Preparation method of multi-layer composite pellet ore and pellet ore thereof

Abstract

The invention relates to a preparation method of a multi-layer composite pellet and the pellet. The preparation method comprises the following steps: mixing hematite powder, pellet return ore powder, boric sludge powder and dolomite powder to prepare pretreated hematite powder; uniformly mixing magnetite powder and bentonite to obtain a mixture; pelletizing the mixed material; spraying atomized water and conveying the pretreated hematite powder into the pelletizer to prepare green pellets; and the green pellets are dried, preheated and roasted to produce finished pellets, and screened unqualified products are made into pellet return ore powder to return to participate in batching again. According to the method, the purpose of producing the high-quality pellets with the hematite as the main raw material is achieved, the problem that in the pellet production process, due to the fact that the proportion of the hematite is too high, the pellet index becomes poor is solved, the pellet melting dripping performance is greatly improved, and the technical problem that magnetite concentrate is mainly used for pellet production is solved; and secondary resources such as pellet return charge and boric sludge are fully utilized, so that the production cost of the pellets is reduced.

Description

Technical Field

[0001] The invention belongs to the technical field of ironmaking production, relates to pellet production technology, and in particular to a preparation method of multi-layer composite pellets and the pellets. Background Art

[0002] As an important raw material for blast furnace ironmaking and direct reduced iron production, pellets are increasingly valued by the industry for their process characteristics and excellent product performance. It is the basic guarantee for steel companies to achieve high-quality production and green development. The use of refined materials for blast furnace ironmaking has always been the direction of research on blast furnace ironmaking in my country. Among them, the use of high-grade raw materials is the core of realizing refined material technology. As a high-quality raw material for blast furnace ironmaking, pellets can better meet the requirements of blast furnace refined materials than sintered ore. At the same time, the pelletizing process is more in line with the needs of the green development strategy of steel. As we all know, the sintering process is the main source of waste gas pollution in the long process of steel production. In addition to emitting conventional pollutants such as particulate matter, sulfur dioxide, and nitrogen oxides, it also emits characteristic pollutants such as dioxins, fluorides, and carbon monoxide. The energy consumption of the pelletizing process is only about 50% of that of the sintering process, and the energy consumption of the pelletizing process of advanced enterprises is only one-third of that of the sintering process. Due to the above advantages of pellets, developed countries have vigorously developed pellet production technology very early, and the proportion of pellets in ironmaking furnace charges has gradually increased, even reaching 100%.

[0003] At present, the production of domestic pellets mainly uses magnetite as raw material, but with the increasing consumption of high-quality magnetite concentrate resources, hematite, which has stable resources and relatively low prices, has received more and more attention from domestic pellet plants. However, the use of a large proportion of hematite to prepare pellets generally has problems such as poor raw ball quality, low strength of finished balls, and poor roasting performance. Pelletizing is the development direction of green ironmaking, and its proportion in my country's blast furnace feed will increase significantly in the future. At present, my country's pellet production process is mainly based on the "chain one return one ring" process to produce acidic pellets. Hematite concentrate and magnetite concentrate are the main raw materials for pelletizing. The proportion of hematite is different, and the roasting mechanism of pellets is different. The spheroidization and reducibility of magnetite meet the current technical requirements. Hematite microcrystals are connected during roasting, and microcrystal bonds are formed as the temperature rises, and recrystallization and polycrystalline growth occur.

[0004] The main raw material for pelletizing in my country is magnetite. However, with the continuous development of the steel industry, the reserves of magnetite in the world are decreasing, and the price of high-quality magnetite is rising. However, hematite is cheap. In order to reduce the production cost of pelletizing and make full use of hematite resources, the pelletizing process of adding some hematite to magnetite has become a hot topic. With the increase of hematite proportion, in order to meet the performance of pellets smelted in blast furnace, it is necessary to increase the preheating and roasting temperature of the pellets and extend the preheating and roasting time. Therefore, it is very necessary to develop a method that can increase the proportion of hematite used as a raw material in pellet production. Summary of the invention

[0005] The invention provides a method for preparing a multi-layer composite pellet and the pellet, which realizes the purpose of producing high-quality pellets with hematite as the main raw material, overcomes the problem of poor pellet index due to excessively high hematite ratio in the pellet production process, greatly improves the molten dripping performance of the pellet, solves the technical problem of pellet production mainly based on magnetite concentrate, broadens the raw material conditions for pellet production, makes full use of secondary resources such as pellet return material and boron mud, and directly reduces the production cost of the pellet.

