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Topic 2: Mining




                                      From a series of 5 lectures on
              Metals, minerals, mining and (some of) its problems
                                prepared for London Mining Network
                                                                  by
                                                       Mark Muller
                                       mmuller.earthsci@gmail.com
                                                      24 April 2009
Outline of Topic 2:

• Surface mining methods

• Open-pit mines
        Slope failure in open-pit mines

• Open-cast mines

• Underground mining methods:
        Room-and-pillar mining
        Longwall mining
         Rockburst hazard in deep longwall mining
         Surface subsidence above shallow longwall mining
        Block-cave mining

• Mining using in-situ leaching

• Other mining methods: hydraulic mining, dredging
Anatomy of a mine:
                                               Grasberg, West Papua




0.2 million tons of
tailings are dumped
into the Ajkwa river
system every day,
causing massive
sedimentation on
coastal floodplains
(Lottermoser, 2007)
                                                  No smelter and refinery.
                                                  This project delivers
                                                  mineral-concentrate.




      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 maximised 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!)
Mineral extraction: from mining to metal
Mining




                                           Mineral
                                           concentrate




         METAL EXTRACTION




                                           Metal




                                    Figure from Spitz and Trudinger, 2009.
Schematic of common mining methods


Simple in concept, highly engineered for efficiency.
Very high waste rock volume.
Better safety record.




Used for laterally extensive deposits.
Overburden cast directly back into mined out panels.
Rehabilitation keeps pace with mining.




Reduced waste rock production.
Poor safety record.




Used for soluble ores: uranium, salt, potash.
Minimal waste production: only water wastes, no solids.



                                 Figure from Spitz and Trudinger, 2009.
Choice of mining method:

The choice of mining method depends on many factors, including:
(i) Shape of the orebody: tabular, cylindrical, spherical.
(ii) Orientation of the orebody: sub-horizontal, sub-vertical.
(iii) Continuity of the orebody.
(iv) Ore-grade: high-grade, low-grade.
(v) Distribution of ore-bearing minerals within the orebody: massive or

         disseminated (with a cut-off grade).
(vi)     Depth to the orebody.
(vii)    Strength of the orebody and overburden/host-rocks rocks.
(viii)    Area of land available for waste disposal – open-pit mines cover a
         larger surface area and generate a greater volume of wastes.
(ix)     Impacts on surface: environmental, surface drainage and sub-surface

       aquifers, land-use changes, social.
(x) Rehabilitation concerns.
(xi) Projected production rates.
(xii) Capital costs, rate of (financial recovery), cash-flow.
(xiii) Safety concerns – surface mining methods have a better safety
       record.
Mining methods:

Surface mining
   Open-pit mining
   Strip or open-cast mining
         includes superficial deposit mining: nickel laterite,
         bauxite, mineral sands, alluvial diamonds

Underground mining
   Block-caving
   Sub-level block-caving
   Longwall
   Room-and-pillar (Bord-and-pillar), Stope-and-pillar
   Longwall Top Coal Caving (LTCC) (China).

In-situ leaching

Dredging from floating vessels: alluvial deposits, mineral sands.

Hydraulic mining: often associated with placer deposits and tailings
   reprocessing.
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).

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
Comparison of waste production for surface and underground mining:

Data are for USA in 1997 (from Hartman and Mutmansky, 2002), in million tons.




  Surface mining                                     Underground mining
  Waste = 73% of total rock tonnage extracted        Waste = 7% of total rock tonnage extracted
          266% of ore tonnage extracted                      9% of ore tonnage extracted



   Pit excavation initially generates huge volumes of waste rock that must be
   removed to allow access the orebody, and to allow stable pit slopes to be
   developed.
Various open-pit and orebody configurations

  (i)                               Flat lying seam or bed, flat terrain. Example
                                    platinum reefs, coal.




 (ii)                               Massive deposit, flat terrain. Example iron-
                                    ore or sulphide deposits.



(iii)                               Dipping seam or bed, flat terrain. Example
                                    anthracite.




(iv)                                Massive deposit, high relief. Example
                                    copper sulphide.



(v)                                 Thick bedded deposits, little overburden, flat
                                    terrain. Example iron ore, coal.



                                                      Figure from Hartman and Mutmansky, 2002.
Open-pit mine: Chuquicamata copper mine, Región de Antofagasta, Chile

                                                         http://en.wikipedia.org/wiki/File:Chuquicamata_panorama.jpg
   Benches


   Access ramps




Photo credit:Till Niermann
11 September 2008.                                                                                     Dust
                                                                                       Slope failure


    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).
Pit slope versus rock strength                   Pit depth versus pit diameter




Greater rock strength can support greater       A greater final pit depth requires a larger
bench heights – resulting in a steeper pit, a   diameter pit (assuming rock strength and pit
lower stripping ratio and less waste rock.      slope remains unchanged)

                                                – resulting in a higher stripping ratio and
                                                more waste rock.




                                                                     Figure from Spitz and Trudinger, 2009.
Open-pit slope failure – case study – groundwater problems

      A slope failure occurred at the Cleo
      Open Pit (Sunrise Dam Gold Mine,
      Western Australia) in December 2000.
      At the time of failure the pit-floor was at                                              Water table
      100 m depth below surface.




