Drilling is a very common unit operation in mining activities, whether during the exploration phase for sampling or even during mining for blasting or grade control. In the collective imagination, it may appear to be a trivial aspect, but this is far from being the case. Several factors come into play when a borehole is drilled. In the following paragraphs, we will explore these factors and identify their influences.
Bit pressure
Pressure (as stated previously) is often expressed in kilopascals, tonnes per square centimetre, or pounds per square inch. When a driller refers to ‘bit pressure’, he usually means thrust on the bit. ‘Thrust’ is a force measured in newtons, although tonnes (force) and kilograms (force) are also common. Vertical (or near-vertical) holes are often drilled using the weight of the drill collars or pipe/rods to provide thrust. Angled or upward holes require the rig to provide the thrust.
Rotary speed
Speeds of rotation are measured in revolutions per minute (rpm). Speeds vary from very low (i.e. 5–10 rpm) for large holes and rigs using air percussion, to more than 1000 rpm for small holes and diamond drill rigs.
Torque capacity
Torque is measured in newton metres, or kilogram (force) metres, as described earlier. Smaller hammer and diamond bits are often used if only small torques are required (i.e. about 500 Nm). The largest torques are required to turn large augers, and bucket auger machines may have a torque capacity exceeding 50,000 Nm (50 kNm or 8000 ft lb).
Hole-clearing capacity
The capacity of a rig to make hole and its hole-making efficiency will often be limited by the rig’s capability to clear cuttings from the hole. If the cuttings are not cleared from the face and sides of the bit, they will be ground up. This recutting or regrinding of cuttings requires additional energy, reduces the energy available to produce new cuttings, and results in lower drilling efficiency. How well the cuttings are removed from a cable tool hole depends entirely on the bailer and the driller. Clearing cuttings by circulation is controlled by fluid:
- up-hole velocity
- viscosity
- density and low characteristics.
All drills depend on good bit design to achieve bottom hole clearing. Rigs employing fluid circulation rely mainly on fluid energy, and adequate pump or compressor capacity must be available to supply it.
Hole-clearing capacity will therefore depend on the capacity of the pump or compressor (i.e. its low and pressure rating).
