Banner 11

 

74. Mining and Quarrying

Chapter Editors:  James R. Armstrong and Raji Menon


 

Table of Contents 

Figures and Tables

Mining: An Overview
Norman S. Jennings

Exploration
William S. Mitchell and Courtney S. Mitchell

Types of Coal Mining
Fred W. Hermann

Techniques in Underground Mining
Hans Hamrin

Underground Coal Mining
Simon Walker

Surface Mining Methods
Thomas A. Hethmon and Kyle B. Dotson

Surface Coal Mining Management
Paul Westcott

Processing Ore
Sydney Allison

Coal Preparation
Anthony D. Walters

Ground Control in Underground Mines
Luc Beauchamp

Ventilation and Cooling in Underground Mines
M.J. Howes

Lighting in Underground Mines
Don Trotter

Personal Protective Equipment in Mining
Peter W. Pickerill

Fires and Explosions in Mines
Casey C. Grant

Detection of Gases
Paul MacKenzie-Wood

Emergency Preparedness
Gary A. Gibson

Health Hazards of Mining and Quarrying
James L. Weeks

Tables

Click a link below to view table in article context.

1. Design air quantity factors
2. Clothing-corrected air cooling powers
3. Comparison of mine light sources
4. Heating of coal-hierarchy of temperatures
5. Critical elements/sub-elements of emergency preparedness
6. Emergency facilities, equipment & materials
7. Emergency preparedness training matrix
8. Examples of horizontal auditing of emergency plans
9. Common names & health effects of hazardous gases

Figures

Point to a thumbnail to see figure caption, click to see figure in article context.

MIN010F3MIN010F4MIN020F2MIN020F7MIN020F4MIN020F6MIN20F13MIN20F10MIN040F4 MIN040F3MIN040F7MIN040F1MIN040F2MIN040F8MIN040F5


Click to return to top of page

Sunday, 13 March 2011 16:36

Detection of Gases

Written by

All who work in underground mines should have a sound knowledge of mine gases and be aware of the dangers they may present. A general knowledge of gas detection instruments and systems is also necessary. For those assigned to use these instruments, detailed knowledge of their limitations and the gases they measure is essential.

Even without instruments, the human senses may be able to detect the progressive appearance of the chemical and physical phenomena associated with spontaneous combustion. The heating warms the ventilating air and saturates it with both surface and integral moisture driven off by the heating. When this air meets colder air at the ventilation split, condensation occurs resulting in a haze and the appearance of sweating on surfaces in the returns. A characteristic oily or petrol smell is the next indication, followed eventually by smoke and, finally, visible flames.

Carbon monoxide (CO), which is odourless, appears in measurable concentrations some 50 to 60 °C before the characteristic smell of a spontaneous combustion appears. Consequently, most fire detection systems rely on the detection of a rise in carbon monoxide concentration above the normal background for the particular part of the mine.

Sometimes, a heating is first detected by an individual who notices a faint smell for a fleeting instant. Thorough examination of the area may have to be repeated a number of times before a measurable sustained increase in the concentration of carbon monoxide can be detected. Accordingly, vigilance by all those in the mine should never be relaxed and a prearranged intervention process should be implemented as soon as the presence of an indicator has been suspected or detected and reported. Fortunately, thanks to considerable progress in the technology of fire detection and monitoring made since the 1970s (e.g., detector tubes, pocket-sized electronic detectors, and computerized fixed systems), it is no longer necessary to rely on the human senses alone.

Portable Instruments for Gas Detection

The gas detection instrument is designed to detect and monitor the presence of a wide range of gas types and concentrations that could result in a fire, an explosion and a toxic or oxygen-deficient atmosphere as well as to provide early warning of an outbreak of spontaneous combustion. Gases for which they are used include CO, carbon dioxide (CO2), nitrogen dioxide (NO2), hydrogen sulphide (H2S) and sulphur dioxide (SO2). Different types of instrument are available, but before deciding which to use in a particular situation, the following questions must be answered:

 

  • Why is the detection of a particular gas or gases required?
  • What are the properties of these gases?
  • Where and in what circumstances do they occur?
  • Which gas detecting instrument or device is most suitable for those circumstances?
  • How does this instrument work?
  • What are its limitations?
  • How should the results it provides be interpreted?

 

Workers must be trained in the correct use of portable gas detectors. Instruments must be maintained according to the manufacturer’s specifications.

Universal detector kits

A dectector kit consists of a spring-loaded piston- or bellows-type of pump and a range of replaceable glass indicating tubes that contain chemicals specific for a particular gas. The pump has a capacity of 100 cc and can be operated with one hand. This allows a sample of that size to be drawn through the indicator tube before passing to the bellows. The warning indicator on the graduated scale corresponds to the lowest level of general discolouration, not the deepest point of colour penetration.

The device is easy to use and does not require calibration. However, certain precautions are applicable:

  • Indicator tubes (which should be dated) generally have a shelf-life of two years.
  • An indicator tube may be re-used ten times provided there has been no discolouration.
  • The general accuracy of each determination is usually within ± 20%.
  • Hydrogen tubes are not approved for use underground because of the intense heat developed.
  • A “pre-tube” filled with activated charcoal is required when estimating low levels of carbon monoxide in the presence of diesel exhausts or the higher hydrocarbons that may be present in afterdamp.
  • Exhaust gas should be passed through a cooling device to make sure the temperature is below 40 °C before passing though the indicator tube.
  • Oxygen and methane tubes are not approved for use underground because of their inaccuracy.

 

Catalytic-type methanometers

The catalytic-type methanometer is used in underground mines to measure the concentration of methane in the air. It has a sensor based on the principle of a network of four resistance-matched spiral wires, usually catalytic filaments, arranged in a symmetrical form known as a Wheatstone-bridge. Normally, two filaments are active and the other two are passive. The active filaments or beads are usually coated with a palladium oxide catalyst to cause oxidation of the flammable gas at a lower temperature.

Methane in the atmosphere reaches the sample chamber either by diffusion through a sintered disc or by being drawn in by an aspirator or internal pump. Pressing the operating button of the methanometer closes the circuit and the current flowing through the Wheatstone-bridge oxidizes the methane on the catalytic (active) filaments in the sample chamber. The heat of this reaction raises the temperature of the catalytic filaments, increasing their electrical resistance and electrically unbalancing the bridge. The electric current that flows is proportional to the resistance of the element and, hence, the amount of methane present. This is shown on an output indicator graduated in percentages of methane. The reference elements in the Wheatstone-bridge circuit serve to compensate for variations in environmental conditions such as ambient temperature and barometric pressure.

This instrument has a number of significant limitations:

  • Both methane and oxygen must be present to get a response. If the oxygen level in the sample chamber is below 10%, not all the methane reaching the detector will be oxidized and a false low reading will be obtained. For this reason, this instrument should not be used to measure methane levels in afterdamp or in sealed off areas where the oxygen concentration is low. If the chamber contains pure methane, there will be no reading at all. Accordingly, the operating button must be depressed before moving the instrument into a suspected methane layer in order to draw some oxygen-containing air into the chamber. The presence of a layer will be confirmed by a greater than full scale reading followed by a return to scale when the oxygen in consumed.
  • The catalytic type of methanometer will respond to flammable gases other than methane, for example, hydrogen and carbon monoxide. Ambiguous reading, therefore, may be obtained in post-fire or explosion gases (afterdamp).
  • Instruments with diffusion heads should be sheltered from high air velocities to avoid false readings. This may be accomplished by shielding it with a hand or some other object.
  • Instruments with catalytic filaments may fail to respond to methane if the filament comes in contact with the vapours of known poisons when being calibrated or used (e.g., silicones in furniture polish, floor polish and paints, phosphate esters present in hydraulic fluids, and fluorocarbons used as the propellant in aerosol sprays).
  • Methanometers based on the Wheatstone-bridge principle may give erroneous readings at variable angles of inclination. Such inaccuracies will be minimized if the instrument is held at an angle of 45° when it is calibrated or used.
  • Methanometers may give inaccurate readings at variable ambient temperatures. These inaccuracies will be minimized by calibrating the instrument under temperature conditions similar to those found underground.

