Topic 5: Mining Methods-Part I-Surface mining

Topic 5: Mining Methods
Part I-Surface mining
Hassan Z. Harraz
hharraz2006@yahoo.com
2010- 2011
Prof. Dr. H.Z. Harraz Presentation
This material is intended for use in lectures, presentations and
as handouts to students, and is provided in Power point format so
as to allow customization for the individual needs of course
instructors. Permission of the author and publisher is required for
any other usage. Please see hharraz2006@yahoo.com for contact
details.
The concepts indicated in these slides are considered common
knowledge to those familiar with the field. Many of these ideas have
been published in a variety of different texts and papers over time – no
one of which was specifically used as an outline for this work.

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:
We will explore all of the above in Topic 5.
In this Topic, you will learn the various surface mining methods used
to extract ore from near surface deposits.
About This Topic
 Open-pit mining.
 Terrace mining.
 Strip mining.
 Contour strip mining.
 Auger Mining
 Glory Holing
 Quarrying
 Panning
 Sluicing
 Dredging
 Hydraulic Mining
 Heap Leaching
 In-situ leaching (ISL)

Anatomy of a mine:
Grasberg, West Papua
Figure from Spitz and Trudinger, 2009

Choice of mining and processing methods:
“The simple aim in selecting and implementing a
particular mine plan is always to mine a mineral
deposit so that profit is maximized given the unique
characteristics of the deposit and its location, current
market prices for the mined mineral, and the limits
imposed by safety, economy, environment” (Text book
definition: Spitz and Trudinger, 2009, my italics) (Social
“limits” are not mentioned specifically!)
2 February 2016 4Prof. Dr. H.Z. Harraz Presentation

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 roof
support.

The following outline lists the basic factors which must be taken into account for evaluation of a prospective surface mine :
 Geography
 Legal status of land and mining rights
 Historical, political, and socialogical factors
 Geology
 Mining conditions
 Ore treatment requirements
 Economic analysis
Geography:
Topography, a function of location, affects cost of development and operation of a surface mine. Geographic location establishes
climate.
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 shops.
Legal status of land and mining 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 socialogical 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 unions.
Evaluation of Surface Deposits

Geology
Geological evaluation may include wide-spaced drilling, drill-sample logging, testing and processing, plotting of the data on
maps and cross-sections, preparation of specialized interpretive maps, calculation of reserves by grades, calculation of
stripping requirements, groundwater studies, and economic analysis.
Mining conditions
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 deemed advisable.
Economic analysis
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 calculated.
Many new surface mines require very high capital investments. There are 3 commonly used yardsticks to value investment
worth :
i) Degree of necessity,
ii) Payback period, and
iii) Rate of return (IRR) .
Evaluation of Surface Deposits (cont.)

History
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.

Surface mining:
Surface mining is the predominant exploitation method worldwide.
In the USA, surface mining contributes about 85% of all minerals
exploitation (excluding petroleum and natural gas). Almost all metallic
ore (98%) and non-metallic ore (97%), and 61% of the coal is mined using
surface methods in the USA (Hartman and Mutmansky, 2002).
 Mineral deposits are on or near the surface of the Earth and are removed.
 This is the traditional cone-shaped excavation (although it can be any shape, depending on the
size and shape of the orebody) that is used when the ore body is typically pipe-shaped, vein-type,
steeply dipping stratified or irregular.
 Although it is most often associated with metallic orebodies, (e.g., Palabora copper, Mamatwan
and Sishen iron-ore), it can be used for any deposit that suits the geometry – most typically
diamond pipes – (e.g., Venetia, Koffiefontein and Finsch).
Surface mining requires large capital investment
(primarily expensive transportation equipment), but generally results in:
 High productivity (i.e., high output rate of ore).
 Low operating costs.
 Safer working conditions and a better safety record than
underground mining.

