first step in surface mining

1. First step in Surface Mining
Technologies

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Mining, process of extracting useful minerals from the surface of the Earth, including
the seas. A mineral, with a few exceptions, is an inorganic substance occurring in
nature that has a definite chemical composition and distinctive physical properties or
molecular structure. (One organic substance, coal, is often discussed as a mineral as well.)
Ore is a metalliferous mineral, or an aggregate of metalliferous minerals and gangue
(associated rock of no economic value), that can be mined at a profit.Mineral deposit
designates a natural occurrence of a useful mineral, while ore deposit denotes a mineral
deposit of sufficient extent and concentration to invite exploitation.
When evaluating mineral deposits, it is extremely important to keep profit in mind. The total
quantity of mineral in a given deposit is referred to as the mineral inventory, but only
that quantity which can be mined at a profit is termed the ore reserve. As the selling
price of the mineral rises or the extraction costs fall, the proportion of the mineral
inventory classified as ore increases. Obviously, the opposite is also true, and a mine
may cease production because the mineral is exhausted or the prices have dropped or
costs risen so much that what was once ore is now only mineral.

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Prospecting and exploration
Various techniques are used in the search for a mineral deposit, an activity
calledprospecting. Once a discovery has been made, the property containing a
deposit, called the prospect, is explored to determine some of the more
important characteristics of the deposit. Among these are its size, shape, orientation in
space, and location with respect to the surface, as well as the mineral quality and
quality distribution and the quantities of these different qualities.

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Prospecting
In searching for valuable minerals, the traditional prospector relied primarily on
the direct observation of mineralization in outcrops, sediments, and soil. Although
direct observation is still widely practiced, the modern prospector also employs
a combination of geologic, geophysical, and geochemical tools to provide indirect
indications for reducing the search radius. The object of modern techniques is to
findanomalies—i.e., differences between what is observed at a particular location and
what would normally be expected. Aerial and satellite imagery provides one means
of quickly examining large land areas and of identifying mineralizations that may be
indicated by differences in geologic structure or in rock, soil, and vegetation type.
Ingeophysical prospecting gravity, magnetic, electrical, seismic, and radiometric
methods are used to distinguish such rock properties as density, magnetic
susceptibility, natural remanent magnetization, electrical conductivity, dielectric
permittivity, magnetic permeability, seismic wave velocity, and radioactive decay.
Ingeochemical prospecting the search for anomalies is based on the systematic
measurement of trace elements or chemically influenced properties. Samples of soils,
lake sediments and water, glacial deposits, rocks, vegetation and humus, animal tissues,
microorganisms, gases and air, and particulates are collected and tested so that unusual
concentrations can be identified.

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Exploration
On the basis of such studies, a number of prospects are identified. The most promising
of these becomes the focus of a field exploration program. Several exploration techniques
are used, depending on the type of deposit and its proximity to the surface. When the top
of a deposit intersects the surface, or outcrops, shallowtrenches may be excavated with
a bulldozer or backhoe. Trenching provides accurate near-surface data and the
possibility of collecting samples of large volume for testing. The technique is obviously
limited to the cutting depth of the equipment involved. Sometimes special drifts are
driven in order to explore a deposit, but this is a very expensive and time-consuming
practice. In general, the purpose of driving such drifts is to provide drilling sites
from which a large volume can be explored and a three-dimensional model of the
potential ore body developed. Old shafts and drifts often provide a valuable and
convenient way of sampling existing reserves and exploring extensions.
The most widely used exploration technique is the drilling of probe holes. In
this practice a drill with a diamond-tipped bit cuts a narrow kerf of rock, extracting
intact a cylindrical core of rock in the centre (see core sampling). These core holes may
be hundreds or even thousands of metres in length; the most common diameter is
about 50 mm (2 inches). The cores are placed in special core boxes in the order in which
they were removed from the hole. Geologists then carefully describe, or log, the core in
order to determine the location and kinds of rock and mineral present; the different
structural features such as joints, faults, and bedding planes; and the strength of the
rock material. Cores are often split lengthwise, with one half being sent to a laboratory so
that the grade, or content, of mineralization can be determined.

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Delineation
Normally, core holes are drilled in a more or less regular pattern, and the locations of the holes are
plotted on plan maps. In order to visualize how the deposit appears at depth, holes are also
plotted along a series of vertical planes called sections. The geologist then examines each
section and, on the basis of information collected from the maps and core logs as well as his
knowledge of the structures present, fills in the regions lying between holes and between planes. This
method of constructing an ore body is widely used where the boundaries between ore and waste are
sharp and where medium to small deposits are mined by underground techniques, but, in the case of
large deposits mined by open-pit methods, it has largely been replaced by the use of block models.
These will be discussed in more detail below (see Surface mining).
Mineral deposits have different shapes, depending on how they were deposited. The most common
shape is tabular, with the mineral deposit lying as a filling between more or less parallel layers of rock.
The orientation of such an ore body can be described by its dip (the angle that it makes with the
horizontal) and its strike (the position it takes with respect to the four points of the compass). Rock
lying above the ore body is called the hanging wall, and rock located below the ore body is called the
footwall.
The concentration of a valuable mineral within an ore is often referred to as itsgrade.
Grade may exhibit considerable variation throughout a deposit. Moreover, there is a certain grade
below which it is not profitable to mine a mineral even though it is still present in the ore. This is called
the mine cutoff grade. And, if the material has already been mined, there is a certain grade below
which it is not profitable to process it; this is the mill cutoff grade. The grade at which the costs
associated with mining and mineral processing just equal the revenues is called thebreak-even grade.
Material having a higher grade than this would be considered ore, and anything below that would be
waste.
Therefore, in determining which portion of a mineral can be considered an exploitable ore
reserve, it is necessary to estimate extraction costs and the price that can be expected for the
commodity. Extraction costs depend on the type of mining system selected, the level of
mechanization, mine life, and many other factors. This makes selecting the best system for a given
deposit a complex process. For example, deposits outcropping at the surface may initially be mined as
open pits, but at a certain depth the decision to switch to underground mining may have to be made.
Even then, the overall cost per ton of ore delivered to the processing plant would be significantly
higher than from the open pit; to pay for these extra costs, the grade of the underground ore would
have to be correspondingly higher.

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Surface mining
It has been estimated that more than two-thirds of the world’s yearly mineral production
is extracted by surface mining. There are several types of surface mining, but the three
most common are open-pit mining, strip mining, and quarrying. These differ from one
another in the mine geometries created, the techniques used, and the minerals produced.
Open-pit mining often (but not always) results in a large hole, or pit, being formed in the
process of extracting a mineral. It can also result in a portion of a hilltop being removed.
In strip mining a long, narrow strip of mineral is uncovered by a dragline, large shovel, or
similar type of excavator. After the mineral has been removed, an adjacent strip is
uncovered and its overlying waste material deposited in the excavation of the first strip.
Since strip mining is primarily applied to thin, flat deposits of coal, it is not discussed here
(see coal mining).
There are two types of quarrying. There is the extraction of ornamental stone blocks of
specific colour, size, shape, and quality—an operation requiring special and expensive
production procedures. In addition, the term quarrying has been applied to the recovery of
sand, gravel, and crushed stone for the production of road base, cement, concrete, and
macadam. However, since the practices followed in these operations are similar to those
of open-pit mines, the discussion of quarrying here is limited to the excavation of
ornamental stone.

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Open-pit mining, surface mining to obtain minerals other than coal.

