A process for the resource utilization of cathode carbon blocks of overhaul slag of electrolytic aluminum in electric furnace steelmaking

Abstract

The present invention discloses a process for the resource utilization of cathode carbon blocks of electrolytic aluminum overhaul slag in electric furnace steelmaking, which relates to two technical fields: the electric furnace steelmaking process and the resource utilization of cathode carbon blocks in electrolytic aluminum overhaul slag. By using an innovative process, the present invention eliminates the restrictive process problems in the resource utilization of cathode carbon blocks of electrolytic aluminum overhaul slag during the electric furnace smelting process, utilizes the fluorine-containing cathode carbon blocks in electrolytic aluminum overhaul slag as furnace charges for electric furnace steelmaking, and improves the quality of the crude steel water in the electric furnace while realizing resource utilization, thus developing a highly competitive process method for the high-value resource utilization of hazardous wastes from electrolytic aluminum.

Description

Technical Field

[0001] The present invention relates to a process for the resource utilization of electrolytic aluminum overhaul slag cathode carbon blocks in electric furnace steelmaking. Background Art

[0002] Modern electrolytic aluminum industry production adopts the cryolite-aluminum oxide molten salt electrolysis process. Molten cryolite is the solvent, aluminum oxide is the solute, a carbon body is used as the anode, and aluminum liquid is used as the cathode. After passing a strong direct current, electrochemical reactions occur at the two electrodes in the electrolytic cell at 950 °C to 970 °C, that is, electrolysis. The aluminum electrolysis process is carried out in an electrolytic cell. The electrolytic cell is composed of carbon materials and refractory materials. During the production of electrolytic aluminum, due to the penetration and corrosion of the high-temperature electrolyte on the lining material, the cathode carbon block is deformed and cracked, and the aluminum liquid and electrolyte in the cell penetrate downward along the cracks until they reach the bottom of the furnace, resulting in the inability of the electrolytic cell to produce normally and requiring a shutdown for major repairs. During major repairs, the carbon materials of the electrolytic cell are removed and discarded, which is the main component of the hazardous waste overhaul slag generated by the electrolytic aluminum industry.

[0003] By performing a phase analysis on the waste cathode carbon blocks of the aluminum electrolytic cell, the specific composition of the waste cathode is obtained: carbon (C), cryolite (Na3AlF6), aluminum oxide (α-Al2O3, β-Al2O3), sodium fluoride (NaF), calcium fluoride (CaF2), etc. The content and composition of the substances contained in different parts of the waste cathode carbon blocks of a certain factory are shown in the following table.

[0004] 。

[0005] Since there is easily soluble sodium fluoride and sodium fluoroaluminate in the cathode carbon block, they are the main factors polluting the environment. The small amount of cyanide formed in the carbon block is a highly toxic substance that can cause death.

[0006] Literature review: (1) Liang Wenqiang published a paper titled "Disposal and Recycling Plan for Aluminum Ash, Carbon Slag, and Overhaul Slag" in the fourth issue of "Gansu Metallurgy" in 2017. In the paper, it is stated that "During the process of treating waste cell linings, harmless treatment should be carried out on the anti-seepage materials, thermal insulation materials, and waste cathode materials in the waste cell linings. During the treatment process, curing agents and additives are added to convert the NaF contained in the anti-seepage materials and the thermal insulation materials at the bottom of the electrolytic cell into insoluble fluorinated salts, achieving the purpose of solidifying the fluoride ions in the waste thermal insulation materials. The cyanide in the waste cell linings is removed through the oxidation of oxidants and additives, which is also the purpose of adding oxidants."; (2) Li Hong published a paper titled "Pollution Prevention and Comprehensive Utilization of Aluminum Electrolysis Cell Overhaul Slag" in the 55th volume of the "Nonferrous Metals" magazine in 2003. In the paper, it is stated that "Aluminum electrolysis cell overhaul slag is a valuable material and should not be easily discarded because it contains up to 70% C and its calorific value is estimated to be 7000 - 12000 kJ / kg. The most valuable compound among them is fluorinated salts, accounting for approximately 30%. Therefore, some aluminum plants, such as the Ranshoffen Aluminum Plant in Austria and the Lister Aluminum Plant in the United States, leach the electrolyte with lye, and the leachate is used to synthesize cryolite, while the carbon is used as fuel." At the same time, in this paper, it is also stated that "The composition of cement is the CaO - SiO2 - Al2O3 - Fe2O3 system, which is a large - scale produced and inexpensive building material. The carbon in the electrolysis cell overhaul slag just serves as a supplementary fuel in cement production. The alkali metal fluorides can act as catalysts in the sintering reaction of the furnace charge, thus reducing the sintering temperature of the clinker and reducing the fuel consumption. After the waste carbon blocks are crushed and added to the cement clinker kiln, they can replace part of the fuel, saving energy. The fluorine contained in the carbon blocks can be used as a mineralizer to improve the firing conditions in the kiln. The fluorine forms solid CaF2 and enters the cement, without polluting the environment, achieving the purpose of comprehensive utilization. And there are cement plants everywhere, so there are relatively wide utilization conditions."; (3) Li Chaonan published a paper titled "Harmless Treatment of Aluminum Electrolysis Cell Overhaul Slag" in the ninth issue of the "Industrial Safety and Dust Prevention" magazine in 2000. In the middle of the paper, it is stated that "(1) The use of waste cathode carbon blocks in the production of alumina by the sintering method has been successful in Shandong Aluminum Plant. The waste carbon blocks are crushed to less than 25 mm and enter the alumina production process together with anthracite, which can improve the digestion conditions; (2) After the waste cathode carbon blocks are crushed and added to the cement clinker kiln to replace part of the fuel, due to the relatively large hardness of the carbon blocks, the crushing and grinding consume more. Unless encouragement measures are taken, the application is subject to certain limitations."

