Metals 3: Recycling Of Zinc And Lead Flashcards Preview

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Flashcards in Metals 3: Recycling Of Zinc And Lead Deck (8)
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1
Q

Where does zinc scrap and residues come from?

A
  • Manufacturing and processing operations (process- or new scrap): zinc sheet and galvanised steel offcuts and trimmings, galvanising residues, die casting foundry returns, brass machining scrap
  • Discarded products (post consumer waste/old scrap): automobiles, tyres, household appliances, electronics components, street furniture, galvanised parts
  • Buildings: dismantles zinc roofs and guttering, etc.
  • Dusts from steelmaking
2
Q

What are the main sources of secondary zinc, and how are they recovered?

A
  • Brass scrap (Cu-Zn alloys): remelting
  • Die casted zinc: remelting
  • Galvanising: remelting (new scrap), refining (old scrap)
  • Rolled zinc: remelting
  • Other: remelting (NS), refining (OS)
3
Q

What are the different steps to recycle scrap zinc, or zinc containing residues, to the final metallic zinc or zinc alloys?

A

Zinc flue dust, retrieved from steelmaking, or zinc evaporated and vacuumed off. The zinc oxide is reduced into zinc in a rotary kiln.

4
Q

What are the principal chemical reactions that takes place in a Waelz rotary kiln process?

A

Oxidation: Zn + CO + O2 -> ZnO + CO2

Escaping CO gas an Zn vapour

Reduction:
ZnO + CO -> Zn + CO2
FeO + CO -> Fe + CO2

5
Q

What kind of residues are generated from the steel galvanising industry? How to recycle these residues?

A

ZnO, Fe-Zn alloys.

Liquidation: heat up the impure Zn to just above the melting point of Zn, then remove lead and iron.

Zinc evaporation: heat up above boiling point of 906 °C => Zn vapour, and ZnO. Then retrieve the zinc.

6
Q

What are the sources of lead scrap?

A
  • Lead acid batteries (80 %, 95 % recycled)

- Lead plumbing, plates and Pb alloys

7
Q

How are lead containing EoL products mechanically treated for lead scrap preparation?

A
  • Removal of sulphuric acid
  • Hammer mill or roll crusher, followed by sink/float separators to remove and sort plastics
  • Further separation of the metals from the paste, based on their size
8
Q

How is lead scrap metallurgically recycled?

A

The lead is melted => everything floats on top of lead, so impurities are skimmed off.