With auger drilling, the rotation of a helical screw causes the blade of the screw to lift the sample to the surface. This can be used for shallow environmental drilling, geotechnical drilling, soil engineering and mineral deposits where the formation is soft and the hole does not collapse.
Percussion rotary air blast (RAB) drilling
Percussion drilling is used most frequently for mineral pilot drilling, water-bore drilling and blast-hole drilling.
To drill holes in fairly soft ground, air-core drilling generally uses hardened steel and tungsten blades in coal formations. The head of the drill is surrounded by blades, which cut the coal. The rods are hollow and the sample is fed through the rod.
Reverse-circulation (RC) drilling
With RC drilling, cuttings are returned to the surface within the rods. The drilling mechanism is a downhole hammer and cuttings are collected through a cyclone and bagged. This method is generally used to a maximum depth of 250m (groundwater will affect efficiency).
For exploration drilling worldwide, the mining industry uses diamond coring almost exclusively with wireline systems that enable the cave to be retrieved without retracting the drill rods.
Core sample can be obtained in different sizes and vary from 3mm diameter to 116mm. The core is removed from the tube and washed, measured, marked and placed into sample trays.
Direct-push is done with Selby tubes, which advance a drill string by pushing or hammering without rotating the drill string. While this does not meet the proper definition of drilling, it does achieve the same result of core recovery. This method is ideal for very shallow sampling in soft material.
Hydraulic rotary drilling
Tri-cone, carbide-embedded or diamond-impregnated drill bits are used. With this method no sample is recovered. It is normally used for exploration of oil and gas.
Sonic (vibratory) drilling
The sonic drill head uses high-frequency resonant vibrations of 50Hz to 120Hz (cycles per second). This technology is ideal for dumpssites and wetland drilling environments.
Underground core drilling is conducted to identify sufficient ore reserves above the main haulage level and for the safety of the mines in establishing the position of possible gas or water intersections.
Blasthole rigs are used to drill blastholes into benches in quarries and opencast mines. They achieve high production rates and are used for either rotary or down-the-hole drilling.
During exploration, grade control is the process used to check quality and control the grade and variability of ore. The purpose of grade control is to develop mining plans and the budgeting process. Drilling samples are further analysed to establish the mineral content of the drill area. The grade of an ore body and the mineral being mined are rarely constant. It can be more concentrated near the surface and dissipate in other areas. Grade control drilling helps to define the economic limits and mine planning.
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