Arch, which announced the deal late Tuesday but did not disclose financial details, said it would gain 2 million tonnes of coal capacity this year and up to 2.5Mt for 2012 through 2015.

"This transaction is another important step in accomplishing our strategic objective of expanding Powder River Basin coal sales into the Asia-Pacific region," Arch chairman and chief executive Steven Leer said.

"This throughput agreement gives us direct, immediate access to the growing seaborne thermal market. It also complements our recently announced investment in the Millennium Bulk Terminal in Longview, Washington, and other continuing terminal negotiations."

Arch announced last week that it had acquired a 38% interest in MBT in a transaction valued at $US25 million plus additional consideration when project milestones are met.

RTI can load up to 12Mt of coal per annum, and expansion plans could increase that capacity up to 24Mtpa by 2015. Coal makes up more than 80% of the terminal’s total volume and Asia is its primary destination, with 8.3Mt of coal shipped last year.

"RTI's vision is to provide value to its parent company and expand its role as a leading trade gateway between North American and world markets," Ridley Terminals president George Dorsey said.

"This agreement is a very important contract for the terminal. Arch Coal's guaranteed US coal volumes will support our goal of doubling our capacity by 2015."

RTI can handle Panamax as well as Cape-size vessel loadings of up to 280,000t.