                                                                                                                  100 m
      Two critical factors played a role in
      the failure:
      • The top of the water table is at a very
         high level: only 30 m below surface
      • A strong layer of younger clay
        sediments overlies weaker weathered
        bedrock.
                                                     Seepage and mineral
                                                     precipitation
      The failure is thought to be due to very                                      Top of water table
      high pore fluid pressures in the
      weathered bedrock that created an                                    Original configuration
      instability at the interface between the                Mud pile
                                                                                                              Stiff clay
      bedrock and the overlying clays, allowing
      a slippage to occur (Speight, 2002).
                                                                                                    Weathered bedrock
                                                                                                    high pore pressures

                                                                                Plane of failure located at boundary
                                                                                between bedrock and clay
                                                    Distance in meters

Figures modified from Speight, 2002.
Open-pit slope failure – structural problems

Pre-mining geological structures, particularly fault planes, represent zones of potential
weakness in the rock mass, and are therefore zones of potential slope failure, and should be
taken into account when designing the mine.

Fault planes dipping towards the pit (as shown in the figure) present a greater risk than faults
dipping away from the pit.

Faults planes often provide passage-ways for water movement, and these waters, through the
process of weathering and chemical alteration of minerals, may reduce the strength of the rocks
on either side of the fault plane, and reduce the “coefficient of friction” along it.

The coefficient of friction (the “traction” or
“grip”) along the fault will determine
whether failure and slippage of rock down
the fault plane is likely.

The coefficient of friction may change with
time:
• as water-flow patterns are affected by
  mining
• as faults are exposed by the removal of
  rock, opening fluid pathways into faults
• by the reduction of the mass of the rock
  located above the fault plane.                  Computer model of a potential failure plane in an
                                                  open-pit mine (From Little, 2006)
Schematic of open-cast coal mine




                                       OVERBU
                                              RDEN




                                                               Dragline gathers overburden
                                    DIRE                       and “casts” it back onto spoil
                                         CTIO
                                              NO   F AD        banks located behind the
                                                       VANC
                                                           E   current working face



       • Significant “permanent” waste dumps are not needed.
       • Mine rehabilitation can be carried out progressively at the same rate as mining.




Figure from Hartman and Mutmansky, 2002.
Open-cast or 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).

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.
Open-cast coal mining, Rhine Westphalia Germany

Simulated natural-color satellite
(ASTER) image of the Garzweiler
open-cast lignite (brown coal) mine
in North Rhine Westphalia, Germany.


The mine is named after the town
Garzweiler, which was located at the
center of the area being mined.




                                             AD
                                                V
                                                AN
                                                                                        8.5 km




                                                  CE
                                           Active mining




                                       Photo credit: NASA/GSFC/METI/ERSDAC/JAROS,
                                       and US/Japan ASTER Science Team, August 26, 2000.
                                       http://asterweb.jpl.nasa.gov/gallery-detail.asp?name=Coal
Underground mining:

Generally underground mining is adopted when the orebody is too deep and
   it’s not economically or technically feasible to use an open-pit:

Deepest “hard-rock” open pits are over 700 m deep (e.g., Palabora in South
   Africa and Chuquicamata in Chile).

It is increasingly common to progress from open-pit to underground mining
     of the same orebody.

Used where surface land use prohibits surface disruption (e.g., towns,
   agricultural land, lakes, near-surface aquifers). Not always prioritised by
   miners!



The major distinction between the different underground mining
   methods is whether the mined out areas remain supported after
   mining, or if they are allowed to collapse.
General anatomy of a deep underground mine

Both ore-rock and water are
allowed to feed to the bottom of the
mine under the force of gravity, and
from there are transported or
pumped to the surface.


Note the use of “backfill” in mined-
out areas to provide support for the
overlying rock. Backfill allows ore
recovery to be maximised, because
ore is not left in-place as support
pillars.


Backfill is generally a mixture of
cement with waste rock, sand or
tailings.




                                             Figure from Spitz and Trudinger, 2009.
Supported underground mining: room-and-pillar layout


                 Pillars have been mined-out
                 in this area

                                                   Note the control of ventilation, i.e.,
                                                   the separation of contaminated
                                                   (used) and uncontaminated (fresh)
                                                   air using a variety of devices.




Figure from Hartman and Mutmansky, 2002.
Supported underground mining – Room-and-Pillar method:

The mining cavity is supported (kept open) by the strength of remnants
   (pillars) of the orebody that are left un-mined.

Room-and-pillar mining method has a low recovery rate (a large
   percentage of ore remains in place underground).

Used for tabular orebodies, with moderate dip: for example, coal and
   evaporite (salt and potash) deposits.

It is an advantageous mining method for shallow orebodies – as a means
     of preventing surface subsidence. Historic, ultra-shallow underground
     coal mines (< 30 m) nevertheless are characterised by surface
     subsidence in the areas between pillars (e.g., Witbank coal field,
     South Africa).

Pillars are sometimes mined on retreat from a working area, inducing
     closure and caving of these working panels, and raising the risk of
     surface subsidence.
Underground mining: room-and-pillar mining of thick seams – “benching”




                                      ll
                                 wa
                           g ing
                       n
                    Ha




                                       ll
                                   t wa
                                Foo



Different approaches allow either the top or
bottom part of the seam to be mined out first.

Note the “hangingwall” is above the mining
cavity, and the “footwall” is below it.



                                                 Figures from Hartman and Mutmansky, 2002.
Unsupported underground mining – longwall mining method:

Longwall mining is suitable for tabular orebodies, with moderate dip (e.g.,
   coal and stratiform hard-rock ores).

In “unsupported” mining, the mine-workings are supported temporarily
    only for as long as needed to keep the active face open to mining.
    After mining, the support (e.g. hydraulic props or wood packs) is
    removed (or becomes crushed), and the mining cavities close up
    under the pressure of the overburden material. The cavity closure
    is either partial, for shallow mining, or complete, for deep level
    mining.

