AU2009272126A1

AU2009272126A1 - Method for producing iron ore pellets

Info

Publication number: AU2009272126A1
Application number: AU2009272126A
Authority: AU
Inventors: Nobuhiro Hasegawa; Nobuyuki Iwasaki; Takeshi Maki; Mitsuru Sakamoto
Original assignee: Kobe Steel Ltd
Current assignee: Kobe Steel Ltd
Priority date: 2008-07-16
Filing date: 2009-06-15
Publication date: 2010-01-21
Anticipated expiration: 2029-06-15
Also published as: CN102066587A; BRPI0915703A2; TWI396750B; UA101209C2; WO2010007849A1; JP2010024477A; CN102066587B; BRPI0915703B1; AU2009272126B2; TW201005101A

Description

DESCRIPTION Title of Invention METHOD FOR PRODUCING IRON-ORE PELLETS Technical Field The present invention relates to a technique of producing iron-ore pellets used as, for example, a raw material in a blast furnace, the technique employing a grate kiln system. Background Art Steps for producing iron-ore pellets include a drying step, a dehydration step, a preheating step, a firing step, and a cooling step. Grate-kiln-system apparatuses for producing iron-ore pellets (hereafter, simply referred to as "grate-kiln-system firing apparatuses"), the apparatuses being used for performing these production steps, are known. In grate-kiln-system apparatuses for producing iron-ore pellets, techniques for suppressing, in the rotary kilns, the generation of kiln rings (powdered pellets in the form of rocks adhering to the surfaces of the brick inner walls of kilns), which cause the instability of operation, are known (refer to Patent Literatures 1 and 2). To deal with the growth in demand for steel in recent years, there has been a demand for a further increase in the production of pellets. In addition, with the degradation of -2 iron-ore material in recent years, there has also been a demand for an increase in the proportion of high combined water ore mixed with pellets. However, to meet these demands, when the production rate of pellets is simply increased or the content of combined water in green pellets GP is simply increased while the production rate of pellets is maintained, combined water is not sufficiently decomposed or removed from pellets in the dehydration step. Thus, pellets in which combined water remains are brought into the preheating step performed at a higher temperature than the dehydration step. The temperature of pellets brought into the preheating step is rapidly increased; combined water remaining in the pellets is rapidly decomposed; the vapor pressure in the pellets is rapidly increased; and bursting of the pellets is caused. Powder generated by the bursting degrades permeability of a pellet layer, which hampers uniform heating of the pellet layer. Thus, for example, pressure loss of the pellet layer is increased and the operation becomes unstable. In addition, the strength of the preheated pellets is decreased. As a result, the generated powder is brought into the kiln and the preheated pellets having a low strength produce powder by the rotation in the kiln. Thus, kiln rings are formed and the operation cannot be continued. Accordingly, to date, to avoid such bursting, there has been no other choice but to decrease the -3 production rate of pellets. Patent Literature 1: Japanese Unexamined Patent Application Publication No. 11-325740 Patent Literature 2: Japanese Unexamined Patent Application Publication No. 2005-60762 Disclosure of Invention Problems to be Solved by the Invention Accordingly, an object of the present invention is to provide a method for producing pellets in which the occurrence of bursting in a preheating chamber of a grate furnace can be prevented with certainty in a grate-kiln system apparatus for producing pellets. Means for Solving the Problems The present invention provides a method for producing iron-ore pellets in accordance with a grate kiln system, the method comprising sequentially heating iron-ore pellets in a drying chamber, a dehydration chamber, and a preheating chamber while the iron-ore pellets are being moved with a grate; and subsequently firing the iron-ore pellets with a rotary kiln including a kiln burner, wherein a condition of a current operation is adjusted such that a temperature difference AT = T 2 - T, between an atmosphere temperature T 2 measured with a preheating-chamber entrance thermometer additionally installed in an upper space of the preheating chamber and in a pellet entrance region of the preheating chamber and a gas temperature T, measured with a dehydration-chamber exit grate thermometer installed in a pellet exit region of the dehydration chamber and immediately below the grate, is smaller than an allowable temperature difference ATmax determined in advance on the basis of a past operation performance. In the method for producing iron-ore pellets, the condition of the current operation is preferably adjusted by adjusting at least one of combustion amount of a dehydration-chamber burner installed in an upper portion of the dehydration chamber, combustion amount of a preheating chamber burner installed in an upper portion of the preheating chamber, a rate at which the grate travels, and a thickness of a layer of the pellets. Advantages According to the present invention, by adjusting a condition of a current operation such that a temperature difference AT = T2 - T 1 between a preheating-chamber entrance temperature T 2 and a dehydration-chamber grate temperature T, is smaller than an allowable temperature difference ATmax determined in advance on the basis of a past operation performance, the temperature increase rate of pellets in a preheating-chamber entrance region is suppressed and, as a result, the occurrence of bursting in the preheating chamber can be prevented with certainty.

