventional shaft furnace operation (the H.sub.2/CO volume ratio of the reducing gas is 80/20 to 66/33), when the blow temperature becomes 900° C., the reducing gas intensity becomes approximately 1200 to 1400 Nm.sup.3/t-Fe. Therefore, the addition of the nitrogen gas 32 to the reducing gas 31 makes it possible to produce reduced iron on substantially the same blow temperature level (for example, 900° C.) and the same reducing gas intensity level (for example, approximately 1500 to 1700 Nm.sup.3/t-Fe) as those in the conventional shaft furnace operation even when the hydrogen gas concentration of the reducing gas 31 is a high concentration (here, 100 volume %).

[0067] Therefore, even in a case where a reducing gas containing a high concentration of a hydrogen gas is used, it is possible to reduce the reducing gas intensity and to drop the blow temperature of the gas mixture 30 (that is, the blow temperature of the reducing gas 31). Furthermore, it is also found that, as the amount of the nitrogen gas 32 added increases, the reducing gas intensity decreases, and the blow temperature of the reducing gas 31 also drops. When the blow temperature of the reducing gas 31 drops, for example, sticking or the like is suppressed.

[0068] FIG. 3 is a view showing the relationship of FIG. 2 arranged to the relationship between the amount of the nitrogen gas 32 added (Nm.sup.3/t-Fe) and the hydrogen gas intensity (Nm.sup.3/t-Fe). That is, FIG. 3 shows the relationship between the amount of the nitrogen gas 32 added (Nm.sup.3/t-Fe) and the hydrogen gas intensity (Nm.sup.3/t-Fe) for each blow temperature (° C.) of the gas mixture 30. The graph L4 indicates the blow temperature becomes 800° C., the graph L5 indicates the above-described relationship when the blow temperature becomes 900° C. and the graph L6 indicates the above-described relationship when the blow temperature becomes 1000° C. According to the graphs L4 to L6, it is found that, in all of the blow temperatures, the hydrogen gas intensity decreases simply by adding a small amount of the nitrogen gas 32.

[0069] Furthermore, when attention is paid to the graphs L5 and L6, addition of 330 Nm.sup.3/t of the nitrogen gas to the reducing gas 31 makes it possible to drop the blow temperature of the gas mixture 30 from 1000° C. to 900° C. while maintaining the hydrogen gas intensity. Therefore, addition of the nitrogen gas 32 to the reducing gas 31 makes it possible to drop the blow temperature and, furthermore, to suppress sticking.

[0070] Furthermore, when attention is paid to the graph L5 (the blow temperature of 900° C.), addition of 330 Nm.sup.3/t-Fe of the nitrogen gas 32 to the reducing gas 31 makes it possible to reduce the hydrogen gas intensity by approximately 300 Nm.sup.3/t-Fe. This means that 330 Nm.sup.3/t of the nitrogen gas 32 and 300 Nm.sup.3/t-Fe of the hydrogen gas are approximately equivalent to each other from the viewpoint of heat and reaction manipulation. Furthermore, from another angle, the amount of the reduced iron 21 produced can also be controlled by adjusting the amount of the nitrogen gas 32 added under a condition where the blow temperature is constant. For example, when the amount (Nm.sup.3/t-Fe) of the nitrogen gas 32 added is increased without changing the blow temperature and the blowing amount of the hydrogen gas per unit time, the amount of the reduced iron 21 produced per unit time increases.

[0071] The reason for the fact that the seemingly incomprehensible equivalence relationship is established or the seemingly mysterious operation manipulation is possible is that, in a case where shaft furnace operation is carried out using a reducing gas containing a high concentration of a hydrogen gas, a reduction reaction, the rate of which is controlled by heat supply, occurs (that is, the temperature level in the furnace is appropriately held by the amount of heat that is taken into the shaft furnace 10, so that a reduction reaction smoothly proceeds) in the shaft furnace 10.

[0072] As described above, according to the first embodiment, since the gas mixture 30 of the reducing gas 31 and the nitrogen gas 32 is blown into the shaft furnace 10, the nitrogen gas 32 can be used as a carrier of sensible heat. This makes it possible to decrease the reducing gas intensity and makes it possible to drop the blow temperature of the gas mixture 30 even in a case where shaft furnace operation is carried out using the reducing gas 31 containing a high concentration of a hydrogen gas as shown in, for example, FIG. 2 and FIG. 3.