While unsupported mining is advantageous in that it maximises ore
   recovery (as little ore as possible is left behind) the method comes
   with significant problems:
    - Surface subsidence in the case of shallow mines
    - Rockbursts underground, causing injury and death in deep level
        mines.
Underground mining: longwall mining

                                                               http://en.wikipedia.org/wiki/File:SL500_
   SCHEMATIC OF LONGWALL PANEL                                 01.jpg
   (HANGINGWALL STRIPPED AWAY FOR
   ILLUSTRATIVE PURPOSES)


                                      Protective screen

                        E
                     ANC
                 ADV


                                       ~1
                                          50
                                           m

                                                               Mechanised cutting machine on a
                                                               longwall coal-mining face.



                                                                In hard-rock minerals mining
  “Permanent” support,                                          a “scraper” is pulled down
  often timber packs, will                                      the length of the stope face
  remain in place after                                         after drilling and blasting, to
  mining. With time, these                                      collect the fragmented ore
  become deformed or                                            rock.
  completely crushed – as
                                     Temporary support near     In coal mining, a mechanised
  part of the “controlled”
                                     the working face: often    cutting device is run along
  closure of the panel.
                                     hydraulic props.           the length of the coal face.

Figure from Hartman and Mutmansky, 2002.
Unsupported underground mining
                                                   σ v = ρgh



Cartoon showing the driving       Depth
                                                                        σv     h     Virgin stress situation
                                  below
                                                                                     at depth h:
    mechanisms of “mining-        surface


    induced seismicity”.                                                                  σ v = ρgh
                                                       h


The creation of a cavity
   underground significantly
   alters the virgin subsurface
   stress (pressure) regime.                       σ v = ρgh



                                  Depth
                                                                       σv      h
                                  below
                                  surface
                                                                                   Mining process:
                                                           Mined out               blast & remove
                                                       h                           material at the
                                                                                   stopes, tunnels …




                                                   σ v = ρgh
                                                                                    Removal of rock causes
                                   Depth
                                                                        σv     h    stresses to redistribute,
                                                                                    stope closure & fracturing
                                   below
                                   surface

                                                                                    Slip on new fractures and
                                                                       Slip!        pre-existing geological
                                                  k!
                                                       h
                                                       h

                                              ra c
                                                                                    features results in seismicity
                                             C
The effect of mining depth on stope (cavity) closure
                                         SURFACE




 Shallow mining: partial closure
 of cavity, and surface
 subsidence above the mining.
                                         “Beam width”




 Deep mining: complete closure
 of cavity, extensive fracturing
 around the cavity, with
 associated rockbursts
 (explosive release of seismic
 energy – effectively
 earthquakes).
Deep level gold mining, South Africa




                                                                     1.5 m




                                                                        Stope face with temporary support




                                               Stopes (yellow):
                                                                                        STOPE
                                                                                        STOPE
                                               on-reef
                                               excavations
                                               where the reef
                                               (orebody) is
                                               mined.

http://www.bullion.org.za/MiningEducation/Images/images/
   Slide 28
CrossSectMine.jpg                           © CSIR 2006       www.csir.co.za
Aftermath of a rockburst in a deep-level tunnel showing complete tunnel closure. The
energy released by this event is equivalent to magnitude M = 3.4 earthquake.
Longwall mining with strike stabilising-pillars at South African gold mine


Plan view of tabular orebody showing mined and un-mined areas         Sub-shaft support pillar
                                                                     Sub-shaft pillar boundary




                                                     SUB-SHAFT
                                                                                                   N




        Dip




                                                                                                 Mined-out
                                                                                                 stopes
                                                                 1km
                                         Mined out
    Geological structures: 1.0 km
             0     0.5 km                       Strike Pillars
    faults and dykes                             Strike pillars (unmined ore rock)
                                                 providing support for hangingwall
Mining strategy can make a difference to rockburst activity:

  Underground seismicity (and rockburst rate) can be influenced by:
     - The rate of advance of a stope face (slower advance is more favourable)
     - The number of adjacent panels being mined simultaneously (smaller
       number is more favourable)
     - The angle at which the advancing face approaches geological
       structures (preferably not parallel)
  The last point highlights the need for detailed advanced knowledge of
  geological structures. Miners are not always prepared to invest in high resolution
  surveys (e.g., geophysical surveys) to achieve the level of detail required for safer mining.

Plan view of mining panels

                                     Seismicity and
             ADVANCE                                          ADVANCE
                                     rockbursts

        Mined out                                         Mined out
                                    Fault                                             Fault

             SUPPORT PILLAR                                 SUPPORT PILLAR

   Unfavourable stoping layout and progression of     More favourable stoping layout and
   mining.                                            progression of mining.
Underground longwall coal mining at Barapukuria Mine, NW Bangladesh


                                         Coal production started in
                                         October 2005.
                                         Mining depth is approximately
                                         400m.
                                         To date 6 panels have been
                                         mined, with a 3 m slice height.

                                         Total production to date has not
                                         exceeded 3 Mt coal (a small
                                         proportion of the total 34 Mt
                                         recovery planned over 30 years).




                                         Plan view Barapukuria Coal Mine
                                         (from levels 260 m to 420 m).

Figure from Islam et al., 2008.
Underground longwall coal mining at Barapukuria Mine, NW Bangladesh


            SURFACE SUBSIDENCE
            (NOT SHOWN TO SCALE)
                                                                                      SURFACE


                                                                                      110 m
                                                                           WATER
               WATER




                                                                                      400 m




                                                             zo ctu e
                                Ri ctur




                                                               fra bsid
                                                               n e re
                                 fra e




COAL SEAM
                                  bs




                                             CAVITY
                                   zon




                                                                  Ri
                                     ide e




   IV
                                                                                      440m




    Schematic cross-section showing subsurface response to longwall mining cavity, and
    subsidence at surface. Arrows show fluid-pathways downwards from the Dupi Tila acquifer
    along mining induced fractures, into the mining panel.

