Rio Tinto has crept closer to getting enough backing for its improved $3.9bn (£2.4bn) offer for coal miner Riversdale, but it still needs at least one of its two big shareholders to say yes.It has received acceptances totalling 13.68% and has a relevant interest in 19.36% of the shares for a total of 33.04%, according to a stock exchange statement.The company recently increased its bid from $16 a share to $16.50, but only if more than 50% accept the deal by 23 March. Still no word though from Riversdale's two largest shareholders - India's Tata Steel and Brazil's CSN - who between them own 47% of the business. Institutions have been reluctant to commit to the deal without first knowing what the big two have decided.