I think it will take interest from a third party to put a fire under RIO they are in no hurry to throw a pile of cash at IOH but they might move faster if Chinese interest bobs up or if FMG / BHP move into some formal negotiation / offer process.
It doesn't make sense because if they wait, they are likely to pay much more in a bidding war... Perhaps RIO have simply decided they don't want IOH?
