Environmental Engineering Reference
In-Depth Information
a smelter producing 1 t Cu/day would have consumed
nearly 5000 ha/a. Demand for metallurgical charcoal
rose sharply once iron began replacing bronze, starting
c. 1000
B
.
C
.
E
. Copper and tin melt, respectively, at
1083
C and 232
C, but iron melts at 1535
C. Unaided
charcoal fire can easily reach 900
C, and even with sim-
ple forced air supply, its temperature can be increased to
close to 2000
C.
Charcoal has only a trace of S and P, but its friability
limited the mass of the smelting charge. This made no
difference as long as the furnaces remained small. The
simplest iron smelting furnaces were just partly enclosed
hearths built on hilltops to maximize natural draft. They
produced small masses (blooms; up to 50-70 kg) of
iron heavily contaminated with slag. This low-carbon
wrought iron had high tensile strength and was directly
malleable; it was easily forged, but it could not be cast.
Bloomery hearths were eventually succeeded by shaft fur-
naces, whose enlargement culminated in the medieval
development of St¨ckofen and then Blasofen. Blast fur-
naces originated in the Rhine-Meuse region in the four-
teenth century and produced cast (pig) iron, an alloy
with up to 4% C that cannot be directly forged or rolled
and is weak in tension but strong in compression.
Bloomery hearths and simple forges needed 3.6-8.8
times more fuel than the mass of charged ore (Johannsen
1953). With good ores averaging 60% Fe (75% of which
ended up in the molten metal), this implies 8-20 kg of
charcoal (240-600 MJ) per kilogram of hot metal. By
1900 a combination of technical advances brought the
typical rate down to about 1.2 kg (36 MJ/kg), and
the best rates in Swedish furnaces were as low as 0.77 kg
(22.9 MJ/kg) (Campbell 1907; Greenwood 1907). In
the year 2000, Brazil was the world's only consumer of
metallurgical charcoal, using about 2.9 m
3
of eucalyptus-derived fuel converted in masonry kilns for
1 t of pig iron, a rate of 21.5 MJ/kg of hot metal (Ferre-
ira 2000). The high energy intensities of charcoal-fueled
smelting caused extensive deforestation, first in copper-
and lead-producing Mediterranean regions, then in the
iron-producing Atlantic Europe. During the early eigh-
teenth century a single English blast furnace, working
from October to May, produced 300 t of pig iron
(Hyde 1977). With as little as 8 kg of charcoal per kilo-
gram of iron and 5 kg of wood per kilogram of charcoal,
it needed some 12,000 t of wood.
By that time deforestation was so common that the
wood was cut in 10-20-year rotations from coppiced
hardwoods, whose annual increment would be 5-10 t/
ha. In 1720, 60 British furnaces produced about 17,000
t of pig iron, requiring about 680,000 t of trees. Forging
added another 150,000 t, for a total of some 830,000 t
of charcoaling wood. At 7.5 t/ha, this represented about
1100 km
2
of forests and coppiced growth (equal to a 33-
km square). Already in 1548 anguished inhabitants of
Sussex wondered how many towns would decay if the
iron mills and furnaces were allowed to continue (people
would have no wood to build houses, watermills, wheels,
barrels, and hundreds of other necessities), and they
asked the king to close down many of the mills (Straker
1931).
Energetic constraints on preindustrial metal smelting
were thus unmistakable, and widespread European defor-
estation was to a large degree a matter of horseshoes,
nails, axes (and mail shirts and guns). Blast furnace loca-
tions were also constrained by the limited radius of
animal-drawn transport used to bring in charcoal and
the necessity for a continuous rapid water flow to power
bellows. Access to ore was also essential, but ore made
up only a fraction of charcoal's charge. Further, the
(725 kg)
