Shares of aluminum producer Alcoa slumped Monday on President Trump’s pledge to double tariffs on imported aluminum and steel, while its U.S. rival Century Aluminum surged on the prospect of a 50% duty starting Wednesday. Alcoa in April estimated its cost for the 25% duty would be $90 million for the current quarter. Chicago-based Century Aluminum, Alcoa’s smaller competitor, supplies the U.S. market from a pair of U.S. smelters.