Influence of disodium hydrogen phosphate dodecahydrate on hydrothermal formation of hemihydrate calcium sulfate whiskers

The influence of Na_2HPO_4·12H_2O on the hydrothermal formation of hemihydrate calcium sulfate(CaSO_4·0.5H_2O) whiskers from dihydrate calcium sulfate(CaSO_4·2H_2O)at 135 ℃ was investigated.Experimental results indicate that the addition of phosphorus accelerates the hydrothermal conversion of CaSO_4·2H_2O to CaSO_4·0.5H_2O via the formation of Ca_3(PO_4)_2 and produces CaSO_4-0.5H_2O whiskers with thinner diameters and shorter lengths.Compared with the blank experiment without Na_2HPO_4·12H_2O,the existence of minor amounts(8.65 ×10~(-4)-4.36 × 10~(-3) mol/L) of Na_2HPO_4·12H_2O led to a decrease in the diameter of CaSO_4·0.5H_2O whiskers from 1.0-10.0 to 0.5-2.0 μm and lengths from 70-300 to50-200 μm.     

Preparation of calcium sulfate whiskers from FGD gypsum via hydrothermal crystallization in the H2SO4-NaCl-H2O system

Little attention has thus far been paid to the potential effect of solution composition on the hydrothermal crystallization of calcium sulfate whiskers prepared from flue-gas desulfurization(FGD) gypsum.When purified FGD gypsum was used as raw material,the morphology and phase structure of the hydrothermal products grown in pure water,H_2SO_4-H_2O,NaCl-H_2O,and H_2SO_4-NaCl-H_2O solutions as well as the solubility of purified FGD gypsum in these solutions were investigated.The results indicate that calcium sulfate whiskers grow favorably in the H_2SO_4-NaCl-H_2O system.When prepared using 10-70 g NaCl/kg gypsum-0.01 M H_2SO_4-H_2O at 130 ℃ for 60 min,the obtained calcium sulfate whiskers had diameters ranging from 3 to 5 |xm and lengths from 200 to 600 |xm,and their phase structure was calcium sulfate hemihydrate(HH).Opposing effects of sulfuric acid and sodium chloride on the solubility of the purified FGD gypsum were observed.With the co-presence of sulfuric acid and sodium chloride in the reaction solution,the concentrations of Ca~(2+) and SO_4~(2-) can be kept relatively stable,which implies that the crystallization of the hydrothermal products can be controlled by changing the concentrations of sulfuric acid and sodium chloride.     

Direct transformation of FGD gypsum to calcium sulfate hemihydrate whiskers: Preparation,simulations, and process analysis

Calcium sulfate hemihydrate (CSH) whiskers were synthesized by phase transition in CaCl₂ solution under atmospheric pressure. Analytical-grade calcium sulfate dihydrate (AR CSD) was used as the raw material for the synthesis of CSH whiskers, according to orthogonal experiments. The effects of reaction temperature, AR CSD content, H₂SO₄ content, and reaction time were investigated, and the crystallization conditions were optimized. The as-prepared CSH whiskers displayed a regular morphology and a highly uniform size, with an aspect ratio of 105. A simulation system was also established by blending various sulfates with AR CSD, to evaluate the effects of impurities in flue gas desulfurization (FGD) gypsum. The main aim was to prepare CSH whiskers directly from FGD gypsum, without any purification, using the optimized conditions. This is a facile potential alternative process for large-scale production of CSH whiskers using abundant FGD gypsum as source materials.     

Influence of temperature and solution composition on the formation of calcium sulfates

T Calcium sulfates(anhydrite and hydrates)were synthesized by mixing CaCl2 and Na2SO4 solutions at room temperature followed by aging the resulting slurries at elevated temperatures.The variation of the morphology and structure of the calcium sulfates with aging temperature was investigated.Experimental results indicated that CaSO4.2H2O plates,CaSO4.0.5H2O whiskers and CaSO4 spindles were formed at≤100℃,130-160℃ and ≥170℃,respectively.The formation and conversion of the calcium sulfates were discussed on the basis of characterization of the products and chemical analysis of the solutions.Compared to NaCl solution,pure water favors one-dimensional hydrothermal growth of CaSO4.0.5H2O whiskers owing to lower supersaturation.     