[0073] The saturation of the effects by the excessive injection of the nitrogen gas 32 will be described based on FIG. 19. The definitions of the vertical axis and horizontal axis of FIG. 19 are the same as those of FIG. 3. However, the horizontal axis of FIG. 19 shows a larger range of the amount of the nitrogen gas added than in FIG. 3. That is, the horizontal axis of FIG. 19 is an extended version of the horizontal axis of FIG. 3. The graphs drawn in FIG. 19 are the same as the graphs L4 to L6 in FIG. 3. The blow temperatures of these graphs are 800° C., 840° C., 860° C., 880° C., 900° C., 920° C., 940° C., 960° C., 980° C., 1000° C., 1020° C., 1050° C. and 1100° C. from above.

[0074] As shown in FIG. 19 and described in the above paragraph, when the nitrogen gas 32 is excessively added to the reducing gas 31, the effects of the present embodiment are saturated. Conditions under which the effects are saturated vary with the condition of the blow temperature, but it can be said that the effects of the present embodiment can be taken advantage of when a condition under which the volume flow rate (amount added) of the nitrogen gas 32 is set to approximately 90 volume % or less of the volume flow rate of the reducing gas 31 (that is, a condition under which the gas mixture 30 contains the nitrogen gas 32 at a proportion of 90 volume % or less of the reducing gas 31) is satisfied.

[0075] Next, an effect from the viewpoint of the thermal efficiency will be described. FIG. 4 is the same graphs as FIG. 3 but shows the relationship between the amount of the nitrogen gas 32 added (Nm.sup.3/t-Fe) and the hydrogen gas intensity (Nm.sup.3/t-Fe) for a wider variety of blow temperatures. The blow temperatures of these graphs are 800° C., 840° C., 860° C., 880° C., 900° C., 920° C., 940° C., 960° C., 980° C., 1000° C., 1020° C., 1050° C. and 1100° C. from above. Simulation conditions at the time of obtaining FIG. 4 were set to be the same as those for FIG. 2 and FIG. 3. According to FIG. 4, for example, in a case where the blow temperature is 900° C. and the amount of the nitrogen gas 32 added becomes 330 Nm.sup.3/t-Fe, the reduced iron 21 can be produced with the reduction degree of 100% by setting the amount of the reducing gas 31 (here, the hydrogen gas) blown in from the tuyere 10c to 1620 Nm.sup.3/t-Fe.

[0076] The present inventors simulated operation in which part of 1620 Nm.sup.3/t-Fe of the reducing gas 31 is blown into the cooling zone 10b at normal temperature (here, 30° C.) and the reducing gas 31 that is flowing up in the cooling zone 10b is used for the reduction of the iron oxide raw material 20. The simulation was carried out using the above-described mathematical model, and the calculation conditions were set to be the same as in Table 1. The amount of the nitrogen gas 32 added was set to 330 Nm.sup.3/t-Fe, the blow temperature was set to 900° C., and the hydrogen gas concentration of the reducing gas 31 was set to 100 volume %. The results are shown in FIG. 5. The horizontal axis of FIG. 5 indicates the blowing amount (Nm.sup.3/t-Fe) of the reducing gas 31 into the cooling zone 10b, and the vertical axis indicates the temperature (° C.) or reduction degree (%) of the reduced iron 21 that is discharged from the lower portion of the shaft furnace 10. The graph L20 shows the correlation between the blowing amount (Nm.sup.3/t-Fe) of the reducing gas 31 into the cooling zone 10b and the reduction degree (%) of the reduced iron 21, and the graph L21 shows the correlation between the blowing amount (Nm.sup.3/t-Fe) of the reducing gas 31 into the cooling zone 10b and the temperature (° C.) of the reduced iron 21.

[0077] As shown in FIG. 5, the reduced iron 21 is reliably cooled to normal temperature by increasing the blowing amount (Nm.sup.3/t-Fe) of the reducing gas 31 into the cooling zone 10b. However, when the blowing amount becomes excessively large, the reducing gas 31 is not sufficiently heated in the cooling zone 10b, the temperature of the reducing gas 31 in the reduction zone 10a drops, and the reduction degree of the reduced iron 21 may become less than 100%.

