In this unit, you will learn the various surface mining
methods used to extract ore from near surface
After completing this unit, you should be able to:
• Understand the geological factors for surface mining
• Understand the engineering factors for surface mining
• Explain what a placer deposit is
•Understand the surface mining methods:
What is Surface Mining?
Surface mine A mine in which the ore lies near the
surface and can be extracted by removing
the covering layers of rock and soil.
Almost all surface mining operations are
exposed to the elements and require no
The history of surface mining is
essentially that of mining coal, copper,
and iron ores, and the nonmetallic
minerals - clays, gypsum, phosphate
rock, sand, gravel, and stone.
Changing public policy is exerting
strong pressure favouring a reduction
or elimination of surface mining; and,
since the economic differences between
surface and underground mining for the
remaining mineral resources is
narrowing, this increasing force may
become the deciding factor in
determining the future trend in surface
vs, underground mining.
Evaluation of Surface Deposits
The following outline lists the basic factors which must be taken
into account for evaluation of a prospective surface mine :
• Legal status of land and mining rights
• Historical, political, and socialogical factors
• Mining conditions
• Ore treatment requirements
• Economic analysis
Topography, a function of location, affects cost of
development and operation of a surface mine. Geographic
Location establishes the condition of remoteness from or
proximity to civilization and its developed facilities such
as transportation systems, power supply, labour pool,
manufacturing and supply services, and specialty repair
Legal Status of Land and Mineral Rights
Land and other necessary rights should be checked, such as water
use rights and the ability to acquire auxiliary land for plant site,
roads, tailings disposal ground etc.
Historical, Political and Sociological Factors
It is important to determine the extent and nature of national and
local laws and regulations in regard to conservation, water use,
water and air pollution, tailings disposal, reclaimation, handling of
explosives, taxes, royalities, import duties, mining safety and health
codes, wage and labour conditions, pension requirements, and
Geological evaluation may
include wide-spaced drilling,
drill-sample logging, testing
and processing, plotting of the
data on maps and crosssections, preparation of
specialized interpretive maps,
calculation of reserves by
grades, calculation of stripping
studies, and economic
The geometry of an ore body and the
topography of the land surface
beneath which the ore body exists
will affect the kind and cost of a
surface mine. The depth and
character of overlying rock and the
physical characteristics of the wall
rock also affect the configuration
and cost of a surface mine.
Ore Treatment Requirements
Almost every potential surface mine
must consider some phase of
product upgrading (benefication).
This may vary from a simple
crushing and sizing operation to a
complex operation including
multiple stages of size reduction,
concentration and agglomeration.
In many cases, pilot-scale testing is
In the broadest sense, economic analysis for a surface
mine involves the determination of market value of the
product and all the elements of cost of production.
By subtraction, a margin of profit (or loss) can be
Many new surface mines require very high capital
investments. There are 3 commonly used yardsticks to
value investment worth :
•1) degree of necessity,
•2) payback period, and
•3) rate of return.
Methods of surface mining can be subdivided into various
classes and subclasses (E. Bohnit, 1992):
Placer deposits are concentrations of heavy minerals,
usually within loose alluvium that can easily be excavated
and washed. Placer minerals such as gold, tin, and tungsten
minerals, are of relatively high value, but the value of the
placer gravel itself may be very low, often less than a dollar
per cubic yard.
For deposits of such low grade to be worked they must be
near water, on or near the surface of the ground, and should
be only loosely consolidated so that drilling and blasting are
Placer mining affects large surface areas for the volume of
material mined, is highly visible and has serious
environmental problems with surface disturbance and
The bulk of placer mining falls into three groups-- panning
and sluicing / hydraulicking, and dredging.
In recent years, gold panning has become a
popular outdoor recreation. There is excitement
and appeal in panning an occasional nugget or
a few small specks of gold. The remote chance
of discovering a rich pocket somehow missed
by the old timers provides a strong incentive.
In general, far more money is made selling
manuals, maps, equipment, and gas and oil to
these hobbyists than is made from the gold
In SLUICING the placer gravel is shoveled,
along with a stream of water, into the head
of an inclined elongated sluice box with
RIFFLES positioned across the bottom.
These trap the heavy minerals and the
lighter minerals are washed over the top
and out as relatively barren waste.
Sometimes fine gold is trapped as an
amalgam when mercury is placed within the
riffles or on a copper plate at the exit of the
sluice box. The gold in the amalgam is
recovered by retorting off the mercury.
Sluicing is dependent upon a
good water supply.
HYDRAULIC MINING involves directing a
high-pressure stream of water, via a
MONITOR or nozzle, against the base of
the placer bank.
The water caves the bank, disintegrates
the ground and washes the material to
and through sluice boxes, and / or jigs,
and / or tables situated down-slope.
