Manufacturing method for fired iron ore pellets

Abstract

To provide a method with which a fired iron ore pellet can be manufactured without any yield decline by preventing occurrence of bursting while using high-crystal-water iron ore powder containing much crystal water.SOLUTION: A method for manufacturing a fired iron ore pellet includes: setting a heating rate from room temperature to 280°C in firing treatment to 70°C / min or less; and setting a heating rate from exceeding 280°C to 1200°C to 200°C / min or less when a green pellet granulated using an iron ore powder as a precursor powder is made into a fired iron ore pellet by a series of firing treatment including a drying zone, a preheating zone, a firing zone, and a cooling zone.SELECTED DRAWING: None

Description

[Technical field]

[0001] The present invention relates to a method for producing fired iron ore pellets, and more particularly to a method for producing fired iron ore pellets by using iron ore powder containing a large amount of water of crystallization. [Background technology]

[0002] Sintered iron ore pellets (sometimes called iron ore pellets, sintered pellets, or simply pellets) are made by granulating fine iron ore powder of 100 μm or less into green pellets, and then sintering the green pellets to form agglomerates of about 10 mm in size. They are used as a raw material in the blast furnace process and direct reduction process.

[0003] The manufacturing process of pellets includes a granulation process in which raw powder containing iron ore powder is granulated to produce green pellets, and a firing process consisting of a series of processes (firing process) in which the obtained green pellets are dried, fired, and cooled to room temperature. In the granulation process, raw powder whose particle size and moisture content have been adjusted is granulated into spheres with a diameter of about 10 mm using a rolling granulator or the like to produce green pellets. In the firing process, the green pellets are heated to a maximum temperature of about 1300°C and fired to produce pellets.

[0004] In the past, high-quality hematite ore and magnetite ore containing almost no water of crystallization have been used as raw materials for pellets, but there are cases where it is necessary to use relatively poor-quality iron ore containing a lot of water of crystallization and gangue. In particular, the proportion of such ore has been increasing in recent years due to factors such as the depletion of high-quality iron ore.

[0005] However, the water of crystallization contained in iron ore is thermally decomposed during the firing process. This generates steam inside the pellets, increasing the pressure and causing the pellets to burst. When such bursting occurs, the pellets are pulverized, lowering the yield and decreasing productivity. Furthermore, if the pellets are pulverized during the firing process, the generated powder blocks the gaps in the pellet packing layer, inhibiting ventilation and causing adverse effects during production. For this reason, several methods (countermeasures) have been considered for obtaining pellets using iron ore that contains a lot of water of crystallization.

[0006] For example, in Patent Document 1, iron ore powder containing a large amount of crystallization water is granulated, dried, dehydrated, preheated, and then fired to produce fired pellets. The problem is that the crystallization water is lost during the preheating stage, creating gaps between the raw material particles in the pellet state, causing the strength of the preheated pellets to decrease and become powdered. That is, in order to prevent the strength of the preheated pellets from decreasing, a method is disclosed in which the preheating temperature and preheating time are increased according to conditions calculated based on a predetermined relational expression. This increases the bonding strength and number of bonding points between the raw material particles, maintaining the strength of the preheated pellets at a normal level, and stabilizing the quality of the fired pellets and their operation.

[0007] Patent Document 2 discloses a method for producing iron ore pellets by drying raw pellets with a moisture content of about 8 to 9% by mass at an atmospheric temperature of about 250°C, then heating the pellets to about 450°C to decompose and remove the water of crystallization mainly in the iron ore (water release), and then heating the pellets to about 1100°C for preheating, followed by firing in a rotary kiln, in which the pressure fluctuation in a wind box at the pellet outlet of the preheating chamber where preheating is performed is detected to suppress the occurrence of bursting in the preheating chamber. In particular, this method discloses that the occurrence of bursting in the preheating chamber can be prevented by controlling the temperature rise rate of the pellets brought from the water release chamber to the preheating chamber to a certain value or less (6 to 7°C / s).