[0078] When actual shaft furnace operation is taken into account, the reduction degree of the reduced iron 21 that is discharged from the shaft furnace 10 becomes 100% (or a value close to 100%, for example, 95%) and the temperature of the reduced iron 21 is cooled to approximately normal temperature (for example, normal temperature to approximately normal temperature+30° C.). Therefore, in the example of FIG. 5, the blowing amount of the reducing gas 31 into the cooling zone 10b is preferably approximately 450 to 550 Nm.sup.3/t-Fe (preferable range). In this case, the reduction degree of the reduced iron 21 can be set to 95% or more, and the temperature of the reduced iron 21 can be set to normal temperature (30° C.)+30° C. or lower. A more preferable blowing amount is 500 Nm.sup.3/t-Fe. In this case, the reduction degree of the reduced iron 21 can be set to 100% or more, and the temperature of the reduced iron 21 can be set to normal temperature (30° C.). That is, it is possible to omit a heating treatment of 500 Nm.sup.3/t-Fe of the reducing gas 31, which accounts for approximately 30% of the total blowing amount of the reducing gas 31 of 1620 Nm.sup.3/t-Fe, without affecting the reduction efficiency in the reduction zone 10a.

[0079] FIG. 6 shows the in-furnace state in a case where the above-described simulation has been carried out. The vertical axis of FIG. 6 indicates the depth (m) of the shaft furnace 10 from the upper end, and the horizontal axis indicates the temperature (° C.) of each component and the reduction degree (%) or E.sub.H2 (%) of the reduced iron 21. Here, E.sub.H2 indicates “hydrogen gas utilization ratio”, which means the H.sub.2O/(H.sub.2+H.sub.2O) volume ratio in the gas; however, in the present simulation, since 100 volume % of a hydrogen gas is blown into the shaft furnace 10, E.sub.H2 simply indicates the H.sub.2O concentration. The graph L30 indicates the temperature of the reducing gas 31 at each depth position in the shaft furnace 10. The graph L31 indicates the temperature of the reduced iron 21 (or the iron oxide raw material 20) at each depth position in the shaft furnace 10. The graph L32 indicates the reduction degree of the reduced iron 21 at each depth position in the shaft furnace 10. The graph L33 indicates E.sub.H2 at each depth position in the shaft furnace 10. “Tuyere” indicates the installation position of the tuyere 10c. According to the graph L30, it is found that the reducing gas 31 blown into the cooling zone 10b flows up in the cooling zone 10b and is heated by the reduced iron 21 and, when reaching the tuyere level, the reducing gas 31 is heated up to approximately the same temperature as the temperature of the reducing gas 31 that is blown in from the tuyere 10c (=the temperature of the gas mixture 30). Therefore, it can be said that the iron oxide raw material 20 can be reduced without affecting the reduction efficiency in the reduction zone 10a. In addition, according to the graph L31, it is found that the temperature of the reduced iron 21 (or the iron oxide raw material 20) is approximately almost the same as the temperature of the reducing gas 31. In addition, it is found that the reduction degree of the reduced iron 21 becomes almost 100% on the tuyere level.

[0080] A preferable blowing amount of the reducing gas 31 that is blown into the cooling zone 10b may vary with operation conditions (for example, the blow temperature, the amount of the nitrogen gas 32 added and the like). Therefore, it is necessary to produce the same graph as FIG. 5 for each operation condition and specify the preferable blowing amount.

[0081] Therefore, when the reducing gas 31 at normal temperature is blown into the cooling zone 10b, it is possible to collect the sensible heat of the reduced iron 21, and the thermal efficiency improves. Furthermore, when the blowing amount of the reducing gas 31 into the cooling zone 10b is adjusted such that the reduction degree of the reduced iron 21 becomes 95% or more and the temperature of the reduced iron 21 becomes normal temperature (30° C.)+30° C. or lower, it is possible to improve the thermal efficiency without affecting the reduction efficiency (and the temperature of the reduced iron 21 to be discharged) in the reduction zone 10a.

1-1. First Modification Example

[0082] Next, a first modification example of the first embodiment will be described based on FIG. 7. In actual shaft furnace operation, it is also important to effectively use the reducing gas 31 and the nitrogen gas 32. Therefore, in the first modification example, an unreacted hydrogen gas 31a and the nitrogen gas 32 are separated and collected from the furnace top gas 40 and reused as part of the gas mixture 30.

[0083] Specifically, the furnace top gas 40 is introduced into a separation and collection device 60, and the furnace top gas 40 is cooled in the separation and collection device 60. The furnace top gas 40 is preferably cooled to room temperature. Furthermore, it is preferable to remove dust from the furnace top gas 40. This removes water vapor from the furnace top gas 40 as water 65 and separates and collects the unreacted hydrogen gas 31a and the nitrogen gas 32 as a circulation gas 70. In a case where the reducing gas 31 contains a reducing gas other than a hydrogen gas (CO or the like), the circulation gas 70 may contain not only the reducing gas that is unreacted but also an oxide of the reducing gas (CO.sub.2 or the like), and there is no problem with operation even when the circulation gas 70 contains these gases. As the separation and collection device 60, it is possible to use, for example, a device or the like that separates and collects an unreacted reducing gas from a furnace top gas of a blast furnace. In addition, the circulation gas 70 is reused as part of the gas mixture 30. That is, the circulation gas 70 is, again, introduced into the heating furnace 50 and heated.

