Trace NaCl and Na₂EDTA Mediated Synthesis of α-Calcium Sulfate Hemihydrate in Glycerol–Water Solution

Abstract

We report a new class of glycerol−water system to transform FGD gypsum into α-calcium sulfate hemihydrate (α-HH) mediated by trace NaCl and Na 2 EDTA to regulate the phase transformation rate as well as the crystal morphology to achieve crystals with high mechanical strength. NaCl plays a role in accelerating the nucleation and crystal growth process, whereas Na 2 EDTA regulates the morphology of α-HH from columnar to lamellar. The paste made from columnar α-HH with higher aspect ratio deserves higher dry bending/compressive strength of 13.6/37.6 MPa, which is comparable with those prepared in nitrate− water medium, chloride−water medium, or by autoclave method. This study provides an improved alternative of glycerol−water system to transform FGD gypsum into α-HH with controlled morphology, high mechanical strength, and less corrosion to the equipment. We also establish the relationship between the mechanical strength of paste and the aspect ratio of α-HH crystals.

Synthesis of α-CaSO4·0.5H2O from flue gas desulfurization gypsum regulated by C4H4O4Na2·6H2O and NaCl in glycerol-water solution

“…For instance, a-HH particles with a low aspect ratio or spherical shape possess better injectability and mechanic strength, and are preferable for use as bone cements. 9,10 Therefore, in order to prepare a-HH products with low aspect ratios, crystal modiers, among which carboxylic acids are highly effective, 5,11,12 tend to be added during the transformation process. However, in alcohol water solutions, the inuence of crystal modiers on the morphology of a-HH crystals and transformation time from DH to a-HH has been rarely reported.…”

Effect of Molecular Structure of Organic Acids on the Crystal Habit of α-CaSO4·0.5H2O from Phosphogypsum

“…Similarly, Tan et al [22] found that long columnar crystals with uniform diameters could be obtained by adding acrylic acid (CH 2 =CH-COOH), which contained only one COOH and could not convert the α-CaSO 4 •0.5H 2 O from long columnar whiskers to short columnar crystals. However, the crystal habit of α-CaSO 4 •0.5H 2 O can be controlled by citric acid and ethylene diamine tetraacetic acid (EDTA) [28], which contain three and four COOH, respectively. Thus, it can be concluded that the crystal modifier must contain two or more COOH, and the separation between two COOH in the crystal modifier molecule cannot be more than two methylene/methine groups.…”

Aspect ratio-controlled preparation of α-CaSO4·0.5H2O from phosphogypsum in potassium tartrate aqueous solution

“…3 In recent years, high-value utilization of I-PG has drawn signicant attention and so far, the most effective way to achieve high-value utilization of I-PG is to produce a-calcium sulphate hemihydrate (a-CaSO 4 $0.5H 2 O). [4][5][6] To date, methods for preparing a-CaSO 4 $0.5H 2 O crystals from CaSO 4 $2H 2 O mainly include autoclave hydrothermal, [7][8][9][10] atmospheric salt solution, [11][12][13][14] reverse microemulsion 15 and alcohol-water solution. [16][17][18] The autoclave hydrothermal method involves high temperature (usually above 120 C) and certain pressure, 19,20 which are not conducive to industrialization.…”