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Fu-xing Lin, Yan-chi Zhong, Shu-feng Yan, Bi-fan Lin, Jian-hua Wang, Zhi-zhong Su. Effects of Soluble Ions on Hydration of Calcined Flue Gas Desulphurization Gypsum[J]. Chinese Journal of Chemical Physics , 2020, 33(6): 764-768. doi: 10.1063/1674-0068/cjcp1907135
Citation: Fu-xing Lin, Yan-chi Zhong, Shu-feng Yan, Bi-fan Lin, Jian-hua Wang, Zhi-zhong Su. Effects of Soluble Ions on Hydration of Calcined Flue Gas Desulphurization Gypsum[J]. Chinese Journal of Chemical Physics , 2020, 33(6): 764-768. doi: 10.1063/1674-0068/cjcp1907135

Effects of Soluble Ions on Hydration of Calcined Flue Gas Desulphurization Gypsum

doi: 10.1063/1674-0068/cjcp1907135
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  • The influence of various water soluble cations (K$^+$, Na$^+$, Ca$^{2+}$, Mg$^{2+}$) on the hydration of calcined flue gas desulphurization gypsum was investigated. The results show that all cations but Ca$^{2+}$ can accelerate the hydration of bassanite. The final crystal size is not largely influenced by different salts, except for Na$^+$, where the giant crystal with length of $>$130 μm is observed. Current study clarifies the influence of different ions on the hydration of bassanite, which could provide sufficient guide for the pre-treatment of original flue gas desulphurization gypsum before actual application.
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Effects of Soluble Ions on Hydration of Calcined Flue Gas Desulphurization Gypsum

doi: 10.1063/1674-0068/cjcp1907135

Abstract: The influence of various water soluble cations (K$^+$, Na$^+$, Ca$^{2+}$, Mg$^{2+}$) on the hydration of calcined flue gas desulphurization gypsum was investigated. The results show that all cations but Ca$^{2+}$ can accelerate the hydration of bassanite. The final crystal size is not largely influenced by different salts, except for Na$^+$, where the giant crystal with length of $>$130 μm is observed. Current study clarifies the influence of different ions on the hydration of bassanite, which could provide sufficient guide for the pre-treatment of original flue gas desulphurization gypsum before actual application.

Fu-xing Lin, Yan-chi Zhong, Shu-feng Yan, Bi-fan Lin, Jian-hua Wang, Zhi-zhong Su. Effects of Soluble Ions on Hydration of Calcined Flue Gas Desulphurization Gypsum[J]. Chinese Journal of Chemical Physics , 2020, 33(6): 764-768. doi: 10.1063/1674-0068/cjcp1907135
Citation: Fu-xing Lin, Yan-chi Zhong, Shu-feng Yan, Bi-fan Lin, Jian-hua Wang, Zhi-zhong Su. Effects of Soluble Ions on Hydration of Calcined Flue Gas Desulphurization Gypsum[J]. Chinese Journal of Chemical Physics , 2020, 33(6): 764-768. doi: 10.1063/1674-0068/cjcp1907135
  • With the increment in economics, there is a great demand for electricity for China. Coal-fired power plants contribute to more than 70% of total electricity generated each year [1]. However, besides the favorable interests, these thermal power plants also produce great amount of pollutants, such as sulfur dioxide, SO$_2$, which can lead to acid rain [2-5]. In order to reduce the emission of SO$_2$, numerous technologies, such as flue gas desulphurization (FGD), have been implanted in these power plants [6, 7]. Gypsum (CaSO$_4$$\cdot$2H$_2$O) is the dominant product of FGD [8] whose annual production has reached more than 7000 million tons.

    FGD gypsum is actually a kind of "green" materials, However, the processing of FGD gypsum is difficult. Generally, FGD gypsum is calcinated at 110-150 ℃ to transform into bassanite (CaSO$_4$$\cdot$0.5H$_2$O) in industry [9, 10], then can be easily processed. After that, bassanite can be rehydrated and transformed to gypsum again to be gypsum plaster [11] and admixture in cement and concrete [12-18]. However, in the hydration process of bassanite, there are other water-soluble ions in the original FGD gypsum, such as Mg$^{2+}$, Na$^+$, and Cl$^-$. The presence of these soluble ions would influence the hydration process of bassanite, and finally could influence the final properties of reproduced gypsum. Therefore, the systematic investigation of these soluble ions on the hydration of bassanite in aqueous solution is definitely necessary for further application of FGD gypsum. From 2006 to 2012, the amount of FGD gypsum waste and the accumulated stockpile are growing rapidly every year. The usage percent of these waste increased from 40.4% in 2006 to 74.73% in 2012, as shown in Ref.[19].

