JP2023033734A - Method for manufacturing fired iron ore pellet - Google Patents

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 The present invention relates to a method for producing calcined iron ore pellets, and more particularly to a method for producing calcined iron ore pellets using iron ore powder containing a large amount of water of crystallization.

Iron ore calcined pellets (sometimes referred to as iron ore pellets, calcined pellets, or simply pellets) are produced by granulating fine iron ore powder of 100 μm or less into raw pellets, and calcining the raw pellets. It is a sintered product that has been agglomerated to a size of about ten and several millimeters, and is used as a raw material for blast furnace processes and direct reduction processes.

The production of pellets consists of a granulation process in which raw material powder containing iron ore powder is granulated into raw pellets, and a series of processes (burning treatment) in which the obtained raw pellets are dried, calcined, and cooled to room temperature. and a baking step consisting of. Among these steps, in the granulation step, the raw material powder whose particle size and water content are adjusted is granulated into spheres having a diameter of about ten and several millimeters by a tumbling granulator or the like to produce raw pellets. In the firing process, pellets are manufactured by heating the raw pellets to a maximum temperature of about 1300° C. and firing them.

Until now, high-quality hematite ore and magnetite ore, which contain almost no water of crystallization, have been mainly used as raw materials for pellets. Sometimes. In particular, in recent years, the proportion has been increasing due to factors such as depletion of good quality iron ore.

However, water of crystallization contained in the iron ore is thermally decomposed during the firing process. As a result, water vapor is generated inside the pellets, increasing the pressure, and there is a risk of bursting (explosion) of the pellets. When such bursting occurs, the pellets are pulverized and the yield is lowered, resulting in a decrease in productivity. In addition, if the pellets are pulverized during the firing process, the generated powder will clog the gaps in the pellet packed layer and impede air permeability, which will adversely affect the manufacturing process. Therefore, several methods (countermeasures) have been studied so far to obtain pellets using iron ore containing a large amount of water of crystallization.

For example, in Patent Document 1, iron ore powder containing a large amount of water of crystallization is granulated, dried, syneresis, and preheated, and then fired to produce fired pellets. This solution solves the problem that voids are generated between raw material particles in a pellet state due to the coming off, and the strength of the preheated pellets is reduced, resulting in pulverization. That is, for the purpose of preventing a decrease in the strength of preheated pellets, a method is disclosed in which preheating is performed by increasing the preheating temperature and preheating time according to conditions calculated based on a predetermined relational expression. As a result, the bonding strength and the number of bonding points between the raw material particles are increased, the strength of the preheated pellets can be maintained normally, and the quality of the fired pellets and its operation can be stabilized.

Further, in Patent Document 2, raw pellets having a moisture content of about 8 to 9% by mass are dried at an ambient temperature of about 250° C., and then heated to about 450° C. to remove mainly water of crystallization in iron ore. It is decomposed and removed (separation of water), further heated to about 1100 ° C and preheated, and then fired in a rotary kiln to produce iron ore pellets. A method of detecting pressure fluctuations and suppressing the occurrence of bursting in the preheating chamber is disclosed. In particular, in this method, bursting occurs in the preheating chamber by controlling the temperature rise rate of the pellets brought from the water separating chamber to the preheating chamber to a certain value or less (6 to 7°C/s). can be prevented.

Furthermore, in Patent Document 3, in obtaining raw pellets by granulating raw material powder containing iron ore powder and an organic binder, the content of water of crystallization is 5% by mass or more depending on the viscosity of the aqueous solution of the organic binder. A method for suppressing bursting is disclosed by adjusting the blending ratio of ore containing high crystal water content.

JP-A-2000-87150 JP 2010-24477 A JP 2020-180371 A

In the production of iron ore calcined pellets, bursting and pulverization due to iron ore powder containing a large amount of water of crystallization have been problems so far, and various countermeasures including the above-mentioned method are being studied. Conventionally, however, when iron ore powder containing a large amount of water of crystallization is used, the amount thereof is limited to some extent, and the rest is made up of good quality iron ore powder. For example, in Patent Document 1, the mixing ratio of the high-crystal-water iron ore powder having a water-of-crystallization content of 9% by mass is 20% at the maximum in the examples. The mixing ratio of 2% by mass of high crystalline water iron ore powder is 47% at maximum in the examples (Patent Document 2 is unknown).