[0007] From the above content, it can be seen that the current treatment processes for waste cathode carbon blocks mainly include two categories: the rotary kiln process for producing cement and the wet flotation process. There is no literature reporting the application process for steelmaking production.

[0008] Electric arc furnace steelmaking uses scrap steel as the main raw material. Electric energy is used to heat and melt part of the scrap steel, and then oxygen is blown into the molten iron to assist in melting. After the electric energy and the chemical heat generated by blowing oxygen can meet the thermodynamic and kinetic conditions required for steelmaking, the harmful elements such as silicon, phosphorus, sulfur, and carbon in the molten steel are oxidized to complete the steelmaking process.

[0009] Modern electric arc furnace steelmaking enterprises basically all use ultra-high power electric arc furnaces and are equipped with enhanced oxygen supply technology to improve the productivity of electric arc furnace steelmaking. The carbon addition process in electric arc furnace steelmaking is the core process technology. Carbon addition in electric arc furnace steelmaking refers to adding raw materials with relatively high carbon content such as coke, pig iron, and pouring molten iron into the electric arc furnace during the melting period. After the electric arc furnace is melted clear (all the steel and iron raw materials added to the electric arc furnace are melted), the molten iron in the electric arc furnace bath contains a certain concentration of carbon element. Through the carbon-oxygen reaction occurring by blowing oxygen in the electric arc furnace, the chemical heat required for electric arc furnace steelmaking is provided. The generated CO / CO2 bubbles escape from the metal bath, achieving the stirring of the bath, removing harmful substances such as inclusions and gases in the scrap steel materials, accelerating the dephosphorization task of electric arc furnace steelmaking, and at the same time, the generated CO / CO2 gas is also an important gas source for forming foamed slag in electric arc furnace smelting. Therefore, the carbon addition process in electric arc furnace steelmaking directly affects the quality of the molten steel and the smelting cost, and is an important process content in electric arc furnace steelmaking.

[0010] Referring to the literature: (1) Yu Shanjun, Feng Wenquan, Su Xiongjie, etc. published a paper titled "Research on Carbon Addition Process Technology in Electric Arc Furnace" in the 4th issue of "Sichuan Metallurgy" magazine in 2012. The paper stated that "Under the circumstances of high price and tight supply of pig iron, Panzhihua Steel used graphite-like materials to replace part of the pig iron for carbon addition. This paper studied the effects of graphite-like carbon addition on the smelting process, power consumption per ton of steel, oxygen consumption per ton of steel, smelting time, and molten steel quality."; (2) Chang Zhenghua and Yang Guang, two authors, published a paper titled "Application of Graphite Ball Carbon Addition in Electric Arc Furnace Steelmaking" in the 6th issue of "Metallurgical Collections" magazine in 2010. The paper stated that "With the improvement of the efficiency of electric arc furnace steelmaking, the oxygen intensity used in the electric arc furnace increases. At the same time as the production efficiency increases, the FeO content in the electric arc furnace slag increases, and the direct result of the increased iron loss is the reduction of the metal recovery rate. Increasing the carbon addition amount can reduce the iron burning loss to a certain extent. Usually, pig iron is used for carbon addition. The carbon content of pig iron is about 4%. During the melting process of steel and iron materials, carbon is released, playing the role of carbon addition. However, using only pig iron for carbon addition will increase the production cost of steelmaking."

[0011] According to the above literature, there is currently no process method for resource utilization of electrolytic aluminum overhaul slag cathode carbon blocks as carbon addition materials in the electric arc furnace smelting process. Summary of the Invention

[0012] The object of the present invention is to provide a process for the resource utilization of cathode carbon blocks of electrolytic aluminum overhaul slag in electric furnace steelmaking, which can resourcefully utilize the cathode carbon blocks of electrolytic aluminum overhaul slag as carbon materials in the electric furnace smelting process, effectively reducing environmental pollution.

