Gold mining in the region around Silverton, Colorado, had been an important part of the local economy until the last mine closed in 1991. The Gold King Mine was abandoned in 1923 and was known to have contaminated mine drainage, a problem common to subsurface mines around the world.
Subsurface mining exposes minerals to water and air, and the resulting drainage must be carefully managed to protect the environment. Gold King had been leaking drainage water for years, resulting in contamination of the upper Animas River basin, which wiped out fish and affected other animal species in the watershed. The U.S. Environmental Protection Agency (EPA) took over the responsibility of remediating the Gold King leaks. Previously, in 2009, the mine’s adits, or passageways, had been plugged and drainage pipes installed to relieve pressure from water buildup, but concern arose that this system might not be enough to prevent a future, more serious leak. The EPA’s goal was to better stabilize the situation.
As work began in 2014, mine water levels were estimated so that a drainage pond could be properly sized and built to treat the expected volume. Returning to the actual mine drainage worksite in 2015, workers found that landslides had compromised the entrance and pipes. Once these were cleared, EPA planned to excavate the entrance starting at the top of the estimated water level and draw water at a controlled rate for treatment in the pond. As operations began, water began spewing out at ever-increasing rates. The levels were much higher than had been estimated. The plug failed, and water rushed into a small creek and then into the Animas River.
The river was closed for recreation and its water banned for drinking, bathing and irrigation. Major towns downstream of the San Juan River were impacted within days, and levels of heavy metals reached hundreds of times their acceptable limits. Levels eventually were diluted enough that, by the time water passed through Lake Powell and further downstream, it was safe for all uses.
The EPA immediately took responsibility and developed a remediation plan. The agency sought an onsite water treatment plant that could be operated at the remote mine location, run all year long with minimum supervision and treat the effluent over a wide range of flow rates. The need was obviously immediate.
Alexco Environmental offered an onsite solution including a 75-ton silo and feed system to feed hydrated lime to treat 200 to 800 gallons per minute of influent. Alexco contacted Vibra Screw Inc. of Totowa, New Jersey, for expertise on equipment for handling bulk powders to see what was needed and what could be shipped within a week. The company offered a volumetric screw feeder with variable speed drive and a controller with 4-20 mA remote speed input, which would allow the lime feed to automatically adjust to the changing pH of the influent. The higher the acidity, the more lime was continuously fed. The system would also treat the heavy metals.
The screw feeder was placed below the silo, which provides the bulk lime supply. Feeder discharge is to the influent feed line. Treatment includes all flow from the mine as well as water stored in the retention ponds.
The feeder uses a patented, controlled-vibration design to ensure even filling of its feed screw with uniformly dense material. This provides for extremely high feed accuracies. The feeder was molded from high-strength composite materials that provide a high degree of corrosion resistance.
The system was up and running within weeks. It operates around the clock, even through the tough Colorado winter. A large supply of lime is maintained at the site since access is difficult. Water continues to flow from the mine at approximately 550 gallons per minute. Without the treatment plant, many settling ponds would have been required.
EPA has spent more than $14 million so far in its response to the Gold King Mine spill. Water is successfully being treated, and EPA is pleased with the plant’s performance and is considering systems at other mine locations.
Ed Giacobbe has been regional sales manager at Vibra Screw Inc. since 2014. Before joining the company, he spent 30 years in the steel construction industry. Giacobbe holds a Bachelor of Engineering from Stevens Institute of Technology.
Vibra Screw Inc.