Chemistry Reference
In-Depth Information
PbS
+
O
=
PbO
+
SO
↑
2
2
galena
lead sulfide
lead oxide
sulfur dioxide
(
)
The lead oxide thus formed then reacts with galena still in the furnace to
yield molten lead, while more sulfur dioxide is evolved:
2PbO
+
PbS
=
3Pb
+
SO
2
↑
The molten lead flows to the bottom of the smelting furnace (which is
usually made of clay) and from there, through a channel and an opening
at the bottom of the furnace, collecting in a lead reservoir. Part of the
sulfur dioxide formed during the reaction is further oxidized by still more
air oxygen into sulfur trioxide:
2SO
+
O
=
2
S
O
2
2
3
sulfur dioxide
sulfur trioxide
The sulfur trioxide reacts with lead oxide created during the roasting
process (see above) to form lead sulfate
SO
PbO
PbSO
+
=
3
4
lead oxide
lead sulfate
and the lead sulfate reacts with still more of the remaining galena to
produce additional molten lead, which also drips to the bottom of the
furnace and into the molten lead reservoir:
PbSO
4
+
PbS
=
2Pb
+
2SO
2
On cooling, the lead solidifies as an
ingot
in the shape of the lead reservoir.
Freshly cast lead has a bright, silvery appearance. On exposure to the
atmosphere, however, lead in the surface layer combines with atmospheric
oxygen and carbon dioxide to form a dark, stable gray coating of mixed lead
oxide and basic lead carbonate. This layer usually protects the metal from
further oxidation and corrosion (see Fig. 38). Protected by a weathered
surface layer, solid lead is stable to further corrosion. Lead is also very ductile
and soft, being the softest metal known in antiquity. It is mainly because of
these properties that lead was widely used for building, to make pipes and
roofs, and in naval construction, for example. Solid lead flows, albeit very
