Blast Furnace Trough and Runners: Why Alumina-SiC-Carbon
The trough refractory carries a river of molten iron and pretends it is fine. It manages this by hiding under a crust of the iron itself.
When the blast furnace is tapped, around 1450 to 1500 °C of liquid iron and slag runs down the main trough and along the runners to the ladles. The refractory in that channel faces iron erosion, slag corrosion, oxidation, mechanical impact, and a thermal shock at the start of every cast. It is one of the hardest jobs in the plant, and the material that does it is almost always alumina-silicon carbide-carbon.
Why three ingredients, not one
- Alumina provides the refractoriness and the base corrosion resistance, as the aggregate and much of the matrix.
- Silicon carbide brings high thermal conductivity, which spreads heat, lowers the hot-face temperature, and improves thermal-shock resistance. It also resists abrasion and slag, and molten iron does not wet it, so the iron does not stick or soak in.
- Carbon, as graphite or pitch, is also non-wetting to iron and slag, has very low thermal expansion, and conducts heat well. Its problem is oxidation above roughly 400 to 500 °C in air, so antioxidants such as silicon, aluminium, or boron carbide are added, and the hot face is kept covered by iron, slag, or a frozen skull.
No single one of these covers all the demands. Together they make a hot face that is conductive, non-wetting, abrasion resistant, and reasonably shock tolerant.
How the trough is built up
- A working lining of alumina-SiC-carbon castable, patched and replaced frequently.
- A safety or permanent lining behind it, lower grade castable or brick, there to contain the iron if the working lining is breached.
- Often a slag-line band with a higher SiC and carbon content, because the slag floating on top attacks a different way than the iron below.
Modern practice leans toward low-cement, ultra-low-cement, or no-cement sol-bonded versions for higher hot strength and lower porosity, installed by casting and vibration against a former, or as a dry vibratable mix.
Installing and baking it
The lining is placed against a former, then given a controlled dry-out and a bake that develops the carbon bond before the first iron arrives. Rushing this leaves free water and an undeveloped bond, and the first cast finds out.
What wears it out
- Slag-line corrosion, as basic slag reacts with the alumina-silica matrix.
- Iron erosion at the impact point and along high-flow zones.
- Oxidation of the carbon wherever the protective skull runs thin.
- Thermal-shock cracking at the start of each cast.
- Alkali and zinc carried down from the burden.
The trough works by keeping a skin of frozen iron and slag on its hot face. The refractory's job is to survive the minutes between casts when that skin is thinnest.
The short version
Life is measured in tonnes of iron through the trough between relines, and main-trough campaigns run into the hundreds of thousands of tonnes with patching. If you are seeing early slag-line or impact-zone wear and want a second opinion on the mix or the practice, get in touch.