 

Electrochemical cells

Instruments using electrochemical cells are used in underground mines to measure oxygen and carbon monoxide concentrations. Two types are available: the composition cell, which responds only to changes in oxygen concentration, and the partial pressure cell, which responds to changes in the partial pressure of oxygen in the atmosphere and, hence, the number of oxygen molecules per unit of volume.

The composition cell employs a capillary diffusion barrier which slows the diffusion of oxygen through the fuel cell so that the speed at which the oxygen can reach the electrode depends solely on the oxygen content of the sample. This cell is unaffected by variations in altitude (i.e., barometric pressure), temperature and relative humidity. The presence of CO2 in the mixture, however, upsets the rate of oxygen diffusion and leads to false high readings. For example, the presence of 1% of CO2 increases the oxygen reading by as much as 0.1%. Although small, this increase may still be significant and not fail-safe. It is particularly important to be aware of this limitation if this instrument is to be used in afterdamp or other atmospheres known to contain CO2.

The partial pressure cell is based on the same electrochemical principle as the concentration cell but lacks the diffusion barrier. It responds only to the number of oxygen molecules per unit volume, making it pressure dependent. CO2 in concentrations below 10% have no short-term effect on the reading, but over the long term, the carbon dioxide will destroy the electrolyte and shorten the life of the cell.

The following conditions affect the reliability of oxygen readings produced by partial pressure cells:

  • Altitude and barometric pressure: The trip from the surface to the bottom of the shaft would increase the oxygen reading by 0.1% for every 40 m travelled. This would also apply to dips, encountered in the underground workings. In addition, the 5 millibar normal daily variations in barometric pressure could alter the oxygen reading by as much as 0.1%. Thunderstorm activity could be accompanied by a 30 millibar drop in pressure that would cause a 0.4% drop in the oxygen reading.
  • Ventilation: The maximum ventilation change at the fan would be 6-8 inches water gauge or 10 millibar. This would cause a drop of 0.4% in the oxygen reading going from the intake to the return at the fan and a drop of 0.2% in travelling from the furthest face from the pit bottom.
  • Temperature: Most detectors have an electronic circuit that senses cell temperature and corrects for the temperature effect on the sensor output.
  • Relative humidity: An increase in relative humidity from dry to saturated at 20 °C would cause approximately a 0.3% decrease in the oxygen reading.

 

Other electrochemical cells

Electrochemical cells have been developed which are capable of measuring concentrations of CO from 1 ppm to an upper limit of 4,000 ppm. They operate by measuring the electric current between electrodes immersed in an acidic electrolyte. CO is oxidized on the anode to form CO2 and the reaction releases electrons in direct proportion to the CO concentration.

Electrochemical cells for hydrogen, hydrogen sulphide, nitric oxide, nitrogen dioxide and sulphur dioxide are also available but suffer from cross-sensitivity.

There are no commercially available electrochemical cells for CO2. The deficiency has been overcome with the development of a portable instrument containing a miniaturized infrared cell that is sensitive to carbon dioxide in concentrations up to 5%.

 

Non-dispersive infrared detectors

Non-dispersive infrared detectors (NDIRs) can measure all gases that contain such chemical groups as -CO, -CO2 and -CH3, which absorb infrared frequencies that are specific to their molecular configuration. These sensors are expensive but they can provide accurate readings for gases such as CO, CO2 and methane in a changing background of other gases and low oxygen levels and are therefore ideal for monitoring gases behind seals. O2, N2 and H2 do not absorb infrared radiation and cannot be detected by this method.

Other portable systems with detectors based on thermal conduction and refractive index have found limited use in the coal mining industry.

Limitations of portable gas detection instruments

The effectiveness of portable gas detection instruments is limited by a number of factors:

  • Calibration is required. This normally involves a daily check on zero and voltage, a weekly span check and a calibration test by an authorized external authority every 6 months.
  • Sensors have a finite life. If not dated by the manufacturer, the date of acquisition should be inscribed.
  • Sensors can be poisoned.
  • Sensors may suffer from cross-sensitivity.
  • Overexposure may saturate the sensor causing its slow recovery.
  • Inclination may affect the reading.
  • Batteries require charging and regular discharging.

 

Centralized Monitoring Systems

Inspections, ventilation and surveys with hand-held instruments often succeed in detecting and locating a small heating with limited makes of CO before the gas is dispersed by the ventilation system or its level exceeds the statutory limits. These do not suffice, however, where a significant risk of combustion is known to occur, methane levels in the returns exceed 1%, or a potential hazard is suspected. Under these circumstances, continuous monitoring at strategic locations is required. A number of different types of centralized continuous monitoring systems are in use.

Tube bundle systems

The tube bundle system was developed in Germany in the 1960s to detect and monitor the progress of spontaneous combustion. It involves a series of as many as 20 plastic tubes made of nylon or polyethylene 1/4 or 3/8 of an inch in diameter that extend from a bank of analysers on the surface to selected locations underground. The tubes are equipped with filters, drains and flame traps; the analysers are usually infrared for CO, CO2 and methane and paramagnetic for oxygen. A scavenger pump pulls a sample through each tube simultaneously and a sequential timer directs the sample from each tube through the analysers in turn. The data logger records the concentration of each gas at each location and automatically triggers an alarm when predetermined levels are exceeded.

This system has a number of advantages:

  • No explosion-proof instruments are required.
  • Maintenance is relatively easy.
  • Underground power is not required.
  • It covers a wide range of gases.
  • Infrared analysers are usually quite stable and reliable; they maintain their specificity in a changing background of fire gases and low oxygen atmospheres (high concentrations of methane and/or carbon dioxide may be cross-sensitive to the carbon monoxide reading in the low ppm range).
  • Instruments can be calibrated on the surface, although calibration samples of gases should be sent through the tubes to test the integrity of the collection system and the system for identifying the locations where particular samples originated.

 

There are also some disadvantages:

  • The results are not in real time.
  • Leaks are not immediately apparent.
  • Condensation may collect in the tubes.
  • Defects in the system are not always immediately apparent and may be difficult to identify.
  • The tubes may be damaged by blasting or in a fire or an explosion.

 

Telemetric (electronic) system

The telemetric automatic gas monitoring system has a control module on the surface and intrinsically safe sensor heads strategically located underground which are connected by phone lines or fibre-optic cables. Sensors are available for methane, CO and air velocity. The sensor for CO is similar to the electrochemical sensor used in portable instruments and is subject to the same limitations. The methane sensor works through the catalytic combustion of methane on the active elements of a Wheatstone-bridge circuit which can be poisoned by sulphur compounds, phosphate esters or silicon compounds and will not work when the oxygen concentration is low.

The unique advantages of this system include:

  • The results are available in real time (i.e., there is rapid indication of fire or a build-up of methane).
  • Long distances between the sensor heads and the control unit are possible without compromising the system.
  • Sensor failure is recognized immediately.

 

There are also some disadvantages:

  • A high level of maintenance is required.
  • The sensor range for CO is limited (0.4%).
  • The variety of sensors is limited; there are none for CO2 or hydrogen.
  • The methane sensor is subject to poisoning.
  • In situ calibration is required.
  • Cross-sensitivity may be a problem.
  • There may be a loss of power (e.g., >1.25% for methane).
  • Sensor life is limited to 1 to 2 years.
  • The system is not suitable for low oxygen atmospheres (e.g., behind seals).

 

Gas chromatograph

The gas chromatograph is a sophisticated piece of equipment that analyses samples with high degrees of accuracy and that, until recently, could only be fully utilized by chemists or specially qualified and trained personnel.

Gas samples from a tube bundle-type of system are injected into the gas chromatograph automatically or they can be manually introduced from bag samples brought out of the mine. A specially packed column is used to separate different gases and a suitable detector, usually thermal conductivity or flame ionization, is used to measure each gas as it elutes from the column. The separation process provides a high degree of specificity.

The gas chromatograph has particular advantages:

  • No cross-sensitivity from other gases occurs.
  • It is capable of measuring hydrogen.
  • It is capable of measuring ethylene and higher hydrocarbons.
  • It can accurately measure from very low to very high concentrations of most of the gases that occur or are produced underground by a heating or a fire.
  • It is well recognized that modern methods of combating fires and heatings in coal mines may be most effectively implemented on the basis of interpretation of gas analyses from strategic locations in the mine. Accurate, reliable and complete results require a gas chromatograph and interpretation by qualified, experienced and fully trained personnel.

