In 2011, for instance, there were five fatalities in underground U.S. metal mines, according to data published by the Mine Safety and Health Administration (MSHA), each of which was an individual event—in other words, only one person was involved, and they were isolated inci-dents. Major mine rescues, as occurred in Chile the previous year, are truly few and far between and, it has to be remem-bered, instances where large groups of miners in underground metal mines are affected by some incident are so rare as to be positively remarkable.

Indeed, when compared to the coal-mining sector, metal mining is intrinsi-cally less prone to fatal accidents, although the potential for serious injury remains. Unlike coal, the risk of explo-sion underground is remote although, as has been shown in the past, nature can throw up some unexpected surprises. The realization (many years ago) that methane was present in deep potash workings in Britain brought about a swift re-appraisal of operating practices there. While water is another major hazard, yet again major incidents have been rare. The Mufulira inflow in 1970, in which an estimated 1 million metric tons of tail-ings slimes flowed into the mine, was a one-off in terms of the circumstances and the loss of life. In South Africa, West Driefontein was saved after the water inrush there in 1968, and remains one of the country’s leading gold producers. Canada’s potash mines have had their own water problems in the past, although potash is one commodity that is amen-able to solution recovery, as well as con-ventional mining.

However, the most dangerous situa-tion underground, without question, is fire, and although the potential causes of fires may have changed as new mining technologies have been introduced, the critical scenario remains. Probably the last time a serious timber-fueled fire occurred in the U.S. was the Homestake incident in February 2001, when fire started in a worked-out, timber-supported area of the mine. With few mines now relying on heavy timbering, today it is much more likely that fire will be caused by either electrical or diesel-powered equipment—indeed, MSHA and NIOSH statistics prove this to be the case.

Whatever the cause, fire in a confined space is a terrifying experience, not only from the threat of the fire itself, but also from the smoke and fumes generated; toxic gas that can easily, and quickly, prove fatal to anyone who is unprotected. As with day-to-day house fires in residen-tial communities, it is invariably the smoke that kills: the fatalities at the Sunshine mine in 1972 resulted from smoke or carbon monoxide inhalation.

Recent coal-mine tragedies have pro-vided the spur for mine safety organiza-tions world-wide to place an increasing focus on fire risk as a significant poten-tial hazard throughout the industry as a whole. In response, mining companies are investigating new ways of providing safe havens for miners in the event of fire, while a growing number of suppliers are now offering equipment that can help people escape by themselves, and give them shelter, if the worst comes to the worst.

Understanding the Mechanics of Fires
Diesel fuel and hydraulic oil may have revolutionized underground mining tech-nology, but their combustibility remains a well-recognized hazard. The replacement of the early, mineral-based hydraulic flu-ids used in the coal-mining industry by emulsions and water-based materials was not mere chance: it was a clear response to the potential danger they posed.

Hard rock mining differs in a number of respects, not least of which is that the equipment that uses flammable fluids is invariably mobile. LHDs, drill rigs, mine trucks and service vehicles are used for their flexibility, in contrast to an essen-tially static longwall face set-up. There are also significant differences in terms of the volumes of fluids needed. The fluid capacity of a longwall installation represents a major fuel source should a fire ever occur, although an individual mine truck can also carry enough diesel and hydraulic fluid to make life more than uncomfortable for any fire crew hav-ing to deal with it. As an example, a 50 t-capacity mine truck from one of the leading manufactures carries an 844-liter (220 gallon) fuel tank and 238 liters (63 gallons) of hydraulic fluid.

Aside from these, other potential sources of smoke and fumes include electrical failures, such as overheated fan or pump motors, jammed belt drives on equipment, and conveyor belts. The use of modern materials has reduced the flammability potential of belt conveyors, yet they still represent a significant risk in that the belt can continue to travel considerable distances when alight.

Either smoldering or open fires will produce both smoke and toxic gas, with the ability to spread along haulages and through raise systems at a truly alarming rate. In addition, there is never any guar-antee that combustion products will behave in a predictable way, even in rela-tion to clearly defined ventilation flows; smoke and gas have been shown to flow back against the airstream if other condi-tions allow it.

A fire needs three components to sur-vive: fuel, heat and oxygen. A hot exhaust system, a leaking hydraulic hose and a good ventilation flow provide all of these without any difficulty, which is one good reason why today’s mobile equipment OEMs invariably fit extinguisher systems on their machines as standard.

Yet the risk remains, and the next part of this article looks at some of the equip-ment and systems that can be put in place to protect and rescue people in the event that underground fires get out of control.

