Bumper iron ore prices have enabled Rio Tinto to pay the second-biggest half-year dividend in its history, but its strong financial results have been marred by $US1 billion ($1.4 billion) of impairments on struggling Australian aluminium smelters and diamond mines.

Rio announced underlying earnings of $US4.75 billion on Wednesday, which was better than the $US4.36 billion expected by analysts surveyed by VUMA.