Figure modified after Islam et al., 2008.
Underground longwall coal mining at Barapukuria Mine, NW
  Bangladesh:

The impact on surface at Barapukuria is typical of many areas around the
   world which have been undermined by longwall coal mining. Subsidence
   problems are particularly associated with the sedimentary basins in
   which coal is found (also evaporite NaCl and KCl deposits) as the
   overburden is weaker than crystalline rocks. Underground coal mining
   also tends to take place at shallower depths than many hard-rock mineral
   mines.

Considering how little of the resource has been mined to date, the impact on the surface above
   the mine at Barapukuria has been devastating:
-  Land subsidence of between 0.6 – 0.9 m has been reported over an area of about 1.2 km 2.
-  The water-table has dropped, leaving commonly used water reservoirs dry in 15 villages.
-  At least 81 houses have developed cracks in 5 villages.
-  Untreated mine water (acknowledged by the mine to contain phosphorous, arsenic and
   magnesium) is passing through canals in farming areas.
-  The scale of the problem has the Bangladesh government currently considering the
   establishment of a new “coal city” near Barapukuria that would provide housing and
   (potential) employment to people whose livelihoods are at risk in 15 villages around the mine.

Barapukuria Mine plans to increase the total panel height mined to 18 m (it is currently
   only 3 m) – it is not clear whether this plan is going to be practically viable!!
Underground mining – Block caving:

Block-caving method is employed generally for steeply dipping ores, and
   thick sub-horizontal seams of ore. The method has application, for
   example in sulphide deposits and underground kimberlite (diamond)
   mining.

                                              An undercut tunnel is driven under the
                                              orebody, with "drawbells" excavated
   SURFACE                                    above. Caving rock falls into the
                             TOP OF OREBODY   drawbells. The orebody is drilled and
                                              blasted above the undercut to initiate the
                                              “caving” process. As ore is continuously
                                              removed from the drawbells, the
                                              orebody continues to cave
                                              spontaneously, providing a steady
                                              stream of ore. If spontaneous caving
                                              stops, and removal of ore from the
                                              drawbells continues, a large void may
                                              form, resulting in the potential for a
   Figure from Hartman and Mutmansky, 2002.   sudden and massive collapse and a
                                              potentially catastrophic windblast
                                              throughout the mine (e.g., the
                                              Northparks Mine disaster, Australia).
Block-cave mining: Mud-rushes – an under-reported hazard

Mud-rushes are sudden inflows of mud from ore drawpoints (or other underground openings), in
block-cave mines that are open to the surface. Considerable violence, in the form of an
airblast, is often associated with mud-rushes. Mud-rushes are (under-reported) hazardous
occurrences that have occurred frequently in mines in South Africa, as well as in Chile and Western
Australia, and have caused fatalities (Butcher et al., 2005).



                                                             Mud is produced by the
                                                             breakdown of rock in the
                                                             near-surface muckpile in the
                                                             presence of rainwater.

                                                             Kimberlite rock on diamond
                             MUD                             mines is particularly
                                                             susceptible to weathering by
                                                             rainwater.




                                                   SCHEMATIC CUT-AWAY VIEW OF SUB-
                                                   LEVEL BLOCK-CAVE MINE


        Figure from Hartman and Mutmansky, 2002.
In-situ leaching (ISL)/ solution mining:

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 oxidised 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.

                                                                    Sodium cyanide: NaCN
   Metals and minerals commonly mined by solution mining methods.
                                                                    Sulphuric acid: H2SO4
   Dissolving agent specified in each case. (From Hartman and
   Mutmansky, 2002, and references therein).                        Hydrochloric acid: HCl
                                                                    Ammonium carbonate
                                                                         (alkali): (NH4)2CO3



                                                                    Aside: The same reagents
                                                                    are often used for
                                                                    processing mined ores in
                                                                    hydrometallurgical plants
In-situ leaching (ISL)/ solution mining:

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).

Acid leaching fluid
sulphuric acid + oxidant
(nitric acid, hydrogen
peroxide or dissolved
oxygen)
or                                          UO22+
Alkali leaching fluid
ammonia, ammonium
carbonate/bicarbonate,
or sodium
carbonate/bicarbonate

The hydrology of the acquifer is
irreversibly changed: its porosity,   UO2
permeability and water quality. It is
regarded as being easier to “restore”
an acquifer after alkali leaching.
                                                    Figure from Hartman and Mutmansky, 2002.
In-situ leaching (ISL)/ solution mining:

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
    crystallise out as the water evaporates.

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).

                                 Evaporation ponds, Arizona
                                                              Figure from Spitz and Trudinger, 2009.
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).
Hydraulic mining:

Generally used for weakly cemented near-surface ore deposits.




                                             Note:
                                             Riffle box
                                             uses mercury
                                             for gold
                                             recovery
                                                                   The “Stang Intelligiant” monitor
Hydraulic mining of a placer gold deposit.                         (operator controlled high
                                                                   pressure water discharge point)
                                                                   mounted on a skid


                                                            Figures from Hartman and Mutmansky, 2002.
Hydraulic mining – tailings dam reprocessing

Kaltails project, Kalgoorlie, Western Australia
The project was established to reprocess and relocate tailings dams from the Boulder and
Lakewood areas of Kalgoorlie. The operations ceased after a decade of work in September
1999. The tailings dumps were hydraulically mined, reprocessed and stored in another
engineered impoundment located 10 km south east of Kalgoorlie. Recovery was by Carbon-in-
Circuit (CIC) and Carbon-in-Pulp (CIP) leach and absorption circuits.


                                                                             Sixty million tons of
                                                                             tailings were mined in
                                                                             an area of 333
                                                                             hectares (3.33 km2),
                                                                             producing 695,000
                                                                             ounces (19.7 tons) of
                                                                             gold at an average ore-
                                                                             grade of 0.33 g/ton.