Controls of Gold Mineralization - LCS
Surface Mining methods
2 February 2016 Prof. Dr. H.Z. Harraz Presentation
Mining Methods, Surface mining
10

Strip out overburden (becomes spoils)
Traditional surface mining methods fall into
two broad categories based on locale:
i) Mechanical excavation methods
{such as: Open-pit (or Open-cut or Open-cast);
Terrace; and Strip mining}.
ii) Aqueous methods {such as: Placer and
In-situ leaching (ISL)/ Solution mining}.
 Clean up (reclamation)
Steps of Surface Mining Operation:
11

Table : Subdivided surface mining methods (Bohnit, 1992)
Method Subclass Method
Mechanical
excavation
 Open-pit (or Open-cut or Open-cast) mining.
 Terrace mining.
 Strip (flat terrain) mining.
 Contour strip (hilly terrain) mining.
 Auger Mining
 Glory Holing
 Quarrying
Aqueous
Placer
 Panning
 Sluicing
 Dredging
 Hydraulic Mining
Solution
 Heap Leaching
 In-situ leaching (ISL)
10

Figure shows major surface mining methods

Mineral Extraction
• Surface Mining: overburden (soil and rock on top of
ore) is removed and becomes spoil.
• 1. open pit mining – digging holes
• 2. Dredging – scraping up underwater mineral
deposits
• 3. Area Strip Mining – on a flat area an earthmover
strips overburden
• 4. Contour Strip Mining – scraping ore from hilly
areas

1) Mechanical Extraction Method
 a thick deposit is generally mined in benches or steps,
 although thin deposits may require only a single bench or
face.
Of all the variations of mechanical surface excavation
mining methods available, the three most common methods
only will be described here, namely:
1.1. Open-pit (or Open-cut or Open-cast or quarry) mining.
1.2. Terrace mining.
1.3. Strip (flat terrain) mining.
1.4. Contour strip (hilly terrain) mining.

Figure refers to Classification of Surface mining methods (Bullivant, 1987)

Open pit
• Used when ore bodies lie near
the surface
• Large hole exposes the ore
body
• Waste rock (overburden) is
removed
• 2nd cheapest method, but has
the largest environmental
impact. Why?

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 methods.
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
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.
Sometimes the preliminary stripping of the waste overburden is contracted to firms specializing in
earthmoving. Mining is usually done by track-mounted 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.
Basic Concept

• Mine working open to the surface.
• Funnel shaped hole in ground, with ramp spiraling down
along sides, allows moderately deep ore to be reached.
• Operation designed to extract minerals that lie close to the
surface
• It is used when the orebody is near the surface and little
overburden (waste rock) needs to be removed.
• It is usually employed to exploit a near-surface deposit or
one that has a low stripping ratio.
• Waste is first removed, then the ore is broken and loaded.
• Generally low grade, shallow ore bodies.
• Non-selective  all high and low grade zones mined
• Mining rate > 20,000 tons mined per day (tpd).
• It often necessitates a large capital investment but generally
results in high productivity, low operating cost, and good
safety conditions.
• Design issues:
Stripping overburden
Location of haul roads
Equipment  size of trucks and fleet
Pit slope angle and stability
Surface Mining methods (Open pit Mining method)
1.1) Open pit Mining method

2 February 2016
21
Typical
Bench Wall
Typical
Haul Road
Typical Non-haul
Road benchOutside Dump
Catch
Berm
Typical Open Pit Mine
Drill rig Drilling Out
a New Pattern
Empty haul truck
returning to shovel
Shovels loading
haul trucks
Drilled out pattern about to
be charged with explosives
Top of Main Ramp
Out of Open Pit
Loaded Haul Truck Going to
Run of Mine Stockpile

open pit mining: funnel shaped hole in
ground, with ramp spiraling down along
sides, allows moderately deep ore to be
reached.
Initial mining for zinc at Franklin
and Ogdensburg, New Jersey-USA.
Photo I took at Bingham. 4 km in diameter 1 km in depth, at
its zenith 400000 tons of rock per day

