A composite additive for improving comprehensive performance of high-alkali-content iron ore oxidized pellets and a method thereof

Abstract

The application discloses a kind of composite additives and methods for improving comprehensive performance of high alkali metal content iron ore oxidized pellets, and belongs to the technical field of iron ore pellet production.The method is that after composite additives containing steel slag, blast furnace slag and bentonite are activated by ball milling, they are mixed with high alkali metal content iron concentrate to form pellets, and the obtained green pellets are sequentially dried, preheated and roasted to obtain pellets.The method effectively improves the compressive strength of the pellets by using special composite additives to increase the liquid phase content in the pellets during the production of pellets from high alkali metal content iron concentrate, while reducing the reduction expansion rate of the pellets.The method promotes the large-scale application of high alkali metal iron concentrate in the preparation of pellets, and realizes the recycling of blast furnace slag and steel slag.

Description

Technical Field

[0001] This invention relates to a composite additive for pelletizing, particularly to a composite additive for improving the overall performance of iron ore oxide pellets with high alkali metal content, and also to a method for using the composite additive to prepare iron ore oxide pellets with high alkali metal content, belonging to the field of iron ore pellet production technology. Background Technology

[0002] As the steel industry moves towards higher efficiency and greener practices, iron ore pellets, as a high-quality blast furnace charge with excellent metallurgical properties, have been widely used in blast furnace ironmaking systems. To further reduce energy consumption in blast furnace ironmaking, increasing the proportion of pellets in the charge has become one of the main directions for future blast furnace ironmaking. Therefore, the demand for iron ore pellets is increasing daily.

[0003] Furthermore, given the increasing scarcity of high-quality iron ore resources, the high-proportion application of iron concentrate with complex gangue composition in pelletizing has become one of the development directions for pelletizing. Based on this, iron ore resources with high alkali metal content are being widely used in pelletizing. Simultaneously, sodium-based bentonite, used as a pelletizing binder, is widely applied in pelletizing due to its low price. Therefore, when using iron concentrate with high alkali metal content and sodium-based bentonite to prepare pellets, the alkali metal content in the prepared pellets increases significantly. Studies have shown that alkali metals in the pellets react with iron oxides such as Fe2O3 during oxidation to form alkali metal ferrates. The formation of these alkali metal ferrates leads to rapid localized growth of metallic iron during subsequent reduction. This causes malignant volume expansion and even pulverization and breakage of the pellets. When these pellets enter the blast furnace, the expansion significantly reduces the permeability of the burden column, disrupting the permeability of the furnace charge structure, leading to abnormal blast furnace operation problems such as hanging, collapse, and furnace bridging, severely restricting smelting efficiency and production stability. Chinese patent (CN106367584A) discloses a method for preparing oxide pellets using high-alkali iron concentrate, but its utilization rate of high-alkali iron concentrate is less than 50%, requiring the mixing with a large amount of low-alkali iron concentrate. Furthermore, it necessitates the use of magnesium additives to reduce the expansion rate of the oxide pellets and improve their metallurgical properties, and the resulting pellets have relatively low compressive strength (greater than 2000 N·P). -1 ), with a relatively high expansion rate (18.9%). Summary of the Invention

[0004] To address the technical problems of low pellet strength and high reduction expansion in the preparation of pellets using high-alkali iron concentrate and sodium-based bentonite in existing technologies, the first objective of this invention is to provide a composite additive for improving the overall performance of oxidized iron ore pellets with high alkali metal content. This composite additive can increase the liquid phase content within the pellets, optimize the distribution of alkali metals within the pellets during oxidative roasting, and avoid the formation of alkali metal ferrates. This effectively inhibits the malignant expansion of the pellets caused by the rapid local growth of metallic iron during subsequent reduction, thereby obtaining high-strength, low-expansion iron ore pellets.

[0005] The second objective of this invention is to provide a method for preparing oxidized iron ore pellets with high alkali metal content. This method involves adding a composite additive composed of steel slag, blast furnace slag, and bentonite during the pelletizing process of high alkali metal content iron concentrate. The SiO2 and CaO contained in the additive are used to regulate the liquid phase formed during the oxidative roasting of the pellets. The liquid phase is used to inhibit the migration of alkali metal ions into the interior of hematite, reduce the amount of alkali metal ferrates generated, and thus improve the reduction expansion behavior of the pellets. At the same time, the liquid phase is used to fill the pores and enhance the structural density, thereby improving the compressive strength of the finished pellets.

