Mullite Insulating Brick

Mullite insulating bricks are lightweight firebricks in which a mullite–alumina aggregate is bonded around a controlled pore network, so the finished brick weighs a fraction of a dense refractory while still holding a mullite crystal phase. XZK supplies four grades — JM23, JM26, JM28 and JM30 — spanning 0.6 to 1.1 g/cm³ bulk density, thermal conductivity from 0.19 to 0.43 W/(m·K) at 350°C and cold crushing strength from 1.5 to 3.5 MPa. The family exists to solve one problem in a glass furnace: cut the heat escaping through the shell without adding deadweight to the structure that sits above the melt.

XZK Group manufactures the JM series at our Xinmi plant in Henan. In glass furnaces these bricks form the insulation course behind dense working linings — crown insulation above a silica or mullite arch, tank bottom insulation under the paving, the external casing of the regenerator, the backing of a suspended wall, the doghouse and the forehearth.

What Is a Mullite Insulating Brick?

The body is built from mullite and alumina aggregate held in a ceramic bond, with the pore network produced deliberately rather than left over from firing. In a dense firebrick porosity is a defect; in an insulating brick it is the functional phase. Lightweight aggregate, a graded fines fraction and a burn-out pore former are combined so that the pores stay small and evenly distributed, because a body of many small closed pores insulates better, and holds together better, than one with a few large open voids.

Alumina content separates the grades. JM23 runs at 42% Al₂O₃ and JM30 at 72%; as alumina rises, service temperature and mechanical strength rise with it, but thermal conductivity rises too, because the denser mullite–corundum structure conducts more heat than a highly porous body. Iron oxide is held between 0.55% and 1.2% so the insulation course does not introduce a low-melting phase of its own.

JM23, JM26, JM28 and JM30 are the industry-standard designations for this family of mullite insulating firebrick. That is commercially useful rather than cosmetic: a specification, a zoning drawing or another supplier's data sheet written around one of these designations can be quoted directly as an equivalent XZK grade, so a furnace rebuild is not re-engineered simply because a material has been withdrawn.

The three governing numbers — bulk density, thermal conductivity and cold crushing strength — move together, and the real design decision is where on that line a given position belongs. Lower density gives lower conductivity and less weight on the arch, but also less strength. Higher density gives a block that can carry a load, at the cost of conducting more heat. In practice the lightest grade goes outermost, where it is cool and nothing rests on it, and the densest grade is placed where the brick has to bear weight.

Key Properties

• Bulk density 0.6–1.1 g/cm³ — JM23 at 0.6, JM28 at 0.7, JM26 at 0.8 and JM30 at 1.1 — against roughly 2.5 g/cm³ for a dense firebrick, so an insulation course adds a small fraction of the deadweight of the working lining.
• Thermal conductivity 0.19–0.43 W/(m·K) at 350°C, with the lowest figure in JM23 and the highest in JM30. This is the property that sets the shell temperature and the fuel bill, not the refractoriness of the hot face.
• Cold crushing strength from ≥ 1.5 MPa (JM23) to ≥ 3.5 MPa (JM30), chosen together with density so that a block in a load-bearing position is not specified lighter than the structure above it can tolerate.
• Alumina content from 42% (JM23) to 72% (JM30), which sets how close to the working lining each grade may be placed and how it behaves in contact with alkali vapour carried in the flue gas.
• Low iron oxide, from ≤ 1.2% in JM23 down to ≤ 0.55% in JM30, so the insulation layer cannot become the source of a liquid phase that would soften it or stain the glass.
• Stable volume and good thermal shock tolerance, because an insulation course behind a cycling crown must not crack and open a path for hot gas to bypass the working lining.