Large-section square ultrahigh-power graphite electrode and preparation method thereof

Abstract

The invention provides a large-section square ultrahigh-power graphite electrode and a preparation method thereof, and belongs to the technical field of graphite electrodes. The preparation method comprises the following steps: mixing and kneading needle coke and a binder, and carrying out extrusion molding on the obtained paste to obtain a green body; roasting the green body for the first time to obtain a first roasted blank; taking asphalt as an impregnant, and impregnating the first roasted blank to obtain an impregnated blank; roasting the impregnated blank for the second time to obtain a second roasted blank; and graphitizing the second roasted blank to obtain the large-section square ultrahigh-power graphite electrode. The large-section square ultrahigh-power graphite electrode prepared by the method is excellent in comprehensive performance and high in yield.

Description

A large-section square ultra-high power graphite electrode and its preparation method Technical Field

[0001] This invention relates to the field of graphite electrode technology, and in particular to a large-section square ultra-high power graphite electrode and its preparation method. Background Technology

[0002] Large-section ultra-high power graphite electrodes with cross-sectional dimensions of 700×700mm and above have a wide range of applications in various industries. For example, they can be used as conductive materials for the furnace head and tail of metallurgical resistance furnaces such as large silicon carbide furnaces and graphitization furnaces, and can also be used as blanks for making large graphite crucible containers.

[0003] Many factors affect the quality of large-section ultra-high power graphite electrodes. For example, the electrode yield of large-section ultra-high power graphite electrodes prepared by vibration molding is low, resulting in a large amount of waste, and the product performance is poor, affecting its use. Therefore, researching and developing a production process for large-section ultra-high power graphite electrodes is of great practical significance for improving their performance characteristics such as bulk density and conductivity, and reducing production costs. Summary of the Invention

[0004] The purpose of this invention is to provide a large-section square ultra-high power graphite electrode and its preparation method. The large-section square ultra-high power graphite electrode prepared by the method of this invention has excellent comprehensive performance and high yield.

[0005] To achieve the above-mentioned objectives, the present invention provides the following technical solution: The present invention provides a method for preparing a large-section square ultra-high power graphite electrode, comprising the following steps: (1) mixing needle coke with a binder, and extruding the resulting paste to obtain a green blank; the extrusion molding includes pre-pressing and extrusion in sequence; the extrusion conditions include: pressure of 6~10MPa, holding time of 6~8min, and extrusion speed of 1.3~1.5mm / s; (2) subjecting the green blank to a first calcination to obtain a first calcined blank; the first calcination... The calcination temperature is 1150~1250℃ and the holding time is 50~60h; (3) the first calcined blank is impregnated with asphalt as an impregnating agent to obtain an impregnated blank; (4) the impregnated blank is calcined for the second time to obtain a second calcined blank; the calcination temperature for the second calcination is 850~950℃ and the holding time is 75~80h; (5) the second calcined blank is graphitized to obtain the large cross-section square ultra-high power graphite electrode; the cross-sectional dimensions of the large cross-section square ultra-high power graphite electrode are ≥700×700mm.

[0006] Preferably, the specifications of the needle coke include: a coefficient of thermal expansion ≤ 1.05 × 10⁻⁶. -7At / ℃, the true density is 2.120~2.153 g / cm³. 3 The tap density is 0.905~0.964 g / cm³ under 8~14 mesh conditions. 3 The volatile matter content is ≤0.48wt%, the compressive strength is 25.0~41.2MPa, the sulfur content is 0.46~0.60wt%, the nitrogen content is 0.18~0.28wt%, the ash content is ≤0.1wt%, the moisture content is ≤0.1wt%, and the powder resistivity is 470~600μΩ·m; the needle coke is graded needle coke; by mass parts, the graded needle coke includes: 6~11 parts of first needle coke, 10~15 parts of second needle coke, 12~16 parts of third needle coke, 16~24 parts of fourth needle coke, and 6~14 parts of fifth needle coke; the particle size of the first needle coke is less than or equal to 0.5mm, and the particle size of the second needle coke is larger than 0.5mm. The particle size of the third needle coke is greater than 1 mm and less than or equal to 2 mm, the particle size of the fourth needle coke is greater than 2 mm and less than or equal to 4 mm, and the particle size of the fifth needle coke is greater than 4 mm and less than or equal to 8 mm; the mass of the binder is 17-26% of the mass of the needle coke; the binder is medium-temperature modified asphalt, and the indicators of the medium-temperature modified asphalt include: softening point of 95-105℃, toluene insoluble content of 24-34 wt%, quinoline insoluble content of 5-14 wt%, β-resin content ≥16 wt%, coking value ≥54%, ash content ≤0.30 wt%, and moisture content ≤4.0 wt%.

