A method for resource utilization of fly ash in steelmaking process

Abstract

The present invention relates to three technical fields: molten steel insulation technology, resource utilization technology of two hazardous wastes, namely, electrolytic aluminum hazardous waste carbon slag and fly ash generated by garbage incineration, and specifically, a method for resource utilization of fly ash in the steelmaking process. The present invention adopts an innovative process to produce fly ash and carbon slag into molten steel insulation covering agent, which is added to the top of the ladle after the molten steel refining process is completed, and plays the role of heat preservation and secondary oxidation prevention for the molten steel, realizing the resource utilization of fly ash and carbon slag in the molten steel refining process, and developing a highly competitive method for large-scale treatment of fly ash generated by garbage incineration in China's steel industry, highlighting the technical advantages of cross-innovation in different technical fields.

Description

Technical Field

[0001] The invention relates to a method for resource utilization of fly ash in a steelmaking process. Background Art

[0002] The solid residue after waste incineration accounts for 30%-35% of the total waste, of which bottom ash accounts for 25%-30% and fly ash accounts for 2%-5%. Waste incineration fly ash refers to the captured materials of the flue gas purification system of the waste incineration plant and the residues settled at the bottom of the flue and chimney. It contains a certain amount of dioxins, soluble heavy metals and salts. It belongs to HW18 hazardous waste (772-002-18 fly ash from the incineration of domestic waste) in the "National List of Hazardous Wastes" and needs to be pre-harmlessly treated before it can be safely landfilled. At present, there are studies on the resource utilization of fly ash in literature and reports. Literature review (1) Luo Renhong published a paper entitled “Overview of Resource Utilization of Fly Ash from Municipal Waste Incineration” in the 4th issue of Environmental Impact Assessment in 2019. The article states that “The resource utilization of fly ash from municipal waste incineration in cement, concrete, ceramic lightweight aggregate and building materials has been launched at home and abroad. Fly ash, as a new type of material, can be considered as a partial base material to replace road construction materials, but it still needs to obtain the support of relevant policies and further research is needed in the future”; (2) Du Jian published a paper entitled “Cement Solidification Technology for the Treatment of Fly Ash from Municipal Waste Incineration” in the 4th issue of Henan Building Materials in 2017. The article states that “The fly ash produced by municipal waste incineration was solidified with silicate cement, and the effect of fly ash content on solidification strength and heavy metal leaching performance was analyzed. The results show that after the fly ash is solidified with silicate cement, the compressive strength of the solidified body decreases with the increase of fly ash content; with the decrease of silicate cement content, the content of Pb and Cd Heavy metal ions such as carbon monoxide and other metals are more likely to leach out of the solidified body. In order to improve the solidification effect, the density of the cement paste needs to be further improved. " (3) Fan Yanling, Zheng Penghui, and Zhu Wen published a paper entitled "A Review of Harmless Treatment and Resource Management Technologies for Waste Incineration Fly Ash" in the 4th issue of "Resource Conservation and Environmental Protection" in 2020. The paper states that "my country is still in the initial stage of harmless treatment and resource utilization of waste incineration fly ash, and many technical difficulties have not yet been overcome, such as the large capacity increase of cement solidification, the incomplete destruction of dioxin-like substances by chemical stabilization method, the unstable and expensive effect of the melting method on heavy metals, the unstable effect of hydrothermal treatment technology, etc., and resource recycling has not been widely used."

[0003] According to the above-mentioned public documents, there is currently no process for utilizing steelmaking fly ash resources.

