Study On The Influencing Factors Of Blast Furnace Hearth Bottom Longevity

The hearth life is one of the determining factors for the campaign life of the large blast furnace.Only by understanding the interaction among the molten iron flow in the hearth,the hearth coke,the carbon brick and its protective layer,can we find out the measures to extend the hearth life.The erosion of sidewall by hot metal is one of the main reasons for the short life of hearth.The key to the long campaign life of the hearth is to bond a protective layer on the hot surface of the carbon brick to isolate the direct contact between the hot metal and the carbon brick.The velocity of liquid iron near the carbon brick and the hot surface temperature of the carbon brick are the main factors affecting the formation of the protective layer.The main factors influcing the velocity of hot metal near the hearth sidewall are:deadman coke behaviors(deadman porosity distribution,coke particle size and coke density,etc.),taphole maintenance program and working state of the hearth(deadman floating height and deadman permeability,etc.).If an embrittlement layer is formed,the thermal conductivity of carbon brick will be reduced,the hot surface temperature of carbon brick will be increased,which is not conducive to the regeneration and stable existence of the protective layer.This is also an important reason that affects the life of blast furnace hearth.In this paper,the influencing factors of the hearth bottom longevity are studied by hearth damage investigation,numerical simulation and hot state experiment.It deepens the understanding of the deadman coke,the embrittlement layer,the skull and the hot metal flow in the hearth,which has certain guiding significance for the hearth design and blast furnace operation.In this paper,firstly,the inorganic mineral composition,graphitization degree,particle size distribution,strength of deadman coke and porosity distribution of deadman in different hearth height and different radial positions between 1.5 m below the tuyere level and bottom of industrial blast furnaces are studied in detail by the method of damage investigation of 2800 m3 and 5500 m3 industrial blast furnace hearth.The results show that a large amount of blast furnace slag is filled in the deadman coke between 2.5 m below the tuyere level and the bottom of 2800 m3 blast furnace.During the hearth damage investigation of the 5500 m3 blast furnace,it is also found that a large amount of blast furnace slag is immersed in the deadman coke from 1.8 m below the tlyere level to the center line of the taphole.The inorganic mineral content of hearth coke increases with the increase of distance from tuyere level and the average inorganic mineral yield of deadman coke is about 45%.The mass of deadman coke by penetration of slag aniron at most of the hearth locations is about 1.43-2.21 times larger than that of the feed coke under the same conditions.The graphitization degree of the deadm coke fines is higher when the coke is close to the bottom of the furnace.The average size of deaman coke particles of 2800m3 blast furnace and 5500 m3 blast furnace in diameter direction distributes in "M-shape" in majority of the hearth-level regions and inversed "V-shape",and the average particle size of coke is 28.7 mm and 23.5 mm,respectively,which is 47%and 56%lower than that of the feed coke.The average porosity of deadman decreases with the increase of the distance to the center line of the taphole and the increasing distance from furnace wall.The overall average porosity of the deadman was 0.3.Secondly,on the basis of the study of deadman coke behavior,the mathematical model of the hot metal flow in the hearth of 5500 m3 blast furnace,including the deadman and the mud mass,is established.The hot metal flow near the sidewall of the hearth is studied under different taphole maintenance program(the taphole depth,the taphole angle and the double tapholes tapping,etc.)and different hearth working conditions(the floating height of the deadman and the deadman central permeability,etc.).The results show that increasing the taphole depth,the taphole angle of 100 and choosing the double taphole with an angle of 1800 can reduce the velocity of molten iron near the sidewall and prolong the hearth life.When the porosity in the center,middle and edge of the deadman is 0.2,0.3 and 0.35 respectively,the liquid iron velocity near the interface between hearth and bottom increases greatly with the decrease of the floating height(0.8 m to 0.1 m)of the deadman,which indicates that the small floating height of the deadman may lead to the "elephant foot" erosion of hearth.The floating height of deadman within 0.6 m to 0.8 m is beneficial to the long campaign life of blast furnace hearth.When the deadman is sitting and floating,the velocity of molten iron near the hearth sidewall will increase only when the poor liquid permeability(porosity equals 0.1)in the center of the deadman develops to 26%and 50%of the hearth diameter,respectively.Then,the erosion mechanism of alkali metals and zinc to the carbon brick and the formation mechanism of the protective layer of carbon brick are analyzed through the damage investigation of the the 2800 m3 blast furnace hearth.The Zn2SiO4,KA1SiO4,ZnO,KAlSi2O6 and a small amount of ZnS as well as ZnA12O4 were found in the embrittlement layer of hearth residual carbon brick.Combined with the mineral phases of the current carbon brick and the embrittlement layer of residual carbon brick,the formation mechanism of the brittle layer is revealed.The existence of blast furnace slag was found in the protective layer of carbon brick and bottom ceramic pad.The blast furnace slag in the protective layer mainly comes from the blast furnace slag immersed in the deadman coke,not from the ash of the feed coke.At last,a hot experimental furnace is designed and built to simulate the smelting process of blast furnace hearth.The cooling water pipe is designed to simulate the cooling stave of blast furnace.The three-phase alternating current electrode is used as the heating source to ensure that the temperature of molten iron is about 1550℃.The bottom blown nitrogen is used to simulate the flow of blast furnace slag and hot metal in the hearth.It is found that when the hot surface temperature of carbon brick is lower than the solidification temperature of blast furnace slag and hot metal,skull can be formed on the hot surface of carbon brick.The continuous erosion of K,Na and Zn vapor on the carbon brick in the industrial blast furnace hearth was simulated by producing K,Na and Zn vapor in simulated blast furnace hearth.In a word,through the study of this paper,it shows that the blast furnace slag can be brought into the hearth area below the taphole through the movement of the deadman coke.The gravity of the deadman is increased because the deadman coke is immersed in a large amount of blast furnace slag.So the salamander depth should be increased to ensure the designed floating height of the deadman.In the smelting process of blast furnace,the hot surface of hearth and bottom linings can form skull under suitable conditions.In order to prolong the life of blast furnace hearth,it is necessary to establish a reasonable tapping maintenance program to reduce the molten iron velocity near the sidewall of hearth,ensure the quality of feed coke to improve the liquid permeability of deadman,strictly control the K and Zn load to avoid the formation of brittle layer of carbon brick.Those measures promote the skull formation on the hot surface of carbon brick to isolate the direct contact betwwen carbon brick and hot metal.