[0006] To achieve the above-mentioned technical objectives, the present invention provides a composite additive for improving the overall performance of iron ore oxide pellets with high alkali metal content. The composite additive is composed of steel slag, blast furnace slag, and bentonite in a mass percentage ratio of 25-65%:20-70%:5-15%. The blast furnace slag contains 30-40% SiO2 and 35-45% CaO by mass, respectively, and the total mass content of other impurities is not higher than 25%. The steel slag contains 10-20% SiO2 and 40-50% CaO by mass, respectively.

[0007] The composite additive of this invention is mainly composed of steel slag, blast furnace slag, and bentonite. Blast furnace slag is characterized by high SiO2 content and low CaO content. Its high SiO2 content helps increase the liquid phase content within the pellets. However, due to its high SiO2 content, the resulting liquid phase has a high melting point. Steel slag, on the other hand, is rich in CaO and has a relatively low SiO2 content. Using it in combination with blast furnace slag can optimize the melting point of the liquid phase. Therefore, this invention combines steel slag and blast furnace slag to effectively regulate the SiO2 and CaO content in the pellet raw materials, thereby controlling the liquid phase content and melting point formed during the oxidative roasting of the pellets. The liquid phase can stabilize the distribution of alkali metal ions, playing a role in capturing and stabilizing them, thus inhibiting their migration into the hematite interior. This effectively reduces the formation of alkali metal ferrates, thereby improving the reduction expansion behavior of the pellets. Simultaneously, the generated liquid phase has excellent wetting properties and fluidity, filling the internal pores of the pellets and enhancing structural density, thus improving the compressive strength of the finished pellets. Bentonite is also introduced into the composite additive, mainly to improve the bonding performance of the composite additive and enhance the compatibility between the composite additive and the iron concentrate with high alkali metal content, thereby improving the pelletizing properties of the iron concentrate with high alkali metal content.

[0008] This invention achieves relative equilibrium of the liquid phase components by adjusting the proportions of steel slag, blast furnace slag, and bentonite within a suitable range. This avoids the problem of reduced liquid phase content due to excessively high melting points. Furthermore, the appropriate proportion design not only improves the wettability and fluidity of the liquid phase but also effectively optimizes its distribution within the pellets. The liquid phase within the pellets is mainly regulated by SiO2 and CaO, with their mass contents preferably controlled within the ranges of 2.5–6.0% and 1.0–5.5%, respectively.

[0009] The blast furnace slag and steel slag used in this invention primarily employ SiO2 and CaO as their main components. This is because SiO2 and CaO facilitate the formation of a stable liquid phase during the pellet oxidative roasting process, effectively encapsulating and fixing alkali metal ions and inhibiting their migration into the hematite interior. This reduces the formation of alkali metal ferrates, thereby improving the reduction and expansion behavior of the pellets. Furthermore, since the SiO2 content in blast furnace slag is relatively higher than that in steel slag, while the CaO content is relatively lower, a suitable ratio of the two can control the mass content of SiO2 and CaO within a wide range.

[0010] The present invention also provides a method for preparing iron ore oxide pellets with high alkali metal content. The method involves activating the composite additives by ball milling, mixing them with iron concentrate with high alkali metal content to form pellets, and then subjecting the resulting green pellets to drying, preheating, and roasting to obtain the pellet ore.

[0011] The key to this invention in preparing high-alkali-metal-content iron ore oxide pellets lies in the addition of a composite additive composed of steel slag, blast furnace slag, and bentonite. This composite additive is characterized by high CaO and SiO2 content. In the pelletizing process of high-alkali-metal-content iron concentrate, it features low addition amount and high liquid phase generation. Furthermore, the CaO and SiO2 components regulate the amount of liquid phase generated, the melting point of the liquid phase, and the uniform distribution of the liquid phase within the pellets. The liquid phase inhibits the migration of alkali metal ions, significantly reducing pellet reduction and expansion. Simultaneously, the liquid phase fills internal pores and enhances structural density, resulting in high-strength pellets. In addition, the introduction of a small amount of bentonite into the composite additive improves the compatibility between steel slag, blast furnace slag, and high-alkali-metal-content iron concentrate, thus improving pelletizing performance.