 

Its disadvantages include:

  • The analyses are relatively slow.
  • A high level of maintenance is required.
  • The hardware and the controls are complex.
  • Expert attention is required periodically.
  • Calibration must be scheduled frequently.
  • High methane concentrations interfere with low level CO measurements.

Choice of system

Tube-bundle systems are preferred for monitoring locations that are not expected to have rapid changes in gas concentrations or, like sealed areas, may have low oxygen environments.

Telemetric systems are preferred in locations such as belt roads or on the face where rapid changes in gas concentrations may have significance.

Gas chromatography does not replace existing monitoring systems but it enhances the range, accuracy and reliability of the analyses. This is particularly important when determination of the risk of explosion is involved or when a heating is reaching an advanced stage.

Sampling considerations

  • The siting of sampling points at strategic locations is of major importance. The information from a single sampling point some distance from the source is only suggestive; without confirmation from other locations it may lead to over- or underestimation of the seriousness of the situation. Consequently, sampling points to detect an outbreak of spontaneous combustion must be sited where heatings are most likely to occur. There must be little dilution of flows between the heating and the detectors. Consideration must be given to the possibility of the layering of methane and warm combustion gases which may rise up the dip in a sealed area. Ideally, the sampling sites should be located in panel returns, behind stoppings and seals, and in the main stream of the ventilation circuit. The following considerations are applicable:
  • The sampling site should be set at least 5 m inbye (i.e., toward the face of) a seal because seals “breathe in” when the atmospheric pressure rises.
  • Samples should be taken from boreholes only when they breathe out and when it can be ensured that the borehole is leak free.
  • Samples should be taken more than 50 m downwind from a fire to ensure mixing (Mitchell and Burns 1979).
  • Samples should be taken up the gradient from a fire near the roof because hot gases rise.
  • Samples should be taken inbye a ventilation door to avoid leakage.
  • All sampling points should be clearly shown on maps of schematics of the mine ventilation system. Taking gas samples underground or from surface boreholes for analysis at another location is difficult and error prone. The sample in the bag or container must truly represent the atmosphere at the sampling point.

 

Plastic bags are now widely used in the industry for taking samples. The plastic minimizes leakage and can keep a sample for 5 days. Hydrogen, if present in the bag, will degrade with a daily loss of about 1.5% of its original concentration. A sample in a football bladder will change concentration in half an hour. Bags are easy to fill and the sample can be squeezed into an analysing instrument or it can be drawn out with a pump.

Metal tubes that are filled under pressure by a pump can store samples for a long time but the size of the sample is limited and leakage is common. Glass is inert to gases but glass containers are fragile and it is difficult to get the sample out without dilution.

In collecting samples, the container should be pre-flushed at least three times to ensure that the previous sample is completely flushed out. Each container should have a tag carrying such information as the date and time of sampling, the exact location, the name of the person collecting the sample and other useful information.

Interpretation of Sampling Data

Interpretation of the results of gas sampling and analysis is a demanding science and should be attempted only by individuals with special training and experience. These data are vital in many emergencies because they provide information on what is happening underground that is needed to plan and implement corrective and preventive actions. During or immediately after an underground heating, fire or explosion, all possible environmental parameters should be monitored in real time to enable those in charge to accurately determine the status of the situation and measure its progress so that they lose no time in initiating any needed rescue activities.

Gas analysis results must meet the following criteria:

  • Accuracy. Instruments must be correctly calibrated.
  • Reliability. Cross-sensitivities must be known
  • Completeness. All gases, including hydrogen and nitrogen, should be measured.
  • Timeliness. If real time is not possible, trending should be carried out.
  • Validity. Sample points must be in and around the site of the incident.

 

The following rules should be followed in interpreting gas analysis results:

  • A few sampling points should be carefully selected and marked on the plan. This is better for trending than taking sample from many points.
  • If a result deviates from a trend, it should be confirmed by resampling or the calibration of the instrument should be checked before taking action. Variations in outside influences, such as ventilation, barometric pressure and temperature or a diesel engine running in the area, are often the reason for the changing result.
  • The gas make or mixture under non-mining conditions should be known and allowed for in the calculations.
  • No analysis result should be accepted on faith; results must be valid and verifiable.
  • It should be borne in mind that isolated figures do not indicate the progress—trends give a more accurate picture.

 

Calculating air-free results

Air-free results are obtained by calculating out the atmospheric air in the sample (Mackenzie-Wood and Strang 1990). This allows samples from a similar area to be properly compared after the dilution effect from air leakage has been removed.

The formula is:

Air-free result = Analysed result / (100 - 4.776 O2)

It is derived as follows:

Atmospheric air = O2 + N2 = O2 + 79.1 O2 / 20.9  =  4.776 O2

Air-free results are useful when trending of results is required and there has been a risk of air dilution between the sample point and the source, air leakage has occurred in sample lines, or bag samples and seals may have breathed in. For example, if the carbon monoxide concentration from a heating is being trended, then air dilution from an increase in ventilation could be misinterpreted as a decrease in carbon monoxide from the source. The trending of air-free concentrations would give the correct results.

Similar calculations are needed if the sampling area is making methane: the increase in methane concentration would dilute the concentration of other the gases that are present. Hence, an increasing carbon oxide level may actually show up as decreasing.

Methane-free results are calculated as follows:

Methane-free result = Analysed result / (100 - CH4 %)

Spontaneous Combustion

Spontaneous combustion is a process whereby a substance can ignite as a result of internal heat which arises spontaneously due to reactions liberating heat faster than it can be lost to the environment. The spontaneous heating of coal is usually slow until the temperature reaches about 70 °C, referred to as the “cross over” temperature. Above this temperature, the reaction usually accelerates. At over 300 °C, the volatiles, also called “coal gas” or “cracked gas”, are given off. These gases (hydrogen, methane and carbon monoxide) will ignite spontaneously at temperatures of approximately 650 °C (it has been reported that the presence of free radicals can result in the appearance of flame in the coal at about 400 °C). The processes involved in a classic case of spontaneous combustion are presented in table 1 (different coals will produce varying pictures).

Table 1. Heating of coal - hierarchy of temperatures

Temperature at which coal absorbs O2 to form a complex and produce heat

30 °C

Complex breaks down to produce CO/CO2

45 °C

True oxidation of coal to produce CO and CO2

70 °C

Cross-over temperature, heating accelerates

110 °C

Moisture, H2 and characteristic smell released

150 °C

Desorbed CH4, unsaturated hydrocarbons released

300 °C

Cracked gases (e.g., H2, CO, CH4) released

400 °C

Open flame

Source: Chamberlain et al. 1970.

Carbon monoxide

CO is actually released some 50 °C before the characteristic smell of combustion is noticed. Most systems designed to detect the onset of spontaneous combustion are based on the detection of carbon monoxide in concentrations above the normal background for a particular area of the mine.

Once a heating has been detected, it must be monitored in order to determine the state of the heating (i.e., its temperature and extent), the rate of accelerations, toxic emissions and explosibility of the atmosphere.

Monitoring a heating

There are a number of indices and parameters available to assist planners to determine the extent, temperature and rate of progression of a heating. These are usually based on changes in the composition of the air passing through a suspected area. Many indicators have been described in the literature over the years and most offer a very limited window of usage and are of minimal value. All are site specific and differ with different coals and conditions. Some of the more popular ones include: carbon monoxide trending; carbon monoxide make (Funkemeyer and Kock 1989); Graham’s ratio (Graham 1921) tracer gases (Chamberlain 1970); Morris ratio (Morris 1988); and the carbon monoxide/carbon dioxide ratio. After sealing, indicators may be difficult to use because of the absence of a defined air flow.

No one indicator affords a precise and sure method of measuring the progress of a heating. Decisions must be based on gathering, tabulating, comparing and analysing all information and interpreting it in the light of training and experience.

