Method for producing iron ore pellets

Abstract

Provided is a method for producing iron ore pellets, the method suppressing dew condensation on the surfaces of green pellets in a low-temperature portion of a drying chamber on the most upstream side of a traveling grate furnace or a grate kiln furnace. The method for producing iron ore pellets uses a traveling grate furnace or a grate kiln furnace, and includes preheating iron ore green pellets before charging the green pellets into a drying chamber so that the temperature of the iron ore green pellets does not drop below the dew point of a gas that comes into contact with the green pellets while the green pellets are passed through the drying chamber on the most upstream side of the traveling grate furnace or the grate kiln furnace.

Description

Method for manufacturing iron ore pellets 【0001】 This disclosure relates to a method for producing iron ore pellets using a traveling great furnace or a great kiln furnace. 【0002】 Iron ore pellets are generally produced by granulating finely crushed iron ore and auxiliary materials into spherical pellets of about 10 to 15 mm in diameter by adding water, and then firing these green pellets (hereinafter sometimes simply referred to as green pellets) in a pellet firing furnace. The green pellets become iron ore pellets when heated, after which the water added during granulation, bound water contained in the iron ore, and carbon dioxide contained in the limestone used as an auxiliary material are removed, and the pellets are further compacted at high temperatures. 【0003】 There are mainly two types of pellet firing furnaces: the traveling grate type and the great kiln type (Non-Patent Literature 1). In both types, iron ore pellets are produced by blowing high-temperature gas (for example, gas at around 300°C in the drying process, and gas at around 1200-1300°C in the firing process) vertically from top to bottom onto a horizontally moving layer of raw pellets formed by stacking raw pellets on a pallet called a grate. In the traveling grate type, a process from drying to preheating and pre-firing up to about 1000°C is carried out on the grate to dry the water added during granulation and to decompose and remove bound water and carbon dioxide, and then the main firing is carried out at about 1300°C to produce iron ore pellets. Furthermore, the produced iron ore pellets are cooled to room temperature. In the great kiln system, the main firing is carried out by rolling the raw pellets in a rotating rotary kiln, but the processes from drying to pre-firing are carried out continuously on the great, similar to the traveling great kiln system. 【0004】Liming Lu: Iron Ore - Mineralogy, Processing and Environmental Sustainability, 2nd Edition. Woodhead Publishing, (2021), pp. 539-541. Akira Shigemi: Iron Ore Handbook, Chijin Shokan (1979), p. 362. Carl Yaws: Chemical Properties Handbook, 1st Edition, (1999), McGRAW-HILL, pp. 33, 52, 79, 131, 181. Warren M. Rohsenow et al., Handbook of Heat Transfer, Third Edition, (1998), pp. 2, 4. 【0005】 This disclosure aims to provide a method for producing iron ore pellets in which condensation on the surface of the raw pellets in the low-temperature section of the drying chamber at the uppermost upstream side of a traveling grate furnace or a great kiln furnace is suppressed. 【0006】The contents of this disclosure include the following embodiments: <Embodiment 1> A method for manufacturing iron ore pellets using a traveling grate furnace or a great kiln furnace, comprising preheating the raw iron ore pellets before charging them into the drying chamber such that the temperature of the raw iron ore pellets while passing through the drying chamber on the uppermost side of the traveling grate furnace or the great kiln furnace does not fall below the dew point of the gas in contact with the raw iron ore pellets. <Embodiment 2> The method for manufacturing iron ore pellets according to Embodiment 1, wherein the temperature of the raw iron ore pellets at the time of charging into the drying chamber is 75°C or higher and less than 100°C. <Embodiment 3> The method for manufacturing iron ore pellets according to Embodiment 1, wherein the raw iron ore pellets are preheated such that the temperature of the raw iron ore pellets at the time of charging into the drying chamber is higher than the upper limit of the dew point of the gas in the drying chamber, which is determined from the moisture content of the gas that has passed through the drying chamber vertically. <Aspect 4> The method for manufacturing iron ore pellets according to Aspect 1, wherein the temperature of the raw iron ore pellets while passing through the drying chamber