Open-pit mine: Chuquicamata copper mine, Región de Antofagasta, Chile
Locality: Región de Antofagasta, Chile.
Pit dimensions: 4.3 km long x 3 km wide x 850 m deep.
Mining dates: 1915 -present
Total production: 29 million tons of copper to the end of 2007 (excluding Radomiro Tomić production). For many
years it was the mine with the largest annual production in the world, but was recently overtaken by Minera
Escondida (Chile). It remains the mine with the largest total cumulative production.
Production 2007: 896,308 fine metric tons of copper (Codelco, 2007).
Mining cost in 2007: 48.5 US¢ per kg (2006), 73.0 US¢ per kg (2007) (Codelco, 2007).
Employees: 8,420 as of 31st 2007 (Codelco, 2007).
Pre-tax profits: US$ 9.215 billion (2006), US$ 8,451 billion (2007) (Codelco, 2007).
Dust
Slope failure
Benches
Access ramps
http://upload.wikimedia.org/wikipedia/commons/2/2a/Chuquicamata_panorama.jpg
2 February 2016
23

Overburden Removal

A Dragline Shovel
Loading ore in pit
Some photos and machinery used in open-pit mining

Haulage is usually by truck, although railroads,
inclined rails, and conveyor belts have been
used.
The conveyance unloads directly into a primary
crusher and crushed material is stored in coarse
ore bins prior to shipment to the mill.
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 original
planning.

Mining Trucks
*To the left is a photograph of a Liebherr
360 ton (327 metric ton) haul truck. This
unit is powered by a 2750 horse power
engine and weighs 443,000 pounds (177
tons) empty...
Crushing in pit
Drilling in pit

Figure shows Open pit Mining method
Benching
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 working face.

Figure 2.8 Open-pit mining sequence (for pipe-like orebody)

Various open-pit and orebody configurations
Massive deposit, flat terrain (Example
iron-ore or sulphide deposits).
Thick bedded deposits, little overburden, flat
terrain (Example iron ore, coal).
Massive deposit, high relief
(Example copper sulphide).
Dipping seam or bed, flat terrain
(Example anthracite).
Flat lying seam or bed, flat terrain
(Example platinum reefs, coal).
Figure from Hartman and Mutmansky, 2002. 33

1.2) Terrace Mining
• Where the overburden is too thick {or the floor of the pit (i.e., The ore
inclination) is too steeply dipping} to allow waste dumping directly over
the pit (as is the case with a dragline and strip mining), it is necessary to
use intermediate cyclic or continuous transport (e.g., trucks or conveyors)
to transport the overburden to where it can be tipped back into the
previously mined void.
• It is a multi-benched sideways-moving method, the whole mine moves
over the ore reserve from one end to the other, but not necessarily in a
single bench. The number of benches used is usually a function of the
excavation depth and type of machinery used (typically between 10-15m
bench height and 1-32 benches in the terrace).
• Where steeply dipping orebodies are encountered, the modified method
is most often applied, a more typical 3 waste bench terrace operation with
steeply dipping orebody. In this case, the pit dimensions are limited by
seam exposure (pit length) and available working area (for mining and
dumping faces) (pit width).

1.3) Strip Mining
Used for near-surface, laterally continuous, bedded deposits such as coal, stratified ores such
as iron ore, and surficial deposits (nickel laterite or bauxite).
When orebodies are flat-lying and close to surface, it is sometimes economical to remove the overlying rock to
expose the orebody.
Strip mining is ideally applied where the surface of the ground and the ore body itself are relatively
horizontal and not too deep under the surface, and a wide area is available to be mined in a series of strips.
The surface soil is stripped off and stockpiled for later land reclaimation.
A stripping dragline with a long-boom or long reach shovels are common.
The pits are shallower that open-pit mines, and the overburden is “hind-cast” directly into adjacent
mined out panels.
It is a very low-cost, high-productivity method of mining.
Typical examples of this type of mining are the larger tonnage coal mining operations in Mpumulanga.
Favourable conditions are:
 Relatively thin overburden (0-50 m maximum otherwise stripping ration and cost of stripping becomes too
high).
 Regular and constant surface topography and coal layers (not more than 20º variation from horizontal on the
coal seam –topography can vary more since pre-stripping can be used to level it – but this is expensive to
apply).
 Extensive area of reserves (to give adequate life of mine (LOM) and to cover all capital loan repayments –
typically more than 20 years life at 4-14mt per annum production).

