Cook the pasta in boilingwater for 11 minutes until tender, then drain and return to the pan.
Meanwhile, cook the beans in boiling water for 3-4 minutes, then drain. Place half in a food processor with the lemon juice and pulse to make a paste. Stir this into the ricotta with the remaining beans, lemon zest and milk.
Stir into the pasta and add the chives. Season to taste.