[0006] In order to achieve the above object, the present invention adopts the following technical solutions:

[0007] A method for preparing multi-layer composite pellets, comprising the following steps:

[0008] 1) Hematite powder, pellet return powder, boron mud powder, and dolomite powder are fully mixed in a powerful mixer according to the weight percentage of 90% to 95%, 0 to 6%, 1% to 2%, and 1% to 3%, respectively, so that the hematite particles are fully in contact with the dolomite powder particles and the boron mud powder particles to obtain pretreated hematite powder;

[0009] 2) fully mixing magnetite powder and bentonite in a powerful mixer according to weight percentages of 99.5% to 99.7% and 0.3% to 0.5%, respectively, to obtain a mixture;

[0010] 3) conveying the mixed material obtained in step 2) to a disc pelletizing machine for pelletizing, spraying a mist of potassium permanganate aqueous solution during the pelletizing process to control the pellet size to be 2 to 3 mm, and controlling the spraying amount of the mist of potassium permanganate aqueous solution to control the mass percentage of the moisture content of the pellets to be 8.0% to 9.5%;

[0011] 4) spraying mist water and continuing to convey the pretreated hematite powder obtained in step 1) into the disc pelletizer to promote the pellets to continue to grow into green pellets;

[0012] 5) The obtained green pellets are dried, preheated and roasted to produce finished pellets. The unqualified products screened out from the finished pellets are screened out through a high pressure roller mill at a pressure of 2 to 7 MPa to prepare pellet return powder as raw material, which is returned to step 1) to participate in the batching again.

[0013] The mass percentage of the hematite powder particle size composition less than 0.044 mm is 100%; the specific surface area of ​​the hematite powder is 1800-2000cm 2 / g; the chemical composition mass fraction of the hematite powder is: TFe: 60% to 67%, FeO: 0.2% to 0.5%, CaO: 0.05% to 0.15%, MgO: 0.05% to 0.2%, SiO 2 :3%~5%,Al 2 O 3 : 0.5% to 2.5%, the remainder being impurity elements.

[0014] The mass percentage of the pelletized ore powder with a particle size composition less than 0.044 mm is 100%; the specific surface area of ​​the pelletized ore powder is 1900-2000 cm 2 / g; the chemical composition of the pelletized ore powder is as follows: TFe: 62% to 66%, FeO: 0.3% to 0.6%, CaO: 3% to 5%, MgO: 1.5% to 3%, SiO 2 :4%~6%,Al 2 O 3 : 0.2% to 1.2%, the remainder being impurity elements.

[0015] The borax powder is prepared from borax; the borax is waste residue produced in the process of producing borax, and the borax is dried at 110°C for 1 hour and then crushed to make the mass percentage of the particle size composition less than 0.044mm particle size 100%, thus obtaining the borax powder; the chemical composition mass fraction of the borax powder is: TFe: 5% to 20%, SiO 2 :20%~32%, MgO: 21%~46%, B 2 O 3 :1%~5%,Al 2 O 3 : 0.5% to 5%, the remainder being impurity elements.

[0016] The mass percentage of the particle size composition of the dolomite powder is less than 0.044 mm, and the mass percentage of the particle size less than 0.074 mm is 100%. The chemical composition of the dolomite powder is as follows: CaO: 30% to 40%, MgO: 25% to 35%, SiO 2 :3%~7%,Al 2 O 3 : 1% to 3%, the remainder is impurity elements.

[0017] The mass percentage of the magnetite powder particle size composition less than 0.044 mm is 75% to 90%, and the mass percentage of the particle size less than 0.074 mm is 100%; the specific surface area of ​​the magnetite powder is 1700 to 1900 cm 2 / g; the chemical composition mass fraction of the magnetite powder is: TFe: 67% to 73%, FeO: 28% to 30%, CaO: 0.1% to 0.3%, MgO: 0.2% to 0.5%, SiO 2 :1.5%~7.0%,Al 2 O 3 : 0.1% to 0.3%, the remainder being impurity elements.

