A process for producing cold-set pellets using agricultural straw and converter sludge

Abstract

This invention discloses a process for producing cold-set pellets using agricultural straw and converter sludge, relating to two fields: the production process of converter sludge pellets and the resource utilization of agricultural straw. This invention uses OG sludge generated during converter steelmaking as the main raw material, adding agricultural straw (limited to straw from gramineous crops) and steelmaking refining slag as auxiliary materials to produce cold-set pellets for steelmaking. This solves the problem of insufficient strength in converter sludge cold-set pellets and also marks the first time that agricultural biomass has been used in steelmaking production, highlighting the advantages of technological innovation across different fields.

Description

Technical Field

[0001] This invention relates to a process for producing cold-solidified pellets using agricultural straw and converter sludge. Background Technology

[0002] OG sludge from converters refers to the large amount of dust removal wastewater generated after the high-temperature flue gas from converters undergoes wet dust removal. After chemical dosing, sedimentation, concentration, and plate and frame filtration, the dust removal wastewater forms a slurry-like solid waste with a water content of 30% to 40%.

[0003] Currently, the most important resource utilization process for converter sludge is the production of chilled pellets, which are mainly used in the converter steelmaking process.

[0004] Converter sludge has a complex composition, with extremely fine particles, low content of polar substances, high water content, and components such as K, Na, and Fe2O3. These characteristics affect the pellet strength of sludge pellets (pellets used in steelmaking processes require a single pellet strength greater than 980N), and also affect the stability of the pellets during storage and transportation, causing cracking and pulverization during storage, transportation, and use. This phenomenon has three serious consequences: ① Pulverized pellets clog the outlet of the converter steelmaking silo, affecting the steelmaking process; ② Excessive dust is generated when adding material from the high-level silo, affecting on-site environmental protection; ③ When added with scrap steel from the scrap hopper, it can cause splashing accidents when molten iron is added.

[0005] To address the strength challenge of converter sludge pellets, some companies have adopted polymer binders to enhance the strength of chilled pellets. While this process improves pellet strength, the high cost of binders increases the overall cost of pellet production, limiting its use to small quantities. The shift in a well-known Northwest company's initial active use of chilled pellets to a de facto refusal demonstrates that improving the strength of steelmaking sludge pellets and resolving pellet pulverization are key technologies for the long-term, large-scale application of converter sludge pellets in steelmaking processes.

[0006] According to the literature (1), Wang Jun of Maanshan Iron and Steel Co., Ltd. published a paper entitled "Research Progress on Resource Utilization Technology of Converter Sludge" in the fourth issue of Metallurgical Power in 2025. The paper (page 90) states: "Lei Haohong et al. developed a cold solidification preparation process, which mixes water glass, lime, bentonite and converter sludge and presses them into 50mm pellets, and then obtains the finished product after drying." (2) Yan Genfa, Jin Yongming, Xing Xianbing and Zhu Peilun published a paper entitled "Discussion on Manufacturing Process of Converter Sludge Pellets" in the third issue of Steelmaking in 2010. The paper states: "The more reasonable resource utilization treatment of converter sludge is..." The recycling mode is a small recycling mode, that is, the converter sludge pellets are made and directly returned to the converter. The converter sludge pellet manufacturing process, especially with the large-scale conversion of converters, the medium-temperature roasting (700-900℃) pellet process has become the better process. "The content description; (3) Chen Jun published a paper entitled "Research on the Recycling Technology of Converter Steelmaking Sludge" in the third issue of Steelmaking magazine in 2014. The paper contains: "In order to realize the recycling of steelmaking sludge, it is proposed to use steelmaking sludge, quartz sand and manganese ore mixed in a certain proportion and pressed into pellets for converter slag making. The metallurgical effect of the new slag-making agent in steelmaking was investigated through industrial test." Based on the publicly available literature information above, there are currently no technological methods to improve the strength of cold-set pellets, nor are there any technological measures to prevent pellet pulverization.