    Numerous studies have been reported to investigate the influence of the soluble ions on the crystallization of gypsum in aqueous solution. The solubility of gypsum is significantly influenced by NaCl, increasing from initial 0.02 mol/L up to 0.07 mol/L accompanying the NaCl concentration of 1.75 mol/L [20]. Besides the change of solubility, the presence of soluble ions could also influence the crystallization mechanism, i.e., growth rate of gypsum [21]. Meanwhile, the transition from gypsum (CaSO$_4$$\cdot$2H$_2$O) to anhydrous calcium sulfate (CaSO$_4$) happens at elevated temperature. In pure water, this transition temperature is 42 ℃. This value decreases dramatically with the increment of sodium chloride (NaCl) [22]. In summary, the study on crystallization mechanism of gypsum under the influence of soluble ions rarely focused on bassanite. Therefore, the detailed mechanism of the influence of various soluble ions on the hydration of bassanite remains unclear.

    In this study, the influence of various soluble ions on the phase transition from bassanite to gypsum was investigated in detail. The ion chromatography (IC) was firstly used to determine various types of ions in bassanite. Based on this, soluble salts, i.e., NaCl, CaCl$_2$, MgCl$_2$, and KCl with different concentration were prepared in aqueous solution before further mixing with bassanite. The ion conductivity was then applied to in situ monitoring the precipitation process. Afterwards, the crystal morphology of the precipitations was observed by the scanning electron microscopy (SEM), and the crystal structure was determined by X-ray diffraction (XRD). Meanwhile, the ion concentration, i.e., $c$(Ca$^{2+}$), in supernatant was characterized by ion chromatography). The specific surface areas obtained by Brunauer-Emmett-Teller (BET) method were also used to clarify the morphological difference of the precipitations obtained in the presence of different salts.

  • Reagents, including $\beta$-bassanite, NaCl, CaCl$_2$ (anhydrous), MgCl$_2$ (anhydrous), and KCl were obtained from Sigma-Aldrich, and used as received without further purification. Ultra-water with resistivity of 18.2 M$\Omega$$\cdot$cm was used.

  • The hydration of $\beta$-bassanite was conducted at 25 ℃ with concentration of 50 g/L. Different concentrations of different salts were controlled in range of 0-6 mol/L before further mix with $\beta$-bassanite. The mixture was then kept for 3 days to ensure complete hydration of original bassanite and filtered later. The supernatant was used for further analysis, while the precipitant was washed three times by the ultra-pure water before other characterizations.

    The water soluble ions in supernatant were characterized by ion chromatography measurement via Thermo Scientific$^\rm{TM}$ Dionex$^\rm{TM}$ ICS-6000, including ion types and weight percent.

    The conductivity meter (INESA DDSJ-318) was used to in situ track the conductivity of different solutions. The salt aqueous solution was firstly placed in a water bath (25 ℃) to maintain a constant measuring temperature. The data acquisition started once the bassanite was added in the solution. 0.01 mol/L KCl solutions (1408.8 μS/cm, 25 ℃) was used for the conductivity calibration.

    The morphologies of different samples were obtained through Philips XL-30 scanning electron microscope with an acceleration voltage of 20 kV. All samples were coated by gold before SEM measurements.

    The XRD measurements were accomplished on a Bruker D8 Advance X-Ray powder diffractometer using a Cu K$\alpha$ radiation source (1.54 Å) at a scan rate of 0.02 °/s in the range of 5°-80°.

    The specific surface areas of all samples were measured in a Micro-meritics ASAP2020m. The multipoint Brunauer-Emmett-Teller (BET) nitrogen adsorption isotherms (77.3 K) was used to obtain such values. During measurements, all samples were degassed at 80 ℃ for 5 h. All samples were measured three times with an average measuring weight of 1 g.

