26 / 66
24 Coal
coal to come out, and for ventilation so that fresh air is brought in and any dangerous
gases, like methane, are removed. Many shaft mines are less than 1,000 feet (300
meters) underground, but some go much deeper.
In
longwall
or
panel
mining, a machine moves along a wall-like coal face, digging
or gouging or scraping off the coal. Usually this falls onto a conveyor that runs
alongside the face and takes it to one end for removal. For safety, the rock surface
above the face is held up by table-like metal plates on hydraulic rams that press on the
roof. These roof supports are put up in stages, with the whole system advancing as the
block-like panel is cut back. Left behind is the unsupported roof over a chamber-like
empty area called the goaf, where coal once was. Here the roof is usually allowed to
collapse in a safe, controlled way. The whole process hardly pauses and is known as
continuous mining.
An open-pit coal mine in the Rocky Mountains.