[0007] Steel refining typically refers to the process of deoxidizing and alloying molten steel during the roughing process in a converter (electric furnace). Further adjustments are made to the steel's composition and temperature range, including deep deoxidation, desulfurization, degassing, and inclusion removal, to improve the steel's cleanliness and meet the performance requirements of the finished steel. These steelmaking processes are called ladle refining or secondary refining processes, or simply refining processes. Steel refining slag is a waste product generated during the steel refining process. The composition range of refining slag varies depending on the steel grade, but the content of the main substances is roughly similar. The composition of refining slag from a steelmaking plant in Northwest China using different deoxidation methods is shown in Table 1 below. ; After the steel refining process is completed, the molten steel in the ladle and the remaining molten steel in the ladle are called refining slag residue. They are poured into the slag pot together and become a type of steel slag.

[0008] The main mineral composition of steel refining slag is dicalcium silicate. As the temperature of the refining slag decreases, the phase transformation reaction of dicalcium silicate causes the refining slag to pulverize into powder, with an f-CaO content of about 15%, making it difficult to directly utilize as a resource.

[0009] With the development of intensive agriculture and the application of continuous cropping technology, the annual output of crop straw has exceeded 1 billion tons. At present, the comprehensive utilization of straw in China has gradually shifted from the initial simple burning treatment to diversified resource utilization, among which various application processes such as straw fertilizer, feed, energy, substrate and raw material have matured.

[0010] The literature (1) Wan Shoumei, Wang Tao, Zhao Jinyu and other authors published a paper entitled "The Status Quo and Countermeasures of Comprehensive Utilization of Straw Resources and Ecological Environment Protection" in the third issue of the journal "Comprehensive Utilization of Resources in China" in 2025. The paper contains the following: "Guizhou Province is rich in straw resources, but the utilization is inefficient and the commercialization is insufficient. Straw resources have a wide range of uses, but comprehensive utilization is limited by technical equipment, enterprise scale and storage costs. With the improvement of the living standards of rural residents and the improvement of the energy structure, coupled with the physical characteristics of straw, the difficulty of comprehensive utilization of straw resources has increased. Measures should be taken to reduce costs, increase income, strengthen publicity, improve the logistics system, and improve the awareness of rural residents. Legal protection should be improved, industrial development should be planned, technical equipment should be updated, processes should be innovated, and the energy structure should be optimized. Through technological innovation, policy support and industrial upgrading, the efficient and sustainable utilization of straw resources can be achieved, and agriculture can be developed in a more efficient and environmentally friendly direction, promoting green growth and contributing Chinese wisdom to global sustainable development."; (2) Bao Mingzhong published a paper entitled "Resource Conservation and Environmental Protection" in the first issue of the journal "Resource Conservation and Environmental Protection" in 2024. The paper, titled "Analysis of Comprehensive Utilization of Straw and Agricultural Ecological Environment Protection," states: "Comprehensive utilization of straw refers to the process of recycling and reusing straw as a resource, utilizing its potential energy and nutritional value to transform waste into resources. Comprehensive utilization of straw not only solves the problem of waste disposal but also provides multiple benefits for agricultural ecological environment protection. First, straw energy utilization technologies (straw power generation and straw heating) can replace the use of fossil fuels, reducing dependence on limited energy resources and lowering greenhouse gas emissions and air pollution. Second, turning straw into organic fertilizer can improve soil structure, increase soil fertility, promote crop growth, and reduce the use of chemical fertilizers, which is conducive to sustainable agricultural development. Furthermore, comprehensive utilization of straw can promote ecosystem stability, protect biodiversity, and provide ecosystem services. Therefore, comprehensive utilization of straw has important background and challenges, but by developing and promoting comprehensive utilization technologies, not only can resource recycling and environmental pollution reduction be achieved, but also the agricultural ecological environment can be protected and sustainable agricultural development promoted."

