Multilayer fluxed pellet as well as preparation method and application thereof
By designing a gradient basicity for multi-layer fluxed pellets, the problem of stress concentration inside the pellets was solved, the compressive strength and reducibility of the pellets were improved, and the synergistic optimization of metallurgical properties was achieved.
Patent Information
- Application Number
- CN202610141721.7
- Authority / Receiving Office
- CN · China
- Patent Type
- Applications(China)
- Current Assignee / Owner
- Filing Date
- 2026-02-02
- Publication Date
- 2026-05-19
AI Technical Summary
The existing flux-modified pellets have a uniform internal composition, which leads to stress concentration during the roasting process, affecting the pellet strength and reduction performance, and making it difficult to coordinate the balance of metallurgical properties.
The pellets are designed with a multi-layer structure, and are divided into three layers from the inside out. The binary basicity is controlled at 0.4-0.6, 0.7-0.9 and 0.9-1.1 respectively. The gradient basicity distribution is formed by step pelletizing and calcination to ensure the strength of the inner layer, the stability of the middle layer and the adhesion of the outer layer.
It significantly improves the compressive strength of pellets, reduces the reduction pulverization rate, optimizes metallurgical properties, and achieves a synergistic improvement in pellet strength, reducibility, and fusibility.
Abstract
Description
Technical Field
[0001] This invention relates to the field of ironmaking raw material technology, specifically to a multi-layer flux pellet, its preparation method, and its application. Background Technology
[0002] In ironmaking, iron ore pellets are an important furnace charge. To improve in-furnace metallurgical performance, fluxes such as limestone are often added to the pellet batch to create flux-treated pellets. These pellets reduce the need for separate flux additions in the blast furnace. However, conventional flux-treated pellets are usually made from a homogeneous mixture with a uniform internal basicity distribution. This homogeneous structure presents some limitations in actual smelting processes.
[0003] During the roasting stage, calcium oxide and ferric oxide readily form low-melting-point compounds such as calcium ferrite. The premature formation of these compounds may weaken the crystal bonds between mineral particles within the pellets, thus adversely affecting the final pellets' room-temperature strength. In the subsequent reduction process, especially at medium and low temperatures, the transformation of hematite to magnetite is accompanied by significant volume changes and lattice reconstruction, generating stress within the pellets. When the overall basicity of the pellets is uniform, this stress distribution may more easily lead to the formation and propagation of microcracks, increasing the tendency for the pellets to pulverize in the furnace. Furthermore, from a metallurgical performance balance perspective, increasing the basicity of the pellets generally improves their high-temperature softening behavior, but may also adversely affect their reduction reaction rate and low-temperature reduction stability. Existing technologies typically address these issues by adjusting the overall batching, introducing trace additives, or altering roasting conditions, but these methods do not change the fundamental characteristic of uniform component distribution within the pellets, thus limiting their effectiveness. Summary of the Invention
[0004] The technical problem to be solved by the present invention is to provide a multilayer flux-sensitive pellet, its preparation method and application, in view of the shortcomings mentioned in the background art.
[0005] To solve the above-mentioned technical problems, the present invention provides the following technical solution: a multilayer flux-dependent pellet, wherein the pellet comprises three layers of gradient alkalinity structure from the inside to the outside: (1) The innermost layer has a diameter of 2-5 mm and a binary alkalinity of 0.4-0.6; (2) The middle layer is wrapped around the innermost layer, so that the pellet diameter reaches 5-10 mm and the binary basicity is 0.7-0.9; (3) The outermost layer is wrapped around the middle layer, so that the overall diameter of the pellet reaches 8-16 mm and the binary alkalinity is 0.9-1.1.
[0006] A method for preparing multilayer flux-dependent pellets includes the following steps: a) Innermost layer pelletizing: Iron-containing raw materials, inner layer flux and inner layer binder are mixed, and the binary basicity of the mixture is controlled to be 0.4-0.6. Pelletizing is carried out to obtain innermost green pellets with a diameter of 2-5mm. b) Intermediate layer coating: The innermost green pellets obtained in step a) are used as mother pellets and fed into the pelletizing machine. During the rolling process, intermediate layer mixture with a binary basicity of 0.7-0.9 and water are added in batches. Pelletizing continues until the pellet diameter increases to 5-10 mm. c) Outermost layer coating: Using the pellets obtained in step b) as mother pellets, continue rolling them in the pelletizing machine, adding the outermost layer mixture with a binary basicity of 0.9-1.1 and water in batches, and continue pelletizing until the overall diameter of the pellets reaches 8-16 mm, to obtain green pellets with a three-layer structure. d) Calcination and solidification: The multi-layer green pellets obtained in step c) are dried, preheated and calcined, and then cooled to obtain multi-layer flux-forming pellets.