does not fall below the dew point of the gas in contact with the raw iron ore pellets, the temperature conditions for the raw iron ore pellets at the time of charging into the drying chamber are determined based on the temperature of the gas supplied to the drying chamber, the moisture content of the gas supplied to the drying chamber, and the moisture content of the raw iron ore pellets before charging, and the raw iron ore pellets are preheated to a temperature that satisfies the determined temperature conditions before charging into the drying chamber. <Aspect 5> The method for manufacturing iron ore pellets according to any one of Aspects 1 to 4, wherein the preheating of the raw iron ore pellets is performed by at least one selected from the following i) and ii): i) Adding at least one selected from water and steam having a temperature higher than the desired preheating temperature when granulating the raw iron ore pellets ii) Heating the raw iron ore pellets between the time of granulation and charging into the drying chamber. 【0007】 According to this disclosure, it is possible to provide a method for producing iron ore pellets in which condensation on the surface of the raw pellets in the low-temperature section of the drying chamber at the uppermost upstream side of a traveling grate furnace or a great kiln furnace is suppressed. 【0008】This is a schematic diagram of an exemplary traveling grate furnace. This is a schematic diagram of an exemplary great kiln furnace. This is a schematic diagram of the firing test furnace used in the examples and comparative examples. 【0009】 The embodiments of the method for manufacturing iron ore pellets according to this disclosure will be described in detail below. However, the method for manufacturing iron ore pellets according to this disclosure is not limited to the following embodiments. 【0010】 A method for producing iron ore pellets according to one embodiment includes a granulation step of granulating a raw material containing iron oxide to obtain raw pellets, a transport step of transporting the raw pellets to a traveling grate furnace or a great kiln furnace, a drying step of drying the raw pellets in a traveling grate furnace or a great kiln furnace, and a firing step of firing the dried raw pellets in a traveling grate furnace or a great kiln furnace. 【0011】 Generally, in the traveling grate and great kiln drying processes, the green pellets on the grate move horizontally within the drying chamber, while high-temperature gas passes vertically through the chamber. At this time, the high-temperature gas is blown vertically from top to bottom into the green pellet layer. Therefore, a temperature gradient is created in the green pellet layer, from the hottest part at the top to the bottom. One problem in the drying process is that water vapor generated during the drying of the green pellets condenses into liquid on the surface of the green pellets in the lower, colder part of the layer. This liquid swells the green pellets, reducing their strength, which can lead to deformation and breakage. In addition, the increased moisture content in the green pellets makes them more prone to bursting in subsequent processes. Bursting is a phenomenon where water vapor evaporates as the temperature rises, causing the green pellets to burst and collapse due to the resulting water vapor pressure. This is a factor that reduces the yield and quality of iron ore pellets. 【0012】 One possible solution to this problem is to reduce the thickness of the raw pellet layer to relatively reduce the temperature difference within the raw pellet layer and suppress the condensation of water vapor in the low-temperature areas. However, reducing the layer thickness inevitably leads to a decrease in productivity. 【0013】Based on the idea that the water vapor in the uppermost drying chamber of a traveling grate furnace or great kiln furnace, which contains both water vapor originally present in the gas supplied to the uppermost drying chamber and water vapor generated by the evaporation of moisture from the raw pellets, condenses on the surface of the raw pellets when it cools below the dew point, the inventors investigated a method to suppress condensation on the surface of raw pellets in the low-temperature section. 【0014】 One embodiment of the method for manufacturing iron ore pellets is a method for manufacturing iron ore pellets using a traveling grate furnace or a great kiln furnace, which includes preheating the raw iron ore pellets before charging them into the drying chamber so that the temperature of the raw iron ore pellets while passing through the drying chamber on the uppermost side of the traveling grate furnace or the great kiln furnace does not fall below the dew point of the gas in contact with the raw iron ore pellets. 