Due to the depletion of good quality iron ore and soaring prices, it is expected that the use of low quality iron ore containing a large amount of water of crystallization will increase more and more in the future.
The present invention has been made in view of such circumstances, focusing on the thermal decomposition behavior of high crystalline water iron ore powder containing a large amount of crystallization water, and the temperature rise in a predetermined temperature range in a series of firing treatments To provide a method capable of suppressing the occurrence of bursting by controlling the speed and producing iron ore sintered pellets without lowering the yield.

That is, the gist of the present invention is as follows.
(1) Manufacture of iron ore calcined pellets, in which raw pellets granulated using iron ore powder as raw material powder are subjected to a series of calcination processes including a drying zone, a preheating zone, a calcination zone, and a cooling zone to produce iron ore calcined pellets. a method,
The iron ore powder has a crystal water content of 3 to 7% by mass,
An iron ore characterized in that the rate of temperature increase from room temperature to 280° C. in the firing treatment is 70° C./min or less and the rate of temperature increase from over 280° C. to 1200° C. is 200° C./min or less. A method for producing calcined pellets.
(2) The method for producing iron ore sintered pellets according to (1), wherein the cooling rate from the highest sintering temperature to 700° C. in the sintering treatment is 200° C./min or more.

According to the present invention, when producing iron ore calcined pellets using high crystalline water iron ore powder containing a large amount of crystallization water, the occurrence of bursting is suppressed to produce iron ore calcined pellets without lowering the yield. can do. In particular, in the present invention, bursting can be suppressed even if the proportion of high-crystalline water iron ore powder in iron ore powder is increased so that it becomes dominant, and iron ore calcined pellets can be produced without reducing productivity. It is possible to Therefore, it can be said that it is an extremely useful invention in the current situation where good quality iron ore is depleted and its price is soaring.

FIG. 1 is an explanatory view schematically showing a traveling-grate-type firing furnace. FIG. 2 is an explanatory view schematically showing a grate kiln type firing furnace. FIG. 3 is a graph showing the thermal decomposition behavior of high crystalline water iron ore. FIG. 4 is an explanatory view schematically showing a test firing furnace used in Experimental Examples.

The present invention will be described in detail below.
In the present invention, iron ore powder is used as a raw material powder to granulate raw pellets, and a series of firing processes including a drying zone, a preheating body, a firing zone, and a cooling zone are performed to obtain iron ore fired pellets. The crystal water content of the stone powder is set to 3 to 7% by mass, the temperature increase rate from room temperature to 280°C in the firing treatment is set to 70°C/min or less, and the temperature increase rate from over 280°C to 1200°C is set. The temperature should be 200°C/min or less.

Here, FIG. 1 shows a traveling grate type (sometimes referred to as a straight grate type), which is one of the typical examples used when obtaining iron ore calcined pellets (hereinafter simply referred to as “pellets”). A firing furnace is shown schematically. In this firing furnace, raw pellets 1 are charged into a grate 2 (moving grid) and fired by successively passing through drying (separation), preheating, firing, and cooling processes. Specifically, the raw pellets 1 obtained by granulation were charged into an endless grate 2 having a pallet with a side plate so as to have a uniform layer thickness of about 300 mm, and held at a preset temperature. It passes through a drying chamber 3, a preheating chamber 4, a baking chamber 5, and a cooling chamber 6, and in each process, heat is exchanged between air and combustion gas, and pellets 9 are recovered as a product (reference 1: Shinichi Yamaguchi et al., Great Kiln Pelletizing Process, Kobe Steel Technical Report Vol.60 No.1 (Apr.2010) pp12-21.).

FIG. 2 schematically shows a grate kiln-type firing furnace, which is another typical example used for obtaining pellets. In this case also, the raw pellets 1 are layered to a predetermined layer thickness on the grate 2 (moving grid), and are sequentially placed in the drying chamber 3 (the drying chamber and the separation chamber may be separated) and the preheating chamber 4. The raw pellets 1 are dried and preheated by heat exchange at a relatively low temperature while passing through the divided space. Next, the dried and preheated raw pellets 1 are calcined by tumbling heating in a rotary kiln 7 and then cooled in an annular cooler 8 to recover pellets 9 (reference document 1). In addition to the traveling grate type and grate kiln type firing furnaces, the shaft furnace method may also be used to manufacture pellets. The point of producing pellets is common.

In conventional pellet production using these firing furnaces, raw pellets are generally heated to about 200°C in the drying zone, then heated to about 1200°C in the preheating zone, and about 1200 to 1350°C in the firing zone. fired. On the other hand, in the present invention, the occurrence of bursting is prevented by increasing the upper limit temperature of the drying zone and suppressing (slowing) the rate of temperature increase. Details will be described later, but this is based on the following findings.

