Influence of disodium hydrogen phosphate dodecahydrate on

G Model PARTIC-617; No. of Pages 5

ARTICLE IN PRESS Particuology xxx (2013) xxx–xxx

Contents lists available at ScienceDirect

Particuology journal homepage: www.elsevier.com/locate/partic

Influence of disodium hydrogen phosphate dodecahydrate on hydrothermal formation of hemihydrate calcium sulfate whiskers Qing Han a,b , Kangbi Luo a,∗ , Huping Li a , Lan Xiang b a b

College of Chemical Engineering, Kunming University of Science and Technology, Kunming 650500, China Department of Chemical Engineering, Tsinghua University, Beijing 100084, China

a r t i c l e

i n f o

Article history: Received 17 May 2013 Received in revised form 30 September 2013 Accepted 12 October 2013 Keywords: Calcium sulfate whiskers Morphology control Phosphorus Hydrothermal technology

a b s t r a c t The influence of Na2 HPO4 ·12H2 O on the hydrothermal formation of hemihydrate calcium sulfate (CaSO4 ·0.5H2 O) whiskers from dihydrate calcium sulfate (CaSO4 ·2H2 O) at 135 ◦ C was investigated. Experimental results indicate that the addition of phosphorus accelerates the hydrothermal conversion of CaSO4 ·2H2 O to CaSO4 ·0.5H2 O via the formation of Ca3 (PO4 )2 and produces CaSO4 ·0.5H2 O whiskers with thinner diameters and shorter lengths. Compared with the blank experiment without Na2 HPO4 ·12H2 O, the existence of minor amounts (8.65 × 10−4 –4.36 × 10−3 mol/L) of Na2 HPO4 ·12H2 O led to a decrease in the diameter of CaSO4 ·0.5H2 O whiskers from 1.0–10.0 to 0.5–2.0 ␮m and lengths from 70–300 to 50–200 ␮m. © 2013 Chinese Society of Particuology and Institute of Process Engineering, Chinese Academy of Sciences. Published by Elsevier B.V. All rights reserved.

1. Introduction Non-toxic, low cost, and environmentally friendly (Freyer & Voigt, 2003; Imahashi & 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, Han, Yuan, & Qiao, 2005; Wang & Li, 2006; Zhu, Xu, Chen, & Li, 2010). Much research has reported on the hydrothermal formation of calcium sulfate whiskers over the last ten years. For example, using CaSO4 ·2H2 O formed by co-precipitation from CaCl2 and Na2 SO4 solutions as the precursor, Luo, Li, Xiang, Li, and Ning (2010) studied the influence of temperature on the formation of CaSO4 ·0.5H2 O 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 ·0.5H2 O whiskers with a diameter of 0.19–2.3 ␮m and length of 70–100 ␮m at 120–140 ◦ C, using natural gypsum with high purity and fine particles (diameter smaller than 18 ␮m) 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 ·0.5H2 O whiskers from CaCO3 -bearing desulfurization gypsum via the acidification-hydrothermal route. They

∗ Corresponding author. E-mail addresses: [email protected] (K. Luo), [email protected] (L. Xiang).

found that acidification rather than sintering of the desulfurization gypsum favored the removal of CaCO3 and the formation of active CaSO4 ·0.5H2 O, which promoted the subsequent hydrothermal formation of CaSO4 .0.5H2 O 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 SO4 2− on the formation of CaSO4 ·2H2 O by mixing Ca10 F2 (PO4 )6 and H2 SO4 at 80 ◦ C. The presence of CTAB, DHBDSA, CMR-100, CPC, Al3+ , and SO4 2− favored the formation of plate- or columnlike CaSO4 ·2H2 O crystals with large size, while the addition of ATMP and TCA decreased the crystal size (Abdel-Aal, Rashad, & El-Shall, 2004; El-Shall, Abdel-Aal, & Moudgil, 2000; Mahmoud, Rashad, Ibrahim, & Abdel-Aal, 2004; Rashad, Baioumy, & AbdelAal, 2003; Rashad, Mahmoud, Ibrahim, & Abdel-Aal, 2004, 2005; Singh & 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 ·0.5H2 O whiskers with high aspect ratio by co-precipitation of CaCl2 and Na2 SO4 solutions at room temperature followed by hydrothermal treatment of the slurry in the presence of minor

