| Steel
production by electric-arc furnace (EAF) technology has been increasing
in importance over the past 20 years at the expense of traditional
open hearth and basic oxygen converter technology, reaching an estimated
33.4% world share in 1999. This trend looks set to continue, and it
is expected, for example, that more than 50% of the steel produced
in the USA will be by this route within the next few years (1999:
46.2%). When steel is produced using an electric arc furnace, about
15 - 20 kg of dust is formed per tonne of steel. This dust is considered
as a toxic waste due to its content of heavy metals. It is estimated
that the world-wide total production of EAF dust could be as high
as several million tonnes, all of which must be treated, recycled
or land-filled. |
|
The
EAF dust forms as a result of volatile metals, like zinc and lead,
passing into the vapour phase at the operating temperature of the
furnace and being oxidized and cooled in the extractive air flow.
These metals are therefore found in the resulting dust both as free
oxides and in the form of composite structures with iron oxides. These
latter compounds are notably of the spinel type, MFe2O4 (M = Fe, Zn,
Ni, Mn or Cd). Overall, EAF dusts contain major elements of the type
iron, zinc, calcium and silicon (these last two from the degradation
of the refractories) in the form of simple or mixed oxides, and minor
elements like copper, manganese, chromium, cadmium and lead present
in the scrap iron raw material or introduced as additives. The dusts
also generally contain significant amounts of chlorides. |