Chemistry Reference
In-Depth Information
TABLE 41
Common Lead Ores
Ore
Composition
Galena
Lead sulfide
Cerusite
Lead carbonate
Anglesite
Lead sulfate
Massicot (lead ocher)
Lead oxide
Minium
Lead oxide
had been known to early humans. Moreover, despite the fact that numerous
minerals include lead in their composition, only a few, listed in Table 41, can
be considered as lead ores to be exploited for the recovery of the metal; in
the table, the minerals are listed in decreasing order of their importance as
a resource of the metal.
Galena
seems to have been the main mineral resource for most of the
metallic lead smelted in antiquity. In addition to lead sulfide, the main com-
ponent of galena, many galena ores also contain appreciable amounts of
valuable impurities such as gold, silver, and copper, which made it eco-
nomical to process the ore for their recuperation. Thus, the ancient metal-
lurgic processing of galena was often designed to extract not only lead but
also copper, silver, and gold.
Smelting lead could have been carried out in simple furnaces fueled with
wood or charcoal and with an ample, regular supply of air. In Europe, before
the fifteenth century C.E., for example, lead-smelting furnaces consisted of
circular or rectangular stone structures with a clay floor, usually built into
the side of a hill. The ore was placed in the furnace on top of the fuel, which,
after being ignited, was fed with a forced air draft; under such conditions
the lead was reduced to the crude, metallic condition. Silver or gold in sig-
nificant quantities in the ore were generally recovered from the crude lead
by the cupellation process.
TEXTBOX
39
LEAD SMELTING
When
galena
, a lead ore (composed of lead sulfide) is roasted in a well-
ventilated, open furnace, part of the lead is oxidized by air oxygen to lead
oxide and the sulfur to sulfur dioxide, which is released into the atmos-
phere (see Textbox 33):
