Solvothermal recrystallization of α-calcium sulfate hemihydrate: Batch reactor experiments and kinetic modelling

Abstract

International audienceUnder appropriate temperature conditions, natural gypsum CaSO4·2H2O, dispersed in an aqueous solution, turns into calcium hemihydrate CaSO4·½H2O. This transformation is performed in a 2 L stirred baffled reactor, where the temperature increase is measured and controlled on line. The water content of the suspension and its size distribution are measured on samples during the transformation. Experiments are achieved at nominal temperature of 140 °C, with three initial solid mass fractions 0.5, 0.33 and 0.25. The transformation takes place through a dissolution followed by re-crystallization. A model is proposed which takes into account the size distribution of the particles of gypsum, their dissolution rate, primary and secondary nucleation and growth rates of calcium hemihydrate. The set of equations is solved with a MATLAB software, which allows to test the assumptions on the kinetics of the transformation and fit their parameters. A satisfying representation of the variations of the extent of transformation and of volume and surface mean diameters of the suspension is obtained

Crystallization Kinetics and Mechanisms of Calcium Sulfate Dihydrate: Experimental Investigation and Theoretical Analysis

“…Currently, all sorts of physicochemical factors have been found to be responsible for the crystallization process of CSD, such as temperature, hydrodynamics, pH value, and polymer. − However, the crystallization process is a complex process of heat and mass transfer, which involves macro instability; the primary nucleation, secondary nucleation, and crystal growth are involved in the stirred tank during the crystallization process, leading to the kinetic analysis more complicated. , In addition, the components in the crystallizer are unevenly distributed, and local supersaturation will greatly affect the crystallization process and product performance . Hence, no consensus about the crystallization process of CSD has been obtained so far.…”

“…Namely, the comprehension of the crystallization process of CSD is often fraught with enormous difficulty, owing to the complexity of the crystallization process involved . Fortunately, the theoretical models may be referred to as a responsible election to surmount the problems. − …”

Novel Synergistic Process of Impurities Extraction and Phophogypsum Crystallization Control in Wet-Process Phosphoric Acid

“…As the reaction time increased to 5 h, the diffraction peak of calcium sulfate dihydrate was strong, and the change of phosphogypsum morphology may be related to the phase leading to the formation of microanhydrite particles . The increase of reaction time leads to the secondary nucleation of the crystal, and these particles may serve as a feedstock for gypsum crystal growth by a dissolution-reprecipitation mechanism . As a consequence, the change of particle size and the generation of tiny particles increase the washing and filtration difficulties of phosphogypsum .…”

“…25 The increase of reaction time leads to the secondary nucleation of the crystal, and these particles may serve as a feedstock for gypsum crystal growth by a dissolutionreprecipitation mechanism. 26 As a consequence, the change of particle size and the generation of tiny particles increase the washing and filtration difficulties of phosphogypsum. 27 Residual P 2 O 5 content in phosphogypsum increased from 2.189 to 3.452% with the increase of reaction time from 5 to 7 h. Under the condition of adding NOA, the particle size of phosphogypsum was distributed in the range of 20−40 μm and 900 μm.…”

Preparation of α-High-Strength Hemihydrate from Flue Gas Desulfurization Gypsum in AlCl3–MgCl2 Solution at Atmospheric Pressure

“…The quality of high-strength gypsum depends on the PSD, the impurity content, and the moisture content. These characteristics will in turn be determined by the crystallization process such as nucleation, crystal growth, crystal agglomeration, and crystal breakage. − Phase transformation and dehydration of calcium sulfate dihydrate in acidic media was carefully studied by Sirota et al with SEM to bring more additional data casting light on the mechanism of phase transformation. In this work, our findings through SEM morphologies reveal that the phase transition mechanisms of reagents DH and FGD gypsum to HH in AlCl 3 solution were similar to the ones proposed by Sirota et al…”