Depending on the geological structure of the Earth’s crust, mining companies can choose between different drill types. I have summarized the most common drilling rigs in the following table:
|Drilling Rig||Maximum Depth||Costs|
|Auger Drilling||Roughly up to 25 meters||Lowest|
|Percussion Rotary Air Blast Drilling (RAB)||Roughly up to 150 meters||Low|
|Reverse Circulation Drilling (RC)||Roughly up to 500 meters||Medium|
|Diamond Drilling||Roughly up to 1800 meters||Highest|
The following information1 explains in more detail what distinguishes the different drilling rigs from each other.
Auger drilling is done with a helical screw which is driven into the ground with rotation; the earth is lifted up the borehole by the blade of the screw. Hollow stem auger drilling is used for softer ground such as swamps where the hole will not stay open by itself for environmental drilling, geotechnical drilling, soil engineering and geochemistry reconnaissance work in exploration for mineral deposits. Solid flight augers / bucket augers are used in harder ground construction drilling. In some cases, mine shafts are dug with auger drills. Small augers can be mounted on the back of a utility truck, with large augers used for sinking piles for bridge foundations.
Auger drilling is restricted to generally soft unconsolidated material or weak weathered rock. It is cheap and fast.
Percussion Rotary Air Blast Drilling (RAB)
The rotary drilling rig uses a pneumatic reciprocating piston-driven “hammer” to energetically drive a heavy drill bit into the rock. The rotary drill bit is hollow, solid steel and has ~20 mm thick tungsten rods protruding from the steel matrix as “buttons”. The tungsten buttons are the cutting face of the bit. The cuttings are blown up the outside of the rods and collected at surface. Air or a combination of air and foam lift the cuttings. (This drilling rig is also know as the Down-the-Hole drill.)
Reverse Circulation Drilling (RC)
The drilling mechanism is a pneumatic reciprocating piston, also 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 drilling; it is cheaper than diamond drilling and is thus preferred for most mineral exploration work.
Diamond drilling utilizes an annular diamond-impregnated drill bit attached to the end of hollow drill rods to cut a cylindrical core of solid rock. The diamonds used are fine to micro fine industrial grade diamonds. They are set within a matrix of varying hardness, from brass to high-grade steel. Matrix hardness, diamond size and dosing can be varied according to the rock which must be cut. Holes within the bit allow water to be delivered to the cutting face. This provides three essential functions: lubrication, cooling, and removal of drill cuttings from the hole.
Diamond drilling is much slower than reverse circulation (RC) drilling due to the hardness of the ground being drilled. Drilling of 1200 to 1800 metres is common and at these depths, ground is mainly hard rock. Diamond drilling rigs need to drill slowly to lengthen the life of drill bits and rods, which are very expensive.
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