The world's largest mining group, BHP Billiton, is considering a rival bid for aluminum producer Alcan and has hired an investment bank, sources familiar with the matter said. The emergence of BHP as a possible white knight in the face of Alcoa's $28.7 billion hostile offer for Alcan came as Alcoa's stock hit a six-year high on a report that the U.S. aluminum company, and not its Canadian rival Alcan, was a target of BHP. Alcoa stock eased to close less than 1% higher.