Remaining part of a waste dump being hydraulically mined (Newmont Mining).

                                                                                         From TAILSAFE, 2004.
Dredging:

Used most often for mineral-sands and some near-shore alluvial diamond
   mining operations.

 Typical bucket-line dredge




                                           www.tmd.ihcholland.com




Figure from Hartman and Mutmansky, 2002.
Oil sand mining:

Blurs the boundary between hydrocarbon and mineral extraction.
    Canada’s oil sands are the second single largest oil deposit on
    Earth, second only to the large reserves in Saudi Arabia.
    Resource has not been exploited significantly to date because of
    the much higher costs associated with extracting the oil compared
    to conventional borehole extraction in normal oil deposits.

The sands are either strip mined or the oil rich sands are heated
   underground (steam) so that oil migrates to recovery boreholes.
   In the case of mining, bitumen is recovered by washing the sands
   in hot water, and is subsequently upgraded to a final “crude” in two
   successive process: hydro-cracking and hydro-treating.

“Ore-grade”: approximately 2 tons of sand needed to produce 1 barrel
   of oil.
Coal-bed methane and Underground Coal Gasification:

Please see my review included with course material:
“It’s not only about coal mining: Coal-bed methane (CBM) and
     underground coal gasification (UCG) potential In Bangladesh”.

For a summary of these methods, their advantages and disadvantages,
   and an quantitative examination of the energy return provided by these
   surface based, non-invasive alternatives to coal mining.

