An Introduction to Mining and Metallurgy - Thanatia: The Destiny of the Earth's Mineral Resources - page 167

7.3 Mining
According to the U.S. Department of Energy (DOE, 2007), 6% of the U.S. primary
energy consumption is used in the mining industry 1 (excluding oil and gas).
Mine size varies from small underground ones, producing less than 100 tonnes of
ore per day, to open pits which daily remove thousands of tonnes. Extracted materi-
als can be coal, metal minerals and industrial minerals. Metal minerals are the raw
materials for metal production whilst industrial minerals are generally extracted for
functional uses such as the production of fillers and pigments.
The opening of a new mine implies a significant investment in machinery and
infrastructures which in turn result in considerable social and environmental impacts
which can either be temporal or definitive in nature. Once a mine starts producing,
necessary activities include not only the extraction of ores and minerals but also
the treatment processes such as grinding, which can take place either on or off-site.
The most common mining method is that of open pit, which is also known as
open cast. In the United States, 97% of all metals are currently mined in open
pits whilst globally this equates to some 75%. Surface mining requires less energy
per tonne of mined material than its underground counterpart but its impact on
land is much higher. Essentially, one chooses one method over another based on
the nature, size, depth and grade of the deposit. Mines are progressively enlarged
until exhaustion unless they become uneconomic, i.e. their stripping costs exceed
ore value.
Depth and grade relate to overburden, defined as the surface material covering
the deposit and waste rock, i.e the rock removed in the mining process to provide
access to the ore body. The problem with the latter, is that the volume can be
several times greater than the amount of ore extracted (stripping ratio).
In both underground and open pit, operations are classified into extraction; ma-
terials handling; beneficiation or processing. Extraction involves: blasting, digging,
drilling, dewatering and ventilation. Blasting uses explosives for fracturing rock.
Digging is a form of direct excavation. Drilling creates holes in the rock and is a
technique mainly used for exploration, blasting preparation or tunnelling. Pump-
ing, de-watering and air ventilation meanwhile are safety operations used solely in
underground mining. Material handling and transport involves the use of vehicles,
conveyors and slurry lines.
The objective of beneficiating is to size and separate ore from unwanted ma-
terials. It predominately consists of crushing, grinding and separation processes.
Crushing is the primary step that converts the run-of-mine into medium size parti-
cles. These coarse materials become further reduced to the desired granulometry in
grinding processes that use abrasives as a cutting medium. Size reduction processes
may also be carried out through wet screening and via different milling operations.
1 The mining industry consumes approximately 1,315 PJ/yr of the 106,800 PJ/yr total U.S.
primary energy consumption (DOE, 2007).