- 5 As a result, by applying the present invention, an increase in the production of pellets and an increase in the proportion of high combined-water ore mixed can be achieved with certainty. Brief Description of Drawings [Fig. 1] Fig. 1 is a longitudinal sectional view of an example of a grate-kiln-system apparatus for producing iron ore pellets according to an embodiment of the present invention. [Fig. 2] Fig. 2 is a graph illustrating the state of variation in a preheating-chamber wind-box pressure over time. [Fig. 3] Fig. 3 is a graph illustrating the relationship between the temperature increase rate of pellets and the powdered proportion of preheated pellets. [Fig. 4] Fig. 4 is a vertical sectional view illustrating the positional relationship in the height direction between a thermocouple for thermocouple running and a dehydration-chamber exit grate thermometer. [Fig. 5] Fig. 5 is a graph illustrating the relationship between a temperature difference AT between a preheating-chamber entrance temperature T 2 and a dehydration-chamber exit grate temperature T 1 and a preheating-chamber wind-box pressure PpHwB. Reference Numerals -6 1 grate furnace 2 traveling grate (grate) 3 drying chamber 4 dehydration chamber 4a dehydration-chamber ceiling wall 4b dehydration-chamber entrance 4c dehydration-chamber exit 5 preheating chamber 6 wind box group for preheating chamber 7 suction fan for preheating chamber 9 rotary kiln 10 kiln burner 11 annular cooler 16 wind box group for dehydration chamber 17 suction fan for dehydration chamber 21 preheating-chamber burner 31 dehydration-chamber burner 41 preheating-chamber wind-box pressure gage 42 dehydration-chamber exit grate thermometer 43 preheating-chamber entrance thermometer 44 preheating-chamber thermometer A preheating-chamber exhaust gas (heating gas) GP green pellets Best Modes for Carrying Out the Invention Fig. 1 illustrates a grate-kiln-system firing apparatus -7 in which a method for producing iron-ore pellets according to the present invention is performed. As illustrated in Fig. 1, this grate-kiln-system firing apparatus includes a grate furnace 1, a rotary kiln (hereafter, also simply referred to as "kiln") 9, and an annular cooler 11. In the grate furnace 1, while a traveling grate (hereafter, simply referred to as "grate") 2 having an endless configuration sequentially moves green pellets GP placed on the grate 2 through a drying chamber 3, a dehydration chamber 4, and a preheating chamber 5 in the longitudinal direction of these chambers, the green pellets GP are subjected to drying, dehydration, and preheating through a down draft of a heating gas to be turned into pellets (hereafter, referred to as "preheated pellets") having sufficient strength to endure rotation in the kiln 9. The green pellets GP are prepared by mixing iron ore serving as a main material with limestone, dolomite, and the like serving as auxiliary materials, further mixing the mixture with water, and pelletizing the mixture. In the drying chamber 3, the green pellets GP having a water content of about 8 to 9 mass% are dried at an atmosphere temperature of about 2500C. Then, in the dehydration chamber 4, the temperature of the dried green pellets is increased to about 4500C so that combined water in the iron ore is mainly decomposed and removed.

