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RC drilling
is similar to air core drilling, in that the drill
cuttings are returned to surface inside the rods. The
drilling mechanism is a pneumatic reciprocating piston
known as a hammer driving a tungsten-steel drill bit. RC
drilling utilises much larger rigs and machinery and
depths of up to 500 metres are routinely achieved. RC
drilling ideally produces dry rock chips, as large air
compressors dry the rock out ahead of the advancing
drill bit. RC drilling is slower and costlier but
achieves better penetration than RAB or air core
drilling; it is cheaper than diamond coring and is thus
preferred for most mineral exploration work.
Reverse circulation is achieved by blowing air down the
rods, the differential pressure creating air lift of the
water and cuttings up the inner tube which is inside
each rod. It reaches the bell at the top of the hole,
then moves through a sample hose which is attached to
the top of the cyclone. The drill cuttings travel around
the inside of the cyclone until they fall through an
opening at the bottom and are collected in a sample bag.
The most commonly used RC drill bits are 5-8 inches
(12.7–20.32 cm) in diameter and have round metal
'buttons' that protrude from the bit, which are required
to drill through rock and shale. As the buttons wear
down, drilling becomes slower and the rod string can
potentially become bogged in the hole. This is a problem
as trying to recover the rods may take hours and in some
cases weeks. The rods and drill bits themselves are very
expensive, often resulting in great cost to drilling
companies when equipment is lost down the bore hole.
Most companies will regularly 'sharpen' the buttons on
their drill bits in order to prevent this, and to speed
up progress. Usually, when something is lost (breaks
off) in the hole, it is not the drill string, but rather
from the bit, hammer, or stabiliser to the bottom of the
drill string (bit). This is usually caused by a blunt
bit getting stuck in fresh rock, over-stressed metal, or
a fresh drill bit getting stuck in a part of the hole
that is too small, due to having used a bit that has
worn to smaller than the desired hole diameter.
Although RC drilling is air-powered, water is also used,
to reduce dust, keep the drill bit cool, and assist in
pushing cutting back upwards, but also when collaring a
new hole. A mud called liqui-pol is mixed with water and
pumped into the rod string, down the hole. This helps to
bring up the sample to the surface by making the sand
stick together. Occasionally, 'super-foam' (AKA 'quik-foam')
is also used, to bring all the very fine cuttings to the
surface, and to clean the hole. When the drill reaches
hard rock, a collar is put down the hole around the rods
which is normally PVC piping. Occasionally the collar
may be made from metal casing. Collaring a hole is
needed to stop the walls from caving in and bogging the
rod string at the top of the hole. Collars may be up to
60 metres deep, depending on the ground, although if
drilling through hard rock a collar may not be
necessary.
Reverse circulation rig setups usually consist of a
support vehicle, an auxiliary vehicle, as well as the
rig itself. The support vehicle, normally a truck, holds
diesel and water tanks for resupplying the rig. It also
holds other supplies needed for maintenance on the rig.
The auxiliary is a vehicle, carrying an auxiliary engine
and a booster engine. These engines are connected to the
rig by high pressure air hoses. Although RC rigs have
their own booster and compressor to generate air
pressure, extra power is needed which usually isn't
supplied by the rig due to lack of space for these large
engines. Instead, the engines are mounted on the
auxiliary vehicle. Compressors on an RC rig have an
output of around 1000 cfm at 500 psi (500 L·s-1 at 3.4
MPa). Alternatively, stand-alone air compressors which
have an output of 900-1150cfm at 300-350 psi each are
used in sets of 2, 3, or 4, which are all routed to the
rig through a multi-valve manifold.
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