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Topic 2: Mining

  • 1. Topic 2: Mining From a series of 5 lectures on Metals, minerals, mining and (some of) its problems prepared for London Mining Network by Mark Muller mmuller.earthsci@gmail.com 24 April 2009
  • 2. Outline of Topic 2: • Surface mining methods • Open-pit mines Slope failure in open-pit mines • Open-cast mines • Underground mining methods: Room-and-pillar mining Longwall mining  Rockburst hazard in deep longwall mining  Surface subsidence above shallow longwall mining Block-cave mining • Mining using in-situ leaching • Other mining methods: hydraulic mining, dredging
  • 3. Anatomy of a mine: Grasberg, West Papua 0.2 million tons of tailings are dumped into the Ajkwa river system every day, causing massive sedimentation on coastal floodplains (Lottermoser, 2007) No smelter and refinery. This project delivers mineral-concentrate. Figure from Spitz and Trudinger, 2009.
  • 4. 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 maximised 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!)
  • 5. Mineral extraction: from mining to metal Mining Mineral concentrate METAL EXTRACTION Metal Figure from Spitz and Trudinger, 2009.
  • 6. Schematic of common mining methods Simple in concept, highly engineered for efficiency. Very high waste rock volume. Better safety record. Used for laterally extensive deposits. Overburden cast directly back into mined out panels. Rehabilitation keeps pace with mining. Reduced waste rock production. Poor safety record. Used for soluble ores: uranium, salt, potash. Minimal waste production: only water wastes, no solids. Figure from Spitz and Trudinger, 2009.
  • 7. Choice of mining method: The choice of mining method depends on many factors, including: (i) Shape of the orebody: tabular, cylindrical, spherical. (ii) Orientation of the orebody: sub-horizontal, sub-vertical. (iii) Continuity of the orebody. (iv) Ore-grade: high-grade, low-grade. (v) Distribution of ore-bearing minerals within the orebody: massive or disseminated (with a cut-off grade). (vi) Depth to the orebody. (vii) Strength of the orebody and overburden/host-rocks rocks. (viii) Area of land available for waste disposal – open-pit mines cover a larger surface area and generate a greater volume of wastes. (ix) Impacts on surface: environmental, surface drainage and sub-surface aquifers, land-use changes, social. (x) Rehabilitation concerns. (xi) Projected production rates. (xii) Capital costs, rate of (financial recovery), cash-flow. (xiii) Safety concerns – surface mining methods have a better safety record.
  • 8. Mining methods: Surface mining Open-pit mining Strip or open-cast mining includes superficial deposit mining: nickel laterite, bauxite, mineral sands, alluvial diamonds Underground mining Block-caving Sub-level block-caving Longwall Room-and-pillar (Bord-and-pillar), Stope-and-pillar Longwall Top Coal Caving (LTCC) (China). In-situ leaching Dredging from floating vessels: alluvial deposits, mineral sands. Hydraulic mining: often associated with placer deposits and tailings reprocessing.
  • 9. 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). 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
  • 10. Comparison of waste production for surface and underground mining: Data are for USA in 1997 (from Hartman and Mutmansky, 2002), in million tons. Surface mining Underground mining Waste = 73% of total rock tonnage extracted Waste = 7% of total rock tonnage extracted 266% of ore tonnage extracted 9% of ore tonnage extracted Pit excavation initially generates huge volumes of waste rock that must be removed to allow access the orebody, and to allow stable pit slopes to be developed.
  • 11. Various open-pit and orebody configurations (i) Flat lying seam or bed, flat terrain. Example platinum reefs, coal. (ii) Massive deposit, flat terrain. Example iron- ore or sulphide deposits. (iii) Dipping seam or bed, flat terrain. Example anthracite. (iv) Massive deposit, high relief. Example copper sulphide. (v) Thick bedded deposits, little overburden, flat terrain. Example iron ore, coal. Figure from Hartman and Mutmansky, 2002.
  • 12. Open-pit mine: Chuquicamata copper mine, Región de Antofagasta, Chile http://en.wikipedia.org/wiki/File:Chuquicamata_panorama.jpg Benches Access ramps Photo credit:Till Niermann 11 September 2008. Dust Slope failure 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).
  • 13. Pit slope versus rock strength Pit depth versus pit diameter Greater rock strength can support greater A greater final pit depth requires a larger bench heights – resulting in a steeper pit, a diameter pit (assuming rock strength and pit lower stripping ratio and less waste rock. slope remains unchanged) – resulting in a higher stripping ratio and more waste rock. Figure from Spitz and Trudinger, 2009.
  • 14. Open-pit slope failure – case study – groundwater problems A slope failure occurred at the Cleo Open Pit (Sunrise Dam Gold Mine, Western Australia) in December 2000. At the time of failure the pit-floor was at Water table 100 m depth below surface. 100 m Two critical factors played a role in the failure: • The top of the water table is at a very high level: only 30 m below surface • A strong layer of younger clay sediments overlies weaker weathered bedrock. Seepage and mineral precipitation The failure is thought to be due to very Top of water table high pore fluid pressures in the weathered bedrock that created an Original configuration instability at the interface between the Mud pile Stiff clay bedrock and the overlying clays, allowing a slippage to occur (Speight, 2002). Weathered bedrock high pore pressures Plane of failure located at boundary between bedrock and clay Distance in meters Figures modified from Speight, 2002.
  • 15. Open-pit slope failure – structural problems Pre-mining geological structures, particularly fault planes, represent zones of potential weakness in the rock mass, and are therefore zones of potential slope failure, and should be taken into account when designing the mine. Fault planes dipping towards the pit (as shown in the figure) present a greater risk than faults dipping away from the pit. Faults planes often provide passage-ways for water movement, and these waters, through the process of weathering and chemical alteration of minerals, may reduce the strength of the rocks on either side of the fault plane, and reduce the “coefficient of friction” along it. The coefficient of friction (the “traction” or “grip”) along the fault will determine whether failure and slippage of rock down the fault plane is likely. The coefficient of friction may change with time: • as water-flow patterns are affected by mining • as faults are exposed by the removal of rock, opening fluid pathways into faults • by the reduction of the mass of the rock located above the fault plane. Computer model of a potential failure plane in an open-pit mine (From Little, 2006)
  • 16. Schematic of open-cast coal mine OVERBU RDEN Dragline gathers overburden DIRE and “casts” it back onto spoil CTIO NO F AD banks located behind the VANC E current working face • Significant “permanent” waste dumps are not needed. • Mine rehabilitation can be carried out progressively at the same rate as mining. Figure from Hartman and Mutmansky, 2002.
  • 17. Open-cast or 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). 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.
  • 18. Open-cast coal mining, Rhine Westphalia Germany Simulated natural-color satellite (ASTER) image of the Garzweiler open-cast lignite (brown coal) mine in North Rhine Westphalia, Germany. The mine is named after the town Garzweiler, which was located at the center of the area being mined. AD V AN 8.5 km CE Active mining Photo credit: NASA/GSFC/METI/ERSDAC/JAROS, and US/Japan ASTER Science Team, August 26, 2000. http://asterweb.jpl.nasa.gov/gallery-detail.asp?name=Coal
