From：Brictec
Date：2025-10-16
Tunnel kilns are continuous high-temperature firing systems characterized by long structures and multiple thermal zones. Each section operates under different temperature, atmosphere, and mechanical stress conditions. Therefore, the proper selection and configuration of refractory bricks are critical for kiln performance, energy efficiency, and service life.
I. Types and Properties of Refractory Bricks Used in Tunnel Kilns
1. Main Types by Material
Refractory Type | Major Composition | Service Temperature (°C) | Main Features | Typical Applications |
High Alumina Brick | Al₂O₃ ≥ 55% | 1300–1600 | High compressive strength, good slag resistance, poor thermal shock resistance | Firing zone roof, kiln door, flame-contact areas |
Mullite Brick | 3Al₂O₃·2SiO₂ | 1350–1700 | Low thermal expansion, excellent thermal shock resistance, no deformation | Firing zone roof and wall, insulation zone |
Cordierite Brick | 2MgO·2Al₂O₃·5SiO₂ | 1250–1400 | Very low thermal expansion, excellent thermal shock resistance | Lower firing zone, transition area |
Lightweight High Alumina Brick | Porous Al₂O₃ ≥ 50% | ≤1350 | Lightweight, excellent insulation | Insulation layer, secondary wall, roof upper layer |
Fire Clay Brick | Al₂O₃ 30–45% | 1200–1350 | Economical, easy to construct, moderate thermal shock resistance | Preheating zone, outer wall, flue lining |
Insulating Brick | SiO₂–Al₂O₃ | ≤1100 | Low thermal conductivity, lightweight | Outer wall insulation layer |
Silica Brick | SiO₂ ≥ 95% | 1650–1700 | Excellent creep resistance at high temperature, acid-resistant | Upper roof of firing zone, kiln head |
Wear-Resistant Brick | High-alumina or mullite-based composite | ≤1400 | Excellent abrasion and impact resistance | Car wheel zone, track edge, kiln car top |
Silicon Carbide Brick (SiC) | SiC ≥ 70% | 1500–1650 | High thermal conductivity, oxidation and erosion resistance | Burner zone, flame impact area, car base |
Castable / Precast Block | High alumina, mullite, or SiC-based | 1300–1600 | Good integrity and air-tightness | Burner ports, arches, sealing joints |
Ceramic Fiber Board / Blanket | Al₂O₃ + SiO₂ | ≤1400 | Lightweight, excellent insulation, easy installation | External insulation, kiln doors, wall lining |
Shaped / Custom Brick | Custom composition | Varies | Precision fit, customized geometry | Burner brick, arch foot, transition pieces |
II. Refractory Configuration and Construction Standards in Tunnel Kiln Design
1. Recommended Material Configuration by Kiln Section
Kiln Section | Recommended Brick Types | Thickness (mm) | Temperature (°C) | Description |
Roof (Firing Zone) | Mullite / Cordierite + Lightweight High Alumina + Ceramic Fiber | 500–550 | 1250–1300 | Combines high strength and insulation |
Wall (Firing Zone) | High Alumina / Mullite + Lightweight Alumina + Fiber Board | 500 | 1200–1300 | Inner heat-resistant, outer insulating |
Wall (Preheating Zone) | Fire Clay + Lightweight Alumina | 400–500 | 900–1100 | Emphasizes thermal shock resistance |
Insulation Zone | Cordierite + Insulating Brick | 400 | 900–1000 | Reduces heat loss |
Flue Lining | Fire Clay / SiC Brick | 250–350 | 800–1000 | High erosion resistance |
Kiln Door / Sealing Panels | Mullite + Fiber Board + Steel Plate | 450–500 | 1100–1200 | Combines insulation and mechanical strength |
Kiln Car Surface | Cordierite / SiC / High Alumina Brick | 230 | 1000–1250 | Load-bearing and wear-resistant |
Kiln Car Insulation Layer | Insulating Brick + Ceramic Fiber | 200–250 | ≤900 | Reduces heat conduction |
Burner Port / Arch Foot | SiC / Castable Blocks | Custom | 1300–1500 | High thermal shock and erosion resistance |
2. Construction and Masonry Standards
Item | Technical Requirements |
Brick Joints | ≤ 2 mm; staggered joints ≥ 1/4 brick length |
Anchoring | Stainless steel anchors every 5 brick layers |
Mortar | Use matching refractory mortar (same base material) |
Construction Sequence | Build walls first, then arches; inner lining before outer layer |
Drying & Heating-Up | Initial heating rate ≤ 30°C/hour to prevent cracks |
Arch Control | Accurate curvature control to avoid stress concentration |
Joints Sealing | High-temp sealing compound or ceramic fiber filling |
III. Standards for Qualified Refractory Materials
1. Appearance and Dimensional Tolerance (per GB/T 2992.1, GB/T 16544)
Item | Requirement |
Surface | Smooth, no cracks, chips, or dense pores |
Dimensional Tolerance | ±2 mm in length, width, and height |
Density Uniformity | ≤ ±0.05 g/cm³ variation within the same batch |
2. Physical and Chemical Properties (Ref. GB/T 3995, GB/T 10325)
Property | High Alumina | Mullite | Cordierite | Fire Clay |
Bulk Density (g/cm³) | 2.3–2.6 | 2.4–2.7 | 1.9–2.2 | 2.0–2.2 |
Apparent Porosity (%) | 18–22 | 15–20 | 25–30 | 22–26 |
Cold Crushing Strength (MPa) | ≥60 | ≥70 | ≥45 | ≥35 |
Permanent Linear Change (%) | ±0.2 | ±0.3 | ±0.3 | ±0.4 |
Refractoriness Under Load (°C) | ≥1450 | ≥1600 | ≥1400 | ≥1350 |
Thermal Shock Resistance (cycles 900°C–water) | ≥20 | ≥25 | ≥30 | ≥15 |
3. Inspection and Acceptance Procedure
Raw Material Inspection
Chemical composition (Al₂O₃, SiO₂, Fe₂O₃ content)
Phase analysis (XRD test)
Finished Product Testing
Dimensional and visual inspection
Fired bulk density and compressive strength test
Thermal shock resistance test
Documentation
Factory test report with chemical and physical data
Quality certificate compliant with GB/T, ISO, or ASTM standards
On-site Verification
Random sampling of ≥10% for re-test before use
Only approved materials may be used in kiln construction
IV. Selection Principles for Refractory Materials
Principle | Description |
Temperature Matching | Select materials according to thermal zones and service temperature |
Thermal Shock Resistance Priority | Roofs and burner zones require mullite or cordierite bricks |
Mechanical Strength Coordination | Use high alumina or SiC bricks for load-bearing areas |
Insulation Coordination | Combine dense inner bricks with lightweight outer layers |
Supplier Qualification | Must hold ISO/GB certification and third-party test reports |
Sample Verification | New suppliers must pass firing performance tests before approval |
Conclusion:
A well-designed refractory system ensures:
Stable tunnel kiln operation
Low energy consumption
Extended kiln service life
Consistent product quality
Proper selection and configuration of refractory bricks are fundamental to the success of modern clay sintered brick plants and to the overall efficiency of tunnel kiln construction projects.
