Mining Techniques and Coal Geology

MINN3022A Course Objectives

• The course explains how to assess an orebody for the selection of an appropriate mining method
• Coal geology and coal marketing/access are reviewed
• Principles of coal mine design are covered
• Different mining techniques such as coal mechanized bord and pillar mining, longwall mining, and mechanised coal pillar extraction are discussed
• Longwall ventilation is explored
• Aspects of narrow reef development infrastructure are studied
• Narrow reef mechanised bord and pillar mining is covered
• Completing a mini-design project involves assessment of a specific mining method
• Application of caving methods is explored
• Caving method layouts and draw control to minimise dilution are discussed
• Advantages and disadvantages of caving methods are detailed

General Information

• The test is scheduled for Wednesday, April 9, 2025, in Richard Ward 5
• The mini-project is due on Wednesday, May 14, 2025
• The exam date is TBA
• Exam is 40%
• Mini-design project is 25%
• The test is 35%
• NQF Level 8 Competency Assessment includes Mining B Project (15%), Mining C Project (15%), and Mining D Project (15%)

Types of Mining

• Three main types of mining are surface, underwater (marine), and underground mining
• Mining operations involve drilling, blasting, loading, transportation, and ore crushing
• These operations occur in open pit, underground, and marine mines
• This analysis excludes underwater or marine mining

Mining Methods

• Compares open-pit and underground mining methods
• Open-pit mines have high productivity but underground mines are ore specialised (expensive)
• Surface mining has a larger environmental impact compared to underground mining
• Open pit mining has low ore losses, whereas the Room and Pillar method (underground) can result in losses of up to 40%
• Open pit mining offers low cost and high ore recovery and allow the extraction of low-grade deposits
• The mining method decision involves orebody characteristics, surface or underground, etc
• Basic mining methods include naturally supported stopes, artificially supported stopes, and caving methods
• Types of natural support include Room and Pillar, Benching, Sublevel Open Stoping (SLOS), and Vertical Crater Retreat (VCR)
• Types of artificial support include Cut and fill, Drift and fill, Post pillar cut and fill mining, Sublevel stoping (SLS), and Longitudinal lateral retreat
• Caving methods include Block Caving, Sublevel Caving (SLC), Open Blast Hole Stoping, Front Caving, and Inclined Caving
• The most common methods for natural support are Room and Pillar, Benching, and Sublevel Open Stoping (SLOS)
• The most common methods for artificial support is Cut and fill, Drift and fill, Post pillar cut and fill mining and Sublevel Stoping (SLS)
• The most common methods for caving are Block Caving and Sublevel Caving (SLC)
• Coal mining differs from hard rock mining as coal is a softer rock
• Coal mining uses cutting methods, drilling, and blasting in room-and-pillar methods
• Underground coal mines contains methane gas and coal dust which requires careful equipment design

Coal Mining Variables

• Seven variables that determine what mining method that will be used:
• Seam height determines method and equipment use
• Floor quality influences equipment operation and selection
• Roof quality determines the type of support needing to be used
• Methane liberation refers to the ventilation needed to ensure safety
• Depth of the coal seam influences pillar safety factors and size
• Hardness of the coal determines the extraction method
• Water presence determines floor conditions, with panels laid up-dip and with water behind in sumps
• Soft coal can be cut with continuous miner or plough on longwall
• Average coal is cut with continuous miner/shearer with longwall
• Moderate coal requires increased horsepower and sharp cutting bits
• Hard coal needs drilling and blasting with conventional room-and-pillar methods

Mining Method Determination

• Factors that determine a mining method include:
• Depth of the orebody
• Size, shape, and dip of the orebody
• Rock strength (orebody and host rock)
• Required mining (production) rate
• Product price
• Depth limits mining by surface or underground means, with surface mining being more economical to 500 meters
• Increasing depth in surface mines increases costs due to the higher volume of mineral (strip ratio)
• Increased depth also results in increased lithostatic pressure, which is critical for safe designs

Orebody Thickness

• Key factors for determining mining
• Orebody thickness is categorised as:
• Thin = less than 6m
• Moderately thick = between 6m and 30m
• Thick = between 30m and 100m
• Massive = greater than 100m

Orebody Dip

• Affects mining methods
• Types of Orebody dip include:
• Flat = less than 7°
• Moderately steep = between 7° and 20°
• Steep = between 20° and 55°
• very steer = greater than 55°

Rock Mass Strength

• Determines which strength to utilise
• Rock strength comes down to uniaxial compressive strength:
• Low strength is 5 – 30 MPa
• Medium strength is 31 – 100 MPa
• High strength is over 100 MPa

Mining Methods - Mass Rock

• Rock class rating determines the need for Natural support & induced caving range or Artificial support & natural caving range

Production Rate

• This often determines which extraction method to utilise.
• Caving and natural support are generally high production methods
• Artificial support tends to be a lower production method
• Production is generally high when secondary/stope development are inside of the orebody, if non caving methods are utilised

Production Pricing

• Product Price affects which mining method is to be utilsed
• High value products can be mined with low production mining methods
• Low value and ferrous metals can be mined with high production methods
• High grade orebodies can be mined with a mix

Techniques

• Dravo mining technique:
  • Can measure: Orebody, Host Rock Characteristics and Orebody Configuration
• Value Ranking Model technique (Nicholas Model)

Cost and Productivity

• Cut and fill and rooming are expensive but highly productive
• Sublevel is more suited for high cave production but is costly

Natural Rock Mining

• Characteristics:
• Ore extraction is low
• Inexpensive
• Adds support when needed
• Low subsidence
• Examples include room and pillar and sublevel opining

Room and Pillar

• Pillars provide roof support
• Pillars must be stable
• Roof bolting is needed between pillars
• Needs to be moderate and firm
• The width dictates sizing
• Pillars need to be of rock strutuctre
• spacing is extraction ratio

Artifical Mining Support

• Ore extraction is high
• Expensive -Needs support
• Subsidence and dilution remains low
• Example support includes; sublevel stope

Cut & Fill

• Applied in steep locations
• Adequate Fill is needed

Advantaages

• 5% dilution
• 100% extraction

Sub-level Cave Mining

• Cave Mining can yield ; 90 Percent Ore extraxtion
• Relatively high tonnage
• low cost and captial cost

SLC Disadvantages

• Continuous development is required
• High Cave Mining Costs exist
• Low Production rates with the caves
• Ventilation is not always ideal

Caving Methods

• High ore
• Extraction is tolerated
• Supports high
• Subsidence and rock breaks