[0084] As described above, the nitrogen gas 32 functions as a carrier that carries sensible heat into the shaft furnace 10 and is thus not consumed in the shaft furnace 10. Therefore, the nitrogen gas 32 circulates in a circulation system that couples the heating furnace 50, the shaft furnace 10 and the separation and collection device 60. Therefore, once a necessary amount of the nitrogen gas 32 for the production of a desired amount of reduced iron is introduced into this circulation system, theoretically, there is no need to introduce the nitrogen gas 32 from the outside afterwards. The nitrogen gas 32 may be further supplied from the outside.

[0085] On the other hand, since the reducing gas 31 is consumed in the shaft furnace 10, only the circulated hydrogen gas 31a cannot make the reducing gas 31 sufficient. Therefore, the reducing gas 31 is supplied from the outside for compensating the insufficiency. Theoretically, this makes it possible to produce the reduced iron 21 with the stoichiometrically minimum amount of the reducing gas 31. The reducing gas 31 may be supplied from the outside more than the stoichiometric amount.

[0086] In a case where the hydrogen gas concentration of the reducing gas 31 becomes 100 volume %, according to the formula (1), the stoichiometrically minimum amount of the reducing gas 31 becomes 600 Nm.sup.3/t-Fe. Therefore, for example, in the case of carrying out operation under the operation conditions under which FIG. 5 is obtained, it is preferable to blow, for example, 500 Nm.sup.3/t-Fe of the reducing gas 31 (here, the hydrogen gas) into the cooling zone 10b from the outside and to supply the remaining 100 Nm.sup.3/t-Fe of the reducing gas 31 to the heating furnace 50 from the outside. In this case, the reduction degree of the reduced iron 21 can be set to 100% or more with the stoichiometrically minimum amount of the reducing gas 31, and the temperature of the reduced iron 21 can be set to normal temperature (30° C.).

[0087] In this case, since it is necessary to blow a total of 1620 Nm.sup.3/t-Fe of the reducing gas 31 into the shaft furnace 10 (refer to FIG. 4), 1120 Nm.sup.3/t-Fe of the reducing gas 31 is blown in from the tuyere 10c. Since 100 Nm.sup.3/t-Fe of the reducing gas 31 is supplied from the outside, the remaining 1020 Nm.sup.3/t-Fe of the reducing gas 31 is covered by the reducing gas 31 that is circulated.

[0088] As described above, according to the first modification example, it is possible to effectively use the reducing gas 31 and the nitrogen gas 32.

1-2. Second Modification Example

[0089] Next, a second modification example of the first embodiment will be described based on FIG. 8. In the second modification example as well, similar to the first modification example, the unreacted hydrogen gas 31a and the nitrogen gas 32 are separated and collected from the furnace top gas 40 and reused as part of the gas mixture 30. However, in the second modification example, the reducing gas 31 that is supplied from the outside is fully blown into the cooling zone 10b. That is, the supply line (b) is omitted. The blowing amount of the reducing gas 31 into the cooling zone 10b is, for example, the above-described stoichiometric lower limit value of 600 Nm.sup.3/t-Fe. The reducing gas 31 blown into the cooling zone 10b cools the reduced iron 21 while flowing up in the cooling zone 10b (the reducing gas 31 itself is heated). In addition, the heated reducing gas 31 reduces the iron oxide raw material 20 in the reduction zone 10a.

[0090] As described above, in a system where the furnace top gas 40 is circulated as in the first modification example, theoretically, the stoichiometrically minimum amount of the reducing gas 31 needs to be supplied from the outside. For example, in the case of applying the operation conditions under which FIG. 5 is obtained as they are to the first modification example (FIG. 7), it is necessary to blow, for example, 500 Nm.sup.3/t-Fe of the reducing gas 31 (here, the hydrogen gas) into the cooling zone 10b from the outside and to supply the remaining 100 Nm.sup.3/t-Fe of the reducing gas 31 to the heating furnace 50 from the outside.

[0091] If it is possible to fully blow the reducing gas 31 that is supplied from the outside into the cooling zone 10b at normal temperature, the thermal efficiency further improves, and, furthermore, the supply line (b) can also be omitted, which is extremely advantageous in terms of operation.

