Reverse Circulation Drill Rig

Overview

A reverse circulation drill rig is a fast sampling machine for mineral exploration and grade control. It does not recover core; instead it hammers the rock to chips and returns those chips to surface as a clean, depth-controlled sample. The defining feature is the flow path. In ordinary drilling the cuttings come up the open annulus between rod and hole wall, where they mix and lag and pick up contamination from the wall. Reverse circulation flips this: air goes down the outer wall of a double-tube rod, and the cuttings return up the sealed inner tube. Because the sample never touches the open hole, each metre drilled gives a representative, uncontaminated sample that can be assayed with confidence.

The rig is built around that dual flow path. The Compressed Air System system supplies the air, the Rotary Head turns the Dual-Wall Drill String, the Down-Hole Hammer cuts and feeds chips inward, the Cyclone and Sampling captures the sample, the Rod Handling adds rods, and the Power and Drive pack drives the machine from the Operator Controls console on the Mast and Feed.

The dual-wall string and sample path

The heart of the method is the Dual-Wall Drill String. Each Dual-Wall RC Rod is a rod within a rod: the outer wall carries torque and the down-going air, while a sealed Inner Sample Tube runs up the center to return the sample. A Rod Joint Seal at every joint keeps the two air paths from mixing, and a Crossover Sub sub at the bottom routes the down-air to the hammer and ports the cuttings into the inner tube. At the top the turning string passes its sample through the hollow Hollow Spindle of the Rotary Head and out through a Discharge Swivel into the discharge hose. Because the chips are pulled into the center right at the bit face, the sample comes from a known depth with almost no contamination.

Hammer and bit

Cutting is done by a pneumatic Down-Hole Hammer. Boosted air drives a steel Hammer Piston inside the Hammer Casing to strike the bit thousands of times a minute, and the RC Face-Sampling Bit is a special face-sampling design. Unlike a normal hammer bit, its ports draw the freshly broken rock straight up into the inner tube before it can escape into the annulus. The face carries pressed Carbide Button tungsten-carbide buttons that crush the rock under each blow, and a Foot Valve check valve stops cuttings from packing the hammer when air is shut off. This combination of percussion and face sampling is what makes the method fast in hard ground while keeping the sample clean.

Air supply

Reverse circulation lives on air. A rotary Screw Compressor screw compressor supplies the bulk volume needed to lift the chip column up the inner tube, and an Air Booster raises pressure further so the hammer keeps working in deep or wet holes where water tries to flood the bit. A Air Receiver tank smooths the supply and drops out condensate, an Aftercooler cools the air, and a High-Pressure Hose carries it to the head. Air pressure and volume set both the penetration rate and the rig's ability to clear water, so the compressor and booster are the largest single power draw on the machine.

Cyclone and sampling

The point of the whole rig is a good sample, and the Cyclone and Sampling system delivers it. The sample arrives at high speed through the Discharge Hose into a Cyclone Body that spins the air-chip mix so the chips drop out the bottom while the air vents the top. From there a Sample Splitter rotary or riffle splitter takes a fixed fraction of each metre as the assay sample, and the split plus bulk reject drop into the Sample Bag Rack rack bagged by interval. A Dust Suppression suppression system keeps the dust down. The quality of the split is what separates a defensible grade-control sample from a noisy one, so the cyclone and splitter are maintained carefully.

Rod handling, power, and operation

Adding and pulling rods by hand would be slow and dangerous, so the Rod Handling uses a Rod Carousel magazine indexed by a Servo Motor, a Breakout Wrench wrench to crack the joints, and a Foot Clamp to hold the string. Power comes from a large Diesel Engine diesel that drives both the compressor and the Hydraulic Pump hydraulic pumps, with the rig riding on a Track Frame crawler. The driller runs the cycle from the Operator Controls console, watching the Gauge Cluster and an LCD Panel for air pressure and penetration rate.

Use, strengths, and limits

Reverse circulation is the workhorse of grade control and early exploration because it is fast, cheap per metre, and gives clean samples in hard or dry ground. It is the right tool when the geologist needs many holes and reliable assays rather than intact rock. Its limits are real: it returns broken chips, so it cannot show structure, true texture, or geotechnical detail, and it struggles below the water table where wet samples smear and lag. When intact rock is needed, the program switches to a Diamond Core Drill Rig, and many exploration holes are drilled RC to a target depth then continued with diamond coring, which gives speed up top and core where it matters.