A process whereby substantially low-valent phosphorus-containing compounds in waste by-products obtained in the manufacture of mineral and organic phosphorus compounds are converted into useful phosphoric acid, polyphosphoric acid, or corresponding alkali metal salts thereof despite the presence of partially noncombustible material, organic pentavalent compounds, and nonphosphorus-containing compounds within said material; by introducing the by-products into an elemental phosphorus-oxygen flame having excess oxygen, the by-products being introduced in an amount so as not to exceed 50 percent by weight of the elemental phosphorus being burned in said flame.