A Chinese company is investing heavily to restore two Maine paper mills it bought last year and turn them into sustainable, long-term businesses. The head of the company’s U.S. operations said his company, ND Paper, wants to improve efficiency and update operations at the former Catalyst Paper Mill in Rumford and the former Expera Old Town pulp mill, both of which ND bought last year. To do so, the company is rolling up its sleeves, fixing oil leaks, repairing equipment and adding more efficient machinery. ND Paper is investing close to $500 million in the four U.S.