1674-2001/$ – see front matter © 2013 Chinese Society of Particuology and Institute of Process Engineering, Chinese Academy of Sciences. Published by Elsevier B.V. All rights reserved.

http://dx.doi.org/10.1016/j.partic.2013.10.002

Please cite this article in press as: Han, Q., et al. Influence of disodium hydrogen phosphate dodecahydrate on hydrothermal formation of hemihydrate calcium sulfate whiskers. Particuology (2013), http://dx.doi.org/10.1016/j.partic.2013.10.002


ARTICLE IN PRESS Q. Han et al. / Particuology xxx (2013) xxx–xxx

2

Table 1 Composition of whiskers and particles. Element

O Ca S P

Fig. 1. Influence of Na2 HPO4 ·12H2 O on XRD spectra of hydrothermal products. Na2 HPO4 ·12H2 O (mol/L): a, b, c, and d denote 0, 8.65 × 10−4 , 4.36 × 10−3 , 8.72 × 10−3 , respectively; time (h): 3.0; temperature (◦ C): 135; -Ca3 (PO4 )2 , -CaSO4 ·0.5H2 O.

amounts of Na2 HPO4 ·12H2 O. The influence of Na2 HPO4 ·12H2 O on the conversion of CaSO4 ·2H2 O to CaSO4 ·0.5H2 O was discussed and a possible mechanism proposed. 2. Experimental 2.1. Experimental procedure Commercial analytical grade Na2 SO4 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 Na2 SO4 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 ·2H2 O precipitate was filtered, washed and dried at 45 ◦ C for 4.0 h. The CaSO4 ·2H2 O (Sinopharm Chemical Reagent Co., Ltd., with a purity more than 99.0%, Beijing, China) precipitate and Na2 HPO4 ·12H2 O (Beijing chemical works, with a purity more than 99.0%, Beijing, China) were mixed with deionized water to form a slurry containing 3.0–10.0 wt% CaSO4 ·2H2 O and 0–8.72 × 10−3 mol/L Na2 HPO4 ·12H2 O. Then the slurry was transferred to an 80 cm3 Teflon-lined stainless steel autoclave and treated at 110–150 ◦ C for 0–3.0 h. After hydrothermal treatment, the autoclave was cooled to room temperature naturally and the suspension was filtered. The solution was sampled for chemical analysis and the precipitate was washed and dried at 60 ◦ C for 4.0 h. 2.2. Analysis The sample morphology was examined with a field emission scanning electron microscope (FESEM, JSM-7401F, JEOL, Japan). The sample elements were measured with an energy dispersive spectrometer (EDS, GENESIS, EDAX, Utah, USA). The sample structure was determined using an X-ray powder diffractometer (XRD, ˚ D/max 2500, Rigaku, Japan) with Cu K␣ radiation ( = 1.54178 A). The concentrations of Ca2+ , SO4 2− and soluble P were analyzed by ethylene diamine tetraacetic acid titration, barium sulfate precipitation and the chinoline molybdic acid method, respectively. 3. Results and discussions 3.1. Influence of Na2 HPO4 ·12H2 O on hydrothermal formation of CaSO4 ·0.5H2 O whiskers Figs. 1 and 2 show the influence of Na2 HPO4 ·12H2 O on the XRD spectra and hydrothermal product morphology,

Mass in whisker (%)

Mass in particle (%)

