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Diamond core
drilling (Exploration
diamond drilling) utilises an annular
diamond-impregnated drill bit attached to the end of
hollow drill rods to cut a cylindrical core of solid
rock. The diamonds used are fine to microfine industrial
grade diamonds. They are set within a matrix of varying
hardness, from brass to high-grade steel. Matrix
hardness, diamond size and dosing can be varied
according to the rock which must be cut. Holes within
the bit allow water to be delivered to the cutting face.
This provides three essential functions; lubrication,
cooling, and removal of drill cuttings from the hole.
Diamond drilling is much slower than reverse circulation
(RC) drilling due to the hardness of the ground being
drilled. Drilling of 1200 to 1800 metres is common and
at these depths, ground is mainly hard rock. Diamond
rigs need to drill slowly to lengthen the life of drill
bits and rods, which are very expensive.
Core samples are retrieved via the use of a lifter tube,
a hollow tube lowered inside the rod string by a winch
cable until it stops inside the core barrel. As the core
is drilled, the core lifter slides over the core as it
is cut. An overshot attached to the end of the winch
cable is lowered inside the rod string and locks on to
the backend, located on the top end of the lifter tube.
The winch is retracted, pulling the lifter tube to the
surface. The core does not drop out the inside of the
lifter tube when lifted because a "core lifter spring,"
located at the bottom of the tube allows the core to
move inside the tube but not fall out.
Once a rod is removed from the hole, the core sample is
then removed from the rod and catalogued. The Driller's
offsider screws the rod apart using tube clamps, then
each part of the rod is taken and the core is shaken out
into core trays. The core is washed, measured and broken
into smaller pieces using a hammer to make it fit into
the sample trays. Once catalogued, the core trays are
retrieved by geologists who then analyse the core and
determine if the drill site is a good location to expand
future mining operations.
Diamond rigs can also be part of a multi-combination
rig. Multi-combination rigs are a dual setup rig capable
of operating in either a reverse circulation (RC) and
diamond drilling role (though not at the same time).
This is a common scenario where exploration drilling is
being performed in a very isolated location. The rig is
first set up to drill as an RC rig and once the desired
metres are drilled, the rig is set up for diamond
drilling. This way the deeper metres of the hole can be
drilled without moving the rig and waiting for a diamond
rig to set up on the pad. |
