Mechanistic insights into CaCl2-assisted chlorination roasting of K-bearing syenite: an in-situ synchrotron X-ray diffraction and thermodynamic study

Potash fertilizers are essential for global agricultural productivity, yet their production relies heavily on limited and unevenly distributed evaporite deposits. Ultrapotassic syenite, a potassium-rich heterogeneous silicate rock (up to 15 wt% K 2 O), represents a promising alternative resource for sustainable potash production. This study investigated the mechanistic aspects of CaCl 2 -assisted chlorination roasting of ultrapotassic syenite through an integrated experimental and thermodynamic approach. Automated mineralogy (TIMA) was used to identify potassium-hosting phases and their distribution within the silicate matrix. Thermodynamic assessment was conducted using the CALPHAD-based FactSage database and isothermal ternary phase diagrams of the CaCl 2 -SiO 2 -K 2 O and CaCl 2 -Al 2 O 3 -K 2 O systems at 700, 800, and 900 °C, which confirmed temperature-dependent expansion of the chloride liquid field and the stability domains of potassium-calcium silicates and aluminates. Furthermore, temperature-resolved in-situ synchrotron X-ray diffraction (SXRD) monitored real-time mineralogical transformations during heating up to 900 °C. The results showed that feldspathic minerals remain structurally stable below 700 °C; above this temperature (∼806 °C), molten CaCl 2 enhanced ionic mobility and promoted the formation of sylvite (KCl) and calcium-bearing silicates, including anorthite (CaAl 2 Si 2 O 8 ), wollastonite (CaSiO 3 ), and grossular (Ca 3 Al 2 Si 3 O 12 ). SEM-EDS observations supported the microstructural evolution, and a conceptual reaction mechanism was proposed, refining the ion-exchange theory through structural disintegration, Ca 2+ /K + ion exchange, and formation of sylvite and secondary calcium silicates, enabling efficient potassium extraction.