[0007] Preferably, the mixing conditions include: dry mixing time of 38-55 min, maximum dry mixing temperature of 125-140℃, wet mixing time of 45-70 min, wet mixing discharge temperature of 160-168℃, and cooling temperature of 125-135℃.

[0008] Preferably, the pre-compression conditions include: a pressure of 25~35MPa and a time of 7~9min.

[0009] Preferably, the first firing includes: raising the temperature from room temperature to the holding temperature for the first firing using a programmed temperature rise; the programmed temperature rise includes: raising the temperature from room temperature to 200°C over 45-50 hours, raising the temperature from 200°C to 300°C over 12-36 hours, raising the temperature from 300°C to 360°C over 24-36 hours, raising the temperature from 360°C to 420°C over 40-50 hours, raising the temperature from 420°C to 480°C over 32-72 hours, and raising the temperature from 480°C to 480°C over 72-80 hours. The temperature is increased from 510℃ to 530℃ over 78-82 hours, then increased from 530℃ to 610℃ over 78-82 hours, then increased from 610℃ to 690℃ over 48-80 hours, then increased from 690℃ to 750℃ over 45-50 hours, then increased from 750℃ to 840℃ over 30-36 hours, then increased from 840℃ to 950℃ over 32-36 hours, and finally increased from 950℃ to the holding temperature of the first firing over 25-30 hours.

[0010] Preferably, the impregnation process further includes preheating the first calcined blank at a temperature of 360-395°C for 9-16 hours; the impregnation conditions include an impregnating agent temperature of 165-185°C, a pressure of 1.5-2.2 MPa, and a pressurization time of 2-4 hours.

[0011] Preferably, the second firing further includes preheating the impregnated blank, wherein the maximum preheating temperature is 290~300℃, the heating rate from room temperature to the maximum preheating temperature is 15~20℃ / h, and the heating rate from the maximum preheating temperature to the holding temperature of the second firing is 8~20℃ / h.

[0012] Preferably, the graphitization power delivery curve includes: an initial power of 2500~5000kW, increasing at a rate of 240~350kW / h for 8~15h, increasing at a rate of 130~180kW / h for 16~28h, increasing at a rate of 70~100kW / h for 22~30h, increasing at a rate of 400~650kW / h for 3~6h, increasing at a rate of 1300~1600kW / h for 2.5~3.5h, increasing at a rate of 1457~1750kW / h for 3~4h, followed by maintaining a constant power for 35~45h.

[0013] Preferably, the graphitization process further includes machining, wherein the machining conditions include: a cutting speed of 80~120m / min, a feed rate of 0.1~0.3mm / r, a thread angle error of ±0.5°, and a pitch error of ±0.05mm.

[0014] This invention provides a large-section square ultra-high power graphite electrode prepared by the preparation method described in the above technical solution.

[0015] Beneficial effects: This invention targets large-section square ultra-high power graphite electrodes with cross-sectional dimensions of 700×700mm and above. By adopting extrusion molding and optimizing extrusion and calcination processes, the resulting large-section square ultra-high power graphite electrodes exhibit excellent comprehensive performance, meet the requirements of "Ultra-high power graphite electrodes YB / T 4090-2015", and have a high yield, facilitating mass production. Detailed Implementation

[0016] This invention provides a method for preparing a large-section square ultra-high power graphite electrode, comprising the following steps: (1) mixing needle coke with a binder, and extruding the resulting paste to obtain a green blank; the extrusion molding includes pre-pressing and extrusion in sequence; the extrusion conditions include: pressure of 6~10MPa, holding time of 6~8min, and extrusion speed of 1.3~1.5mm / s; (2) subjecting the green blank to a first calcination to obtain a first calcined blank; the calcination holding temperature for the first calcination is 115℃. (3) Using asphalt as an impregnating agent, the first calcined blank is impregnated to obtain an impregnated blank; (4) The impregnated blank is calcined a second time to obtain a second calcined blank; the holding temperature of the second calcination is 850~950℃ and the holding time is 75~80h; (5) The second calcined blank is graphitized to obtain the large cross-section square ultra-high power graphite electrode; the cross-sectional dimensions of the large cross-section square ultra-high power graphite electrode are ≥700×700mm.

