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 gas mixture which contains a reducing gas and a nitrogen gas, and has a predetermined temperature, is blown into the shaft furnace. The reducing gas contains 90 volume% or more of a hydrogen gas.

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 is blown into the shaft furnace, the reducing gas containing 90 volume% or more of a hydrogen gas.

2. A method for producing reduced iron that produces reduced iron by reducing iron oxide charged in a shaft furnace, the method comprising: heating a gas mixture which contains a reducing gas and a nitrogen gas, the reducing gas containing 90 volume% or more of a hydrogen gas, and blowing the heated gas mixture into the shaft furnace.

3. The method for producing reduced iron according to claim 2, 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.

4. The method for producing reduced iron according to claim 3, the method comprising: using part of the separated and collected hydrogen gas and nitrogen gas as a fuel gas at a time of heating the gas mixture.

5. A method for producing reduced iron that produces reduced iron by reducing iron oxide charged in a shaft furnace, the method comprising: separately heating a reducing gas and a nitrogen gas, the reducing gas containing 90 volume% or more of a hydrogen gas, mixing the heated reducing gas and the heated nitrogen gas to prepare a gas mixture, and blowing the gas mixture into the shaft furnace.

6. The method for producing reduced iron according to claim 5, wherein a heating temperature of the nitrogen gas is higher than a heating temperature of the reducing gas.

7. The method for producing reduced iron according to claim 5, 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 the reducing gas and the nitrogen gas to be heated.

8. The method for producing reduced iron according to claim 1, wherein, in the gas mixture, a proportion of the nitrogen gas is 90 volume% or less of the reducing gas.

9. The method for producing reduced iron according to claim 1, wherein, at a time of blowing the gas mixture into the shaft furnace, a temperature of the gas mixture is 900° C. or lower.

10. The method for producing reduced iron according to claim 2, wherein, in the gas mixture, a proportion of the nitrogen gas is 90 volume% or less of the reducing gas.

11. The method for producing reduced iron according to claim 2, wherein, at a time of blowing the gas mixture into the shaft furnace, a temperature of the gas mixture is 900° C. or lower.

12. The method for producing reduced iron according to claim 5, wherein, in the gas mixture, a proportion of the nitrogen gas is 90 volume% or less of the reducing gas.

13. The method for producing reduced iron according to claim 5, wherein, at a time of blowing the gas mixture into the shaft furnace, a temperature of the gas mixture is 900° C. or lower.

Description

BRIEF DESCRIPTION OF DRAWINGS

[0029] FIG. 1 is an explanatory view showing the process flow of a method for producing reduced iron according to a first embodiment.

[0030] 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.

[0031] 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.

[0032] 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.

[0033] FIG. 5 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.

[0034] FIG. 6 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.

[0035] FIG. 7 is an explanatory view showing the process flow of a method for producing reduced iron according to a second embodiment.

[0036] FIG. 8 is a graph showing a relationship between the heating temperature (°C) of a hydrogen gas and the heating temperature (°C) of a nitrogen gas.

[0037] FIG. 9 is an explanatory view showing the process flow of a method for producing reduced iron according to a modification example of the second embodiment.

[0038] FIG. 10 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.

[0039] FIG. 11 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.

[0040] FIG. 12 is a graph showing the utilization ratio (%) of the reducing gas for each H.sub.2/Co volume ratio of the reducing gas.

[0041] FIG. 13 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.

[0042] FIG. 14 is an explanatory view showing the process flow of a conventional shaft furnace operation.

EMBODIMENTS FOR IMPLEMENTING THE INVENTION

[0043] 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

[0044] 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 and a nitrogen gas 32 is blown into a shaft furnace 10. The gas mixture 31 contains 90 volume% or more of a hydrogen gas.

[0045] In more detail, 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, and a step of blowing the heated gas mixture 30 into the shaft furnace 10. Steps other than these may be the same as those of the conventional shaft furnace operation.

[0046] For example, as shown in FIG. 1, the reducing gas 31 and the nitrogen gas 32 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.

[0047] 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%. Additionally, as a method for heating the reducing gas, an electric heater is preferably used, and, in a case where the heating is performed by combustion heating, a combustion gas mainly containing hydrogen is preferable.

[0048] 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.

[0049] 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).

[0050] 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.

[0051] 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.

[0052] In the heating furnace 50, the gas mixture 30 is heated up to a predetermined temperature (the temperature of the gas mixture 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.

