A TAIWANESE banking consortium is close to approving a US$5.3 billion syndicated loan for the island's Formosa Plastics group to build a petrochemical complex, a spokesman for the consortium said. Terms for the loan had more or less been settled. ''We will sign an agreement with Formosa Plastics after individual banks formally approve their loans,'' said Lee Yung-san, president of state-owned Chiao Tung Bank. It would be the largest financing facility ever granted to a private Taiwanese business group, bankers said. The US$7.5 billion petrochemical complex, Taiwan's largest private investment, will include the island's first private naphtha cracker and an oil refinery. It is expected to be completed in the late 1990s.