【0015】 Referring to Figure 1, an exemplary configuration of a traveling grate furnace will be described. The traveling grate furnace 100 comprises a first drying chamber 2, a second drying chamber 4, a preheating chamber 6, a first firing chamber 8, a second firing chamber 10, a first cooling chamber 12, and a second cooling chamber 14. Raw pellets are charged onto a grate 16 that passes through the traveling grate furnace 100 to form a raw pellet layer, and are fired by blowing high-temperature gas in a direction perpendicular to the grate 16. In the traveling grate furnace 100, the first drying chamber 2 corresponds to the upstream drying chamber. 【0016】 Referring to Figure 2, an exemplary configuration of a great kiln furnace will be described. The great kiln furnace 200 comprises a drying chamber 18, a water-freezing chamber 20, a pre-firing chamber 22, and a rotary kiln 24. Raw pellets are charged onto a grate 16 that passes through the great kiln furnace 200 to form a raw pellet layer, and high-temperature gas is blown in in a direction perpendicular to the grate 16. In the great kiln furnace 200, the processes from drying to pre-firing are carried out on the grate 16, and the main firing is carried out in the rotary kiln 24. In the great kiln furnace 200, the drying chamber 18 corresponds to the uppermost drying chamber. 【0017】In one embodiment of a method for manufacturing iron ore pellets, the raw iron ore pellets are preheated such that the temperature of the raw iron ore pellets when they are charged into the drying chamber is higher than the upper limit of the dew point of the gas in the drying chamber, which is determined from the moisture content of the gas that has passed through the drying chamber vertically. In this disclosure, the surface temperature of the raw iron ore pellets immediately before they are charged into the upstream drying chamber is considered to be the temperature of the raw iron ore pellets when they are charged into the drying chamber. 【0018】 The dew point is the temperature at which the partial pressure of water vapor equals the saturated vapor pressure of water. Raw iron ore pellets are dried by moving them horizontally through the uppermost drying chamber while passing gas vertically through the chamber. As the gas passing vertically through the drying chamber passes through the raw pellet layer, it picks up water vapor evaporated as the raw pellets dry, so the moisture content of the gas increases from the gas inlet to the gas outlet. Since a higher moisture content results in a higher dew point, the upper limit of the dew point of the gas in the drying chamber can be estimated from the moisture content of the gas that has passed vertically through the drying chamber. Therefore, by preheating the raw pellets so that their temperature is higher than this upper limit of the dew point when they are charged into the drying chamber, the temperature of the raw iron ore pellets as they pass through the uppermost drying chamber can be prevented from falling below the dew point of the gas in contact with them. Note that the temperature of the raw pellets rises as they pass through the drying chamber, so it is sufficient to preheat them so that their temperature is higher than the above upper limit of the dew point when they are charged into the drying chamber. The upper limit of the dew point of the gas in the uppermost drying chamber can be determined, for example, by the following method: Collect the gas that has passed vertically through the drying chamber, and analyze its components using gas chromatography or the like to calculate the moisture content (mol%) of the gas that has passed through the drying chamber. By multiplying the moisture content (mol%) of the gas that has passed through the drying chamber by the total pressure in the uppermost drying chamber, the upper limit of the partial pressure of water vapor in the drying chamber can be calculated. Determine the upper limit of the dew point of the gas in the uppermost drying chamber at the temperature at which the calculated upper limit of the partial pressure of water vapor equals the saturated vapor pressure of water. Alternatively, the value measured by a dew point meter for the collected gas can also be used to determine the upper limit of the dew point of the gas in the uppermost drying chamber. 