A process for converter steelmaking with coordinated use of cathode carbon blocks from electrolytic aluminum overhaul slag

Abstract

The present invention discloses a process for synergistically utilizing cathode carbon blocks from electrolytic aluminum overhaul slag in converter steelmaking, and involves two technical fields: a process for increasing the amount of scrap steel added by increasing chemical heat in converter steelmaking, and resource utilization of fluorine-containing carbon blocks in electrolytic aluminum overhaul slag, a hazardous waste. The present invention utilizes an innovative process to eliminate the problem that the addition of cathode carbon blocks from electrolytic aluminum overhaul slag to the converter affects the early smelting process of the converter, and then utilizes the graphitized carbon element in the electrolytic aluminum overhaul slag to increase the chemical heat of converter smelting, and utilizes the fluoride and alumina in the cathode carbon blocks from the overhaul slag to flux the converter slag, thereby reducing the slag heat required in the early stage of converter smelting. While utilizing the electrolytic aluminum overhaul slag as a resource, it increases the scrap steel ratio of converter steelmaking and reduces the smelting cost, thus developing a highly competitive process method for the high-value resource utilization of electrolytic aluminum hazardous waste.

Description

Technical Field

[0001] The invention relates to a process for converter steelmaking and coordinated use of cathode carbon blocks from electrolytic aluminum overhaul slag. Background Art

[0002] Modern electrolytic aluminum industry production adopts cryolite-alumina molten salt electrolysis process. Molten cryolite is the solvent, alumina is the solute, carbonite is the anode, and aluminum liquid is the cathode. After a strong direct current is passed, an electrochemical reaction is carried out at the two electrodes in the electrolytic cell at 950℃~970℃, that is, electrolysis. The aluminum electrolysis process is carried out in the electrolytic cell. The electrolytic cell is composed of carbon materials and refractory materials. In the electrolytic aluminum production process, 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 and reach the bottom of the furnace, causing the electrolytic cell to fail to produce normally and need to be stopped for overhaul. During the overhaul, the carbon material of the electrolytic cell is dismantled and discarded, which is the main component of the hazardous waste overhaul slag generated by the electrolytic aluminum industry.

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

[0004] ;

[0005] The presence of easily soluble sodium fluoride and sodium fluoroaluminate in the cathode carbon block is the main factor in polluting the environment. The small amount of cyanide formed in the carbon block is a lethal and highly toxic substance.

[0006] References (1) Liang Wenqiang published a paper titled “Disposal and Recycling of Aluminum Ash, Carbon Slag and Overhaul Slag” in the fourth issue of Gansu Metallurgy in 2017. In the paper, it is stated that “In the process of treating the waste tank lining, the anti-seepage material, thermal insulation material and waste cathode material in the waste tank lining must be treated harmlessly. During the treatment process, curing agents and additives are added to make the NaF contained in the anti-seepage material and the thermal insulation material at the bottom of the electrolytic cell Converted into insoluble fluoride salts. The purpose of solidifying the fluoride ions in the waste insulation material is achieved. The cyanide in the waste tank lining is removed by the oxidation of the oxidant and the additive, which is also the purpose of adding the oxidant. "Description of the content; (2) Li Hong published a paper entitled "Pollution Prevention and Comprehensive Utilization of Aluminum Electrolytic Cell Overhaul Slag" in the 55th volume of "Nonferrous Metals" magazine in 2003. The paper states that "Aluminum electrolytic cell overhaul slag is a valuable material that cannot be easily discarded because it contains up to 70% C and the calorific value is estimated to be 7000-12000 kJ / kg. The most valuable compound is fluoride salt, which accounts for about 30%. Therefore, some aluminum plants, such as the Austrian Ranshoffen Aluminum Plant and the American Lister Aluminum Plant, use alkali solution to dissolve the electrolyte, the leaching solution is used to synthesize cryolite, and the carbon is used as fuel." The paper also states that “cement is composed of CaO-SiO2-Al2O3-Fe2O3 and is a cheap building material produced in large quantities. The carbon in the slag from the overhaul of electrolytic cells is just the right supplementary fuel for cement production. The alkali metal fluoride in it can act as a catalyst in the sintering reaction of the charge, thereby reducing the sintering temperature of the clinker and reducing the amount of fuel used. The waste carbon blocks can be crushed and added to the cement clinker kiln to replace part of the fuel and save energy. The fluorine contained in the carbon blocks can be used as a mineralizer to improve the sintering conditions in the kiln. The fluorine generates solid CaF2 and enters the cement, which will not pollute the environment and achieve the purpose of comprehensive utilization. In addition, there are cement plants in various places, so there are relatively wide conditions for utilization.” (3) Li Chaonan published a paper entitled “Harmless Treatment of Aluminum Electrolytic Cell Overhaul Slag” in the 9th issue of the “Industrial Safety and Dust Control” magazine in 2000. In the middle, there is “(1) The use of waste cathode carbon blocks in sintering alumina production has been successful in Shandong Aluminum Plant. Crushing the waste carbon blocks to less than 25 mm and entering the alumina production process together with anthracite can improve the dissolution conditions; (2) Crushing the waste cathode carbon blocks into cement clinker kilns to replace part of the fuel. Due to the high hardness of the carbon blocks, the crushing and grinding process consumes a lot of energy. Unless incentives are taken, the application is subject to certain restrictions.

