SAO PAULO - The board of Vale SA , the world’s largest iron ore producer, on Wednesday approved plans to raise 1 billion reais ($287 million) from the sale of debt notes to expand the railway system at its Carajas ore mine. Vale plans to file a request with the securities industry watchdog CVM on Thursday to approve the sale of the so-called infrastructure notes in two portions, according to a securities filing. The notes will have maturities of five and seven years and could be increased by 350 million reais pending certain conditions, the filing said. Credit ratings agency Moody’s Investors Service rated the senior unsecured notes at Baa2, the second-lowest investment-grade ranking, with a “negative” outlook. Though Vale has a fairly diversified portfolio of mining assets in areas such as nickel and coal, most of its revenue comes from iron ore. Moody’s said its negative outlook was based on the agency’s expectations that iron ore prices “will not experience any meaningful recovery before 2017” due to the slowdown in China’s economic growth and steel production. Vale’s railway investment is part of its expansion of production at its Carajas iron ore mine in Para state in the Amazon Basin.