Production
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.

Why……………..?
 The ore is close to the surface of the land (30m) but has one or more layers of rock
and dirt on top of it. To mine the ore, these layers have to be taken off.
 This mining is done in long, narrow strips. When the ore is done in one strip, the
miners begin to create another strip next to it. The waste, dirt, and rock that they
take off of the top of the next strip is put on top of the last one.
Strip Mining:
The cheapest and safest method,
but can have a significant impact
environmentally on the surface.
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.
Strip Mining:

Strip Mining
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.

 Strip-mining: Blast, scoop off rock
overburden, and then scoop out ore material.
Fairly shallow.
 Economics of strip mining depend on stripping
ratio
 Large land area can be involved, especially for
coal and bauxite.
Strip mining.
Example: Alcoa’s
Sierra de Bahoruco
Aluminum mining
in D.R. Southern
Peninsula until 1985

"Strip mining" is the practice of mining a seam of mineral by first removing a
long strip of overlying soil and rock (the overburden). It is most commonly used to mine
coal or tar sand. Strip mining is only practical when the ore body to be excavated is
relatively near the surface. This type of mining uses some of the largest machines on
earth, including bucket-wheel excavators which can move as much as 12,000 cubic
meters of earth per hour.
There are two forms of strip mining. The more common method is:-
i) "Area stripping", which is used on fairly flat terrain, to extract deposits over a
large area. As each long strip is excavated, the overburden is placed in the
excavation produced by the previous strip.
ii) "Contour stripping" involves removing the overburden above the mineral
seam near the outcrop in hilly terrain, where the mineral outcrop usually follows
the contour of the land. Contour stripping is often followed by auger mining into
the hillside, to remove more of the mineral. This method commonly leaves
behind terraces in mountain sides.
Among others, strip mining is used to extract the oil-impregnated sand in the
Athabasca Tar Sands in Alberta. It is also common in coal mining. Bucket-wheel
excavators are widely used for this purpose, however, they are prone to damage and
require many millions of dollars to repair.

• Significant “permanent” waste dumps are not needed.
• Mine rehabilitation can be carried out
progressively at the same rate as mining.
Area Strip Mine (Coal)
Schematic of strip coal mine
Figure from Hartman and Mutmansky, 2002.
Coal strip mine in Wyoming
http://en.wikipedia.org/wiki/File:Coal_mine_Wyoming.jpg

Strip Mining
Coal Mine Shovel-Truck Operation
Mining Geology, Introduction
42
200 T Haulage Truck
2 February 2016

Strip Mining
Kansas Geological Survey
Example: coal, placer

Figure shows Strip mining with dragline (on overburden) and rope
shovel (below, loading coal)
The Bagger 288 is a bucket-wheel excavator
used in strip mining.
http://en.wikipedia.org/wiki/File:Bagger-garzweiler.jpg

Large bucket wheel extractor being moved through Germany. Moves 10
meters per minute. Takes 5 people to operate. Used in strip mining

Contour Mining in KY
“Highwalls”
1.4. Contour strip (hilly terrain) mining or Contour (Bench) Strip Mining

Selection of
Mining
Equipment
• Stripping Ratio – in case of Opencast.
• Life of the mine.
• Infrastructure available.
• Proposed annual output.
• Technology available.
1. Shovel + Dumper
2. Dragline
3. Surface Miner
4. Bucket Wheel Excavator
5. In-pit crushing + Spreader
Different OC Machinery

1.4) Glory Holing
This kind of operation is uncommon, as it involves a mine opening at the surface,
from which ore is removed by gravity through raises connected to adit haulage
ways beneath, and by tramming the ore to the surface on the haulage level.
The glory hole method is best suited to mining on a hillside, and irregular deposits
can be cleanly mined without dilution by waste wall rock. Narrow veins have been
mined by glory hole; in these cases the “hole” becomes narrow and long. The
benches are mined away as work descends to the bottom of the deposit or to the
haulage way, so that spectacular steep side walls may result if the walls do not
slough in.
Mining can be quite selective, and little waste rock is thrown on the surface dumps.
The principal environmental objection to the method is difficulty in reclamation of
the surface of the mine area.