[0013] The technical solution adopted by the present invention is a process for the resource utilization of cathode carbon blocks of electrolytic aluminum overhaul slag in electric furnace steelmaking, which is implemented according to the following steps:

[0014] 1), Process the cathode carbon blocks of electrolytic aluminum overhaul slag to a particle size of 50 - 100 mm and a carbon content of more than 55%, and transport them to the spraying treatment area of the steel slag ladle for steelmaking;

[0015] 2), Dissolve the powder of the refined steel slag of molten steel in steelmaking in water to make a pulp to form an emulsion, and use the equipment for spraying the slag ladle to spray the powder of the refined steel slag on the surface of the cathode carbon blocks of overhaul slag, with a spraying thickness of more than 20 mm;

[0016] 3), After the spraying is completed and the sprayed coating on the surface of the carbon block is dried, transport the carbon block to the electric furnace production line for standby;

[0017] 4), When adding scrap steel in the electric furnace smelting, add the above carbon blocks and scrap steel into the electric furnace together, and follow the normal smelting process of the electric furnace; where the addition amount W of the carbon blocks is calculated according to the following formula

[0018] W = (M × Q) ÷ (α × β)

[0019] In the formula: M: Carbon addition amount of the electric furnace, %

[0020] Q: Addition amount of steel and iron raw materials in the electric furnace, t

[0021] α: Carbon content in the overhaul slag carbon block, %

[0022] β: Carbon yield in the carbon block, 85% - 90%.

[0023] The innovation points and innovative content of the present invention:

[0024] Most modern electric furnaces adopt the scrap steel preheating process, and only a few electric furnaces do not adopt the scrap steel preheating process. During the carbon addition process in the electric furnace, the addition method of the cathode carbon blocks of electrolytic aluminum overhaul slag is affected. For electric furnaces with scrap steel preheating, chemical reactions occur during the flue gas preheating process of the cathode carbon blocks of overhaul slag (such as the oxidation reaction of carbon and the high-temperature vaporization and overflow of sodium fluoride, etc.), which is harmful to the environment; for electric furnaces without scrap steel preheating, during the storage and feeding process of the cathode carbon blocks of overhaul slag, it is harmful to the operating environment (cyanide is precipitated when encountering water, and combustible and harmful gases are generated). Therefore, eliminating the above harmful factors is the prerequisite for the resource utilization of the cathode carbon blocks of overhaul slag;

[0025] The inventor found that Na3AlF6, α-Al2O3, β-Al2O3, NaF, and CaF2 in the cathode carbon blocks of overhaul slag are slag melting agents and fluxes for lime and dolomite, which can promote the melting of slag in the early stage, facilitate the formation of foamy slag in the early stage of electric furnace blowing, and optimize the process of electric furnace steelmaking;

[0026] The inventor studied the functions and roles of the cathode carbon blocks of electrolytic aluminum overhaul slag in the early stage of electric furnace blowing, as well as the melting characteristics of scrap steel in the early stage of electric furnace blowing. The cathode carbon blocks of overhaul slag were crushed to 50 - 100 mm and added to the electric furnace together with the scrap steel as a carbon material for electric furnace smelting for resource utilization. While achieving resource utilization, harmless conversion was completed, and at the same time, the process of electric furnace smelting was optimized;

[0027] The inventor found that the main component of the low-temperature pulverization part of molten steel refining slag is dicalcium silicate, which has the ability of hydration reaction and is a good thermal insulation spraying refractory material. Spraying it on the surface of the cathode carbon blocks of overhaul slag can play a role in heat insulation and preventing the leakage of harmful substances from the carbon blocks;

[0028] To solve the problem that after the carbon blocks are added to the electric furnace, the reaction between the carbon blocks and the residual steel slag in the electric furnace forms flame spraying and slag overflow, affecting the process of electric furnace smelting, the inventor sprayed molten steel refining slag powder on the surface of the electrolytic aluminum carbon blocks to passivate the carbon blocks. Utilizing the characteristic of poor thermal conductivity of steel slag, the problem that the carbon slag affects the process effect and operation after being added to the electric furnace with the scrap steel was eliminated, and the role of the carbon blocks as a heating material was exerted;

[0029] After the carbon blocks are sprayed with molten steel refining slag powder, it can prevent safety problems caused by chemical reactions of substances such as sodium salts and cyanides in the cathode carbon blocks of overhaul slag during stacking and scrap steel preheating;

