Key Takeaways
- 1Molten aluminium damages furnace linings differently than molten iron or steel does. It soaks into tiny pores in the lining, then reacts inside them and forms a hard mineral that expands and cracks the lining apart from the inside.
- 2Aluminium also reacts with ordinary refractory on contact, pulling silicon out of it and into the metal. That means the standard linings used for steel or iron furnaces simply can't be used anywhere aluminium touches.
- 3The fix is a special "non-wetting" lining material, designed so molten aluminium beads up and rolls off the surface instead of soaking in, stopping the damage before it starts.
- 4Different parts of an aluminium plant wear out differently. The furnace itself faces steady chemical attack and constant heating and cooling, while the channels that carry molten metal and the filter boxes face that same attack plus constant flow and wear at every corner and joint.
- 5Which aluminium alloy you're melting matters as much as how hot the furnace runs. Alloys containing magnesium are noticeably tougher on the lining than plain aluminium and need a stronger grade of lining, not just one rated for higher heat.
Why Does Molten Aluminium Need Different Refractories?
Molten aluminium doesn't fail refractory by heat, it fails it by chemistry: the melt wets and penetrates the pore structure, oxidises inside it, and grows corundum (Al₂O₃) that expands and spalls the hot face apart from the inside out. This is a completely different failure mode from the abrasion and slag attack that governs steel and iron linings, and it's why an aluminium furnace running at a modest 700-1300°C can destroy a lining that would comfortably survive far higher temperatures in a different service.
Aluminium also directly attacks any silica in the refractory: 4Al + 3SiO₂ → 2Al₂O₃ + 3Si. The reaction strips silicon out of the lining and into the melt, contaminating alloy chemistry while destroying the refractory's bond structure. Between penetration-driven corundum growth and direct silica reduction, an aluminium contact lining has to be engineered specifically against these two mechanisms, not simply rated to a high enough temperature.
Quick answer: Aluminium contact refractories are high-alumina, low-silica non-wetting castables. Non-wetting additives stop the melt entering the pore network, which is what prevents corundum growth before it starts. Silica content is kept low, our own grades run 11.5-12%, because aluminium reduces silica on contact. Service temperature is a secondary spec; penetration resistance and alloy compatibility are what actually determine lining life.
What Refractories Line an Aluminium Melting and Holding Furnace?
The melting furnace and holding furnace share the same core requirement, a non-wetting lining across every metal-contact surface, but see somewhat different duty. The melting furnace sees higher thermal cycling as cold charge is added; the holding furnace sees longer, steadier metal contact with less cycling but more time for slow penetration to develop.
| Zone | Primary stress | Refractory type |
|---|---|---|
| Hearth and walls (melting furnace) | Penetration + thermal cycling from charging | Non-wetting low cement castable |
| Hearth and walls (holding furnace) | Sustained penetration, less cycling | Non-wetting low cement castable |
| Belly band / metal line | Oxidation and flux attack at the air-metal interface | Higher-alumina non-wetting grade |
| Roof and burner zone | High temperature, combustion atmosphere, not metal contact | Conventional high alumina castable or brick |
| Launders and troughs | Penetration + continuous flow abrasion | Self-flow non-wetting castable |
| Filter and degassing boxes | Penetration + complex, joint-heavy geometry | Self-flow non-wetting castable |
Note that the roof and burner zone is the one part of the system that isn't in metal contact, and it can generally use conventional high alumina refractory rather than the non-wetting grade, since it never sees the penetration mechanism the rest of the furnace is built to resist.
Why Do Launders and Filter Boxes Fail Differently From the Furnace Itself?
A furnace hearth is a large, relatively simple shape holding a static or slow-moving metal pool. A launder or filter box is the opposite: a narrow, corner-heavy geometry with metal constantly flowing through it, being skimmed, and being mechanically disturbed during cleaning. Both see the same underlying chemical attack, but the launder adds continuous flow abrasion and far more joints and corners where a poorly-installed lining can start to fail first.
This is why launders and filter boxes are usually specified in a self-flow or mild-vibration castable rather than a heavier vibro-cast grade, the installation method has to suit the shape, not just the chemistry. A non-wetting castable that is correct for the furnace hearth but installs poorly into a launder's geometry will still fail early, from installation defects rather than from the material being wrong.
How Does Alloy Chemistry Change the Lining Choice?
