Pebble files revisions to mining plan

Pebble Limited Partnership has made changes to its mine plan that would slow its mining rate but increase its ore processing while potentially lessening the project’s environmental impact, according to a document filed with the Corps of Engineers May 11.

The Corps published the five-page overview of the plan changes on the project EIS website May 21.

The revisions would cut the peak mining rate from 90 million tons of ore per year to 75 million tons; at the same time the milling rate would grow from 160,000 tons per day in the original plan submitted to the Corps to 180,000 tons per day.

Pebble had planned to stockpile up to 330 million tons of low-grade ore mined during the first 14 years for processing in the latter years of the initial 20-year mine.

The mining-milling adjustments mean the project would now mine roughly 1.5 billion tons of material — with about 200 million tons of that being waste rock — up from the original plan of 1.2 billion tons. From that, annual production should increase by about 10 percent to 660,000 tons of copper-gold concentrate and 16,500 tons of molybdenum concentrate.

Mining more material means the pit dimensions “will increase slightly” from the 6,500 feet long; 5,500 feet wide and up to 1,750 feet deep mine contemplated in the plan submitted in December, according to the five-page summary of the changes. The specific changes to the pit dimensions are not detailed.

The onsite power plant will also need to grow from 230 megawatts to 270 megawatts of capacity to accommodate the increased mill throughput, according to Pebble.

By not storing the potential acid-generating low-grade ore Pebble will not have to treat runoff water from the stockpile, the document notes.

Further changes are also being made to the tailings storage facility. Originally, Pebble designed a single storage facility with two segments, one for bulk tailings and another to hold pyritic tailings. The updated plan includes separating the bulk and pyritic storage areas and moving the bulk tailings storage about 2,000 feet south to incorporate more of the existing terrain into the tailings dam construction, according to Pebble.

Oxidization of pyritic tailings can lead to acid rock drainage depending on the amount of sulphur in the tailings.

A new, lined pyritic tailings storage facility located closer to the pit will also house potentially acid-generating mine waste during operations. That waste will then be moved into the pit when the mine is closed, allowing the harmful waste to be permanently stored under water and below ground level.

The tailings storage changes also eliminates the need for perpetual water treatment of the pyritic tailings and storing the material below-ground also removes all risk of downstream impacts related to a pyritic tailings storage facility failure, Pebble notes.

The original plan for two water management ponds has also been changed to a single, much larger lined pond built using rock fill similar to how the tailings storage dams will be built.

Pebble spokesman Mike Heatwole wrote via email that the changes were made to enhance the overall project and make environmental improvements to the plan. Milling the low-grade ore throughout the project versus waiting until the end makes for a more efficient mining process, while the other adjustments will help during mine closure, according to Heatwole.

He said Pebble couldn’t discuss how the changes impact the project’s economics until the company publishes its preliminary economic assessment, which is expected later this year.

Heatwole added that the project’s wetlands impact — pegged at 3,190 acres in the original submittal — has not been fully quantified but Pebble doesn’t expect it to change significantly.

Corps of Engineers Project Manager Shane McCoy said as of May 24 Pebble had not submitted engineering designs of the proposed changes, but agency officials expected to get more detailed documents on the new plans soon.

“Everything that they have proposed to date is a reduction in the proposed impacts to aquatic resources or navigable waters, so the Corps does not believe these are major changes other than the fact that it’s a reduction in scope,” McCoy said.

Finally, Pebble has concluded after further study of its shipping plan that it no longer needs a deepwater port. The company is now proposing to shuttle concentrate containers with barges from the Amakdedori port site to bulk freighter vessels that could moor at two locations 12 and 18 miles offshore from west Cook Inlet port.

Consumable materials and fuel would go directly to the port on barges without being lightered, according to Pebble.

Cargo is lightered to reduce a vessel’s draft, making the individual portions of the overall bulk shipment lighter to allow for travel in shallower water.

Keeping the bulk freighters offshore will end the need for channel dredging and storing up to 20 million cubic yards of dredged material at the port.

Pebble is also evaluating the possibility of a “high-tide only” access port to further reduce dredging requirements, according to the outline document.

Elwood Brehmer can be reached at [email protected].

Updated: 
05/30/2018 - 10:48am

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