Two-stage variable-rotating-speed pelletizing method of iron ore pellets for low-carbon ironmaking

Abstract

The invention discloses a two-stage variable-rotating-speed pelletizing method of iron ore pellets for low-carbon ironmaking, which comprises the following steps: uniformly mixing fine iron powder and a binder as a pelletizing raw material, and carrying out mother pellet pelletizing by using a disc pelletizer at the rotating speed of 25-40r / min; then the rotating speed is adjusted to be 10-25 r / min, final pelletizing is conducted, and green pellets are obtained; wherein the rotating speed of the mother pellet pelletizing stage is greater than the rotating speed of the final pelletizing stage. The method has the characteristics that the pelletizing rate in the pelletizing process is high, the manufactured pellets are excellent in strength, meanwhile, the energy consumption and abrasion of the disc pelletizer can be reduced, and the method has important economic value and wide application prospects.

Description

Technical Field

[0001] The present invention relates to the technical field of metallurgy, and particularly relates to a two-stage variable-speed pelletizing method for iron ore pellets used in low-carbon ironmaking. Background Art

[0002] Pellet ore, as the main iron charge for modern low-carbon ironmaking processes, has the characteristics of high iron grade, stable metallurgical properties, low fuel consumption, and low CO 2 emission. Its quality has a significant impact on the stable operation of the process and the smelting efficiency. Under the background of China's "dual carbon" development strategy, high-proportion pellet ore smelting is one of the important technologies for the low-carbon development of blast furnace ironmaking. Blast furnace production practice shows that for every 10% increase in the proportion of pellet ore charged into the furnace, the coke ratio can be reduced by 4-6%, and CO 2 emission can be reduced by 3-5%. Currently, the proportion of pellet ore in blast furnaces of some domestic steel enterprises and international advanced steel enterprises has been increased from 20% in the traditional burden structure to more than 60%, achieving a reduction in CO 2 emission per ton of iron by more than 15%. As an important process for the iron and steel production to achieve fossil-free energy smelting and reach "zero carbon" emission, the hydrogen-based shaft furnace requires the compressive strength of the pellet ore charged into the furnace to reach 2500 N / piece due to its high-temperature and high-pressure smelting characteristics. However, the oxidized pellets prepared by the existing technologies have poor high-temperature and high-pressure resistance, and are prone to problems such as bursting, pulverization, and adhesion. Although China's pellet ore output reached 216 million tons in 2020, the proportion of high-quality pellets is less than 60%. The unreasonable pelletizing process parameters lead to uneven particle size distribution of green balls, low pelletizing rate, poor compressive strength of pellets, and excessive reduction swelling rate, which not only limit the high-proportion pellet smelting in blast furnaces, but also hinder the large-scale application of the hydrogen-based shaft furnace process. Therefore, improving the pelletizing process to increase the pelletizing rate, uniform the pellet particle size, and increase the compressive strength is the key to promoting the rapid development of low-carbon ironmaking technologies.

[0003] A disk pelletizer is a device in which iron ore concentrate particles roll and agglomerate in a rotating disk to form pellets. The existing pellet production technologies generally use disk pelletizers to produce green balls. However, the green balls prepared by the existing technologies have certain defects, such as uneven pellet particle size distribution, low pelletizing rate, low compressive strength, etc., which will further have an adverse impact on the subsequent smelting effect. Although patents CN119220810A and CN119220810A propose to use different binders in different pelletizing stages to improve the mechanical strength by regulating the pore distribution and density inside the pellets, the mixing process is complex and the binder compatibility is low, resulting in an increase in the pelletizing cost. Patent CN119223182A improves the qualification rate of green balls by installing a lidar in the disk pelletizer to detect the pellet particle size distribution in real time, but it cannot fundamentally improve the pellet properties by regulating the pelletizing mechanism. Summary of the Invention

[0004] Aiming at the above-mentioned deficiencies in the prior art, the purpose of the present invention is to provide a two-stage variable-speed pelletizing method for iron ore pellets used in low-carbon ironmaking, so as to solve the problems of uneven particle size distribution, low pelletization rate, and low compressive strength in the pellets prepared by the prior art.