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PIT GEOMETRY
Deposits mined by open-pit techniques are generally divided into horizontal layers
called benches. The thickness (that is, the height) of the benches depends on the type of
deposit, the mineral being mined, and the equipment being used; for large mines it is on
the order of 12 to 15 metres (about 40 to 50 feet). Mining is generally conducted on a
number of benches at any one time. The top of each bench is equivalent to a working
level, and access to different levels is gained through a system of ramps. The width of a
ramp depends on the equipment being used, but typical widths are from 20 to 40 metres
(65 to 130 feet). Mining on a new level is begun by extending a ramp downward. This
initial, or drop, cut is then progressively widened to form the new pit bottom.
The walls of a pit have a certain slope determined by the strength of the rock mass
and other factors. The stability of these walls, and even of individual benches and groups
of benches, is very important—particularly as the pit gets deeper. Increasing the pit slope
angle by only a few degrees can decrease stripping costs tremendously or increase
revenues through increased ore recovery, but it can also result in a number of slope
failures on a small or large scale. Millions of tons of material may be involved in such
slides. For this reason, mines have ongoing slope-stability programs involving the
collection and analysis of structural data, hydrogeologic information, and operational
practices (blasting, in particular), so that the best slope designs may be achieved. It is
not unusual for five or more different slope angles to be involved in one large pit.
As a pit is deepened, more and more waste rock must be stripped away in order to
uncover the ore. Eventually there comes a point where the revenue from the exposed
ore is less than the costs involved in its recovery.
Mining then ceases. The ratio of the amount of waste rock stripped to ore removed is
called the overallstripping ratio. The break-even stripping ratio is a function of ore value
and the costs involved.

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ORE RESERVES
The first step in the evaluation and design of an open-pit mine is the determination of
reserves. As was explained above, information regarding the deposit is collected through the drilling of
probe holes. The locations of the holes are plotted on a plan map, and sections taken through the
holes give a good idea of the ore body’s vertical extent. From these vertical sections the
tentative locations of the benches are selected. However, since the deposit is to be mined in horizontal
benches, it is also convenient to calculate the ore reserve in horizontal sections, with the thickness of
each section equal to the height of a bench. These horizontal sections are divided along coordinate lines
into a series of blocks, with the plan dimensions (i.e., the length and width) of each block generally being
one to three times the bench height. After the grade of each block has been determined, the
blocks are assembled into a block model representation of the ore body. (This model must be
significantly larger than the actual ore reserve in order to include the eventual pit that must be dug to
expose the ore body.)
Economic factors such as costs and expected revenues, which vary with grade and block location,
are then applied; the result is an economic block model. Some of the blocks in the model will eventually
fall within the pit, but others will lie outside. Of the several techniques for determining which of the blocks
should be included in the final pit, the most common is the floating cone technique. In two dimensions the
removal of a given ore block would require the removal of a set of overlying blocks as well. All of these
would be included in an inverted triangle with its sides corresponding to the slope angle, its base lying on
the surface, and its apex located in the ore block under consideration. In an actual three-dimensional
case, this triangle would be a cone. The economic value of the ore block at the apex of the cone would be
compared with the total cost of removing all of the blocks included in the cone. If the net value proved
positive, then the cone would be mined. This technique would be applied to all of the blocks making up
the block model, and at the end of this process a final pit outline would result.

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UNIT OPERATIONS
The largest open-pit operations can move almost one million tons of material (both ore and waste) per day. In smaller
operations the rate may be only a couple of thousand tons per day. In most of these mines there are four unit operations:
drilling,blasting, loading, and hauling.
In large mines rotary drills are used to drill holes with diameters ranging from 150 to 450 mm (about 6 to 18 inches). The drill bit,
made up of three cones containing either steel or tungsten carbide cutting edges, is rotated against the hole bottom under a
heavy load, breaking the rock by compression and shear. An air compressor on the drilling machine forces air down the centre
of the drill string so that the cuttings are removed. In smaller pits holes are often drilled by pneumatic or hydraulic
percussion machines. These rigs may be truck- or crawler-mounted. Hole diameters are often in the range of 75 to 120
mm (about 3 to 5 inches).
Holes are drilled in special patterns so that blasting produces the types of fragmentation desired for the
subsequent loading, hauling, and crushing operations. These patterns are defined by the burden (the shortest distance
between the hole and the exposed bench face) and the spacing between the holes. Generally, the burden is 25 to 35 times
the diameter of the blasthole, depending on the type of rock and explosive being used, and the spacing is equal to the burden.
There are a number of explosives used, but most are based on a slurry of ammonium nitrate and fuel oil (ANFO), which is
transported by tanker truck and pumped into the holes. When filled with ANFO, a blasthole 400 mm (about 16 inches) in
diameter and 7.5 metres (about 25 feet) deep can develop about one billion horsepower. It is incumbent upon those
involved in the drilling and blasting to turn this power into useful fragmentation work. To achieve the proper
fragmentation, a series of blastholes is generally shot in a carefully controlled sequence.
The object of blasting is to fragment the rock and then displace it into a pile that will facilitate its loading and transport. In
large open pits the main implements for loading are electric, diesel-electric, or hydraulic shovels, while electric or mechanical-
drive trucks are used for transport. The size of the shovels is generally specified by dipper, or bucket, size; those in common
use have dipper capacities ranging from 15 to 50 cubic metres (20 to 65 cubic yards). This means that 30 to 100 tons can be
dug in a single ―bite‖ of the shovel. The size of the trucks is matched to that of the shovel, a common rule of thumb being that
the truck should be filled in four to six swings of the shovel. Thus, for a shovel of 15- cubic-metre capacity, a truck having a
capacity of 120 to 180 tons (four to six swings) should be assigned. The largest trucks have capacities of more than 350 tons
(about 12 swings) and are equipped with engines that produce more than 3,500 horsepower; their tire diameters are often more
than 3 metres (10 feet). Because of their high mobility, very large-capacity wheel loaders (front-end loaders) are also used in
open-pit mines.
As pits became deeper—the deepest pits in the world exceed 800 metres (2,600 feet)—alternate modes of transporting
broken ore and waste rock became more common. One of these is the belt conveyor, but in general this method requires in-pit
crushing of the run-of-mine material prior to transport. For most materials a maximum angle of 18° is possible. To transport
directly up the sides of pit walls, special conveying techniques are under development.
After loading, waste rock is transported to special dumps, while ore is generally hauled to a mineral-processing plant for
further treatment. (In some cases ore is of sufficiently high quality for direct shipment without intermediate processing.)
In some operations separate dumps are created for the various grades of sub-ore material, and these dumps may be re-
mined later and processed in the mill. Certain dumps can be treated by various solutions to extract the contained metals (a
process known as heap leaching or dump leaching).

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Quarrying
Although seldom used to form entire structures, stone is greatly valued
for its aesthetic appeal, durability, and ease of maintenance. The most
popular types include granite, limestone, sandstone, marble,
slate, gneiss, and serpentine. All natural stone used for structural
support, curtain walls, veneer, floor tile, roofing, or strictly ornamental
purposes is called building stone, and building stone that has been
cut and finished for predetermined uses in building
construction and monuments is known as dimension stone. The
characteristics required of good dimension stone are uniformity of texture
and colour, freedom from flaws, suitability for polishing and carving,
and resistance to weathering. This section describes the quarrying of
dimension stone.

