Pebble projection: More gold, faster

Vancouver, British Columbia-based Northern Dynasty Minerals Ltd. is looking to speed up the rate of mining, and to improve gold recovery at its proposed Pebble porphyry copper-gold project near Illiamna in Southwest Alaska.

Because of the enormous resource, the mine can run at a higher rate, using bigger equipment, said Stephen Hodgson, Northern Dynasty engineer.

As Northern Dynasty revises its resource estimates upward, the estimated mine life increases even as the speed of mining increases, Hodgson said Nov. 4 in remarks to the Alaska Miners Association 2005 convention in Anchorage.

"When you add the extension, the potential extension of the inferred resource, we're approaching 40 years - from 27 years, and of course that's before the extensions of the resource that have just been announced," Hodgson said.

Metallurgical test work this season included four stages of grinding test work and four stages of flotation work, two of which are still in progress, he said.

Low to medium hardness

"The grinding work is telling us that the deposit is low to medium hardness," Hodgson said. "We're projecting grinding power consumption in the area of about 12.5 kilowatt hours per ton.

"The flotation work is confirming the results that we projected late last year," he said. "We're showing 26 to 28 percent copper concentrate grades, so what this has allowed us to do is to go with very large equipment to achieve the scale of throughput."

Northern Dynasty engineers are also looking at opportunities to improve gold recovery.

"On the gold side we're achieving good gold recoveries but the test work has shown that we can enhance this by installing a secondary gold recovery circuit, Hodgson said, adding that a number of options are being considered.

"We'll be reviewing the gold recovery circuit and assessing it against economic, environmental and socio-economic factors," Hodgson said.

Superlative size is the rule for the processing components.

"The current design is the ore trucks deliver to two large primary crushers, then two primary grinding circuits and each grinding circuit has a 45,000 horsepower SAG mill, the largest ever made," he said. "From the SAG mill (ore) would discharge directly into two 30-foot diameter, 32,000 horsepower ball mills - again the largest ever made - and from this circuit the ore will flow to the flotation circuit at about 140 microns."

After flotation, rougher concentrate would be reground to about 45 microns, cleaned in three stages, and then routed to a molybdenum separation circuit to separate the copper and molybdenum concentrates.

"We're big and getting bigger," Hodgson said. "More importantly is that the higher grade that we're finding is opening up opportunities and options for us that we didn't have even at the end of last year, and those opportunities are something that we're going to be spending some time evaluating."