[0007] Furthermore, in steps a), b), and c), the iron-containing raw material is one or more of iron concentrate, rolled steel scale, and red mud; the flux is one or more of limestone powder, quicklime, and dolomite powder; and the binder is one or more of bentonite, carboxymethyl cellulose, and alginate.
[0008] Furthermore, in steps b) and c), the humidity of the pelleting process is controlled by atomizing water, with a water pressure of 0.2-0.5 MPa and a water droplet size of 50-200 μm.
[0009] Furthermore, in step d), the calcination process includes: drying temperature 200-400℃, preheating temperature 800-1000℃, calcination temperature 1200-1300℃, and calcination time 15-30 minutes.
[0010] Furthermore, the mass percentages of the innermost layer, the middle layer, and the outermost layer are 5%-15%, 25%-40%, and 45%-65%, respectively.
[0011] Application of a multi-layer flux-forming pellet in blast furnace ironmaking or direct reduction ironmaking.
[0012] With the above structure, the present invention has the following advantages: synergistic optimization of metallurgical performance: through gradient basicity design, the functions of different basicities are spatially separated and organically combined. The inner layer ensures a high-strength substrate, the outer layer optimizes reducibility and softening properties, and the middle layer ensures structural stability, thus achieving synergistic optimization of pellet strength, reducibility, and softening properties.
[0013] Significantly improves calcination strength: The low basicity of the inner layer prevents excessive melting at high temperatures, maintaining the integrity of the crystal framework; the high basicity of the outer layer produces a calcium ferrite liquid phase that acts as a strong binder. The combination of these two factors increases the compressive strength of the pellets by more than 10% compared to homogeneous pellets with the same average basicity.
[0014] Effective suppression of reduction pulverization: The gradient structure can guide and buffer the phase transformation stress during the reduction process, avoiding the concentrated release of stress in the homogeneous body, thereby significantly reducing the low-temperature reduction pulverization rate (RDI+3.15) of the pellets.
[0015] High feasibility of the process: This invention is based on conventional pellet production process and can be achieved simply by improving the ingredient formulation and pelletizing process. It does not require major equipment investment and is easy to promote and apply in existing pellet production lines. Detailed Implementation
[0016] The present invention will now be described in further detail.
[0017] A multilayer flux-dependent pellet. The pellet comprises three layers from the inside out, each with a different binary basicity value. The innermost layer has a diameter of 2 to 5 mm and its binary basicity is controlled within the range of 0.4 to 0.6. The middle layer covers the outermost layer, increasing the pellet diameter to 5 to 10 mm, and this layer has a binary basicity of 0.7 to 0.9. The outermost layer covers the middle layer, forming the final pellet with an overall diameter between 8 and 16 mm and a binary basicity of 0.9 to 1.1. This structure results in a stepwise increase in basicity from the core to the surface of the pellet.
[0018] Accordingly, the present invention also provides a method for preparing the above-mentioned multilayer pellets. This method includes a stepwise pelletizing and coating process. First, a mixture of iron-containing raw materials, flux, and binder is prepared with an alkalinity of 0.4 to 0.6 to form core green pellets with a diameter of 2 to 5 mm. Then, using this core green pellet as a mother pellet, a binary alkalinity intermediate layer mixture and water are added while rolling in a pelletizing device, causing the pellet diameter to grow to 5 to 10 mm. Next, using this pellet as a mother pellet, an outermost layer mixture with a binary alkalinity of 0.9 to 1.1 and water are added until the pellet reaches a final diameter of 8 to 16 mm, thus obtaining green pellets with a three-layer structure. Finally, the green pellets are dried, preheated, calcined, and cooled to obtain the finished pellets.
[0019] During pelleting, water can be added via atomization, with a water pressure ranging from 0.2 to 0.5 MPa and a droplet size of 50 to 200 micrometers, to facilitate uniform wetting of the material and layer formation. The calcination process can be carried out under specific temperature regimes, such as a drying temperature of 200 to 400 degrees Celsius, a preheating temperature of 800 to 1000 degrees Celsius, a calcination temperature of 1200 to 1300 degrees Celsius, and a calcination time of 15 to 30 minutes.