[0005] To solve the above technical problems, the present invention adopts the following technical solutions:

[0006] A two-stage variable-speed pelletizing method for iron ore pellets used in low-carbon ironmaking, comprising the following steps:

[0007] Mix the iron concentrate powder and the binder evenly as the pelletizing raw material, use a disc pelletizing machine, and carry out mother pellet making under the condition that the rotational speed is 25 - 40 r / min; then adjust the rotational speed to 10 - 25 r / min for final pelletizing to obtain green pellets; wherein, the rotational speed in the mother pellet making stage > the rotational speed in the final pelletizing stage. In the present invention, mother pellet making is carried out under the condition that the rotational speed is 25 - 40 r / min, and this rotational speed range refers to the rotational speed in the mother pellet making stage. Then the rotational speed is adjusted to 10 - 25 r / min for final pelletizing, and this rotational speed range refers to the rotational speed in the final pelletizing stage.

[0008] Preferably, the rotational speed in the mother pellet making stage - the rotational speed in the final pelletizing stage ≥ 8 r / min.

[0009] Preferably, the binder is bentonite, and in the pelletizing raw material, the addition amount of bentonite is 1.2 - 3.0 wt%.

[0010] Preferably, calculated by mass percentage, in the pelletizing raw material, the pelletizing raw material for mother pellet making accounts for 20 - 50 wt% of the total amount of the pelletizing raw material.

[0011] Preferably, the time for the mother pellet making process is 1 - 3 min to obtain mother pellets with a particle size of 2 - 5 mm.

[0012] Preferably, the final pelletizing includes two stages: the mother pellet growth stage and the dense pellet stage; wherein, under the condition that the rotational speed remains 10 - 25 r / min, the time for the mother pellet growth stage is 3 - 8 min, and the time for the dense pellet stage is 2 - 6 min; green pellets with a particle size of 10 - 13 mm are obtained.

[0013] Preferably, during the mother pellet making process, water is added in a dropping manner; during the final pelletizing process, water is added in a spraying manner; finally, the water content in the obtained green pellets is controlled at 7 - 9 wt%.

[0014] Compared with the prior art, the present invention has the following beneficial effects:

[0015] 1. The method of the present invention adjusts the rotational speed during the pelletizing process. During the nucleation stage, the disk pelletizer is adjusted to a relatively high rotational speed to form a relatively high shear force, which can accelerate the collision frequency between iron ore concentrate particles, thereby facilitating the extrusion, adhesion, and nucleation between particles. This improvement can fundamentally enhance the stability of the green pellet structure, promote the full fusion of particles, and enable rapid pelletization. During the growth stage of the green pellet, the rotational speed is adjusted to a relatively low value, which is conducive to promoting the orderly stacking of particles and forming a uniform pore structure inside the pellet. This can not only accelerate the growth rate of the pellet but also enhance the binding force between particles during pellet growth, thereby improving and strengthening the pellet strength.

[0016] 2. The pellets prepared by the method of the present invention have a high qualification rate. The proportion of qualified pellets has increased by 24% compared with the prior art, significantly improving the proportion of qualified pellets. At the same time, only the rotational speed at different pelletizing stages is changed during the pelletizing process, which can reduce the energy consumption of the pelletizer while increasing the pelletization rate. Especially during the low-speed growth stage, the wear rate of the disk liner can be significantly reduced, achieving the dual optimization of production efficiency and energy consumption, and having good industrial application prospects. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] Figure 1 It is a schematic block diagram of the process flow of the two-stage variable rotational speed pelletizing method of the present invention.

[0018] In the figure: 1 is the pelletizing raw material for green pellet production, and 2 is the pelletizing raw material for green pellet growth. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0019] The present invention will clearly and completely describe the technical solutions in the embodiments of the present invention with reference to the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all of the embodiments. All other embodiments obtained by those of ordinary skill in the art based on the present invention belong to the scope of protection of the present invention.

[0020] Unless otherwise specified in specific cases, the numerical ranges listed herein include the upper and lower limit values, as well as all integers and fractions within the range, rather than the specific values listed when defining the range.

[0021] I. A two-stage variable rotational speed pelletizing method for iron ore pellets used in low-carbon ironmaking

[0022] The method of the present invention includes the following steps:

[0023] Mix the iron ore concentrate and the binder evenly as the pelletizing raw material, and use a disk pelletizer to pelletize the green pellets at a rotational speed of 25 - 40 r / min; then adjust the rotational speed to 10 - 25 r / min for final pelletizing to obtain green pellets; wherein, the rotational speed during the green pellet pelletizing stage > the rotational speed during the final pelletizing stage.