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PIT GEOMETRY
• Although quarrying is also done underground, using room-and-pillar techniques,
most quarries involve the removal of blocks from hillsides or from an open-pit type
of geometry. The first step in developing such a quarry is the removal of the
vegetative cover of trees and underbrush. Next, the overburden of topsoil and
subsoil is removed and stockpiled for future reclamation. The rock is quarried in a
series of benches or slices corresponding to the thickness of the desired blocks.
This is often on the order of 4.5 to 6 metres (about 15 to 20 feet), but, since it is
actual quarry practice to take advantage of any natural horizontal seams, block
thickness may vary.
• The quarrying process consists of separating large blocks, sometimes called loafs,
from the surrounding rock. These blocks may be 6 metres high by 6 metres deep
and 12 to 18 metres (about 40 to 60 feet) long, and they may weigh in the range of
1,200 to 2,000 tons. (Such large blocks are subsequently divided into mill blocks
weighing 15 to 70 tons.) The removal of blocks from the quarry has traditionally
been done by one or more fixed derricks. As a result, the plan area of a quarry has
been determined not only by the geometry of the deposit and the amount of
overburden but also by the reach of the derrick boom. However, derricks are
gradually being replaced by highly mobile front-end loaders of sufficient capacity to
move, lift, and carry 30-ton mill blocks, and the layout, design, and operating
procedures of quarries are being modified accordingly.
• There is a very high waste factor in the quarrying of dimension stone. For some
quarries the amount of usable stone is only 15 to 20 percent of that quarried. For
this reason an important aspect of quarry planning is the location of the waste or
―grout‖ pile.

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• UNIT OPERATIONS
There are a number of techniques for separating a mass of stone from the parent mass. For many
years the primary technique was the wire saw, which consists of a single-, double-, or triple-stranded
helicoidal steel wire about 6 mm (0.2 inch) in diameter into which sand, aluminum oxide, silicon
carbide, or other abrasive is fed in a water slurry. As the wire is pulled across the surface, a groove or
channel is worn in the stone.
Although the wire does not do the cutting itself (this is done by the abrasive), it does wear in the
process so that the width of the cut continuously decreases. If the wire breaks prior to the completion of
a cut, there will be great difficulty in beginning again; hence, the wire must be sufficiently long to
complete the cut. In granite quarrying, a rule of thumb is that about 27 metres (about 89 feet) of wire
are used for each square metre of stone that is cut (8 feet of wire per square foot). Completing a 6-
metre-high by 9-metre- (30-foot-) long cut thus requires approximately 1,450 metres (about 4,800 feet)
of wire; indeed, a typical wire saw setup may require 3 to 5 km (2 to 3 miles) of wire driven by an
electric motor or diesel engine and directed around the quarry by a system of sheave wheels. A single
wire may make several cuts at one time by suitable sheave direction.
The advantage of wire sawing is that it produces a smooth cut that minimizes later processing and
does not damage adjacent rock. The technique has largely been superseded by others, however. In
hard rocks such as granite that have a significant quartz content, channels may be cut by handheld or
automated jet burners. A pressurized mixture of fuel oil and air or of fuel oil and oxygen is burned in a
combustion chamber similar to a miniature rocket engine, producing a high-temperature, high-velocity
flame. A channel 75 to 150 mm (3 to 6 inches) wide and up to 6 metres deep can be formed.
Another technique for cutting slots involves drilling a series of long parallel holes, using
pneumatically or hydraulically powered percussion drills. In line drilling, closely spaced pilot holes may
be drilled first and the intervening material then removed by reaming with a larger-diameter bit. Other
arrangements using special guides are also available. For softer, less-abrasive rocks, the remaining
rock web between holes may simply be chipped or broached out.

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Rock between less closely spaced holes (125 to 250 mm [about 5 to 10 inches] apart) can be
broken rather than removed. One technique for doing this involves the use of special explosives to exert a
high gas pressure against the hole walls and thereby produce a crack along the firing line. A mechanical
technique for accomplishing this is the use of feathers and wedges. Feathers are two half-round pieces of
steel that are inserted into all of the holes forming a side of the block. The quarry worker works down the
row, inserting a wedge between each pair of feathers and then tapping the wedges with a
sledgehammer. This forces pressure from the wedge to the feathers so that eventually a crack line forms.
This procedure is commonly followed to form the bottom of a block and for dividing large blocks into
smaller blocks. In the latter case a line of small- diameter holes only a few centimetres deep is required. In
addition, special cement grouts that expand during curing, as well as special hydraulic pressurization
techniques, have also been used.
A relatively new development is the diamond wire saw. This consists of a 6-mm steel carrier cable
on which diamond-impregnated beads and injection-molded plastic spacers are alternately fixed. The
plastic spacers protect the cable against the abrasiveness of the rock and also maintain the diamond
segments on the cable. Relatively clean water serves both as the flushing medium and to cool the wire.
The initiation of a cut requires two boreholes 40 to 90 mm (1.6 to 3.5 inches) in diameter. One hole is
drilled down from the upper corner of the block, and the other is drilled horizontally along the bottom to
intersect the vertical hole. The wire is strung through the holes, and a driving mechanism supplies the
power to move the wire and apply the proper tension. The diamond wire cut is very narrow (thus reducing
waste), and it does not produce cracks or fissures in the stone. Moreover, once the saw is set up, an
operator is not required.
Large chain saws, similar to those used for cutting trees but equipped with tungsten carbide or
diamond-tipped cutters, are applicable to marbles, limestones,travertines, shales such as slate, and some
types of sandstone. The chain, made up of removable links that carry the tool holders, rides in a channel
with replaceable walls and bottom. The machine is self-propelled through a rack-and-pinion mechanism
along modular track sections.
Channels may be cut in the stone by high-pressure jets of water with or without the addition of
an abrasive substance. Water is forced through a small-diameter nozzle at extremely high velocity, creating
new cracks and penetrating small natural cracks. In the process, thin layers of rock are sliced away. The
advantages of water-jet channeling are that it cuts narrow, straight channels with very little noise and that it
does not damage the wall surface.

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Underground mining
When any ore body lies a considerable distance below the surface, the amount of waste that has to be removed in order to
uncover the ore through surface mining becomes prohibitive, and underground techniques must be considered. Counting
against underground mining are the costs, which, for each ton of material mined, are much higher underground than on the
surface. There are a number of reasons for this, not the least of which is that the size of underground mining equipment—
because of ground conditions, ore body geometry, and other factors—is much smaller than in the open pit. Also, access is much
more limited. All of this means that productivity, as measured in tons produced per worker per shift, can be 5 to 50 times lower,
depending on the mining technique, than on the surface. Balanced against this is the fact that underground only ore is mined,
Once a decision has been made to go underground, the specific mining method selected depends on the size, shape, and
orientation of the ore body, the grade of mineralization, the strength of the rock materials, and the depths involved. For example,
if the ore is very high grade or carries a high price, then a higher cost method can be used. In order to minimize the
mixing of ore and waste, highly selective extraction methods are available, but if ore and waste can be separated easily later
(for example, by using magnets in the case of magnetite), then a less- selective bulk mining method may be chosen.
The orientation, specifically the dip, of the ore body is particularly important in method selection. If the dip is greater than
about 50°, then systems using gravity to move the ore can be considered. If the dip is less than about 25°, then systems using
rubber-tired equipment for ore transport can be considered. For ore bodies having dips in between these, special designs are
required.
The openings made in the process of extracting ore are called stopes or rooms. There are two steps involved
in stoping. The first is development—that is, preparing the ore blocks for mining—and the second is production, or stoping,
itself. Ore development is generally much more expensive on a per-ton basis than stoping, so that every effort is made
to maximize the amount of stoping for a given amount of development. For steeply dipping ore bodies, such as the one
illustrated in the figure, this means having as large a distance as possible between production levels. The resulting larger
openings would offer an opportunity to use larger, more productive equipment, and fewer machines and workplaces would be
needed to achieve a given production level.
In stoping, the geometry—that is, the size and shape—of the ore body imposes one constraint on the size of openings that
can be constructed, and the strength of the ore and wall rocks imposes another. Most rock materials are inherently much
stronger than the concrete used in the construction of highways, bridges, and buildings, but they also contain structural defects
of various types, and it is these defects that determine the strength of the rock structure. If the defects are very close
together, filled with crushed materials, and unfavourably oriented, then the underground openings must be kept small.
As one goes deeper into the Earth, the thickness and, consequently, weight of the overlying rock increase. Pressure from
the sides also increases with depth; the amount of this pressure depends on the rock type and the geologic situation, but it can
range from about one-third of the vertical pressure to as high as three times the vertical. In the world’s deepest mines, which are
more than 4 km (2.5 miles) below the surface, pressure becomes so intense that the rock literally explodes. These rock
bursts are major limitations to mining at depth. A specialized field of engineering known as rock mechanics deals with the
interaction between rock mass and mine openings.

