Rio Tinto's A$3.9bn (£2.4bn) bid for Australian coal miner Riversdale has been declared unconditional.Eager shareholders who've already sent off their acceptance forms will get the $16 a share offered by 5 April, but will be watching closely to see if Rio gets enough backing to see that increased to $16.50.In an effort to get things moving, Rio has said it will pay the higher price if more than 47% of shareholders agree to the terms by 6 April. It had previously been 50%.By 9am in Sydney, Australia, Rio had secured an interest in 41.04% of Riversdale shares, up from 34.94% a week ago. But that still leaves it just under 6% short of the price hike trigger.It's thought Rio is in talks with Brazilian steelmaker CSN about winning its support. CSN and India's Tata Steel own about 47% of Riversdale. Institutions have been reluctant to commit to the deal without first knowing what the big two have decided.LW