[0053] The gas mixture 30 is heated up to the predetermined temperature and then blown into the shaft furnace 10. Meanwhile, 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 gas mixture 30 blown into the shaft furnace 10 flows up in the shaft furnace 10. The reducing gas 31 in the gas mixture 30 reduces the iron oxide raw material 20 in the shaft furnace 10, whereby the reduced iron 21 is produced. A reduction reaction by the hydrogen gas is an endothermic reaction, but the reduction reaction heat is supplied not only by sensible heat from the reducing gas 31 but also by sensible heat from the nitrogen gas 32. The reduced iron 21 is discharged from the bottom of the shaft furnace 10 and then cooled. On the other hand, a furnace top gas 40 is exhausted from the 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.

[0054] Next, effect of the first embodiment 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. It should be noted that these simulations correspond to Example and Comparative Example of the first embodiment. 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%.

[0055] 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. 11 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%).

[0056] 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.

[0057] 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.

[0058] 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.

[0059] 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 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 produced per unit time increases.

[0060] 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.

[0061] 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.

[0062] The saturation of the effects by the excessive injection of the nitrogen gas 32 will be described based on FIG. 4. The definitions of the vertical axis and horizontal axis of FIG. 4 are the same as those of FIG. 3. However, the horizontal axis of FIG. 4 shows a larger range of the amount of the nitrogen gas added than in FIG. 3. That is, the horizontal axis of FIG. 4 is an extended version of the horizontal axis of FIG. 3. The graphs drawn in FIG. 4 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.

[0063] As shown in FIG. 4 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.

1-1. First Modification Example

[0064] Next, a first modification example of the first embodiment will be described based on FIG. 5. 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.

[0065] 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. 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.

[0066] 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.

[0067] 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.

[0068] 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

[0069] Next, a second modification example of the first embodiment will be described based on FIG. 6. In the second modification example, as same as 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. In the second modification example, while performing the treatments similar to the first modification example, it is possible to control the amount of the nitrogen gas 32 circulating in the circulation system.

[0070] More specifically, in the second 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. 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, as same as the first modification example, 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. If the nitrogen gas 32 is not introduced from the outside, the nitrogen gas 32 that circulates in a circulation system (the circulation system in which the circulation gas 70 circulates) gradually decrease. On the other hand, by introducing the nitrogen gas 32 from the outside, it is possible to maintain or increase the amount of the nitrogen gas 32 that circulates in the circulation system.

[0071] As explained above, according to the second modification example, it is possible to effectively use the reducing gas 31 and the nitrogen gas 32, and to control the amount of the nitrogen gas 32 which circulates in the circulation system.

2. Second Embodiment

[0072] Next, the process flow of a method for producing reduced iron (shaft furnace operation) according to a second embodiment will be described based on the FIG. 7. In the second embodiment, as similar to the first embodiment, a gas mixture 30 which contains a reducing gas 31 and a nitrogen gas 32, the reducing gas 31 containing 90 volume% or more of a hydrogen gas, and which has a certain temperature, is blown into a shaft furnace 100. However, the method for producing reduced iron according to the second embodiment differs from the first embodiment in the point that the reducing gas 31 and the nitrogen gas 32 are separately heated and thereafter mixed.

[0073] More specifically, the method for producing reduced iron according to the second embodiment includes: a step of separately heating the reducing gas 31 and the nitrogen gas 32, a step of mixing the heated reducing gas 31 and the heated nitrogen gas 32 to prepare a gas mixture 30 having a predetermined temperature, and a step of blowing the gas mixture 30 having a predetermined temperature into the shaft furnace 10. Other steps may be the same as those employed in the conventional shaft furnace operation.

[0074] For example, as shown in FIG. 7, the reducing gas 31 is introduced into a reducing gas heating furnace 51 and then is heated, and the nitrogen gas 32 is introduced into a nitrogen gas heating furnace 52 and then is heated. This makes it possible to separately heat the reducing gas 31 and the nitrogen gas 32. The properties of the reducing gas 31 and the nitrogen gas 32 are the same as those explained in the first embodiment. For example, the reducing gas 31 contains the hydrogen gas by 90 volume%or more (volume% with respect to the total volume of the reducing gas 31). That is, the hydrogen gas concentration of the reducing gas 31 is 90 volume% or more. Within a range of 90 volume% or more, it is preferable that the hydrogen gas concentration of the reducing gas 31 is high, and it is preferable that the hydrogen gas concentration of the reducing gas 31 is 100 volume%. If the hydrogen gas concentration of the reducing gas 31 is within a range of 90 volume% or more and less than 100 volume%, the reducing gas 31 may contain a reducing gas other than the hydrogen gas. An example of this reducing gas is not only CO gas, but also a hydrocarbon gas and the like. 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.