[0008] Furthermore, Patent Document 3 discloses a method for suppressing bursting by granulating raw material powder containing iron ore powder and an organic binder to obtain green pellets, by adjusting the blending ratio of a high crystal water content ore having a crystal water content of 5 mass% or more depending on the viscosity of the organic binder in an aqueous solution. [Prior art documents] [Patent documents]

[0009] [Patent Document 1] JP 2000-87150 A [Patent Document 2] JP 2010-24477 A [Patent Document 3] JP 2020-180371 A Summary of the Invention [Problem to be solved by the invention]

[0010] In the production of iron ore fired pellets, bursting and powdering caused by iron ore powder containing a large amount of crystal water have been problems, and various countermeasures have been considered, including the above-mentioned method. However, in the past, when using iron ore powder containing a large amount of crystal water, the amount of the powder was limited to a certain extent, and the rest was made of high-quality iron ore powder. For example, in the above-mentioned Patent Document 1, the blending ratio of high crystal water iron ore powder with a crystal water content of 9 mass% is up to 20% in the examples, and in the same Patent Document 3, the blending ratio of high crystal water iron ore powder with a crystal water content of 5.2 mass% is up to 47% in the examples (Patent Document 2 is unknown).

[0011] Due to the depletion of high-quality iron ore and the soaring prices, it is expected that the use of inferior iron ore containing a large amount of water of crystallization will continue to increase in the future. The present invention has been made in consideration of these circumstances, and focuses on the thermal decomposition behavior of high water of crystallization iron ore powder, which contains a large amount of water of crystallization, to provide a method for producing iron ore fired pellets without reducing yield by controlling the rate of temperature rise within a predetermined temperature range in a series of firing processes, thereby suppressing the occurrence of bursting. [Means for solving the problem]

[0012] That is, the gist of the present invention is as follows. (1) A method for producing iron ore sintered pellets, comprising the steps of: granulating raw pellets using iron ore powder as a raw material powder; and subjecting the raw pellets to a series of firing treatments including a drying zone, a preheating zone, a firing zone, and a cooling zone to produce iron ore sintered pellets, The iron ore powder has a crystal water content of 3 to 7 mass %, The method for producing iron ore fired pellets is characterized in that the heating rate from room temperature to 280°C in the firing treatment is 70°C / min or less, and the heating rate from over 280°C to 1200°C is 200°C / min or less. (2) The method for producing iron ore sintered pellets according to (1), wherein the cooling rate from the maximum sintering temperature to 700°C in the sintering treatment is 200°C / min or more. Effect of the Invention

[0013] According to the present invention, when producing iron ore fired pellets using high water of crystallization iron ore powder containing a large amount of water of crystallization, the occurrence of bursting can be suppressed and the iron ore fired pellets can be produced without reducing the yield. In particular, in the present invention, even if the ratio of high water of crystallization iron ore powder in the iron ore powder is increased so that it becomes dominant, bursting can be suppressed, and it is possible to produce iron ore fired pellets without reducing productivity. Therefore, it can be said that this invention is extremely useful in the current situation where high quality iron ore is being depleted and its price is rising. [Brief description of the drawings]

[0014] [Figure 1]FIG. 1 is an explanatory diagram that shows a schematic diagram of a traveling grate type firing furnace. [Diagram 2] FIG. 2 is an explanatory diagram showing a schematic diagram of a grate kiln-type firing furnace. [Diagram 3] FIG. 3 is a graph showing the thermal decomposition behavior of high crystallinity iron ore. [Figure 4] FIG. 4 is an explanatory diagram that shows a schematic diagram of the test firing furnace used in the experimental examples. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0015] The present invention will be described in detail below. In the present invention, iron ore powder is used as a raw material powder and granulated into green pellets, and then subjected to a series of firing processes including a drying zone, a preheater, a firing zone, and a cooling zone to obtain fired iron ore pellets. The crystal water content of the iron ore powder is set to 3 to 7 mass%, and the heating rate from room temperature to 280°C in the firing process is set to 70°C / min or less, and the heating rate from over 280°C to 1200°C is set to 200°C / min or less.

