Topic 2: Mining

Topic 2: Mining

From a series of 5 lectures onMetals, minerals, mining and (some of) its problems

prepared for London Mining Networkby

Mark Muller [email protected]

24 April 2009

Outline of Topic 2:

• Surface mining methods

• Open-pit minesSlope failure in open-pit mines

• Open-cast mines

• Underground mining methods: Room-and-pillar miningLongwall mining Rockburst hazard in deep longwall mining Surface subsidence above shallow longwall miningBlock-cave mining

• Mining using in-situ leaching

• Other mining methods: hydraulic mining, dredging

Figure from Spitz and Trudinger, 2009.

Anatomy of a mine:Grasberg, West Papua

No smelter and refinery.This project delivers mineral-concentrate.

0.2 million tons of tailings are dumped into the Ajkwa river system every day, causing massive sedimentation on coastal floodplains (Lottermoser, 2007)

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

Mineralconcentrate

Metal

METAL EXTRACTION

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.


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 miningOpen-pit miningStrip or open-cast mining

includes superficial deposit mining: nickel laterite, bauxite, mineral sands, alluvial diamonds

Underground miningBlock-cavingSub-level block-cavingLongwallRoom-and-pillar (Bord-and-pillar), Stope-and-pillarLongwall 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 miningWaste = 73% of total rock tonnage extracted 266% of ore tonnage extracted

Underground miningWaste = 7% of total rock 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

Figure from Hartman and Mutmansky, 2002.

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

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

Dipping seam or bed, flat terrain. Example anthracite.

Massive deposit, high relief. Example copper sulphide.

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

(i)

(ii)

(iii)

(iv)

(v)

Open-pit mine: Chuquicamata copper mine, Región de Antofagasta, Chile

DustPhoto credit:Till Niermann11 September 2008. Slope failure

Locality: Región de Antofagasta, Chile.Pit dimensions: 4.3 km long x 3 km wide x 850 m deep.Mining dates: 1915 - presentTotal 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).

http://en.wikipedia.org/wiki/File:Chuquicamata_panorama.jpgBenches

Access ramps

Pit slope versus rock strength


Pit depth versus pit diameter

Greater rock strength can support greater bench heights – resulting in a steeper pit, a lower stripping ratio and less waste rock.

A greater final pit depth requires a larger diameter pit (assuming rock strength and pit slope remains unchanged)

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

Open-pit slope failure – case study – groundwater problems

Figures modified from Speight, 2002.

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 100 m depth below surface.

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.

The failure is thought to be due to very high pore fluid pressures in the weathered bedrock that created an instability at the interface between the bedrock and the overlying clays, allowing a slippage to occur (Speight, 2002).

Top of water table

Water table

Distance in meters

Mud pile

Original configuration

Plane of failure located at boundary between bedrock and clay

Weathered bedrock high pore pressures

Stiff clay

Seepage and mineral precipitation

100 m

Open-pit slope failure – structural problems

Computer model of a potential failure plane in an open-pit mine (From Little, 2006)

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.

Schematic of open-cast coal mine

Dragline gathers overburden and “casts” it back onto spoil banks located behind the current working face

OVERBURDEN


DIRECTION OF ADVANCE

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

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.

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

8.5 km

Active mining

ADVANCE

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.

Supported underground mining: room-and-pillar layout


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

Pillars have been mined-out in this area

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.

Figures from Hartman and Mutmansky, 2002.

Underground mining: room-and-pillar mining of thick seams – “benching”

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.

Hangingwall

Footwall

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.

In hard-rock minerals mining a “scraper” is pulled down the length of the stope face after drilling and blasting, to collect the fragmented ore rock.

In coal mining, a mechanised cutting device is run along the length of the coal face.


Underground mining: longwall mining

Temporary support near the working face: often hydraulic props.

Protective screen

SCHEMATIC OF LONGWALL PANEL(HANGINGWALL STRIPPED AWAY FOR ILLUSTRATIVE PURPOSES)

http://en.wikipedia.org/wiki/File:SL500_01.jpg

“Permanent” support, often timber packs, will remain in place after mining. With time, these become deformed or completely crushed – as part of the “controlled” closure of the panel.

Mechanised cutting machine on a longwall coal-mining face.

