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Materials selection and design

Take the production energy of aluminum alloy as 200 MJ per kg for primary metal made from ore and 15 MJ per kg for secondary (recycled) metal made by remelting scrap. Which value is closest to the production energy per kg of a material containing 60% secondary metal by mass?

  1. AAbout 108 MJ
  2. BAbout 126 MJ
  3. CAbout 89 MJ
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Answer: C. About 89 MJ

With 40% primary and 60% secondary metal, the weighted average is 0.40 × 200 + 0.60 × 15 = 80 + 9 = 89 MJ/kg. The larger the share of secondary metal, the further the production energy falls below that of primary metal alone. About 108 MJ is the simple average (200 + 15)/2, ignoring the proportions, and about 126 MJ reverses the proportions, 0.60 × 200 + 0.40 × 15. Aluminum uses large amounts of electricity in smelting, while remelting needs less than a tenth of the energy of primary production, so recycling is especially effective for this metal. However, wrought alloys are sensitive to elements mixed in with scrap, so collecting scrap sorted by alloy type is important for maintaining quality.

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