-8 Furthermore, in the preheating chamber 5, the temperature of the pellets is increased to about 11000C so that carbonate contained in limestone, dolomite, and the like is decomposed and CO 2 is removed and magnetite in the iron ore is oxidized. By performing such steps, preheated pellets having sufficient strength to endure rotation in the kiln 9 are prepared. As a result, the productivity of a grate-kiln system firing apparatus can be enhanced. The rotary kiln 9, which is directly connected to the grate furnace 1, is a cylindrical rotary kiln placed so as to be inclined. In the rotary kiln 9, the pellets having been subjected to drying, dehydration, and preheating and introduced into the rotary kiln 9 through the preheating chamber 5 of the grate furnace 1 are fired by combustion with a kiln burner 10 installed on the exit side of the rotary kiln 9. In addition, the rotary kiln 9 is configured to feed the high-temperature combustion exhaust gas from the firing of the pellets into the preheating chamber 5 where the gas serves as a heating gas. To date, fuel such as powdered coal or coke-oven gas has been blown into the rotary kiln 9 and subjected to combustion together with air for combustion with the kiln burner 10. In an upper portion of the preheating chamber 5, preheating-chamber burners 21 serving as kiln-combustion exhaust-gas temperature-increasing means for increasing the -9 temperature of kiln combustion exhaust gas from the rotary kiln 9 are provided. Coke-oven gas (hereafter, abbreviated as "COG") or powdered coal is used as fuel for the preheating-chamber burners 21. Such COG or powdered coal is subjected to combustion in the preheating chamber 5 with remaining oxygen in kiln combustion exhaust gas to increase the temperature of the kiln combustion exhaust gas. As a result, the strength of preheated pellets can be enhanced and the generation of kiln rings (powdered pellets in the form of rocks adhering to the surface of the brick inner wall of a kiln), which cause the instability of operation, in the rotary kiln 9 is suppressed (refer to Patent Literatures 1 and 2). The reference numeral 6 denotes a wind box group for the preheating chamber. The space under the grate 2 is sectioned into a plurality of chambers in the direction in which pellets are moved. These chambers are referred to as wind boxes. That is, the wind box group 6 for the preheating chamber includes a plurality of wind boxes. For example, nine wind boxes are arranged in a line in the longitudinal direction of the preheating chamber 5 (in the direction in which pellets are moved) . The reference numeral 7 denotes a suction fan for the preheating chamber. The suction fan 7 includes a fan damper (omitted in the figure) for adjusting the volume of suction draft (the - 10 volume of down draft). The suction fan 7 is configured to suck kiln exhaust gas serving as a heating gas downward through a pellet layer on the grate 2 and the wind box group 6 and then to feed the kiln exhaust gas to the dehydration chamber 4. The reference numeral 16 denotes a wind box group for the dehydration chamber. For example, five wind boxes are arranged in a line in the longitudinal direction of the dehydration chamber 4 (in the direction in which pellets are moved). The reference numeral 17 denotes a suction fan for the dehydration chamber. The suction fan 17 includes a fan damper (omitted in the figure) for adjusting the volume of suction draft (the volume of down draft). The suction fan 17 is configured to guide preheating-chamber exhaust gas A to the dehydration chamber 4, the exhaust gas A serving as a heating gas; to suck this heating gas A downward through the pellet layer on the grate 2 and the wind box group 16; and then to feed the heating gas A to the drying chamber 3. The technique of controlling the atmosphere temperature of the preheating chamber with the preheating-chamber burners 21 installed is very effective for enhancing the strength of preheated pellets when the production rate of pellets is constant and the combined water content of green pellets GP is also constant. In the dehydration chamber 4, to sufficiently remove - 11 combined water from pellets in the dehydration chamber 4 even in increased production, burners (hereafter, referred to as "dehydration-chamber burners") 31 for increasing the temperature of exhaust gas from the preheating chamber 5 are installed (refer to Japanese Patent Application No. 2008 84178). However, even after the dehydration-chamber burners 31 are installed, the occurrence of bursting in the preheating chamber 5 cannot be completely prevented. Here, the inventors have found that the occurrence of bursting in the preheating chamber S can be detected with variation in the pressure PPHWB (hereafter, referred to as "preheating-chamber wind-box pressure") of the wind box that is positioned in the pellet exit region of the preheating