  • 19. Underground mining: Generally underground mining is adopted when the orebody is too deep and it’s not economically or technically feasible to use an open-pit: Deepest “hard-rock” open pits are over 700 m deep (e.g., Palabora in South Africa and Chuquicamata in Chile). It is increasingly common to progress from open-pit to underground mining of the same orebody. Used where surface land use prohibits surface disruption (e.g., towns, agricultural land, lakes, near-surface aquifers). Not always prioritised by miners! The major distinction between the different underground mining methods is whether the mined out areas remain supported after mining, or if they are allowed to collapse.
  • 20. General anatomy of a deep underground mine Both ore-rock and water are allowed to feed to the bottom of the mine under the force of gravity, and from there are transported or pumped to the surface. Note the use of “backfill” in mined- out areas to provide support for the overlying rock. Backfill allows ore recovery to be maximised, because ore is not left in-place as support pillars. Backfill is generally a mixture of cement with waste rock, sand or tailings. Figure from Spitz and Trudinger, 2009.
  • 21. Supported underground mining: room-and-pillar layout Pillars have been mined-out in this area Note the control of ventilation, i.e., the separation of contaminated (used) and uncontaminated (fresh) air using a variety of devices. Figure from Hartman and Mutmansky, 2002.
  • 22. Supported underground mining – Room-and-Pillar method: The mining cavity is supported (kept open) by the strength of remnants (pillars) of the orebody that are left un-mined. Room-and-pillar mining method has a low recovery rate (a large percentage of ore remains in place underground). Used for tabular orebodies, with moderate dip: for example, coal and evaporite (salt and potash) deposits. It is an advantageous mining method for shallow orebodies – as a means of preventing surface subsidence. Historic, ultra-shallow underground coal mines (< 30 m) nevertheless are characterised by surface subsidence in the areas between pillars (e.g., Witbank coal field, South Africa). Pillars are sometimes mined on retreat from a working area, inducing closure and caving of these working panels, and raising the risk of surface subsidence.
  • 23. Underground mining: room-and-pillar mining of thick seams – “benching” ll wa g ing n Ha ll t wa Foo Different approaches allow either the top or bottom part of the seam to be mined out first. Note the “hangingwall” is above the mining cavity, and the “footwall” is below it. Figures from Hartman and Mutmansky, 2002.
  • 24. Unsupported underground mining – longwall mining method: Longwall mining is suitable for tabular orebodies, with moderate dip (e.g., coal and stratiform hard-rock ores). In “unsupported” mining, the mine-workings are supported temporarily only for as long as needed to keep the active face open to mining. After mining, the support (e.g. hydraulic props or wood packs) is removed (or becomes crushed), and the mining cavities close up under the pressure of the overburden material. The cavity closure is either partial, for shallow mining, or complete, for deep level mining. While unsupported mining is advantageous in that it maximises ore recovery (as little ore as possible is left behind) the method comes with significant problems: - Surface subsidence in the case of shallow mines - Rockbursts underground, causing injury and death in deep level mines.
  • 25. Underground mining: longwall mining http://en.wikipedia.org/wiki/File:SL500_ SCHEMATIC OF LONGWALL PANEL 01.jpg (HANGINGWALL STRIPPED AWAY FOR ILLUSTRATIVE PURPOSES) Protective screen E ANC ADV ~1 50 m Mechanised cutting machine on a longwall coal-mining face. In hard-rock minerals mining “Permanent” support, a “scraper” is pulled down often timber packs, will the length of the stope face remain in place after after drilling and blasting, to mining. With time, these collect the fragmented ore become deformed or rock. completely crushed – as Temporary support near In coal mining, a mechanised part of the “controlled” the working face: often cutting device is run along closure of the panel. hydraulic props. the length of the coal face. Figure from Hartman and Mutmansky, 2002.
  • 26. Unsupported underground mining σ v = ρgh Cartoon showing the driving Depth σv h Virgin stress situation below at depth h: mechanisms of “mining- surface induced seismicity”. σ v = ρgh h The creation of a cavity underground significantly alters the virgin subsurface stress (pressure) regime. σ v = ρgh Depth σv h below surface Mining process: Mined out blast & remove h material at the stopes, tunnels … σ v = ρgh Removal of rock causes Depth σv h stresses to redistribute, stope closure & fracturing below surface Slip on new fractures and Slip! pre-existing geological k! h h ra c features results in seismicity C
  • 27. The effect of mining depth on stope (cavity) closure SURFACE Shallow mining: partial closure of cavity, and surface subsidence above the mining. “Beam width” Deep mining: complete closure of cavity, extensive fracturing around the cavity, with associated rockbursts (explosive release of seismic energy – effectively earthquakes).
  • 28. Deep level gold mining, South Africa 1.5 m Stope face with temporary support Stopes (yellow): STOPE STOPE on-reef excavations where the reef (orebody) is mined. http://www.bullion.org.za/MiningEducation/Images/images/ Slide 28 CrossSectMine.jpg © CSIR 2006 www.csir.co.za
  • 29. Aftermath of a rockburst in a deep-level tunnel showing complete tunnel closure. The energy released by this event is equivalent to magnitude M = 3.4 earthquake.
  • 30. Longwall mining with strike stabilising-pillars at South African gold mine Plan view of tabular orebody showing mined and un-mined areas Sub-shaft support pillar Sub-shaft pillar boundary SUB-SHAFT N Dip Mined-out stopes 1km Mined out Geological structures: 1.0 km 0 0.5 km Strike Pillars faults and dykes Strike pillars (unmined ore rock) providing support for hangingwall
  • 31. Mining strategy can make a difference to rockburst activity: Underground seismicity (and rockburst rate) can be influenced by: - The rate of advance of a stope face (slower advance is more favourable) - The number of adjacent panels being mined simultaneously (smaller number is more favourable) - The angle at which the advancing face approaches geological structures (preferably not parallel) The last point highlights the need for detailed advanced knowledge of geological structures. Miners are not always prepared to invest in high resolution surveys (e.g., geophysical surveys) to achieve the level of detail required for safer mining. Plan view of mining panels Seismicity and ADVANCE ADVANCE rockbursts Mined out Mined out Fault Fault SUPPORT PILLAR SUPPORT PILLAR Unfavourable stoping layout and progression of More favourable stoping layout and mining. progression of mining.
  • 32. Underground longwall coal mining at Barapukuria Mine, NW Bangladesh Coal production started in October 2005. Mining depth is approximately 400m. To date 6 panels have been mined, with a 3 m slice height. Total production to date has not exceeded 3 Mt coal (a small proportion of the total 34 Mt recovery planned over 30 years). Plan view Barapukuria Coal Mine (from levels 260 m to 420 m). Figure from Islam et al., 2008.