[0018] The mass percentage of moisture content of the green pellets is 6.5% to 8.5%.

[0019] The mass percentage of the green pellets with a particle size of 7 to 11 mm is 70% to 85%, and the mass percentage of the particle size of 12 to 14 mm is 15% to 30%.

[0020] The pellet roasting system is as follows: drying at a temperature of 250-350°C for 3-5 minutes, preheating at a temperature of 850-950°C for 5-7 minutes, roasting at a temperature of 1220-1240°C for 5-8 minutes, roasting at a temperature of 1250-1270°C for 10-13 minutes, roasting at a temperature of 1320-1340°C for 3-7 minutes, and naturally cooling.

[0021] The pellets are two-layer composite pellets, the inner layer of which is a mixture of the following raw materials in weight percentage: hematite powder: 90% to 95%, pellet return ore powder: 0 to 6%, boron mud powder: 1% to 2%, dolomite powder: 1% to 3%, and the particle size of the inner layer is 2 to 3 mm; the outer layer of which is a mixture of the following raw materials in weight percentage: magnetite powder: 99.5% to 99.7%, bentonite: 0.3% to 0.5%; the mass percentage of the particle size composition of the pellets of 7 to 11 mm is 70% to 85%, and the mass percentage of the particle size composition of 12 to 14 mm is 15% to 30%.

[0022] Compared with the prior art, the present invention has the following beneficial effects:

[0023] 1. The present invention provides a method for preparing a multi-layer composite pellet and a pellet thereof, which realizes the addition of a large proportion of hematite into the pellet production, overcomes the phenomenon that the strength of the pellet deteriorates due to the excessively high proportion of hematite in the pellet production process, and the finished pellet compressive strength can reach 3215N / ball to 3563N / ball. The low-temperature reduction pulverization index and the reduction expansion rate of the pellet deteriorate. The low-temperature reduction pulverization index RDI+3.15mm of the pellet can reach 97.65% to 98.68%, and the reduction expansion rate of the pellet can reach 4.3% to 6.3%. The melting dripping performance of the pellet is greatly improved, and the softening interval T40-T10 of the soft melting dripping index can reach 36°C to 45°C, thereby providing a high-quality pellet for blast furnace production.

[0024] 2. The present invention processes the unqualified products screened out from the finished pellets to obtain pellet return ore powder, which is used as a raw material for pellet production. Since the pellet return ore powder has been fully oxidized and roasted, it is also a kind of return ore and can be regarded as a cheap hematite powder, which directly reduces the production cost of the pellets and achieves the purpose of recycling the return material.

[0025] 3. The borax powder used in the present invention is prepared from borax powder as waste material. By using it as a raw material to participate in the pretreatment process of hematite powder, not only can the additional addition of a binder be avoided to achieve the purpose of cost saving, but also the beneficial elements in the borax powder can be fully utilized to achieve the effect of improving the reduction expansion rate index of the finished pellet ore, and realize the comprehensive utilization of secondary resources to reduce its environmental pollution, with significant economic and social benefits. DETAILED DESCRIPTION

[0026] In order to make the purpose, technical solutions and advantages of the present invention more clearly understood, the specific implementation methods of the present invention are further described below in conjunction with embodiments. The following embodiments are used to specifically illustrate the contents of the present invention. These embodiments are only general descriptions of the contents of the present invention and do not limit the contents of the present invention.