chamber 5 and closest to the kiln 9. Fig. 2 is an example illustrating the state of variation in the preheating chamber wind-box pressure PpIwB over time under certain operation conditions. Although the preheating-chamber wind box pressure PpHis usually varies between -340 to -380 mmAq (gage pressure; hereafter, the same definition. Note: 1 mmAq = 9.80665 Pa), there are cases where the preheating chamber wind-box pressure Pp.wB suddenly sharply drops to a pressure less than -400 mmAq. Such a drop of the preheating-chamber wind-box pressure PprwB to a pressure considerably lower than usual pressures is probably caused because bursting occurs in the pellet layer in the - 12 preheating chamber.5 and permeability of the pellet layer is degraded and pressure loss of the pellet layer sharply increases. Thus, it has been found that the occurrence of bursting in the preheating chamber 4 can be detected by continuously monitoring variation in the preheating-chamber wind-box pressure PPHWB. However, such a detection is achieved after the occurrence. Then, a measure with which the occurrence of bursting can be prevented with certainty has been developed. The inventors considered that the occurrence of bursting in the preheating chamber 5 is most influenced by the temperature increase rate of pellets having been brought from the dehydration chamber 4 into the preheating chamber 5. The inventors have examined this influence through the following laboratory tests. The blended raw material that is used in a pelletizing apparatus installed in the Kakogawa Works of the applicant was used and pelletized with a tier type pelletizer into green pellets having a size of 10 to 12 mm and a water content of about 8.5 mass%. Then, with reference to the temperature pattern of a pellet bottom layer portion determined by thermocouple running (refer to what is described below) in the grate of the pelletizing apparatus, the green pellets were dried with a small drying apparatus - 13 at 105 0 C for 20 minutes into dried pellets having a water content of about 0.2 mass% (equivalent to the drying chamber) and then the dried pellets were further heated with the small drying apparatus at 300 0 C for 5 minutes into dehydrated pellets (equivalent to the dehydration chamber). Then, the dehydrated pellets were charged into a small heating furnace adjusted to have a predetermined atmosphere temperature and were maintained for 2 minutes to be turned into preheated pellets (equivalent to preheating chamber). A temperature transition measured with a thermocouple set immediately above the pellets was subjected to linear approximation to determine the temperature increase rate of the pellets. In addition, the mass proportion of the preheated pellets that had a size of 5 mm or less was determined and defined as the powdered proportion of the preheated pellets. The occurrence of bursting was determined on the basis of the powdered proportion of the preheated pellets. Fig. 3 illustrates the relationship between the temperature increase rate of the pellets and the powdered proportion of the preheated pellets. As illustrated in Fig. 3, it has been found that, when the temperature increase rate of the pellets is a certain value (6 to 7 0 C/s) or less, the powdered proportion of the preheated pellets is always suppressed to less than 0.5 mass%; whereas, when the - 14 temperature increase rate of the pellets exceeds the certain value, the powdered proportion of the preheated pellets sharply increases and bursting starts to occur. Accordingly, from the results of the laboratory tests, it has been confirmed that, by controlling the temperature increase rate of pellets having been brought from the dehydration chamber into the preheating chamber so as to be a certain value or less, the occurrence of bursting in the preheating chamber can be prevented. However, in actual pelletizing apparatuses, it is not easy to directly measure the temperature increase rate of a pellet layer on a grate that is traveling. For example, a technique (hereafter, referred to as "thermocouple running") of placing, on a grate, a wire basket charged with green pellets in which a long thermocouple is inserted into the green-pellet charged layer and measuring the temperature transition of the pellet layer with the traveling of the grate is performed in spots. However, this technique incurs large cost and requires large effort and hence cannot be continuously performed. Then, the inventors have conceived, instead of the direct measurement of the temperature increase rate of a pellet layer, as a parameter corresponding to the temperature increase rate of a pellet layer, use of the temperature difference AT = T 2 - T 1 between a preheating- - 15 chamber entrance temperature T 2 and a dehydration-chamber exit grate temperature T 1 that can be continuously and easily measured. Here, the preheating-chamber