Perpetua previews plans for tailings storage facility

Perpetua Resources, formerly known as Midas Gold, publicly previewed its tailings storage facility (TSF) for the Stibnite mine project in a  May 13 webinar. The mining company is in the permitting process to mine the previously abandoned Stibnite mine for both gold and antimony.

Tailings are the residue left over after processing ore. Many mines older than the Clean Water Act of 1970, the Surface Mining Act of 1977 and Superfund legislation took care of their tailings by just piling them on the ground, without any kind of  lining or barrier between these mine wastes and surface water or groundwater. Because many ore bodies are rich in sulfides, the interaction between water and tailings often results in the scourge of acid mine drainage.

Perpetua’s ore processing, from going into the plant through disposal of the tailings, is designed so that heavy metals leaching or acid mine drainage will not be a problem.

“The gold is disseminated in the sulfides so one of our first steps is a pyrite floatation to grab the pyrites and the arsenopyrites where the gold is,” Gene Bosley told the Idaho Business Review. Bosley is a senior civil engineer who has worked on designing the TSF for Perpetua since 2013. “That floatation takes out most of the sulfides, which then are pressure oxidized.” Floatation is a separation method that uses density differences to separate minerals in a slurry of ground-up rocks, flocculating and frothing agents, and water.

Perpetua is also capturing stibnite, the ore mineral of antimony, in its own flotation step. Stibnite is the mineral form of antimony trisulfide, which is also a sulfide. The current plan is to ship stibnite concentrates off-site for processing, so the the removal of this additional sulfide from the site will be by truck.

“The rest of our rocks are net neutralizing,” Bosley said, remarking that Perpetua has done exhaustive humidity cell tests to determine the chemical behavior of the tailings over time.

“We (will) add lime so there will be zero acid (producing material) in the tailings,” Bosley explained what happened to the leftover material  after the two-floatation step to remove the gold and antimony concentrates. The tailings will then be pumped as a thickened slurry with 50% water to the TSF.

Perpetua spent eight years picking the site for the TSF. The firm’s engineers looked at several potential sites before picking the site where legacy tailings from previous mining operations are piled without any liner. “The problem you have to solve is where can you put the TSF and what do you use to build it,” Bosley remarked.

“The legacy tailings are from the ’30s through the ’80s,” Bosley said during the webinar, discussing the four sites the company evaluated. “We didn’t want to use Blowout (Valley) because it was the site of a prior dam failure where there was no (dam) buttress, hence its name.” Fiddle Valley was rejected because the valley sides were too steep and the last valley was rejected because it was the location a large landslide. Meadow Creek Valley became the TSF site for these plus other reasons,  including valley walls of stable bedrock.

Between the tailings and the mouth of the valley, Bosley has designed a dam with a buttress that grows as the tailings grow, using a design that is “more stable in a seismic situation.” Perpetua will use clean igneous rocks from the pit to build the dam and buttress.

Perpetua will process the legacy tailings along Meadow Creek, to remove them from the TSF site and to extract economic mineralization left behind by older, less efficient mineral processing methods. The legacy tailing would need to be moved regardless because Perpetua will install a liner under the news tailings pile.

“We know from our testing that the legacy tailings have elevated arsenic and antimony,” Bosley commented. “We will spoon-feed them into the mill, mixed in with the material from the Yellow Mine pit. After four years, the majority of the legacy tailings will be gone.”

The dammed and buttressed acid-neutral tailings will then rest on their geotextile and bentonite liner. As they consolidate under their own weight, the slurry water will be expelled, collected and reused in the mill. Any water left over will not be able to penetrate through the liner to infiltrate groundwater.

“When the TSF is complete, 10% of its perimeter is dam and the rest is hillside,” Bosley said. “After post-tailings closure, it will take an estimated 25 years for the tailings to consolidate,” Bosley said. Consolidation effectively turns the loose sediment into a coherent soil. “The liner has an estimated half-life in excess of 450 years, so the tailings will be consolidated long before than.”

Perpetua will divert surface water in the Meadow Creek Valley around the TSF using lined channels designed to withstand a 100-year flood. “We have 40 years of stream gauge data so we know what kind of flows we can have,” Bosley concluded.

This article was updated on May 20 to clarify the flotation process at the mill. It was also corrected to remove U.S. Antimony as the smelter of the antimony ore. U.S. Antimony has agreed to test the antimony concentrates but has not committed to smelting them.