UPDATE 2-Rio Tinto flags uncertain future at Australia's largest aluminium smelter

(Rewrites paragraph 1, updates with more details and background throughout) MELBOURNE, Oct 28 (Reuters) – Rio Tinto warned on Tuesday that Australia's largest aluminium smelter, Tomago, may be forced to shut down as it struggles to source power at commercially viable rates beyond 2028 when its current power deal expires. Tomago Aluminium is the biggest power user in New South Wales state, and like a slew of Australian smelters that are struggling with high energy prices as the country transitions to renewables, it was built last century to take advantage of Australia's plentiful and cheap coal. Power makes up more than 40% of Tomago's operating costs, and both coal-fired and renewable options are expected to rise sharply in price once its existing contract expires, jeopardising the smelter