That is, FIG. 3 shows the thermal decomposition behavior of Australian high crystal water ore, which is high crystal water iron ore having a crystal water content of 4.70% by mass. This high crystalline water ore shows a gradual weight loss due to thermal decomposition from around 200°C, and large thermal decomposition occurs at around 300°C. FIG. 3 also shows the thermal decomposition behavior of the α-FeOOH reagent. The thermal decomposition of this high-crystal water ore at around 300° C. is mainly due to the thermal decomposition of FeOOH contained in the ore (2FeOOH→Fe ₂ O ₃ +H ₂ O). In other words, it is presumed that the cause of the bursting is that rapid thermal decomposition of water of crystallization generates water vapor inside the pellets, increasing the pressure. Therefore, in the present invention, bursting is suppressed by controlling the heating rate in the thermal decomposition temperature range of water of crystallization based on the experimental results of the following examples.

First, in the present invention, the rate of temperature increase from room temperature to 280° C. in a series of firing treatments is 70° C./min or less, preferably 50° C./min or less. The treatment in this temperature range corresponds to the drying zone of raw pellets. The main purpose of the conventional heat treatment in the drying zone is to dry the moisture used in granulating the raw pellets (separation), but as shown in FIG. Pyrolysis of high crystalline water iron ore occurs. In the present invention, in consideration of this point, the temperature of the drying zone is set to 280° C. and the rate of temperature increase is controlled to 70° C./min or less. If the heating rate exceeds 70° C./min, bursting may occur. If the rate of temperature rise is suppressed, the occurrence of bursting can be more reliably suppressed, but in consideration of productivity, the rate of temperature rise in the drying zone is desirably 40° C./min or more.

Further, in the present invention, the rate of temperature increase from over 280° C. to 1200° C. in a series of firing treatments is 200° C./min or less, preferably 150° C./min or less. Most of the water of crystallization in the high-crystal-water iron ore should be decomposed (evaporated) in the drying zone, but some remaining water of crystallization is removed in this temperature zone, which corresponds to the preheating zone. At that time, if the heating rate exceeds 200° C./min, the water of crystallization that remains without being thermally decomposed in the drying zone will rapidly thermally decompose, possibly causing bursting in the preheating zone. In addition, as in the case of the dry zone, if the temperature rise rate is suppressed, the occurrence of bursting can be suppressed more reliably, but considering productivity, the temperature rise rate in the preheat zone should be 100°C/min or more. It is desirable to

In addition, in the present invention, since the water of crystallization of the iron ore with high crystal water content is removed in the drying zone and the preheating zone, there are no particular restrictions on the temperature of the sintering zone, the sintering time, and the like. The temperature is about 1200 to 1350° C., and the firing time can be about 5 to 30 minutes.

On the other hand, in the cooling zone, it is preferable to increase the cooling rate in order to compensate for the reduced temperature rise rate in the dry zone and preheat zone. Specifically, the cooling rate from the maximum firing temperature (maximum temperature in the drying zone) to 700°C in the firing process is preferably 200°C/min or more, more preferably 250°C/min or more. In this way, it is possible to prevent the extension of the entire processing time due to the gradual temperature rise in the dry zone or preheat zone, thereby maintaining productivity. Here, the reason why the cooling rate up to 700°C is stipulated is that the cooling temperature is set within a range that can be controlled from the viewpoint of facility capacity by a cooling device (cooling means) generally employed in the cooling zone, such as an annular cooler. It is taken into consideration. That is, in general, when the temperature 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. Further, the higher the cooling rate, the more advantageous it is because the productivity can be increased accordingly, but the upper limit of the cooling rate is about 300° C./min due to the capacity of the generally used cooling device.

ここで、Ｔ_１はある位置１で測定された温度を表し、Ｔ_２はある位置２で測定された温度を表す。また、Ｌは位置１と位置２の進行方向の距離を表し、Ｖはパレットの移動速度である。 When the heating rate and cooling rate are specified as in the present invention, strictly speaking, the temperature is measured by a thermocouple installed so as to follow the movement of the raw pellet (the positional relationship between the raw pellet and the thermocouple is constant). Therefore, it is necessary to obtain from the temperature change per unit time at this time, but such control is actually difficult when producing pellets in the above-mentioned traveling grate type or grate kiln type kiln. Therefore, for example, a fixed thermocouple may be installed directly above or below the pellet layer stacked on the pallet (pallet truck) in the grate, and the temperature may be calculated based on the temperature measurement result. Specifically, it can be obtained from the following formula.

where T ₁ represents the temperature measured at some location 1 and T ₂ represents the temperature measured at some location 2 . Also, L represents the distance in the traveling direction between positions 1 and 2, and V represents the moving speed of the pallet.

