A blast furnace lining is designed zone by zone, because the hearth, bosh, belly, stack and top each see a different combination of temperature, abrasion, chemical attack and thermal cycling. The choice of refractory in each zone is made together with the cooling system, so the lining and the staves or cooling plates are designed as one thermal and mechanical assembly rather than as separate items.
Hearth, Bottom and Taphole Zone
The hearth and bottom carry the highest risk in the furnace, because a failure here cannot be repaired without a major shutdown. The modern configuration combines carbon and graphite materials with a ceramic cup. Carbon ramming material is used in the base area above the foundation and around the water-cooling tubes embedded below the bottom bricks, and the lining above it is built from semi-graphite carbon blocks, microporous carbon bricks and, in higher-duty designs, ultra-microporous carbon bricks.
The ceramic cup is the second line of defence. Two layers of ceramic pad are laid on the furnace bottom, and the hearth wall is built as a ceramic cup structure. Because the ceramic material resists the chemical attack of iron and slag far better than carbon, the cup slows the erosion of the carbon below it. It also improves heat retention in the hearth, which supports a stable slag and iron temperature, reduces coke consumption and helps keep the hearth active. When the bottom bricks are combined with buried water-cooling tubes and a carbon hearth with ceramic cup masonry, the corrosion rate of the whole assembly drops and the campaign life is extended.
The taphole area is built with carbon composite bricks, which combine resistance to hot metal washing with the thermal shock tolerance needed for repeated drilling and plugging.
Tuyere Zone
The tuyere zone is exposed to intense heat, slag and iron splashing, and to the mechanical load transmitted from the brickwork above. Composite corundum-based composite bricks are used here because they combine high refractoriness with good resistance to slag and iron erosion and to thermal shock. The result is a compact, load-bearing structure that protects the tuyere assembly and the surrounding blowing equipment, and that carries the belly lining above it.
Belly, Bosh and Lower Stack
The belly, bosh and lower stack are built with an integrated brick wall in a thin lining configuration, and the brickwork is locked to the cast iron cooling staves by a cold inlay method. Production experience with this design shows that it can support the normal 12 to 15 year service life expected from a modern blast furnace.
| Zone | Typical refractory | Main duty |
|---|---|---|
| Hearth bottom | carbon ramming, semi-graphite and microporous carbon blocks, ceramic pad | hot metal penetration, chemical attack, thermal load |
| Hearth wall | ceramic cup wall, ultra-microporous carbon bricks | erosion by circulating iron and slag |
| Taphole | carbon composite bricks | hot metal wash and thermal shock |
| Tuyere zone | corundum-based composite bricks | slag and iron erosion, thermal shock, load bearing |
| Belly and bosh | nitride-bonded silicon carbide bricks | alkali attack, abrasion, thermal cycling |
| Lower and middle stack | microporous alumina-carbon bricks | chemical attack, temperature fluctuation |
| Middle and upper stack | high-density clay bricks impregnated with phosphoric acid | abrasion by burden and gas stream |
| Top and riser tube | spray materials with high CO resistance | carbon monoxide attack and thermal shock |
| Stave to shell gap | waterless press-in grouting material | filling voids, gas tightness, heat transfer |
Copper Staves, Silicon Carbide and Microporous Carbon
The belly, bosh and lower stack are inlaid with silicon nitride bonded silicon carbide bricks. Silicon carbide offers high thermal conductivity and good resistance to abrasion and to alkali attack, while the nitride bond keeps the brick stable under thermal cycling and gives it the strength needed to survive the stave gap. Microporous aluminium carbon bricks are inlaid through the middle of the furnace body, where their pore structure limits the penetration of slag and alkali, and high-density clay bricks impregnated with phosphoric acid are used in the middle and upper body, where abrasion by the descending burden is the controlling mechanism.
Furnace Top, Riser Tube and the Stave to Shell Gap
The furnace top and riser tube are protected by spray materials formulated for strong resistance to carbon monoxide attack and for high hot modulus of rupture, because the off-gas atmosphere in this zone can disintegrate refractories that are otherwise perfectly serviceable. In the gap between the cooling stave and the furnace shell, a waterless press-in grouting material is used. Filling this gap properly is essential: voids reduce heat transfer from the brickwork to the stave, allow gas to circulate behind the lining and produce local hot spots on the shell.
Frequently Asked Questions
Q: What is a ceramic cup in a blast furnace hearth?
A: It is a wall of ceramic material placed inside the carbon hearth so that iron and slag meet a chemically resistant surface before reaching the carbon bricks. It slows erosion and improves hearth heat retention.
Q: Why is silicon carbide used in the bosh and belly?
A: Nitride-bonded silicon carbide resists alkali attack and abrasion and tolerates thermal cycling, which are the dominant loads in this zone. It also transfers heat well to the cooling staves.
Q: What is the purpose of microporous carbon bricks?
A: Their pore structure is fine enough to restrict the penetration of slag, iron and alkali, which slows chemical erosion compared with conventional carbon blocks.
Q: How long should a blast furnace lining last?
A: With the integrated thin wall design and correct stave grouting, service lives in the range of 12 to 15 years are achievable; the hearth lining is normally the factor that sets the campaign length.
Q: Why does the stave to shell gap need grouting?
A: Voids in that gap reduce heat transfer, allow gas circulation and create hot spots on the shell. A waterless press-in grout fills the gap and restores thermal contact.
Q: What attacks the furnace top lining?
A: Carbon monoxide in the off-gas, combined with thermal cycling. Spray materials selected for CO resistance and high hot flexural strength are used in the top and riser tube for this reason.
