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A typical open pit mine near Tuscon, Arizona. The benches are about 20 feet high. Note
the red zone parallel to the land surface, showing the depth of weathering, and the bluish
tint of the fresh rock. This is a porphyry copper deposit: copper minerals
disseminated in a fine-grained granitic rock. The fresh rock is not economically minable,
but weathering leaches copper from the surface and concentrates the ores at deeper levels,
making the deposit rich enough to mine. |
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A typical mining truck, capacity 50 tons or so. The capital costs of even a small mine
are in the millions. |
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As Crocodile Dundee would put it: "That's not a mine. This is a
mine." This is the Bingham Canyon copper mine west of Salt Lake City. From bottom to
top the mine reaches 2,500 vertical feet and has its own climate zones. Note the haze in
the bottom and the snow on the higher levels. |
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Everything at Bingham Canyon is on a grand scale. The shovel cab is the size of a
house - it fills a rail car with one scoop. Tires for the earth movers are
twelve feet in diameter and cost $20,000. |
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An open pit at Sudbury, Ontario. |
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Where do you go from here? The bottom of the pit is 700 feet below. The pit can't be
deepened without widening it, and can't be widened without beginning to move huge amounts
of barren rock. Solution: start underground mining from the bottom, linking to existing
tunnels from other mines. |
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The device here revolutionized iron mining. In the 1950's, we only had a century or so
of iron ore reserves. Low-grade ore (called taconite)was abundant, but too finely
mixed with silicate rock. It could be separated, but the resulting ore was too
fine-grained - it would blow right out of a blast furnace. |
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This is how the problem was solved. The ground ore is mixed with clay and sprayed onto
a rotating conical drum, where it rolls up into balls. When the balls reach a certain size
they roll off the drum onto a conveyor belt. The result is the familiar gray pellets found
along every railroad track in the country. Thanks to this process taconite can be mined
readily and our iron reserves, for all practical purposes, are unlimited. |
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The perfect tread impressions in this sand near Ripon, Wisconsin give some clue as to
its use. Most of the sand from this pit (in the St. Peter Sandstone) is exported to Mexico
for sand casting automobile engine blocks. Any time you see a metal part with
rounded corners and a pebbly finish, it was probably sand-cast. The sand can be re-used,
of course, but eventually the grains become too rounded to hold impressions well. |
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The open areas among the wooded hills are coal strip mines in eastern
Kentucky. |
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Everyone's mental picture of a mine: the molybdenum mine at Climax, Colorado. Although
the scene is 2,000 feet underground, it's still 10,000 feet above sea level. Older mines
had rails, but modern mines tend to use diesel earth movers. To do that, you need adequate
ventilation, and most large mines have a noticeable wind blowing through them. Some mines
move more air, pound for pound, than they do ore. |
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All mining is like a military retreat - you pull back with careful
planning all the way. The mining at Climax was once done by stoping
- caving in the ore from below. By 1969 the mine had collapsed the top of
the mountain to produce this "glory hole", looking quite a bit
like the crater of Mount Saint Helens.
The mine is sitting right on the Continental Divide at 11,000 feet
elevation. This picture was taken in May. |
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Later on, mining was carried on by surface mining. This is the mine in
the summer of 1998. |
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One of the most unusual mines in the United States - an underground sand mine
near Pepin Wisconsin. The mine produces fracturing sand used in oil drilling to
hold fractures open to speed oil flow. Such sand has to be coarse, very hard (to withstand
enormous pressure), and very round (so it can penetrate deep into fractures without
sticking). The coarse and poorly-cemented Cambrian sandstone here is ideal. |
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The first stage in processing ore is to crush it to roughly head-sized chunks. This
zinc mine near Knoxville, Tennessee has its crusher underground in the mine itself. |
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After crushing, the ore goes to a mill, such as this one near Sudbury, Ontario. |
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Big chunks of ore are ground to sand size in ball mills like these, filled with
ore and large steel balls. Yes, they do make a racket! They are not spinning as fast as
they appear - this is a long exposure in dim light. The stationary mill shows the studs
that help keep the ore from slipping as the mill rotates. |
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This large mill complex is operated by only about 100 workers and run fromthis control
room. |
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Once milled, the ore is separated from the silicate residue by flotation.
Surprisingly, it's the dense ore that floats. Legend has it a miner's wife noted that ore
particles stuck to her soapsuds while washing her husband's clothes. The milled ore is
mixed with a surfactant (fancy name for suds) sticks to the foam and is skimmed off while
the barren silicate rock sinks. |
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Pulverized silicate residue from the flotation process is called tailings.
Tailings at Sudbury, Ontario are deposited here. In places the tailings are over 200 feet
deep. |
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Bingham Canyon, Utah, has a vast waste pile. |
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Writers of the "ancient mysteries" school claim that even today we could not
build the Great Pyramid. At peak production, Bingham Canyon moved the volume of the Great
Pyramid every month. To get an idea of the scale of the waste piles,
look at the buildings at the base of the pile, bottom center. |
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Some tailings present particular health problems, like these uranium mine tailings in
New Mexico. |
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A common environmental hazard of mining is iron contamination, leached out of the mine
itself or mining wastes. A little iron is essential to life; too much is toxic. The rocks
in the bottom of this stream near Boulder, Colorado have been colored orange by iron from
old mines. |
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Small but very deadly. The mine here on the slopes of Mount Diablo, near San
Francisco, CA, is barely the size of a football field but produced mercury. Associated
minerals in the deposit contain arsenic, antimony, and other delectables. The pond catches
the polluted runoff - we hope. |
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The mine at Climax, Colorado, sends its waste water over the Continental
Divide to the Colorado River, with a long chain of settling and filtration
ponds extending down-valley. Note the iron staining around this pond, the
highest in the chain. |
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The smelter complex at Sudbury, Ontario. You are looking at well over a billion
dollars worth of industrial facilities. The dark, medium-sized stack is nearly as tall as
the Washington Monument. The tall stack is 1250 feet high - tallest in the world when it
was built. The metal tubes leading to its base are almost 20 feet in diameter. |
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The Sudbury Superstack. This single stack emits one per cent of the entire planet's
sulfur emissions - up to 40,000 tons per day. |
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Smelting separates metal ores from silicate slag. Here slag is being poured off a
crucible. To tip it, someone has to attach a chain to the rear of the crucible. I talked
to a retired smelter worker who told me of a friend who was attaching the chain when the
shackle broke and dumped the contents of the crucible on him. |
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Slag is poured into these cars and taken to the slag disposal area. The white material
is lime to prevent slag from sticking to the metal of the cars. |
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The slag is dumped on a disposal heap. |