In the present invention, by adopting a series of firing treatments as described above, the content of water of crystallization in the iron ore powder to be blended as the raw material powder can be made higher than before, and the content of water of crystallization in the iron ore powder is 3. ~7% by mass, preferably 4.5 to 7% by mass. In general, the content of water of crystallization in high-quality iron ore varies depending on the type of ore, but is at most about 0.1 to 1% by mass. Therefore, the iron ore powder having a water of crystallization content of 3% by mass or more means that the iron ore powder having a high water of crystallization of inferior quality containing a large amount of water of crystallization is blended in a dominant amount as the iron ore powder. On the other hand, when the content of water of crystallization exceeds 7% by mass, it becomes difficult to suppress the occurrence of bursting. The content of water of crystallization in iron ore can be measured according to JIS M8211:1995 "Iron ore-compound water determination method". In addition, the presence or absence of bursting can be evaluated by visual observation of the pellets after firing as in the examples described later. You may make it evaluate from.

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 consist of a single ore species, or may be a blend of two or more ore species. may That is, the content of water of crystallization in the entire iron ore powder should be within this range. For example, high-crystal water iron ore powder having a water of crystallization content of more than 7% by mass may be used. As the rest of the iron ore powder, a powder having a lower content of water of crystallization may be blended so that the content of water of crystallization (weighted average) in the entire iron ore powder falls within the above range. In this case, as the iron ore powder, in addition to high-crystalline water iron ore powder, for example, hematite ore or magnetite ore, which are high-quality iron ores, can be mixed and used.

In the present invention, the raw material powder can include not only iron ore powder, but also auxiliary materials such as limestone and dolomite, binders such as bentonite, and the like. It is preferable to use powdered raw materials, including these, which are pulverized individually or collectively by a ball mill or the like so that 90% or more of each powder has a particle size of 100 μm or less. In addition, the pulverized raw material powder is adjusted to a moisture content suitable for granulation, and can be formed into a spherical shape (raw pellet) with a diameter of about ten and several mm by a granulator such as a pan pelletizer or a drum pelletizer. The method can be the same as the known method except for the calcination treatment and the content of water of crystallization in the iron ore powder.

The present invention will be described below based on examples. In addition, this invention is not restricted to these contents.

(Experimental example)
Table 1 shows the components of the iron ore powder used in this experimental example. Ore A is a high grade hematite ore from Brazil, and ores B and C are high crystal water ores from Australia. Here, CW in Table 1 indicates the content of water of crystallization, which is measured according to JIS M8211:1995 "Iron ore-compound water determination method".

In addition, Table 2 shows the blending conditions (mixed raw material No.) of the raw material powder in this experimental example. In all cases, bentonite was used as a binder, and 0.5% by mass was added under all compounding conditions. In addition, each of these blended raw materials was pulverized to 100 μm or less in a ball mill, and 8% by mass (external number) of water was added to each of the blended raw materials, and granulated with a pan pelletizer to obtain grains of 12.5 to 15 mm. A raw pellet with a diameter was produced. Note that Table 2 shows the weighted average value of the crystal water content (CW) in the entire iron ore powder for each blended raw material.

The raw pellets prepared above were fired using a test firing furnace capable of controlling the heating rate, and the presence or absence of bursting was evaluated. FIG. 4 shows a schematic diagram of the test firing furnace used in this experimental example. In this test firing furnace, a sample basket 13 is installed in an alumina tube 11 with an inner diameter of 42 mmφ, raw pellets 14 are placed in the sample basket 13, and the alumina tube 11 is heated to 1360° C. by a heater 12. Further, the temperature of the raw pellet 14 put in the sample basket 13 can be measured by the thermocouple 15 installed right above it, and the sample basket 13 can move up and down within the alumina tube 11. Therefore, it is possible to control the temperature of the green pellets during firing.