[0012] As a preferred embodiment, the ball milling activation conditions are: a rotation speed of 250-300 rpm, a time of 30-60 min, and a ball-to-material mass ratio of 2:1-5:1. Under these preferred ball milling activation conditions, mineral particles can be effectively broken down, their specific surface area increased, and their reactivity stimulated, making them more likely to participate in the liquid-phase generation reaction during the roasting process. The generated liquid phase is beneficial for timely capturing and solidifying migrating alkali metals during the oxidative roasting stage, thus playing a key role in stabilizing alkali metals and inhibiting expansion.

[0013] As a preferred embodiment, the ball milling activation is used to control the particle size of the composite additive to ensure that the mass percentage of particles in the -38 μm size range is above 98%.

[0014] This invention activates components such as steel slag, blast furnace slag, and bentonite through ball milling. Ball milling activation can reduce the reaction energy of these components and refine their particle size. This fine particle size distribution can improve the contact efficiency between the components and iron ore particles, promote the uniform formation of the liquid phase during oxidative roasting, effectively fill pores, and enhance structural compactness. Thus, without affecting the quality of green pellets, the amount of bentonite used can be reduced, and the compressive strength and reduction expansion of the finished pellets can be improved.

[0015] As a preferred embodiment, the mass of the composite additive is 0.8-2.5% of the mass of the high alkali metal content iron concentrate. Within the preferred range of composite additive dosage, sufficient liquid phase can be formed to stabilize the alkali metal distribution without significantly reducing the TFe grade of the pellets, while also enhancing the compressive strength and reducibility of the pellets.

[0016] As a preferred embodiment, the alkali metal content in the high alkali metal content iron concentrate is 0.25~0.42% by mass.

[0017] As a preferred embodiment, the preheating conditions are: temperature of 920~950℃ and time of 8~12 minutes. Preheating is carried out in an air atmosphere.

[0018] As a preferred embodiment, the calcination conditions are: a temperature of 1250~1280℃ and a time of 15~20 minutes. Calcination is carried out in an air atmosphere.

[0019] Compared with existing technologies, the beneficial technical effects of the present invention are as follows:

[0020] The composite additive of the present invention is used in the production of iron concentrate pellets with high alkali metal content. It has the characteristics of low addition amount and large liquid phase generation. While significantly reducing the reduction expansion of pellets, it can obtain pellets with high strength.

[0021] The composite additive of this invention enables the reuse of steel slag and blast furnace slag, reducing the environmental pollution caused by their large-scale stockpiling, while also achieving resource recovery.

[0022] The preparation method of the composite additive of the present invention is simple to operate, has a short process, uses a wide range of raw materials, and is low in cost, thus meeting the requirements of industrial production. Attached Figure Description

[0023] Figure 1 (a) shows the microstructure of the pellets described in Comparative Example 1 of the present invention; (b) shows the microstructure of the pellets described in Example 2 of the present invention. Detailed Implementation

[0024] The following examples are intended to further illustrate the present invention, but not to limit the scope of protection of the claims of the present invention.

[0025] In the following examples and comparative examples, the high-alkali metal content iron concentrate, steel slag, blast furnace slag and bentonite used were all commercially available raw materials.

[0026] The total mass content of K2O+Na2O in the high-alkali metal iron concentrate is 0.29%.

[0027] The SiO2 and CaO contents in the blast furnace slag are 40% and 35%, respectively, and the total content of other impurities is 25%.

[0028] The SiO2 and CaO contents in the steel slag are 13% and 45%, respectively.

[0029] Comparative Example 1:

[0030] Blast furnace slag, steel slag, and bentonite were mixed and ball-milled at a weight ratio of 40:52:8 for 30 minutes at 280 rpm. The ball-to-material mass ratio was 3:1. The ball-milling process resulted in approximately 98.5% of the material having a particle size of -38 μm, yielding a composite additive. This composite additive was then mixed with high-alkali metal content iron concentrate at a weight ratio of 0.4:99.6 and pelletized. The mixture was oxidatively roasted at a preheated temperature of 940℃ for 10 minutes, followed by roasting at 1250℃ for 20 minutes. The resulting pellets had a compressive strength of 1600 N and a reduction expansion index of 89.87%.

[0031] Compared to Example 2, under the same process conditions, the amount of composite additive added was reduced by approximately 1.0%, resulting in a decrease of 950 N in the strength of the pellets and an increase of 73.42% in the reduction expansion index. This is because a lower content of composite additive leads to a lower amount of liquid phase generated within the pellets, resulting in increased porosity and reduced compressive strength. Furthermore, a lower liquid phase content leads to the formation of more alkali metal ferrates within the pellets, promoting the growth of localized metallic iron during subsequent reduction processes, thus resulting in higher reduction expansion.

