Analysis and Discussion on activity of blast furnace hearth

　　Modern blast furnaces are developing towards large furnace capacity and high production efficiency. As a resource and energy intensive blast furnace ironmaking process, the essence of improving the activity of blast furnace hearth lies in clarifying the space-time scale characteristics of slag, iron and coke in the blast furnace hearth and the multiphase interface migration behavior. At present, the research on the behavior of slag, iron and coke in the blast furnace hearth and the activity of the hearth only depends on the surface cognition obtained from production experience, and the basic theoretical research on the activity of the blast furnace hearth is relatively weak. As it is difficult to obtain relevant parameters in the actual process and the hearth status cannot be monitored in real time, the actual production conditions of each blast furnace are also different. The judgment of hearth activity is mainly based on actual production experience and indirectly reflects the hearth activity status with the help of characterization indicators, which has certain lag and theoretical limitations. With the acceleration of the large-scale process of blast furnace, the larger the volume of blast furnace, the greater the lag and limitation of various parameters of blast furnace. If the fluctuations of blast furnace hearth state are not found in time, it will lead to a long recovery cycle of blast furnace conditions, large economic losses and other adverse effects.

　　1. Variation characteristics of blast furnace hearth activity

　　The working state inside the hearth is mainly affected by three aspects: the state of dead material column, the discharge process of slag and iron and the distribution of air volume at the tuyere. When the hearth activity is good, the blast furnace condition is stable and smooth; When the activity of blast furnace decreases, the furnace condition often fluctuates and is not smooth. The main manifestations of the poor activity of the hearth are as follows: the temperature of the bottom center decreases, and the temperature of the side walls on both sides of the hearth increases; With the increase of oxygen potential, the desulfurization reaction in molten iron weakens and the sulfur content increases; Slag ratio decreases; The carbon content in molten iron decreases; The tuyere raceway is shortened, and the inner wall surface is hard through the rod test; The air pressure increases and the air volume decreases; The fuel ratio increases and the output decreases; The tapping time is unstable and the taphole becomes shorter. In the actual production process, if the above phenomena are observed, measures should be taken immediately to find out the causes, so as to avoid major economic losses caused by the continuous deterioration of hearth activity.

　　2. Factors affecting hearth activity

　　2.1 adjustment of lower part of blast furnace

　　The lower adjustment parameters of blast furnace mainly include air volume, air temperature, air pressure, tuyere area and tuyere depth length. The lower adjustment mainly refers to the control technology of controlling blast furnace temperature and air supply. The size of the dead material column has a great relationship with the blast intensity. The greater the blast intensity, the smaller the dead material column. The good performance of the hearth working condition is that the slag iron temperature is high and the fluidity is good. However, controlling the higher tuyere combustion temperature is conducive to better heating the hearth and improving the exchange heat between the hearth gas and slag iron.

　　According to the above research, the air distribution can be improved by reducing the tuyere area, maintaining the appropriate wind speed, increasing the blast kinetic energy, extending the raceway depth, increasing the air temperature, appropriately reducing the air pressure and other means, so that the hot gas can better pass through the dead material column in front of the tuyere, better penetrate the high furnace center, increase the hearth center temperature, and activate the hearth center. Properly control the development of edge gas flow, find the appropriate gas flow distribution, and prevent the slag skin on the furnace wall from falling off. If the slag skin falls off and enters the hearth, the hearth temperature will drop sharply and the hearth activity will be damaged.

　　2.2 upper adjustment of blast furnace

　　The upper adjustment of blast furnace refers to the adjustment of blast furnace distribution system. The ore and coke present a layered overlapping structure. Blast furnace gas is generated in the lower part of the blast furnace, and then rises through the charge layer; The burden drops from the top and acts with the gas to complete the smelting processes such as heating, reduction, slagging and melting. The temperature of furnace charge rises gradually from top to bottom in the furnace, and the layered structure of furnace throat distribution is maintained until melting.

　　When the charge reaches the soft melting zone, the gas resistance increases because the gap between the charges decreases continuously when the charge begins to soften. Due to the large resistance of the ore soft melting layer, most of the gas flow passes through the coke layer (coke window). In this area, the ores begin to melt, and only coke is in solid state. The initial slag of the formed ore is distributed between coke and coke, making the gas have the greatest resistance to pass through this area. The gas in the soft melt zone mainly depends on the interlayer of coke, i.e. the coke window. The gap between the coke blocks and the coke pores in the drip zone and the hearth mainly depend on the liquid permeability and air permeability. Therefore, the better the gas permeability in the upper area, the better the liquid permeability of the lower hearth, the easier the slag iron passes through the coke, the less the slag iron retained in the dead column, and the stronger the hearth activity.

　　With the increase of coke batch weight and coke layer thickness, the resistance of gas passing through will be reduced, which is conducive to the permeability of charge column and the development of blast furnace gas flow.

　　With the increase of ore batch weight, as the ore stacking angle is larger than that of coke, the proportion of ore distribution to the center will increase, which is conducive to the development of edge gas flow and the suppression of center gas flow. This situation will lead to the uniform distribution of furnace burden and the improvement of gas utilization. However, if the ore batch is too heavy, there will be insufficient central air flow and excessive development of edge air flow, which may cause accumulation in the center of the hearth.

　　2.3 physical properties of blast furnace slag iron

　　The basicity of blast furnace slag plays an important role in the metallurgical properties of blast furnace slag. When the alkali is too low, the CaO content is small, which can not destroy the network structure formed by SiO2 and Al2O3 in blast furnace slag, resulting in the increase of its viscosity. When the basicity increases, the viscosity of blast furnace slag decreases; When the alkali is too high, the viscosity and melting temperature of the slag rise sharply, resulting in poor fluidity of the slag, which is not conducive to the smelting of the slag. Therefore, properly increasing the basicity of the blast furnace slag is conducive to improving the fluidity of the slag, the permeability of the hearth and the activity of the hearth.

