Bountiful black bean soup

1 lb Dried black beans
      4 qt chicken stock
      1 bn Celery w/ leaves, chopped
      1 lb Carrots, peeled and chopped
      2 lg Onions, chopped
           Grated zest of 1 large lemon
    1/4 c  Lemon juice
      1 ts Salt
    1/4 ts Black pepper
      1 lg Lemon, sliced
 
  Place beans in large saucepan. Add enough water to cover beans by 1 inch.
  Bring to boil over high heat. Boil 1 minute. Remove pan from heat; cover
  tightly and let stand 1 hour. (Or soak beans overnight in large bowl with
  enough cold water to cover by 1 inch.) Drain. Combine beans in 5-quart
  Dutch oven or soup kettle with chicken broth, celery, carrots, onions, and
  lemon zest. Bring to boil over high heat. Reduce heat to low and simmer,
  uncovered, until liquid is just below surface of beans and ingredients are
  very tender, 2 1/2 to 3 hours. Using slotted spoon, transfer soup solids in
  batches to food processor and puree. Transfer puree to large bowl and add
  cooking liquid. (Or puree solids and liquids together in blender. Or force
  soup through coarse sieve or pass it through food mill, discarding skins.)
  Return soup to Dutch oven. Stir in lemon juice, salt and pepper and cook
  over medium heat, stirring constantly, until heated through. Place lemon
  quarter in bottom of each bowl. Pour in soup and serve immediately. Makes
  about 3 quarts of 10-12 servings.