Explosions

Explosions are the greatest single hazard in coal mining. It has the potential to kill the entire underground workforce, destroy all the equipment and services and prevent any further working of the mine. And, all this can happen in 2 to 3 seconds.

The explosibility of the atmosphere in the mine must be monitored at all times. It is especially urgent when workers are engaged in a rescue operation in a gassy mine.

As in the case of indicators for evaluating a heating, there are a number of techniques for calculating the explosibility of the atmosphere in an underground mine. They include: Coward’s triangle (Greuer 1974); Hughes and Raybold’s triangle (Hughes and Raybold 1960); Elicott’s diagram (Elicott 1981); and Trickett’s ratio (Jones and Trickett 1955). Because of the complexity and variability of the conditions and circumstances, there is no single formula that can be relied on as a guarantee that an explosion will not occur at a particular time in a particular mine. One must rely on a high and unremitting level of vigilance, a high index of suspicion and an unhesitating initiation of appropriate action at the slightest indication that an explosion might be imminent. A temporary halt in production is a relatively small premium to pay for assurance that an explosion will not occur.

Conclusion

This article has summarized the detection of gases that might be involved in fires and explosions in underground mines. The other health and safety implications of the gaseous environment in mines (e.g., dust diseases, asphyxia, toxic effects, etc.) are discussed in other articles in this chapter and elsewhere in this Encyclopaedia.

 

Back

Sunday, 13 March 2011 16:41

Emergency Preparedness

Written by

Mine emergencies often occur as the result of a lack of systems, or failures in existing systems, to limit, control or prevent circumstances that trigger incidents which, when ineffectively managed, lead to disasters. An emergency may then be defined as an unplanned event that impacts upon the safety or welfare of personnel, or the continuity of operations, which requires an effective and timely response in order to contain, control or mitigate the situation.

All forms of mining operations have particular hazards and risks that may lead to an emergency situation. Hazards in underground coal mining include methane liberation and coal dust generation, high-energy mining systems and coal’s propensity to spontaneous combustion. Emergencies can occur in underground metalliferous mining due to strata failure (rock bursts, rock falls, hangingwall and pillar failures), unplanned initiation of explosives and sulphide ore dusts. Surface mining operations involve risks relating to, large-scale high-speed mobile equipment, unplanned initiation of explosives, and slope stability. Hazardous chemical exposure, spill or leak, and tailing dam failure can take place in minerals processing.

Good mining and operational practices have evolved that incorporate relevant measures to control or mitigate these risks. However, mine disasters continue to occur regularly throughout the world, even though formal risk management techniques have been adopted in some countries as a pro-active strategy to improve mine safety and reduce the likelihood and consequence of mine emergencies.

Accident investigations and inquiries continue to identify failures to apply the lessons of the past and failures to apply effective barriers and control measures to known hazards and risks. These failures are often compounded by a lack of adequate measures to intervene, control and manage the emergency situation.

This article outlines an approach to emergency preparedness that can be utilized as a framework to both control and mitigate mining hazards and risks and to develop effective measures to ensure control of the emergency and the continuity of mine operations.

Emergency Preparedness Management System

The emergency preparedness management system proposed comprises an integrated systems approach to the prevention and management of emergencies. It includes:

  • organizational intent and commitment (corporate policy, management commitment and leadership)
  • risk management (identification, assessment and control of hazards and risks)
  • definition of measures to manage an unplanned event, incident or emergency
  • definition of emergency organization (strategies, structure, staffing, skills, systems and procedures)
  • provision of facilities, equipment, supplies and materials
  • training of personnel in the identification, containment and notification of incidents and their roles in the mobilization, deployment and post-incident activities
  • evaluation and enhancement of the overall system through regular auditing procedures and trials
  • periodic risk and capability reassessment
  • critique and evaluation of the response in the event of an emergency, coupled with necessary system enhancement.

 

Incorporation of emergency preparedness within the ISO 9000 quality management system framework provides a structured approach to contain and control emergency situations in a timely, effective and safe manner.

Organizational Intent and Commitment

Few people will be convinced of the need for emergency preparedness unless a potential danger is recognized and it is seen as directly threatening, highly possible if not probable and likely to occur in a relatively short time span. However, the nature of emergencies is that this recognition generally does not occur prior to the event or is rationalized as non-threatening. The lack of adequate systems, or failures in existing systems, results in an incident or emergency situation.

Commitment to and investment in effective emergency preparedness planning provides an organization with the capability, expertise and systems to provide a safe work environment, meet moral and legal obligations and enhance prospects for business continuity in an emergency. In coal mine fires and explosions, including non-fatal incidents, business continuity losses are often significant due to the extent of damage, the type and nature of control measures employed or even loss of the mine. Investigative processes also impact considerably. Failure to have effective measures in place to manage and control an incident will further compound overall losses.

Development and implementation of an effective emergency preparedness system requires management leadership, commitment and support. Consequently it will be necessary to:

  • provide and ensure continuing management leadership, commitment and support
  • establish long-term goals and purpose
  • guarantee financial support
  • guarantee availability of personnel and their access to and involvement in training
  • provide appropriate organizational resources to develop, implement and maintain the system.

 

The necessary leadership and commitment can be demonstrated through the appointment of an experienced, capable and highly respected officer as Emergency Preparedness Coordinator, with the authority to ensure participation and cooperation at all levels and within all units of the organization. Formation of an Emergency Preparedness Planning Committee, under the Coordinator’s leadership, will provide the necessary resources to plan, organize and implement an integrated and effective emergency preparedness capability throughout the organization.

Risk Assessment

The risk management process enables the type of risks facing the organization to be identified and analysed to determine the likelihood and the consequence of their occurrence. This framework then enables the risks to be assessed against established criteria to determine if the risks are acceptable or what form of treatment must be applied to reduce those risks (e.g., reducing likelihood of occurrence, reducing consequence of occurrence, transferring all or part of the risks or avoiding the risks). Targeted implementation plans are then developed, implemented and managed to control the identified risks.

This framework can be similarly applied to develop emergency plans that enable effective controls to be implemented, should a contingent situation arise. Identification and analysis of risks enables likely scenarios to be predicted with a high degree of accuracy. Control measures can then be identified to address each of the recognized emergency scenarios, which then form the basis of emergency preparedness strategies.

Scenarios that are likely to be identified may include some or all of those listed in table 1. Alternatively national standards, such as the Australian Standard AS/NZS 4360: 1995—Risk Management, may provide a listing of generic sources of risk, other classifications of risk, and the areas of impact of risk that provides a comprehensive structure for hazard analysis in emergency preparedness.

Table 1. Critical elements/sub-elements of emergency preparedness

Fires

  • Underground
  • Plant and surface
  • Bushfires
  • Community
  • Vehicle

 

Chemical spills/leaks

  • Oil spills
  • Ruptured gas main
  • Containment of spill
  • Offsite/onsite
  • Storage capabilities

 

Injuries

  • Onsite
  • Multiple
  • Fatal
  • Critical

 

Natural disasters

  • Flooding
  • Cyclone
  • Earthquake
  • Severe storm
  • Ruptured dam
  • Mud or land slide

 

Community evacuation

  • Planned
  • Unplanned

Explosions/implosions

  • Dust
  • Chemicals
  • Blasting agents
  • Petroleum
  • Nitrogen
  • Gas line explosion

 

Civil disturbance

  • Strike
  • Protest
  • Bomb threat
  • Kidnap/extortion
  • Sabotage
  • Other threats

 

Power failure

  • Electrical blackout
  • Gas shortage
  • Water shortage
  • Communication systems
    failure

 

Water in-rush

  • Exploration drill hole
  • Bulkheads
  • Pillar failure
  • Unplanned holing of old workings
  • Tailings
  • Ruptured dam
  • Fractured ground
  • Water main failure

Exposures

  • Heat/cold
  • Noise
  • Vibration
  • Radiation
  • Chemical
  • Biological

 

Environmental

  • Air pollution
  • Water pollution
  • Soil pollution
  • Waste material (disposal
    problem)

 

Cave-in

  • Underground
  • Surface subsidence
  • Highwall failure/slip
  • Surface excavation
    failure
  • Structural (building)

 

Transportation

  • Automobile accident
  • Train accident
  • Boat/shipping accident
  • Aeroplane accident
  • Hazardous materials in
    transport accident

 

Extrication

  • System/resources
  • Unplanned

Source: Mines Accident Prevention Association Ontario (undated).