[0024] In the research on the existing pellet production technology, it is found that the existing technology often simply determines the optimal rotational speed of the disc pelletizer based on the properties of the prepared pellets, without studying the relationship between the rotational speed of the disc pelletizer and the pelletizing mechanism. Therefore, the existing technology generally uses a constant rotational speed (20 - 30 r / min) for pelletizing production. Since the rotational speed is constant throughout the pelletizing process, this leads to a mismatch in the mechanical environment during the nucleation and growth stages: insufficient rotational speed during the nucleation stage results in low particle kinetic energy, making it difficult to promote the release of the activation energy on the particle surface. This not only causes a decrease in the nucleation rate of the green balls but also affects subsequent growth, leading to a too-high porosity inside the pellets, an unstable structure, weak binding force between particles, and difficult improvement in the compressive strength of the pellets. Moreover, during this production process, high-speed pelletizing is maintained throughout, resulting in redundant motor power consumption, and the proportion of effective energy consumption during the nucleation stage is only about 40%. The disc pelletizer not only has high energy consumption but also high wear. Based on this, the present invention improves the existing pellet production technology and further discovers that a relatively high and reasonable rotational speed needs to be maintained during the nucleation stage to form a sufficiently high shear force, accelerate the collision frequency between iron ore concentrate particles, and achieve a breakthrough in the activation energy on the particle surface through such high-speed collisions, effectively overcoming the van der Waals force barrier, ensuring sufficient contact between particles and enabling them to bond strongly into nuclei. This can not only form balls quickly but also fundamentally enhance the stability of the green ball structure, laying a good foundation for the subsequent growth of the green balls; however, the rotational speed in this stage should not be too high, as too high a speed will be disadvantageous to the formation of the mother nuclei. In the growth stage of the green balls, the rotational speed is adjusted to a relatively low level, which is conducive to promoting the orderly stacking of particles on the green balls, enabling the green balls to grow in an orderly laminar manner, and forming a uniform pore structure inside the pellets, making the overall structure of the pellets more uniform. This can not only accelerate the growth rate of the pellets but also enhance the binding force between particles during the growth of the pellets, thereby improving and strengthening the strength of the pellets; at the same time, a relatively low rotational speed can avoid excessive centrifugal force, resulting in too-high collision energy between the spheres and causing the spheres to break, making it impossible to form a sufficient amount of qualified pellets. The present invention uses a two-stage variable rotational speed mode for pelletizing, which can fundamentally solve the problems existing in the pellets prepared by the existing technology, such as uneven particle size distribution, low pelletizing rate, and low compressive strength, significantly increasing the number of qualified pellets and improving the properties of the pellets, so as not to have an adverse impact on the subsequent smelting process; in particular, the method of the present invention only changes the rotational speed in different pelletizing stages, can reduce the energy consumption of the pelletizer while increasing the pelletizing rate, and can significantly reduce the wear rate of the disc liner, especially in the low-speed growth stage, achieving a double optimization of production efficiency and energy consumption, and having good industrial application prospects.

[0025] In some embodiments of the present invention, the mother ball pelletizing is carried out under the condition that the rotational speed is 25 - 40 r / min. This speed should not be too low or too high. If it is too low or too high, it will be difficult for the mother ball to form. Even if the mother ball is formed, there will be problems with its own insufficient stability. Therefore, the rotational speed can be 25 r / min, 27 r / min, 28 r / min, 30 r / min, 34 r / min, 35 r / min, 38 r / min or 40 r / min, or within the numerical range formed by any two of the above specific values as endpoints. It should be understood that in the implementation scheme, any of the above ranges can be combined with any other range.

[0026] In some embodiments of the present invention, the final pelletizing is carried out under the condition that the rotational speed is 10 - 25 r / min. This speed should also not be too low or too high. If it is too low, it will be unfavorable for the growth of the mother ball, while if it is too high, it will cause the mother ball to break. Both will affect the formation of qualified pellets. Therefore, the rotational speed can be 10 r / min, 11 r / min, 12 r / min, 13 r / min, 14 r / min, 15 r / min, 16 r / min, 18 r / min, 20 r / min, 21 r / min, 24 r / min or 25 r / min, or within the numerical range formed by any two of the above specific values as endpoints. It should be understood that in the implementation scheme, any of the above ranges can be combined with any other range.