1

2

1

2

48.94 28.17 22.89 –

48.69 28.83 23.47 –

55.43 24.24 8.15 12.17

53.30 24.12 7.08 15.50

respectively. The presence of Na2 HPO4 ·12H2 O led to the formation of CaSO4 ·0.5H2 O with poor crystallinity and a Ca3 (PO4 )2 impurity phase. In the absence of Na2 HPO4 ·12H2 O (Fig. 2(a1)–(a3)), irregular CaSO4 ·2H2 O plates were produced within 2.0 h and CaSO4 ·0.5H2 O whiskers with a length of 70–300 ␮m, diameter of 1.0–10.0 ␮m and aspect ratio of 40–200 were produced at 3.0 h. In the presence of 8.65 × 10−4 –4.36 × 10−3 mol/L Na2 HPO4 ·12H2 O, CaSO4 ·0.5H2 O whiskers with lengths of 50–200 ␮m, diameters of 0.5–2.0 ␮m and aspect ratios of 50–200 were produced after 2.0 h of reaction (Fig. 2(b1)–(b3) and (c1)–(c3)). This indicates that the presence of Na2 HPO4 ·12H2 O accelerated the hydrothermal conversion of CaSO4 ·2H2 O to CaSO4 ·0.5H2 O. The whisker length and diameter decreased with increase in Na2 HPO4 ·12H2 O concentration. The whiskers formed in the presence of 8.72 × 10−3 mol/L Na2 HPO4 ·12H2 O were unstable and redissolved partially with increase in reaction time from 2.0 to 3.0 h, accompanied by a decrease in whisker length from 70–150 to 30–70 ␮m (Fig. 2(d2)–(d3)). The hydrothermal products formed in the presence of Na2 HPO4 ·12H2 O were composed mainly of whiskers and a minor amount of irregular particles with typical morphology shown in Fig. 3. An energy spectrum analysis of the selected areas showed that no P was detected in the whiskers and the mass ratio of Ca:S:O was 0.97:1.00:4.22. Ca, S, O, and P were detected in the particles with a Ca:S:O:P mass ratio of 1.65:0.49:6.97:0.90 (Table 1). The occurrence of P in the particle is attributed mainly to the formation of Ca3 (PO4 )2 . 3.2. Influence of Na2 HPO4 ·12H2 O on solution composition Fig. 4 shows the variation of soluble P and the formation of Ca3 (PO4 )2 with hydrothermal reaction time. The gradual decrease in concentration of soluble P with reaction time should be connected with the formation of Ca3 (PO4 )2 . The increase in initial Na2 HPO4 ·12H2 O concentration favored the formation of Ca3 (PO4 )2 . Fig. 5 shows the variation of soluble Ca2+ ([Ca2+ ]) and SO4 2− ([SO4 2− ]) with hydrothermal reaction time. In the blank experiment without Na2 HPO4 ·12H2 O, [Ca2+ ] (0.016–0.017 mol/L) and [SO4 2− ] (0.013–0.015 mol/L) were quite similar and stable and their low concentration is attributed to the low solubility of CaSO4 ·2H2 O under the experimental conditions (Luo et al., 2010). In the presence of Na2 HPO4 ·12H2 O, [Ca2+ ] and [SO4 2− ] increased gradually with increase in reaction time, indicating a faster dissolution of CaSO4 ·2H2 O than precipitation of CaSO4 ·0.5H2 O. Moreover, [SO4 2− ] was higher than [Ca2+ ] and the difference between the [SO4 2− ] and [Ca2+ ] became more obvious at higher initial Na2 HPO4 ·12H2 O concentration. This should be linked to the formation of Ca3 (PO4 )2 precipitate. Compared with CaSO4 ·2H2 O and CaSO4 ·0.5H2 O, Ca3 (PO4 )2 was more stable under the experimental conditions owing to the different Ksp at 135 ◦ C (5.16 × 10−6 for CaSO4 ·2H2 O, 4.53 × 10−6 for CaSO4 ·0.5H2 O, and 9.41 × 10−40 for Ca3 (PO4 )2 ). As shown in Eqs. (1)–(4), Ca2+ and SO4 2− were produced mainly by the dissolution of CaSO4 ·2H2 O