[0020] The iron-containing raw material may include one or more of iron concentrate, rolled steel scale, or red mud. The flux used may include one or more of limestone powder, quicklime, or dolomite powder. The binder may be one or more of bentonite, carboxymethyl cellulose, or alginate.
[0021] The mass ratio of each layer in the final pellet can be within a certain range: the innermost layer accounts for about 5% to 15%, the middle layer accounts for about 25% to 40%, and the outermost layer accounts for about 45% to 65%.
[0022] Compared to conventional homogeneous pellets with the same average basicity, the pellet structure provided by this invention may bring about the following changes: the lower basicity in the core region of the pellet helps to reduce the large-scale generation of low-melting-point liquid phase in this region during roasting, thereby supporting the formation of a solid skeleton mainly composed of recrystallized iron oxides; the higher basicity in the outer layer helps to form an appropriate amount of binder phase in the outer region of the pellet during roasting, which may improve the overall strength; during the reduction process, the varying basicity gradient from the inside to the outside may affect the generation and transmission path of phase transformation stress, or help to alleviate stress concentration; at the same time, the higher basicity in the outer layer may be beneficial to improving the reduction reaction kinetics and high-temperature performance of the pellet surface. This method of spatially distinguishing and combining different basicity functions provides a way to coordinate multiple metallurgical properties of pellets.
[0023] In specific implementation of this invention: Implementation Method 1 Product parameters: The target is to prepare multi-layer pellets with a total diameter of 12-14 mm, with the innermost layer (diameter 3-4 mm, binary basicity 0.5, mass percentage 10%), the middle layer (diameter 7-8 mm, binary basicity 0.8, mass percentage 35%), and the outermost layer (diameter 12-14 mm, binary basicity 1.0, mass percentage 55%). Raw material selection: The iron-containing raw material is iron concentrate, the flux is limestone powder, the binder is bentonite, and the auxiliary raw material is silica sand; Preparation steps: a) Innermost layer pelletizing: Calculate the ingredient ratio based on a binary alkalinity of 0.5, add 1% (by mass) bentonite, mix evenly, and then pelletize on a disc pelletizing machine to obtain core green pellets with a diameter of 3-4 mm. b) Intermediate layer coating: The core green pellets are fed into the pelletizing machine as mother pellets, and an intermediate layer mixture with a binary alkalinity of 0.8 (0.5% bentonite) is prepared. The mixture is continuously added during the operation of the pelletizing machine, and water is added at a pressure of 0.3 MPa (100 μm droplet size) until the pellet diameter reaches 7-8 mm. c) Outermost layer coating: Prepare an outermost layer mixture with a binary alkalinity of 1.0 (0.5% bentonite addition). Using the above pellets as the mother pellets, add the mixture using the same atomized water addition method until the pellet diameter reaches 12-14 mm to obtain multi-layer green pellets. d) Calcination and curing: The multi-layer green pellets are placed in a chain grate-rotary kiln system and processed according to the drying temperature of 300℃, the preheating temperature of 950℃, the calcination temperature of 1250℃, and the calcination time of 20 minutes. After cooling, the finished pellets are obtained. Performance testing: The finished pellets have a single pellet compressive strength of 2800N, a low-temperature reduction pulverization index of 75%, and a reduction degree of 68%, all of which are superior to conventional homogeneous pellets with an average basicity of 0.9 (compressive strength of 2350N, reduction pulverization index of 65%, and reduction degree of 65%).
