Mullite Insulating Fire Brick (JM Series) for High-Temperature Backup Linings

Mullite insulating brick is the top of the insulating range, and it is specified for one reason: it keeps working at temperatures where cheaper insulation starts to shrink. Above roughly 1350 °C, lightweight high-alumina and fireclay insulation lose dimensional stability, joints open, and the insulation layer stops insulating. Mullite does not.

Insulation Is a System, Not a Product

The objective is not minimum thermal conductivity everywhere — it is the lowest total cost of heat loss, shell protection and lining life at each depth. In practice that means a graded system:

• Nearest the working lining — the densest insulation (XZK-JM30 at 1.1 g/cm³, CCS ≥3.5 MPa, 72% Al₂O₃) because it may carry incidental load and must resist higher temperature.
• Toward the shell — progressively lower density (JM23 at 0.6 g/cm³, thermal conductivity ≤0.19 W/(m·K)) where only insulation matters.

A single-density backup is either over-strong and under-insulating, or the reverse. Grading is where the value is.

What Low Conductivity Actually Buys

Lower shell temperature — commonly 40–80 °C on a reheating furnace or stove backup — and a measurable fuel saving. It also protects the shell steel from creep and from acid condensation, which is a structural issue rather than an energy one.

We will calculate the saving from your measured shell temperatures rather than quoting a generic percentage, because the answer depends entirely on where you are starting from.

Respect Classification Temperature

Every insulating grade has a classification temperature — the point at which it undergoes a defined permanent linear change over 24 hours. Select with at least a 100 °C margin at each depth. Run closer than that and the material shrinks, joints open, and heat finds a path to the shell.

Handling: The Practical Weakness

Insulating brick is fragile relative to dense refractory, and damage in transit or during installation is the most common reason an insulation layer underperforms. Low-density grades are crated or boxed rather than simply palletised, and cut pieces are numbered so the layer closes properly. Specify edge protection and keep traffic off installed insulation.

Supply

Minimum order 10 metric tons for standard shapes and 20 metric tons for cut-to-fit work, with 20–35 days lead time where cutting is involved. Bulk density, crushing strength, thermal conductivity, permanent linear change, classification temperature and chemistry are tested per batch, with a Certificate of Analysis on every shipment.

Where This Grade Sits in the Lining System

Insulating grades are specified backwards, from the shell temperature or the fuel figure you need to hit, and then checked against the mechanical load the backup lining actually carries. Picking a density out of a catalogue is the usual reason insulation underperforms its own datasheet.

XZK supplies mullite insulating brick as one zone of a zoned package rather than as a standalone item: the grades normally zoned alongside it — among them high-alumina insulating brick — are documented on the insulating refractories page, and the vessel-level architecture — where each zone starts and ends, and which mechanism actually limits the campaign — is set out on the blast furnace and hot blast stove pages.

The MMK Magnitogorsk No.10 BF and 375 t ladle programme reference documents how this class of material performed in full service, with the measured campaign figures rather than datasheet values.