Discussion on several hot issues in the development of ironmaking technology

      1. Treatment of dust and mud with high harmful impurities produced in iron and steel enterprises

      Most enterprises use the dust and mud with high harmful impurities to the sintering plant, which leads to the enrichment of harmful impurities and is not conducive to the production of blast furnace. The construction of rotary hearth furnace can remove the harmful impurities in the furnace burden. However, the reduction degree of its product (direct reduced iron) iron is less than 90% and the FeO content is high. Therefore, it can only be used for blast furnace ironmaking and cannot be used as the raw material of electric furnace. The construction cost of the rotary hearth furnace is relatively high. The 7 production lines of the rotary hearth furnace that have been built have not reached the ideal state, and no new rotary hearth furnace has been built abroad.

      For the dust sludge with high harmful impurities, such as blast furnace bag dust removal ash and sintering machine head ash, it is recommended to add lime, mix, pelletize, dry them, and then use them for the converter; This can cut off the cycle chain of enrichment of harmful impurities.
 

      2.Technical analysis of coke oven gas injection into blast furnace

      Injecting coke oven gas into blast furnace can reduce coke ratio, but the effect is not ideal. Coke oven gas has high methane content (for direct reduction of iron ore, gas reforming is required to achieve co+h2 content greater than 90%), which is a good gas source for indirect reduction of iron ore; However, the coke oven gas injected from the blast furnace tuyere can only play the role of combustion, and can not carry out the indirect reduction of iron ore (the indirect reduction of iron ore is an exothermic reaction). If the gas is injected from the furnace body and cannot be burned with oxygen, the indirect reduction of iron ore can be carried out. This will bring high economic benefits.

     3 smelting technology of iron ore with high Al2O3 content

     When the Al2O3 content of iron ore used in blast furnace ironmaking is high, MgO needs to be added to improve the fluidity of slag and desulfurization efficiency. Generally, it is scientific to add MgO to iron ore with high Al2O3 content to realize slag with mg / Al ratio of 0.5. However, after using low MgO, the smelting range of slag phase diagram is very narrow. If the quality of burden is very stable, smelting can still go smoothly; However, if the quality of burden fluctuates greatly, short slag is easy to occur, resulting in increased viscosity of slag, poor fluidity and hearth freezing. It is suggested that the slag operation system with mg / Al ratio greater than 0.5 should be adopted in enterprises with large fluctuations in charge quality.

     Most enterprises add MgO to sinter, and the MgO content in sinter exceeds 2.0%. This will reduce the quality of sinter and increase the pulverization rate. To solve this problem, it is suggested that MgO should not be added to sinter, but to pellet or directly to blast furnace.

     The low MgO content of slag is conducive to reducing the amount of slag, improving the permeability of charge, reducing the fuel ratio, and reducing the production cost. The content of MgO in sinter increases by 1%, the iron grade decreases by 1%, and the amount of blast furnace slag increases by 10%. Therefore, MgO should be used scientifically according to specific production conditions.

     4. Change operation concept and operate blast furnace scientifically

     At present, a number of small and medium-sized blast furnaces are operated with large air volume and high smelting intensity, sacrificing the fuel ratio to achieve high production. In general, the utilization factor is equal to the smelting intensity divided by the fuel ratio. The scientific operation policy of blast furnace is to obtain high utilization coefficient by reducing fuel ratio. The good smelting strength of blast furnace is about 1.2t/ (M3 · d). Above this value, the blast furnace output will increase, but the fuel ratio and cost will increase. Some medium and small blast furnaces adopt large air volume and high smelting intensity to achieve high utilization factor, which is an incorrect operation method. The high air consumption per ton of iron will inevitably lead to high fuel ratio (0.93m3 of oxygen and about 4.42m3 of air are required to burn 1kg of carbon), which also leads to high energy consumption in ironmaking process (the blast energy consumption accounts for about 4.5% of the total energy consumption in ironmaking). Therefore, medium smelting intensity can obtain good indicators for ironmaking. Blast furnaces in industrialized countries are operated with medium smelting intensity and medium utilization coefficient, so as to achieve low fuel ratio and long blast furnace life.

     5 scientific evaluation of tamping coke

     The pores of tamping coke are oblate, with many caecum pores and high density, but it is an illusion that the coke index obtained from the test is better than that of the top loaded coke (compared with the top loaded coke, it should be multiplied by 0.85%); The combustibility of tamping coke is not as good as that of top loaded coke, resulting in the decrease of CO content in blast furnace gas and indirect reduction of ore, resulting in the increase of fuel ratio and the decrease of output. Tamping coke is not resistant to alkali metal corrosion and is easy to pulverize. The operation of blast furnace will be greatly affected by the use of burden with high harmful impurities. Tamping coke is used for blast furnaces below 1000m3, which has no great impact on blast furnace operation. However, when tamping coke is used for blast furnaces above 3000m3, some problems will occur in operation, and the technical and economic indicators are not good.

     Tamping coke is a production that reduces the cost by reducing the proportion of main coking coal. Tamping coke has high density, many cecal pores and poor combustibility.

     Tamping coke is basically not used in large blast furnaces abroad. Lianyuan 3200m3 blast furnace uses tamping coke, which is produced by the company's internal coking plant. The main coking coal for coking and coal blending is the same as the top charging coke, so the quality of tamping coke has been improved. In addition, it is recommended not to use tamping coke when the blast furnace is started and shut down, so as to quickly resume production.

     Increasing the proportion of pellets in blast furnace burden is the development direction of ironmaking technology
     Increasing the proportion of pellets in blast furnace burden has the following advantages, which is the development direction of ironmaking technology.

     1) Increasing the proportion of iron making burden pellets can effectively improve the iron grade of iron making ore.

     In general, the iron grade of pellets is 7-8% higher than that of sinter. Therefore, the iron grade of ore can be effectively improved by using more pellets in blast furnace.

2) Increasing the iron grade of the charging material can reduce the fuel ratio and increase the output.

Generally, the iron grade of the ore is increased by 1%, the blast furnace fuel ratio is decreased by 1.5%, the pig iron output is increased by 2.5%, and the slag amount is reduced by 1.5%. If pellet ore is used for 100% iron making, compared with sinter ore, the fuel ratio will decrease by 11.52% and the pig iron output will increase by 19.2%. This benefit is very considerable.

3) Increasing the proportion of iron making burden pellets can reduce the energy consumption of iron making system.

According to statistics, in 2018, the energy consumption of pellet process and sintering process in key iron and steel enterprises was 25.36kgce/t and 48.60kgce/t respectively; The energy consumption of pelletizing process is 23.24kgce/t lower than that of sintering process. If one ton of pellet is used to replace one ton of sinter for ironmaking, the energy consumption of ironmaking system will be reduced by 23.24kgce.

4) Increasing the proportion of iron making burden pellets can reduce the emission of pollutants.

Sintering production produces a large amount of SO2, NOx, dioxin, dust and other harmful substances. In 2012, the state published the standard for comprehensive treatment of sintering flue gas (gb28662-2012 emission standard of air pollutants for iron and steel sintering and pelletizing industry), and the indicators are basically in line with the international level. This has increased the cost of environmental protection management in iron and steel enterprises, and increased the environmental protection investment and operating costs of enterprises. At present, the environmental protection treatment cost per ton of sinter is about 50 yuan. If pellet is used to replace sintering, the pollutant treatment cost can be greatly reduced. Natural gas is used for pellet production without coal. The emission of pellet waste gas is half lower than that of sintering. The content of dioxin, SO2 and NOx in pellet waste gas is less than that of sintering. It is simple and easy for environmental protection and treatment, and the plant is clean.