[0017] This invention involves mixing needle coke with a binder to obtain a paste. This invention analyzes and selects raw materials for the preparation of large-section square ultra-high-power graphite electrodes. Through small-scale tests on various raw materials, needle coke with suitable true density, volatile matter, ash content, and sulfur content was selected, making it economical and reasonable. As one embodiment of this invention, the specifications of the needle coke include: coefficient of thermal expansion ≤ 1.05 × 10⁻⁶. -7 At / ℃, the true density is 2.120~2.153 g / cm³. 3 The tap density is 0.905~0.964 g / cm³ under 8~14 mesh conditions. 3 The volatile matter content is ≤0.48wt%, the compressive strength is 25.0~41.2MPa, the sulfur content is 0.46~0.60wt%, the nitrogen content is 0.18~0.28wt%, the ash content is ≤0.1wt%, the moisture content is ≤0.1wt%, and the powder resistivity is 470~600μΩ·m. Specifically, this invention can use different grades of needle coke as raw materials. The needle coke can include needle coke 1, needle coke 2, needle coke 3, or needle coke 4. In this embodiment of the invention, needle coke 3 is specifically used because it has a higher cost-performance ratio. The indicators of each grade of needle coke are shown in Table 1.

[0018] Table 1. Indicators of needle coke at different grades

[0019] In one embodiment of the present invention, the needle coke is graded needle coke; by mass fraction, the graded needle coke comprises: 6-11 parts of first needle coke, specifically 6, 8, 10, or 11 parts; 10-15 parts of second needle coke, specifically 10, 12, or 15 parts; 12-16 parts of third needle coke, specifically 12, 14, or 16 parts; 16-24 parts of fourth needle coke, specifically 16, 20, or 24 parts; and 6-14 parts of fifth needle coke, specifically 6, 10, or 14 parts. The particle size of the first needle coke is less than or equal to 0.5 mm, the particle size of the second needle coke is greater than 0.5 mm and less than or equal to 1 mm, the particle size of the third needle coke is greater than 1 mm and less than or equal to 2 mm, the particle size of the fourth needle coke is greater than 2 mm and less than or equal to 4 mm, and the particle size of the fifth needle coke is greater than 4 mm and less than or equal to 8 mm. This invention prepares raw materials of various particle sizes by crushing, grinding and sieving according to the raw material formula, configures the raw materials according to the formula through the batching system, and then puts them into the mixing system and adds a binder to prepare a paste.

[0020] In one embodiment of the present invention, the mass of the binder can be 17-26% of the mass of needle coke, specifically 17%, 19%, 22%, 24%, or 26%. In another embodiment, the binder can be medium-temperature modified asphalt, and the indicators of the medium-temperature modified asphalt include: softening point of 95-105℃, toluene-insoluble content of 24-34wt%, quinoline-insoluble content of 5-14wt%, β-resin content ≥16wt%, coking value ≥54%, ash content ≤0.30wt%, and moisture content ≤4.0wt%. Specifically, the present invention can use different grades of medium-temperature modified asphalt as binders. According to YB / T 5194-2024 "Modified Asphalt", the medium-temperature modified asphalt includes Grade I or Grade II medium-temperature modified asphalt. In this embodiment, Grade I medium-temperature modified asphalt is specifically used. The specific indicator requirements for each grade of medium-temperature modified asphalt are shown in Table 2.

[0021] Table 2. Indicators of medium-temperature modified asphalt of various grades

[0022] In one embodiment of the present invention, the mixing conditions include: dry mixing time of 38-55 min, specifically 38 min, 45 min, or 55 min; maximum dry mixing temperature of 125-140℃, specifically 125℃, 135℃, or 140℃; wet mixing time of 45-70 min, specifically 45 min, 60 min, or 70 min; wet mixing discharge temperature of 160-168℃, specifically 160℃, 165℃, or 168℃; and cooling temperature of 125-135℃, specifically 125℃, 130℃, or 135℃.

[0023] After obtaining the paste, the present invention extrudes and molds the paste to obtain a green body. In the present invention, the extrusion molding includes sequential pre-pressing and extrusion; the pre-pressing conditions include: pressure of 25~35MPa, time of 7~9min; the extrusion conditions include: pressure of 6~10MPa, holding time of 6~8min, and extrusion speed of 1.3~1.5mm / s. In one embodiment of the present invention, the extrusion molding is performed under a vacuum of -0.09MPa. The present invention transports the paste prepared on the batching production line to the material chamber of a 50MN hydraulic extruder. The hydraulic extruder is equipped with a square nozzle with a cross-sectional size of 725×725mm or larger. Through a vertical and horizontal pressing process, a green body is obtained. The appearance quality, bulk density, flexural strength, resistivity, etc., of the green body are tested. For any problems found during testing, the holding pressure and holding time of the hydraulic extruder are adjusted to ensure that all indicators of the green body are qualified, and the yield is ≥82%.