  • Before further investigation of the influence of ions on the hydration of bassanite, it is necessary to obtain the species of various ions. Besides Ca$^{2+}$ and SO$_4$$^{2-}$, the original bassanite also contains other ions. The ion chromatography was used to screen different species of ions in original bassanite before further experiments. As shown in FIG. 1(a), more than 9 soluble ions are found in original bassanite, including Mg$^{2+}$ (195.9 ppm), K$^+$ (28.32 ppm), Na$^+$ (16.59 ppm), NH$_4$$^+$ (1.4 ppm), PO$_4$$^{3-}$ (5 ppm), Br$^-$ (5 ppm), NO$_3$$^-$ (4.6 ppm), Cl$^-$ (116 ppm), and F$^-$ (119.1 ppm). The combinations of cation and anion are also important for experimental design. FIG. 1(b) summarizes different combinations of these soluble ions, and it indicates that for fluorine salts, the solubility is quite low except KF. The solubility of all chlorine salts is larger than 20 g (20 ℃). Therefore, in following experiment, the chlorine salts are selected as model soluble salts to investigate the influence of ions on the hydration of bassanite.

    Figure 1.  (a) Ions concentration in original bassanite. (b) Solubility of various salts in water.

  • FIG. 2 summarizes the concentration of Ca$^{2+}$ in supernatant as a function of salt adding level. Since CaCl$_2$ itself contains Ca$^{2+}$, the corresponding supernatant is not characterized. For NaCl and MgCl$_2$, the concentration of Ca$^{2+}$ increases first but subsequently decreases. However, for KCl, such concentration increases dramatically from 20 ppm to 120 ppm, which is one order higher than that of other salts. Such a phenomenon again suggests that the presence of KCl leads to a completely different precipitation mechanism. Actually, there is an equilibrium reaction between CaSO$_4$$\cdot$2H$_2$O and KCl in an aqueous solution, as shown below: [9]:

    Figure 2.  (a) The concentration of Ca$^{2+}$ in supernatant as a function of concentration of different salts, and (b) the magnified figure of the dependence of the concentration of Ca$^{2+}$ on the concentration of Na$^+$ and Mg$^{2+}$.

    When the concentration of KCl is very high, it is favorable for the equilibrium reaction to proceed to the right, which means gypsum will dissolve and K$_2$SO$_4$ precipitates from the solution.

    The kinetics study of the hydration process of bassanite was also investigated. The hydration process of bassanite follows the dissolution of original bassanite and the precipitation of new gypsum. As a result, the conductivity would subsequently decrease. Therefore, the conductivity of the solution could be used as an effective indicator for in situ observing the hydration of FGD gyspum.

    As shown in FIG. 3, all salts show an accelerating effect rather than retarding one. Here, the retarding time $t_\rm{ret}$ is defined as the inflection point of the conductivity curve. As shown in FIG. 3, NaCl (1 mol/L) ($t_\rm{ret}$$=$ca. 4.2 min) shows the strongest accelerating effect among all samples, whereas the control sample presents $t_\rm{ret}$$=$ca. 12.1 min. Such accelerating effect is well consistent with previously reported results [20, 23, 24]. The accelerating effect of various salts is: NaCl ($t_\rm{ret}$$=$ca. 4.2 min)$>$KCl ($t_\rm{ret}$$=$ca. 4.5 min)$>$CaCl$_2$ ($t_\rm{ret}$$=$ca. 8.2 min)$>$MgCl$_2$ ($t_\rm{ret}$$=$ca. 8.5 min) in current concentration (1 mol/L). The influence of different additives on the hydration of bassanite has been studied extensively. Different additives, such as organic acids [25] and potassium salt [26], have been found that they can influence the precipitation of gypsum through the influence of dissolution of original bassanite. The ions used here are thought to interact with Ca$^{2+}$, which accelerates the precipitation process.

    Figure 3.  Conductivity measurements of the gypsum precipitation in the presence of various chloride salts.

  • The morphology of crystal precipitations obtained from the aqueous solutions with different salts is shown in FIG. 4. For CaCl$_2$ and MgCl$_2$, the crystal morphology does not significantly change even at the highest salt adding level condition. But for NaCl (4.5 mol/L), the giant crystal with length of $>$130 μm is observed. For KCl, 0.5 mol/L adding level causes minor influence of crystal morphology whereas 4.5 mol/L KCl leads to plate-like structure. As mentioned above, the presence of K$^+$ could react with gypsum to form K$_2$SO$_4$ precipitations. Further detailed interior structure needs to be investigated through XRD, as shown in FIG. 5. Finally, KCl (4.5 mol/L) results in a completely difference XRD pattern. Based on Jade analysis, the crystal structure is determined to be K$_2$Ca(SO$_4$)$_2$$\cdot$H$_2$O (Jade PDF No.28-0739). Therefore, this result further confirms above conclusion that the precipitation obtained in the presence of KCl is not gypsum.

    Figure 4.  SEM photos of the precipitations obtained with different salts in different concentration: (a, b) NaCl, (c, d) KCl, (e, f) MgCl$_2$, and (g, h) CaCl$_2$.