【0019】The preheating method of green pellets of iron ore is not particularly limited. The preheating of green pellets of iron ore can be carried out, for example, in the pelletizing process of green pellets by raising the temperature of at least one of the raw iron ore, auxiliary raw materials, and moisture, adding heat-generating substances such as quicklime and slaked lime to the raw materials, and pelletizing while heating the raw materials on a pelletizing device. Examples of the pelletizing device include a pan pelletizer and a rolling drum. The heating on the pelletizing device can be carried out by using convection by hot air, radiant heat transfer from a heat source, or conduction heat transfer. By conveying the green pellets heated in the pelletizing process under conditions where heat loss is suppressed and charging them into the drying chamber on the most upstream side, the required temperature can be ensured. The preheating of green pellets of iron ore can also be carried out in the conveying process of the pelletized green pellets. Specifically, the green pellets on the conveyor can be conveyed while being heated by using convection by hot air, radiant heat transfer from a heat source, or conduction heat transfer. The preheating of green pellets of iron ore may also be carried out using microwaves. The preheating of green pellets may be carried out by one preheating method or a combination of multiple preheating methods. 【0020】In one embodiment, preheating of raw iron ore pellets is performed by at least one selected from the following i) and ii): i) Adding at least one selected from water and steam having a temperature higher than the desired preheating temperature when granulating the raw iron ore pellets; ii) Heating the raw iron ore pellets between the time of granulation and charging into the upstream drying chamber. The desired preheating temperature is a temperature that takes into account the temperature decrease between granulation and charging, and is, for example, a temperature higher than the minimum preheating temperature T* described later. This makes it possible to raise the temperature of the raw pellets to a temperature higher than the minimum preheating temperature T* when charging into the drying chamber. Furthermore, it is preferable that the desired preheating temperature is a temperature higher than the minimum preheating temperature T* and is appropriate from the viewpoint of equipment or operation. The water having a temperature higher than the desired preheating temperature may be, for example, water at 40 to 100°C. The above i) may be carried out by using water having a temperature higher than the desired preheating temperature as the moisture content of the raw pellet material, or by blowing heated steam supplied from within or outside the process into the granulation section of the granulation apparatus. 【0021】 The temperature conditions for the raw iron ore pellets at the time of charging into the drying chamber, such that the temperature of the raw iron ore pellets passing through the upstream drying chamber does not fall below the dew point of the gas in contact with the raw iron ore pellets, can be determined, for example, based on the temperature of the gas supplied to the upstream drying chamber, the moisture content (water vapor concentration) of the gas supplied to the upstream drying chamber, and the moisture content of the raw iron ore pellets before charging. By preheating the raw iron ore pellets to a temperature that satisfies the determined temperature conditions before charging into the upstream drying chamber, condensation on the surface of the raw iron ore pellets in the drying chamber can be suppressed. Such temperature conditions can be determined, for example, by the following method. 【0022】Assuming that heat transfer occurs in an adiabatic state between the gas (also referred to as the blowing gas) supplied to the drying chamber on the far upstream side and the green pellets, and that an equilibrium state is reached at the adiabatic equilibrium temperature Te, when the blowing gas and the green pellets exchange heat and the blowing gas temperature and the green pellet temperature reach the adiabatic equilibrium temperature Te, the evaporation amount of the moisture in the green pellets can be estimated by Equation (1) without considering the constraint of the saturated vapor pressure of water. [Heat transfer amount from the blowing gas to the green pellets] = [Latent heat of evaporation of the moisture in the green pellets] + [Increment of sensible heat of the green pellets] In the formula, Tg is the initial temperature (°C) of the blowing gas, Ts is the initial temperature (°C) of the green pellets, Te is the adiabatic equilibrium temperature (°C), M is the initial moisture content (mass%) of the green pellets, Cg(T) is the heat capacity (J / mol·K) of the blowing gas, Cs(T) is the heat capacity (J / kg·K) of the green pellets, ΔH(Te) is the latent heat of evaporation of water (J / kg) at the adiabatic equilibrium temperature Te, and W is the amount (kg) of green pellets dried by 1 mol of the blowing gas. Since heat transfer occurs between the blowing gas and the green pellets, Tg, Ts, and Te satisfy the relationship Tg > Te > Ts. The initial temperature and the initial moisture content of the green pellets are the surface temperature and the moisture content of the green pellets at the time of charging into the drying chamber on the far upstream side. Note that the moisture content does not include crystal water. The initial moisture content M of the green pellets can be determined by the drying loss method of JIS K 0068:2001. The surface temperature of the green pellets can be measured with a radiation thermometer. The evaporation amount (kg) of the moisture in the green pellets dried by 1 mol of the blowing gas is represented by WM / 100. Note that "mol·K" means the product of mol and K, and "kg·K" means the product of kg and K. 【0023】 The virtual water vapor partial pressure Pm in the gas in the drying chamber on the far upstream side when the blowing gas and the green pellets reach the adiabatic equilibrium temperature Te is calculated by Equation (2). The virtual water vapor partial pressure means the water vapor partial pressure when the constraint of the saturated vapor pressure of water is not considered. In the formula, Pm is the virtual water vapor partial pressure (Pa) in the gas in the drying chamber on the far upstream side, P ０ is the total pressure (Pa) in the drying chamber on the far upstream side, m is the initial moisture content (vol%) of the blowing gas, and a is water (H ２O) has a molecular weight of 0.018 kg / mol. W and M are the same as in formula (1). 【0024】 If the hypothetical partial pressure of water vapor Pm exceeds the saturated vapor pressure of water Psv(Te) at Te, then some of the water vapor in the gas in the uppermost drying chamber will condense. Therefore, The temperature of the raw pellets when they are charged into the drying chamber should be set so as to satisfy the following condition. Therefore, if the temperature of the raw pellets when they are charged into the drying chamber such that Pm = Psv(Te) ... (4) is taken as the minimum preheating temperature T*, then the raw pellets should be preheated so that the temperature of the raw pellets when they are charged into the drying chamber, i.e., the initial temperature Ts of the raw pellets, is higher than T*. The minimum preheating temperature T* can be calculated using equations (1), (2), and (4), with Cg(T), Cs(T), ΔH(Te), and the saturated vapor pressure of water as parameters, from the initial moisture content M of the raw pellets, the initial moisture content m of the blown gas, and the initial temperature Tg of the blown gas. 【0025】 Here, the Ts that maximizes Pm, i.e., the Ts that maximizes W, is T*, and in this case, T* coincides with the adiabatic equilibrium temperature Te. Therefore, equation (1) can be simplified to equation (1'). 【0026】 The heat capacity Cs(T) of raw pellets can be calculated by weighting and averaging the heat capacity Cs'(T) of dry pellets and the heat capacity Cw(T) of liquid water as follows: Cs(T) = ((100 - M) / 100)Cs'(T) + (M / 100)Cw(T) In the formula, Cs(T) is the heat capacity of raw pellets (J / kg·K), Cs'(T) is the heat capacity of dry pellets (J / kg·K), Cw(T) is the heat capacity of liquid water (J / kg·K), and M is the initial moisture content of raw pellets (mass%). 【0027】 For the heat capacity Cs'(T) of the dry pellet, for example, the heat capacity of iron ore may be used. For the heat capacity of iron ore, for example, the following formula, which is converted from the formula described in Non-Patent Document 2, can be used: Cs'(T) = 4.186 × (0.1998 + 1.12 × 10) －５ (T+273.15))×10 ３ (J / kg.K) 【0028】 As the specific heat capacity Cw(T) of liquid water, for example, the following equation converted from the equation described in Non-Patent Document 3, p. 79 can be used. Cw(T) = ((92.053 - 3.9953×10 －２ (T + 273.15) - 2.1103×10 －４ (T + 273.15) ２ + 5.3469×10 －７ (T + 273.15) ３ ) / 18) × 1000 (J / kg·K) 【0029】 As the latent heat of vaporization ΔH(T) of water, for example, the following equation converted from the equation described in Non-Patent Document 3, p. 131 can be used. ΔH(T) = (52.053×(1 - (T + 273.15) / 647.13) ０．