Emergency Control Measures and Strategies

Three levels of response measures should be identified, evaluated and developed within the emergency preparedness system. Individual or primary response comprises the actions of individuals upon the identification of hazardous situations or an incident, including:

  • notifying appropriate supervisors, controllers or management personnel of the situation, circumstances or incident
  • containment (basic fire-fighting, life support or extrication)
  • evacuation, escape or refuge.

 

Secondary response comprises the actions of trained responders upon notification of the incident, including fire teams, search and rescue teams and special casualty access teams (SCAT), all utilizing advanced skills, competencies and equipment.

Tertiary response comprises the deployment of specialized systems, equipment and technologies in situations where primary and secondary response cannot be safely or effectively utilized, including:

  • personnel locating devices and seismic event detectors
  • large diameter borehole rescue
  • inertization, remote sealing or flooding
  • surveillance/exploration vehicles and systems (e.g., borehole cameras and atmospheric sampling).

 

Defining the Emergency Organization

Emergency conditions grow more serious the longer the situation is allowed to proceed. Onsite personnel must be prepared to respond appropriately to emergencies. A multitude of activities must be coordinated and managed to ensure that the situation is rapidly and effectively controlled.

Emergency organization provides a structured framework that defines and integrates the emergency strategies, management structure (or chain of command), personnel resources, roles and responsibilities, equipment and facilities, systems and procedures. It encompasses all phases of an emergency, from the initial identification and containment activities, to notification, mobilization, deployment and recovery (re-establishment of normal operations).

The emergency organization should address a number of key elements, including:

  • capability for primary and secondary response to an emergency
  • capability to manage and control an emergency
  • coordination and communications, including gathering, assessing and evaluating data, decision making and implementation
  • the breadth of procedures necessary for effective control, including identification and containment, notification and early reporting, declaration of an emergency, specific operational procedures, fire-fighting, evacuation, extrication and life support, monitoring and review
  • identification and assignment of key functional responsibilities
  • control, advisory, technical, administration and support services
  • transitional arrangements from normal to emergency operations in terms of lines of communication, authority levels, accountability, compliance, liaison and policy
  • capability and capacity to maintain emergency operations for an extended period and provide for shift changes
  • impact of organizational changes in an emergency situation, including supervision and control of personnel; re-allocation or re-assignment of personnel; motivation, commitment and discipline; role of experts and specialists, external agencies and corporate officers
  • contingency provisions to address situations such as those arising after hours or where key organizational members are unavailable or affected by the emergency
  • integration and deployment of tertiary response systems, equipment and technologies.

 

Emergency Facilities, Equipment and Materials

The nature, extent and scope of facilities, equipment and materials required to control and mitigate emergencies will be identified through application and extension of the risk management process and determination of the emergency control strategies. For example, a high-level risk of fire will necessitate the provision of adequate fire-fighting facilities and equipment. These would be deployed consistently with the risk profile. Similarly, the facilities, equipment and materials necessary to address effectively life support and first aid or evacuation, escape and rescue can be identified as illustrated in table 2.

Table 2. Emergency facilities, equipment and materials

Emergency

Response level

   
 

Primary

Secondary

Tertiary

Fire

Fire extinguishers, hydrants and hoses installed adjacent to high risk areas, such as conveyors, fuelling stations, electrical transformers and sub-stations, and on mobile equipment

Breathing apparatus and protective clothing provided in central areas to enable a “fire team” response with advanced apparatus such as foam generators and multiple hoses

Provision for remote sealing or inertization.

Life support and first aid

Life support, respiration and circulation

First aid, triage, stabilization and extrication

Paramedical, forensic, legal

Evacuation, escape and rescue

Provision of warning or notification systems, secure escapeways, oxygen-based self rescuers, lifelines and communication systems, availability of transportation vehicles

Provision of suitably equipped refuge chambers, trained and equipped mines rescue teams, personnel locating devices

Large diameter borehole rescue systems, inertization, purpose-designed rescue vehicles

 

Other facilities and equipment that may be necessary in an emergency include incident management and control facilities, employee and rescue muster areas, site security and access controls, facilities for next of kin and the media, materials and consumables, transport and logistics. These facilities and equipment are provided for prior to an incident. Recent mine emergencies have reinforced the necessity to focus on three specific infrastructure issues, refuge chambers, communications, and atmospheric monitoring.

Refuge chambers

Refuge chambers are being increasingly utilized as a means of enhancing escape and rescue of underground personnel. Some are designed to permit persons to be self-rescuers and communicate with the surface in safety; others have been designed to effect refuge for an extended period so as to permit assisted rescue.

The decision to install refuge chambers is dependent upon the overall escape and rescue system for the mine. The following factors need to be evaluated when considering the need for and design of refuges:

  • the likelihood of entrapment
  • the time taken for people underground to evacuate through the normal means of egress, which may be excessive in mines with extensive workings or difficult conditions such as low heights or steep grades
  • the capability of persons underground to escape unassisted (e.g., pre-existing medical conditions or fitness levels and injuries sustained in the incident)
  • the discipline required to maintain and utilize refuge chambers
  • the means to assist personnel to locate the refuge chambers in conditions of extremely low visibility and duress
  • the required resistance to explosions and fire
  • the necessary size and capacity
  • the services provided (e.g., ventilation/air purification, cooling, communications, sanitation, and sustenance)
  • the potential application of inertization as a control strategy
  • the options for final recovery of personnel (e.g., mine rescue teams and large diameter boreholes).

 

Communications

Communications infrastructure is generally in place in all mines to facilitate management and control of operations as well as contribute to the safety of the mine through calls for support. Unfortunately, the infrastructure is usually not robust enough to survive a significant fire or explosion, disrupting communication when it would be most beneficial. Furthermore conventional systems incorporate handsets which cannot be safely used with most breathing apparatus and are usually deployed in main intake airways adjacent to fixed plant, rather than in escapeways.

The need for post-incident communications should be closely evaluated. While it is preferable that a post-incident communications system is part of the pre-incident system, to enhance maintainability, cost and reliability, a stand-alone emergency communications system may be warranted. Regardless, the communications system should be integrated within the overall escape, rescue and emergency management strategies.

Atmospheric monitoring

Knowledge of conditions in a mine following an incident is essential to enable the most appropriate measures to control a situation to be identified and implemented and to assist escaping workers and protect rescuers. The need for post-incident atmospheric monitoring should be closely evaluated and systems should be provided that meet mine-specific needs, possibly incorporating:

  • the location and design of fixed station atmospheric and ventilation sampling points for normal and potentially abnormal atmospheric conditions
  • the maintenance of capabilities to analyse, trend and interpret the mine atmosphere, particularly where explosive mixtures may be present post-incident
  • modularization of tube-bundle systems around boreholes to minimize sampling delays and improve the system’s robustness
  • provision of systems to verify integrity of tube-bundle systems post-incident
  • utilization of gas chromatography where explosive mixtures are possible after the incident and rescuers may be required to enter the mine.

 

Emergency Preparedness Skills, Competencies and Training

The skills and competencies required to cope effectively with an emergency can be readily determined by identification of core risks and emergency control measures, development of emergency organization and procedures and identification of necessary facilities and equipment.

Emergency preparedness skills and competencies include not only planning and management of an emergency, but a diverse range of basic skills associated with the primary and secondary response initiatives that should be incorporated in a comprehensive training strategy, including:

  • the identification and containment of the incident (e.g., fire-fighting, life support, evacuation and extrication)
  • notification (e.g., radio and telephone procedures)
  • mobilization and deployment activities (e.g., search and rescue, fire-fighting, casualty management and recovering bodies).