3

Fig. 2. Influence of Na2 HPO4 ·12H2 O on hydrothermal product morphology. Na2 HPO4 ·12H2 O (mol/L): a, b, c, and d denote 0, 8.65 × 10−4 , 4.36 × 10−3 , and 8.72 × 10−3 , respectively; time (h): 1, 2, and 3 denote 0.5, 2.0, and 3.0, respectively; temperature (◦ C): 135.

and consumed by the formation of CaSO4 ·0.5H2 O. The formation of Ca3 (PO4 )2 decreased the Ca2+ concentration and consumed no SO4 2− , thus promoting the further dissolution of CaSO4 ·2H2 O and also leading to an accumulation of SO4 2− . CaSO4 ·2H2 O → Ca2+ + SO4 2− + 2H2 O

(1)

Ca2+ + SO4 2− + 0.5H2 O → CaSO4 ·0.5H2 O

(2)

Na2 HPO4 → 2Na+ + H+ + PO4 3−

(3)

3Ca2+ + 2PO4 3− → Ca3 (PO4 )2

(4)

Fig. 6 shows the variation in super-saturation (abbreviated as S) with reaction time based on the data in Fig. 5. S was defined as the ratio of [Ca2+ ][SO4 2− ] to the Ksp of CaSO4 ·0.5H2 O at 135 ◦ C. The data in Fig. 6 indicates that the presence of Na2 HPO4 ·12H2 O led to an increase in super-saturation of CaSO4 ·0.5H2 O. This favored the nucleation of CaSO4 ·0.5H2 O and promoted the formation of CaSO4 ·0.5H2 O whiskers with thinner diameter and shorter length (El-Shall, Rashad, & Abdel-Aal, 2002, 2005).



4


Fig. 3. Morphology of whiskers and particles. Na2 HPO4 ·12H2 O (mol/L): 8.72 × 10−3 ; time (h): 2.0.

Fig. 4. Variation of soluble P (I) and Ca3 (PO4 )2 (II) with reaction time. Na2 HPO4 ·12H2 O (mol/L): a, b, and c denote 8.65 × 10−4 , 4.36 × 10−3 , and 8.72 × 10−3 , respectively; temperature (◦ C): 135.

Fig. 5. Variation of [Ca2+ ] (I) and [SO4 2− ] (II) with reaction time. Na2 HPO4 ·12H2 O (mol/L): a, b, c, and d denote 0, 8.65 × 10−4 , 4.36 × 10−3 , and 8.72 × 10−3 , respectively; temperature (◦ C): 135.

4. Conclusions

Fig. 6. Variation of S with reaction time. Na2 HPO4 ·12H2 O (mol/L): a, b, c, and d denote 0, 8.65 × 10−4 , 4.36 × 10−3 , and 8.72 × 10−3 , respectively.

The hydrothermal formation of CaSO4 ·0.5H2 O whiskers from the CaSO4 ·2H2 O precursor was influenced by the existence of Na2 HPO4 ·12H2 O. The presence of a minor amount of Na2 HPO4 ·12H2 O led to the formation of Ca3 (PO4 )2 precipitate in the hydrothermal conversion of CaSO4 ·2H2 O to CaSO4 ·0.5H2 O. This accelerated the dissolution of CaSO4 ·2H2 O, led to an increase in super-saturation for the formation of CaSO4 ·0.5H2 O and thus the production of CaSO4 ·0.5H2 O with thinner diameters and shorter lengths. Compared with the blank experiment without Na2 HPO4 ·12H2 O, the existence of 8.65 × 10−4 –4.36 × 10−3 mol/L of Na2 HPO4 ·12H2 O led to a decrease in CaSO4 ·0.5H2 O whisker diameter from 1.0–10.0 to 0.5–2.0 ␮m and length from 70–300 to 50–200 ␮m.




