U.S. coal producer Cloud Peak Energy Inc. said Friday its initial public offering of 30.6 million common shares priced at $15 per share, just below its expected range.

The company had expected to price the offering between $16 per share and $18 per share.

The offering generated gross proceeds of $459 million. Underwriters have the option to purchase up to an additional 4.6 million shares to cover over-allotments, which could boost proceeds.

Prior to its public launch, Cloud Peak had been a Rio Tinto PLC subsidiary overseeing mines in a coal-rich region of the West. A little more than a year ago, London-based Rio Tinto began considering options to divest its U.S. coal mining assets and formed the holding company, Cloud Peak.