Structural characteristic and formation mechanism of hemihydrate calcium sulfate whiskers prepared using FGD gypsum

Hemihydrate calcium sulfate whiskers (HH-CSWs) were hydrothermally synthesized in a sulfuric acid solution at 120 °C for different holding times (20, 40, and 60 min). The phase structures and morphologies were characterized by XRD and SEM, respectively. The XRD pattern of the sample under 60 min was refined via the Rietveld fitting method. The structure models of the HH-CSW sample under a 60-min holding time was established based on Rietveld fitting results. No difference in the positions of diffraction peaks was determined. The as-prepared holding time increased the intensity and aspect ratio of the diffraction peaks of both samples. In the prepared HH-CSW structure, Ca–O polyhedron is a 12-sided polyhedron similar to that in the gypsum structure; the Ca atom is located in two positions in the one-unit cell; the H₂O channel along the c-axis similar is O-shaped and bigger than that in hexagonal and monoclinic CaSO₄·0.5H₂O structures. Therefore, although the prepared HH-CSW crystals’ structure are similar to that of the reported hexagonal and monoclinic CaSO₄·0.5H₂O structures, they are not the same. The formation mechanism of HH-CSW from flue gas desulfurization (FGD) gypsum is discussed based on the analysis of gypsum structure.     

Structural characteristic and formation mechanism of hemihydrate calcium sulfate whiskers prepared using FGD gypsum

Hemihydrate calcium sulfate whiskers (HH-CSWs) were hydrothermally synthesized in a sulfuric acid solution at 120 ℃ for different holding times (20,40,and 60 mi...     

A readily monitored and controllable hydrothermal system for the facile,cost-effective transformation of FGD gypsum to calcium sulfate hemihydrate whiskers

The controllable transformation of flue gas desulfurization(FGD)gypsum to calcium sulfate hemihydrate(HH)whiskers under facile,readily monitored,and cost-effective hydrothermal conditions could play a vital role in the preparation of high quality HH whiskers and improve our understanding of the transformation process.This work assessed the conversion of FGD gypsum to HH whiskers in 5×10^-4mol/L H₂SO₄and 1.5 wt%CuCl₂at 120℃while determining the effects of temperature as well as H₂SO₄and CuCl₂concentrations on transformation kinetics.The preparation of HH whiskers was found to involve a solution-mediated transformation from the dihydrate(DH)to theα-HH.This transition was determined to proceed via a dissolution crystallization mechanism,the rate of which was controlled by nucleation and growth of the HH whiskers.An autocatalytic kinetic model was established based on variations in the HH whiskers mole fraction over time,and this model accurately fit the experimental data with R2=0.990.Increasing the temperature or H₂SO₄concentration accelerated the transformation by modifying the super-saturation and water activity in the reaction solution,while increasing the CuCl₂concentration had the opposite effect.The hydrothermal conditions had an important effect on the transformation from FGD gypsum to HH whiskers.     

Effects of anionic additives on the rheological behavior of aqueous calcium montmorillonite suspensions

Three different experimental measurements, namely, rheology, particle sizing, and x-ray diffraction (XRD), were used to study the effect of anionic additives on the properties of bentonite suspensions. The three additives were sodium carboxymethylcellulose, xanthan gum, and sodium dodecyl sulfate. Flow curves were obtained from shear stress–shear rate measurements, and the viscoelastic properties were determined from oscillatory and transient measurements. Mineralogical data were evaluated by XRD and the particle size analysis performed by light scattering technique. The presence of the surfactant modifies the face-to-face interactions and yields changes of the mixtures rheological behavior at low deformation rates. Polymers act by coating each clay particle and prevent their agglomeration. Therefore, the additives are responsible for the mechanisms of destructuration and structure reorganization as well as the mixtures viscous and viscoelastic behavior.