[0004] The refining of molten steel is to deoxidize, desulfurize and degas the molten steel through LF, LFV, VD, RH and other process methods, and these process contents are achieved through slag making. Each ton of molten steel refining produces 8-15kg of refining slag, which has a high basicity and is easy to solidify and form a shell after the refining process, causing part of the molten steel to be exposed, which can cause secondary oxidation of the molten steel and a decrease in temperature. Therefore, after the molten steel refining is completed, adding molten steel insulation covering agent to the ladle is a process method to reduce the temperature loss of molten steel and prevent secondary oxidation of molten steel;

[0005] References (1) In the article “Development of a New Type of Ladle Covering Agent” published in “Steelmaking” in 2006 (3), Xu Yongbin and Xu Bing stated that “the mass percentage of the main components of the new type of ladle insulation agent is CaO: 5-10; SiO2: 40-50; Al2O3<10; Fe2O3<3; fixed carbon<20. It is a typical acidic ladle covering agent with a binary basicity R (CaO / SiO2<1)”. Related content; (2) Yu Huacai and Li Shuqing published a paper entitled “Application and Research of Ladle Covering Agents” in the 4th issue of “Steelmaking” magazine in 2001. The paper stated that “the covering agent is acidic, and its alkalinity before use is about 0.7, while the residue in the ladle is alkaline, with an alkalinity of about 2.6-3.0. We tested the residue in the ladle after casting, and the alkalinity was about 1.5. This shows that the covering agent can react chemically with the slag, changing the properties of the slag and making it brittle, which is beneficial to the ladle slag turning, significantly improving the cleanliness of the ladle, and also helping to increase the life of the ladle.”

[0006] According to the above literature discussion, there is currently no precedent for the use of fly ash to produce molten steel insulation covering agents.

[0007] During the electrolytic aluminum production process, the anode carbon blocks and cathode carbon linings can peel off from the carbon blocks or linings under the thermodynamic conditions of electrochemistry and smelting, coupled with the erosion of molten aluminum, and enter the electrolyte to form carbon slag.

[0008] In the aluminum electrolysis production process, carbon slag will burn on the electrolyte surface, but in the case of excessive production (for self-baking cells), it is necessary to manually remove it from the cell in time to reduce the adverse effects of carbon slag on the electrolysis production process. The main components of the removed carbon slag are sodium aluminum fluoride, mainly ice stone (Na3AlF6), α-A1203 and carbon; the carbon content is about 40%, and the electrolyte fluoride is about 60%. The main components of a typical carbon slag are shown in Table 1 below:

[0009] ;

[0010] The components of carbon slag produced by electrolytic aluminum plants are not much different. The components of carbon slag from a plant in Xinjiang are shown in the following table:

[0011] .

[0012] Carbon slag is a hazardous waste, with the national hazardous waste catalog code 321-026-48, and is defined as "flammable skimming slag produced during the aluminum pyrometallurgical smelting process."

[0013] Literature review (1) Li Hong published a paper entitled “Flotation of Carbon Slag from Aluminum Electrolysis” in the 6th issue of Light Metals in 2002. The abstract of the paper states that “Carbon slag is a hazardous waste generated in the aluminum electrolysis production process. Carbon slag contains a large amount of fluoride salts. The flotation method is used to recycle the fluoride salts in carbon slag, which can not only reduce the loss of fluoride salts and improve the utilization rate of resources. It also avoids environmental pollution and has significant economic and social benefits.” (2) Liang Wenqiang published a paper entitled “Disposal and Recycling Plan for Aluminum Ash, Carbon Slag and Overhaul Slag” in the 4th issue of Gansu Metallurgy in 2017. The abstract states that “: The hazardous solid wastes such as aluminum ash, carbon slag, overhaul slag, etc. generated by Dongxing Aluminum Company of Jiugang Group in the process of aluminum electrolysis production every year are as follows: The solution is to sort and crush the hazardous solid waste generated by electrolytic aluminum, and then use flotation, chemical reaction and other methods to reuse the hazardous solid waste that can be recycled, and treat the hazardous solid waste that cannot be recycled harmlessly to eliminate the pollution to the ecological environment. After implementation, it will have certain economic benefits and significant social benefits. " (3) Liu Haixia published a paper entitled "Research on the Process of Regenerating Cryolite from Fluorine-Containing Waste Slag" in the 11th issue of "Inorganic Salt Industry" magazine in 2013. In the middle, there is "In the process of electrolytic aluminum production, in order to ensure normal production, the electrolyte in the electrolytic cell must be maintained at a reasonable height and cleanliness. The traditional method is to extract excess electrolyte during the electrolysis process and salvage the carbon blocks floating on the top of the electrolyte. These objects form fluorine-containing waste slag after condensation. It is estimated that 10 to 12 kg of fluorine-containing waste slag will be produced for every ton of aluminum produced, which contains more than 30% cryolite, about 1% calcium oxide, about 1% aluminum oxide, and the rest are impurities such as carbon particles, graphite carbon, aluminum carbide and silicon iron oxide. Research in recent years has found that these fluorine-containing waste slags can be obtained into cryolite crude products after preliminary manual screening, ball milling, flotation and other physical treatments, but due to the high impurity content, it still cannot meet the raw material requirements for electrolytic aluminum and urgently needs to be improved in quality. "