[0092] However, as shown in FIG. 5, simply when the reducing gas 31 that is supplied from the outside is fully blown into the cooling zone 10b at normal temperature, there is a concern of a decrease in the reduction degree of the reduced iron 21. In order to fully blow the reducing gas 31 that is supplied from the outside into the cooling zone 10b at normal temperature while maintaining the reduction degree of the reduced iron 21 at 95% or more, it is preferable that, for example, a preferable range of the blowing amount as shown in FIG. 5 (a preferable range of the blowing amount of the reducing gas 31 into the cooling zone 10b, 450 to 550 Nm.sup.3/t-Fe in the example of FIG. 5) can be shifted to the right such that the preferable range includes the stoichiometric lower limit value of 600 Nm.sup.3/t-Fe.

[0093] By the way, the reason for the above-described preferable range being determined is that, when the reducing gas 31 is excessively blown into the cooling zone 10b, the temperature of the reduction zone 10a drops and the reduction degree of the reduced iron 21 decreases. Therefore, if it is possible to compensate for such a temperature drop with separate means, it is possible to increase the blowing amount of the reducing gas 31 into the cooling zone 10b, and, furthermore, it is possible to include the stoichiometric value in the preferable range of the blowing amount. As such means, the present inventors paid attention to an increase in the amount of the nitrogen gas 32 added (in other words, the amount of the nitrogen gas 32 circulating in the circulation system including the shaft furnace 10).

[0094] For example, in a case where the amount of the nitrogen gas 32 added is added under the operation conditions of FIG. 5, the graphs shown in FIG. 9 are obtained. The graph L40 indicates the correlation between the blowing amount (Nm.sup.3/t-Fe) of the reducing gas 31 into the cooling zone 10b and the reduction degree (%) of the reduced iron 21 in a case where the amount of the nitrogen gas added becomes 500 Nm.sup.3/t-Fe. The graph L41 indicates the correlation between the blowing amount (Nm.sup.3/t-Fe) of the reducing gas 31 into the cooling zone 10b and the reduction degree (%) of the reduced iron 21 in a case where the amount of the nitrogen gas added becomes 800 Nm.sup.3/t-Fe. The graph L42 indicates the correlation between the blowing amount (Nm.sup.3/t-Fe) of the reducing gas 31 into the cooling zone 10b and the temperature (° C.) of the reduced iron 21 in a case where the blowing amount of the nitrogen gas becomes 800 Nm.sup.3/t-Fe.

[0095] According to FIG. 9, the reduction degree of the reduced iron 21 with respect to the blowing amount of the reducing gas 31 into the cooling zone 10b can be increased by changing the blowing amount of the nitrogen gas 32 like 330.fwdarw.500.fwdarw.800 Nm.sup.3/t-Fe. That is, when the blowing amount of the nitrogen gas 32 is set to 500 Nm.sup.3/t-Fe, it is possible to set the reduction degree of the reduced iron 21 at the time of having blown 600 Nm.sup.3/t-Fe of the reducing gas 31 into the cooling zone 10b to 95%. Furthermore, when the blowing amount of the nitrogen gas 32 is set to 800 Nm.sup.3/t-Fe, it is possible to set the reduction degree of the reduced iron 21 at the time of having blown 600 Nm.sup.3/t-Fe of the reducing gas 31 into the cooling zone 10b to 100%. As described above, an increase in the amount of the nitrogen gas 32 added makes it possible to include the stoichiometric value in the preferable range of the blowing amount. That is, when the amount of the nitrogen gas 32 added is set to 500 Nm.sup.3/t-Fe, it is possible to set the reduction degree of the reduced iron 21 to 95% or more even when the stoichiometric value of the reducing gas 31 is fully blown into the cooling zone 10b. When the amount of the nitrogen gas 32 added is set to 800 Nm.sup.3/t-Fe, it is possible to set the reduction degree of the reduced iron 21 to 100%. The stoichiometric value or more of the reducing gas 31 may be blown into the cooling zone 10b; however, in this case, it is preferable to further increase the amount of the nitrogen gas 32 added and maintain the reduction degree of the reduced iron 21 at 95% or more and preferably 100%.

[0096] A preferable amount of the nitrogen gas 32 added varies with the operation conditions. Therefore, it is preferable to produce graphs as shown in FIG. 9 for each operation condition and specify the preferable amount of the nitrogen gas 32 added. That is, even when the stoichiometric value (or more) of the reducing gas 31 is blown into the cooling zone 10b, the amount of the nitrogen gas 32 added at which the reduction degree of the reduced iron 21 can be maintained at 95% or more may be adjusted.