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). References Abdel-Aal, E. A., Rashad, M. M., & El-Shall, H. (2004). Crystallization of calcium sulfate dihydrate at different supersaturation ratios and different free sulfate concentrations. Crystal Research and Technology, 39, 313–321. El-Shall, H., Abdel-Aal, E. A., & Moudgil, B. M. (2000). Effect of surfactants on phosphogypsum crystallization and filtration during wet-process phosphoric acid production. Separation Science and Technology, 35, 395–410. El-Shall, H., Rashad, M. M., & Abdel-Aal, E. A. (2002). Effect of phosphonate additive on crystallization of gypsum in phosphoric and sulfuric acid medium. Crystal Research and Technology, 37, 1264–1273. El-Shall, H., Rashad, M. M., & Abdel-Aal, E. A. (2005). Effect of cetyl pyridinium chloride additive on crystallization of gypsum in phosphoric and sulfuric acids medium. Crystal Research and Technology, 40, 860–866. Freyer, D., & Voigt, W. (2003). Crystallization and phase stability of CaSO4 and CaSO4 based salts. Monatshefte für Chemie/Chemical Monthly, 134, 693–719. Imahashi, M., & Miyoshi, T. (1994). Transformations of gypsum to calcium sulfate hemihydrate and hemihydrate to gypsum in NaCl solutions. Bulletin of the Chemical Society of Japan, 67, 1961–1964. Luo, K. B., Li, C. M., Xiang, L., Li, H. P., & Ning, P. (2010). Influence of temperature and solution composition on the formation of calcium sulfates. Particuology, 8, 240–244. Mahmoud, M. H. H., Rashad, M. M., Ibrahim, I. A., & Abdel-Aal, E. A. (2004). Crystal modification of calcium sulfate dihydrate in the presence of some surface-active agents. Journal of Colloid and Interface Science, 270, 99–105.

5

Rashad, M. M., Baioumy, H. M., & Abdel-Aal, E. A. (2003). Structural and spectral studies on gypsum crystals under simulated conditions of phosphoric acid production with and without organic and inorganic additives. Crystal Research and Technology, 38, 433–439. Rashad, M. M., Mahmoud, M. H. H., Ibrahim, I. A., & Abdel-Aal, E. A. (2004). Crystallization of calcium sulfate dihydrate under simulated conditions of phosphoric acid production in the presence of aluminum and magnesium ions. Journal of Crystal Growth, 267, 372–379. Rashad, M. M., Mahmoud, M. H. H., Ibrahim, I. A., & Abdel-Aal, E. A. (2005). Effect of citric acid and 1,2-dihydroxybenzene 3,5-disulfonic acid on crystallization of calcium sulfate dihydrate under simulated conditions of phosphoric acid production. Crystal Research and Technology, 40, 741–747. Singh, N. B., & Middendorf, B. (2007). Calcium sulphate hemihydrate hydration leading to gypsum crystallization. Progress in Crystal Growth and Characterization of Materials, 53, 57–77. Wang, Z. H., Han, Y. X., Yuan, Z. T., & Qiao, J. H. (2005). Calcium sulfate whiskers preparation and its application. Mining and Metallurgy, 14(2), 38–41 (in Chinese). Wang, Y., & Li, Y. S. (2006). Research situation and development of calcium sulphate whikers. New Chemical Materials, 34, 30–34 (in Chinese). Xu, A. Y., Li, H. P., Luo, K. B., & Xiang, L. (2011). Formation of calcium sulfate whiskers from CaCO3 -bearing desulfurization gypsum. Research on Chemical Intermediates, 37, 449–455. Xu, Z. Y. (2005). Research progress of whisker and its application. Technology and Development of Chemical Industry, 34(2), 11–15 (in Chinese). Yuan, Z. T., Wang, X. L., Han, Y. X., & Yin, W. Z. (2008). Preparation of ultrafine calcium sulfate whiskers by hydrothermal synthesis. Journal of Northeastern University (Natural Science), 29(4), 573–576 (in Chinese). Zhu, Z. C., Xu, L., Chen, G. A., & Li, Y. L. (2010). Optimization on tribological properties of aramid fibre and CaSO4 whisker reinforced non-metallic friction material with analytic hierarchy process and preference ranking organization method for enrichment evaluations. Materials and Design, 31, 551–555.