[0032] Comparative Example 2:

[0033] Blast furnace slag, steel slag, and bentonite were mixed and ball-milled in a weight ratio of 45:45:10. The mixture was then ball-milled at 150 rpm for 15 minutes, with a ball-to-material mass ratio of 3:1. The ball-milling continued until approximately 80% of the particles were of the -38 μm size, yielding a composite additive. This composite additive was then mixed with high-alkali metal content iron concentrate in a weight ratio of 1.0:99.0, and pelletized. The pellets were oxidatively roasted at a preheated temperature of 950℃ for 15 minutes, followed by roasting at 1260℃ for 20 minutes. The resulting pellets had a compressive strength of 1600 N and a reduction expansion index of 56.95%.

[0034] Compared to Example 3, with the same raw material addition and roasting conditions, the grinding speed and time of the composite additive were reduced, resulting in a 763 N decrease in the strength of the pellets and a 46.15% increase in the reduction expansion index. This is because when the additive particle size does not meet the requirements, large particles of additive are less likely to generate a suitable amount of liquid phase during the oxidative roasting process, thus affecting the distribution of alkali metals within the pellets. Consequently, this leads to a decrease in the compressive strength of the pellets and an increase in reduction expansion.

[0035] Comparative Example 3:

[0036] Blast furnace slag, steel slag, and bentonite were mixed in a weight ratio of 15:75:10 to obtain a mixture. This mixture was then ball-milled at 260 rpm for 40 min with a ball-to-material mass ratio of 3:1. The ball-milling was continued until approximately 98.7% of the particles met the -38 μm particle size requirement, yielding a composite additive. This composite additive was then mixed with high-alkali metal content iron concentrate in a weight ratio of 0.9:99.1, and the mixture was pelletized. The pellets were oxidatively roasted at a preheating temperature of 950℃ for 15 min, followed by roasting at 1260℃ for 20 min. The resulting pellets had a compressive strength of 2200 N and a reduction expansion index of 32.16%.

[0037] Compared to Example 1, under the same process conditions, the low proportion of blast furnace slag and high proportion of steel slag in the additives led to a decrease in the introduced SiO2 content, resulting in a 700N reduction in the strength of the pellets and an 18.75% increase in the reduction expansion index. Because the low SiO2 content in the additives resulted in a low liquid phase content within the pellets, the compressive strength of the pellets decreased and the reduction expansion increased.

[0038] Comparative Example 4:

[0039] Blast furnace slag, steel slag, and bentonite were mixed in a weight ratio of 35:55:10 to obtain a mixture. This mixture was then ball-milled at 260 rpm for 40 min with a ball-to-material mass ratio of 3:1. The ball-milling was continued until approximately 98.7% of the particles met the -38 μm particle size requirement, yielding a composite additive. This composite additive was then mixed with high-alkali metal content iron concentrate at a weight ratio of 1.1:98.1, and the mixture was pelletized. The pellets were oxidatively roasted at a preheating temperature of 950℃ for 10 min, followed by roasting at 1150℃ for 20 min. The resulting pellets had a compressive strength of 1689 N and a reduction expansion index of 63.16%.

[0040] Compared to Example 4, with the same raw material addition and roasting conditions, the roasting temperature was reduced by 120°C. This resulted in a decrease of 846 N in the strength of the pellets and an increase of 48.98% in the reduction expansion index. At the lower roasting temperature, the additives failed to react with the gangue minerals in the iron concentrate to form a liquid phase, causing the additives to remain primarily in the pellets as SiO2 and CaO. This resulted in insufficient liquid phase formation within the pellets, allowing the alkali metals to further react with iron oxides to form ferrates, leading to a decrease in pellet strength and an increase in reduction expansion.

[0041] Example 1:

[0042] Blast furnace slag, steel slag, and bentonite were mixed in a weight ratio of 35:50:15 to obtain a mixture. This mixture was then ball-milled at 260 rpm for 40 min with a ball-to-material mass ratio of 3:1. The ball-milling was continued until approximately 98.7% of the particles were of the -38 μm size, yielding a composite additive. This composite additive was then mixed with high-alkali metal content iron concentrate at a weight ratio of 0.9:99.1, and the mixture was pelletized. The pellets were oxidatively roasted at a preheating temperature of 950℃ for 15 min, followed by roasting at 1260℃ for 20 min. The resulting pellets had a compressive strength of 2900 N and a reduction expansion index of 13.41%.

