Rio Tinto Group, fending off a $189 billion hostile bid from BHP Billiton Ltd., will expand capacity at its Kennecott U.S. copper unit by 20 percent and more than double the life of the Bingham Canyon mine.

Upgrades to a conveyer in the second half will boost refinery capacity to 327,000 metric tons a year, London-based Rio said Friday. Capacity is currently about 272,000 tons.

Rio plans to extend mining at Bingham Canyon in Utah underground, beneath the open pit. The indicated and inferred resource below the pit is 637 million tons, an increase of 628 million tons from the company's previous estimate in December, Rio said in a separate statement.

The company is continuing pre-feasibility studies to examine how to develop the Utah deposit with expansion options to extend the mine life to at least 2036, said Bret Clayton, chief executive officer of Rio Tinto Copper. The previous extent of the mine's life was 2019, company spokesman Nick Cobban said.