Soak the navy beans for 6-8 hours or overnight. Drain and rinse. Place in a pot with enough fresh water to cover them by 7.5cm (3 inches). Bring to a boil, remove from the heat, and drain. Place the beans back in the pot with enough fresh water to cover them by 7.5cm (3 inches), bring to a boil, reduce the heat to medium-low, and simmer for 30 minutes.

Meanwhile, heat 1/3 cup of the olive oil in a large, heavy skillet over medium-low heat. Cook the onion, celery, and carrot, stirring with a wooden spoon, until softened, about 10 minutes.

Add the sautéed vegetables to the beans, together with the tomatoes and another 1/3 cup of the olive oil. Simmer slowly until the beans are very tender and the soup is thick and creamy, 2 to 2 ½ hours, depending on the age and condition of the beans. About 20 minutes before removing from the heat, add the orange zest and season with salt. Just before removing from the heat, pour in the remaining olive oil.