[0011] Based on the above publicly available literature, there are currently no reports on the application of agricultural straw in the production of cold-pressed steel pellets. Summary of the Invention

[0012] The purpose of this invention is to provide a process for producing cold-solidified pellets using agricultural straw and converter sludge. By adding solid waste produced by the steel plant and straw from the agricultural sector without using polymer binders, the problem of low strength and easy breakage and pulverization of converter sludge pellets can be solved, resulting in significant cost advantages. Furthermore, it can also make resource-efficient use of straw generated in the agricultural sector, which is beneficial to environmental protection.

[0013] The technical solution adopted in this invention is a process for producing cold-set pellets using agricultural straw and converter sludge, the process flow of which is as follows: S1. Following the normal process flow, the OG sludge from the converter is dewatered by filter pressing, and the moisture content is less than 15%. It is then transported to the dry powder pelletizing production line for use. S2. Transport the pulverized refining slag powder to the dry powder pelletizing production line for later use; S3. For agricultural gramineous crops: limited to corn, rice, wheat, millet, highland barley, and barley straw, the straw should be crushed until the fiber length is less than 50mm, and then transported to the dry powder pelletizing production line for later use. S4. Mix the materials from steps S1, S2, and S3 in a mass percentage ratio of 75%:15%:10% evenly. It is best to use a twin-shaft mixer. Do not add water. Process the mixture into cold-solidified pellets on a dry powder pelletizing machine. The pellet diameter should be controlled between 30 and 50 mm. S5. Spread the above pellets in the factory to dry naturally. After the surface of the pellets is dry, transport them to the converter production line for cold solidification and resource utilization.

[0014] The inventor discovered the following scientific phenomena through experimentation: 1. During the inactivation treatment of straw from gramineous crops using Ca(OH)2 (straw composting), it was found that the surface of ordinary straw fibers was smooth before treatment, but after treatment, a dense groove structure formed on the surface of the straw fibers, the surface roughness was significantly increased, the specific surface area increased, and the average diameter was reduced by 15% to 25%. 2. Using Ca(OH)2 to inactivate straw reduces the water absorption rate of straw fibers by 25% to 40% after treatment, while increasing tensile strength. 3. When the straw of gramineous crops is inactivated by Ca(OH)2, non-cellulose components such as lignin are dissolved from the straw. The straw fibers are oriented along the direction of force, which improves mechanical properties and reduces hydrophilicity, thereby enhancing the stability of the straw fibers. 4. Converter sludge contains a large amount of silicates and ferrates, as well as K and Na components. Therefore, converter sludge is generally strongly alkaline and has extremely fine particles, which is beneficial for the inactivation of agricultural straw. 5. Steelmaking refining slag contains a large amount of cementing components such as f-CaO, dicalcium silicate and calcium aluminate, and the heat of hydration reaction generated during its hydration process is relatively large.

[0015] Based on the above findings, the inventors used converter sludge as the main raw material, adding 15% straw and 5% to 10% refining slag, to produce cold-solidified pellets for converter steelmaking on a dry powder briquetting machine production line (replacing scrap steel raw materials and ferrite slag agents). This not only meets the process requirements for improving pellet strength, but also utilizes the C and H elements in the straw to reduce iron oxide in the pellets, reducing the heat expenditure of converter steelmaking and facilitating the smooth operation of the converter steelmaking process.

