Rosemont Copper says it plans to reduce truck traffic, energy use and air emissions at its proposed mine, while keeping the number of jobs the same at about 450.
That's due to changes in the technologies and production levels it now plans for the Rosemont Mine southeast of Tucson.
But the company's statements, made in its latest letter to the U.S. Forest Service, have drawn skepticism from critics.
On truck traffic, a table included in the letter from Rosemont Copper Vice President Kathy Arnold predicts the total number of daily mine truck shipments will drop from 78 to 69.
One reason is that 81 weekly shipments of copper cathodes and sulfuric acid will no longer be made because the company is no longer going to produce copper cathodes through heap leaching.
She also credits the reduction in shipments to the fact that the copper concentrates produced at the mine now will have a higher grade compared with earlier estimates.
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The table shows the total amount of copper concentrate shipped declining from 484,700 to 439,000 tons per year, although it also shows the number of daily concentrate truckloads would rise from 46 to 50.
It's unclear how the company determined its copper concentrate grade improved. Rosemont didn't respond to a question from the Star about the concentrate grades.
If you divide the total amount of copper produced by the total amount of copper concentrate as shown in a 2009 feasibility study and a new study released in July, the concentrate grade comes out roughly 30 percent in both cases.
Retired Tucson mineralogist Gary Kordosky said, "If you took total copper they will produce and divide it by the total concentrate, and they are roughly the same in both cases, I don't know how you haul less concentrate out of the mine." Kordosky said he doesn't have a strong opinion on the mine.
Fergus Graham, a retired mining exploration geologist active in the mine opposition group Save the Scenic Santa Ritas, said that due to the company's plan to increase its total copper production, there would also likely be more shipments of pebble lime, diesel fuel, ammonium nitrate and other materials than before.
Arnold's letter shows those shipments staying the same.
On air emissions, Arnold's letter predicts that the cancellation of heap leaching and related facilities will reduce emissions by 4,852 tons annually, mostly through reductions in carbon dioxide. The letter said nothing about impacts on emissions of the mine's increased sulfide production.
Graham predicts increased air emissions. But Kordosky said that even with the sulfide production increase, total emissions shouldn't increase much if at all. The new Rosemont feasibility study said some rock that was going to be disposed of as waste rock will now be processed into concentrates, Kordosky explained. Total rock mined will rise only 3.4 percent, while total ore processed will go up 21 percent. And the bulk of air emissions comes from vehicles hauling copper ore, he said.
A two-day report
Sunday: Water use.
Today: Other impacts.
Contact reporter Tony Davis at tdavis@azstarnet.com or 806-7746.