Model-Based Construction of Calcium Sulfate Phase-Transition Diagrams in the HCl−CaCl₂−H₂O System between 0 and 100 °C

Abstract

Phase-transition diagrams of three CaSO4 phases, namely, dihydrate, hemihydrate, and anhydrate, in the HCl−CaCl2−H2O system are successfully constructed making use of a recently developed OLI-based chemical model. After initial validation of the model by comparison to experimentally determined CaSO4 phase-transition points in H2O or pure HCl solutions, the phase-transition border between dihydrate and anhydrite and that between dihydrate and hemihydrate was obtained by calculating the solution supersaturation (or scaling tendency according to OLI). The constructed phase-transition diagrams show three main regions, namely, regions I, II, and III. In region I, dihydrate is stable, while anhydrite is stable in regions II and III. Dihydrate is metastable in region II, while hemihydrate is metastable in region III. An increase in HCl and/or CaCl2 concentration causes the metastable region of HH to expand at the expense of DH. This has the result of lowering the corresponding transition temperatures.

The crystal growth kinetics of alpha calcium sulfate hemihydrate in concentrated CaCl2–HCl solutions

“…HH has limited metastability, quickly converting to anhydrite, a condition that again is undesirable as not only leads to a loss of α-HH but also creates crystal de-watering/separation problems. Background experimental and modeling results on calcium sulfate phase transitions in CaCl 2 -HCl solutions can be found in previous publications [19,23].…”

Phase transformation kinetics of calcium sulfate phases in strong CaCl2HCl solutions

“…Research on the determination of process conditions (or operating windows) such as operating temperature and concentration levels for the production of high quality calcium sulfate dihydrate (DH) or the valuable calcium sulfate α-hemihydrate (α-HH) phase has been reported previously (Al-Othman and Demopoulos, 2009;Girgin and Demopoulos, 2004). However, thermodynamic modeling and experiments showed that especially in the presence of high strength HCl and modestly high temperatures (>~40°C) calcium sulfate anhydrite (AH) is the thermodynamically stable phase (Li and Demopoulos, 2006). However, the formation of AH is undesirable due to its fibrous morphology, which results to extremely poor filtration rates compared to high filtration rates obtained for DH or α-HH (Al-Othman and Demopoulos, 2009;Girgin, 2006).…”

“…Since DH and HH are metastable phases in the CaCl 2 \HCl\CaSO 4 aqueous system over a wide range of conditions (Li and Demopoulos, 2006), it is necessary to investigate the transformation kinetics of these phases.…”

Preparation of calcium sulfate whiskers using waste calcium chloride by reactive crystallization

“…With the given concentrations of reactants, different reaction temperature may result in different phase state of calcium sulfate [16]. CaSO 4 ·2H 2 O can be formed below 40 °C and CaSO 4 becomes the stable form with the increase of temperature.…”