　　MgO and Cao in slag belong to basic oxides, while Al2O3 and SiO2 in slag belong to acid oxides. Increasing the mg / Al ratio is equivalent to increasing the MgO content under the condition of keeping the Al2O3 content unchanged, which is conducive to improving the slag fluidity. However, if the mg / Al ratio is too high, MgO and Al2O3 will form complex high melting point compounds, which will increase the melting temperature of the slag and deteriorate the fluidity of the slag.

　　Physical heat of hot metal in blast furnace is an important index to judge furnace temperature. The temperature of molten iron in the hearth is low, and the heat of slag is insufficient, so it can not flow freely between the coke in the furnace core. A large amount of slag iron can not pass through the dead column smoothly, and it will stay in the dead column in the hearth, resulting in inactive dead column area in the center of the hearth. As the inactive area gradually expands, hearth accumulation will be formed. To restore the activity of the hearth, the molten iron temperature of the hearth should be ensured first.

　　2.4 blast furnace raw fuel properties

　　With the addition of coke from the top of the blast furnace to the blast furnace, the particle size of Coke will gradually decrease due to friction, gasification reaction and carburization. When the coke is in the lump zone of the furnace body, because the upper temperature is low, the coke is only affected by the friction between the furnace charges, and the particle size changes little.

　　As the coke enters the soft melt zone and the drip zone, a severe dissolution loss reaction occurs, the pores on the coke surface gradually become larger, the pore wall becomes thinner, and the coke strength gradually decreases. On the other hand, the particle size of coke drops sharply, the porosity becomes larger, accompanied by gasification, reduction of FeO and other reactions. When the coke reaches the raceway of the blast furnace tuyere, the coke reacts rapidly. The coke rotates violently with the hot air at the tuyere and reacts with the oxygen in the hot air, resulting in rapid pulverization of the coke. Coke far away from the tuyere raceway slowly forms a dead column due to the slower reaction speed. At the top of the hearth, the carbon unsaturation of molten iron is large, and the carburization reaction occurs when the coke contacts with molten iron, which makes the particle size of coke continue to decrease, and the voids of the dead column become larger. In the center of the hearth, the iron coke reaction is the most intense, and the voids of the dead column reach the maximum; Further down, due to the buoyancy of molten iron, the coke accumulates slightly, making the void of the dead column slightly reduced. However, because the effect of buoyancy of molten iron is weaker than that of coke, the rate of porosity reduction decreases gradually. The coke particle size continues to decrease with the increase of the distance from the top of the residual iron, because the coke continues to react with molten iron in different height areas of the dead column and consumes coke continuously.

　　The evolution behavior of coke in the blast furnace, and the coke with good performance provides strong support for the voids of the hearth dead column. Assuming that the gasification degree of coke and the burning loss ratio of tuyere are the same, the larger the coke particle size into the furnace, the larger the coke particle size in the hearth after a series of upper reactions. The larger the coke particle size in the hearth, the greater the void of the dead column, the larger the space for the slag iron in the hearth to pass through the coke bed, the smaller the slag iron retention, and the hearth is more active.

　　3. Conclusion

　　3.1 lower dispensing

　　By means of reducing tuyere area, maintaining appropriate wind speed, increasing blast kinetic energy, extending raceway depth, increasing air temperature, appropriately reducing air pressure and coal injection, and increasing oxygen enrichment rate, the air distribution and pulverized coal combustion are improved, so that hot gas can better pass through the dead column in front of tuyere, better penetrate the center of blast furnace, increase the temperature of hearth center, and activate the hearth center.

　　3.2 upper dispensing

　　The gas in the soft melt zone is mainly transmitted through coke, and the liquid and gas permeability in the drip zone is mainly achieved by the gap between the coke blocks and the coke pores. The better the gas permeability of the upper area, the better the liquid permeability of the lower hearth, the easier the slag iron passes through the coke, and the stronger the hearth activity. From the point of view of slag iron retention, hearth activity and blast furnace longevity, the blast furnace operation should appropriately reduce the ore batch weight, appropriately increase the coke batch weight and improve the upper permeability index.

　　3.3 physical properties of slag iron

　　The activity of the hearth is in direct proportion to the temperature in the center of the hearth. The low temperature in the center of the hearth makes the flow of slag in the coke dead column worse, and a large amount of slag iron can not pass through the dead column smoothly, which slowly leads to the inactivity of the hearth, resulting in the accumulation of the hearth. In order to improve the activity of the blast furnace hearth, the blast furnace operation should appropriately increase the slag basicity, magnesium aluminum ratio and TiO2 content, ensure a sufficiently high hot metal temperature, and try not to use titanium ore to protect the furnace.

　　3.4 raw material conditions

　　The larger void of the dead material column is the foundation to ensure the good activity of the hearth. The particle size of coke, CSR, CRI and the content of harmful elements in raw materials and fuels are important indicators of the voids in the dead column. The larger the comprehensive particle size and CSR of coke into the furnace, the smaller the CRI, the larger the particle size of coke in the hearth, and the larger the void of the dead column; The less harmful elements in the raw fuel, the better the fluidity of the hearth slag, and the easier the hot metal carburization. The larger the space and capacity of the hearth hot metal and slag passing through the coke bed, the smaller the slag iron retention, and the more active the hearth.