Hydraulic mining totally disturbs large
areas and puts much debris into the
drainage system. Presently,
hydraulicking is used primarily in Third
World countries. It is closely controlled
or prohibited in the U.S.
Large alluvial deposits are mined by floating
washing plants capable of excavating the
gravel, processing it in the washing plant, and
stacking the tailings away from the dredge
A Dredge floats in water and digs the gravel by
an endless string of buckets. Coarse material
is screened out and dumped out the back. The
fine material passes into a series of sluices
where the gold in recovered.
Several types of excavation methods are in use:
DRAGLINE and BACKHOE PLANTS. Dragline
use in placer mining with washing plants is
limited to shallow digging depths. Its bucket is
less controllable on the bottom than the
backhoe, and it is less able to dig into the
bottom to clean up all the ore that may be there.
However, it has the advantage of a longer
The digging reach of the backhoe extends to as
much as 70 feet below the surface. It has the
advantage of relatively low first cost, excellent
mobility, and an ability to excavate hard
BUCKET WHEEL HYDRAULIC
DREDGES are becoming more
popular for underwater
excavation, except where a high
content of soft clay exists or
where excessive oversize material
occurs. It is dependent upon
flooded pump openings that
convey the material mined to the
washing plant, and therefore it
cannot work above water level.
Placement of the pump suction is
BUCKETLINE DREDGES are
capable of continuous excavation
and are very efficient. They mine,
process, and discard tailings to
waste in one continuous stream.
However, no storage
opportunities exist, and the
stream moves through the
system by the force of gravity.
Buckets, supported by a
LADDER, dig the mine face.
Material moves up the ladder and
dumps into a hopper that feeds
the washing plant. They are
capable of high excavation rates.
Various methods are used to
position the dredge --anchored
by wire ropes or piling (SPUDS)
at the rear of the dredge.
Boulders can cause serious
SUCTION CUTTER DREDGES are
similar to the Bucket Wheet Dredge
except the digging device consists
of a series of cutting arms rotating
in a basket about a suction intake.
The rotating arms break up the
bank material, slurrying it so it can
be drawn into the dredge suction. It
has proven to be successful in
mining unconsolidated beach
sands and offshore placers.
PLACER MINING COSTS
Operating Costs (1990):
Capital Cost of Bucketline Dredge (1990):
Because large placer deposits can be thoroughly explored before floating a
dredge, such operations can lend themselves to thorough planning, and it
is possible to carry out reclamation as mining progresses at only a slight
increase in operating costs.
When orebodies are flat-lying and
close to surface, it is sometimes
economical to remove the overlying
rock to expose the orebody. The
surface soil is stripped off and
stockpiled for later land reclaimation.
A stripping dragline with a longboom or long reach shovels are
Large-scale continuous bucket
excavators are gaining popularity.
These large scale machines are
designed for high capacity output
and are tremendous in size, highly
productive, and very expensive.
1 Electric drills prepare the overlying
strata for blasting.
2 Removal of broken ore.
3 Removal of broken rock.
4 Extraction of upper ore seam.
5 Removal of upper ore.
Open Pit Mining
Although the basic concept of an open pit
is quite simple, the planning required to
develop a large deposit for surface mining
is a very complex and costly undertaking.
At one mine, it may be desirable to plan for
blending variations in the ore so as to
maintain, as nearly as possible, a uniform
feed to the mill. At another operation it may
be desirable to completely separate two
kinds of ore, as for example, a low- grade
deposit where one kind of "oxide" ore must
be treated by acid leach, but a second kind
of "sulfide" ore must be treated by different
Open Pit Mining
The grade and tonnage of material
available will determine how much
waste rock can be stripped, and there
is often an ultimate limit to the pit that
is determined more by the economics
of removing overburden than a
sudden change in the ore deposit
from mineral to non-mineral bearing
material. The ultimate pit limit and the
slope of the pit walls are therefore
determined as much by economics
and engineering as by geological
structure. Material that is relatively
high grade may be left unmined in
some awkward spot extending back
too deeply beneath waste.
Open Pit Mining
The typical large open pit mining
operation that has been in
production for 10 years and more is
operating under conditions that
could not possibly have been
foreseen by the original planners of
the mine. Metal prices, machinery,
and milling methods are constantly
changing so that the larger
operations must be periodically
reevaluated, and several have been
completely redeveloped from time to
time as entirely different kinds of
mining and milling operations.
Open Pit Mining
Sometimes the preliminary stripping
of the waste overburden is contracted
to firms specializing in earthmoving.
Mining is usually done by trackmounted electric shovels in the large
operations, and by rubber-tired diesel
front-end loaders in the smaller
operations. Scrapers are sometimes
used in special situations.