entrance temperature T 2 is an atmosphere temperature measured with a preheating-chamber entrance thermometer installed in the pellet entrance region of the preheating chamber 5. The dehydration-chamber exit grate temperature T 1 is a gas temperature measured with a dehydration-chamber exit grate thermometer installed in the pellet exit region of the dehydration chamber and immediately below the grate. The inventors have examined the relationship between the dehydration-chamber exit grate temperature Ti and the temperature of the pellet bottom layer portion measured by thermocouple running (Note that, since a pellet layer is heated by a down draft, the temperature of the pellet bottom layer portion is increased last and bursting tends to occur in the pellet bottom layer portion. Thus, thermocouple running is generally used to measure the temperature of the pellet bottom layer portion.). As illustrated in Fig. 4, a thermocouple 42 serving as a dehydration-chamber exit grate temperature measurement device was set at a position that was as close as possible to the pellet bottom layer portion in the height direction, that is, at a position 200 mm immediately below the grate 2. The thermocouple for thermocouple running was set at a central position in the - 16 pellet bottom layer portion in the height direction, that is, at a position 35 mm immediately above the grate 2. Then, when the thermocouple for thermocouple running reached a position immediately above the dehydration-chamber exit grate thermometer 42, the temperatures measured with the two thermocouples (thermometers) were compared with each other. As described in Table 1 below, the dehydration-chamber exit grate temperature T 1 was a little lower than the temperature of the pellet bottom layer portion measured by thermocouple running, and the temperature difference therebetween was always in the neighborhood of 25 0 C and was substantially constant. Accordingly, it has been confirmed that the temperature of the pellet bottom layer portion can be evaluated on the basis of the dehydration-chamber exit grate temperature T 1 . [Table 1] Year, month, and day Dehydration-chamber Pellet bottom layer TB - T1 of measurement exit grate temperature T, portion temperature (*C) (*C) TB (*C) 2007/04/24 188 210 22 2007/04/25 186 214 28 2007/04/26 191 215 24 Average 188.3 213.0 24.7 Then, the relationship between the temperature difference AT between the preheating-chamber entrance - 17 temperature T 2 and the dehydration-chamber exit grate temperature Ti and the preheating-chamber wind-box pressure PPHWB in a pelletizing apparatus installed in the Kakogawa Works of the applicant has been examined. As a result, the relationship illustrated in Fig. 5 was provided. As illustrated in Fig. 5, although the temperature difference AT generally has a strong correlation with the preheating-chamber wind-box pressure PPHWB (the line in the figure is a regression line), the number of cases (corresponding to the occurrence of bursting) where the preheating-chamber wind-box pressure PPHWB is considerably low relative to the regression line increases as the temperature difference AT increases. Accordingly, it has been found that, by maintaining the temperature difference AT at a predetermined temperature (for example, 850 0 C) or less, the probability of such a considerable decrease in the preheating-chamber wind-box pressure Perma can be sufficiently reduced and there is a possibility of preventing the occurrence of bursting. As illustrated in Fig. 1, to increase the temperature of the preheating-chamber exhaust gas A, the plurality of dehydration-chamber burners 31 are installed in the dehydration chamber 4, for blowing gaseous fuel such as COG into the dehydration chamber 5. Gaseous fuel instead of powdered coal is employed as the fuel for the dehydration- - 18 chamber burners 31. This is because the preheating-chamber exhaust gas A blown into the dehydration chamber 4 has a low temperature of about 400 0 C to 450 0 C and hence combustion of powdered coal does not continue without an ignition source. In contrast, combustion of gaseous fuel spontaneously continues without an ignition source. In addition, in the case of installing the dehydration-chamber burners 31 on a ceiling wall 4a as illustrated in Fig. 1 as an example, when powdered coal burners are used, the burner flames are long and hence pellets in the uppermost surface of the pellet layer are overheated and bursting tends to occur. In view of this, gaseous fuel, which provides short burner flames, is preferably used. In the descriptions below, the "entrance" and the "exit" of the "dehydration-chamber entrance" and the "dehydration-chamber exit" are based on the direction in which pellets are moved. The plurality of burners 31 are preferably installed in the range of from a position corresponding to (1/3)LDH to a position corresponding to 0.

