Refractory Lining Basics

Overview

Refractory materials withstand extreme temperatures (up to 1800°C) inside furnaces, boilers, kilns, incinerators, reactors, and flue gas systems. They contain heat, resist thermal shock, chemical attack from slag/ash, and mechanical abrasion. Refractories are selected based on temperature, chemistry of the process, abrasion exposure, and insulation requirements. The three main forms are brick, monolithic (castable/gunite), and ceramic fiber.

Refractory Classifications

By chemistry:

• Acidic (silica, fireclay, high-alumina): resistant to acid slags; for steel, iron, general furnaces
• Basic (magnesite, dolomite, chrome-magnesia): resistant to basic slags; for cement kilns, steel converters, non-ferrous
• Neutral (chrome, alumina >70%, carbon, silicon carbide): for severe environments; carbon black reactors, aluminum melting
• Insulating (lightweight, low k): back-up insulation behind dense refractory, low heat loss

By form:

• Brick (shaped): pre-formed, fired, laid with mortar joints
• Monolithic (castable, plastic, ramming, gunning): installed in place as a single monolith (no joints)
• Ceramic fiber: blanket/module/board — lightweight, excellent insulation, low heat storage

Material Types and Properties

Fireclay Brick

• 30-45% Al₂O₃; classification by duty: low (1400°C), medium (1500°C), high (1600°C), super-duty (1700°C)
• Good general purpose; low cost; moderate thermal shock resistance
• Used in boilers, furnaces, kilns, chimneys
• Thermal conductivity ~1.0-1.5 W/m·K

High-Alumina Brick

• 50-99% Al₂O₃; higher temperature and chemical resistance
• Better abrasion and slag resistance than fireclay
• Used in aluminum furnaces, boiler burner throats, cement kilns, slagging applications
• k ~1.5-2.5 W/m·K

Silica Brick

• >93% SiO₂; very high temperature (1700°C); high strength at temperature
• Good for glass furnaces, coke ovens, acid steel processes
• Poor thermal shock resistance (cracks on rapid heating/cooling below 600°C due to cristobalite inversion)

Magnesite (Basic) Brick

• >85% MgO; high temperature (1700°C+); resists basic slags and iron oxide
• For steel converters, cement rotary kilns, lime kilns
• Poor thermal shock; requires careful heating schedule

Insulating Firebrick (IFB)

• Lightweight (0.5-1.0 g/cm³); low k (0.15-0.4 W/m·K); low heat storage
• Hot face up to 1600°C; not for severe slag/abrasion
• Back-up insulation behind dense brick or castable
• Reduces wall heat loss dramatically

Castables (Monolithic)

Conventional refractory castables:

• Low cement (LCC): <2.5% CaO; high strength, low porosity; most common modern monolithic
• Ultra-low cement (ULCC): <1.0% CaO; better high-temperature properties
• No-cement (NCC): colloidal silica bond; high temperature, good thermal shock
• Insulating castable: lightweight aggregate (vermiculite, perlite, bubble alumina); for back-up insulation
• Installed by casting (forms), pumping, gunning (pneumatic projection), shotcreting

Ceramic Fiber

• Alumino-silicate fiber blanket/module; temperature classes 1000-1600°C
• Very low k (0.05-0.2 W/m·K); extremely low heat storage (fast furnace heat-up/cool-down)
• Advantages: energy savings on cyclic furnaces; light weight (no heavy structural support); easy installation
• Limitations: poor abrasion resistance; cannot withstand direct flame impingement, slag contact, high velocity gas; limited to 20-40 m/s; fiber degradation in reducing atmospheres; health concerns (respirable fibers — handle with PPE)
• Used in furnace liners, boiler liners, ducting, kiln cars, backup to dense refractory

Multi-Layer Lining Design

Modern refractory systems use a multi-layer design:

• Hot-face layer (100-250 mm): dense refractory (brick or castable) — resists temperature, abrasion, chemical attack
• Backup/insulating layer (50-150 mm): insulating castable, IFB, or fiber — reduces heat loss and shell temperature
• Steel shell or casing: structural containment

Example multi-layer boiler wall:

1. Hot face: 150 mm high-alumina low-cement castable (k = 1.8 W/m·K)
2. Backup: 75 mm insulating castable (k = 0.3 W/m·K)
3. Steel shell (6 mm)

Heat loss through wall for T_hot = 1100°C, T_ambient = 25°C: Q ≈ 450 W/m²; shell temperature ≈ 60-70°C (safe for personnel).

Anchors

Refractory must be anchored to the steel shell with metallic or ceramic anchors:

Metallic Anchors (V, Y, U shapes)

• Carbon steel: up to 400°C shell temperature
• 304/310 stainless: up to 800-900°C
• Inconel/Haynes alloys: up to 1100°C
• Spacing: typically 300-600 mm grid, closer for dense/heavy refractory; staggered
• Length: through back-up layer + 70-80% of hot-face thickness (do NOT go through to hot face — acts as heat sink/cracking point)
• Apply plastic cap on tip to allow shrinkage (the anchor tip burns off and creates a void for thermal expansion)

Ceramic Anchors

• For temperatures above metallic anchor limits (>1100°C)
• Alumina, mullite, silicon carbide shapes
• Used in brick linings (brick ties) or with ceramic fiber modules

Installation Best Practices

Castable

• Mix strictly per manufacturer instructions; correct water content (excess water destroys strength and density)
• Use forced mixer; mix no longer than needed
• Place within 30 minutes after mixing; vibrate during placement to eliminate voids
• Cure: keep moist for 24 hours (cement hydration); air cure 24 hours
• DRYOUT is the most critical step: controlled heating schedule to remove water without explosive spalling (typically 25°C/hr to 100°C, hold 8 hours, ramp up)

Brick

• Use correct mortar (air-setting, heat-setting, or phosphate-bonded)
• Brick joints: maximum 1.5-2 mm for precision brick; staggered joints in running bond
• Expansion joints in brickwork every 2-3 m (ceramic fiber paper or board filler); 3-5 mm per meter of wall
• Coursing: alternate header and stretcher courses for structural bond

Ceramic Fiber

• Modules anchored to shell with center stud; compress slightly to ensure tight joints
• Adjacent modules expand against each other (10-15% compression at installation)
• Vapor barrier on cold face if process produces condensable gases
• Coat with rigidizer/hardener on hot face if subject to air velocity

Dry-Out / Pre-Heat Schedule

Refractory contains mix water and water of hydration in cement. If heated too fast, water flashes to steam internally and spalls the lining explosively.

Typical dry-out schedule for castable lining:

1. 20°C → 100°C at 25°C/h — free water removal
2. Hold at 100°C for 12-24 hours (per thickness)
3. 100°C → 300°C at 25°C/h — hydrated water removal
4. Hold at 300°C for 8 hours
5. 300°C → operating T at 50-75°C/h — ceramic bond formation

Forced cooling after operation is equally damaging; cool-down at same rates to avoid thermal shock.

Failure Modes

Refractory Selection by Application

Summary

Refractory lining selection balances temperature capability, chemical resistance (acid vs basic slag), abrasion/velocity, and insulation. Multi-layer designs (dense hot face + insulating back-up) give best service and energy efficiency. Low-cement castables (LCC) are the modern default monolithic; brick is used for rotary kilns and high-temperature processes; ceramic fiber saves energy in cyclic furnaces but cannot take abrasion or direct flame. The most common causes of refractory failure are improper dry-out (explosive spalling), wrong material chemistry for slag, and anchor failures — not material under-specification. Always follow the manufacturer's dry-out temperature schedule strictly.