Africa, largely ignored in a US-China trade war that could roil economies worldwide, is quietly piecing together the world’s largest free-trade zone. The African Continental Free Trade Area comes into force on paper on Thursday after the required 22 countries ratified the deal a month ago. Once it’s passed by all 55 nations recognised as part of the African Union, it would cover a market of 1.2-billion people, with a combined gross domestic product of $2.5-trillion.