[0024] After obtaining the green blank, the present invention subjectes the green blank to a first firing to obtain a first fired blank. In the present invention, the holding temperature of the first firing is 1150~1250℃, specifically 1200℃; the holding time is 50~60h. As one embodiment of the present invention, the first firing includes: using a programmed temperature rise from room temperature to the holding temperature of the first firing; the programmed temperature rise includes: rising from room temperature to 200℃ over 45~50h, rising from 200℃ to 300℃ over 12~36h, rising from 300℃ to 360℃ over 24~36h, rising from 360℃ to 420℃ over 40~50h, rising from 420℃ to 480℃ over 32~72h, and then... The temperature is increased from 480℃ to 510℃, then increased from 510℃ to 530℃ over 78-82 hours, then increased from 530℃ to 610℃ over 78-82 hours, then increased from 610℃ to 690℃ over 48-80 hours, then increased from 690℃ to 750℃ over 45-50 hours, then increased from 750℃ to 840℃ over 30-36 hours, then increased from 840℃ to 950℃ over 32-36 hours, and finally increased from 950℃ to the holding temperature of the first firing over 25-30 hours. Specifically, in this embodiment of the invention, qualified green blanks are placed in a firing furnace and fired for the first time under a set firing curve to obtain the first fired blank. Its appearance quality (no cracks, deformation, missing corners, pitting, hollows, short lengths, or material adhesion defects) and bulk density (≥1.70 g / cm³) are then tested. 3 Compressive strength (≥25MPa), resistivity (≤1500μΩ·m), coefficient of linear expansion (≤3.5×10⁻⁶) -6 The invention addresses issues related to non-compliance of test indicators, such as carbon content (≥93wt%), ash content (≤0.5wt%), and volatile matter content (≤0.5wt%). By adjusting the roasting curve, defects are eliminated, increasing the first roasted billet pass rate to ≥86%. Through the first roasting, volatile matter is expelled, the binder is coked, the geometric shape is fixed, the billet resistivity is reduced, and the billet volume is further reduced. In one embodiment of the invention, the roasting furnace can include a ring-type roasting furnace or a box-type roasting furnace. The ring-type roasting furnace can be a covered ring-type furnace or an uncovered ring-type furnace; in this embodiment, an uncovered ring-type furnace is specifically used.

[0025] After obtaining the first calcined billet, this invention uses asphalt as an impregnating agent to impregnate the first calcined billet, obtaining an impregnated billet. In one embodiment of this invention, the first calcined billet is preheated before impregnation at a temperature of 360-395°C for 9-16 hours. In another embodiment of this invention, the impregnation conditions include an impregnating agent temperature of 165-185°C, a pressure of 1.5-2.2 MPa, and a pressurization time of 2-4 hours. This invention introduces a qualified first calcined billet into the impregnation process. Specifically, the first calcined billet is preheated in a preheating furnace to fully open the inner pores of the billet, and then placed in an impregnation tank. Under vacuum conditions, asphalt is injected, and under a certain pressure, the asphalt is fully impregnated into the inner pores of the billet to improve its density, thereby improving its bulk density, compressive strength, and other indicators. After impregnation, the asphalt in the tank is withdrawn, the impregnation tank is opened, and the impregnated blanks are taken out and cooled. The weight gain (≥15% for primary impregnation, ≥10% for secondary impregnation), impregnation depth (no unimpregnated sections), and other indicators are tested. For issues with unqualified indicators, measures such as extending the preheating time, increasing the impregnation pressure, and increasing the holding time are taken, ultimately improving the pass rate to over 95%. This invention reduces the porosity of the product through impregnation, increases its bulk density and mechanical strength, and improves its electrical and thermal conductivity. To effectively impregnate the liquid impregnating agent into the pores of the first calcined blank, it is preheated in a preheating furnace to approximately 400°C (the highest temperature at which oxidation does not begin) before impregnation. This heat opens the pores inside the first calcined blank, facilitating thorough impregnation by the impregnating agent.

[0026] After obtaining the impregnated blank, the present invention subjectes the impregnated blank to a second firing to obtain a second fired blank. In one embodiment of the present invention, the second firing further includes preheating the impregnated blank to a maximum preheating temperature of 290-300°C, with a heating rate of 15-20°C / h from room temperature to the maximum preheating temperature. In the present invention, the holding temperature for the second firing is 850-950°C, and the holding time is 75-80 hours; the heating rate from the maximum preheating temperature to the holding temperature for the second firing is 8-20°C / h. This invention involves placing qualified impregnated blanks into a roasting furnace for a second roasting process. This process removes the volatile components of the impregnating agent, asphalt, from the blank, causing the asphalt to carbonize and form a fixed carbon network. Simultaneously, it further shrinks the blank structure, increasing density and mechanical strength, resulting in a second-roasted blank. The blank's appearance quality (cracks, deformation, material adhesion, etc.), bulk density, compressive strength, and resistivity are then tested. For any non-compliant test indicators, defects are eliminated by adjusting the process curve of the second roasting, ultimately increasing the pass rate to over 95%. This invention uses a second roasting process to carbonize the asphalt embedded in the blank's pores, further improving the bulk density and mechanical strength of the product. In one embodiment of this invention, the heat source for the second roasting is natural gas and volatile components within the furnace.

