A phosphorous pentoxide producing method includes forming a kiln bed using feed agglomerates with a calcium-to-silica mole ratio of less than 1.0 and maintaining a bed temperature at or above 1180.degree. C. along at least 50% of the bed length without exceeding 1380.degree. C. along the entire bed length. Less than 10% of the agglomerates' phosphate input to the kiln remains in the kiln residue as phosphate. Another method includes maintaining a bed temperature at or above 1180.degree. C. along a portion of the bed length and establishing a bed surface-to-volume ratio multiplied by a time for bed heat up to 1180.degree. C. of less than 50 minutes-ft.sup.2/ft.sup.3.