**ii. Thermal process phosphoric acid (TPA).**

**Fig. 9.** The scheme of phosphate processes [57].

Since the concentration and the purity grade of phosphoric acid depend on the purpose, the content of waterless H3PO4 in the product is usually expressed as P4O10, where waterless H3PO4 = 1.38 × P4O10. For example, the product of TPA with the content of 66.33% P4O10 contains 66.33 × 1.38 = 91.54% of waterless H3PO4. With increasing content of P4O10 in the system, pyrophos‐ phoric acid (diphosphoric acid, H4P2O4) becomes the dominant species.
