Mining in focus
determine the best recovery methods,
which will influence the design of the
plant and the required plant equipment.
In the case of quarries, such as dimension
stone, the samples will be used to
determine the physical appearance and
dominant grain orientation, to establish
the best extraction layout direction.
The geological and assay or quality data
are then used to construct a model. In
the case of an opencast mining operation,
a geological- and block model, with the
results of the assays, is constructed. This
will determine the capital footprint and
the most suitable area to make the first
box cut. It will also give an indication of
the amount of material that will have to
be removed as part of the overburden
or as waste. The block model will give
an indication of the tonnages and areal
extent of ore reserves.
The pit shell
The next step would be to determine the
pit shell. This could be done by means
of various geological or mining software,
such as running a Whittle Optimisation
of various scenarios, taking the modifying
factors into consideration. Such an
exercise enables the planners to generate
a mine plan and schedule. The schedules
usually cover short-term, mid-term,
long-term, and life of mine, which can be
adjusted according to changing economic
or demand conditions.
The design and layout of an open pit
or quarry need to take various factors
into consideration and will determine
the pit limits. The first is the types of ore
or material that can be mined in the case
of an opencast operation, for example
oxidised and/or fresh ore. Quarries and
opencast operations have to consider
geological losses, including intrusions
such as dolerite dykes or fault zones.
The pit shell dimensions are largely
influenced by the cut-off grade, as it
will determine the depth or distance
from the plant that material can still be
economically mined.
The physical design and layout of the
open pit or quarry consider the following
factors: stripping ratio, pit slopes, bench
heights, road grades, dewatering, and
property lines. The stripping ratio refers
to the amount of overburden or waste
material that needs to be removed
before accessing the ore (mineralised
material) or the saleable quarry
product. Overburden usually refers to
the residual or transported soils that
cover the outcrop. Most environmental
management plans require this top soil
not to be mined, but to be stockpiled
and used during the mine closure and
rehabilitation phase.
The mine layout needs to make
provision for areas where waste rock
and low-grade stock piles can be built,
without influencing mining activities or
sterilising mineable ground. The ground
control domains determine the angle of
the pit slopes and the bench heights to
meet the minimum factors of safety. The
road grades and width of the permanent
SEPTEMBER 2018 MINING MIRROR
[35]