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Mine development
Prior to the production of ore, a certain capital investment in mine development work is
required. In open-pit mines this consists of building access roads and stripping the
overlying waste material in order to expose the ore and establish the initial bench
geometries. For an underground mine the development stage is considerably more
complicated. Some of the development components of an underground mine
are illustrated in the figure.

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VERTICAL OPENINGS:
SHAFTS AND RAISES
The principal means of access to an underground ore body is a vertical opening called a shaft. The shaft is excavated, or
sunk, from the surface downward to a depth somewhat below the deepest planned mining horizon. At regular intervals along
the shaft, horizontal openings called drifts are driven toward the ore body. Each of these major working horizons is called a
level. The shaft is equipped with elevators (called cages) by which workers, machines, and material enter the mine. Ore is
transported to the surface in special conveyances called skips.
Shafts generally have compartments in which the media lines (e.g., compressed air, electric power, or water) are
contained. They also serve as one component in the overall system of ventilating the mine. Fresh air may enter the
mine through the production shaft and leave through another shaft, or vice versa.
Another way of gaining access to the underground is through a ramp—that is, a tunnel driven downward from the surface.
Internal ramps going from one level to another are also quite common. If the topography is mountainous, it may be
possible to reach the ore body by driving horizontal or near-horizontal openings from the side of the mountain; in metal
mining these openings are called adits.
Ore that is mined on the different levels is dumped into vertical or near-vertical openings called ore passes, through which
it falls by gravity to the lowest level in the mine. There it is crushed, stored in an ore bin, and charged into skips at a skip-filling
station. In the head frame on the surface, the skips dump their loads and then return to repeat the cycle. Some common
alternative techniques for ore transport are conveyor belts and truck haulage. Vertical or near-vertical openings are also
sometimes driven for the transport of waste rock, although most mines try to leave waste rock underground.
Vertical or subvertical connections between levels generally are driven from a lower level upward through a process called
raising. Raises with diameters of 2 to 5 metres (7 to 16 feet) and lengths up to several hundred metres are often drilled by
powerful raise-boring machines. The openings so created may be used as ore passes, waste passes, or ventilation openings.
An underground vertical opening driven from an upper level downward is called a winze; this is an internal shaft.

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HORIZONTAL OPENINGS: DRIFTS
All horizontal or subhorizontal development openings made in a mine have the generic name of drift.
These are simply tunnels made in the rock, with a size and shape depending on their use—for example,
haulage, ventilation, or exploration. A drift running parallel to the ore body and lying in the footwall is
called a footwall drift, and drifts driven from the footwall across the ore body are called crosscuts. A
ramp is also a type of drift.
Because the drift is such a fundamental construction unit in underground mining, the process by
which it is made should be described. There are five separate operations involved in extending the
length of the drift by one round, or unit volume of rock. Listed in the order in which they are done, these
are drilling, blasting, loading and hauling, scaling, and reinforcing. Drilling is done in various ways
depending on the size of the opening being driven, the type of rock, and the level of mechanization.
Most mines use diesel-powered, rubber-tired carriers on which several drills are mounted; these
machines are called drill jumbos. The drills themselves may be powered by compressed air or hydraulic
fluid. In percussive drilling a piston is propelled back and forth in the cylinder of the drilling machine.
On the forward stroke it strikes the back end of a steel bar or drill rod, to the front of which is attached a
special cutter, or bit. The cutter’s edges are pushed into the bottom of the hole with great force, and, as
the piston moves to the back of the cylinder, the bit is rotated to a new position for the next stroke.
Through the action of high energy, frequency (2,000 to 3,000 blows per minute), and rotation speed,
holes may be drilled in even the hardest rock at a high rate.
A pattern of parallel blastholes is drilled into the rock face at the end of the drift. The diameter of
these holes ranges from 38 to 64 mm (1.5 to 2.5 inches), but in general one or more larger-diameter
uncharged holes are also drilled as part of the initial opening. These latter serve as free surface for the
other holes to break as well as expansion room for rock broken by the blast.
Explosives may be placed in the blastholes in the form of sticks or cartridges wrapped in paper or
plastic, or they may be blown or pumped in. They are composed of chemical ingredients that, when
properly initiated, generate extremely high gas pressures; these in turn induce new fractures in the
surrounding rock and encourage old fractures to grow. In the process rock is broken and displaced.
For many years dynamite was the primary explosive used underground, but this has largely been
replaced by blasting agents based on ammonium nitrate (AN; chemical formula NH4NO3) and fuel oil
(FO; chemical formula CH2). Neither of these components is explosive by itself, but, when mixed in the
proper weight ratio (94.5 percent AN, 5.5 percent FO) and ignited, they cause the following chemical
reaction:

20. - 20
The products of the above reaction (carbon dioxide, water, and nitrogen, respectively) are commonly
present in air. If there is too much fuel oil in the mixture, however, the poisonous gas carbon monoxide
will be formed; with too little fuel oil, nitrous oxides, also poisonous, are formed. For this reason gases are
carried out of the mine through the ventilation system, and blasting is normally done between shifts or at the end
of the last shift, when the miners are out of the mine.
Blastholes must be fired in a certain order so that there is sufficient space to accommodate the broken rock.
Those closest to the large empty holes are fired first, followed by those next to the resulting larger hole. This
continues until the holes at the contour are reached. To create such an expanding pattern, the timing of
explosions is very important. There are both electric and nonelectric systems for doing this. In the electric system
an electric current is passed through a resistive element contained in the blasting cap. When this heats up, it
initiates a fuse head, which in turn ignites a chemical compound that burns at a known rate. This combination
serves as the timing or delay element within the cap. At the other end of the delay is the primer, an explosive
(generally lead azide, mercury fulminate, or pentaerythritol tetranitrate [PETN]) that, upon detonation, releases a
great deal of energy in a very short time. This is sufficient to ignite the larger amount of ANFO explosive packed
into the hole. The most common time interval between adjacent delays is 25 milliseconds. Other caps are
available in which the delays are introduced electrically through the use of microcircuitry. These have the
advantage of extremely little variation among caps of the same delay period; also, the number of delay periods
available is much greater than with burning-compound caps.