[0027] In some embodiments of the present invention, the rotational speed in the mother ball pelletizing stage > the rotational speed in the final pelletizing stage. Preferably: the rotational speed in the mother ball pelletizing stage - the rotational speed in the final pelletizing stage ≥ 8 r / min. If the rotational speeds in these two stages do not differ much, it will not have a favorable effect. If the difference is smaller, it will instead have an adverse effect. This adverse effect may even make the final effect inferior to the properties of the green balls prepared by the constant speed preparation method.

[0028] In some embodiments of the present invention, the binder is bentonite. Calculated by mass percentage, in the pelletizing raw materials, the addition amount of bentonite is 1.2 - 3.0 wt%. In the method of the present invention, the dosage of bentonite is controlled at a level lower than that of the prior art because the preparation method of the present invention has been adjusted, which can reduce the dependence on the dosage of the bentonite binder, enabling it to prepare a sufficient amount of qualified pellets with only a lower dosage. In the present invention, if the addition amount of bentonite is too low, it is difficult to achieve a suitable bonding effect, and if it is too high, it will significantly reduce the iron grade of the pellets, which has an adverse impact on subsequent ironmaking. Therefore, the addition amount of bentonite can be 1.2 wt%, 1.4 wt%, 1.5 wt%, 1.7 wt%, 1.9 wt%, 2.0 wt%, 2.1 wt%, 2.4 wt%, 2.7 wt%, 2.9 wt% or 3.0 wt%, or within the numerical range formed by any two of the above specific values as endpoints. It should be understood that in the implementation scheme, any of the above ranges can be combined with any other range.

[0029] In some embodiments of the present invention, the pelletizing raw materials are added to the disc pelletizer in batches for production. As Figure 1 shown, the pelletizing raw materials are divided into two parts: pelletizing raw material 1 for mother ball pelletizing and pelletizing raw material 2 for mother ball growth. Among them, calculated by mass percentage, in the pelletizing raw materials, the pelletizing raw material for mother ball pelletizing accounts for 20 - 50 wt% of the total amount of pelletizing raw materials. The proportion of the pelletizing raw material for mother ball pelletizing can be 20 wt%, 25 wt%, 30 wt%, 35 wt%, 40 wt%, 45 wt% or 50 wt%, or within the numerical range formed by any two of the above specific values as endpoints. It should be understood that in the implementation scheme, any of the above ranges can be combined with any other range.

[0030] In some embodiments of the present invention, the time of the mother ball pelletizing process is 1 - 3 min, and mother balls with a particle size of 2 - 5 mm are obtained. If the time of the mother ball pelletizing process is too short, the number of particle collisions will be insufficient, the nucleation rate of mother balls will be low, and the structure will be loose, which is not conducive to subsequent growth. However, if the mother ball pelletizing time is too long, due to continuous high-speed collisions, the formed mother balls will be broken, and some mother balls will continue to grow, resulting in too high a particle size and ultimately too many large balls, which is not conducive to subsequent smelting. Therefore, the time of the mother ball pelletizing process can be 1 min, 1.5 min, 2 min, 2.5 min or 3 min, or within the numerical range formed by any two of the above specific values as endpoints. It should be understood that in the implementation scheme, any of the above ranges can be combined with any other range.

[0031] In some embodiments of the present invention, the final pelletizing includes two stages: the green ball growth stage and the dense pellet stage; wherein, under the condition that the rotation speed is maintained at 10 - 25 r / min, the time of the green ball growth stage is 3 - 8 min, and the time of the dense pellet stage is 2 - 6 min; green balls with a particle size of 10 - 13 mm are obtained. The present invention discovers that after the growth of green balls, a relatively low rotation speed can be maintained for a certain period of time, which helps to make the internal structure of the pellets compact and reduce the porosity. Therefore, the time of the green ball growth stage can be 3 min, 4 min, 5 min, 6 min, 7 min or 8 min, or within the numerical range formed by any two of the above specific values as endpoints; the time of the dense pellet stage can be 2 min, 3 min, 4 min, 5 min or 6 min, or within the numerical range formed by any two of the above specific values as endpoints. It should be understood that in the implementation scheme, any of the above ranges can be combined with any other range.

[0032] In some embodiments of the present invention, during the green ball pelletizing process, water is added in a dripping manner, and green balls are formed by dripping water droplets into the pelletizing raw materials. During this process, the addition amount of water needs to be controlled. Calculated by mass percentage, the addition amount of water in this process is about 1 - 9 wt% of the iron ore concentrate used for green ball pelletizing. While in the final pelletizing process, water is added in a spraying manner, and an appropriate amount of water is sprayed in a misty and batch-by-batch form, and the addition amount of water in this step is regulated according to the water content requirement in the final green balls, so that the water content in the obtained green balls is controlled at 7 - 9 wt%.