[0007] From the above description, it can be seen that the current treatment processes for waste carbon blocks mainly include two types: rotary kiln production of cement and wet flotation process. There is no literature report on the application of the process in steelmaking production.

[0008] As a recyclable metal material, steel products play an irreplaceable role in the modernization of the national economy and national defense. During the processing and use of steel materials, periodic scrap steel can be generated. As an energy-carrying resource, scrap steel re-enters the steel production process, which is of great significance to reducing costs, reducing energy consumption and controlling pollution. According to the literature, using 1t of scrap steel instead of 1t of molten iron in converter steelmaking can save about 559kg of standard coal. Every 1% increase in the scrap steel ratio of the converter can reduce the blowing loss by about 0.065%. Therefore, the scrap steel ratio has become an important factor affecting energy conservation and consumption reduction in the steel industry and improving the environment. Under the condition that the scrap steel market price has certain advantages, maximizing the scrap steel ratio of the converter is one of the effective means for enterprises to reduce costs and increase efficiency.

[0009] The process methods to increase the scrap steel ratio in converter smelting mainly utilize three main process methods: scrap steel preheating and baking technology, converter gas injection preheating scrap steel technology, and converter using heating material technology. Literature review (1) Feng Yaogang and Hu Liang published a paper entitled “Production Practice of Using Heat Enhancer to Increase Scrap Steel Ratio in Converter” in the 2nd issue of China Metallurgy in 2019. The paper states that “Combined with the molten iron conditions, the feasibility of using thermal coal as a heat enhancer to increase the scrap steel ratio in the converter was analyzed. At the same time, the effect of adding thermal coal on the converter smelting cycle, smelting operation and sulfur content of molten steel was studied. A process of first adding scrap steel and molten iron, and then adding thermal coal with the first batch of slag was formed. The scrap steel ratio of a 50t converter was successfully increased by 6% to 7% in a short period of time. Practice has proved that it is feasible to use thermal coal as a heat enhancer to increase the scrap steel ratio in the converter. At the same time, the economic and social benefits brought by this process are significant, which is conducive to alleviating the huge cost and environmental pressure of the current steel enterprises”; (2) Yang Wenyuan, Zhang Xiangui, Yang Yong, et al. published a paper entitled “A Review of Research on the Utilization of Scrap Steel in Converter Steelmaking” in China Metallurgy in 2012. The paper states that “The Second Steelmaking Plant of Anshan Iron and Steel Company has adopted a new method to improve the scrap steel ratio of the converter and to improve the scrap steel ratio of the converter”. According to the specific conditions of the molten iron composition, temperature, charging method, and smelting steel type of the three steelmaking plants, measures such as reducing heat loss, lowering the tapping temperature, and using temperature-raising agents (silicon carbide, coke blocks) to preheat scrap steel were proposed. (3) Wei Chunxin, Fei Peng, Li Zhen, et al. published a paper entitled "Analysis and Countermeasures of the Factors Affecting the Scrap Steel Ratio of the Converter" in the 2nd issue of Angang Technology in 2006. The paper stated that "the main factors affecting the scrap steel ratio of the converter are molten iron conditions, smelting steel type, production organization compactness, and smelting operation level. Under the condition of a certain smelting steel type, improving the scrap steel ratio of the converter should mainly start from the following two aspects: on the one hand, providing an external heat source, that is, adding temperature-raising agents or preheating scrap steel; on the other hand, improving the operation level, that is, improving the compactness of production organization, increasing the slag retention operation ratio, and making rational use of existing resources. After the above measures were adopted by the North District of Angang Second Steelmaking Plant, the scrap steel ratio of the converter increased from 7.6% to about 12%.

