resmelt

Resmelting is the practice of melting down metal that has already been melted once, typically because the first casting failed, contained defects, or consisted of mixed or contaminated scrap. The process sends the solid metal back through the furnace to achieve a molten state again, allowing for removal of oxides, gases, and foreign material before pouring into a new mold or ingot. This differs from initial smelting, which extracts ore; resmelting works with already-refined or partially refined metal.

The most common use is processing scrap from rejected castings, machining chips, gates, risers, and runner systems from foundries. Aluminum alloys are frequently remelted because they oxidize readily during the first pour and can trap air; resmelting under controlled flux cover reduces oxide content and improves mechanical properties. Steel and copper scrap streams also rely heavily on resmelting as a cost-effective way to upgrade material quality and reduce waste to landfill.

Why resmelting matters

In aluminum foundries, the oxide film that forms during casting can trap hydrogen and create porosity that causes tensile and fatigue failures. A second melt under inert atmosphere or with proper flux allows degassing and oxide removal that a single melt cannot always achieve. The furnace charge is also visually inspected and foreign material removed before resmelting, which catches contamination that initial smelting may have missed.

The economics are significant. Scrap aluminum can lose 5 to 15 percent of its mass during resmelting due to oxidation and dross formation, but the recovered metal commands a premium over buy-in cost because of improved purity. Conversely, repeated resmelting degrades alloy composition; elements with low boiling points volatilize, and grain structure coarsens. Most specifications limit the number of remelts allowed, typically two or three, before material must be downgraded or discarded.

Temperature control and hold time are critical. Holding molten metal too long increases grain size and oxide formation; too brief a melt means incomplete gas removal. Induction furnaces and crucible furnaces are standard for resmelting small batches, while larger operations use reverberatory or electric arc furnaces with ceramic or graphite crucibles. The furnace charge is usually covered with a protective flux or inert gas blanket to minimize atmospheric oxidation.