AUGER MINING refers to a method of
removing coal, clay, phosphate, oil-shale,
etc. from thin seams exposed in deep
trenches or high-walls in strip mines.
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 low-
cost and highly productive, but
recovery ranges from 40 to
60%. It can be implemented
with relatively low capital
costs.
1.5) Auger Mining

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 systems.
1.6) Quarrying

Depend on water or another liquid (e.g., dilute sulfuric acid, weak
cyanide solution, or ammonium carbonate) to extract the mineral.
2.1) Placer mining:
2.1.1) Hydraulic mining
2.1.2) Dredging mining
2.2) In-situ leaching (ISL)/ Solution mining
2.3) Undersea Mining
Placer and solution mining are among the most economical of all mining
methods but can only be applied to limited categories of mineral deposits.
2) Aqueous Extraction Methods

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 not necessary.
Placer mining affects large surface areas for the volume of material mined, is highly visible and has
serious environmental problems with surface disturbance and stream pollution.
Placer mining is used to exploit loosely consolidated deposits like common sand and gravel or gravels containing
gold, tin, diamonds, platinum, titanium, or gems.
There are two types of placer mining:-
2.1.1) Hydraulic mining: Generally used for weakly cemented near-surface ore deposits. Hydraulic mining
of a placer gold deposit. Hydraulic king utilizes a high-pressure stream of water that is directed
against the mineral deposit (normally but not always a placer), under-cutting it, and causing its
removal by the erosive actions of the water.
2.1.2) Dredging mining: Generally used most often for mineral-sands and some near-shore alluvial
diamond mining operations. Dredging performed from floating vessels, accomplishes the
extraction of the minerals mechanically or hydraulically.
"Dredging" is a method often used to bring up underwater mineral deposits. Although dredging is
usually employed to clear or enlarge waterways for boats, it can also recover significant amounts
of underwater minerals relatively efficiently and cheaply.
2.1) Placer mining

The “Stang Intelligiant” monitor
(operator controlled high pressure
water discharge point) mounted
on a skid
Figure shows Hydraulic mining of a placer gold deposit
Figures from Hartman and Mutmansky, 2002.

i) Hydraulic Mining (or Hydraulic king)
 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.

Dredging mining
"Dredging" is a method often used to bring up
underwater mineral deposits. Although
dredging is usually employed to clear or
enlarge waterways for boats, it can also
recover significant amounts of underwater
minerals relatively efficiently and cheaply.

 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 pond.
 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 reach.
 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 material.
ii) Dredging

i) 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 critical.
ii) 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.
iii) 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.
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 problems.
Types of Excavation Methods Using Dredging:

Placer Mining Costs
Capital Cost of Bucketline Dredge (1990): Operating Costs (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.

Solution Mining
Basic concept
 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.

In-situ leaching (ISL)/ Solution mining
Solution mining (in-situ recovery) = resources in a
deep deposit are dissolved in a liquid and
siphoned out.
Salts, lithium, boron, bromine, potash, copper,
uranium.
Less environmental impact than other methods:
Less surface area is disturbed.
Acids, heavy metals, uranium can accidentally leak.
2 February 2016
63