[0033] II. Examples and Comparative Examples

[0034] Example 1

[0035] Step 1: 1000 g of iron ore concentrate and 17 g of bentonite (1.70 wt%) are mixed evenly and used as the pelletizing raw materials. The chemical composition analysis and water content of the iron ore concentrate are shown in Table 1;

[0036] Step 2: As Figure 1 shown, the pelletizing raw materials are divided into two parts. Among them, 1 / 5 (by weight) of the pelletizing raw materials is taken and placed in a disk pelletizer with a rotation speed of 34 r / min. 10 g of water is dripped into it within 2 min to form green balls until the green balls nucleate to a suitable particle size;

[0037] Step 3: The rotation speed of the disk pelletizer is adjusted to 24 r / min, and the remaining pelletizing raw materials are added to the green balls made in Step 2 in 4 batches within 5 min, 200 g of pelletizing raw materials are added each time, and about 2.5 g of water mist is sprayed until the green balls grow to a suitable particle size;

[0038] Step 4: Keep the rotational speed of the disc pelletizer at 24 r / min, and continue to rotate the prepared pellets tightly at this speed for 4 min and then take them out, which are used as the green pellets of Example 1.

[0039] Example 2

[0040] Step 1: Mix 1000 g of iron ore concentrate with 17 g of bentonite (1.70 wt%) and use it as the pelletizing raw material. The chemical composition analysis and water content of the iron ore concentrate are shown in Table 1;

[0041] Step 2: Take out 1 / 5 (by weight) of the pelletizing raw material and place it in a disc pelletizer with a rotational speed of 25 r / min. Drop 10 g of water into it within 2 min to form mother pellets until the mother pellets nucleate to a suitable particle size;

[0042] Step 3: Adjust the rotational speed of the disc pelletizer to 10 r / min, and add the remaining pelletizing raw material to the mother pellets prepared in Step 2 in 4 portions within 5 min, with 200 g of pelletizing raw material added each time, and spray about 2.5 g of water mist until the mother pellets grow to a suitable particle size;

[0043] Step 4: Keep the rotational speed of the disc pelletizer at 10 r / min, and continue to rotate the prepared pellets tightly at this speed for 4 min and then take them out, which are used as the green pellets of Example 2.

[0044] Example 3

[0045] Step 1: Mix 1000 g of iron ore concentrate with 17 g of bentonite (1.70 wt%) and use it as the pelletizing raw material. The chemical composition analysis and water content of the iron ore concentrate are shown in Table 1;

[0046] Step 2: Take out 1 / 5 (by weight) of the pelletizing raw material and place it in a disc pelletizer with a rotational speed of 40 r / min. Drop 10 g of water into it within 2 min to form mother pellets until the mother pellets nucleate to a suitable particle size;

[0047] Step 3: Adjust the rotational speed of the disc pelletizer to 25 r / min, and add the remaining pelletizing raw material to the mother pellets prepared in Step 2 in 4 portions within 5 min, with 200 g of pelletizing raw material added each time, and spray about 2.5 g of water mist until the mother pellets grow to a suitable particle size;

[0048] Step 4: Keep the rotational speed of the disc pelletizer at 25 r / min, and continue to rotate the prepared pellets tightly at this speed for 4 min and then take them out, which are used as the green pellets of Example 3.

[0049] Example 4

[0050] Step 1: Mix 1000 g of iron ore concentrate with 17 g of bentonite (1.70 wt%) and use it as the pelletizing raw material. The chemical composition analysis and water content of the iron ore concentrate are shown in Table 1;

[0051] Step 2: Take out 1 / 5 (by weight) of the pelletizing raw materials and place them in a disk pelletizer with a rotation speed of 40 r / min. Drop 10 g of water into them within 2 min to form mother balls until the mother balls nucleate to a suitable particle size;

[0052] Step 3: Adjust the rotation speed of the disk pelletizer to 10 r / min. Add the remaining pelletizing raw materials to the mother balls prepared in Step 2 in 4 portions within 5 min, with 200 g of pelletizing raw materials added each time, and spray about 2.5 g of water mist until the mother balls grow to a suitable particle size;

[0053] Step 4: Still set the rotation speed of the disk pelletizer to 10 r / min. Rotate the prepared pellets tightly at this speed for another 4 min and then take them out to obtain the green balls of Example 4.