[0016] The innovative aspects of this invention are as follows: 1. The inventors discovered that when gramineous crop straw is added to converter steelmaking sludge and steelmaking refining slag, the K and Na components in the converter sludge and the f-CaO in the refining slag can inactivate the straw fibers. The treated straw fibers can increase the bonding strength between the fibers and sludge particles, which is beneficial to the rapid pelletizing of cold-fixed pellets. 2. By controlling the amount of straw added, the inventors utilize the interfacial friction between the straw fibers and the steelmaking sludge to play a bridging role, thereby weakening the deformation and crack expansion of the pellet skeleton under water conditions, thus reducing the disintegration and damage of the cold-solidified pellets in the early stage (before drying). 3. The inventors utilize the interwoven distribution characteristics of straw fibers in the pellets to divide most of the original large pores in the cold-solidified pellets. At the same time, the rough surface morphology of the fibers and the micro-interface gaps formed by the sludge particles promote the generation of a large number of tiny penetrating pores inside the pellets, which is conducive to the escape of water from the pellets by heat without causing the pellets to break. 4. K and Na in converter steelmaking sludge are alkaline activators that promote the hydration reaction of cementitious materials in refining slag, which can increase the hydration reaction rate of refining slag and is beneficial to the formation of early strength of steelmaking sludge pellets. 5. The inventors utilize the characteristic that refining slag contains a large amount of aluminate cement to make it a binder for cold-setting pellets. During the hydration reaction, it promotes the release of free water from the steelmaking sludge pellets by heating, thereby reducing the moisture content in the sludge. 6. The dicalcium silicate and calcium aluminate in the refining slag can undergo hydration reaction with the water in the converter sludge to form hydration reaction products. After further carbonation, the structure is dense, which can improve the strength of the pellets in the later stage and prevent the pellets from pulverizing (the volume expansion and breakage caused by the transformation of low-valent iron oxide to high-valent iron oxide in the pellets). 7. During the steelmaking process, plant fibers dissociate into elements such as C and H, which can undergo reduction reactions with iron oxide in pellets and converter slag. This can reduce the heat expenditure of converter steelmaking, which is conducive to the large-scale use of sludge pellets in converter steelmaking and reduces the consumption of steel materials in converter steelmaking.

[0017] The beneficial effects of this invention are as follows: This invention solves the problem of low strength and easy breakage of converter sludge pellets by adding solid waste produced by steel mills and straw from the agricultural sector without using polymer binders, resulting in significant cost advantages; currently, the steel industry lacks technology for the resource utilization of agricultural straw, and this project can solve the problem of straw generated in the agricultural sector, which is beneficial to environmental protection and also to increasing income in the agricultural sector. Detailed Implementation

[0018] The implementation of this invention is illustrated by specific embodiments: Taking a 65-ton converter production line as an example, the process flow for producing cold-set pellets using agricultural straw and converter sludge is as follows: S1. Following the normal process flow, the OG sludge from the converter is dewatered by filter pressing, and the moisture content is less than 15%. It is then transported to the dry powder pelletizing production line for use. S2. Transport the pulverized refining slag powder to the dry powder pelletizing production line for later use; S3. For agricultural gramineous crops: limited to corn, rice, wheat, millet, highland barley, and barley straw, the straw should be crushed until the fiber length is less than 50mm, and then transported to the dry powder pelletizing production line for later use. S4. Mix the materials from steps S1, S2, and S3 in a mass percentage ratio of 75%:15%:10% evenly. It is best to use a twin-shaft mixer. Do not add water. Process the mixture into cold-solidified pellets on a dry powder pelletizing machine. The pellet diameter should be controlled between 30 and 50 mm. S5. Spread the above pellets in the factory to dry naturally. After the surface of the pellets is dry, transport them to the converter production line for cold solidification and resource utilization.

Claims

1. A process for producing cold-set pellets using agricultural straw and converter sludge, characterized in that... The process flow is as follows: S1. Following the normal process flow, the OG sludge from the converter is dewatered by filter pressing, and the moisture content is less than 15%. It is then transported to the dry powder pelletizing production line for use. S2. Transport the pulverized refining slag powder to the dry powder pelletizing production line for later use; S3. For agricultural gramineous crops: limited to corn, rice, wheat, millet, highland barley, and barley straw, the straw should be crushed until the fiber length is less than 50mm, and then transported to the dry powder pelletizing production line for later use. S4. Mix the materials from steps S1, S2 and S3 above evenly in a mass percentage ratio of 75%:15%:10%, without adding water, and process them into cold solid pellets on a dry powder pelletizing machine. The pellet diameter is controlled at 30-50mm. S5. Spread the above pellets in the factory to dry naturally. After the surface of the pellets is dry, transport them to the converter production line for cold solidification and resource utilization.