[0027] After obtaining the second calcined blank, the present invention graphitizes the second calcined blank to obtain the large-section square ultra-high power graphite electrode. As one embodiment of the present invention, the power delivery curve of the graphitization includes: an initial power of 2500~5000kW, a rise rate of 240~350kW / h for 8~15h, a rise rate of 130~180kW / h for 16~28h, a rise rate of 70~100kW / h for 22~30h, a rise rate of 400~650kW / h for 3~6h, a rise rate of 1300~1600kW / h for 2.5~3.5h, a rise rate of 1457~1750kW / h for 3~4h, followed by constant power processing for 35~45h. This invention involves placing a qualified second-stage calcined blank into a graphitization furnace, surrounding it with filler material, and then graphitizing it according to a predetermined power supply curve. Specifically, the product is gradually heated to 2300-2800℃, held at that temperature for a certain period, and then the power is cut off for cooling. After cooling, the product is removed from the graphitization furnace, achieving the transformation of carbon into graphite. Simultaneously, impurities are purified, volatiles are removed, and the bulk density, high-temperature resistance, and oxidation resistance are improved, while resistivity is reduced, resulting in a graphitized blank. Its appearance quality (free from cracks, deformation, missing corners, pitting, hollow areas, short lengths, and material adhesion), bulk density, and resistivity are tested. For any non-compliant test indicators, defects are eliminated by adjusting the graphitization power supply and heating curve, achieving a yield rate ≥85%. In this invention, graphitization is a high-temperature heat treatment process that transforms the disordered overlapping of hexagonal carbon atoms in two-dimensional space into an ordered folding in three-dimensional space, resulting in a graphite structure. In one embodiment of the present invention, the graphitization furnace is a resistance furnace with a furnace core as the resistor, and its heat source is electricity. The electricity consumption of this process accounts for about 75% of the total electricity consumption of the entire process, and it is the main energy-consuming process.

[0028] As one embodiment of the present invention, the graphitization process further includes machining. The core of the machining described in this invention is to ensure the dimensional accuracy and surface quality of the electrode, ensuring that the electrode can accurately adapt to the requirements of steelmaking electric arc furnaces, etc., and that parameters strictly match the electrode specifications (such as diameter and length). Specifically, the technical requirements for machining include: 1. Dimensional accuracy: Key dimensional tolerances must be controlled within ±0.1mm, such as electrode diameter and thread hole depth, to avoid misalignment or poor conductivity due to dimensional deviations. 2. Surface quality: The surface roughness (Ra) after machining must be ≤6.3μm, free of burrs, chipped corners, and cracks, to prevent graphite dust or tip discharge during use. 3. Geometric tolerances: The overall straightness error of the electrode must be ≤0.5mm / m, and the end face flatness must be ≤0.1mm, ensuring tight fit when docking with other electrodes. The process parameters for machining include: 1. Machining accuracy parameters: Thread accuracy must conform to ISO 4042 standard, with the thread angle and pitch errors controlled within ±0.5° and ±0.05mm, respectively. 2. Cutting parameters: Adjusted according to graphite hardness (typically Shore hardness 35~65HS), cutting speed is 80~120m / min, feed rate is 0.1~0.3mm / r, to avoid chipping. 3. Inspection parameters: Sampling inspection of each batch, using a laser diameter gauge and coordinate measuring machine to verify dimensions, and ultrasonic testing to check for internal hidden cracks. This invention introduces qualified graphitized billets into the machining process, processing them into final products with precise dimensions, smooth surfaces, and regular structures according to customer requirements or finished product standards. The dimensional error, surface roughness, and appearance defects of the finished products are tested. For any unqualified indicators, defects are eliminated by adjusting the processing technology and machine tooling, increasing the finished product qualification rate to over 97%, meeting product design requirements. After machining, sampled verification of the finished products shows resistivity ≤7.0μΩ·m and bulk density ≥1.75g / cm³. 3 Compressive strength ≥20MPa.

