معدن کاری کانسارهای زیر زمینی
Underground mining
The extraction of ore from beneath
the surface of the ground. Underground mining is also applied to deposits of
industrial (nonmetallic) minerals and rocks, and underground or deep methods are
used in coal mining. Some ores and industrial minerals can be recovered from
beneath the ground surface by solution mining or in-place leaching using
boreholes. See also: Coal mining;
Solution mining
Underground mining involves a larger
capital investment and higher production cost per ton of ore than open pit
mining. It is done where mineral deposits are situated beyond the economic depth
of open pit mining; it is generally applied to steeply dipping or thin deposits
and to disseminated or massive deposits for which the cost of removing the
overburden and the maintaining of a slope angle in adjacent waste rock would be
prohibitive. In some situations, the shallower portion of a large orebody will
be mined by open pit methods, and the deeper portion will be mined by
underground methods. See also:
Open-pit mining
Underground mine entries are by
shaft, adit, incline, or spiral ramp (Fig. 1). Development workings, passageways
for gaining access to the orebody from stations on individual mine levels, are
called drifts if they follow the trend of the mineralization, and cross-cuts if
they are driven across the mineralization. Workings on successive mine levels
are connected by raises, passageways that are driven upward. Winzes are
passageways that are sunk downward, generally from a lowermost mine level.
Fig. 1 Underground mining entries and
workings.
In a fully developed mine with a
network of levels, sublevels, and raises for access, haulage, pumping, and
ventilation, the ore is mined from excavations referred to as stopes. Pillars of
unmined material are left between stopes and other workings for temporary or
permanent natural support. In large-scale mining methods and in methods where an
orebody and its overlying waste rock are allowed to break and cave under their
own weight, the ore is extracted in large collective units called blocks,
panels, or slices. See also: Mining
Exploration
Exploration and development
constitute the preproduction stage of underground mining. Exploration refers to
the delineation of a newly discovered mineral deposit or an extension of a known
deposit and to its evaluation as a prospect. During exploration, the deposit is
investigated in sufficient detail to estimate its tonnage and grade, its
metallurgical recovery characteristics, and its suitability for mining by
various methods.
Information on the size, shape, and
attitude of a deposit and information for estimating the tonnage and grade of
the ore is taken from drill holes and underground exploration workings. Diamond
core drilling provides intact samples of ore and rock for assaying and for
detailed geologic and geotechnical study; percussion drilling provides chips of
material for the recognition of ore and waste boundaries and for additional
sampling. Underground exploration workings are used for bulk and detailed
sampling, rock mechanics testing, and the siting of machinery for underground
drilling. See also: Drilling,
geotechnical
The tonnage and the grade of the
material available in a mineral deposit are interrelated. The cutoff grade is
the weakest mineralization that can be mined at a profit. Ore reserves are
calculated in respect to the amount of ore in place at potential cutoff grades,
the tonnages and average grades in identified blocks of ore, and the ultimate
tonnage and grade of ore that should be available under projected conditions of
recovery and wall rock dilution in mining. The suitability of a deposit for
mining is determined in testing and evaluation work related to the physical and
chemical nature of the ore, hydrologic conditions, and the needs for ground
control. See also: Rock mechanics
Mine
Development
Where high topographic relief allows
for an acceptable tonnage of ore above a horizontal entry site, an adit or blind
tunnel is driven as a cross-cut to the deposit or as a drift following the
deposit from a portal at a favorable location for the surface plant, drainage
facilities, and waste disposal. In situations where the deposit lies below or at
a great distance from any portal site for an adit, entry must be made from a
shaft collar or from an incline or decline portal. A large mine will commonly
have a main multipurpose entry and several more shafts or adits to accommodate
personnel, supplies, ventilation, communication, and additional production.
