GILLETTE, Wyo. (

TheStreet

) --

Cloud Peak Energy's

initial public offering of 30.6 million shares was priced at $15 a share, below the expected range of $16 to $18 a share.

Cloud Peak, a subsidiary of

Rio Tinto

( RTP), raised about $459 million in the IPO. The proceeds will go to Rio Tinto, which will retain a stake of just above 48% in Cloud Peak.

Cloud Peak

is the third-largest U.S. coal producer, overseeing mines in the coal-rich regions of Montana, Wyoming and Colorado.

Shares of Cloud Peak will begin trading Friday on the

New York Stock Exchange

under symbol "CLD."

-- Reported by Joseph Woelfel in New York

.

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