The major miner also freed its revised offer from all conditions apart from the 50% minimum acceptance condition.

Rio is in the running for at least a 17.86% stake of Riversdale based on the acceptance notifications received under its institutional acceptance facility.

Tata Steel and Brazilian steelmaker Cia Siderurgica Nacional have both recently increased their stakes in Riversdale and collectively hold 44.72% of the company.

Riversdale’s flagship Benga coking coal mine in Mozambique is a joint venture development, with Tata owning 35%.

At full production under stage-two development, the Benga mine aims to export 6 million tonnes per annum of prime hard coking coal and 4Mtpa of export thermal coal by 2013.

Riversdale is also advancing a prefeasibility study for its nearby and bigger $2 billion Zambeze hard coking coal project, which holds more than 9 billion tonnes of resources.

Last year the company started investigating the long-term possibility that Zambeze could become a 90Mtpa run-of-mine operation.

Riversdale shares are down 31c to $14.82 this morning while Rio shares are down 2% to $80.72.