[0016] FIG. 1 shows a schematic diagram of a traveling grate type (sometimes called a straight grate type) calcination furnace, which is one of the typical examples used to obtain iron ore calcined pellets (hereinafter simply referred to as "pellets"). In this calcination furnace, raw pellets 1 are charged into a grate 2 (moving grate) and calcined by passing through each process of drying (water release), preheating, calcination, and cooling in sequence. In detail, the raw pellets 1 obtained by granulation are charged into an endless grate 2 with side plates on a pallet so as to have a uniform layer thickness of about 300 mm, and pass through a drying chamber 3, a preheating chamber 4, a calcination chamber 5, and a cooling chamber 6, which are maintained at a preset temperature, and heat exchange occurs between the pellets and air or combustion gas during each process, and pellets 9 are collected as products (Reference 1: Shinichi Yamaguchi et al., Grate Kiln Pelletizing Process, Kobe Steel Technical Review Vol. 60 No. 1 (Apr. 2010) pp 12-21.).

[0017] FIG. 2 is a schematic diagram of a grate kiln type firing furnace, which is another typical example used to obtain pellets. In this case, too, raw pellets 1 are stacked in layers on a grate 2 (moving grate) to a specified layer thickness, and the raw pellets 1 are dried and preheated by heat exchange at a relatively low temperature while passing through a drying chamber 3 (sometimes the drying chamber and the water-removing chamber are separated) and a preheating chamber 4, which are divided in sequence. Next, the dried and preheated raw pellets 1 are fired by rolling heating in a rotary kiln 7, and then cooled in a circular cooler 8, and the pellets 9 are collected (Reference 1). In addition to the traveling grate type and grate kiln type firing furnaces, pellets may also be produced by a shaft furnace type, but they are common in that they are produced by a continuous series of firing processes including a drying zone, a preheating zone, a firing zone, and a cooling zone.

[0018] In conventional pellet production using these firing furnaces, green pellets are generally heated to about 200°C in the drying zone, then heated to about 1200°C in the preheating zone, and fired at about 1200 to 1350°C in the firing zone. In contrast, in the present invention, the occurrence of bursting is prevented by increasing the upper limit temperature of the drying zone and suppressing (slowing down) the rate of temperature rise. As will be described in detail later, this is based on the following findings.

[0019] That is, FIG. 3 shows the thermal decomposition behavior of high water of crystallization ore from Australia, which is a high water of crystallization iron ore with a water of crystallization content of 4.70 mass%. This high water of crystallization ore gradually loses weight due to thermal decomposition from around 200°C, and large thermal decomposition occurs around 300°C. FIG. 3 also shows the thermal decomposition behavior of the α-FeOOH reagent, and the thermal decomposition of this high water of crystallization ore around 300°C is thought to be mainly due to the thermal decomposition of FeOOH contained in the ore (2FeOOH → Fe2O3 + H2O). In other words, it is speculated that the cause of bursting is that steam is generated inside the pellet due to the rapid thermal decomposition of water of crystallization, which increases the pressure. Therefore, in the present invention, based on the experimental results in the following examples, bursting is suppressed by controlling the heating rate in the thermal decomposition temperature range of water of crystallization.

[0020] First, in the present invention, the heating rate from room temperature to 280°C in the series of firing treatments is 70°C / min or less, preferably 50°C / min or less. Treatment in this temperature range corresponds to the drying zone of green pellets. The main purpose of the conventional heat treatment in the drying zone is to dry the water used in granulating the green pellets (water release), but as shown in Figure 3, high crystal water iron ore thermal decomposition occurs at temperatures between 200°C and 280°C. In the present invention, taking this into consideration, the temperature of the drying zone is set to 280°C and the heating rate is controlled to 70°C / min or less. If the heating rate exceeds 70°C / min, bursting may occur. In addition, if the heating rate is suppressed, the occurrence of bursting can be more reliably suppressed, but in consideration of productivity, the heating rate in this drying zone is preferably 40°C / min or more.

