Transcript KillaraHighSchool_CareersMorning_070307

A review of heat issues in
underground metalliferous mines
Tom Payne
Rudrajit Mitra
Overview
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Introduction
Major sources of underground heat
Underground heat issues
Heat Controls
Conclusions
Introduction
• Heat and humidity encountered in tropical
locations and underground mines
• Both natural and ‘man-made’ sources
contribute to the heat load
• Some heat sources better managed than
others
• Management of heat issues required through
better engineering and work practice controls
Introduction
• Major impediment for deep mining –
ambient high stress and temperature
• Humans maintain reasonably constant
body temperature
• Preliminary study to review heat issues in
underground Australian mines
Major Sources of Underground Heat
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Geothermal gradient
Autocompression
Other natural sources
Explosives
Mechanical processes
Geothermal Gradient
• Within 50m of earth’s surface maintain
average air temperature
• Between 50 – 100m – gradient is variable
• > 100m – increases with depth
• Rate of increase varies with tectonic
setting and rock thermal properties
• Typical geothermal gradient of the upper
crust: 15oC/km – 40oC/km
Geothermal Gradient
Australian average surface rock and air temperature
(Source: Australian Bureau of Meteorology)
Geothermal Gradient
Hot rock temperature in Australia at 5 km depth
(Source: Geoscience Australia)
Geothermal Gradient
Geothermal gradients at several Australian mining locations
(Source: AMC Consultants, 2003)
Autocompression
• Air travelling down to workings undergo
compression
• Temperature increases up to 10oC per km of
vertical depth
• Will become a significant source of underground
heat
Other Natural Sources
• Groundwater
• Service water
• Oxidation (negligible)
Explosives
• As much as 90 – 95% of energy released
during blasting
heat
• Some of the heat produced carried away
with the blasting fumes out of the
development end
• Remaining in the broken rock which may
be released prior to and during rock
removal
Mechanical Process
• Use of electricity and other mechanical
processes add heat
• Diesel engine efficiency estimated at 33%
• Remaining released as heat
• Loaders, trucks, jumbos, explosive
transport vehicles and 4 wheel drives – all
use diesel powered engine
Example of heat source
Mt Isa Mines – Underground heat contributors
Underground Heat Issues
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Thermal stress in workers
Heat stroke
Heat exhaustion
Dehydration and rehydration
Acclimatisation
Related issues
Thermal stress in workers
• Exposure to hot conditions – unhealthy
and unproductive
• Persons working in hot, humid work sites
tend to be inefficient
• Dexterity and coordination adversely
affected by heat strain
Heat Stroke
• Often fatal
• Caused by more severe or prolonged
elevations in body temperature
• Incidence of heat stroke 10 times higher
between 28.9 – 31.1oC than between 26.7
– 28.8oC
Heat Exhaustion
• Caused by the inability of the circulatory
system to simultaneously supply sufficient
blood flow to the skin to achieve adequate
heat loss and to supply the vital organs
and exercising skeletal muscles
Heat Exhaustion
• One year study (2000) observed 106
cases
– Seasonal occurrence with a spike of 147
cases per million man-hours during February
(Similar in US)
– Higher incidence at depth – 3.17:1 for depths
> 1200m
– Median wet bulb temperature – 29.8oC; less
than 5% below 25oC wet bulb
Dehydration & Rehydration
• Dehydration
– 1 – 2% of body weight: 6 – 7% reduction in
physical work rate
– 3 – 4% of body weight: 22 – 50% reduction in
physical work rate
– Mental performance decreases at 2%
dehydration and after that proportional to the
degree of further dehydration
• Underground mine workers typically only
replace half of water they lose as sweat
Acclimatisation
• Repeated or continuous exposure to hot
conditions induces physiological
adaptations, which reduced strain caused
by hot underground conditions
Acclimatisation
• 1998 study on acclimatisation for working in
heat:
– Reduction in heart rate from 153 to 127 beats per
minute
– Core temperature when reduces from 38.8oC to
38.1oC
– Sweat becomes more dilute with sodium
concentrations down by 29%
– More rapid onset of sweating, up by 15%
– Blood volume increases by 21%
Related issues
• Safety
• Productivity
• Morale
• Cost
Heat Controls
• Ventilation
• Refrigeration
• Localised cooling
• Avoiding/reducing heat problems
Ventilation
• Presently strategy for heat control –
ventilation, cooling and refrigeration
• Extremely expensive approach
• Limited air cooling effect on increasing the
air velocity once temperature > 32oC wet
bulb
• Depths > 1000m the ability to remove heat
and cool the underground workforce
reduces rapidly
Refrigeration
• General rule of thumb: any workplace in
mine below critical depth requires external
cooling
• Critical depth: depth below surface at
which air will exceed the underground
target wet bulb temperature solely through
autocompression without taking into
account any other heat loads at all
Localised cooling
• Air-conditioned cabins on equipment and
cooling vests – reduce heat load on
individual workers
• Localised underground refrigeration
considered ineffective
Avoiding/reducing heat problems
• Mining excavation as large as required
• Ventilation on-demand system shown to
result in significant cost savings
• Industry needs to consider how equipment
can be used at optimal efficiency and with
minimal production of waste heat and
pollutants
Avoiding/reducing heat problems
• Education of underground workers
• Discussion at pre-shift meetings
• Formal protocols put in place
accompanied by some form of testing
regime
Conclusion
• Preliminary study to review heat issues in
underground Australian mines
• Best solution to control underground heat
– mixture of available technologies
• Coupled with a strong campaign of worker
education
• Successfully implemented at Mt Isa Mines
Thank You
Dr Rudrajit Mitra
School of Mining Engineering,
The University of New South Wales,
Ph: +61 (02) 9385 5161
Email: [email protected]