[0075] Then, the heated reducing gas 31 and the heated nitrogen gas 32 are mixed at a mixing part 55 (for example, at a junction of a pipe of the reducing gas 31 and a pipe of the nitrogen gas 32). In other words, the heated nitrogen gas 32 is added to the heated reducing gas 31. This makes it possible to prepare the gas mixture 30 having a predetermined temperature. Then, the gas mixture 30 is blown into the shaft furnace 10. Steps performed thereafter are the same as those of the first embodiment. Therefore, the heating temperature of the reducing gas 31 and the heating temperature of the nitrogen gas 32 are controlled such that the temperature of the gas mixture 30 after the mixing step becomes the predetermined temperature. As described above, 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 second embodiment is possible and may be, for example, approximately 750° C.

[0076] The heating temperature of the reducing gas 31 and the heating temperature of the nitrogen gas 32 are not particularly limited if the temperature of the gas mixture 30 is within a range of the predetermined temperature, but it is preferable that the heating temperature of the nitrogen gas 32 is set higher than the heating temperature of the reducing gas 31. In this case, it is possible to drop the heating temperature of the reducing gas 31, and thus further reduce a heating load applied to the reducing gas 31. As described in the first embodiment, when the amount of the nitrogen gas 32 added is large, the blow temperature of the gas mixture 30 (that is, the predetermined temperature) can be dropped. Therefore, if the amount of the nitrogen gas 32 added is large, a heating load applied to the reducing gas 31 can be further reduced. In addition, as will be described below in detail, if the amount of the nitrogen gas 32 added is large, the dropped amount of the heating temperature of the reducing gas 31 increases with respect to the increase amount of the heating temperature of the nitrogen gas 32. That is, if the amount of the nitrogen gas 32 added is large, it is possible to significantly drop the heating temperature of the reducing gas 31 by only slightly increasing the heating temperature of the nitrogen gas 32. Thus, if the amount of the nitrogen gas 32 added is large, the effect of separate heating becomes significant. It should be noted that, as explained in the first embodiment, it is preferable that the amount of the nitrogen gas 32 added is 90 volume% or less with respect to the total volume of the reducing gas 31.

[0077] Next, effects obtained by the second embodiment will be explained. Present inventors simulated a shaft furnace operation according to the second embodiment using the above-described mathematical model. In addition, for the purpose of comparison, present inventors simulated a shaft furnace operation which is the same as the first embodiment, and a shaft furnace operation in which the nitrogen gas 32 is not added. These simulations correspond to Example and Comparative Examples. FIG. 8 shows the results. The calculation conditions are the same as those shown in Table 1. The hydrogen gas concentration of the reducing gas 31 was 100 volume%.

[0078] FIG. 8 is a graph showing a relationship between the heating temperature of the hydrogen gas (that is, the heating temperature of the reducing gas 31) (°C) and the heating temperature of the nitrogen gas (°C). In FIG. 8, the hydrogen gas intensity (the reducing gas intensity) is a constant value of 1620 Nm.sup.3/t-Fe. The graph L10 represents this relationship in a case that the reducing gas 31 and the nitrogen gas 32 are mixed and then heated (that is, the heating treatment which is the same as the treatment in the first embodiment). As the hydrogen gas intensity is constant, in each point in the graph L10, the amounts of the nitrogen gas 32 added are different respectively (see FIG. 2 and FIG. 3). At the point P1 on the graph L10, the amount of the nitrogen gas added is zero. The graph L11 and the graph L 12 show relationship between the heating temperature of the hydrogen gas when the reducing gas 31 and the nitrogen gas 32 are separately heated (that is, the heating temperature of the reducing gas 31) (°C), and the heating temperature of the nitrogen gas (°C). It should be noted that, in the graph L11, the amount of the nitrogen gas 32 added is 330 Nm.sup.3/t-Fe, and in the graph L12, the amount of the nitrogen gas 32 added is 500 Nm.sup.3/t-Fe.

[0079] If the hydrogen gas intensity is 1620 Nm.sup.3/t-Fe, and the amount of the nitrogen gas 32 added is 330 Nm.sup.3/t-Fe, as shown in FIG. 3, the blow temperature of the gas mixture 30 becomes approximately 900° C. Therefore, in a case that the reducing gas 31 and the nitrogen gas 32 are mixed and then heated, the heating temperature of the reducing gas 31 and the nitrogen gas 32 becomes 900° C., as same as the blow temperature of the gas mixture 30 (point P2).