21. - 21
After blasting, the broken ore is loaded and transported by machines that may be powered by
compressed air, diesel fuel, or electricity. Highly mechanized mines employ units that load themselves, haul
the rock to an ore pass, and dump it. Known as LHD units, these come in various sizes denoted by the
volume or weight of the load that they can carry. The smallest ones have a capacity of less than 1 cubic
metre (1 ton), whereas the largest have a 25-ton capacity. In small, narrow vein deposits, tracked or
rubber-tired overshot loaders are often employed. After the bucket of this machine is filled by being forced
into the pile, it is lifted and rotated backward so that it dumps into a built-in dump box or attached railcar.
Overshot loaders are commonly powered by compressed air.
Another type of loading machine features special gathering arms that sweep or scrape the
broken material into a feeder, whence it is fed via an armoured conveyor belt into waiting trucks or railcars.
Although most loading machines have an onboard operator-driver, some are controlled remotely via
television monitor.
After the broken rock has been removed (and sometimes even during the loading process),
the roof, walls, and face are cleaned of loose rock. This process is calledscaling. In small openings scaling
is normally done by hand, with a special steel or aluminum tool resembling a long crowbar being used to
―bar down‖ loose material. In larger openings and mechanized mines, a special machine with an impact
hammer or scaling claw mounted on a boom is used. Scaling is an extremely important step in making the
workplace safe.
Depending on the ground conditions and the permanence of the openings, various means of
rock reinforcement may be employed before beginning a new round of drifting. The ideal is for the rock to
support itself; this is accomplished by keeping rock blocks in place, thereby allowing rock arches or
beams to form, but often these blocks need to be reinforced by various implements, the most common
being rock bolts inserted into holes drilled around the opening. In one technique a steel bolt equipped with
an expansion anchor at the end is inserted into the hole. Rotation of the bolt causes the anchor to expand
against the wall of the hole, and further rotation compresses a large steel faceplate, or washer, against the
rock, effectively locking the blocks together. A pattern of such bolts around and along an opening creates a
rock arch. If the rock pieces are quite small, a steel net (much like a chain-link fence) or steel straps can be
placed between the bolts. Some mines simply cement reinforcing bar or steel cables in the boreholes.
Shotcrete, concrete sprayed in layers onto the rock surfaces, has also proved to be a very satisfactory
means of rock reinforcement.

22. - 22
VENTILATION AND LIGHTING
Ventilation is an important consideration in underground mining. In addition to the
obvious requirement of providing fresh air for those working underground, there are
other demands. For example, diesel-powered equipment is important in many mining
systems, and fresh air is required both for combustion and to dilute exhaust contaminants. In
addition, when explosives are used to break hard rock, ventilation air carries away and dilutes
the gases produced.
Special fans, controls, and openings are used to direct fresh air to the working
places and spent or contaminated air out of the mine. In very cold climates incoming
ventilation air must first be warmed by gas- or oil-fired heaters. On the other hand, in very deep
mines, because of high rock temperatures, the air must be cooled by elaborate refrigeration
systems. This makes the energy costs associated with ventilation systems very high, which in
turn has created a trend toward sealing unused sections of the mine and changing from diesel
to electric machines.
Properly lighted working places are very important for both safety and productivity. Each
underground miner is equipped with a hard-hat-mounted lamp with the battery worn on the
belt. In some mines this is the primary source of lighting under which the various jobs are
done. In others, however, many jobs have been taken over by machinery equipped with high-
powered lights that fully illuminate the working areas.
Fixed lighting is installed along travel ways and at shaft stations, dumping points,
and other important locations.

23. - 23
WATER CONTROL
The amount of water encountered in underground mining operations
varies greatly, depending on the type of deposit and the geologic setting. Some
mines must be prepared only to reuse the water introduced in such operations as
drilling; others must contend with large inflows from the surrounding rock. In extreme
cases special water doors and underground chambers must be constructed in order
to control sudden large inflows. Typically, mine water flows or is pumped to a central
collection point called a settling basin, or sump. From there it is pumped through pipes
located in the shaft to the surface for treatment and disposal.

24. - 24
Mining flat-lying deposits
Many of the ore deposits mined today had their origins in an ocean, lake, or
swamp environment, and, although they may have been pressed, compacted, and
perhaps somewhat distorted over time, they still retain the basic horizontal orientation in
which the minerals were originally deposited. Such deposits are mined by means of
either of two basic techniques, longwall or room-and- pillar, depending on the thickness,
uniformity, and depth of the seam, the strength of the overlying layers, and whether
surface disturbance is permitted.

25. - 25
ROOM-AND-PILLAR MINING
The most common mining system is room-and-pillar. In this system a series of
parallel drifts are driven, with connections made between these drifts at regular
intervals. When the distance between connecting drifts is the same as that between the
parallel drifts, then a checkerboard pattern of rooms and pillars is created, as shown in the
figure. The pillars of ore are left to support the overlying rock, but in some mines, after
mining has reached the deposit’s boundary, some or all of the pillars may be
removed.

26. - 26
LONGWALL MINING
In the longwall system the ore body is divided into rectangular panels or blocks. In
each panel two or more parallel drifts (for ventilation and ore transport) are driven
along the opposite long sides to provide access, and at the end of the panel a
singlecrosscut drift is driven to connect the two sides. In the crosscut drift, which
is the ―longwall,‖ movable hydraulic supports are installed to provide a safe
canopy under which the seam can be mined. A cutting machine moves back and
forth under this protective canopy, cutting the mineral from the longwall face, and an
armoured conveyor carries the mineral to the access drifts, where it is
transferred onto other conveyor belts and out of the panel. As the mineral is
removed, the supports are moved up, allowing the overlying layers of rock to
cave in back of the canopy.
The process as described above is for softer rocks—such as trona, salt, potash,
mineral-bearing shale, and coal—which can be cut by machine. (Longwall mining of
coal is discussed in greater detail in coal mining: Underground mining.) In hard
rocks, such as the gold- and platinum- bearing reefs of South Africa, the same
basic pattern is followed, but in these cases the seam is removed by drilling and
blasting, and the ore is scraped along the face to a collection point. Roof
support is provided by hydraulic props, wooden packs, and rock or sand fill.

27. - 27
Mining steeply dipping deposits
Many vein-type deposits are not flat-lying but, because of the way they were
emplaced or distortions that have taken place, are found in various vertical or
near-vertical orientations. Often there are sharp boundaries between ore and gangue—as
will be assumed in this discussion.

28. - 28
BLASTHOLE STOPING
When the dip of a deposit is steep (greater than about 55°), ore and waste strong, ore
boundaries regular, and the deposit relatively thick, a system called blasthole
stoping is used. A drift is driven along the bottom of the ore body, and this is
eventually enlarged into the shape of a trough. At the end of the trough, a raise is
driven to the drilling level above. This raise is enlarged by blasting into a vertical slot
extending across the width of the ore body. From the drilling level, long, parallel blastholes
are drilled, typically 100 to 150 mm (about 4 to 6 inches) in diameter. Blasting is then
conducted, beginning at the slot; as the miners retreat down the drilling drift, blasting
successive slices from the slot, a large room develops. Several techniques are available
for extracting blasted ore from the trough bottom.
There are a number of variations on blasthole stoping. In sublevel stoping, shorter
blastholes are drilled from sublevels located at shorter vertical intervals along the
vertical stope. A fairly typical layout is shown in the figure. In vertical retreat miningthe
stope does not take the shape of a vertical slot. Instead, the trough serves as a horizontal
slot, and only short lengths at the bottoms of the blastholes are charged with
explosives, blowing a horizontal slice of ore downward into the trough. Another
short section of the blastholes is then charged, and the process is repeated until the
upper level has been reached.

29. - 29
SHRINKAGE STOPING
Shrinkage stoping is used in steeply dipping, relatively narrow ore bodies with regular
boundaries. Ore and waste (both the hanging wall and the footwall) should be
strong, and the ore should not be affected by storage in the stope.
The miners, working upward off of broken ore, drill blastholes in a slice of intact ore to
be mined from the ceiling of the stope, and the holes are charged with explosives.
From 30 to 40 percent of the broken ore is withdrawn from the bottom of the stope,
and the ore in the slice is blasted down, replacing the volume withdrawn. The miners then
reenter the stope and work off the newly blasted ore.
Shrinkage stoping is rather difficult to mechanize; in addition, a significant period can
elapse between the commencement of mining in the stope and the final withdrawal of all
the broken ore.