[0027] Embodiment 1:

[0028] A multi-layer composite pellet and its preparation method, hematite powder, pellet return ore powder, boron mud powder, dolomite powder according to the weight percentage of 90%, 6%, 1%, 3% respectively, are fully mixed in a strong mixer, so that the hematite particles are fully in contact with the dolomite powder particles and the boron mud powder particles, and the pre-treated hematite powder is obtained. The mass percentage of the hematite powder particle size composition less than 0.044mm is 100%, and its surface area is 1800cm 2 / g, chemical composition (mass fraction): TFe: 60%, FeO: 0.2%, CaO: 0.05%, MgO: 0.05%, SiO 2: 5%, Al 2 O 3 : 0.5%, the remainder is impurity elements. The mass percentage of the pelletized ore powder with a particle size of less than 0.044 mm is 100%, and its specific surface area is 1900 cm 2 / g, chemical composition (mass fraction): TFe: 62%, FeO: 0.3%, CaO: 3%, MgO: 1.5%, SiO 2 : 4%, Al 2 O 3 Boron mud is the waste residue produced in the process of producing borax. Boron mud is dried at 110℃ for 1 hour and then crushed to make the mass percentage of particle size less than 0.044mm to be 100%, thus obtaining the required boron mud powder. The chemical composition (mass fraction) of boron mud powder is: TFe: 5%, SiO 2 :29%,MgO:46%,B 2 O 3 : 3%,Al 2 O 3 :0.5%, the rest is impurity elements. The mass percentage of dolomite powder with a particle size of less than 0.044mm is 90%, and the mass percentage of particle size less than 0.074mm is 100%. Its chemical composition (mass fraction) is: CaO: 30%, MgO: 25%, SiO 2 : 3%,Al 2 O 3 : 1%, the remainder is impurity elements.

[0029] The magnetite powder and bentonite are mixed in a powerful mixer according to the weight percentage of 99.5% and 0.5% respectively to obtain a mixture. The mass percentage of the magnetite powder particle size composition less than 0.044mm is 75%, the mass percentage of the particle size less than 0.074mm is 100%, and the surface area is 1700cm 2 / g, chemical composition (mass fraction): TFe: 67%, FeO: 28%, CaO: 0.1%, MgO: 0.2%, SiO 2 : 1.5%, Al 2 O 3 : 0.1%, the remainder is impurity elements.

[0030] The obtained mixture is conveyed to a disc pelletizer for pelletization. During the pelletization process, a mist of potassium permanganate aqueous solution is sprayed to control the pellet size to 2 mm. The spraying amount of the mist of potassium permanganate aqueous solution is controlled to control the mass percentage of the moisture content of the pellets to 8.0%. The mass percentage of the raw pellet size composition of 7-11 mm is 70%, and the mass percentage of the particle size composition of 12-14 mm is 30%. Spraying mist water and continuing to convey pre-treated hematite powder to the disc pelletizer promotes the pellets to continue to grow into raw pellets. The mass percentage of the moisture content of the raw pellets is 6.5%. The obtained green pellets are dried at 250°C for 5 minutes, preheated at 850°C for 7 minutes, roasted at 1220°C for 8 minutes, roasted at 1250°C for 13 minutes, roasted at 1320°C for 7 minutes, and naturally cooled to produce finished pellets. The unqualified products screened out from the finished pellets are passed through a high-pressure roller mill at a pressure of 2MPa to prepare pellet return ore powder as raw material for re-batch.

[0031] After the implementation of the present invention, the finished pellet has a compressive strength of 3215N / ball, a low-temperature reduction powdering index RDI+3.15mm of 97.65%, a reduction expansion rate of 6.3%, and a softening interval T40-T10 of a soft melting dripping index of 45°C. The pellet is a high-quality pellet that can meet the needs of blast furnace production.

[0032] Embodiment 2:

[0033] A multi-layer composite pellet and its preparation method, hematite powder, pellet return ore powder, boron mud powder, dolomite powder according to the weight percentage of 93%, 2%, 2%, 3% respectively, are fully mixed in a strong mixer, so that the hematite particles are fully in contact with the dolomite powder particles and the boron mud powder particles, and the pre-treated hematite powder is obtained. The mass percentage of the hematite powder particle size composition less than 0.044mm is 100%, and its surface area is 1900cm 2 / g, chemical composition (mass fraction): TFe: 65%, FeO: 0.4%, CaO: 0.11%, MgO: 0.17%, SiO 2 : 3.5%, Al 2 O 3 : 1.3%, the remainder is impurity elements. The mass percentage of the pelletized ore powder with a particle size of less than 0.044 mm is 100%, and its specific surface area is 1950 cm 2 / g, chemical composition (mass fraction): TFe: 64%, FeO: 0.45%, CaO: 4.1%, MgO: 2.3%, SiO 2 : 5.2%,Al 2 O 3:1.1%, the remainder is impurity elements. Boron mud is the waste residue produced in the process of producing borax. Boron mud is dried at 110℃ for 1 hour and then crushed to make its particle size composition less than 0.044mm particle size content of 100%, that is, the required borax mud powder is obtained. The chemical composition (mass fraction) of borax mud powder is: TFe: 16%, SiO 2 :40%,MgO:44%,B 2 O 3 : 4.5%, Al 2 O 3 : 3.5%, the rest is impurity elements. The mass percentage of dolomite powder with a particle size of less than 0.044mm is 90%, and the mass percentage of particle size less than 0.074mm is 100%. Its chemical composition (mass fraction) is: CaO: 36%, MgO: 33%, SiO 2 : 6%, Al 2 O 3 : 2.5%, the remainder is impurity elements.

