Effects of Impurities on CaSO 4 Crystallization in the Ca(H 2 PO 4 ) 2 -H 2 SO 4 -H 3 PO 4 -H 2 O System

Wet-process phosphoric acid is a fundamental process in the fertilizer industry. The influence of impurities on crystallization kinetics of CaSO was investigated in the Ca(H PO ) -H SO -H PO -H O system using a mixed suspension mixed product removal crystallizer. Effects of Si, Al, Fe, Mg, K, and Na on crystal morphology and structure were examined in the highly acidic system through scanning electron microscopy and high-resolution transmission electron microscopy. Results show that the increase of Mg, K, and Na content facilitates crystal growth. Si, Al, and Fe are beneficial to CaSO crystal growth at a certain concentration range. Impurities also affect the crystal morphology, and the addition of Si, Fe, and Na promotes the formation of needle-like crystals compared with other impurities. X-ray diffraction results show that the preferred crystal growth direction is (020), and the interplanar spacing of the crystals is affected by the element radius of the impurities.