３２１ / 18) × 10 ６ (J / kg) 【0030】 The specific heat capacity Cg(T) of the blowing gas can be approximated by the following equation using the H ２ O content of the blowing gas and the total content in the blowing gas of CO ２ and SO ２ which are linear triatomic molecules with similar specific heat capacities. Cg(T) = CpCO ２ × n / 100 + CpH ２ O × m / 100 + CpAir(100 - n - m) / 100 (J / mol·K) CpCO ２ is the specific heat capacity of CO ２ (J / mol·K), CpH ２ O is the specific heat capacity of H ２ O (J / mol·K), CpAir is the specific heat capacity of air (J / mol·K), m is the H ２ O content (vol%) of the blowing gas (i.e., the initial moisture content of the blowing gas), and n is the total content (vol%) of CO ２ and SO ２ in the blowing gas. Note that m and n can be determined, for example, by component analysis such as gas chromatography analysis of the blowing gas. m and n can also be calculated stoichiometrically from the blowing gas production conditions. 【0031】 CpCO２ (T), CpH ２ For O(T) and CpAir(T), for example, the following formulas can be used, which are derived by converting the formulas described in Non-Patent Document 3, p. 33 and p. 52, and the formula described in Non-Patent Document 4, p. 2.4, respectively: CpCO ２ (T)=27.437+4.2315×10 －２ (T+273.15)-1.9555×10 －５ (T+273.15) ２ +3.9968 × 10 －９ (T+273.15) ３ -2.9872 × 10 －１３ (T+273.15) ４ (J / mol.K) CpH ２ O(T)=33.933-8.4186×10 －３ (T+273.15)+2.9906×10 －５ (T+273.15) ２ -1.7825 × 10 －８ (T+273.15) ３ +3.6934 × 10 －１２ (T+273.15) ４ (J / mol.K) CpAir(T)=27.434+6.180×10 －３ (T+273.15)-8.987×10 －７ (T+273.15) ２ (J / mol.K) 【0032】 For the saturated vapor pressure Psv(T) of water, the following formula can be used, for example, by converting the formula described in Non-Patent Document 3, p. 181: Psv(T) = (10^(29.8605 - 3.1522 × 10) ３ / (T+273.15)-7.3037log １０ (T+273.15)+2.4247×10 －９ (T+273.15)+1.8090×10 －６ (T+273.15) ２ ) / 760)×101325(Pa) 【0033】The above shows examples of approximate formulas for heat capacity, latent heat of vaporization, and saturated vapor pressure of water, but the method for calculating these parameters is not limited to the above specific example. These parameters may be calculated using computational thermodynamics software such as FactSage, for example. 【0034】 Using the above equations (1), (2), and (4), as well as the approximate formulas for Cg(T), Cs(T), ΔH(T), and the saturated vapor pressure of water shown above, the minimum preheating temperature T* was calculated for typically assumed operating conditions. Specifically, the CO2 gas used for blowing gas was used. ２ The content was set to 0 vol%, 10 vol%, or 30 vol%, and the initial moisture content M of the raw pellet was varied from 0 to 12 mass%, the initial blown gas temperature Tg from 100 to 400°C, and the initial moisture content m of the blown gas from 0 to 20 vol%. The calculation results are shown in Tables 1 to 3. 【0035】 【0036】 【0037】 【0038】 Tables 1-3 show that under typical operating conditions, the minimum preheating temperature T* is less than 75°C. Therefore, preheating the raw iron ore pellets to a temperature of 75°C or higher when charging into the drying chamber can suppress the condensation of water vapor on the surface of the raw pellets. 【0039】 The temperature of the gas supplied to the upstream drying chamber is, for example, 100 to 400°C. The moisture content of the gas supplied to the upstream drying chamber is, for example, 0 to 20 vol%. The moisture content of the raw pellets when charged into the upstream drying chamber is, for example, 6 to 12 mass%, or 9 to 12 mass%. 【0040】In one embodiment, the temperature of the raw iron ore pellets when they are charged into the drying chamber is 75°C or higher and less than 100°C. Since this is preheating for the drying process of the raw pellets, the temperature of the raw pellets when they are charged into the drying chamber is less than 100°C. From the viewpoint of suppressing the decrease in strength of the raw pellets due to the evaporation of moisture, it is preferable that the upper limit of the temperature of the raw pellets when they are charged into the drying chamber is T* + 20°C. The temperature of the raw pellets when they are charged may be, for example, less than 90°C or less than 80°C. 【0041】 The present invention will be described in detail below based on examples and comparative examples, but the present invention is not limited to these examples. 