[0010] According to the above literature, there is currently no process for using electrolytic aluminum overhaul slag to increase the scrap steel ratio of the converter. Summary of the invention

[0011] The purpose of the present invention is to provide a process for converter steelmaking that utilizes cathode carbon blocks from electrolytic aluminum overhaul slag in a coordinated manner, utilizing fluoride and alumina in the cathode carbon blocks from the overhaul slag to flux the converter slag, thereby reducing the slagging heat required in the early stage of converter smelting, and while utilizing the electrolytic aluminum overhaul slag as a resource, increasing the scrap steel ratio of converter steelmaking and reducing smelting costs.

[0012] The object of the present invention is achieved by a process for converter steelmaking in cooperation with the cathode carbon block of electrolytic aluminum overhaul slag, which is implemented according to the following steps:

[0013] 1) The overhaul slag cathode carbon blocks are processed to a particle size of 40-60 mm, with a carbon content greater than 55% by mass, and transported to the steelmaking slag tank spraying treatment area for standby use;

[0014] 2) Add water to dissolve the steelmaking slag powder to make slurry, and use the equipment of spraying slag pot to spray the slurry on the surface of the carbon block in step 1), and the thickness of the surface spray layer formed by spraying is greater than 20mm;

[0015] 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 converter production line for standby use;

[0016] 4) When adding scrap steel to the converter, add the carbon blocks from step 3) together with the scrap steel into the converter, and proceed according to the normal smelting process of the converter; the amount of carbon blocks added accounts for 1% of the amount of steel added to the converter, and the amount of scrap steel added is increased by 1.5% accordingly.

[0017] The present invention has found the following innovative features through research and testing:

[0018] 1. The inventors have found that the main component of the low-temperature pulverized part of molten steel refined slag is dicalcium silicate, which has hydration reaction ability and is a good thermal insulation spray refractory material. Adding it to the converter can optimize the slag making operation in the early stage of the converter;

[0019] 2. The inventors found that Na3AlF6, α-Al2O3, β-Al2O3, NaF, and CaF2 in the overhaul slag carbon block are slagging agents and fluxes for lime and dolomite, which can promote the early melting of slag and the formation of foamed slag in the early stage of converter blowing, which is beneficial to reducing the heat expenditure in the converter slag making process and improving the scrap steel ratio of the converter;

[0020] 3. The inventor studied the role and function of the cathode carbon blocks of the electrolytic aluminum overhaul slag in the early stage of converter blowing, as well as the melting characteristics of scrap steel in the early stage of converter blowing. The cathode carbon blocks of the overhaul slag were crushed into 40-60 mm and added into the converter together with the scrap steel, and used as a resource for heating materials in converter smelting. While utilizing the resources, harmless transformation was completed, and the amount of scrap steel added to the converter smelting was increased.

[0021] 4. In order to solve the contradiction that after the carbon blocks are added to the converter, the carbon blocks react with the residual steel slag in the converter to form flame spraying and slag overflow, and the carbon blocks gather on the upper part of the converter after adding molten iron, affecting the process of converter smelting, the inventors sprayed the surface of the electrolytic aluminum cathode carbon block with molten steel refining slag powder, passivated the carbon block, and used the poor thermal conductivity of steel slag to eliminate the problem that the carbon slag affects the process effect and operation after adding the scrap steel to the converter, and played the role of the carbon block as a heating material;

[0022] 5. After the carbon block is sprayed with molten steel refined slag powder, it can prevent safety problems caused by chemical reactions of sodium salts, cyanides and other substances in the carbon block during storage;

[0023] 6. The inventors have discovered a mechanism for utilizing harmful substances in cathode carbon blocks from overhaul slag to complete harmless transformation in the early stage of converter blowing. After spraying steel slag powder on the surface of the carbon blocks for passivation treatment, the fluoride and sodium salt in the carbon blocks can quickly participate in the melting of converter lime and dolomite to form low-alkalinity foam slag, which increases the efficiency of the oxygen blowing reaction and reduces the expenditure of physical heat for converter slagging, creating conditions for the carbon blocks to accelerate the melting of scrap steel during the silicon-manganese oxidation period. The presence of foam slag also promotes the conversion of CO→CO2 after the early carbon block reaction, thereby improving the utilization of chemical reaction heat.

