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Magnesium alloy melting furnaces typically use three main heating methods: resistance heating, gas heating, and fuel heating. Among these, resistance heating is considered the safest option. It comes in single-chamber, double-chamber, and triple-chamber configurations, each with distinct advantages.
A single-chamber furnace combines both melting and insulation within one unit, making it simple but less efficient for continuous operations. A double-chamber furnace separates the melting and holding functions, using a U-shaped tube to transfer the melt between chambers. This design helps maintain stable temperatures during charging, reduces slag formation in the melting chamber, and ensures higher purity of the magnesium alloy melt.
The triple-chamber furnace features a lower-temperature melting zone (around 650°C), which helps protect the protective CO₂ atmosphere and facilitates slag removal. The fluid supply chamber provides a high-temperature melt suitable for die casting. These furnaces are widely used for smelting cast magnesium alloys, similar in structure to aluminum alloy melting furnaces, but with modifications due to the different physical and chemical properties of magnesium.
Scale formation on the inner walls of the furnace can lead to dangerous reactions between iron oxide and molten magnesium, releasing large amounts of heat and potentially causing explosions. To prevent this, the furnace floor should be free of scale debris, and a leak-proof tray should be placed at the bottom to contain any spills. In areas where scale formation is hard to detect, a Ni-Cr alloy coating on the steel lining can help reduce scaling without affecting thermal efficiency.
Fuel-fired furnaces may experience gradual thinning of the refractory walls due to flame exposure, so regular inspections are necessary to avoid leaks. Magnesium melts can react violently with certain refractories, so selecting appropriate materials is crucial. High-alumina and ultra-high-temperature alumino-silica bricks (with 57% SiO₂ and 43% Al₂O₃) have proven effective in practice.
When designing fuel furnaces, the slag door should be easily accessible. Resistance heating systems often use low-melting materials like zinc sheets to seal the door. In case of a leak, zinc does not block the magnesium melt but helps suppress the "chimney effect," which could otherwise accelerate oxidation.
At or above the melting point, magnesium melts can ignite, so fluxing or using a 1% S mixed gas can help suppress combustion. With stricter environmental regulations, older fuel-heated furnaces are being phased out in favor of more efficient and eco-friendly alternatives.
The choice of furnace type and size depends largely on production scale. Small foundries often use updraft crucible furnaces for batch processing, while large-scale magnesium alloy production benefits from larger, more controlled melting units.
Alloy fluxes are added to crucibles and processed in coil furnaces, which handle melting, stabilization, and storage. Melt transfer should be done carefully to avoid turbulence, which can increase oxidation and inclusions in the final product. Direct-fired reverberatory furnaces are now obsolete due to excessive oxidation issues, while indirect-heating systems are rarely used due to low efficiency. Coreless induction furnaces, though more expensive upfront, offer lower operating costs and a smaller footprint, making them a preferred choice for modern applications.