[0054] Comparative Example 1

[0055] Based on Example 1 with improvements. The difference is that in Steps 2, 3, and 4, the rotation speed of the disk pelletizer remains unchanged at 34 r / min, and other steps are exactly the same as those in Example 1 to obtain the green balls of Comparative Example 1.

[0056] Comparative Example 2

[0057] Based on Example 1 with improvements. The difference is that in Steps 2, 3, and 4, the rotation speed of the disk pelletizer remains unchanged at 24 r / min, and other steps are exactly the same as those in Example 1 to obtain the green balls of Comparative Example 2.

[0058] Comparative Example 3

[0059] Based on Example 1 with improvements. The difference is that in Step 2, the rotation speed is 25 r / min, in Step 3, the rotation speed is 24 r / min, and in Step 4, the rotation speed of the disk pelletizer remains unchanged at 24 r / min, and other steps are exactly the same as those in Example 1 to obtain the green balls of Comparative Example 3.

[0060] Table 1 Chemical composition and water content of iron ore concentrate (mass percentage / %)

[0061] Iron concentrate TFe CaO <![CDATA[SiO 2 > MgO <![CDATA[Al 2 O 3 > <![CDATA[SO 3 > <![CDATA[TiO 2 > <![CDATA[V 2 O 5 > <![CDATA[H 2 O]]> Content 52.33 0.29 4.31 4.54 4.02 0.94 9.76 0.72 7.54

[0062] Table 2 Nucleation rate and balling rate of examples and comparative examples

[0063]

[0064] Nucleation rate of mother balls: Balls with a particle size of 2 - 5 mm are regarded as mother balls, and the proportion of the mass of this part in the mass used for making mother balls is counted as the nucleation rate; Pelletization rate: Pellets with a particle size of 9 - 16 mm that have formed balls, and the proportion of the mass of all pelletizing raw materials is counted as the pelletization rate.

[0065] It can be seen from Table 2 that:

[0066] (1) In the examples, during the nucleation stage of mother balls, the rotational speed of the disc pelletizer is faster, and during the growth stage of mother balls, the rotational speed of the disc pelletizer is lower. This method not only has a high nucleation rate but also a higher pelletization rate for the prepared pellets; In Examples 1 - 4 with variable rotational speeds, when the rotational speed of the disc pelletizer is set to 40 r / min when the mother balls form and 10 r / min when the mother balls grow, the nucleation rate and pelletization rate are the highest, significantly superior to the comparative examples.

[0067] (2) In the comparative examples, in Comparative Example 1, the preparation is carried out at a relatively high rotational speed all the time. Although its nucleation rate is not low, the pelletization rate is significantly inferior to that of the examples. This shows that a constant high rotational speed can provide a relatively high nucleation rate but is not conducive to the growth of mother balls, resulting in a decrease in the pelletization rate; In Comparative Example 2, the preparation is carried out at a relatively low rotational speed all the time, and both the nucleation rate and the pelletization rate are not ideal, indicating that a constant low rotational speed is not conducive to the nucleation and growth of mother balls; In Comparative Example 3, although different rotational speeds are used in different stages, the rotational speed difference between the nucleation stage and the growth stage is not large, resulting in its nucleation rate and pelletization rate being inferior to those of Comparative Example 1 and Comparative Example 2 with constant rotational speeds. This also shows that if the rotational speed difference between the nucleation stage and the growth stage is not large, it is not conducive to the preparation of pellets. Compared with Example 2, although both Comparative Example 3 and Example 2 use the same rotational speed, in fact, their nucleation rates are not high, and Example 2 is also lower than other examples. However, the rotational speed difference between the nucleation stage and the growth stage in Example 2 is greater than 8, which makes the pelletization rate of Example 2 significantly higher than that of Comparative Example 3, further proving that the control of the rotational speed in the nucleation stage and the growth stage of the present invention is beneficial to the preparation of pellets.

[0068] The green balls prepared in the above Examples 1 - 4 and Comparative Examples 1 - 3 were tested. The green ball strength, water content, and dry ball strength after drying at 105°C for 24 h are shown in Table 3. Among them, the compressive strength, drop strength, and dry ball strength of the green balls were measured according to the national standard YB / T4848 - 2020; The water content was measured as follows: The green balls were placed in a drying oven at a temperature of 105°C, and after constant-temperature drying for 24 h, the weight change of the green balls was recorded. Each time, 7 green ball samples were measured. After individual measurements, one maximum value and one minimum value were removed, and the average value of the remaining 5 was calculated and recorded as the water content of the green ball sample.