Used most commonly on evaporite (e.g. salt and potash) and sediment-hosted uranium deposits,
and also to a far lesser extent to recover copper from low-grade oxidized ore.
The dissolving solution is pumped into the orebody from a series of injection wells, and is then
pumped out, together with salts dissolved from the orebody from a series of extraction
(production) wells.
Aside: The same
reagents are often
used for processing
mined ores in
hydrometallurgical
plants
Metals and minerals commonly mined by solution mining methods.
Dissolving agent specified in each case. (From Hartman and Mutmansky,
2002, and references therein).
Metal or Mineral
Approximate
Primary production
Dissolution Agent/ Method
Gold 35% Sodium cyanide (NaCN)
Silver 25% Sodium cyanide (NaCN)
Copper 30% Sulphuric acid (H2SO4); Ammonium carbonate (alkali)
{(NH4)2CO3}
Uranium 75% Sulphuric acid (H2SO4); Ammonium carbonate (alkali)
{(NH4)2CO3}
Common Salt 50% Water
Potash 20% Water
Trona 20% Water
Boron 20% Hydrochloric acid (HCl)
Magnesium 85% Seawater, lake brine processing
Sulfur 35% Hot water (melting)
Lithium 100% Lake brine processing
2.2. In-situ leaching (ISL)/ Solution Mining
2 February 2016
64

In-situ leaching (ISL)/ Solution Mining
Solution mining includes both borehole mining, such as the methods used to extract sodium
chloride or sulfur, and leaching, either through drillholes or in dumps or heaps on the surface.
2 February 2016
65
ISL salt mine ISL sulfur mine
Hot water Compressed air
Sulfur, Water & air
Brine out

In-situ leaching (ISL)/ solution mining:
Advantages:
 No solid wastes.
 Liquid wastes (low concentration brines with no market value) can be re-injected
into the stratum being leached. Also reported that wastes are sometimes injected
into a separate acquifer (not good practice).
Problems:
 Little control of the solution underground and difficulty in ensuring the process
solutions do not migrate away from the immediate area of leaching.
 Main impact of evaporite ISL is derived from surface or shallow groundwater
contamination in the vicinity of evaporation ponds. Pregnant solutions can be
highly corrosive and pyhto-toxic, and can react with the soil materials used in
pond construction, and may migrate to surrounding areas through seepage,
overflow (both bad practice),and windblown spray.
 Surface subsidence and the development of sink-holes may also occur after
prolonged solution mining if inadequate un-mined material is left to support
the overburden (bad practice).
2 February 2016
66

Example
Evaporite deposits
Has been used for many decades to extract soluble evaporite salts such as halite (NaCl),
trona (3Na2O · 4CO2), nahcolite (NaHCO3), epsomite (MgSO4 · 7H2O), carnallite
(KMgCl3 · 6H2O), borax (Na2B4O7 · 10H2O) from buried evaporite deposits in UK,
Russia, Germany, Turkey, Thailand and USA).
A low salinity fluid, either heated or not, is injected underground directly into the
evaporite layer; the “pregnant” solutions (brines) are withdrawn from recovery
boreholes and are pumped into evaporation ponds, to allow the salts to crystallize
out as the water evaporates.
Evaporation ponds, Arizona
Figure from Spitz and Trudinger, 2009.
Old underground mines, consisting typically of room-
and-pillar workings, are often further mined using
solutions to recover what remains of the deposit,
i.e., the pillars (with associated surface subsidence
risk).

Heap Leaching
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 heap.
'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 counter currently 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 solvent phase.

Uranium heap leaching
 Occurs in tetravalent and hexavalent forms
 Tetravalent uranium requires oxidation during
leaching
 Leaching in acid or carbonate medium,
depending on gangue acid consumption. Lower
recoveries in carbonate medium.
 Addition of suitable oxidising agent such as,
H2O2, MnO2, NaClO3 for regeneration of Fe3+,
or by bacterial oxidation. Typically 0.5g/L Fe,
ORP 475-425 mV, which may be produced from
gangue dissolution.
 Bacterial leaching offers advantage of reduced
oxidising agent cost and generation of acid
from sulphide minerals such as pyrite, as well
as liberation of mineral from sulphide host.
 “Readily leachable” minerals are acid leached
at pH 1.5-2.0 and 35-60oC, which are suitable
conditions for bioleaching. “Refractory”
minerals require higher temperature (60-80oC)
and stronger acid (up to 50g/L).

