Life cycles of common metals

This post is from my notes for the first week of lecture in Wheels of Metals: Urban Mining for a Circular Economy on Coursera, and it highlights—for common metals (iron, copper, aluminum)—useful properties that makes the metal attractive for applications, what applications the metal is used in, where and how the metal is mined, how the metal is processed, and how it is recycled.

Iron

Mixed metals. Image credit: nde-ed.org

Useful Properties

• Malleable, soft
• Strong with the addition of carbon (to form steel)

Uses

• 98% as steel: Fe + <2% C
  • Pig iron: >2% iron, becomes brittle
• 11-16 tons steel per capita
• Most steel in buildings, infrastructure

Mining

• Large-scale open pits
  • China: 44%
  • Australia: 18%
  • Brazil: 13%
• Transported by ship and railway

Processing

• Iron oxide ores converted to pure iron by reducing iron oxide by heating and oxygenating
  • Heated with coke (coal-derived carbon-rich fuel)
  • Oxygen steel making: pure oxygen blown over hot ore to produce low carbon steel
  • Electric arc furnace: scrap metal and pig iron heated together to produce low carbon steal
• Add up to 50% other elements for specialty steel: up to 30% Cr in stainless steel, with Ni, Mn; rare earth elements add heat resistance

Recycling

• Most iron still in active use
• Collected from demolished buildings
• Easily collected from landfills with magnets

Copper

Useful Properties

• Very high conductivity
• More readily available than other good conductors like silver
• Doesn't form oxides, which will spark, as readily as aluminum, so preferable inside buildings
• Biostatic, preventing bacterial growth
• Very strong with the addition of tin (to form bronze)

Uses

• Electric wiring: 60%
• Plumbing: 20%
• Industrial machines: 15%

Mining

• Mostly open pits (75%), some underground mines (25%)
  • Chile: 32%
  • China: 9%
  • Peru + US: 7%

Processing

• 50% from chalcopyrite ores (CuFeS₂)
  • Low Cu concentration in ore (25-35%)
• Grind ore, then refine
  • Pyrometallurgical for sulfide ores: oxidation at high temperatures via smelting, then analytic refining
  • Hydrometallurgical for oxide ores: leaching to generate copper ions, extraction via electrowinning, conversion into pure copper by electrolysis

Recycling

• Most copper still in active use
• Used in applications that makes accessibility, recovery easy
• Eddy current separators
• Common target of theft

Aluminum

Useful Properties

• High conductivity
• Very lightweight, but strong
• Most abundant metal in crust

Uses

• Transportation
• Construction (outer shells of buildings)
• Overhead power lines

Mining

• Bauxite contains 40-60% aluminum oxide
• Large-scale open pits
• Purified on site, transported to refinery
  • Waste product: red mud

Processing

• Requires plentiful electricity
• Electrolysis reduces oxide to create pure aluminum

Recycling

• Most still in use
• Used in applications where easy to recover
• Eddy current separators

Rare Earth Elements

Properties

• 17 elements of very similar chemical and physical properties
• Called "rare" because before special technology was developed, similar properties meant individual REEs could not be separated

Uses

• Energy-efficient light bulbs
• Magnets
• Electric motors & wind turbines

Mining

• Large-scale open pits
  • China: 90%
    • By-product of iron mining, clay minerals

Processing

• Concentrated via acid roasting and leaching
• Separated via solvent extraction into metal oxides for transport
• Oxides reduced onsite via electrolysis to create pure metals

Recycling

• Low recycling rate due to difficulty of recovery
• Technology improving