[0030] The inventor discovered the mechanism of harmless conversion of harmful substances in the cathode carbon blocks of overhaul slag in the early stage of electric furnace blowing. After passivating the surface of the carbon blocks by spraying steel slag powder, the fluorides, sodium salts in the carbon blocks can quickly participate in the melting of lime and dolomite added to the electric furnace smelting, quickly form foamy slag, increase the efficiency of the oxygen blowing reaction, and optimize the process content of electric furnace carbon addition smelting;

[0031] The present invention utilizes the process principle of electric furnace smelting, and takes advantage of the characteristics that the phosphorus and sulfur content in the cathode carbon blocks of overhaul slag is low and various compounds in the carbon blocks are beneficial to the electric furnace smelting process. The cathode carbon blocks in the electrolytic aluminum overhaul slag are used as a carbon material for electric furnace steelmaking for resource utilization, optimizing the process of electric furnace steelmaking. While solving the problem of resource utilization of the cathode carbon blocks of overhaul slag, the process of electric furnace steelmaking is optimized, the cost of electric furnace steelmaking is reduced, and an operable process method is explored for the integrated development of short-process steelmaking and the resource utilization of hazardous waste.

[0032] The benefits of the present invention are as follows: the present invention utilizes overhaul slag cathode carbon blocks as resourceful carbon-matching materials for electric furnace steelmaking, thereby reducing the amount of phosphorus and sulfur brought into the electric furnace by the carbon-matching materials compared with the same period last year, increasing the chemical heat of electric furnace carbon-matching smelting, improving the quality of molten steel in the electric furnace, and reducing the cost of electric furnace smelting; various substances in the overhaul slag cathode carbon blocks can participate in the smelting of the electric furnace in the early stage of electric furnace smelting, reducing the consumption of slag auxiliary materials in the electric furnace smelting process, and realizing the maximum value utilization of the cathode carbon blocks of electrolytic aluminum overhaul slag; the process of the present invention can quickly and massively process the cathode carbon blocks of overhaul slag produced by electrolytic aluminum plants, which is of great significance for eliminating the environmental pollution problem caused by the storage of overhaul slag cathode carbon blocks. DETAILED DESCRIPTION

[0033] The implementation of the present invention is described with a 100-ton electric furnace production line:

[0034] A process for utilizing cathode carbon blocks from electrolytic aluminum overhaul slag in electric furnace steelmaking as a resource is implemented according to the following steps:

[0035] 1) Process the cathode carbon blocks of electrolytic aluminum overhaul slag into a particle size of 50-100mm and a carbon content greater than 55%, and transport them to the steelmaking slag tank spraying treatment area;

[0036] 2) Add water to dissolve the steelmaking slag powder to form an emulsion, and use the slag spraying tank equipment to spray the slag powder onto the surface of the overhaul slag cathode carbon block, with a spraying thickness of more than 20mm;

[0037] 3) After spraying is completed and the spray layer on the surface of the carbon block is dry, the carbon block is transported to the electric furnace production line for standby use;

[0038] 4) When adding scrap steel to the electric furnace for smelting, add the above carbon blocks and scrap steel to the electric furnace and carry out the smelting process according to the normal process of the electric furnace; the amount of carbon blocks added W is calculated according to the following formula:

[0039] W = (M × Q) ÷ (α × β)

[0040] Where: M: Carbon content of electric furnace, %

[0041] Q: Amount of steel raw materials added to the electric furnace, t

[0042] α: Carbon content in overhaul slag carbon blocks, %

[0043] β: The carbon recovery rate in the carbon block is 85% to 90%.

Claims

1. A process for the resource utilization of electrolytic aluminum overhaul slag cathode carbon blocks in electric furnace steelmaking, characterized in that It is implemented according to the following steps: 1), Process the cathode carbon block of the spent potlining of electrolytic aluminum to a particle size of 50 - 100 mm and a carbon content of more than 55%, and transport it to the spraying treatment area of the steelmaking slag ladle; 2), Dissolve the powder of the steelmaking refining slag in water to make a pulp to form an emulsion, and use the equipment for spraying the slag ladle to spray the powder of the steelmaking refining slag on the surface of the spent potlining cathode carbon block, with a spraying thickness of more than 20 mm; 3), After the spraying is completed and the sprayed coating on the surface of the carbon block is dried, transport the carbon block to the electric furnace production line for standby; 4), When adding scrap steel in the electric furnace smelting, add the above carbon block and scrap steel into the electric furnace together, and carry out according to the normal smelting process of the electric furnace; among them, the addition amount W of the carbon block is calculated according to the following formula: W = (M × Q) ÷ (α × β) In the formula: M: Carbon addition amount of the electric furnace, % Q: Addition amount of the steel and iron raw materials in the electric furnace, t α: Carbon content in the spent potlining cathode carbon block, % β: Recovery rate of carbon in the carbon block, 85% - 90%.