【0042】(Test apparatus) The firing test of raw pellets was carried out in a pot-type pellet firing test furnace that mimicked the traveling grate firing method. Figure 3 shows a schematic diagram of the firing test furnace 300 used in the examples and comparative examples. The firing test furnace 300 comprises a cylindrical furnace body 26 made of refractory material and a grate 28 installed inside the furnace body 26 for stacking the sample pellets 44. Furthermore, the firing test furnace 300 includes a main burner 50 for heating the blown gas during firing, a main burner combustion gas inlet 30 for introducing combustion gas from the main burner 50, a main burner combustion gas inlet valve 36, an exhaust gas blower side gas outlet 34-2 for discharging combustion gas from the main burner 50, a lower gas outlet valve 42, a sub-burner 52 for heating the blown gas during preheating, a sub-burner combustion gas inlet 32 ​​for introducing combustion gas from the sub-burner 52, a sub-burner combustion gas inlet valve 40, an exhaust gas blower side gas outlet 34-1 for discharging combustion gas from the sub-burner 52, and an upper gas outlet valve 38. In the firing test furnace 300, alumina spheres are arranged on the grate 28 as a side layer 46 and a hearth layer 48 to prevent direct contact between the sample pellets 44 and the furnace body 26. The side layer 46 and the hearth layer 48 are arranged on the side and bottom of the furnace body 26, respectively. Sample pellets 44 are loaded inside the side layer 46 and hearth layer 48. In the firing test furnace 300, firing proceeds as follows. During preheating, the main burner 50 is stopped and the sub-burner 52 is ignited. During preheating, the sub-burner combustion gas inlet valve 40 and the upper gas outlet valve 38 are opened, and the main burner combustion gas inlet valve 36 and the lower gas outlet valve 42 are closed. In this state, the combustion gas is introduced from the sub-burner combustion gas inlet 32, flows from the bottom to the top of the sample pellet layer, preheats the sample pellets 44, and is discharged from the exhaust gas blower side gas outlet 34-1. When preheating is complete, the sub-burner 52 is stopped and the main burner 50 is ignited. During firing, the main burner combustion gas inlet valve 36 and the lower gas outlet valve 42 are opened, and the sub-burner combustion gas inlet valve 40 and the upper gas outlet valve 38 are closed. In this state, the combustion gas is introduced from the main burner combustion gas inlet 30, flows from the top to the bottom of the sample pellet layer, scorching the sample pellets 44, and is discharged from the exhaust gas blower side gas outlet 34-2.By performing these operations and adjusting the combustion conditions of the main burner 50 and the sub-burner 52, pellet firing is simulated. 【0043】 (Preparation of raw pellets) The properties of the iron ore used in the examples and comparative examples are shown in Table 4. 【0044】 【0045】 Iron ore is processed using a ball mill, resulting in a specific surface area of ​​1600 cm². ２ The iron ore was crushed to a density of 1 / g. The specific surface area was measured using a Blaine specific surface area analyzer. To the crushed iron ore, 2.5 mass% of water and 0.25 mass% of bentonite as a binder were added and kneaded for 10 minutes. Then, granulation was performed at 18 rpm using a φ1000 mm pan pelletizer. The initial moisture content M of the raw pellets was 10 mass%. Raw pellets with a particle size of 10-15 mm were obtained from the raw pellets produced from the iron ore by sieving. 【0046】 (Examples 1-3, Comparative Examples 1-3) The raw pellets and alumina spheres used as side layers and hearth layers were heated according to the initial raw pellet temperature Ts described in Table 5. Specifically, to prevent evaporation of moisture, the surface of the raw pellets, which were spread thinly on a stainless steel tray, was protected with a vinyl sheet. The tray on which the raw pellets were placed was held in an electric furnace for about 3 hours to preheat the raw pellets. After confirming that the raw pellets had reached the predetermined temperature, the raw pellets were quickly loaded into a pot-type pellet firing test furnace. However, no preheating was performed in Comparative Examples 1 and 3. The layer thickness of the raw pellets was 450 mm. In the examples and comparative examples, the initial raw pellet temperature Ts refers to the surface temperature (°C) of the raw pellets at the start of the test. The raw pellets were dried by holding them for 10 minutes under the blown gas conditions described in Table 5, then fired by holding them at a pre-firing temperature of 900°C for 5 minutes and a firing temperature of 1250°C for 15 minutes, and then cooled with nitrogen at room temperature. The temperature of the blown gas was controlled by burning LPG with air. 【0047】The obtained iron ore pellets were sieved to determine the weight fraction of iron ore pellets with a particle size of 10-15 mm, and this was determined as the final yield. The crushing strength of the iron ore pellets with a particle size of 10-15 mm obtained by sieving was evaluated in accordance with JIS M8718:2017. The results are shown in Table 5. 