Mullite (3Al₂O₃·2SiO₂) refractories are prized in foundry linings for their thermal shock resistance, chemical inertness to siliceous slags, and cost-effectiveness. Foundry environments expose linings to molten iron and steel (~1,450 °C), cyclic thermal loads, and abrasive slag–metal interactions. Selecting the optimal mullite grade—pressed bricks, castables, or cast-in-place modules—maximizes lining life and minimizes downtime.
1. Dense Pressed Mullite Bricks
Composition: ≥ 85 % mullite crystalline phase, low glassy matrix (< 10 %).
Key Properties:
Cold Modulus of Rupture ≥ 25 MPa
HMOR ≥ 10 MPa at 1,200 °C
Thermal Shock ΔT ≥ 1,200 °C
Applications: Hot face in slagline and tapping zones; mechanical wear areas under ladle slides.
Advantages: High density (3.0 g/cm³) affords robust abrasion resistance and long campaigns (2–3 years).
2. Insulating Mullite Castables
Composition: 60–70 % mullite aggregates, 30–40 % aluminosilicate binder; engineered porosity 15–25 %.
Key Properties:
Thermal Conductivity 1.2 W/m·K at 800 °C
CCS ≥ 50 MPa at ambient
Perm. Linear Change < 0.4 % after thermal cycling
Applications: Backup insulation behind dense bricks; arch insulation and preheater linings.
Advantages: Monolithic installation eliminates joints, reduces heat loss, and protects backing steel.
3. Cast-in-Place Mullite Modules
Composition: Fiber–mullite modules with ceramic coating.
Key Properties:
Extremely low thermal conductivity (< 0.3 W/m·K)
ΔT shock ≥ 1,300 °C
Low mass for rapid heat-up and cool-down
Applications: Expansion joint seals, burner blocks, area requiring flexible insulation.
Advantages: Modular design for ease of replacement; minimizes thermal bridging.
4. Mullite-Bonded Monolithic Castables
Composition: 70 % mullite aggregates, 30 % reactive alumina binder; optional SiC addition.
Key Properties:
CCS ≥ 80 MPa at ambient
HMOR ≥ 8 MPa at 1,300 °C
Abrasion loss < 15 mg (G65)
Applications: Riser ducts, slide-gate liners, transition zones.
Advantages: Combines ease of installation with performance close to dense bricks.
Selection and Installation Best Practices
Zone Mapping: Identify slag attack areas, thermal gradients, and mechanical load points to assign the correct mullite grade.
Joint Design: Use compatible alumina-silicate mortars; control joint thickness to ≤ 5 mm to prevent slag infiltration.
Heat-Up Schedule: Stage ramp from ambient to 800 °C over 8–10 h, then to full temperature at ≤ 50 °C/hr to cure binders and avoid spalling.
Preventive Maintenance: Thermographic surveys and routine inspection of slagline thickness guide timely repairs with castables or modules.
Conclusion
Mullite refractories offer versatile grading for foundry linings—from dense pressed bricks in high-wear zones, insulating castables for heat conservation, to flexible cast-in-place modules for expansion areas. By mapping service conditions to the appropriate mullite grade and following best practices in installation and maintenance, foundries in the US and Canada can extend lining campaigns, optimize energy use, and maintain safe, efficient molten metal operations.