Security and Shelter
While the initial priority for any under-ground fire incident is to put it out as quickly as possible, the reality is that peo-ple who are in the immediate vicinity when it starts may not be the best equipped to react, or may have no opportunity to do so because of the intensity. In this case, and the decision-making time may be a matter of seconds, the absolute priority is to ensure their safety, and that of anyone else in the mine at the time. Since smoke and toxic gas can overcome a person faster than they can try to escape from it, the obvious—and increasingly widely used— solution is to bring safety to the miner, rather than have the miner try to reach a place of safety on his or her own.

A number of companies have devel-oped mine refuges, initially for use in coal but with a rapidly accelerating take-up within hard rock mining as well. For example, the Australian company, MineARC, offers both mobile and perma-nent mine refuges that are designed to keep trapped miners alive until they can be rescued, and reports in its latest newsletter that an increasing number of mines are now installing and equipping permanent chambers that can offer secu-rity for larger numbers of people than was previously the case

. MineARC’s moveable hard rock mine refuge chambers are available in four, eight, 12, 16, 20 or 26-person capaci-ties, with the company noting it can also design custom models to meet individual needs. Its refuges are built from 5-mm (!-in.) steel plate, and are equipped with three sources of breathable air sup-ply, initially from the mine air supply-system. Should this connection be lost or contaminated, medical-grade oxygen cylinders and oxygen candles continue to supply air for a minimum of 36 hours.

Fire is not the only potential hazard for which personal gas detection may be needed. Chemicals can also
pose a risk, and MSA’s Altair 4X detector can be equipped with a sampling probe to check confined
spaces or containers.

Air conditioning maintains a comfort-able temperature inside the chamber, with carbon monoxide and carbon dioxide being scrubbed from the air inside it by active chemicals and MineARC’s patent-ed electrical scrubbing system. Digital gas monitoring constantly alerts occu-pants to the levels of dangerous gases in the chamber, which also has a back-up battery system that can power its internal life-support systems for a minimum of 36 hours should the mine’s main power sup-ply be lost.

MineARC also reports it has recently seen a large rise in the number of mines world-wide that are installing its perma-nent hard rock refuge chamber technolo-gy, with seven units having been com-missioned in the past few months in Australia, Turkey and the Philippines. These refuges have been built in both new and existing excavations, and are often doubling as lunch rooms, the com-pany adds. Permanent refuge chamber offer a practical alternative to standard ‘portable’ refuge units, and have advan-tages where refuges do not need to be moved regularly, or where large numbers of miners might need to find safety.

In this context, MineARC’s permanent refuge technology can sustain up to 150 or more people in a single confined space. Recent installations include two 80-person lunch rooms at Argyle Diamond Mines’ new underground opera-tion in Australia, while in Turkey, Inmet Mining has installed two 30-person sys-tems at is Cayeli base-metals mine. Other companies with MineARC refuges in-clude BHP Billiton, Newmont, Xstrata, Barrick Gold, Newcrest Mining and Freeport McMoRan Copper & Gold.

Other suppliers that offer mine refuge systems include Rana Mine Refuge Systems in Canada, and U.S.-based Strata Safety Products and Mining Health and Safety Solutions. Rana’s Tommyknocker is a portable, completely self-sustaining refuge chamber that maintains a controlled positive pressure level inside to keep dangerous gases from entering. Available in various sizes to take up to 20 people, its features include a double door airlock entry system, emer-gency battery back-up power, seating and bench storage space, and a self-con-tained toilet system, together with options such as air-conditioning, a gas monitor, a carbon monoxide scrubber and other custom add-ons.

The Tommyknocker uses Rana’s Re-fuge One Breathable Air Center to main-tain safe breathable air levels for emer-gency durations of up to 96 hours. These centers remove carbon dioxide from the sealed chamber air and replenish oxygen to maintain safe breathable air levels. They run independently on mine electrical power, and have an emergency battery back-up. The number of centers required and the quantities of batteries, chemical and oxygen depend on the size of the refuge station, the number of people it is designed to accommodate, and the time-span for which protection is designed.

Designed for handling with a regular industrial forklift, the standard Tommy-knocker measures 4.3 m (14 ft.) by 2.2 m (88 in.) and weighs nearly 2 mt. The personnel compartment measures 3 m (10 ft.) long, with the double-door air-lock taking up the remaining length. This size will accommodate ten people, Rana says, with greater capacity being gained by adding 610-mm (2-ft) sec-tions to the length.

Keeping Watch on the Air
Providing refuges for use in emergencies is just one aspect of evolving safety practice. There is also an increasing reliance on indi-viduals being able to monitor the air around them, using small belt- or harness-worn detectors that can give an immediate warning should toxic gases appear.

