Influence of disodium hydrogen phosphate dodecahydrate on hydrothermal formation of hemihydrate calcium sulfate whiskers
Graphical abstract
Introduction
Non-toxic, low cost, and environmentally friendly (Freyer and Voigt, 2003, Imahashi and Miyoshi, 1994, Xu, 2005) calcium sulfate whiskers are used widely as the filler or reinforcing material in plastics, rubber, paper-making, cement, and so on (Wang et al., 2005, Wang and Li, 2006, Zhu et al., 2010). Much research has reported on the hydrothermal formation of calcium sulfate whiskers over the last ten years. For example, using CaSO4·2H2O formed by co-precipitation from CaCl2 and Na2SO4 solutions as the precursor, Luo, Li, Xiang, Li, and Ning (2010) studied the influence of temperature on the formation of CaSO4·0.5H2O whiskers and found that at 130–160 °C, whiskers were produced via the dissolution-precipitation route. Yuan, Wang, Han, and Yin (2008) reported on the hydrothermal formation of CaSO4·0.5H2O whiskers with a diameter of 0.19–2.3 μm and length of 70–100 μm at 120–140 °C, using natural gypsum with high purity and fine particles (diameter smaller than 18 μm) as the raw material. However, the process is costly because a long grinding time is required for the gypsum ore. Xu, Li, Luo, and Xiang (2011) reported on the formation of CaSO4·0.5H2O whiskers from CaCO3-bearing desulfurization gypsum via the acidification-hydrothermal route. They found that acidification rather than sintering of the desulfurization gypsum favored the removal of CaCO3 and the formation of active CaSO4·0.5H2O, which promoted the subsequent hydrothermal formation of CaSO4.0.5H2O whiskers with high aspect ratio.
Some researchers studied the influence of organic surfactants amino trimethylene phosphonic acid (ATMP), cetyltrimethyl ammonium bromide (CTAB), 1,2-dihydroxybenzene 3,5-disulfonic acid (DHBDSA), a mixture of C6–C22 sorbitan esters (CMR-100), citric acid (TCA), sodium dodecyl sulfate, cetyl pyridinium chloride (CPC) and so on or inorganic ions such as Al3+, Mg2+, and SO42− on the formation of CaSO4·2H2O by mixing Ca10F2(PO4)6 and H2SO4 at 80 °C. The presence of CTAB, DHBDSA, CMR-100, CPC, Al3+, and SO42− favored the formation of plate- or column-like CaSO4·2H2O crystals with large size, while the addition of ATMP and TCA decreased the crystal size (Abdel-Aal et al., 2004, El-Shall et al., 2000, Mahmoud et al., 2004, Rashad et al., 2003, Rashad et al., 2004, Rashad et al., 2005, Singh and Middendorf, 2007). Former studies reveal that the growth environment, structure and surface properties of the crystals varied with the addition of organic or inorganic additives, which led to a change in nucleation and growth speeds and produced crystals with different shapes.
This paper reports on an alternative way to produce CaSO4·0.5H2O whiskers with high aspect ratio by co-precipitation of CaCl2 and Na2SO4 solutions at room temperature followed by hydrothermal treatment of the slurry in the presence of minor amounts of Na2HPO4·12H2O. The influence of Na2HPO4·12H2O on the conversion of CaSO4·2H2O to CaSO4·0.5H2O was discussed and a possible mechanism proposed.
Section snippets
Experimental procedure
Commercial analytical grade Na2SO4 and CaCl2 reagents were provided by Beijing Chemical Reagent Factory (Beijing, China). In a typical experiment, 80 mL of 0.2–1.2 mol/L Na2SO4 was added (3.0–6.0 mL/min) to 160 mL of 0.2–1.2 mol/L CaCl2 at room temperature, keeping the stirring speed at 200–350 min−1. The slurry was stirred for 0.5 h and then the CaSO4·2H2O precipitate was filtered, washed and dried at 45 °C for 4.0 h.
The CaSO4·2H2O (Sinopharm Chemical Reagent Co., Ltd., with a purity more than 99.0%,
Influence of Na2HPO4·12H2O on hydrothermal formation of CaSO4·0.5H2O whiskers
Fig. 1, Fig. 2 show the influence of Na2HPO4·12H2O on the XRD spectra and hydrothermal product morphology, respectively. The presence of Na2HPO4·12H2O led to the formation of CaSO4·0.5H2O with poor crystallinity and a Ca3(PO4)2 impurity phase. In the absence of Na2HPO4·12H2O (Fig. 2(a1)–(a3)), irregular CaSO4·2H2O plates were produced within 2.0 h and CaSO4·0.5H2O whiskers with a length of 70–300 μm, diameter of 1.0–10.0 μm and aspect ratio of 40–200 were produced at 3.0 h. In the presence of 8.65 ×
Conclusions
The hydrothermal formation of CaSO4·0.5H2O whiskers from the CaSO4·2H2O precursor was influenced by the existence of Na2HPO4·12H2O. The presence of a minor amount of Na2HPO4·12H2O led to the formation of Ca3(PO4)2 precipitate in the hydrothermal conversion of CaSO4·2H2O to CaSO4·0.5H2O. This accelerated the dissolution of CaSO4·2H2O, led to an increase in super-saturation for the formation of CaSO4·0.5H2O and thus the production of CaSO4·0.5H2O with thinner diameters and shorter lengths.
Acknowledgments
This work was supported by the National Science Foundation of China (No. 51234003, No. 51174125 and No. 51374138) and National Hi-Tech Research and Development Program of China (863 Program, 2012AA061602).
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