[0029] This invention, through exploration and adjustment of raw materials, formulas, equipment parameters for each production process, and process parameters, has developed an economical and reasonable process for preparing large-section square ultra-high-power graphite electrodes based on extrusion molding. This process ensures production safety, high product quality, and high production efficiency. The large-section square ultra-high-power graphite electrodes produced using a 50MN vertical-horizontal press and needle coke as raw material exhibit significant differences in final product quality compared to similar products produced by vibration molding. Specifically: 1. The long axis of the aggregate particles in extrusion-molded products is mostly aligned along the extrusion direction, resulting in significant anisotropy in some physical and mechanical properties (such as resistivity, thermal conductivity, and coefficient of linear expansion). This is highly stable for conductive electrodes that conduct current along the long axis. Therefore, the graphite electrodes used in smelting electric furnaces are all produced using extrusion molding, whose physicochemical properties are far superior to those of other molded products.

[0030] 2. Vibration molding does not cause directional alignment of aggregate particles, resulting in less anisotropy in the finished product. The radial resistivity of the vibration molding electrode is low, leading to a higher elastic modulus in the product. However, the tensile strength of the final product is lower during horizontal vibration, making vibration molding unsuitable for producing blanks requiring high tensile and compressive strength. Vibration-molded products exhibit density inhomogeneity along the height direction (during vertical vibration) or along the radial direction (during horizontal vibration).

[0031] This invention provides a large-section square ultra-high power graphite electrode prepared by the preparation method described above. The cross-sectional dimensions of the large-section square ultra-high power graphite electrode of this invention are ≥700×700mm, and more preferably ≥825×825mm.

[0032] The technical solutions of this invention will be clearly and completely described below with reference to the embodiments thereof. Obviously, the described embodiments are only a part of the embodiments of this invention, and not all of them. All other embodiments obtained by those skilled in the art based on the embodiments of this invention without creative effort are within the scope of protection of this invention.

[0033] Examples 1-2 and Comparative Example 1 prepared a square ultra-high power graphite electrode with a cross section of 825×825mm based on extrusion molding, including the following steps: (1) Needle coke (specifically needle coke 3 as shown in Table 1) was crushed, ground and sieved to obtain needle coke of different grades, and then mixed with medium-temperature modified asphalt (specifically medium-temperature modified asphalt grade 1 as shown in Table 2). The resulting mixture was placed in a kneading device for kneading to obtain a paste. The paste was transported to the material chamber of a 50MN hydraulic extruder (specifically a vertical tamping press) for extrusion molding to obtain a green body. The formula of the mixture is shown in Table 3, and the operating conditions of kneading and extrusion molding and the indicators of the green body are shown in Table 4 (Example 1 is the best, and Example 2 is the second best).

[0034] Table 3. Formulation of the mixture (parts by weight)

[0035] Table 4. Operating conditions and green body parameters for kneading and extrusion molding

[0036] (2) The green blank is placed in the filling material in the roasting furnace and roasted for the first time under the condition of air isolation to obtain the first roasted blank; the roasting furnace is a ring-type roasting furnace without a cover, which uses natural gas to provide energy, and the filling material is metallurgical coke powder; the conditions of the first roasting and the yield are shown in Table 5 (among which Example 1 is the best).

[0037] Table 5. Conditions for the first roasting and yield.

[0038] (3) The first calcined blank is placed in a preheating furnace for preheating, and then placed in an impregnation tank. Vacuum is drawn, and liquid asphalt is added to the impregnation tank as an impregnating agent. The first calcined blank is impregnated so that the impregnating agent penetrates into the pores of the first calcined blank. During the impregnation process, liquid asphalt is continuously added and pressurized to increase the pressure inside the impregnation tank. After impregnation, the asphalt is extracted from the impregnation tank, the product is removed, and it is cooled with water to obtain an impregnated blank. The preheating and impregnation conditions and the index of the impregnated blank are shown in Table 6 (Among them, Example 1 is the best and Example 2 is the second best).

[0039] Table 6. Preheating and impregnation conditions and indexes of impregnated blanks.

[0040] (4) The impregnated blank is placed in the filling material in the calcining furnace and calcined for the second time under the condition of air isolation to obtain the second calcined blank; the calcining furnace is a ring-type calcining furnace without a cover, which uses natural gas to provide energy, and the filling material is metallurgical coke powder; the conditions of the second calcination and the indicators of the second calcined blank are shown in Table 7 (among which Example 1 is the best and Example 2 is the second best).

[0041] Table 7. Conditions for the second firing and the properties of the second-fired billet.

