Rio Tinto's US coal business Cloud Peak Energy has filed an application with the Securities and Exchange Commission (SEC) for an IPO worth as much as $500m.The move follows a previous application for a $1bn float about a year ago, then one for a less ambitious $250m fundraising last autumn.Cloud Peak includes most of Rio Tinto Energy America's (RTEA's) western US coal assets, operating two of the five largest coal mines in the region. No other details were made available.