9 8 LDH (LDH: the entire length of the dehydration chamber) with respect to a dehydration-chamber entrance 4b serving as the starting point of LDH. The reason for this is as follows. When the burners 31 are installed at positions corresponding to less than (1/3)LDH with respect to the dehydration-chamber entrance 4b serving as the starting point, the atmosphere - 19 temperature near the dehydration-chamber entrance 4b is increased. Thus, when pellets are not sufficiently dried in the drying chamber 3 and the pellets in which adhesion water remains are brought into the dehydration chamber 4, bursting tends to occur. When the burners 21 are installed at positions corresponding to more than 0

.

9 8 LDH with respect to the dehydration-chamber entrance 4b serving as the starting point (that is, at positions corresponding to less than 0.0 2 LDH with respect to a dehydration-chamber exit 4c serving as the starting point), the burners 21 are too close to a partition wall at the dehydration-chamber exit 4c. Thus, heat of radiation from the burner flames tends to damage the refractory of the partition wall. The plurality of burners 31 are more preferably installed in the range of from a position corresponding to (1/2)LDH to a position corresponding to 0.

9 5 LDH with the dehydration-chamber entrance 4b serving as the starting point and, in particular, preferably, in the range of from a position corresponding to (1/3)LDH to a position corresponding to 0.

9 2 LDH. The thermocouple 42 serving as a dehydration-chamber exit grate thermometer is installed in the pellet exit region of the dehydration chamber 4 (for example, at a central position, in the direction in which the grate travels, in the wind box for the dehydration chamber 4 that is closest to the kiln 9) and immediately below the grate 2.

- 20 In addition, aside from a preheating-chamber thermometer 44, a thermocouple 43 serving as a preheating-chamber entrance thermometer is installed in the pellet entrance region of the preheating chamber 5 (for example, at a central position, in the direction in which the grate travels, in the wind box for the preheating chamber 5 that is closest to the entrance) and in a space above the pellet layer. Here, the reason for installing the thermocouple 42 at the position that is in the pellet exit of the dehydration chamber 4 and immediately below the grate 2 is that, as described above, the temperature that is in closest correlation with the temperature of the pellet bottom layer in the exit region of the dehydration chamber 4 is measured at an accuracy as high as possible. The reason for installing the thermocouple 43 in the pellet entrance region of the preheating chamber 5 and in the space above the pellet layer is that the temperature of the atmosphere gas that heats the pellet layer immediately after being brought into the preheating chamber 5 is measured at an accuracy as high as possible. Here, to sufficiently ensure the measurement accuracy of the temperatures, the "pellet exit region of the dehydration chamber 4" refers to the range of from the exit 4c of the dehydration chamber 4 to a position corresponding to 0.

2 LDH (preferably 0.1LDH), and the "pellet entrance region of the preheating chamber 5" refers to the range of from the - 21 entrance of the preheating chamber 5 to a position corresponding to 0.2Lpy (preferably 0.1LPH; note that LPH represents the entire length of the preheating chamber.). The dehydration-chamber exit grate temperature Ti and the preheating-chamber entrance temperature T 2 are continuously measured with the thermocouples 42 and 43. As described above, the preheating-chamber wind-box pressure Prima is continuously measured with a pressure gage (preheating-chamber wind-box pressure gage) 41 installed in the wind box for the preheating chamber 5 that is closest to the kiln 9. Note that the reason for measuring the pressure in the wind box for the preheating chamber 5 that is closest to the kiln 9 is that variation in the pressure caused by the occurrence of bursting in whichever position of the preheating chamber 5 can be detected. The relationship between the temperature difference AT and the preheating-chamber wind-box pressure Ppims as a result of collection in an operation performed in the past is plotted in advance in a scatter diagram, for example, in Fig. 5. The allowable temperature difference ATma is determined with the diagram. For example, when there is a relationship illustrated in Fig. 5, as described above, 850 0 C at which the number of plots considerably deviated from the regression line is relatively small is determined as the allowable temperature difference ATmax.