[0097] As described above, according to the second modification example, the adjustment of the amount of the nitrogen gas 32 added makes it possible to fully blow the reducing gas 31 from the outside into the cooling zone 10b and, furthermore, makes it possible to set the reduction degree of the reduced iron 21 to 95% or more and preferably 100%.

2. Second Embodiment

[0098] Next, a second embodiment of the present invention will be described. FIG. 10 is an explanatory view showing the process flow of a method for producing reduced iron according to the second embodiment. As shown in FIG. 10, in the second embodiment, schematically, an ammonia gas 33 is blown into the cooling zone 10b instead of the reducing gas 31. The ammonia gas 33 is blown into the cooling zone 10b at normal temperature. That is, in the second embodiment, the ammonia gas 33 is used as a hydrogen gas supply source.

[0099] In more detail, the ammonia gas is industrially produced in large quantities as a chemical fertilizer and also can be easily liquefied and thus can be said as a hydrogen carrier having excellent portability. The fact that ammonia can be used for the reduction of iron oxide is shown in Patent Document 1 and Non Patent Document 2. However, in these documents, the basic phenomenon was simply verified in laboratories, and no specific process images were shown. The present inventors paid attention not to the fact that ammonia is capable of directly reducing iron oxide but to the fact that reduced iron serves as a catalyst and ammonia is decomposed (formula (a)).

[00001]$\begin{array}{cc}\left[\mathrm{Chem}.1\right]& \\ {\mathrm{NH}}_3.fwdarw.\frac{1}{2}{N}_2+\frac{3}{2}{H}_2& \left(a\right)\end{array}$

[0100] In the shaft furnace 10, a countercurrent moving bed is formed (that is, the reducing gas 31 and the nitrogen gas 32 flow up in the shaft furnace 10, and the iron oxide raw material 20 and the reduced iron 21 descends in the shaft furnace 10). Therefore, when the ammonia gas 33 is blown in from the cooling zone 10b, the ammonia gas 33 can be decomposed with the reduced iron 21 serving as a catalyst while flowing up in the shaft furnace 10. In more detail, the ammonia gas 33 blown into the cooling zone 10b cools the reduced iron 21 while flowing up in the cooling zone 10b. Accordingly, the ammonia gas 33 is heated by the sensible heat of the reduced iron 21. After that, the ammonia gas 33 is decomposed into a nitrogen gas and a hydrogen gas with the reduced iron 21 serving as a catalyst. That is, the decomposition reaction of the ammonia gas 33 is an endothermic reaction, but heat necessary for the decomposition reaction is supplied from the reduced iron 21. In addition, the reduced iron 21 itself serves as a catalyst to accelerate the decomposition of the ammonia gas. The nitrogen gas 32 and the hydrogen gas, that is, the gas mixture 30, generated by the decomposition of the ammonia gas 33 flow up while being heated by the sensible heat of the reduced iron 21 and reach the tuyere level. The gas mixture 30 has been heated up to, for example, the blow temperature level when reached the tuyere level. This gas mixture 30, similar to the gas mixture 30 blown in from the tuyere 10c, flows up in the reduction zone 10a and reduces the iron oxide raw material 20 in the reduction zone 10a. That is, the hydrogen gas and the nitrogen gas 32 generated by the decomposition of the ammonia gas 33 while flowing up in the cooling zone 10b are used as part of the gas mixture 30. Therefore, the blowing of the ammonia gas 33 into the cooling zone 10b makes it possible to use the ammonia gas 33 as a hydrogen gas supply source and also makes it possible to improve the thermal efficiency. Furthermore, part of the reducing gas 31 that is supplied from the outside (mainly a hydrogen gas) can be replaced by the ammonia gas 33, which makes it possible to reduce the amount of the reducing gas 31 supplied from the outside.

[0101] Next, an effect of the second embodiment will be studied in detail. The equilibrium constant Kp in the formula (a) is defined by the following formula (b). The hydrogen gas concentration of the reducing gas 31 was set to 100 volume %.

[00002]$\begin{array}{cc}\left[\mathrm{Math}.1\right]& \\ K_p\equiv \frac{p_{N2}^{1/2}.Math.p_{H2}^{3/2}}{p_{\mathrm{NH}3}}=\exp \left[-\frac{\Delta G^0\left(T\right)}{\mathrm{RT}}\right]& \left(b\right)\end{array}$

[0102] Here, P.sub.NH3, P.sub.H2 and P.sub.N2 each indicate the partial pressure of the ammonia gas, the hydrogen gas and the nitrogen gas, T indicates the reaction temperature, and R is the gas constant. In addition, ΔG.sup.0 (T) is the standard free energy of the decomposition reaction of the ammonia gas 33 and can be calculated based on the thermochemical data of the ammonia gas, the hydrogen gas and the nitrogen gas.