[0034] The magnetite powder and bentonite are mixed in a powerful mixer according to the weight percentage of 99.6% and 0.4% respectively to obtain a mixture. The mass percentage of the magnetite powder particle size composition less than 0.044mm is 85%, the mass percentage of the particle size less than 0.074mm is 100%, and the surface area is 1800cm 2 / g, chemical composition (mass fraction): TFe: 71%, FeO: 29%, CaO: 0.2%, MgO: 0.4%, SiO 2 : 4.5%, Al 2 O 3 : 0.2%, the remainder is impurity elements.

[0035] The obtained mixture is conveyed to a disc pelletizer for pelletization. During the pelletization process, a mist of potassium permanganate aqueous solution is sprayed to control the pellet size to 2 mm. The spray amount of the mist of potassium permanganate aqueous solution is controlled to control the mass percentage of the moisture content of the pellets to 8.0%. The mass percentage of the raw pellet size composition of 7-11 mm is 80%, and the mass percentage of the particle size composition of 12-14 mm is 20%. Spraying mist water and continuing to convey pre-treated hematite powder to the disc pelletizer promotes the pellets to continue to grow into raw pellets. The mass percentage of the moisture content of the raw pellets is 7.5%. The obtained green pellets are dried at 300°C for 4 minutes, preheated at 900°C for 6 minutes, roasted at 1230°C for 6 minutes, roasted at 1260°C for 11 minutes, roasted at 1330°C for 4 minutes, and naturally cooled to produce finished pellets. The unqualified products screened out from the finished pellets are passed through a high-pressure roller mill at a pressure of 2MPa to prepare pellet return ore powder as raw material for re-batch.

[0036] After the implementation of the present invention, the finished pellet has a compressive strength of 3423N / ball, a low-temperature reduction powdering index RDI+3.15mm of 98.32%, a reduction expansion rate of 5.7%, and a softening interval T40-T10 of a soft melting dripping index of 42°C. The present invention is a high-quality pellet that can meet the needs of blast furnace production.

[0037] Embodiment 3:

[0038] A multi-layer composite pellet and its preparation method, hematite powder, pellet return ore powder, boron mud powder, dolomite powder according to the weight percentage of 95%, 3%, 1%, 1% respectively in a strong mixer, so that the hematite particles are fully in contact with the dolomite powder particles and the boron mud powder particles, and the pre-treated hematite powder is obtained. The mass percentage of the hematite powder particle size composition less than 0.044mm is 100%, and its surface area is 2000cm 2 / g, chemical composition (mass fraction): TFe: 67%, FeO: 0.5%, CaO: 0.15%, MgO: 0.2%, SiO 2 : 3%,Al 2 O 3 : 2.5%, the remainder is impurity elements. The mass percentage of the pelletized ore powder with a particle size of less than 0.044 mm is 100%, and its specific surface area is 2000 cm 2 / g, chemical composition (mass fraction): TFe: 66%, FeO: 0.6%, CaO: 5%, MgO: 3%, SiO 2 : 6%, Al 2 O 3:1.2%, the remainder is impurity elements. Boron mud is the waste residue produced in the process of producing borax. Boron mud is dried at 110℃ for 1 hour and then crushed to make its particle size composition less than 0.044mm particle size content of 100%, and the required borax mud powder is obtained. The chemical composition (mass fraction) of borax mud powder is: TFe: 20%, SiO 2 :25%,MgO:36%,B 2 O 3 : 5%, Al 2 O 3 : 5%, the remainder is impurity elements. The mass percentage of dolomite powder with a particle size of less than 0.044mm is 95%, and the mass percentage of particle size less than 0.074mm is 100%. Its chemical composition (mass fraction) is: CaO: 40%, MgO: 35%, SiO 2 :7%,Al 2 O 3 : 3%, the remainder is impurity elements.

