Cement plants rely on preheater towers to dry and preheat raw meal using hot combustion gases. Mullite bricks—composed of 3Al₂O₃·2SiO₂—are favored in preheaters for their thermal shock resistance, chemical stability, and mechanical strength. Proper grade selection enhances uptime, reduces replacement frequency, and maintains process efficiency.
1. High-Purity Dense Mullite Bricks
Composition: ≥ 85 % mullite phase, minimal glassy phase (< 10 %) for maximum hot strength.
Performance: Cold modulus of rupture (CMOR) ≥ 30 MPa; HMOR ≥ 10 MPa at 1,200 °C; thermal shock ΔT ≥ 1,200 °C.
Use Case: Hot-face lining of preheater riser ducts and cyclones where abrasive dust and temperature cycling are severe.
2. Insulating Mullite Bricks
Composition: Porosity 25–35 %, bulk density 1.8–2.1 g/cm³.
Performance: Thermal conductivity < 1.5 W/m·K at 800 °C; adequate strength (CMOR ≥ 15 MPa).
Use Case: Cold-side linings behind dense mullite to conserve heat and protect steel structure from gas temperatures (~600 °C).
3. Mullite Castable Refractories
Composition: Pre-cast in situ castables with 70–80 % mullite aggregates and alumina matrix.
Performance: Monolithic application eliminates cold joints; strength and thermal shock properties comparable to dense bricks.
Use Case: Transition zones, refractory patching, and areas with complex geometry where brick installation is impractical.
4. Fiber–Mullite Modules
Composition: Mullite–silica fibers bonded into lightweight modules with controlled permeability.
Performance: Excellent thermal shock (ΔT > 1,300 °C); low mass reduces thermal inertia.
Use Case: Preheater duct expansion joints and expansion bellows liners, where flexibility and insulation are crucial.
Selection Guidelines:
Operating Profile: Analyze gas temperatures (up to 1,200 °C), particulate load, and cycling frequency to determine required ΔT and strength.
Chemical Environment: Ensure low Fe₂O₃/TiO₂ (< 1 %) to prevent liquid-phase formation and phase instability in the mullite matrix.
Mechanical Load: Evaluate dust velocity and impact; dense mullite or castables in high erosion zones, insulating grades elsewhere.
Lifecycle Planning: Balance upfront cost against expected campaign length (typically 2–4 years for preheaters).