[0069] Table 3 Green Ball Properties of Examples and Comparative Examples

[0070] Performance item Green pellet compressive strength (N / pellet) Green pellet drop strength (times / pellet) Water content (%) Dry pellet compressive strength (N / pellet) Example 1 15.9 9.0 7.9 55.8 Example 2 14.8 8.6 7.6 52.3 Example 3 16.5 9.1 7.3 58.7 Example 4 18.3 9.7 6.9 63.5 Comparative example 1 13.6 8.4 7.4 48.1 Comparative example 2 9.7 6.3 8.7 36.9 Comparative example 3 10.1 6.6 8.8 37.5

[0071] As can be seen from Table 3:

[0072] (1) Compared with the comparative examples, the green balls prepared in the examples are superior to the comparative examples in terms of the strength of the green balls and the strength of the dry balls after drying. Using different rotation speeds for pelletizing in stages can increase the compressive strength of the green balls by up to 64% and the drop strength by 43%, and most of the water content can be in the appropriate range (it is best to control the water content between 7.50 and 8.50 wt% to meet the subsequent smelting requirements).

[0073] (2) The strength of the green balls prepared in the comparative examples is much lower than that in the examples. Moreover, the control effect on the final water content is also poor. Comparative examples 2 and 3 both fail to reach the appropriate range of water content, which will also have an adverse impact on the subsequent smelting effect.

[0074] The green balls prepared in the above Examples 1-4 and Comparative Examples 1-3 were subjected to the same heat treatment, and the strength of the preheated pellets, the strength of the roasted pellets, and the porosity obtained during the heat treatment were detected. The results are shown in Table 4. Among them, the compressive strength of the pellet preheating and roasting was measured according to the national standard YB / T4848-2020; the pellet porosity was measured according to the national standard GB / T24586-2009.

[0075] Table 4 Porosity and preheating and roasting compressive strength of pellets after heat treatment of green balls in examples and comparative examples

[0076] Performance item Preheated pellet compressive strength (N / pellet) Roasted pellet compressive strength (N / pellet) Porosity (%) Example 1 525 2650 19 Example 2 563 2353 20 Example 3 613 2856 17 Example 4 684 3059 15 Comparative example 1 482 1952 25 Comparative example 2 428 1459 29 Comparative example 3 451 1601 25

[0077] As can be seen from Table 4:

[0078] (1) For the green balls prepared in the examples, after heat treatment, the compressive strength of the preheated pellets and the roasted pellets is significantly better than that of the comparative examples, and the porosity is also much lower than that of the comparative examples. This shows that using the method described in the present invention can significantly improve the performance of the pellets and make them have excellent mechanical strength after heat treatment.

[0079] (2) For the green balls prepared in Comparative Examples 1-3, after heat treatment, the compressive strength of the preheated pellets and the roasted pellets did not meet the standards (the required compressive strength of the preheated pellets is 500 N / piece, and the required compressive strength of the roasted pellets is 2500 N / piece), and the porosity is also much higher than that of the examples.

[0080] In summary:

[0081] (1)In Example 1, when the rotation speed during the nucleation stage was 34 r / min, the high centrifugal force promoted the mutual collision between particles and rapid nucleation, resulting in a high nucleation rate of mother balls. After the rotation speed was reduced to 24 r / min during the growth stage, the decrease in centrifugal force prolonged the residence time for particle bonding, slowed down the rolling speed of the spheres, and thus achieved orderly laminated growth, further increasing the ball forming rate. This behavior also promoted the enhancement of the bonding force between iron concentrate particles, improved the compressive strength of the pellets. And this behavior became stronger as the rotation speed difference between the two stages increased, but it was necessary to consider whether the moisture content of the pellets was within a reasonable range. In Comparative Example 1, when the disk pelletizer maintained a high rotation speed of 34 r / min throughout the process, the mother balls that had already nucleated and grown would have an increased collision frequency between pellets under the action of high-speed centrifugal force, resulting in pellet breakage. Comparing the results of Example 1 and Comparative Example 1, it can be seen that if the disk pelletizer always maintains a high rotation speed during the pelletizing process, the nucleation rate of the obtained mother balls and the final ball forming rate are both low, and the green ball strength and dry ball strength of the prepared green balls are also lower than the pellet properties obtained by staged pelletizing, and the moisture content is not within the appropriate moisture content range for green balls. Comparing the results of Example 1 and Comparative Example 2, it can be seen that if the disk pelletizer always maintains a low rotation speed during the pelletizing process, the nucleation rate of the obtained mother balls and the final ball forming rate are too low, and the dry ball strength of the prepared green balls is also much lower than the pellet properties obtained by staged pelletizing, and the moisture content of the pellets is too high and not within the appropriate moisture content range for green balls. The green balls prepared by the two-stage variable rotation speed pelletizing method not only increase the ball forming rate, but also enhance the green ball and dry ball strength, contribute to improving the pelletizing efficiency and pellet properties, and can reduce the energy consumption and wear of the disk pelletizer.

