What is mineral exploration drilling? It is a technique used in the mineral mining sector to (1) explore for new mineral prospects (2) evaluate land for economic mining (3) augment additional ore reserves and resources in the mine. Exploration drilling enables discovery and acquisition of new mineral deposits amenable to economic extractive operations now or in future. Mineral exploration aims to discover deposits of minerals or rocks that can be used to meet the resource needs of society, for example:

  • Industrial raw materials – clay, rock, limestone, sulphur, salts, and fertilizer minerals
  • Ores from which metals are extracted – iron, copper, and zinc ores 
  • Gemstones – diamonds, sapphires, and opals 
  • Solid fuels – coal, oil shale, and uranium 

Mineral exploration requires high investment, sustained cash inflow, and considerable time with inherent risk. Mineral exploration drilling is planned in a grid pattern to be conducted in sequence to facilitate the midterm assessment of quality, quantity and reliability of estimates. World over, mineral exploration drilling demand has increased. It is attributed to rise in demand for metals, growth of industrial output, new ore types and greatly improved geological knowledge and exploration technology. The article focuses on mineral exploration drilling from a mineral exploration methods and mineral exploration drilling methods similarities, advantages and differences. 

Mineral exploration drilling methods – auger, air-core and diamond core drilling. 

Drilling is used to obtain detailed information about rock types, mineral content, rock fabric and the relationship between the rock layers close to the surface and at depth. Auger drilling uses a hand-held auger, or an auger mounted on a light vehicle. The rock is cut and broken with a simple blade bit mounted on the end of a rotating string of rods. As the drill advances, extra rod sections are added to the top of the drill string. An auger is quite similar to a posthole digger used by farmers when fencing. This drilling method is generally used for shallow boreholes and requires little site preparation. Air-core drilling is a specialized reverse circulation drilling where a small, annular bit is used to cut a solid core of rock from relatively soft or easily broken material. The bit produces short sections of core which are recovered, along with broken rock chips, up the center of the drill stem in the manner of a standard reverse circulation rig. The system is often capable of penetrating and coring soft sticky clays with might bind a normal blade bit. Diamond core drilling uses an annular, diamond-impregnated bit mounted on the end of a rotating string of rods. Interestingly, these diamonds are not useful as jewelry but are used in the drill bits for their hardness and the bit is suitable for the hardest rocks. The rod cuts a solid core which passes up inside the drill rods as the bit advances. The bit is lubricated with water and drilling fluid or water/mud mixture which is pumped to the cutting face down the inside of the rods. It then returns to the surface between the rods and the sides of the hole. At the surface, the return water is collected in a sump where fine suspended ground rock material can settle. 

Drilling methods – reverse circulation, rotary air-blast and sonic drilling

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Reverse circulation drilling is a form of percussion drilling, by which the rock is made to fail through the use of a piston that delivers rapid impacts to the drill stem, transferring energy to the drill bit. These blows to the rock are delivered by the bit, while a rotational device makes sure that the bit hits a new rock surface with each blow. A feed force is applied to maintain rock/bit contact, and compressed air is used to remove or flush the drill cutting from the hole, advancing the hole depth efficiently. The piston can be mounted out of the hole or down the hole for quiet and efficient drilling. Rotary air blasting drilling is a type of rotary drilling, where a blade bit or roller-cone bit is used to cut and break rock with a simple blade mounted on the end of a rotating drill stem. This stem is rotated while a thrust is applied to it by a pulldown mechanism that uses up to 65% of the weight of the machine. As the bit breaks and removes the rock using either a crushing and chipping action or a ploughing-scraping action, compressed air is supplied to the bit. The air cools the bit and flushes the cutting back to the surface. Sonic drilling uses high frequency, resonate energy to advance the core barrel or casing. During drilling, the resonant energy is transferred down the drill string to the bit face at various sonic frequencies. Simultaneously rotating the drill sting evenly distributes the energy and impact at the bit face. Sonic drilling is particularly useful when hard-to-drill formations are encountered, and a high level of sample recovery is required. 

What are the similarities, advantages and differences across these methods?

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Let’s firstly assess similarities of mineral exploration drilling methods. A sonic drill rig is similar in size and set up to a conventional air-core drill rig. Reverse circulation drilling is similar to air-core drilling in that the drill cuttings are returned to the surface via an inner tube inside the drill rods. Advantages of auger drilling are in obtaining samples in soft unconsolidated ground during the reconnaissance stage of mineral exploration. It is effective in environmental and geotechnical sampling. Rotary air-blast drilling is primarily used for first-pass mineral exploration and development. The incentives to using air-blast drilling are that it can be applied in water bore drilling and blast-hole drilling in mines. For mineral exploration, air-core drilling is generally preferred over rotary air-blast as it provides cleaner samples. Cuttings are removed inside the drill rods and are less prone to contamination than those obtained by auger or unstabilized rotary air-blast drilling. Contamination of cuttings happens as they are brought to the surface between the outside of the drill rod and the walls of the hole. Diamond core drilling differs from other drilling methods used in that a solid core of rock rather than cuttings, are extracted from depth. Reverse circulation drilling has larger rigs than air-core rigs. However the method is slower and more expensive than rotary air-blast or air-core drill, but it achieves better penetration in harder rocks. 

Best Practice Dust Control for Drilling

Different particle sizes and chemistries of drill cuttings and shavings are generated from the mineral exploration drilling methods given above. GRT takes a non-negotiable stance on the importance of providing effective dust control at the source. Superactivation of water using surfactants renders it hydrophobic and hydrophilic. In essence this increases its binding capacity of the drill cuttings regardless of the chemical nature. GRT products offer 12X more binding capabilities and water savings for the overall mineral exploration operations. Recently GRT introduced new products – GRT: 12X and GRT DC Binder for drilling dust control. Lookout for the product brochure on our social media platforms. Be sure to read the recent article on drill and blast dust control. As the weeks progress we commit to bringing you insightful articles on the journey into understanding holistic aspects of drilling and blasting.

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