[0021] In the present invention, the heating rate from 280°C to 1200°C in the series of firing processes is set to 200°C / min or less, preferably 150°C / min or less. Most of the crystallization water in high crystallization water iron ore should be decomposed (evaporated) in the drying zone, but some remaining crystallization water is removed by treatment in this temperature range corresponding to the preheating zone. In this case, if the heating rate exceeds 200°C / min, the crystallization water that was not thermally decomposed in the drying zone and remained may be rapidly thermally decomposed, causing bursting in the preheating zone. In addition, as in the case of the drying zone, the occurrence of bursting can be more reliably suppressed by suppressing the heating rate, but considering productivity, it is desirable to set the heating rate in the preheating zone to 100°C / min or more.

[0022] In the present invention, since the crystal water of the high crystal water iron ore is removed in the drying zone and the preheating zone, there are no particular limitations on the temperature of the calcination zone or the calcination time, and, as in the conventional case, for example, the temperature of the calcination zone can be about 1200 to 1350°C and the calcination time can be about 5 to 30 minutes.

[0023] On the other hand, in the cooling zone, it is preferable to increase the cooling rate in order to recover the reduced heating rate in the drying zone and preheating zone. Specifically, it is preferable to set the cooling rate from the maximum firing temperature (maximum temperature in the drying zone) to 700°C in the firing process to 200°C / min or more, more preferably 250°C / min or more, thereby preventing the extension of the overall processing time due to the gradual heating in the drying zone and preheating zone and maintaining productivity. Here, the cooling rate up to 700°C is specified in consideration of the cooling temperature range that can be controlled from the viewpoint of equipment capacity with a cooling device (cooling means) generally adopted in the cooling zone, such as a circular cooler. In other words, in general, when the temperature range is lower than 700°C, the temperature difference between the cooling gas (air) and the pellets becomes small, making it difficult to maintain a high cooling rate. In addition, the faster the cooling rate, the more the productivity can be increased, which is advantageous, but the cooling rate is limited to about 300°C / min due to the capacity of a generally used cooling device.

[0024] When the heating rate and cooling rate are specified as in the present invention, strictly speaking, the temperature must be measured using a thermocouple (the positional relationship between the raw pellets and the thermocouple is constant) installed to follow the movement of the raw pellets, and the temperature must be calculated from the temperature change per unit time. However, in actuality, such control is difficult when producing pellets in a traveling grate type or grate kiln type firing furnace as described above. Therefore, for example, fixed thermocouples may be installed directly above or below the pellet layer stacked on the pallet (pallet cart) on the grate, and the temperature may be calculated based on the results of the temperature measurement. Specifically, the temperature can be calculated from the following formula.

number

[0025] In the present invention, by adopting the above-mentioned series of firing treatments, the crystal water content of the iron ore powder mixed as the raw material powder can be made higher than ever before, and the crystal water content of the iron ore powder is 3 to 7 mass%, preferably 4.5 to 7 mass%. In general, the crystal water content of good quality iron ore is at most about 0.1 to 1 mass%, depending on the type of ore. Therefore, when the crystal water content of the iron ore powder is 3 mass% or more, it means that inferior high crystal water iron ore powder containing a lot of crystal water is mixed in a dominant amount as the iron ore powder. On the other hand, when the crystal water content exceeds 7 mass%, it becomes difficult to suppress the occurrence of bursting, and even if it can be avoided, there is a risk that cracks will occur in the pellets after firing, resulting in a decrease in strength. The crystal water content in the iron ore can be measured by JIS M8211:1995 "Iron ore-compound water determination method". The presence or absence of bursting can be evaluated by visually observing the pellets after firing as in the examples described later, but it may also be evaluated from the amount of powder generated by sieving the pellets after firing through a sieve of a specified mesh size.

