The hearth and bottom of a blast furnace should adopt a structure made entirely of carbon bricks or composite carbon bricks, using high-quality carbon bricks for construction. For large blast furnaces, the use of carbon bricks and SiC bricks is crucial for extending furnace lifespan. With the adoption of copper cooling walls, the weak point for longevity has shifted from the lower furnace body, belly, and waist to the hearth area. Therefore, prolonging the hearth's lifespan has become a key focus in achieving a long service life for blast furnaces.
In recent years, several blast furnaces in China have experienced increased water temperature differences or even burn-through in the hearth, requiring comprehensive measures to address these issues. High-quality carbon bricks and blocks used in blast furnaces should meet conventional performance standards while also fulfilling requirements for thermal conductivity, permeability, oxidation resistance, alkali resistance, and resistance to molten iron erosion.
The tuyere area is recommended to use a composite brick structure, typically corundum-mullite bricks, brown corundum bricks, or hot-pressed carbon bricks like NMA or NMD. Common causes of erosion in the hearth include mechanical abrasion from coke and slag, chemical corrosion, oxidation by water vapor, attack from zinc and alkali metals, and thermal stress damage.
To address these issues, blast furnaces can adopt microporous carbon bricks with high thermal conductivity and strengthen cooling measures on the hearth’s cooling wall. This ensures that the solidification temperature of slag and molten iron (1150°C) remains within the carbon bricks, keeping it away from the cooling wall.
Currently, there are three basic structures for hearths and bottoms in domestic and international blast furnaces:
- Large carbon bricks with a ceramic pad.
- Hot-pressed small carbon bricks with a ceramic pad.
- Large or small carbon bricks with a ceramic cup at the bottom.
All three structures have proven to enhance furnace lifespan. High thermal conductivity carbon bricks, microporous carbon bricks, and ceramic pad structures have been widely adopted.
In high coal-injection blast furnaces, operations emphasize maintaining an active hearth center while ensuring an appropriate bottom center temperature. As a result, attention has shifted to the thermal resistance of the ceramic pad and improving its lifespan to achieve suitable bottom center temperatures.
Theory of Solidified Layer Formation via Enhanced Cooling:
The use of refractory materials with high thermal conductivity (18.4 W/(m·K) at 600°C, 60-80 W/(m·K) at 20°C) on the hearth sidewall, combined with intensive cooling, prevents slag and molten iron penetration. These materials also offer high alkali resistance, absorb thermal stress, and rapidly transfer heat to the cooling water through an efficient water-cooling system. This creates a stable solidified protective layer on the hot surface of the refractory lining (with isothermal lines above 1150°C), forming an "iron shell" on the hearth bottom and resisting sidewall erosion caused by "elephant foot" effects.
The key to hearth longevity lies in the thermal conductivity of the hearth sidewall materials. The focus is on selecting materials with excellent thermal conductivity, impermeability, and resistance to annular cracking. Proper maintenance emphasizes cooling effectiveness, monitoring water temperature differences on the cooling wall, and injecting grout into porous areas.
The integrated bottom with cooling is considered a reasonable structure. It includes carbon ramming material above the cooling pipes and 2-3 layers of carbon bricks. Different parts require carbon bricks with specific properties. Below the tap hole, microporous carbon bricks with high impermeability should be used, while SiC bricks with high thermal conductivity should be used for the furnace bottom's lowest layer. Other areas may use standard or microporous carbon bricks.
Incorporating longer carbon bricks below the furnace bottom perimeter beneath the tap hole enhances resistance to molten iron and alkali metal penetration and erosion. The gaps between bricks should be reduced to less than 0.5 mm for construction.
Regarding the "ceramic cup" structure, academic opinions are divided. Some argue that the ceramic cup plays a significant role and should be strengthened, while others believe it will eventually deteriorate, leaving the carbon bricks as the primary structure. High-density, hot-pressed small carbon bricks with resistance to molten iron penetration and high thermal conductivity can also be used on the hearth sidewall.
Both approaches have advantages and disadvantages, and either can achieve a long furnace lifespan, though with differing economic costs. The adoption of high-quality microporous and ultra-microporous carbon bricks, as well as hot-pressed small carbon bricks, has significantly improved the lifespan of blast furnaces in China. Most medium and large blast furnaces in China have abandoned ramming carbon material bottoms and self-baking production processes.
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