Adits
Access by adit generally provides
for relatively low-cost underground mining. The broken ore from above the adit
level can be brought to the portal in trains, conveyor belts, and rubber-tired
trucks without the need for hoisting, and the workings can be drained without
pumping. The driving of an adit is generally less expensive per unit distance of
advance than the sinking of a shaft or the driving of an inclined access. In
areas of low topographic relief and in the mining of deep orebodies, the sinking
of a shaft will often be a more economical approach than the driving and
maintaining of a considerably longer incline or adit from a remote part of the
site.
Shafts
Production shafts are generally
located in stable ground on the footwall side of a dipping deposit rather than
in the deposit itself or in the hanging-wall side, where protective pillars
would be needed to maintain stability as mining progresses. A shaft may be
inclined to follow the dip of the deposit and avoid increasingly longer
cross-cuts to the ore at greater depth, but vertical shafts are more common
because of their lower construction and maintenance cost per unit of depth and
their better efficiency for hoisting ore. Shafts are sunk as rectangular or
circular openings 15–30 ft (5–9 m) in diameter; they are equipped with a
headframe and hoisting system and are lined with timber, steel forms, or
concrete for ground support. Smaller shafts 5– 15 ft (1.5–5 m) in diameter,
generally for escapeways and ventilation, may be bored by mechanical drilling
machines.
Inclines
Inclines equipped with hoists,
declines for access by rubber-tired equipment, and gently inclined spiral ramps
for diesel-powered truck haulage allow for direct access to relatively deep mine
levels without having to transfer the ore and materials to hoisting systems.
Development
workings
Development workings in the deposit
consist of mine levels and sublevels, with drifts in the ore zone or in the more
stable rock on the footwall side of the ore zone. Level workings serve as
passageways for miners and low-profile equipment and as haulageways. In broken
or unstable ground, passageways and haulageways are supported by timber sets and
steel beams or arches; further stabilization is given by rock bolts, sometimes
in combination with cable bolting and wire mesh, and the walls may be lined with
concrete or spray-on shotcrete.
The raises that connect levels and
sublevels provide for the removal of broken ore (chutes and ore passes), for
access by miners, and for ventilation and supply routes.
In conventional mining and in the
most common development procedures, headings are advanced in a cyclic sequence
of drilling, blasting, mucking (removal of broken rock), and installing ground
support. In continuous mining, the cycle is replaced by rapid excavation, a
single operation in which headings are advanced by powerful tunnel boring and
road header machines with teeth that break rock from the face. In situations
where the uniformity and texture of the rock and ore permit development by
continuous mining, the walls of the resulting passageways are smoother and more
stable than would be provided by conventional cyclic operations involving
blasting. See also: Tunnel
The continuous mining procedure of
raise boring is well established. Shaft boring is used in the sinking of
small-diameter ventilation shafts and escapeways. The driving of mine level
development headings by cutting and boring machinery is more common in coal,
potash, and salt deposits and in relatively soft sandstones and shales than in
hard ore and rock.
Hydraulic breakers provide
successively smaller rock sizes at development headings, and the broken rock is
collected at the face by mechanical loading machinery and transferred to the
mine haulage system by mobile conveyors or rubber-tired load-haul-dump machines.
Haulage beyond the transfer point has been done by electric-powered locomotives
with trains of cars but now is increasingly done by rubber-tired electric- or
diesel-powered shuttle cars or trucks and by conveyor belt systems. In shaft
mines, the broken rock is collected in underground storage pockets and loaded
into skips for hoisting to the surface.
The entire sequence in mine
development—the advance of headings, breaking of rock, loading, haulage, and
hoisting—is increasingly automated. Teleoperated and autonomous machines have
become central to every stage in mining, and new mines are developed with the
use of geographic information systems (GIS) technology to accommodate the
extensive communication systems and mining methods that relate to operations by
remote control. See also:
Geographic information systems
Mining
Methods
A fundamental condition in the
choice of mining method is the strength of the ore and wall rock. Strong ore and
rock permit relatively low-cost methods with naturally supported openings or
with a minimum of artificial support. Weaker ore and wall rock necessitate more
costly methods requiring wide-spread temporary or permanent artificial support
such as rock bolting. Large deposits with weak ore and weak walls that collapse
readily and provide suitably broken material for extraction may be mined by
low-cost caving methods. Few mineral deposits are so uniform that a single
method can be used without modification in all parts of the mine. Mining to an
increasing depth with higher stress conditions and mining from a thicker portion
of an orebody into thinner or less uniform portions will especially call for
changes in method.
