For an introduction to Mining, or to refresh your understanding of
the basic industry rules of thumb, begin here with
Mining 101
The aim of this section is to create the definitive source of practical mining
information. Beginning with the two sections below, Mining Explained will be
regularly updated with contributions from companies, institutions and industry
experts.
Readers of all of our publications are encouraged to both contribute and refer
to this expanding guide by contacting
webmaster@mining-journal.com
Drilling & Explosives
Percussion drilling is required for drilling blastholes in rocks with a hardness
of 4 or greater on Mohs' scale. These are mainly the volcanic rocks. Rotary
drilling is satisfactory for softer rocks, mainly sedimentary.
Button bits normally give higher penetration rates but are more prone to
deviation in long holes than cross bits.
For long-hole drilling, rod life, thread reversing and redistributing the rods
in the drill string considerably prolong wear life.
Typical powder consumption in hard rock:
Shaft Sinking -
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1.25 kg/t (2.5 lb/s.ton)
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Drifting -
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0.9 kg/t (1.8 lb/s.ton)
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Raising -
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0.75 kg/t (1.5 lb/s.ton)
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Shrink Stope -
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0.25 kg/t (0.5 lb/s.ton)
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Cut and Fill O/H -
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0.25 kg/t (0.5 lb/s.ton)
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Bulk Mining -
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0.2 kg/t (0.4 lb/s.ton)
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Block Cave U/C -
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0.05 kg/t (0.1 lb/s.ton)
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Open Pit Cut -
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0.45 kg/t (0.9 lb/s.ton)
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Open Pit Bench -
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0.3 kg/t (0.6 lb/s.ton)
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Blasting strength is a direct function of density, other things being equal.
Typical explosives for dry ground (ANFO) may have a blasthole density (specific
gravity) of 0.8 to 1.3, while for wet ground (slurry or emulsion) it varies
from 1.1 to 1.3. Developments in explosive technology make it possible to
choose any density desired, within the given ranges.
For hard-rock open-pit mines, or backfill rock quarries, the burden between
rows can vary from 25 to 40 blasthole diameters. Spacing between holes in a row
can vary between 25 and 80 blasthole diameters.
To obtain optimum fragmentation and minimum overbreak for stripping hard rock
open pits or quarrying rock fill, the burden should be about 25 times the bench
blasthole diameter for ANFO and about 30 times the blasthole diameter for high
explosives.
The burden required in an open-pit hard-rock operation is about 25 times the
hole diameter for ANFO, and about 30 times for high explosives; and the ANFO
ratio is 30:1 and 35:1 for medium and soft rock, respectively. The spacing is 1
to 1.5 times the burden and the timing is a minimum of 16.4 ms/m (5 ms/ft) of
burden.
When 'smooth wall' blasting techniques are employed underground, the accepted
standard spacing between the trim (perimeter) holes is 15-16 times the hole
diameter and the charge in perimeter holes is 1/3 that of the regular
blastholes. The burden between breast holes and trim holes is 1.25 times the
spacing between trim holes.
Maintenance & Utilisation
In a trackless mine operating round the clock, there should be 0.8 journeyman
mechanic or electrician on the payroll for each major unit of mobile equipment
in the underground fleet.
LHDs at a shallow mine with ramp entry should have a utilisation of 5,000 -
6,000 hours per year.
LHDs in production service should have a useful life of at least 12,000 hours,
including one rebuild at 7,500 hours. A longer life can be presumed from LHDs
at the high end of the market with onboard diagnostics.
Underground haul trucks should have a useful life of 20,000 hours; more if they
are electric (trolley system).
With ramp entry, a satellite workshop is required when the mean mining depth
reaches 200 m below surface. A second one is required at a vertical depth of
400 m.
A main workshop facility underground is required when the average mining depth
reaches 500 m below surface, and should have the capacity to handle 10% of the
underground fleet.
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