Mining and Quarrying

• • Mining - the underground extraction of minerals from economically viable deposits.
  • Quarrying - the surface extraction of a wide range of minerals.
• • large surface operations are 'open pit mines'; used for less complex but larger mineral deposits near the surface (involves extraction of mineral/overlying material, and often cheaper)
  • underground mining used for more complex, and smaller/deeper mineral deposits; produce less waste rock, but more expensive and dangerous than open-pit mining
• Potential problems can be designed out at planning stage, if sufficient information is gathered.

  Mining Hazards Associated with:

  • strength/weakness of rock being dug
  • nature of the rock on which structures are built/material placed
  • influence of water at the surface or underground
• Problems of Open Pit Mining:

  • stability of sides of the quarry; working faces are unstable/steep
  • lots of waste material remains after processing ore (cannot be put back into pit before mining operation over)
  • water may flood up through porous/permeable deposits (if pit extends below water table)
  • loss of habitat (woodland/hedgerows/fields destroyed) + inconvenience to local people (noise/vibrations/dust)
• Management of Open Pit Mining:

  • stability improved by planning a stepped profile for face with horizontal working platforms alternating with verticasl working face of restricted height + rock bolts prevent slabs slipping
  • sand/gravel pits pumped to prevent water flooding + drainage system for old faces prevents lubrication of joints/bedding planes and wetting of clay
• Longwall Retreat (Underground Mining):

  • main shaft dug from surface with tunnels/roadways to furthest point of extraction
  • 'longwall' is between 2 roadways and extracting machine transfers material removing slices of material onto conveyor belt
  • roof held up by mobile hydraulic steel chocks + system of deliberate collaspe as mine moves backwards towards shaft
• Underground Mining Methods:

  • Slope Mining - mining along a slope where vertical shafts are drilled downwards and horizontal levels through ore body + slope may be backfilled or controll collasped.
  • Room and Pillar Mining - mining/extracting materials to create rooms but leaving pillars as foundations to support rock + wall of rooms may have shotcrete/rock bolts used for reinforcements.
• Problems of Underground Mining:

  • Flooding - water accumulates in weathered strata near surface, porous superficial deposits and in old worked-out seams (danger from breaking into water-filled voids releasing water).
  • Gas/Cold Dust - compression of coal seam squeezes out volatiles + explosions need gas/oxygen/spark (deadly if channelled along tunnels; CO is toxic) and worsened by coal dust.
  • pressure, permeability, geothermal gradient, and groundwater pollution more problems
• Problems of Underground Mining:

  • Collaspe of Roof/Rockfalls - weight from rock above extracted area makes collaspe likely + release of pressure when tunnels open allows joints to widen loosening rocks.
  • Subsidence - collaspe of mines causes subsidence at surface + long wall mining causes areas to subiside evenly.
• Management of Underground Mining:

  • opening shaft lowers air pressure in void which water percolates towards; gradual seepage of water controlled by pumps + risk of flooding reduced by research
  • ventilation prevents gas buildup + monitor atmosphere in mine for gas + enclose electric parts of machinery/avoid iron on iron contact (limit sparks) + spray water on coal dust
• Management of Underground Mining:

  • room/pillar methods prevent collaspe + roof supported by closely spaced steel semi-circular girders + shotcrete on walls + steel hydraulic rams support longwall method
  • don't mine coal seam directly below sensitive areas + pillar/room method intended to prevent subsidence
• • mineral veins/ore contain mixture of minerals but only one wanted to be sold comercially (valuable ore minerals); common ore minerals are sulfides (chalcopyrite, sphalerite and galena)
  • gangue minerals aren't wanted as no industrial use/uneconomical to process or contain elements that make processes difficult/expensive; so tipped as heaps of waste
• Surface/Groundwater Pollution:

  • mine waste tips are mixture of elements not usually found at high concentrations at surface (contain toxic heavy metals; lead, mercury, cadmium and arsenic)
  • mining breaks up rock/opens up passages and brings chemicals to surface allow access to air/oxygen/water + some bacteria oxidise iron as metabollic process
  • chemical reactions at surface; sulfur becomes sulphur dioxide then sulfuric acid which dissolves heavy metals from ore/gangue - poisonous solutions percolate through soil/bedrock to water table contaminating groundwater
• Acid Mine Drainage (AMD):

  • controlling AMD at source of abandoned mine; dewatering mine/treating pumped out water (expensive/effective) + flood mine then seal with impermeable concrete/clay (becomes anaerobic once oxygen in water used by microorganisms)
  • AMD treatment outside of mine; neutralise acidity (precipitate metals as chemical salts; non-toxic/disposable)
• Active Treatment of AMD:

  • neutralise with base (lime, calcium carbonate, and sodium carbonate/hydroxide); expensive as requires constant chemical supply/appropriate disposal of salts + reliable/effective for high rates of flow
  • plants permanent or portable + oxidising agents added to accelerate precipitation of metal hydroxides/carbonates from iron-rich sludge
• Passive Treatment of AMD:

  • wetland ecosystems set up with shallow pondss filled with limestone gravel covered by organic compost creates anaerobic environment (bacteria produce metal sulfides)
  • expensive to set up but cheap to run; requires land but minimal maintenance and no hazardous chemicals
  • used if AMD low acidity/rate of flow + in remote area where no electricity is needed
  • natural oxidation precipitates iron, manganese, and other metal salts
• Mineral Processing (harmful chemicals produced so pollution of soil/groundwater needs preventing) creates problems:

  • solutions in heap leaching are acidic
  • tailings from crushing operations are fine grained sediments with toxic metals/chemicals + flotation tailings are waste rock materials including heavy metals (don't attach to bubbles in froth flotation process)
  • spoil heaps unstable if slopes too steep; their toxic heavy metals are subject to same chemical processes as AMD but reactions quicker as spoil is fine-grained