Solid-liquid phase equilibria of ternary system NH4+, Ca2+ // SO42-- H2O at 323.2 and 348.2 K, and its application in the comprehensive utilization of waste secondary gypsum

To solve the massive accumulation of gypsum generated from polyhalite leaching and secondary gypsum, this study investigated phase equilibria of the ternary system NH 4 + , Ca 2+ // SO 4 2- - H 2 O at 323.2   K and 348.2   K. The phase diagrams included 2 invariant points, 3 univariate curves, and 3 crystallization regions corresponding to (NH 4 ) 2 SO 4 , CaSO 4 ·2H 2 O, and double salt koktaite (NH 4 ) 2 Ca(SO 4 ) 2 ·H 2 O. As temperature increased, the crystallization region of CaSO 4 ·2H 2 O contracted, whereas those of (NH 4 ) 2 SO 4 and koktaite expanded, indicating that the temperature-enhanced was beneficial to improve the crystallization efficiency of koktaite. Guided by multitemperature phase diagram, the influencing factors and optimal conditions of the comprehensive utilization process of phosphogypsum were explored. The synthesis of koktaite achieved maximum efficiency under conditions of 1: 1.5 molar ratio, 348.2   K reaction temperature, 90   min duration, and 250   rpm agitation. Concurrently, under solid-liquid ratio of 1: 0.5 and after three leaching cycles of 30   seconds each, the yield of CaSO 4 ·2H 2 O production reached 99.80% with a purity of 99.52%. However, the purity and yield of product decreased due to the accumulation of impurities during the cycles, stabilizing after 6 cycles (koktaite yield: 98.50%; purity: 95.69%; (NH 4 ) 2 SO 4 yield: 97.69%; purity: 95.42%). Granulated koktaite released NH 4 + through layer-by-layer dissolution, which prolonged the dissolution time, and the complete dissolution time was 6 times and 24 times longer than that of ungranulated koktaite and (NH 4 ) 2 SO 4 . This work integrates theoretical phase behavior analysis with process engineering to establish a circular resource recovery strategy for synthesizing high-value products from phosphogypsum.