Geology Reference
In-Depth Information
Currently, the separation of ore from waste is achieved by taking advantage of some
of the physical properties of materials. Centrifugal separation is typically used for
coal, whilst for metals and minerals flotation is common. Other possible methods
are suspension, dispersion, screening, gravity separation, magnetic and a number
of other ad-hoc separations. All are customarily highly intensive with huge energy,
water and material demands.
Energy, water and chemical consumption in mining is highly dependent on ore
grade and thus varies mine to mine, depending on rock hardness, stripping ratio
and whether the mine is open pit or underground. All factors greatly influence
energy costs. A large proportion of energy consumption is associated with milling
operations that are responsible for approximately 40%, albeit with great variations
that typically accompany the given type of milled material (DOE, 2007). This is
because the comminution processes (see Sec. 9.5.2.3) are irreversible. Thus, the
energy required to grind minerals (also referred to as work index) varies from 1.43
kWh/t for calcined clay to 134.5 kWh/t for mica. Meanwhile, the energy inten-
sive separation processes are centrifugation in coal mining and flotation for metals
and minerals. The energy requirement per tonne of ore is significantly higher in
underground mining as opposed to open pit with a typical ratio of 25:1 but with
important deviations depending on local geology, the size of the mine and the qual-
ity of the equipment. The U.S. Department of Energy (DOE, 2007) for instance,
claims that the best available practices for eight mined commodities (coal; potash,
soda ash and borate; iron; copper; lead and zinc; gold and silver; phosphate rock;
and limestone, which collectively account for 78% of the total energy use in the U.S.
mining industry) could reduce their associated energy consumption by up to 50%.
In open pit, it is the transfer of materials, after grinding, which consumes the most
energy (in the form of diesel oil accounting for some 17%) whilst in underground
mining, the highest energy consumption occurs in the dewatering and ventilation
processes. In cases where water needs to be pumped out of the mine, the process
of dewatering may have a high energy cost due to the large volume of water to
be pumped and the elevated height to overcome. Water of course can be treated
and reclaimed but this requires a yet further use of energy and chemicals. Water,
generally not just in dewatering, plays an important role in mining (see for instance
Mavis (2003)). It is required for drilling, size reduction, the transport of slurries
or even in separation processes, such as flotation in which water is required at a
certain quality. It is also employed for dust control and the prevention of explosions
in underground mining.
According to the Network on European Sustainable Minerals Industries Report
(NESMI, 2005), there is a need for technological and fundamental research in mining
processes. In particular, a development of new technologies was proposed in conti-
nuous mining systems; remote mining methods; integrated transportation systems;
new equipment for the excavation of hard rocks; high pressure roller and vertical
mill systems; and variable speed control pumps.
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