Naturally supported
openings
The stopes remain open, essentially
by their own strength, during ore extraction. Stability may be maintained to
some extent by timbers, rock bolts, and accumulations of broken ore. The
workings may collapse with time or may eventually need to be filled with waste
material to protect workings in adjacent areas. Backfilling involves the
placement of a paste of cemented waste rock or mill tailings. The methods range
from gophering, an unsystematic small-scale practice, to carefully planned and
executed systems using limits determined by rock mechanics investigations.
Open
stoping
This is used in steeply dipping and
thin orebodies with relatively strong ore and wall rock. In overhand methods the
ore is stoped upward from a sill pillar by miners working on a staging composed
of stulls (round timbers) and lagging (planks). With the drilling and blasting
of successive small blocks of ore from the back (roof), the broken ore falls
onto lower stagings and to the bottom of the stope; it is collected on the
haulage level through draw points or chutes. In underhand stoping the ore is
mined downward in a series of benches, and the broken ore is scraped or hauled
into a raise or ore pass for collection on a lower mine level. The width of an
open stope is limited by the strength of the ore and its capability to stand
unsupported. Occasional pillars, generally of waste or low-grade zones in a
vein, are left for support; timber stulls may be wedged between the stope walls
for stability as well as for access, and rock bolts may also be used to maintain
wall stability.
Sublevel
stoping
Also referred to as longhole or
blasthole stoping, sublevel stoping is practiced in steeply dipping and somewhat
wider orebodies with strong ore and strong walls (Fig. 2). Sublevel drifts and
raises or slots are driven at the ends of a large block of ore so that a series
of thinner horizontal slices can be provided. Miners in the sublevels drill
patterns of radial holes (ring or fan drilling) or quarrylike parallel holes
(slashing). Beginning at the open face of the initial slot, the ore is blasted
in successive increments, and the broken ore falls directly to the bottom of the
stope. A crown pillar is generally left unmined at the top of the stope to
support the next major level.
Fig. 2 Sublevel stoping, with ring
drilling.
Vertical crater
retreat
This is a method of sublevel stoping
in which large-diameter blastholes are drilled in a parallel pattern between
major levels, and the ore is broken from the bottom of the stope in a sequence
of localized blasts. All of the drilling, loading, and blasting are done by
miners and teleoperated machinery in the upper level, so there is no need for
access to the ore from below as the stope progresses upward.
Room-and-pillar
mining
This is also referred to as
stope-and-pillar mining when done in a less regular pattern. Room-and-pillar
mining is done in coal seams and in flat-lying or gently dipping ore and
industrial mineral deposits (Fig. 3). It is a low-cost method of underground
mining because fast-moving rubber-tired equipment can operate freely, especially
in large rooms and haulageways. Thin-bedded deposits are generally mined in a
single stage (pass) by conventional or continuous mining; thicker deposits are
mined in a two-stage benching operation. In deposits of considerable thickness,
an underground quarrying operation follows the first-stage opening of a
development level for sufficient access by open-pit-type blasthole drills.
Room-and-pillar mining is generally limited to depths on the order of 3000 ft
(914 m) in hard-rock mines and to lesser depths in coal mines because of rock
bursts and similar manifestations of high-stress concentration on the pillars.
Extraction in mining generally amounts to about two-thirds of the ore in a
bedded deposit, with the remaining ore being left in pillars; in places where
pillars can be “robbed” and the roof allowed to settle, extraction can be
increased to 90% or more. See also:
Rock burst
Fig. 3 Room-and-pillar mining; two-stage
benching operation.