30. - 30
CUT-AND-FILL MINING
This system can be adapted to many different ore body shapes and ground conditions.
Together with room-and- pillar mining, it is the most flexible of underground methods. In
cut-and-fill mining, the ore is removed in a series of horizontal drifting slices. When each
slice is removed, the void is filled (generally with waste material from the mineral-
processing plant), and the next slice of ore is mined. In overhand cut-and-fill
mining, the most common variation, mining starts at the lower level and works upward. In
underhand cut-and-fill mining, work progresses from the top downward. In this latter case
cement must be added to the fill to form a strong roof under which to work.
Overhand cut-and-fill mining in a stope with access provided by a ramp is
illustrated in the figure. In this particular design raises are constructed in the fill as
mining proceeds upward. These perform various functions, such as manways or ore
passes, but an alternative would be to load and haul the rock by LHD to an ore pass
located in the footwall.
Where ground conditions permit, it is possible to use a combination of cut-and-fill
mining and sublevel stoping called rill mining. In this method drifts are driven in the ore
separated by a slice of ore two or three normal slices high. As in sublevel stoping, vertical
slices are removed by longhole drilling and blasting, but, as the slices are extracted, filling
is carried out. In this way the amount of open ground is kept small.

31. - 31
SUBLEVEL CAVING
This method owes the first part of its name to the fact that work is carried out
on many intermediate levels (that is, sublevels) between the main levels. The second
half of the name derives from the caving of the hanging wall and surface that takes place
as ore is removed.
In the transverse sublevel caving system shown in the figure, parallel crosscuts
are driven through the ore body on each sublevel from the footwall drift to the hanging
wall. Drifts on the next sublevel down are driven in the same way, but they are positioned
between those above. Blastholes are then drilled in a fan pattern at regular intervals
along the crosscuts. Blasting begins at the hanging wall on the uppermost sublevel.
As the broken ore is removed, caved material from the hanging wall and above
follows, so that, as more and more ore is drawn, the amount of waste removed with it
increases. When the amount of waste reaches a certain level, loading is stopped and the
next fan is blasted. For certain minerals such asmagnetite, in which ore and waste can be
easily and inexpensively separated, dilution of the ore is less of a problem than for other
minerals.

32. - 32
Mining massive deposits
Several of the methods described above (e.g., blasthole stoping, sublevel caving)
can be applied to the extraction of massive deposits, but the method specifically
developed for such deposits is called panel/block caving. It is used under the following
conditions: (1) large ore bodies of steep dip, (2) massive ore bodies of large vertical
extension, (3) rock that will cave and break into manageable fragments, and (4)
surface that permits subsidence.
Two development levels—the production level and, 15 metres (50 feet) higher,
the undercut level—are established at some distance (100 to 300 metres [330 to 980
feet]) below the top of the ore. A series of parallel drifts are driven at the undercut level,
and the rock between the drifts is blasted. This forms a large horizontal slot that removes
the support from the overlying ore so that it caves. In the caving process the ore body
breaks into pieces small enough to be easily removed from the bottom troughs, or
drawbells, which are located at the production level. LHD machines or similar
conveyances transport the ore to ore passes.
As ore is withdrawn from the troughs, caving progresses upward, eventually reaching
the surface. Only the ore initially extracted in creating the troughs and undercuts has
to be drilled and blasted; the remaining ore is broken as it moves its way
downward to the production level. The challenge is obviously to maintain the
troughs and draw points during the drawing period.

33. - 33
Placer mining
Placers are unconsolidated deposits of detrital material containing valuable minerals. The
natural processes by which they form range from chemical weathering to stream, marine,
and wind action. Typical minerals recovered in placers are gold, tin, platinum, diamonds,
titaniferous and ferrous iron sands, gemstones (rubies,emeralds, and sapphires), and
abrasives (rutile, zircon, garnet, and monazite). These are minerals of high specific gravity
and physical toughness.
Although there are several different types of placer deposits, the two most economically
important are stream and beach placers. Stream (or alluvial) placers are formed by running
water, while beach placers are formed by the action of shore waves on preexisting or currently
forming stream placers. Because of the shifting of sea and land throughout geologic time,
placers can be found at any elevation above or below sea level. The particular techniques
chosen to mine them depend on a number of physical conditions: the extent, thickness,
and character of the deposit and bedrock; the orientation of the deposit; the thickness of the
overburden; the source and quantity of water available; and the value per unit volume of
material. For placers that are too thin or too deeply buried to be mined by surface
techniques, an underground system based on shaft sinking and drift driving may be
considered. In this case, because of the unconsolidated nature of the material, heavy support is
often required. Nevertheless, most placers are excavated by surface techniques; broadly
speaking, these may be classified by whether the operations are based on land or on a floating
plant.

34. - 34
Land-based operations
• PANNING
Of the land-based techniques, panning is the simplest and most labour-intensive. Usually, a
pan is filled with placer dirt, and then it is submerged in still water. While underwater the
contents of the pan are kneaded with both hands until all the clay has dissolved and the lumps
of dirt are thoroughly broken. Stones and pebbles are also picked out. Then the pan is held
flat and shaken under water to permit the valuable mineral to settle to the bottom, and, in a
series of quick motions, the pan is tilted and raised repeatedly until the lighter top material is
washed off and only the valuable heavy mineral is left. Good prospects for panning include
unworked ground in or around old workings, crevices in the bedrock of river channels, old river
bars, and dry creek beds.
• SLUICING
Another hand method involves the use of a sluice box. This is a sturdy rectangular box, nearly
always built of lumber, with an open top and a bottom roughened by a set of riffles. The most
common riffles are transversely mounted wooden bars, but they may also be made of wooden
poles, stone, iron, or rubber. Water and placer dirt are introduced at the upper end of the
inclined sluice box, and, as they flow downward, the specially shaped riffles agitate the
current, preventing lighter material from settling while retaining the valuable heavy mineral.
• MECHANIZED METHODS
Mechanized land-based placer operations excavate placer material with draglines, shovels,
backhoes, front-end loaders, and dozers. The material is then delivered to concentrating
plants or sluice boxes for mineral recovery. Such methods are suitable to narrow, shallow, or
bouldery deposits and to irregular and steep topography that is not easily mined by other
techniques.
Ground sluicing is a special technique for the mining of natural placers as well as artificial
ones (tailings piles, for example). A natural flow of water is used to disintegrate and then
transport the material through a sluice, where the valuable mineral is concentrated. In a
method known as hydraulicking, in-place material is excavated by moving a stream of
high-pressure water through a nozzle over the mining face. The resulting slurry then moves
into a downgrade channel and into a contained circuit for concentrating. Although hydraulic
mining is sometimes used to mine coal underground, its primary application is on the surface,
where it is a practical way to mine relatively fine-grained, unconsolidated material from
placers, tailings, alluvium, and lateritic deposits. A major application is in stripping
overburden for the development of open-pit mines.

35. - 35
• Floating-plant operations
• DRAGLINE OR BACKHOE
In certain cases placer material is most economically excavated with a shore-mounted dragline or
backhoe and a floating (barge-mounted) concentrating plant. (The digging equipment may also
be mounted on a separate barge or on the same barge as the plant.) Material is dug from the sides and
bottom of the mining pond and deposited into the washing plant’s hopper. Oversized material is
rejected by screening and placed in waste piles, while the undersized material is distributed to a
gravity-separation system consisting of riffled sluices, jigs, or similar equipment. After treatment, as
much waste as possible is returned to the pond, but, because of swell, some waste may be deposited
outside the pond area. The pond moves along with the mining front.
The backhoe technique has the advantages of powerful digging and good control.

