Reverse circulation (RC) drilling has become standard practise in most mines across the world.
First developed in Australia in the 1970’s, the drilling technique was originally conceived as a solution to drilling difficulties encountered when using open hole techniques in soft iron ore and mineral sands.
The first RC drill rods were adapted from the US oil industry and manufactured in Western Australia in 1972 by Bruce Metzke and John Humphries.
A RC drilling is usually a large piece of apparatus, that requires a lot of space, not just for the rig itself, but the supporting vehicles and the pit for collecting waste runoff.
The drill cutting are transferred to the surface inside drill rods, which are linked together to create a ‘drill string’. Drill bits attached to the end of the hammer are made from tungsten-steel, and are usually around 13-20cm in diameter. These also have metal nodules attached at the end to allow cutting through particularly tough rock.
Most RC drilling uses a dual-tube drill rods, with one tube inside another. The tubes inside overlap and provide a path for drilled rock from the ground to the surface.
Inner tubes can be sealed together, meaning that the RC drill can sample up to very large depths, often around 500m.
Another type of RC drilling is ‘center sample’ drilling. This is a modern variation, in which a central hammer, with a hollow center, allows the sample to immediately enter the drill pipe, without the need to travel past the hammer.
The samples produced from RC drilling are dry chips of the drilled rock.
To create the sample, the hammer acts like a pneumatic piston and pushes a tungsten-steel drill bit on to the rock, breaking it up. Before the drill bit hits the rock it is dried out using an air compressor, so that the rock chips are dry at the surface.
Water is often used down the hole to cool the drill bit and reduce dust, as well as assisting with the transportation of sample bits to the surface.
Air is blown down the drill rods to create a pressure difference, allowing the sample chips and water to rise through the inner tube. The sample then reaches a bell at ground level, which transports the sample to a cyclone where it dries out and is deposited into sacks.
When at the surface, an exploration geologist will analysis and log the sample to ascertain the prospective mineralisation within the drilled orebody. A representative sample is taken from the sack, and is the sieved to remove excess dust. Once the desired sample has been washed and dried, the exploration geologist will note the weight percentage of the desired minerals in the sample. The samples will then be stored in plastic cutting boxes before further chemical analysis offsite.
RC drills can occasionally get stuck parts in the ground as the drill bit wears down and rods need to be recovered as they are very expensive. This can be a time consuming process and can on occasions set a project back by weeks. To combat this, many companies will consistently re-grind the tungsten nodules of the drill bit.
RC drilling is a technique used in the majority of stages of mine development.
As it is cheaper than diamond core drilling, it is often used in first stage exploration mining to delineate a potentially extractable ore body. It is also preferable to RAB or air-core drilling when trying to reach great depths, but RC drilling is slower and more expensive than either of these two methods.
RC drilling is also consistently used during in-pit grade control and the development stage of an ore body.
Health and Safety
There are many inherent risks involved in operating large mining equipment and RC drilling is no exception. PPE, or personal protective equipment, must be worn at all times. This includes hearing protection, gloves, goggles, suitable footwear and dust masks when necessary.
Companies Involved in RC Core Drilling
Here are several examples of RC drilling contractors, who specialise in RC drilling.
Midnight Sun Drilling Co. Ltd.: Founded in 1970, MSD was the first to use the Center Sample Hammer in Canada and still under the same management.
SBD Group: A large group based in Western Australia, which undertakes drilling in Western Australia and West Africa.
Leon Marsh Drilling: Based in Western Australia and specialising in exploration RC drilling.
Atlas Copco: A large drilling company, involved in exploration drilling, geotechnical engineering and rock reinforcement.
Marjoribanks, R.W. Geological Methods in Mineral Exploration and Mining. 1st ed. (Chapman and Hall, 1997)