Environmental Engineering Reference
In-Depth Information
t, the Bucyrus-Erie 4250W walking dragline at Ohio
Power Company's Muskingum mine was not far behind.
Such machines allowed the maximum overburden to coal
ratio to rise from 1-2 before WW II to 5-6.
Mining mechanization resulted in the consolidation of
coal extraction into a smaller number of larger opera-
tions, and the rising productivity of labor led to falling
labor force totals and plummeting occupational deaths
and injuries. In Germany's Ruhr region the share of me-
chanical coal extraction rose from less than 30% in the
early 1950s to more than 95% by 1975 while the total
number of operations fell by about 90% (Erasmus
1975). The average productivity of U.S. underground
mining rose from less than 1 t per work shift in 1900 to
more than 3.5 t per labor hour by 2003. In surface oper-
ations it approached 10 t per labor hour, with productiv-
ities as high as 6 t per labor hour in longwalls and 35 t
per labor hour in the surface mines in Wyoming (EIA
2006b). By the year 2000 U.S. coal mining produced al-
most four times more fuel than it had in 1900, with less
than 20% of the former labor force. At the same time, ac-
cidental deaths declined by 90% since the early 1930s
(MSHA 2000).
Extraction productivities span a broad range, from no
more than a few hundred kilograms (2-6 GJ) in primi-
tive small rural Chinese mines to more than 2,000 t per
workday (
>
40 TJ) in the largest U.S. surface mine. Dif-
ferences in seams and extraction techniques result in un-
derground power densities as high as 1-2 kW/m
2
in
longwall mining but only 100-200 W/m
2
in smaller
pits with thin seams. Surface mining needs more land
than just the area overlying the worked seam, for tempo-
rary displacement of overburden before eventual recla-
mation and for on-site transportation and buildings. The
power densities of extraction in opencast mines are com-
8.4
Comparison of traditional room-and-pillar extraction and
modern longwall mining. From Smil (2003).
mining health risks are additional factors responsible for
the growing adoption of surface mining. By 2000 it pro-
duced 65% of U.S. output, about 40% of Russian output,
but less than 10% of Chinese extraction.
Remarkable economies of scale were achieved by using
larger shovels and walking draglines to remove thick
overburden layers (Hollingsworth 1966). In the United
States, Bucyrus-Erie and Marion were the dominant
makers of these machines (G. B. Anderson 1980).
Capacities of the largest shovel dippers rose from 2 m
3
in 1904 to 138 m
3
in 1965. Similarly, bucket volumes
of the largest walking draglines increased from less than
3m
3
before WW I to 168 m
3
. These machines had to
be erected on site, and they consumed as much electricity
as a city of 15,000-20,000 people (the world's largest
one, as much as 100,000 people). With 12,700 t, the
Marion 6360 shovel, built in 1965 for Captain Mine in
Illinois, was the world's heaviest excavator. With 12,247