36. - 36
• DREDGING
• Dredging is the underwater excavation of a placer deposit by floating equipment. Dredging
systems are classified as mechanical or hydraulic, depending on the method of material transport.
• The bucket-ladder, or bucket-line, dredge has been the traditional placer-mining tool, and it is still the
most flexible method for dredging under varying conditions. It consists of a single hull supporting an
excavating and lifting mechanism, beneficiation circuits, and waste-disposal systems. The excavation
equipment consists of an endless chain of open buckets that travel around a truss or ladder. The
lower end of the ladder rests on the mine face—that is, the bottom of the pond where excavation
takes place—and the top end is located near the centre of the dredge, at the feed hopper of the
treatment plant. The chain of buckets passes around the upper end of the ladder at a drive
sprocket (called the upper tumbler) and loops downward to an idler sprocket (the lower
tumbler) at the bottom. The filled buckets, supported by rollers, are pulled up the ladder and dump
their load into the hopper. After the valuable material has been removed by the treatment plant,
waste is dumped off the back end of the dredge.
• The clamshell dredge, another mechanical system, is characterized by a large single bucket
operating at the end of cables. Although it can operate in deeper water than other systems and
handles large particles and trash well, it has the disadvantage of being a discontinuous, batch-type
system, taking approximately one bite per minute.
• In pure hydraulic dredging systems, the digging and lifting force is either pure suction, suction with
hydrojet assistance, or entirely hydrojet. They are best suited to digging relatively small-sized loose
material such as sand and gravel, marine shell deposits, mill tailings, and unconsolidated
overburden. Hydraulic dredging has also been applied to the mining of deposits containing
diamonds, tin, tungsten, niobium-tantalum, titanium, monazite, and rare earths.
• The digging power of hydraulic systems has been greatly increased by the addition of underwater
cutting heads. The cutter suction dredge has a rotary cutting head or other excavating tool for
loosening and mixing soil at the face of the mine. The material falls downward to the mouth of a
centrifugal pump, and this transports the slurry (containing 20 to 25 percent solids) to the processing
plant. Normally, the dredge is held in place during cutting by a pile called a spud. Winches and wire
ropes are used to swing the dredge in an arc around the spud until all the material in the arc has
been removed. The dredge is then moved ahead and the process repeated. The cutter suction
dredge is most suitable for mining softer deposits where the material is of a relatively low specific
gravity or fine particle size—for example, in sand and gravel pits, phosphate mines, and various salt
deposits.
• The bucket-wheel dredge is identical to the cutter suction dredge except that a wheel excavator is
used in place of the rotary cutter. It is better at excavating harder materials, has better digging
characteristics at the bottom of the cut, and traps heavy minerals such as gold or tin that might fall
away from the standard cutter. However, it is more expensive and mechanically complex than the
cutter suction dredge.

37. - 37
Marine mining
• Although the sea is a major storehouse of minerals, it has been little exploited; given the
relative ease with which minerals can be obtained above sea level, there is no pressing
need to exploit the sea at the present time. In addition, the technology required to exploit
the sea and seafloor economically has not been developed, and there is also a general
lack of knowledge regarding the resource. Nevertheless, as a potential source of
mineral wealth, the sea can be divided into three regions—seawater, beaches and
continental shelves, and the seafloor.

38. - 38
• Seawater
Seawater contains by weight an average of 3.5 percent dissolved solids. The most important constituents, in
decreasing order, are chloride, sodium, sulfate,magnesium, calcium, potassium, bromine, and bicarbonate. (In addition
to the oceans, minerals are also recovered from the waters of inland salt seas, the Dead Seaand the Great Salt Lake
being two notable examples.) While seawater is an important source of magnesium, by far the most common minerals
extracted from seawater are salts—especially common table salt (sodium chloride, NaCl), the chlorides of
potassium and magnesium, and the sulfates of potassium and magnesium. These minerals are mined by
evaporation, very often in large shallow ponds with energy being supplied by the Sun.
• EVAPORATION OF SEAWATER
The criteria for the production of salt by the evaporation of seawater are (1) a hot, dry climate with dry winds, (2) land
available and the sea nearby, (3) a soil that is almost impermeable, (4) large areas of flat ground at or below sea level,
(5) little rainfall during the evaporating months, (6) no possibility of dilution from freshwater streams, and (7)
inexpensive transportation or nearby markets. The main features of pond facilities constructed to exploit these criteria
include (1) impervious base soils and dikes to retain the brine, (2) canals to transmit brine from the source to the
appropriate ponds, (3) pumps to elevate the brine over dikes and existing land gradients, and (4) structures to facilitate
flow between ponds.
In a modern system of solar ponds, raw brines are pumped or channeled into pre-concentration ponds, where
evaporation brings the sodium chloride level to saturation. The brines, which then contain 19–21 percent sodium
chloride and 28–30 percent total dissolved solids, are transferred to another pond to crystallize the salt. The dwell
time in this pond varies (in one operation at the Great Salt Lake, it takes about one year). The sodium chloride
crystallizes and precipitates out prior to the time when the other dissolved constituents become concentrated to
saturation. Companies producing only sodium chloride will discard the brine well before reaching the saturation point of
other salts in order to avoid contamination, but producers of potassium salts will continue the evaporation process in
order to extract as much of the sodium ion as possible before their desired product reaches saturation. After the desired
salt has crystallized and collected on the pond floors, it is removed, or harvested, with graders, front-end loaders, and
haulage trucks and taken to the processing plant.
• EVAPORATION OF EFFLUENTS
Increasing attention has been devoted to the extraction of salts from brines discharged as effluent after the distillation
of fresh water from seawater. By using these brines for the extraction of minerals, several important advantages
are gained. First, the cost of pumping is carried by the conversion plant; second, the brine temperature is relatively
high, which aids in evaporation; and, third, the concentrations of salts in these effluents are as much as four times the
concentrations in primary seawater.

39. - 39
Marine beaches and continental shelves
Although micas, feldspars, and other silicates, as well as quartz, form the bulk of the
material on most beaches, considerable quantities of valuable minerals such ascolumbite,
magnetite, ilmenite, rutile, and zircon are also commonly found. All these are classified
as heavy minerals, and all are generally resistant to chemical weathering and
mechanical erosion. Less commonly found in minable concentrations are gold,
diamonds, cassiterite, scheelite, wolframite, monazite, and platinum.
For the mining of beach deposits above sea level, conventional surface techniques are
sufficient. Draglines are commonly used, since they can work in the surf zone as well.
Offshore beach and placer deposits are mined by wire line or dredge. In wire line
methods the digging tools or buckets are suspended on a steel cable and lowered
to the sediment surface, where they are loaded and retrieved. Grab buckets (going by
such names as clamshells and orange peels) consist of a hinged digging device that, in
closing, bites into the sediment and contains it inside the closed shell. The bucket and its
load are then hoisted to the surface, where the shell is opened to dump the load.
Dredges come in many varieties similar to those used to mine placer deposits (see
above Dredging). Being a continuous process, bucket-ladder dredging can produce at high
rates, depending on bucket size, power, and digging conditions. Dredges of this type have
been used successfully all over the world for mining gold, tin, and platinum placers as well
as diamond deposits. Their offshore use has been limited to gold and tin. The hydraulic
suction dredge has been mainly used by mining companies to remove overburden from
ore deposits. Its greatest application is in moving unconsolidated sediments of low
specific gravity over long distances where a continuous supply of water is available.
For digging in semiconsolidated sediments,bucket-wheel suction dredges and cutter
suction dredges are used. Also effective areair-lift dredges, which operate by
injecting compressed air into a submerged pipe at about 60 percent of the depth of
submergence. This reduces the density of the fluid column inside the pipe so that, if the
top of the pipe is not too far above the surface of the water, the air-water mixture will
overflow it. Water and sediment rush into the bottom of the pipe to replace that lost in the
overflow at the top. The capacity of these air-lift dredges for lifting solids can be
substantial; they are also extremely simple because they have no submerged moving
parts.

