Solution-driven processing of calcium sulfate: The mechanism of the reversible transformation of gypsum to bassanite in brines Stawski, Tomasz M. Karafiludis, Stephanos Pimentel, Carlos Montes Hernández, German Kochovski, Zdravko Bienert, Ralf Weimann, Karin Emmerling, Franziska Scoppola, Ernesto Van Driessche, Alexander Edgard Suzanne Gypsum Bassanite Calcium sulfate Here, we show that calcium sulfate dihydrate (gypsum) can be directly, rapidly and reversibly converted to calcium sulfate hemihydrate (bassanite) in high salinity solutions (brines). The optimum conditions for the efficient production of bassanite in a short time (<5 min) involve the use of brines with c(NaCl) > 4 M and maintaining a temperature, T > 80 ºC. When the solution containing bassanite crystals is cooled down to around room temperature, eventually gypsum is formed. When the temperature is raised again to T > 80 ºC, bassanite is rapidly re-precipitated. This contrasts with the better-known behaviour of the bassanite phase in low-salt environments. In low-salinity aqueous solutions, bassanite is considered to be metastable with respect to gypsum and anhydrite, and therefore gypsum-to-bassanite conversion does not occur in pure water. Interestingly, the high-salinity transformation of gypsum-to-bassanite has been reported by many authors and used in practice for several decades, although its very occurrence actually contradicts numerical thermodynamic predictions regarding solubility of calcium sulfate phases. By following the evolution of crystalline phases with in situ and time-resolved X-ray diffraction/scattering and Raman spectroscopy, we demonstrated that the phase stability in brines at elevated temperatures was inaccurately represented in the thermodynamic databases. Most notably for c(NaCl) > 4 M, and T > 80 ºC gypsum becomes readily more soluble than bassanite, which induces the direct precipitation of the latter from gypsum. The fact that these transformations are controlled by the solution provides extensive opportunities for precise manipulation of crystal formation. Our experiments confirmed that bassanite remained the sole crystalline phase for many hours before reverting into gypsum. This property is extremely advantageous for practical processing and efficient crystal extraction in industrial scenarios. 2024-06-13T07:42:32Z 2024-06-13T07:42:32Z 2024-01-29 journal article Stawski, Tomasz M., et al. Solution-driven processing of calcium sulfate: The mechanism of the reversible transformation of gypsum to bassanite in brines. Journal of Cleaner Production 440 (2024) 141012 [10.1016/j.jclepro.2024.141012] https://hdl.handle.net/10481/92555 10.1016/j.jclepro.2024.141012 eng info:eu-repo/grantAgreement/EC/H2020/MSC 101021894 http://creativecommons.org/licenses/by/4.0/ open access Atribución 4.0 Internacional Elsevier