Metals 2: Recycling Of Aluminium & Copper

Question 1

What are the benefits of recycling aluminium?

Answer 1

• Recycling 1 kg of Al saves up to 6 kg of bauxite, 4 kg of chemical products, and 14 kWh of electricity.
• Requires only 5 % of the energy, and only produces 5 % of CO2 emissions, compared with primary production and reduces the waste going to landfill.
• Al can be recycled indefinitely, as reprocessing doesn’t damage its structure.

Question 2

How is aluminium recycled?

Answer 2

New scrap: re-melting and direct use (in plant or external)

Old scrap:

• Pre-treatment and cleaning
• Alloy recipe preparation
• Melting and refining
  
  • Box furnace
  • Rotary furnace
  • With or w/o salt protection (Na3AlF6)
• Alloying and casting (=> ingot: 4 - 25 kg)

Question 3

When to use refineries and remelters?

Answer 3

A refiner is used when dealing with old and contaminated scrap, casting alloys. A remelter is used for new and cleaner scrap, wrought alloys.

Question 4

Why is salt fluxes sometimes used in aluminium refining from scrap? What are the main components?

Answer 4

• Protection from metal oxidation and cleaning. Any oxidation of Al is a permanent metal loss!
• 30 % KCl and 70 % NaCl, or alternatively 50-50.
  
  • Melting point is 645 °C
• Max 5 % flouride; NaF, AlF3, or Na3AlF6 (cryolite)
  
  • F breaks the oxide layer
  • Promotes separation of Al from the salt

Question 5

What are the economic and environmental issues regarding the use of salt flux?

Answer 5

Using salt is more expensive than not using it. Salt needs to be processed to be used again. Carbon content in scrap => carbide formation in slags => more methane generation.

Question 6

What are the secondary resources of copper for recycling?

Answer 6

The copper resources that are currently locked up in urban mines.

Urban mine = the stockpile of rare metals in discarded WEEE.

Question 7

How are copper wires and cables recycled?

Answer 7

• Chopping: 0.1 - 1 mm
• Separation from polymers by air classification (vibrating screen, heavy parts fall down)
• Steel wire: separation by magnetic separation
• Aluminium wire: air classification
• Final metals recovery:
  
  • Cu: P-S converters or anode furnace
  • Al: melting and refining in Al refineries

Question 8

How is copper metallurgically recycled/recovered?

Answer 8

New scrap: pure Cu or Cu alloys (brass, bronze)

• Direct re-use
• Remelting or fire refining, and/or alloying
• Casting, extrusion

Old scrap: various forms and grades
In secondary smelters:
- Smelting: blast furnace, electric arc furnace, top blown rotary converter(TBRC)/Kaldo => black copper (Cu-Fe alloy w/ 70 - 90 % Cu)
- Converting: P-S converter, TBRC/Kaldo (Fe-oxidation) => blister copper (96 - 98 % Cu)
- Fire refining: reverberatory/anode furnace (99 % Cu)
- Electrorefining: H2SO4 tank house (99.99 % Cu) - same quality as primary Cu!
- Melting and casting => pure Cu, Cu alloys