BHP Billiton Ltd. signage is mounted on a wall in the lobby of the company’s headquarters in Melbourne, Australia, on Friday, July 15, 2011. BHP Billiton Ltd., the world’s largest mining company, agreed to acquire Petrohawk Energy Corp. for about $12.1 billion in cash in its biggest acquisition, betting natural gas demand will gain in the U.S. Photographer: Carla Gottgens/Bloomberg