Combine flour, sugar, baking powder, and salt in a medium mixing bowl. Cut in shortening with a pastry blender until mixture resembles coarse meal. Stir in orange rind.
Dissolve soda in buttermilk; stir well. Pour buttermilk mixture evenly over flour mixture, stirring until dry ingredients are moistened. Turn dough out onto a lightly floured surface; knead 4 to 5 times.
Roll dough to 1/2-inch thickness; cut with a 2-inch biscuit cutter. Place biscuits on a greased baking sheet; brush with melted butter. Bake at 450° for 10 minutes or until lightly browned.