The lifespan of the blast furnace hearth is a decisive factor affecting blast furnace overhauls. To extend the hearth's lifespan, research has been conducted from various aspects, including design, carbon brick material, construction quality, smelting intensity, and molten iron circulation during tapping. The consensus is to establish a stable protective layer on the carbon bricks in the hearth to isolate the molten iron, protect the carbon lining, and prevent rapid dissolution of the carbon bricks. Therefore, understanding the formation mechanism of the protective layer on the hearth carbon bricks is essential.
Blast furnace smelting is carried out in a closed vertical furnace, and the process is quite complex. Different researchers and producers have come to various conclusions when studying and discussing it. The protective layer on the hearth carbon bricks is something that many researchers and producers have observed during shutdown and overhaul investigations.
The main process of the protective layer formation on the hearth carbon bricks involves the hearth's cooling system, including the cooling wall, cooling water, gaps, and carbon bricks. This cools the molten iron in contact with the hot surface of the carbon bricks, increasing its viscosity and causing graphite to precipitate. High-melting-point compounds also adhere to the carbon brick surface, gradually forming a protective film. During this process, residual coke particles and ash floating in the molten iron may also adhere to the carbon brick surface, forming a protective layer.
Due to the different conditions in different parts of the hearth, the different fuel properties used, and the different compositions of the pig iron produced, the thickness of the protective layer varies. A thicker protective layer is not formed in a single process, but rather through multiple accumulations. Therefore, the structure and composition of each layer often differ depending on the time and reasons for its formation, and there may even be gaps between the layers.
Generally, the conditions for the formation of the protective layer on the hearth carbon bricks are:
① A complete hearth cooling system that reduces the temperature of the carbon brick's hot surface, thus lowering the temperature of the molten iron in contact with it to a temperature suitable for protective layer formation;
② Maintaining a smelting intensity commensurate with the smelting conditions, ensuring sufficient reduction of the ore in the upper part of the blast furnace, and ensuring a suitable dripping speed of the slag and iron, minimizing (FeO) in the slag, and increasing the carbon saturation of the iron, which is conducive to the precipitation of graphitic carbon;
③ An active hearth with abundant high-temperature heat, allowing [Si], [Ti], TiC, and TiN to reach the conditions necessary for protective layer formation. After extensive research on blast furnace lining damage, four types of carbon brick protective layers were identified: iron-rich layer, slag-rich layer, graphite-rich layer, and titanium-rich layer.
01 Iron-Rich Layer
Generally, as the blast furnace hearth lining is gradually eroded, the carbon brick thickness decreases, the thermal resistance of the carbon brick decreases, and the total thermal resistance between the cooling system and the molten iron decreases. The temperature of the refractory material's hot surface then decreases. When its temperature drops to the solidification temperature of molten iron, the high-temperature molten iron solidifies on the hot surface of the refractory material, forming an iron-rich protective layer. There are generally two types of blast furnace hearth and bottom structures: one is a full carbon brick hearth and bottom structure, and the other is a composite structure of carbon bricks combined with ceramic cups. Regardless of the structure, the vertically penetrating brick joints in the hearth can allow molten iron, coal gas, and alkali metals to penetrate. In the presence of ceramic cups, these substances can also pass through the ceramic material and erode the carbon bricks on its hot surface. As alkali vapors and other substances enter the carbon brick area, at a temperature range of 750-850°C, alkali metals and thermal stress cause embrittlement reactions in the carbon bricks. Once the embrittlement layer of the hearth is formed, under the scouring action of the molten iron flow, the carbon bricks on the hot surface of the embrittlement layer are easily detached, allowing the molten iron to come into contact with the carbon bricks in the embrittlement layer. At this point, the temperature of the molten iron is below its solidification temperature, with a certain degree of supercooling. The molten iron then solidifies, and the iron-rich layer gradually forms. The formation of the iron-rich layer prevents direct contact between the high-temperature molten iron and the carbon bricks, and also prevents alkali metals and other substances from penetrating the carbon bricks, thus protecting the brick lining from erosion. In fact, the iron-rich layer and the liquid adhesion layer are of the same type; the difference is that the former is considered to be a solid protective layer condensed on the surface of the carbon brick, while the latter is considered to be a liquid protective layer adhering to the hot surface of the carbon brick.