[0042] (5) The second calcined blank is placed in the filler of the graphitization furnace and graphitized under air-isolated conditions to obtain a graphitized blank; the graphitization furnace is a resistance furnace with the furnace core as the resistor, the heat source is electricity, and the filler is metallurgical coke powder; the graphitization conditions and graphitized blank indicators in Examples 1-2 and Comparative Example 1 are shown in Tables 8-10 (Example 1 is the best, while Example 2 has a large power consumption and Comparative Example 1 has a low yield).

[0043] Table 8. Graphitization conditions and indices of the graphitized preform in Example 1.

[0044] Table 9. Graphitization conditions and indices of the graphitized preform in Example 2.

[0045] Table 10. Graphitization conditions and indices of the graphitized preform in Comparative Example 1

[0046] (6) The graphitized blank is machined to obtain the 825×825mm square ultra-high power graphite electrode; the technical requirements and process parameters of the machining are shown in Table 11.

[0047] Table 11 Conditions and technical requirements for machining

[0048] This invention is based on the extrusion molding process to prepare large-section square ultra-high power graphite electrodes. Its core advantages are as follows: 1. Size and process adaptability: It can achieve continuous molding of cross-sections of 700×700mm and above, can stably control the dimensional tolerance of the green section (within ±2mm), and the longitudinal length can be set as needed (up to 4m), which fully matches the electrode specifications required by ultra-large electric arc furnaces.

[0049] 2. Controllable production efficiency and cost: The continuous extrusion molding process can produce an average daily output of 30 to 50 tons per unit (calculated based on a single electrode weight of about 1.5 tons), which can meet the batch order requirements of ultra-large electrodes; and the high unit weight of large-size products can reduce the cost of molds and energy, with the unit production cost only 8 to 12% higher than that of small-size extruded products.

[0050] 3. Finished product performance meets standards: Although slight transverse layering exists, by optimizing the extrusion speed and calcination curve, the bulk density of the finished product can be stably maintained at ≥1.72 g / cm³. 3 With a resistivity of ≤5.5μΩ·m, it fully meets the requirements for conductivity and bending resistance of ultra-large electrodes, and the transverse cracking rate can be controlled within 1% in industrial applications.

[0051] Vibration molding is unsuitable for fabricating large-section square ultra-high-power graphite electrodes, mainly because: 1. The process principle cannot be overcome: Vibration molding relies on the compaction of powder within the mold by the excitation force. For ultra-large molds of 700×700mm and above, the excitation force is difficult to uniformly transmit to the mold cavity, resulting in a density difference between the center and edge of the green body exceeding 0.08 g / cm³. 3 (far exceeding the acceptable standard of 0.05g / cm) 3 The finished product is extremely prone to cracking due to uneven density during the baking and graphitization processes, with a pass rate of less than 30%.

[0052] 2. Equipment and molds cannot achieve this: Currently, the largest vibration forming mold cross-section in the industry can only be 500×500mm, while there is no mature equipment to match the rigidity and excitation system power of 700×700mm molds; moreover, the mold manufacturing cost is as high as several million yuan, and the lifespan is only 1 / 5 of that of the extrusion die head, which is completely uneconomical.

[0053] 3. Extremely low production efficiency: The weight of a single 700×700mm green blank exceeds 2 tons, and the vibration compaction time is 40~60 minutes per mold. The average daily output is only 3~5 tons, which is only 1 / 10 of the extrusion molding process, and it is completely unable to meet the batch delivery requirements of ultra-large electrodes.

[0054] In summary, although the extrusion molding process has a slight shortcoming in lateral performance for large cross-section square ultra-high power graphite electrodes of 700×700mm and above, it can be completely avoided through process optimization; while the vibration molding process has no breakthrough potential in terms of size adaptation, yield rate and efficiency, and is not suitable for preparing large cross-section square ultra-high power graphite electrodes of 700×700mm and above.

[0055] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the principle of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A method for fabricating a large-section square ultra-high power graphite electrode, comprising the following steps: (1) Mix the needle coke with the binder, and then extrude the resulting paste to form a green body; The extrusion molding process includes sequential pre-compression and extrusion. The extrusion conditions include: a pressure of 6~10MPa, a holding time of 6~8min, and an extrusion speed of 1.3~1.5mm / s; (2) The green blank is firstly roasted to obtain a first roasted blank; the holding temperature of the first roasting is 1150~1250℃ and the holding time is 50~60h; (3) The first roasted blank is impregnated with asphalt as an impregnating agent to obtain an impregnated blank; (4) The impregnated blank is roasted a second time to obtain a second roasted blank; the holding temperature of the second roasting is 850~950℃ and the holding time is 75~80h; (5) The second roasted blank is graphitized to obtain the large cross-section square ultra-high power graphite electrode; the cross-sectional dimensions of the large cross-section square ultra-high power graphite electrode are ≥700×700mm.