40. - 40
• The seafloor
The floors of the great ocean basins consist to large extent of gently rolling hills, where slopes
generally do not exceed a few degrees and the relief does not vary by more than a few hundred
metres. The mean depth of the ocean is 3,800 metres (about 12,500 feet). The dominant
seafloor sediments are oozes and clays.
An estimated 1016 tons of calcareous oozes, formed by the deposition of calcareous shells
and skeletons of planktonic organisms, cover some 130 million square km (50 million square miles) of
the ocean floor. In a few instances these oozes, which occur within a few hundred kilometres of most
countries bordering the sea, are almost pure calcium carbonate; however, they often show a
composition similar to that of the limestones used in the manufacture of portland cement.
An estimated 1016 tons of red clay covers about 104 million square km (40 million square miles) of
the ocean floor. Although compositional analyses are not particularly exciting, red clay may possess some
value as a raw material in the clay products industries, or it may serve as a source of metals in the future.
The average assay for alumina is about 15 percent, but red clays from specific locations have assayed as
high as 25 percent alumina; copper contents as high as 0.20 percent also have been found. A few
hundredths of a percent of such metals as nickel and cobalt and a percent or so of manganese also are
generally present in a micronodular fraction of the clays and in all likelihood can be separated and
concentrated from the other materials by a screening process or by some other physical method.
Underlying the hot brines in the Red Sea are basins containing metal-rich sediments that
potentially may prove to be of considerable significance. It has been estimated that the largest of
several such pools, the Atlantis II Deep, contains rich deposits of copper, zinc, silver, and gold in relatively
high grades. These pools lie in about 2,000 metres (about 6,600 feet) of water midway between Sudan
and the Arabian Peninsula. Because of their gel-like nature, pumping these sediments to the surface may
prove relatively uncomplicated. These deposits are forming today under present geochemical conditions
and are similar in character to certain major ore deposits on land.

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The most important mineral deposits known (but not yet exploited) are phosphoriteand manganese
nodules. From an economic standpoint the manganese nodules (actually concretions of manganese
dioxide) are more important. These nodules are found in a variety of physical forms, but the average size is
about 3 cm (1.2 inches). An estimated 1.5 trillion tons of manganese nodules lie on the Pacific Ocean floor
alone. Averaging about 4 cm (1.6 inches) in diameter and found in concentrations as high as 38,600 tons per
square km, these manganese nodules contain as much as 2.5 percent copper, 2.0 percent nickel, 0.2 percent
cobalt, and 35 percent manganese. In some deposits, the content of cobalt and manganese is as high as 2.5
percent and 50 percent, respectively. Such concentrations would be considered high-grade ores if found on
land, and, because of the large horizontal extent of the deposit, they are a potential source of many important
industrial metals.
Two means of bringing nodules to the surface on a commercial scale seem to have merit. These are the
deep-sea drag dredge and the deep-sea hydraulic dredge. The deep-sea drag dredge would be designed to
skim only a thin layer of material from the seafloor until its bucket is filled with nodules. The dredge would then
be retrieved, the bucket drawn up over a track on the back of the dredging ship, and the load dumped into a
hopper. Such a system, along with its associated submerged motors and pumps, could be used to mine the
nodules at rates as high as 10,000 to 15,000 tons per day, from depths as great as 6,000 metres (about 19,700
feet).
As an intermittent operation that would require significant nonproductive time periods for lowering and
raising the bucket, drag dredging would have serious economic disadvantages. Any large-scale operation
for mining seafloor sediments would have to be continuous in order to be efficient, and the hydraulic
dredge could be a solution to this challenge. A hydraulic dredge arrangement might involve a pump, an
air-lift system, and a self-propelled bottom nodule collector. Different nodule-pickup principles would involve a
variety of buckets, scrapers, brushes, and water jets. The location of the pump with respect to the surface of the
ocean would depend on the fluid-solids ratio of the material in the pipe as well as the fluid velocity.
Although the recovery of manganese nodules from the seafloor has been too costly to mount an operation,
diamonds and other minerals have been successfully extracted from the seafloor using remotely operated
vehicles (ROVs) and vertical tunnel cutters.

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Brine solution mining
Natural brine wells are the source of a large percentage of the world’s bromine,lithium, and boron and lesser amounts of
potash, trona (sodium carbonate), Glauber’s salt (sodium sulfate), and magnesium. In addition, artificial brines are produced by
dissolving formations containing soluble minerals such as halite (rock salt; sodium chloride), potash, trona, and boron. This
latter activity is known as brine solution mining, and this section focuses on the solution mining of salt.
All techniques begin with the successful drilling of a borehole to the top of the salt formation. The well is cased, or lined, with
one or more pipes of steel or another material, and the hole is then extended to the bottom of the formation. At this point any
one of four different production configurations is used. In the top injection technique, tubing is suspended inside the well to the
bottom of the hole. Water injected into the annulus, or open ring, between the inner tube and the casing emerges at the top of
the salt formation and dissolves the salt nearest the entrypoint. The brine sinks to the bottom of the cavity, where it is pushed
out of the well through the tube. The result is a cavern with a ―morning glory‖ shape (that is, wide at the top and narrow at the
bottom). In the bottom injection technique, the same basic geometry is used, but the fresh water is injected through the
suspended tube at the bottom of the formation, and the brine is extracted through the annulus at the top. The cavern begins as
―pear-shaped‖ (that is, wide at the bottom) and changes into a barrel shape; if the process is continued, a mature morning
glory shape results. In the bottom annular injection technique, water is injected through the casing annulus, which is
positioned near the bottom of the salt formation, and brine is withdrawn through the tubing, which is set slightly deeper.

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This creates a barrel-shaped cavern. A variation
of bottom annular injection is to suspend two
concentric tubes in the cased well. Water is injected
through the annulus between the first and second
tubes, and brine is extracted from the lower inner tube.
Oil and air are injected through the annulus between
the casing and the first tube and, being lighter than
water or brine, float to the top of the cavern, where they
inhibit upward growth of the cavern while allowing
lateral growth. When the desired cavern diameter
at a particular elevation has been achieved, the oil
or air pad is withdrawn, allowing upward cavern
growth.
Caverns of 100 metres (330 feet) or more in diameter
can be produced in both bedded and dome salt by
using the above techniques. Production is markedly
increased when the caverns from adjacent wells can be
made to coalesce. In such cases one well becomes the
injection well and the other the production well. Indeed,
it is common to have an injection well in the centre
surrounded by several production wells—typically a
five-spot pattern with the injection well surrounded by
four production wells. The brine is pumped to a plant or
solar pond, where it is condensed through evaporation.

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Frasch sulfur recovery
Although the Frasch process is used to recover sulfur from both bedded and salt-dome-
related deposits, only the latter type is described here. Within the capstone
sequence overlying a salt dome, sulfur can be found disseminated in porous or
fractured limestone that is sandwiched between barren, impervious, and insoluble layers
of rock. The well is started by drilling a borehole in the top of the caprock and setting a
casing with a diameter of 200 to 250 mm (about 8 to 10 inches). A hole is then drilled from
this casing to the bottom of the limestone-sulfur formation, and a 150-mm (6-inch) pipe is
set. This pipe is perforated at two levels. Inside the pipe is yet another pipe, this one 75
mm (3 inches) in diameter, which extends almost to the bottom of the sulfur-bearing
limestone. Finally, a 25-mm (1-inch) pipe is suspended from the surface inside the 75-mm
pipe.
Superheated water (about 170 °C [340 °F]) is injected down the annular space between
the 150-mm pipe and the 75-mm pipe. It is forced out of the upper set of perforations into
the porous formation, which is heated to a temperature above the melting point of sulfur
(about 115 °C [240 °F]). The liquid sulfur, being heavier than water, sinks to the bottom of
the formation, where it flows into the 75-mm pipe through the lower perforations in the
150-mm pipe. The molten sulfur is taken all the way to the surface by reducing its density
through the injection of compressed air via the 25-mm tube.

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