In the first step, powerful rotary drills will bore a pattern of approximately 45-feet-deep, 12-inch-diameter holes into the bedrock. The holes are then loaded with an explosive compound of ammonium nitrate and fuel oil (commonly referred to as ANFO). At carefully scheduled times, the explosives are discharged remotely, breaking thousands of tons of rock into more manageable-sized pieces. Blasting is expected to occur about every two to three days. Through carefully orchestrated blasting and removal of the loose rock, 40-foot-high benches are formed within the pit on which ramps and roads are built. These rock benches allow for the work of deepening and expanding the mine to proceed so that all the ore can be accessed and removed safely and efficiently over the life of the mine. Waste rock material that has no economic value and is not processed will be categorized, and stored in nearby rock stockpiles according to the amount of sulfur it contains.

Each stockpile will have an engineered containment system to collect rainwater that may come in contact with the rock and pump it to a water treatment plant. Containment systems such as ours are used widely and effectively at modern mining and mineral processing sites and landfills worldwide. They ensure that none of the water that comes in contact with waste rock leaves the facility without first being treated to meet state and federal water quality standards.

Some of the waste rock with the lowest sulfur content, called category 1 rock, will be placed in a permanent stockpile and surrounded by a containment system. Some of it may be used for building roads and berms in the mine, and some of it will be back filled into the East Pit.

Once the East Pit has been mined out, waste rock with the highest sulfur content will be returned to the empty mine pit for underwater storage where the potential to weather and oxidize is greatly reduced. This higher-sulfide content rock represents about 6 percent of the total waste rock that will be mined. Extensive testing demonstrates that the other 94 percent of waste rock is not capable of generating acid.

Importantly, even before treatment, water on site will be in the pH-neutral range – about the same pH as milk. The Minnesota Department of Natural Resources has determined that the mine will not create acid rock drainage as a result of mine construction, operations and closure