[0014] According to the above literature introduction, there is currently no process technology for producing molten steel insulation covering agent using electrolytic aluminum carbon slag. Summary of the invention

[0015] The purpose of the present invention is to provide a method for resource utilization of fly ash in the steelmaking process, which can effectively utilize the fly ash generated by the incineration of urban waste, and at the same time fully digest the electrolytic aluminum carbon slag, turning waste into treasure, and greatly improving the environmental pollution situation.

[0016] The technical solution adopted by the present invention is a method for resource utilization of fly ash in a steelmaking process, which is implemented according to the following steps:

[0017] 1) First, the hazardous waste carbon slag from the electrolytic aluminum plant is crushed into 0.5-1.0 mm using a crusher, and then transported to the dry powder pelletizing production line for standby use;

[0018] 2) Transport the fly ash generated by the municipal waste incineration plant to the dry powder pelletizing production line for standby use;

[0019] 3) After the fly ash and carbon slag are uniformly mixed in a mass percentage of 85:15, they are pressed into pellets with a particle size of 15-25 mm using a high-pressure dry powder pelletizing machine, and transported to the steelmaking production line of the steel plant for standby use;

[0020] 4) After the molten steel is refined, the pellets are added into the ladle top slag as a molten steel heat preservation covering agent, the amount of the pellets added is 0.5-1.5 kg per ton of molten steel, and then the molten steel is cast according to the normal process, and the rest of the smelting process remains unchanged.

[0021] The inventors have discovered the following scientific phenomena through research: 1. After the molten steel refining process is completed, the basicity of the refined slag is high and the temperature is above 1400°C, which is the best process platform for the harmless transformation of fly ash. In particular, under this temperature condition, the heavy metals in the fly ash can be reduced and enter the molten steel. Those that are not reduced can form alkali metal compounds, become a component of the refined slag, and remain in the steel slag. After the steel slag is recycled, it can undergo a hydration reaction to form a cementitious material, realize the mineralization and storage of harmful substances, and achieve the process purpose of harmless transformation; 2. The high alkalinity and high temperature of molten steel refining slag can cause irreversible decomposition reactions of dioxins in fly ash, and the chlorine in dioxins can exist in the slag in the form of chlorides, which improves the gelling activity of converter slag and helps eliminate the unstable factors of f-MgO in slag; 3. Fly ash has a small specific gravity and low alkalinity. Adding some acidic substances and insulation materials to fly ash can make fly ash an ideal thermal insulation covering agent after molten steel refining; 4. Fluoride, potassium salt and sodium salt in hazardous waste carbon slag of electrolytic aluminum can promote The molten steel thermal insulation covering agent quickly forms a liquid phase, and the fluoride in it is converted from a soluble fluoride to a sparingly soluble fluoride and a gelling material during the solidification process of the steel slag, thereby realizing the harmless transformation of the carbon slag; 5. The graphite in the carbon slag is a good deoxidation material and heat insulation material, which can promote the molten steel thermal insulation covering agent to form a molten layer on the upper part of the liquid phase layer and an ideal thermal insulation structure of a sintered layer on the upper part of the molten layer; 6. The C and Al in the carbon slag can react with the FeO+MnO remaining in the slag to reduce the probability of secondary oxidation of the molten steel;

[0022] Based on the above findings, the inventors used fly ash as the main raw material, added some carbon slag powder to the fly ash, and used a dry powder briquette machine to produce 15-25 mm pellets, which were used as a thermal insulation covering agent after molten steel refining, realizing the integrated development of the steelmaking industry's coordinated treatment of hazardous waste. The composition (mass percentage) of the thermal insulation covering agent for molten steel is shown in the following table:

