INDUSTRY EQUIPMENT: MINING
induced, for example, by a dozer moving on
stockpiled material,” says Fennell.
“Nevertheless, significant material
degradation was measured during the
simulation, leaving one to assume that the
material degradation that takes place during
stockpiling results in an even higher increase
of the fine fractions,” he adds.
Keeping in mind that even this level of
‘soft’ rehandling causes significant material
degradation, it is important to minimise the
number of rehandling steps, each of which
contribute to processing costs. This is
where Wirtgen surface miners offer a two-
fold advantage,” says Fennell. “Material
mined with a surface miner has not been
blasted, but cut out of the ground by the
rotating cutting drum, which means that
the level of fines is already low; and as the
material is loaded for transport, rehandling
is kept to a minimum, contributing further
to a low level of fines. With a view of the
complete mining operation, it is clear
that a surface miner simplifies mining
operations and reduces the number of
process steps tremendously, resulting in
immediate cost savings.”
There is no doubt that the Wirtgen
surface miner is a good long-term
investment, especially for coal mines. The
challenge is to change the mindset of an
industry set in their ways.
optimising the process for target size
material, enabling most of the material to
be processed in the cheaper coarse circuit
of the processing plant. Reducing crushing
costs was also an objective.
“Once again, the test proved the suitability
of the Wirtgen surface miner for this type
of operation,” says Fennell, explaining, “The
machine delivered coal with a lower fines
level and more target size material than the
conventional dozer rip-and-stack method.
In fact, out of the 1 500t/h of coal delivered
from the dozer to the processing plant,
225t/h were fines smaller than 2mm, but
with the surface miner, this figure is closer
to 185t/h. This means that 22% less coal had
to be washed in the fines circuit and could
be washed in the cheaper coarse circuit. In
terms of fines smaller than 1mm, by using
the Wirtgen surface miner, the plant could
process 33% less fine material.”
The Wirtgen surface miner delivers
more than 70% target size coal (2-40 mm),
while the dozer ranges at less than 58%.
Savings are also enjoyed in the crushing
stage, where only 17% of the material
from the surface miner has to be crushed,
as opposed to more than 26% when
processing dozer coal, resulting in more
fines that have to be processed.
In another trial location with sedimentary
ore deposit mined using surface miners
as well as the conventional drill-and-blast
method, the surface miner was able to
continually feed material with a < 1mm fines
level as low as 15% to the processing plant.
The normal plant feed (includes material of
drill-and-blast and surface miner operation)
contained 25% and more fines < 1mm.
The nature of South Africa’s coal seams make it difficult for Wirtgen’s surface
miner to gain traction.
Rehandling contributes significantly to
material degradation occurring during the
mining process. Simulating this process,
Wirtgen conducted a study to establish the
amount of material degradation that occurs
during rehandling.
For simulation purposes, 80 tons of
material was loaded by a wheel loader
and run through a screening plant several
times. With every throughput, the amount
of fine material increased significantly and
during five test cycles in coal, the amount
of material < 4mm increased from 19% to
26%, representing an increase of 34%. A
similar result was found with sedimentary
ore in that the fine fraction increased by
24% during five test cycles.
“None of the steps in the simulation
(loading, transport to the screen deck,
sizing on the screen deck itself) involved the
high material stress levels that would be
www.equipmentandhire.co.za
Impact of rehandling
South Africa has been slow to invest in large surface mining machines even
though the benefits could outweigh the costs.
JANUARY/FEBRUARY 2020
23