Metals 3: Recycling Of Zinc And Lead

Question 1

Where does zinc scrap and residues come from?

Answer 1

• 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

Question 2

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

Answer 2

• 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)

Question 3

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

Answer 3

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

Question 4

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

Answer 4

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

Escaping CO gas an Zn vapour

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

Question 5

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

Answer 5

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.

Question 6

What are the sources of lead scrap?

Answer 6

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

- Lead plumbing, plates and Pb alloys

Question 7

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

Answer 7

• 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

Question 8

How is lead scrap metallurgically recycled?

Answer 8

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