Application of X-ray diffractometry and scanning electron microscopy to study the transformation of carnallite and thenardite to schoenite at 25 °C

AbstractAn experimental study was carried out on the preparation of schoenite from carnallite and thenardite by the double-decomposition method. According to the metastable phase diagram for the quinary system, Na+, K+, Mg2+//Cl–, and SO42−-H2O at 25 °C, three theoretical models were established for the double-decomposition processes. The mineralogy and morphology of schoenite, sylvite, epsomite/hexahydrite, and halite were studied by X-ray powder diffractometry and scanning electron microscopy during the double-decomposition processes. It is confirmed the preparation of schoenite via carnallite and thenardite is feasible. The double-decomposition reactions processes were described by three model equations. First, sylvite was obtained by carnallite decomposition in aqueous solution; second, epsomite/hexahydrite was obtained by a conversion reaction via thenardite and an MgCl2-rich mother liquor; third, schoenite was obtained via sylvite and epsomite/hexahydrite. The results suggest that (1) sylvite exists...