Surface And Underground Iron Ore Mining
Surface Mining
When the iron ore lies close to the surface, it often can be
uncovered by stripping away a layer of dirt, sometimes only a few
feet thick. The ore is mined from large open pits by progressive
extraction along steps or benches. The benches provide access to
progressively deeper ore, as upper-level ore is removed. After the
soil and overlying rock are cleared, the ore is drilled and
blasted. The portion of the ore body to be removed is first drilled
in a specific pattern, and the holes are loaded with explosive
mixtures and blasted. Following blasting, the fractured ore is
loaded by huge electrical shovels, hydraulic excavators, or
front-end loaders onto large dump trucks. The wide holes in the
ground created by drilling, blasting, and ore removal are referred
to as "open pits"
Underground Mining
Underground mines are established in areas with promising ore
deposits. Iron ore deposits may lie deep underground. A shaft must
be dug from the surface and an elevator or hoist must be installed.
The shaft is the primary vertical channel through which people and
ore are transported in and out of the mine. It may be desirable to
sink a vertical shaft in barren wall rock at one end or to one side
of the ore deposit to keep haulage and hoisting facilities clear of
actual underground mining while minimizing haulage of ore
underground to the mine exit. The miner's elevator is called a cage
and the ore reaches the surface via a car called a skip. A
ventilation system near the main shaft ensures that miners receive
fresh air and prevents the accumulation of dangerous gases. Miners
cut tunnels (drifts) branching out from the shaft at various levels
to access the veins of ore. These levels are, in turn, connected by
openings called raises. Stopes are the chambers in which ore is
broken and mined. Cars or other conveyors carry the ore to the
shaft, where it is hoisted to the surface. The basic mine plant for
underground mining operations consists of headframe, hoist, timber
framing and storage area, miner's change house, compressor house,
machine shops, warehouse, office, ore storage, ore loading, and
shipping facilities. Most iron ore leaves the mine by rail, after
which, much is transferred to ships. A much larger proportion of
ore is moved by water in the United States than elsewhere because
of the proximity of the mines to the Great Lakes, which offer
low-cost transportation. No U.S. taconite mine is more than about
100 miles from the Great Lakes, and most are much closer. A
lesser-used method of transportation is an iron ore slurry
pipeline.
