value of zinc process, page-6

Zinc has a much lower melting and vaporization temperature than iron.

Iron melting point = 1540 deg C
Iron boiling point = 3000 deg C
Zinc boiling point = 609 deg C

The b.p. of zinc is below the melting point of iron, so when it is charged into the furnace, it vaporizes off, condenses and solidifies to dust as it leaves the furnace, then oxidizes to zinc oxide. You end up getting a much higher concentration of zinc in the fume dust than you do in the initial charge.

The same thing happens with any lead in the scrap.

May not be exactly 50%, but I recall that sort of figure from somewhere.