[0023] ;

[0024] The innovative points of the present invention are as follows: 1. The inventor discovered the scientific phenomenon that high-temperature alkaline reducing slag can promote the harmless transformation of harmful heavy metal elements in fly ash, and that alkaline high-temperature steel slag can decompose dioxins and organic matter. The fly ash is used as the main raw material to produce a molten steel insulation covering agent. While utilizing the fly ash as a resource, the harmless transformation of the fly ash is completed, which is an industry-first technology; 2. Fly ash has a large specific surface area and a small specific gravity, and is a good thermal insulation material. By adding a portion of carbon slag powder, the fly ash is produced into a granular molten steel insulation covering agent. In the process of resource utilization, the heavy metal elements in the fly ash can be recovered, the alkalinity of the refined slag can be reduced, and the process performance of the refined slag resource utilization can be optimized; 3. Carbon slag powder is added, and potassium-sodium carbon thermal reaction is used to inhibit the metal cadmium, lead, and zinc from being reduced in the top slag to form metal vapor and escape; 4. The inventor replaces the fluorine element in fluorite with the fluorine element in the carbon slag, and uses the refined slag to achieve the harmless transformation of the hazardous waste carbon slag of electrolytic aluminum.

[0025] The beneficial contributions of the present invention are as follows: 1. The fly ash of hazardous waste is used to produce thermal insulation covering agents for molten steel in steelmaking. While utilizing the resources of steelmaking, the harmless transformation of fly ash and carbon slag is achieved, saving the treatment cost of hazardous waste and solving the land occupation contradiction caused by the fact that most of the fly ash is currently used for landfill, which is of great significance to the development of society; 2. Traditional thermal insulation covering agents for molten steel use carbonized rice husks, which have high production costs and generate a lot of dust during use. The thermal insulation covering agents for molten steel produced by the present invention have simple processing technology, less dust during use, and can optimize the on-site environment of steelmaking production. DETAILED DESCRIPTION

[0026] The embodiment of the present invention is described by taking the converter production line of China Baowu Group Yili Iron and Steel Co., Ltd. as an example.

[0027] A method for resource utilization of fly ash in a steelmaking process is implemented according to the following steps:

[0028] 1) First, the hazardous waste carbon slag from the electrolytic aluminum plant is crushed into 0.5-1.0 mm using a crusher, and then transported to the dry powder pelletizing production line for standby use;

[0029] 2) The fly ash generated by the Yining City Waste Incineration Plant is transported to the dry powder pelletizing production line for standby use;

[0030] 3) After the fly ash and carbon slag are uniformly mixed in a mass percentage of 85:15, they are pressed into pellets with a particle size of 15-25 mm using a high-pressure dry powder briquetting machine, and transported to the Yigang steelmaking production line for standby use;

[0031] 4) After the molten steel is refined, the pellets are added into the ladle top slag as a molten steel heat preservation covering agent, the amount of the pellets added is 0.5-1.5 kg per ton of molten steel, and then the molten steel is cast according to the normal process, and the rest of the smelting process remains unchanged.

Claims

1. A method for recycling fly ash in a steelmaking process, characterized in that Follow these steps to implement: 1) First, the hazardous waste carbon slag from the electrolytic aluminum plant is crushed into 0.5-1.0 mm using a crusher, and then transported to the dry powder pelletizing production line for standby use; 2) Transport the fly ash generated by the municipal waste incineration plant to the dry powder pelletizing production line for standby use; 3) After the fly ash and carbon slag are uniformly mixed in a mass percentage of 85:15, they are pressed into pellets with a particle size of 15-25 mm using a high-pressure dry powder pelletizing machine, and transported to the steelmaking production line of the steel plant for standby use; 4) After the molten steel is refined, the pellets are added into the ladle top slag as a molten steel heat preservation covering agent, the amount of the pellets added is 0.5-1.5 kg per ton of molten steel, and then the molten steel is cast according to the normal process, and the rest of the smelting process remains unchanged.