    Figure 5.  XRD patterns of final precipitations in the presence of various salts.

    As compared with pure gypsum (obtained from Shanghai Waigaoqiao Power Generation Co., Ltd.) [27-29], the precipitations with KF only show two broad peaks rather than sharp diffraction peaks. This suggests no gypsum, or even non-crystalline structure is obtained in the presence of KF. MgCl$_2$ (2.5 mol/L) precipitation shows almost the same diffraction peak as compared with pure gypsum, where one minor difference is that the peak is broader. The peak width of XRD is usually correlated with the particle size, and such broad peak suggests the obtained crystal structure is not so uniform. NaCl (4.5 mol/L) presents the same XRD diffraction peak position as pure gypsum. Therefore, the giant crystal obtained in the presence of NaCl (4.5 mol/L) is indeed gypsum. The introduction of NaCl could help us obtain crystal size comparable to that obtained from supersaturated solution (Ca$^{2+}$/SO$_4$$^{2-}$). CaCl$_2$ (4 mol/L) shows the same XRD pattern except the relative intensity, which is correlated with crystal shape [30]. The relative intensity of (021) planes is found to be closely related to the gypsum crystal morphology: the increment of this intensity is associated with the decrement of average longitude and aspect ratio [30].

  • Despite crystal morphology and interior structure, another parameter quantifying the mesoscopic difference among various systems is the specific surface area. FIG. 6 shows the selected nitrogen adsorption isotherms of different samples, as obtained by BET. One caution should be kept in mind is the setting of the degassing temperature. As a hydrated crystal, the gypsum crystal will be dehydrated at high temperature (i.e., $>$100 ℃), whereas it shows almost no specific surface area at low temperature (i.e., $<$40 ℃) [31]. Herein, the degassing temperature is set as 80 ℃ during BET measurements [25, 32]. As a comparison, the specific surface area of original bassanite and pure gypsum (control) were firstly characterized to be 1.73 and 13.85 m$^2$/g, respectively. Since CaCl$_2$ does not significantly alter the crystal morphology of the precipitation, whereas NaCl leads to giant crystal under high adding ratio (4.5 mol/L, FIG. 4), these two samples were chosen as representatives, and the results are shown in FIG. 6. The specific surface area of precipitations in the presence of 4 mol/L CaCl$_2$ and 4.5 mol/L NaCl are 12.66 and 7.39 m$^2$/g, respectively. For 4 mol/L CaCl$_2$, there is no significant difference in specific surface area as compared with that of pure gypsum. Such a result is well consistent with above results where the addition of CaCl$_2$ will not significantly change either crystal morphology or interior crystal structure of the precipitation. And the introduction of NaCl has significantly decreased the specific surface area of the precipitation, whose origin comes from the formation of the giant crystal.

    Figure 6.  The nitrogen adsorption isotherms of different precipitations in the presence or absence of various additives.

  • The influence of various water soluble ions including K$^+$, Ca$^{2+}$, Na$^+$, and Mg$^{2+}$ on the hydration of bassanite was investigated through various characterization techniques.

    (i) All ions here could accelerate the hydration of bassanite into FGD gypsum. (ii) The crystal morphology of final gypsum is not largely influenced by different ions, except for Na$^+$, where crystals with length of several hundred micrometers were observed. (iii) The presence of K$^+$ can increase the solubility of gypsum through the precipitation of corresponding sulphate salt. (iv) The fluorine based salts could lead to the amorphous, or low crystalline precipitation, which is attributed to the low solubility of CaF$_2$.

    Current results provide guidance for the pre-treatment of original FGD gypsum, and partially clarify the influence of different ions on the hydration of bassanite. This is expected to highlight the importance of removal of certain soluble ions before further application of gypsum products.

  • This work was supported by the National Natural Science Foundation of China (No.51473152), Scientific research foundation for Young Talents from Fujian Provincial Department of Education (No.JT180494), Start-up Foundation for Advanced Talents in Sanming University (No.18YG07), Industry-University-Research Cooperation Fund from Sanming Institute of Fluorine Chemical Industry Technology (FCIT20180105), and Scientific research Platform Construction Pproject from Fujian Provincial Department of Science and Technology (No.2018H2002). We also sincerely thank Dr. Wen-xiu Yang, Kun Zeng, Wei Chen, and Prof. Liang-bin Li, Mo-zhen Wang, Xue-wu Ge from University of Science and Technology of China (USTC) for their kind help in both instructions and instrument.

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