2. The preparation method according to claim 1, characterized in that, The specifications for the needle coke include: coefficient of thermal expansion ≤ 1.05 × 10⁻⁶. -7 At / ℃, the true density is 2.120~2.153 g / cm³. 3 The tap density is 0.905~0.964 g / cm³ under 8~14 mesh conditions. 3 The volatile matter content is ≤0.48wt%, the compressive strength is 25.0~41.2MPa, the sulfur content is 0.46~0.60wt%, the nitrogen content is 0.18~0.28wt%, the ash content is ≤0.1wt%, the moisture content is ≤0.1wt%, and the powder resistivity is 470~600μΩ·m; the needle coke is graded needle coke; by mass parts, the graded needle coke includes: 6~11 parts of first needle coke, 10~15 parts of second needle coke, 12~16 parts of third needle coke, 16~24 parts of fourth needle coke, and 6~14 parts of fifth needle coke; the particle size of the first needle coke is less than or equal to 0.5mm, and the particle size of the second needle coke is larger than 0.5mm. The particle size of the third needle coke is greater than 1 mm and less than or equal to 2 mm, the particle size of the fourth needle coke is greater than 2 mm and less than or equal to 4 mm, and the particle size of the fifth needle coke is greater than 4 mm and less than or equal to 8 mm; the mass of the binder is 17-26% of the mass of the needle coke; the binder is medium-temperature modified asphalt, and the indicators of the medium-temperature modified asphalt include: softening point of 95-105℃, toluene insoluble content of 24-34 wt%, quinoline insoluble content of 5-14 wt%, β-resin content ≥16 wt%, coking value ≥54%, ash content ≤0.30 wt%, and moisture content ≤4.0 wt%.

3. The preparation method according to claim 1, characterized in that, The mixing conditions include: dry mixing time of 38-55 min, maximum dry mixing temperature of 125-140℃, wet mixing time of 45-70 min, wet mixing paste temperature of 160-168℃, and cooling temperature of 125-135℃.

4. The preparation method according to claim 1, characterized in that, The pre-compression conditions include: pressure of 25~35MPa and time of 7~9min.

5. The preparation method according to claim 1, characterized in that, The first firing includes: using a programmed temperature increase to raise the temperature from room temperature to the holding temperature for the first firing; the programmed temperature increase includes: raising the temperature from room temperature to 200℃ over 45-50 hours, raising the temperature from 200℃ to 300℃ over 12-36 hours, raising the temperature from 300℃ to 360℃ over 24-36 hours, raising the temperature from 360℃ to 420℃ over 40-50 hours, raising the temperature from 420℃ to 480℃ over 32-72 hours, and raising the temperature from 480℃ to 72-80 hours. The temperature is raised to 510℃, then increased to 530℃ over 78-82 hours, then to 610℃ over 78-82 hours, then to 690℃ over 48-80 hours, then to 750℃ over 45-50 hours, then to 840℃ over 30-36 hours, then to 950℃ over 32-36 hours, and finally to the holding temperature of the first firing over 25-30 hours.

6. The preparation method according to claim 1, characterized in that, The impregnation process includes preheating the first calcined blank at a temperature of 360-395°C for 9-16 hours. The impregnation conditions include an impregnating agent temperature of 165-185°C, a pressure of 1.5-2.2 MPa, and a pressurization time of 2-4 hours.

7. The preparation method according to claim 1, characterized in that, The second firing also includes preheating the impregnated blank, the maximum preheating temperature being 290~300℃, the heating rate from room temperature to the maximum preheating temperature being 15~20℃ / h; and the heating rate from the maximum preheating temperature to the holding temperature of the second firing being 8~20℃ / h.

8. The preparation method according to claim 1, characterized in that, The graphitization power delivery curve includes: an initial power of 2500~5000kW, increasing at a rate of 240~350kW / h for 8~15h, increasing at a rate of 130~180kW / h for 16~28h, increasing at a rate of 70~100kW / h for 22~30h, increasing at a rate of 400~650kW / h for 3~6h, increasing at a rate of 1300~1600kW / h for 2.5~3.5h, increasing at a rate of 1457~1750kW / h for 3~4h, followed by maintaining a constant power for 35~45h.

9. The preparation method according to claim 1, characterized in that, The graphitization process also includes machining, with the machining conditions including: a cutting speed of 80~120m / min, a feed rate of 0.1~0.3mm / r, a thread angle error of ±0.5°, and a pitch error of ±0.05mm.

10. A large-section square ultra-high power graphite electrode prepared by the preparation method according to any one of claims 1 to 9.

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