[0043] Example 2:

[0044] Blast furnace slag, steel slag, and bentonite were mixed in a weight ratio of 40:52:8 to obtain a mixture. This mixture was then ball-milled at 280 rpm for 30 min with a ball-to-material mass ratio of 3:1 until approximately 98.5% of the particles were of the -38 μm size, yielding a composite additive. This composite additive was then mixed with high-alkali metal content iron concentrate in a weight ratio of 1.4:98.6, and pelletized. The pellets were oxidatively roasted at a preheating temperature of 940℃ for 10 min, followed by roasting at 1250℃ for 20 min. The resulting pellets had a compressive strength of 2500 N and a reduction expansion index of 16.45%.

[0045] Example 3:

[0046] Blast furnace slag, steel slag, and bentonite were mixed in a weight ratio of 45:45:10 to obtain a mixture. This mixture was then ball-milled at 270 rpm for 40 min with a ball-to-material mass ratio of 3:1. The ball-milling was continued until approximately 98.8% of the particles met the -38 μm particle size requirement, yielding a composite additive. This composite additive was then mixed with high-alkali metal content iron concentrate at a weight ratio of 1.0:99.0, and pelletized. The pellets were oxidatively roasted at a preheating temperature of 950℃ for 15 min, followed by roasting at 1260℃ for 20 min. The resulting pellets had a compressive strength of 2363 N and a reduction expansion index of 10.80%.

[0047] Example 4:

[0048] Blast furnace slag, steel slag, and bentonite were mixed in a weight ratio of 35:55:10 to obtain a mixture. This mixture was then ball-milled at 260 rpm for 40 min with a ball-to-material mass ratio of 3:1. The ball-milling was continued until approximately 98.7% of the particles were of the -38 μm size, yielding a composite additive. This composite additive was then mixed with high-alkali metal content iron concentrate at a weight ratio of 1.1:98.1, and pelletized. The pellets were oxidatively roasted at a preheating temperature of 950℃ for 10 min, followed by roasting at 1270℃ for 20 min. The resulting pellets had a compressive strength of 2535 N and a reduction expansion index of 14.18%.

Claims

1. A composite additive for improving the overall performance of iron ore oxide pellets with high alkali metal content, characterized in that: It is composed of steel slag, blast furnace slag and bentonite in a mass percentage ratio of 25~65%: 20~70%: 5~15%; The mass content of SiO2 and CaO in the blast furnace slag is 30-40% and 35-45% respectively, and the total mass content of other impurities is not higher than 25%. The mass contents of SiO2 and CaO in the steel slag are 10%~20% and 40%~50%, respectively. The high-alkali metal content iron ore oxide pellets are prepared from high-alkali metal content iron concentrate, wherein the mass content of alkali metal in the high-alkali metal content iron concentrate is 0.25~0.42%.

2. A method for preparing iron ore oxide pellets with high alkali metal content, characterized in that: The composite additive described in claim 1 is activated by ball milling and then mixed with iron concentrate with high alkali metal content to form pellets. The resulting green pellets are then dried, preheated, and roasted to obtain pellet ore.

3. The method for preparing high-alkali metal content iron ore oxide pellets according to claim 2, characterized in that: The conditions for ball mill activation are: rotation speed of 250~300 rpm, time of 30~60 min, and ball-to-material mass ratio of 2:1~5:

1.

4. The method for preparing high-alkali metal content iron ore oxide pellets according to claim 3, characterized in that: The ball milling activation is used to control the particle size of the composite additives to ensure that the mass percentage of -38 μm particles is above 98%.

5. A method for preparing high-alkali metal content iron ore oxide pellets according to claim 2, 3 or 4, characterized in that: The mass of the composite additive is 0.8 to 2.5% of the mass of the high alkali metal content iron concentrate.

6. The method for preparing high-alkali metal content iron ore oxide pellets according to claim 2, characterized in that: The preheating conditions are: temperature 920~950℃, time 8~12min.

7. The method for preparing high-alkali metal content iron ore oxide pellets according to claim 2, characterized in that: The calcination conditions are: temperature of 1250~1280℃ and time of 15~20min.

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