Temperature alone does not distinguish between the two non-wetting grades commonly specified in a cast house, both are typically rated to the same service temperature. What changes is alloy chemistry. Al-Mg alloys are measurably more aggressive toward refractory than commercial-purity aluminium, and a furnace running Al-Mg needs a higher cold crushing strength, higher-alumina grade to hold the same campaign life a lower-grade castable would achieve on commercial-purity metal.
| Factor | Self-flow non-wetting grade | High-strength Al-Mg resistant grade |
|---|---|---|
| Al₂O₃ | 77.2% | 79.5% |
| SiO₂ | 11.5% | 12% |
| Service temperature | 1300°C | 1300°C |
| Cold crushing strength (110/816/1200°C) | 55 / 65 / 80 N/mm² | 80 / 110 / 120 N/mm² |
| Installation | Self-flow / mild vibration | Vibro cast, traditional or pump cast |
| Best fit | Launders, holding furnace, general contact | Melting furnace, Al-Mg alloys, high-impact zones |
Specifying by temperature alone and ignoring alloy chemistry is one of the most common reasons a cast house lining fails well short of its expected campaign, particularly in remelt and secondary aluminium operations where scrap-derived alloy chemistry varies from batch to batch.
SAPL Supply for Aluminium Cast House Refractories
Why this matters: a lining specified against peak temperature alone, with alloy chemistry treated as an afterthought, is the single most common reason an aluminium cast house reline comes early. Tell us your alloy (commercial-purity or Al-Mg) and the zone you're lining, and we'll match the grade rather than defaulting to whichever castable happens to be in stock.
Shanker Agencies supplies non-wetting aluminium contact castables for melting and holding furnace hearths, belly bands, launders, filter and degassing boxes, transfer ladles and cast house floors, in both self-flow and high-strength Al-Mg resistant grades. Send us your furnace capacity, alloy chemistry and the zone you're lining and our engineering team will confirm grade and quantity from your drawings.
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Frequently Asked Questions
Why does molten aluminium destroy refractory that survives molten steel fine?
Because the failure mechanism is chemical, not thermal. Aluminium furnaces run at 700-1300°C, well below steel-melting temperatures, so raw heat isn't the problem. The problem is that molten aluminium wets ordinary refractory and penetrates its pore structure, then oxidises inside those pores and grows corundum (Al₂O₃). Corundum takes up more volume than the metal that formed it, so the growth expands inside the lining and spalls the hot face away from the inside out, a mechanism that has nothing to do with peak temperature and everything to do with pore structure and chemistry.
Why can't silica-bearing castable or mortar be used in an aluminium furnace?
Molten aluminium chemically reduces silica on direct contact: 4Al + 3SiO₂ → 2Al₂O₃ + 3Si. The reaction strips silicon out of the refractory and into the melt, contaminating alloy chemistry, while destroying the lining's bond structure from within. This is why aluminium contact refractories are specified by low silica content (our own grades run 11.5-12% SiO₂) rather than by temperature rating alone, and why a patch made with the wrong material, even a small one, can fail within a single campaign.
What is a non-wetting castable and how does it stop corundum growth?
A non-wetting castable is a high-alumina, low-silica castable formulated with additives, commonly proprietary wetting-angle modifiers, that increase the contact angle between the melt and the refractory surface, so the metal beads rather than soaking in. Because the melt cannot enter the pore network in the first place, it cannot oxidise and grow corundum inside the lining. The castable's job is prevention, not resistance, once penetration has already happened, no amount of refractoriness stops the expansion that follows.
Do launders need a different refractory than the melting furnace?
Launders and filter boxes see the same chemical attack as the furnace hearth, but on top of it they see constant metal flow, more corners and joints, and more mechanical wear from skimming and cleaning. A furnace hearth can tolerate a slower-flowing, larger-mass lining; a launder needs a castable that installs cleanly into a narrow, geometrically complex shape and holds up under continuous flow abrasion, not just static contact. Self-flow non-wetting grades are generally the practical choice here because they install into that geometry without heavy vibration equipment.
Does the aluminium alloy change which refractory grade I need?
Yes, materially. Al-Mg alloys are more chemically aggressive toward refractory than commercial-purity aluminium, so a furnace running Al-Mg needs a higher cold crushing strength, higher-alumina grade even at the same 1300°C service temperature. Specifying by temperature alone and ignoring alloy chemistry is a common cause of linings that fail well before their expected campaign life, particularly in remelt and secondary aluminium operations where alloy chemistry varies batch to batch.
Can a worn aluminium furnace lining be patched, or does it need a full reline?
Localised wear, a worn ramp, a spot of erosion at the metal line, can often be patched with the same non-wetting castable family used for the original lining, provided the surrounding lining is still structurally sound. Never patch with a generic or silica-bearing material even for a small repair, it will be reduced by the melt and fail faster than the surrounding lining, undermining the patch. Corundum growth that has already penetrated deep into the lining, visible as a hard, expanded, cracked hot face rather than surface wear, is a reline indicator rather than a patching job.