[0103] The evaluation results of the decomposition ratio of the ammonia gas and the generation ratio of hydrogen in an equilibrium state with respect to the reaction temperature are FIG. 11. The horizontal axis of FIG. 11 indicates the reaction temperature, and the vertical axis indicates the volume ratio (the volume of each gas/the total volume of all gases) or equilibrium constant Kp of each gas. The graph L50 indicates the volume ratio of the ammonia gas at each reaction temperature, the graph L51 indicates the volume ratio of the hydrogen gas at each reaction temperature and the graph L52 indicates the volume ratio of the nitrogen gas at each reaction temperature. The graph L53 indicates the equilibrium constant Kp at each reaction temperature.

[0104] According to FIG. 11, almost all of the ammonia gas can be decomposed into a hydrogen gas and a nitrogen gas at reaction temperatures of 600° C. or higher. In addition, in the shaft furnace 10, the reduced iron catalyst satisfying this temperature condition is present in the cooling zone 10b, for example, as shown in FIG. 6. Therefore, in a case where the ammonia gas 33 has been blown into the cooling zone 10b, the ammonia gas 33 can be decomposed into a hydrogen gas and the nitrogen gas 32 in the cooling zone 10b.

[0105] Therefore, in consideration of the fact that this decomposition reaction is an endothermic reaction (ΔH=46110 J/mol/K), the amount of an ammonia gas that can be blown into the cooling zone 10b was simulated based on the heat and mass balance in the cooling zone 10b under the prerequisites of (1) the ammonia gas 33 completely decomposes in the cooling zone 10b, (2) heat exchange between the reduced iron 21 and the ammonia gas 33 in the cooling zone 10b proceeds ideally and (3) the in-furnace state of the reduction zone 10a is not affected (that is, the temperature of the gas mixture 30 generated by the decomposition reaction on the outlet side (tuyere level) of the cooling zone 10b approximately coincides with the blow temperature and the reduction degree of the reduced iron 21 becomes 100% on the tuyere level). The simulation was carried out using the above-described mathematical model, and the calculation conditions were set to be the same as in Table 1. The amount of the nitrogen gas 32 added was set to 330 Nm.sup.3/t-Fe, and the hydrogen gas concentration of the reducing gas 31 that was supplied from the outside was set to 100 volume %. The results are shown in FIG. 12.

[0106] The horizontal axis of FIG. 12 indicates the blow temperature of the gas mixture 30 that is blown into the shaft furnace 10 from the tuyere 10c, and the vertical axis indicates the intensity (Nm.sup.3/t-Fe) of the ammonia gas 33 that can be blown into the cooling zone 10b (that is, can be decomposed in the cooling zone 10b), the intensity (Nm.sup.3/t-Fe) of a hydrogen gas that is generated by the decomposition of the ammonia gas 33 or the intensity (Nm.sup.3/t-Fe) of the reducing gas 31 (here, the hydrogen gas) that is supplied from the outside. The graph L60 indicates the correlation between the intensity (Nm.sup.3/t-Fe) of the ammonia gas 33 that can be blown into the cooling zone 10b and the blow temperature, and the graph L61 indicates the correlation between the intensity (Nm.sup.3/t-Fe) of the hydrogen gas that is generated by the decomposition of the ammonia gas 33 and the blow temperature. The graph L62 indicates the correlation between the intensity (Nm.sup.3/t-Fe) of the reducing gas 31 (here, the hydrogen gas) that is supplied from the outside and the blow temperature.

[0107] As shown in FIG. 12, since the amount of the ammonia gas that can be decomposed in the cooling zone 10b is proportional to sensible heat that is taken by the reduced iron 21, as the blow temperature becomes higher, the amount of the decomposable ammonia gas increases (that is, the amount of the ammonia gas that can be blown into the cooling zone 10b increases), and the hydrogen gas that is generated by the decomposition of the ammonia gas 33 increases. Accordingly, the amount of the hydrogen gas that is supplied from the outside decreases. Since approximately 200 Nm.sup.3/t-Fe of a hydrogen gas is generated by the decomposition of the ammonia gas 33, if there is no heat loss from the furnace wall, approximately 30% of the minimum amount (600 Nm.sup.3/t-Fe) of the hydrogen gas necessary for reduction can be covered by the decomposition of the ammonia gas 33.