[0024] The present invention utilizes the process principle of converter smelting and adds cathode carbon blocks in electrolytic aluminum overhaul slag to increase the amount of chemical heat in the converter steelmaking process, improve the slag-melting capacity of the converter in the early stage, and reduce the expenditure of physical heat for slag-melting, thereby increasing the scrap steel ratio in the converter steelmaking process. While solving the problem of resource utilization of cathode carbon blocks in overhaul slag, the cost of converter steelmaking is reduced.

[0025] Compared with the prior art, the present invention: 1. utilizes overhaul slag cathode carbon blocks to increase chemical heat in the converter blowing process, improves the scrap steel ratio, and can reduce the converter smelting cost; 2. Various substances in the overhaul slag cathode carbon blocks can participate in the smelting of the converter in the early stage of converter smelting, reducing the consumption of slag auxiliary materials in the converter smelting process, and realizing the maximum value utilization of the electrolytic aluminum overhaul slag cathode carbon blocks; 3. The process of the present invention can quickly and massively process the hazardous waste overhaul slag cathode carbon blocks generated 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. Implementation

[0026] The implementation of the present invention is illustrated by a 65-ton converter production line:

[0027] A process for converter steelmaking and coordinated use of cathode carbon blocks from electrolytic aluminum overhaul slag is implemented according to the following steps:

[0028] 1) The overhaul slag cathode carbon blocks are processed to a particle size of 40-60 mm, with a carbon content greater than 55% by mass, and transported to the steelmaking slag tank spraying treatment area for standby use;

[0029] 2) Add water to dissolve the steelmaking slag powder to make slurry, and use the equipment of spraying slag pot to spray the slurry on the surface of the carbon block in step 1), and the thickness of the surface spray layer formed by spraying is greater than 20mm;

[0030] 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 converter production line for standby use;

[0031] 4) When adding scrap steel to the converter, add the carbon blocks from step 3) together with the scrap steel into the converter, and proceed according to the normal smelting process of the converter; the amount of carbon blocks added accounts for 1% of the amount of steel added to the converter, and the amount of scrap steel added is increased by 1.5% accordingly.

[0032] This process can increase the scrap steel ratio by 1.5%: the scrap steel ratio refers to the mass percentage of scrap steel added to the total steel material added to the converter.

[0033] For example, a 100-ton converter normally adds 15 tons of scrap steel and 85 tons of molten iron to each furnace. By adopting the process of the present invention, 1% of carbon blocks are added, that is, after adding 1 ton of carbon blocks, the amount of scrap steel added can be increased by 1.5 tons, that is, 16.5 tons of scrap steel and 83.5 tons of molten iron can be added to each furnace.

Claims

1. A process for converter steelmaking in synergistic use of cathode carbon blocks from electrolytic aluminum overhaul slag, characterized in that: Follow these steps to implement: 1) The overhaul slag cathode carbon blocks are processed to a particle size of 40-60 mm, with a carbon content greater than 55% by mass, and transported to the steelmaking slag tank spraying treatment area for standby use; 2) Add water to dissolve the steelmaking slag powder to make slurry, and use the equipment of spraying slag pot to spray the slurry on the surface of the carbon block in step 1), and the thickness of the surface spray layer formed by spraying is greater than 20mm; the main component of the low-temperature pulverized part of the molten steel slag is dicalcium silicate; 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 converter production line for standby use; 4) When adding scrap steel to the converter, add the carbon blocks from step 3) together with the scrap steel into the converter, and proceed according to the normal smelting process of the converter; the amount of carbon blocks added accounts for 1% of the amount of steel added to the converter, and the amount of scrap steel added is increased by 1.5% accordingly.