As the German gas-detection equip-ment manufacturer, Dräger, points out, gases that occur naturally or are generat-ed during mining remain a major hazard and continue to place miners at risk of poisoning through toxic gas exposure, or suffocation because of oxygen deficiency. Since toxic or combustible gases can be colorless and odorless, and reduced oxygen levels are invisible, measurement and environmental control are needed to make sure that these potential dangers are recognized in time.

Dräger’s product range includes a comprehensive line of gas-detection pro-ducts for personal air monitoring and workplace exposure monitoring, both in day-to-day mining operations and during mine rescues. Fitted with the new CatEx 125 PR sensor, which is three-to-five times more resistant to silicone and H 2 S poisoning than other portable gas detec-tors, its X-am 5000 is perfectly suited for mining applications, the company says. The unit’s different sensors can be cus-tomized, allowing this one-to-five-gas detector to measure combustible gases and vapors as well as oxygen and harmful concentrations of CO, H 2 S, CO 2 , HCN, NH 3 , NO 2 and SO 2 that are common to hard-rock mining.

Dräger says it recently developed a new nitrogen dioxide (NO 2 ) sensor to improve the ambient air monitoring of potential diesel emission contaminants. The sensor is able to measure NO 2 at very low concentrations, starting at 0.04 ppm, while its very low cross-sensitivity to H 2 S, SO 2 , NO and CO, which are often found in mines and diesel emis-sions, permits the selective measure-ment of NO 2

. Water- and dust-resistant, the X-am 5000 detector remains fully functional even after falling into water, Dräger adds. It can be used with either standard alka-line or rechargeable NiMH batteries, and has an energy-saving function that allows the normal operation time of 12 hours-plus to be extended to more than 40 hours.

The X-am 5000 is equipped as stan-dard with a data logger for more than 1,000 hours of data storage. The data can be transmitted via infrared interface to a PC and analyzed using Dräger’s GasVision software, which can also be used to customize reporting.

Another gas-monitoring system suppli-er, U.S.-based MSA, introduced its Altair 4X personal multi-gas detector in mid-2010. The company claims a number of advantages for the instrument, including a four-year sensor life, faster response time, increased stability, and less than 60-sec-onds calibration time. The Altair 4X uses MSA’s XCell sensors for combustible gas, O 2 , H 2 S and CO, and has optional Motion-Alert and InstantAlert features.

According to MSA, the instrument’s other major benefits include its ability to withstand a 6-m (20-ft) drop, its dust and water resistance, the option of a phosphorescent cover that makes it easi-er to locate in the dark, and the use of big buttons that can be operated even when the holder is wearing work gloves. Meanwhile, the MotionAlert feature is designed to react in the event that the user becomes incapacitated (‘man down’), and sounds an alarm after 30 seconds without movement, something that could be vital when working in con-fined spaces, the company suggests. InstantAlert, on the other hand, involves the user pressing a button on the instru-ment to alert other people nearby to a sudden hazard.

Dräger’s X-arm 5000 is small and light enough to be worn all of the time, and is capable of measuring
concentrations of up to five toxic gases.

MSA’s gas-detector range also in-cludes the Solaris model, which weighs under 225 g (8 oz) and measures just 115 " 64" 32 mm (4#"2 #"1 ! in.). This is also available in one-, two-, three- and four-gas configurations for O 2 , H 2 S, CO and combustible gas, and has three alarm systems built in: visual (flashing LEDs), audible (a 100 dB-plus horn) and vibrating. Its rechargeable lithium-ion battery has a duration of more than 14 hours, while an alkaline battery-powered version is also avail-able. The instrument is protected by a rubberized armored case, and has an optional powered sampling-pump probe that can be used for remote sampling up to 15 m (50 ft) from the user.

As Safe as Can Be
It is an unfortunate truth that mining will never be injury- or fatality-free, no matter how much effort is put into improving safety. On the other hand, adopting a fatalistic viewpoint provides no solutions either, and there is an industry-wide responsibility to continue reducing the risk even further.

As this article has shown, major progress has been achieved in making gas monitoring more accessible to the individual, and in providing secure re-fuges for people in the event that some-thing goes badly wrong. Here, however, no less than with other technologies used in mining, it is not just a case of relying on each person to make the right deci-sions, and to take the right action, should they be confronted with a hazardous sit-uation. People need to be trained what to do in an emergency, and that training kept up to date, if the systems that are provided are to be fully effective.

Fear and panic can be overwhelming when someone is faced with conditions that are beyond their personal control. Confusion and disorientation mean that actions that are normally simple to per-form can quickly become impossible. As MineARC notes in its early-2012 newsletter, it is now working on a new virtual reality training program that is designed to simulate potential emer-gency situations. This allows people to learn how to operate its hard-rock refuge systems in a risk-free environ-ment, before even entering the mine, the company says.