 

The emergency preparedness system provides a framework for the development of an effective training strategy by identifying the necessity, extent and scope of specific, predictable and reliable workplace outcomes in an emergency situation and the underpinning competencies. The system includes:

  • a statement of intent that details why the necessary expertise, skills and competencies are to be developed and provides the organizational commitment and leadership to succeed
  • risk management and measures to manage emergencies that identify key content elements (e.g., fires, explosions, hazardous materials, unplanned movements and discharges, sabotage, bomb threats, security breaches, etc.)
  • a definition of the emergency organization (strategies, structure, staffing, skills, systems and procedures) that identifies who is to be trained, their role in an emergency and the necessary skills and competencies
  • identification of training resources that determines what aids, equipment, facilities and personnel are necessary
  • training of personnel in identification and containment, notification, mobilization, deployment and post-incident activities that develops the necessary skills and competency base
  • routine testing, evaluation and enhancement of the overall system, coupled with periodic risk and capability reassessment, that completes the learning process and ensures that an effective emergency preparedness system exists.

 

Emergency preparedness training can be structured into a number of categories as illustrated in table 3.

Table 3. Emergency preparedness training matrix

Training response level

 

 

Educational primary

Procedural/secondary

Functional/tertiary

Designed to ensure employees understand the nature of mine emergencies and how specific aspects of the overall emergency plan may involve or affect the individual, including primary response measures.

Skills and competencies to successfully complete specific procedures defined under the emergency response plans and the secondary response measures associated with specific emergency scenarios.

Development of skills and competencies necessary for the management and control of emergencies.

Knowledge and competence elements

  • Knowledge of key indicators of mine incidents
  • Knowledge of key indicators of mine incidents
  • Knowledge of key indicators of mine emergencies and detailed knowledge of trigger events to initiate emergency response
  • Environmental conditions following an incident (e.g., temperature, visibility and gases)
  • Ability to detect, monitor and evaluate environmental conditions following an incident (e.g., mine gases, ventilation, smoke)
  • Detailed knowledge of mine design, mine ventilation and monitoring systems
  • Ability to respond to adverse changes in environmental conditions (e.g., smoke, ventilation disruption)
  • Ability to assess and interpret changes to mine ventilation systems (e.g., destruction of stoppings, seals and air crossings, damage to main fans)
  • Ability to assess and interpret current information systems at the mine (e.g., ventilation and environmental monitoring data)
  • Ability to perform notification and communications required post-incident
  • Knowledge of response measures that can be used to manage and mitigate an emergency (e.g., fire-fighting, search and rescue, restoration of ventilation, first aid, triage and extrication)
  • Awareness of control measures that can be used to manage and mitigate an emergency
  • Knowledge of appropriate emergency response options to environmental conditions
  • Knowledge of roles and responsibilities of all mine personnel under the emergency response plans and the capability to perform their nominated role
  • Ability to operate and manage emergency response plans and procedures, conducting simulated emergencies
  • Awareness of use and limitations of escape apparatus, routes and systems
  • Awareness of use and limitations of escape apparatus, routes and systems (e.g., self-rescuers, refuge chambers, breathing apparatus)
  • Ability to implement emergency communications and protocols, both internally and externally
  • Knowledge of roles and responsibilities of all mine personnel under emergency response plans including specific roles and responsibilities
  • Ability to implement internal emergency communications and protocols
  • Capability of mine rescue and other emergency services and access support from these services
  • Possession of primary response skills and competencies associated with specific emergency scenarios (e.g., basic fire-fighting, life support, escape and refuge
  • Awareness of use and limitations of escape and rescue apparatus and systems (e.g., self-rescuers, refuge chambers, breathing apparatus)
  • Ability to establish and support critical incident team
  • Knowledge about mine rescue and other emergency services
  • Capability of mine rescue and other emergency services
  • Knowledge of the capability and deployment of tertiary response systems (e.g., locating systems, inertization, remote sealing, large diameter borehole rescue, mobile laboratories)
  • Participation in simulated emergencies
  • Initiation of call out and mutual assistance schemes
  • Ability to use specialist resources (e.g., paramedical, forensic, legal, critical incident stress debriefing, technologists)

 

  • Participating in simulated exercises and emergencies
  • Crisis management and leadership

 

Audit, Review and Evaluation

Audit and review processes need to be adopted to assess and evaluate the effectiveness of the overall emergency systems, procedures, facilities, maintenance programmes, equipment, training and individual competencies. The conduct of an audit or simulation provides, without exception, opportunities for improvement, constructive criticism and verification of satisfactory performance levels of key activities.

Every organization should test its overall emergency plan at least once per year for each operating shift. Critical elements of the plan, such as emergency power or remote alarm systems, should be tested separately and more frequently.

Two basic forms of auditing are available. Horizontal auditing involves the testing of small, specific elements of the overall emergency plan to identify deficiencies. Seemingly minor deficiencies could become critical in the event of an actual emergency. Examples of such elements and related deficiencies are listed in table 4. Vertical auditing tests multiple elements of a plan simultaneously through simulation of an emergency event. Activities such as activation of the plan, search and rescue procedures, life support, fire-fighting and the logistics related to an emergency response at a remote mine or facility can be audited in this manner.

Table 4. Examples of horizontal auditing of emergency plans

Element

Deficiency

Indicators of incipient incident or event

Failure to recognize, notify, record and action

Alert/evacuation procedures

Employees unfamiliar with evacuation procedures

Donning of emergency respirators

Employees unfamiliar with respirators

Fire-fighting equipment

Fire extinguishers discharged, sprinkler heads painted over, fire hydrants concealed or buried

Emergency alarms

Alarms ignored

Gas testing instruments

Not regularly maintained, serviced or calibrated

 

Simulations may involve personnel from more than one department and perhaps personnel from other companies, mutual aid organizations, or even emergency services such as police and fire departments. Involvement of external emergency service organizations provides all parties with an invaluable opportunity to enhance and integrate emergency preparedness operations, procedures and equipment and tailor response capabilities to major risks and hazards at specific sites.

A formal critique should be conducted as soon as possible, preferably immediately following the audit or simulation. Recognition should be extended to those individuals or teams that performed well. Weaknesses must be described as specifically as possible and procedures reviewed to incorporate systemic improvements where necessary. Necessary changes must be implemented and performance must be monitored for improvements.

A sustained programme emphasizing planning, practice, discipline and teamwork are necessary elements of well-balanced simulations and training drills. Experience has proven repeatedly that every drill is a good drill; every drill is beneficial and presents opportunities to demonstrate strengths and expose areas that require improvement.

Periodic Risk and Capability Reassessment

Few risks remain static. Consequently, risks and the capability of control and emergency preparedness measures needs to be monitored and evaluated to ensure that changing circumstances (e.g., people, systems, processes, facilities or equipment) do not alter risk priorities or diminish system capabilities.

Conclusions

Emergencies are often regarded as unforeseen occurrences. However, in this day and age of advanced communication and technology there are few events that can be truly called unforeseen and few misfortunes that have not been already experienced. Newspapers, hazard alerts, accident statistics and technical reports all provide sound historical data and images of what the future may hold for the ill-prepared.

Still, the nature of emergencies changes as industry changes. Relying on techniques and emergency measures adopted from past experience will not always provide the same degree of security for future events.

Risk management provides a comprehensive and structured approach to the understanding of mine hazards and risks and the development of effective emergency response capabilities and systems. The process of risk management must be understood and continuously applied, particularly when deploying mine rescue personnel into a potentially hazardous or explosive environment.

Underpinning competent emergency preparedness is the training of all mine personnel in basic hazard awareness, the early recognition and notification of incipient incidents and trigger events and primary response and escape skills. Expectations-training under conditions of heat, humidity, smoke and low visibility is also essential. Failure to adequately train personnel in these basic skills has often been the difference between an incident and a disaster.

Training provides the mechanism for operationalizing emergency preparedness organization and planning. Integration of emergency preparedness within a quality systems framework coupled with routine auditing and simulation provides the mechanism to improve and enhance emergency preparedness.

The ILO Safety and Health in Mines Convention, 1955 (No. 176), and Recommendation, 1995 (No. 183), provide an overall framework for improving safety and health in mines. The emergency preparedness system proposed provides a methodology for achieving the outcomes identified in the Convention and Recommendation.

Acknowledgement: The assistance of Mr Paul MacKenzie-Wood, Manager Coal Mines Technical Services (Mines Rescue Service NSW, Australia) in the preparation and critique of this article is gratefully acknowledged.