[0026] Here, in order for the iron ore powder to have a crystal water content of 3 to 7% by mass, the iron ore powder may be made of a single ore type, or may be made of two or more ore types. That is, the crystal water content of the entire iron ore powder should be within this range. For example, a high crystal water iron ore powder with a crystal water content of more than 7% by mass can be used, and in that case, the remaining iron ore powder should be blended with a crystal water content lower than this to make the crystal water content of the entire iron ore powder (weighted average) within the above range. In this case, in addition to the high crystal water iron ore powder, for example, hematite ore ore ore, which is a high-quality iron ore, can be mixed and used as the iron ore powder.

[0027] In the present invention, the raw material powder may include, in addition to iron ore powder, auxiliary materials such as limestone and dolomite, binders such as bentonite, etc. It is preferable to use raw material powders including these, which are crushed by a ball mill or the like so that 90% or more of each is 100 μm or less. The crushed raw material powder is adjusted to a moisture content suitable for granulation, and can be formed into spherical shapes (green pellets) with a diameter of about 10 mm or so by a granulator such as a pan pelletizer or a drum pelletizer, and can be similar to known methods except for the above-mentioned firing treatment and the content of water of crystallization of the iron ore powder. EXAMPLES

[0028] The present invention will be described below based on examples, but the present invention is not limited to these examples.

[0029] (Experimental Example) The components of the iron ore powder used in this experiment are shown in Table 1. Ore A is a high-grade hematite ore from Brazil, and ores B and C are high-water-of-crystallization ores from Australia. Here, CW in Table 1 indicates the water of crystallization content, which was measured according to JIS M8211:1995 "Iron ore - Method for determining water of crystallization."

[0030] [Table 1]

[0031] Table 2 shows the blending conditions of the raw material powder in this experimental example (raw material blend No.). In all cases, bentonite was used as a binder, and 0.5 mass% was added in all blending conditions. Each of these blended raw materials was pulverized to 100 μm or less in a ball mill, and 8 mass% (excl. number) of water was added to each blended raw material, which was then granulated in a pan pelletizer to produce raw pellets with a particle size of 12.5 to 15 mm. Table 2 shows the weighted average value of the crystal water content (CW) of the entire iron ore powder in each blended raw material.

[0032] [Table 2]

[0033] The green pellets prepared above were sintered in a test sintering furnace capable of controlling the heating rate, and the presence or absence of bursting was evaluated. Figure 4 shows a schematic diagram of the test sintering furnace used in this experimental example. In this test sintering furnace, a sample cage 13 is installed in an alumina tube 11 with an inner diameter of 42 mmφ, green pellets 14 are placed in the sample cage 13, and the alumina tube 11 is heated to 1360°C by a heater 12. The temperature of the green pellets 14 placed in the sample cage 13 can be measured by a thermocouple 15 installed directly above the sample cage 13, and the sample cage 13 can be moved up and down within the alumina tube 11, so that the temperature of the green pellets during sintering can be controlled.

[0034] Using the raw pellets (No. 1 to 5) obtained from each raw material blend and the above-mentioned test calciner, calcination treatment was performed in the drying zone, pre-heat zone, calcination zone, and cooling zone shown in Table 3 to produce calcined iron ore pellets. For example, in Comparative Example 1, raw pellets of blended raw material No. 1 were placed in a sample cage 13 of a test firing furnace, and the sample cage was lowered so as to approach the heater 12 in the furnace, and the raw pellets were heated from room temperature to 200°C at a heating rate of 50°C / min (drying zone), and then the lowering speed of the sample cage 13 was changed to heat the raw pellets from 200°C to 1300°C at a heating rate of 200°C / min (preheating zone to firing zone). Then, when the temperature of the raw pellets 14 in the sample cage 13 reached 1300°C, it was held there for 5 minutes (firing zone).