- 22 Then, the temperature difference AT (= T 2 - Ti) is calculated from Ti and T 2 that are measured in a current operation. A condition of the current operation is adjusted such that the temperature difference AT is smaller than the allowable temperature difference ATma. determined in advance as described above. As a specific technique of adjusting a condition of the current operation, a technique of adjusting the combustion amount of the dehydration-chamber burners 31, the combustion amount of the preheating-chamber burners 21, the rate at which the grate travels, the thickness of the pellet layer, or the like may be employed. These techniques may be employed alone or in combination. Hereinafter, these techniques will be described. [Combustion amount of dehydration-chamber burners 31] When AT is larger than ATm, the adjustment is performed such that the combustion amount of the dehydration-chamber burners 31 is increased. In this way, Ti is increased and, as a result, AT can be decreased. The combustion amount is adjusted by adjusting the amount of fuel supplied to the dehydration-chamber burners 31. However, when the combustion amount of the dehydration chamber burners 31 is made too large, the temperature of the dehydration chamber is increased and some regions in the - 23 pellet layer reach the temperature at which combined water in ore is decomposed, which may cause bursting. Accordingly, the technique of increasing the combustion amount of the dehydration-chamber burners 31 has a limitation. [Combustion amount of preheating-chamber burners 21] When AT is larger than ATm, the adjustment is performed such that the combustion amount of the preheating chamber burners 21 is decreased. In this way, T 2 is decreased and, as a result, AT can be decreased. The combustion amount is adjusted by adjusting the amount of fuel supplied to the combustion amount of the preheating-chamber burners 21. [Grate traveling rate] When AT is larger than ATmax, the adjustment is performed such that the grate traveling rate is increased. In this way, the time for which the pellet layer is moved from the T 1 measurement position to the T 2 measurement position is decreased and the amount of heat received by the pellet layer during the time is decreased and, as a result, AT can be decreased. [Thickness of pellet layer] When AT is larger than ATm,, the adjustment is performed such that the thickness of the pellet layer is increased. In this way, even when the amount of heat supplied to the pellet layer is unchanged, the time for - 24 which the temperature of the pellet layer increases becomes longer and an increase in the temperature of the pellet layer being moved from the T 1 measurement position to the T 2 measurement position is reduced and, as a result, AT can be decreased. The thickness of the pellet layer is adjusted in the state of green pellets immediately before being brought into the drying chamber 3. The pellet layer has a temperature distribution in the thickness direction. The larger the thickness of the pellet layer is, the larger the temperature distribution in the thickness direction of the layer is. Such an increase in the temperature distribution may cause bursting in some regions. Accordingly, the technique of increasing the thickness of the pellet layer also has a limitation. As described above, by adjusting a condition of the current operation such that the temperature difference AT is smaller than the allowable temperature difference ATmax, the probability of a considerable decrease in the preheating chamber wind-box pressure PPHwB is reduced and the occurrence of bursting in the preheating chamber 5 can be prevented with certainty. As a result, good permeability of the pellet layer is maintained and uniform heating for the pellet layer is ensured and the strength of the preheated pellets is - 25 enhanced. Then, these preheated pellets having a high strength are less likely to produce powder under rotation in the kiln 9 and hence the generation of kiln rings is suppressed. Therefore, the production of pellets having high quality can be achieved with more stability and high productivity.

Claims

1. A method for producing iron-ore pellets in accordance with a grate kiln system, the method comprising sequentially heating iron-ore pellets in a drying chamber, a dehydration chamber, and a preheating chamber while the iron-ore pellets are being moved with a grate; and subsequently firing the iron-ore pellets with a rotary kiln including a kiln burner, wherein a condition of a current operation is adjusted such that a temperature difference AT = T 2 - T 1 between an atmosphere temperature T 2 measured with a preheating-chamber entrance thermometer additionally installed in an upper space of the preheating chamber and in a pellet entrance region of the preheating chamber and a gas temperature T 1 measured with a dehydration-chamber exit grate thermometer installed in a pellet exit region of the dehydration chamber and immediately below the grate, is smaller than an allowable temperature difference ATmax determined in advance on the basis of a past operation performance.

2. The method for producing iron-ore pellets according to Claim 1, wherein the condition of the current operation is adjusted by adjusting at least one of combustion amount of a dehydration-chamber burner installed in an upper portion of the dehydration chamber, combustion amount of a preheating chamber burner installed in an upper portion of the preheating chamber, a rate at which the grate travels, and a - 27 thickness of a layer of the pellets.