Using the raw pellets (No. 1 to 5) obtained with each blended raw material and the above test firing furnace, the firing process consisting of the drying zone, preheating zone, firing zone, and cooling zone shown in Table 3 Then, iron ore calcined pellets were produced respectively.
For example, in Comparative Example 1, blended raw material No. The raw pellets of 1 are placed in the sample basket 13 of the test firing furnace, the sample basket is lowered so as to approach the heater 12 in the furnace, and the raw pellets are heated from room temperature to 200 ° C. at a temperature increase rate of 50 ° C./min (drying zone), and then the raw pellets were heated from 200° C. to 1300° C. at a temperature rate of 200° C./min by changing the lowering speed of the sample basket 13 (preheating zone to firing zone). When the temperature of the raw pellets 14 in the sample basket 13 reached 1300° C., it was held for 5 minutes (firing zone).

Next, the sample basket is raised away from the heater 12 in the furnace, and the fired pellets are cooled from 1300° C. to 700° C. at a cooling rate of 150° C./min. , and naturally cooled to room temperature in the atmosphere to obtain iron ore calcined pellets according to Comparative Example 1. In addition, in Table 3, the time required from when the raw pellets were placed in the sample basket and heating in the drying zone to when they were cooled to 700°C in the cooling zone is shown as the total firing time.

Regarding the iron ore calcined pellets according to Invention Examples 1 to 5 and Comparative Examples 1 to 6 obtained as described above, the presence or absence of bursting was visually confirmed from the appearance to determine whether cracks occurred. evaluated. The results are shown in Table 3.

First, in Comparative Example 1, iron ore calcined pellets were obtained only from ore A having a low content of water of crystallization by conventional typical calcination treatment, and bursting did not occur.
Comparative Example 2 is the same calcination treatment as Comparative Example 1, but ore B having a high content of water of crystallization is blended, and the content of water of crystallization as a whole iron ore powder is 3.0% by mass, and bursting occurs. bottom.
In Comparative Example 3, the ultimate temperature of the drying zone was raised to 240° C. as compared with Comparative Example 2, but bursting occurred.
In Invention Example 1, the ultimate temperature of the drying zone was set to 280° C., which is higher than in Comparative Example 3, and bursting did not occur in this case.
In Invention Example 2, the heating rate of the drying zone was increased to 70° C./min as compared with Invention Example 1, and bursting did not occur in this case either.
In Comparative Example 4, the heating rate in the drying zone was increased to 100° C./min as compared to Inventive Example 2, but bursting occurred.
In Invention Examples 3 and 4, ore B and ore C were used, and the crystal water contents of the iron ore powder as a whole were set to 4.7% by mass and 7.0% by mass. However, bursting did not occur.
In Comparative Example 5, only ore C having the highest content of water of crystallization was used and the content of water of crystallization was 8.5% by mass, which was higher than that of Invention Example 4. Bursting occurred.
In Comparative Example 6, the same raw materials as in Invention Example 2 were used, but when the heating rate in the preheating zone was increased to 250°C/min, bursting occurred.
In Invention Example 5, the cooling rate was set to 200° C./min, which was higher than in Invention Example 2, but bursting did not occur.

From the results of these experimental examples, it was found that the rate of temperature increase from room temperature to 280°C in the firing treatment should be 70°C/min or less, and the rate of temperature increase from over 280°C to 1200°C should be 200°C/min or less. By using iron ore powder having a water of crystallization content of 3 to 7% by mass, it is possible to produce fired iron ore pellets while suppressing the occurrence of bursting and without lowering the yield. Moreover, since the productivity is not particularly reduced as compared with the conventional method, it becomes possible to manufacture iron ore calcined pellets using inferior quality iron ore, which is expected to increase in the future.

1: raw pellets, 2: grate, 3: drying chamber, 4: preheating chamber, 5: firing chamber, 6: cooling chamber, 7: rotary kiln, 8: annular cooler, 9: iron ore firing pellets, 11: alumina tube, 12: heater, 13: sample basket, 14: raw pellet, 15: thermocouple.

Claims (2)

A method for manufacturing iron ore calcined pellets, wherein raw pellets granulated using iron ore powder as raw material powder are subjected to a series of calcination treatments including a drying zone, a preheating zone, a calcination zone, and a cooling zone to produce iron ore calcined pellets. hand,
The iron ore powder has a crystal water content of 3 to 7% by mass,
An iron ore characterized in that the rate of temperature increase from room temperature to 280° C. in the firing treatment is 70° C./min or less and the rate of temperature increase from over 280° C. to 1200° C. is 200° C./min or less. A method for producing calcined pellets. 2. The method for producing iron ore sintered pellets according to claim 1, wherein the cooling rate from the highest sintering temperature to 700[deg.] C. in said sintering treatment is 200[deg.] C./min or more.

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