[0108] The amount of the ammonia gas that can be blown into the cooling zone 10b can vary with each operation condition, but the amount of the ammonia gas is preferably determined such that the reduction degree of the reduced iron 21 becomes 95% or more and preferably 100%.

[0109] According to the second embodiment, the blowing of the ammonia gas 33 into the cooling zone 10b makes it possible to use the ammonia gas 33 as a hydrogen gas supply source and also makes it possible to improve the thermal efficiency. Furthermore, part of the reducing gas 31 that is supplied from the outside (mainly a hydrogen gas) can be replaced by the ammonia gas 33, which makes it possible to reduce the amount of the reducing gas 31 supplied from the outside.

2-1. Modification Example

[0110] Next, a modification example of the second embodiment will be described based on FIG. 13. The present modification example is schematically a combination of the first modification example of the first embodiment and the second embodiment. That is, in the present modification example, the unreacted hydrogen gas 31a and the nitrogen gas 32 are separated and collected from the furnace top gas 40 and reused as part of the gas mixture 30 in the second embodiment.

[0111] However, in the present modification example, since the ammonia gas-derived nitrogen gas 32 flows into the system, it is necessary to steadily exhaust the nitrogen gas as much as the inflow amount to hold the amount of the nitrogen gas that circulates in the system to be constant.

[0112] Therefore, in the present modification example, the circulation gas 70 separated and collected with the separation and collection device 60 is introduced into a branching pipe 80. In the branching pipe 80, part of the circulation gas 70 is supplied to the heating furnace 50 as a fuel gas 85 in the heating furnace 50 (in a state of containing the hydrogen gas). The amount of the nitrogen gas 32 that is contained in the fuel gas 85 is set to be approximately the same as the amount of the nitrogen gas 32 that has flowed into the system due to the decomposition of the ammonia gas 33 (that is, ½ of the volume of the ammonia gas blown into the cooling zone 10b). The amount of the nitrogen gas 32 in the fuel gas 85 can be found by, for example, the measurement of the flow rate (generally, volume flow rate) of the fuel gas and the analysis of the nitrogen gas. The heating furnace 50 generates heat by combusting the fuel gas 85 and heats the gases in the heating furnace 50 with this heat. A combusted exhaust gas 85a (containing water vapor and the nitrogen gas 32) is dissipated to the outside. The rest of the circulation gas 70 is reused as part of the gas mixture 30. That is, the circulation gas 70 is, again, introduced into the heating furnace 50. Other treatments are the same as those in the first modification example of the first embodiment and the second embodiment. This makes it possible to obtain the same effects as those of the first modification example of the first embodiment and the second embodiment while holding the amount of the nitrogen gas that circulates in the system to be constant.

[0113] Furthermore, from the viewpoint of avoiding the inhibition of reduction by the excessive injection of the nitrogen gas 32, which has been mentioned in the above paragraphs, the rate of the amount of the nitrogen gas that circulates in the system with respect to the reducing gas 31 is desirably maintained in the appropriate range shown in FIG. 19.

[0114] In the above-described examples, the reducing gas 31 and the nitrogen gas 32 are heated together, but the present invention is not limited to such examples. For example, the reducing gas 31 and the nitrogen gas 32 may be separately heated, then, mixed and blown into the shaft furnace 10 from the tuyere 10c. In this case, it is preferable to set the heating temperature of the nitrogen gas 32 than the heating temperature of the reducing gas 31. This makes it possible to reduce the heating load of the reducing gas 31.

[0115] Hitherto, the preferred embodiments of the present invention have been described in detail with reference to the accompanying drawings, but the present invention is not limited to such examples. It is evident that a person skilled in the art of the present invention is able to consider a variety of modification examples or correction examples within the scope of the technical concept described in the claims, and such examples are understood to be in the technical scope of the present invention.

BRIEF DESCRIPTION OF THE REFERENCE SYMBOLS

[0116] 10 Shaft furnace [0117] 10a Reduction zone [0118] 10b Cooling zone [0119] 20 Iron oxide raw material [0120] 30 Gas mixture [0121] 31 Reducing gas [0122] 32 Nitrogen gas [0123] 33 Ammonia gas [0124] 40 Furnace top gas [0125] 50 Heating furnace [0126] 60 Separation and collection device [0127] 70 Circulation gas [0128] 80 Branching pipe [0129] 85 Fuel gas