 

Back

Sunday, 13 March 2011 16:50

Health Hazards of Mining and Quarrying

Written by

The principal airborne hazards in the mining industry include several types of particulates, naturally occurring gases, engine exhaust and some chemical vapours; the principal physical hazards are noise, segmental vibration, heat, changes in barometric pressure and ionizing radiation. These occur in varying combinations depending on the mine or quarry, its depth, the composition of the ore and surrounding rock, and the method(s) of mining. Among some groups of miners who live together in isolated locations, there is also risk of transmitting some infectious diseases such as tuberculosis, hepatitis (B and E), and the human-immunodeficiency virus (HIV). Miners’ exposure varies with the job, its proximity to the source of hazards and the effectiveness of hazard control methods.

Airborne Particulate Hazards

Free crystalline silica is the most abundant compound in the earth’s crust and, consequently, is the most common airborne dust that miners and quarry-workers face. Free silica is silicon dioxide that is not chemically bonded with any other compound as a silicate. The most common form of silica is quartz although it can also appear as trydimite or christobalite. Respirable particles are formed whenever silica-bearing rock is drilled, blasted, crushed or otherwise pulverized into fine particles. The amount of silica in different species of rock varies but is not a reliable indicator of how much respirable silica dust may be found in an air sample. It is not uncommon, for example, to find 30% free silica in a rock but 10% in an air sample, and vice versa. Sandstone can be up to 100% silica, granite up to 40%, slate, 30%, with lesser proportions in other minerals. Exposure can occur in any mining operation, surface or underground, where silica is found in the overburden of a surface mine or the ceiling, floor or ore deposit of an underground mine. Silica can be dispersed by the wind, by vehicular traffic or by earth-moving machinery.

With sufficient exposure, silica can cause silicosis, a typical pneumoconiosis that develops insidiously after years of exposure. Exceptionally high exposure can cause acute or accelerated silicosis within months with significant impairment or death occurring within a few years. Exposure to silica is also associated with an increased risk of tuberculosis, lung cancer and of some autoimmune diseases, including scleroderma, systemic lupus erythematosus and rheumatoid arthritis. Freshly fractured silica dust appears to be more reactive and more hazardous than old or stale dust. This may be a consequence of a relatively higher surface charge on freshly formed particles.

The most common processes that produce respirable silica dust in mining and quarrying are drilling, blasting and cutting silica-containing rock. Most holes drilled for blasting are done with an air powered percussion drill mounted on a tractor crawler. The hole is made with a combination of rotation, impact and thrust of the drill bit. As the hole deepens, steel drill rods are added to connect the drill bit to the power source. Air not only powers the drilling, it also blows the chips and dust out of the hole which, if uncontrolled, injects large amounts of dust into the environment. The hand-held jack-hammer or sinker drill operates on the same principle but on a smaller scale. This device conveys a significant amount of vibration to the operator and with it, the risk of vibration white finger. Vibration white finger has been found among miners in India, Japan, Canada and elsewhere. The track drill and the jack-hammer are also used in construction projects where rock must be drilled or broken to make a highway, to break rock for a foundation, for road repair work and other purposes.

Dust controls for these drills have been developed and are effective. A water mist, sometimes with a detergent, is injected into the blow air which helps the dust particles to coalesce and drop out. Too much water results in a bridge or collar forming between the drill steel and the side of the hole. These often have to be broken in order to remove the bit; too little water is ineffective. Problems with this type of control include reduction in the drilling rate, lack of reliable water supply and displacement of oil resulting in increased wear on lubricated parts.

The other type of dust control on drills is a type of local exhaust ventilation. Reverse air-flow through the drill steel withdraws some of the dust and a collar around the drill bit with ductwork and a fan to remove the dust. These perform better than the wet systems described above: drill bits last longer and the drilling rate is higher. However, these methods are more expensive and require more maintenance.

Other controls that provide protection are cabs with filtered and possibly air-conditioned air supply for drill operators, bulldozer operators and vehicle drivers. The appropriate respirator, correctly fitted, may be used for worker protection as a temporary solution or if all others prove to be ineffective.

Silica exposure also occurs at stone quarries that must cut the stone to specified dimensions. The most common contemporary method of cutting stone is with the use of a channel burner fuelled by diesel fuel and compressed air. This results in some silica particulate. The most significant problem with channel burners is the noise: when the burner is first ignited and when it emerges from a cut, sound level can exceed 120 dBA. Even when it is immersed in a cut, noise is around 115 dBA. An alternative method of cutting stone is to use very high-pressure water.

Often attached to or nearby a stone quarry is a mill where pieces are sculpted into a more finished product. Unless there is very good local exhaust ventilation, exposure to silica can be high because vibrating and rotating hand tools are used to shape the stone into the desired form.

Respirable coal mine dust is a hazard in underground and surface coal mines and in coal-processing facilities. It is a mixed dust, consisting mostly of coal, but can also include silica, clay, limestone and other mineral dusts. The composition of coal mine dust varies with the coal seam, the composition of the surrounding strata and mining methods. Coal mine dust is generated by blasting, drilling, cutting and transporting coal.

More dust is generated with mechanized mining than with manual methods, and some methods of mechanized mining produce more dust than others. Cutting machines that remove coal with rotating drums studded with picks are the principal sources of dust in mechanized mining operations. These include so-called continuous miners and longwall mining machines. Longwall mining machines usually produce larger amounts of dust than do other methods of mining. Dust dispersion can also occur with the movement of shields in longwall mining and with the transfer of coal from a vehicle or conveyor belt to some other means of transport.

Coal mine dust causes coal workers’ pneumoconiosis (CWP) and contributes to the occurrence of chronic airways disease such as chronic bronchitis and emphysema. Coal of high rank (e.g., high carbon content such as anthracite) is associated with a higher risk of CWP. There are some rheumatoid-like reactions to coal mine dust as well.

The generation of coal mine dust can be reduced by changes in coal cutting techniques and its dispersion can be controlled with the use of adequate ventilation and water sprays. If the speed of rotation of cutting drums is reduced and the tram speed (the speed with which the drum advances into the coal seam) is increased, dust generation can be reduced without losses in productivity. In longwall mining, dust generation can be reduced by cutting coal in one pass (rather than two) across the face and tramming back without cutting or by a clean-up cut. Dust dispersion on longwall sections can be reduced with homotropal mining (i.e., the chain-conveyor at the face, the cutter head and the air all travelling in the same direction). A novel method of cutting coal, using an eccentric cutter head that continuously cuts perpendicular to the grain of a deposit, seems to generate less dust than the conventional circular cutting head.

Adequate mechanical ventilation flowing first over a mining crew and then to and across the mining face can reduce exposure. Auxiliary local ventilation at the working face, using a fan with ductwork and scrubber, can also reduce exposure by providing local exhaust ventilation.

Water sprays, strategically placed close to the cutterhead and forcing dust away from the miner and towards the face, also assist in reducing exposure. Surfactants provide some benefit in reducing the concentration of coal dust.

Asbestos exposure occurs among asbestos miners and in other mines where asbestos is found in the ore. Among miners throughout the world, exposure to asbestos has elevated the risk of lung cancer and of mesothelioma. It has also elevated the risk of asbestosis (another pneumoconiosis) and of airways disease.

Diesel engine exhaust is a complex mixture of gases, vapours and particulate matter. The most hazardous gases are carbon monoxide, nitrogen oxide, nitrogen dioxide and sulphur dioxide. There are many volatile organic compounds (VOCs), such as aldehydes and unburned hydrocarbons, polycyclic aromatic hydrocarbons (PAHs) and nitro-PAH compounds (N-PAHs). PAH and N-PAH compounds are also adsorbed onto diesel particulate matter. Nitrogen oxides, sulphur dioxide and aldehydes are all acute respiratory irritants. Many of the PAH and N-PAH compounds are carcinogenic.

Diesel particulate matter consists of small diameter (<1 mm in diameter) carbon particles that are condensed from the exhaust fume and often aggregate in air in clumps or strings. These particles are all respirable. Diesel particulate matter and other particles of similar size are carcinogenic in laboratory animals and appear to increase the risk of lung cancer in exposed workers at concentrations above about 0.1 mg/m3. Miners in underground mines experience exposure to diesel particulate matter at significantly higher levels. The International Agency for Research on Cancer (IARC) considers diesel particulate matter to be a probable carcinogen.