Shrinkage
stoping
This is an overhand method in which
broken ore accumulates in the stope, affording temporary support for the walls
and a working platform for miners (Fig. 4). Shrinkage stoping is most applicable
to steeply dipping veins with strong ore that will stand across a span and with
relatively strong wall rock that would slough into the stope in places if left
completely unsupported. When ore is broken, it has an expansion or swell factor;
this necessitates a periodic drawing (shrinking) of some of the broken ore from
the draw points and chutes to allow for continued access to the top of the
stope. When all of the ore has been broken except for that left in pillars to
protect the adjacent raises and mine levels, the entire content (the magazine)
of the stope is drawn. The empty stope may be left open or filled with waste
rock, and the pillars may eventually be mined.
Fig. 4 Shrinkage stoping, longitudinal
section.
Artificially supported
openings
In these methods, workings are kept
open during mining by using waste material, timber, and hydraulic props. After
the ore is extracted, the workings are filled to maintain stability or are
allowed to cave.
Cut-and-fill
stoping
This method, also referred to as
drift-and-fill, is used in steeply dipping orebodies in which the ore has
sufficient strength to be self-supporting but the walls are too weak to stand
entirely without support (Fig. 5). Most cut-and-fill stoping is done overhand,
with the drilling and blasting phase similar to that in shrinkage stoping; the
broken ore, however, is removed from each new cut or slice along the back, and
the floor of the stope is built up of waste material such as sand or mill
tailings brought in by pipeline as a water slurry. The smooth and compacted or
cemented fill material provides an especially suitable floor for rubber-tired
machinery. Variations in cut-and-fill mining include the ramp-in-stope system,
in which load-haul-dump equipment can move rapidly in and out of the stope on an
inclined surface of fill material, and the less-mechanized system of resuing in
narrow veins. In resuing, ore and waste material are broken separately and the
waste material is left to accumulate as fill. One additional system,
undercut-and-fill, is applied to bodies of weaker ore. It provides a solid
artificial back of reinforced and cemented fill for the mining of successively
underlying slices of ore.
Fig. 5 Cut-and-fill stoping with sand slurry
and ramp.
Square set
stoping
This is a labor-intensive and
high-cost method that has been classically used in situations where the ore is
too weak to stand across a wide or long back and the walls are not strong enough
to support themselves. A square set, a skeletal box of keyed timbers, is filled
and wedged into the available space as each small block of ore is removed by
drilling and blasting. Mining continues by overhand or underhand stoping, and
the stope becomes a network of interlocked square sets. The sets in the mined
portion of the stope are filled with mill tailings or waste rock and pillars are
left between mined-out stopes for additional wall support while the remainder of
the deposit is being mined. Because of its high cost, square setting is no
longer in use; it has been superseded in many mines by cut-and-fill, top
slicing, and sublevel caving methods.
Longwall
mining
This method is applicable to uniform
and extensive but relatively thin deposits. Primarily a highly mechanized and
increasingly automated coal mining method at depths where rock pressures are too
high for safe room-and-pillar mining, it has also been used in potash deposits
and to some extent in bedded iron, copper, and uranium orebodies. In the South
African deep gold mines, a form of longwall mining is used in the thin-bedded
ore zones.
In longwall mining, practically all
of the coal or ore is recovered except for that left in safety pillars to
protect surface structures.
The basic practice is to maintain a
temporary opening in a uniform line along a working face and then to allow the
roof to cave onto the floor or waste fill (gob) behind the active area. In a
typical mechanized longwall coal operation, the roof support units are canopies
with hydraulic-powered adjustable legs or chocks that are moved ahead as the
coal is shaved into slices by shearing and plowing machinery with integrated
conveyor systems. In the mining of South African gold reef deposits,
longwall-type mining is done by drilling and blasting; the active area is kept
open by hydraulic props and timber-concrete packs, and the mined-out areas are
filled to some extent by waste rock or cemented mill tailings.
