Underground Mine Gases and Control Study Guide

Introduction to Underground Mine Gases and Control

• Maintaining clean air in mining operations for surface and underground environments is critical for operational success and the health, safety, and comfort of workers.
• An adequate mine environment is generally defined as:
  • Providing circulating air at velocities that maintain at least $19.5\%$ oxygen ($O_2$) in work areas.
  • Diluting gaseous pollutants such as methane ($CH_4$), carbon dioxide ($CO_2$), and respirable dust below specified legal limits.
  • Maintaining temperature and humidity below specified thresholds.
• Sources of underground mine gases include:
  • Blasting operations.
  • Diesel engine equipment usage.
  • Naturally occurring gases emanating from rock strata.

Kinetic Theory and Physical Properties of Gases

• Kinetic Theory Assumptions (Chang & Thoman, 2014):
  • A gas consists of a very large number of atoms or molecules separated by distances large compared to their actual size.
  • The molecules possess mass, but their volume is considered negligibly small.
  • Molecules are in constant, random motion.
  • Collisions between molecules and against container walls are elastic; kinetic energy is transferred but not converted to other energy forms.
  • There is no interaction (attractive or repulsive) between molecules.
• States of Matter Comparison:
  • Solids: Molecules are at rest and form a definite shape.
  • Liquids: Molecules move slightly and change positions relative to each other.
  • Gases: Molecules are in rapid, random motion in all directions, constantly colliding.
• Diffusion:
  • When two or more gases contact each other, they mix automatically (diffusion).
  • Light gases (low density) diffuse faster than heavy gases (high density).
  • The rate of diffusion is inversely proportional to the square root of relative densities. Example: Hydrogen (atomic mass = $1.0$) is $16$ times lighter than oxygen (atomic mass = $16.0$) and diffuses four times faster.
• Gravity and Buoyancy:
  • Gases lighter than air rise toward the roof.
  • Gases heavier than air gravitate toward the floor.
• Avogadro's Hypothesis: Equal volumes of all gases at the same temperature and pressure contain the same number of molecules.
• Relative Density (Specific Gravity): Proportional to the mass of the gas molecule (sum of atomic masses).

Atomic Mass and Specific Gravity Calculations

• Atomic Masses of Common Mine Elements:
  • Hydrogen ($H$): $1.0$
  • Carbon ($C$): $12.0$
  • Nitrogen ($N$): $14.0$
  • Oxygen ($O$): $16.0$
  • Fluorine ($F$): $19.0$
  • Sulphur ($S$): $32.0$
  • Chlorine ($Cl$): $35.5$
• Specific Gravity Formula:
  • $S = \frac{MW}{29}$
  • Where $S$ is specific gravity and $MW$ is molecular weight (relative to air, where air is approx. $29$\,$\text{g/mol}$).
• Calculated Specific Densities (STP):
  • Carbon dioxide ($CO_2$): $MW = 44$, $S = \frac{44}{29} = 1.52$
  • Carbon monoxide ($CO$): $MW = 28$, $S = \frac{28}{29} = 0.97$
  • Nitric oxide ($NO$): $MW = 30$, $S = \frac{30}{29} = 1.04$
  • Nitrogen dioxide ($NO_2$): $MW = 46$, $S = \frac{46}{29} = 1.59$
  • Hydrogen ($H_2$): $MW = 2$, $S = \frac{2}{29} = 0.07$

Detailed Properties and Sources of Mine Gases

• Fresh Air Composition (Moisture-free): Approximately $78\%$ Nitrogen, $21\%$ Oxygen, and $1\%$ other gases.
• Classification Table (Name, Symbol, Specific Gravity, OEL, Source):
  • Carbon dioxide ($CO_2$): $1.5$; $5\,000$\,$\text{ppm}$; Sources: Breathing, oxidation, blasting, diesel exhaust, fires, fermentation.
  • Carbon monoxide ($CO$): $0.97$; $25$\,$\text{ppm}$; Sources: Blasting, diesel, fires, incomplete combustion.
  • Nitrous fumes ($NO$): $1.04$; $25$\,$\text{ppm}$; Sources: Blasting, diesel, welding.
  • Nitrous fumes ($NO_2$): $1.60$; $3$\,$\text{ppm}$; Sources: Blasting, diesel, welding.
  • Methane ($CH_4$): $0.55$; Simple asphyxiant; Sources: Fissures, coal seams, organic matter decomposition.
  • Hydrogen ($H_2$): $0.07$; Simple asphyxiant; Sources: Battery charging, fissures, electric motors, fires.
  • Hydrogen sulphide ($H_2S$): $1.20$; $10$\,$\text{ppm}$; Sources: Fissures, stagnant water.
  • Chlorine ($Cl_2$): $2.50$; $10$\,$\text{ppm}$; Sources: Chlorination of water.
  • Aldehydes ($HCHO$, etc.): $1.04$; Source: Diesel exhaust.
  • Ammonia ($NH_3$): $0.60$; $25$\,$\text{ppm}$; Sources: Cooling plants, blasting, chemical reactions (ANFO + cement).
  • Acetylene ($C_2H_2$): $0.93$; Source: Welding.
  • Freon 11 ($CCl_3F$): $4.80$; Source: Cooling plants.
  • Freon 12 ($CCl_2F_2$): $4.20$; Source: Cooling plants.
  • Hydrocyanic acid gas ($HCN$): $0.94$; Source: Sand filling.
  • Oxygen ($O_2$): $1.10$; Required $> 19.5\%$
  • Nitrogen ($N_2$): $0.97$; Source: Normal air, blasting.
  • Helium ($He$): $0.28$; Source: Sometimes with $CH_4$ in fissures.

