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Topic 2: Mining


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Topic 2: Mining

  1. 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 24 April 2009
  2. 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. 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. 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. 5. Mineral extraction: from mining to metal Mining Mineral concentrate METAL EXTRACTION Metal Figure from Spitz and Trudinger, 2009.
  6. 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. 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. 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. 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. 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. 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. 12. Open-pit mine: Chuquicamata copper mine, Región de Antofagasta, Chile 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. 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. 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. 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. 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. 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. 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.
  19. 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. 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. 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. 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. 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. 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. 25. Underground mining: longwall mining 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. 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. 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. 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. Slide 28 CrossSectMine.jpg © CSIR 2006
  29. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 43. Dredging: Used most often for mineral-sands and some near-shore alluvial diamond mining operations. Typical bucket-line dredge Figure from Hartman and Mutmansky, 2002.
  44. 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. 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

  • 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.