Longwall mining systems allow for a
high abutment pressure to build up in solid ore or coal in advance of the face,
a low-pressure zone to exist in the working area just behind the face, and a
normal lithostatic pressure to build up again in the mined-out and caved or
gob-filled area as the face is moved ahead.
Top slice
mining
Seldom used today, this method has
been applied to wide and steeply dipping deposits with weak ore and weak walls.
It has been of use in recovering pillars that have been left between filled
stopes. It is a relatively expensive and labor-intensive method with a
requirement for abundant timber, but it permits nearly total extraction of the
ore. Top slicing is ultimately a caving method of mining, but the ore must first
be drilled and blasted, and temporary support is needed between the taking of
each successive downward slice or horizontal cut of ore. Working begins in
drifts and cross-cuts on a mining floor at the top of a raise; after the driving
of a series of adjacent cross-cuts so that a slice of sufficient width has been
taken, a mat of timber and scrap lumber is laid down on the floor and the
supporting timbers are blasted to cave the overlying rock. A new slice is mined
laterally from drifts and cross-cuts under the mat, with the mat supported by
timber props (stulls). A mat is again laid down, supports are blasted, and
subsequent slices are mined beneath the subsiding accumulation of timber mats
and waste rock.
Caving
methods
These methods are used in large
orebodies with relatively weak ore and with weak wallrock that will collapse as
the ore is removed. Geologic conditions must permit subsidence, and the ore must
be sufficiently jointed or fractured to form fragments small enough to be
handled in drawpoints and raises. Ore recovery in mining is generally quite
high, but a certain amount of dilution from waste rock must be accepted.
Sublevel
caving
This type is most suited to large
and steeply dipping orebodies with weak walls and with ore that has enough
stability to maintain sublevels (Fig. 6). It is similar to sublevel open
stoping, but in this method the walls and the back are allowed to collapse. The
ore is mined in downward increments that are drilled, blasted, and drawn from
levels below the ore. Access drifts are driven on the footwall side of the
orebody, sublevel cross-cuts are driven in ore, and fans of blastholes are
drilled at intervals in the cross-cuts. A steplike succession of slices is mined
in retreat from the hanging wall, with the wall rock collapsing and following
the extraction of the ore. As each fan of holes is blasted, the broken ore caves
into the sublevel, where it is loaded and transported to the ore pass. Broken
waste rock fills the void as the ore is drawn. When an excess of waste rock
begins to dilute the broken ore, the drawing is stopped and the next fan of
holes is blasted.
Fig. 6 Sublevel caving, with stages of
development and mining.
Block
caving
This is applied to large and
relatively uniform bodies in which both ore and waste will cave readily (Fig.
7). Production on the order of 50,000–75,000 tons (45,000–68,000 metric tons)
per day can be achieved at a very low mining cost, but the capital cost of a
block-caving mine is high. A mine is prepared for block-caving operations by
establishing a principal haulage level, driving raises to production levels
(slusher or grizzly levels), and driving a larger number of raises to workings
on an undercut level beneath the orebody or block to be mined. Caving is
initiated by drilling and blasting a slice of ore above the undercut level and,
if necessary, by excavating narrow stopes at the boundaries of the block. With
the drawing of the initially broken ore, the block begins to cave under its own
weight. With further drawing, the entire column of ore and overburden rock
continues to subside and break upward for as much as 4000 ft (1220 m) to the
surface, where a depression forms. The ore, broken and crushed in caving, flows
through cone-shaped draw holes and finger raises. The finger raises are
carefully monitored at draw points on the grizzly level so that the caving
action is kept uniform and salient channels of subsiding waste rock are not
allowed to form prematurely. Broken ore collected from finger raises reaches the
haulage level through transfer raises.
See also: Explosive; Mining; Prospecting
William C. Peters
Fig. 7 Block caving, with principal haulage
level, driving raises to production (grizzly) levels, and raises to
workings.
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