Chemistry Reference
In-Depth Information
5.2.
METALLIFEROUS ORES
The overall abundance of most metals in the upper crust of the earth is rather
small (see Table 32). At such low concentrations, separating them from other
associated matter could be quite a monumental task. Fortunately, natural
geochemical processes have resulted in the uneven distribution and abun-
dance of the metals on the outer part of the planet. As a result of geologic
and natural weathering processes, metals occur in high concentration in geo-
logic beds usually referred to as
metalliferous ores
, metal-bearing minerals,
valuable enough to be mined (Brimhall 1990; Ixer 1990). In the
ores
, the
metals are combined with other elements, mainly with oxygen, sulfur,
carbon, or silicon. Common ores of iron and tin, for example, are minerals
composed of the oxides of these metals, such as
magnetite
(composed of iron
oxide) or
cassiterite
(composed of tin oxide). Copper, lead, mercury, silver,
and zinc, and also some iron, often occur in minerals in which the metal is
combined with sulfur (in the form of metal sulfides) and even with other
nonmetals. It is from these ores that practically all metals have been recov-
ered, since antiquity (Brimhall 1990).
Between the fifth and third millennia B.C.E., while already familiar with
the native metals, humans began to realize that there was a relationship
between metals and metalliferous ores. It was probably at this time that the
process of
smelting
, for the production of metals from metalliferous ores, was
discovered. In retrospect, it is impossible to pinpoint with any degree of cer-
tainty precisely when, or where, the smelting process was first practiced. Nor
is it clear how the process was discovered. What is certain is that the dis-
covery of the smelting processes brought about the gradual replacement of
native metals by metalliferous ores as the main sources of metals. By the end
of the third millennium B.C.E., nearly all the metals that could be recovered
from metalliferous ores had been smelted and put to use (Maddin 1988;
Forbes 1997a).
5.3.
MINING
The procedures necessary for excavating and removing metalliferous ores
from their earthy surroundings developed quite early into an organized
“
mining technology
.” Ores on or near the earth's surface required relatively
easy mining techniques, while those far beneath the surface, could be
extracted only after digging, sometimes deep underground. Still others ores
had to be extracted from the bottom of lakes or rivers (Shepherd 1993;
Craddock 1990). The ores exploited in early antiquity were probably easily
