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In this present work, the transformation of the Moroccan phosphogypsum (PG) waste, considered as a potential source of sulfate, into potassium sulfate compound could help to reduce environmental impact and to create a new value chain for the phosphate industry. Generally, solidliquid equilibria are frequently applied in chemical industries. They are a valuable aid for visualizing the precipitation, separation, purification of a solid phase and pathways through which crystallization can proceed. The aim of this process is to produce potassium sulfate, a high-value fertilizer, from sulfate solutions obtained after dissolving Moroccan phosphogypsum in NaOH medium. The quaternary phase diagram Na + , K + // Cl - , SO 4 2- - H 2 O at 25°C was especially used to determine the operating conditions and the design of a crystallization process during the phosphogypsum conversion into potassium sulfate. The Jänecke representation of this system enables the determination of the optimal trajectory in the phase diagram for the double decomposition reaction. X-ray Fluorescent (XRF) and X-ray diffraction (XRD) techniques were conducted to identify the crystalline phases formed. The results of this study could be contributing to the development of a sustainable valorization of Moroccan phosphogypsum. Furthermore, the potassium sulfate represents a good alternative to potassium chloride for chloride-sensitive crops.