Page 239 - Introduction to Mineral Exploration

222 M.K.G. WHATELEY & B. SCOTT

FIG. 10.10 A diamond drill. From left to right the components are the power, control, drill, and water pump
units. (Reproduced by permission of Diamond Boart Craelius Ltd.)

TABLE 10.6 Diamond drilling rigs.
Drill unit weight 890 kg Length of holes: In mineral exploration most drill
Power unit weight 790 kg AQ (48 mm) 425 m holes are less than 450 m in length.
Motor rating 40 kW at 2200 rpm −1 BQ (60 mm) 350 m A typical drill rig for this purpose is
Flush pump flow 76 liters min −1 NQ (76 mm) 250 m shown in Fig. 10.10 but many
Flush pump pressure 5 Mpa models are in use. Some of the
specifications for the drill in
The drill is a fully hydraulic rig. It is suitable for both underground and surface Fig. 10.10 are given in this table.
units and is powered by either an air, diesel or electric motor. To a certain extent
penetration rates can be increased by an increase in pressure on the drill bit. This
is regulated by hydraulic rams in the drill unit at surface and transmitted to the
bit by the drill rods. A limit is reached when all the diamond chips in the bit’s
surface have been pressed as far as possible into the rock. Higher penetration
rates can then only be achieved by increasing the bit rotation speed. Excessively
high bit pressure, with low rotation speed, results in abnormally high wear of the
bit. Conversely, low pressure with high rotation results in low penetration and
polishing (i.e. blunting) of the diamonds.
In the above unit, drill water would flush through a 60-mm-diameter hole at
−1
about 50 cm s . In a 350-m-length hole return water would take about 12
minutes to pass from the drill bit to the surface. The ascending velocity of the
fluid should be greater than the settling velocity of the largest rock particles
generated by the cutting and grinding action of the bit.