[0039] The magnetite powder and bentonite are mixed in a strong mixer according to the weight percentage of 99.7% and 0.3% respectively to obtain a mixture. The mass percentage of the magnetite powder particle size composition less than 0.044mm is 90%, the mass percentage of the particle size less than 0.074mm is 100%, and the surface area is 1900cm 2 / g, chemical composition (mass fraction): TFe: 69.3%, FeO: 28.5%, CaO: 0.3%, MgO: 0.5%, SiO 2 : 1.5%, Al 2 O 3 : 0.3%, the remainder is impurity elements.

[0040] The obtained mixture is conveyed to a disc pelletizer for pelletization. During the pelletization process, a mist of potassium permanganate aqueous solution is sprayed to control the pellet size to 3 mm. The spraying amount of the mist of potassium permanganate aqueous solution is controlled to control the mass percentage of the moisture content of the pellets to 9.5%. The mass percentage of the raw pellet size composition of 7-11 mm is 85%, and the mass percentage of the particle size composition of 12-14 mm is 15%. Spraying mist water and continuing to convey pre-treated hematite powder to the disc pelletizer promotes the pellets to continue to grow into raw pellets. The mass percentage of the moisture content of the raw pellets is 8.5%. The obtained green pellets are dried at 350°C for 3 minutes, preheated at 950°C for 5 minutes, roasted at 1240°C for 3 minutes, roasted at 1270°C for 10 minutes, roasted at 1340°C for 3 minutes, and naturally cooled to produce finished pellets. The unqualified products screened out from the finished pellets are passed through a high-pressure roller mill at a pressure of 7MPa to prepare pellet return ore powder as raw material for re-batch.

[0041] After the implementation of the present invention, the finished pellet has a compressive strength of 3563N / ball, a low-temperature reduction powdering index RDI+3.15mm of 98.68%, a reduction expansion rate of 4.3%, and a softening range T40-T10 of a soft melting dripping index of 36°C. The present invention is a high-quality pellet that can meet the needs of blast furnace production.

Claims

1. A method for preparing multi-layer composite pellets, characterized in that: The method comprises the following steps: 1) fully mixing hematite powder, pellet return ore powder, boron mud powder, and dolomite powder according to the weight percentages of 90% to 95%, 0 to 6%, 1% to 2%, and 1% to 3%, respectively, to obtain pretreated hematite powder; 2) fully mixing magnetite powder and bentonite in weight percentages of 99.5% to 99.7% and 0.3% to 0.5%, respectively, to obtain a mixture; 3) conveying the mixed material obtained in step 2) to a pelletizing machine for pelletizing, spraying a mist of potassium permanganate aqueous solution during the pelletizing process, controlling the pellet size to be 2 to 3 mm, and controlling the spraying amount of the mist of potassium permanganate aqueous solution to control the mass percentage of the moisture content of the pellets to be 8.0% to 9.5%; 4) spraying mist water and continuing to convey the pretreated hematite powder obtained in step 1) into the pelletizer to promote the pellets to continue to grow into green pellets; 5) The obtained green pellets are dried, preheated and roasted to produce finished pellets. The unqualified products screened out from the finished pellets are screened out and pellet return powder is prepared under a pressure of 2 to 7 MPa as raw material and returned to step 1) to participate in the batching again.

2. The method for preparing a multi-layer composite pellet according to claim 1, characterized in that: The mass percentage of the hematite powder particle size composition less than 0.044 mm is 100%; the specific surface area of ​​the hematite powder is 1800-2000cm 2 / g; the chemical composition mass fraction of the hematite powder is: TFe: 60% ~ 67%, FeO: 0.2% ~ 0.5%, CaO: 0.05% ~ 0.15%, MgO: 0.05% ~ 0.2%, SiO2: 3% ~ 5%, Al2O3: 0.5% ~ 2.5%, and the remainder is impurity elements.