[0024] Implementation Method 2 Product parameters: The target is to prepare multi-layer pellets with a total diameter of 15-16 mm, with the innermost layer (diameter 4-5 mm, binary basicity 0.45, mass percentage 12%), the middle layer (diameter 9-10 mm, binary basicity 0.85, mass percentage 38%), and the outermost layer (diameter 15-16 mm, binary basicity 0.95, mass percentage 50%). Raw material selection: The iron-containing raw material is a mixture of iron concentrate and rolled steel scale (mass ratio 1:1), the flux is dolomite powder, and the binder is carboxymethyl cellulose; Preparation steps: a) Innermost layer pelletizing: Prepare the raw materials according to the binary basicity of 0.45, add 0.8% carboxymethyl cellulose, mix evenly and then pelletize to obtain core green pellets with a diameter of 4-5mm; b) Intermediate layer coating: The core green pellets are fed into the pelletizing machine as mother pellets, and an intermediate layer mixture with a binary alkalinity of 0.85 (carboxymethyl cellulose addition of 0.5%) is prepared. Water is added by atomization at a pressure of 0.4 MPa (water droplet size of 150 μm), and the mixture is continuously added until the pellet diameter is 9-10 mm. c) Outermost layer coating: Prepare an outermost layer mixture with a binary alkalinity of 0.95 (carboxymethyl cellulose addition of 0.5%), add the mixture using the same atomization water addition method until the pellet diameter is 15-16 mm; d) Calcination and curing: Drying temperature 350℃, preheating temperature 900℃, calcination temperature 1280℃, calcination time 25 minutes, and the finished product is obtained after cooling; Performance testing: The finished pellets have a single pellet compressive strength of 2750N, a low-temperature reduction pulverization index of 73%, and a reduction degree of 67%, which is superior to conventional homogeneous pellets with the same average alkalinity.
[0025] The method described in this invention is based on the conventional pellet production process and achieves a multi-layer structure through step-by-step batching and segmented pelletizing, without requiring major modifications to existing production lines.
[0026] The present invention and its embodiments have been described above. This description is not restrictive, and the actual structure is not limited thereto. In conclusion, if those skilled in the art, inspired by this description, design similar structures and embodiments without departing from the spirit of the invention, such designs should fall within the scope of protection of this invention.
Claims
1. A multilayer flux-dependent pellet, characterized in that, The pellets consist of a three-layer gradient alkalinity structure from the inside out: (1) the innermost layer, with a diameter of 2-5 mm and a binary alkalinity of 0.4-0.6; (2) the middle layer, which wraps around the innermost layer, so that the pellet diameter reaches 5-10 mm and the binary alkalinity is 0.7-0.9; and (3) the outermost layer, which wraps around the middle layer, so that the overall diameter of the pellet reaches 8-16 mm and the binary alkalinity is 0.9-1.
1.
2. A method for preparing multilayer flux-dependent pellets as described in claim 1, characterized in that, Includes the following steps: a) Innermost layer pelletizing: Iron-containing raw materials, inner layer flux and inner layer binder are mixed, and the binary basicity of the mixture is controlled to be 0.4-0.
6. Pelletizing is carried out to obtain innermost green pellets with a diameter of 2-5mm. b) Intermediate layer coating: The innermost green pellets obtained in step a) are used as mother pellets and fed into the pelletizing machine. During the rolling process, intermediate layer mixture with a binary basicity of 0.7-0.9 and water are added in batches. Pelletizing continues until the pellet diameter increases to 5-10 mm. c) Outermost layer coating: Using the pellets obtained in step b) as mother pellets, continue rolling them in the pelletizing machine, adding the outermost layer mixture with a binary basicity of 0.9-1.1 and water in batches, and continue pelletizing until the overall diameter of the pellets reaches 8-16 mm, to obtain green pellets with a three-layer structure. d) Calcination and solidification: The multi-layer green pellets obtained in step c) are dried, preheated and calcined, and then cooled to obtain multi-layer flux-forming pellets.
3. The method for preparing multilayer flux-dependent pellets according to claim 2, characterized in that: In steps a), b), and c), the iron-containing raw material is one or more of iron concentrate, rolled steel scale, and red mud; the flux is one or more of limestone powder, quicklime, and dolomite powder; and the binder is one or more of bentonite, carboxymethyl cellulose, and alginate.
4. The method for preparing a multilayer flux-dependent pellet according to claim 2, characterized in that: In steps b) and c), the humidity of the pelleting process is controlled by atomizing water, with a water pressure of 0.2-0.5 MPa and a water droplet size of 50-200 μm.
5. The method for preparing a multilayer flux-dependent pellet according to claim 2, characterized in that: In step d), the calcination process includes: drying temperature 200-400℃, preheating temperature 800-1000℃, calcination temperature 1200-1300℃, and calcination time 15-30 minutes.
6. The method for preparing a multilayer flux-dependent pellet according to claim 2, characterized in that: The innermost, middle, and outermost layers account for 5%-15%, 25%-40%, and 45%-65% of the total mass, respectively.
7. The application of the multilayer flux-forming pellets as described in claim 1 in blast furnace ironmaking or direct reduction ironmaking.