02 Slag-Rich Layer
The cooling system reduces the surface temperature of the molten iron in contact with the carbon bricks to below the solidification temperature of molten iron (1150℃), causing a iron-rich layer to form on the surface of the carbon bricks, thus isolating the molten iron from direct contact with the carbon bricks. However, during actual blast furnace damage investigations, it was found that the protective layer in some blast furnaces not only contains an iron-rich layer but also a slag-rich layer for protection. Generally, the density of liquid slag in the hearth is around 2.4 g/cm³, while the density of liquid iron is around 7.0 g/cm³. The slag density is far less than the iron density. Therefore, blast furnace slag has less protective effect on the critical parts of the hearth. However, investigations of different blast furnace hearths and bottoms show that the hearth protective layer does contain some slag phase, and the conditions for slag phase formation exist in the area below the centerline of the taphole. In the area below the taphole centerline, only molten iron and coke exist, and the carbon in the molten iron is unsaturated. On the one hand, the molten iron will further undergo a carburization reaction with the coke, and a portion of the remaining coke ash will float up to form slag, while another portion may accumulate on the side wall of the hearth; on the other hand, the refractory material contains some ash, and as the carbon bricks are gradually eroded, the ash gradually remains behind, interacting with the coke ash and other substances, thus forming a slag-rich layer. The slag phase has a certain fluidity under high-temperature conditions, and in the presence of other phases, a small amount of slag phase will be dispersed within them. The slag-rich layer is likely to be generated during normal production. During production, due to the density difference, it gradually separates from the molten iron and floats into the slag layer in the hearth. It can be observed during investigations after furnace shutdown and cooling, but most of it is residual slag that did not have time to float up or slag that dripped down from the upper material column during the tapping process and solidified on the carbon bricks at the low-temperature side wall.
03 Graphite-Rich Layer
During blast furnace damage investigations, it was found that almost all blast furnace hearth protective layers contain a graphite carbon phase. In actual production, a graphite carbon phase is also present on the surface of the iron trough and pig iron. In blast furnace operation, hearth buildup is a common phenomenon, and the large amount of graphitic carbon phase precipitated on the hearth sidewalls and the cooler bottom is one of the causes of hearth buildup. This indicates that carbon in the molten iron can precipitate in the form of graphitic carbon. Generally, when the blast furnace hearth temperature fluctuates, and the molten iron temperature at the refractory surface is below the liquidus line of the molten iron, in addition to crystallization, graphitic carbon often precipitates. The amount of precipitated graphitic carbon mainly depends on the ability of carbon in the molten iron to transform into graphitic carbon. The precipitation of graphitic carbon in molten iron is closely related to the supercooling degree of the molten iron, and the cooling rate of the molten iron also determines the amount of graphitic carbon precipitation. As graphitic carbon gradually precipitates, the precipitated graphitic carbon continuously crystallizes and grows, forming a graphite-rich layer.
04 Titanium-Rich Layer
In blast furnace damage investigations, analysis of samples taken from the protective layer on the hearth bottom refractory lining revealed that the titanium-rich layer has a relatively dense structure. It is a multiphase material composed of a large amount of high-melting-point titanium compounds, metallic iron, and other slag phase minerals, mainly containing a considerable amount of Ti(C,N). Titanium ore protection has become one of the most effective measures to extend the service life of blast furnaces in the later stages of their operation, and its excellent protective effect has become a consensus among blast furnace operators. It is generally believed that the formation mechanism of titanium compounds is as follows: TiO2 in the ore is reduced to TiC or directly reduced to Ti, which dissolves in the molten iron, but its solubility is extremely low, and most of it is suspended in the molten iron as TiC particles. This [Ti]-containing molten iron migrates to the low-temperature areas of the hearth, or changes with changes in blast furnace operating conditions, or due to changes in molten iron temperature and N2 pressure caused by shutdowns. If the concentration product of [Ti], [C], and [N] in the molten iron reaches saturation, it can crystallize and precipitate in the form of TiC, TiN, or Ti(C,N) in the abnormally corroded areas around the hearth, or where the molten iron flow is relatively slow. The formed TiC and TiN can undergo phase reconstruction and evolution to form Ti(C,N) solid solutions. Conclusion
These four types of protective layers form the basis of the protective layer on the hearth carbon bricks. In actual blast furnace production, the protective layer formed in the hearth may also be a combination of these four types. During blast furnace operation, cooling reduces the temperature of the molten iron in contact with the hot surface of the carbon bricks, increasing the viscosity of the molten iron, causing graphite to precipitate and adhere to the surface of the carbon bricks along with high-melting-point compounds (such as TiC, TiN, etc.), forming a film-like protective layer.
Wechat Us 