Common Uranium minerals
Mineral Formula Operation
leachable oxides UraniniteTL U+4
1-xU+6
xO2+x Rossing, Dominion
Reefs, Ezulwini
PitchblendeTL UO2 to UO2.25 Narbalek, Kintyre
leachable silicates CoffiniteTL U(SiO4)1-x(OH)4x Rystkuil
refractory
complex oxides
Brannerite TR (U,Ca,Fe,Th,Y)(Ti,Fe)2O6 Elliot Lake
DaviditeTR (La, Ce, Ca)(Y, U)(Ti, Fe3+)20O38 Radium Hill
hydrated oxides BecquereliteHL 7UO2.11H2O
Gummite HL UO3.nH2O
Silicates Uranophane HL Ca(UO2)2Si2O7.6H2O Rossing
UranothoriteTL (UTh)SiO4 Dominion Reefs
Sklodowskite HL (H3O2)Mg(UO2)2(SiO4)22H2O
Vanadates Carnotite HL K2(UO2)2(VO4)2.3H2O Langer Heinrich
Tyuyamunite HL Ca(UO2)2(VO4)2.8H2O
Phosphates Torbernite HL Cu(UO2)2(PO4)2.10H2O Rum Jungle
Autunite HL Ca(UO2)2(PO4)2.11H2O Rum Jungle
Carbonates Schroekingerite HL NaCa3(UO)2(CO3)3(SO4)F.10H2O
Arsenates Zeunarite HL Cu(UO2)2(AsO4)2.10-12H2O
Hydrocarbons ThucholiteTL
HL- hexavalent readily acid leachable without oxidation
TL - tetravalent readily acid leachable with oxidation
TR - tetravalent refractory

Uranium deposits
Uranium minerals are soluble in acidic or alkaline solutions.
The production (“pregnant”) fluid consisting of the water soluble uranyl
oxyanion (UO22+) is subject to further processing on surface to precipitate
the concentrated mineral product U3O8 or UO3(yellowcake).
2 February 2016 71
Acid leaching fluid
sulphuric acid + oxidant (nitric acid,
hydrogen peroxide or dissolved oxygen)
or
Alkali leaching fluid
ammonia, ammonium
carbonate/bicarbonate,
or sodium carbonate/bicarbonate
The hydrology of the acquifer is irreversibly
changed: its porosity, permeability and
water quality. It is regarded as being easier
to “restore” an acquifer after alkali
leaching. Figure from Hartman and Mutmansky, 2002.

Heap Leach Operation
Installing a Plastic Membrane Liner

Layout of copper bio-heap pilot plant
Heaps
Auxiliary, Ponds
PLS,
Raffinate
Ponds
Crushing,
Agglomeration
SX-EW (off
photo)
Drum agglomeration
Humidification layer with drainage pipes

Advantages/disadvantages of heap
leaching
•Advantages
• Low capital and operating costs
• Absence of milling step, may require crushing and agglomeration
• Simplicity of atmospheric leach processes
• Can be used to treat low-grade ores, wastes and small deposits
• Absence of liquid-solid separation step allows counter-current
operation
• Metal tenor may be built up by recycling solution over heaps
•Disadvantages
•Lower recoveries than mill/float or mill/leach
•Long leach cycles and hold-up
•Lengthy experimental programmes
•Large footprint
•Acid-mine drainage of wastes
Advantages/disadvantages of heap leaching

Undersea (or Oceans) Mining
• We extract minerals (e.g., magnesium) from seawater
• Minerals are dredged from the ocean floor
 Sulfur, phosphate, calcium carbonate (for cement), silica (insulation and glass), copper, zinc, silver, gold
• Manganese nodules = small, ball-shaped ores scattered across the ocean floor
 Mining them is currently uneconomical
 Manganese Nodules (pacific ocean)– ore nodules crystallized from hot solutions arising from volcanic
activity. Contain manganese, iron copper and nickel.
• Hydrothermal vents may have gold, silver, zinc
• Mining would destroy habitats and organisms and release toxic metals that could enter the food
chain
1) Minerals are found in seawater, but occur in too low of a concentration
2) Continental shelf can be mined
3) Deep Ocean are extremely expensive to extract (not currently viable)

Topic 5: Mining Methods-Part I-Surface mining

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