Sensory Characteristics of Gases

• CO: Odourless, Tasteless, Colourless.
• CO2: Odourless, Acidic (at high concentrations), Colourless.
• CH4: Odourless, Tasteless, Colourless.
• SO2: Sulphur/harsh odour, Acidic/bitter taste, Colourless.
• N2: Odourless, Tasteless, Colourless.
• NH3: Harsh odour, Soapy/rotten taste, Colourless.
• NO: Odourless/sharp sweet smell, Tasteless, Colourless (orange at high concentrations).
• NO2: Harsh (chlorine-like) odour, Flavourless (causes burning sensation), Orange color.
• H2S: Rotten eggs smell, Sweet taste, Colourless.
• H2: Odourless, Tasteless, Colourless.

Respiration and Gas Concentration (Worked Example)

• Scenario: $10$ workers in a mine heading; ventilation air at $5$\,$\text{m}^3/s$ (intake: $20.6\%\,O_2$, $0.1\%\,CO_2$).
• Consumption/Production Rates (Moderate Activity):
  • $O_2$ consumed: $0.03$\,$\text{L/s}$
  • $CO_2$ produced: $0.027$\,$\text{L/s}$
• Calculations for 10 People:
  • Total $O_2$ consumed: $10 \times 0.03 \times 10^{-3} = 0.0003$\,$\text{m}^3/s$
  • Total $CO_2$ produced: $10 \times 0.027 \times 10^{-3} = 0.00027$\,$\text{m}^3/s$
• Final Concentrations in Exhaust Air:
  • $O_2$ Concentration: $\frac{1.03 - 0.0003}{5} \times 100 = 20.594\%$
  • $CO_2$ Concentration: $\frac{0.005 + 0.00027}{5} \times 100 = 0.1054\%$
• Conclusion: Respiration has a minimal effect compared to other factors like mineral oxidation and diesel exhaust.

Threshold Limit Values (TLV) and Occupational Exposure Limits (OEL)

• TLV Types:
  1. Time-Weighted Average (TWA): Average concentration for an 8-hour shift and 40-hour week without adverse effects.
  2. Short-Term Exposure Limit (STEL): 15-minute TWA concentration; should not occur more than 4 times daily with at least 1 hour between intervals.
  3. Ceiling Limit (C): Concentration NEVER to be exceeded at any time.
• OEL Categories and Actions:
  • Category A: Exposure $> OEL$. Action: Verify results, issue RPE/stop work, notify inspector, amend action plan. Sample $5\%$ of HEG monthly.
  • Category B: Exposure between $50\% - 100\%$ of OEL. Action: Sample $5\%$ of HEG every 6 months.
  • Category C: Exposure between $10\% - 50\%$ of OEL. Action: Sample $5\%$ of HEG annually.
• Synonyms for OEL: TLV (ACGIH), REL (NIOSH), BOELV/IOELV (EU), CL/RL (South Africa/ILO).