[0035] Next, the sample cage was raised away from the heater 12 in the furnace, and the fired pellets were cooled from 1300°C to 700°C at a cooling rate of 150°C / min, at which point the pellets were taken out of the sample cage 13 and naturally cooled to room temperature in the air to obtain fired iron ore pellets according to Comparative Example 1. In Table 3, the time required from when the raw pellets were placed in the sample cage and heating in the drying zone was started to when the pellets were cooled to 700°C in the cooling zone is recorded as the total firing time.

[0036] [Table 3]

[0037] The iron ore fired pellets according to Examples 1 to 5 and Comparative Examples 1 to 6 obtained as described above were visually inspected for the presence or absence of bursting, and were evaluated based on whether or not cracks occurred. The results are shown in Table 3.

[0038] First, in Comparative Example 1, iron ore fired pellets were obtained by a typical conventional firing process using only ore A having a low content of water of crystallization, and no bursting occurred. In Comparative Example 2, the firing treatment was the same as in Comparative Example 1, but Ore B, which had a high crystallization water content, was added, so that the crystallization water content of the iron ore powder as a whole was 3.0 mass %, and bursting occurred. In Comparative Example 3, the temperature reached in the drying zone was increased to 240° C. compared to Comparative Example 2, but bursting still occurred. In Example 1 of the present invention, the temperature reached in the drying zone was 280° C., which was higher than that in Comparative Example 3, and in this case no bursting occurred. In Example 2, the temperature rise rate in the drying zone was increased to 70° C. / min, which was faster than that in Example 1, and in this case too, no bursting occurred. In Comparative Example 4, the temperature rise rate in the drying zone was increased to 100° C. / min, which was higher than that in Inventive Example 2, but bursting still occurred. In Examples 3 and 4, Ore B and Ore C were used to increase the crystal water content of the iron ore powder as a whole to 4.7% by mass and 7.0% by mass, respectively, which is an even higher crystal water content than in Example 2, but bursting did not occur. In Comparative Example 5, the crystallization water content was 8.5 mass %, which was even higher than in Invention Example 4, using only Ore C, which had the highest crystallization water content, and bursting occurred. In Comparative Example 6, the same raw material blend as in Invention Example 2 was used, but when the heating rate in the preheating zone was increased to 250° C. / min, bursting occurred. In Example 5, the cooling rate was 200° C. / min, which was faster than that in Example 2, but no bursting occurred.

[0039] From the results of these experimental examples, by setting the heating rate from room temperature to 280°C in the firing process to 70°C / min or less, and setting the heating rate from over 280°C to 1200°C to 200°C / min or less, it is possible to produce fired iron ore pellets using iron ore powder with a crystal water content of 3 to 7 mass% without reducing yield while suppressing the occurrence of bursting. Moreover, since there is no particular decrease in productivity compared to the conventional method, it becomes possible to produce fired iron ore pellets using inferior iron ore, which is expected to increase in the future. [Explanation of symbols]

[0040] 1: raw pellets, 2: grate, 3: drying chamber, 4: preheating chamber, 5: calcination chamber, 6: cooling chamber, 7: rotary kiln, 8: annular cooler, 9: iron ore calcined pellets, 11: alumina tube, 12: heater, 13: sample cage, 14: raw pellets, 15: thermocouple.

Claims

1. A method for producing iron ore fired pellets, comprising the steps of: granulating raw pellets using iron ore powder as a raw material powder; and subjecting the raw pellets to a series of firing processes including a drying zone, a preheating zone, a firing zone, and a cooling zone to produce fired iron ore pellets; The iron ore powder has a crystal water content of 3 to 7 mass %, The method for producing iron ore fired pellets is characterized in that the heating rate from room temperature to 280°C in the firing treatment is 70°C / min or less, and the heating rate from over 280°C to 1200°C is 200°C / min or less.

2. A method for producing iron ore sintered pellets as described in claim 1, wherein the cooling rate from the maximum sintering temperature to 700°C in the sintering treatment is 200°C / min or more.

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