ADVANCE

~150 m

Virgin stress situation at depth h:

ghv

h

h

Depth below surface

v

ghv

Virgin stress situation at depth h:

ghv

h

h

Depth below surface

v

ghv

ghv

h

h

Depth below surface

v

Mining process: blast & remove material at the stopes, tunnels …

Mined out

ghv

h

h

Depth below surface

v


ghv

h

h

Depth below surface

v


Mined out

h

h

Depth below surface

vRemoval of rock causes stresses to redistribute, stope closure & fracturing

ghv

h Slip!

Crack!

Slip on new fractures and pre-existing geological features results in seismicity

h

h

Depth below surface

vRemoval of rock causes stresses to redistribute, stope closure & fracturing

ghv

h Slip!

Crack!

Slip on new fractures and pre-existing geological features results in seismicity

Cartoon showing the driving mechanisms of “mining-induced seismicity”.

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

Unsupported underground mining

The effect of mining depth on stope (cavity) closure

Shallow mining: partial closure of cavity, and surface subsidence above the mining.

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

“Beam width”

SURFACE

Slide 28 © CSIR 2006 www.csir.co.za

STOPE

http://www.bullion.org.za/MiningEducation/Images/images/CrossSectMine.jpg

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

Deep level gold mining, South Africa

Stope face with temporary support

1.5 m

STOPE

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.

Strike Pillars

Dip

0 1.0 km0.5 km

N

Sub-shaft pillar boundary

Mined out1km

Strike Pillars

Dip

0 1.0 km0.5 km

N

Sub-shaft pillar boundary

Mined outMined out1km1km

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

Strike pillars (unmined ore rock) providing support for hangingwall

Mined-out stopes

Geological structures: faults and dykes

SUB-SHAFT

Fault

Mined out

Unfavourable stoping layout and progression of mining.

Mined out

Fault

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)

More favourable stoping layout and progression of mining.

Seismicity and rockbursts

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

SUPPORT PILLAR SUPPORT PILLAR

ADVANCE ADVANCE

Figure from Islam et al., 2008.

Underground longwall coal mining at Barapukuria Mine, NW Bangladesh

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

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).

Figure modified after Islam et al., 2008.

Underground longwall coal mining at Barapukuria Mine, NW Bangladesh

COAL SEAMIV

110 m

SURFACE

400 m

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.

SURFACE SUBSIDENCE (NOT SHOWN TO SCALE)

Ribside

fracture

zone

Rib

side

frac

ture

zone

CAVITY

WATERWATER

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 km2.- 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 above. Caving rock falls into the 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 sudden and massive collapse and a potentially catastrophic windblast throughout the mine (e.g., the Northparks Mine disaster, Australia).

SURFACE

TOP OF OREBODY

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 mines is particularly susceptible to weathering by rainwater.

MUD

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

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.

Metals and minerals commonly mined by solution mining methods.Dissolving agent specified in each case. (From Hartman and Mutmansky, 2002, and references therein).

Sodium cyanide: NaCNSulphuric acid: H2SO4

Hydrochloric acid: HClAmmonium 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 depositsUranium minerals are soluble in acidic or alkaline solutions.

The production (“pregnant”) fluid consisting of the water soluble uranyl oxyanion (UO2

2+) is subject to further processing on surface to precipitate the concentrated mineral product U3O8 or UO3 (yellowcake).


Acid leaching fluidsulphuric acid + oxidant (nitric acid, hydrogen peroxide or dissolved oxygen)

or

Alkali leaching fluidammonia, ammonium carbonate/bicarbonate,or sodium carbonate/bicarbonate

UO22+

UO2

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.

In-situ leaching (ISL)/ solution mining: Evaporite depositsHas 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

Evaporation ponds, Arizona

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).


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.

Figures from Hartman and Mutmansky, 2002.

Hydraulic mining of a placer gold deposit. The “Stang Intelligiant” monitor (operator controlled high pressure water discharge point) mounted on a skid

Note:Riffle box uses mercury for gold recovery

Hydraulic mining – tailings dam reprocessing

Kaltails project, Kalgoorlie, Western AustraliaThe 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.

From TAILSAFE, 2004.

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).

Dredging:

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

Typical bucket-line dredge

www.tmd.ihcholland.com


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.