The generation of diesel exhaust can be reduced by engine design and with high-quality, clean and low-sulphur fuel. De-rated engines and fuel with a low cetane number and low sulphur content produce less particulate matter. Use of low sulphur fuel reduces the generation of SO2 and of particulate matter. Filters are effective and feasible and can remove more than 90% of diesel particulate matter from the exhaust stream. Filters are available for engines without scrubbers and for engines with either water or dry scrubbers. Carbon monoxide can be significantly reduced with a catalytic converter. Nitrogen oxides form whenever nitrogen and oxygen are under conditions of high pressure and temperature (i.e., inside the diesel cylinder) and, consequently, they are more difficult to eliminate.

The concentration of dispersed diesel particulate matter can be reduced in an underground mine by adequate mechanical ventilation and restrictions on the use of diesel equipment. Any diesel powered vehicle or other machine will require a minimum amount of ventilation to dilute and remove the exhaust products. The amount of ventilation depends on the size of the engine and its uses. If more than one diesel powered piece of equipment is operating in one air course, ventilation will have to be increased to dilute and remove the exhaust.

Diesel powered equipment may increase the risk of fire or explosion since it emits a hot exhaust, with flame and sparks, and its high surface temperatures may ignite any accumulated coal dust or other combustible material. Surface temperature of diesel engines have to be kept below 305 °F (150 °C) in coal mines in order to prevent the combustion of coal. Flame and sparks from the exhaust can be controlled by a scrubber to prevent ignition of coal dust and of methane.

Gases and Vapours

Table 1 lists gases commonly found in mines. The most important naturally occurring gases are methane and hydrogen sulphide in coal mines and radon in uranium and other mines. Oxygen deficiency is possible in either. Methane is combustible. Most coal mine explosions result from ignitions of methane and are often followed by more violent explosions caused by coal dust that has been suspended by the shock of the original explosion. Throughout the history of coal mining, fires and explosions have been the principal cause of death of thousands of miners. Risk of explosion can be reduced by diluting methane to below its lower explosive limit and by prohibiting potential ignition sources in the face areas, where the concentration is usually the highest. Dusting the mine ribs (wall), floor and ceiling with incombustible limestone (or other silica-free incombustible rock dust) helps to prevent dust explosions; if dust suspended by the shock of a methane explosion is not combustible, a secondary explosion will not occur.

Table 1. Common names and health effects of hazardous gases occurring in coal mines

Gas

Common name

Health effects

Methane (CH4)

Fire damp

Flammable, explosive; simple asphyxiation

Carbon monoxide (CO)

White damp

Chemical asphyxiation

Hydrogen sulphide (H2S)

Stink damp

Eye, nose, throat irritation; acute respiratory depression

Oxygen deficiency

Black damp

Anoxia

Blasting by-products

After damp

Respiratory irritants

Diesel engine exhaust

Same

Respiratory irritant; lung cancer

 

Radon is a naturally occurring radioactive gas that has been found in uranium mines, tin mines and some other mines. It has not been found in coal mines. The primary hazard associated with radon is its being a source of ionizing radiation, which is discussed below.

Other gaseous hazards include respiratory irritants found in diesel engine exhaust and blasting by-products. Carbon monoxide is found not only in engine exhaust but also as a result of mine fires. During mine fires, CO can reach not only lethal concentrations but also can become an explosion hazard.

Nitrogen oxides (NOx), primarily NO and NO2, are formed by diesel engines and as a by-product of blasting. In engines, NOx are formed as an inherent by-product of putting air, 79% of which is nitrogen and 20% of which is oxygen, under conditions of high temperature and pressure, the very conditions necessary to the functioning of a diesel engine. The production of NOx can be reduced to some extent by keeping the engine as cool as possible and by increasing ventilation to dilute and remove the exhaust.

NOx is also a blasting by-product. During blasting, miners are removed from an area where blasting will occur. The conventional practice to avoid excessive exposure to nitrogen oxides, dust and other results of blasting is to wait until mine ventilation removes a sufficient amount of blasting by-products from the mine before re-entering the area in an intake airway.

Oxygen deficiency can occur in many ways. Oxygen can be displaced by some other gas, such as methane, or it may be consumed either by combustion or by microbes in an air space with no ventilation.

There is a variety of other airborne hazards to which particular groups of miners are exposed. Exposure to mercury vapour, and thus risk of mercury poisoning, is a hazard among gold miners and millers and among mercury miners. Exposure to arsenic, and risk of lung cancer, occurs among gold miners and lead miners. Exposure to nickel, and thus to risk of lung cancer and skin allergies, occurs among nickel miners.

Some plastics are finding use in mines also. These include urea-formaldehyde and polyurethane foams, both of which are plastics made in-place. They are used to plug up holes and improve ventilation and to provide a better anchor for roof supports. Formaldehyde and isocyanates, two starting materials for these two foams, are respiratory irritants and both can cause allergic sensitization making it nearly impossible for sensitized miners to work around either ingredient. Formaldehyde is a human carcinogen (IARC Group 1).

Physical Hazards

Noise is ubiquitous in mining. It is generated by powerful machines, fans, blasting and transportation of the ore. The underground mine usually has limited space and thus creates a reverberant field. Noise exposure is greater than if the same sources were in a more open environment.

Exposure to noise can be reduced by using conventional means of noise control on mining machinery. Transmissions can be quieted, engines can be muffled better, and hydraulic machinery can be quieted as well. Chutes can be insulated or lined with sound-absorbing materials. Hearing protectors combined with regular audiometric testing is often necessary to preserve miners’ hearing.

Ionizing radiation is a hazard in the mining industry. Radon can be liberated from stone while it is loosened by blasting, but it may also enter a mine through underground streams. It is a gas and therefore it is airborne. Radon and its decay products emit ionizing radiation, some of which have enough energy to produce cancer cells in the lung. As a result, death rates from lung cancer among uranium miners are elevated. For miners who smoke, the death rate is very much higher.

Heat is a hazard for both underground and surface miners. In underground mines, the principal source of heat is from the rock itself. The temperature of the rock goes up about 1 °C for every 100 m in depth. Other sources of heat stress include the amount of physical activity workers are doing, the amount of air circulated, the ambient air temperature and humidity and the heat generated by mining equipment, principally diesel powered equipment. Very deep mines (deeper than 1,000 m) can pose significant heat problems, with the temperature of mine ribs about 40 °C. For surface workers, physical activity, the proximity to hot engines, air temperature, humidity and sunlight are the principal sources of heat.

Reduction of heat stress can be accomplished by cooling high temperature machinery, limiting physical activity and providing adequate amounts of potable water, shelter from the sun and adequate ventilation. For surface machinery, air-conditioned cabs can protect the equipment operator. At deep mines in South Africa, for example, underground air-conditioning units are used to provide some relief, and first aid supplies are available to deal with heat stress.

Many mines operate at high altitudes (e.g., greater than 4,600 m), and because of this, miners may experience altitude sickness. This can be aggravated if they travel back and forth between a mine at a high altitude and a more normal atmospheric pressure.

 

Back

Page 2 of 2

" DISCLAIMER: The ILO does not take responsibility for content presented on this web portal that is presented in any language other than English, which is the language used for the initial production and peer-review of original content. Certain statistics have not been updated since the production of the 4th edition of the Encyclopaedia (1998)."

Contents

Preface
Part I. The Body
Part II. Health Care
Part III. Management & Policy
Part IV. Tools and Approaches
Part V. Psychosocial and Organizational Factors
Part VI. General Hazards
Part VII. The Environment
Part VIII. Accidents and Safety Management
Part IX. Chemicals
Part X. Industries Based on Biological Resources
Part XI. Industries Based on Natural Resources
Part XII. Chemical Industries
Part XIII. Manufacturing Industries
Part XIV. Textile and Apparel Industries
Part XV. Transport Industries
Part XVI. Construction
Part XVII. Services and Trade
Part XVIII. Guides