【0048】 The minimum preheating temperature T* in Table 5 is the value of the adiabatic equilibrium temperature Te calculated using the above-mentioned equations (1'), (2), and (4), as well as the approximate formulas for Cg(T), ΔH(T), and the saturated vapor pressure of water shown above. 【0049】 【0050】 In Comparative Example 1, where the raw pellets were not preheated before charging, the product yield was 62% and the crushing strength was a low 224 daN due to deformation and bursting caused by pellet wetting. In Examples 1 and 2, where the raw pellets were preheated so that the initial raw pellet temperature Ts was higher than the minimum preheating temperature T*, condensation on the surface of the raw pellets in the drying chamber was suppressed, and a significant improvement in product yield and crushing strength was observed compared to Comparative Example 1. However, if the initial raw pellet temperature Ts is higher than T*, there is almost no difference in the effect due to the difference in the initial raw pellet temperature Ts. On the other hand, in Comparative Example 2, where the raw pellets were preheated so that the initial raw pellet temperature Ts was T* or lower, condensation on the surface of the raw pellets in the drying chamber was not suppressed, and the product yield and crushing strength were almost the same as in Comparative Example 1. A comparison between Comparative Example 3, where the initial blowing gas temperature Tg was 200°C, and Example 3 also showed that preheating the raw pellets so that the initial raw pellet temperature Ts is higher than T* improves product yield and crushing strength. 【0051】2. First drying chamber 4. Second drying chamber 6. Preheating chamber 8. First firing chamber 10. Second firing chamber 12. First cooling chamber 14. Second cooling chamber 16. Grate 18. Drying chamber 20. Water removal chamber 22. Pre-firing chamber 24. Rotary kiln 26. Furnace body 28. Grate 30. Main burner combustion gas inlet 32. Sub-burner combustion gas inlet 34-1. Exhaust gas blower side gas outlet 34-2. Exhaust gas blower side gas outlet 36. Main burner combustion gas inlet valve 38. Upper gas outlet valve 40. Sub-burner combustion gas inlet valve 42. Lower gas outlet valve 44. Sample pellet 46. Side layer 48. Hearth layer 50. Main burner 52. Sub-burner 100. Traveling grate furnace 200. Grate kiln furnace 300. Firing test furnace

Claims

1. A method for manufacturing iron ore pellets using a traveling grate furnace or a great kiln furnace, comprising: preheating the raw iron ore pellets before charging them into the drying chamber so that the temperature of the raw iron ore pellets while passing through the drying chamber on the uppermost side of the traveling grate furnace or the great kiln furnace does not fall below the dew point of the gas in contact with the raw iron ore pellets.

2. The method for producing iron ore pellets according to claim 1, wherein the temperature of the raw iron ore pellets when they are charged into the drying chamber is 75°C or higher and less than 100°C.

3. The method for manufacturing iron ore pellets according to claim 1, wherein the raw iron ore pellets are preheated so that the temperature of the raw iron ore pellets when they are charged into the drying chamber is higher than the upper limit of the dew point of the gas in the drying chamber, which is determined from the moisture content of the gas that has passed through the drying chamber vertically.

4. The method for manufacturing iron ore pellets according to claim 1, wherein the temperature of the raw iron ore pellets while passing through the drying chamber does not fall below the dew point of the gas in contact with the raw iron ore pellets, the temperature conditions for the raw iron ore pellets at the time of charging into the drying chamber are determined based on the temperature of the gas supplied to the drying chamber, the moisture content of the gas supplied to the drying chamber, and the moisture content of the raw iron ore pellets before charging, and the raw iron ore pellets are preheated to a temperature that satisfies the determined temperature conditions before charging into the drying chamber.

5. A method for producing iron ore pellets according to any one of claims 1 to 4, wherein the preheating of the raw iron ore pellets is performed by at least one selected from i) and ii) below. i) ii) Adding at least one selected from water and steam having a temperature higher than the desired preheating temperature when granulating the raw iron ore pellets; ii) Heating the raw iron ore pellets between the time of granulation and charging into the drying chamber.

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