A blast furnace is a type of vertically oriented iron-smelting furnace with a circular cross-section. The furnace shell is constructed from steel plates and lined with refractory bricks. It is divided into five sections from top to bottom: furnace throat, body, waist, belly, and hearth. Due to its favorable economic indicators, simple technology, high labor productivity, large production capacity, and low energy consumption, the iron produced using this method is the world’s primary source of total iron production.
The blast furnace production process involves loading iron ore, coke, and fluxing agents, such as limestone, from the top, and blowing preheated air in from tuyeres located at the furnace’s base circumference. At high temperatures, carbon from the coke reacts with oxygen in the air, creating carbon monoxide and hydrogen, which rise in the furnace, removing oxygen from the iron ore and reducing it to iron.
From the tap hole of the blast furnace, molten iron is extracted, while the unreduced impurities in iron ore and fluxing agents combine to create slag, which is tapped out from the slag hole. The gas generated during the process is discharged from the furnace’s top and then filtered for use as fuel in hot blast stoves, heating furnaces, boilers, coke ovens, and other applications. The primary output of blast furnace smelting is pig iron, along with by-products like blast furnace slag and gas.
| Part | Brick |
| Throat | Dense clay bricks High alumina bricks |
| Upper and lower parts of the furnace body | Common clay bricks High alumina bricks Sillimanite bricks Dense clay bricks |
| Lower part of the furnace body and waist | Aluminum-carbon bricks Silicon carbide bricks Silicon nitride bonded silicon carbide Carbon bricks |
| Belly | High alumina bricks Aluminum-carbon bricks Silicon carbide bricks Silicon nitride bonded silicon carbide Carbon bricks |
| Bosh | Carbon bricks Mullite, corundum mullite Plastic composite corundum bricks Corundum silicon carbide |
| Hearth bottom | Clay bricks High alumina bricks Carbon bricks |
Furnace Bottom
Based on the detection results of the furnace bottom damage condition and the temperature of the furnace bottom during production before the blast furnace shutdown for overhaul, it is known that the furnace bottom damage can be divided into two stages. The initial stage is when molten iron seeps into the bricks, causing them to float and form deep pits. The second stage is the chemical erosion that occurs after the formation of the sinter layer.
The conditions for molten iron seepage are: First, the brick lining of the furnace bottom withstands 10% to 20% of the pressure from the liquid slag and iron, gas pressure, and the weight of the column of materials. Second, there are gaps and cracks in the bricks.
In the sinter layer, the bricks have been sintered together into a whole, which can resist the seepage of molten iron. Moreover, the temperature of the molten iron at the bottom of the pit is lower, and the cracks in the bricks are no longer the weak points for molten iron seepage. At this time, the main cause of damage to the furnace lining is the chemical erosion caused by the carbon in the molten iron reducing the silica in the bricks and being absorbed by the molten iron.
The reaction equation is as follows: SiOz+2[C]+[Fe]–[FeSi]+2CO From the above furnace bottom damage mechanism, it can be seen that the factors affecting the furnace bottom life are: first, the high pressure it withstands, second, the high temperature, and third, the erosion of the furnace bottom by the flow of molten iron and slag during tapping.
Furnace hearth
The lower part of the furnace hearth is where the slag and molten iron are collected and discharged periodically, so the flow of slag and iron, the fluctuation of the molten iron level in the furnace, and the high-temperature flow of a large amount of gas are the main factors that erode the furnace lining.
The blast furnace slag is alkaline, while the commonly used refractory bricks are acidic, which leads to chemical erosion of the furnace lining at high temperatures and is an important factor in its damage. The highest temperature area of the entire blast furnace is the tuyere zone in the upper part of the furnace hearth, where the inner surface temperature of the lining can reach up to 1300℃~1900℃, so the high-temperature resistance of the brick lining and corresponding cooling measures are very important.
Belly of the furnace
This area is close to the tuyere zone, so it is subject to high-temperature thermal stress. Due to the inclination of the furnace belly, it is also affected by the pressure of the burden column and the impact of material collapse and settling. It also suffers from chemical erosion from the initial slag.
Because the FeO, MnO, and free CaO content is high in the initial slag, FeO, MnO, CaO in the initial slag reacts with SiO in the brick lining to generate low-melting-point compounds, which cause the surface of the brick lining to soften and fall off under the erosion of the liquid slag and gas flow. In actual production, this part of the lining is often completely eroded shortly after the furnace is opened. Increasing the thickness of the lining is useless, and production is maintained by the slag skin on the cooling wall.
Furnace body
The lower part of the furnace body has a higher temperature, so the influence of thermal stress is greater. It is also subject to chemical erosion from the initial slag and alkali metal and zinc. In addition, carbon deposition is also a cause of damage to the lining in this area.
In the upper part of the furnace body, the burden is relatively hard and angular, and the main cause of damage to the lining in this area is the wear and tear caused by the falling burden and the high-speed gas flow carrying a large amount of dust.
Throat of the furnace
The throat of the furnace is subject to the impact of the falling burden, so it is protected by a metal protective plate, also known as a throat steel brick. Even so, it will still lose strength at high temperatures and experience thermal deformation due to uneven temperature distribution. Damage is more severe when the gas flow inside the furnace changes frequently. For large and medium-sized blast furnaces, the furnace body is the weak link of the entire furnace.
Although the working conditions here are better than those in the lower part, the lifespan is shorter because there is no protection from the slag skin. For small and medium-sized blast furnaces, the furnace hearth is the weak link, and furnace hearth burn-through accidents often occur due to poor cooling of the furnace hearth and a small amount of mud bubbles in the iron-blocking hole.
