Copper North Mining Corp. July 2 reported that metallurgical work shows that a finer crush and higher temperatures improve the efficiency for recovering the copper, gold and silver at its Carmacks project in Yukon Territory.

A new processing plan consists of crushing to minus-19 millimeters, followed by rod mill grinding to minus-1 then acid leaching copper in an agitated leach tank, followed by gold and silver leaching in similar tanks.

The copper leachate is to be processed in a standard solvent extraction electro-winning plant to produce cathode copper sheets, and gold and silver will be recovered using Merrill Crowe to produce doré bars.

Test work indicates that raising the temperature of the acid leach for copper improves the overall recovery and significantly shortens the leach time.

With a grind size of minus-1mm and a leach temperature of 40 degrees Celsius, recoveries of 88 percent were achieved within four to six hours, less than half the time for a coarser grind at ambient temperature.

Copper North said waste heat from the manufacturing of sulfuric acid will be used to heat the leach tanks.

Washed slurry is pumped into the gold-silver circuit for cyanidation; also an agitated tank leach.

Optimal leach time for a two-stage cyanidation circuit is about 12 hours with recoveries of some 80 percent for gold and 62 percent for silver.

With this new information, Copper North said the design work for a process plan as part of a new preliminary economic assessment can resume.

The improvements based on the new metallurgical test results should lead to a reduction of equipment size in the leach circuit as compared to the previous studies and are expected to have a significant positive impact on project economics.

The company has set a new initial capital cost target of US$150 million for Carmacks, roughly 30 percent lower than costs outlined in a 2014 PEA.

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Shane Lasley, Publisher

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