Blast Furnace Refractory Lining: Which Refractory Goes in Each Zone

Why a Blast Furnace Needs Different Refractories in Each Zone

A blast furnace runs a campaign of many years without a full reline, so its lining has to last as long as the furnace itself. No single refractory can do that, because conditions change completely from top to bottom. Near the top, the lining is scraped by descending ore, coke and sinter. In the middle, alkali vapours and early slag attack it while the temperature swings. At the bottom it sits in molten iron and slag. Each failure mode needs a different material.

This guide follows the furnace from top to bottom, then covers the cast house and the hot blast stoves. The zone-by-zone practice summarised here follows the iron and steel refractory literature, notably Biswas and Sarkar's Introduction to Refractories for Iron- and Steelmaking (Springer, 2020).

Upper and Middle Stack: Abrasion From the Burden

The stack carries the burden down toward the hot zone. The main enemy here is abrasion and impact from the charge, plus carbon monoxide disintegration in the cooler upper part. Dense, abrasion-resistant alumino-silicate bricks are the traditional choice: high alumina bricks and dense high alumina bricks, which have lower porosity and higher crushing strength than standard grades of the same alumina content. On larger modern furnaces with stave cooling, the lower stack increasingly moves to silicon carbide and graphite, for the reasons below.

Lower Stack, Belly and Bosh: Slag, Alkali and Thermal Shock

This is the hardest-working zone above the hearth. Primary slag forms here, alkali vapours condense into the lining, and the temperature cycles as the furnace operates. Two facts drove the change away from high alumina:

• Slag attack. FeO-bearing slag forms low-melting phases with alumino-silicate brick: from about 1200°C with fireclay, and around 1380°C even with bricks above 60% Al₂O₃.
• Thermal shock. Bricks above about 65% Al₂O₃ are not practical in the bosh because their higher thermal expansion makes them crack.

The answer is silicon carbide: Si₃N₄-bonded and SiAlON-bonded SiC bricks resist slag and alkali, and their high thermal conductivity carries heat to the cooling staves. That lets a thin layer of slag freeze on the hot face and protect the brick. SiC-graphite combinations perform best of all, provided they are protected from oxidation. For running repairs on worn stack and bosh linings, gunning mixes are sprayed in during short stoppages.

Tuyere Zone: Peak Temperature

The tuyeres blow the hot blast into the furnace, creating the hottest region in the process. The tuyere belt is built around the water-cooled copper tuyere coolers using carbon blocks or SiAlON-bonded SiC shapes, with carbonaceous ramming mass packed into the gaps between the coolers and the bricks.

Hearth: Carbon Blocks and the Ceramic Cup

The hearth holds liquid iron and slag between casts. Early furnaces lined the hearth wall and bottom with high alumina bricks, and hearth campaigns lasted only about six to eight years: the brick shrank in service and breakouts were a recurring risk. Moving to carbon blocks in the hearth wall, and later the bottom, was the breakthrough. Carbon doesn't soften or melt, isn't wetted by iron or slag, handles thermal shock and conducts heat to the cooling system. Hearth life rose to more than 20 years.

Modern hearths combine carbon blocks with a ceramic cup of high-alumina or corundum-type bricks on the hot face, which shields the carbon from dissolution in carbon-unsaturated iron. Carbon hearth blocks are specialist products ordered direct from their manufacturers as part of a reline package.

Taphole and Cast House

Iron and slag leave through the taphole, which is plugged between casts with taphole clay pushed in by the mud gun and drilled out for each cast. The wear area around it needs a different, repair-grade product: see our guide to taphole filling mass.

From the taphole, iron runs through the main trough (where slag separates) and the iron and slag runners. The permanent lining is high alumina brick. The working lining is a castable containing silicon carbide and carbon for resistance to iron and slag, usually a low-cement or ultra-low-cement castable. Large troughs are increasingly cast with self-flow or pumpable castables, and ramming mass is used for local repairs.

Hot Blast Stoves: Graded by Temperature

Each blast furnace has three or four hot blast stoves that preheat the air blown through the tuyeres. A stove has a combustion chamber, a dome and a checker chamber full of refractory checker bricks that store and release heat. The lining is graded by temperature from bottom to top:

• Bottom zone (below about 800°C): standard fireclay bricks and checkers with 39–45% Al₂O₃.
• Middle zone (about 600–900°C): 50–55% Al₂O₃ bricks and checkers.
• Top zone, dome and upper checkers: 62–74% Al₂O₃ high alumina bricks, or andalusite and mullite bricks for creep resistance and volume stability. Where the hot blast runs above about 1100°C, silica bricks are used in the dome and upper checkers, because silica has excellent creep and thermal shock resistance above 600°C (the stove must then never be cooled below that).
• Ceramic burners: andalusite and mullite shapes, chosen for volume stability, high softening temperature and resistance to the fuel gas.

Behind the working lining, insulating fire bricks and other insulation layers keep the stove shell cool and cut heat loss. The hot blast main that carries air to the furnace is lined the same way: high alumina working bricks with insulating bricks and castables behind.

Quick Reference: Blast Furnace Refractory by Zone

Zone	Main wear mechanism	Typical refractory
Upper and middle stack	Abrasion, CO disintegration	High alumina and dense high alumina bricks
Lower stack, belly, bosh	Slag and alkali attack, thermal shock	Si₃N₄- or SiAlON-bonded SiC, SiC-graphite
Tuyere belt	Peak temperature, erosion	Carbon blocks or SiAlON-bonded SiC, carbonaceous ramming mass
Hearth wall and bottom	Hot metal dissolution, penetration	Carbon blocks with a ceramic cup
Taphole	Erosion during casting	Taphole clay; filling mass for the free-flow area
Main trough and runners	Iron and slag erosion at the slag line	High alumina brick backup; SiC-carbon LCC/ULCC working lining
Hot blast stoves	Creep, thermal cycling	Fireclay (bottom), 50–55% Al₂O₃ (middle), 62–74% Al₂O₃, andalusite, mullite or silica (top)

What SAPL Can Supply

From our Delhi warehouse and partner mills we supply the high alumina, dense high alumina, mullite, andalusite and silicon carbide bricks, insulating fire bricks, trough and runner castables, gunning mixes and taphole filling mass used in the zones above. Carbon hearth blocks and silica stove bricks come direct from specialist manufacturers and are outside our stocked range. Share your furnace volume, zone and current lining and we will match the grade and send the manufacturer's datasheet before you order.