Large bucket-wheel excavators of the
kind used in European coal mines
have not been applied to metal
mining, because this equipment is
best adapted to softer bedded,
relatively flat-lying strata..
Open Pit Mining
Haulage is usually by truck,
although railroads, inclined
rails, and conveyor belts have
The conveyance unloads
directly into a primary crusher
and crushed material is stored
in coarse ore bins prior to
shipment to the mill.
Open Pit Mining
Bench level intervals are to a
large measure determined by
the type of shovel or loader
used, and these are selected
on the basis of the character
of the ore and the manner in
which it breaks upon blasting
and supports itself on the
Open Pit Mining
Blastholes are usually drilled vertically
by self-propelled, track-mounted
pneumatic or rotary drills. Bulk
explosives are loaded in the holes and
large volumes of ore are broken in a
single blast. Sometimes the drill holes
are routinely sampled and assayed to
help plan the position of the shovels in
advance of mining.
Blasthole assay control is especially
desirable when exploration data are
incomplete or lacking as in the case in
the older pits which have long been
mined past the limits of "ore" used in
QUARRYING or Quarry Mining is usually
restricted to mining dimension stone prismatic blocks of marble, granite,
limestone, sandstone, slate, etc. that are
used for primary construction of buildings
or decorative facing materials for exterior
and interior portions of buildings.
Quarries generally have benches with
vertical faces from a few feet to 200 feet in
height. Blocks are drilled and wedged free
in a highly selective manner using time
consuming and expensive methods.
Planning of the excavation is based
primarily on geological factors such as the
direction and attitude of bedding and joint
GLORY HOLING involves a mine opening at the surface from which
ore is removed by gravity through raises connected to adit
haulageways beneath, and tramming the ore to the surface. It is
suited to mining on a hillside, and irregular deposits can be mined
without dilution by waste wall rock. Mining can be quite selective
and little waste rock accumulates on the surface. However,
reclamation is difficult.
AUGER MINING refers to a method of
removing coal, clay, phosphate, oilshale, etc. from thin seams exposed in
deep trenches or high-walls in strip
The auger consists of two principal
pieces. The first is a cutting head,
generally from 1.5 to 8 feet in diameter.
It may be single or multiple. The second
is a prime mover, usually a skid
mounted carriage, providing a mounting
for the engine, drive head, and controls.
As coal arrives at the surface it is
transported via a conveyor belt or a
front-end loader to a waiting truck.
Operations are usually lowcost and highly productive,
but recovery ranges from 40
to 60%. It can be
implemented with relatively
low capital costs.
Heap leaching is also
used in recovering metals
from their ores.
Bacterial leaching is first
used to oxidize sulphide
minerals. Cyanide solution
is then used to leach the
metals from the mineral
The theory and practice of leaching are well-developed because
for many years leaching has been used to separate metals from
their ores and to extract sugar from sugar beets. Environmental
engineers have become concerned with leaching more recently
because of the multitude of dumps and landfills that contain
hazardous and toxic wastes. Sometimes the natural breakdown
of a toxic chemical results in another chemical that is even more
toxic. Rain that passes through these materials enters ground
water, lakes, streams, wells, ponds, and the like.
Although many toxic materials have low solubility in water, the
concentrations that are deemed hazardous are also very low.
Furthermore, many toxic compounds are accumulated by living
cells and can be more concentrated inside than outside a cell.
This is why long-term exposure is a serious problem;
encountering a low concentration of a toxic material a few
times may not be dangerous, but having it in your drinking
water day after day and year after year can be deadly.
The main theory of leaching neglects mechanisms for holding
the material on the solid. Although adsorption and ion
exchange can bind materials tightly to solids, we will simplify
the analysis and consider only dissolving a soluble constituent
away from an insoluble solid. An example is removing salt
from sand by extraction with water.
Countercurrent stage wise processes are frequently used in
industrial leaching because they can deliver the highest possible
concentration in the extract and can minimize the amount of
solvent needed. The solvent phase becomes concentrated as it
contacts in a stage wise fashion the increasing solute-rich solid.
The raffinate becomes less concentrated in soluble material as it
moves toward the fresh solvent stage.
'Heap leaching' is a countercurrent process where the solid is in
a stationary heap and the solvent percolates through the solid.
An example is a dump or landfill. This leaching is essentially
countercurrent. In industrial leaching, solvent and solid are
mixed, allowed to approach equilibrium, and the two phases are
separated. Liquid and solids move countercurrently to the
adjacent stages. The solvent phase, called the extract, becomes
more concentrated as it contacts in stagewise fashion the
increasingly solute-rich solid. The raffinate becomes less
concentrated in soluble material as it moves toward the fresh