Sonic Sampling and Well Installation of Gold Heap Leach Pad

Heap leach pads are large piles of rock waste from a mine operation that receive further processing through a leaching solution to remove the mineral ore from the rock. The leaching solution is applied via an irrigation or sprinkler system to the top of the heap leach pad. The solution penetrates the heap leach pad and separates the mineral ore from the rocks. At the bottom of the heap leach pad is an impermeable liner that is angled to allow the solution to be collected in a reservoir and then be sent for further processing.

Using one of their sonic rigs, the LS™600, Boart Longyear can collect highly representative samples and achieve excellent sample recovery. Leach pads are made up mostly of unconsolidated material and the LS600 has proven effective in these conditions by producing 100 percent accurate in-situ core samples. The rig is also used to install wells in the heap leach pad.

Boart Longyear was contracted to sample and install solution wells for the largest gold heap leach pad in the USA, located in Northwest Nevada. The heap leach pad is over 400 feet tall. To achieve that height the mine operation built up the heap leach pad via 50 foot lifts.

When constructing each level, the haulers unavoidably crush and compress the previous top layer of the pad while adding material to the top of the pile. This creates a dense layer that causes a dam effect and does not allow the leaching solution to penetrate through the heap leach pad properly. The heap leach pad can then develop dry pockets where no leaching activity takes place — lowering the efficiency and return on investment.

Boart Longyear was hired to detect dry zones in the leach pad and to reactivate the zones via solution enhancement wells. First the Boart Longyear team drilled test holes to determine the depths of the dry zones and the specific lithological (physical characteristics of the ground) information.

These boreholes were 107 meters (350 ft) deep and were 15 to 23 meters (50 to 75 ft) above the heap leach pad liner. A continuous core sample was obtained by using the LS600 sonic rig. This allowed the Boart Longyear team to determine moisture concentration at accurate depths and to identify exposed ore.

Once depths of ore and moisture levels of the dry zones were indicated, Boart Longyear installed solution enhancement wells using an 11.5-centimeter (4.5 in) flush threaded well casing. The casings allowed the borehole to stay intact as an upper and lower k-packer were installed at specific depths. A 100-foot injection screen was placed between the k-packers to direct the leaching solution into targeted dry zones.

This process was repeated on an eight-week cycle with four weeks of solution and four weeks of rest. Once the cycle was completed, the k-packers and screen were moved to a new depth. The well would receive the solution at three heights, spanning 100 feet each time — bottom, middle, and top.

Over a three-year period, the team, using the LS600 sonic rig, drilled 55 gravity flow solution wells to alleviate the fine unprocessed gold from the heap leach pad. This resulted in 6,100 ounces of gold that equated to $8 million ($1,300 per ounce average). The project paid for itself within the first 70 days and the client received a 400 percent internal rate of return (IRR).