[0080] If the hydrogen gas intensity is 1620 Nm.sup.3/t-Fe, and the amount of the nitrogen gas 32 added is 500 Nm.sup.3/t-Fe, as shown in FIG. 3, the blow temperature of the gas mixture 30 becomes 900° C. or lower (the calculated value by the mathematical model is 872° C.). Therefore, in a case that the reducing gas 31 and the nitrogen gas 32 are mixed and then heated, the heating temperature of the reducing gas 31 and the nitrogen gas 32 becomes 872° C., as same as the blow temperature of the gas mixture 30 (point P4).

[0081] Therefore, considering a case in which 100 volume% hydrogen gas is blown into the shaft furnace 10 at the blow temperature 1000° C. as a starting point (point P1), by setting the amount of the nitrogen gas 32 added to be 330 Nm.sup.3/t-Fe, without increasing the hydrogen gas intensity, it is possible to drop the blow temperature of the gas mixture 30 to 900° C. (point P2). Further, as shown in the graph L11, by separately heating the reducing gas 31 and the nitrogen gas 32 and increasing the heating temperature of the nitrogen gas 32 to be approximately 1350° C., it is possible to drop the heating temperature of the reducing gas 31 to be approximately 800° C. (point P3).

[0082] Further, by setting the amount of the nitrogen gas 32 added to be 500 Nm.sup.3/t-Fe, without increasing the hydrogen gas intensity, it is possible to drop the blow temperature of the gas mixture 30 to be 872° C., which is not higher than 900° C. (point P4). Further, as shown in the graph L12, by separately heating the reducing gas 31 and the nitrogen gas 32 and increasing the heating temperature of the nitrogen gas 32 to be approximately 1080° C., it is possible to drop the heating temperature of the reducing gas 31 to be approximately 800° C. (point P5).

[0083] Further, comparing the graph L 11 and the graph L12, the slope in the graph L12 is smaller than the slope in the graph L11. This indicates that when the amount of the nitrogen gas 32 added is large, the dropped amount of the heating temperature of the reducing gas 31 increases with respect to the increased amount of the heating temperature of the nitrogen gas 32.

[0084] As explained above, according to the second embodiment, the reducing gas 31 and the nitrogen gas 32 are separately heated, therefore, it is possible to further reduce a heating load applied to the reducing gas 31. This effect obtained by the separate heating becomes strong when the heating temperature of the nitrogen gas 32 is set higher than the heating temperature of the reducing gas 31. Further, this effect obtained by the separate heating becomes strong when the amount of the nitrogen gas 32 added is large.

2-1. Modification Example

[0085] Next, a modification example of the second embodiment will be described based on FIG. 9. Schematically, this modification example applies the first modification example of the first embodiment to the second embodiment. That is, in this modification example, from the furnace top gas 40, the unreacted hydrogen gas 31a and the nitrogen gas 32 are respectively separated and collected and are reused as the reducing gas 31 and the nitrogen gas 32 to be heated.

[0086] 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. 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. Then, the gas separated and collected forms a gas mixture of the unreacted hydrogen gas 31a and the nitrogen gas 32, thus, to this gas mixture, hydrogen gas separating treatment, or nitrogen gas separating treatment is performed. By this treatment, the unreacted hydrogen gas 31a and the nitrogen gas 32 are respectively separated and collected. As the hydrogen gas separating treatment and the nitrogen gas separating treatment, for example, Pressure Swing Adsorption method (PSA method) (https://www.jstage.jst.go.jp/article/jvsj 1958/43/12/43_12_1088/_pdf) may be used. The unreacted hydrogen gas 31a separated and collected is introduced into the reducing gas heating furnace 51, and the nitrogen gas 32 separated and collected is introduced into the nitrogen gas heating furnace 52. This makes it possible to reuse the unreacted hydrogen gas 31a and the nitrogen gas 32.

[0087] As same as the first modification example of the first embodiment, the nitrogen gas 32 is not consumed in the circulation system (in this case, the circulation system that couples the nitrogen gas heating furnace 52, 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. On the other hand, since the reducing gas 31 is consumed in the shaft furnace, 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.

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

[0089] 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.

TABLE-US-00002 Brief Description of the Reference Symbols 10 Shaft furnace 20 Iron oxide raw material 30 Gas mixture 31 Reducing gas 32 Nitrogen gas 40 Furnace top gas 50 Heating furnace 60 Separation and 70 collection device 80 Circulation gas 85 Branching pipe Fuel gas