Lightweight mullite castables are insulating refractories built on a mullite crystal phase rather than a fireclay one, which lifts the useful service temperature well above that of an ordinary insulating castable while keeping density and thermal conductivity low. They fill the gap between light...
Lightweight mullite castables are insulating refractories built on a mullite crystal phase rather than a fireclay one, which lifts the useful service temperature well above that of an ordinary insulating castable while keeping density and thermal conductivity low. They fill the gap between light insulating castables, which cannot survive much above 1200°C, and dense high alumina castables, which insulate poorly and add weight.
XZK Group supplies lightweight mullite castables with an Al2O3 content of roughly 55–70% and bulk density from about 1.5 to 2.2 g/cm3. In a glass furnace they are a backing and insulation material for the hotter parts of the structure.
The material combines a mullite-rich matrix with porous lightweight aggregate, usually synthetic mullite or bubble aggregate rather than expanded clay. The matrix is a low-cement mullite bond so that the lime content, and therefore the amount of low-melting phase, stays low. Al2O3 runs about 55–70% with iron kept low. Density is set by how much porous aggregate is used; the refractoriness comes from the mullite phase, not from the aggregate filler.
Mullite is the reason the material can take higher temperature than an insulating castable. Mullite has a low and stable coefficient of thermal expansion and deforms only slowly under load, so a mullite matrix holds its shape and volume at temperatures where a fireclay-bonded lightweight body would begin to shrink and slump. Keeping iron low also keeps thermal conductivity down and avoids the fluxing that shortens life in a reducing or alkali-bearing atmosphere.
Placement follows the standard castable sequence and needs a slow, staged dry-out, because a lightweight body holds water and releases it gradually. The material is a backing refractory: it insulates a hot face that is protected by a denser lining, and it is not intended for melt contact or for carrying structural load.
| Project Index | ZKM - 1.8 | ZKM - 1.5 | ZKM - 1.2 | |
| Aggregate | Lightweight Mullite Aggregate | Lightweight Mullite Aggregate | Lightweight Mullite Aggregate | |
| Al₂O₃ /% | ≥55 | ≥50 | ≥45 | |
| SiO₂ /% | ≤40 | ≤45 | ≤50 | |
| Maximum service temperature /℃ | 1500 | 1400 | 1300 | |
| Bulk density /g/cm³ | After drying at 110℃×24h | 1.8 ± 0.1 | 1.5 ± 0.1 | 1.2 ± 0.1 |
| Cold compressive strength /MPa ≥ | 110℃×24h | 10 | 5 | 4 |
| 1400℃×3h | 25 (at 1350℃) | 20 (at 1300℃) | 8 (at 1200℃) | |
| Linear change after firing /% ≤ | 1400℃×3h | ±1.0 (at 1350℃) | ±1.0 (at 1300℃) | ±1.0 (at 1200℃) |
| Thermal conductivity /W/m·K ≤ | 500℃ | 0.45 | 0.40 | 0.30 |
| 800℃ | 0.55 | 0.50 | 0.35 | |
Grades are separated by density and mullite content. A lower-density grade around 1.5 g/cm3 gives the best insulation and is used where the hot face is fully protected; a higher-density grade around 2.0–2.2 g/cm3 is more robust under mechanical contact and thermal cycling while still insulating well. Higher mullite content raises the service ceiling and improves volume stability.
Choose by comparing the temperature the backing will actually see with the limits of each option. Below about 1200°C an insulating castable is usually sufficient and lower in cost. Where the backing runs hotter, a lightweight mullite castable is the appropriate upgrade. Where the position is a dense hot face rather than a backing, a high alumina castable or a brick working lining is the right choice, with the lightweight mullite castable kept behind it.
Lightweight mullite castable is specified where a lining must insulate at a temperature an ordinary insulating castable cannot survive:
Manufacturing is controlled from raw material through to finished brick. Four automatic batching lines feed 20 CNC presses ranging from 630 to 2000 tonnes, so density is set by press force rather than operator judgement, and firing runs through four 108-metre fully automatic tunnel kilns and five ultra-high-temperature shuttle kilns for a uniform soak.
Every batch is tested for bulk density, apparent porosity, cold crushing strength and refractoriness under load, and ships with a certificate of analysis. XZK Group holds ISO 9001, ISO 14001, ISO 45001 and ISO 50001 certification and operates the Henan Provincial Engineering Technology Research Center for Sintered Refractories for Glass Furnaces.
Lightweight mullite castables complete the monolithic refractory range with high alumina castable, insulating castable, silicon carbide castable and mullite insulating bricks. Browse all refractory products or the insulation range.
Al₂O₃:46%-60%;
Fe₂O₃: 0.7%-1.0%;
Operating Temp:1300-1500℃;
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