  • 33. Underground longwall coal mining at Barapukuria Mine, NW Bangladesh SURFACE SUBSIDENCE (NOT SHOWN TO SCALE) SURFACE 110 m WATER WATER 400 m zo ctu e Ri ctur fra bsid n e re fra e COAL SEAM bs CAVITY zon Ri ide e IV 440m Schematic cross-section showing subsurface response to longwall mining cavity, and subsidence at surface. Arrows show fluid-pathways downwards from the Dupi Tila acquifer along mining induced fractures, into the mining panel. Figure modified after Islam et al., 2008.
  • 34. Underground longwall coal mining at Barapukuria Mine, NW Bangladesh: The impact on surface at Barapukuria is typical of many areas around the world which have been undermined by longwall coal mining. Subsidence problems are particularly associated with the sedimentary basins in which coal is found (also evaporite NaCl and KCl deposits) as the overburden is weaker than crystalline rocks. Underground coal mining also tends to take place at shallower depths than many hard-rock mineral mines. Considering how little of the resource has been mined to date, the impact on the surface above the mine at Barapukuria has been devastating: - Land subsidence of between 0.6 – 0.9 m has been reported over an area of about 1.2 km 2. - The water-table has dropped, leaving commonly used water reservoirs dry in 15 villages. - At least 81 houses have developed cracks in 5 villages. - Untreated mine water (acknowledged by the mine to contain phosphorous, arsenic and magnesium) is passing through canals in farming areas. - The scale of the problem has the Bangladesh government currently considering the establishment of a new “coal city” near Barapukuria that would provide housing and (potential) employment to people whose livelihoods are at risk in 15 villages around the mine. Barapukuria Mine plans to increase the total panel height mined to 18 m (it is currently only 3 m) – it is not clear whether this plan is going to be practically viable!!
  • 35. Underground mining – Block caving: Block-caving method is employed generally for steeply dipping ores, and thick sub-horizontal seams of ore. The method has application, for example in sulphide deposits and underground kimberlite (diamond) mining. An undercut tunnel is driven under the orebody, with "drawbells" excavated SURFACE above. Caving rock falls into the TOP OF OREBODY drawbells. The orebody is drilled and blasted above the undercut to initiate the “caving” process. As ore is continuously removed from the drawbells, the orebody continues to cave spontaneously, providing a steady stream of ore. If spontaneous caving stops, and removal of ore from the drawbells continues, a large void may form, resulting in the potential for a Figure from Hartman and Mutmansky, 2002. sudden and massive collapse and a potentially catastrophic windblast throughout the mine (e.g., the Northparks Mine disaster, Australia).
  • 36. Block-cave mining: Mud-rushes – an under-reported hazard Mud-rushes are sudden inflows of mud from ore drawpoints (or other underground openings), in block-cave mines that are open to the surface. Considerable violence, in the form of an airblast, is often associated with mud-rushes. Mud-rushes are (under-reported) hazardous occurrences that have occurred frequently in mines in South Africa, as well as in Chile and Western Australia, and have caused fatalities (Butcher et al., 2005). Mud is produced by the breakdown of rock in the near-surface muckpile in the presence of rainwater. Kimberlite rock on diamond MUD mines is particularly susceptible to weathering by rainwater. SCHEMATIC CUT-AWAY VIEW OF SUB- LEVEL BLOCK-CAVE MINE Figure from Hartman and Mutmansky, 2002.
  • 37. In-situ leaching (ISL)/ solution mining: 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 oxidised 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. Sodium cyanide: NaCN Metals and minerals commonly mined by solution mining methods. Sulphuric acid: H2SO4 Dissolving agent specified in each case. (From Hartman and Mutmansky, 2002, and references therein). Hydrochloric acid: HCl Ammonium carbonate (alkali): (NH4)2CO3 Aside: The same reagents are often used for processing mined ores in hydrometallurgical plants
  • 38. In-situ leaching (ISL)/ solution mining: 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). Acid leaching fluid sulphuric acid + oxidant (nitric acid, hydrogen peroxide or dissolved oxygen) or UO22+ Alkali leaching fluid ammonia, ammonium carbonate/bicarbonate, or sodium carbonate/bicarbonate The hydrology of the acquifer is irreversibly changed: its porosity, UO2 permeability and water quality. It is regarded as being easier to “restore” an acquifer after alkali leaching. Figure from Hartman and Mutmansky, 2002.
  • 39. In-situ leaching (ISL)/ solution mining: 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 crystallise out as the water evaporates. 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). Evaporation ponds, Arizona Figure from Spitz and Trudinger, 2009.
  • 40. 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).
  • 41. Hydraulic mining: Generally used for weakly cemented near-surface ore deposits. Note: Riffle box uses mercury for gold recovery The “Stang Intelligiant” monitor Hydraulic mining of a placer gold deposit. (operator controlled high pressure water discharge point) mounted on a skid Figures from Hartman and Mutmansky, 2002.
  • 42. Hydraulic mining – tailings dam reprocessing Kaltails project, Kalgoorlie, Western Australia The project was established to reprocess and relocate tailings dams from the Boulder and Lakewood areas of Kalgoorlie. The operations ceased after a decade of work in September 1999. The tailings dumps were hydraulically mined, reprocessed and stored in another engineered impoundment located 10 km south east of Kalgoorlie. Recovery was by Carbon-in- Circuit (CIC) and Carbon-in-Pulp (CIP) leach and absorption circuits. Sixty million tons of tailings were mined in an area of 333 hectares (3.33 km2), producing 695,000 ounces (19.7 tons) of gold at an average ore- grade of 0.33 g/ton. Remaining part of a waste dump being hydraulically mined (Newmont Mining). From TAILSAFE, 2004.
  • 43. Dredging: Used most often for mineral-sands and some near-shore alluvial diamond mining operations. Typical bucket-line dredge www.tmd.ihcholland.com Figure from Hartman and Mutmansky, 2002.
  • 44. Oil sand mining: Blurs the boundary between hydrocarbon and mineral extraction. Canada’s oil sands are the second single largest oil deposit on Earth, second only to the large reserves in Saudi Arabia. Resource has not been exploited significantly to date because of the much higher costs associated with extracting the oil compared to conventional borehole extraction in normal oil deposits. The sands are either strip mined or the oil rich sands are heated underground (steam) so that oil migrates to recovery boreholes. In the case of mining, bitumen is recovered by washing the sands in hot water, and is subsequently upgraded to a final “crude” in two successive process: hydro-cracking and hydro-treating. “Ore-grade”: approximately 2 tons of sand needed to produce 1 barrel of oil.
  • 45. Coal-bed methane and Underground Coal Gasification: Please see my review included with course material: “It’s not only about coal mining: Coal-bed methane (CBM) and underground coal gasification (UCG) potential In Bangladesh”. For a summary of these methods, their advantages and disadvantages, and an quantitative examination of the energy return provided by these surface based, non-invasive alternatives to coal mining.

Editor's Notes

  1. Here’s a cross section of a deep level gold mine, looks like Western Deeps. There’s a twin shaft system that has been sunk through the reef. Horizontal tunnels called haulages are mined from the shaft to the reef. The important thing to note here are the stopes. These are on reef excavations where the reef or stof is mined. The plans I’ll be showing just now are of stopes.