Health Effects by Gas and Concentration

• Carbon Monoxide ($CO$): High affinity for haemoglobin.
  • $0.005\%$: daily 8-hour limit.
  • $0.02\%$: headache within 2-3 hours.
  • $0.04\%$: death within 3 hours.
  • $0.16\%$: death within 1 hour.
• Oxygen ($O_2$):
  • $0.50\%$: displacement leads to deeper breathing.
  • $3.00\%$: sweats, rapid pulse.
  • $5.00\%$: respiratory rhythm 3x higher.
  • $7.00\%$: death.
• Nitrous Fumes ($NO$, $NO_2$):
  • $NO$: Irritates moist surfaces; toxic effects may be delayed for up to $72$ hours.
  • $200$\,$\text{ppm}$ of $NO$ is toxic/fatal.
  • $NO_2$: Irritation at $20$\,$\text{ppm}$; fatal at $250$\,$\text{ppm}$ even for short periods.
• Hydrogen Sulphide ($H_2S$): Paralyzes respiratory system.
  • $10 - 50$\,$\text{ppm}$: Nausea, vomiting, shortness of breath.
  • $50 - 200$\,$\text{ppm}$: Seizures, coma, death.
• Ammonia ($NH_3$):
  • $700$\,$\text{ppm}$: Immediate severe irritation.
  • $> 10\,000$\,$\text{ppm}$: Pulmonary oedema, death.

Gas Control Strategies

1. Prevention: Proper blasting techniques and internal combustion engine maintenance.
2. Extraction: Methane drainage holes in coal mines.
3. Chemical Absorption: Catalytic converters in diesel engines.
4. Isolation: Sealing off areas to contain emissions.
5. Ventilation: Local dilution with auxiliary fans and removal via main airstreams.

Gas Detection and Measurement Instruments

• Handheld Flammable Gas Measuring Instrument (FGMI): Measures concentration range (e.g., $0 - 5\%\,v/v$ of methane). Includes audiovisual alarms.
• Flammable Gas Warning Device (FGWD): Continuous duty, no display, provides only alarm when set point is reached. Usually in cap lamp batteries.
• Sensor Types:
  • Reactive Tubes: Glass ampules with color-changing reagents drawn via bellows pump. Discoloration length indicates concentration.
  • Electrochemical Sensors: Most common. Gases penetrate a membrane to electrodes, causing oxidation/reduction that discharges measurable electrons.
  • Catalytic Sensors: Uses two platinum pellistors in a Wheatstone bridge. One pellistor with a catalyst oxidizes gas at $450^{\circ}\text{C}$, unbalancing the bridge via resistance changes. Used for LEL detection.
  • Infrared Sensors: Uses gas molecule excitation by specific IR wavelengths. Concentration deduced by energy absorption differences between sample and reference tubes.
  • Thermal Conductivity Sensors: Measures heat loss of a heater (approx. $250^{\circ}\text{C}$) in gas paths. Resistance change indicates concentration. Useful for high concentrations and inert gases.

Atmospheric Monitoring Systems (AMS)

• Consists of an underground network of sensors sending real-time data to a surface control room.
• Processes data to actuate alarms, configure ventilation, and monitor electrical installation fires.
• Originally designed for $CH_4$ in coal mines but now used in all underground mining.

Measurement Procedures and Strata Positioning

• Testing Requirements:
  • Start of shift, after ventilation/power failure, and at specific risk-based intervals.
  • Before blasting, before starting equipment, when intersecting geological features (dikes, fissures).
  • Hourly at diamond drilling/cementation sites.
• Sensor Placement Strategy (Stratification):
  • Roof: $H_2$ and $CH_4$ (lighter than air).
  • Floor: $SO_2$, $NO_2$, $CO_2$, $H_2S$ (denser than air).
  • Head Height: $CO$, $O_2$, and toxic gases (essential for human exposure monitoring).
  • Avoidance: Do not place sensors directly in fresh air supply zones as it dilutes results.

Diesel Particulate Matter (DPM)

• Composition: Solids (burned/unburned hydrocarbons, sulphates, metal fragments), gases ($C$, $S$, $NO$), and vapours.
• Nano Diesel Particulate Matter (nDPM): Particles $< 100$\,$\text{nm}$. These can pass from lungs to blood and into the brain.
• Chronic Health Effects: Cardiopulmonary disease and lung cancer.
• Exhaust Treatment:
  • Catalytic Converters: Granulated agents convert $90\%$ of $CO$ and $50\%$ of fuel to $CO_2$ and water.
  • Water Scrubbers: Remove $SO_2$ and large particulates.
  • Exhaust Filters: Remove fine particulate matter.
• Mitigation Measures: New engine technologies, biodiesel usage, sealed operator cabins (environmental cabs), and maintenance programs.

Risk Management and Legal Framework

• Mine Health and Safety Act (South Africa):
  • Section 5: Mandates employer maintains a safe work environment.
  • Section 11: Requires hazard identification (11.1), risk assessment (11.1), and following a hierarchy of control (11.2): Elimination, Control at source, Minimisation, PPE, and Monitoring.
  • Section 9: Requires a mandatory Code of Practice (COP).
• COP Content: Ventilation network controls, identification of gas-prone areas, airflow rate documentation, and procedures for geological features.