3. The method for preparing a multi-layer composite pellet according to claim 1, characterized in that: The mass percentage of the pelletized ore powder with a particle size composition less than 0.044 mm is 100%; the specific surface area of ​​the pelletized ore powder is 1900-2000 cm 2 / g; the chemical composition mass fraction of the pellet return ore powder is: TFe: 62% ~ 66%, FeO: 0.3% ~ 0.6%, CaO: 3% ~ 5%, MgO: 1.5% ~ 3%, SiO2: 4% ~ 6%, Al2O3: 0.2% ~ 1.2%, and the remainder is impurity elements.

4. The method for preparing a multi-layer composite pellet according to claim 1, characterized in that: The borax powder is prepared from borax; the borax is waste residue produced in the process of producing borax, and the borax is dried at 110°C and crushed so that the mass percentage of the particle size composition less than 0.044mm is 100%, thus obtaining the borax powder; the chemical composition mass fraction of the borax powder is: TFe: 5% to 20%, SiO2: 20% to 32%, MgO: 21% to 46%, B2O3: 1% to 5%, Al2O3: 0.5% to 5%, and the remainder is impurity elements.

5. The method for preparing a multi-layer composite pellet according to claim 1, characterized in that: The mass percentage of the dolomite powder with a particle size composition less than 0.044 mm is 90% to 95%, and the mass percentage of the particle size less than 0.074 mm is 100%; the chemical composition mass fraction of the dolomite powder is: CaO: 30% to 40%, MgO: 25% to 35%, SiO2: 3% to 7%, Al2O3: 1% to 3%, and the remainder is impurity elements.

6. The method for preparing a multi-layer composite pellet according to claim 1, characterized in that: The mass percentage of the magnetite powder particle size composition less than 0.044 mm is 75% to 90%, and the mass percentage of the particle size less than 0.074 mm is 100%; the specific surface area of ​​the magnetite powder is 1700 to 1900 cm 2 / g; the chemical composition mass fraction of the magnetite powder is: TFe: 67% ~ 73%, FeO: 28% ~ 30%, CaO: 0.1% ~ 0.3%, MgO: 0.2% ~ 0.5%, SiO2: 1.5% ~ 7.0%, Al2O3: 0.1% ~ 0.3%, and the remainder is impurity elements.

7. The method for preparing a multi-layer composite pellet according to claim 1, characterized in that: The mass percentage of moisture content of the green pellets is 6.5% to 8.5%.

8. The method for preparing multi-layer composite pellets according to claim 1, characterized in that: The mass percentage of the green pellets with a particle size of 7 to 11 mm is 70% to 85%, and the mass percentage of the particle size of 12 to 14 mm is 15% to 30%.

9. The method for preparing multi-layer composite pellets according to claim 1, characterized in that: The pellet roasting system is as follows: drying at a temperature of 250-350°C for 3-5 minutes, preheating at a temperature of 850-950°C for 5-7 minutes, roasting at a temperature of 1220-1240°C for 5-8 minutes, roasting at a temperature of 1250-1270°C for 10-13 minutes, roasting at a temperature of 1320-1340°C for 3-7 minutes, and naturally cooling.

10. A multi-layer composite pellet prepared by the method for preparing a multi-layer composite pellet according to any one of claims 1 to 9, characterized in that: The pellets are two-layer composite pellets, the inner layer of which is a mixture of the following raw materials in weight percentage: hematite powder: 90% to 95%, pellet return ore powder: 0 to 6%, boron mud powder: 1% to 2%, dolomite powder: 1% to 3%, and the particle size of the inner layer is 2 to 3 mm; the outer layer of which is a mixture of the following raw materials in weight percentage: magnetite powder: 99.5% to 99.7%, bentonite: 0.3% to 0.5%; the mass percentage of the particle size composition of the pellets of 7 to 11 mm is 70% to 85%, and the mass percentage of the particle size composition of 12 to 14 mm is 15% to 30%.