[0082] (2)Comparing the results of Comparative Example 3 and Example 4, it can be seen that if the rotation speed used for mother ball nucleation and the rotation speed used for mother ball growth differ more, the effect of improving the green ball strength and the properties of preheated and fired pellets is the best. However, if the rotation speed difference is too large, the moisture escape speed inside the pellets will increase, resulting in a low moisture content of the pellets. When using the two-stage variable rotation speed pelletizing method of the present invention to prepare pellets, the greater the rotation speed difference between the two stages of pelletizing, the better the process improves the pellet properties. However, the moisture migration speed inside the pellets will be faster. Therefore, it should be considered comprehensively.

[0083] (3) In Comparative Examples 1-2, when the disc pelletizer maintained a low rotational speed of 24 r / min throughout the process, the fine particles had insufficient collisions during the initial stage of green ball nucleation, resulting in a decrease in the number of effective nucleations and thus affecting the overall pelletization rate. When the disc pelletizer maintained a high rotational speed of 34 r / min throughout the process, the green balls that had already nucleated and grown would have an increased collision frequency between pellets under the action of high-speed centrifugal force, leading to pellet breakage. Both of these processes would cause the internal moisture migration of the green balls to get out of control, making the water content difficult to control and resulting in insufficient green ball strength. When the disc pelletizer uses a constant rotational speed for pelletization, problems such as the water content of the green balls exceeding the optimal range due to the mismatch of the mechanical environment, low pelletization rate, and insufficient pellet strength will occur.

[0084] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and not to limit them. Those of ordinary skill in the art should understand that any modifications or equivalent replacements made to the technical solutions of the present invention without departing from the purpose and scope of the present technical solution should be covered within the scope of the claims of the present invention.

Claims

1. A two-stage variable speed pelletizing method for low-carbon iron ore pellets for iron ore making, characterized in that: The following steps are involved: The iron ore concentrate and the binder are uniformly mixed as the raw material for pelletizing, and a disc pelletizing machine is used to pelletize the mother ball at a rotation speed of 25 to 40 r / min; then the rotation speed is adjusted to 10 to 25 r / min, and the final pelletizing is performed to obtain raw balls; wherein, the rotation speed of the mother ball in the pelletizing stage is greater than the rotation speed of the final pelletizing stage.

2. The pelletizing method according to claim 1, characterized in that: The rotation speed of the cue ball in the ball-making stage - the rotation speed of the final ball-making stage ≥ 8r / min.

3. The pelletizing method according to any one of claims 1 to 2, characterized in that: The adhesive is bentonite. Calculated by weight percentage, the amount of bentonite added to the pelletizing raw material is 1.2-3.0wt%.

4. The pelletizing method according to any one of claims 1 to 2, characterized in that: Among the pelletizing raw materials, the pelletizing raw materials used for the mother ball account for 20 to 50 wt % of the total amount of the pelletizing raw materials.

5. The pelletizing method according to any one of claims 1 to 2, characterized in that: The time of the mother ball making process is 1 to 3 minutes, and the mother ball with a particle size of 2 to 5 mm is obtained.

6. The pelletizing method according to any one of claims 1 to 2, characterized in that: The final ball making includes two stages: a mother ball growth stage and a compact ball agglomeration stage; wherein, under the condition of maintaining a rotation speed of 10 to 25 r / min, the mother ball growth stage lasts for 3 to 8 minutes, and the compact ball agglomeration stage lasts for 2 to 6 minutes; and green balls with a particle size of 10 to 13 mm are obtained.

7. The pelletizing method according to any one of claims 1 to 2, characterized in that: In the process of making the mother ball, water is added by dripping; in the final ball making process, water is added by spraying; and finally the water content of the obtained raw ball is controlled at 7-9wt%.

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