Reverse Circulation, or RC, drilling is a fast and cost efficient
method of retrieving high quality samples from exploration and mine
drilling. The system has been continuously developed since its
inception in Australia in the early 1970s, and is now a preferred
method for initial exploration, ore body development and in-pit
grade control.
The RC method employs dual wall drill rods that comprise an outer
drill rod, with an inner tube located inside the drill rod. The
inner tubes overlap, and seal on the tube below with O rings when
the drill rods are screwed together. These inner tubes provide a
continuous sealed pathway for the drill cuttings to be transported
from the bit face to the surface. The circulating medium, in most
cases high-pressure air, enters the annulus between the rod and
tube via the air swivel, which is normally part of the drill
string, or sometimes mounted on top of the rotation head. The air
travels down the annulus to the drilling tool,
which is usually an RC hammer, or can be a blade bit or tricone
roller bit. As in conventional open hole drilling, the air powers
the drilling tool and the exhaust air carries the cuttings. In RC
drilling the cuttings are returned to the surface through the inner
tubes in the drill string and rotation head. Once through the
rotation head, the air and cuttings comprising the sample change
direction at the discharge blast box and are transported through
the sample hose to the cyclone. The cyclone slows the sample,
separates it from the air, and collects it.
(2) RC(reverse circulation) benefits?
RC drilling provides virtually uncontaminated cuttings to the
cyclone. As the cuttings travel directly from the drill bit through
the steel inner tubes and sample hose, there is no cross
contamination from other areas of the hole. Using good sample
splitters and sampling procedures, RC results rival the accuracy of
diamond core assays. Drilling penetration rates are similar to open
hole drilling, and are often faster at greater depths. The sample
velocity through the inner tubes can be up to 250 m/sec, so
retrieval of the sample and hole cleaning is rapid. Production
rates of up to 200-300m/ day are common at rates exceeding 10 m/h,
many times faster than diamond drilling, and achieving rapid
results for the customer. Unconsolidated formations can often be
drilled and sampled without casing. Washing and scouring of the
hole is minimized, because there is normally no fluid or cuttings
flow against the walls after the drill bit has passed. Low impact
bits, such as RC blade or RC roller, are ideal in these soft or
loose formations. With good drilling techniques, samples can be
kept dry, even several hundred metres below the water table. Dry
samples are preferred as they split more accurately for assay, and
are easier to handle. RC sample content ranges from dust to 25 mm
chips, and is already partially processed for analysis. Wireline
surveying of the hole is still possible through the drill rods.
With the use of a stainless steel rod at the bottom of the string,
hole azimuth
readings are also possible.
1. Drop Center Bit
For high penetration rates in soft to medium hard and fissured rock
formations. Low to medium air pressures. Maximum hole deviation
control.
2. Concave Face
The all-round application bit face specifically for medium hard and
homogenerous rock formations. Good hole deviation control and good
flushing capacity.
3. Flat Face Bit
This kind of face shape is suitable for hard to very hard and
abrasive rock formations in applications with high air pressures.
Good penetration rates an resistance to steel wash.
4. Double Gauge Face
This kind of face shape is suitable for fast penetration rates in
medium to hard rock formations. Designed for high air pressures and
good resistance to steel wash step gauge bit.
5. Convex Face
For high penetration rates in soft to medium-hard with low to
medium air pressures. It is the most resistance to steel wash, and
may reduce the load and wear on the gauge buttons, but poor hole
deviation control.