Underground mining represents a complicated transport environment.
Far fewer people are involved at the face than are involved diatomaceous earth food grade in kenyain
getting the ore to surface, and payload efficiency is therefore an
important contributor to profitability, Dr Tony Paterson of the
Aluminium Federation of South Africa (Afsa) reports. According to
Paterson, mining skips are required to carry the rotary drum screening equipment manufacturers indiaore to the
surface. “Generally, these have a capacity of around 26 tons
and a dead mass of about ten tons. To lift this to the surface, a
wire rope of some 120 tons is required, so about 146 tons is lifted
to gain 26 tons,” jaw crusher for barite crushing brochureshe explains. Paterson says the winder
capacity is a restriction, but a trade-off between skip mass and
payload is possible. “While the skip overall dimensions are
normally restricted by specific shaft steelwork, additional payload
could be carried in length or the height,” he adds.
The aluminium skip offers possible savings of about 30% to 40% of
the steel equivalent.
“This equates to about three to four tons and, as this
represents an increased payload of some 15%, it is
attractive.” According to Paterson, bolted aluminium man
cages have been in use in South African mines since the 1920s.
However, while alu- minium skips had been in use in Canada and the
US for many years it was not until 1986 that they were introduced
into South Africa. The original designs were based on bolted
construction and operated at Vaal Reefs Number 9 shaft – an
experiment which proved to be economically successful. By 1997,
sufficient confidence had been built to go to the next step, that
of all-welded construction, a method that was estimated to yield an
additional one ton payload over the previous bolted fabrication.
The base design, Paterson reveals, was in steel converted to
aluminium. “The Aluminium Federation was involved in
assessing the design from an aluminium point of view and proposed
method-ologies through which the anticipated gains could be
achieved,” Paterson recalls. The main differences between
aluminium and steel are: n Aluminium is one-third of the mass of
mild steel, which helps with payload.
n Aluminium has a rate of corrosion of around 1% that of steel and
10% that of zinc.
n Aluminium’s Young’s modulus, at around 70 GPa, is
some three times lower than mild steel (which implies greater
deflection under the same stress – in practice a structure
with the same stiffness weighs half that of a steel
structure).
n Auminium also has a different reaction to welding, in that
welding tends to reduce the stress capacity of joints by about 40%.
This is not the case with mild steel, and aluminium requires more
care in weld joint selection and positioning.
Paterson says the last aspect was the most important consideration.
“The effects of welding aluminium required a total review of
the design and fabrication practices normally used with steel.
“While, practically, the use of some bolted connections in
some areas would have made engineering sense, the client was keen
to test welding,” he notes.
Joint work with the SAIW met with a successful project. Particular
attention was paid to the need to increase the value of the end
product. This revealed that the fabrication procedures were more
expensive than would have been the case had the same shape been
produced without attention to welding detail at highly stressed
corners.
“Subsequent to the Vaal Reefs development of welded skips, a
number of other mines and fabrica-tors took an active interest and
many more welded alu-minium skips have been introduced into
underground mines such as Union, Rustenburg and other
platinum-mines,” Paterson says.
He says that one of the reasons for caution was due to concerns
about the possibilities of thermit reactions, an active fear
resulting from UK postulated experiences of the 1950s. “The
thermit reaction is real, but overstated in impact,” Paterson
notes. “If freshly-abraded aluminium is struck lightly by
slightly rusty steel at an angle of around 45 degrees with the
right force, an exothermic reaction is possible. “That
exothermic reaction is sufficiently intense to probably ignite
methane of a concentration of around 8%, with a decreasing
probability to zero down to 4,5% and towards 11%. “In
context, mines are evacuated at about 1,5%. “If the gas
ignites in the presence of a carrying medium, such as the correct
concentration of suspended coal dust, that medium may ignite and
carry the reaction further,” Paterson explains. He says that
from the mid- eighties aluminium was used to an increasing extent
on offshore oilrigs in the North Sea, mainly to limit the top side
mass and to prevent overturning. “Before this was done,
extensive work was undertaken on understanding the thermit reaction
and on risk analysis. “Following a similar method-ology as
that used by the offshore sector, risk assessment profiles for
aluminium use underground have been developed,” notes
Paterson, adding that these are currently under consideration by
the Department of Minerals and Energy.
“Reports to date suggest a favourable view of the
risk-assessment approach and more extensive use of aluminium in
underground transport is anticipated,” he concludes.