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Ingot Casting

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Parent
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Casting and Forming
Turning molten aluminum into useful shapes
Current
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Ingot Casting
Pouring aluminum into molds for storage or remelting

Remelting scrap aluminum into ingots is the practical way a village gets the metal: sort the scrap, dry it completely, melt it at 660 °C in a crucible, skim the dross, and pour at 700-750 °C into steel or cast-iron moulds preheated to 200 °C, making ingots of 5-20 kg for storage or later casting. The heat needed is about 0.27 kWh per kilogram, a fiftieth of making new metal, but a damp piece of scrap or a cold mould can throw the whole melt.

Part of Aluminum Smelting

Melting scrap or smelter output and casting it into ingots, the step a settlement will actually use.

This Is The Step Most Villages Will Reach

The carrier describes ingot casting as pouring tapped aluminum into steel or cast-iron moulds preheated to 200 °C, in ingots of 5-20 kg for easy handling. For a village without a smelter the feed is scrap, not tapped metal (Why Aluminum Matters). Melting takes only an ordinary crucible furnace: aluminum melts at 660 °C (Metalworking and Soldering), well within the range of a charcoal forge (Metalworking) and crucibles (Crucibles).

What You Need

  • Scrap, sorted. Remove steel, plastic, wood and rubber; a magnet pulls steel inserts and rivets (Metal Identification). Keep wire, castings and sheet in separate piles, because each may be a different alloy and remelting cannot sort them.
  • A dry crucible and furnace.
  • Cast-iron or steel ingot moulds, dry and preheated to 200 °C (carrier).
  • A skimmer, a ladle and a dry sand bed for spills.
  • Tongs, face shield, leather apron and gloves.

Method: One Melt

Step 1: Dry everything. Heat the scrap gently on the furnace edge before it goes in, and warm the mould to 200 °C. Metalworking and Soldering gives the same rule: dry the crucible, scrap and mould completely.

Step 2: Melt. Start with a little metal, then feed scrap in small pieces so it sinks into liquid; do not drop a large cold piece onto a deep pool.

Step 3: Skim the dross (oxide skin and floating dirt) with a dry tool.

Step 4: Hold at 700-750 °C, the pouring range the carrier gives for aluminum casting (Sand Casting).

Step 5: Pour steadily into the preheated mould. The carrier says to let the ingot cool slowly. As I recall aluminum shrinks when it freezes, so the top of an ingot sinks; fill the mould slightly proud and do not use the sunken top as sound metal.

Step 6: Mark each ingot with its source and alloy class. Without analysis, the label is the only record.

Worked Example: Energy For A Batch

The corpus gives 971 kJ to heat and melt 1 kg of aluminum from 20 °C, which is 971 / 3,600 = 0.27 kWh/kg.

  • 5 kg ingot: 5 × 971 = 4,855 kJ = 4,855 / 3,600 = 1.35 kWh.
  • 20 kg ingot: 20 × 971 = 19,420 kJ = 5.39 kWh, about 5.4 kWh.

That is the heat that must reach the metal. A charcoal furnace delivers only a fraction to the charge; if it delivers 20% (an assumed value), the 20 kg ingot needs 27 kWh of fuel heat. The crucible also takes heat. For comparison, new metal takes about 14 kWh per kilogram electrically (Operating Parameters), or 14.4 × 20 = 288 kWh for the same 20 kg.

What Remelting Cannot Do

  • It cannot purify. Alloy in, alloy out; mixed scrap gives a mixed alloy fit for castings but not for anodizing (Aluminum Anodizing rates cast alloys poor) or for conductors.
  • It loses metal to oxide, especially from thin scrap with a large surface; the corpus gives no loss figure.
  • It does not remove paint or coating, which burn off as fume (Metalworking and Soldering warns of poisonous fume from coated scrap).

Common Mistakes

  • Melting sealed or hollow pieces. Closed tubes, cans and sealed containers can burst.
  • Using a rusty steel pot or ladle. I recall that molten aluminum can react violently with iron rust; unsourced.
  • Pouring onto bare concrete. I recall that moisture in concrete can spall explosively under molten metal; unsourced. Pour over dry sand.
  • Pouring into a cold, damp mould.

Safety

Molten aluminum and steam

Water or damp scrap in a crucible of molten metal turns to steam instantly and throws the melt, and a cold mould does the same. Dry and preheat scrap, tools, ladles and moulds; never add scrap to a deep pool in a rush; keep spills on dry sand. A 20 kg pour is heavy: use a lifting arrangement and a clear path. Work outdoors or under a flue: coated scrap gives poisonous fume. Wear a face shield, leather gloves and apron, and closed boots.

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